Recent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications a review

Review ArticleRecent developments in manganese oxide based nanomaterials with oxygen reduction reaction functionalities for energy conversion and storage applications: A review a Departm

Review Article

Recent developments in manganese oxide based nanomaterials with

oxygen reduction reaction functionalities for energy conversion and

storage applications: A review

a Department of Applied Chemistry, Adama Science and Technology University, Adama, Ethiopia

b Department of Chemistry, Hawassa University, Hawassa, Ethiopia

a r t i c l e i n f o

Article history:

Received 22 February 2019

Received in revised form

29 June 2019

Accepted 9 July 2019

Available online xxx

Keywords:

Manganese oxide nanomaterials

Microbial fuel cell

Bioremediations

Batteries

Oxygen reduction reaction

a b s t r a c t

In this article, a brief overview of manganese oxide nanomaterials (NMs) potential towards oxygen reduction reaction (ORR) for microbial fuel cell (MFC), bioremediations, and battery applications is discussed It's known that using non-renewable fossil fuels as a direct energy source causes greenhouse gas emissions Safe, sustainable and renewable energy sources for biofuel cell (BFC) and metal-air bat-teries hold considerable potential for clean electrical energy generators without the need for a thermal cycle In an electrochemical reaction system, the four-electron reduction from molecular oxygen at the air-cathode surface to hydroxide ion or water at a reasonably low overpotential was the ultimate goal of many investigations and plays a vital role in metal-air batteries and fuel cell device systems Different

MnxOy nanostructured materials, from Biofunctional structural catalysts up to their electrocatalytic contributions towards ORR are discussed Brief descriptions of ORR, principle strategy and mechanism, as well as recent developments of cationic dopants and electrolytic media, effect on the air-cathode surface

of manganese oxide nanocatalyst are also discussed Finally, challenges associated with platinum and carbon support platinum in improving electron and charge transfer between biocatalyst and air-cathode electrode are summarized

© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an

open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Recently, the use of fossil fuels (e.g., coal, natural gas, and oil) as a

direct energy source has led to a global energy crisis[1] This crisis

increased focus on greenhouse gas emissions to the environment,

and the limited and unstable supply of fossil fuel resources for

future forecast makes unsustainability of energy resources [2]

Renewable energy is thus considered as a sustainable way to reduce

the current global warming crisis However, various efforts have

been devoted to developing an alternative mechanism for

renew-able energy generation[3]

Safe and sustainable futures can be ensured by making new

innovations and modifications to the existing energy generation

and storage device technologies The best way to do this is by

utilizing energy from the fuel cell, batteries, supercapacitors, etc

via oxygen reduction reaction (ORR) concerning the best mecha-nism for a variety of infrastructure applications[4] Among these, fuel cells and metal-air batteries are energy generators that hold considerable potential for future application and relatively clean electrical energy generators These electrochemical devices transform chemical energy from a specific fuel into electricity without the need for a thermal cycle [5] During this electro-chemical reaction, a four-electron reduction reaction of molecular oxygen to either hydroxide ion or water at a reasonably low overpotential is the ultimate goal of many investigations It also plays a vital role in electrochemical energy-conversion systems in metal-air batteries and fuel cells [6] Therefore, in this review, a detailed electron transfer and potentials of the ORR mechanisms are going to be visualized to the reader with a clear and concise manner

Currently, fuel cells (either biotic or abiotic) are considered to be

a value add source of energy due to their high gravimetric and volumetric energy efficiency Mild operation process, zero emission and most importantly, unlimited renewable source of reactants especially biotic fuel cells are commonly known as biological or

* Corresponding author.

E-mail address: yilikaldessie@gmail.com (Y Dessie).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2019.07.001

2468-2179/© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license ( http:// creativecommons.org/licenses/by/4.0/ ).

MFC It converts the chemical energy of different fuels into a useful

form of electrical energy similar to that of batteries However, fuel

cells do not stop working until the fuels and oxidants are

contin-uously fed The main areas of fuel cell technology are from

trans-portation, stationary to the small-scale portable power source[7]

Goswami et al reviewed a typical conventional fuel cell in which its

fuel is oxidized at the anode and oxygen as an oxidant is reduced at

the cathode (Fig 1)[4] This review aims to investigate the

funda-mentals and recent progress of manganese oxide-based

electro-catalysts towards ORR capability for remediation, electricity

harvesting, and energy conversion at the air-cathode electrode in

the bioelectrochemical device systems According to the references

cited in each section, a clear and brief summary of the principle,

mechanism, and synthesis strategy for manganese oxide based

NMs (bare, doped or composites, as well as, supported.) involved in

the ORR is presented

2 Oxygen reduction reaction (ORR) on biocathode

ORR is the most important reaction that occurs in the cathode

surface of fuel cells[4] Interconnecting living systems or their

parts with simple and low-cost transition metal oxide to speed

up chemical reactions on the surface of an electronic conductor

for energy conversion and waste treatment is known as a

bio-catalyst[8] Besides their cost-effectiveness, the durability of the

ORR catalysts in MFC is another major challenge, because the

cathode is constantly exposed to waste effluents containing a

variety of contaminants (either organic or inorganic) and

mi-croorganisms The common assembly of this cell is

single-chamber and double single-chamber system In single-single-chamber MFCs

(Fig 2a) the catalysts directly contact the waste and may be

poisoned by intermediate products such as methanol, chloride,

sulfide, etc Catalyst poisoning by such intermediates leads to

high potential loss and reduced power production Furthermore,

organisms can form a biofilm on the cathode surface and

degenerate catalytic performance by blocking the O2transport

In some circumstances, the biofilm may serve as a biocatalyst,

however, similar problems may happen in two-chamber system MFCs (Fig 2b)[8]

