Highly graphitized nitrogen-doped ordered mesoporous carbon supported Ni nanocrystals for efficient hydrazine-assisted CO₂ splitting

Abstract

The reaction activity and product selectivity of the CO₂ electroreduction process are hindered by intense hydrogen evolution and substantial *CO adsorption when nickel particle size is increased to the nanoscale dimension. This study introduces a highly graphitized nitrogen-doped ordered mesoporous carbon (Ni-NC) as a support for dispersing Ni nanocrystals, which not only enhances mass transfer but also exposes abundant catalytic Ni sites. In situ electrochemical measurements, characterization, and density functional theory calculations revealed that the synergistic interaction between nickel nanocrystals and the nitrogen-doped carbon matrix promoted CO₂ adsorption and the formation of the *COOH intermediate. Ni-NC achieved a peak CO faradaic efficiency (FE_CO) of ∼100% at −0.8 V vs. RHE, maintaining FE_CO above 90% across a wide potential window from −0.7 to −1.0 V vs. RHE. Additionally, Ni-NC efficiently catalyzed hydrazine-assisted CO₂ decomposition, offering up to 69% theoretical energy savings compared to traditional water oxidation-coupled systems in MEA while consistently maintaining a cathodic FE_CO above 90% across a broad cell voltage range, offering valuable insights for the development of more cost-effective CO₂ splitting systems.

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1 Introduction

In the context of rapid advancements in modern society, excessive anthropogenic CO₂ emissions have caused a multitude of climate and environmental crises, thereby posing a substantial threat to the sustainable development of human society.^1–5 Fortunately, the advancement of renewable energy utilization has facilitated the adoption of green electricity to drive the CO₂ reduction reaction (CO₂RR), which has emerged as a pivotal strategy for emission reduction.^6–8 This approach effectively converts CO₂ into fuels or value-added chemicals under mild conditions with relatively low energy expenditures, thereby contributing to the mitigation of the CO₂ crisis and the closure of the anthropogenic carbon cycle.^9,10 However, the practical application of the CO₂RR remains hindered by the lack of efficient catalysts specifically designed for target products and the absence of suitable low-energy reaction systems.

As one of the principal products of CO₂RR, CO not only serves as a direct fuel but also plays a pivotal role in various industrial applications, including carbonylation reactions and Fischer–Tropsch synthesis.¹¹ Recent studies have demonstrated that despite the exceptional catalytic performance of precious metal catalysts, particularly Au and Ag, in the electroreduction of CO₂-to-CO, their high costs and scarcity significantly impede their practical implementation.^12–14 In contrast, non-precious metal nickel-based catalysts have garnered considerable interest due to their abundant resources and impressive catalytic properties, particularly atomic dispersed Ni, which exhibits substantial catalytic activity in converting CO₂ to CO.^15–17 However, as the metal size increases further, nano-sized Ni-based catalysts, despite demonstrating conductivity and stability comparable to conventional metal catalysts, face limitations in their applications for CO₂RR due to their pronounced competitive hydrogen evolution capability and significant adsorption of the reactive intermediate *CO.^18–20 To mitigate this limitation, the integration of metal with carbon materials to modulate the active metal sites has emerged as a promising strategy.^21,22 This approach facilitates electron transfer between the metal and the carbon substrate, enabling the precise tuning of the electronic interactions between the catalyst surface and reactants.^23,24 Such modulation influences the desorption energy during the adsorption reaction process, ultimately enhancing CO₂RR activity. Consequently, the rational design of efficient nano-Ni and carbon composite catalysts for CO₂RR represents a critical and promising direction for future research.

