Mössbauer Analysis of S-Doped Nickel Nitroprusside for Oxygen Evolution Reaction

Nitroprussides, salts containing pentacyanonitrosylferrate(II) anions and cations with alkali and transition metals, have been useful for myriad purposes. Recently, they have continued to attract interest for the role their porosities could play in small molecule storage and gas sensing, the ability to tune spin-coupling via metal to ligand charge transfer, and in the oxygen evolution reaction (OER). Specifically, nickel nitroprusside is an effective OER catalyst and its performance is improved by sulfur doping. It has been proposed that the doping causes an isothiocyanate substitution for one of the cyanide ligands in the nitroprusside molecule. We present Mössbauer spectra of the undoped nickel nitroprusside sample and samples with increasing S doping in pursuit of understanding the mechanism responsible for the increased performance of the S-doped samples in OER. Mössbauer data exhibit two distinct modifications to the undoped spectra as doping proceeds through superstoichiometric S dose, consistent with the creation of more than one novel iron site.