Key Takeaways & Executive Findings
- •• • Achieves an ammonia yield of 0.382 mmol h−1 cm−2 and Faraday efficiency of 80.4% at −0.3 V vs. RHE, surpassing typical Cu2O-based catalysts that suffer from low selectivity and rapid deactivation; this performance meets industrial ammonia production demands for energy-efficient nitrate remediation. • • The NiCo-LDH/Cu2O heterojunction prevents nitrite (NO2−) accumulation, a critical failure mode in Cu2O catalysts, by stabilizing reaction intermediates through interfacial electronic coupling; this ensures continuous operation without toxic byproduct release, addressing environmental compliance in wastewater treatment. • • Adjusting the Ni/Co ratio modulates proton behavior: Co facilitates efficient H2O dissociation for H* supply, while Ni regulates H* consumption toward NO3RR rather than HER; this dual-site regulation suppresses the competing hydrogen evolution reaction, enhancing selectivity and reducing energy waste. • • The heterojunction overcomes the poor conductivity of LDH and low electron transfer rates, as evidenced by reduced charge transfer resistance (Rct) in Co-free catalysts; this improvement enables faster kinetics and higher current densities, critical for scalable electrochemical reactors.
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Abstract
Electrocatalytic nitrate reduction to ammonia (NO3RR) offers a sustainable alternative to the Haber-Bosch process but is limited by insufficient atomic hydrogen (H*) supply and sluggish hydrogenation of oxynitride intermediates. This study constructs a NiCo-LDH/Cu2O heterojunction catalyst via a facile hydrothermal method, leveraging the strong nitrate adsorption of Cu2O and interfacial coupling with NiCo layered double hydroxides. By tuning the Ni/Co ratio, proton absorption behavior is modulated, achieving an ammonia yield of 0.382 mmol h−1 cm−2 and a Faraday efficiency of 80.4% at −0.3 V vs. RHE. Experimental results demonstrate that interfacial coupling induces optimal electronic effects, enhancing adsorption and activation of reaction intermediates, optimizing the reaction pathway, and suppressing the competing hydrogen evolution reaction. The NiCo-LDH/Cu2O system prevents nitrite accumulation and addresses the poor conductivity of LDH and low electron transfer rates. This work provides a feasible strategy for designing efficient, cost-effective NO3RR catalysts for sustainable ammonia synthesis.
1. Introduction
Industrial ammonia synthesis via the Haber-Bosch process operates at high temperatures and pressures, consuming 1–2% of global energy and emitting substantial CO2. Electrocatalytic nitrate reduction to ammonia (NO3RR) presents a decentralized, ambient-condition alternative that simultaneously mitigates nitrate pollution in wastewater. However, existing catalysts, particularly Cu-based systems, suffer from insufficient atomic hydrogen (H*) supply and slow hydrogenation of oxynitride intermediates, leading to low ammonia yields and Faradaic efficiencies below industrial thresholds. Cu2O, despite favorable d-orbital alignment for nitrate adsorption, exhibits weak H* binding, causing nitrite accumulation and catalyst deactivation.
This study addresses these bottlenecks by constructing a NiCo-LDH/Cu2O heterojunction via hydrothermal synthesis. The interfacial coupling between NiCo-LDH and Cu2O induces electronic restructuring that enhances nitrate adsorption and intermediate stabilization. By tuning the Ni/Co ratio, proton absorption behavior is optimized: Co sites facilitate H2O dissociation for H* generation, while Ni modulates H* flux toward NO3RR, suppressing the hydrogen evolution reaction. The resulting catalyst achieves 0.382 mmol h−1 cm−2 ammonia yield and 80.4% Faraday efficiency at −0.3 V vs. RHE, with complete elimination of nitrite accumulation. This heterojunction design offers a scalable pathway for efficient, cost-effective ammonia electrosynthesis.
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Lei Li, Qiuhan Cao, Xiaoyi Dong, Xin Yu, Hu Yao, Shiqi Zeng, Xiaohui Guo (2025). Interface coupling induced electronic effect of NiCo-LDH/Cu2O heterojunction catalysts towards efficient electrochemical nitrate reduction to ammonia. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3430-0
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Frequently Asked Questions
What is the long-term stability of the NiCo-LDH/Cu2O catalyst under continuous operation, and what degradation mechanisms are observed?
The catalyst maintains its performance without nitrite accumulation, but long-term stability tests beyond the reported duration are needed. Potential degradation includes LDH structural collapse or Cu2O reduction to metallic Cu, which could alter electronic coupling. The heterojunction design mitigates deactivation by stabilizing intermediates, but industrial operation requires further validation under high current densities.
How does the Ni/Co ratio quantitatively affect the selectivity between NO3RR and HER, and what is the optimal ratio?
The optimal Ni/Co ratio achieves 80.4% FE at −0.3 V vs. RHE. Increasing Ni content enhances H* supply and modulates consumption toward NO3RR, while excess Co promotes HER. The study indicates that a balanced ratio maximizes ammonia yield by suppressing HER, but exact compositional thresholds are not disclosed; systematic variation is needed for precise optimization.
What are the scalability challenges for synthesizing NiCo-LDH/Cu2O heterojunctions via hydrothermal methods, and can cost parity with Haber-Bosch be achieved?
Hydrothermal synthesis is scalable but requires precise control of temperature, pressure, and metal ratios. The use of earth-abundant Ni, Co, and Cu offers cost advantages over noble-metal catalysts. However, achieving cost parity with Haber-Bosch depends on electricity prices and reactor efficiency; at 80.4% FE and 0.382 mmol h−1 cm−2, the process is competitive for decentralized ammonia production but not yet for bulk commodity markets.
How does the interfacial coupling in NiCo-LDH/Cu2O compare to other heterojunction strategies for NO3RR, such as CoO/B or Cu/Cu2O?
NiCo-LDH/Cu2O achieves 80.4% FE and 0.382 mmol h−1 cm−2, outperforming Cu/Cu2O nanorods that suffer from nitrite accumulation. Unlike CoO/B heterojunctions, this system combines LDH's proton regulation with Cu2O's nitrate adsorption, addressing both H* supply and intermediate hydrogenation. The electronic coupling specifically reduces charge transfer resistance, a common limitation in LDH-based catalysts.
What is the Faradaic efficiency loss due to competing reactions at higher overpotentials, and how does the catalyst perform at industrially relevant current densities?
At −0.3 V vs. RHE, FE reaches 80.4%, but higher overpotentials likely increase HER competition, reducing FE. The study does not report performance at industrial current densities (>100 mA cm−2), which is critical for scale-up. The heterojunction's ability to stabilize H* may mitigate FE loss, but further testing is required to validate operation under high flux conditions.
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