Pioneering practical direct sea water splitting via an intrinsically-selective chlorine-phobic nickel polysulphide nanostructured electrocatalyst for pure oxygen evolution

Direct sea water splitting as a source of clean and uninterrupted energy source is an unparalleled approach to sustainable development. Chlorine oxidation, however, strongly limits its usage and thus requests novel avenues to create chlorine-repelling electrocatalysts for pure oxygen evolution under direct-sea water conditions. Herein, for the first time, a NiS₂pS_x^surface (catenated sulphur type Ni polysulfide)-based binder-free, 3D electrocatalyst is established as an excellent oxygen evolution reaction (OER) catalyst under un-buffered neutral water conditions, exhibiting a remarkably decreased overpotential (Δη∼320 mV) in comparison to nowadays accepted noble metal IrO₂ catalyst. This overpotential value (∼ 360 mV, lowest ever reported in the literature) is lower than the limit for chlorine evolution in neutral conditions, thus furnish a potential platform for intrinsically chlorine-phobic OER electrocatalysis for direct sea water splitting. Surprisingly, quantitative analysis of electro-oxidation products in 0.5 M NaCl using our NiS₂pS_x^surface electrocatalyst demonstrates the sole formation of pure oxygen, without any chlorine, which is inevitable when using IrO₂ and Pt as catalysts. The intrinsic ion-selective behavior of our electrocatayst is related to limited exposure of the sterically and electrostatically hindered Ni metal centers to large Cl^- ions, thus selectively evolving oxygen. Furthermore, the fast electrochemical evolution of the NiS₂pS_x^surface surface to form catenated sulphur type polysulphides species (pS_n^2-/ S^2-=2.1) further enhances the intrinsic chlorine-phobicity of the catalyst, displaying pure oxygen evolution even at noteworthy current densities up to 300 mA/cm². This is the first demonstration of intrinsically chlorine-phobic catalytic electrodes for the “direct” sea water splitting, displaying unprecedented electrochemical performances and stability, which further opens new paths towards engineering the electrocatalysts surface aiming for intrinsically ion-selective electrodes for various applications.