Single-Electrode Tandem Electrocatalysis with Dual TiO₂@Cu and Cu Surfaces Enables Energy-Efficient Ammonia Production from Nitrate

Abstract

Electrochemical ammonia synthesis from nitrate can complement a hydrogen-based energy system with a liquid, shippable energy carrier. Ammonia (NH₃) is attractive because it serves both as an energy vector and as an essential feedstock. However, nitrate reduction is an eight-electron, proton-coupled pathway that forms multiple intermediates; the NO₃ ^– → NO₂ ^– step is widely viewed as rate-determining, followed by nitrite reduction and hydrogenation steps, until the formation of NH₃. Here, we implement a tandem strategy in a flow cell using a double-sided electrode with Cu and TiO₂@Cu surfaces that cooperatively boost the NO₃ ^– → NO₂ ^– and NO₂ ^– → NH₃ conversions. The system achieves a Faradaic efficiency toward NH₃ of 97%, with an NH₃ selectivity of 80%, and a productivity of 7.7 mg h^–1 cm^–2 (0.45 mmol h^–1 cm^–2), corresponding to a full-cell energy efficiency of 29%. This integrated electrode–reactor design expands the accessible performance space for the NO₃RR by combining nitrate adsorption with selective nitrite hydrogenation. These results establish a practical route for the efficient electrochemical synthesis of NH₃ from nitrate.

1. Introduction

The energy transition demands carbon-free carriers that are storable, shippable, and compatible with the existing infrastructure. Ammonia (NH₃) is attractive because it functions both as an energy vector and as a major industrial feedstock. In addition to serving as a hydrogen carrier, NH₃ offers a high volumetric energy density and established organization for storage and transport. Electrochemical dinitrogen reduction (e-N₂RR) could enable sustainable NH₃ synthesis without the energy-intensive Haber–Bosch process, − but practical efficiencies and productivities remain low. Consequently, attention has shifted to the electroreduction of nitrogen oxyanions (NO _x RR), in which nitrate (NO₃ ^–) or nitrite (NO₂ ^–) is reduced to NH₃, offering a pathway that can simultaneously contribute to NH₃ production and remove aqueous pollutants.

Outcomes in the NO _x RR depend strongly on the electrocatalyst, operating conditions, and reactor design, which together govern intermediate binding and the local availability of NO _x ^– species. Diverse catalystsincluding metals, metal oxides, single-atom catalysts, and other heterogeneous materialshave been explored to maximize NO _x toNH₃ conversion. − A key challenge is suppressing competing pathways that generate NO₂ ^–, NO, N₂O, N₂, N₂H₄, and NH₂OH. − Most mechanistic proposals identify NO₂ ^– and NO as semistable intermediates, motivating tandem strategies in which reduction proceeds in two sequential steps under different potentials or on complementary catalytic surfaces to steer selectivity toward NH₃. ,

Because NO₂ ^– mediates the eight-electron pathway, maximizing NH₃ formation requires architectures that pair efficient nitrate activation with selective nitrite conversion under minimal mass transport limitations. In the NO₃ ^–RR to NH₃, the 2e^– + 6e^– cascade directly governs specific energy consumption and thus both half-cell and full-cell energy efficiency (EE). Only a few tandem NO₃RR systems have been reported, typically employing complex bimetallic motifs with dual sites to promote the reaction of NO₃ ^– → NO₂ ^– → NH₃ at low overpotentials. In most cases, Cu facilitates the initial NO₃ ^– → NO₂ ^– step, while metals such as Co or Ru, , which exhibit stronger NO₂RR → NH₃ activity, drive subsequent hydrogenation. Parallel efforts have focused on developing materials to promote H* generation by accelerating water dissociation. This approach favors *NO hydrogenation, further delivering high NH₃ productivities with heterostructures like NiNNi₃/Cu and Ru/Co­(OH)₂-CoP.

Tandem strategies can also be realized through spatially segmented designs. Macroscopically separated regions are simpler to fabricate and can offer improved durability because the performance does not rely on preserving nanoconfined structures over long operations. Such segmentation has proven effective in tandem CO₂ reduction, yet for NO₃ ^–RRespecially in flow-cell reactors, configurations using separated electrodes or deliberately segmented surfaces remain largely unexplored.

Here, we propose a tandem NO₃ ^–RR approach that integrates dual-surface electrodes into a modified flow cell. We select Cu and TiO₂ as complementary active materials: beyond Co and Ru, titanium-based materials show high NH₃ selectivity owing to strong adsorption of the *NO₂ ^– intermediate, , and in TiO₂, nonintrinsic defects such as oxygen vacancies further enhance activity toward both NO₃ ^–RR and NO₂ ^–RR. , As illustrated in Figure , the first surface is bare Cu, and the second comprises TiO₂ nanoparticles supported on Cu, enabling a cascade reduction in which nitrate activation and nitrite hydrogenation occur on distinct faces. We evaluate Faradaic efficiency, product selectivity among N-containing species, and preliminary scale-up metrics for alkaline NO₃ ^–RR. By combining reactor engineering with catalysts of dissimilar intrinsic properties and optimized operating conditions, we achieveto the best of our knowledgethe highest reported cell EE with reduced specific energy consumption for NH₃ generation via NO₃ ^– reduction.

1.

