Regulating the hydrogen transfer pathways for electrochemical nitrate-to-ammonia at industrial current density

The motivation

Ammonia is an essential chemical feedstock and a promising carbon-free energy carrier. However, the Haber-Bosch process remains highly energy-intensive and contributes significantly to global CO₂ emissions. Electrochemical nitrate reduction reaction (NO₃RR) offers a sustainable alternative by converting nitrate pollutants in wastewater into ammonia under ambient conditions using renewable electricity.

Despite rapid progress, achieving high NH₃ selectivity at industrial current densities remains extremely challenging. One of the major bottlenecks originates from the imbalance in active hydrogen (*H) utilization. During NO₃RR, surface *H species are required for stepwise hydrogenation of nitrogen intermediates, yet excessive *H tends to participate in HER instead of nitrate reduction.

Moreover, pristine Cu₂O catalysts exhibit a hydrogenation mechanism: the early-stage nitrate reduction follows a Langmuir–Hinshelwood (L-H) pathway involving surface *H, while the later NO-to-NH₃ conversion mainly proceeds through an Eley–Rideal (E-R) pathway independent of adsorbed hydrogen. This may lead to residual *H accumulation and accelerated HER under high current densities.

We therefore asked: can we redesign the catalytic active sites to continuously channel active hydrogen toward nitrogen intermediate hydrogenation rather than hydrogen evolution?

 

The origin of the idea

Copper-based catalysts are widely regarded as one of the most promising systems for NO₃RR because of their favorable nitrate adsorption and moderate binding strength toward nitrogen intermediates. However, their hydrogenation pathways are difficult to regulate.

We hypothesized that introducing rare-earth elements with strong electronic modulation capability could alter the adsorption configuration and hydrogenation behavior of key intermediates. Cerium, with its dynamic Ce redox properties and strong oxygen affinity, emerged as an ideal candidate.

This concept led us to design a Ce-doped Cu₂O catalyst, where atomically dispersed Ce induces the formation of Cu-O-Ce active centers alongside the intrinsic Cu⁰-Cu⁺ sites. We envisioned that these new interfacial sites could activate an additional *H-mediated hydrogenation route and improve active hydrogen utilization efficiency.

 

Our approach: dual active-site for hydrogen pathway regulation

We developed a Ce-Cu₂O catalyst containing synergistic Cu⁰-Cu⁺ and Cu-O-Ce dual active centers. Structural characterization confirmed that Ce atoms were uniformly incorporated into the Cu₂O lattice without forming CeO₂ aggregates.

The newly formed Cu-O-Ce sites introduced an additional hydrogenation pathway involving the *NHOH intermediate, fundamentally changing the reaction mechanism of NO₃RR. Instead of relying solely on the traditional E-R hydrogenation route, Ce-Cu₂O enabled cooperative L-H and E-R hydrogenation processes throughout nitrate reduction.

This design effectively redirected excessive surface *H toward nitrogen intermediate hydrogenation, thereby suppressing HER and enhancing ammonia selectivity under high current densities.

 

What we did and key experiments

To validate the mechanism and catalytic origin, we combined advanced operando spectroscopy with theoretical calculations:

1. In situ DEMS experiments tracked reaction intermediates under NO₃⁻, NO₂⁻, and NO feeding conditions, revealing the emergence of the *NHOH intermediate exclusively on Ce-Cu₂O.
2. The t-BuOH trapping experiments confirmed that the newly introduced hydrogenation pathway strongly depended on surface active hydrogen, indicating the dominant L-H mechanism.
3. Operando Raman and ATR-FTIR spectroscopy monitored the dynamic evolution of nitrogen-containing intermediates during electrocatalysis.
4. In situ XAFS demonstrated that the Cu⁰-Cu⁺ ratio remained stable during long-term electrolysis, indicating that the dynamic Ce redox couple stabilized catalytically active sites under reducing conditions.
5. DFT calculations showed that Cu-O-Ce sites significantly lowered the energy barriers for key hydrogenation steps, including *NO₃ → *NO₂OH and *NO → *NOH. Electronic structure analysis further revealed the stronger orbital coupling between Ce 4f states and adsorbed *NO intermediates.
6. Electrochemical measurements demonstrated excellent catalytic activity, achieving a Faradaic efficiency of 96.48% and long-term stability at industrial current densities up to 1 A cm⁻².

 

Why it matters

This work introduces a new catalyst design principle based on active hydrogen pathway regulation. Rather than simply increasing catalytic activity, we demonstrate that controlling how surface hydrogen participates in elementary reaction steps is crucial for achieving efficient nitrate-to-ammonia conversion.

The discovery of cooperative L-H and E-R hydrogenation pathways provides new mechanistic insight into multi-step electrocatalytic hydrogenation reactions. More importantly, the Cu-O-Ce dual-site strategy offers a general framework for regulating hydrogen utilization in other electrochemical systems, including CO2 reduction, nitrite reduction, and biomass upgrading.

From a sustainability perspective, this work advances the development of decentralized ammonia production technologies powered by renewable electricity while simultaneously enabling nitrate wastewater remediation.

Outlook

We envision several promising future directions:

1. Expanding rare-earth modulation strategies to other transition-metal catalysts for tuning hydrogenation pathways.
2. Designing adaptive catalytic interfaces capable of dynamically regulating active hydrogen coverage under industrial operating conditions.
3. Integrating NO₃RR systems into scalable flow-cell electrolyzers for practical wastewater-to-ammonia conversion.

We hope this work encourages the community to view active hydrogen not merely as a reaction intermediate, but as a controllable reaction resource whose transfer pathway can fundamentally determine catalytic efficiency and selectivity.

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