Overestimation of Photoelectrochemical Hydrogen Evolution Reactivity Induced by Noble Metal Impurities Dissolved from Counter/Reference Electrodes

Abstract

A three-electrode system is typically utilized in many voltammetry studies to understand the behavior of an analyte at the electrode/electrolyte interface. A bulk Pt piece is usually used as a counter electrode in such systems because of its high activity and stability in many electrochemical reactions. However, the dissolution of the Pt counter electrode led to growing concern about inaccurate evaluation of the inherent characteristics of the analyte. In the present study, we have demonstrated that strong interferences emerged from the conventional Pt counter and Ag/AgCl reference electrodes in the photo electrochemical (PEC) hydrogen evolution reaction (HER) with a model photocathode of p-type silicon (p-Si). Under light illumination, the Pt counter electrode is polarized to as high as 1.6-2.0 V-RHE, which leads to a non-negligible Pt dissolution from the oxidized surface, as monitored by operando inductively coupled plasma-mass spectrometry (ICP-MS). Postreaction spectroscopy and microscopy studies confirm the formation of Pt and Ag particles on p-Si photocathode, resulting in erroneous overestimation of the HER activity of p-Si. Various configurations of the three-electrode system, e.g., an H-type cell with a Nafion membrane, have been studied to find a suitable cell structure for prohibiting undesirable contamination of p-Si. Isolation of p-Si from the Pt counter and Ag/AgCl reference electrodes using the Nafion membrane effectively alleviates the contamination of p-Si but, toward the end, the metallic ions can be slowly deposited on p-Si by diffusion through the membrane. Consequently, this work highlights that the careful caution is necessary when the conventional Pt counter and Ag/AgCl reference electrodes are employed; it is recommended to use a graphite counter electrode and separate reference electrode to prevent artifacts originating from the dissolved Pt and Ag species during PEC cathode evaluations.