Enhanced activity and chlorine protection in prolonged seawater electrolysis using MoS2/sulfonated reduced graphene oxide

Abstract

Electrolyzer technology necessitates the use of seawater instead of freshwater to achieve a comprehensive supply of clean and economical energy. However, the tendency of chloride ions (Cl⁻) to significantly erode the metal surface is a major challenge during seawater electrolysis. Therefore, designing an electrode that is resistant to chloride ions is of great importance to develop an efficient seawater electrolyser. In this work, we present a double layer anode consisting of a molybdenum sulfide electrocatalyst uniformly deposited over sulfonated graphene sheets coated over an Ni foam. The developed electrode (GNiMoOS) helps selectively convert H₂O into H₂ and O₂ rather than chloride (Cl⁻) ions into ClO⁻ in a seawater environment by resisting corrosion due to the Cl⁻ ions in seawater. The chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB) provides solid evidence that the GNiMoOS electrocatalyst blocks the chloride oxidation reaction owing to its distinct resistance to Cl⁻. In addition, density functional theory (DFT) calculations clearly validated the preference of sulfonic moieties towards OH⁻ compared with Cl⁻ ions, confirming the chlorine repelling properties of the GNiMoOS electrode. The successful in situ functionalisation of sulfonic moieties into the reduced graphene oxide (RGO) skeleton with simultaneous development of flower-like MoS₂ was well confirmed using XPS, Raman, SEM, TEM, and FT-IR techniques. GNiMoOS delivered an impressive current density of 100 mA cm⁻² for OER and HER at room temperature, requiring remarkably low overpotentials of just 180 mV and 201 mV, respectively. Industrial faradaic current densities (400–600 mA cm⁻²) were reported with the active electrode at combined overpotentials of ≤600 mV at room temperature. The unique morphology of MoS₂ provides more active sites for the HER/OER, while sulfonated functional groups over graphene impart much-needed anticorrosion properties to the system. Moreover, the electrical coupling between MoS₂ and RGO can make the electron transfer to RGO easier. Therefore, the synergistic interactions among MoS₂, SO₃H and RGO lead to improved catalytic activity and prolonged stability.