Herein we have shown that electrodeposited NiSe2 can be used as a bifunctional electrocatalyst under alkaline conditions to split water at very low potential by catalyzing both oxygen evolution and hydrogen evolution reactions at anode and cathode, respectively, achieving a very high electrolysis energy efficiency exceeding 80% at considerably high current densities (100 mA cm−2). The OER catalytic activity as well as electrolysis energy efficiency surpasses any previously reported OER electrocatalyst in alkaline medium and energy efficiency of an electrolyzer using state-of-the-art Pt and RuO2 as the HER and OER catalyst, respectively. Through detailed electrochemical and structural characterization, we have shown that the enhanced catalytic activity is attributed to directional growth of the electrodeposited film that exposes a Ni-rich lattice plane as the terminating plane, as well as increased covalency of the selenide lattice which decreases the Ni(II) to Ni(III) oxidation potential. Thereby, the high efficiency along with extended stability makes NiSe2 as the most efficient water electrolyzer known to-date.

Ultrasmall FeP nanoparticles have been reported as an efficient oxygen evolution electrocatalyst in alkaline medium with low onset potential for oxygen evolution and require low overpotential to reach 10 mA cm−2 exchange current density.

Cobalt telluride has been identified as an efficient multifunctional electrocatalyst for oxygen and hydrogen evolution reactions and oxygen reduction reaction in alkaline medium. Both hydrothermally synthesized and electrodeposited, CoTe and CoTe2 show efficient electrocatalytic activities. CoTe shows better efficiency for OER with a low Tafel slope (43.8 mV dec–1) and lower overpotential (200 mV) at 10 mA cm–2 compared to CoTe2. DFT studies have also been performed which revealed that CoTe showed higher adsorption energy for intermediate −OH adsorption on the catalyst surface, which corresponds to the catalyst activation step. Comparison of the −OH adsorption energies (E ads) on different catalyst surfaces with the observed overpotential also revealed that this E ads can be used as an appropriate descriptor for benchmarking catalytic efficiencies. Both CoTe and CoTe2 exhibited improved OER catalytic efficiency compared to Co3O4, confirming the primary hypothesis that decreasing anion electronegativity enhances catalytic efficiency by virtue of increasing lattice covalency around the catalytically active site. The difference in OER catalytic activity between CoTe and CoTe2 could be explained from fundamental materials chemistry concepts by comparing their lattice structures which showed different packing density of catalytically active Co sites as well as varying unsaturation of Co-terminated surfaces. Band structure calculations also corroborated such differences and could potentially explain the difference in activity due to observed differences in electron density distribution around the catalytically active Co site. The cobalt telluride compositions also showed moderate HER and ORR activity in alkaline medium, making them trifunctional catalysts which can be used in practical devices. Both CoTe and CoTe2 showed extensive functional and compositional stability for OER, HER, and ORR, under continuous operation in alkaline medium for over 24 h with less than 5% degradation of current density. The excellent compositional stability of each catalyst was revealed by detailed electrochemical measurements and surface and bulk analytical characterizations, which confirmed that there was no catalyst leaching even with long-term operation and no other impurity enrichment in the electrolyte.

Water splitting reaction using earth abundant and environmentally benign catalysts is critical for renewable energy technologies. Herein we report a hybrid composite, FeNi 2 Se 4 nanoparticles supported on nitrogen doped reduced graphene oxide (FeNi 2 Se 4 -NrGO) as an efficient and dependable bifunctional electrocatalyst for oxygen evolution and oxygen reduction reactions (OER and ORR, respectively) under alkaline conditions. While FeNi 2 Se 4 nanoparticles themselves showed good catalytic activity for water oxidation, the constructed hybrid nanocomposite with NrGO as the supporting matrix, showed enhanced catalytic activity with a small overpotential of 170 mV @ 10 mAcm -2 , small Tafel slope of 62.1 mV/decade, and high current density. The ORR catalytic activity of This article is protected by copyright. All rights reserved. 2 the nanocomposite catalyst was also good with an onset potential of 0.93 V. This is possibly due to the synergistic chemical coupling effects between the FeNi 2 Se 4 and NrGO matrix.Chronoamperometric studies showed that the catalyst is stable under conditions of continuous O 2 evolution and reduction with very less degradation. Apart from reporting highly efficient OER-ORR bifunctional catalyst, this study also provides more proof for the effect of anion coordination on the catalyst performance, as well as the synergistic role of nanoscale interactions between the catalyst particles and graphene matrix to enhance catalytic activity.

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