Mechanistic studies of pyridinium electrochemistry: alternative chemical pathways in the presence of CO2

Abstract

Protonated heterocyclic amines, such as pyridinium, have been utilized as catalysts in the electrocatalytic reduction of carbon dioxide. While these represent a new and exciting class of electrocatalysts, the details of the mechanism and faradaic processes occurring in solution are unclear. We report a series of cyclic voltammetry experiments involving Pt, Ag, Au, and Cu electrodes, under both aqueous and nonaqueous conditions, directed towards gaining an improved mechanistic understanding of pyridinium electrochemistry. Surface-enhanced Raman (SER) spectroelectrochemistry was also performed on Cu film-over-nanosphere electrodes in order to identify adsorbed species. It was found that the reduction potential of pyridinium (−0.58 V vs. SCE) and its electrochemical reversibility are unique features of platinum electrodes. In contrast, the reduction potentials on Ag, Au, and Cu electrodes are ∼400 mV more negative than Pt in both the presence and the absence of CO₂. SER spectroelectrochemistry of pyridinium solutions shows no evidence for a pyridinium radical or a pyridinium ion. Increased cathodic current in the presence of CO₂ is only detected at scan rates less than 10 mV s⁻¹ in aqueous solutions. The addition of CO₂ resulted in a shift in the potential for the hydrogen evolution reaction. Pyridinium electrochemistry was observed under nonaqueous conditions; however no increase in cathodic current was observed when CO₂ was added to the solution. Based on this set of results it is concluded that the reduction potential of pyridinium is surface dependent, CO₂ acts as a pseudo-reserve of H⁺, and pyridinium and CO₂ create an alternative mechanism for hydrogen evolution.