Cobalt-based electrocatalysts for sustainable nitrate conversion: structural design and mechanistic advancements

With agricultural run-off and industrial effluents pushing nitrate levels to record highs, electrocatalytic nitrate reduction (NO₃RR) offers a rare “two-birds-one-stone” opportunity: clean water + green NH₃. A multinational team—Zhengzhou University, Wenzhou University & Nankai University—led by Prof. Zhijie Kong and Prof. Zheng-Jun Wang has now delivered the first comprehensive roadmap for cobalt-based catalysts that can hit industrial current densities while keeping Faradaic efficiencies > 95 %.

Why Co Matters

• Earth-abundant & recyclable: Co price ≈ 1/10 of Ru, yet its 3d-electron manifold delivers near-optimal d-band centres for N–O scission and H* supply.
• HER sweet-spot: Moderate H-chemisorption suppresses H₂ loss, funnelling protons into the 8 e⁻/9 H⁺ NH₃ pathway.
• Coordination versatility: Co–Co, Co–O, Co–P, Co–N, Co–B & Co-single-atom motifs enable atom-scale tuning that Cu/Fe simply cannot match.

Design Toolkit Highlighted

• Electronic descriptor: Work-function ≤ 4.7 eV plus E{d} ≈ E{F} maximises NO₃⁻ adsorption while starving the Heyrovsky step—Co foils already give 96 % FE at −0.24 V vs RHE.
• Alloying & tandem catalysis: Ru₁₅Co₈₅ hollow nanododecahedra push onset to +0.4 V vs RHE (3.2 mmol h^-1 mg^-1, 97 % FE) via a three-step relay (spontaneous redox → electrochemical → hydrogenation).
• Oxygen-vacancy engineering: Mn-doped Co₃O₄ nanotubes lower the PDS barrier from 0.83 → 0.42 eV, hitting 99.5 % NH₃ selectivity in neutral media.
• Single-atom & molecular platforms: CoP₁N₃/C boosts NH₄⁺ FE to 92 % by breaking N–N coupling; Co-phthalocyanine/CNT co-reduces CO₂ + NO₃⁻ to methylamine with 84 % yield—opening a C–N circular economy.

From Lab to Factory

• Flow-cell validation: Poly-cobalt-cluster coordination polymer NJUZ-2 sustains 470 mA cm^-2 at −0.5 V, yielding 3.4 mol h^-1 g^-1 NH₃—20× H-cell output.
• Real wastewater: Co foam remains best among 15 commercial metals even with 11 773 ppm Ba²⁺, 3 824 ppm SO₄^2-; long-term Co-leaching < 0.05 mg L^-1.
• Reactor roadmap: Membrane-electrode assemblies (MEA) and Zn–NO₃⁻ batteries are spotlighted as the shortest path to kA-scale plants, yet demand catalyst layers < 80 µm and Cl⁻-tolerant active sites.

Challenges & Next Steps

1. Scale-up synthesis: Continuous spray-drying or carbothermal shock for kg-batch Co-alloy nanocages.
2. Durability: Suppress surface reconstruction (Co³⁺ → CoOOH) via protective carbon or B-doped skins.
3. Product diversification: Target urea, methylamine or hydrazine by coupling CO₂ or CH₂O streams.
4. Machine-learning: Fast-screen Co–M–X ternaries (M = Cu, Fe, Ni, Al; X = O, P, S) against 10⁵ crystal facets.
5. System integration: Pair catalyst innovation with membrane-free or MEA reactors; recover NH₃ by gas-permeable membranes or stripping columns.

By bridging surface-science insights with reactor engineering, this review positions cobalt at the heart of next-generation nitrogen-recycling plants—turning an environmental liability into a carbon-free feedstock for fuels, fertilisers and fine chemicals. Stay tuned for pilot-scale demonstrations from the Henan Green Catalysis Center and Wenzhou Bay Batteries Hub!