Unveiling the In Situ Dissolution and Reconstruction of CoMo for Electrocatalytic Nitrate Reduction to Ammonia

Why It Matters

The breakthrough resolves the long‑standing trade‑off between nitrate adsorption and competing hydrogen evolution, enabling efficient, durable ammonia synthesis from waste streams and advancing sustainable nitrogen management.

Key Takeaways

• Mo doping improves nitrate adsorption.
• Reconstructed β‑Co(OH)₂ generates active hydrogen.
• Faradaic efficiency reaches ~96% at –0.1 V.
• 95% ammonia selectivity sustained over 1000 h.
• 660 mA cm⁻² current density shows industrial relevance.

Pulse Analysis

The electrochemical conversion of nitrate (NO₃⁻) to ammonia (NH₃) offers a low‑energy pathway to recover valuable nitrogen from waste streams while avoiding the carbon footprint of the Haber‑Bosch process. However, the reaction is hampered by the hydrogen evolution reaction (HER), which consumes electrons and protons, especially at the low nitrate concentrations typical of municipal wastewater. Cobalt‑based catalysts have shown promise because of their moderate binding to nitrogen intermediates, yet their weak affinity for nitrate limits overall activity. Incorporating a second metal that can both attract nitrate ions and promote water splitting is therefore a logical strategy.

The study introduces a reconstructed Mo‑doped β‑Co(OH)₂/Co₈₅Mo₁₅ (R‑Co₈₅Mo₁₅) catalyst prepared by electrochemical activation. In situ Raman and X‑ray absorption spectroscopy reveal multiple Moδ⁺ oxidation states that facilitate water dissociation, delivering abundant surface hydrogen. Mo sites preferentially adsorb NO₃⁻, while adjacent Co atoms accelerate the conversion of *NO₃ to *NO₂. The doped hydroxide layer supplies the active hydrogen needed for subsequent hydrogenation of *NO₂ to NH₃. The resulting electrode achieves a Faradaic efficiency of ≈96 % at –0.1 V vs RHE, 95 % NH₃ selectivity, and operates stably for 1 000 h at a current density of 660 mA cm⁻².

These metrics place R‑Co₈₅Mo₁₅ among the few electrocatalysts that combine high selectivity, low overpotential, and industrial‑scale durability, making it a strong candidate for decentralized ammonia production and nitrate remediation. The ability to sustain 660 mA cm⁻² suggests compatibility with existing electro‑chemical modules, potentially lowering capital costs for fertilizer manufacturers and water utilities. Moreover, the modular nature of Mo‑induced surface reconstruction could be transferred to other transition‑metal systems, opening avenues for tailored catalysts that address competing HER in various reduction reactions. Continued scale‑up studies and techno‑economic analyses will determine its commercial viability.