Underestimation of Platinum Electrocatalysis Induced by Carbon Monoxide Evolved from Graphite Counter Electrodes

In the study of electrocatalysis, precise measurement of voltammetric responses for electrode materials is an essential step in evaluating them accurately and understanding electron transfer at the electrode/electrolyte interfaces. Due to growing concerns regarding the validity of Pt counter electrodes in such measurements, largely attributed to their dissolution and redeposition, graphite is increasingly being employed as a versatile counter electrode in conventional three-electrode systems. However, the reliability of graphite in this role has not been fully investigated. Herein, we have demonstrated that using graphite as a counter electrode can significantly hinder the precise evaluations of electrocatalytic materials. For a polycrystalline Pt surface coupled with a graphite counter electrode, the rapid loss of catalytic activity in the hydrogen evolution (HER) and oxygen reduction reactions (ORR) is observed within a few voltammetric evaluation cycles. Online differential electrochemical mass spectroscopy (DEMS) and in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) revealed that the catalytic activity loss is mainly due to the formation of poisonous carbon monoxide (CO) and its subsequent strong adsorption on the catalytic Pt sites. CO coverage on the Pt surface becomes almost saturated within just 50 cycles of HER polarization measurements. The resultant underestimation of activity is effectively avoidable by physically separating the graphite counter electrode from the analyte in an H-type cell. Consequently, our findings highlight that, similar to Pt counter electrodes, graphite counter electrodes must be carefully applied to eliminate any experimental artifacts originating from CO evolved during electrocatalytic reactions.