Disentangling multistep kinetics by combining electrochemical Arrhenius analysis with micro-kinetic modelling

Electrocatalytic reactions often involve several steps and intermediates, along with changes in the catalyst structure and chemistry under reaction conditions, making their mechanistic understanding very challenging. As a way to extract maximum information about the kinetics, temperature-dependent electrochemistry enables access to the apparent activation energy and pre-exponential factor as a function of the electrochemical bias. Recently, many reactions were shown to exhibit rich structures in their bias-dependent activation parameters, structures which cannot be accounted for by the traditional, single-step Butler-Volmer theory. Here, we study the overpotential-dependent activation parameters of a 2-step microkinetic model featuring an electrochemical adsorption followed by a chemical recombination step. We show that the electrochemical bias drives transitions across several kinetic regimes where the degree of rate control of each step varies. A key finding is that the bias dependent Arrhenius signatures constrain the underlying phase space of intermediate binding and activation enthalpies, even for such a seemingly simple model. From the close fit of our model with our experiments on the oxygen reduction reaction, we find that for a wide overpotential range, one kinetically relevant intermediate - and thus two partially rate determining steps - are controlling the kinetics on platinum and ruthenium nanoparticles. From the fits, we extract the corresponding binding and activation energies along with bias-dependent coverage. We argue that combining temperature dependent electrochemistry with minimalistic micro-kinetic models allows a direct comparison with DFT calculations and operando spectroscopy measurements.