Besides optimizing electrocatalyst design, enhancing the reaction system has emerged as pivotal for improving CO₂RR performance. Traditionally, CO₂RR has been coupled with the energy-intensive water oxidation reaction (OER) at the anode, consuming about 90% of total input energy and significantly limiting cathodic CO₂ electroreduction efficiency.^25–27 To address these challenges, a promising approach involves replacing OER with a more thermodynamically favorable oxidation reaction, thereby forming a hybrid electrocatalytic CO₂RR (HECR) system.²⁸ Among potential anode replacement reactions, the hydrazine oxidation reaction (HzOR) stands out due to its substantially lower onset potential compared to OER, enabling significant energy savings.²⁹ This strategy, previously demonstrated in hydrogen electrolysis, also offers environmental benefits by leveraging the hydrazine commonly found in wastewater.^30–32

In this work, we introduce nanoscale nickel dispersed on highly graphitized nitrogen-doped ordered mesoporous carbon (Ni-NC). The highly graphitized nitrogen-doped ordered mesoporous carbon provides efficient mass transfer channels and exposes catalytic Ni sites, which, in synergy with Ni nanocrystals (Ni NCs), promote CO₂ adsorption and the formation of the *COOH intermediate during CO₂RR. Furthermore, the electrocatalytic HzOR on Ni-NC exhibits thermodynamically favorable characteristics. When applied to both the anode and cathode, Ni-NC facilitates an efficient HzOR-assisted CO₂RR system, significantly reducing the reaction voltage compared to conventional OER-coupled CO₂RR. This leads to improved CO₂ reduction activity at a lower cell voltage and, consequently, a substantial increase in the energy utilization efficiency of the CO₂ electrolysis system.

2 Results and discussion

The synthesis procedure for Ni-NC is illustrated in Fig. 1a, in which nickel chloride hexahydrate (NiCl₂·6H₂O), ethylenediamine (EDA), and carbon tetrachloride (CTC) serve as the sources of nickel, nitrogen, and carbon, respectively. The aforementioned precursors were filled into the pore structure of the mesoporous molecular sieve (SBA-15), followed by a series of processes, including refluxing, calcination, and HF etching, ultimately yielding a target catalyst that exhibits a morphology resembling a reverse replica of the SBA-15. Scanning electron microscopy (SEM, Fig. S1†) images demonstrate that both Ni-NC and NC exhibit densely packed carbon nanofibers with a consistent orientation. This arrangement provided Ni-NC with a carbon support possessing a large specific surface area, thereby facilitating the effective dispersion of Ni nanocrystals (Ni NCs). Transmission electron microscopy (TEM, Fig. 1b and S2†) images indicated that the aligned carbon nanofibers in Ni-NC and NC formed ordered mesoporous voids, confirming the successful reverse replication of the SBA-15 template. Additionally, Fig. 1b displays the lattice fringes of Ni NCs on Ni-NC with an interplanar spacing of 0.209 nm, corresponding to Ni (111). Fig. 1c illustrates high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images, which highlight the effective dispersion of Ni NCs on the carbon substrate. From the corresponding particle size distribution graph, as illustrated in Fig. 1d, it is evident that the average particle size of the Ni NCs is approximately 27.40 nm. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping of Ni-NC, shown in Fig. 1e, revealed a uniform distribution of C, N, and O elements, with Ni elements exhibiting distinct aggregation.

Fig. 1 (a) Synthesis of the Ni-NC. (b) TEM, and (c) HAADF-STEM of the Ni-NC. (d) Corresponding particle size distribution of Ni NCs. (e) EDS elemental mapping of the Ni-NC. Inset in (b): locally magnified TEM image of Ni NCs.

