DSpace at VNU: The development of mixture, alloy, and core-shell nanocatalysts with nanomaterial supports for energy conversion in low-temperature fuel cells

DSpace at VNU: The development of mixture, alloy, and core-shell nanocatalysts with nanomaterial supports for energy con...

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journal homepage: www.elsevier.com/locate/nanoenergyAvailable online at www.sciencedirect.com

REVIEW

The development of mixture, alloy, and

core-shell nanocatalysts with nanomaterial

supports for energy conversion in

low-temperature fuel cells

Nguyen Viet Longa,b,c,d,e,h,n, Yong Yanga, Cao Minh Thif,

Nguyen Van Minhh, Yanqin Caoa, Masayuki Nogamia,d,g

aState Key Laboratory of High Performance Ceramics and Superﬁne Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Science,1295, Dingxi Road, Shanghai 200050, China

b

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences,

Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka 816-8580, Japan

c

Department of Education and Training, Posts and Telecommunications Institute of Technology, Nguyen Trai,

Ha Dong, Hanoi, Vietnam

Ho Chi Minh City University of Technology, 144/24 Dien Bien Phu, Ward 25, Binh Thach, Ho Chi Minh City, Vietnam

gNagoya Industrial Science Research Institute, Yotsuya, Chikusa-ku, Nagoya 464-0819, Japan

h

Hanoi National University of Education, Vietnam

Received 15 March 2013; received in revised form 16 May 2013; accepted 6 June 2013

Available online 25 June 2013

http://dx.doi.org/10.1016/j.nanoen.2013.06.001

n Corresponding author Tel.: +86 21 52414321;

fax: 86 21 52414219; Mob.: +81(0)90 9930 9504; +84 (0)94 6293304

E-mail addresses: nguyenviet_long@yahoo.com, nguyenviet

long01@gmail.com, nguyenvietlong01@yahoo.com (N.V Long),

mnogami@mtj.biglobe.ne.jp (M Nogami).

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Direct methanol fuel

cell (DMFC)

proton-exchange membrane FCs (PEMFCs) and direct methanol FCs (DMFCs) On the basis of the latest scientific reports and research results, new catalytic models of the possibilities and relations of both Pt-based catalysts and supports, which are typically carbon, glasses, oxides, ceramics, and composite nanosized nanomaterials, are proposed for the further investigation of catalytic surface roles to achieve crucial improvements of Pt-based catalysts The various applications of Pt-based catalysts with specific supports in PEMFCs and DMFCs are also discussed The nanosystems of as-prepared Pt nanoparticles as well as Pt-based nanoparticles with various mixture, alloy, and core-shell structures are of great importance to next-generation FCs Low-cost Pt-based mixture, alloy, and core-shell nanoparticles have been shown to have the advantages of excellently durability, reliability, and stability for realizing FCs and their large-scale commercialization The latest trend in the use of new non-Pt alloys or new alloys without Pt but they have high catalytic activity as the same as to that of Pt catalyst has been discussed We propose a new method of atomic deformation, and surface deformation as well as nanoparticle and structure deformation together with plastic and elastic deformation at the micro- and nano-scale ranges by heat treatments at high temperature can be applied for enhancement of catalytic activity, stability and durability of Pt catalyst and new non-Pt alloy and oxide catalysts in future while the characteristics of size and shape can be retained Finally, there has been a great deal of demand to produce catalytic nanosystems of homogeneous Pt-based nanoparticles because of their ultra-high stability, long-term durability, and high reliability as well as the durable and stable nanostructures of Pt-based catalysts with carbon, oxide and ceramic supports Such materials can be utilized in FCs, and they pose new challenges to scientists and researchers in thefields of energy materials and FCs In addition, the importance of Pt based nanoparticle heat treatment with, and without the nanoparticle surface deformation or nuclei surface deformation is very crucial to discover a new robust Pt based catalyst for alcohol FCs The new urgently trend of producing various novel alloy catalysts replacing Pt catalyst but similar catalytic activity is confirmed in the avoidance of the dependence of Pt-noble-metal catalyst in both the cathode and the anode of FCs

Contents

Introduction 637

Low-temperature fuel cells 638

Fuel cells 638

Proton-exchange membrane fuel cell (PEMFC) 638

Direct methanol fuel cells (DMFC) 639

Platinum catalyst 640

Characterization of Pt- and Pd-based nanoparticles 643

Development of Pt-based catalysts 644

Development of Pt-Ru-based catalysts (PtxRuyand PtxRuy/support) 646

Development of Pt-Rh-based catalysts (PtxRhyand PtxRhy/support) 647

Development of Pt-Au-based catalysts (PtxAuyand PtxAuy/support) 647

Development of Pt-Cu-based catalysts (PtxCuyand PtxCuy/support) 647

Development of Pt-Ni-based catalysts (PtxNiyand PtxNiy/support) 648

Development of Pt-Co-based catalysts (PtxCoyand PtxCoy/support) 648

Development of Pt-Sn-based catalysts (PtxSnyand PtxSny/support) 648

Development of Pt-Fe-based catalysts (PtxFeyand PtxFey/support) 648

Development of Pt-and-Pd-based nanoparticles (PtxPdyand PtxPdy/support) 649

Development of Pt- and Pd-based catalysts with carbon and oxide supports 661

Development of novel alloy-based catalysts (alloy and alloy/support) without Pt 663

Stability and durability 663

Conclusion 664

Acknowledgments 664

References 665

Introduction

Several known direct chemcal-electrical energy conversion

processes in various fuel cells (FCs) with high efﬁciency and

low pollutant emission have been studied[1,2] Recently, the

U S Department of Energy Fuel Cell Technologies Program (DOE Program) and the New Energy and Industrial Technology Development Organization (NEDO Program) in Japan have ﬁnancially supported large research and development pro-grams (R&D) concerning FCs and FC systems for stationary,

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portable, and transportation applications, such as FCs for cars

and vehicles as well as for portable devices, such as laptop

computers and mobile devices[3–7] In PEMFCs and DMFCs, Pt

catalysts are mainly used to provide catalytic activity,

typi-cally for reactions such as oxygen reduction reactions (ORRs)

or hydrogen evolution reactions (HERs)/hydrogen oxidation

reactions (HORs) In addition, very important ORRs have also

been examined in studies of Pt-based catalysts with transition

metals, typically Pt, Ir, Os, Pd, Rh, and Ru, for their

applica-tions for the enhancement of current density in FCs[8–10] Ru,

Os, Rh, Ir, Pd, Pt, Ag, and Au are precious metals of great

importance in catalysis [11] A Pt-based catalyst in the

catalyst layer is one of the main elements of

power-generation membrane-electrode-assembly (MEA) technology

in PEMFCs and DMFCs [7], which utilizes various

proton-conducting membranes, such as Naﬁon-type membranes

[7,18] The roles of the dissolved gases (O2, H2, and N2) in

the solvent medium have been proven via their interactions

and reactions to certain shapes and morphologies of Pt and Pd

nanoparticles[12] The speciﬁc catalytic properties of metal

nanoparticles on various supports with respect to the effects

of size, shape, morphology, porosity, surface, structure,

support, composition, and oxidation state have been discussed

previously [13] Besides, it has been established that the

ability to control particle sizes is very crucial to create good

and robust Pd-based catalysts in place of Pt[14] At present,

various proton-exchange membranes with high quality and

long-term stability are used for FC applications below or above

1001C [15] The best efforts toward improving the

electro-catalytic activity of pure Pt catalysts have been conducted in

the process of testing Pt-based catalysts in DMFCs and

PEMFCs [16] In addition, a large number of the various

support materials for PEMFC and DMFC electrocatalysts have

been reviewed [17,18] Analogously, other noble-metal

electrocatalysts can be used in a promising structural

paradigm for DMFCs [19] The catalyst layer is of great

importance to efforts to decrease the very high cost of FC

products, as it constitutes more than 50% of the cost For

this reason, Pt and Pt-based alloys have been developed for

next-generation PEMFCs and DMFCs[6,20] Pt-based

nano-wires can be used as potential electrocatalysts in PEMFCs

[21] The improved manufacture of Pt nanoparticles with a

well-deﬁned size, composition, and shape via chemistry can

lead to a very good catalyst with high selectivity and thermal

stability, especially in future FCs[22,23] The challenges of

designing Pt-based electrocatalysts have been considered in

the context of automotive FC applications [24] Moreover,

various novel Pt-based catalysts have been proposed for

PEMFCs [25] Proposals and ideas for novel low-Pt-loading

catalysts in PEMFC or DMFC systems have been presented,

and the catalytic activity and stability of ORR catalysts that

use metal and bimetal nanoparticles in various FCs have

been compared [26,27] The avenues for improving Pt- or

Pd-based catalysts involve shape- or size-dependent

cataly-tic activity, instability and surface-area loss, dealloying

phenomena, and the synergistic effects of bimetallic

cata-lysts Incredible differences between the catalytic activity

of a homogeneous catalytic system of synthesized Pt

nano-particles and that of an inhomogeneous catalytic system of

synthesized Pt nanoparticles have been observed in the

relations between their preparation processes, structures

and properties The rapid development of direct alcohol FCs

(DAFCs) has primarily involved the design and discoveries ofnew materials and catalysts [28] The HOR and MORmechanisms have been intensively studied with the goals

of improving the long-term durability, stability and cost ofPEMFCs and DMFCs In particular, the price of FCs mainlydepends on the price of the Pt-based catalysts

or on the design of catalysts that use a low Pt weight or no

Pt at all

In this review, we present the latest developments in prepared Pt-based nanoparticles for use as the Pt-basedcatalysts for alcohol FCs, particularly PEMFCs and DMFCs.The issues of nanosized ranges of Pt-based nanoparticles arediscussed with respect to the related degrees of stabilityand durability in alcohol FCs The advantages of polyhedral-like and spherical-like shapes and morphologies are alsodiscussed for the purpose of identifying the best Pt-basedcatalysts for various applications of growing concern It iscertain that the issues of tuning, controlling, and shapingPt-based nanostructures within certain size and shaperanges are usually much more difﬁcult than controlling themetal compositions of Pt-based nanostructures To achievelarge-scale commercialization of FCs, various designed Ptnanoparticles in the mixture, alloy and core-shell categorieshave been tested to determine whether they meet thedemands of high catalytic activity Importantly, in thisreview, we propose new catalytic models for the use ofpure Pt-based nanoparticles on and inside of supports,leading to improvement in the catalytic activity and sensi-tivity of Pt-based nanoparticles that are loaded on varioussupports, such as carbon, oxide, and ceramic, to maximizetheir durability and stability The excellent recentlyachieved advantages of Pt-based catalysts with mixture,alloy and core-shell nanostructures have been conﬁrmed intesting and measurements

as-Low-temperature fuel cells

Fuel cells

Proton-exchange membrane fuel cell (PEMFC)

In principle, a PEMFC with a polymer membrane electrolyteand a pure Pt catalyst has a low operation temperature ofo90 1C The electrochemical reactions that occur in a low-temperature PEMFC are as follows[29,52]

Cathode: 1/2 O2+2H++2e--H2O (ORR);

Overall reaction: 1/2 O2+H2-H2O (Fuel cell reaction) (2)

At present, Pt-metal catalysts are the most active toward thehydrogen oxidation reaction (HOR) that occurs at the anode inPEMFCs To achieve a low-cost FC design, the very high Ptcatalyst loading must be decreased Two strategies are underinvestigation for reducing the Pt loading in PEMFCs: thefabrication of binary and ternary Pt-based alloyed nanoma-terials and the dispersion of Pt-based nanomaterials ontohigh-surface-area substrates, such as carbon nanomaterials

To reduce the cost associated with pure Pt catalysts, Pt-basedcatalysts have been widely developed At present, carbonmonoxide (CO) poisoning still occurs at the anode, and CO canheavily adsorb on a Pt-based catalyst and block the hydrogen

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oxidation To improve the stability and activity of the HOR on

a pure Pt catalyst, different base metals can be added to

reduce CO poisoning Because of such efforts, Pt- and

Pd-based catalysts with various mixture, alloy, and core-shell

nanostructures have been developed In addition, novel

CO-tolerant catalysts will necessarily be developed in large

amounts but at low cost Thus far, the Pt-based bimetallic

nanoparticles for Pt-based bimetallic catalysts that have been

studied are Pt-Ru, Pt-Fe, Pt-Cu, Pt-Mo, Pt-Ni, Pt-Sn, Pt-Re,

Pt-W, Pt-Ir, Pt-Os, Pt-Rh, Pt-Pd, Pt-Au, and Pt-Ag in mixture,

alloy and core-shell structures[55,376,377]

