Nitrate Electroreduction Research Articles

Engineering the Co/CoO heterostructure to trigger the in-situ generation of abundant high-valent cobalt species for enhanced electroreduction of nitrate to ammonia

The CoO-based materials are promising candidates for electrochemical nitrate reduction reaction to ammonia (eNO3RR). Herein, Zn/Co bimetallic MOFs are adopted to construct Co/CoO Schottky heterostructures, where the Co-CoO interfaces are engineered by controlling the Zn/Co ratio. The interface-optimized Co/CoO displays an NH3 yield of 713 µmol/h cm−2 and a maximum Faradaic efficiency of 99.16 % due to the synergistic effect between CoO and Co and abundant oxygen vacancies. The more the Co-CoO interfaces, the more the in-situ generated high-valent cobalt species (CoOOH), and the higher the catalytic performance. Therefore, the high-valent cobalt species are considered the true active sites. When used as a cathode in a rechargeable Zn-NO3− battery, the Co/CoO heterostructure achieves a power density of 4.7 mW cm−2 and an NH3 yield of 131.7 µmol/h cm−2 with robust working stability.

Extending Click Chemistry to Fabricate Th-MOF-Supported Dynamic Single-Site Metal Catalysts toward Nitrate Electroreduction to Ammonia

Extending Click Chemistry to Fabricate Th-MOF-Supported Dynamic Single-Site Metal Catalysts toward Nitrate Electroreduction to Ammonia

A Theoretical Perspective for Ammonia Synthesis: Nitric Oxide or Nitrate Electroreduction?

Ammonia is an important raw material for agricultural production, playing a key role in global food production. However, conventional ammonia synthesis resulted in extensive greenhouse gas emissions and huge energy consumption. Recently, researchers have proposed electrocatalytic reverse artificial nitrogen cycle (eRANC) routes to circumvent these issues, which can be driven by electrocatalysis and sustainable electricity. Here, a theoretical and computational perspective on the challenges and opportunities with the comparison with experimental results: electrochemical reduction of nitrate (eNO3RR) and nitrite (eNO2RR), electrochemical reduction of nitric oxide (eNORR) combined with oxidative nitrogen fixation are presented. By comparison, the N2→NO→NH3 route is proposed as the most promising in case the NO solubility can be solved well in reactor design. Its high efficiency of ammonia production is demonstrated. Instead, the eNO3RR can be another choice because it is non-toxic and the solid-liquid interface is usually efficient for electrochemical reactions, while its low selectivity at low overpotentials is an issue. These fundamentals highlight the potential and key factors of eRANC as an efficient and sustainable route for ammonia production.

Unveiling the Dual Role of Oxophilic Cr4+ in Cr-Cu2O Nanosheet Arrays for Enhanced Nitrate Electroreduction to Ammonia.

Cuprous oxide (Cu2O)-based catalysts present a promising activity for the electrochemical nitrate (NO3-) reduction to ammonia (eNO3RA), but the electrochemical instability of Cu+ species may lead to an unsatisfactory durability, hindering the exploration of the structure-performance relationship. Herein, we propose an efficient strategy to stabilize Cu+ through the incorporation of Cr4+ into the Cu2O matrix to construct a Cr4+-O-Cu+ network structure. In situ and quasi-in situ characterizations reveal that the Cu+ species are well maintained via the strong Cr4+-O-Cu+ interaction that inhibits the leaching of lattice oxygen. Importantly, in situ generated Cr3+-O-Cu+ from Cr4+-O-Cu+ is identified as a dual-active site for eNO3RA, wherein the Cu+ sites are responsible for the activation of N-containing intermediates, while the assisting Cr3+ centers serve as the electron-proton mediators for rapid water dissociation. Theoretical investigations further demonstrated that the metastable state Cr3+-O-Cu+ favors the conversion from the endoergic hydrogenation of the key *ON intermediate to an exoergic reaction in an ONH pathway, and facilitates the subsequent NH3 desorption with a low energy barrier. The superior eNO3RA with a maximum 91.6% Faradaic efficiency could also be coupled with anodic sulfion oxidation to achieve concurrent NH3 production and sulfur recovery with reduced energy input.

