Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications

Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications Bioenergy systems for the future 12 nanocomposites for “nano green energy” applications

Nanocomposites for “nano

green energy” applications

Liangdong Fan*, Muhammad Afzal†, Chuanxin He*, Bin Zhu† , ‡

*Shenzhen University, Shenzhen, PR China,†KTH Royal Institute of Technology, Stockholm,Sweden,‡Hubei University, Wuhan, PR China

NANOCOFC nanocomposite for advanced fuel-cell technology

Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00012-0

© 2017 Elsevier Ltd All rights reserved.

12.1 Introduction

Green energy resources are urgently demanded in recent years considering the largefossil-fuel consumption and increasing worldwide environmental concerns, especiallythe developing countries those are facing serious air pollution Though the internalcombustion engine has been well-recognized to contribute to the societies for manydecades, the low energy utilization efficiency has required for alternative candidates.Among several kinds of energy conversion devices and technologies, fuel cell showsunique characteristics of high efficiency and low environmental impact The electricefficiency normally reaches above 40%, which is already higher than the internal com-bustion engine If combined with the highly valuable exhausted heat due to the internalloss, the energy efficiency could be doubled (Minh, 1993)

A fuel cell is an energy conversion device that can directly convert chemical energyinto electricity by electrochemical process (Winter and Brodd, 2004) It is distinctfrom the conventional engine that the chemical energy should be converted to heatfirst, then to mechanical energy, and finally to the electricity Therefore, the fuel-cellefficiency is independent from the Carnot cycle and much higher than those of otherconventional energy conversion devices Generally, a fuel cell contains three keycomponents of porous anode and cathode while among them the sandwiched denseelectrolyte (Ormerod, 2003) A schematic illustration of fuel cell is presented inFig 12.1 The anode is the place where the fuel oxidation reaction takes place, releas-ing the electrons and producing the protons, while, in the cathode, the oxygen gains theelectron and is reduced to oxygen ions Depending on the applied electrolyte, the pro-ton or the oxygen ion transports through the electrolyte and meets with its counterpart

ions, to produce the water The electrons released at the anode move through the nal circuit providing the electric power for practical applications The detailed reac-tions in a fuel cell with proton-conducting electrolyte or with oxygen-ion-conductingelectrolyte are also presented inFig 12.1AandB, respectively.

exter-There are already several kinds of fuel cell; however, the first discovery of the fuelcell was in 1839 by Sir Grove (Grove, 1839) According to the applied electrolytes,they are divided into alkaline fuel cell (AFC), proton exchange membrane fuel cell(PEMFC), phosphoric acid fuel cell (PAFC), molten-carbonate fuel cell (MCFC),and solid oxide fuel cell (SOFC) PAFC was the first fuel-cell type that was success-fully employed for commercialization, while the AFC has been the focus at the middle

of the 20th century for military/space applications

PEMFC and SOFC have been put on the agenda in the recent years because of thesignificant development of the material and technology evolution (Wachsman andLee, 2011) The operational temperature of PEMFC with the simple methanol fuel

is suitable for vehicle applications and portable or electronic devices, while the bined heat and power resulted in high efficiency and unique characteristics of fuelflexibility make SOFC ready for stationary power plant applications or other auxiliarypower unit (APU) Interestingly, there is a contrary development tendency for PEMFCand SOFC The research activity in PEMFC tries to enhance the operational temper-ature from below 100°C to above 100°C by developing the suitable electrolyte mem-brane to overcome the constraint of the water-involved proton conduction and toimprove the electrode activity While SOFC community intentionally reduces theoperation temperature from the conventional 800–1000°C to 500–700°C or evenlower to overcome the high-temperature-related problems like electrode microstruc-ture change, element diffusion, and cell performance degradation issues

com-Compared with the PEMFC, SOFC shows the unique property of adoption of ious fuels including biogas, natural gas, hydrocarbon fuel, CO, H2S, and solid-statecarbon, besides H2 While the PEMFC can only use very limited fuel, such as H2

var-and methanol (Wachsman and Lee, 2011) Moreover, CO is the poisoning gas for

Pt in PEMFC The direct use of hydrocarbon fuel such as natural gas makes SOFC

as a candidate of transition power source to replace the current internal combustionengine to reach the hydrogen-powered society in near future Therefore, more andmore government and industrial investment and academic research efforts have beenput in SOFC field to promote the commercialization of this green energy technology

