Co(OH)2 catalyst gives 40.67 µg h⁻¹ cm⁻² of ammonia at 24.9% Faradaic efficiency, lab stage

A team from Qingdao Agricultural University (China), Politecnico di Milano (Italy) and Trinity College Dublin (Ireland) reported a cobalt hydroxide, Co(OH)2, electrocatalyst for nitrogen reduction to ammonia in a paper in eScience. At −0.90 V (vs. RHE) the material gave an NH3 yield of 40.67 ± 0.43 μg h⁻¹ cm⁻² with a Faradaic efficiency of 24.9 ± 0.61% sciencedirect.comeSciencesciencedirect.comOpen source ↗. The authors describe this as a balance between production rate and electrochemical efficiency. The results come from electrochemical experiments on the catalyst itself.

Why nitrogen reduction is hard

Ammonia is essential for agriculture. According to the release accompanying the paper, the Haber–Bosch process supplies nearly all of it, operates at high temperature and pressure, uses about 1–2% of global energy and emits roughly 1.6 tons of CO2 per ton of ammonia. Electrocatalytic nitrogen reduction driven by electricity is an alternative, but the N≡N bond has a dissociation energy of 941 kJ/mol. The authors note that existing associative pathways suffer from premature hydrazine release or high activation barriers, which gives poor selectivity and slow kinetics.

Hydroxide-activated hydrogen spillover

The team proposes a mechanism it calls hydroxide-activated hydrogen spillover (HAHS). By controlling the synthesis temperature, the researchers introduced lattice distortion into Co(OH)2, which elongates Co–O bonds and weakens the adjacent O–H bonds. Hydrogen then detaches from the surface hydroxyl groups, forming surface hydrogen vacancies, and takes part directly in nitrogen reduction. According to the authors, this lowers the N≡N cleavage energy barrier. The vacancies are refilled by hydrogen species from the electrolyte, so the cycle sustains itself.

The authors say the pathway simplifies the conventional protonation step: it bypasses one elementary step, speeds up intermediate turnover and suppresses the competing hydrogen evolution reaction. Density functional theory calculations gave a nearly barrierless initial N2 activation on Co(OH)2 (ΔG = 0.01 eV). The catalyst kept its structure over 180 hours of continuous operation, the authors report. In situ electron spin resonance and isotope-labeled differential electrochemical mass spectrometry were used as experimental evidence for hydrogen spillover and vacancy formation sciencedirect.comeSciencesciencedirect.comOpen source ↗.