Unveiling the Dual Role of Oxophilic Cr4+ in Cr‐Cu2O Nanosheet Arrays for Enhanced Nitrate Electroreduction to Ammonia

Cuprous oxide (Cu2O)‐based catalysts present a promising activity for the electrochemical nitrate (NO3‐) reduction to ammonia (eNO3RA), but the electrochemical instability of Cu+ species may lead to an unsatisfactory durability, hindering the exploration of the structure‐performance relationship. Herein, we propose an efficient strategy to stabilize Cu+ through the incorporation of Cr4+ into the Cu2O matrix to construct a Cr4+‐O‐Cu+ network structure. In situ and quasi‐in situ characterizations reveal that the Cu+ species are well maintained via the strong Cr4+‐O‐Cu+ interaction that inhibits the leaching of lattice oxygen. Importantly, in situ generated Cr3+‐O‐Cu+ from Cr4+‐O‐Cu+ is identified as a dual‐active site for eNO3RA, wherein the Cu+ sites are responsible for the activation of N‐containing intermediates, while the assisting Cr3+ centers serve as the electron‐proton mediators for rapid water dissociation. Theoretical investigations further demonstrated that the metastable state Cr3+‐O‐Cu+ favors the conversion from the endoergic hydrogenation of the key *ON intermediate to an exoergic reaction in an ONH pathway, and facilitates the subsequent NH3 desorption with a low energy barrier. The superior eNO3RA with a maximum 91.6% Faradaic efficiency could also be coupled with anodic sulfion oxidation to achieve concurrent NH3 production and sulfur recovery with reduced energy input.

In-situ electrochemical reconstruction and modulation of adsorbed hydrogen coverage in cobalt/ruthenium-based catalyst boost electroreduction of nitrate to ammonia

The electroreduction of nitrate offers a promising, sustainable, and decentralized route to generate valuable ammonia. However, a key challenge in the nitrate reduction reaction is the energy efficiency of the reaction, which requires both a high ammonia yield rate and a high Faradaic efficiency of ammonia at a low working potential (≥−0.2 V versus reversible hydrogen electrode). We propose a bimetallic Co–B/Ru12 electrocatalyst which utilizes complementary effects of Co–B and Ru to modulate the quantity of adsorbed hydrogen and to favor the specific hydrogenation for initiating nitrate reduction reaction at a low overpotential. This effect enables the catalyst to achieve a Faradaic efficiency for ammonia of 90.4 ± 9.2% and a remarkable half-cell energy efficiency of 40.9 ± 4% at 0 V versus reversible hydrogen electrode. The in-situ electrochemical reconstruction of the catalyst contributes to boosting the ammonia yield rate to a high level of 15.0 ± 0.7 mg h−1 cm−2 at −0.2 V versus reversible hydrogen electrode. More importantly, by employing single-entity electrochemistry coupled with identical location transmission electron microscopy, we gain systematic insights into the correlation between the increase in the catalyst’s active sites and its structural transformations during the nitrate reduction reaction.

Role of Structural and Compositional Changes of Cu2O Nanocubes in Nitrate Electroreduction to Ammonia.

Nitrate electroreduction reaction (NO3RR) to ammonia (NH3) still faces fundamental and technological challenges. While Cu-based catalysts have been widely explored, their activity and stability relationship are still not fully understood. Here, we systematically monitored the dynamic alterations in the chemical and morphological characteristics of Cu2O nanocubes (NCs) during NO3RR in an alkaline electrolyte. In 1 h of electrolysis from -0.10 to -0.60 V vs RHE, the electrocatalyst achieved the maximum NH3 faradaic efficiency (FE) and yield rate at -0.3 V (94% and 149 μmol h-1 cm-2, respectively). Similar efficiency could be found at a lower overpotential (-0.20 V vs RHE) in long-term electrolysis. At -0.20 V vs RHE, the catalyst FE increased from 73% in the first 2 h to ∼90% in 10 h of electrolysis. Electron microscopy revealed the loss of the cubic shape with the formation of sintered domains. In situ Raman, X-ray diffraction (XRD), and in situ Cu K-edge X-ray absorption near-edge spectroscopy (XANES) indicated the reduction of Cu2O to oxide-derived Cu0 (OD-Cu). Nevertheless, a remaining Cu2O phase was noticed after 1 h of electrolysis at -0.3 V vs RHE. This observation indicates that the activity and selectivity of the initially well-defined Cu2O NCs are not solely dependent on the initial structure. Instead, it underscores the emergence of an OD-Cu-rich surface, evolving from near-surface to underlying layers over time and playing a crucial role in the reaction pathways. By employing online differential electrochemical mass spectrometry (DEMS) and in situ Fourier transform infrared spectroscopy (FTIR), we experimentally probed the presence of key intermediates (NO and NH2OH) and byproducts of NO3RR (N2 and N2H x ) for NH3 formation. These results show a complex relationship between activity and stability of the nanostructured Cu2O oxide catalyst for NO3RR.

