Monitoring Mechanical Modulation of Reactivity in Electrocatalysis

Many modern catalyst materials exploit a strained surface layer as the active component, exploiting that reaction rates in heterogeneous catalysis vary when the substrate is elastically strained in the tangent plane. Here we report a direct and quantitative experimental monitoring of the modulation of the electrocatalytic reaction current while the electrode is strained. The hydrogen evolution reaction (HER) on Au and Pt electrodes in H₂SO₄ is studied as a model process.

Our experiment uses a lock-in technique for following the modulation of the electrocatalytic reaction current in situ while a small cyclic elastic strain (~ 10^-4) is imposed on the electrode material. The approach yields well-defined and precisely quantifiable coupling coefficients linking the reactivity to strain. The coefficients are measured as the function of the overpotential. We advertise the fact, ignored in previous work on strained-layer catalysis, that the coupling strength depends on the overpotential by magnitude and, as we show, even by sign. Specifically, tensile strain enhances the reactivity at low overpotential, whereas the trend is inverted and the reactivity diminished at higher overpotential. Furthermore, we show that the data affords a quantification of strain-dependence of the adsorption and barrier energies. This quantification is another distinguishing feature of our approach.

The overpotential-dependent coupling is readily modeled by kinetic rate theory. To this end, we take the impact of strain for, e.g., Heyrowsky kinetics to be mediated by (i) a strain dependence of the activation enthalpy for the ion + atom reaction step, and (ii) an independent strain dependence of the hydrogen adsorption enthalpy. In agreement with experiment, the rate equation predicts the reaction current-strain coupling coefficient to invert its sign upon transition from dilute to concentrated hydrogen adsorbate layer.

Our method appears applicable quite generally to electrocatalytic reactions. Therefore it promises a new tool for studying strain-dependent catalysis and for identifying the underlying microscopic processes.