HER in NaOH – Investigations Using an Inverted Rotating Disc Electrode

Abstract

Hydrogen is one of the promising sources of renewable energy to meet the impending energy crisis. Hydrogen can be produced using techniques such as steam reforming of methane, coal gasification, water electrolysis etc.[ 1 ] Among these techniques, water electrolysis produces purest H2 without any harmful by-products, where water is split in to H2 and O2 gas with the supply of a DC power.[ 2 ] Hydrogen Evolution Reaction (HER) is the cathodic reaction in water splitting, and attracts wide attention among the researchers. HER on various noble and transition metals has been studied well.[ 3 ] Pt is the best known catalyst for HER and located at the top of volcano curve while Au is found to have low activity for HER.[ 4 ] HER is oftentimes performed in alkaline media due to the higher stability of the electrode materials though the rates of the reaction are relatively slow to those in acidic media.[ 5 ] The mechanism of HER is proposed to be a combination of three elementary steps (Eqns. 1-3). The Volmer (V) step is the first step of the reduction of water molecule on the catalytic surface into an adsorbed hydrogen atom and negatively charged hydroxide anion (Eqn. 1). The adsorbed hydrogen atom can either be attacked by a water molecule to produce a hydrogen molecule and a hydroxide anion (the Heyrovsky step, Eqn. 2), or combine with another adsorbed hydrogen atom (formed by the Volmer step) to generate a hydrogen molecule that leaves the surface (the Tafel step, Eqn. 3). Though several studies have been published on HER studies, the investigations were limited to Tafel slope analysis in a stationery electrode or a rotating disc electrode(RDE) in a narrow potential range.[ 6 ] A major reason for this constrain is that in a normal RDE, many bubbles are produced at high over-potentials and block the electrode surface, since the electrode surface faces downward. Thus, it restricts the experimental range and the confidence in the model parameters. In this work, cathodic polarization of HER on Pt and Au disc electrode at different concentration and rotational speeds are performed in an inverted RDE (IRDE) experimental arrangement. In IRDE, the electrode surface is faced upward and the gas bubbles from the electrode are released rapidly. This allow us to perform experiments in a wide potential window and increase the confidence in the mechanism identified. In all the experiments NaOH is used as the electrolyte and sufficient NaClO4 was added so that the total concentration of anions/ cations was maintained at 1 M. MillQ water (Millipore) of high purity was used to prepare the solutions. A three-electrode setup with Pt mesh as a counter electrode and Ag/AgCl in 3MNaCl as a reference electrode was used. The experimental results are compared with model predicted results of VH, VT and VHT mechanisms and the best fit kinetic parameters are determined. HER studies were conducted at 10 and 100mM of NaOH on Pt and Au IRDE at 900 rpm and the cathodic polarization results are shown in Fig. 1. The polarization current density is more or less independent of concentration of alkali and rotational speed of the electrode (not shown). References [1] E. Fakioğlu, Y. Yürüm, T. Nejat Veziroğlu, International Journal of Hydrogen Energy 2004, 29, 1371-1376. [2] C. A. C. S. Diogo M. F. Santos, José L. Figueiredo, Quím. Nova 2013, vol.36, 1176-1193. [3] aJ. Durst, C. Simon, F. Hasché, H. A. Gasteiger, Journal of The Electrochemical Society 2015, 162, F190-F203; bD. M. Delgado, M. Kim, D-J, International Journal of Electrochemical Science 2015, 11, 9379-9394. [4] aB. Conway, L. Bai, Electrochimica Acta 1986, 31, 1013-1024; bJ. M. H. Ludwig A. Kibler, Areeg Abdelrahman, Ahmed A. El-Aziz and Timo Jacob, Current Opinion in Electrochemistry 2018, 9:265–27 2018, 9, 265-270. [5] J. Rossmeisl, K. Chan, E. Skúlason, M. E. Björketun, V. Tripkovic, Catalysis Today 2016, 262, 36-40. [6] L. Chen, A. Lasia, Journal of The Electrochemical Society 1991, 138, 3321-3328. Figure 1