Efficient nitrate electroreduction to ammonia over copper catalysts supported on electron-delocalized covalent organic frameworks

Electrochemical nitrate reduction to ammonia (NRA) has recently emerged as a promising route to convert nitrate in wastewater into high-value-added products. Nevertheless, the development of NRA has been hampered by reaction involves multi-proton and electron transfer, posing a low Faradaic efficiency (FE). Here, we develop a composite catalyst of Cu/Cu2O nanoparticles on covalent organic frameworks (COFs) with highly ordered and porous topological structures. The high electron delocalization feature of COFs arising from their two-dimensional structure and rich π-bond increase the electron density of Cu active site, which is benefit for the adsorption and activation of NO3–. The composite catalyst can deliver a satisfactory FE of 95% and a yield rate of 3.4 mmol/h mg−1 for ammonia synthesis. Experimental and theoretical studies both reveals that the COF support can effectively increase the local concentration of NO3– at catalyst surface and decrease the energy barrier, leading to superior NRA performance.

Chloride doping of Co4S3 via methanol solvothermal synthesis for enhanced electrocatalytic nitrate reduction to ammonia

The electrocatalytic reduction of nitrate offers an environmentally friendly means of eliminating hazardous nitrate from wastewater while simultaneously yielding ammonia, a vital fertilizer precursor. Given the importance of nitrate reduction for environmental remediation and the potential of electrochemical methods, the development of a highly efficient and selective electrocatalyst specifically for nitrate reduction to ammonia (NO3ER) is crucial. The Cl-doped Co4S3 (Cl-Co4S3) catalyst was successfully synthesized via a solvothermal method in methanol, aiming to enhance nitrate electroreduction. The incorporation of chlorine was found to modulate the local electronic environment around cobalt atoms, enhancing their reactivity and leading to exceptional catalytic performance for nitrate reduction, leading to exceptional catalytic performance. At an operating potential of −0.61 V vs. the reversible hydrogen electrode, Cl-Co4S3 exhibited remarkable Faradaic efficiency (97.64 ± 2.03 %) and yield rate (0.652 ± 0.012 mmol h−1 cm−2) for ammonia production, surpassing the majority of reported electrocatalysts for NO3ER.

Achieving over 90% Faradaic Efficiency in Cyclohexanone Oxime Electrosynthesis Using the Cu-Mo Dual-Site Catalyst.

Coupling with the nitrate electroreduction reaction (NitRR), the electrosynthesis of cyclohexanone oxime (CHO, the vital feedstock in the nylon-6 industry) from cyclohexanone provides a promising alternative to the traditional energy consumption process. However, it still suffers from low efficiency because selective production of *NH2OH intermediate from NitRR under large current densities is challenging. We here report a Cu1MoOx/nitrogen-doped carbon (NC) electrocatalyst with high-density Cu-Mo dual sites for NitRR to selectively produce and stabilize *NH2OH, with the subsequent cyclohexanone oximation achieving the highest CHO Faradaic efficiency of 94.5% and a yield rate of 3.0 mol g-1 h-1 at an industrially relevant current density of 0.5 A cm-2. Furthermore, in situ characterizations evidenced that the Cu-Mo dual sites in Cu1MoOx/NC effectively inhibited hydrodeoxygenation of hydroxyl-containing intermediates of NitRR, selectively producing *NH2OH and thus achieving cyclohexanone oximation with high efficiency. This work provides a high-performance catalyst for CHO electrosynthesis from nitrogenous waste, showing promising application potential in industrial production of CHO.

Interfacial Electronic Interactions Promoted Activation for Nitrate Electroreduction to Ammonia over Ag‐Modified Co3O4

