Disentangling Chemical Interaction and Electric Fields at Electrochemical Interfaces

The specific binding of solution components to a catalytic surface and the interfacial electric field are critical to electrocatalysis, but the two factors are entangled. Distinguishing whether adsorbate binding to the surface is governed by through-bond charge transfer that is modulated by a field or through-space electrostatic effects is essential for rational catalyst design. Here, we probed the binding of thiocyanate (SCN–) to Au and Pd as a function of applied potential (ϕapp) using surface-enhanced infrared absorption spectroscopy (SEIRAS). The IR data showed distinct shifts of the CN stretching wavenumbers (ν̃CN) with ϕapp for S- versus N-bound SCN– and on Au versus Pd. These ν̃CN shifts with ϕapp are the result of different extents of through-space electric field (Stark) effects and through-bond chemical binding effects. The presence of two binding modes of SCN– on the metal surfaces provided two separate interfacial probes, which enabled estimating the contributions of the two effects to the overall ν̃CN versus ϕapp slopes quantitatively. Competitive adsorption experiments using SCN– and different halides (Cl–, Br–, and I–) support our conclusions. While the through-bond effect for S-bound SCN– is more pronounced on Pd than on Au, that of N-bound SCN– is similar. Our data also suggest a similar electric field change with ϕapp on Pd and Au. This work offers a method to differentiate key contributors to interfacial chemistry in an external electric field and provides important groundwork for the development of future electrocatalysts.