In fact, the demonstration of SOFC for power generation has been presented; the mostfamous case is the Siemens-Westinghouse tubular SOFC power plant though the pro-ject is abandoned after several years’ operation However, the successful demonstra-tion has inspired more effort and continuous input to this promising field Currently,

“Bloom Energy” has installed more than 150–200 MW of its fuel-cell systems in theUnited States since 2001 (Singhal, 2013) The fuel cell uses scandium-stabilized zir-conia as electrolyte, operating at 700°C with natural gas as the fuel The customersinclude Adobe, FedEx, Staples, Google, Coca-Cola, and Wal-Mart Another example

of SOFC success application is the Japan ENE-FARM project; there are more than150,000 fuel-cell units that had been installed for resident application The ENE-FARM system is a combined heat and power system; the total system efficiency couldNanocomposites for “nano green energy” applications 423

reach to 90%, still using natural gas as the fuel precursor Therefore, SOFC has beenthe edge of the wide application A recent work by Blum et al reported that the shortstack with anode-supported cell based on standard materials of Ni-cermet anode, YSZelectrolyte, and LSM-YSZ cathode presented a continuous operation time of 70,000 h(8 years) without significantly cell performance degradation (Blum et al., 2016).Therefore, we believed that SOFC technology will be the next-generation energy con-version device soon with wide applications.

One of the agreed targets between PEMFC and SOFC is to improve the fuel-cellperformance through developing the novel cell components with improved electricproperties and/or optimizing cell microstructure based on the existing materials Thelatter is even important since the newly exploited material should not only be highlyactive but also be robust and chemically and thermally compatible with the fuel-cellsystem Their wide application asks for lengthy, extensive practical atmosphereapplication Therefore, in parallel to the development of new materials, the researchactivities on cell component microstructure to improve the ionic conductivity of thepresented electrolyte and to enhance the triple phase boundary of the electrode layerare widely performed One of the recognized technologies to optimize the micro-structure of the cell comments is the nanotechnology In fact, the application ofnanotechnology has been widely used in low-temperature or high-temperaturePEMFC (Arico et al., 2005) For example, the state-of-the art electrode catalyticmaterial in PEMFC is platinum because of the high catalytic activities both for oxy-gen reduction and hydrogen oxidation reactions and high electronic conductivity.However, due to the limited material resources, the price of Pt has been increasedsignificantly in the recent years Hence, application of nanosized Pt particle loaded

on the porous carbon cloth/particle has been employed to reduce the Pt use whilemaintaining the electrode activity for fuel-cell power generation The particle size

of the Pt has been decreased <5 nm in some cases, and the loading amount is

43μg cm
2, reaching to the US DOE target (0.125 mg cm
2 by 2020) (Kaplan

et al., 2016) The recent work on nanotechnology in SOFC has also been improved

in recent years because of the progress made in reducing the working temperature(from<800°C to around 600°C) In response, the use of nanostructured cell compo-nents has enormously improved the electrode activity, though no consistent agree-ment on the nanostructure on the ionic conductivity of electrolyte was achieved(Cavallaro et al., 2010; Garcia-Barriocanal et al., 2008; Infortuna et al., 2008) More-over, nanocomposites have been widely used in improving the cell component per-formance and overcoming the nanomaterials instability issue in the harsh fuel-cellatmospheres, especially for SOFC at elevated temperatures Factually, the applica-tion of nanocomposite materials for SOFC key components has got huge attention

in recent years due to the large surface-to-volume ratio, distinct surficial, or cial or space charge layer property of nanomaterials and the synergic effect in com-posite with positive benefits to the current SOFC research and commercializationwork (Duan et al., 2015; Kim et al., 2006; Lee and Wachsman, 2014; Zhu, 2006,

interfa-2011) There have already been many papers summarized about the advances inapplication of nanostructure and nanocomposite materials in PEMFC and otherlow-temperature energy conversion and storage devices, but without including SOFC

(Arico et al., 2005) Therefore, this chapter aims to provide an overview of currentresearch in the field of SOFC on nanocomposite to improve the performance at thegiven temperature or to reduce the temperature at the certain fuel-cell power output,with the ultimate target of cost-effective and robust SOFC.

