Single-atom-catalysts (SACs) afford a fascinating activity with respect to other nanomaterials for hydrogen evolution reaction (HER), yet the simplicity of single-atom center limits its further modification and utilization. Obtaining bimetallic single-atom-dimer (SAD) structures can reform the electronic structure of SACs with added atomic-level synergistic effect, further improving HER kinetics beyond SACs. However, the synthesis and identification of such SAD structure remains conceptually challenging. Herein, systematic first-principle screening reveals that the synergistic interaction at the NiCo-SAD atomic interface can upshift the d-band center, thereby, facilitate rapid water-dissociation and optimal proton adsorption, accelerating alkaline/acidic HER kinetics. Inspired by theoretical predictions, we develop a facile strategy to obtain NiCo-SAD on N-doped carbon (NiCo-SAD-NC) via in-situ trapping of metal ions followed by pyrolysis with precisely controlled N-moieties. X-ray absorption spectroscopy indicates the emergence of Ni-Co coordination at the atomic-level. The obtained NiCo-SAD-NC exhibits exceptional pH-universal HER-activity, demanding only 54.7 and 61 mV overpotentials at −10 mA cm−2 in acidic and alkaline media, respectively. This work provides a facile synthetic strategy for SAD catalysts and sheds light on the fundamentals of structure-activity relationships for future applications.

Recently, the MXene itself and its composites with various metal oxides have shown excellent electrochemical performance due to the presence of multiple oxidation states. However, the restacking of MXene layers and poor electrical conductivity of metal oxides are major bottlenecks in their effective electrochemical transport, when they are applied individually. Herein, we report a novel manganese oxide/MXene (MnO 2 /MXene) composite material to overcome these critical issues. Sub-50 nm-thick MnO 2 nanowires (NWRs) were introduced inside the MXene to effectively stop the restacking as well as to increase the surface area of the supercapacitor (SC) electrode material. The special control on the thickness of NWRs is not only providing an opportunity to adjust them inside the MXene layers but also giving a high surface area. Electrochemical studies suggested that the MnO 2 /MXene composite behaves as an excellent electrode material for hybrid SCs, as compared to individual MXene and MnO 2 . Maximum specific capacitance (C sp ) of MXene, MnO 2 NWRs, and MnO 2 /MXene composite was observed to be about 527.8, 337.5, and 611.5 F/g, respectively. The calculated specific capacity of the MnO 2 /MXene composite was about 489.5 C/g at 1 A/g, which shows better performance as an electrode material for energy storage devices. The synthesized electrode material demonstrated excellent capacitance retention of about 96% up to 1000 cycles.

Designing an efficient oxygen evolution reaction (OER) electrocatalysts based on single-atom catalysts is a highly promising option for cost-effective alkaline water electrolyzers. However, the instability of the OOH* intermediate and...