Cascade NO₃ ^– to NH₃ process involving a 2 + 6 electron transfer. First stage (2-electron transfer): NO₃ ^– to NO₂ ^– using a Cu-based active support. Second stage (6-electron transfer): NO₂ ^– to NH₃ was carried out using a TiO₂@Cu catalyst.

2. Experimental Section

2.1. Chemical and Materials

Copper foil (Cu) and sodium carbonate (Na₂SO₄) were obtained from Thermo Scientific Chemicals (Alfa-Aesar). Potassium hydroxide (KOH) and ethanol (C₂H₆O) were purchased from Sigma-Aldrich. Potassium nitrate (KNO₃), potassium nitrite (KNO₂), Nessler’s reagent, ammonium chloride (NH₄Cl), and sodium hydrogen carbonate (NaHCO₃) were supplied by Merck. A titanium foil target (1 mm thick foil, 99.6% purity) was purchased from MaTecK Material-Technologie & Kristalle GmbH.

2.2. Synthesis of TiO₂ Nanoparticles

Titania nanoparticles (TiO₂ NPs) were fabricated by pulsed laser ablation in liquid (PLAL method), as it offers catalytic enhancement due to the formation of defect-rich NPs. , A titanium target was ablated using a 1064 nm neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with a pulse duration of 10 ps, a repetition rate of 1000 kHz, and a laser power of 43 W. The laser was coupled to a galvanometer scanner (scanning speed 20 m/s) and an f-theta lens (focal length 167 mm) to steer and focus the laser beam into an Archimedean spiral pattern. The PLAL process was performed in a flow chamber − with ethanol as the liquid carrier, selected for its role as a capping agent to hinder the nanoparticle growth (size quenching). The resulting colloidal dispersion of TiO₂ NPs in ethanol was partially evaporated until it achieved a concentration of 0.125 mg mL^–1.

2.3. TiO₂@Cu Electrode Preparation

Copper (Cu) supports were pretreated to remove surface oxides. Cu foil was sonicated for 15 min in a 1:1:1 mixture of acetone, isopropanol, and ethanol, followed by immersion in an acidic solution (10% H₂SO₄ + 36 g L^–1 C₆H₈O₇) under sonication for 5 min. After thorough rinsing with Milli-Q water for 10 min, the catalyst ink was prepared using the TiO₂ dispersion described in Section . The dispersion was diluted to 0.0625 mg mL^–1 with anhydrous ethanol, and 2 wt % Nafion perfluorinated solution was added as a binder. The final ink consisted of 0.50 mL of TiO₂ dispersion (0.125 mg mL^–1), 0.48 mL of ethanol, and 0.02 mL of binder solution. This ink was drop-cast onto a pretreated Cu substrate (1 cm² area) and dried in a vacuum oven for 3 h. The resulting electrodes contained 0.0625 mg of cm^–2 TiO₂. The fabrication process is illustrated in Figure S1.

2.4. Electrode Characterization

The morphology of TiO₂ nanoparticles and the Cu surface was examined by using a field emission scanning electron microscope (Zeiss Auriga series). Elemental composition was determined by energy-dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advanced diffractometer using Cu Kα radiation (λ = 1.5418 Å) to analyze the crystallographic structures. The crystal structure of the nanoparticles was analyzed by means of high-resolution transmission electron microscopy (HRTEM) by using a FEI Tecnai F20 TEM with a field emission gun. The TiO₂ structure was also analyzed by Raman spectroscopy with an iHR320 spectrometer from HORIBA Scientific with a green laser (λ = 532 nm). X-ray photoemission spectroscopy (XPS) was employed to prove the chemical and electronic states of the electrode components, using a SPECS system equipped with an XR50 source operating at 300 W and a Phoibos 150 MCD-9 detector with an Al Kα source. The pass energy of the hemispherical analyzer was set at 20 eV, and the energy step of the high-resolution spectra was 0.1 eV.

Continuous wave electron paramagnetic resonance (EPR) spectra of TiO₂ were acquired on a Bruker EMX Micro X-band spectrometer operating at 9.85 GHz using a Bruker ER 1164 HS resonator. Individual capillaries were filled with the samples and were placed into EPR tubes at the same position of the resonant cavity for EPR spectral acquisition. The spectral data were collected at room temperature with the following spectrometer settings: microwave power = 0.6479 mW; center field = 3200 G, sweep width = 6000 G, sweep time = 60 s, modulation frequency = 100 kHz, modulation amplitude = 4 G, power attenuation = 25 dB, and time constant = 0.01 ms.

2.5. Electrochemical Measurements

All electrochemical experiments were performed at room temperature using a BioLogic electrochemical workstation. Three-electrode cell tests with pristine and TiO₂-loaded Cu disk electrodes were conducted for NO₃ ^–RR and NO₂ ^–RR studies in 0.1 M KNO₃ + 1 M KOH, 0.1 M KNO₂ + 1 M KOH, and 1 M KOH electrolytes. A Hg/HgO electrode served as the reference electrode and a Pt mesh as the counter electrode (CE). Prior to each experiment, the electrolyte was purged with Ar for 20 min. Linear sweep voltammetry (LSV) was conducted at a scan rate of 20 mV s^–1.

