The HER on Pt: revisiting tafel slope analyses when deducing reaction mechanisms

A common convention in electrochemical kinetics is that the symmetry factor (β) of an elementary electron transfer is 0.5. While justified for outer-sphere reactions, for reactions involving forming and breaking bonds with a catalyst surface, as required by electrocatalytic processes, there is no foundation for such an assumption. Nonetheless, conclusions regarding the mechanism of electrocatalytic reactions are often based on the coincidence between the measured Tafel slope and that calculated based on the assumption that β of the rate-determining step (rds) is 0.5. Similarly, the fact that reaction mechanisms involving the formation of an adsorbed intermediate will normally lead to a changing Tafel slope with increasing overpotential, due first to the deviation of the adsorbed intermediate from ideal behaviour and then to reaching saturation coverage, is often ignored. Instead, Tafel analysis is usually limited to identifying a more or less linear region in the Tafel plot, from which a Tafel slope on which all mechanistic conclusions are based can be obtained. We show here, using the hydrogen-evolution reaction (HER) as an illustrative example, and by quantitatively monitoring the potential-dependent coverage of the reaction intermediate (overpotential deposited hydrogen, Hopd) using surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS), how these assumptions can lead to wrong conclusions, or coincidentally to the right conclusions without any solid scientific basis. In doing so, we provide experimental evidence that the HER proceeds on polycrystalline Pt via the Volmer-Heyrovsky mechanism. We also show experimentally that, on Pt, the standard equilibrium potentials of the Volmer ( $${E}_{{\text{H}}^{+}/{\text{H}}_{\text{o}\text{p}\text{d}}}^{\text{o}}$$ ) and Heyrovsky ( $${E}_{{{\text{H}}_{\text{o}\text{p}\text{d}},\text{H}}^{+}/{\text{H}}_{2}}^{\text{o}}$$ ) steps are only approximately 20 mV away from that of the H+/H2 couple ( $${E}_{{\text{H}}^{+}/{\text{H}}_{2}}^{\text{o}}$$ ). In other words, we experimentally prove that Pt is very near to the ideal catalyst for the HER (that for which $${E}_{{\text{H}}^{+}/{\text{H}}_{\text{o}\text{p}\text{d}}}^{\text{o}}={E}_{{{\text{H}}_{\text{o}\text{p}\text{d}},\text{H}}^{+}/{\text{H}}_{2}}^{\text{o}}={E}_{{\text{H}}^{+}/{\text{H}}_{2}}^{\text{o}}$$ ).