Joint Kinetic/In Situ Spectrometric Investigation of the Multielectron/Multiproton-Transfer-Based Adsorption Electrode Process of Phosphate Anions on the Ir(111) Surface across a Comprehensive pH Range
Tomoaki Kumeda, Ken Sakaushi
Abstract
Understanding the intriguing nature of multi-electron/-proton transfer electrode processes is essential for designing advanced electrode materials. Phosphate buffer is a typical electrolyte system for studying electrocatalysis, such as water electrolysis and fuel cell reactions over a wide pH range. The phosphate anions are known to exhibit a remarkable site-blocking effect toward various electrode processes, which is comparable to or greater than sulfate anions. However, less is known about the effect of phosphate adsorption on electrode processes because only a few comprehensive analyses on the phosphate adsorption effects for electrocatalysis have been carried out over a wide range of pH. Here, we show a combination of kinetic analysis and in situ FTIR spectrometry for uncovering the electrode process based on the phosphate adsorption dynamics on Ir(111) under acidic, neutral, and alkaline conditions, from pH 1.7 to 12.1. The spectroscopic study aims to unveil the correlation between phosphate adsorption in different pH and electrocatalytic performance for the oxygen evolution reaction. In situ spectrometry revealed the pH- and potential-dependent polyprotic nature and binding strength of adsorbed phosphate anions. We observed that the phosphate adsorption occurs at a more negative potential under low pH (acidic) conditions compared to higher pH (neutral and alkaline) conditions. The onset potential of the phosphate adsorption becomes lower in the order of Ir < Pt < Au, indicating strong binding of the phosphate adlayer on Ir. The phosphate adsorption on Ir(111) leads to remarkable site-blocking effects on surface oxide formation. Furthermore, phosphate anions are adsorbed on Ir(111) at the OER potential region (>1.23 V vs RHE), and the inhibiting and buffering effects of phosphate anions influence OER performance on the Ir surface.