A flow-cell configuration was employed for the chronoamperometry and chronopotentiometry tests. Nafion-117 membrane separates the cathode and anode compartments. To capture NH₃ evolved during electrolysis, a gas absorption chamber filled with 0.01 M H₂SO₄ was connected to both catholyte and anolyte bottles. Ar was circulated through the headspace of the two bottles. Total NH₃ content (catholyte, anolyte, and trap) was used for the yield calculations. The flow-cell configurations are shown in Figure S2.

Pristine Cu and TiO₂@Cu electrodes were used as working electrodes (WEs), while leak-free Ag/AgCl and a commercial dimensionally stable anode (DSA) were employed as a reference electrode (RE) and CE, respectively.

Chronoamperometry and chronopotentiometry tests were performed for the NO₃ ^–RR and the NO₂ ^–RR under various conditions (E _W = −0.4 to −0.8 V vs RHE; j = 35 to 110 mA cm^–2). Faradaic efficiencies (FE_i) toward NH₃, NO₂ ^–, and H₂ and selectivities (SE_i) toward NH₃ and NO₂ ^–, and NH₃ yield (productivity) were evaluated as direct efficiency parameters. Full cell EE (EE_cell), half-cell EE (EE_half‑cell), and energy consumption per mass of NH₃ or NO₃ ^– ( $\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{H}}_3}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ respectively) were also evaluated. Equations – define all metrics −

$$\mathrm{F}{\mathrm{E}}_{\mathrm{i}}(\%)=\frac{\alpha ·C_{\mathrm{i}}·F}{M·Q}·100$$ 1

$$\mathrm{N}{\mathrm{H}}_3\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}=\frac{C_{\mathrm{N}{\mathrm{H}}_3}·V·M}{A·t}$$ 2

$$\mathrm{S}{\mathrm{E}}_{\mathrm{i}}(\%)=\frac{C_{\mathrm{i}}}{C_{\mathrm{N}{\mathrm{O}}_{3_0}^{-}}·C_{\mathrm{N}{\mathrm{O}}_{3_t}^{-}}}·100$$ 3

$$\mathrm{E}{\mathrm{E}}_{\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}}(\%)=\frac{|\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{H}}_3}|}{|{\mathrm{E}}_{\mathrm{C}\mathrm{E}\mathrm{L}\mathrm{L}}·Q|}·100$$ 4

$$\mathrm{E}{\mathrm{E}}_{\mathrm{h}\mathrm{a}\mathrm{l}\mathrm{f}‐\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}}(\%)=\frac{\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{H}}_3}}{(0.4-E_{\mathrm{w}})·Q}·100$$ 5

$$\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{H}}_3}=[\mathrm{\Delta }{G}_{\mathrm{N}{\mathrm{H}}_3}^0+R·T·\ln \left(\frac{{C_{\mathrm{N}{\mathrm{H}}_3}}_t·{C_{\mathrm{O}{\mathrm{H}}^{-}}}_t^9}{{C_{\mathrm{N}{\mathrm{O}}_x^{-}}}_t}\right)]·{C_{\mathrm{N}{\mathrm{H}}_3}}_t·V$$ 6

$$\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{O}}_3^{-}}=\frac{|E_{\mathrm{C}\mathrm{E}\mathrm{L}\mathrm{L}}Q|}{m_{\mathrm{N}{\mathrm{O}}_3^{-}}}$$ 7

where C _i (M) is the molar concentration of a target product or evaluated reactant; V (L) is the volume of the electrolyte; M (g·mol^–1) is the molar mass of the target product or reactant; A (cm²) is the geometric surface area of the electrode; α is the number of transferred electrons to produce a defined product; F is the Faradaic constant expressed in SI units; Q is the total transferred charge to the system; C _NH3 (M) is the measured ammonia concentration at the end of NO₃ ^–RR and NO₂ ^–RR; t (h) is the electrolysis time; $C_{\mathrm{N}{\mathrm{O}}_{3_0}^{-}}$ and $C_{\mathrm{N}{\mathrm{O}}_{3_t}^{-}}$ (M) represent the initial and final concentration of NO₃ ^– ion in the electrolyte. $\mathrm{\Delta }{G}_{\mathrm{N}\mathrm{H}3}^0$ (J or kWh) is the standard Gibbs free energy to form NH₃ from NO₃ ^– or NO₂ ^–; E _cell (V) is the applied cell potential to carry out the reaction; E _W (V) represents the applied potential; m _NH3 (kg) is the mass of NH₃ produced during electrolysis, and $m_{\mathrm{N}{\mathrm{O}}_3^{-}}$ (kg) is the mass of NO₃ ^– consumed during the electrolysis.

2.6. Analytical Instrumentation and Measurements

Hydrogen (H₂) was analyzed by using a gas chromatography system (GC, Agilent Technologies 490 Micro-GC). Nitrate and nitrite concentrations were determined by ion chromatography (DIONEX 1100) with a Dionex Ion Pack AS-22 anion exchange column and a chemical suppressor (ASR-ultra 4 mm). The eluent (4.5 mM Na₂CO₃ + 1.4 mM NaHCO₃) was delivered at a flow rate of 1.5 mL min^–1. Ammonia (NH₃/NH₄ ⁺) was quantified by UV–vis spectrophotometry (PerkinElmer Lambda 950) after complexation of NH₄ ⁺ ions with Nessler reagent. Calibration curves are shown in Figure S3.