Among all cell components, electrolyte is considered as the most important part since

it determines the operational temperature of the constructed SOFC and related trode material Therefore, to develop low-temperature SOFC, research activities onthe new electrolyte material with improved ionic conductivity has always been thehot topic The state-of-the art electrolyte, yttrium-stabilized zirconia (YSZ) showedsufficient ionic conductivity above 800°C However, it does not qualify to be goodionic conductor for intermediate and low-temperature SOFCs based on commonand scalable fabrication technology Though reducing the thickness of the YSZ elec-trolyte layer up to nanometer level allows the system operate at 300–500°C, the com-plex cell fabrication technology and its high fabrication cost and the low power outputhinder its further applications (Garbayo et al., 2014) Therefore, the development ofnew material with improved ionic conductivity is needed

elec-In the last decades, many kinds of the electrolyte materials with interesting electricproperties have been investigated Nevertheless, the practical application to replaceYSZ is a question due to their distinct problems, like the electronic conduction of ceriaoxide-based electrolyte, high reactive activity with normally used electrolyte and easyformed second phase during fabrication of LSGM electrolyte, and the trade-offbetween ionic conductivity and the chemical stability of proton-conducting electro-lytes, not mentioned other newly developed ionic conductors with even more issues

In addition, the ionic conductivities of these developed new electrolyte materials arestill less than the target value of 0.1 S cm
1below 600°C (Etsell and Flengas, 1970).The following section will present several approaches to overcome the existing issues

or improve the ionic conductivity of the conventional electrolyte by nanocompositeapproach, to meet at least one of the requirements of a good ionic conductor

12.2.1 Conventional electrolyte based nanocomposite

Ceria-based solid electrolyte has much better ionic conductivity than YSZ, it alsoshows good catalytic activity toward ORR and HOR, and it has the additional merit

of compatibility with the currently most used electrode material Therefore, it hasreceived particular attention in SOFC field However, the low oxygen partial pressureinduced electronic conduction, and the lattice expansion has limited its wide applica-tion How to overcome the electronic conduction in ceria has become a challenge toSOFC researcher YSZ is a good ionic conductor with ionic transport number almostunity at wide operational oxygen partial pressure.Bellino et al (2008)then preparedYSZ-yttrium-doped ceria nanocomposite through a fast firing process (from 200°

C min
1up to 1100–1200°C and dwelled for 3 min) with nanograin precursor Theyfound that the electronic conductivity is negligible in the studied nanocomposites withNanocomposites for “nano green energy” applications 425

much improved mechanical properties Moreover, the ionic conductivity ment with the decrease of crystal size of the nanocomposite is observed, which isthought to be the enhanced grain boundary conductivity in this nanocomposite DopedBaCeO3oxide is one of the best proton-conducting ceramic electrolytes However, thechemical instability in H2O- or CO2-containing atmosphere is a big barrier for itsdeployment in SOFC real condition Sun et al (2011) recently fabricated dopedBaCeO3-SDC nanocomposite by one-step gel combustion method The compositeshowed much improved open-circuit voltage (OCV) in fuel-cell condition, 0.2 Vhigher than SDC-based fuel cell in the same atmosphere More recently,Sun et al.(2014)also reported a SDC@Ba(Ce, Zr)1
x(Sm, Y)xO3
δcore/shell electron-blocking

enhance-nanocomposite layer that was formed when cosintering SDC electrolyte withNi-BaZr0.8Y0.2O3
δfunctional anode The resulted cell gave an OCV close to the bestvalue of ceria-electrolyte-based SOFC and presented a stable and high performanceduring 50 h testing In fact, before this work,Zhu et al (2004)reported that the low-temperature hot-pressed doped ceria-BaCeO3-based nanocomposite SOFC presented