AbstractElectrocatalytic nitrate (NO3−) reduction to ammonia (NRA) offers a promising pathway for ammonia synthesis. The interfacial electronic interactions (IEIs) can regulate the physicochemical capabilities of catalysts in electrochemical applications, while the impact of IEIs on electrocatalytic NRA remains largely unexplored in current literature. In this study, the high‐efficiency electrode Ag‐modified Co3O4 (Ag1.5Co/CC) is prepared for NRA in neutral media, exhibiting an impressive nitrate conversion rate of 96.86 %, ammonia Faradaic efficiency of 96.11 %, and ammonia selectivity of ~100 %. Notably, the intrinsic activity of Ag1.5Co/CC is ~81 times that of Ag nanoparticles (Ag/CC). Multiple characterizations and theoretical computations confirm the presence of IEIs between Ag and Co3O4, which stabilize the CoO6 octahedrons within Co3O4 and significantly promote the adsorption of reactants (NO3−) as well as intermediates (NO2− and NO), while suppressing the Heyrovsky step, thereby improving nitrate electroreduction efficiency. Furthermore, our findings reveal a synergistic effect between different active sites that enables tandem catalysis for NRA: NO3− reduction to NO2− predominantly occurs at Ag sites while NO2− tends to hydrogenate to ammonia at Co sites. This study offers valuable insights for the development of high‐performance NRA electrocatalysts.

Boosting Electroreduction of Nitrate and CO2 to Urea on a Tandem Fe1/MoS2 Catalyst.

Urea electrosynthesis by coelectrolysis of NO3- and CO2 (UENC) holds enormous promise for sustainable urea production, while the efficient UENC process relies on the rational design of high-performance catalysts to facilitate the electrocatalytic C-N coupling efficiency and the hydrogenation reaction process. Herein, Fe single atoms supported on MoS2 (Fe1/MoS2) are developed as a highly effective and robust catalyst for UENC. Theoretical calculations and operando spectroscopic measurements reveal a tandem catalysis mechanism of the Fe1-S3 motif and MoS2-edge to jointly promote the UENC process, where the Fe1-S3 motif drives the early C-N coupling and subsequent *CO2NO2-to-*CO2NH2 step. The generated *CO2NH2 is then migrated from the Fe1-S3 motif to the nearby MoS2-edge, which facilitates the *CO2NH2 → *COOHNH2 step for urea formation. Noticeably, Fe1/MoS2 assembled in a flow cell reaches a maximum urea Faraday efficiency of 54.98% with a corresponding urea yield rate of 18.98 mmol h-1 g-1, performing at the top level among all of the UENC catalysts reported to date.

Tandem Electroreduction of Nitrate to Ammonia Using a Cobalt–Copper Mixed Single‐Atom/Cluster Catalyst with Synergistic Effects

AbstractElectrochemical conversion of waste nitrate (NO3−) to ammonia (NH3) for environmental applications, such as carbon‐neutral energy sources and hydrogen carriers, is a promising alternative to the energy‐intensive Haber–Bosch process. However, increasing the energy efficiency is limited by the high overpotential and selectivity. Herein, a Co─Cu mixed single‐atom/cluster catalyst (Co─Cu SCC) is demonstrated—with well‐dispersed Co and Cu active sites anchored on a carbon support—that delivers high NH3 Faradaic efficiency of 91.2% at low potential (–0.3 V vs. RHE) due to synergism between the heterogenous active sites. Electrochemical analyses reveal that Cu in Co─Cu SCC preferentially catalyzes the NO3−‐to‐NO2− pathway, whereupon Co catalyzes the NO2−‐to‐NH3 pathway. This tandem electroreduction bypasses the rate‐determining steps (RDSs) for Co and Cu to lower the reaction energy barrier and surpass scaling relationship limitations. The electrocatalytic performance is amplified by the subnanoscale catalyst to increase the partial current density of NH3 by 2.3 and 5.4 times compared to those of individual Co, Cu single‐atom/cluster catalysts (Co SCC, Cu SCC), respectively. This Co─Cu SCC is operated stably for 32 h in a long‐term bipolar membrane (BPM)‐based membrane electrode assembly (MEA) system for high‐concentration NH3 synthesis to produce over 1 m NH3 for conversion into high‐purity NH4Cl at 2.1 g day−1.

Efficient and selective electroreduction of nitrate to ammonia via interfacial engineering of B-doped Cu nanoneedles

Electrocatalytic nitrate reduction to ammonia is a promising method to mitigate nitrate contamination and produce valuable chemical. However, it still suffers from slow active hydrogen (*H) transfer kinetics and unfavorable thermodynamics. Here the *H transfer and reaction energy barrier of nitrate reduction reaction were regulated on B-doped Cu nanoneedles (Cu NNs-B) to enhance ammonia electrosynthesis via interfacial engineering. The high-curvature nanoneedles showed locally enhanced electric fields, which promoted *H supply from water dissociation. B-doping provided Cu/Cu+ active sites for the activation of nitrate and intermediates. Due to the simultaneously improved *H supply kinetics and reaction thermodynamics, Cu NNs-B was efficient for reducing nitrate to ammonia, achieving high Faradaic efficiencies (FEs) of 95.1-98.6% and ammonia yields of 0.12-1.33mmol·h−1·cm−2 at 50-1500mg·L−1 NO3−-N. Nitrate was selectively converted to ammonia with the remaining nitrate and nitrite concentrations below drinking water standards. Experimental and DFT results revealed Cu NNs-B with properly higher nanotip curvature was more favorable for boosting ammonia electrosynthesis from both kinetics and thermodynamics.