In addition to various alloy and core-shell nanostructures,

a wide variety of material mixtures can be used for the

development of PEMFCs As a result, the catalytic activity and

stability of Pt-based alloy and core-shell catalysts are greatly

enhanced with respect to those of pure Pt catalysts

In recent research, various binary, ternary and quaternary

Pt-based catalysts using metals such as Pt, Ru, Rh, Pd, Ir, Os,

Au, Ag, Cu, Ni, Fe, Co, Mn, Zn, Mo, Sn, and W have been

prepared for the purpose of obtaining catalysts with higher

catalytic activity and stability[2,56,57] A shape

transforma-tion from Pt nanocubes to tetrahexahedra with a size of near

10 nm, leading to an inﬂuence on the catalytic activity of a Pt

nanoparticle catalyst, was observed in one study [58] In

general, such tetrahexahedral Pt nanoparticles in this size

range had a high density of step atoms They exhibited an

enhancement of electrocatalytic activity toward ethanol

oxidation [58] However, the complexity of the preparation

processes was high These catalysts can be used on various

carbon nanomaterials, such as CNT and Vulcan-XC-72R, to

provide a signiﬁcant enhancement of catalytic activity

Modiﬁed catalysts that do not contain Pt are being considered

to avoid the dependence on Pt metal The use of various

Pt-Ru-alloy/C catalysts in the anodes of DMFCs has been

reviewed previously [56] These catalysts can also be used

as CO-tolerant catalysts in the anodes of PEMFCs In addition,

Pt-alloy catalysts have exhibited improved catalyst behavior

for novel cathodes of both PEFCs and DMFCs[56] Because of

their high stability and durability, Pt-alloy catalysts can be

used for the large-scale commercialization of automotive FCs

[57] The potential PEMFC applications of Pt- and Pt-Ru-based

catalysts of mixed metal nanoparticles have been discussed

previously [2,56], especially those of Pt/C and Pt-Ru/C

catalysts that use ordered mesoporous carbon[2,56,57]

Direct methanol fuel cells (DMFC)

Instead of hydrogen fuel, methanol is used in DMFCs It is

known that DMFCs have a low operation temperature of 40–

1001C when they are constructed using MEA technology

with a proton-exchange membrane, such as Naﬁon, as the

electrolyte, and there is a direct MOR at the anode of

DMFCs Methanol offers advantages over hydrogen as a fuel,

including ease of transportation and storage and a high

theoretical energy density Pt is the most promising

candi-date among the pure basic metals for application in DMFCs

because Pt exhibits the highest activity to the dissociative

adsorption of methanol However, a pure Pt catalyst is

easily poisoned by CO, which is produced as a by-product of

the MOR at room temperature Pt-based catalysts can be

used in the electrodes, both the anode and the cathode In

principle, the operation of a DMFC primarily depends on thechemical reactions at the electrodes, as follows

Cathode: 3/2O2+6H++6e--3H2O (ORR);

Anode: CH3OH+H2O-CO2+6H++6e

To improve the performance of DMFCs and reduce theircosts, Pt-based catalysts must yet be considerably furtherdeveloped Thus far, Pt-Ru-based catalysts have been verysuccessfully used for the cathode reaction in DMFCs, butonly at high cost[59–62] The CO poisoning that is stronglyadsorbed on Pt atoms on the surfaces of Pt-Ru is addressed

by reduction by the Ru metal atoms, leading to the poisoned

Pt surface becoming very active to the MOR with thefollowing bi-functional mechanism[18,59,61]

mechan-Fe, Cu, Cr, or other cheap and abundant metals Thus, thereare many available Pt-based mixture catalysts, includingvarious binary, ternary and quaternary Pt-based catalysts.Accordingly, a Pt-Ru-Rh-Ni-based catalyst has been preparedfor the sake of achieving a high MOR rate in DMFCs, although

it suffers from the high complexity of its composition.Similar trends have led to a reduction in the amount of Ptmetal used [63] and the development of new Pt-basedcatalysts with carbon nanomaterials, such as Pt/C, PtSn/C,PtRu/C, and Pt/CeO2/C [381] as well as Pt/FeRu/C, Pt/NiRu/C, and Pt/CoRu/C[382], with the goal of reducing thetotal cost of DMFCs We must develop Pt-based catalysts, orgood catalysts without Pt, that not only resist CO poisoningbut also prevent it and maintain CO poisoning in the MOR at

a suitable minimal level, or we must develop CO-tolerant based catalysts In CO poisoning, intermediates that aregenerated by oxygen reduction, such as hydroxyl and oxidegroups, canﬁrmly adsorb on the surfaces of the nanocata-lysts, which will decrease the overall performance of thecatalytic activity The issue of CO poisoning can be under-stood by the mechanisms of Pt-COad and a second metalatom, a third metal atom, and so forth reacting with their

Pt-OH groups, e.g., M-OH reacts to form CO This

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characterization is a so-called bi-functional mechanism

(Pt-based bimetallic nanoparticles with alloy and mixture

struc-tures) or a complex multi-functional mechanism (Pt-based

multi-metal nanoparticles) Accordingly, a simple solution is

toﬁnd a suitable metal that acts against CO poisoning and

can be used in based-alloy catalysts The known Pt/C,

Pt-oxides/C, Pt-Ru/C, and Pt-Ru-oxides catalysts are promising

candidates for novel DMFC electrodes The various carbon

nanomaterials used are carbon (C) black, C-nanotubes

(CNTs), C-nanoﬁbers (CNFs), C-nanowires (CNWs), etc., and

the other various supports are oxides, glasses, ceramics,

composites, or mixtures, typically WO3, SnO2, SiO2, etc[64]

The primary issue facing Pt/C-based catalysts is the

corro-sion of the carbon by water over time[65] Therefore, the

investigation of the catalytic behavior of metal-, bimetal-,

and multi-metal-based nanoparticles on carbon supports as

well as novel supports such as glass and ceramics is

important for future developments in nanocatalysis, energy

conversion, PEMFCs and DMFCs At present, single metal

nanoparticles, such as Cu, Ag, Au, Pt, Pd, Ru, Rh, Ir, Os, Ni,

Fe, and Co, and the chemical and physical methods by which

they can be synthesized are of importance to various FC

sciences and technologies[66–69]

FC materials with non-homogeneous sizes and

hetero-morphology from several nm toμm can be synthesized with

the ultrasound method However, the challenge is to obtain

a homogeneous size and morphology[70] At present, pure

ultraﬁne Pt clusters of approximately 0.88 nm on

commer-cial carbon can be used for the DMFC reactions [71]

However, a quick collapse in the nanostructure of Pt

nanoparticles and a corresponding decline in catalytic

activity have been discovered in the particle-size limit of

o1 nm[72] This phenomenon affects the catalytic

sensi-tivity and acsensi-tivity of Pt-based nanostructures Most ultraﬁne

or very small metal nanoclusters exhibit very high catalytic

activity but no conﬁrmed stability or durability; for

exam-ple, very tiny Au and Pt clusters can be used to achieve

much higher ORR rates, but their stability and durability

cannot be reliably conﬁrmed[73] In addition, Pt

nanopar-ticles of approximately 2–5 nm in size on carbon supports

are known to be the best catalysts for ORRs, and very small

Pt clusters exhibit very high catalytic activity for the

four-electron reduction of oxygen molecules [74] At present,

metal and bimetal nanoparticles of different shapes have

high potential for applications in catalysis and energy

[75,76] Among the noble nanoparticles, platinum (Pt) and

palladium (Pd) are of importance for their utilization in the

catalyst layers of PEMFCs and DMFCs in both the cathode

and the anode In general, most noble-metal nanoparticles

can be shaped in size and morphology below or above

1000 nm; sizes of less than 10 or 20 nm are of particular

interest for their excellent potential for application in

catalysis, biology and medicine because of their large

quantum-size and surface effects Small, strong adsorbates

or adsorbents (e.g., I , CO, amines) are crucial for

provid-ing size and shape control durprovid-ing the synthesis of Pd and Pt

nanoparticles[77–80] A Pt/C catalyst that uses

functiona-lized ordered mesoporous carbon has been utifunctiona-lized for

DMFCs[81] In addition, a graphene-nanoplate-Pt catalyst

has been demonstrated to serve as a high-performance

catalyst for DMFCs [82] At the same time, various types

of new membranes have been developed for DMFCs with the

use of the above modiﬁed catalyst layers [83] In manycases, Pd nanoparticles can be replaced with Pt nanoparti-cles, despite alcohol the lower catalytic activity of thelatter The effect of pseudo-halide thiocyanate ions on theseed-mediated growth of Pd nanocrystals has been investi-gated [84,85] In the synthesis of metal nanoparticles viachemistry and physics, most of the metal nanoparticles areshaped to size and morphology ranges of less than 10 nm,approximately 100 nm, and 1000 nm or more [86] In gen-eral, nanoparticles of a homogeneous size and morphologyoffer excellent properties in practical applications Wesuggest that the ability to shape metal nanoparticles orbimetallic nanoparticles of both noble and cheap metals tosize and morphology ranges of approximately 10 nm,approximately 20 nm, and approximately 30 nm is extre-mely important to catalysis and FCs The size and morphol-ogy of Pt nanoparticles or Pt-based nanoparticles can bemaintained by storing these nanosystems in various suitablesolvents The issues of the dissolution of noble metals (Pt,

Au, Pd, Rh, and Ru) from the nanoparticles or the catalysts

in electro-chemical measurements and the experimentalconditions of DMFCs or PEMFCs must be further investi-gated To improve the catalytic activity and durability ofpure Pt catalysts, the speciﬁc effects of synergistic, de-alloying, Janus, and composition effects have been givenparticular consideration in Pt- and Pd-based mixture, alloyand core-shell nanoparticles The most popular nanosystems

of Pt-based catalysts for alcohol FCs, PEMFCs and DMFCs arethose that utilize bimetallic nanoparticles They include

PtxAuy, PtxRhy, PtxPdy, PtxCuy, PtxNiy, PtxFey, and PtxSnyaswell as polymetal nanoparticles, such as PtRuRh Thereduction of the very high cost of pure Pt catalysts thefocus of much current research At the same time, it mayalso be possible to enhance the catalytic activity for theHOR, reduce CO poisoning (by using the various base metalsdiscussed above), and increase the ORR rate (for the sake of

a large current density), all of which contribute to the highstability and durability of FCs The composition of Pt-noble-metal- and cheap-metal-based catalysts can be adjusted toobtain various desirable Pt-based catalysts of high catalyticactivity, high durability and high sensitivity by reducing thelevel of CO poisoning

Platinum catalyst

To date, pure Pt catalysts have been the most commonlyused catalysts for FCs Novel FC catalysts (Pt-based cata-lysts and carbon or oxide supports) have been undercontinuous development, but they have not completelyreplaced Pt catalysts In recent years, Pt nanoparticlecatalysts have played a key role in the sustainable hydrogeneconomy because Pt is the best catalyst for the hydrogenoxidation and oxygen reduction reactions at the anodes andcathodes of FCs [23,29] At present, many metal nanopar-ticles, such as Au, Ag, Pt, Pd, Cu, Rh, Rh, Ru, Ni, Co, Fe, and

Mo nanoparticles, are used in electrocatalysis The trolled synthesis of noble- and cheap-metal nanoparticles inparticular size ranges of 10 nm (1–10 nm), 20 nm (1–20 nm),

con-30 nm (1–con-30 nm), etc with well-controlled shapes andmorphologies, such as the polyhedral and polyhedral-likecategories (tetrahedra, octahedra, cubes, etc.) and the

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spherical and spherical-like categories, is critical for FC

electrocatalysts in DMFCs and PEMFCs, especially the

synth-esis of Pt nanoparticles in the range of 10 nm[1,2,29–31]

However, among the above-listed elements, Pt noble

metal is known to be the best metal electrocatalyst for

the FC reactions, which include the hydrogen evolution

reaction (HER)/hydrogen oxidation reaction (HOR), the

oxygen oxidation reaction (ORR), and the

electro-oxidation of carbon monoxide (CO) The issue of CO

poison-ing in the HOR on pure Pt catalysts is well known The effect

of such CO poisoning of Pt-based catalysts at the anode

should be signiﬁcantly reduced to achieve the best FC

performance To minimize CO poisoning, Pt-based catalysts

can be heated at high temperatures in N2/H2[134,135] As

another method of dealing with this problem, Pt-based

bimetallic and multi-metal nanoparticles have been

devel-oped for use in the catalyst layer[389] In this way, the CO

poisoning will be controlled by reduction by a second metal,

a third metal, or even more This is an excellent method of

avoiding and preventing anode failure In addition, highly

CO-poisoning-resistant catalysts must be developed for the

anode or fuel electrode The surfaces of the Pt

nanoparti-cles are very important to nanocatalysis Polyhedral Pt

nanoparticles typically exhibit mainly low-index facets of

(100), (110), and (111), although they do include high-index

facets, (h k l) Nevertheless, in the typical TEM method, a

certain number of the (h k l) planes of the low- and

high-index planes was determined in the selection rule for the

various types of fcc crystal structures because of the

limitations of the TEM method In addition, certain peaks

have been characterized as (111), (200), (220), (311), and

(222) peaks by the XRD method[32] The dependence of the

catalytic activity on the surfaces of the prepared Pt

nanoparticles has been determined for various categories

of the Pt nanostructures[33–35] Furthermore, catalytically

active Pt atoms belonging to the low-index facets of (111),

(100) and (110) of the Pt nanoparticles have been shown to

have high stability and durability in nanocatalysis, as

indicated by electrochemical measurements, and good

reconstruction in the highest catalytic reactions in various

FCs[36–38]

