Electrochemical oxidation of H2S on polycrystalline Ni electrodes

We have evaluated the applicability of Ni anodes in electrochemical conversion of H2S to form sulfur (polysulfides) and H-2. Two different electrolytes containing sulfide were evaluated: a buffered solution of Na2HPO4 at pH 9.2, and a NaOH solution at pH 13. At pH 9.2, deposition of sulfur on the Ni anode was observed, resulting in a significant decrease in electrochemical performance. The composition, morphology, and thickness of the sulfur deposit, as determined by Raman spectroscopy and SEM, was found to strongly depend on the applied potential, and ranged from dense S-8 films to highly porous spherical sulfur structures. Oxidation of the anode was also observed by conversion of Ni to NiS2. The formation of the sulfur film was prevented by performing the reaction at pH 13 in NaOH in the range of - 1.0 V to + 0.6 V versus Hg/HgO. It is proposed that at these highly basic pH values, sulfur is dissolved in the electrolyte in the form of polysulfides, such as S-2(2-) or S-8(2-). When using Ni anodes some oxygen evolution was observed at the anode, in particular at pH 13, resulting in a Faradaic efficiency for sulfur removal of similar to 90%. Graphic