3. Results and Discussion

3.1. TiO₂@Cu Characterization

Figure a shows the XRD patterns of the Cu substrate and the TiO₂@Cu electrodes. For the Cu substrate, four characteristic peaks of face-centered cubic Cu (JCPDS 04-0836) are observed: two intense peaks at 2θ = 43.3° and 73.99°, corresponding to the (111) and (220) planes, and two less pronounced peaks at 50.4° and 90.2°, assigned to the (200) and (300) planes. The TiO₂@Cu electrode exhibits the same Cu-related peaks, indicating minimal changes in the Cu-based component. No diffraction peaks associated with Ti-containing phases are detected, which is consistent with the small size of the TiO₂ nanoparticles. Additional structural characterization by Raman spectroscopy is shown in Figure b. Five distinct Raman active modes characteristic of anatase TiO₂ were identified at frequencies of 143, 193, 392, 510, and 639 cm^–1, corresponding to the symmetries of Eg, Eg, B1g, A1g, and Eg, respectively. These vibrational frequencies and their relative intensities confirm the anatase phase of TiO₂.

2.

(a) XRD patterns for the Cu and TiO₂@Cu electrodes; (b) Raman spectrum of TiO₂ nanoparticles with the corresponding anatase Raman shifts.

HR-TEM analysis of the TiO₂ nanoparticles is included in Figure a–e. The particles display a range of particle sizes, predominantly a few nanometers in diameter, and crystallize in the anatase tetragonal phase of TiO₂, which has also been confirmed by the d-spacing measured in Figure d, corresponding to the (101) and (105)/(112) lattice planes of anatase.

3.

TEM analyses of TiO₂ nanoparticles: (a,c) HRTEM general views of the nanoparticles. (d) HRTEM magnified detail of the squared region in (a); (e) corresponding indexed power spectra of the magnified detail in (d). (f,g) SEM general view images of the TiO₂@Cu electrode.

SEM images of the TiO₂@Cu electrode are presented in Figures f,g, revealing aggregated porous layers on the electrode surface both before (Figure f) and after (Figure g) electrolysis. EDX analysis shows an estimated atomic composition of 4.3% Ti, 35.2% Cu, and 20.8% O, along with notable amounts of C and F originating from the Nafion ionomer. Elemental mapping (Figure S5) confirms the uniform distribution of these elements, with O, F, and C exhibiting similar spatial distribution patterns to Ti.

The presence of oxygen vacancies in TiO₂ was determined by XPS and EPR analyses. Figure presents the XPS spectra of the main components of the TiO₂@Cu electrodes before and after electrolysis. In the high-resolution Cu 2p spectrum (Figure a), peaks at 933.2 and 955.9 eV correspond to the 2p_3/2 and 2p_1/2 sublevels of Cu⁰/Cu⁺. Additional satellite peaks at 943.2 and 964.2 eV, typically associated with surface Cu²⁺, are also observed. ,, After electrolysis (Figure b), the intensity of these satellite peaks decreases, indicating a partial electrochemical reduction of surface Cu²⁺. Both Cu₂O and CuO have been reported to exhibit electrocatalytic activity for NO₃ ^–-to-NH₃ conversion. , Postelectrolysis analysis in Figure S5 shows no evidence of bulk Cu oxidation in the TiO₂@Cu electrode according to the XRD results.

4.

XPS analysis of the TiO₂@Cu electrode: (a) before and (b) after electrolysis at the Cu 2p level; (c) before and (d) after electrolysis at the Ti 2p level.

Figure c,d displays the high-resolution Ti 2p spectra before and after electrolysis, showing four distinct peaks assigned to titanium dioxide (Ti⁴⁺) and titanium subdioxide (Ti³⁺) in the 2p_3/2 and 2p_1/2 sublevels, respectively. Specifically, Ti⁴⁺ 2p_1/2 appears at 464.4 eV, Ti⁴⁺ 2p_3/2 at 458 eV, Ti³⁺ 2p_1/2 at 461.8 eV, and Ti³⁺ 2p_3/2 at 460.2 eV. The separation of 5.75 eV between each pair of 2p_3/2 and 2p_1/2 peaks matches the standard binding energy difference for TiO₂. The detection of Ti³⁺ peaks confirms the presence of oxygen vacancies on the surface, further supported by the EPR spectrum shown in Figure S6. A distinct resonance signal is observed with a g-value of 2.0018, which can be attributed to Ti³⁺ centers formed as a result of oxygen deficiency. These Ti³⁺ species arise when oxygen vacancies trap electrons, leading to the reduction of neighboring Ti⁴⁺ to Ti³⁺. The obtained g-value is in agreement with the reported values for oxygen-deficient TiO₂, confirming that the nanoparticles contain oxygen-related defects. ,