Pt nanoparticles that have been engineered via facile and

successful preparation methods based on chemistry and

physics can be used for FC applications, such as PEMFCs

and DMFCs[66–70,155] In typical electrochemical

measure-ments of pure Pt catalysts, the electrode is usually swept

from E=- 0.2 to E=1.0 V with respect to the saturated

hydrogen electrode (SHE) In such measurements, speciﬁc

regions are observed in the cyclic voltammogram that

exhibit catalytic activity and surface kinetics for the case

of pure Pt catalysts Regarding hydrogen catalytic activity,

the HER on the Pt catalyst is described by the important

Volmer, Tafel, and Heyrovsky mechanisms In addition, the

Volmer-Tafel and Volmer-Heyrovsky mechanisms also occur

in the complex combinations of the basic mechanisms As a

rule, the surface kinetics and chemical activity that occur at

the surface of electrodes that contain pure Pt catalysts as

well as those that contain mixtures of Pt/supports are

characterized by the catalytic activity of Pt with respect

to hydrogen and oxygen atoms and to water[1–10,39–41] In

the mechanisms of the catalytic activity, selectivity and

sensitivity of pure Pt nanoparticles with respect to

hydrogen, there are surface chemical and physical ences that arise as the pure Pt-metal catalyst changes intoPtO oxide, but these differences may exist only on thesurfaces The important effects of the thickness of the PtOoxide, which are relevant to the inverse change of the PtOoxide into pure Pt-metal catalyst during the general pro-cesses at the electrodes, have not yet been thoroughlyexamined Therefore, further study of the formation of Pt-Hand Pt-O through the catalytic activity of Pt with H and O isvery crucial and may lead to the enhancement of thecatalytic activity of Pt-based catalysts, which, in turn, willimprove low-temperature FCs, PEMFCs and DMFCs In acidicelectrolytes, the ORR is observed to follow two mainpathways, one with four electrons transferred directly andone with two electrons transferred consecutively[39–41] Toevaluate the catalytic activity of the prepared nanocata-lysts, the electrochemical active surface area (ECA) of thevarious pure Pt-based catalysts has been calculated to be

differ-QH0.21 LPt[39] Here, the speciﬁc charge transfer (QH)due to hydrogen adsorption and desorption is calculated as

QH=(QT QDL)/2, where QT denotes the total amount ofcharge during hydrogen adsorption and desorption on Ptsites, and QDL is related to the charge due to the double-layer capacitance The area within the curve in the relevantregion can provide QTand QDLand can be obtained by takingthe area under the same region, but with upper and lowerboundaries of horizontal lines passing through a data pointoutset of the hydrogen desorption/adsorption waves Aconversion factor of approximately 0.21 (in mC cm 2) can

be used for a monolayer of hydrogen In this context, thevalue of LPtcorresponds to the loading of the Pt catalyst onthe glassy carbon surface (in mg cm 2)[39–41]

As discussed previously, the catalytic characterization of

Pt nanoparticles has involved investigation into the size,shape, morphology, and structure of Pt nanostructures aswell as various effects of composition modiﬁcations Toobtain a performance enhancement with respect to pure Pt

Figure 1 Trends in oxygen reduction activity as a function ofthe oxygen binding energy The highest ORR of Pt and Pd basemetals are theoretically calculated in the proof as importantfactor to enhance the current density in fuel cells, PEMFC andDMFC Reprinted with permission from: J.K Nørskov, J Ross-meisl, A Logadottir, L Lindqvist, J.R Kitchin, T Bligaard,

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catalysts, Pt-based catalysts of various categories, such as

mixture, alloy, and core-shell structures, have been

devel-oped In the operation of FCs, the most important concerns

are that the ECA and the QHshould be high and that the

loading LPt should be low At present, the phenomena of

the ORR kinetics and mechanisms that occur on Pt catalysts

have been intensively investigated, but a very high

over-potential loss has been observed, indicating that hydrogen

peroxide (H2O2) is formed before the formation of water

molecules Therefore, very high loadings of Pt must be

used for the operation of FCs with large currents It is

known that the Pt catalyst has exhibited the highest

activity with respect to the ORR mechanism In recent

years, much research has been conducted with the goal of

understanding the ORR in catalytic systems with Pt

cata-lysts that are designed to use the minimal level of

ultra-low Pt loading However, the challenges of ultra-low Pt-catalyst

loading, high performance, durability, and cost-effective

design in FC systems remain very crucial for the large-scale

commercialization of such systems The cyclic

voltamme-try (CV) results of various Pt nanoparticles (spherical,

cubic, hexagonal and tetrahedral-octahedral

morpholo-gies) in HClO4 or H2SO4 have illustrated the strong

struc-tural sensitivity of as-prepared Pt nanoparticles The most

basic and stable (111), (100) and (110) planes with high

densities of highly active Pt atoms have been conﬁrmed in

the active sites of speciﬁc catalytic activity, such as in the

edges, corners, and terraces [36,111,112] Further

inves-tigations of the HER, HOR, and ORR mechanisms in

as-prepared catalyst layers are crucial to obtaining high

currents Nanostructured catalysts must have high

hydro-gen solubility and reactivity In addition, fast, sensitive,

and stable hydrogen desorption/adsorption could be very

crucial for FCs Currently, catalysts that are designed for

FCs must have a high and stable ORR rate To obtain large

current densities in PEMFCs and DMFCs, we must study the

ORR mechanism in detail and develop novel Pt-based

catalysts Interesting studies have performed

density-functional-theory (DFT) calculations of the energies of

the surface intermediates for a number of metals, both

expensive, rare, noble metals such as Pt, Pd, Au, and Ag

and abundant, cheap metals such as Cu, Ni, Fe, and Co, as

shown in Figure 1 [42,43] As a result, a clear

volcano-shaped relationship was established between the rate of

the cathode reaction and the oxygen-adsorption (Oad)

energy From this useful model, which involves the

d-band center or d-state of various base metals, Pt and Pd

were determined to be the two elements that are the best

choices for cathode materials (Figure 1) It is likely that a

Pd-based catalyst can replace a Pt-based catalyst in the

cathode for the ORR in PEMFCs and DMFCs, reducing the

dependence on Pt, which is an expensive and rare precious

metal[42–44] To enhance the catalytic activity, stability

and durability of the catalysts, various Pt/support

cata-lysts have been studied as part of the continuous

devel-opment of low-temperature FCs, such as PEMFCs and

DMFCs Pt, Pd, and Pt- and Pd-based bimetallic

nanopar-ticles with sizes of 10 nm and 20 nm on

carbon-nanomaterial supports have been evaluated for potential

applications involving the direct methanol oxidation

reac-tion (MOR) It has been found that pure Pt nanoparticles

must be highly dispersed on the supports to obtain the best

catalytic activity for the operation of FCs In catalystengineering, the microwave-assisted polyol method hasbeen used for the preparation of Pt/C, Ru/C and PtRu/Ccatalysts for the MOR [45] PtRu/C electrocatalysts andPtRu-graphitic mesoporous carbons (GMCs) have beensynthesized for evaluation for MOR applications Theresults indicate that the role of the various pore sizes ofthe GMCs is especially important in determining theperformance of DMFCs [46] In one study, it was foundthat the electrodeposition of Au, Pt, and Pd metal nano-particles on carbon nanotubes (CNTs), such as single-walled CNTs (SW-CNTs), could be performed via a two-electrode arrangement An issue of concern for Pt/Ccatalysts is the corrosion of the carbon by water, whichcan cause a signiﬁcant decrease in the catalytic activity inboth PEMFCs and DMFCs In the future, we expect thatnovel supports (metals, alloys, oxides, and ceramics) withthe same catalytic activity as carbon will be developedthat can replace the carbon supports However, carbonsupports are of importance to low-temperature FC cata-lysts [47] Accordingly, a microwave-heated polyol synth-esis of a Pt/CNTs catalyst for methanol electro-oxidationhas been presented [48] Pt is expensive, and Pd can beused to replace Pt in many cases [49–51] Some authorshave proven that by adding a very small amount of Pt (5 at

%) to a Pd-based catalyst, the HOR activity of the Pd-basedcatalyst can increase to nearly the same as that of a pure

Pt catalyst These results can serve as a foundation for thesuitable utilization of noble Pt metal in FCs[52,378], forexample, Pd-based electrocatalysts with a thin layer of Pt(5 wt%) [378] The catalytic properties of as-prepared Ptnanoparticles are strongly affected by the nature of theirsurface structure and internal structure, including factorssuch as roughness, sharpness,ﬂatness, smoothness, poros-ity, the atomic density of the particle surface, chemicalbonding, and chemical and structural changes[53] In therecent research, Pt-based metallic and bimetallic nano-particles with alloy, core-shell, and mixture nanostructureshave been synthesized and developed for the purposes ofcatalysis, energy conversion, environmental friendliness,and FCs In Figure 2, pure as-prepared Pt nanoparticlesthat were prepared with shape-controlled synthesis areshown to be in the size range of 10 nm with a highlyhomogeneous distribution in size and morphology, which isrequired for a good characterization of a catalytic system

In most cases, the chemical reactivity can be increasedthrough nanostructuring because of the resultant increase

in the ratio of reactive surface atoms to non-participatingbulk atoms In a particle with a diameter of 20 nm, onlyapproximately 10% of the atoms are on the surface, while

in a particle with a diameter of 1 nm, the proportion ofreactive surface atoms is approximately 99% [54] Innature, a given number of larger Pt particles has a muchhigher durability and stability than the (larger) number ofsmall Pt nanoparticles with the same total weight of Ptmetal, but their catalytic activity is much smaller thanthat of the small Pt particles In other words, a nanopar-ticle with a very small particle size, in the range of 10 nm,has a larger relative number of surface atoms and there-fore a higher catalytic activity compared to a largerparticle, but it is clear that very small particles have lessstructural durability and stability Because of the high

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quantum-size effect, the optimization of these factors is

extremely important to improve the catalytic activity and

the cost of the Pt-metal loading of catalysts

Characterization of Pt- and Pd-based

nanoparticles

At present, Pt- and Pd-based nanosystems that can be

prepared with simple chemical methods with homogeneous

morphology, shape, size, and structure in the nanosized range

of 10 nm are extremely important to catalysis and

next-generation FCs [87–89] The shape-controlled synthesis of

single metal nanoparticles, with an emphasis on various Pt

nanostructures, is discussed, along with its crucial role in the

electrocatalysis of anodic reactions in PEMFCs A clear shape

and size–dependence of catalytic activity has been

demon-strated [33] Various methanol-tolerant Pd nanocubes have

been compared in terms of their catalytic activity for ORR in

H2SO4 electrolyte, showing that the catalytic activity of all

types of Pd nanocubes is better than that of normal Pd

nanoparticles[88] The MOR can be used as a probe reaction

on Pt dendrites and cubes to determine their effect on the

MOR as a function of particle shape and morphology[90] Pt/C

catalysts have been very successfully used for PEMFCs[91] A

more active Pt/C catalyst for DMFCs has been developed[92]

However, for the 10-nm size range, hetero-characterizations

of shape, morphology, size, structure, and surface are alsovery crucial to understanding the natures of nanoclusters,nanocrystals and homogeneous and non-homogeneous nano-systems for FCs The ability to control the size and shape of Pt

or Pd nanoparticles is very crucial to catalysis and FCs,especially DMFCs and PEMFCs[93–96] A nanoporous Pd rodcatalyst was developed for MOR [97] In general, ethyleneglycol (EG) (and other various alcohols) has been shown to beone of the most important organic compounds used as achemical intermediate in a large number of industrial pro-cesses EG can be used as the reducing agent or the solvent forthe successful syntheses of various metal, bimetal, or multi-metal nanoparticles [98] The kinetics of the ORR on a Pdcatalyst in acid media is very crucial to enhance the perfor-mance of PEMFCs [99] The effect of the nature of theprecursor has been investigated with regard to the perfor-mance of Pd-Co catalysts for DMFCs [100] A strategy forcontrolling bimetallic nanostructures, e.g., Pd-Au, by seed-mediated co-reduction has been proposed [101] Structure-sensitive catalysis in Pt catalysts has been conﬁrmed in the(111), (100), and (110) low-index facets[102,103] The shape-dependent catalytic properties of Pt nanoparticles depend ontheir speciﬁc crystal nanostructures[104] However, the high-index facets of metal nanoparticles, such as the (557) and(730) surfaces of Pt nanoparticles, also play a crucial role inmorphology-dependent catalysts [105] Tetrahexahedral Ptnanoparticles of high catalytic activity with average sizes of

Figure 2 (a)-(f) TEM images of the uniform Pt nanoparticles by a modiﬁed polyol method The homogeneous polyhedral Ptnanoparticles of main tetrahedral, cubic, and octahedral morphologies and truncated shapes and morphologies were clearly observed inthe size range of 10 nm This proves that highest quantum-size effect in catalytic activity, sensitivity, and selectivity are achieved in thesize range of 10 nm The surface attachments among the Pt nanoparticles were observed Scale bars: (a)-(c) 100 nm; (d) 50 nm; (e) and(f) 20 nm Reprinted with permission from: N.V Long, C.M Thi, M Nogami, M Ohtaki, Novel issues of morphology, size, and structure of

Pt nanoparticles in chemical engineering: surface attachment, aggregation or agglomeration, assembly, and structural changes, New J.Chem 36 (2012) 1320-1334[53] Copyright © 2012 Royal Society of Chemistry (RSC), Thomas Graham House

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53, 100, 126, and 144 nm exhibit 24 high-index facets, such as

the {730}, {210}, and/or {520} surfaces, with a large density of

atomic steps and dangling bonds[106] Because of the higher

concentration of surface steps, kinks, islands, terraces, and

corners in their surfaces and morphologies, superior catalytic

performance has been obtained for un-sharp Pt nanoparticles

[107,108] The instability and surface-area loss of Pt/C

electrocatalysts at high voltages in low-temperature FCs have

been shown[109] Pt and Pt-based catalysts have both been

considered for use in low-temperature FCs Meso/nanoporous

metal structures possess much higher surface areas and higher

catalytic activities than non-porous catalysts, but their

stabi-lity and durabistabi-lity have yet to be proven[110] Highly active

selective Pt-based catalysts have been proposed, with an

emphasis on the various roles of the kinks and steps[35]and

the active sites on Pt nanoparticles[108,109]

