Sulfur incorporation into NiFe oxygen evolution electrocatalysts for improved high current density operation

Abstract

The efficient production of green hydrogen via electrochemical water splitting is important for improving the sustainability and enabling the electrification of the chemical industry. One of the major goals of water electrolysis is to utilize non-precious metal catalysts, which can be accomplished with alkaline electrolyzer technologies. However, there is a continuing need for designing catalysts that can operate in alkaline environments with high efficiencies under high current densities. Here we describe a simple, aqueous-based synthesis method to incorporate sulfur into NiFe-based electrocatalysts for the oxygen evolution reaction (OER). Sulfur incorporation was able to reduce the overpotential for the OER from ca. 350 mV on a NiFe catalyst to ca. 290 mV on the NiFeS catalyst at 100 mA cm⁻² on a flat supporting electrode. Electrochemical impedance spectroscopy data showed a small decrease in the charge transfer resistance of the NiFeS catalysts, showing that sulfur incorporation may improve the electronic conductivity. Surface-interrogation scanning electrochemical microscopy (SI-SECM) studies combined with Tafel slope analysis suggested that the NiFeS catalyst was able to have vacant surface sites available under OER conditions and was able to maintain a Tafel slope of 39 mV dec⁻¹. This is in contrast to the NiFe catalyst, for which the SI-SECM studies showed a saturated surface under OER conditions with the Tafel slope transitioning from 39 mV dec⁻¹ to 118 mV dec⁻¹. The low Tafel slope enabled the NiFeS catalyst to maintain low overpotentials under high current densities, which we attribute to the ability of the NiFeS catalyst to maintain vacant sites during the OER.