When an appropriate and environmentally safe half potential electron acceptor (e.g., oxygen) is present in the cathode, the cell is then thermodynamically favorable, so that the electronflow across the whole system is spontaneous The most important reactions that take place in the cathode of the fuel cell are the ORR Such a reaction is a challenge in thefield of catalysis and electrochemistry Normally, the reaction is a complex four-electron transfer reaction that involves the breaking of a double bond from oxygen molecule and the formation of 4 OH-bonds through several elementary steps and intermediate species To understand such a complex process in ORR, Jiang et al developed manganese oxide catalyst using carbon

as a supported material to understand its intrinsic catalytic behavior in alkaline electrolytes[9] A general classical scheme for this reaction and hydrogen peroxide (H2O2) stability as a reaction intermediate species was described inFig 3 [9] Therefore, other approaches due to its broad reaction pathways, a theoretical calculation are needed to shed light on the microscopic structures and processes taking place at the surface during the reaction[10] Oxygen is an ideal electron acceptor molecule for MFCs due to its high redox potential, availability, and sustainability However, the ORR is kinetically sluggish, resulting in a large proportion of potential loss known as activation loss next to concentration and ohmic losses To reduce such loss, MnO2 as a nanocatalyst has been successfully used as a cathode material in both aqueous and non-aqueous fuel cells However, most of these oxides as ORR achieved only half of that with platinum/carbon (Pt/C) electrode, regardless of structure modifications or doping with other tran-sition metals Therefore, this review also summarizes how this different nanostructured component of MnXOY (MnO2, Mn2O3,

Mn3O4, Mn5O8, and MnOOH)[8], coexists and their contributions would be clearly discussed to ORR Beyond ORR some manganese oxides such as MnO2, Mn2O3and Mn3O4, which were synthesized

by a hydrothermally technique, exhibit the best catalytic activity performance towards NO oxidation with its maximum conversion

efficiency of 91.4% [11] Thermally decomposed ε-MnO2 from manganese nitrate using carbon powder as a support mixture was

Fig 1 A typical fuel cell Adapted with permission from Elsevier [4]

tested for ORR in an alkaline electrolyte The complete 4e

reduction pathway via a 2 plus 2e
reduction process were

pro-ceeds on MnO2catalysts through involving H2O2as an

interme-diate The close performance forε-MnO2/C material in comparison

with 20% (w/w) Pt/C catalyst as a benchmark was observed in the

kinetic control region due to the presence of structural defects in

this oxide This structurally modified catalyst thus has higher

electrochemical activity for proton insertion kinetics[12] The art

of these structural defects plus electrical conductivity ona-MnO2

effectively reduce active polarization followed by maximizing

kinetics [13] Thermal reduction of Mn2O3 to MnO had also

occurred for hydrogen production in solar energy concentration

devices[14] All this catalytic activity is facilitating due to increase

in activity of the surface amorphous MnOx, especially in a highly

monodisperse amorphous MnXOYnanosphere[15] NiOx hybrid

could tune the performance for its catalytic superiority towards

reversible oxygen evolution reaction (OER) and ORR, due to the

synergistic effect of NiOx and amorphous MnXOYon the surface of

graphene nanosheets after being synthesized with a

self-assembly method[16]

Less ecological impact, low operating temperature and high

energy density proton exchange membrane fuel cells (PEMFCs)

have gained much attention In such a fuel-cell device, the fuel can

be any of hydrogen, methanol, ethanol, or formic acid, whereas, highly electronegative oxygen molecule is chosen to receive the electrons released from the fuel on the cathode electrode Overall, between the fuel oxidation reaction and the ORR, electronsflow outside the cell to power electronic devices and protons migrate from anode to cathode inside through the Nafion membrane to complete the chargeflow in the circuit[17] Despite their great potential, PEMFCs do have their own serious limitations that pre-vent them from being scaled-up for commercial applications due to using expensive noble metal[18]

ORR is a multi-electron transfer process that follows an elec-trocatalytic inner sphere mechanism The reaction is highly dependent on the nature of the electrode surface ORR is, in general,

a suitable mechanism that employed for the formation of different oxygen-containing intermediates (such as OH, O2
, O, H2O2 and

HO2
) under both acidic and alkaline media [4] In an aqueous

medium, ORR is highly reversible The possibility of rapidly reversible redox transformation in nanophase MnOx at room temperature triggered by changes in hydration[19]and potential surface Mn(IV)/Mn(III) redox couple[20,21] However, the appro-priate mechanisms associated with ORR have not been well

Fig 2 Schematics of (a) single-chamber MFCs and (b) two-chamber MFCs Adapted with permission from The Royal Society of Chemistry [8]

Fig 3 Reaction pathways proposed for the ORR This figure by G
omez-Marín et al is reproduced from Ref [10] and is licensed under CC BY 2.0 ( http://creativecommons.org/ licenses/by/2.0 ).

understood in spite of extensive experimentation From this point

of view, ORR is regarded as the kinetically limiting element of the

electrochemical devices due to slow reaction rate This inspired the

researchers to fabricate a novel mesoporous MnXOYelectrocatalyst

support due to their large specific surface area and unique pore

structure which can permit more active sites for the contact

be-tween catalysts and electrolytes Such contact clearly demonstrated

a good competitive ORR activity at the cathode in fuel cells, which

can be used to reduce the loss of cell energy storage and conversion

efficiency by overcoming the slow kinetics during ORR[22]