The preliminary electrochemical characterization of TiO₂@Cu for the NO₃ ^–RR and NO₂ ^–RR was conducted in a three-electrode cell. TiO₂ nanoparticles were deposited onto a polished copper disk, and LSV was performed in three electrolytes: 0.1 M KNO₃ + 1 M KOH for the NO₃ ^–RR, 0.1 M KNO₂ + 1 M KOH for the NO₂ ^–RR, and 1 M KOH as a blank. Four distinct peaks (P₁–P₄) are identified in Figure : P₁ between 0 and 0.1 V vs RHE, P₂ between −0.2 and −0.1 V vs RHE, P₃ between −0.4 and −0.35 V vs RHE, and P₄ between −0.55 and −0.5 V vs RHE. Peak P₁ exclusively appeared during NO₃ ^–RR and corresponds to the RDS, a two-electron transfer reaction associated with the conversion of NO₃ ^– to NO₂ ^–. Peak P₂, observed for both NO₃ ^–RR and NO₂ ^–RR, is assigned to a four-electron transfer reducing NO₂ ^– to hydroxylamine (NH₂OH), which can subsequently be reduced to NH₃. Peak P₃, also present in both reactions, corresponds to the six-electron reduction of NO₂ ^– to NH₃, in agreement with previous reports. , No discernible peaks were detected in the blank electrolyte, confirming that these electrochemical features arise specifically from the NO₃ ^– and NO₂ ^– species.

5.

Preliminary electrochemical characterization of TiO₂ deposited on a Cu disk electrode in a three-electrode cell. Linear scan voltammetries under different electrolyte compositions.

During NO₃ ^–RR, the current densities for P₂ and P₃ were higher than in NO₂ ^–RR. This can be attributed to the elevated interfacial concentration of NO₂ ^– generated in situ during the RDS, which shows limited desorption from the Cu-based electrodes, thereby reducing diffusion limitations. In contrast, during the NO₂ ^–RR, NO₂ ^– ions must diffuse from the bulk electrolyte to the electrode surface.

The increased HER overpotential observed for NO₃ ^–RR aligns with mechanistic insights from previous studies. , These studies show that the NO₃ ^–RR involves sequential deoxygenation and hydrogenation steps mediated by adsorbed hydrogen (*H). The availability of *H depends on NO₃ ^– concentration, since all nitrogen intermediates formed from NO₃ ^– react with *H until NH₃ is produced. At lower overpotentials (P1), the sluggish generation rate of *H favors the accumulation of non-hydrogenated intermediates such as NO₂ ^–, regardless of $C_{\mathrm{N}{\mathrm{O}}_3^{-}}$ . At higher overpotentials (P₃ or P₄), the rapid generation of *H promotes further hydrogenation of nitrogen intermediates, particularly at high $C_{\mathrm{N}{\mathrm{O}}_3^{-}}$ . As a result, the *H balance is maintained, and competitive HER is suppressed.

3.2. Catalytic Performance of Individual Electrodes

The flow cell (Figure S2a) was assembled using 1 cm² TiO₂@Cu or pristine Cu as WE, a DSA serving as CE, and an Ag/AgCl RE. To compare the individual performance of bare Cu and TiO₂@Cu for NO₃ ^–RR and NO₂ ^–RR, chronoamperometry tests were performed for 2 h at each potential in 0.1 M KNO₃ + 1 M KOH (NO₃ ^–RR) and 0.1 M KNO₂ + 1 M KOH (NO₂ ^–RR). For NO₃ ^–RR, only FE toward NO₂ ^– and the specific nitrate conversion per unit charge ( $\mathrm{\Delta }{C}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ ) were determined.

The NO₃ ^–RR results (Figure a) show that both Cu and TiO₂@Cu electrodes display decreasing ${\mathrm{F}\mathrm{E}}_{\mathrm{N}{\mathrm{O}}_2^{-}}$ values at more negative potentials, with Cu consistently achieving higher values. The maximum FE of 65% was obtained at −0.4 V vs RHE for Cu, decreasing to below 10% at potentials more negative than −0.7 V vs RHE. Previous studies have linked high overpotentials to enhanced HER, particularly on materials with weak hydrogen adsorption, which limits NO₃ ^–RR. At lower overpotentials, cleavage of N–O bonds (deoxygenation) in adsorbed NO₂ ^– becomes rate-limiting due to insufficient H_ads, leading to NO₂ ^– release and accumulation in the bulk electrolyte. , In terms of NO₃ ^– conversion, Cu again outperformed TiO₂@Cu, reaching $\mathrm{\Delta }{C}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ = 0.6 $\mathrm{m}{\mathrm{g}}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ C^1– at −0.4 V vs RHE. TiO₂@Cu followed similar trends of decreasing specific conversion and ${\mathrm{F}\mathrm{E}}_{\mathrm{N}{\mathrm{O}}_2^{-}}$ at higher overpotentials but with lower values across all conditions.

6.

Electrocatalytic activity of the single-side TiO₂@Cu and Cu electrodes: (a) specific NO₃ ^– conversion and Faradaic efficiency to NO₂ ^– during NO₃ ^–RR-to-NO₂ ^– tests; (b) Faradaic efficiency to NH₃; and (c) EE_half‑cell and NH₃ productivity in NO₂ ^–RR-to-NH₃ tests.

