An Electrochemical-Chemical Cascading Strategy for the Efficient Conversions of CO 2 and Nitrate Pollutants to NaHCO 3 and NH 4 Cl Chemicals

Open AccessCCS ChemistryRESEARCH ARTICLES19 Aug 2025An Electrochemical-Chemical Cascading Strategy for the Efficient Conversions of CO2 and Nitrate Pollutants to NaHCO3 and NH4Cl Chemicals Heng Xu†, Xuanyi Wang†, Xue Teng, Shaozhen Liang, Lisong Chen and Jianlin Shi Heng Xu† Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Xuanyi Wang† Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Xue Teng Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Shaozhen Liang Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 , Lisong Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Shanghai Key Laboratory of Green Chemistry and Chemical Processes, State Key Laboratory of Petroleum Molecular & Process Engineering, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062 Institute of Eco-Chongming, Shanghai 202162 and Jianlin Shi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of High-Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 https://doi.org/10.31635/ccschem.025.202506072 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookXBlueskyLinked InEmail Nitrogen and carbon are the main elements of industrial pollutants threatening the health of human beings. Electrocatalytic nitrate reduction reaction (NO3RR) has been widely recognized as a promising technology for nitrate-containing water purification and value-added NH3 chemical production. However, the by-products of NO3RR, OH−, and certain cations remain in the solution as contaminants after NO3RR. Here we report an electrochemical-chemical cascading strategy for the efficient conversions of CO2 and nitrate pollutants into value-added NaHCO3 and NH4Cl chemicals. Briefly, by pouring CO2 gas into NH3- and OH−-enriched electrolyte after NO3RR, the produced NH3 was stripped into HCl solution successively, while NaHCO3 and NH4Cl were obtained at a maximum atom utilization without any waste. To demonstrate the potential industrial applications of the proposed system, concentrated NaNO3 solution (1 M) was adopted as an electrolyte, and an Ru-Cu catalyst was developed to achieve NO3RR at industrial-level current density. Impressively, the Ru-Cu catalyst offered over 82% Faradaic efficiency (FE) at 1.0 A cm−2 current density under neutral conditions for as long as 600 h. The loaded Ru species were determined to be capable of promoting H2O activation, assisting the Cu species to enhance the kinetics of NO2−-to-NH3 conversion. Download figure Download PowerPoint Introduction Nitrogen and carbon, the essential and major elements of proteins and tissues, play an indispensable role in life; however, they also exist in various forms as contaminants such as CO2, NO3−, NO2−, and so on, in nature, threatening the health of human beings and other organisms.1–8 Great efforts have been made in converting these contaminants into value-added chemicals. For instance, ammonia (NH3), one of the most common industrial chemicals for nitrogenous fertilizer production, can be fabricated by electrocatalytic NO3− reduction reaction (NO3RR), which benefits both the wastewater purification and ammonia production.9 Compared with the traditional Haber–Bosch process necessitating harsh conditions, the NO3RR can be operated under ambient atmosphere. Theoretically, the thermodynamic potential of NO3RR is 0.69 V versus the reversible hydrogen electrode (RHE) in the half-cell reaction, which offers feasible thermodynamics by using renewable electricity (Equation 1), though the issues of product separation and collection remain.10 Recently, an approach of separating ammonia from electrolyte by Ar gas has been reported.11 However, no attention has been paid to the OH− generation during ammonia production, which is also a common pollutant in the water and needs further purification, as shown in equation 1.11,12 NO 3 − + 2 H 2 O → N H 3 + O H − + 2 O 2 ↑ (1) NaHCO3, also referred to as baking soda, is another highly important commodity chemical applied in the fields of food, medicine, agriculture, and others, the production technology of which has been continuously studied and updated over the past centuries.13,14 In 1943, Chinese scientists, Debang Hou and his coworkers15 invented the union system alkaline process (Hou's combined alkali process) by stripping the NH3 and CO2 gases into a saturated NaCl solution successively, which resulted in greatly magnified industrial NaHCO3 production globally. Most recently, NO3RR has achieved much enhanced selectivity under varying conditions and ever-lower applied potentials, which holds great promise to participate in the ammonia-soda chemical industry. As mentioned above, one OH− molecule will be generated accompanying the synthesis of one NH3 molecule (Equation 1). Alternatively, one mole of OH−, as is well-known, can bind one mole of CO2 to form one mole of HCO3−, which can then precipitate out as NaHCO3 with one mole of introduced Na+. Here, inspired by Hou's process, we report an electrochemical-chemical cascading system for NaHCO3 and NH4Cl productions from NO3− and CO2 pollutants (Figure 1). Briefly, high concentration NaNO3 (1.0 M) solution was applied as the electrolyte for the generation of NH3 via electrocatalytic NO3RR, and then CO2 was introduced to produce NaHCO3 by binding with OH− and Na+ in cathodic electrolyte; in the meantime, the produced NH3 was separated from the system and introduced into tandem HCl solution, resulting in NH4Cl production. Figure 1 | Comparison of the reaction conditions of different routes for little soda (NaHCO3) and ammonium chloride (NH4Cl). The cascading electrochemical-chemical strategy for NaHCO3 and NH4Cl productions by nitrate reduction reaction (NO3RR) to NH3, followed by CO2 introduction, is more efficient and yields less pollutant than the current industrial NaHCO3 and NH4Cl production processes. Download figure Download PowerPoint To achieve efficient NH3 generation by NO3RR, a Cu confined Ru cluster (Ru-Cu) electrocatalyst featuring high catalytic activity of NO3RR was prepared. Under a neutral condition, the Ru-Cu achieved 85.2% of Faradaic efficiency of NH3 (FENH3) under an applied industrial current density of 1 A cm−2 for at least 600 h, and the production rate of NH3 (Prod.NH3) was over 45.5 mg h−1 cm−1 under the Ru-Cu catalysis. In this system, all atoms were converted into value-added NaHCO3 and NH4Cl theoretically, and the elimination rate of the obtained NH3 in electrolyte was over 96% by CO2 stripping. As revealed by a series of characterizations, including in-situ Fourier transform infrared (FT-IR) spectra and Nyquist plots, Ru sites were proposed to accelerate the Volmer step of water splitting for OH− generation, favorable for NO* formation by binding of NO2* intermediate with OH−. Moreover, considering the diversity of industrial wastewater, the NO3RR electrocatalytic performance of Ru-Cu nanowire (NW) catalyst was also tested in acidic and alkali conditions, which exhibited considerably high Prod.NH3 of 62.6 and 47.1 mg h−1 cm−1, respectively. Experimental Methods Preparation of Cu nanowires First, Cu foam (3 × 3.5 cm2) was ultrasonically cleaned sequentially in ethanol, HCl, and deionized water for 30 min. Then the cleaned Cu foam was placed in 3.0 M NaOH and 0.1 M (NH4)2S2O8 mixed solution at 25 °C for 0.5 h to fabricate Cu foam-supported Cu(OH)2 nanowires (Cu(OH)2 NW/Cu foam). The Cu(OH)2 NW/Cu foam was heat-treated at 200 °C for 2 h to obtain the CuO NW/Cu foam. The obtained CuO NW/Cu foam was washed with deionized water and dried under vacuum at 60 °C for 10 h. Finally, a pure Cu NW/Cu foam was obtained by electro-reducing CuO NW/Cu foam under galvanostatic control (−50 mA cm−2) for 25 min. Preparation of Ru-Cu nanowires The Cu(OH)2 NW/Cu foam was electro-reduced under −3 A cm−2 current density