It is remarkable that the complexity of the surface,

surface structure and morphology of Pt nanoparticles has

been clearly conﬁrmed In addition, various possible visible

(h k l) indices have been assigned to fcc structures of

speciﬁc (h k l) low and high indices, including (111), (200),

(220), (311), (222), (400), (311), (420), (422), (333), (511),

(440), (531), (442), (600), (620) and (533)[32] The

synth-esis methods for nanoparticles of Pd and its alloys have been

discussed in the context of FC applications [52,113] The

general methods for the shape-controlled synthesis of Pd

nanocrystals in aqueous solutions with size and morphology

control have been presented previously [114] Clearly, Pt

and Pd nanoparticles are of importance to catalysis Their

size, shape and morphological transformations are crucial

for the long-term stability of Pt- and Pd-based FC

technol-ogies The complex morphologies of Pt and Pd nanoparticle

catalysts over the entire nanoscale have been studied from

both theoretical and experimental perspectives The

rela-tive stability of nanoparticles of various shapes and sizes has

been found for various Pt and Pd nanostructures [115]

Hollow-structured nanoparticles with an appropriate

void-to-total-volume ratio can be stable at high temperatures

with respect to an increasing stable-void size with

increas-ing temperature[116,117] Pd-based catalysts are used for

alcohol oxidation in half-cells and in direct alcohol FCs

[118] In the DFT method, computational results of the

catalytic reactions at the surfaces are used for comparison

with experiments The catalytic activity may be tuned by

engineering the electronic structure of the active surface by

changing its composition and structure [43,44] Bimetallic

Pd-Pt nanoparticles have exhibited a signiﬁcantly enhanced

electrocatalytic activity with respect to pure Pt or Pd

nanoparticles [119] At present, DMFCs are a key enabling

technology for the direct conversion of chemical energy into

electrical energy Using DFT calculations, the ORR on model

electrodes has been studied The reactivity has been found

for a set of monometallic and bimetallic transition-metal

surfaces, both ﬂat and stepped, that included Pt-based

alloys with Ru, Sn, and Cu as well as non-precious alloys,

overlayer structures, and modiﬁed edges Pt-Cu surfaces are

promising anode catalysts for DMFCs [120] At present,

various metals (Pt, Ru, Rh, Pd, Os, Ir, Au, Ag, Fe, Co, Ni,

Cu, and Mn) are recognized as promising electrode materials

for FC anodes because of the predictions of quantum

mechanical calculations and DFT, especially in SOFCs

[121] Because of the modiﬁcations of the surface catalytic

properties of noble-metal surfaces induced by the loying phenomenon, the electrocatalytic Pt mass activity ofdealloyed Pt-Cu core-shell particles for the ORR is higherthan that of a Pt electrocatalyst by more than a factor of 4,and it therefore meets the performance targets for FCcathodes[122,123] The catalytic activity and selectivity forhydrogen of various Pt(h k l) facets of Pt catalysts have beenconﬁrmed, especially the hydrogen adsorption on Pt(100),Pt(111), and Pt(110), because of the speciﬁc characteriza-tion of the corresponding Pt-metal nanoparticles [36,124–

deal-127] Meso-structured Pt ﬁlms have been prepared thatexhibit high catalytic activity and stability for the ORR

[128] Nanostructured tungsten-carbide/carbon compositeshave been synthesized with a microwave-heating method toserve as supports for platinum catalysts for methanoloxidation[129] Highly dispersed Pt nanoparticles supported

on poly(ionic liquid)-derived hollow carbon spheres havebeen studied for the enhancement of the MOR[130] Hollowgraphite carbon spheres have also been used as Pt-catalystsupports in DMFCs [131] The signiﬁcant inﬂuence of theproperties of CNF supports on the ORR behavior has beenobserved in proton-conducting-electrolyte-based DMFCs

[132] At present, the design of lyst-support materials with “dense-erythrocyte-like (DEL)”and “hollow-porous-microsphere (HPM)” morphologiessynthesized by spray drying is under development for high-performance PEMFC applications[133]

Pt/C-based-electrocata-Development of Pt-based catalysts

In all research on Pt-based catalysts in FCs, the catalyticactivity, reliability, durability and stability of the preparedPt-based catalysts should be evaluated alongside theiraccompanying reduction of the high cost of such catalysts

In one work, the authors have successfully synthesized Ptnanoparticles with various modified polyol methods withsize-and morphology-control processes[53] However, thesize of the Pt nanoparticles must be controlled within thesize range of 10 nm or 20 nm, while the uniform shapes andmorphologies must be controlled to manifest in spherical-like and polyhedral-like shapes and morphologies forbetter catalytic behavior In one study, pure Pt catalystswith as-prepared Pt nanoparticles of both polyhedral-likeand spherical-like morphologies were investigated for theircatalytic properties in methanol The results showed thatthe electrocatalytic performance of spherical-like andrough Pt nanoparticles is better than that of polyhedraland sharp Pt nanoparticles Pt nanoparticles have specificfringe lattices, 0.910 nm for the distance of the (100)planes, and 0.235 nm for the distance of the (111) planes.Special surface-dependence catalytic properties have beenconfirmed for various polyhedral and polyhedral-likemorphologies as well as spherical and spherical-likemorphologies in the size range of 8-16 nm (20 nm) for both

of the two cases above, with ECA values of10.53 m2

/g forthe sharp and polyhedral-like Pt nanoparticles and 14.370

m2/g for the un-sharp and spherical-like Pt nanoparticles.The catalytic activity of the as-prepared Pt nanoparticleswas measured in a 0.1 M HClO4+1 M CH3OH solution Astable voltammogram was attained after 10 cycles ofsweeping a potential range of -0.2 to 1.0 V for both

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samples The two typical oxidation peaks can be clearly

observed; one is between 0.6 and 0.7 V in the forward

scan, and the other is at approximately 0.50 V in the

reverse scan The two peaks are directly related to the

oxidation of methanol and its associated intermediate

species In the reverse scan, the oxidation peak at 0.50 V

can be related to the removal of the residual carbon

species formed in the forward scan The peak current

density in the forward scan represents the activity of a

catalyst during CH3OH dehydrogenation For the prepared

catalyst samples, the peak current density for the Pt

nanoparticles with non-sharp and spherical shapes

(14.90x10-4A/cm2) was conﬁrmed to be 1.51 times higher

than that for the Pt nanoparticles with sharp and

poly-hedral shapes (9.90x10-4 A/cm2) The heat treatment of

as-prepared Pt nanoparticles has been conﬁrmed to

pro-mote advantageous electrochemical features [134] Heat

treatment plays an important role in improving catalytic

activity First, the as-prepared PVP-Pt nanoparticles must

be washed and cleaned to remove the PVP Then, the pure

Pt nanoparticles must be heated at 3001C or higher In this

way, the sizes, shapes, and morphologies of the polyhedral

Pt nanoparticles can be maintained during such high heat

treatment However, the removal of PVP only by heat

treatment at 300 1C without washing causes a signiﬁcant

variation in the Pt nanoparticles, creating a sharply

poly-hedral shape and morphology Therefore, methods that are

capable of removing the PVP without changing the

char-acterization of the as-prepared Pt nanoparticles are

strin-gently required to obtain good catalytic performance The

self-aggregation and assembly of the as-prepared

polyhe-dral Pt nanoparticles have been studied in the formation of

large Pt particles accompanied by the decrease of catalytic

activity [135,136] Therefore, the physics and chemistry

methods for controlling the size and morphology of Pt

nanoparticles for electrocatalysis in FCs are very important

to the overall performance of the catalytic system [137–

139], for example, to ensure that the prepared Pt

nano-particles are within the desirable size range of 10 nm

(1-10 nm), 20 nm (1-20 nm), or 30 nm Therefore, the

synth-esis and characterization of metallic and bimetallic

nano-particles with alloy, core-shell, and mixture nanostructures

are of great importance to electrocatalysis, energy

con-version, and FCs Thus far, we have successfully achieved

size and shape control during the synthesis of various Pt,

Rh, Pd NPs or Pt-Pd and Pt-Au bimetallic nanoparticles for

catalysis and direct energy conversion by utilizing modiﬁed

polyol methods Certainly, both precious and cheaper

metal nanoparticles, typically Au, Ag, Fe, Co, etc., can

be synthesized for catalysis, energy and FCs Therefore,

metal-, bi-metal- and multi-metal-based NPs show promise

for practical applications Facile methods of synthesis

based on chemistry and physics can be used to prepare

NPs with controlled sizes, shapes, morphologies, and

structures In particular, Pt- and Pd-based nanoparticles

as well as the combination of Pt and Pd NPs of certain

sizes, shapes, structures, and compositions have great

prospects for use in PEMFCs, DMFCs, and other energy

applications Catalysts that use Pt- or Pd-based NPs can

improve future FCs Therefore, continuous efforts are

ongoing to engineer Pt- and Pd-based alloy and core-shell

NPs in a variety of compositions using multi-metals (Co, Ni,

Fe, Cu …) as well as oxides, ceramics, and glasses.Scientists must study the catalytic activity, selectivity,durability, and stability of these materials for application

to next-generation FCs Pt clusters, nanoclusters, andnanoparticles can be synthesized with simple chemistryand physics methods, such as the modiﬁed polyol method,with or without the assistance of chemical compounds(typically sodium iodide or silver nitrate) for controllingthe synthesized nanostructures Pt clusters, nanoclustersand Pt nanoparticles can be easily synthesized in thenanosize range of approximately 10 nm or 20 nm Cer-tainly, the issues of size and morphology must be inten-sively studied in further catalytic investigations of Ptnanoparticles of 10 nm and 20 nm, or even 30 nm or larger,

to conﬁrm the catalytic durability, stability, and activity ofPt-NP-based catalysts In this respect, the research results

on the use of a low-weight loading of Pt metal in novelrobust and efﬁciently designed catalysts provide a goodfoundation for the large-scale commercialization of FCs.Therefore, the controlled synthesis of Pt- and Pd-basednanoparticles is crucial to reducing the Pt and Pd loadingcatalysts while preserving large quantum-size or shapeeffects Such nanoparticles can be used as catalytic Ptand Pd shells for important bimetallic nanosystems toreduce the high costs of FC systems that use noble-metalcatalysts In addition, the controlled synthesis of metal,bimetal, multi-metal, and multi-component NPs has beenstudied in the ﬁelds of catalysis, biology, and medicine.Here, we mainly focus on the controlled synthesis of novelPt- or Pd-based alloy or core-shell nanoparticles, or

“metal-based core-shell nanosystems” with metal or oxide shells, and their potential applications It iscertain that bimetallic or multi-metallic nanoparticleswith novel homogeneous alloys and core-shell nanostruc-tures (e.g., Pt-Fe-, Pt-Cu-, and Pt-Ni-based catalysts) can

noble-be easily synthesized However, at present, homogeneouscore-shell nanosystems pose considerable challenges toresearchers and scientists Accordingly, other authors havereported a study of the lattice-strain control of thecatalytic activity in the dealloying of core-shell Pt-Cucatalysts that was conducted with the goal of reducingthe Pt loading signiﬁcantly or developing a new method oflattice-strain control[140] A synergistic effect of Pt-Pdcore-shell bimetallic nanoparticles has been discovered toenhance catalytic activity and sensitivity Thus, the syner-gistic core-shell effects of Pt- and Pd-based bimetalliccatalysts and the dealloying effects of Pt-based core-shellcatalysts are very important to the creation of efﬁcient Pt-and Pd-based catalysts for developing sustainable andrenewable energy sources with various FC technologies.Therefore, it is still necessary to develop novel metal and/

or oxide nanoparticles, nanosized structures, and various

FC materials Similarly, multi-metal NPs are promisingcatalysts for next-generation FCs that can be synthesizedwith simple chemical and physical methods Such NPs can

be used in catalysis, energy conversion, and FCs, and theyalso have prospects for low-cost applications in theﬁelds

of thermoelectric materials, biology and medicine Wesuggest that the use of noble-metal thin shells (Pt, Pd,

Rh, and Ru, possibly in combination with Ag and Au) andcheap-metal thin shells is of great importance to core-shellnanoparticles that are engineered for catalysis, catalysts,

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and FCs because such thin shells protect the cores of the

nanosystems and provide an avenue for reducing the cost

of expensive catalysts The recent development of non-Pt

catalysts for the ORR has become important to the

large-scale commercialization of various FCs [141] Synthesis

methods that are capable of producing Pt- or Pd-based

bimetallic nanoparticles with various sizes and shapes as

well as various alloy and core-shell nanostructures have

been used to produce such nanoparticles for catalysis The

core-shell structures of bimetallic nanoparticles are very

important in Pt- and Pd-based catalysts for FCs[142–149]