The crystal phases of the materials were elucidated through X-ray diffraction (XRD) analysis, as depicted in Fig. 2a. Unlike the metal-free NC, the Ni-NC displayed three prominent peaks corresponding to metallic Ni, aligning with the cubic nickel phase (JCPDS PDF #04-0850). Additionally, at approximately 26°, Ni-NC exhibited a diffraction peak with higher intensity and sharpness compared to NC, which corresponded to the (002) plane of graphitic carbon.³³ This enhancement is likely attributable to the catalytic effect of Ni during the thermal synthesis process, which increases the graphitization degree of Ni-NC.^34–36 Further investigation of the carbon structure via Raman spectroscopy confirmed this finding, as shown in Fig. 2b. The intensity ratios of the D band (sp³ disordered carbon, 1349 cm⁻¹) to the G band (sp² graphitic carbon, 1583 cm⁻¹) were 0.83 and 1.20 for Ni-NC and NC, respectively.^37,38 Furthermore, the pronounced 2D band at 2675 cm⁻¹ in the Ni-NC spectrum corroborated its graphitization characteristics. The increased degree of graphitization is beneficial for enhancing the material's electronic conductivity and chemical stability, which is crucial for electrochemical applications.^39,40

Fig. 2 (a) XRD patterns and (b) Raman spectra of Ni-NC and NC. XPS spectra of (c) N 1s for Ni-NC and NC, and (d) Ni 2p for Ni-NC.

The specific surface area and pore size distribution of the two materials were determined using automated gas adsorption methods (Fig. S3 and Table S1†). Both Ni-NC and NC exhibited typical type IV adsorption isotherms. Given that both materials utilized SBA-15 as a template, the Brunauer–Emmet–Teller method was employed to ascertain their specific surface areas, which were found to be similar: 827.3 m² g⁻¹ for Ni-NC and 816.5 m² g⁻¹ for NC. Additionally, pore size distribution calculations using density functional theory revealed that both materials exhibited a similar multi-level structure predominantly composed of mesopores, consistent with the results obtained from TEM. It has been reported that this unique porous matrix not only facilitated the dispersion of Ni NCs but also possessed excellent reactant/product mass transfer capabilities and significantly enhanced the accessibility of catalytic active Ni sites.^35,41

To investigate the surface chemical states of the materials, X-ray photoelectron spectroscopy (XPS) was conducted. The comprehensive XPS spectra revealed the presence of C, N, and O in both materials, with a distinctive Ni 2p signal observed in Ni-NC (Fig. S4†). As shown in Fig. 2c, the high-resolution N 1s spectra of both materials could be deconvoluted into pyridinic-N (398.7 eV), pyrrolic-N (400.2 eV), graphitic-N (401.2 eV), and oxidized-N (402.9 eV), confirming the successful incorporation of nitrogen into the carbon matrix.^42,43 It is noteworthy that the peak intensity of pyridine-N in Ni-NC is significantly enhanced, which, considering its potential as an effective active site for CO₂RR, indicates that Ni-NC holds great promise for CO₂RR applications.⁴⁴ Furthermore, the binding energy of pyridine-N in Ni-NC is reduced by 0.1 eV compared to NC. In conjunction with the XPS spectrum of Ni 2p shown in Fig. 2d, the binding energy of Ni⁰ in Ni-NC (853.7 eV) exhibits a noticeable shift from the reported value of Ni⁰ (853.5 eV), suggesting a strong interaction between the Ni NCs and the nitrogen-doped carbon matrix.^35,45

The electrocatalytic performance of Ni-NC was evaluated through a series of electrochemical measurements utilizing a three-electrode configuration. The material's CO₂RR capabilities were assessed in an H-type cell containing 0.5 M KHCO₃ electrolyte. Through linear sweep voltammetry (LSV, Fig. 3a) analysis, Ni-NC exhibited an enhanced current density in CO₂-saturated electrolyte compared to that in an Ar atmosphere, indicating its substantial electrocatalytic activity for CO₂RR.^46,47 Additionally, the corresponding Tafel slope (Fig. S5†) in CO₂-saturated electrolyte was 242.0 mV dec⁻¹, demonstrating the favorable kinetic advantage of Ni-NC.⁴⁸ Combined with the performance analysis of NC, this enhanced electrochemical activity could be attributed to the synergistic interaction between the catalytically active Ni sites and the highly graphitized carbon support in Ni-NC. Through gas chromatography and ¹H nuclear magnetic resonance analysis of the reaction products (Fig. S6†), only gaseous products CO and H₂ were detected, and no liquid-phase formation was observed. Specifically, as shown in Fig. 3b, Ni-NC consistently maintained a FE_CO above 90% across the broad potential ranging from −0.7 V to −1.0 V vs. RHE, with the highest value approaching 100% at −0.8 V vs. RHE, indicating an exceptionally high selectivity of Ni-NC for the sole carbon product CO. In contrast, NC achieved a maximum FE_CO of only 70% at −0.5 V vs. RHE. To further optimize the CO₂RR performance of Ni-NC, a series of Ni-NC-X (where X represents the multiple of Ni content in the original Ni-NC, taking values of 0.1, 0.5, and 1.5) materials was synthesized with varying nickel contents. Performance evaluations revealed that Ni-NC exhibited the highest current density along with the maximum FE_CO (Fig. S7†). Consequently, Ni-NC was selected for all subsequent electrochemical measurements of CO₂RR under the optimal synthesis conditions.

