The Role of Iron Impurities on Nickel Cathodes for Hydrogen Evolution Reaction (HER)

Abstract

Over the last decades, several studies have reported the effect of traces of iron species on the improvement of the oxygen evolution reaction (OER) kinetics for nickel-based anode materials in alkaline media. [1] However, the effect of iron impurities on the hydrogen evolution reaction (HER) is still a subject to debate. For instance, at low iron concentrations (0.03 – 0.5 ppm) a loss of the HER activity for Ni surface is observed due to the electrochemical deposition of less active Fe. [2,3] Other studies showed less deactivation behavior of the Ni cathode at 3 and 14 ppm iron ion concentration over time. [2,4] This improvement of the lifetime of the Ni electrode in the presence of iron cations is very likely related to the increased surface roughness by Fe electrodeposition and thereby the decrease in the formation of inactive NiH species. [2] Systematic investigations on Fe sputtered Ni surfaces with different coverages showed that ≥ 60% of Fe coverage stabilizes the Ni cathode by preventing hydrogen diffusion through the Ni lattice at constant current density of -100 mA cm-2. [5] Generally, the role of Fe impurities on the HER activity for nickel cathode surface in alkaline environments is still unclear to date.In this work, we comprehensively evaluated the influence of the iron ion concentration on the long-term HER performance for a polycrystalline Ni electrode by using rotating disc electrode (RDE) setup. The changes in HER activity as a function of the iron ion concentration was correlated with resulting surface roughness and layer thickness/coverage of the electrodeposited iron. Highly purified 0.1 M KOH was used as basic electrolyte solution and mixed with iron ion concentrations in the range of <1, 6 and 14 ppm. The initial HER activity on the Ni surface is unchanged irrespective of the iron ion concentration added into the electrolyte. However, at iron ion concentrations of <1 ppm and 6 ppm, the chronopotentiometric (CP) experiments at -10 mA cm-2 for 24 hours show a potential drop of ~70 mV and ~53 mV, respectively, resulting in a deactivation of the Ni surface by forming NiH species. Very interestingly, high iron ion concentrations (e.g. ~14 ppm) lead to an almost constant potential (~ 1 - 7 mV drop) during the CP experiment, by reducing the formation of inactive NiH. The electrochemically treated Ni electrodes were then characterized by X-ray fluorescence spectroscopy (XRF) and scanning electron microscopy in combination of energy dispersive X-ray spectroscopy (SEM-EDX). The SEM micrographs show iron particles electrodeposited uniformly on the polycrystalline Ni surface after the CP experiments with < 1, 6 and 14 ppm of iron ion concentrations in 0.1 M KOH. The analysis of the EDX maps reveals a coverage of 17, 54 and 58 % on the Ni surface at iron ion concentrations of < 1, 6 and 14 ppm, respectively. Obviously, iron ion concentrations of 6 and 14 pm show very similar coverages on the Ni surface, indicating their minor role on the HER activity. We suggest that during the HER the hydrogen bubbles suppress the formation of a dense and complete iron layer on the Ni electrode surface. The spatial distribution and thickness of the electrodeposited Fe particles were also evaluated after 1 h and 24 h of CP experiments with < 1, 6 and 14 ppm iron ion concentrations by using micro-XRF equipped with a polycapillary lens of 20 µm. Thicker iron layers after 24 hours are formed at high iron ion concentrations, preventing the hydrogen diffusion into Ni lattice to form inactive NiH species and to maintain the HER activity over time under alkaline conditions.In summary, we show that traces of iron impurities have a huge impact on the long-term durability of polycrystalline Ni electrodes for HER in alkaline environments. A critical iron ion concentration is required to suppress the formation of inactive NiH species during the HER at -10 mA cm-2 for 24 h and/or to electrodeposite enough iron for maintaining the HER kinetics on the polycrystalline Ni electrodes in alkaline environments.