At present, the alloy surfaces of Pt- and Pd-based

bime-tallic alloys, for example, the important formula of Pt3M

catalysts (where M stands for Fe, Ni, Co, V, or Ti), are the

subject of intensive study[150–152] The catalytic activity

of Pt/GC electrodes for CO monolayer oxidation has been

tested The CO stripping voltammetry provided a

ﬁnger-print of the particle-size distribution and the extent of

particle agglomeration in carbon-supported Pt catalysts

[153] The issue of CO poisoning can addressed with the use

of Pt/CeO2/ZrO2 in H2 FCs Core-shell nanoparticles are

classiﬁed into various core-shell structures, including

inor-ganic-inorganic, inorganic-organic, organic-inorganic,

organic-organic, core/multishell, and

movable-core/hol-low-shell Among these nanoparticles, bimetallic

nanopar-ticles are some of the most important nanoparnanopar-ticles that

can serve as catalysts for various FC reactions[154] The

synergistic effects of bimetallic catalysts have been

inves-tigated Pt-based core-shell bimetallic nanoparticles have

been shown to demonstrate much better catalytic activity

than Pt nanoparticles Alloyed and core-shell bimetallic

nanoparticles have higher catalytic activities than metallic

nanoparticles of single metals because of synergistic

effects or the bi-functional mechanism that acts between

two different metals or within bimetallic nanoparticles and

catalysts[155–157] The nanoshell provides sites for

cata-lytic activity At the same time, the core element exerts an

electronic effect (a ligand effect) on the nanoshell

ele-ment because the surface atoms of the nanoshell are

coordinated to the nanocore in their catalytic reactions

Therefore, the nanoshell is an important factor in

control-ling the catalytic properties Core-shell bimetallic

struc-tures induce a greater suppression of adsorbed poisonous

species (CO species) They modify the electronic band

structure to create better surface adsorption Therefore,

the electrocatalysis at the surfaces of a pure Pt catalyst

must be further studied with respect to the combinations

and mixtures of various elements that can be used in

nanostructured catalysts for FCs[155] Clear evidence of

catalytic enhancement can be observed in the

electro-chemical data of Pt-Pd core-shell bimetallic nanoparticles

with respect to pure Pt-Pd alloy and core-shell catalysts

Bimetallic Pt-M/CNTs catalysts (where M stands for Fe, Co,

or Ni) for DMFCs have been developed to enhance the

catalytic activity with respect to the MOR Among them,

the Pt-Co/CNT catalyst exhibits the best catalytic activity

and stability, especially in terms of its anti-poisoning and

long-term cycle abilities [159] In addition, Pt-Co/CNT

catalysts have been prepared by microwave-assisted

synth-esis [158], and the synergistic activity of Au-Pt alloy

catalysts has been studied[160] The important synergistic

effects of Pt-based core-shell bimetallic nanoparticles,

which are capable of providing signiﬁcant enhancements

to catalytic activity, must be intensively investigated inboth theory and practice to evaluate potential catalystsfor future FCs

Development of Pt-Ru-based catalysts (PtxRuyand

PtxRuy/support)

The most successful but high-cost present Pt-Ru-basedcatalysts with various supports for DMFCs have been dis-cussed with respect to improving their activity and optimiz-ing the exploration of new catalysts with a low Pt-metalcontent[59,161] The segregation of core-shell Ru-Pt nano-particles has been discovered in NMR studies[162] PAMAM-stabilized Pt-Ru nanoparticles have been used to catalyzethe MOR [163] Pt/Ru nanoparticles (2.5 nm) can be easilysynthesized in high-temperature and high-pressure ﬂuids

[164] The MOR of Pt-Ru catalysts deposited on a HOPGsubstrate by sequential and simultaneous electrodepositionhas been studied in aqueous sulfuric acid [165] PtRu/Ccatalysts have been prepared for DMFC applications[166].CO-tolerant Pt-Ru–MoOx/carbon nanoﬁbers have been usedfor DMFCs [167] The catalytic activity of a Pt-Ru catalystfor DMFCs has been enhanced by repetitive redox treat-ments[168] The relative advantages and disadvantages ofmetal shells versus alloy shells primarily arise from theissues involved in their synthesis Pt/C and Pt-Ru/C cata-lysts show high catalytic activity but carry high costs[169].Pt-Ru electrocatalysts supported on ordered mesoporouscarbon have been evaluated for use in DMFCs [170] NovelPt-based catalysts with Pt/mesoporous carbon nanocompo-sites imbued with Ni or Co nanoparticles have been used forFCs [171] From the perspective of their electrocatalyticactivity, Pt- and Ru-based bimetallic catalysts with 30 at%

Ru has been found to be the most active electrode formethanol, ethanol and ethylene-glycol oxidation [172].Methanol crossover has been discussed in the context ofDMFCs In the past, Pt-Ru catalysts have exhibited goodperformance for use in DMFCs[173] Pt-Ru nanowire cata-lyst material in the anode of a DMFC has been used toenhance the performance of the DMFC [174] Electroche-mical impedance studies on PtRuNi/C and PtRu/C anodecatalysts in an acid medium have demonstrated that thesecatalysts can be used for DMFCs The catalytic activity ofPt-Ru-Ni/C with respect to the MOR is higher than that

of Pt-Ru/C, and its tolerance to CO is also better than that

of Pt-Ru/C[175] In one example of the effects of tion on Pt-based catalysts, the quaternary Pt-Ru-Sn-W/Ccatalyst for DMFCs can deliver signiﬁcant currents underhalf-cell operation, but it can also cause ohmic lossesbecause of the presence of semi-insulating metal oxides,which limit the single-cell performance[176]

composi-In general, Pt/SWCNTs and PtRu/SWCNTs have exhibitedhigh ORR rates[177] The performance of PtRu/C catalystswith respect to the MOR has been improved with sensitiza-tion and activation treatments[178] Pt–Ru/C catalysts thatuse carbon ﬁbers a with stacked-cup-type structure havealso exhibited good MOR catalytic behavior [179] VariousPtRu/C catalysts were fabricated using surfactants forstudies of the MOR [180] A novel type ofpolyoxometalate-stabilized Pt-Ru/MWCNTs catalyst has

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been prepared for use in DMFCs[181] Pt-Ru catalysts have

been stabilized against the dissolution of Ru with the

incorporation of Au[182] A Pt-Ru-TiO2photoelectrocatalyst

has also been used for the MOR [183] The

microwave-assisted polyol method has been investigated for the

pre-paration of Pt/C, Ru/C and PtRu/C nanoparticles and its

application in the electro-oxidation of methanol Pt-Ru/C

catalysts for the MOR have also been synthesized with a

two-stage polyol reduction process [45,184] A novel

CO-tolerant PtRu core-shell-structured electrocatalyst with a

Ru-rich core and a Pt-rich shell has been studied with

respect to the hydrogen oxidation reaction and its

implica-tions for PEMFCs [185] Additionally, Pt and Pt-Ru

electro-catalysts supported on carbon xerogels have been evaluated

for use in DMFCs [186] For DMFCs, a kinetic analysis of

carbon-monoxide and methanol oxidation has been

per-formed on high performance carbon-supported Pt-Ru

elec-trocatalysts[187] The studies discussed above demonstrate

that alloy and core-shell structures are of interest for the

improvement of the overall performance of DMFCs

Meso-porous Pt and Pt/Ru alloy electrocatalysts for the MOR have

been prepared [188] In addition, the performance of

Pt-based core-shell catalysts has been studied for methanol

and ethylene-glycol oxidation[189] Recently, nanosized

Pt-Ru/C catalysts have also been shown to exhibit superior

catalytic activities for alcohol reactions [190], such as

methanol and ethanol oxidation

Development of Pt-Rh-based catalysts (PtxRhyand

PtxRhy/support)

At present, Rh-Pt bimetallic catalysts with various core-shell,

alloy, and monometallic nanoparticle structures can be

successfully synthesized by polyol reduction[191]to produce

robust Rh-Pt-based catalysts for PEMFCs and DMFCs, but only

at high cost In this type of Rh-Pt-based catalyst, the size

effect of the Rh-Pt bimetallic nanoparticles is sensitive to the

catalytic activity of CO oxidation It has been discovered that

surface segregation plays an important role[192] Hence, Rh

metal can play a beneﬁcial role in reducing CO poisoning or

enhancing catalytic activity and stability via the synergistic

effect

Development of Pt-Au-based catalysts (PtxAuy and

PtxAuy/support)

We suggest that Pt-Au catalysts (Pt-Au mixture, alloy, and

core-shell catalysts or catalysts that use Au clusters) may be

very promising candidates for FCs because of the enormous

availability by weight of Au metal on our planet[193–195]

An unalloyed bimetallic Au-Pt/C catalyst has been found to

be effective for the ORR[196] The MOR mechanism on Pt

that has been spontaneously deposited on unsupported and

carbon-supported Ru nanoparticles has been presented for

Pt-Ru/C catalysts [197,198] Pt-Au nanoparticles can be

used as catalysts for the ORR in the presence of methanol

[199] The electrochemical stability of mesoporous Pt-Au

alloys toward the MOR has been found to be highly improved

relative to that of nonporous Pt and mesoporous Pt ﬁlms

Therefore, Pt-Au alloyﬁlms may be good catalysts to use for

DMFCs[200-202] In one different work, Pt-Au nanoparticles

with the random self-assembly of Pt-Au nanoparticles havesuccessfully been synthesized and discovered[203] Pt-Au/CNT catalysts for the ORR have been investigated in acomparison between Pt/Au and PtAu nanoparticles for thedetermination of methanol-tolerance issues[204] The MORhas been studied with Au-Pt core-shell nanoparticles/Csynthesized with the epitaxial growth method [205], andAu-Pt/C catalysts have also been studied for the ORR[206].Various catalysts with Pt monolayers (or a thin coating or athin skin of Pt monolayers) have been developed and proven

to exhibit high stability and durability [207,208] monolayer Pt-shell/Pd-core nanoparticles have exhibitedhigh HOR activity [209] At present, Pt-covered-MWCNTsare under development for the investigation of the ORR in

Sub-FC applications[210] PtRu electrocatalysts with ultra-low

Pt and Ru content are under development for the MOR

[211] An efﬁcient Pd-Co-Pt core-shell catalyst/C with a thin

Pt shell has been developed for the MOR[212], and Pd-Pt/Celectrocatalysts have been developed that show goodmodiﬁcations of the Pt or Pt-Pd monolayer but also demon-strate the CO-stripping voltammetry phenomenon [285].The preferential CO oxidation in hydrogen has been studiedwith respect to the reactivity of Pt-based core-shell nano-particles[213], and excellent results on the use of Pt-basedalloy and core-shell nanostructures for PEMFCs and DMFCshave been obtained [213–217] Pt/M catalysts with core-shell structures (where M stands for Ru, Rh, Ir, Pd, or Au)have been prepared and investigated for the determination

of both the CO coverage and the reduction of CO poisoning.The shells were very thin Pt layers In a comparison ofcatalytic activity, the CO poisoning on the surfaces of Pt-Mcatalysts in hydrogen-rich environments showed the follow-ing relation, which can be attributed to the compositioneffect: Ru/PtoRh/PtoIr/PtoPd/PtoPtoAu/Pt The Pt/Ru-based catalyst exhibited the highest catalytic activitybecause it had the weakest CO binding on its surface[213]

In the effort to improve the electrocatalytic properties of

FC cathodes, a volcano-shaped relationship was conﬁrmedbetween the rate of the cathode reaction and the oxygenadsorption energy for the best catalytic activity of the Pt-Pdbimetal [218,219] The ORR of Pd-Co-Pt/C catalysts hasbeen studied; these catalysts exhibit a high ORR ratebecause of their enhanced activity and stability withrespect to monolayer Pt[220] It has been proven that theevidence for a synergistic effect that has been discovered inbimetallic nanoparticles indicates an enhancement of cat-alytic activity and selectivity[156,155,294,295]

Development of Pt-Cu-based catalysts (PtxCuyand

PtxCuy/support)

Thus far, Pt-Cu-based catalysts have been developed forlow-temperature FCs, PEMFCs, and DMFCs They havebecome promising electrocatalyst candidates because oftheir high durability and stability during operation andtesting[221–224] The catalytic activity can be controlledvia the lattice-strain effects in dealloyed core-shell FCcatalysts[140,221] Pt-Cu catalysts have exhibited catalyticactivity four times higher than that of a Pt catalyst with thesame Pt mass activity for the ORR because of dealloying intothe core-shell structure Cu-Pt core-shell catalysts with

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nano-carbon supports can be used for PEMFCs[222,223] In

most cases, supported Cu-M (where M stands for Pt, Pd, Ru,

or Rh) bimetal nanocatalysts have been studied

simulta-neously in an effort to identify the best catalytic

perfor-mances[224]

Development of Pt-Ni-based catalysts (PtxNiyand

PtxNiy/support)

In recent years, PtxNiy- and PtxNiy/support-based catalysts

have been considered for their potential use in PEMFCs and

DMFCs because of their high catalytic stability and durability

[224–229] Ni-Pt core-shell nanoparticles with thin Pt shells

prepared with the polyol method are 10 nm in total size, and

they are promising cathode catalysts for use in PEMFCs that

have a low Pt content but high catalytic activity [225]

Nanoporous Pt/Ni surface alloys of approximately 3 nm in

size have been prepared, which exhibited superior ORR

activity and long-term durability compared to a commercial

Pt/C catalyst [226] The effect of the stabilizer on the

properties of a synthetic Ni-Pt core-shell catalyst for PEMFCs

has been investigated[227] The effects of acid treatment of

Pt-Ni alloy nanoparticles-graphene on the kinetics of the ORR

in acidic and alkaline solutions have been studied[228] Pt-Ni

alloy/C catalysts have shown high catalytic activity toward

the ORR[229] The scientists show that Pt-Ni alloy nanoporous

nanoparticles have been prepared with a facile chemical

dealloying process using nanocrystalline alloys as precursors

[230] As a result, Pt-Ni alloy/C has shown superior catalytic

properties compared to alloyed nanoparticles because of its

large surface area and small pores, which provide a signiﬁcant

catalytic enhancement, which is very crucial to the operation

of alcohol FCs, PEMFCs and DMFCs

Development of Pt-Co-based catalysts (PtxCoyand

PtxCoy/support)

