Hydrogen evolution reaction (HER) in alkaline medium plays an important role in producing green hydrogen but suffers from sluggish reaction kinetics owing to additional water dissociation step. Extensive research interest has been placed on engineering dual active sites (i.e., water-dissociation sites and hydrogen-adsorption/recombination sites) within a catalyst to enhance the HER activity. This article reviews recent progress in developing alkaline HER catalysts with high-efficiency dual active sites via strategies of heterogeneous interfaces constructing and heteroatoms doping or alloying. The latest advances in the component design, synthetic strategy, catalytic performance, and mechanistic understanding are discussed with selective examples of the hybrid between metal/alloy or metal phosphide/nitride/sulfide and transition metal hydroxides, oxyhydroxide or bicarbonates. Furthermore, remaining challenges and perspectives in the field of dual-site engineering are highlighted for future development of better alkaline HER electrocatalysts.

The conventional ceramic synthesis of perovskite oxides involves extended high-temperature annealing in air and is unfavorable to the in situ hybridization of the conductive agent, thus resulting in large particle sizes, low surface area and limited electrochemical activities. Here we report a rapid gel auto-combustion approach for the synthesis of a perovskite/carbon hybrid at a low temperature of 180 ℃. The energy-saving synthetic strategy allows the formation of small and homogeneously dispersed La_xMnO_3±δ/C nanocomposites. Remarkably, the synthesized La_0.99MnO_3.03/C nanocomposite exhibits comparable oxygen reduction reaction (ORR) activity (with onset and peak potentials of 0.97 and 0.88 V, respectively) to the benchmark Pt/C due to the facilitated charge transfer, optimal e_g electron filling of Mn, and coupled C–O–Mn bonding. Furthermore, the nanocomposite efficiently catalyzes a Zn-air battery that delivers a peak power density of 430 mW·cm^-2, an energy density of 837 W·h·kg_Zn^-1 and 340 h stability at a current rate of 10 mA·cm^-2.

A combined hot-injection and heat-up method was developed to synthesize monodisperse and uniform CoMn₂O₄ quantum dots (CMO QDs). CMO QDs with average size of 2.0, 3.9, and 5.4 nm were selectively obtained at 80, 90, and 105 ℃, respectively. The CMO QDs supported on carbon nanotubes (CNTs) were employed as catalysts for the oxygen reduction/evolution reaction (ORR/OER) in alkaline solution to investigate their size-performance relationship. The results revealed that the amount of surface-adsorbed oxygen and the band gap energy, which affect the charge transfer in the oxygen electrocatalysis processes, strongly depend on the size of the CMO QDs. The CMO-3.9/CNT hybrid, consisting of CNT-supported CMO QDs of 3.9 nm size, possesses a moderate amount of surface- adsorbed oxygen, a lower band gap energy, and a larger charge carrier concentration, and exhibits the highest electrocatalytic activity among the hybrid materials investigated. Moreover, the CMO-3.9/CNT hybrid displays ORR and OER performances similar to those of the benchmark Pt/C and RuO₂ catalysts, respectively, due to the strong carbon-oxide interactions and the high dispersion of CoMn₂O₄ QDs on the carbon substrate; this reveals the huge potential of the CMO-3.9/CNT hybrid as a bifunctional OER/ORR electrocatalyst. The present results highlight the importance of controlling the size of metal oxide nanodots in the design of active oxygen electrocatalysts based on spinel-type, nonprecious metal oxides.

Through in situ redox deposition and growth of MnO₂ nanostructures on hierarchically porous carbon (HPC), a MnO₂/HPC hybrid has been synthesized and employed as cathode catalyst for non-aqueous Li-O₂ batteries. Owing to the mild synthetic conditions, MnO₂ was uniformly distributed on the surface of the carbon support, without destroying the hierarchical porous nanostructure. As a result, the as-prepared MnO₂/HPC nanocomposite exhibits excellent Li-O₂ battery performance, including low charge overpotential, good rate capacity and long cycle stability up to 300 cycles with controlling capacity of 1, 000 mAh·g^-1. A combination of the multi-scale porous network of the shell-connected carbon support and the highly dispersed MnO₂ nanostructure benefits the transportation of ions, oxygen and electrons and contributes to the excellent electrode performance.

Since the high-voltage spinel LiNi_0.5Mn_1.5O₄ (LNMO) is one of the most attractive cathode materials for lithium-ion batteries, how to improve the cycling and rate performance simultaneously has become a critical question. Nanosizing is a typical strategy to achieve high rate capability due to drastically shortened Li-ion diffusion distances. However, the high surface area of nanosized particles increases the side reaction with the electrolyte, which leads to poor cycling performance. Spinels with disordered structures could also lead to improved rate capability, but the cyclability is low due to the presence of Mn³⁺ ions. Herein, we systematically investigated the synergic interaction between particle size and cation ordering. Our results indicated that a microsized disordered phase and a nanosized ordered phase of LNMO materials exhibited the best combination of high rate capability and cycling performance.