Hydrogen Evolution Reaction Activity of Heterogeneous Materials: A Theoretical Model

In
this study, we present a new comprehensive methodology to quantify
the catalytic activity of heterogeneous materials for the hydrogen
evolution reaction (HER) using ab initio simulations. The model is
composed of two parts. First, the equilibrium hydrogen coverage is
obtained by an iterative evaluation of the hydrogen adsorption free
energies (ΔGH) using density functional
theory calculations. Afterward, the ΔGH are used in a microkinetic model to provide detailed characterizations
of the entire HER considering all three elementary steps, i.e., the
discharge, atom + ion, and combination reactions, without any prior
assumptions of rate-determining steps. The microkinetic model takes
the equilibrium and potential-dependent characteristics into account,
and thus both exchange current densities and Tafel slopes are evaluated.
The model is tested on several systems, from polycrystalline metals
to heterogeneous molybdenum disulfide (MoS2), and by comparing
to experimental data, we verify that our model accurately predicts
their experimental exchange current densities and Tafel slopes. Finally,
we present an extended volcano plot that correlates the electrical
current densities of each elementary reaction step to the coverage-dependent
ΔGH.