Kinetics of the hydrogen evolution on nickel in alkaline solution: new insight from rotating disk electrode and impedance spectroscopy analysis

Abstract

Nickel catalysts were characterized for the hydrogen evolution reaction (HER) using rotating disk electrode (RDE) and electrochemical impedance spectroscopy (EIS). The theoretical Levich slope was calculated on the basis of the HER mechanism in alkaline solution. Kinetic and thermodynamic parameters of the reaction were obtained for fresh polished nickel electrode (Ni_f), and for nickel after a quick chronoamperometric aging procedure (Ni_pc). Koutecky-Levich analysis indicated that the rate determining step on Ni_f is the one-electron Volmer reaction, while for Ni_pc decreases from one to 0.68 and exhibits a strong temperature dependence. A loss of catalytic activity, corresponding to an increase of 0.3 V of the HER onset potential, was observed after aging. Furthermore, from the EIS-Tafel analysis we concluded that the H adsorption changes from a Langmuir type for Ni_f, to a Temkin type for Ni_pc.

Introduction

The hydrogen evolution reaction (HER) is one of the most frequently studied electrochemical reactions for different reasons. First, the reaction takes place through a limited number of reaction steps with only one reaction intermediate involved [1], [2], [3], [4], [5]. Second, it has an industrial/technological interest in the alkaline water electrolysis [6], [7], [8], where nickel-based materials are used as a cathode.

There are a large number of works directed toward characterize the mechanism involved in the hydrogen evolution. The general mechanism of HER in alkaline solution on nickel electrodes, is based on the following three steps [2], [9], [10]:

$$\begin{array}{c}\begin{array}{c}\mathrm{Ni}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\underset{}{\overset{}{\underset{k_{-V}}{\overset{k_V}{\rightleftharpoons }}\mathrm{Ni}···{\mathrm{H}}_{\mathrm{ad}}+{\mathrm{HO}}^{-}}}\end{array}(\mathrm{Vo}\mathrm{lmer}\text{Reaction})\\ \begin{array}{c}2\mathrm{Ni}···{\mathrm{H}}_{\mathrm{ad}}\underset{k_{-T}}{\overset{k_T}{\rightleftharpoons }}2\mathrm{Ni}+{\mathrm{H}}_2\end{array}(\mathrm{Tafel}\text{Reaction})\\ \mathrm{Ni}···{\mathrm{H}}_{\mathrm{ad}}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\underset{k_{-H}}{\overset{k_H}{\rightleftharpoons }}\mathrm{Ni}+{\mathrm{HO}}^{-}+{\mathrm{H}}_2(\mathrm{Heyrovsky}\text{Reaction})\end{array}$$

In a simple thermodynamic analysis, entropy change for the Volmer reaction is negative, since the H adsorption orders the system, while Heyrovsky and Tafel reactions have positive entropy changes due to the desorption of one and two H_ad, respectively, to release H_2(g). For the Heyrovsky reaction an additional decrease of entropy, as compared to the Tafel reaction, is promoted by the hydration of the HO⁻ ions [11].

There are several results reported for HER kinetics on nickel in alkaline solutions, where a Tafel slope close to −0.120 V dec⁻¹ is commonly observed [11], [12], [13], [14], [15]. This indicates that, under these conditions, the Volmer reaction with the transference of one electron is the rate-determining step, making irrelevant the analysis of the Tafel and Heyrovsky steps [11], [16].

RDE technique provides a large number of highly reproducible kinetics parameters, including those related to surface electrode passivation under controlled conditions. As far as we know, there are few RDE studies of the HER on nickel electrodes [17], [18], [19], and there is no information of Koutecky-Levich and Conway transfer coefficients in literature. Those parameters would allow establish the mechanistic and thermodynamic parameters affecting HER catalysis.

Ni cathodes exhibit a significant decrease of their HER activity after several hours of electrolysis [20], [21], [22], [23], [24], [25]. Polished Ni surfaces exposed to air become coated with a bilayer composed of α-Ni(OH)₂ and NiO_x. At potentials lower than the reversible hydrogen electrode (RHE), the bilayer reduces to Ni metal, and forming α-NiH_x and β-NiH_x [22], [23], [24], [26] contributing to the decrease of HER activity [22], [24]. Small amounts of β-NiH_x was detected by galvanostatic pulses applied to nickel electrodes in alkaline solutions [24]. Hall et al. [27] found that Ni polished surfaces in this media are covered by a α-Ni(OH)₂/NiO_x bilayer that incorporates H atoms deep into the electrode. In concentrated alkaline solutions and large cathodic current densities, α-NiH_x and β-NiH_x can be formed at the electrode surface, while NiO_x, α-Ni(OH)₂, β-Ni(OH)₂, and β-NiOOH can be reduced to Ni on subsequent cathodic polarization. However, repeated oxidation and reduction cycles introduce strains and mechanical failures are found on the electrode surface.

Although the mechanism of HER on nickel electrodes has been widely studied, the kinetics and electrochemical impedance spectroscopy parameters are not reported in the literature under controlled hydrodynamic conditions and kinetic and thermodynamic parameters under these conditions necessary to compare catalysts were not calculated.

In this work, the results of RDE and electrochemical impedance spectroscopy (EIS) analysis for a polycrystalline nickel catalyst, before and after being subjected to hydrogen evolution conditions for 4 hours in alkaline solution, are reported. Koutecky-Levich, Tafel, and Conway analysis of the RDE results, along with the EIS analysis at different temperatures are discussed with the aim to show how the kinetics and thermodynamic parameters of HER are modified when the nickel catalyst is electrochemically aged under controlled conditions. The theoretical Levich slope was calculated on the basis of the HER mechanism in alkaline solution and used to establish changes in the reaction mechanism with temperature and aging. Calculated kinetic and thermodynamic values are necessary to establish catalysts comparison parameters that help to understand the reason for the improvement or decay of electrocatalytic activity of a catalyst (including alloys) in HER.