English

• Direct methanol (CH₃OH) fuel cells (DMFCs) have demonstrated significant application potential in the fields of portable power sources and electric vehicles due to their high energy conversion efficiency and low pollution emission characteristics^[1-4]. The anodic methanol oxidation reaction (MOR), as the core process in DMFCs, has kinetic performance that directly affects the device′s overall efficiency. Therefore, developing a catalyst that combines high activity, low cost, and resistance to carbon monoxide (CO) poisoning has become a key scientific issue^[5-6]. At present, the mainstream MOR electrocatalysts are primarily based on precious metals such as platinum (Pt)^[7], palladium (Pd)^[8], and ruthenium (Ru)^[9-10], and their composite materials. However, their high cost, resource scarcity, and severe CO poisoning effect have seriously restricted their commercial application^[11-12]. Against this backdrop, transition metal-based catalysts represented by nickel (Ni), cobalt (Co), copper (Cu), and iron (Fe) have gradually become research hotspots due to their cost advantages and potential for resistance to CO poisoning^[13-16]. Ni-based materials have attracted much attention owing to their abundant reserves and considerable catalytic activity^[17-18].

  Traditional nano-powder MOR catalysts are mostly loaded on carbon-based carriers (such as porous carbon, carbon nanotubes, graphene oxide, etc.). The weak interaction between carbon materials and metals limits electron transport at the interface^[19-21]. Nitrogen (N) doping can effectively enhance chemical coupling between the carrier and the metal, improve the charge transfer efficiency, and stabilize the active components^[22]. Compared with the post-treatment doping process, N-rich precursors can directly construct carbon matrices with high nitrogen doping levels.

  Graphitized carbon nitride (g-C₃N₄, named as CN), as a typical N-rich carbon material, with its two-dimensional layered structure^[23-24], excellent thermal stability, and unique N-C-N heterocyclic system, can not only serve as a nitrogen source precursor but also anchor metal nanostructures through coordination, providing an ideal platform for the construction of efficient metal/CN composite catalysts^[25-26]. However, the electron transport bottleneck, arising from the insufficient intrinsic conductivity of CN, remains a key factor limiting its electrocatalytic performance. This limitation prompted researchers to explore synergistic optimization strategies within M-C-N (M: metal) composite systems^[27-29].

  In response to the above challenges, a composite (Ni-NiO@CN) comprising Ni and NiO anchored on CN was fabricated via a two-step hydrothermal and calcination process. Among them, the Ni-NiO heterojunction serves as the active center. Meanwhile, CN not only acts as a N-rich carrier to promote electron delocalization and transport, but also effectively encapsulates the active components in its two-dimensional network structure to prevent agglomeration. The electrochemical results indicated that the mass activity of the Ni-NiO@CN-500 catalyst prepared at 500 ℃ in alkaline methanol solution reached 164 mA·cm^-2 at 1.67 V (vs RHE). Meanwhile, it demonstrated excellent stability at high current density. More importantly, under CO saturation, its activity could still be maintained at 94.5% of the initial level. The characteristics of low cost, high activity, and strong resistance to CO poisoning make Ni-NiO@CN-500 a promising MOR electrocatalytic material.

  1.   Experimental

  1.1   Chemical reagents

  Melamine (C₃H₆N₆), hydrochloric acid (HCl), urea (CO(NH₂)₂), and nickel chloride hexahydrate (NiCl₂·6H₂O) are all analytical grades and were purchased from Sinopharm Group Chemical Reagent Co., Ltd. The other reagents were used directly. Carbon cloth (CC, 0.32 mm) was purchased from Shanghai Hesen Electric Co., Ltd., China.

  1.2   Preparation of Ni-NiO@CN

  The method reported in the literature^[30] was slightly improved, and two-dimensional CN nanosheets were successfully prepared. At room temperature, 2 g of C₃H₆N₆ and 2 g of CO(NH₂)₂ were ground into fine powder, dissolved in water (50 mL), and then ultrasonicated for 30 min. Subsequently, dilute HCl was slowly added to adjust the pH of the mixed solution to 4-5. The mixture was heated at a constant temperature of 60 ℃ until all the water had evaporated entirely. The obtained precursor was then transferred to a porcelain boat, which was placed in a muffle furnace, calcinated at 550 ℃, and kept for 3 h to obtain CN nanosheets.