For NO₂ ^–RR to NH₃ (Figure b,c), TiO₂@Cu achieved FE_NH3 of 99% at −0.4 V and −0.5 V vs RHE. As the potential became more cathodic, the values gradually decreased and stabilized around 90%. In contrast, Cu exhibited the opposite trend, with FE_NH3 increasing from 80% at −0.4 V to 90% at −0.8 V vs RHE. The incorporation of TiO₂ into Cu thus lowers the overpotential, enabling high FE_NH3 under relatively mild cathodic conditions. In alkaline media, *H species originate from water dissociation, making this process a critical step in subsequent hydrogenation reactions. Computational studies have extensively investigated Cu, TiO₂, and bimetallic surfaces in this context. On Cu surfaces, water dissociation generally exhibits moderate barriers, strongly influenced by the degree of water coverage. Accordingly, surface properties such as oxophilicity have been identified as key parameters for enhancing water binding and promoting water dissociation. ,, In contrast, studies on rutile- and anatase–water interfaces indicate a certain probability of spontaneous dissociative water adsorption, particularly on defective surfaces. , Consequently, the lower FE observed for Cu at less negative potentials can be attributed to the decreased hydrogen adsorption, resulting in low hydrogen coverage and a slower *NH _x hydrogenation rate.

Across the entire potential range, TiO₂@Cu also delivered higher EE_half‑cell values than Cu, starting at 38% at −0.4 V vs RHE and gradually decreasing to 28% at −0.8 V vs RHE (calculation details are given in Supporting Information). NH₃ productivity for the NO₂ ^–RR (Figure c) increased with potential, reaching 0.45 mmol h^–1 cm^–2 for TiO₂@Cu and 0.21 mmol h^–1 cm^–2 for Cu under optimal conditions. These results demonstrate the superior NO₂ ^–RR electrocatalytic performance of TiO₂@Cu over bare Cu at low overpotentials.

3.3. Cell Configuration: Tandem Approach

Building on the previous results where Cu outperformed TiO₂@Cu for the initial NO₃ ^–RR to NO₂ ^–, while TiO₂@Cu was more efficient for the NO₂ ^–RR to NH₃ (Figure a), we propose a tandem electrocatalytic system designed to cascade the initial accumulation of NO₂ ^– intermediates into their final conversion to NH₃ at lower overpotentials. For this purpose, the electrochemical cell was adapted to expose two faces (each with a 1 cm² geometric area). This configuration leverages the individual strengths of each catalyst while mitigating their respective limitations. Surface 1 consisted of bare Cu, selected for its superior activity in the RDS of NO₃ ^– reduction. In this role, Cu facilitates the generation and desorption of NO₂ ^– as the first intermediate in the cascade pathway. This partial reduction of NO₃ ^– to NO₂ ^– sets the stage for the subsequent conversion to NH₃. Surface 2 consisted of TiO₂@Cu, which was responsible for driving the NO₂ ^–RR (and NO₃ ^–RR) toward NH₃ formation. This second stage is optimized for lower overpotentials, with the added benefit of suppressing the HER.

The cathode arrangement and cell layout are illustrated in Figure . By pairing Cu and TiO₂@Cu in this tandem configuration, the system aims to maximize the NO₃ ^– and NO₂ ^– conversions at lower overpotentials, thereby improving the overall EE. The performance of this setup was evaluated by chronoamperometry tests across a range of applied potentials in a flow-cell configuration. In all tests, both electrolytes consisted of 0.1 M KNO₃ + 1 M KOH aqueous solutions.

7.

Three-channel electrochemical flow cell adapted with a current collector that exposes two sides of the WE.

Figure a shows the FE distribution for the tandem configuration over the applied potential range. Across all potentials, NH₃ was the predominant product. A volcano-shaped trend was observed, with the maximum FE_NH3 (91%) at −0.6 V vs RHE, and values remaining above 85% at −0.4 V, −0.5 V, and −0.7 V vs RHE. In contrast, ${\mathrm{F}\mathrm{E}}_{\mathrm{N}{\mathrm{O}}_2^{-}}$ decreased progressively with increasing overpotential, dropping from 13% at −0.4 V to 3% at −0.8 V vs RHE. The remaining FE toward other products aligned with HER trends as determined by GC analysis. Importantly, FE toward H₂ was significantly suppressed by the high bulk concentrations of NO₃ ^– and K⁺ ions, as well as by the presence of NO₂ ^– intermediates, whose faster reduction kinetics outcompete H₂O reduction to H₂. , Previous studies have also reported more favorable *NO₂ hydrogenation in the presence of O.V., which can effectively inhibit the formation of undesired nitrogenated byproducts. These findings are consistent with the selectivity trends in Figure b: peak selectivity toward NH₃ was achieved at −0.4 V vs RHE (86%), followed by a gradual decline at more cathodic potentials, although it remained above 80% across the studied potential range. Conversely, the selectivity toward NO₂ ^– decreased steadily, reaching its maximum of 14% at −0.5 V vs RHE.

8.

NO₃ ^–RR to NH₃ for the tandem configuration: (a) FE distribution of products, (b) SE of products, and (c) EE_half‑cell and NH₃ productivity as a function of E _W.

Figure c presents the corresponding EE_half‑cell values. These closely follow the FE_NH3 tendency, exceeding 35% at −0.5 V and −0.6 V vs RHE, being among the highest half-cell energy efficiencies reported. , At higher overpotentials, EE_half‑cell declined, likely due to the increasing contribution of the HER to the overall energy consumption. The NH₃ yield increased with applied potential, approaching a plateau at −0.8 V vs RHE, where it reached 0.45 mmol h^–1 cm^–2. Although higher yields were attainable at more negative potentials, the volcano-shaped FE_NH3 indicates a reduced conversion efficiency of NO₃ ^– to NH₃ under these conditions.