in 0.05 M RuCl3 and 0.5 M Na2SO4 mixed solution for 30 s, and then the obtained Ru-Cu(OH)2 NW/Cu foam was heat-treated at 200 °C for 2 h to obtain the RuOx-CuO NW/Cu foam. The RuOx-CuO NW/Cu foam was electro-reduced under 100 mA cm−2 for 1 h and 800 mA cm−2 to obtain Ru-Cu NW/Cu foam. Computational methods We have employed the Vienna Ab initio Simulation Package (VASP; https://cmp.univie.ac.at/research/vasp/) to perform all the spin-polarized density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) formulation. We chose the projected augmented wave (PAW) potentials to describe the ionic cores, taking valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 400 eV. Partial occupancies of the Kohn–Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10−6 eV. A geometry optimization was considered convergent when the force change was smaller than −0.05 eV/Å2. Grimme's DFT-D3 methodology was used to describe the dispersion interactions among all the atoms. During structural optimizations of the surface models, the 2×2×1 gamma-point centered k-point grid for the Brillouin zone was used. Results and Discussion Electrocatalyst designs and synthesis Cu-based materials have been proven as efficient electro-nitrate reduction reaction (NO3RR) catalysts.16 However, due to the weak H* absorption on the Cu active sites, the hydrogen evolution reaction will take place at the electrocatalytic interface as a competing reaction. By using a multistep reconstruction method (Figure 2a), we prepared a Cu-confined Ru cluster catalyst featuring fast kinetics of H2O molecule activation and H* generation. With a strong electrochemical reduction potential under high current density (−800 mA cm−2), the bulk RuO could be reduced and reconstructed as an Ru cluster dispersed on Cu nanowires substrate (Figure 2b–e). The evolution of chemical structures during the catalyst synthetic process has been demonstrated by X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) spectra, as shown in Supporting Information Figure S1a–c, exhibiting the chemical states of Ru and Cu elements in Ru-Cu NW mainly being Ru0 and Cu0, respectively.17,18 As demonstrated in Supporting Information Figure S2a,b, the Cu 2p peaks of Cu NW and Ru-Cu NW were located at 932.58 and 933.14 eV, respectively. The increase of Cu binding energy after combining with Ru clusters further led to the electron density decrease of Cu0 species. Figure 2 | Electrocatalyst design and the proposed mechanism for H* generation and NO3RR. (a) Scheme of Cu NW and Ru-Cu NW preparation. (b) scanning electron microscopy (SEM) image of Cu NW, (c) High-resolution transmission electron microscopy (HR-TEM) image of Cu NW, (d) SEM image of Ru-Cu NW, (e) HR-TEM image of Ru-Cu NW. (f) Bode plots of Cu NW and Ru-Cu NW catalysts measured in 0.5 M Na2SO4 solution. (g) Equivalent circuit model used in fitting the electrochemical impedance spectra and schematic diagram of the proposed HER mechanism. (h) Correlation between the equivalent resistances of the intermediate (H*) generation resistance (R2) and charge transfer resistance (R3) of interface reaction (Tafel step or Heyrovsky step) under the potential window from 0.1 V to −0.6 V (vs RHE) in 0.5 M Na2SO4 solution over Cu NW and Ru-Cu NW catalysts. (i) Scheme of hypothesized NO3RR on the surface of Cu NW and Ru-Cu NW catalysts. Download figure Download PowerPoint The catalytic performance of Cu and Ru-Cu NW to generate H* was carefully verified by electrochemical impedance spectroscopy (EIS) measurements in 0.5 M Na2SO4 solution. Under a wide potential window (0.1 to −0.6 V), only the hydrogen evolution reaction (HER), also named H2O molecule activation, takes place at the cathode during EIS measurements. The Nyquist plots revealed that the semicircles occurring in the low frequency (10−2–100), corresponding to the Ru-Cu NW-catalyzed water activation (WA) were much smaller than that of Cu-catalyzed WA ( Supporting