Hydrogen Evolution and Oxidation over Pt Electrode under Neutral Condition

Tremendous attentions have been recently paid to refocus electrocatalysis on H₂O/H₂/O₂ chemistries for the conversion of solar energy into electricity or chemical energy[1-2]. Hydrogen can be used as a fuel in fuel cells that directly convert the chemical energy into electricity at high efficiency. The fundamental reaction for proton exchange membrane fuel cells is the hydrogen oxidation reaction (HOR). On the other hand the hydrogen has been produced by electrolysis, which is known as hydrogen evolution reaction (HER). For both reactions Pt is a well-known active and durable catalyst at various pH and temperature.

A large number of investigations have been conducted for the understanding of the Pt properties especially over low index single crystal electrodes to explicitly understand the structure-activity relationship. In practice, however, polycrystalline electrode composed of a mixture of various facets is generally used. And almost all of these studies on Pt electrocatalysts were conducted in pH-extreme conditions: acidic (H₂SO₄ or HClO₄) or alkaline (e.g., KOH or NaOH) electrolyte. To the best of our knowledge, there are only a few reports for the comprehensive kinetic study on Pt activity with various pH[3]. For HER and HOR chemistries, the activity of hydronium ion and hydroxyl ion (pH) play crucial roles as reactants or products on the electrocatalysis. Systematic understanding of the dependence of each reaction on their activities over the same catalyst would lead to further understanding of how hydronium and hydroxyl ions behave. We trust that this kind of fundamental work must be useful for, e.g., photocatalytic or photoelectrochemical water splitting and biological process because suitable pH is determined not by the electrocatalytic performance but by available water source, the stability of semiconductors or bacteria, some of which exhibit great performance in neither acidic nor alkaline but more neutral conditions. It is thus useful to understand electrochemical performance of well-known electrocatalysts, such as Pt, at various conditions (pH).

In the present study, we demonstrate detailed kinetic study on HER and HOR using polycrystalline Pt rotating disk electrode in 0.5 M Na₂SO₄ of pH ranging from pH 1 to 13. The systematic investigation focusing on the concentration effect of hydronium and hydroxyl ions revealed that there is a distinct mechanism switching because of the different activity of hydronium ions, surface coverage of the intermediates and also relative activity of ions from the supporting electrolyte. Especially in neutral conditions of pH 5-9, substantial overpotentials were required to evolve or oxidize hydrogen using the Pt electrodes. Effects of different supporting electrolytes on HER/HOR kinetics were investigated to understand how kinetic rate and mass transportation were affected by different anions and cations of supporting electrolyte. As a result kinetics of hydrogen depended on the kinds and concentration of ions from supporting electrolyte: SO₄^2- gave lower Levich slope value and higher Tafel slope compared to other ions such as ClO₄^-, Cl^- . The universal effects of the pH on HER/HOR reaction rates, irrespective of different electrolytes, suggest that the different pH and ions mainly affect the surface nature and environment of Pt, which in turn exclusively determine the electrocatalytic performance.

[1] K. Takanabe, K. Domen, Green 2011, 1, 313-322.

[2] A. Wieckowski, J. K. Norskov, FUEL CELL SCIENCE: Theory, Fundamentals, and Biocatalysis, John Wiley & Sons, Inc., 2010.

[3] D. Stmcnik, et. al., Nature Chem. 2013, 5, 300-306.