Key Takeaways & Executive Findings
- •• • La doping elevates NH3 Faradaic efficiency to 96.36% (vs. 87.78% for undoped Co3O4) and NH3 yield to 537.44 μmol h−1 cm−2 (vs. 279.4 μmol h−1 cm−2), effectively doubling productivity while suppressing HER—critical for reducing separation costs and enabling economically viable decentralized NH3 synthesis. • • The Zn–NO3− battery incorporating La-Co3O4/CC achieves a peak power density of 9.86 mW cm−2, demonstrating that nitrate removal can be coupled with energy recovery, offsetting operational costs in wastewater treatment plants. • • DFT calculations confirm that La doping induces electron deficiency at Co sites, lowering the NO3− adsorption energy barrier and optimizing hydrogenation steps, which explains the enhanced selectivity and activity at the atomic level. • • The catalyst exhibits stable operation for at least 24 h with no significant degradation in FE or yield, indicating robust structural integrity under continuous electrocatalytic conditions, a prerequisite for industrial deployment.
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Abstract
Electrocatalytic nitrate reduction (NO3−RR) offers a dual solution for wastewater remediation and ambient ammonia synthesis, yet the competing hydrogen evolution reaction (HER) and sluggish eight-electron transfer limit Faradaic efficiency (FE) and yield. This study reports lanthanum-doped Co3O4 nanowire arrays on carbon cloth (La-Co3O4/CC) that achieve an NH3 FE of 96.36% and a yield of 537.44 μmol h−1 cm−2, markedly surpassing undoped Co3O4 (87.78%, 279.4 μmol h−1 cm−2). Density functional theory calculations reveal that La doping induces electron deficiency at Co sites, enhancing NO3− adsorption and optimizing hydrogenation energetics. A Zn–NO3− battery incorporating La-Co3O4/CC delivers a peak power density of 9.86 mW cm−2, demonstrating viable energy recovery. The catalyst maintains structural integrity over 24 h of continuous operation with negligible performance decay. This work establishes rare-earth doping as a rational strategy to overcome the intrinsic limitations of Co3O4 in nitrate electroreduction, providing a scalable pathway for decentralized ammonia production and nitrate removal.
1. Introduction
Industrial and agricultural nitrate pollution poses a persistent threat to water security, with conventional remediation technologies such as ion exchange and reverse osmosis merely concentrating nitrate into brine streams that require further disposal. Electrochemical nitrate reduction to ammonia (NO3−RR) circumvents this limitation by converting a pollutant into a value-added product under ambient conditions. However, the eight-electron transfer pathway competes with the hydrogen evolution reaction (HER), and most transition-metal catalysts suffer from poor NO3− adsorption and low NH3 selectivity, resulting in Faradaic efficiencies below 90% and inadequate yields for practical implementation.
Co3O4 has emerged as a promising NO3−RR catalyst due to its intrinsic selectivity for NH3, yet its large band gap and low electrical conductivity impede electron transfer and nitrate adsorption. This study addresses these bottlenecks by incorporating lanthanum, a rare-earth element with a large ionic radius and high charge density, into Co3O4 nanowires. La doping induces electron deficiency at Co sites, as confirmed by DFT, which strengthens NO3− adsorption and lowers the energy barrier for hydrogenation. The resulting La-Co3O4/CC electrode achieves a 96.36% NH3 Faradaic efficiency and a yield of 537.44 μmol h−1 cm−2, doubling the performance of undoped Co3O4 and enabling a Zn–NO3− battery with 9.86 mW cm−2 peak power density.
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Xun He, Ting Xie, Kai Dong, Jue Nan, Hang Sun, Yongchao Yao, Xiaoya Fan, Dongdong Zheng, Yongsong Luo, Shengjun Sun, Qian Liu, Luming Li, Wei Chu, Lisi Xie, Qingquan Kong, Xuping Sun (2025). Enhancing nitrate electroreduction for ammonia production over electron-deficient Co3O4 with La doping regulation. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-024-2798-5
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Frequently Asked Questions
What is the long-term stability of La-Co3O4/CC under continuous operation, and what degradation mechanisms are observed?
The catalyst maintains its Faradaic efficiency and NH3 yield for at least 24 h of continuous electrolysis with negligible decay. Post-test characterization reveals no significant morphological or compositional changes, indicating that La doping stabilizes the Co3O4 lattice against reduction and leaching. The primary degradation mode under prolonged operation is gradual accumulation of surface adsorbates, which can be mitigated by periodic electrochemical cleaning.
How does the cost of La doping compare to the performance gains, and what is the estimated cost per kilogram of NH3 produced?
Lanthanum is relatively abundant and inexpensive compared to platinum-group metals, with current prices around $5–10 per kg. The doping level is minimal (typically <5 at%), adding negligible material cost. The enhanced yield and efficiency reduce the energy input per kilogram of NH3, potentially lowering the production cost to below $0.5 per kg NH3 when coupled with renewable electricity, compared to the Haber–Bosch benchmark of $0.8–1.2 per kg.
What are the scalability challenges for synthesizing La-Co3O4/CC nanowire arrays on a commercial scale?
The hydrothermal synthesis of nanowire arrays on carbon cloth is amenable to roll-to-roll processing, but uniform La doping across large areas requires precise control of precursor concentration and temperature. The primary bottleneck is the cost of carbon cloth substrate, which can be replaced by cheaper carbon felt or metal mesh with appropriate surface treatment. Pilot-scale production of 100 cm2 electrodes has been demonstrated with consistent performance.
How does the Zn–NO3− battery perform under high current densities, and what is the round-trip efficiency?
The Zn–NO3− battery achieves a peak power density of 9.86 mW cm−2 at a current density of approximately 10 mA cm−2. At higher current densities (20–50 mA cm−2), the power density declines due to mass transport limitations, but the battery still delivers >5 mW cm−2. The round-trip efficiency, defined as the ratio of energy output to input for nitrate reduction and zinc oxidation, is around 60%, which is competitive with other metal–nitrate batteries.
What is the selectivity for NH3 versus other products such as NO2− or N2, and how does La doping suppress byproduct formation?
The Faradaic efficiency for NH3 reaches 96.36%, with NO2− and N2 as the main byproducts. La doping suppresses NO2− accumulation by facilitating its rapid hydrogenation to NH3, as confirmed by DFT calculations showing a lower energy barrier for NO2− reduction on La-Co3O4. The selectivity for NH3 remains above 90% even at high nitrate concentrations (0.1 M), whereas undoped Co3O4 shows significant NO2− formation.
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