context: Besides NiFe-based catalysts, the performance of transition metal sulfides can also be enhanced by Fe incorporation, similar to the effect of Fe-ion doping. For instance, Liu's group reported a novel HER candidate, namely, a Fe-doped NiS2 nanosheet, with high activity and long-time durability. The theoretical and experimental results show that Fe3+ doping into the surface lattice of the NiS2 (002) crystal plane can reduce the activation energy of H2 formation. Further, Fe-doped NiS2 nanosheets can be a bifunctional electrocatalyst with high activities toward both OER and HER in the same media. Sun's group reported the exploitation of iron-doped nickel disulfide nanoarrays on Ti substrate via subsequent sulfidation treatment and used it as a bifunctional electrocatalyst with overall water-splitting capacity. In addition, Cao's group prepared a vertically oriented Fe-doped Ni3S2-nanosheet-based bifunctional electrocatalyst toward both HER and OER with outstanding activity and stability in alkaline electrolytes. Based on detailed experiments and theoretical simulations, it was proved that the electrochemically active surface area, water adsorption ability, and hydrogen adsorption energy of Ni3S2 can be regulated via Fe doping (Fig. 5). For transition metal selenides, the effect of Fe incorporation is also evident. Yu and co-workers described ultrathin Fe-doped NiSe2 nanowires (diameter: <1.7 nm) via a soft-template-mediated colloidal synthesis strategy, further revealing the amorphous hydroxide layers formed in situ that contributed toward the enhanced OER activity. Moreover, Zhao et al. incorporated both Fe dopants and Co vacancies into atomically thin CoSe2 nanobelts for oxygen evolution catalysis, and the resultant CoSe2–DFe–VCo exhibits much higher catalytic activity than other defect-activated CoSe2 and previously reported FeCo compounds. Deeper characterizations and theoretical calculations identify the most active centers of Co2 sites that are adjacent to the VCo-nearest Fe site. Fe doping and Co vacancy can synergistically tune the electronic states of Co2 to a near-optimal value, resulting in considerably decreased binding energy of OH* and consequently lowering the catalytic overpotential.
answer: alkaline 