Electrochemical Performance of Borate‐Doped Nickel Sulfide: Enhancement of the Bifunctional Activity for Total Water Splitting

Abstract Electrolytic water splitting with solar electricity is one of the few technologies which can produce hydrogen with nearly zero carbon emission. The viability of the technology depends on the availability of low‐cost alternatives to the platinum group metals (PGMs); which are the best known electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In this study, borate doping of nickel sulfide was found to yield catalysts which are effective for total water electrolysis. For the borate‐doped nickel sulfides ((BO 3 ) x −Ni 3 S 4 , x=0, 0.57, 0.78 and 1.08), the strong affinity of borate for hydroxyl groups facilitates the dissociation of adsorbed water molecules to oxygen. Consequently, the borate‐doped nickel sulfide catalyst with the best performance in this study, (BO 3 ) 1.08 −Ni 3 S 4 , requires only a small overpotential of 306 mV to support a current density of 10 mA cm −2 in OER. This, together with a small Tafel slope of 36.5 mV decade −1 and a stable extended operation; places (BO 3 ) 1.08 −Ni 3 S 4 among the top non‐PGM OER electrocatalysts reported to date. At the same time (BO 3 ) 1.08 −Ni 3 S 4 also shows good HER activity and stability for it to be used as a standalone catalyst for efficient total water splitting in alkaline electrolyte (1.69 V at 10 mA cm −2 ).