An inverted rotating disc electrode (IRDE) setup was fabricated to study the hydrogen evolution reaction (HER) on a Pt electrode. Potentiodynamic polarization studies were conducted in solutions from 5 to 100 mM HClO4 on Pt at three different rotational speeds. Periodic anodic activation of the electrode yielded repeatable results. At high currents, bubble formation adversely affects current measurement in the rotating disc electrode, but not in IRDE. At large cathodic overpotentials, HER current in IRDE is limited by the mass transfer of H+ ions. Simulations show that the polarization results of HER on Pt IRDE are described well by the Volmer Heyrovsky mechanism.
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).
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
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