In situ bulk hydrogen intercalation in a mirror-symmetric Ru/WO3-x nanoarray boosts neutral electrocatalytic nitrate reduction to ammonia

The electrocatalytic nitrate reduction reaction (NO3RR) to ammonia is deemed as an ideal strategy to balance the global nitrogen cycle. However, the cycling of active sites by proton transfer is highly likely to result in poor faradaic efficiency (FE) of catalysts at potentials relevant to the hydrogen evolution reaction, especially in neutral solutions. Herein, we report the construction of an unprecedented mirror-symmetric nanoarray (MSN) assembled by c-axis-oriented single-crystalline WO3 nanoneedles, and design oxygen-deficient MSN-WO3-x to anchor ultrasmall Ru nanoclusters for the neutral NO3RR. Impressively, the resultant Ru/MSN-WO3-x exhibits an outstanding ammonia FE of 95.1% at 0 V vs. RHE and delivers an excellent ammonia production rate of 12.38 mg h-1 cm-2 at a low potential of -0.6 V in a neutral electrolyte, which is 6.32 times that of commercial Ru/C (1.96 mg h-1 cm-2). Additionally, the Ru mass activity of Ru/MSN-WO3-x is 4.6-9.5 times that of commercial Ru/C at various potentials. In situ surface enhanced Raman spectroscopy (SERS) combined with multiple characterization reveal that the electrochemically induced hydrogen intercalation occurs before the NO3RR on Ru/MSN-WO3-x, which can trigger the phase transformation to generate the real active species (Ru/MSN-HyWO3-x) with an accelerated hydrogenation process to ammonia. Further theoretical calculations indicate that bulk hydrogen intercalation is accompanied by altered electronic structures with band repositioning in HyWO3-x, which also accounts for the boosted hydrogenation process during the NO3RR.