Combined theoretical and experimental analysis of processes determining cathode performance in solid oxide fuel cells

Abstract

Solid oxide fuel cells (SOFC) are under intensive investigation since the 1980's as these devices open the way for ecologically clean direct conversion of the chemical energy into electricity, avoiding the efficiency limitation by Carnot's cycle for thermochemical conversion. However, the practical development of SOFC faces a number of unresolved fundamental problems, in particular concerning the kinetics of the electrode reactions, especially oxygen reduction reaction. We review recent experimental and theoretical achievements in the current understanding of the cathode performance by exploring and comparing mostly three materials: (La,Sr)MnO3 (LSM), (La,Sr)(Co,Fe)O3 (LSCF) and (Ba,Sr)(Co,Fe)O3 (BSCF). Special attention is paid to a critical evaluation of advantages and disadvantages of BSCF, which shows the best cathode kinetics known so far for oxides. We demonstrate that it is the combined experimental and theoretical analysis of all major elementary steps of the oxygen reduction reaction which allows us to predict the rate determining steps for a given material under specific operational conditions and thus control and improve SOFC performance.

Reversible nano-structuring of SrCrO3-δ through oxidation and reduction at low temperature

“…According to these calculations, forming an isolated V O requires 1.3 eV, a very low value compared with La-based perovskites, for example, LaCrO 3 (3.9 eV), and lower than that found for other SrBO 3 perovskites with an incomplete B t 2g shell, for example, SrCoO 3 (B1.6 eV; ref. 21). When there are r2 V O per supercell (dr0.10), the perovskite structure is preserved even if the vacancies are located in the same {111}-oriented SrO 3 lattice plane; the lattice parameters expand by 0.4% on average with respect to those of stoichiometric SrCrO 3 .…”

Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

“…For MIEC oxygen-electrode materials the 2PB pathway dominates. For most perovskites, the electrode polarisation resistance (Rp) is dominated by the oxygen surface exchange reaction 23 with the rate determining step involving both oxygen vacancies and molecular oxygen 24 . Recent experimental findings show that near surface oxygen anions demonstrate significant redox activity in relation to oxygen adsorption, associated with strong hybridization between oxygen 2p and transition metal 3d electronic states 25 .…”

B-Site Metal (Pd, Pt, Ag, Cu, Zn, Ni) Promoted La1−xSrxCo1−yFeyO3–δ Perovskite Oxides as Cathodes for IT-SOFCs

“…As previously mentioned, DFT investigations have been successfully applied to investigate processes determining cathode performance and oxygen vacancy formation and migration in LSM, BSCF and LSCF [70][71][72][73]. Since such investigation has been not yet reported on metal (Pd, Pt, Ag, Cu, Zn, Ni) substituted LSCF, it will be meaningful to carry out such studies on perovskite metal substituted at the B-site.…”

“…Such materials can be advantageous, helping to decrease long-time degradation processes caused by cation segregation and poisoning by gaseous chromium oxide or SOx species, forming SrCrO4 or SrSO4 precipitates that may block the active surface [70].…”

“…Another investigation focuses more on the strain caused at the hetero interface leading to increased oxygen vacancy mobility [69]. There is also an idea that the transition state structure of the perovskite may play an important role in the course of the vacancy diffusion [70][71][72][73]. Variations in structural disorder also were pointed out as a factor.…”