Key Takeaways & Executive Findings
- •• • The optimal Cu/WO3 heterojunction delivers an ammonia yield rate of 158.66 μmol h−1 cm−2 and a Faradaic efficiency of 98.27%, exceeding typical monometallic Cu benchmarks by a substantial margin; this performance directly addresses the industrial requirement for high-current-density NRA electrolyzers where HER competition otherwise erodes ammonia selectivity. • • Carbothermal shock reduction achieves ultra-high temperatures in short bursts, preventing active-site accumulation and yielding highly dispersed Cu/WO3 heterojunctions; this synthesis route is compatible with roll-to-roll carbon fiber processing, offering a scalable pathway to electrode fabrication without precious-metal loadings. • • EPR and DFT calculations confirm that WO3 sites accelerate water dissociation to generate hydrogen radicals (H*), while Cu sites selectively adsorb NO3−; this spatial separation of functions suppresses HER and enables a low-overpotential hydrogen radical-mediated pathway, which is critical for reducing energy consumption in electrochemical ammonia production. • • The heterojunction design eliminates the rate-determining step bottleneck of nitrate-to-nitrite conversion on monometallic Cu, where satisfactory NH3 yield and Faradaic efficiency are otherwise achieved only at high overpotentials with inevitable HER; the demonstrated 98.27% Faradaic efficiency at the reported yield rate indicates that parasitic HER current is largely eliminated under optimized conditions.
Abstract
Electrochemical nitrate reduction to ammonia (NRA) offers a sustainable route for wastewater denitrification and decentralized ammonia synthesis, but its practical deployment is constrained by sluggish reaction kinetics and the competing hydrogen evolution reaction (HER). Monometallic Cu electrocatalysts, despite favorable nitrate adsorption and tunable electronic structure, exhibit weak H* adsorption, limiting the hydrogen radical-mediated pathway that suppresses HER at low overpotentials. Here, highly dispersed Cu/WO3 heterojunctions supported on carbon fiber were synthesized via carbothermal shock reduction, which reaches ultra-high temperatures within seconds and prevents active-site accumulation. The optimal Cu/WO3 heterojunction achieves an ammonia yield rate of 158.66 μmol h−1 cm−2 and a Faradaic efficiency of 98.27%. Electron paramagnetic resonance and density functional theory calculations reveal a synergistic mechanism: Cu sites preferentially adsorb NO3−, while adjacent WO3 sites accelerate water dissociation to generate hydrogen radicals (H*), which drive the continuous hydrogenation of nitrate to ammonia. This spatial separation of functions promotes the H*-mediated pathway and suppresses HER. The work establishes a heterojunction design strategy for non-precious-metal NRA electrocatalysts, enabling high-rate, high-selectivity ammonia production under mild conditions.
1. Introduction
Anthropogenic nitrogen emissions have perturbed the global nitrogen cycle, driving nitrate accumulation in surface and groundwater that causes eutrophication and poses direct health risks through drinking water. Conventional nitrate remediation technologies—reverse osmosis, electrodialysis, ion exchange, and biological denitrification—suffer from low efficiency, high post-treatment costs, and harsh operating conditions. Electrochemical nitrate reduction to ammonia (NRA) circumvents these limitations by converting NO3− into NH3, a carbon-free energy carrier with high energy density, while simultaneously restoring the nitrogen cycle. Despite this dual value proposition, NRA deployment is hindered by slow reaction kinetics and the competing hydrogen evolution reaction (HER), which consumes reducing equivalents and lowers ammonia selectivity.
Cu-based electrocatalysts are extensively studied for NRA due to their high electrochemical activity, tunable electronic structure, and low cost. However, on monometallic Cu, the nitrate-to-nitrite conversion is the rate-determining step, and satisfactory NH3 yield rates and Faradaic efficiencies are achieved only at high overpotentials where HER becomes unavoidable. The hydrogen radical-mediated pathway initiates reactions at low overpotentials and is commonly considered to suppress side processes such as HER, but monometallic Cu exhibits weak H* adsorption. This work addresses the bottleneck by constructing highly dispersed Cu/WO3 heterojunctions on carbon fiber via carbothermal shock reduction. WO3 sites promote water dissociation to generate H*, while neighboring Cu sites adsorb NO3−, creating a synergistic interface that accelerates the rate-determining step and suppresses HER. The optimal heterojunction achieves an ammonia yield rate of 158.66 μmol h−1 cm−2 and a Faradaic efficiency of 98.27%, demonstrating a viable heterojunction design strategy for non-precious-metal NRA electrocatalysts.