Pt based materials have been used as a common ORR owing to

its high electrocatalytic activity, stability and high exchange current

density However, their high price/or cost, scarcity and low

dura-bility limit them from extensive commercialization In addition, the

stability of Pt is low as it suffers from dissolution, agglomeration,

coalescence, poisoning during fuel cell reaction conditions, and

sintering whichfinally results in an unusual rise in overpotential

for ORR, thus reducing both the active catalyst surface area and the

catalytic efficiency, which leads to an undesired increase in

over-potentials for fuel-cell device system, especially for ORR Therefore,

exploring highly abundant, low cost and durable electrocatalysts

with comparable or even higher catalytic performance than that of

Pt-based electrocatalysts have become a key interest [23] Zeng

et al have proved by hybridizing MnO2with Ag4Bi2O5that exhibits

superior long-term durability and stronger methanol tolerance

than commercial Pt/C for ORR in alkaline solution Based on this

concept one way to make PEMFCs inexpensive and durable is to

incorporate Pt more in the catalyst layers It can be done by using

effective support materials (e.g., carbon materials) along with Pt

nanoparticles or alloying Pt with other inexpensive metals like Co,

Ni, Fe, etc.[24] However, this approach didn't work well on a

long-term basis due to the ever-growing price of Pt[25] Therefore, this

review gives attention to non-precious metal or non-noble metal

oxides (e.g., manganese oxide and its nanocomposites) based

cat-alysts in detail as cost-effective alternatives to Pt For current ORR,

Konev et al described effective electrocatalytic NMs with good

properties from manganese and cobalt polymorphing films[26]

Other than this mixed metal polymorphing, a highly active and new

microporous based manganese porphyrin-polymer networks

catalyst was successfully fabricated for ORR catalytic activity and its

selectivity[27]

3 Manganese oxide based nanomaterials

Transition metal oxides (TMOs) are attractive noble metal-free

catalysts for oxygen reduction application at the cathode of

alka-line based membrane fuel cells or metal-air batteries [28]

Theo-retical and experimental studies revealed that TMO based catalysts

having a spinel structure can act as a proficient cathode electrode

material for energy conversion and storage devices For example,

manganese mixed oxides[29], manganese-iron mixed oxide doped

with TiO2[30], layered manganese-cobalt-nickel mixed oxides[31],

layered copper-manganese oxide[11], well dispersed spinel

cobalt-manganese oxides [32], cubic Mn2O3-carbon [33], and bond

competition control manganese oxide [34] have attracted much

attention for excellent Bifunctional catalytic activity They also have

higher electrical conductivity than single TMOs This unique

prop-erty of TMOs facilitates better ORR performance, due to their

vari-able oxidation states and better mixing ability into one material

Comparatively, by forming a nanosized bimetallic cluster[35], and

bimetallic oxide, its oxygen reduction and oxidation performance

could be also performed[36] Furthermore, TMOs are commercially

affordable due to their low price and high abundance, which makes

them to be used often as electrocatalyst[37] The review extended

more focuses on manganese oxide; one part of TMO based catalyst synthesis, principle, and mechanism activities towards ORR

In the last few years, the study of nanoparticles has acquired enormous interest due to their variation in physicochemical, elec-tronic, and morphological properties As a functional material, they can be synthesized in the nanometric scale ranges Due to changes

in their structure and bonds, they have also displayed interesting electronic and catalytic properties Controlling the structure of catalysts at the atomic level provides an opportunity to establish a detailed understanding of the catalytic form-to-function and realize new non-equilibrium catalytic structures[38] Molecular-level by itself is a factor that determines reaction mechanisms and electrocatalytic activity[39] Due to these properties, manga-nese oxides can be considered to be the most complex of the metallic oxide compound Afterward, manganese oxides as NMs have been studied due to their efficient uses in rechargeable lithium-ion batteries [40e42], a simple energy conversion[43], catalysts[44,45], capacitors[46], sensors[47,48], remediation[49], flame retardants[50],fire safety[51], alkaline fuel cells[52], radical scavenging and cytoprotection[53], pharmaceutically active com-pounds removal [54,55], biofilters[56], oxidative transformation

[57], and water splitting[58] For an effective application, the synthesis of nanocrystalline manganese oxides has been used more widely within the context of solution chemistry For example, synthesis using thermolysis from organometallic precursors[59], directly mixing of potassium per-manganate [60] and polyelectrolyte aqueous solutions [61], co-precipitation [62e64], room-temperature synthesis [65], hydro-thermal[66,67], solvothermal[22], plant[68], biological[69e71], wet chemical method[72,73], electrospinning[74], solegel[75,76], sonochemical [77e79], microwave-assisted [80e82], complex decomposition [83], chemical reduction [84], electrochemical method[85,86], direct electrodeposition[87], sulfur-based reduc-tion followed by acid leaching[88], are the most common and simple types of techniques to fabricate MnXOYnanocatalyst Obvi-ously by simple thermal treatment method different forms of manganese oxides (MnO, Mn3O4, Mn2O3, MnO2, as well as the metastable Mn5O8) could be fabricated at different conditions[89] These oxides as shown fromTable 1could coexist and progressively change one into the other during the oxidation process, which is usually controlled by the diffusion of oxygen There are also several relations between these manganese oxides[59,90]

Manganese monoxide, MnO, occurs as the mineral manganosite Nanocrystals of this oxide are capped with organic ligands and highly dispersible in nonpolar solvents It can generate particle sizes between 7 and 20 nm by controlling the reaction conditions and from 5 to 40 nm particle sizes were controlled by changing the surfactant Structural characterization showed that the nano-particles had core/shell structures with a thin Mn3O4shell[90] The manganese oxide hausmannite, Mn3O4, is a black mineral that forms the spinel structure with tetragonal distortion due to a JahneTeller effect on Mn3þ In the Mn2þ(Mn3þ)2O4structure, the

Mn2þand Mn3þions occupy the tetrahedral and octahedral sites, respectively, and Mn3O4 is ferrimagnetic below 43 K Nano-crystalline Mn3O4has been synthesized by a number of methods

Table 1 Different manganese oxide nanostructural products obtained at different calcination temperature ranges.