  20 mg CN, 2 mmol NiCl₂·6H₂O, and 6 mmol CO(NH₂)₂ were dissolved in water (15 mL). After magnetic stirring at room temperature for 30 min, the resulting mixture was put into a 25 mL reactor and reacted at 100 ℃ for 18 h. After natural cooling to 25 ℃, the product was thoroughly washed with deionized water, freeze-dried, and then pyrolyzed at a gradient temperature (400, 500, and 600 ℃) for 2 h in a N₂ atmosphere to prepare the composites. The resulting composites (400, 500, and 600 ℃) were denoted as NiO/@CN-400, Ni-NiO@CN-500, and Ni-NiO@CN-600, respectively.

  1.3   Preparation of the working electrode

  5 mg of the powder catalyst was dispersed in a mixed solution of CH₃OH and deionized water (50 μL, 2∶3, V/V) to form a slurry, which was evenly coated on the surface of CC (0.5 cm×0.5 cm). After drying at 60 ℃, the working electrode was obtained. Similarly, the Pt/C electrode was also prepared using the method described above.

  1.4   Characterization

  X-ray diffraction (XRD) tests were performed using a Bruker D8 X-ray diffractometer with Mo Kα radiation (λ=0.071 076 nm) operating at 50 kV and 30 mA. The diffraction patterns were recorded in a 2θ range of 10°-90° with a scanning rate of 2 (°)·min^-1. The microscopic morphology of the samples was analyzed by a transmission electron microscope (TEM, FEI Talos F200X) operating at an acceleration voltage of 200 kV and a field-emission scanning electron microscope (SEM, Hitachi S4800), equipped with a field emission gun and energy-dispersive X-ray spectroscopy (EDS) for elemental analysis. The VG ESCALAB MKII X-ray photoelectron spectrometer was used to obtain X-ray photoelectron spectroscopy (XPS) data based on the Al Kα (1 486.6 eV).

  1.5   Electrochemical measurements

  All electrochemical tests were conducted using a three-electrode system based on the CHI660E electrochemical workstation. The working electrode, counter electrode, and reference electrode were selected as the prepared catalysts, a piece of stainless steel (4 cm×3 cm), and a saturated calomel electrode (SCE), respectively. The electrolyte system was a 1.0 mol·L^-1 KOH solution or a mixed 1.0 mol·L^-1 KOH+1.0 mol·L^-1 CH₃OH solution. Cyclic voltammetry (CV) tests were conducted in the potential range of 1.07 to 1.67 V (vs RHE). In the frequency range of 0.01 Hz to 100 kHz, electrochemical impedance spectroscopy (EIS) was performed at a voltage amplitude of 5 mV. The long-term stability of the MOR catalyst was tested by continuously monitoring the current response in a mixed solution of 1.0 mol·L^-1 KOH and 1.0 mol·L^-1 CH₃OH at 1.67 V (vs RHE) using the chronoamperometry (CA) method.