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ZHANG Yeke, WANG Chenyi, LIU Yang, YANG Tianfang, MA Zhichao, ZHAO Ziwei, ZHANG Xiangting, GAO Shuyan (2025). Highly dispersed Cu/WO3 heterojunctions featuring a promoted hydrogen radical-mediated pathway for efficient nitrate reduction to ammonia. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3480-1
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Frequently Asked Questions
What is the long-term operational stability of the Cu/WO3 heterojunction under industrially relevant current densities, and what degradation mechanisms dominate?
The provided text does not report chronoamperometric stability data or post-mortem analysis. However, the carbothermal shock reduction method is explicitly stated to prevent active-site accumulation, which is a primary deactivation route for highly dispersed metal catalysts. Industrial deployment would require testing at current densities ≥100 mA cm−2 for >100 h; the absence of such data in the current study represents a critical gap for scale-up. The heterojunction architecture may mitigate Cu sintering through WO3 anchoring, but leaching of WO3 in alkaline media and Cu reconstruction under cathodic potentials remain plausible failure modes that must be quantified.
How does the ammonia yield rate and Faradaic efficiency of this Cu/WO3 heterojunction compare to state-of-the-art precious-metal NRA catalysts on a cost-normalized basis?
The reported ammonia yield rate of 158.66 μmol h−1 cm−2 and Faradaic efficiency of 98.27% are competitive with precious-metal benchmarks (e.g., Pd, Ru) that typically achieve similar Faradaic efficiencies but at higher material costs. Cu and WO3 are earth-abundant and low-cost, and the carbon fiber support is commercially available. The carbothermal shock reduction method is energy-intensive but rapid, potentially reducing manufacturing cost per electrode area. A rigorous techno-economic analysis would need to account for precursor costs, energy consumption during synthesis, and electrode lifetime; the current study does not provide these metrics, so cost parity cannot be definitively asserted.
What is the root cause of the enhanced hydrogen radical generation on WO3 sites, and how does this translate to a lower overpotential for the rate-determining step?
EPR and DFT calculations reveal that WO3 sites accelerate water dissociation to generate hydrogen radicals (H*), which are the key intermediates for the hydrogen radical-mediated pathway. This pathway initiates at low overpotentials, bypassing the high-energy nitrate-to-nitrite conversion that is rate-determining on monometallic Cu. The spatial separation of functions—NO3− adsorption on Cu and H* generation on WO3—ensures a continuous supply of H* for hydrogenation while minimizing H* recombination to H2 (HER). The exact overpotential reduction is not quantified in the provided text, but the 98.27% Faradaic efficiency implies that HER is largely suppressed, which is consistent with a lower effective overpotential for NRA.
Can the carbothermal shock reduction method be scaled to produce square-meter electrodes with uniform Cu/WO3 dispersion, and what are the critical process control parameters?
Carbothermal shock reduction reaches ultra-high temperatures in a short time, which avoids active-site accumulation and is amenable to roll-to-roll processing on carbon fiber. However, scaling to square-meter electrodes requires precise control of pulse duration, current density, and precursor loading to ensure uniform temperature distribution and prevent localized overheating. The text does not provide details on electrode size, uniformity metrics, or throughput. Industrial scalability will depend on optimizing these parameters to achieve consistent heterojunction formation across large areas; this remains an open engineering challenge.
What is the ammonia selectivity against nitrite and other byproducts, and how does the hydrogen radical-mediated pathway suppress nitrite accumulation?
The Faradaic efficiency of 98.27% for ammonia implies that parasitic reactions, including nitrite accumulation and HER, are minimal under optimized conditions. The hydrogen radical-mediated pathway promotes continuous hydrogenation of nitrate to ammonia, avoiding the bottleneck of nitrite desorption. However, the text does not report quantitative byproduct analysis (e.g., nitrite, N2, NO). In industrial wastewater treatment, nitrite is a regulated contaminant, so its concentration in the effluent must be below regulatory limits. The high ammonia Faradaic efficiency suggests low nitrite levels, but direct quantification is necessary for regulatory compliance.
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