Information Figure S3a,b), indicating the reduced resistance of mass transfer during the H2O activation process by Ru introduction. Moreover, the charge transfer process could be described by Bode plots. As shown in Figure 2f, the peak of Ru-Cu NW-catalyzed WA at a rather low potential of −0.1 V was located at a lower frequency than that of Cu-catalyzed WA, also suggesting that Ru-Cu NW had a faster kinetics of Volmer step (H2O + e + * ↔ H* + OH−) in promoting H* generation. With an increase in measurement potential, the peaks of both Ru-Cu NW- and Cu-catalyzed WA shifted to higher frequency (100–102), accompanied by peak intensity declines, resulting from the Heyrovsky step (H* + H2O + e− + * ↔ H2 + OH− + *) dominated H* desorption and H2 generation.19–21 Thus, the rate-determining step of HER changed from mass transfer to charge transfer. Interestingly, the gap between the two peaks of Cu and Ru-Cu NW-catalyzed WA reduced significantly at the measurement potential being increased to −0.6 V, though the Cu-related peak was still located at a higher frequency. This confirmed that the Ru-Cu NW featured a marked H* desorption, which was in favor of reacting with other intermediates in the next step. An equivalent circuit was proposed to describe the generation and consumption of H* (Figure 2g), and the resistances of corresponding intermediate steps were calculated using fitting Nyquist plots of Cu and Ru-Cu NW catalyzed WA. As shown in Figure 2h, Ru-Cu NW offered a smaller R2 in Na2SO4 solution in the whole measurement potential range, proving its stronger activity for H* generation and absorption than Cu NW. Also, R2 was always lower than R3 for Ru-Cu NW, implying the Volmer step was more favorable than the Heyrovsky step for the Ru-Cu NW-catalyzed WA. In contrast, the R3 dropped sharply as the potential increased for Cu NW, and the Volmer step became the rate-determining step at elevated potentials. The cyclic voltammograms (CV) in 0.5 M Na2SO4 ( Supporting Information Figure S4a,b) showed the peaks located at 0.12 V (vs RHE) in the cathodic scan belonging to the H* absorption on Cu and Ru-Cu NW catalysts. The H* absorption peak of Ru-Cu NW was significantly stronger, suggesting that more H* was generated by the Ru-Cu NW catalyst than by Cu NW.19 Based on the above results on Cu and Ru-Cu NW catalysts, the electrochemical NO3RR processes at both catalysts were suggested (Figure 2i). A previous literature reported that the Cu active sites possessed an ideal property of converting NO3− to NO2−.11 However, the NO2−-to-NH3 conversion was a 6e− process which could be largely retarded by the low density of H* at the reaction interface. Compared with the nitrite reduction reaction (NO2RR) on Cu NW, the generated NO2− could be rapidly consumed by adjacent H* at the Ru-Cu NW surface for NH3 production. Electrocatalytic ammonia synthesis under industrial conditions As exhibited in the linear sweep voltammetry (LSV) curves in Figure 3a, the Ru-Cu NW catalyst showed higher current density (j) than Cu catalyst in various solutions (0.5 M Na2SO4, 1.0 M NaNO2, 1.0 M NaNO3), and the j values of both catalysts increased after adding NO2− or NO3−, indicating the occurrence of NO2RR and NO3RR. Notably, the jNO2RR was lower than the jNO3RR at relatively low potentials over both Cu and Ru-Cu NW. However, the jNO2RR became higher than jNO3RR at elevated potentials, implying that the NO3−-to-NO2− conversion contributed a large portion to the current density increment at lowered potentials. Further, the large gap between jCu-NO2RR and jCu-NO3RR demonstrated the limited activity of Cu for NO2−-to-NH3 conversion. This phenomenon could be more intuitively described by using the Tafel slope (Figure 3b). The Tafel slope of NO3RR (337.1 mV dec−1) was much lower than that of NO2RR (436.1 mV dec−1) for the Cu catalyst, which indicated the significantly inhibited NO2−-to-NH3 conversion on the Cu surface. After