  2.   Results and discussion

  2.1   Characterization of materials

  The crystal phase structure of the sample was analyzed using XRD. As shown in Fig.S1 (Supporting information), the characteristic diffraction peaks at 12.8° and 27.3° correspond to the (100) and (002) crystal planes of g-C₃N₄ (PDF No.87-1526), respectively, confirming the successful synthesis of CN. As summarized in Fig.1a, the peaks of the (111), (200), and (220) crystal planes of the rhombic NiO (PDF No.47-1049) were situated at 37.2°, 43.3°, and 62.9°, respectively. The peaks of the (111), (200), and (220) crystal planes of the face-centered cubic structure Ni (PDF No.04-0850) were situated at 44.5°, 51.8°, and 76.4°, respectively. The peak of NiO@CN-400 at 27.4° is attributed to the standard peak of the g-C₃N₄ (002) crystal plane. The peak of Ni-NiO@CN-500 around 15° is the thermally expanded shifted peak of the in-plane ordered (100) peak of g-C₃N₄ (13.1°). At 500 ℃, interlayer expansion occurs, which results in the enlargement of the lattice parameter, leading to a shift of the peak position toward higher angles. XRD patterns showed that in the 2θ range of 43°-45°, the sample at 400 ℃ exhibited only a single diffraction peak, which corresponds to the (200) crystal plane of NiO (PDF No.47-1049). In contrast, Ni-NiO@CN-500 and Ni-NiO@CN-600 showed obvious double peaks (43.3°, 44.5°) in this region, which are assigned to the (200) crystal plane of NiO (PDF No.47-1049) and the (111) crystal plane of metal Ni (PDF No.04-0850), respectively. This directly confirms that high-temperature calcination promotes the in-situ reduction of part of NiO to metal Ni by the CN support. It is worth noting that as the pyrolysis temperature rose from 400 to 600 ℃, the intensities of each characteristic diffraction peak gradually increased, referring to the fact that the crystallinity of the sample increased with rising temperature.

  Figure 1

  

  Figure 1.  (a) XRD patterns of NiO@CN-400, Ni-NiO@CN-500, and Ni-NiO@CN-600; (b) Survey, (c) C1s, and (d) Ni2p XPS spectra of Ni-NiO@CN-500

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  XPS was used to characterize the surface chemical composition and valence states of elements in samples. The survey XPS spectrum of Ni-NiO@CN-500 consists of C1s, O1s, N1s, and Ni2p (Fig.1b). As exhibited in Fig.1c, the characteristic peaks at 284.7, 287.1, and 288.5 eV correspond respectively to graphite carbon (C=C), N—C—N bond, and N—C=N bond^[31]. The peaks of Ni²⁺2p_1/2 and Ni²⁺2p_3/2 were situated at 873.4 and 855.8 eV, respectively, while the satellite peaks of Ni were at 879.9 and 861.6 eV (Fig.1d). The spin-orbit splitting energy of 17.6 eV (873.4 to 855.8 eV) between two Ni2p peaks indicates the formation of the Ni(OH)₂ phase, confirming that the Ni element in the composite is dominant in the form of Ni^{2+ [32-33]}.

  SEM and TEM were used to characterize the microstructure of Ni-NiO@CN-500. As shown in Fig.2a and 2b, Ni-NiO@CN-500 was a bulk structure composed of nanoparticle packing. Furthermore, as shown in Fig.2c, the interior of the Ni-NiO@CN-500 nano- sheets was composed of Ni-NiO nanocrystalline particles stacked together. As illustrated in Fig.2d, the lattice spacings of 0.205 and 0.243 nm belong to the crystal planes of NiO (111) and Ni (111), respectively, which are consistent with the XRD results, indicating the successful fabrication of the composite. In the EDS images (Fig.2e), the C, N, O, and Ni elements were uniformly distributed, and the atomic ratio of C, N, O, and Ni was 21.57∶7.65∶29.62∶41.16 (Fig.S2).

  Figure 2

  

  Figure 2.  (a, b) SEM images, (c) TEM image, (d) high-resolution TEM image, and (e) SEM-EDS mapping images of Ni-NiO@CN-500

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  2.2   Electrocatalytic properties of composites

  The influence of CH₃OH concentration gradient (0.2-2.0 mol·L^-1) on the activity of Ni-NiO@CN-500 for MOR was systematically studied in 1.0 mol·L^-1 KOH solution. With the increase of CH₃OH concentration, the current density of MOR significantly increased (Fig.S3). Still, the increase slowed down when the concentration exceeded 1.0 mol·L^-1, which is ascribed to the increased ionic conduction resistance of the electrolyte caused by high-concentration CH₃OH.

