Plasma-Induced S-Vacancy and Mn-Doping Synergistically Boost the Catalytic Activity of NiCo2S4@Co2N Heterostructure for Rechargeable Zinc-Air Batteries and Alkaline Methanol Fuel Cells.

Here, we introduce a plasma-assisted sulfurization strategy to synthesize Mn-doped NiCo2S4@Co2N heterostructures enriched with sulfur-vacancies (VS). These heterostructures are incorporated into a 3D network of nitrogen-doped carbon-nanotubes (NCNTs) supported by carbon-cloth, resulting in the p-Mn-NiCo2S4@Co2N/NCNTs. It demonstrates excellent bifunctional catalytic activity, with a potential difference (ΔE = EOER,50-EORR,1/2) as low as 0.695 V. The half-wave potential of oxygen-reduction reaction (ORR) is 0.828 V, and the overpotential for oxygen-evolution reaction (OER) to achieve 50 mA cm⁻2 is only 293 mV. Density-Functional-Theory (DFT) calculations confirm that the exceptional catalytic activity originates from the VS and Mn-doping structure. The assembled liquid zinc-air battery (ZAB) has an open-circuit voltage (OCV) of 1.54 V, a peak power-density of 139.9 mW cm-2, and can undergo stable charge/discharge cycles for 1500 cycles at 10 mA cm-2. The assembled flexible all-solid-state ZAB also features a stable OCV of 1.486 V and exhibits a peak power-density superior to that of commercial 20 wt.% Pt/C+RuO2, accompanied by stable charging/discharging cycles of up to 700 cycles. The performance remains stable across various bending angles, demonstrating good flexibility and durability. The maximum power-density exhibited by alkaline direct methanol fuel-cells (ADMFCs) assembled with p-Mn-NiCo2S4@Co2N/NCNTs is 75.6 mW cm⁻2, exceeding that of the commercial Pt/C catalyst.