The use of reactive electropositive metal contacts is proposed to lower contact resistance in MoS₂ devices, based on calculations of the sulfur vacancy in MoS₂ by the screened exchange (sX) hybrid functional. sX gives band gaps of 1.88 eV and 1.34 eV for monolayer and bulk MoS₂. The S vacancy has a formation energy of 2.35 eV in S-rich conditions, while the Mo vacancy has a large formation energy of 8.02 eV in Mo-rich conditions. The S vacancy introduces defect levels 0/−1 at 1.23 eV and −1/−2 at 1.28 eV in the upper gap. Its formation energy becomes small or negative for E_F near the conduction band edge, leading to E_F pinning near the conduction band for reactive metal contacts and lower contact resistances.

Efficient water electrolyzers are constrained by the lack of low-cost and earth-abundant hydrogen evolution reaction (HER) catalysts that can operate at industry-level conditions and be prepared with a facile process. Here we report a self-standing MoC–Mo₂C catalytic electrode prepared via a one-step electro-carbiding approach using CO₂ as the feedstock. The outstanding HER performances of the MoC–Mo₂C electrode with low overpotentials at 500 mA cm⁻² in both acidic (256 mV) and alkaline electrolytes (292 mV), long-lasting lifetime of over 2400 h (100 d), and high-temperature performance (70 ^oC) are due to the self-standing hydrophilic porous surface, intrinsic mechanical strength and self-grown MoC (001)–Mo₂C (101) heterojunctions that have a ΔG_H* value of −0.13 eV in acidic condition, and the energy barrier of 1.15 eV for water dissociation in alkaline solution. The preparation of a large electrode (3 cm × 11.5 cm) demonstrates the possibility of scaling up this process to prepare various carbide electrodes with rationally designed structures, tunable compositions, and favorable properties.