loading Ru, the corresponding Tafel slopes were noticeably reduced, especially for NO2RR (313.6 mV dec−1), demonstrating the stronger absorption of NO2− on Ru-Cu than on Cu. The electrochemical surface areas (ECSA) of Cu and Ru-Cu NW in various electrolytes were calculated, as shown in Figure 3c. The ECSA of the Ru-Cu NW catalyst was close to that of the Cu NW catalyst at elevated scan rates (>60 mV s−1), implying that the interfacial capacitance was primarily suppressed by the transport of ions. However, at lowered scan rates (>50 mV s−1), the Ru-Cu NW catalyst demonstrated a significantly elevated ECSA (101.6 and 85.4 mF cm−2) in NaNO3 and NaNO2 solutions. This signified that the inclusion of Ru resulted in the enhanced inherent adsorption capacity of NO3− and NO2− on the catalyst surface and/or at the catalyst interface. Figure 3 | Electrocatalytic NO3RR performance. (a) LSV curves, (b) Tafel slopes, and (c) ECSA plots measured in 0.5 M Na2SO4, 1.0 M NaNO2, and 1.0 M NaNO3 solution under the catalysis by Cu and Ru-Cu NW. (d) FE and Prod.NH3 values of NO3RR tested by CP measurement from −100 to −1000 mA cm−2 for 1 h over Cu and Ru-Cu NW catalysts. (e) FE and Prod.NH3 values tested by long-term CP measurement at −1000 mA cm−2 for 6 h over Ru-Cu NW under neutral conditions. (f) FE of NO2RR tested by CP measurement from −100 to −1000 mA cm−2 for 1 h over Cu and Ru-Cu NW catalysts. (g) Stability test of Ru-Cu NW at 1.0 A cm−2 in neutral 1 M NaNO3 by recycling the CP method. (h) Comparisons of NO3RR performances between Ru-Cu NW and recently literature-reported catalysts. The volume of the sphere: relative NH3 production rates of related catalysts.11,22–33 Download figure Download PowerPoint The NO3RR and NO2RR performances of Cu and Ru-Cu NW were measured by the chronoamperometry (CA) method ( Supporting Information Figure S37) under the potential window of −0.1 to −0.6 V (vs RHE), as shown in Supporting Information Figure S5. We found that the Ru-Cu NW had an impressively high activity for NO3− conversion with a high selectivity of 99.9% to generate NH3 and NO2−. Similarly, Ru-Cu NW also exhibited a 99.9% FE for NO2RR to NH3 at an operation potential of −0.6 V (vs RHE) and the current densities of 400–500 mA cm−2 ( Supporting Information Figure S6). The selectivity of NO3− conversion to Cu and Ru-Cu NW catalysts was also detected by CA measurement at −0.6 V (vs RHE) in 50 mM NaNO3 and 0.5 M Na2SO4 mixed electrolyte, as shown in Supporting Information Figure S7. The Ru-Cu catalyst exhibited an excellent NO3RR performance composed of FERuCu-NH3 and FERuCu-NO2− of 52.8% and 49.2%, respectively, and the NH3 production rate (Prod.NH3) reached 2.84 mg cm−2cat. h−1. More detailed discussion has been attached in Supporting Information Note S1. Compared with the operating potential, j is a more crucial parameter for industrial production. Figure 3d exhibits the chronopotentiometric (CP) measurement results under industrial j values of −100 to −1000 mA cm−2. At j = 1000 mA cm−2, the Ru-Cu NW performed a markedly high FERuCu-NO3RR of 87.4% composed of FERuCu-NH3 and FERuCu-NO2− of 64.6% and 22.8%, respectively. The catalytic performance of Ru-Cu NW was evaluated by prolonging CP durations to 6 h. As shown in Figure 3e, the FERuCu-NO3RR was maintained at over 80% during 6 h CP operation, and the Prod.NH3 was calculated to be 45.5 mg cm−2cat. h−1. Additionally, the Prod.NH3 plots were fitted using a polynomial curve, indicating that the production rate of NH3continued to increase. Both Cu and Ru-Cu NW demonstrated satisfactory performance in NO2RR in CP measurement (Figure 3f), with Prod.NH3 being 81 and 108.2 mg cm−2cat. h−1 and the corresponding FEs being 67.8% and 86.7%, respectively, at the current density of 1000 mA cm−2. Interestingly, the FENO3RR was higher than FENO2RR for Cu NW catalyst at relatively low current densities (>200 mA cm−2), but became much lower than that in elevated current