Subnanometric metal clusters usually have unique electronic structures and may display electrocatalytic performance distinctive from single atoms (SAs) and larger nanoparticles (NPs). However, the electrocatalytic performance of clusters, especially the size-activity relationship at the sub-nanoscale, is largely unexplored. Here, we synthesize a series of Ru nanocrystals from single atoms, subnanometric clusters to larger nanoparticles, aiming at investigating the size-dependent activity of hydrogen evolution in alkaline media. It is found that the d band center of Ru downshifts in a nearly linear relationship with the increase of diameter, and the subnanometric Ru clusters with d band center closer to Femi level display a stronger water dissociation ability and thus superior hydrogen evolution activity than SAs and larger nanoparticles. Benefiting from the high metal utilization and strong water dissociation ability, the Ru clusters manifest an ultrahigh turnover frequency of 43.3 s−1 at the overpotential of 100 mV, 36.1-fold larger than the commercial Pt/C.

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.

The electrochemical nitrate reduction reaction (NO3−RR) provides a promising route to produce ammonia (NH3) while addressing environmental issues of NO3−. Although great success has been achieved on the development of efficient NO3−RR electrocatalysts, few has concerned about how to capture NH3 from the electrolyte, despite that the production and capture of NH3 are equally important to the practical application of NO3−RR. Here, inspired by the fact that nearly all in situ generated NH3 is gaseous at the electrode surface during the small‐current NO3−RR, a “two‐in‐one” flow cell electrolyzer is smartly designed that integrates the chambers of NO3−RR electrolysis and NH3 capture through a commercial gas diffusion electrode, aiming at achieving the synchronization of NH3 production and capture. Remarkably, this electrolyzer also enables rapid transport of NH3 products away from the three‐phase reaction interfaces, thereby significantly promoting the conversion of NO3− to NH3. By using robust electrocatalysts with the function like nanoreactors, the electrolyzer delivers a maximum ammonia Faradaic efficiency of 90.2%, along with a large current density (−528.0 mA cm−2) and NH3 capture rate of 90.4%. Clearly, this work provides opportunities to concurrently produce and capture NH3 and thus help the realization of nitrogen cycle.

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As the much larger mass of proton than electron, the oxygen evolution reaction (OER) on heterogeneous electrocatalysts usually undergoes slow proton/fast electron transfer, which greatly limits the OER performance. Boosting...