In air, N 2 or O 2 MnO 2 ƒƒƒƒƒƒƒƒƒƒƒ!550oC
600oCMn 5 O 8 ƒƒƒƒƒƒƒƒƒƒƒƒ!850oC
1050oCMn 2 O 3 ƒƒƒƒƒƒƒƒƒƒƒƒ!950oC
1050oCMn 3 O 4

In H 2 MnO 2 ƒƒƒƒƒƒƒƒƒƒƒ!400oC
500oCMnO

In pure O 2 gas MnOƒƒƒƒƒ!190oC

Mn 3 O 4 ƒƒƒƒƒƒƒƒƒƒƒƒ!430oC
470oC

Mn 5 O 8 ƒƒƒƒƒ!510oC

Mn 2 O 3

MnOƒƒƒƒƒ!390oCMn 3 O 4 ƒƒƒƒƒ!450oCMn 2 O 3

that produce relatively monodisperse particles 6e15 nm

nano-particles were obtained by thermal decomposition of manganese

(II) acetylacetonate in oleylamine under an inert atmosphere They

manipulate the particle size changing the employed reaction

temperature Pure MnO with size in between 11 and 22 nm can also

be obtained when a small amount of water was added to the

re-action slurry; therefore, the metal-oxide phase is controlled by the

presence or absence of water Mn2O3exists in two forms,a-Mn2O3

andg-Mn2O3 Almost purea-Mn2O3occurs as the mineral bixbyite

with black and crystallizes form in a cubic structure[90]

The manganite, g-MnOOH is the most stable and abundant

mineral of the three polymorphs of manganese oxyhydroxide The

other two are feitknechtite and groutite Both of them have the

same chemical formula, Mn3þO(OH) but differ with their crystal

system The crystal system of feitknechtite is a hexagonal while

groutite is orthorhombic The manganite crystal structure is similar

to that of pyrolusite, but, in all the Mn is trivalent and one-half of

the oxygen atoms are replaced by hydroxyl anions The Mn(III)

octahedral are quite distorted because of JahneTeller effects In air

manganite alters at 300 C to pyrolusite The metastable oxide,

Mn5O8is not a well-known compound of manganese It may be

formed together with pyrolusite during the diagenetic

decompo-sition of manganite Mn5O8is established as an intermediate phase

too, forming between MnO2and Mn2O3at or above 300C The unit

cells of pyrolusite, manganite, and Mn5O8are closely related, their

crystallographic axes remain in nearly the same relative

orienta-tions Mn5O8 crystallizes in a monoclinic structure containing

mixed valences of Mn2þand Mn4þas Mn2 þMn3þO8[90] Doping

manganese with latest transition metals such as cobalt encased

within bamboo-like N-doped carbon nanotubes [91] and

lanthanum at a time can increase the stability and improving the

catalytic activity for ORR[92,93]

One of the challenges in the synthesis of oxide nanoparticles is

obtaining monodisperse nanoparticles Furthermore, it is also

necessary to control the overall sizes of nanoparticles, as well as to

know its exact composition In particular for manganese oxides,

one of the most important challenges is to obtain a single phase,

because in almost all procedures the obtained result is significant

for coreeshell structures All these challenges are not easy to solve

since nanoparticles are unstable during long periods of time

Furthermore, since nanoparticles are highly reactive, they oxidize

easily in air, losing catalytic activity and dispersibility Because of

this, protection strategies are used, like capping with surfactants,

organic products or inorganic membranes[90]

4 Principle and mechanism of ORR on MnxOysurface

Based on the principle of physical adsorption of oxygen

mole-cule (O2) on manganese oxides surface due to high contact between

electrolyte and active catalyst, O2is converted to either OH
or H2O

During this conversion, manganese oxides are known active

cata-lysts in a given media [94] Practically, in alkaline solution, the

mechanism of oxygen reduction at MnO2catalyzed air cathode was

investigated by measurements of polarization curves in a wide

range hydroxide ion (OH
) concentration, oxygen pressures, and

using different crystalline MnO2catalysts[95] ORR at the cathode

surface precedes either partially or two electron reduction

path-ways results in the formation of adsorbed H2O2species and direct

four-electron reduction pathways Direct four electron pathways

are more desirable for ORR than the partial reduction pathway

since the reactivity of H2O2is comparatively higher than that of the

stability of H2O[96] The direct conversion of O2into H2O involves a

dissociative mechanism, where thefirst step is the adsorption of O2

on the metal/catalyst surface followed by breaking off the

oxygeneoxygen bond to give adsorbed oxygen atoms

Subsequently, transfer of electrons to the adsorbed oxygen atoms in the form of hydrogen addition, yields surface-bound hydroxyl groups Further reduction and protonation of the hydroxyl group produce the H2O molecule leaving behind the metal/catalyst sur-face On the other hand, partial reduction of O2follows an asso-ciative mechanism in which the adsorption of O2 on the metal surface doesn't lead to the cleavage of oxygeneoxygen This alter-native two-electron reduction pathwayfinally generates H2O2[17] For more clarifications, Table 2 shows the pathways of ORR in alkaline and acidic medium[97] It is interesting that graphene-oxide-intercalated layered manganese oxide enhanced four elec-tron transfer activity towards ORR in alkaline media at 0.8 V vs RHE

