Energy storage technologies, such as fuel cells, ammonia production and lithium–air batteries, are important strategies for addressing the global challenge of energy crisis and environmental pollution. Taking overpotential as a direct criterion, we illustrate in theory and experiment that the adsorption energies of charged species such as Li⁺+e⁻ and H⁺+e⁻ are a central parameter to describe catalytic activities related to electricity-in/electricity-out efficiencies. The essence of catalytic activity is revealed to relate with electronic coupling between catalysts and charged species. Based on adsorption energy, some activity descriptors such as d-band center, e_g-electron number and charge-transfer capacity are further defined by electronic properties of catalysts that directly affect interaction between catalysts and charged species. The present review is helpful for understanding the catalytic mechanisms of these electrocatalytic reactions and developing accurate catalytic descriptors, which can be employed to screen high-activity catalysts in future high-throughput calculations and experiments.

Active electronic states in transition metal dichalcogenides are able to prompt hydrogen evolution by improving hydrogen absorption. However, the development of thermodynamically stable hexagonal 2H-MoS₂ as hydrogen evolution catalyst is likely to be shadowed by its limited active electronic state. Herein, the charge self-regulation effect mediated by tuning Mo−Mo bonds and S vacancies is revealed in metastable trigonal MoS₂ (1T'''-MoS₂) structure, which is favarable for the generation of active electronic states to boost the hydrogen evolution reaction activity. The optimal 1T'''-MoS₂ sample exhibits a low overpotential of 158 mV at 10 mA cm⁻² and a Tafel slope of 74.5 mV dec⁻¹ in acidic conditions, which are far exceeding the 2H-MoS₂ counterpart (369 mV and 137 mV dec⁻¹). Theoretical modeling indicates that the boosted performance is attributed to the formation of massive active electronic states induced by the charge self-regulation effect of Mo−Mo bonds in defective 1T'''-MoS₂ with rich S vacancies.

Non-noble transition metal oxides are abundant in nature. However, they are widely regarded as catalytically inert for hydrogen evolution reaction (HER) due to their scarce active electronic states near the Fermi-level. How to largely improve the HER activity of these kinds of materials remains a great challenge. Herein, as a proof-of-concept, we design a non-solvent strategy to achieve phosphate substitution and the subsequent crystal phase stabilization of metastable β-NiMoO₄. Phosphate substitution is proved to be imperative for the stabilization and activation of β-NiMoO₄, which can efficiently generate the active electronic states and promote the intrinsic HER activity. As a result, phosphate substituted β-NiMoO₄ exhibits the optimal hydrogen adsorption free energy (−0.046 eV) and ultralow overpotential of −23 mV at 10 mA cm⁻² in 1 M KOH for HER. Especially, it maintains long-term stability for 200 h at the large current density of 1000 mA cm⁻² with an overpotential of only −210 mV. This work provides a route for activating transition metal oxides for HER by stabilizing the metastable phase with abundant active electronic states.

N₂ reduction to ammonia by solar light represents a green and sustainable ammonia synthesis approach which helps to suppress the global warming and energy crisis. However, conventional semiconductors usually suffer from low activity or poor stability, largely suppressing the application of this technology. Here, we report that bismuth monoxide (BiO) quantum dots with an average size of 2–5 nm exhibited efficient photocatalytic activity for ammonia synthesis under simulated solar light. A highly efficient ammonia synthesis rate of 1226 μmol g⁻¹ h⁻¹ is achieved without the assistance of any sacrificial agent or co-catalyst, which is about 1000 times higher than that using the traditional Fe-TiO₂ photocatalyst. Kinetic analysis reveals that the synergy of three low valence surface Bi(II) species markedly enhances N₂ activation by electron donation, which finally resulted in the highly efficient N₂ photoreduction performance. This work will shed light on designing efficient and robust N₂ reduction photocatalysts.