Electrochemical H2 Production using Polypyrazole based Zinc(II) Complex in Alkaline Medium

Mohamed M. Ibrahim1,*,, Gaber A.M. Mersal1,, Ahmed M. Fallatah1,, Rabah Boukherroub2,, Safaa N. Abdou3, and Mohammed A. Amin1,*,

1Department of Chemistry, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia

2Univ. Lille, CNRS, Centrale Lille, Université Polytechnique Hauts-de-France, UMR 8520-IEMN, F 59000 Lille, France

3Chemistry Department, Khourma University College, University of Taif, Taif, Saudi Arabia

*Corresponding author: E-mail: ibrahim@tu.edu.sa

Abstract

A zinc(II) complex of polypyazole containing ligand namely, [Tp*ZnCl] (Zn1) {Tp* = tris(3,5-dimethylpyrazolyl)borate}, along with its zinc(II) bound hydroxo complex [Tp*Zn–OH] (Zn2) were synthesized and characterized by FT-IR, Raman, 1H NMR techniques and elemental analysis. In alkaline solution (0.1 M KOH), the Zn2 modified glassy carbon (GC), namely GC–Zn2, was investigated as a molecular electrocatalyst for the hydrogen evolution reaction (HER). Different quantities (ca. 0.1–0.5 mg cm–2) of Zn2 were loaded on GC electrodes to make various GC–Zn2 electrodes. These electrodes were utilized as cathodes in 0.1 M KOH to produce H2 using linear sweep voltammetry (LSV) measurements. The HER electrocatalytic activity of the GC–Zn2 catalyst was found to be quite high and it increased with the catalyst loading density. The top performing electrocatalyst, GC–Zn2 (0.5 mg cm–2), demonstrated considerable HER catalytic performance with a low HER onset potential (EHER) of –33 mV vs. RHE and a high exchange current density of 0.59 mA cm–2. Moreover, the top performing electrocatalyst had a Tafel slope of –152 mV dec–1 and consumed an overpotential of 135 mV to generate a current density of 10 mA cm–2. Under the same operating conditions, these HER electrochemical kinetic parameters were found to be not remarkably far from those determined for commercial Pt/C (–10 mV vs. RHE, 0.88 mA cm–2, 108 mV dec–1 and 110 mV to yield a current density of 10 mA cm–2). In addition, the most active molecular electro-catalysts for H2 production from aqueous alkaline electrolytes were found to be comparable to the HER electrochemical kinetic parameters reported here for the GC–Zn2 electrocatalyst. Using 5000 cycles of repetitive cyclic voltammetry and 48 h of chronoamperometry measurements, the best electrocatalyst’s stability and long term durability were tested. It exhibited high stability for the HER catalytic activity.

Keywords

Tripod ligand, Zinc(II) complex, Molecular electrocatalyst, Hydrogen evolution reaction.

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