Investigations of Hydrogen Evolution Reaction on Platinum in Alkaline Medium Using an Inverted Rotating Disc Electrode

Abstract

Electrochemical energy storage and conversion using hydrogen has become important in the area of sustainable energy and development1. HER is one of the key reactions in alkaline water electrolysis and chlor-alkali industry2. Though water electrolysis is not a cost-effective method yet,it finds attention as it produces high purity hydrogen3.Platinum is a very active catalyst for HER.The mechanism of HER is believed to be a combination of three elementary steps, 1. the Volmer (V) step, where water molecules are reduced to produce adsorbed hydrogen, followed by either the Heyrovsky (H) step or the Tafel (T) recombination step. 2. In H step, a hydrogen molecule is formed by an electron transfer reaction between an adsorbed hydrogen and another water molecule to produce a hydrogen molecule in the desorbed state. 3. In T step,two adsorbed surface hydrogen next to each other react to form a hydrogen molecule by chemical desorption4.Until now, the published HER studies were restricted to Tafel slope analysis under a small over potential range and were conducted in a stationary electrode or a rotating disc electrode(RDE) set up. One difficulty in using RDE is the formation of gas bubbles which block the electrode surface. At small overpotentials, the bubble formation rate is very small and RDE is sufficient for performing HER. This restriction of small overpotential limits the experimental range and confidence in the model. Besides, using Tafel slope to determine the underlying mechanism is fraught with pitfalls, and it is desirable to fit the entire polarization data to the complete model.In this work, cathodic polarization of HER on inverted Pt disc electrode at different concentrations of KOH/NaOH are performed. In inverted RDE experimental arrangement,the electrode surface is faced upward and the gas bubbles from the electrode are released quickly.This expands the experimental range for HER studies. In all the experiments, sufficient NaClO4 was added so that the total concentration of anions/ cations was maintained to 1 M, and the solution resistance effects are minimized. MillQ water (Millipore) was used to prepare the solutions. A three electrode setup with Pt mesh as counter electrode and Ag/AgCl in 3 M NaCl as reference electrode was used. Prior to acquisition of polarization current at each cathodic potential, the electrode was activated to increase the catalytic activity and reproducibility. The experimental results are compared with model predicted results of VH, VHT and VT models and the best fit kinetic parameter values are determined. References A. Eftekhari, International Journal of Hydrogen Energy, 42 (16), 11053-11077 (2017).F. Safizadeh, E. Ghali, and G. Houlachi, International Journal of Hydrogen Energy, 40 (1), 256-274 (2015).A. Lasia, in Handbook of fuel cells: fundamentals technology and applications (eds W. Vielstich, A. Lamm, H. A. Gasteiger and H. Yokokawa), Vol. 2, p. 414-440, (2010).D. A. Harrington and B. E. Conway, Electrochimica Acta, 32 (12), 1703-1712 (1987).