3.4. Energy AnalysisIncreased Cell EE

To further assess the advantages of the tandem configuration, comparative tests were performed using single- and double-sided TiO₂@Cu cathodes (1 and 2 cm² geometric areas, respectively). Figure a presents the estimated EE_cell values, which exhibited a characteristic volcano-shaped trend for all three configurations. The tandem system delivered the highest efficiencies, reaching 28.7 and 28.5% at −0.5 V and −0.6 V vs RHE, respectively, highlighting the efficient utilization of supplied energy for conversion of NO₃ ^– to NH₃. At higher cathodic potentials, EE_cell gradually decreased, reaching a minimum of 23% at −0.8 V vs RHE, due to the growing influence of HER and other side reactions. Both 1 cm² and 2 cm² TiO₂@Cu electrodes exhibited lower EE_cell values, with maxima centered at −0.5 V vs RHE. The slightly higher EE_cell observed for the 1 cm² TiO₂@Cu compared to the 2 cm² version can be attributed to the cell disposition (Figure ), in which the reference electrode faces only surface 2.

9.

(a) EE_cell as a function of E _W; (b) variation of EC with respect to EE_cell, and (c) NH₃ productivity as a function of EC per kg of NH₃ produced.

Figure b shows the relationship between EC_NH3 and EE_cell for the three configurations. EC_NH3 , expressed in kWh kg^–1, quantifies the energy required to produce 1 kg of NH₃. For the 1 cm² and 2 cm² TiO₂@Cu electrodes, EC_NH3 ranged from 43 kWh kg^–1 at the lowest EE_cell value (16%) to 30 kWh kg^–1 at the highest (20%), with intermediate data points overlapping (red and blue regions). The tandem configuration achieved EC_NH3 values between 29 kWh kg^–1 (at EE_cell = 23%) and 23 kWh kg^–1 (at EE_cell = 28%), demonstrating its superior energy performance.

For context, the minimum theoretical energy requirement for NH₃ production, given by its lower heating value, is 5.92 kWh kg^–1, while the minimum required for the Haber-Bosch process ranges from 16.7 to 8.8 kWh kg^–1, depending on the technology used. The tandem system, therefore, represents an energetically favorable approach for the electrochemical NO₃ ^– to NH₃ conversion. This improvement is attributed to the lower activation energy for the NO₃ ^– → NO₂ ^– step on the Cu surface, which provides kinetic benefits and ultimately enhances the overall NO₃ ^–RR to NH₃ efficiency.

The decrease in EC_NH3 directly impacts the energy requirements for achieving higher productivities. As shown in Figure c, the tandem system consistently attains higher NH₃ productivities at lower EC_NH3 values compared to 1 cm² and 2 cm² TiO₂@Cu cathodes. For example, to reach a productivity of ∼0.1 mmol h^–1 cm^–2, the tandem configuration requires only 22.10 kWh kg^–1, whereas the single- and double-sided TiO₂@Cu electrodes demand 30–32 kWh kg^–1. Increasing the working potential from −0.4 to −0.7 V vs RHE enhanced the tandem productivity by ∼ 3.5x (up to 0.38 mmol h^–1 cm^–2) while raising EC_NH3 by only 2.5 kWh kg^–1, consistent with the EE_cell vs E _W trends. However, further increasing productivity to 0.44 mmol h^–1 cm^–2 (at −0.8 V vs RHE) required an EC_NH3 of 29 kWh kg^–1, reflecting the drop in EE decrease at high overpotentials due to HER and other competing reactions. By contrast, the 1 cm² and 2 cm² TiO₂@Cu configurations require 38–43 kWh kg^–1 to 0.41 mmol h^–1 cm^–2, representing over a 40% increase in energy consumption with respect to the tandem approach.

3.5. Scaling-Up ParametersGalvanostatic Conditions

Figure summarizes the performance of the tandem configuration under galvanostatic operation at constant current densities from 35 mA to 110 mA cm^–2. The FE_NH3 exhibits a clear volcano-shaped trend, peaking at 97% at 90 mA cm^–2 and maintaining values above 80% across all tested conditions. The corresponding potentials at the highest FE (−0.7 and −0.8 V vs RHE for 70 and 90 mA cm^–2, respectively) differ slightly from potentiostatic tests (Figure S5b), where the maximum FE was observed at −0.6 V vs RHE. The highest FE toward NO₂ ^– occurred at the lowest current density.

10.

(a) FE distribution, (b) NH₃ productivities and specific NO₃ ^– conversion, and (c) energy consumption per kg of NH₃ at different current densities with the tandem approach. The numbers next to the points in c correspond to the FE_NH3 .

NH₃ productivity increased linearly with current density, reaching 0.43 mmol h^–1 cm^–2 at 110 mA cm^–2, representing the apparent performance limit under these conditions. This limit likely arises from a transient depletion of available NO₃ ^–, as up to 45% of the initial NO₃ ^– was continuously converted (Figure S7a). Indeed, the specific NO₃ ^– conversion per unit charge decreased with current density, from 0.15 to 0.10 mg C^1–, consistent with the lower limits observed in our previous Cu/Ti-based NH₃ generation system.