4 times and 2.6 times higher peak power outputs than the doped BaCeO3and dopedceria-based fuel cells at 550°C and the same fuel-cell condition, respectively There-fore, the application of nanocomposite is a well-demonstrated method to overcomethe shortcoming of the constitute phase and to improve the electric properties (OCVand peak power output) compared with the single-phase materials

12.2.2 Hetero-structured nancomposite

Versatile electrolyte materials have been exploited with improved ionic conductivity.However, these materials have not yet reached the target value of 0.1 S cm
1at thelow-temperature range or not been able to construct a real fuel cell using theabovementioned heterostructured nanocomposite with super conductivity(Pergolesi et al., 2010) The development of LT-SOFCs has been limited by the lack

of electrolyte systems that could exhibit the necessary elevated conductivities to vide acceptable power outputs Considering most of the current ionic conducting elec-trolyte materials, the ionic conductivity is limited due to structure factor: the dopinginduced vacancy defects with an optimized content In addition, the hopping conduc-tion of ions in these electrolytes asks for large activation energy to activate the ionictransfer Therefore, novel electrolyte design and synthesis should be invented to over-come the above barriers According to the ionic conduction theory, the conductivity isproportional to the charge-carrier concentration (C) and the charge mobility (μ) based

pro-on the following equatipro-on:

Since the single-phase material has the above constrains of the charge-carrier tration and mobility, the development of two-phase or multiphase composite materialhas become the alternate route to realize the super ionic conduction Liang (Liang,

concen-1973) in 1973 experimentally found that the lithium ionic conductivity of LiI

increased by 50 times when 30%–45% of Al2O3was added to form the compositeelectrolyte Some later work also found that F-conductivity of epitaxy at the well-defined heterolayer films of CaF2and BaF2increased proportionally with interfacedensity for interfacial spacing >50 nm (Guo and Maier, 2009) Both works reflectthe important role of interfacial conduction for improving the ionic conduction There-fore, many efforts are put on building heterostructure and try to utilize the interface orspace charge carrier to improve the ionic conductivity of the existing electrolyte mate-rials.Azad et al (2005)then tried to identify the possible interface effects of layer-by-layer structures of gadolinium-doped ceria and zirconia on Al2O3(001) using oxygen-plasma-assisted molecular beam epitaxy The total film thickness was kept 150 nmwhile changing the number of discrete layers (Fig 12.2) The conductivity increasedwith the rise in discrete layer first, then reached a maximum at 10 layers, and thenreduced with the continuous increase of the layers (Fig 12.2) However, we cansee that the conductivities were much higher than YSZ single crystal The authorssuggested to the combined contribution of lattice strain and extended defects due

to lattice mismatch between the two different constitutes Similar strategy was alsoadapted by Sanna et al (2015) to develop Er0.4Bi1.6O3 (EBO) and Ce0.8Gd0.2O2(GDC) in alternate layers onto a MgO (001) single crystal by pulsed-laser-depositionmethod with the total thickness of the 60 nm while changing the individual layerthickness The authors found that such a layered material showed anomalous high con-ductivity, equal or better than the pure Bi2O3in air at low oxygen partial pressure,which is never observed previously due to the easily reduction of bismuth oxide

In addition, an exceptionally high chemical stability in reducing conditions and redoxcycles at high temperature of Bi2O3was also achieved This work presented a newmethod to design and synthesize functional material using multilayered hetero-structures consisting of alternate layer to overcome the inherent drawbacks,suggesting various technological perspectives

Fig 12.2 Gadolinia-doped ceria and zirconia on Al2O3(001) and the dependence of theconductivity on the number of discrete layers (Azad et al., 2005)