[98] Because, electrodeposition of manganese oxide into a gra-phene hydrogel not only improves the carbon material's capacitive performance, but also affects the surface chemical environment of the graphene-oxide framework [99] Controllable growth from uniform nanoparticles with specific morphology to obtain a high active electrocatalyst is a key common problem in developing

efficient energy conversion and storage devices [91,100] Even though to reduce such challenge, Sun and his coworker Liu have fabricated a nanoflake oxygen reduction ternary composite catalyst from manganese oxide and CNTs-graphene support for an elevated power performance in pilot scale manufacturing technology[101]

A typical ORR polarization curve (Fig 4) is generally divided into three regions, these are kinetically controlled region, diffusion controlled region and mixed kinetic and diffusion controlled re-gions On the kinetically controlled region the rate of O2reduction

is slow with a small increase in the current density as decreasing potential A substantial rise in the current density is observed in the mixed kinetic and diffusion controlled area In this region, accel-eration of the reaction takes place with a marked drop in the po-tential value In the diffusion controlled region, the current density

is determined by the rate at which diffusion of the reactants occurs Quantitative analysis of the catalyst in terms of its activity can be done from the two parameters i.e., the onset potential (Eonset) and the half-wave potential (E1/2) The more positive is the potential, the more active will be the catalyst towards ORR JLdenotes the diffusion limited current density[102]

Achieving efficient catalysis for ORR plays an important role in energy conversion, even if manganese oxides have attracted enor-mous interest due to their unique catalytic properties, manganese

as an element in a higher extent may cause a potential limitation to plant growth on acidic and poorly drained soils[103] Due to the high theoretical capacitance of manganese oxide nanomaterials,

1370 F g
1, a huge number of works is devoted to these materials

[46] Due to its variable oxidation state of manganese (þ2, þ3, þ4, þ6 and þ7), it contains various morphologies and crystallographic forms The structuralflexibility in its oxides form (e.g., available in binary oxide type or capable of incorporating) with another metal can also form a composite structures perovskite

[104], three-dimensionally ordered macroporous perovskite LaMnO3with increased specific surface area and pore volume[105], and spinels [106] Moreover, the Mn2O3 nanoparticles catalyst withþ3 oxidation state exhibits higher ORR activity compared to

Table 2 ORR pathways in alkaline and acidic medium [97] Electrolyte Pathway ORR Alkaline aqueous solution 4 e

2 e

O 2 þ H 2 O þ 4 e
/ 4OH

O 2 þ H 2 O þ 2 e
/ HO 2
þ OH

HO 2
þ H 2 O þ 2 e
/ 3OH

Acidic aqueous solution 4 e

2 e

O 2 þ 4H þ þ 4 e
/ H 2 O

O 2 þ 2H þ þ 2 e
/ H 2 O 2

H 2 O 2 þ 2H þ þ 2 e
/ 2H 2 O

the Mn3O4nanoparticles catalyst with mixed (þ2, þ3) oxidation

state[107] Tang et al have reported a nanobelt bundles manganese

oxide with its specific surface area of 160 m2 g
1 The nanobelt

bundle exhibits good capacitive behavior and cycling stability in a

neutral electrolyte system, and its initial capacitance value is

268 F g
1[108] A nanostructured manganese oxide which is

syn-thesized by a simple hydrothermal route at a very low temperature

of 60C using potassium permanganate as oxidant and ethanol as

reductant succeeded with a maximum specific capacitance of

198 F g
1[109] Aflexible and binder-free cathodic electrode for

electrochemical capacitors was prepared from electrodeposition of

manganese oxide onto reduced graphene oxide paper From (Fig 5)

hausmannite phase manganese oxide coated electrodes exhibit a

promising performance at a specific capacitance of 546 F g
1with

current density 0.5 A g
1and 308 F g
1with a scan rate of 1 mV s
1

in chargeedischarge and cyclic voltammetry measurements,

respectively During potential cycling, phase transformation of

Mn3O4to mixed-valent MnOxwas observed Consequently, MnOx

nanostructures on self-standing reduced graphene oxide electrodes

have succeeded with 154% capacitance retention at 10,000 cycles

from cyclic voltammetric data[110]

In order to fully leverage their potential application, a precise

control over particle size, surface area, and Mnxþoxidation state

properties is required Here, the inverse micelle solegel method

which is categorized by the heat treatment can control such

properties followed by keeping tenability and crystallinity[111]

and calcination of Mn(II) glycolate nanoparticles using polyol

technique was used to synthesize a mesoporousa-Mn2O3, Mn3O4,

and Mn5O8 nanoparticles The authors conclude that these

different oxidation states of manganese oxide nanoparticles using

such route can facilitate their actual structuraleproperty

rela-tionship In situ X-ray diffractometer measurements suggested

that different MnOxphases were observed From the analysis, it is

conclude that a complete time and temperature dependent phase

transformations were occurred successfully from Mn(II) glycolate

precursor oxidation toa-Mn2O3via Mn3O4and Mn5O8in O2

at-mosphere From sweep voltammetry measurements, mesoporous

a-Mn2O3showed a good kinetic enhancement potentials for ORR

in aprotic media[59]