It has been shown that Pt-Co-based catalysts can

success-fully substitute for pure Pt catalysts Most research

regard-ing PtxCoyand PtxCoy/support for PEMFCs and DMFCs has led

to the discovery of the suitable composition of Pt and Co for

the highest catalytic activity, considering the type of

supports used [321–351] In these catalysts, Co can play

the role of reducing the CO poisoning or act as a synergistic

agent for the enhancement of the catalytic activity Co-Pt

core-shell nanoparticles can be used as cathode catalysts

(Co-Pt/C catalysts) for PEMFCs or DMFCs [231–251] The

utilization of Pt metal in conjunction with cheap Co metal is

important to reducing the cost of FCs[231–233,390] Pt-Co/

C catalysts with low Pt content have been prepared, and

they can be used for PEMFCs and DMFCs [224] Pt-Co

catalysts with polyphosphazene-coated CNT supports have

been studied for use in DMFCs[235] The effect of thermal

treatment on the structure and surface composition of

Pt-Co electrocatalysts can be exploited for application in

PEMFCs operating under automotive conditions[236]

More-over, Pt, Pt-Ni and Pt-Co supported catalysts can be used to

achieve high ORR rates in PEMFCs [237] Pt-Ni-Co-based

catalysts have been prepared for the ORR in which the

electrocatalytic activity has been enhanced through the

manipulation of structure parameters, such as the lattice

strain, the surface oxidation state, and the distribution

[238] Pt-Ni and Pt-Co alloy catalysts have been studied foruse in PEMFCs and DMFCs[239–242] Ni-Pt core-shell nano-catalysts have exhibited an enhancement of catalyticactivity for the ORR [243] Pt-Ni bimetallic bundles pre-pared with a seed-based diffusion method exhibited metha-nol oxidation and catalytic activity that was 3.6-fold higherthan that observed for conventional Pt nanoparticles[244],and the same method has been used to producePt-Co/C catalysts for PEMFCs[245] The main role of Co inthe MOR mechanism of the Pt/C catalyst has been presented

[246] The effects of thermal annealing on the properties of

a Corich core-Ptrich shell/C catalyst with respect to theimplications for the ORR have been presented [247] Theelectrodeposition of PVA-protected PtCo electrocatalystshas been developed to achieve high ORR rates in H2SO4

[248] and the synthesis of PtCo nanowires for the MOR

[249] Among the Pt-based catalysts, PtxCoy/C is well suited

to high-performance use in the cathode of temperature PEMFCs[250] The catalytic activity of Pt-Co-alloy-nanoparticle-decorated functionalized MWCNTs can beused to achieve high ORR rates in PEMFCs [251] In adifferent study [395], the authors proved that Pt-Co/C-based catalysts that use graphitic carbon supports can beranked in order of durability as follows: Pt3Co/GrC4Pt/GrC4Pt3Co/Non-GrC4Pt/Non-GrC Research results regard-ing catalytic activity and sensitivity should always be con-sidered alongside the corresponding results regardingdurability, stability, and reliability when evaluating thesuitability of Pt-based catalysts for use in low-temperatureFCs, PEMFCs and DMFCs For example, based on the values ofECSA and ORR activity of as-synthesized PtxCoyalloys afterelectrochemical treatment in 0.1 M HClO4, the Pt-mass-based activities (jmass) increase in the order of Pt(HT) (heattreatment)oPtCooPt3CooPtCo3 at comparable particlesizes[397]

high-Development of Pt-Sn-based catalysts(PtxSnyand PtxSny/support)

Pt-Sn nanoparticles with different particle sizes (1-9 nm)and metal compositions (Sn content of 10-80 mol%) and withvarious organic capping agents have been synthesized[252]

In the case of Pt-Sn catalysts with supports, the Pt-Sn/Ccatalysts were directly applied to DMFCs[253] The MOR at

a lead (Pb) electrode modified with Pt, Pt-Ru and Pt-Snmicroparticles dispersed in poly(o-phenylenediamine)(PoPD)film has been investigated The catalytic activity ofthe Pt particles was found to be enhanced when Ru was co-deposited in the polymerfilm, and the enhancement effectwas even greater in the case of Sn [254] So far, thedevelopment of PtxSny, and PtxSny/support has beenfocused on the discovery of their relative catalytic activity

as well as on the optimal catalytic activity for PEMFCsand DMFCs

Development of Pt-Fe-based catalysts (PtxFeyand

PtxFey/support)

At present, PtxFeyand PtxFey/support are of great tance because of their ultra-high durability, ultra-high

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impor-stability, and high reliability Fe nanoparticles can be used

as durable cores for Pt that will survive on very long time

scales, but there are few reports regarding such uses of

PtxFeyand PtxFey/support They have potential for

applica-tion in both low- and high-temperature FCs A signiﬁcant

improvement of the Pt-Fe catalyst has been achieved by

adjusting the Fe composition to obtain the optimal

con-centrations for pure Pt-Fe catalysts with mixture, alloy and

core-shell structures [255–259] In addition, Pt-Fe-based

catalysts are of interest for use in PEMFCs and DMFCs

because of their great durability, stability, and reliability

in the FC reactions Among the combinations and mixtures

of Pt, Pt/C, and Pt-Fe/C, a category of Pt-Fe/C with aﬁxed

Pt:Fe ratio of 1.2:1 exhibits the highest FC performance at

90 1C and an oxygen back pressure in the cell of 0.2 MPa

[255–259] Pt-Fe/C catalysts of high catalytic activity for

the ORR have been prepared [260] In addition, Pt-Fe

catalysts have been used for the ORR in low-temperature

DMFCs[261] The stability of Pt–Fe/C catalysts for the ORR

has been discussed in relation to the catalyst composition

[262], and a similar study has been performed with the

addition of Co [394] Ternary Pt-Fe-Co alloy multi-metal

catalysts prepared by electrodeposition exhibited the best

catalytic mass activity for a structure of Pt85Fe10Co5 when

Pt-Fe-Co catalysts such as Pt, Pt97Fe3, Pt94Fe5Co1,

Pt94Fe6Co2, Pt88Fe8Co4, Pt86Fe10Co5, Pt83Fe12Co5, and

Pt78Fe15Co7 were considered [394] A similar investigation

of Pt1-xMx-based catalysts (where M stands for Fe or Ni and

0oxo1) prepared with the sputtering method for use in

PEMFCs has been conducted[396] Importantly, the relative

catalytic activities are very crucial to determine the types

of Pt-based catalysts that can be used for low-temperature

FCs, PEMFCs, and DMFCs

Development of Pt-and-Pd-based nanoparticles

(PtxPdyand PtxPdy/support)

At present, Pt-and-Pd-based nanoparticles can be very

successfully prepared with various chemistry and physics

methods The synthesized nanoparticles can be binary,

ternary, multi-metal, and multi-component and can have

various mixture, alloy, and core-shell nanostructures with

various sizes, shapes and morphologies Scientists have

commonly investigated such nanoparticles in the most

suitable experimental and theoretical ranges of

character-ization, preparation and synthesis, structure, and properties

of desirable based catalysts Among the types of

Pt-based catalysts that are suitable for use in PEMFCs and

DMFCs, the Pt-and-Pd-based catalyst is one of the most

important catalysts that can be used in both the cathodes

and the anodes Pt-and-Pd-based catalysts have been

con-tinuously developed for testing in FCs, PEMFCs and DMFCs

for large-scale commercialization [263–301,391] In recent

years, a Pt/Co-Ru/C catalyst has been shown to exhibit a

higher catalytic activity for the MOR than Pt on Fe-Ru/C or

Ni-Ru/C, and its performance is closer to that of a

com-mercial Pt-Ru catalyst with a slightly higher metal loading

and a high cost[263] Pt-Pdx-Cuy/C core-shell catalysts have

exhibited high catalytic activity for the ORR in PEMFCs

[264] In addition, a low-Pt-content Pd45Pt5Sn50 cathode

catalyst has been developed for use in PEMFCs [265] The

high performance and stability of Pd-Pt-Ni nanoalloy trocatalysts have been tested in PEMFCs [266] Ru-free,carbon-supported, Co- and W-containing binary and ternary

elec-Pt catalysts have been developed for the anodes of DMFCs

[267] Carbon-Nb0.07Ti0.93O2-composite-supported Pt-Pdcatalysts of good catalytic activity and stability have beenprepared for the ORR in PEMFCs [268], as have efficientpseudo-core-shell PdCu-Pt/C catalysts for DMFCs[269]andPd-Co-Mo catalysts for the ORR in PEMFCs[270] Pt-basedternary catalysts for low-temperature FCs have beenreported, and the very good characterization of Pt-Pd-based catalysts has been confirmed, according to theirelectrochemical properties The complexity of multi-metaland multi-component catalysts should be significantly sim-plified by synthesis and preparation methods A significantchallenge is tofind novel cheap catalysts that can replacethe high-cost Pt catalysts Among the known Pt-basedcatalysts, Pt-Pd-based catalysts have proven to be success-ful for use in commercial PEMFCs and DMFCs In particular,core-shell Pt modified Pd/C is an active and durableelectrocatalyst for the oxygen reduction reaction in PEMFCs

[273] The enhancement of the ORR activity of Pt-Pd/Cbimetallic catalysts can be achieved the preparation of Pt-enriched surfaces in acid media and the use of a core-shellstructure[274,275] In this context, Pd and Pt-Pd catalystscan be used in DMFCs[276] Some evidence of an epitaxialovergrowth of Pt-on-Pd nanocrystals has been observed

[277] Pd-Pt core-shell nanowire catalysts with ultra-thin

Pt shells on Vulcan XC-72 carbon supports have exhibited asignificant enhancement of the electrocatalytic perfor-mance with respect to the ORR [278] The preparation ofPt-Pd alloys with the one-pot solvothermal method withselective shapes and enhanced electrocatalytic activitieshas been presented[279], and Pt-Pd core-shell nanoparti-cles have been easily synthesized via the supermolecularroute [278] Pd-Pt bimetallic nanodendrites with highactivity for the ORR have been reported [281] The con-trolled synthesis of Pd-Pt alloy hollow structures withenhancement of the catalytic activities for the ORR hasbeen confirmed[282,283] It has been observed that PAMAMdendrimer-encapsulated Pd-Pt nanoparticles exhibiting aslight core-shell morphology can be synthesized with thesuccessive method using an aqueous NaBH4 solution[284].The ORR was significantly enhanced by the Pt monolayers on

a Pd-Au alloy [285] In an extensive study, the driven restructuring of Rh-Pd and Pt-Pd core-shell nanopar-ticles was investigated with respect to the changes in thestructure, size, shape, and morphology of the nanoparticles

reaction-[286] The important discovery of the high hydrogen storage

of Pd and Pt nanoparticles and their core-shell structureshas been reported [287–289] With the addition of Ptcoating, Pd nanotubes can be used as excellent electro-catalysts for the ORR[290] In many investigations, variousPt-and-Pd-based catalysts with speciﬁc alloy nanostructuresand bimetallic core-shell nanoparticles have successfullybeen synthesized[291–295]

In typical works, Pt and Pt-Pd bimetallic nanoparticleswith polyhedral core-shell morphologies are preciselysynthesized by the reduction of Pt and Pd precursors at acertain temperature in ethylene glycol with silver nitrate asthe structure-controlling agent, as shown inFigure 3 [291–

295] Such Pt nanoparticles exhibit well-shaped polyhedral

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Figure 3 (A)-(C) Pt nanoparticles in the range of 20 nm, and Pt-Pd core-shell nanoparticles in the range of 30 nm The bestsynergistic effect is found in the Pt-Pd bimetallic core-shell nanoparticles Reprinted with permission from: V.L Nguyen, M Ohtaki,

T Matsubara, M.T Cao, M Nogami, New experimental evidences of Pt-Pd bimetallic nanoparticles with core-shell conﬁguration andhighlyﬁne-ordered nanostructures by high-resolution electron transmission microscopy, J Phy Chem C 116 (2012) 12265-12274

V Long, T Asaka, T Matsubara, M Nogami, Shape-controlled synthesis of Pt-Pd core-shell nanoparticles exhibiting polyhedralmorphologies by modiﬁed polyol method, Acta Materialia, 59 (7), 2011, 2901-2907[291] Copyright © (2011) Elsvier Publishers

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morphology withﬁne and speciﬁc nanostructures in the size

range of 20 nm Important evidence of core-shell

conﬁgura-tions of Pt-Pd core-shell nanoparticles can be clearly

obtained with HRTEM measurements The results of HRTEM

imaging have shown that core-shell Pt-Pd nanoparticles in

the size range of 25 nm with polyhedral morphology form

thin Pd shells of 3 nm in thickness as atomic Pd layers grow

on the Pt cores during synthesis High-resolution TEM images

of Pt-Pd bimetallic nanoparticles have shown that the

Frank-van der Merwe and Stranski-Krastanov growth modes

coexist in the nucleation and growth of the Pd shells on the

as-prepared Pt cores, indicating a good lattice match

Experimental evidence of the deformations of lattice

fringes and lattice-fringe patterns has been found in Pt

and Pt-Pd core-shell nanoparticles The interesting

renu-cleation and recrystallization phenomena at the

attach-ments, connections, and bondings between the

nanoparticles have been revealed to form a very good

lattice match

Several specialists indicated that the porous structures of

hollow spherical sandwich PtPd/C catalysts for the MOR can

be fabricated by electrostatic self-assembly in polyol solution

[296] The role of a composition with a Pd:Pt ratio of 3:1 has

been studied in Pd3Pt1/C for highly methanol-tolerant ORR

catalysis [297] The effect of the Pt precursor on the

morphology and catalytic performance of Pt-impregnated

Pd/C for the ORR in PEMFCs has been investigated [298]

