Correlation between hydration properties and electrochemical performances on Ln cation size effect in layered perovskite for protonic ceramic fuel cells

PrBa_(0.5)Sr_(0.5)Co_(1.5)Fe_(0.5)O_(5+δ)(PrBSCF) has attracted much research interest as a potential triple ionic and electronic conductor(TIEC) electrode for protonic ceramic fuel cells(PCFCs). The chemical formula for Pr BSCF is AA'B_(2)O_(5+δ), with Pr(A-site) and Ba/Sr(A'-site) alternately stacked along the c-axis. Due to these structural features, the bulk oxygen ion diffusivity is significantly enhanced through the disorder-free channels in the PrO layer;thus, the A site cations(lanthanide ions) play a pivotal role in determining the overall electrochemical properties of layered perovskites. Consequently, previous research has predominantly focused on the electrical properties and oxygen bulk/surface kinetics of Ln cation effects,whereas the hydration properties for PCFC systems remain unidentified. Here, we thoroughly examined the proton uptake behavior and thermodynamic parameters for the hydration reaction to conclusively determine the changes in the electrochemical performances depending on LnBa_(0.5)Sr_(0.5)Co_(1.5)Fe_(0.5)O_(5+δ)(LnBSCF,Ln=Pr, Nd, and Gd) cathodes. At 500 ℃, the quantitative proton concentration of PrBSCF was 2.04 mol% and progressively decreased as the Ln cation size decreased. Similarly, the Gibbs free energy indicated that less energy was required for the formation of protonic defects in the order of Pr BSCF < Nd BSCF < Gd BSCF. To elucidate the close relationship between hydration properties and electrochemical performances in LnBSCF cathodes, PCFC single cell measurements and analysis of the distribution of relaxation time were further investigated.

Al^(3+) doped CeO_(2) for proton conducting fuel cells

Developing high ionic conducting electrolytes is crucial for applying proton-conducting fuel cell(PCFCs)practically.The cur-rent study investigates the effect of alumina on the structural,morphological,electrical,and electrochemical properties of CeO_(2).Lattice oxygen vacancies are induced in CeO_(2) by a general doping concept that enables fast ionic conduction at low-temperature ranges(300-500℃)for PCFCs.Rietveld refinement of the X-ray diffraction(XRD)patterns established the pure cubic fluorite structure of Al-doped CeO_(2)(ADC)samples and confirmed Al ions’fruitful integration in the CeO_(2) lattice.The electronic structure of the alumina-doped ceria of the materials(10ADC,20ADC,and 30ADC)has been investigated.As a result,it was found that the best composition of 30ADC-based electrolytes induced maximum lattice oxygen vacancies.The corresponding PCFC exhibited a maximum power output of 923 mW/cm^(2)at 500℃.Moreover,the investigation proves the proton-conducting ability of alumina-doped ceria-based fuel cells by using an oxide ion-blocking layer.

Simultaneous generation of electricity, ethylene and decomposition of nitrous oxide via protonic ceramic fuel cell membrane reactor

Ethylene,one of the most widely produced building blocks in the petrochemical industry,has received intense attention.Ethylene production,using electrochemical hydrogen pump-facilitated nonoxidative dehydrogenation of ethane(NDE)to ethylene,is an emerging and promising route,promoting the transformation of the ethylene industry from energy-intensive steam cracking process to new electrochemical membrane reactor technology.In this work,the NDE reaction is incorporated into a BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3-δ)electrolyte-supported protonic ceramic fuel cell membrane reactor to co-generate electricity and ethylene,utilizing the Nb and Cu doped perovskite oxide Pr_(0.6)Sr_(0.4)Fe_(0.8)Nb_(0.1)Cu_(0.1)O_(3-δ)(PSFNCu)as anode catalytic layer.Due to the doping of Nb and Cu,PSFNCu was endowed with high reduction tolerance and rich oxygen vacancies,showing excellent NDE catalytic performance.The maximum power density of the assembled reactor reaches 200 mW cm^(-2)at 750℃,with high ethane conversion(44.9%)and ethylene selectivity(92.7%).Moreover,the nitrous oxide decomposition was first coupled in the protonic ceramic fuel cell membrane reactor to consume the permeated protons.As a result,the generation of electricity,ethylene and decomposition of nitrous oxide can be simultaneously obtained by a single reactor.Specifically,the maximum power density of the cell reaches 208 mW cm^(-2)at 750℃,with high ethane conversion(45.2%),ethylene selectivity(92.5%),and nitrous oxide conversion(19,0%).This multi-win technology is promising for not only the production of chemicals and energy but also greenhouse gas reduction.