Nanocomposites for “nano green energy” applications 427

The above cases are focused on the composite electrolyte constructed by the two ofthe single ionic conductors As seen from the first composite materials for energyconversion and storage application, the addition of insulator also helps to improvethe ionic conductivity In fact, there are a lot of successful demonstrations of hete-rostructure thin films those have showed magnitude enhancement of the electricproperties over the constituted ionic conductive phase For example, in 2008,Garcia-Barriocanal et al (2008) reported that the lateral ionic conductivity of epitaxialSrTiO3/ZrO2:Y2O3/SrTiO3heterostructures with different sandwiched YSZ thicknesswas improved by eight orders of magnitude at room temperature They also confirmedthat the enhanced ionic conductivity is ascribed to the interface process due to the YSZlayer thickness-independent conductance characteristics and the larger charge-carrierconcentration and mobility of the sandwiched thin film because of the lattice dissim-ilarity between YSZ and SrTiO3 However, the enhanced claimed ionic conductivitystimulated considerable debate over the origin of the conductivity enhancement.Cavallaro et al (2010)found that the enhanced conductivity belonged to electron con-tribution rather than the ionic conductivity based on the results of the combined char-acterization of the conductivity dependence on oxygen partial pressure and directoxygen diffusion by means of tracer experiments Anyway, this work suggested animportant aspect to design artificial nanostructures with enhanced electric conductiv-ity for electronic/ionic devices, such as sensors and other ionotronic devices.Pergolesi

et al (2010)recently reported that the highly textured, epitaxially oriented BZY films

on (100)-oriented MgO substrates by pulsed laser deposition (PLD) showed the largestproton conductivity of 0.11 S cm
1at 500°C, which is ever attainable for polycrystalBZY samples The conductivity is two orders of magnitude higher than that of BZYpolycrystal material and equal to the latter’s bulk conductivity and higher than those ofmost commonly used electrolyte materials including the oxygen ionic conducting one

In other words, the grain boundary resistance of the BZY material has been removedusing the epitaxial thin-film fabrication by PLD method The single-chamber fuel cell(two electrodes on the same side of BZY electrolyte) was constructed, and its short-circuit current density was one order of magnitude larger than the value measured forthe sintered BZY pellet tested in a double-chamber configuration The unique ionicconductivity and good chemical stability suggest its promising application for low-temperature SOFC The above two epitaxial thin films showed improved ionic con-ductivities than the corresponding polycrystalline ionic conductors However, the epi-taxial thin film on insulator substrate makes hard to construct normal two chamberfuel-cell configuration based on the buried interface.Yang et al (2015)created a prac-tical geometry for device miniaturization with highly crystalline micrometer-thickvertical nanocolumns of SDC embedded in supporting matrices of SrTiO3(Fig 12.3A) The ionic conductivity, 0.03 S cm
1 at 400°C, was higher at leastone order of magnitude than plain SDC films and other ionic conductors and SrTiO3(Fig 12.3B) In addition, they performed FORC-IV measurement to probe the fast iontransport channels in the nanocomposite and indicated that SDC column core wasmore mobile than SrTiO3and the interface Therefore, the fast ion-conducting chan-nels not only are exclusively restricted to the interface but also are localized at thewell-crystalline SDC nanopillars, which is different from the conventional knowledge

of high ionic conduction through the heterostructure This work also suggested a novelstructure of fast ionic conductor composite heterostructure for low-temperature SOFCapplications.

12.2.3 Ceria-carbonate/oxide nanocomposite

Nanocomposites of heterostructured two-phase or multiphase nanocomposite or singleionic conductive phase and one insulator showed improved electric conductivities thanthe single-phase electrolytes However, the buried interface or the expensive/complexthin-film fabrication techniques restrict their wide applications The research reports ofconstruction real fuel cell are still rare Fortunately, in SOFC field, a research groupleading by Dr Zhu from Royal Institute of Technology (KTH, Sweden) developed aserious of ceria-salt composites that are capable of much higher ionic conductivity thancommonly used single-phase electrolytes (Zhu, 2003; Zhu et al., 2003) The ionic con-ductivity could reach to 0.1 S cm
1below 600°C and showed unique hybrid oxygenionic and proton conduction in fuel-cell condition and extremely low ionic conductionactivation energy The ceria-salt composite contained two phases of doped ceria oxidephase that served as the solid matrix with sufficient mechanical strength and at least onemolten (liquid-like) state in fuel-cell operational condition (elevated temperature and

