Theory of multiple proton–electron transfer reactions and its implications for electrocatalysis

Abstract

This Perspective article outlines a simple but general theoretical analysis for multiple proton-electron transfer reactions, based on the microscopic theory of proton-coupled electron transfer reactions, recent developments in the thermodynamic theory of multistep electron transfer reactions, and the experimental realization that many multiple proton-coupled electron transfer reactions feature decoupled proton-electron steps in their mechanism. It is shown that decoupling of proton and electron transfer leads to a strong pH dependence of the overall catalytic reaction, implying an optimal pH for high catalytic turnover, and an associated optimal catalyst at the optimal pH. When more than one catalytic intermediate is involved, scaling relationships between intermediates may dictate the optimal catalyst and limit the extent of reversibility that may be achievable for a multiple proton-electron-transfer reaction. The theory is discussed in relation to the experimental results for a number of redox reactions that are of importance for sustainable energy conversion, primarily focusing on their pH dependence.

Ab initio GGA+U study of oxygen evolution and oxygen reduction electrocatalysis on the (001) surfaces of lanthanum transition metal perovskites LaBO3(B = Cr, Mn, Fe, Co and Ni)

“…The physical origin of such linear scaling relationship has been explained by Rossmeisl et al based on the same single bonding nature of HO* and HOO* 16 and the 3.4 eV difference in the OH -and HO 2 -and in aqueous solution 29 .…”

“…We note that our thermodynamic analysis is within the scheme of the concerted proton-electron transfer 29 , where transfer of single (or multiple) proton and electron occurs in a concerted elementary step, instead of stepwise mechanisms in which the electron and proton are transferred sequentially. Consequently, all equilibrium potentials of (electrochemical) reaction steps contain the same G(H + )/e 0 term (G(H + ) is the free energy of proton and e 0 is the magnitude of its charge) and the activities/overpotentials of catalysts exhibit no pH dependences 23 .…”

“…In this study, we have utilized the DFT-GGA+U method to self-consistently predict the ORR and OER activities of LaBO 3 (B=Cr, Mn, Fe, Co, and Ni) (001) surfaces at stable surface coverage under the ORR/OER conditions in alkaline solution within the concerted proton-electron transfer scheme 29 . In our DFT surface modeling, we demonstrated that the bare (001) AO surfaces generally bind oxidizing species more strongly than the bare BO 2 surfaces.…”

Gold Nanofilm Redox Catalysis for Oxygen Reduction at Soft Interfaces

“…Thus, their system also expressly precluded the possibility of IT in the mechanism and highlighted the feasibility of O 2 reduction on the aqueous side of an adsorbed interfacial AuNP nanofilm. Reduction of O 2 at the surface of a gold electrode is known to be strongly dependent on the pH of the medium [49,50,52]. The catalytic activity of gold towards the ORR is higher in alkaline rather than acidic media as under these conditions the ratedetermining step (formation of the superoxide anion by outer sphere electron transfer) does not depend on pH and overpotentials towards 2 e -and 4 e -reductions are lower [49].…”

“…The latter leads to the formation of weakly bound intermediates, thus increasing the catalytic activity of gold in comparison to acidic conditions. However, this behaviour is usually a sign of decoupled proton-electron transfer step in the mechanism [52].…”