densities (>500 mA cm−2). In contrast, the Ru-Cu NW exhibited similar FE values between NO2RR and NO3RR, which indicated that the Ru species played a key role in moderating intermediate absorption. The ampere-level current density (1.0 A cm−2) was achieved for Ru-Cu NW-catalyzed NO3RR. Moreover, the Ru-Cu NW catalyst showed especially better stability than Cu NW ( Supporting Information Figure S8) during the successive tests for at least 600 h in over 50 cycles in 1.0 M NaNO3 solution, as shown in Figure 3g, and the FE of NH3 production was sustained at ∼82% during the stability tests. Moreover, Ru-Cu NW showed no observable changes in the morphology and element distribution after stability measurement, as displayed in Supporting Information Figures S9 and S10. As shown in Supporting Information Figure S11a,b, the binding energy of Cu and Ru exerted a negative shift, proving a slight surface reduction during the NO3RR process. Afterward, inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis ( Supporting Information Table S1) demonstrated that the superior stability of Ru-Cu NW originated from the slight dissolution of metal sites. In addition, the full utilization of the N element in NO3− was verified by CP measurement at 500 mA cm−2 and 50 mM NO3− ( Supporting Information Figure S12). During a 75-min operation, the N-total percentage was kept at ∼99.9%, indicating no N-based chemicals other than NO2− and NH3 were produced. Considering the complicated composition of industrial wastewater, acid and alkaline electrolytes were also employed; the results and discussions of related detailed measurements have been attached in Supporting Information Figures S13–S17 and Notes S2–S3. Meanwhile, a comparison was made between the NO3RR performances of this work and recent literature reports, as displayed in Figure 3h. Notably, the Ru-Cu NW exhibited the longest operating duration (600 h) under industrial-level current density (1.0 A cm−2), and an impressive NH3 production rate in neutral conditions (2.68 mmol h−1 cm−1). Moreover, this Ru-Cu NW electrocatalyst showed great applicability under acidic and alkaline conditions, with the NH3 production rate being 3.68 and 2.77 mmol h−1 cm−1, respectively, as summarized in Supporting Information Table S2. NO3RR mechanism investigation The kinetics of NO3RR and NO2RR catalyzed by Cu and Ru-Cu NW were further investigated by in-situ EIS at varied potentials. In Figure 4a, the Nyquist plots obtained at a rather low potential (−0.1 V vs RHE) showed a larger semicircle radius in the high-frequency region than those obtained at an elevated potential (−0.6 V vs RHE) for both NO2RR and NO3RR, indicating the faster reaction kinetics at the enhanced potentials. More than that, the NO2RR (RuCu-NO2RR) exhibited much higher charge transfer resistance (Rct) and series resistance (Rs) than NO3RR (RuCu-NO3RR) under the catalysis by Ru-Cu NW at −0.1 V (vs RHE), which indicated that the Ru-Cu NW interface contributed significantly to the NO3− conversion at the low potential. However, as the operating potential increased to −0.6 V versus RHE, the RuCu-NO2RR showed lowered resistance and promoted mass diffusion in the low-frequency region, compared with RuCu-NO3RR. This suggested that the faster kinetics of NO2RR compared with NO3RR occurred not only in the charge transfer step but also in the mass diffusion step over Ru-Cu NW. A similar phenomenon was observed on the Cu NW catalytic interface. This phenomenon explained why the FE-NO3RR was higher than FE-NO2RR at the low voltage, but FE-NO2RR became higher than FE-NO3RR at the elevated voltage for Cu and Ru-Cu NW catalysts, as presented in Supporting Information Figure S5a,b. The Bode plots of these reactions are shown in Figure 4b,c. In the case of NO3RR, the Bode plot exhibited a shift of the peak toward lower frequencies when Ru atoms were introduced (Figure 4b). Conversely, for NO2RR, the Bode peak of Ru-Cu