In the electrochemical ORR, H2O2has been detected as a reac-tion intermediate on TMO and other electrode materials Hence, the electrocatalytic and catalytic reactions of H2O2on a set of manga-nese oxides such as Mn2O3, MnOOH, LaMnO3, MnO2, and Mn3O4, were studied All of these different crystal structures were adopted

to shed light on ORR mechanisms Among MnO2has attracted great attention due to its high catalytic activity, thermal stability, facile synthesis with low-cost materials and availability in various crystal morphologies [112] Kinetic modeling and experiment objective correlates the differences in the ORR activity to the kinetics of the elementary reaction steps displayed that the catalytic activity of

Mn2O3was better in the ORR due to its high catalytic activity both

in the reduction of oxygen to H2O2detection with its unique crystal structure and reactivity shown from the tentative mechanisms (Fig 6)[47] Previously, aggregates of gold nanoparticles (AuNPs)

on manganese dioxide nanoparticles (nano-MnO2) was developed for better H2O2 amperometric sensing [113] Electrodeposited manganese oxide in the average size range of 21e40 nm was identified with different phases (MnO, MnO2, and Mn3O4) for H2O2

detection[114] Till now, a represented divalent alkaline-earth metal ion or trivalent rare-earth metal ion (such as perovskite (AMnO3) or spinel (AMn2O4) structure) adopted by Mn-based oxides displays an

efficient ORR activity [115] To improve the activity of Mn-based oxides oxygen defects have been introduced by thermal reduction which reduces Mn4þto more active Mn3þ, and improves the elec-trical conductivity However, the overall ORR activity of Mn-based oxides has been still higher than that of Pt/C To design an active ORR catalyst the oxidation state of manganese centers is critical The intermediate species for this reaction is Mn3þ, which plays a significant role in the success of catalytic activity in ORR To boost the catalytic activity, numerous approaches have been made to generate the active Mn3þ species The high catalytic activity of

Mn3þspecies is attributed to the presence of one electron resulting

in JahneTeller (JeT) distortion [116] Therefore, to achieve high

Fig 4 A characteristic ORR curve of an individual catalyst Adapted with permission from the Wiley publishing group [102]

specific ORR activities, the presence of Mn3þwith some Mn4þis the

key in perovskites

Cao et al also showed the dependence of electrochemical

ac-tivity on the crystalline structure of manganese oxides The

ob-tained result shows that, the ORR current of different MnO2

catalysts increase in the following order:b-MnO2<l-MnO2<g

-MnO2 < a-MnO2 < d-MnO2 [95] The specific morphology and

crystalline structure effect ona-MnO2nanowires,a-MnO2 nano-rods, b-MnO2 nanowires, and b-MnO2 nanorods have been suc-cessfully synthesized via a hydrothermal process, and their microstructures and electrocatalytic activities were investigated for ORR Among the four different types of one-dimension MnO2, the

a-MnO2 nanowires exhibited significantly larger electrocatalytic property than the others This is due to the highest electron transfer

Fig 6 A tentative mechanism for the oxygen reduction/H 2 O 2 reactions Adapted with permission from the Wiley publishing group [47]

Fig 5 (a) Cyclic voltammograms of Mn 3 O 4 /rGO with different mass loads at 20 mV s
1; (b) Cyclic voltammograms of Mn 3 O 4 /rGO deposited at
1.1 V with a fixed charge of

500 mC at different scan rates; (c) Capacitance retention of the film as a function of cycle number; (d) Nyquist diagram of Mn 3 O 4 /rGO along with the equivalent circuit to fit the experimental data (solid line represents the fitted curve) Adapted with permission from Elsevier [110]

number, which may contribute to the special crystalline structure

and larger specific surface area As a result, more active sites could

be exposed in the three-phase (electrolyte, oxygen, and catalyst)

interfaces during the whole reaction process and thus enhance the

ORR catalytic performance[117] Lower valence state manganese

oxides are also targeted for ORR; Mn3O4is rich in electrochemical

properties due to the mixed valence of Mn However, because of its

poor electrochemical structural stability and low electrical

con-ductivity, its use as ORR catalyst is diminished Goswami et al have

reviewed carbon-coated tubular monolayer superlattices (TMSLs)

of hollow Mn3O4NCs (h-Mn3O4-TMSLs) by exploiting the structural

evolution of MnO nanocomposites They have characterized the

catalyst by various techniques From transmission electron

micro-scopy and x-ray diffractometer characterization result (Fig 7a), it

can be seen that the average diameter of the particles is 18 nm

Fig 7b and c shows the effectiveness of this in achieving

high-quality nanocrystal monolayers within anodized aluminum-oxide

channels The XRD pattern of MnO@Mn3O4@AAO mainly ascribed

to the cubic MnO phase shown inFig 7d The presence of Mn2þand

Mn3þcan be clearly showed from x-ray photoemission

spectros-copy result (Fig 7e)[4] Due to lack of accepted protocols for its

precise catalytic activity measurement, Mn/polypyrrole (PPy)

nanocomposite has a unique quantitative assessment for the ORR

electrocatalytic activity in alkaline aqueous solutions based on the

rotating risk electrode method[118]

