Key Takeaways & Executive Findings
- •• • Co0.5Cu1.5(OH)PO4 achieves 99.0% Faradaic efficiency for NH3 at 200 mA cm−2 in 1 M NO3−, with a production rate of 9.18 mg h−1 cm−2, outperforming other Co/Cu ratios and sustaining >200 h continuous operation, indicating industrial viability for high-strength nitrate wastewater treatment. • • The energy barrier difference between NO3RR and H* generation on Co0.5Cu1.5(OH)PO4 is 0.18 eV, versus 0.38 eV on Cu2(OH)PO4, demonstrating a 53% reduction in HER competition, which is critical for achieving high selectivity at industrially relevant current densities. • • DFT calculations reveal that Co sites dissociate water to supply H*, while Cu sites drive deoxygenation and hydrogenation of intermediates (*NO2, *NO), lowering the rate-determining step energy barrier; this tandem mechanism enables efficient nitrate-to-ammonia conversion at high concentrations. • • The volcano-shaped activity dependence on Co/Cu ratio underscores the necessity of balancing H* generation and consumption; optimal performance at Co0.5Cu1.5(OH)PO4 highlights the importance of precise atomic-ratio control for circumventing scaling relations in electrocatalysis.
Abstract
Electrochemical nitrate reduction (NO3RR) to ammonia offers a sustainable route for nitrogen recovery from wastewater, yet its efficiency is constrained by complex multi-step proton-electron transfers and competitive hydrogen evolution. Here, we report a series of binary cobalt-copper phosphates with precisely tuned Co/Cu ratios, revealing a volcano-type relationship between composition and catalytic activity. The optimized Co0.5Cu1.5(OH)PO4 catalyst, supported on a Ni3Co1OxHy/Ni foam substrate, achieves a Faradaic efficiency of 99.0% for ammonia at a high current density of 200 mA cm−2 in 1 M nitrate electrolyte, with a production rate of 9.18 mg h−1 cm−2 and sustained stability over 200 hours. In-situ ATR-FTIR spectroscopy and density functional theory calculations elucidate a tandem mechanism: Co sites promote water dissociation to generate active hydrogen (H*), while adjacent Cu sites facilitate nitrate adsorption and subsequent hydrogenation steps. This synergistic division of labor lowers the energy barrier for the rate-determining step, effectively suppressing HER and enhancing intrinsic kinetics. The work demonstrates that precise atomic-ratio engineering in dual-site transition metal phosphates provides a viable strategy to overcome activity-selectivity trade-offs in electrocatalytic nitrate-to-ammonia conversion.
1. Introduction
Nitrate pollution from industrial and agricultural sources poses severe environmental and health risks, while the electrochemical reduction of nitrate to ammonia (NO3RR) offers a dual benefit of wastewater treatment and sustainable ammonia synthesis. However, the 8-electron transfer process is kinetically sluggish and plagued by side reactions, particularly the competing hydrogen evolution reaction (HER) at negative potentials. Conventional Cu-based catalysts, though selective for nitrate adsorption, suffer from strong intermediate binding leading to deactivation and require high overpotentials or concentrated electrolytes to achieve practical rates. The scaling relations between adsorption energies of intermediates further limit the activity-selectivity trade-off, necessitating innovative catalyst designs that decouple these constraints.
Transition metal phosphates, with their tunable electronic structures and ability to facilitate water dissociation, present a promising platform. This work introduces a binary CoCu phosphate catalyst, Co0.5Cu1.5(OH)PO4, which leverages a tandem mechanism: Co sites generate active hydrogen (H*) from water, while Cu sites utilize this H* for nitrate reduction. By precisely controlling the Co/Cu ratio, the catalyst achieves a remarkable Faradaic efficiency of 99.0% at 200 mA cm−2 in 1 M nitrate, with a production rate of 9.18 mg h−1 cm−2 and stability exceeding 200 hours. This rational design directly addresses the bottleneck of HER competition and scaling relations, offering a scalable pathway for high-concentration nitrate treatment and ammonia recovery.
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ZHAO Jingsi, YANG Yifei, ZHU Chun, ZHANG Hao, LIU Peng, LI Jing, YANG Fei, SUN Wei (2026). Activated water molecular dissociation enhances nitrate electrochemical reduction activity by rational design of binary CoCu-Pi catalyst. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4236-8
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Frequently Asked Questions
What is the long-term stability of Co0.5Cu1.5(OH)PO4 under industrial-scale current densities, and what degradation mechanisms might limit its operational lifetime?
The catalyst maintains stable performance for over 200 hours of continuous electrolysis at 200 mA cm−2, indicating robust structural integrity. However, potential degradation mechanisms include phosphate dissolution in alkaline media, mechanical stress from gas evolution, and possible Cu site poisoning by strongly adsorbed intermediates. Further post-electrolysis characterization is required to assess morphological and compositional changes.
How does the energy barrier difference of 0.18 eV between NO3RR and H* generation translate to selectivity improvements in practical electrolyzers, and what are the implications for energy consumption?
The 0.18 eV barrier difference, compared to 0.38 eV for Cu2(OH)PO4, indicates that HER is significantly suppressed, leading to higher Faradaic efficiency (99.0%). In practical terms, this reduces parasitic hydrogen production, lowering energy consumption per kg of ammonia produced. At 200 mA cm−2, the cell voltage would be lower than that for less selective catalysts, improving overall energy efficiency.
What is the cost and scalability of the Co0.5Cu1.5(OH)PO4 catalyst compared to conventional Cu-based or noble metal catalysts?
The catalyst uses earth-abundant Co and Cu, making it cost-effective relative to noble metals like Ru or Pt. The synthesis involves simple hydrothermal or electrodeposition methods, which are scalable. However, the Ni3Co1OxHy/Ni foam substrate adds cost, but the high activity and stability may offset this by reducing catalyst loading and increasing throughput.
How does the catalyst perform in real wastewater matrices containing other ions (e.g., chloride, sulfate) that could compete or poison active sites?
The study was conducted in 1 M nitrate electrolyte, which is a high-concentration model system. Real wastewater often contains chloride, which can oxidize to chlorine species and compete for active sites, or sulfate, which may adsorb and block sites. The catalyst's selectivity and stability in such complex matrices remain to be tested; however, the high nitrate concentration and strong adsorption on Cu sites may mitigate interference.
What is the mechanistic evidence for the tandem catalysis, and how does the Co/Cu ratio influence the rate-determining step?
In-situ ATR-FTIR and DFT calculations reveal that Co sites dissociate water to generate H*, while Cu sites adsorb nitrate and reduce it to nitrite, followed by hydrogenation steps. The volcano-shaped activity dependence on Co/Cu ratio indicates that an optimal balance between H* supply and consumption is necessary. At Co0.5Cu1.5(OH)PO4, the energy barrier for the rate-determining step is minimized, as evidenced by the 0.18 eV barrier difference, leading to enhanced kinetics.
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