Prediction of impedance responses of protonic ceramic cells using artificial neural network tuned with the distribution of relaxation times

A deep-learning-based framework is proposed to predict the impedance response and underlying electrochemical behavior of the reversible protonic ceramic cell(PCC) across a wide variety of different operating conditions.Electrochemical impedance spectra(EIS) of PCCs were first acquired under a variety of opera ting conditions to provide a dataset containing 36 sets of EIS spectra for the model.An artificial neural network(ANN) was then trained to model the relationship between the cell operating condition and EIS response.Finally,ANN model-predicted EIS spectra were analyzed by the distribution of relaxation times(DRT) and compared to DRT spectra obtained from the experimental EIS data,enabling an assessment of the accumulative errors from the predicted EIS data vs the predicted DRT.We show that in certain cases,although the R^(2)of the predicted EIS curve may be> 0.98,the R^(2)of the predicted DRT may be as low as~0.3.This can lead to an inaccurate ANN prediction of the underlying time-resolved electrochemical response,although the apparent accuracy as evaluated from the EIS prediction may seem acceptable.After adjustment of the parameters of the ANN framework,the average R^(2)of the DRTs derived from the predicted EIS can be improved to 0.9667.Thus,we demonstrate that a properly tuned ANN model can be used as an effective tool to predict not only the EIS,but also the DRT of complex electrochemical systems.

Electrolyte materials for protonic ceramic electrochemical cells:Main limitations and potential solutions

Solid oxide fuel cells(SOFCs)and electrolysis cells(SOECs)are promising energy conversion devices,on whose basis green hydrogen energy technologies can be developed to support the transition to a carbon-free future.As compared with oxygen-conducting cells,the operational temperatures of protonic ceramic fuel cells(PCFCs)and electrolysis cells(PCECs)can be reduced by several hundreds of degrees(down to low-and intermediatetemperature ranges of 400–700C)while maintaining high performance and efficiency.This is due to the distinctive characteristics of charge carriers for proton-conducting electrolytes.However,despite achieving outstanding lab-scale performance,the prospects for industrial scaling of PCFCs and PCECs remain hazy,at least in the near future,in contrast to commercially available SOFCs and SOECs.In this review,we reveal the reasons for the delayed technological development,which need to be addressed in order to transfer fundamental findings into industrial processes.Possible solutions to the identified problems are also highlighted.

Highly active and durable triple conducting composite air electrode for low-temperature protonic ceramic fuel cells