Hazarika et al have synthesized mesoporous cubic Mn2O3

nanoparticles supported on carbon (Vulcan XC 72-R) for both ORR

and OER They have shown that the ORR activity of Mn2O3/C

material is much better compared to the commercially available Pt/

C and Pd/C in alkaline media However, Mn2O3without the carbon support shows less ORR activity compared to Mn2O3/C, Pt/C and Pd/

C From the parallelfitting lines of the KeL plots the average elec-tron transfer number was found to bez1.2 and z4.1 for Mn2O3 and Mn2O3/C, respectively The high catalytic activity is due to the synergistic influence of Mn2O3and carbon interface They have also proved that Mn2O3/C is quite stable up to 1000 cycles and the re-action follows a 4-electron pathway for ORR[33]

The combination of Mn oxide with other TMOs (such as Co, Fe,

Cu oxides, etc.) provides excellent ORR activity useful for a range

of applications The high catalytic activity is due to the synergistic effect of the mixed TMOs Li et al have prepared ultra-small cobalt manganese spinels using simple solution-based oxidation pre-cipitation and insertion-crystallization process at the mild con-dition They have studied the catalyzation of nanocrystalline spinels for ORR Furthermore, strongly coupled carbon support spinel nanocomposites exhibit similar activity except superior durability to carbon support platinum catalyst[42] Even struc-tural and surface changes can happen on manganese oxide after modification using cobalt during activation within ethanol steam reforming reaction[119]

5 Effect of cationic dopants and media on MnOxstructure

A series of calcium-manganese oxides (CaMnO3, Ca2Mn3O8, CaMn2O4, and CaMn3O6) and the detailed investigation of their electrocatalytic properties were reported through a simple

Fig 7 (a) TEM image of octahedral MnO NCs used for constructing h-Mn 3 O 4 -TMSLs; Cross section SEM images of MnO NC monolayers self-assembled within the AAO template having (b) circular and (c) hexagonal channels, respectively; (d) XRD patterns of MnO@Mn 3 O 4 @AAO and h-Mn 3 O 4 -TMSLs, respectively The blue asterisks denote the reflections of

Mn 3 O 4 ; (e) High-resolution Mn 2p XPS spectra of MnO@Mn 3 O 4 NCs and h-Mn 3 O 4 -TMSLs, respectively Adapted with permission from Elsevier [4]

calcination route using Ca1
xMnxCO3solid-solution precursors The

parallel formation of highly crystalline CaeMneO porous

micro-spheres with similar textures and its ORR catalytic activities of the

synthesized CaeMneO compounds were compared with MnOxin

alkaline conditions and Pt/C was used as a benchmark The

exper-imental and theoretical study demonstrated that the surface Mn

oxidation state and crystal structure are influential factors to

determine CaeMneO electrocatalysts activity In recent years,

CaeMneO has captured strong scientific interest due to

excep-tional catalytic activities, inexpensive method of synthesis,

abun-dant, and environmentally benign nature of elements In particular,

the catalytic properties of CaeMneO systems have been reviewed

as a potentially useful new tool in addressing energy and

envi-ronmental problems[120]

Structurally, cation dopants ona-MnO2have a vital role for ORR

Hydrothermally synthesized nickel-dopeda-MnO2nanowires

(Ni-a-MnO2) at different weight ratio in general, had higher n values

(n¼ 3.6), faster kinetics (k ¼ 0.015 cm s
1), and lower in charge

transfer resistance (RCT¼ 2264 U at the half-wave) values than

MnO2or Cu-a-MnO2 This was happened due to the effective

sur-face defect functionality between nickel and manganese Therefore,

the overall catalytic activity for Ni-a-MnO2trended with increasing

Ni content, i.e., Ni-4.9%> Ni-3.4% was increasing[121] Off course

other than cation doping, surface topography or shape of a catalyst

have had different catalytic activity towards ORR For example,

Affandi and Setyawan reported that nanocatalysts were prepared

electrochemically from KMnO4precursor in different media

con-ditions at a temperature of 60C The result surface morphology

was nanorod at pH¼ 0.2 and nanoflake at pH ¼ 9, respectively The

acidity of the solution systematically influenced the particle

morphology As shown in Fig 8, the particles had nano-rod

morphology at a very acid solution whereas they had a

nano-flake shape at the base condition XRD revealed that the particle

generated at very acid condition wasa-MnO2while at basic

con-dition MnO2 was amorphous So, the electrocatalytic activity for

nanorod and nanoflake MnO2towards ORR of the materials was

studied in oxygen saturated 0.6 M KOH solution Thus, the number

of electrons transferred during ORR was 2.23 and 1.75, respectively This result suggested that nanorod MnO2particles was exhibited better ORR activity than nanoflake MnO2[86] Surface manganese valence of manganese oxides exhibits better catalytic activity to-ward the ORR than those with lower Mn valences on the activity of ORR in alkaline media[122]