Fig. 3 CO₂RR tests: (a) LSV profiles of Ni-NC and NC under Ar and CO₂-saturated conditions in 0.5 M KHCO₃, (b) FE_CO of Ni-NC and NC, (c) comparison of j_CO and FE_CO of Ni-NC with other reported CO₂-to-CO performance, and (d) stability test of Ni-NC at −0.8 V vs. RHE. HzOR tests: (e) LSV profiles of Ni-NC and NC during the HzOR test in 1 M KOH with and without 0.5 M N₂H₄, and (f) stability test of Ni-NC at 10 mA cm⁻². Inset in (f): comparison of LSV profiles before and after the stability test.

The j_CO (Fig. S8†) of NC exhibited a low and gentle slope in stark contrast to Ni-NC, which maintained 94% FE_CO at −1.0 V vs. RHE while achieving a j_CO of 16.4 mA cm⁻². This performance exceeded that of several previously reported electrocatalysts for CO production from CO₂RR, as illustrated in Fig. 3c (and Table S2†). The exceptional catalytic performance of Ni-NC was further elucidated through electrochemical impedance spectroscopy (EIS) and electrochemical surface area (ECSA) analysis. It was distinctly observed from the low-frequency region of the EIS (Fig. S9†) that Ni-NC exhibited the lowest charge transfer resistance (R_ct), indicating that the exposed Ni NCs and highly graphitized carbon support collectively endowed Ni-NC with superior charge transport properties.^49,50 Furthermore, through double-layer capacitance measurements (Fig. S10†), the ECSA values of Ni-NC and NC were determined to be 41.3 cm² and 12.6 cm², respectively.⁵¹ This indicates that the ordered mesoporous structure provides a substantial active surface area for the Ni NCs, while the introduced Ni NCs may play a critical role in the adsorption and activation of the reactants. Considering the aforementioned characteristics and the significant advantages demonstrated by the Tafel slope, the substantial potential of Ni-NC as a CO₂RR electrocatalyst was confirmed. At −0.8 V vs. RHE, a 52 hour durability test was conducted, during which Ni-NC maintained a relatively stable current density and a FE_CO exceeding 95%, as shown in Fig. 3d. The structural and chemical composition of the material exhibited minimal changes before and after the test (Fig. S11†), confirming the exceptional stability of Ni-NC.

To reduce the anode energy consumption in the CO₂RR system, we employed HzOR as a replacement for the sluggish OER.^52,53 Simultaneously, to control the catalyst cost, the catalytic performance of Ni-NC for HzOR was evaluated in a single-pool reactor containing 1 M KOH with 0.5 M N₂H₄ as the electrolyte. Initially, a comparative analysis of the LSV profiles for the Ni-NC and Ni-NC-X series was performed to optimize the nickel content (Fig. S12†). The results demonstrate that Ni-NC exhibits superior HzOR performance. Accordingly, under the optimal synthesis conditions, Ni-NC was utilized for all subsequent electrochemical measurements related to HzOR. As shown in Fig. 3e, the LSV analysis of NC indicates that the ordered mesoporous structure exhibits commendable HzOR activity. The introduction of Ni NCs led to a synergistic interaction with the highly graphitized carbon support, which further enhanced the HzOR activity of Ni-NC, allowing it to achieve a current density of 10 mA cm⁻² at only 0.66 V vs. RHE, which was significantly lower than the 1.70 V vs. RHE required to drive the same current density in water oxidation reactions (1 M KOH as the electrolyte). The corresponding Tafel slope demonstrates that Ni-NC has a slope of only 211.3 mV dec⁻¹ (Fig. S13†), highlighting its excellent HzOR kinetics.^54,55