  To further study the influence of KOH concentration on the performance of MOR, as shown in Fig.S4, the current density gradually rose with the increase of KOH concentration, which is attributed to the fact that the rise of KOH concentration enhances the electrical conductivity of the solution.

  As shown in Fig.3a, in the KOH solution (1.0 mol·L^-1), the CV curves of all samples presented the Ni²⁺/Ni³⁺ redox pairs at a scan rate of 50 mV·s^{-1 [34]}. Considering the area surrounded by the CV curves, the order was Ni-NiO@CN-500 > Ni-NiO@CN-600 > NiO@CN-400, which indicates that Ni-NiO@CN-500 showed the largest specific capacitance^[5]. Meanwhile, the data in Table S1 show that both the anode peak current density (j_pa) and the cathode peak current density (j_pc) of Ni-NiO@CN-500 were superior to those of the control samples, which confirms that it had the optimal electrocatalytic activity. Furthermore, the redox peak potential difference [ΔE_p=0.24 V (vs RHE)] of Ni-NiO@CN-500 exceeded that of NiO@CN-400 (0.17 V) and Ni-NiO@CN-600 (0.21 V), which indicates that its electron transfer kinetic rate is the fastest. The initial oxidation potential [1.35 V (vs RHE)] of Ni-NiO@CN-500 was lower than that of NiO@CN-400 [1.36 V (vs RHE)] and Ni-NiO@CN-600 [1.40 V (vs RHE)], further confirming that it had the fastest MOR kinetic rate.

  Figure 3

  

  Figure 3.  Electrochemical performance: (a) CV curves at 50 mV·s^-1 in 1.0 mol·L^-1 KOH solution; (b) calculation of the double layer capacitance of different materials; (c) CV curves at 50 mV·s^-1 in 1.0 mol·L^-1 KOH+1.0 mol·L^-1 CH₃OH solution; (d) EIS

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  In the non-Faraday potential range of 1.11-1.17 V (vs RHE) and 1.0 mol·L^-1 KOH solution, CV tests were measured to obtain the sample′s electrochemically active surface area (ECSA) (Fig.S5). Based on the relationship between the scanning rate (v) and the current density (j), the double-layer capacitance (C_dl) values were calculated to be 2.7 mF·cm^-2 (NiO@CN-400), 11.9 mF·cm^-2 (Ni-NiO@CN-500), and 8.7 mF·cm^-2 (Ni-NiO@CN-600). According to the linear relationship between ECSA and C_dl, the ECSA values were 67.5 cm² (NiO@CN-400), 297.5 cm² (Ni-NiO@CN-500), and 217.5 cm² (Ni-NiO@CN-600), which indicated that Ni-NiO@CN-500 had the optimal MOR catalytic activity (Fig.3b). In the mixed electrolyte of 1.0 mol·L^-1 KOH+1.0 mol·L^-1 CH₃OH solution, the activity (164 mA·cm^-2) of Ni-NiO@CN-500 for MOR exceeded that of NiO@CN-400 (96 mA·cm^-2) and Ni-NiO@CN-600 (131 mA·cm^-2) (Fig.3c). This activity difference perfectly agreed with the ECSA trend measured in the CH₃OH-free alkaline electrolyte system. The dual experimental evidence jointly confirmed that Ni-NiO@ CN-500 was the optimal MOR electrocatalyst.

  As shown in Fig.3d, the Nyquist curves of all samples had similar characteristics. The solution resistance (R_s) of Ni-NiO@CN-500 was significantly inferior to that of NiO@CN-400 and Ni-NiO@CN-600 in the high-frequency region, indicating that it had a lower interfacial contact resistance^[35]. The Warburg impedance of Ni-NiO@CN-500 was significantly inferior to that of the control samples in the low-frequency region, reflecting the minimal ion diffusion resistance^[36]. Combining high-frequency/low-frequency region characteristics, Ni-NiO@CN-500 exhibited optimal MOR electrocatalytic activity.

