Catalyzing hydrogen evolution reaction in alkali media is challenging owing to the sluggish kinetics, originated from the water dissociation process. In this context, synergistic coupling between Ni/Co-based materials with transition metal dichalcogenides (TMDs) often accelerates the alkaline hydrogen evolution reaction (HER). Significant interaction between the two components and active-site density are the keys for achieving a promising catalytic activity. This report emphasizes a two-step selenization approach to prepare a Ni0.85Se/MoSe2 interfacial structure with abundant active sites. Initially, Ni0.75Se nanoparticles were prepared using the solvothermal method and subsequently employed them as a support for the growth of MoSe2 under hydrothermal conditions. This resulted in the formation of a Ni0.85Se/MoSe2 interfacial structure. The results of physical characterization techniques confirm the significant interaction between Ni0.85Se and MoSe2. The interfacial structures showed a superior HER activity in alkali media compared to the individual components; especially, Ni0.85Se/MoSe2 (20) delivers a current density of 10 mA cm–2 at an overpotential of 108 mV. The improved HER activity of the interfacial structure is attributed to the (i) efficient water dissociation process over the Ni0.85Se promoter and (ii) exposure of more catalytic active sites (edges) of MoSe2. In addition, as-prepared Ni0.75Se exhibits a better oxygen evolution reaction (OER) activity by delivering a current density of 10 mA cm–2 at an overpotential of 340 mV. Furthermore, overall water splitting has been demonstrated by constructing an electrolyzer using Ni0.85Se/MoSe2 (20) and Ni0.75Se as a cathode and anode, respectively. The electrolyzer delivers a current density of 10 mA cm–2 at a cell potential of 1.7 V. The long-term stability experiment and the post catalytic characterization reveals the high robustness of the Ni0.85Se/MoSe2 interfacial structure.

Despite predictions of high electrocatalytic OER activity by selenide-rich phases, such as NiCo 2 Se 4 and Co 3 Se 4 , their synthesis through a wet-chemical route remains a challenge because of the high sensitivity of the various oxidation states of selenium to the reaction conditions. In this work, we have determined the contribution of individual reactants behind the maintenance of conducive solvothermal reaction conditions to produce phase-pure NiCo 2 Se 4 and Co 3 Se 4 from elemental selenium. The maintenance of reductive conditions throughout the reaction was found to be crucial for their synthesis, as a decrease in the reductive conditions over time was found to produce nickel/cobalt selenites as the primary product. Further, the reluctance of Ni(II) to oxidize into Ni(III) in comparison to the proneness of Co(II) to Co(III) oxidation was found to have a profound effect on the final product composition, as a deficiency of ions in the III oxidation state under nickel-rich reaction conditions hindered the formation of a monoclinic "Co 3 Se 4 -type" phase. Despite its lower intrinsic OER activity, Co 3 Se 4 was found to show geometric performance on a par with NiCo 2 Se 4 by virtue of its higher textural and microstructural properties.

The electrocatalytic oxygen evolution reaction (OER) demands an efficient catalyst with low overpotential, rapid kinetics, and long-term stability. Herein, we demonstrate the activity of molybdenum oxide (MoO 2 )-embedded cobalt oxalate (CoC 2 O 4 • 2H 2 O) nanostructures for the OER process. The excellent performance of the microrod-like MoO 2 /CoC 2 O 4 •2H 2 O composite is reflected in just 330 mV overpotential for 10 mA/cm geo 2 , low Tafel slope (78 mV/dec), 90% faradaic efficiency, and 24 h stability in 1.0 (M) KOH. The as-prepared electrocatalyst requires a significantly lower overpotential wrt CoC 2 O 4 •2H 2 O. Incorporation of MoO 2 elegantly modified the textural property, such as surface area and porosity, of the as-prepared material. Furthermore, MoO 2 / CoC 2 O 4 •2H 2 O was found to follow the proton-decoupled electrontransfer mechanism for electrocatalyzing OER. Postcatalytic characterization revealed the electrochemical transformation of a one-dimensional (1-D) MoO 2 /CoC 2 O 4 •2H 2 O microrod into a sheetlike two-dimensional α-Co(OH) 2 /CoOOH during alkaline OER. Interestingly, postcatalytic X-ray photoelectron spectroscopy, inductively coupled plasma, and energy-dispersive X-ray spectroscopy analyses suggest MoO 2 etching from the material, leading to exposure of a higher number of electrochemically active sites that otherwise lay inactive because of their presence in the bulk. Both CoC 2 O 4 •2H 2 O-and MoO 2 /CoC 2 O 4 •2H 2 O-integrated 1-D nanostructures showed an ∼0.01 s −1 turnover frequency value at 400 mV overpotential.We believe that the enhancement in geometrical electrocatalytic activity is not due to the direct participation of MoO 2 in catalysis but due to its electrochemical etching, which makes a higher number of catalytically active sites accessible to the electrolyte. This study conveys the in situ electrochemical activation strategy through etching of pore additive for the alkaline OER process.

Nickel–cobalt oxalate (Ni2.5Co5C2O4–nH2O) based block-like nanostructure has been introduced as superior electrocatalyst compared to nickel–cobalt oxide (NiCo2O4) for alkaline water oxidation.

Developing electrocatalysts with abundant active sites is a substantial challenge to reduce the overpotentials for the water splitting reaction. Toward the oxygen evolution reaction (OER) in alkaline medium, designing precatalysts that could provide abundant catalytic active centers can significantly improve the electrocatalytic activity. This work demonstrates the production of active α-Co(OH) 2 /CoOOH species using the sacrificial pre-catalyst Zn x /Co 3−x (PO 4 ) 2 •4H 2 O (ZCP) (x = 0.15−0.9), which was developed by the one-step hydrothermal method. Electrochemical activation of ZCP results in the formation of a porous cobalt catalyst with abundant active centers. Elemental analysis revealed that both Zn 2+ and PO 4 3− ions etch away during the activation process, thereby exposing a higher number of catalytically active cobalt centers that otherwise stay inactive due to their location in the bulk. The resulting materials showed promising electrocatalytic activities toward OER; especially, a material (ZCP-0.3) consisting of Zn 2+ and Co 2+ in a 1:7 ratio was found to deliver the best activity. The carbon paper-supported ZCP-0.3 exhibited a high current density of 50 mA/cm 2 at an overpotential of 390 mV. In contrast, the material prepared without zinc (i.e., Co 3 (OH) 2 (HPO 4 ) 2 ) showed inferior catalytic activity, thus demonstrating the necessity for Zn 2+ etching to expose the additional active sites for improved electrocatalytic activity of ZCP-0.3.