The HzOR activity of Ni-NC was further assessed using turnover frequency (TOF), EIS, and ECSA. A comparison of the two sets of TOF curves indicates (Fig. S14†) that the intrinsic HzOR activity of Ni-NC surpasses that of NC, consistent with the HzOR measurement results.^56,57 Additionally, EIS analysis (Fig. S15†) reveals that Ni-NC exhibits the smallest curvature radius, confirming its exceptional alkaline HzOR activity.⁵⁸ Furthermore, the significant differences observed in ECSA (Fig. S16†) are analogous to the results obtained from CO₂RR, highlighting that the introduction of Ni NCs can facilitate the rapid occurrence of HzOR.⁵⁹ A larger ECSA also confirms that it has more catalytic active sites. The long-term stability of the material was assessed through the examination of LSV profiles before and after prolonged electrolysis, as well as the characterization of the Ni-NC's morphology and composition (Fig. 3f and S17†). The results indicated that, at a current density of 10 mA cm⁻², Ni-NC provided a relatively stable current response for HzOR over 12 hours, with minimal changes observed in the LSV profiles before and after durability testing. The material retained its original structure and chemical composition even after extended electrolysis, demonstrating that Ni-NC possesses stable catalytic performance.

The application of in situ FTIR spectroscopy enables the continuous monitoring of the behavior of reaction intermediates adsorbed on the Ni-NC catalytic interface. As illustrated in Fig. 4a, all prominent bands between 1275 and 1668 cm⁻¹ is attributed to the *COOH.⁶⁰ As the negative reduction potential increases, the *COOH signal progressively intensifies, indicating that *CO₂ can be continuously converted into *COOH on the Ni-NC. Moreover, the infrared absorption band at 2078 cm⁻¹, associated with the chemisorption of CO/*CO, further elucidated the subsequent transformation process of *COOH.⁶¹ For HzOR, as shown in Fig. 4b, the H–N–H bending vibration appears around 1442 cm⁻¹, while the –NH₂ is observed at approximately 1281 cm⁻¹.⁶² With the increase in positive oxidation potential, both signals gradually strengthen, suggesting that *N₂H₄ can be continuously converted into *N₂ at the Ni-NC catalytic interface.

Fig. 4 In situ FTIR curves of Ni-NC for (a) CO₂RR under −0.5 to −1.0 V vs. RHE, and (b) HzOR under 0.8 to 1.2 V vs. RHE. Free energy curves of reaction intermediates in (c) CO₂RR and (d) HzOR on Ni-NC and NC.

