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Twin boundary (TB) engineering provides exciting opportunities to tune the performance levels of metal-based electrocatalysts. However, the controllable construction of TB greatly relies on surfactants, blocking active sites, and electron transfer by surfactants. Here, a surfactant-free and facile strategy is proposed for synthesizing copper (Cu) nanocatalysts with dense hierarchical TB networks (HTBs) by the rapid thermal reductions in metastable CuO nanosheets in H 2 . As revealed by in situ transmission electron microscopy, the formation of HTBs is associated with the fragmentation of nanosheets in different directions to generate abundant crystal nuclei and subsequently unconventional crystal growth through the collision and coalescence of nuclei. Impressively, the HTBs endow Cu with excellent electrocatalytic performance for direct nitrate-ammonia conversion, superior to that of Cu with a single-oriented TB and without TB. It is discovered that the HTBs induce the formation of compressive strains, thereby creating a synergistic effect of TBs and strains to efficiently tune the binding energies of Cu with nitrogen intermediates (i.e., NO 2 *) and thus promote the tandem reaction process of NO 3 - -to-NO 2 - and subsequent NO 2 - -to-NH 3 electrocatalysis. This work demonstrates the crucial role of HTBs for boosting electrocatalysis via the synergistic effect of TBs and strains., (© 2023 Wiley-VCH GmbH.)

Heteroatom doping has emerged as a highly effective strategy to enhance the activity of metal-based electrocatalysts toward the oxygen evolution reaction (OER). It is widely accepted that the doping does not switch the OER mechanism from the adsorbate evolution mechanism (AEM) to the lattice-oxygen-mediated mechanism (LOM), and the enhanced activity is attributed to the optimized binding energies toward oxygen intermediates. However, this seems inconsistent with the fact that the overpotential of doped OER electrocatalysts (<300 mV) is considerably smaller than the limit of AEM (>370 mV). To determine the origin of this inconsistency, we select phosphorus (P)-doped nickel-iron mixed oxides as the model electrocatalysts and observe that the doping enhances the covalency of the metal-oxygen bonds to drive the OER pathway transition from the AEM to the LOM, thereby breaking the adsorption linear relation between *OH and *OOH in the AEM. Consequently, the obtained P-doped oxides display a small overpotential of 237 mV at 10 mA cm -2 . Beyond P, the similar pathway transition is also observed on the sulfur doping. These findings offer new insights into the substantially enhanced OER activity originating from heteroatom doping., (© 2023 Wiley-VCH GmbH.)

Hydrazine-assisted water electrolysis provides new opportunities to enable energy-saving hydrogen production while solving the issue of hydrazine pollution. Here, the synthesis of compressively strained Ni 2 P as a bifunctional electrocatalyst for boosting both the anodic hydrazine oxidation reaction (HzOR) and cathodic hydrogen evolution reaction (HER) is reported. Different from a multistep synthetic method that induces lattice strain by creating core-shell structures, a facile strategy is developed to tune the strain of Ni 2 P via dual-cation co-doping. The obtained Ni 2 P with a compressive strain of -3.62% exhibits significantly enhanced activity for both the HzOR and HER than counterparts with tensile strain and without strain. Consequently, the optimized Ni 2 P delivers current densities of 10 and 100 mA cm -2 at small cell voltages of 0.16 and 0.39 V for hydrazine-assisted water electrolysis, respectively. Density functional theory (DFT) calculations reveal that the compressive strain promotes water dissociation and concurrently tunes the adsorption strength of hydrogen intermediates, thereby facilitating the HER process on Ni 2 P. As for the HzOR, the compressive strain reduces the energy barrier of the potential-determining step for the dehydrogenation of *N 2 H 4 to *N 2 H 3 . Clearly, this work paves a facile pathway to the synthesis of lattice-strained electrocatalysts via the dual-cation co-doping., (© 2023 Wiley-VCH GmbH.)