Scanning electrochemical microscopy (SECM) study of pH changes at Pt electrode surfaces in Na2So4 solution (pH 4) under potential cycling conditions

Abstract

pH Changes in the vicinity of a platinum electrode immersed in a 10^–4 mol dm^–3 H₂SO₄+ 0.5 mol dm^–3 Na₂SO₄ solution have been probed with a scanning electrochemical microscope (SECM). Experiments were carried out in the tip substrate voltammetric mode where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate. The tip current was made pH-sensitive by holding the tip potential in a region where a pH-dependent reaction occurs. Three reactions were considered: hydrogen evolution, Pt oxide formation and oxygen evolution. The results provide direct evidence of a pH decrease close to the substrate during hydrogen desorption, oxide formation and oxygen evolution. Conversely, a pH increase is observed during oxide reduction, hydrogen adsorption and hydrogen evolution. A quantitative assessment of pH changes is proposed; variations as large as one pH unit were observed.