Protonic ceramic fuel cells(PCFCs)are more suitable for operation at low temperatures due to their smaller activation energy(Ea).Unfortunately,the utilization of PCFC technology at reduced temperatures is limited by the lack of durable and high-activity air electrodes.A lot number of cobalt-based oxides have been developed as air electrodes for PCFCs,due to their high oxygen reduction reaction(ORR)activity.However,cobalt-based oxides usually have more significant thermal expansion coefficients(TECs)and poor thermomechanical compatibility with electrolytes.These characteristics can lead to cell delamination and degradation.Herein,we rationally design a novel cobalt-containing composite cathode material with the nominal composition of Sr_(4)Fe_(4)Co_(2)O_(13)+δ(SFC).SFC is composed of tetragonal perovskite phase(Sr_(8)Fe_(8)O_(23)+δ,I4/mmm,81 wt.%)and spinel phase(Co_(3)O_(4),Fd3m,19 wt.%).The SFC composite cathode displays an ultra-high oxygen ionic conductivity(0.053 S·cm^(-1)at 550℃),superior CO_(2)tolerance,and suitable TEC value(17.01×10^(-6)K^(-1)).SFC has both the O_(2)^(-)/e^(-)conduction function,and the triple conducting(H^(+)/O_(2)^(-)/e^(-))capability was achieved by introducing the protonic conduction phase(BaZr_(0.2)Ce_(0.7)Y_(0.1)O_(3-δ),BZCY)to form SFC+BZCY(70 wt.%:30 wt.%).The SFC+BZCY composite electrode exhibits superior ORR activity at a reduced temperature with extremely low area-specific resistance(ASR,0.677Ω·cm^(2)at 550℃),profound peak power density(PPD,535 mW·cm^(-2)and 1.065 V at 550℃),extraordinarily long-term durability(>500 h for symmetrical cell and 350 h for single cell).Moreover,the composite has an ultra-low TEC value(15.96×10^(-6)K^(-1)).This study proves that SFC+BZCY with triple conducting capacity is an excellent cathode for low-temperature PCFCs.

Functional layer engineering to improve performance of protonic ceramic fuel cells

Protonic ceramic fuel cells(PCFCs)have been attracting increasing attention because of their advances in high-efficiency power generation in an intermediate-temperature range,as compared to the high-temperature solid oxide fuel cells(SOFCs).The greatest difference between PCFCs and SOFCs is the specific requirement of protonic(H+)conductivity at the PCFC cathode,in addition to the electronic(e^(-))and oxide-ion(O^(2-))conductivity.The development of a triple H^(+)/e^(-)/O^(2-)conductor for PCFC cathode is still challenging.Thus,the most-widely used cathode material is based on the mature e^(-)/O^(2-)conductor.However,this leads to insufficient triple phase boundary(TPB),i.e.,reaction area.Herein,an efficient strategy that uses a~100 nm-thick proton conductive functional layer(La_(0.5)Sr_(0.5)CoO_(3-δ),LSC55)in-between the typical La_(0.8)Sr_(0.2)CoO_(3-δ)cathode(a mature e-/O^(2-)conductor,LS C 82)and B aZr_(0.4)Ce_(0.4)Y_(0.1)Yb_(0.)1O_(3-δ)elec trolyte(11 mm in diameter,20μm in thickness)is proposed to significantly enhance the reaction area.Reasonably,the ohmic resistance and polarization resistance are both decreased by 47%and 62%,respectively,compared with that of PCFCs without the functional layer.The power density of the PCFC with such a functional layer can be raised by up to 2.24 times,superior to those described in previous reports.The enhanced PCFC performances are attributed to the well-built TPB and enhanced reaction area via the functional layer engineering strategy.

Hydrogen production from methane and carbon dioxide mixture using all-solid-state electrochemical cell based on a proton-conducting membrane and redox-robust composite electrodes

In recent years, interest in hydrogen as a fuel has sharply increased in the field of alternative and green energy due to its high energy capability and zero-emission behaviour. As a result, research in the development of new highly efficient methods for producing high-purity hydrogen is relevant. This paper presents, for the first time, the test results of an electrochemical cell with a proton-conducting La_(0.9)Sr_(0.1)ScO_(3-δ) electrolyte and symmetrical Sr_(1.95)Fe_(1.4)Ni_(0.1)Mo_(0.5)O_(6-δ)+ La_(0.9)Sr_(0.1)Sc_(0.9)Co_(0.1)O_(3-δ) electrodes as a hybrid setup for electricity generation in proton ceramic fuel cell mode, for hydrogen separation from H_(2)+ Ar mixture and the production of high-purity hydrogen from methane with simultaneous CO_(2) utilization.It was found that this electrochemical cell generates high flow rates of hydrogen during its separation through a proton-conducting membrane from H_(2)+ Ar mixture, about 500 cm^(3)h^(-1)cm^(-2)at a current density of 0.6 A cm^(-2)as well as about 370 cm^(3) h^(-1)cm^(-2)at a current density of 0.5 A cm^(-2) from CH_(4)+ CO_(2) mixture at 800 ℃ which shows that these cells are promising for hydrogen production.

