A Revolutionary Approach to Study the Hydrogen Electrode Formation on Different Noble Metals Exposed to Monolayer to Sub-Monolayers Range of Water Using Scanning Kelvin Probe

Abstract

The inadequacy of modern surface analytical tools to probe the electrochemical double layer (EDL) which governs the catalytic reactions at the interface hampers its in-depth investigation. The reason for this is that the EDL is usually buried under the bulk electrolyte. Though great progress was made in computational modeling the EDL [1,2], at the experimental level similar progress is necessary. One approach is to study the EDL of electrodes covered by electrolyte of nano-scale thickness. Till date, such studies of electrodes covered by electrolyte in the monolayer to sub-monolayer range are scarce, although they were at the center of the birth of electrochemical surface science [3]. Recently the “hydrogen electrode in the dry” concept which was established on palladium surface under dry nitrogen conditions could be a novel approach to understand that facet of the electrode interface activities [4]. The amount of water in this case is confined to just 1-2 monolayers to adsorbed water or even below. Similarly, this approach was successful applied for studying the current-potential relationships for oxygen reduction reaction exposed to different amounts of water ranging from micro meter to monolayers of thickness [5]. In this study, this concept was explored to other noble metal systems such as Pd, Au, Pt, and Ir to understand the hydrogen electrode formation.The present study focuses on hydrogen electrode formation on palladium, iridium, gold, and platinum exposed to inert conditions carried out by scanning Kelvin probe method (see fig) at different hydrogen activities. The activities of hydrogen on the surface was defined by electrochemically (entry side from a=0 to a=0.2) cathodic hydrogen charging method. Firstly, the hydrogen activity of 0.2 was defined on the palladium side by increasing the cathodic potential stepwise by potentiostatic hydrogen charging and measuring the corresponding potential on the exit side by Kelvin probe. This potential on palladium in both wet (>95% rh) and dry (<0.1% rh) conditions follows a 1:1 correlation with the applied entry potential, in accordance with the Nernst equation. Iridium samples show 1:1 in humid conditions, but not in dry conditions, i.e. in presence of a much thinner layer of adsorbed water molecules. At intermediate partial pressures of water, a linear response with slope lower than 1 was observed, that increased with increasing humidity. Whereas, gold samples show no correlation in both wet and dry conditions. Additionally, hydrogen electrode formation on emersed surface was checked to understand the effect of an already existing EDL on the formation of the hydrogen electrode. Also, ellipsometry spectroscopy measurements were carried out to determine the water layer thickness at different partial pressures of water. Overall, the current experimental approach of measuring the potential of these different “dry” electrodes gives more insights about hydrogen electrode formation on noble metals at different hydrogen activities, effect of adsorbed anions, and the effect of different amounts of adsorbed water.