Abstract
The electrochemical nitrate-to-ammonia conversion process (eNO3RR) represents a sustainable pathway as a viable, eco-benign alternative to the traditional Haber-Bosch methodology for synthesizing NH3. Metal electrocatalysts have attracted intense attention in eNO3RR because their transition metal active sites enabling enhanced intrinsic activity and selectivity for nitrate-to-ammonia conversion through d-orbital electron modulation and multisite synergy. However, these catalysts face significant challenges under industry-relevant current densities, primarily due to three interrelated stability issues: structural degradation through electrochemical reconstruction at high overpotentials, causing accelerated activity decay; chemical instability manifesting as electrolyte corrosion, intermediate poisoning, and parasitic side reactions; mechanical failure from delamination or support corrosion, limiting operational lifetime. It is thus urgent to explore promising strategies for developing robust self-supported catalysts met a dilemma for simultaneously promoting activity and stability.We first introduce an in-situ nucleation strategy to fabricate CuO/NiO heterostructures directly on Ni foam, explicitly addressing the activity-stability trade-off. In-situ Raman spectroscopy confirms that the electrochemically reconstructed Cu₂O/NiO heterostructure functions as the genuine active phase, maintaining structural stability at low overpotentials throughout the NO₃RR operational window. Moreover, these complementary phases form a tandem system that breaks adsorption-energy scaling relations: Cu₂O facilitates strong NO₃⁻ adsorption while NiO enables rapid NH₃ desorption. Consequently, this heterostructure achieves superior eNO₃RR performance at −0.2 V vs. RHE, delivering 95.6% NH3 Faradaic efficiency (FENH3), 88.5% NH₃ selectivity, and 2.1 mol h⁻¹ m⁻² yield rate, surpassing most catalysts at ultralow potentials.
We then present MnFeCoNiCu high-entropy alloy (HEA) catalysts synthesized on carbon paper via rapid direct current (DC) magnetron sputtering. This technique ensures strong film-substrate adhesion, preventing delamination during operation. Critically, the transition to quinary composition induces entropic stabilization, elevating kinetic barriers to enhance structural integrity. Moreover, multi-element synergy modulates adsorption energies beyond conventional scaling relations, while electronic structure variations create a broad adsorption landscape with cooperative multifunctional sites. The catalyst achieves 94.5 ± 4.3% FE and 10.2 ± 0.5 mg h⁻¹ mgcat⁻¹ NH₃ yield, maintaining >250-hour stability in a three-chamber system with integrated ammonia recovery.
While the aforementioned binder-free self-supporting catalysts, fabricated via direct growth on substrates through electrostatic adsorption or van der Waals forces, partially improve interfacial binding strength, they remain constrained by heterointerface limitations that impede full resolution of interfacial instability issues. To overcome this fundamental challenge, we introduce a scalable integrated manufacturing strategy for metamaterial catalysts. The monolithic architecture eliminates nanomaterial peeling while dislocation-induced strain fields enhance intrinsic activity by facilitating NO₃⁻ adsorption and reducing reaction barriers. The resulting FeCoNi metamaterial achieves 95.4% FE, 20.58 mg h⁻¹ cm⁻² yield, and >500-hour stability. When integrated into a flow-through electrolyzer with acid absorption, it produces solid NH₄Cl fertilizer, establishing a new paradigm for functional catalyst manufacturing.
Collectively, we establish three paradigm-shifting strategies to transcend the fundamental activity-stability trade-off in industrial-scale eNO₃RR. These approaches collectively address structural degradation, chemical instability, and mechanical failure, yielding catalysts with unprecedented faradaic efficiency, durability, and NH₃ yield rates under industry-relevant currents. Crucially, the successful demonstration of integrated electrolyzer-absorption systems for direct NH₄Cl fertilizer production validates their practical viability. In all, this work not only advances mechanistic understanding of dynamic catalyst evolution under operando conditions but also provides a scalable materials design blueprint for sustainable nitrogen-cycle electrification.
| Date of Award | 6 Oct 2025 |
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| Original language | English |
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| Supervisor | Johnny Chung Yin HO (Supervisor) |