A real proton-conductive,robust,and cobalt-free cathode for proton-conducting solid oxide fuel cells with exceptional performance

The development of proton,oxygen-ion,and electron mixed conducting materials,known as triple-conduction materials,as cathodes for proton-conducting solid oxide fuel cells(H-SOFCs)is highly desired because they can increase fuel cell performance by extending the reaction active area.Although oxygen-ion and electron conductions can be measured directly,proton conduction in these oxides is usually estimated indirectly.Because of the instability of cathode materials in a reducing environment,direct measurement of proton conduction in cathode oxide is difficult.The La0.8Sr0.2Sc0.5Fe0.5O3–δ(LSSF)cathode material is proposed for H-SOFCs in this study,which can survive in an H_(2)-containing atmosphere,allowing measurement of proton conduction in LSSF by hydrogen permeation technology.Furthermore,LSSF is discovered to be a unique proton and electron mixed-conductive material with limited oxygen diffusion capability that is specifically designed for H-SOFCs.The LSSF is an appealing cathode choice for H-SOFCs due to its outstanding CO_(2)tolerance and matched thermal expansion coefficient,producing a record-high performance of 2032 mW cm^(−2)at 700℃and good long-term stability under operational conditions.The current study reveals that a new type of proton–electron mixed conducting cathode can provide promising performance for H-SOFCs,opening the way for developing high-performance cathodes.

K-doped BaCo_(0.4)Fe_(0.4)Zr_(0.2)O_(3−δ) as a promising cathode material for protonic ceramic fuel cells

Slow oxygen reduction reaction(ORR)involving proton transport remains the limiting factor for electrochemical performance of proton-conducting cathodes.To further reduce the operating temperature of protonic ceramic fuel cells(PCFCs),developing triple-conducting cathodes with excellent electrochemical performance is required.In this study,K-doped BaCo_(0.4)Fe_(0.4)Zr_(0.2)O_(3−δ)(BCFZ442)series were developed and used as the cathodes of the PCFCs,and their crystal structure,conductivity,hydration capability,and electrochemical performance were characterized in detail.Among them,Ba_(0.9)K_(0.1)Co_(0.4)Fe_(0.4)Zr_(0.2)O_(3−δ)(K10)cathode has the best electrochemical performance,which can be attributed to its high electron(e^(−))/oxygen ion(O^(2−))/H^(+)conductivity and proton uptake capacity.At 750℃,the polarization resistance of the K10 cathode is only 0.009Ω·cm^(2),the peak power density(PPD)of the single cell with the K10 cathode is close to 1 W·cm^(−2),and there is no significant degradation within 150 h.Excellent electrochemical performance and durability make K10 a promising cathode material for the PCFCs.This work can provide a guidance for further improving the proton transport capability of the triple-conducting oxides,which is of great significance for developing the PCFC cathodes with excellent electrochemical performance.

