Cascade Electrocatalytic Nitrate Reduction Reaching 100% Nitrate-N to Ammonia-N Conversion over Cu₂O@CoO Yolk-Shell Nanocubes

The electroreduction of nitrate to ammonia via a selective eight-electron transfer nitrate reduction reaction offers a promising, low energy consumption, pollution-free, green NH₃ synthesis strategy alternative to the Haber-Bosch method. However, it remains a great challenge to achieve high NH₄⁺ selectivity and complete conversion from NO₃^--N to NH₄⁺-N. Herein, we report ingredients adjustable Cu₂O@CoO yolk-shell nanocubes featured with tunable inner void spaces and diverse activity centers, favoring the rapid cascade conversion of NO₃^- into NO₂^- on Cu₂O and NO₂^- into NH₄⁺ on CoO. Cu₂O@CoO yolk-shell nanocubes exhibit super NH₄⁺ Faradaic efficiencies (>99%) over a wide potential window (−0.2 V to −0.9 V versus RHE) with a considerable NH₄⁺ yield rate of 15.27 mg h^-1 cm^-2 and fantastic cycling stability and long-term chronoamperometric durability. Cu₂O@CoO yolk-shell nanocubes exhibited glorious NO₃^--N to NH₄⁺-N conversion efficiency in both dilute (500 ppm) and highly concentrated (0.1 and 1 M) NO₃^- electrolytes, respectively. The nitrate electrolysis membrane electrode assembly system equipped with Cu₂O@CoO yolk-shell nanocubes delivers over 99.8% NH₄⁺ Faradaic efficiency at cell voltages of 1.9-2.3 V.