Resolving the chemical identity of H2SO4 derived anions on Pt(111) electrodes: they're sulfate.

Igor Ying Zhang,Martin Wolf,Yujin Tong,R Kramer Campen,Gregor Zwaschka,Zhenhua Wang

doi:10.1039/c9cp03397a

Abstract

Understanding how electrolyte composition controls electrocatalytic reactions requires molecular-level insight into electrode/electrolyte interaction. Perhaps the most basic aspect of this interaction, the speciation of the interfacial ion, is often controversial for even relatively simple systems. For example, for Pt(111) in 0.5 M H2SO4 it has long been debated whether the adsorbed anion is SO42-, HSO4- or an H3O+SO42- ion pair. Here we apply interface-specific vibrational sum frequency (VSF) spectroscopy and theory to this problem and perform an isotope exchange study: we collect VSF spectra of Pt(111) in H2SO4(H2O) and D2SO4(D2O) as a function of bias and show that at all potentials they are identical. This is the most direct spectroscopic evidence to date that SO42- is the dominant adsorbate, despite the fact that at 0.5 M H2SO4 bulk solution is dominated by HSO4-. This approach is based on the unique selection rule of the VSF spectroscopy and thus offers a new way of accessing general electrode/electrolyte interaction in electrocatalysis.

Highlights

Much recent work has shown that the efficiency of such important electrocatalytic reactions as CO2 reduction, hydrogen evolution and oxygen evolution or reduction, depends on the electrode electronic structure and the composition of the supporting electrolyte.[1,2,3,4] While it is clear that electrolyte effects on reaction efficiency are related to the potential dependent interaction of electrode, ion and solvent, molecular level insight into how electrolyte interacts with the electrode surface is required
While the data are similar in all studies, the assignment of the observed spectral features, principally the intense, narrow resonance near 1250 cmÀ1 that is higher in frequency than any vibrations of SO42À or HSO4À in bulk solution, has varied between adsorbed SO42À, adsorbed HSO4À and/or adsorbed SO42ÀÁ Á ÁH3O+ ion pairs
Feliu, Lipkowski and coworkers showed that some of the challenges in assigning peaks observed in Infrared Reflection–Absorption Spectroscopy (IRRAS) could be overcome by conducting Subtractively Normalized Interfacial Fourier Transform Infrared Reflection Spectroscopy (SNIFTIRS) and quantitatively correcting for IR absorption in bulk solution.[23]. Using this approach they quantified the pH dependence of the 1250 cmÀ1 peak and suggested that SO42À is the dominant adsorbate based on the invariance of the spectral feature in a wide range of pH (1–5.6)

Summary

Introduction

Much recent work has shown that the efficiency of such important electrocatalytic reactions as CO2 reduction, hydrogen evolution and oxygen evolution or reduction, depends on the electrode electronic structure and the composition of the supporting electrolyte.[1,2,3,4] While it is clear that electrolyte effects on reaction efficiency are related to the potential dependent interaction of electrode, ion and solvent, molecular level insight into how electrolyte interacts with the electrode surface is required Gaining such insight has proven challenging: despite much recent effort[5,6,7,8] very basic questions regarding the link between electrolyte, electrode and reactivity remain for even well studied systems. The most direct measurement to confirm surface speciation is an isotope exchange: the spectral response of adsorbed HSO4À modes involving H should shift dramatically on deuteration while that of adsorbed SO42À should not Performing this experiment in IRRAS or SNIFTIRS is extremely challenging, as the strong D2O bending absorption at 1200 cmÀ1 substantially complicates data interpretation.[21]

Methods

Results

Conclusion