Study On Hydration Reaction And Proton Transport Mechanisms Of Y,Nd Co-doped BaZrO₃ And Sr₃Fe_1.75Co_0.25O_7-?

As a kind of energy conversion device,solid oxide fuel cell?SOFC?has attracted wide attention due to its diverse fuel selection,high energy conversion efficiency,and negligible toxic gas emissions.According to the type of electrolyte conducting ions,SOFC can be divided into two categories:oxygen-ion-conducting solid oxide fuel cells?O-SOFC?and proton-conducting solid oxide fuel cells?P-SOFC?.High operating temperature has always been a major obstacle to the commercialization of solid oxide fuel cells.In order to reduce the operating temperature,researchers have turned their attention to proton-conducting solid oxide fuel cells because protons are much smaller and more mobile than oxygen ions.The ion diffusion activation energy of the proton conductor is lower than the ion diffusion activation energy of the oxygen ion conductor,which also makes the conductivity of the proton conductor material at intermediate or lower temperatures greater than that of most oxygen ion conductor materials.This article uses the first-principles calculations method based on density functional theory,focusing on the following two aspects of electrolytes and cathode materials for proton solid oxide fuel cells:?1?Study on hydration reaction and proton transport mechanisms of Y,Nd co-doped BaZrO₃.For the good sinterability,excellent proton conductivity and high chemical stability,Y and Nd co-doped BaZrO₃?BZNY?is considered as a new potential electrolyte for proton-conducting solid oxide fuel cells.In order to investigate the changes in the properties of BaZrO₃ co-doped by Y and Nd,oxygen vacancy formation energies,hydration energies,dopant-proton association energies and proton migration energy barriers in BZNY are investigated by DFT+U method.The calculation research shows that Y,Nd co-doping has a synergistic effect on the improvement of the performance of BaZrO₃ electrolyte.The addition of Nd further reduces the hydration energy on the basis of Y-doped BaZrO₃,and the addition of Y helps reduce the oxygen vacancy formation energy.Therefore,Y,Nd co-doping helps to enhance the BZNY hydration process.We further conclude that the oxygen vacancy formation energy in BZNY is inversely proportional to the hydration energy of the corresponding site.In addition,the presence of Nd also helps to reduce the trapping effect of Y on protons,reduces the proton migration energy,and is more conducive to proton migration.In summary,our research reveals that the synergistic effect of Y and Nd makes BZNY have excellent hydration performance and good proton conductivity,which fully supports the potential application of BZNY in solid oxide fuel cells?SOFC?,and is useful for developing other high performance electrolytes.?2?Study on hydration reaction and proton transport mechanisms of Sr₃Fe_1.75Co_0.25O_7-?.Sr₃Fe₂O_7-??SFO?with two-layer Ruddlesden-Popper?R-P?structure has recently been proved to have long-term stability,high proton conductivity,however relatively low O₂ absorption activity.In order to improve SFO's properties as a single phase cathode in P-SOFCs,partially doping cobalt into SFO could be a good choice.For the purpose of investigating the hydration process and proton conducting mechanisms of SFO and cobalt doped SFO?SFCO?,both bulk and surface properties were calculated by DFT+U method,including:electronic structures,formation and migration energies of oxygen vacancies,proton stability and migration,surface energies and surface oxygen vacancy formation energies.We conclude that cobalt doping can not only improve the electronic conductivity but also enhance the oxygen vacancy formation ability of SFCO.Although the Sr-O-F and Sr-O-N layers in SFCO are difficult to form oxygen vacancies and are obstacles for the migrations of oxygen vacancies,protons are stable in these rock salt layers.For the Sr-O-M layer,oxygen vacancies are most easily formed,and protons are relatively stable.Furthermore,cobalt doping can obviously decrease surface oxygen vacancy formation energies and the easily exposed Co-Fe-O surface can facilitate the hydration reactions on the surface.Our work provides a microscopic perspective to study the hydration and proton conducting mechanisms in both SFO and SFCO.It could also give an informative insight into the relationships among the doped elements,the R-P structures and the relative properties which could be useful in developing high performance R-P structure materials as a single phase cathode in P-SOFCs.