  A comparative experimental analysis was conducted to explore the MOR activity of Ni-NiO@CN-500. In 1.0 mol·L^-1 KOH solution, the CV curve of the composite showed typical Ni²⁺/Ni³⁺ redox peak characteristics (Fig.4a). When 1.0 mol·L^-1 CH₃OH was introduced into the system, the anode current density value increased significantly, confirming that the MOR was dominant on the Ni-NiO@CN-500 surface (Fig.4a). As shown in Fig.S6, the Ni-NiO@CN-500 for the MOR in alkaline solution was much better than that of the Pt/C catalyst. According to literature report^[5], the electrocatalytic process of MOR on Ni-based catalysts follows the following three-step reaction mechanism, among which the NiOOH species have the dual functions of adsorption sites and active species for CH₃OH oxidation.

  Figure 4

  

  Figure 4.  CV tests of Ni-NiO@CN-500: (a) CV curves at 50 mV·s^-1 in different solutions; (b) CV curves at different scanning rates (10, 20, 30, 40, 50, 80, and 100 mV·s^-1) in 1.0 mol·L^-1 KOH solution; (c) Linear fitting of anodic and cathodic peak current densities to the square roots of the scan rates for Ni-NiO@CN-500; (d) CV curves in 1.0 mol·L^-1 KOH+ 1.0 mol·L^-1 CH₃OH solution at different scanning rates (10, 20, 30, 40, 50, 80, and 100 mV·s^-1; inset: the magnified view of a local region)

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  As shown in Fig.4b, with the increase of the scan rate, the anode peak position shifted positively while the cathode peak position shifted negatively. As shown in Fig.4c, the linear relationship between the anode peak current density (I_pa) and the cathode peak current density (I_pc) of electrodes and the square root of the scan rate (v^1/2) indicates that the Ni(OH)₂⇌NiOOH conversion process is regulated by the diffusion mechanism^[37-38]. Significantly, a greater slope of the curve indicated that the material not only had a stronger OH^- diffusion flux but also could generate more active substance NiOOH (Fig.4c). The linear slope of Ni-NiO@ CN-500 exceeded those of other samples (Fig.S7), indicating that its Ni site had a more substantial adsorption effect on OH^-. As illustrated in Fig.4d, in the mixed solution of 1.0 mol·L^-1 KOH+1.0 mol·L^-1 CH₃OH solution, with the rise of the scan rate, the current density of Ni-NiO@CN-500 did not show significant fluctuations, revealing that a kinetic process dominates the MOR.

  Long-term durability is a key indicator to assess the performance of composites. The durability tests were conducted on Ni-NiO@CN-500 at 1.67 V (vs RHE) in the mixed solution of 1.0 mol·L^-1 KOH+1.0 mol·L^-1 CH₃OH solution. As shown in Fig.5a, after 1 500 CV cycles (50 mV·s^-1), the current density decayed by 16%. When the fresh electrolyte was replaced, the current density returned to 94% of the initial one, indicating that the attenuation of the current density was mainly due to the consumption of CH₃OH. Meanwhile, the shapes of EIS before and after the CV cycle (Fig.5b) were highly similar, a semi-circle and a straight line correspond to the charge-transfer resistance and the capacitive behavior of the electrodes^[39]. The equivalent contact resistance, namely the intercept on the X-axis, increased gradually from 4.0 Ω·cm² (1st) to 4.5 Ω·cm² (1 500th), indicating that the change in electrode impedance can be ignored. Furthermore, after the 12 h CA tests (Fig.5c), the current density decreased by only 10.1% (from 169 to 152 mA·cm^-2), and the current density on the Pt/C catalyst quickly decreased by 33.1% (from 60 to 20 mA·cm^-2). The above results confirm that Ni-NiO@CN-500 has both high activity and good tolerance, and is an excellent MOR catalyst.