H2/air condition) for the mobile ion species within a rigid lattice framework

Among all the ceria-salt-composite series, ceria-carbonate composites gave themost promising characteristics as electrolytes for low-temperature SOFC Presently,the ceria-carbonate not only shows super ionic conductivity higher than 0.1 S cm
1above 300°C, which can never be achieved by the conventional material throughstructure design, but also gives interesting multiionic conduction, that is, H+,

O2
and CO3
This unique property makes ceria-carbonate not only successfully

work for SOFC but also for other advanced energy and environmental applications

Fig 12.3 (A) Crystal structure and (B) transport properties of nanoscaffold SDC-STO films.TEM scale bar—100 nm The ionic conductivity of STO, YSZ, and SDC thin films areincluded for comparison (Yang et al., 2015)

Nanocomposites for “nano green energy” applications 429

such as CO2 separation, electrochemical ammonia synthesis, and highly efficientwater electrolysis for fuel and chemical production The existence of carbonates inthe composite acts as glue and sintering aid, which significantly reduces electrolytedensification temperature from conventional at least 1300°C to current 600°C In fact,these have stimulated a worldwide research activity (Fan et al., 2013; Zhao et al.,2013; Zhu et al., 2013a) It has become a star material to replace YSZ to operate atlow temperature even down to 300°C with enough fuel-cell performance if compatibleelectrode material is developed.

Highly improved ionic conductivity is ascribed to the interfacial ionic conductionbetween ceria and carbonate, besides the intrinsic ionic conduction through the con-stituent phases (Zhu, 2003, 2006; Zhu et al., 2008; Zhu and Mat, 2006) Moreover, thehigh-concentration oxygen ions/defects on ceria particle surface and interactionsbetween two phases offer favorable features for ionic transportation by increasingion conduction charge carrier and long-term migration distance Therefore, according

to Eq.(12.1), the ionic conduction can be much higher than the normal single-phaseelectrolyte It is recognized that the larger the interface between ceria oxide and car-bonate, the higher the ionic conductivity Therefore, in recent years, the research efforthas been put on the development of nanocomposite ceria-carbonate aiming at higherionic conductivity and better thermal stability to put this novel material in practicalapplication

Wang et al (2008)first reported the synthesis of SDC-Na2CO3by a wet-mixingmethod with newly prepared SDC nanoparticle and Na2CO3solution as precursor.The composite presented homogeneous grain distribution where the grain size wasfound<100 nm (Fig 12.4A and B) The SDC nanoparticle was surrounded by a car-bonate nanothin layer of 4–6 nm to form a unique core-shell structure ofSDC@Na2CO3as shown inFig 12.4A The fine structure resulted in an exceptionalionic conductivity >0.1 S cm
1 above 300°C, two orders of magnitude higherthan pure SDC electrolyte Such a high value can only be achieved by traditionalYSZ above 1000°C Therefore, ceria-carbonate nanocomposite presented itself the firstcandidate to operate SOFC below 500°C The excellent ionic conductivity contributed

to the outstanding fuel-cell performance For example, peak power output of

800 mW cm
2has been achieved at 580°C The cell based on this novel nanocompositeelectrolyte also showed a super performance of>500 mW cm
2at 400°C

The same group latterly developed even a simple method to prepareSDC@Na2CO3 nanocomposite through one-step coprecipitation (Raza et al.,

2010) The nanocomposite was obtained when Sm3+and Ce3+were coprecipitated

by Na2CO3solution The newly formed nanoparticle in the precursor slurry absorbed

a lot of resident Na2CO3, and it was kept at the final sample Even homogeneous particle with thinner carbonate-coated nanocomposite was achieved, and the peak out-put was raised to 1100 mW cm
2at 470°C Such a remarkable fuel-cell performance

nano-is still the highest value until to date as per the best knowledge of authors

