Perovskite Oxide Based Electrodes for the Oxygen Reduction and Evolution Reactions: The Underlying Mechanism

Abstract

One hindrance to the development of fuel cells and electrolyzers are the oxygen electrodes, which suffer from high overpotentials and slow kinetics. Perovskite oxides have been shown to be promising oxygen electrode catalysts because of their low cost, flexibility, and tailorable properties. In order to improve perovskite catalysts for the oxygen reduction (ORR) and oxygen evolution (OER) reactions, a better understanding of their reaction mechanisms is needed. This Perspective aims to inform researchers of the current proposed reaction mechanisms for ORR and OER on perovskites and perovskite/carbon composites in order to guide future catalyst development. Additionally, important experimental practices will be recommended. A recent development for OER is the lattice oxygen evolution reaction, which is a possible addition to the conventional four consecutive proton-coupled electron transfer mechanism. Carbon additives are consistently added to perovskites to enhance conductivity and ORR/OER activity. However, carbon plays an active role in ORR, and there is evidence of a synergistic relationship between perovskite and carbon in perovskite/carbon composites.

Correlation between Oxygen Vacancies and Oxygen Evolution Reaction Activity for a Model Electrode: PrBaCo2O5+δ

“…This suggests that the lattice oxygen evolution reaction (LOER), besides the conventional OER mechanism, takes place on the surface of the PBCO samples [4, 7c–f] . The participation of perovskite lattice oxygen to the OER generally leads to the catalyst surface reconstruction, specifically to the formation of a superficial oxyhydroxide layer [5a, 6, 7a,b,e, 32] . Furthermore, the LOER and the formation of the oxyhydroxide layer are also accompanied by cation dissolution as expressed in Equation 1. …”

Dynamic Surface Reconstruction Unifies the Electrocatalytic Oxygen Evolution Performance of Nonstoichiometric Mixed Metal Oxides

“…As alternatives, crystalline, nonstoichiometric, mixed ionic–electronic conducting oxides belonging to the perovskite family of the general form A n +1 B n O 3 n +1 ( n = 1, 2, 3, ... ∞; A and B represent alkaline-earth-/rare-earth-metal and transition-metal (TM) cations, respectively) have been explored. 18 , 19 , 24 , 32 − 35 These oxides can accommodate >90% of the metals in the periodic table in their structure, giving rise to a significant number of opportunities to tune their catalytic performance. 36 For instance, variations in the A-site composition can be used to tune the electronic structure and catalytic properties of TM cations in these oxides.…”

Correlation between Oxygen Vacancies and Oxygen Evolution Reaction Activity for a Model Electrode: PrBaCo2O5+δ

“…The formation of a superficial oxyhydroxide layer does not necessarily leads to catalyst instability under OER conditions. Indeed, recent results suggest that for some OER catalysts a dynamic equilibrium between cation dissolution and re‐deposition can be reached, leading to a co‐existence of the perovskite structure and the superficial oxyhydroxide phase [5a, 6, 7a,b,e, 32] . Furthermore, the self‐assembled oxyhydroxide layer can itself undergo LOER and metal cation dissolution, as described in ref.…”

“…This suggests that the lattice oxygen evolution reaction (LOER), besides the conventional OER mechanism, takes place on the surface of the PBCO samples [4, 7c–f] . The participation of perovskite lattice oxygen to the OER generally leads to the catalyst surface reconstruction, specifically to the formation of a superficial oxyhydroxide layer [5a, 6, 7a,b,e, 32] . Furthermore, the LOER and the formation of the oxyhydroxide layer are also accompanied by cation dissolution as expressed in Equation 1. …”