New two-layer Ruddlesden-Popper cathode materials for protonic ceramics fuel cells

New two-layer Ruddlesden-Popper(RP)oxide La_(0.25)Sr_(2.75)FeNiO_(7-δ)(LSFN)in the combination of Sr_(3)Fe_(2)O_(7-δ) and La_(3)Ni_(2)O_(7-δ) was successfully synthesized and studied as the potential active single-phase and composite cathode for protonic ceramics fuel cells(PCFCs).LSFN with the tetragonal symmetrical structure(IMmmm)is confinned,and the co-existence of Fe^(3+)/Fe^(4+) and Ni^(3+)/Ni^(2+) couples is demonstrated by X-ray photoelectron spectrometer(XPS)analysis.The LSFN conductivity is apparently enhanced after Ni doping in Fe-site,and nearly three times those of Sr_(3)Fe_(2)O_(7-δ),which is directly related to the carrier concentration and conductor mechanism.Importantly,anode supported PCFCs using LSFN-BaZr_(0.1)Ce_(0.7)Y_(0.2)O_(3-δ)(LSFN-BZCY)composite cathode achieved high power density(426 mW·cm^(-2) at 650℃)and low electrode interface polarization resistance(0.26Ω·cm^(2)).Besides,distribution of relaxation time(DRT)function technology was further used to analyse the electrode polarization processes.The observed three peaks(Pl,P2,and P3)separated by DRT shifted to the high frequency region with the decreasing temperature,suggesting that the charge transfer at the electrode-electrolyte interfaces becomes more difficult at reduced temperatures.Preliminary results demonstrate that new two-layer RP phase LSFN can be a promising cathode candidate for PCFCs.

High-temperature transport properties of BaSn_(1−x)Sc_(x)O_(3−δ) ceramic materials as promising electrolytes for protonic ceramic fuel cells

Protonic ceramic fuel cells(PCFCs)offer a convenient means for electrochemical conversion of chemical energy into electricity at intermediate temperatures with very high efficiency.Although BaCeO_(3)-and BaZrO_(3)-based complex oxides have been positioned as the most promising PCFC electrolytes,the design of new protonic conductors with improved properties is of paramount importance.Within the present work,we studied transport properties of scandium-doped barium stannate(Sc-doped BaSnO_(3)).Our analysis included the fabrication of porous and dense BaSn_(1−x)Sc_(x)O_(3−δ)ceramic materials(0≤x≤0.37),as well as a comprehensive analysis of their total,ionic,and electronic conductivities across all the experimental conditions realized under the PCFC operation:both air and hydrogen atmospheres with various water vapor partial pressures(p(H2O)),and a temperature range of 500–900℃.This work reports on electrolyte domain boundaries of the undoped and doped BaSnO_(3)for the first time,revealing that pure BaSnO_(3)exhibits mixed ionic–electronic conduction behavior under both oxidizing and reducing conditions,while the Sc-doping results in the gradual improvement of ionic(including protonic)conductivity,extending the electrolyte domain boundaries towards reduced atmospheres.This latter property makes the heavilydoped BaSnO_(3)representatives attractive for PCFC applications.

Silver particle interlayer with high dislocation density for improving the joining of BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3-δ) electrolyte and AISI 441 interconnect

One of the critical challenges for the protonic ceramic fuel cell stack is sealing electrolytes and interconnects.However,the traditional AgeCuO sealant will aggravate the oxidation along the interconnect surface and result in brittle compound layers at the BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3-δ)(BZCYYb)electrolyte interface.The present work demonstrates that a silver particle interlayer with high dislocation density can be adopted to join BZCYYb electrolyte to the interconnect(AISI 441 stainless)in air instead of traditional AgeCuO sealant.Elevating temperatures result in a sufficient bonding at the Ag/BZCYYb interface,and a defect-free joint is obtained at 950℃.Atomic bonding at Ag/BZCYYb interface is confirmed by TEM.Also,a dense and thin oxide layer(2-3 μm)is formed along the AISI 441 interface.Ag particles in the interlayer provide the main driving force for the sintering joining.The massive dislocations promote the recovery and recrystallization of the Ag interlayer,as well as the interdiffusion of BZCYYb/Ag.After aging in the wet oxidizing atmosphere at 600℃ for 300 h,joints remain intact and dense,indicating superior oxidation resistance and aging performance.Besides,the joint shear strength(25.3 MPa)is 59%higher than that of the joint brazed by traditional AgeCuO.