Energy consumption for producing 1 kg of NH₃ (EC_NH3 , kWh kg^–1) was also evaluated for three cathode configurations: tandem, two-side Cu, and two-side TiO₂@Cu (Figure c). For the single-material electrodes, the minimum energy consumption occurred at 50 mA cm^–2, coinciding with their maximum FE. In contrast, the tandem system achieved its lowest energy requirement at 90 mA cm^–2, where it also delivered 97% FE to NH₃. Notably, the pristine Cu electrode exhibited low EC_NH3 at the low current densities but suffered from poor NH₃ selectivity, whereas TiO₂@Cu consumed more energy but provided a higher selectivity than Cu.

Besides the demonstration of the improved EE achieved with the tandem configuration, additional cycling experiments were performed to evaluate the stability of both the electrode and the overall system. The tests were conducted under identical experimental conditions, with a constant current density of 90 mA cm^–2. Individual cycles were analyzed, and the electrolyte was replaced between cycles. As shown in Figure S8, stable cell voltages between 2.1 and 1.9 V were observed, together with a consistent FE of 92–95% and NH₃ production rates of approximately 0.4 mmol h^–1 cm^–2.

Figure shows $\mathrm{\Delta }{C}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ as a function of $\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ , offering a complementary figure of merit that quantifies the energy required to convert 1 kg of NO₃ ^–. This parameter is particularly valuable for estimating and comparing the operation costs of energy-intensive water treatment systems. Here, it is applied to benchmark the tandem configuration against various catalysts reported for electrochemical denitrification. Under potentiostatic conditions, the tandem system achieved an $\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ as low as 2.89 kWh kg^–1 cm^–2 and 4.95 kWh kg^–1 cm^–2 under galvanostatic operation, representing the lowest reported values among recent studies. By contrast, other Cu-based catalysts, such as Cu/Ni or Cu/Zn, exhibited higher specific NO₃ ^– conversions but at substantially greater energy costs. It should be noted that some literature values correspond to systems operating under different conditions (e.g., lower initial NO₃ ^– concentrations), which contributes to the widespread of $\mathrm{E}{\mathrm{C}}_{\mathrm{N}{\mathrm{O}}_3^{-}}$ values shown in the figure.

11.

Specific NO₃ ^– conversion by unit charge as a function of the energy consumption to convert 1 kg of NO₃ ^– for different electrode materials used in NO₃ ^–RR, and a comparative with other reported values in literature.

Overall, the outstanding performance of the tandem configuration highlights its potential for energy-efficient denitrification, either for complete NO₃ ^– removal from water or for NH₃ as a production as the main product. These results provide preliminary scaling parameters that indicate significantly reduced energy requirements compared to conventional approaches. Future mechanistic studies aimed at elucidating the role of each individual surface and the influence of operating conditions could further guide the optimization of the electrode architecture.

4. Conclusion

In summary, we demonstrate a cascade electrolysis strategy for the conversion of nitrate to ammonia in alkaline media, employing a tandem system with two distinct active surfaces, Cu and TiO₂@Cu, operating under identical polarization conditions. This configuration yields markedly enhanced energy efficiencies for ammonia production, with energy consumption reduced by more than 32% compared to single-surface TiO₂@Cu electrodes, particularly at moderate applied potentials as low as −0.7 V. Mechanistic investigations reveal that Cu preferentially accelerates the rate-determining step nitrate-to-nitrite conversion, while TiO₂@Cu excels in nitrite-to-ammonia hydrogenation, thereby reducing the overall overpotential for global NO₃ ^–RR. Energy consumption analysis further highlights the competitiveness of the tandem cascade approach, offering lower energy requirements than comparable systems.

Overall, this work establishes cascade cell configurations as an effective platform for tandem catalysis of reactive intermediates across spatially distinct active sites, significantly boosting the EE. Beyond advancing sustainable ammonia electrosynthesis, our findings exemplify how rational electrode and cell design can overcome intrinsic scaling relations in electrocatalysis and inspire innovative pathways for other multielectron reactions.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssuschemeng.5c14004.

• Additional experimental details on electrode preparation, cell configuration, analytical results, and efficiency calculations, additional results: EDX analysis, EPR spectroscopy, and XRD analysis, and additional results of galvanostatic and cycling tests (PDF)

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M.E. Chávez: The Institute of Photonic Sciences (ICFO), Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain. Email: marcelo.chavez@icfo.eu

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S. Murcia-López: Catalan Institute of Nanoscience and Nanotechnology (ICN2), Edifici ICN2, UAB Campus, 08193 Bellaterra (Barcelona), Spain. Email: sebastian.murcia@icn2.cat.

M.E. Chávez: Conceptualization, formal analysis, investigation, methodology, and writingoriginal draft. I.Y. Khairani: Investigation, methodology, and writingreview and editing. M. Biset-Peiró: Data curation, investigation, methodology, resources, and writingreview and editing. S. Martí-Sánchez: Investigation and methodology. K. Villa: Investigation, supervision, and writingreview and editing. J. Arbiol: Investigation, supervision, and writingreview and editing. J.R. Morante: Funding acquisition, supervision, and writingreview and editing. B. Gökce: Funding acquisition, project administration, supervision, and writingreview and editing. S. Murcia-López: Conceptualization, funding acquisition, methodology, supervision, and writingreview and editing.

The authors declare no competing financial interest.