A porous spinel-type of magnetic iron-manganese oxide nano-cubes with a hollow structure deposited on the reduced graphene oxide nanoflakes nanocomposite [123] and (CoMn2O4 and MnCo2O4) spinel microspheres reflect a high efficient catalyst for OER, as well as for the ORR The as-prepared cubic MnCo2O4 dis-plays better OER activity compared to the tetragonal CoMn2O4

material in an alkaline medium However, the tetragonal CoMn2O4 material display better ORR activity and stability compared to cubic MnCo2O4and also Pt catalysts Spinels structural features such as microspherical morphology and their unique porous results in the higher catalytic activity and stability of the material[106] Its spinel arrangements are continued because manganese oxides structure flexibility under working conditions remains a great challenge for identifying their active structures [124] Liang et al reported a manganese-cobalt spinel MnCo2O4/graphene hybrid is highly ef fi-cient in electrocatalyst for ORR in alkaline conditions They have suggested from the X-ray absorption near edge structure of Co L-edge and Mn L-L-edge that substitution of Co3þ sites by Mn3þ resulting in higher catalytic sites that enhance the ORR activity compared to the pure cobalt oxide hybrid Mechanically, such hybrid material possesses greater activity and durability than the physical mixture of nanoparticles and N-rmGO and the MnCo2O4/ N-graphene hybrid displays higher ORR current density and sta-bility compared to Pt/C in alkaline solutions at the same mass loading[109] In addition, the use of manganese ore as an oxygen carrier has recently gained interest, primarily due to the combi-nation of low cost and moderate to high reactivity The possibility of

an oxygen uncoupling reaction enhancing reactivity may be an additional advantage[125]

Fig 8 Tafel plots of electrodes for KMnO 4 solution at pH 0.2 and pH 9 electrolysis Adapted with permission from the Ceramic Society of Japan [86]

6 Applications of manganese oxide nanomaterial

6.1 Microbial fuel cells (MFCs)

The need to reduce the costs of renewable energy conversion

sources such as bioelectrochemical systems (BES) has pushed the

research towards alternative cathodes performing ORR to maintain

a catalytic efficiency close to that of platinum or platinum-based

catalysts[126] Conventional MFCs set up consist of biological

an-odes and abiotic cathan-odes Abiotic cathode usually requires a

catalyst or an electron mediator to achieve high electron transfer,

increasing the cost and lowering operational sustainability Such

disadvantages can be overcome by low cost biocathodes, which use

microorganisms to assist cathodic reactions The classification of

biocathodes is based on which terminal electron acceptor is

avail-able For aerobic biocathodes with oxygen as the terminal electron

acceptor, electron mediators, such as iron and manganese arefirst

reduced by the cathode (abiotically) and then reoxidized by

bteria Anaerobic biocathodes directly reduce terminal electron

ac-ceptors, such as nitrate and sulfate, by accepting electrons from a

cathode electrode through microbial metabolism[127] Manganese

by itself as a manganese peroxidase enzymes as catalyst could

apply as an enzymatic electrode in the cathode chamber of an MFC

Its output power density was 100% higher than that for the

con-ventional graphite electrode As a biocathode, its activation

over-potential loss was diminished during H2O2reduction (Fig 9)[128]

Air cathode (open air at the cathode) in a single chambered MFC

is the one which uses oxygen (O2) as a direct electron acceptor

species and is reduced by the electrons coming from the anode and

the protons via the membrane into water However, ORR on the

surface of air-cathode is one of the main drawbacks in MFCs The

reaction kinetics is limited by an activation energy barrier

(activa-tion polariza(activa-tion loss) which impedes the conversion of oxygen into

the reduced form at the cathode surface; hence, it requires an

efficient and effective catalyst for ORR Even at the laboratory scale

platinum (Pt) is the most practically used catalyst for the ORR in

MFC But due to its special case, Pt-based catalyst in large scale air

cathode MFCs is limited due to high-cost and dissolution at a short

lifetime in a given media In addition, other external factors also

directly affect MFC performance Therefore, to reduce cost followed

by increasing ORR rate, synthesizing a non-preciousa-MnO2 cata-lyst using a hydrothermal method is being a unique strategy for air-cathode application [67] Results from Table 3 found that 28.57 mg cm
2a-MnO2was going to be an optimum catalyst load with 13.40mW power output Later, nanostructured Mn2O3/Pt/CNTs was also used as a selective electrode for ORR and membrane less micro-direct methanol fuel cells (DMFC) in alkaline media Inter-estingly, during the cell reaction, there is no activity for methanol oxidation reaction, in contrast with Pt Even the bilayer cathode was tested in this membrane less micro fuel cell under mixed-reactant conditions, producing an open circuit voltage (OCV) with 0.54 V and a maximum power density of 2.16 mW cm
2[129] Tan et al have manufactured MnO containing mesoporous nitrogen-doped carbon (m-N-C) nanocomposite which was low-cost non-precious metal catalysts that perform high ORR in alka-line solution with four-electron transferred per molecule This nanocomposite involves the one-pot hydrothermal synthesis of

Mn3O4@polyaniline core/shell nanoparticles from a mixture con-taining aniline, Mn(NO3)2, and KMnO4, conducting polymer with metal precursors; and followed by heat treatment to produce N-doped ultrathin graphitic carbon coated MnO hybrids partial acid leaching of MnO The composite exhibits superior stability and methanol tolerance to commercial Pt/C catalyst, making it a promising cathode catalyst for alkaline containing methanol fuel cell applications The synergetic effect between MnO and N-doped carbon described, provides a new route to design advanced cata-lysts for such energy conversion device[130] Later, a 4þoxidation states of MnO2nanostructured material was fabricated by hydro-thermal technique; it was potentially applicable as cathode catalyst

in MFC due to their unique properties Hydrothermally synthesized MnO2 (HSM) was one-dimensional nanorod structured that accomplish a noticeable oxygen reduction peak current due to high

Fig 9 Open circuit voltageetime curve of MFC after H 2 O 2 addition to the cathode in the presence of MnP Adapted with permission from the World Renewable Energy Congress

[128]

Table 3 Maximum current and power produced by the MFCs with different catalyst loadings Catalyst loadings (mg cm
2) Maximum current (mA) Maximum power (mW)

28.57 130.14 13.40