Scientists have demonstrated the temperature dependence of

methanol oxidation and product formation on Pt- and

Pd-modiﬁed Pt electrodes in an alkaline medium [299] In

general, Pd-Pt/C catalysts have shown high durability and

stability for the MOR in DMFCs[300] Recently, the nanocage

effect has been found in hollow Pt-Pd nanoparticles during

electrocatalysis testing[301] Further computations regarding

catalysis have been developed to identify the interactions and

catalytic activities of Pt-O and Pt-H as well as Pt and various

other species Through the DFT approach, it has been found

that the level of Pt content in catalysts that use Pd-Pt alloys

should be appropriately modiﬁed to achieve an efﬁcient ORR

[283] The maximum of the ORR volcano curve is predicted to

occur for Pd3Fe-Pd3Pt and Pd3Mn-Pd3Pt core-shell catalysts by

DFT calculations Compared with commercial Pt catalysts,

Pd3M/Pd3Pt catalysts have a lower Pt content but show

favorable ORR activity and selectivity[302] A computational

and experimental study has been carried out to understand

the volcano behavior of the ORR of PdM-PdPt/C (where M

stands for Pt, Ni, Co, Fe, or Cr) core-shell catalysts The result

is that the core-shell catalysts exhibit a high methanol

tolerance, which is important for use in PEMFCs and DMFCs,

in the volcano approach [303] To study the kinetics andmechanisms of the ORR, DFT calculations and moleculardynamics simulations have been widely used in designing Pt-and Pd-based catalysts for catalysis and FCs Based on theGupta empirical potential and DFT calculations, the adsorp-tion of CO, O, OH, and O2on Pd-Pt clusters with 55 atoms hasbeen studied using molecular simulations In this study,

Pd43Pt12 with a three-shell onion-like structure exhibitedthe highest relative stability with respect to both DFT andGupta levels In addition, these clusters showed the weakest

CO, O, OH, and O2 adsorption strength compared with the

Pt55, Pd55, and Pd13Pt42 clusters, indicating good catalyticactivities toward the ORR for the better Pt-based electro-catalysts[304] The issues surrounding O2on icosahedral Ni-

Pt12core-shell nanoparticles have been investigated using abinitio DFT calculations A high catalytic activity of the Ni-Ptcore-shell nanoparticles was predicted for the highly activeORR [305] Molecular dynamics simulations can be used toinvestigate the thermal stabilities of Pt-Pd core-shell nano-particles with different core sizes and shell thicknesses Two-stage melting occurs during the continuous heating of bime-tallic nanoparticles The melting behaviors at the atomiclevel of bimetallic and multi-metallic nanoparticles have beenidentified [306,307] In a study of the size effect, Pd-Ptnanoparticles of 1.5-5.5 nm with controllable core-shellstructures were successfully prepared with a hydrogen-sacrificial protective strategy [308] A study of methanoldecomposition over Pt-M bimetallic catalysts (where M standsfor Au, Pd, Ru, or Fe) has been presented The oxidation stateand activity of the Pt were found to be influenced by theaddition of the secondary metal In this study, PtO was found

to be highly stable [13,309] The structural conversion andformation of Pt, Pt-O, and PtO2needs to be further studiedwith respect to the catalytic activity in the oxidation states;this is of particular interest for the MOR mechanism Pt andPt-Ru can be supported on porous nanostructured materials,such as mesoporous carbon and metal oxides, for use in DMFCsand PEMFCs [310] Porous Pt nanocubes of 20 to 80 nmoriginating from small nanoparticles of 10 to 20 nm have alsoexhibited high catalytic activity for the methanol oxidationreaction (MOR)[311] It is certain that bimetals such as Au-Pt,Pt-Au, and Fe3O4-Au-Pt nanoparticles are catalytically activefor the ORR and the MOR, and their behavior must be studied

in further catalytic investigations to obtain better catalyticcharacterizations[312–314] The metal monolayers on Pt-WCcatalysts (Pt-WC and Pt-W2C) used for hydrogen productionfrom water electrolysis can be supported on low-cost transi-tion-metal carbides [315] The kinetics of the hydrogenoxidation reaction have been determined for a Pt/WC

Figure 4 (A) HRTEM images of Pt-Pd core-shell The thin Pd shells protect polyhedral Pt cores The nucleation and growth of Pdshells are controlled by a chemical synthesis Scale bars: (a)-(c) 20 nm (d) 5 nm (e) 5 nm (f) 2 nm (B) Surface kinetics andmechanism of cyclic voltammogram of Pt nanocatalysts, and Pt-Pd core-shell nanocatalysts on glassy carbon electrode in N2-bubbled0.5 M H2SO4electrolyte (scan rate: 50 mV s 1) (C) Cyclic voltammograms of as-prepared Pt-Pd core-shell nanocatalysts in 0.5 M

H2SO4in the ranges of E= 0.2 V to E=1.0 V and E= 0.2 V to E=0.2 V Cyclic voltammograms of as-prepared Pt nanocatalysts in0.5 M H2SO4in the ranges of E= 0.2 V to E=1.0 V and E= 0.2 V to E=0.2 V Cyclic voltammograms of as-prepared Pt-Pd core-shellnanocatalysts in 0.5 M H2SO4in the ranges of E= 0.2 V to E=1.0 V and E= 0.2 V to E=0.2 V (D) Cyclic voltammograms towardsmethanol electro-oxidation of Pt nanocatalyst and Pt-Pd nanocatalyst (E) Chronoamperometry data of Pt and Pt-Pd nanoparticles.Electrolyte solution of 0.5 M H2SO4+1.0 M CH3OH and polarization potential about E=0.5 V Reprinted with permission from: Long, N.V., Ohtaki, M., Hien, T.D., Randy, J., Nogami, M., A comparative study of Pt and Pt-Pd core-shell nanocatalysts, 2011, ElectrochimicaActa 56 (25), pp 9133-9143[294] Copyright © (2011) Elsvier Publishers

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catalyst with a low content of Pt nanoparticles[316]

Noble-metal (Pt, Pd, Ru, Au, Ag) nanoparticles and engineered

nanoparticles that use combinations of noble metals with

cheaper metals (Ni, Fe, Cu ) can be used as mixture, alloy,

and core-shell catalysts for FCs For high-temperature

cata-lytic reactions up to 750 1C, Pt-metal cores coated with

mesoporous silica shells of Pt-SiO2can be used as highly active

catalysts that are equally as effective as bare Pt metal for

ethylene hydrogenation and CO oxidation [317] Potential

applications of metal and bimetal nanoparticles to energy

conversion have been discussed with a particular focus on

electrocatalysts for PEMFCs and for thermoelectric energy

conversion For this purpose, Pd-Pt core-shell nanoalloys

protected by a perﬂuorinated sulfonic acid ionomer can be

used[318–320] The evolution of the structure and chemistry

of bimetallic nanoparticle catalysts under catalytic reaction

conditions must be intensively investigated[321]because it

exerts an important inﬂuence on the durability and stability

of the FC as a whole Core-shell PdPt-Pt/C catalysts have

been observed to exhibit high catalytic activity because of

the structure of their Pd-Pt alloy cores and its synergistic

effects with the thin Pt coatings[322,323] In addition, Pd-Pt

core-shell nanowire catalysts with high catalytic activity for

the MOR have been used for PEMFCs and DMFCs [324] The

catalytic activities of Pt and Pt-Pd nanoparticles in

polypyr-roleﬁlms for the ORR have been compared against the very

beneﬁcial advantages of Pt-Pd nanoparticles[325] The role

of Pd loading in Pd-Pt catalysts used to dope mesoporous

hollow core-shell carbon has been discussed in the context of

the performance of PEMFCs [326] This study illustrated the

great advantage of a high porosity accompanied by relatively

high stability and durability [326] A novel

hydrogen-absorption site in the hetero-interface of the Pd core and

the Pt shell of Pd-Pt core-shell bimetallic nanoparticles has

been identiﬁed[227], and hydrogen insertion in

Pd-core/Pt-shell cubo-octahedral nanoparticles has been observed[328]

In a study of the composition effect, Co5Pt95 and Pd16Pt84

were synthesized and found to be excellent electrocatalysts

for the MOR with mass activities of 1417 and 1790 mA/mg Pt,

respectively, which are much higher than that of a high-cost

Ru-Pt catalyst[329] Another study investigated Pd-Pt alloys

with controlled compositions ranging from Pd88Pt12 to

Pd34Pt66 These alloy NPs were found to be much more active

and stable for the MOR, and their activities were

Pd/Pt-composition-dependent, with alloys containing 40-60% Pt

demonstrating the optimal activity and stability [330] In

most cases, Pt-Pd/C has been shown to exhibit better

durability than Pt/C[383], and Pt-Pd/C demonstrates

super-ior catalytic activity for methanol and ethanol oxidation

[384] In the better surface C modiﬁcation of the catalysts,

Pd/C or Pt-Pd/C catalyst can be prepared with the

modiﬁca-tion of the Pt monolayers or the Pt shell, as well as Pt/C or

Pd-Pt/C catalyst with the modiﬁcation of Pd monolayers or

the Pd shell[385] Pd-Pt alloy catalysts have been used for

methanol-tolerant catalysis of the ORR The highest mass and

speciﬁc activities for the ORR using Pd-Pt/C catalysts have

been found for a Pd:Pt atomic ratio of 1:2 Pd-Pt alloy

catalysts of this atomic ratio have exhibited enhanced

methanol tolerance during the ORR with respect to Pt/C

catalysts [331] Pd-Co/C catalysts have been developed for

use for the ORR in place of Pt-Pd/C catalysts[332] A study of

potential oscillations in PEMFCs using a Pd-Pt/C anode has

provided an improved understanding of the catalytic activity

of Pd-Pt/C[333]

To date, the considerable advantages of core-shell tures have been established in various reports Therefore,scientists and researchers are using the advantages of core-shell structures to improve the catalytic activity and theefﬁcient utilization of Pt in PEMFCs and DMFCs, which isimportant to the low-cost, large-scale commercialization ofsuch FCs Through ultrasound-assisted polyol synthesis,

struc-Pd4Co core-shell catalysts have been produced that showhigh catalytic activity towards the ORR, comparable to that

of Pt catalysts[334] In one work, the charge redistribution

in Pt-based core-shell nanoparticles was found to promotethe ORR[335] The ORR activity of well-deﬁned core-shellnanocatalysts has also been investigated in relation toparticle-size, facet, and Pt-shell-thickness effects [336].Recently, a simple route has been used to synthesize novel

Fe3O4-Pt core-shell nanostructures with high tic activity, which has promising implications for core-shellcatalysts with Pt layers on oxide- and ceramics-basednanoparticles; however, the ability to ensure the homoge-neity of core-shell catalysts and the size range of Pt-basedcatalysts during synthesis still poses a challenge toresearchers, even though Pt nanoparticles anchored ongraphene-encapsulated Fe3O4 magnetic nanospheres(38075 nm) have been prepared to serve as robust cata-lysts for the MOR [337,338] The performance of low-Pt-content electrodes of PEMFCs can be appropriately con-trolled with the use of a Fe2O3-Pt/C core-shell catalystprepared with an in situ anchoring strategy [339] Inparticular, an Fe-based cathode catalyst has been found

electrocataly-to perform very well, with an enhanced power density of0.75 W cm 2at 0.6 V, in PEMFCs[340]

In this work, the synthesis of Pt (4-8 nm) and Pt-Pd shell nanoparticles (15-25 nm) is presented Pt-Pd core-shellcatalysts possess catalytic properties far superior to those of

core-Pt catalysts core-Pt-Pd core-shell catalysts exhibit fast andhighly stable catalytic activity for hydrogen Methanoloxidation is signiﬁcantly enhanced by Pt-Pd core-shellcatalysts, with a current density much higher than that of

Pt catalysts Fascinatingly, the size effect is not as tant as the nanostructure effect The improvement of fast,stable, sensitive hydrogen adsorption is very crucial for FCs

impor-[294,295] According to the scholars' predictions andassumptions, particular Pt-Pd core-shell catalysts with thin

Pt shells, thin Pd shells, or thin Pt-Pd alloy shells over thick

Pt cores or thick Pd cores have the special property that the

Pt or Pd cores cause an inherently preferential synergisticeffect with the thin Pt, Pd, or Pt-Pd shells, leading to highcatalytic activity, sensitivity, and selectivity with respect to

to the fast, efﬁcient, and robust HOR, ORR, and MOR inPEMFCs, and DMFCs Such behavior is very crucial for thedesign of desirable Pt-based catalysts In one interestingresearch, Pt-Pd core-shell nanostructures with high cataly-tic activity for methanol oxidation have been successfullyprepared We have used pure Pt catalysts produced from as-prepared Pt nanoparticles of 10 nm (4-8 nm) and pure Pt-Pdcore-shell catalysts produced from as-prepared Pt-Pd core-shell nanoparticles of 30 nm (15-25 nm) to perform acomparison of catalytic activity (Figure 4) We have therebyproven that the Pt-Pd core-shell catalysts possess catalyticproperties that are far superior to those of the Pt catalysts

