High-efficiency ammonia electrosynthesis from nitrate on ruthenium-induced trivalent cobalt sites

Electrocatalytic nitrate reduction to ammonia holds significant potential for sustainable ammonia production and nitrate-rich wastewater treatment. However, the low catalytic efficiency and limited understanding of catalyst evolution hinder its further advancement, particularly at low nitrate concentrations. Here, we report a Ru-doped Co(OH)₂ nanoarray that achieves a high ammonia faradaic efficiency of ∼96% and a large ammonia yield rate of ∼56 501 μg h⁻¹ cm_geo⁻² at −0.48 V versus reversible hydrogen electrode under wastewater-relevant nitrate concentrations. Ex situ X-ray absorption spectroscopy and X-ray diffraction measurements reveal the dynamic redox behavior of Co(ii)/Co(iii) sites, driven by Ru-catalyzed hydroxide oxidation and electroreduction. Mechanistic insights from in situ Raman spectroscopy and electron paramagnetic resonance spectroscopy indicate that Ru doping generates more hydrogen radicals, thus facilitating the formation of intermediate HNO species at Co(iii) sites. Additionally, the practical applicability and economic feasibility of electrocatalytic nitrate reduction to ammonia are underscored by an integrated membrane electrode assembly system and a techno-economic analysis.