The mechanisms of CO₂RR and HzOR on Ni-NC were systematically investigated using DFT calculations. The model for the active site of Ni-NC is constructed with a single Ni NC anchored on the N-doped carbon substrate, whose density of states demonstrates favorable bonding interactions and electrical conductivity (Fig. S18†). Through differential charge density analysis (Fig. S19†), a detailed understanding of the electronic interactions at the interface between the Ni NCs and the N-doped carbon support was gained. The differential charge density distribution revealed a significant charge transfer from the Ni NCs to the N-doped carbon support in the optimized Ni-NC structure, suggesting a strong interfacial interaction between the metal and the support. Subsequently, the free energy changes of steps in the CO₂RR and HzOR were evaluated on the Ni-NC and N-doped carbon (NC) model surfaces based on reaction pathways established from the relevant literature and in situ FTIR data.^62,63 As shown in Fig. 4c, the rate-limiting step for CO₂RR on Ni-NC is the desorption of *CO, with a Gibbs free energy (ΔG) of 0.96 eV, which is lower than the rate-determining reaction (*COOH formation, 1.20 eV) on NC. Furthermore, compared to NC, the synergy between Ni NCs and the N-doped carbon substrate significantly reduces the ΔG for CO₂ adsorption and *COOH formation on Ni-NC. For HzOR, extensive model optimization calculations (Fig. S20†) indicate that NC has a weak adsorption affinity for N₂H₄ and N₂.^64,65 In contrast, the incorporation of Ni NCs renders the HzOR steps on Ni-NC thermodynamically spontaneous, as illustrated in Fig. 4d, underscoring its superior HzOR activity.

Given the excellent performance of Ni-NC in the independent CO₂RR and HzOR processes, it was used as both the anode and cathode materials to assemble a two-electrode HECR system in an H-type cell (Fig. S21†) to verify the feasibility of the energy-efficient HzOR‖CO₂RR. Fig. 5a illustrates the polarization curves comparing HzOR‖CO₂RR with the conventional OER‖CO₂RR. Compared to OER‖CO₂RR, the voltage required to drive a current density of 10 mA cm⁻² in HzOR‖CO₂RR under cathodic CO₂ saturation is significantly reduced by 1.1 V, theoretically saving a remarkable 33% of energy consumption.^66,67 Further verification through polarization curves under cathodic Ar saturation confirms that HzOR, as an alternative reaction to OER, plays a dominant role in lowering the cell voltage of the HECR system. To ensure the exceptional electrocatalytic performance of the cathodic Ni-NC in HzOR‖CO₂RR, precise voltage control for regulating FE_CO is imperative. As shown in Fig. 5b, HzOR‖CO₂RR maintains a cathodic FE_CO above 83% within the voltage range of 1.2 to 2.2 V, reaching 91% at 1.8 V. In contrast, the FE_CO of OER‖CO₂RR barely reaches 79% at a higher voltage of 2.2 V. The above results confirm that substituting the traditional OER with the thermodynamically more favorable HzOR significantly reduces the overall reaction voltage of CO₂RR. This results in improved cathodic CO selectivity at low cell voltages, leading to a substantial increase in the system's CO₂ electroreduction efficiency.⁶⁵ In addition, a HzOR‖CO₂RR system in an H-type cell was prepared using NC as both the anode and cathode materials, whose polarization curve showed a significant lag compared to the HECR system employing Ni-NC as the electrode material (Fig. S22†). Furthermore, at the same cell voltage, the FE_CO of the Ni-NC-based HECR system surpassed that of the NC-based system by about 79%, as illustrated in Fig. 5c. This result strongly demonstrates the superior electrocatalytic activity of Ni-NC as an electrode material for HECR. At a cell voltage of 1.8 V, the durability of Ni-NC as an electrode material in the HECR system was validated (Fig. S23†). Over a prolonged 60 h stability test, its cathodic FE_CO consistently remained above 90%, with periodic electrolyte refreshes having a negligible impact on this system's stability. To the best of our knowledge, this work is one of the rare documented instances of HzOR-assisted energy-saving CO₂RR, thereby validating the feasibility of such HECR systems.⁶⁸

Fig. 5 In H-type cell: (a) polarization curves and (b) cathodic FE_CO comparison of HzOR‖CO₂RR and OER‖CO₂RR using Ni-NC as both anode and cathode, and (c) cathodic FE_CO comparison of Ni-NC-based HzOR‖CO₂RR and NC-based HzOR‖CO₂RR. In MEA: (d) schematic of HzOR‖CO₂RR, (e) polarization curves comparison of HzOR‖CO₂RR and OER‖CO₂RR using Ni-NC as both anode and cathode, and (f) cathodic FE_CO of Ni-NC-based HzOR‖CO₂RR.