The research of the hybrid ionic conductivity and its possible ionic conductionmechanism is also widely investigated However, it is out of the scope of this work;the readers are suggested to refer to the literature (Maheshwari and Wiemh€ofer, 2016;Wang et al., 2011) and the recent published review article (Fan et al., 2013)

Besides the above ceria-carbonate nanocomposite, the same research group alsotried to use the nanocomposite approach (NANOCOFC) to develop ceria oxidenanocomposite with improved ionic conductivity over single-phase ceria oxide and

to overcome the possible thermal instability issue of carbonate during harsh SOFC ditions (Fan et al., 2014; Raza et al., 2011; Wu et al., 2012) In2011, Raza et al prepared

con-a new oxide ncon-anocomposite of SDC-Y2O3 synthesized by coprecipitation method.Core-shell structure with SDC core and Y2O3shell was obtained The electric conduc-tivity is in the range of 0.44–0.92 S cm
1in 300–600°C with ionic transport numberabove 0.965 and ionic conduction activation energy of 0.27 eV at these temperatures.The unique microstructure is ascribed to the excellent electric properties of SDC@Y2O3all oxide nanocomposite The stability of oxide in fuel-cell condition at the reducedtemperature suggests the promising practical application

12.2.4 Semiionic nanocomposite electrolyte

The above sections on improving the ionic conductivity are based on the present trolytes; the other phase is either insulator or ionic conductive molten phase While inthis section, a new science based on recent 6 years of research progress mainly from

elec-Fig 12.4 TEM (A) and SEM (B) images of the the wet-mixing method synthesizedSDC@Na2CO3nancomposite (C) conductivity measurement (D) fuel-cell performance

Nanocomposites for “nano green energy” applications 431

the results of the authors’ group will be revealed (Wu et al., 2016; Zhu et al., 2011b,c,2013b, 2015, 2016a) It is called “semiionic nanocomposite” as functional material forsingle-layer fuel cell, as schematically illustrated inFig 12.5 The nanocompositecontains a normally ionic conductive phase, like the conventional electrolyte mate-rials The uniqueness of semiionic nanocomposite is its second phase of semiconduc-tors, which could be originated the electrode materials of the current SOFCs (Singh

et al., 2013) or other simple transition metal oxide or composite, such as commonlyused perovskite oxides, LSCF (Wu et al., 2016; Zhu et al., 2013b), Sr2Fe1.5Mo0.5O6
δ

(Dong et al., 2014; Liu et al., 2015b), La2NiO4(Li et al., 2016), and lithiated transitionmetal (Ni, Cu, Zn, Fe, and Mn) oxides or composite (Fan et al., 2012; Zhu et al., 2011a,b,c, 2013a, 2015) Such a semiionic conductor could serve as anode, electrolyte, andcathode simultaneously in a single-layer fuel cell Since it is originated from the con-ventional electrode materials, it has good electrode activity for electrode reactions

It is well recognized that the fuel-cell investment cost, system loss, and system radation issues are mainly caused by the complex fuel-cell structure and materials.The simplicity of the material system could effectively alleviate the chemical andthermal compatibility issues and reduce the material and fuel-cell manufacturing pro-cess and related cost and energy consumption It is expected from elimination of theelectrode/electrolyte interface to improve the fuel-cell performance tremendouslysince the major interfacial voltage loss has been removed Therefore, SLFC based

deg-on the semiideg-onic composite provides a new path for fuel cell and innovative energytechnologies and may speed up the FC commercialization

It is interesting to see that the composite with semiconductor can act as the trolyte material to separate the electrons produced and consumed in the fuel-cell elec-trode According to the traditional knowledge, electronic conduction in the electrolyteshould be limited as much as possible since it will cause the cell internal short circuit,reduce the fuel-cell voltage and the energy efficiency (Shen et al., 2014) The typical

example is the ceria oxide Though it shows higher ionic conductivity than YSZ, thereducible property of ceria from Ce4+to Ce3+has largely hindered its wide applica-tions The OCV of the SOFCs with ceria electrolytes never surpass 0.95 V when thetemperature is higher than 450°C (Matsui et al., 2005) Therefore, it is not acceptable

to use the highly electronic conductive phase in the electrolyte layer While in theabove semiionic conductor, the normal weight composition ratio of ion to semicon-ductor is 60:40 to reach optimal conductivity and electrode properties The semicon-ductor formed an interconnected 3D structure for efficient electronic and ionicconduction Therefore, the OCV of the cell should be lower than that of the theoreticalNernst value While it is a wonder to see that the OCV of the cell based on this semi-ionic conductor gave OCVs close to the theoretical values in H2/air fuel-cell condition