  Figure 5

  

  Figure 5.  (a) CV cycling stability tests and (b) EIS (inset: the corresponding enlarged spectra and the fitted circuit diagram) of Ni-NiO@CN-500; (c) CA curves of Ni-NiO@CN-500 at 1.67 V (vs RHE) and Pt/C at 0.97 V (vs RHE) in the mixed solution of 1.0 mol·L^-1 KOH and 1.0 mol·L^-1 CH₃OH solution

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  During the MOR process, the generation of CO was inevitable, which can lead to catalytic deactivation by occupying the active sites of the catalyst^[40]. As displayed in Fig.6a, at the potential of 0.97 V (vs RHE), the current density of the Pt/C catalyst significantly decayed from 59 mA·cm^-2 (without CO) to 35 mA·cm^-2 (CO saturated), and the retention rate of the current density was only 59%. In the comparative experiment, the CO tolerance test of Ni-NiO@CN-500 at the potential of 1.67 V (vs RHE) showed that its current density only slightly decreased from 164 mA·cm^-2 (in the CO-free system) to 155 mA·cm^-2 (in the CO-saturated system) with a retention rate of 94.5% (Fig.6b). Meanwhile, the EIS shapes of Ni-NiO@CN-500 in KOH+CH₃OH solution with and without CO were highly consistent (Fig.6c), indicating that its charge transfer resistance was not significantly affected by CO. The above comparative experiments confirm that Ni-NiO@CN-500 has significant anti-CO poisoning performance compared with commercial Pt/C catalysts.

  Figure 6

  

  Figure 6.  CO-resistance tests of Pt/C and Ni-NiO@CN-500: (a, b) CV curves at 50 mV·s^-1 and (c) EIS

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  3.   Conclusions

  In summary, a composite of Ni-NiO nanoparticles anchored on two-dimensional N-rich CN was successfully constructed in this work. The optimally prepared Ni-NiO@CN-500 demonstrated significantly enhanced MOR catalytic activity, excellent chemical stability, and CO-tolerance. The superb performance of the composite is primarily determined by the fact that the uniformly dispersed Ni-NiO nanoparticles have excellent activity, and the N-rich CN two-dimensional carrier not only promotes electron transfer but also protects the active components from loss. This work provides new methods and ideas for preparing low-cost, high-activity, and anti-CO poisoning MOR electrocatalysts.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  (a) XRD patterns of NiO@CN-400, Ni-NiO@CN-500, and Ni-NiO@CN-600; (b) Survey, (c) C1s, and (d) Ni2p XPS spectra of Ni-NiO@CN-500

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  Figure 2  (a, b) SEM images, (c) TEM image, (d) high-resolution TEM image, and (e) SEM-EDS mapping images of Ni-NiO@CN-500

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  Figure 3  Electrochemical performance: (a) CV curves at 50 mV·s^-1 in 1.0 mol·L^-1 KOH solution; (b) calculation of the double layer capacitance of different materials; (c) CV curves at 50 mV·s^-1 in 1.0 mol·L^-1 KOH+1.0 mol·L^-1 CH₃OH solution; (d) EIS

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  Figure 4  CV tests of Ni-NiO@CN-500: (a) CV curves at 50 mV·s^-1 in different solutions; (b) CV curves at different scanning rates (10, 20, 30, 40, 50, 80, and 100 mV·s^-1) in 1.0 mol·L^-1 KOH solution; (c) Linear fitting of anodic and cathodic peak current densities to the square roots of the scan rates for Ni-NiO@CN-500; (d) CV curves in 1.0 mol·L^-1 KOH+ 1.0 mol·L^-1 CH₃OH solution at different scanning rates (10, 20, 30, 40, 50, 80, and 100 mV·s^-1; inset: the magnified view of a local region)

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  Figure 5  (a) CV cycling stability tests and (b) EIS (inset: the corresponding enlarged spectra and the fitted circuit diagram) of Ni-NiO@CN-500; (c) CA curves of Ni-NiO@CN-500 at 1.67 V (vs RHE) and Pt/C at 0.97 V (vs RHE) in the mixed solution of 1.0 mol·L^-1 KOH and 1.0 mol·L^-1 CH₃OH solution

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  Figure 6  CO-resistance tests of Pt/C and Ni-NiO@CN-500: (a, b) CV curves at 50 mV·s^-1 and (c) EIS

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