NW appeared at a higher frequency than that of Cu NW, suggesting that the presence of Ru species benefited the charge transfer involved in the NO2−-to-NH3 conversion. On the other hand, the presence of Ru facilitated the diffusion and adsorption of H2O molecules on the catalyst interface. Figure 4 | The in-situ EIS measurement of NO3RR and NO2RR under the catalysis by Cu NW and Ru-Cu NW. (a) Nyquist plots of Cu- and Ru-Cu NW-assisted NO3RR and NO2RR at −0.1 and −0.6 V vs RHE. (b, c) Corresponding Bode plots of Cu- and Ru-Cu NW-assisted NO3RR (b) and NO2RR (c) under varied potentials. (d) Equivalent circuit diagram used in fitting the electrochemical impedance spectra and the schematics of the proposed NO3RR mechanism. (e) Correlation between equivalent resistances of R2 and charge transfer resistances of interface reactions, including NO3−-to-NO2− conversion (R4) and NO2−-to-NH3 conversion (R5) in the potential window from 0.1 V to −0.6 V (vs RHE) in 1.0 M NaNO3 solution over Cu NW and Ru-Cu NW catalysts. (f) Correlation between equivalent resistances of R2 and R5 in NO2RR (1.0 M NaNO2 solution). Download figure Download PowerPoint The NO3RR is a complicated 8-electron process that generates various intermediates. One of these intermediates is NO2−, which is stable in the electrolyte; therefore, the NO3RR could be simplified to a two-step reaction: the conversions of NO3−-to-NO2− and NO2−-to-NH3. Figure 4d shows the equivalent circuit diagram depicting the electron transfer and chemical conversion processes of NO3RR. The resistances (R) related to the reactions (similar to Figure 2h) were used to describe the kinetics. Figure 4e shows the fitted results of the Nyquist plots of NO3RR. We observed that the R2 of Cu-NO3RR was higher than that of RuCu-NO3RR, consistent with the HER, indicating that Ru-Cu NW could provide more H* for subsequent reactions than Cu NW. The R4 was smaller than R5 for both Cu NW and Ru-Cu NW catalysts, suggesting that the NO2−-to-NH3 step (R5), rather than the NO3−-to-NO2− (R4) step, is the rate-determining step. However, in the Cu catalyzed reaction, the R3Cu-HER was close to R4Cu-NO3RR, but lower than R5Cu-NO3RR, resulting in a relatively low FE of Cu NW catalyzed and a low Prod.NH3. This was consistent with the results of the CA measurements (Figure 3d). On the other hand, the Ru-Cu NW catalyzed NO3RR enhanced and sustained the high level of FERuCu-NO3RR. Interestingly, in the NO2RR, the R5 was lower than that in the NO3RR on both Cu and Ru-Cu NW catalysts (Figure 4f). This observation suggested that the R5 in the reaction was not solely determined by interfacial charge mass transfer but also by local spatial intermediate concentration and mass transfer. In the case of abundant NO2−, which could be considered as NO3RR intermediates, R5 was significantly reduced, while at the constant ion concentration the charge transfer resistance in the NO3− conversion in NaNO3 solution (R4 in Figure 4e) was found to be smaller than that of NO2− conversion in NaNO2 solution (R5 in Figure 4f). This suggested that the intrinsic reaction kinetics of the NO3−-to-NO2− step was faster at the Ru-Cu NW interface. Meanwhile, the R5 in Cu NW and Ru-Cu NW catalyzed NO2RR was obviously smaller than that in the NO3RR process. In other words, the NO2−-to-NH3 step exhibited faster kinetics in NO2RR instead of NO3RR, indicating that the transition from NO2−-to-NH3 step was primarily governed by the interfacial concentration of NO2−, rather than the intrinsic reaction kinetics. Furthermore, DFT calculations were performed to better understand the reaction pathways for NO3RR/NO2RR and the synergistic catalysis effect of Ru and Cu active sites. First, the adsorption strength of *NOx at different catalysts was analyzed by the partial density of states (PDOS). As shown in Supporting Information Figure S18, the d-band center of Cu NW adsorbed with NO3− (−2.374) and NO2− (−2.416) were closer t