Spatial-confined effect of CuOx microneedles bundles on TiO2 nanotubes: Reinforcing the adsorption and enrichment of ultralow concentration nitrate for efficient NH3 electrosynthesis

Nitrate (NO3−) Electroreduction to produce NH3 offers an alternative approach to sustainable production of NH3. Yet it remains challenging to simultaneously improve the adsorption and enrichment of low-concentration NO3−. Here, bundles of CuOx microneedles on TiO2 nanotubes (NTs) were developed by a spatial-confined electrodeposition strategy, achieving 87.7 % or 98.4 % NH3 FE at −0.25 V or −0.45 V vs RHE in 2 or 10 mM NO3− electrolytes, respectively. Both in-situ spectroscopy and finite element analysis simulation demonstrated that CuOx microneedle bundles and its interface between TiO2 NTs not only greatly improve the enrichment of low concentration NO3− and its derived-NO2− and the diffusion of NH3 product, but also enhance the adsorption of NO2− and H2O to facilitate the hydrodeoxygenation of NO3− as well as inhibit hydrogen evolution reaction competition. This work will provide a promising avenue to develop electrocatalysts to realize efficient catalytic conversion of reactants at low concentrations.

Insights into the multiple active sites in Bi-Co bimetallic oxide for a deeper understanding of nitrate electroreduction to ammonia

The electrochemical nitrate reduction reaction (NO3RR) is a promising approach for nitrate removal and NH3 production at ambient conditions. The development of various types of efficient electrocatalysts, especially bimetallic oxides with the advantages of superior durability and excellent catalytic activity, have become a research focus. However, the analysis of active sites for bimetallic oxide catalysts either emphasizes the role of bimetallic interactions or focuses on the role of oxygen vacancies (OVs), resulting in a lack of thoroughness. Herein, a Bi-Co bimetallic oxide (BCO) was constructed for NO3RR, illustrating 1.5∼7.8 folds higher NH3 yield rate than those of commercial single metal oxide (Bi2O3, Co3O4). The NO3RR pathway by BCO was explored according to H* capture test, DEMS, and isotope labeling experiment. An in-situ Raman test showed the structure evolution of BCO during NO3RR, and therefore the catalytic sites of BCO were further identified according to physical characteristics before and after NO3RR. Results suggest that Bi0, OVs and amorphous Co oxide as the real active sites play a vital role in nitrate electroreduction. Furthermore, research on 25 articles also indicates the presence of multiple catalytic sites in bimetallic oxides for NO3RR. This study provides a new and comprehensive insight into the active sites identification for the type of bimetallic oxide NO3RR electrocatalysts.

Electroreduction of nitrate to ammonia on graphyne-based single-atom catalysts by combined density functional theory and machine learning study

Nitrate pollution has emerged as a significant global issue, prompting increased interests in electrocatalytic reduction of nitrate to ammonia (NO3RR). This study explores the use of graphyne as the substrate to anchor single transition metal (TM) atoms from the 3d and 4d series on acetylene corners, creating single-atom catalysts (SAC) for the NO3RR reaction. Two catalysts, CrGY and MoGY, were finally identified as promising catalysts for NO3RR with low limiting potential and high selectivity for NH3. To further investigate the origin of catalyst activity, in addition to traditional electronic structrue analysis, machine learning models of the least solution shrinkage and selection operator (LASSO), the sure independence screening and sparsifying operator (SISSO) and the gradient boosting regression (GBR) algorithm are adopt to reveal the intrinsic properties of catalysts that determine the NO3RR performance. After preliminary evaluation of the 13 prepared features with LASSO algorithm, GBR models revealed that the nitrate adsorption free energy ΔGNO3- and the NO bond length are the two most important descriptors for NO3RR performance. A general equation for the relationship between the limiting potential and the characteristics of the catalysts are obtained by SISSO model. Finally, with the assitance of combined density functional calculations and machine learning, this study provides new enlightening insights into practical application of graphyne as SACs for NO3RR.