At the initial attempt, the performance of the fuel cell is comparable to the conventionalfuel-cell based on the pure ionic conductive electrolyte A continuous work on theunderstanding of the working principle, new material development, and new cell struc-ture design and optimization has notably improved the fuel-cell performance

A maximum power density of 1080 mW cm
2has been obtained at 550°C in authors’group by a careful optimization of cell structure for easy current collection and design ofelectronic blocking junction (Zhu et al., 2016a) The prominent fuel-cell performancecombined with low material and fabrication cost, single-layer fuel cell based onsemiionic conductor shows promising application to promote the commercialization

of fuel-cell technology

Fuel cell based on semiionic conductor goes beyond the conventional knowledge

of the fuel cell Therefore, besides the extensive effort on developing new materials,effort is also devoted to figure out the possible mechanism of single-layer fuel cell Atthe initial stage of its development, the solar-cell bulk p-n junction principle isborrowed The semiionic composite contained n-type conductor, such as ZnO, andp-type conductors of NiO and CuO The in situ formed anode zone (n-typeconduction)-cathode zone (p-type conduction), and the electronic charge-carrierdepletion zone realizes the charge separation (Zhu et al., 2012) The ions were notblocked by the bulk p-n junction and could transport with the constructed percolationnetwork Integration of the double catalytic activity of semiionic composite, the fuel-cell functionality was then guaranteed Later work found that the Schottky junctionformed that the anode zone plays an important role to improve the semiioniccomposite-based single-layer fuel cell The transition metal oxides of NiO and CoO

or the lithiated oxide could be partially reduced to metal, which, nevertheless, formsmetal-semiconductor interface, termed as “Schottky junction” (Zhu et al., 2015) Based

on this in situ formed Schottky junction, a potential is built up simply at the interface,which helps effectively to separate electrons/holes pairs by building up internal devicevoltage (Fig 12.6) In addition, the metal-semiconductor Schottky junction acceleratesthe electrode reaction and ionic transport at the interface Therefore, the electronicinternal short-circuit issue was overcome, evidenced by the high OCVs higher than1.0 V at the temperature range of 500–550°C The integrated merits resulted in a supe-rior fuel-cell performance, 1000 mW cm
2at 550°C (Zhu et al., 2015)

A recent work presented byZhu et al (2016a)ingeniously assembled a fuel cellusing the perovskite solar-cell principle The perovskite solar cell, with hybridNanocomposites for “nano green energy” applications 433

organic-inorganic perovskite structured CH3NH3PbI3as the core component, wiched between electronically conductive n-type and hole conductive p-type layer(Fig 12.7A), gives a recorded photovoltaic efficiency of 19.3% in recent work(Zhou et al., 2014) Similar to the above solar-cell structure, Zhu used perovskite oxideLSCF-ionic conductor composite as the core material, La0.2Sr0.25Ca0.45TiO3as then-type layer, and Ni0.8Co0.15Al0.05Li-oxide as the p-type material (Fig 12.7B) Thesandwiched pellet presented a fuel-cell performance of 405 mW cm
2at 550°C in

sand-H2/air atmosphere, 1.3 times higher than the conventional three-layer fuel cells withpure ionic conductive electrolyte Moreover, the replacement of La0.2Sr0.25Ca0.45TiO3

by Ni0.8Co0.15Al0.05Li oxide to form a symmetrical structure (Fig 12.7C) increased thefuel-cell peak output to 1080 mW cm
2because of the improved electrode reactionand ionic conduction kinetics caused by the online formed Schottky junction

A detailed study of the electrochemical reaction process by electrochemical impedance