Niobium Doped Perovskite Ferrates As SOFC Anode Materials

Solid oxide fuel cell?SOFC?,as a third-generation fuel cell technology,can directly convert chemical energy of fuels into electricity and heat through electrochemical reactions with high efficiency,fuel flexibility and environmental-friendliness.Some ABO3-structured perovskite oxides with mixed ionic and electronic conductivity could be anode candidates for SOFCs with enhanced tolerance to coking,sulfur poisoning and redox cycling.Catalytic activity of perovskites is strongly associated with B-site cations,perovskite titanates,vanadates and chromates are structurally stable in reducing environment with quite low catalytic activity.Perovskite ferrates are widely used for SOFC cath odes with good catalytic activity toward oxygen reduction reaction?ORR?,but they suffer from phase decomposition and cannot maintain ABO3 perovskite structure in reducing atmosphere.In this work,we perform investigations on incorporation of high-valent cations?Nb,Ta?and exsolution of metallic nanoparticles to promote the structural stability and electrochemical performance of perovskite anodes for SOFCs.Firstly,pentavalent niobium is doped on the B-site of La_0.8Sr_0.2FeO_3-??LSF?to prepare novel perovskite oxide La0.8Sr0.2Fe0.9Nb0.1O3-??LSFNb?.Nb doping significantly promote the structurally stability and electrical conductivity of LSF in reducing atmosphere and LSFNb anode exhibits high electrochemical pe rformance and long-term stability.Then,investigations are carried out on sulfur poisoning behaviors of LSFNb anode and the corresponding mechanisms,during which we find that LSFNb anode can maintain performance stable when H2 with high concentration of H2 S is fed to anode chamber,indicative of good sulfur tolerance.Meanwhile,performance degradation of whole cell is due to the sulfur poisoning of?La0.75Sr0.25?0.95 Mn O3-?-?Sc2O3?0.1?Ce O2?0.01?Zr O2?0.89 composite cathode,which is caused by the sulfur diffusion from anode chamber to cathode chamber through sealant in ambient air.TEM study reveals that Fe S nanoparticles and ultrathin sulfur species layer are dynamically formed on the LSFNb surface during sulfur poisoning,while they are disappeared after stopping the H2 S.Furthermore,sulfur poisoning process of the whole cell could be divided into three stages: instant performance promotion,rapid degradation and slow degradation,which are related to rapid generation of Fe S nanoparticles,sulfur poisoning of LSFNb anode and sulfur poisoning of LSM-Sc SZ cathode,respectively.Using LSFNb as anode for direct carbon-solid oxide fuel cells?DC-SOFCs?,initial maximum power densities?MPDs?of single cells reach 302.8 and 218.5 m W cm-2 at 850 ? fueled by activated carbon and corn straw derived carbon,respectively.Meanwhile,the discharging time reaches 17 h and 22 h respectively under constant current of 25 m A,and the cell fueled by corn straw carbon delivers more stable performance and higher fuel utilization rate.Corn straw carbon has low content of ash with better thermal reaction activity while activated carbon has higher specific surface area and better carbon gasification reaction rate,which have a more significant impact on the performance of single cell.Nonstoichiometric amount of Pd is introduced onto the B-site of LSFNb to further promote the electrochemical performance of LSFNb perovskite anode and perovskite oxide La0.8Sr0.2Fe0.9Nb0.1Pd0.04O3-??LSFNP?is prepared.In reducing environment,Fe and Pd nanoparticles are exsolved to enhance the electrochemical performance of parent anode.H2-TPD experiments demonstrate that introduction of Pd can promote the gas adsorption ability of LSFNb perovskite anode significantly,leading to the MPD of LSFNP anode cell with Pd exsolutions is 18.1 times of that for LSFNb anode cell in low temperature and low concentration of H2?650 ??5% H2/Ar?.Performance promotion of LSFNb anode contributed by Pd exsolution is evaluated quantitively by calculating the activation energy.To study the influence of lattice structure on the stability of Nb doped ferrat e system,we design and prepare novel double-perovskite oxide Pr Ba Fe1.75Nb0.25O5+??PBFNO?.PBFNO is gradually decomposed in reducing atmosphere whereas its electrical conductivity and H2 adsorption ability increase with the decomposition degree.MPD of single cell with PBFNO anode reaches 700 m W cm-2 at 800 ? fueled by wet H2 and electrochemical performance of PBFNO anode increases continuously during long-term operation.Huge amounts of nanoparticles are generated after short-term operation,but they are agglomerated significantly during long-term operation,revealing that nanoparticles exsolved from stable perovskites and nanoparticles formed by decomposition have different generation mechanisms and properties.Metal-oxide interface for nanoparticles formed by decomposition is weak,leading to the poor morphological stability.Different contents of tantalum are doped onto B-site of LSF to investigate the effects of high-valent Ta cations doping and to optimize the doping amount,therefore,parent material La_0.8Sr_0.2FeO_3-??LSF?and novel La0.8Sr0.2Fe0.95Ta0.05O3-??LSFTa05?and La0.8Sr0.2Fe0.9Ta0.1O3-??LSFTa10?perovskite oxides are prepared by solid-state reaction method.We find that LSF anode is unstable in wet hydrogen and could deliver high performance merely in the beginning of the test.LSF anode degrades during long-term operation and LSF anode particles are agglomerated significantly.LSFTa05 and LSFTa10 remain structurally stable in reducing environment,indicating that incorporation of Ta can stabilize the perovskite lattice.Electrochemical performance of LSFTa05 anode is always higher than that for LSFTa10 anode,suggesting that too much Ta doping would lead to the decreasing of oxygen vacancy content and deteriorate the electrochemical performance.MPD of LSFTa05 anode cell reaches 441.7 m W cm-2 after discharging in wet H2 at 800 ? for 50 h.Using DFT calculations,we verify that Ta doping can lead to th e increasing of formation enthalpy and oxygen vacancy formation energy of LSF perovskite oxide,which is benefit to stabilize the perovskite lattice.Furthermore,oxygen vacancy formation energy and density of states calculations illustrate the electrical conductivities of Ta doped LSF system successfully.