Dynamic electrocatalysis: Examining resonant catalytic rate enhancement under oscillating electrochemical potential

Abstract

A microkinetic analysis is presented for a generalized electrocatalytic reaction mechanism to evaluate whether oscillating electrochemical potential can be used to achieve resonant catalytic rate enhancement. It is illustrated that because changing the potential changes the free energy of reaction, this approach is conceptually distinct from oscillating binding energies of catalytic intermediates as within catalytic resonance theory. For faradaic reactions in series, no enhancements relative to the maximum steady-state turnover rate (within the potential range spanned by oscillation) are achievable, even in cases where the potential limits favor adsorption and desorption, respectively. It is possible to exceed a time-averaged steady-state rate (weighted by time at each condition), although only if the elementary reactions show disparate responses to potential. In contrast, if a faradaically driven parallel reaction controls surface coverage of a strongly adsorbed blocking species, significant dynamic enhancements over the maximum steady-state can be achieved, albeit at a cost of thermodynamic efficiency. • Electrocatalytic reactions are evaluated under conditions of oscillating potential • Pure series reactions cannot exceed the steady-state rate at the highest potential • Parallel reactions can allow dynamic enhancement, but energy efficiency is sacrificed Recent simulations have shown that heterogeneous catalysts with dynamic properties—for example, the ability to vary adsorbate binding energy with time—could, in principle, give higher rates than an optimized catalyst operating at steady state (i.e., the Sabatier principle or “volcano curve” maximum, assuming typical correlations shaping the energy landscape of the reaction). Enhancements may be realized by oscillating catalyst properties at frequencies near the timescales of the elementary steps. Variation of electrochemical potential has been proposed as a possible method to induce such rate enhancements in electrocatalytic systems, but the extent to which this method could be effective has not been fully explored. Deeper understanding of electrochemical systems is also broadly critical, as electrocatalysis is well suited to utilize energy from renewables such as wind and solar power for the production of fuels, chemicals, and materials with low carbon intensity. Certain electrocatalytic reactions can be accelerated by varying the applied potential with time. Requirements for this promotional effect are identified and contrasted with other approaches attempting to create catalysts that accelerate reaction rates with dynamically tunable surface properties.