Rational Design of Metal Nanomaterials for Nitrate Electro-reduction to Ammonia

Abstract

Nitrogen (N) cycle is important to all kinds of life forms on the planet, including agriculture growth, ecosystem sustainability and biodiversity. However, anthropogenic activities have resulted in disturbed N cycle. For example, the industrial nitrogen fixation process (Haber-Bosch process) has produced large quantity of ammonium (NH₃), which is not fully digested by plants or animals, leading to severe nitrate (NO₃^-) pollution and eutrophication in water bodies. As the World Health Organization stipulated, the maximum level of NO₃^--N in drinking water is 10 mg mL^-1. To meet the standard toward water security, electrochemical nitrate reduction (NO₃RR) is proposed as a “kill two birds with one stone” strategy, which can degrade NO₃^- and produce value-added NH₃ simultaneously. Currently, electrocatalyst plays an important role in manipulating the conversion pathway in NO₃RR and achieving high NH₃ production rate plus selectivity. Herein, metal nanomaterials are the focus considering their high intrinsic catalytic activity and electron conductivity. With rational design of metal nanostructures, NO₃^--to-NH₃ performance has been enhanced with mechanism clearly uncovered. The detailed results are shown in three sub-sections.

First, facile element doping is adopted to tune the catalytic behavior of IrNi alloy nanobranches with unconventional hexagonal close-packed (hcp) phase towards NO₃RR. In particular, the obtained hcp IrNiCu nanobranches favor the ammonia production in neutral electrolyte with high Faradaic efficiency (FE) of 85.6% and large yield rate of 1253 μg cm^-2 h^-1 at -0.4 and -0.6 V (vs. reversible hydrogen electrode (RHE)), respectively. In contrast, the resultant hcp IrNiCo nanobranches promote the formation of nitrite, with a peak FE of 33.1% at -0.1 V (vs. RHE). Furthermore, a hybrid electrolysis cell consisting of NO₃RR and formaldehyde oxidation is constructed, which are both catalyzed by hcp IrNiCu nanobranches. This electrolyzer exhibits lower overpotential and holds the potential to treat polluted air and wastewater simultaneously, shedding lights on green chemical production based on contaminants degradation.

Second, although great progress has been achieved, the crystal phase effect of electrocatalysts on NO₃RR remains rarely explored. Here we report the epitaxial growth of unconventional 2H Cu on hexagonal close-packed (hcp) IrNi template, resulting in the formation of three IrNiCu@Cu nanostructures. IrNiCu@Cu-20 shows superior catalytic performance, with NH₃ Faradaic efficiency (FE) of 86% at -0.1 (vs reversible hydrogen electrode (RHE)) and NH₃ yield rate of 687.3 mmol gCu^-1 h^-1, far better than common face-centered cubic (fcc) Cu. In sharp contrast, IrNiCu@Cu-30 and IrNiCu@Cu-50 covered by hcp Cu shell display high selectivity towards nitrite (NO₂^-), with NO₂^- FE above 60% at 0.1 (vs RHE). Theoretical calculations have demonstrated that the IrNiCu@Cu-20 has the optimal electronic structures for NO₃RR due to the highest d-band center and strongest reaction trend with the lowest energy barriers. The high electroactivity of IrNiCu@Cu-20 originates from the abundant low coordination of Cu sites on the surface, which guarantees the fast electron transfer to accelerate the intermediate conversions. This work provides a feasible tactic to regulate the product distribution of NO₃RR by crystal phase engineering of electrocatalysts.

Third, we report on the rational design of RuNi alloy nanostructures. Benefitted from the synergism effect between Ru and Ni, Ru₂₀Ni₈₀ alloy exhibits a high NH₃ Faradaic efficiency of 98.02% at -0.35 V (vs. reversible hydrogen electrode (RHE)) and a large NH₃ yield rate of 27.88 mg mgcat^-1 h^-1 at -0.65 V (vs. RHE). Importantly, the atomic scale cooperation between Ru and Ni active sites endows RuNi alloy a close-to-unity NH₃ selectivity via HNO* pathway. Theoretical calculations have revealed that the interactions between Ru and Ni optimize the electronic structures of Ru₂₀Ni₈₀ alloy, where Ru sites with enhanced electroactivity improve the generation of active hydrogens and more electron-rich Ni sites benefit the reduction of nitrate. Accordingly, the adsorption strengths of key intermediates become stronger and the energy barriers of NO₃RR are reduced to guarantee the efficient NO₃RR. Furthermore, a flow-type reactor coupled with coprecipitation is established to achieve continuous NH₃ generation and recovery as struvite.

Date of Award	6 Aug 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhanxi FAN (Supervisor)