In Situ Investigation on Cl- Repellency Effect of Electrolyte Additive on NiOOH for Alkaline Sea Water Splitting

Abstract

Sea water splitting can produce green hydrogen by feeding abundant sea water directly without high-qualified water purification process. However, excess cation such as Mg2+ and Ca2+ in sea water absorbs on the cathode surface, and it causes the loss of active site and activity toward hydrogen evolution reaction. At the anode side, oxygen evolution reaction (OER) should be competed with chlorine evolution reaction due to huge amount of Cl- ion, so operating in high pH electrolyte at low overpotential has been suggested to increase the selectivity toward OER. To further inhibit the adsorption of Cl-, some researchers have suggested using anion additive such as PO4 3- and SO4 2- in the alkaline electrolyte, but both adsorption stability of additive anion and the interaction between anion and metal oxide catalyst are still needed to be studied.In this study, we investigated the Cl- repellency effect of additive anion in the electrolyte to NiOOH catalyst, known as state-of-the-art OER catalyst, for alkaline sea water splitting. To observe the change in adsorbate species by additive anion, we tracked the mass change of the catalyst during the reaction through in situ electrochemical quartz crystal microbalance (EQCM). Furthermore, Cl- can change (degrade) the NiOOH structure by formation of metal-chloride complex. Through in situ Raman spectroscopy, we observed the structural stability of NiOOH in presence of anion additive as well as the adsorption stability of additive anion depending on the applied potential.