Nickel-cobalt alloy nanosheet-decorated three-dimensional titanium dioxide nanobelts electrodeposited on titanium meshes for boosting selective nitrate electroreduction to ammonia

Electrocatalytic nitrate reduction reaction presents a promising avenue for environmentally friendly ammonia (NH3) synthesis and wastewater treatment. An essential aspect to consider is the meticulous design of electrocatalysts. This study explores the utilization of a Ni-Co alloy nanosheet-decorated three-dimensional titanium dioxide (3D-TiO2) nanobelts electrodeposited on titanium meshes (NixCoy@TiO2/TM) for efficient electrocatalytic NH3 production. The optimized Ni1Co3@TiO2/TM electrode achieves a significant NH3 yield of 676.3 ± 27.1 umol h−1 cm−2 with an impressive Faradaic efficiency (FE) of 95.1 % ± 2.1 % in a 0.1 M KOH solution containing 0.1 M NO3− at −0.4 V versus the reversible hydrogen electrode. Additionally, the electrode demonstrates exceptional electrochemical activity for NH3 synthesis in simulated wastewater, delivering an outstanding NH3 yield of 751.6 ± 44.3 umol h−1 cm−2 with a FE of 96.8 % ± 0.4 % at the same potential of −0.4 V. Moreover, the electrode exhibits minimal variation in current density, NH3 yields and FEs throughout the 24-h stability test and the 20-cycle test, demonstrating its excellent stability and durability. This study offers a straightforward electrodeposited approach for the development of 3D-nanostructured alloys as catalysts for NH3 electrosynthesis from nitrates at room temperature.

(Invited) Curvature-Enhanced Hydrophobicity: A Counter-Intuitive Strategy for Efficient Electroreduction of Nitrate into Ammonia

The electrochemical upcycling of nitrate (NO3 −) to ammonia (NH3) holds promise for synergizing both wastewater treatment and NH3 synthesis. Efficient stripping of gaseous byproducts (NH3, H2, and N2) from electrocatalysts is crucial for continuous and stable electrochemical reactions. This study evaluated a layered electrocatalyst structure using copper (Cu) dendrite to enable a high curvature and hydrophobicity and achieve a stratified liquid contact at the gas-liquid interface of the electrocatalyst layer. As such, gaseous product desorption or displacement from electrocatalysts was enhanced due to the separation of a wetted reaction zone and a non-wetted zone for gas-transfer. Consequently, this electrocatalyst structure yielded a 1.9-fold boost in per active site activity compared to that with a low curvature and high hydrophilic counterpart. Moreover, a high NH3 Faradaic efficiency of 90.9 ± 2.3% was achieved with nearly 100% NO3 − conversion. This high-curvature hydrophobic Cu dendrite was further integrated with a gas-extraction membrane, which demonstrated outstanding NH3 yield from the real reverse osmosis retentate brine. Figure 1

(Invited) Efficient Recycling of Dilute Nitrate to Ammonia Using Cu-Based Electrocatalyst

The use of nitrogen fertilizers has contributed significantly to the growth of agricultural production, but the nitrogen use efficiency is typically lower than 40%, so most of the nitrogen readily leaches into ground water, rivers, and lakes, causing an increase of nitrate (NO3 −) concentration and severe environmental problems. Electrochemical reduction of nitrate has emerged as a promising route for the recycling of nitrate from wastewater and sustainable production of ammonia when powered by renewable electricity. Here I present our recent studies of the intrinsic activity and selectivity of Cu for nitrate electroreduction, as well as a rational design of Cu-based catalysts for single-pass conversion of nitrate to ammonia. Using polycrystalline Cu foils for benchmarking, we elucidated the impact of often overlooked factors on nitrate reduction, including Cu facet exposure, nitrate concentration, and electrode surface area. We find that an electropolished Cu foil exhibits a higher activity and selectivity for nitrate reduction to ammonia than a wet-etched Cu foil, benefiting from a greater exposure of Cu(100) facets that are more favorable for the reaction. The NH3 selectivity shows no apparent dependence on the nitrate concentration, but it increases monotonically with Cu electrode area, which is attributed to a promoted conversion of intermediately produced NO2 − to NH3 on a larger electrode. Based on the understandings, we further developed a Cu/Co-based tandem electrocatalyst, which achieved nearly 100% single-pass conversion of dilute nitrate (5−50 mM) to NH3 at a low overpotential using a flow cell.