Predicting the Stability of Single-Atom Catalysts in Electrochemical Reactions

The attention toward single-atom catalysts (SACs) for electrochemical processes is growing at an impressive pace. Electronic structure calculations play an important role in this race by providing proposals of potentially relevant catalysts based on screening studies or on the identification of descriptors of the chemical activity. So far, almost all of these predictions ignore a crucial aspect in the design of a catalyst: its stability. We propose a simple yet general first-principles approach to predict the stability of SACs under working conditions of pH and applied voltage. This is based on the construction of a thermodynamic cycle, where part of the information is taken from experiment and the rest from density functional theory (DFT) calculations. In particular, we make use of the formalism of Pourbaix diagrams to investigate the stability of SACs in reductive or oxidative conditions and we identify those that show a pronounced tendency to dissolve or to form other chemical species. Applying the procedure to four transition metal atoms, Cr, Mn, Fe, and Co, and to three supports, N-doped graphene, carbon nitride, and covalent organic frameworks, we show that a key factor in determining the final stability is the binding energy of the free metal atom to the support. The results show that several potentially very good catalysts in key electrochemical reactions are, in fact, dramatically prone to dissolution or transformation in other chemical species, suggesting that every prediction of the SAC's catalytic activity should be accompanied by a parallel investigation of the stability.