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because of the advantages provided by core-shell

struc-tures, shapes, and morphologies We also found that the

Pt-Pd core-shell catalysts exhibited fast and highly stable

catalytic activity for hydrogen, leading to the possible

improvement of Pt- based catalysts via a novel phenomenon

of fast hydrogen adsorption through their speciﬁc core-shell

structures The MOR activity was signiﬁcantly enhanced by

the Pt-Pd core-shell catalysts, with a current density much

higher than that of the Pt catalysts It was also discovered

that the size effect in the size limit of 10 nm is not as

important as the core-shell nanostructuring effect in the

size range of 30 nm (15-25 nm) Therefore, the fast, stable,

and sensitive hydrogen adsorption of Pt-based core-shell

structures is very crucial for PEMFCs and DMFCs In addition,

the problem of CO poisoning was not observed in cyclic

voltammetry (CV) measurements because of the effective

reduction of weak and strong CO intermediates by the Pt

and core-shell Pt-Pd catalysts The observed ORR activity of

the Pt catalyst was more sluggish than that of the Pt-Pd

core-shell catalyst In this case, the fast enhancement of

the ORR on the electrode with Pt-Pd core-shell catalyst

because of its fast hydro-adsorption/desorption was clearly

observed The fast hydro-absorption/desorption rates are

the most signiﬁcant advantages of the core-shell bimetallic

nanoparticles Therefore, Pt-Pd core-shell catalysts can

signiﬁcantly increase the rates of the HER, HOR, and ORR

as well as providing some defense against CO poisoning The

mechanism of the reduction of the CO poisoning coverage

occurred at the active sites of the catalytic activity of the

Pt and Pt-Pd core-shell catalysts because of proper catalyst

preparation The mechanism of the reduction of the CO

poisoning coverage on Pt-Pd core-shell nanoparticles is

known to arise from the synergistic effects between the

core (Pt or Pd) and the shell (Pt or Pd) of Pt-based

bimetallic nanoparticles; these effects are the reason that

based core-shell bimetallic nanoparticles are the best

Pt-based nanostructures for the electrocatalysis of hydrogen

and methanol in PEMCs and DMFCs In the catalyst

prepara-tion, the as-prepared Pt, Pd, and Pt-Pd nanoparticles were

heated at approximately 3001C in H2/N2, which resulted in

a good characterization of the size, surface, structure, and

morphology of the catalysts The suitable temperature

range for the heat treatment should be chosen depending

on the intended application in various FCs, for example, the

ORR catalytic activity This choice of the temperature range

depends on the operating temperature of the FCs, which are

characterized by the chemical activity occurring at the

electrode surface In the forward sweep, the ﬁrst region

assigned to the hydrogen desorption is crucial to conﬁrm the

catalytic activity of Pt catalysts The slow kinetics of the

hydrogen desorption in the case of the Pt catalyst was

conﬁrmed in the cell before the stabilization of the CV was

achieved, from theﬁrst cycle to the twentieth cycle, and

the fast kinetics of the hydrogen desorption was determined

in the case of the Pt-Pd core-shell catalyst Indeed, the

results demonstrated the good desorption and adsorption of

hydrogen for both the Pt catalyst and the Pt-Pd core-shell

catalyst, providing evidence of good catalytic activity in

both catalysts following the preparation process and heat

treatment To evaluate the catalytic activity of the

pre-pared catalysts, the ECSA of the Pt catalyst was calculated

to be 10.5 m2g-1, and that of the Pt-Pd core-shell catalyst

was found to be 27.7 m2g-1in the catalytic investigations.The Pt-Pd core-shell catalyst, which had a stable and highcatalytic activity, showed a very high initial current ofj=1.29 10 3

A cm 2, and 30.01% of the current remainedafter 2 h of polarization This was much higher than the Ptcatalyst, which had an initial current of j=4.33 10 4

A

cm 2, and 3.67% of the current remained after 2 h ofpolarization Therefore, the more stable Pt-Pd core-shellcatalyst (15 min) showed the higher initial current, and itstill had a current of approximately 1.5 x 10 3A/cm2after

2 h of polarization In this research, the as-prepared Pt, Pd,Pt-Pd catalysts were heat treated at approximately 300 1C

to obtain high catalytic activity and stability because thenanostructures of the treated nanoparticles possess veryhigh hardness However, the good characterization of theas-prepared nanoparticles in terms of their surface, struc-ture, size, shape, and morphology should be maintainedduring heat treatment at high temperature After heattreatment (or sintering) at 300 1C, the Pt-Pd alloy andcore-shell catalysts exhibited good activity and stability inthe desirable nanostructures The heat treatment is neces-sary to ensure the highly robust catalytic activity, long-termstability and durability of the as-prepared nanoparticles.The pure Pt-Pd core-shell catalysts demonstrated a signiﬁ-cant enhancement of activity and selectivity for the ORR inFCs compared to those of the pure Pt catalysts

In all the interesting research, the catalytic activity andstability of Pt-based catalysts has strongly depended on theeffects of heat treatment at 3001C and the removal of thepoly(vinylpyrrolidone) (PVP) polymer from the surfaces ofthe Pt-based nanoparticles A signiﬁcant enhancement ofthe electrocatalytic activity toward the MOR of polyhedral

Pt nanoparticles has been achieved by removing the cappingagents A pure Pt catalyst was obtained by heat treatment

at 3001C while maintaining a good characterization in terms

of size, structure, shape and morphology According to theobserved electrocatalytic property and activity of the Ptnanoparticles, the ECA was approximately 6.75 m2/g for thewashed-only Pt nanoparticles, 8.56 m2/g for the directly-heated-only Pt nanoparticles, and 10.53 m2/g for thewashed and heated Pt nanoparticles The cyclic voltammo-grams of the methanol electro-oxidation of polyhedral-shaped Pt nanoparticles were investigated for differentmethods of PVP removal by heat treatment The electrolytesolution was 0.1 M HClO4+1.0 M CH3OH, and the scan ratewas 50 mV/s We discovered that the peak current density

in the forward scan (i(f)) serves as a benchmark for thecatalytic activity of the Pt nanoparticles during methanoldehydrogenation For the prepared catalyst samples, the i(f) values were 7.62 10 4, 8.75 10 4, and 9.90 10 4A/

cm2 for the washed-only, heated-only, and washed andheated samples, respectively [135] When a new catalyst

is prepared and tested for catalytic activity, the prepared polyhedral Pt nanoparticles in the size range of

as-10 nm can be used as the standard catalyst for anyconﬁrmation of catalytic activity because of their highcatalytic activity [136] In these works, Pt-Pd alloy andcore-shell bimetallic nanoparticles were synthesized, andthe epitaxial growth mode of the Pd-monolayer shells on the

Pt nanocores was observed Pt-Pd and Pd-Pd core-shellnanoparticles with thin Pt or Pd nanoshells in the form ofmonolayers exhibit excellent electrocatalytic behavior for

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DMFCs Interestingly, the size effects on the catalytic

activity, selectivity, and sensitivity are not as important as

the effects of morphology and nanostructure

In previous research, it has also been observed that Pt-Pd

alloy and core-shell bimetallic nanoparticles can be simply

synthesized (Figure 8) The epitaxial growth mode of the

Pd-monolayer shells on the Pt nanocores can be controlled

Pt-Pd and Pd-Pd core-shell nanoparticles with controllable

thin Pt or Pd nanoshells in various monolayer forms exhibit

excellent electrocatalytic behavior for DMFCs Interestingly,

the size effects on the catalytic activity are not as

important as the effects of morphology and nanostructure

The Pt-and-Pd-based core-shell catalysts in the range of

30 nm demonstrated suitable properties, such as a high

current density, and resisted CO poisoning much more

effectively than various other Pt, Pd, Pt-Pd, and Pd-Pd

bimetallic catalysts in the various forms of single metal (Pt

or Pd) nanoparticles and bimetallic nanoparticles with alloy,

core-shell, and mixture structures [293,294,295] In

addi-tion, the phenomenon of the CO stripping effect can not be

observed in any of electrochemical experiments because of

the careful preparation processes and heat treatments of

the pure Pt and Pt-Pd catalysts that were used The past

experiments include the following: Category (1) - alloy

nanoparticles with polyhedral, spherical, near-polyhedral,

and near-spherical shapes and morphologies of 10 nm

(5-10 nm) in size; Category (2) – mono-nanoparticles (Pt, Pd)

and bimetallic nanoparticles (Pt-Pd) with polyhedral,

sphe-rical, near-polyhedral, and near-spherical shapes and

morphologies of 10 nm (5-10 nm) in size for small

nanopar-ticles and 30 nm (20-30 nm) in size for large nanoparnanopar-ticles;

Category (3) – Pt-Pd core-shell nanoparticles with good

polyhedral shapes and morphologies of 30 nm (15-25 nm)

in size with thin shells of approximately 3 nm in thickness;

Category (4) – Pd-Pt core-shell nanoparticles with good

spherical or near-spherical shapes and morphologies of

20 nm (6-16 nm) in size; Category (5) – Pt-Pd alloy and

core-shell nanoparticles with large and irregular shapes and

morphologies of various size ranges of 10 nm, 20 nm, 30 nm,

and up to 40 nm; and Category (6) – Pd-Pt alloy and

core-shell nanoparticles with large and irregular shapes and

morphologies of 30 nm (15-30 nm) in size Importantly, the

Pt-based bimetallic nanoparticles with core-shell structures

appear to be the most advantageous in terms of their

signiﬁcantly enhanced catalytic activity and sensitivity for

both the HER and/or HOR mechanisms and the ORR and/or

MOR mechanisms (Figure 5) In comparison to the Pt-Pd

nanostructures in alloy, cluster and mixture forms, the

electrocatalytic properties were signiﬁcantly enhanced for

the observed Pt-Pd bimetallic core-shell nanoparticlesbecause of the very strong synergistic effect that appears

in well-shaped core-shell morphologies and nanostructures

[294,295] and various other advantages of the core-shellstructure[215,341,342,370]

In our electrochemical data and measurements of Pt-Pdcatalyst with various nanostructures, we have found thatPt-Pd alloy, Pt-Pd cluster and mixture, Pt-Pd core-shell(15 min), Pd-Pt core-shell, Pt-Pd core-shell, and Pd-Pt core-shell catalysts have ECA values (m2/g) of 12.7, 11.5, 27.7,17.7, 13.6, and 14.2 m2/g, respectively They exhibited E(V) values of 0.7, 0.67, 0.63, 0.65, 0.77, and 0.73 V,respectively, according to the current response The peakcurrent in the forward scan, if or i(forward) (A/cm2), wasfound to be 1.4 10 3

/g)and the highest initial current (1.29 x 10 3 A/cm2or 1.5 x

10 3 A/cm2) in two separate experiments, and mately 30% of the current remained after 2 h of polarization

approxi-[295] The as-prepared Pt-Pd catalysts with uniform shell structures exhibited far superior catalytic, selective,sensitive, and quick activity to the as-prepared catalystswith single, alloy and mixture structures

core-Observations of most of the core-shell bimetallic particles indicated that the Frank-van der Merwe (FM) andStranski-Krastanov (SK) growth modes coexist in the nuclea-tion, growth, and formation of the shells on the cores It ispredicted that one of these two growth modes will becomedistinctly more favorable than the other in the formation ofthe thin shells of core-shell nanoparticles and nanostruc-tures In our research, we have described a strategy forimproving the catalytic activity of Pt-based catalyststhrough the use of Pt-and-Pd-based alloy and core-shellnanoparticles [294,295] In addition, Pd-Pt nanoparticleshave shown high hydrogen solubility because of the co-existence of the Pd(2H), Pt(2H), Pt/Pd(2H) and Pt-Pd(2H)hydride phases in the very tiny Pt-Pd nanoparticles This is

nano-an inherent property of Pd nano-and Pt metals[287,288] In thiscontext, Pt and Pd metals exhibit a very high stronginherent interaction with hydrogen For example, at roomtemperature, Pd metal has the unusual property of absorb-ing up to 900 times its own volume of hydrogen [53,392].Therefore, a crucial question for scientists is how to explain

Figure 5 The pure Pt-Pd mixture, alloy and core-shell catalysts for direct methanol fuel cells The best catalytic activity andsensitivity is found in the Pt-Pd bimetallic core-shell catalyst (A1) (a)-(c) TEM images of alloy Pt-Pd nanoparticles (A1) (d)-(i) TEMimages of mixture Pt-Pd nanoparticles (A2)-(A3) (a)-(c) TEM and HRTEM images of Pt-Pd core-shell nanoparticles (Category 3) (A3)(d)-(i) TEM and HRTEM images of Pd-Pt core-shell nanoparticles (Category 4) (B) Cyclic voltammograms of Pt-Pd nanoparticles withtheir different conﬁgurations Electrolyte solution was 0.5 M H2SO4 (Scan rate: 50 mV/s) (C) Cyclic voltammograms towardsmethanol electro-oxidation of Pt-Pd nanoparticles with their different conﬁgurations Electrolyte solution was 0.5 M H2SO4+1.0M

CH3OH (Scan rate: 50 mV/s) (D) Chronoamperometry data of Pt-Pd nanoparticles with their different conﬁgurations Electrolytesolution is 0.5 M H2SO4+1.0 M CH3OH The polarization potential was 0.5 V Reprinted with permission from: N.V Long, T.D Hien, T.Asaka, M Ohtaki, M., Nogami, M., Synthesis and characterization of Pt-Pd alloy and core-shell bimetallic nanoparticles for directmethanol fuel cells (DMFCs): Enhanced electrocatalytic properties of well-shaped core-shell morphologies and nanostructures, Int

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