As shown in Fig. 5d, the HzOR‖CO₂RR system was further upgraded by employing Ni-NC as the anode and cathode catalysts, integrated into a membrane electrode assembly (MEA) with a zero-gap configuration, utilizing an anion exchange membrane (AEM). The polarization curves of the conventional OER‖CO₂RR and HzOR‖CO₂RR systems obtained on this setup, as depicted in Fig. 5e, revealed that under the minimization of ohmic resistance by the MEA, the onset voltage of HzOR‖CO₂RR significantly decreased with the introduction of N₂H₄ into the anode feed, and the current density was much higher than that of OER‖CO₂RR. Within a given voltage range, the required cell voltages for OER‖CO₂RR to achieve current densities of 10, 20, and 30 mA cm⁻² were 3.9, 4.4, and 4.8 V, respectively. In contrast, the HzOR‖CO₂RR system reached the same current densities with voltage reductions of 2.7, 2.6, and 2.4 V, achieving a maximum energy savings of 69% at 10 mA cm⁻². As shown in Fig. 5f, analysis of the cathodic products of HzOR‖CO₂RR indicated that the FE_CO remained nearly constant across different current densities, maintaining approximately 90%, and reached a peak value of 93.8% at 30 mA cm⁻². To assess the feasibility at higher current densities, the electrochemical stability of the HzOR‖CO₂RR system was investigated (Fig. S24†). The HzOR‖CO₂RR system, with Ni-NC as the catalyst, exhibited excellent stability, maintaining FE_CO above 91% over 300 minutes at a higher current density of 30 mA cm⁻². It is worth noting that the simple implementation of the HzOR‖CO₂RR system in the MEA setup in this work did not include the design of other coupling system components, such as the cathodic gas diffusion electrode, anode membrane, solid electrolyte, and interfaces.^69,70 Future efforts will focus on addressing these issues.

3 Conclusions

In summary, we report the development of Ni nanocrystals supported on nitrogen-doped ordered mesoporous carbon for CO production via CO₂RR. The Ni-NC exhibits exceptional CO₂RR performance, achieving nearly 100% FE_CO at −0.8 V vs. RHE, and demonstrates robust stability over 52 hours of operation. XPS, in situ FTIR, and DFT calculations reveal that the synergistic interaction between Ni nanocrystals and the nitrogen-doped carbon matrix enhances CO₂ adsorption and promotes the formation of the *COOH intermediate, facilitating CO₂ electroreduction. Additionally, the ordered mesoporous structure and enhanced graphitization of the carbon substrate improve the accessibility of catalytic Ni sites and boost electronic conductivity. Moreover, leveraging the thermodynamically spontaneous properties of HzOR on Ni-NC and considering catalyst cost, we constructed an efficient hydrazine-assisted CO₂RR system with Ni-NC as both the anode and cathode. Compared to conventional water oxidation coupled with CO₂RR, the HzOR‖CO₂RR system achieves 69% energy savings, a FE_CO of over 90%, and excellent electrochemical stability in MEA. This work provides a valuable strategy for designing efficient CO₂-to-CO electrocatalysts and offers a promising solution for profitable CO₂ electrolysis systems.

Data availability

Data will be made available on request.

Author contributions

Kang Lian: writing – original draft, data curation. Kang Lian: data curation. Junyang Ding: data curation. J. Zhang: data curation, investigation, writing – review & editing. Quan Zhang: investigation, writing – review & editing. Yifan Liu: investigation. Guangzhi Hu: validation. Jin Zhang: writing – review & editing, conceptualization. Quan Zhang: writing – review & editing, software. Jia He: data curation, software. Xijun Liu: writing – review & editing, conceptualization.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Guangxi Natural Science Fund for Distinguished Young Scholars (2024GXNSFFA010008), National Natural Science Foundation of China (22469002), and the Shenzhen Science and Technology Program (JCYJ20230807112503008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06503d

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