Durable Ni₃N porous nanosheets array for non-noble metal methanol oxidation reaction

Methanol is one of the most interesting fuels due to its high energy density, portability, safety while handling, solubility in aqueous solutions, accessibility, and potential generation from renewable energies.^1–3 With the rising availability of methanol, the obstacle will be creating a reliable device to transform methanol into electricity. Direct-methanol fuel cells (DMFCs), which utilize methanol as a fuel and convert chemical energy into electrical energy by directly oxidizing with oxygen (O₂) from the air to carbon dioxide (CO₂) and water, have drawn considerable research interest owing to their easy mobility and energy density.⁴ One of the key components of DMFCs is the electrocatalyst, which improves the methanol oxidation reaction (MOR) efficiency on the anode.⁵ Sustainable energy storage and conversion is a key driver for the development of nanostructured electrocatalysts, but it is crucial to design catalysts with high activity and durability for MOR.  Pt or Pt-based alloys have been reported to be the most active catalysts for some electrochemical reactions such as biosonsing,⁶ hydrogen evolution reaction,^7,8 oxygen reduction reaction,⁹ and MOR, but CO poisoning and the high cost have limited their commercialization.¹⁰ Therefore, Pt-free electrocatalysts with high activity, high reliability, and stability are required for practical applications and the market acceptance of DMFC technologies.

Among non-platinum metal catalysts, nickel-based catalysts are considered the most promising to replace Pt because of their low cost, resistance to poisoning, good surface oxidation properties, and high catalytic activity toward electro-oxidation of alcohols.¹¹ Among them, the Ni₃N catalyst has attracted considerable interest due to abundant active sites, rapid charge transfer rate, and notable catalytic performance.^12,13 Theoretical calculations predict that metal nitride electrocatalysts possess many advantages over pure metals.¹⁴ Moreover, the presence of nitrogen strongly influences the electronic properties of the metal by increasing the density of electrons on the surface. However, Ni₃N is vulnerable to passivation under alkaline environments, and the activity and durability of Ni₃N catalysts in alkaline solutions are substantially lowered.¹⁵ Previous research has exploited activated carbonaceous additives and metal foil/foam/mesh substrates as the catalyst support that enhanced the conductivity and stability of the electrode materials.^16,17

Notably, catalysts with accessible active surface sites promote electrocatalytic activity in a single direction.¹⁸ Nickel is also recognized as a outstanding flexible support, and researchers have used Ni foam as a substrate.¹⁷ Also, Ni foam (NF) is extraordinarily stable in alkaline solutions and can be mass-produced with diverse surface and structural modifications.  Herein, we design unique self-supported nanosheets of Ni₃N anchored on a 3D Ni foam skeleton by using a simple two-step method. This unique in situ growth of Ni₃N NSAs on Ni foam (Ni₃N NSAs/NF) is beneficial in many ways; for example, no additional binder or conducting agent is required to prepare the electrode, making handling very easy and cheap. With a large specific and electrochemically accessible surface area, Ni foam provides abundant catalytic active sites by dispersing the active centers, resulting in a high loading of Ni₃N NSAs. Last but not least, the stability of the catalyst improves due to the seamless contact between the nanosheet arrays of Ni₃N and Ni foam, ensuring tight integration that prevents leaching and triggers rapid charge transfer. Detailed catalytic tests show that the Ni₃N NSAs/NF not only has enhanced electrocatalytic properties in MOR but also has an improved stability for 20 h in methanol oxidation under alkaline conditions. The exceptional electrochemical activity of the Ni₃N NSAs/NF system originates from the high conductivity, exposed edges of Ni₃N nanosheets, and synergistic effects between Ni₃N NSAs and Ni foam.

A simple hydrothermal technique is used to create the Ni(OH)₂ NSAs/NF hybrid structure. Typically, 26.25 mmol urea, 10.5 mmol NH₄F, and 3.93 mmol Ni(NO₃)₂·6H₂O were mixed in 70 mL H₂O and stirred at room temperature for 1 hour to ensure the homogenous mixing. The mixture was transferred to a 100 ml Teflon reactor. A piece of nickel foam (NF) with a size of 2 × 3 cm² was cleaned by soaking in 1M HCl for 30 min, washed with water, and placed vertically in the Teflon reactor containing the mixture. The reactor was put in an oven at 100 °C for 8 h to produce the Ni(OH)₂ nanosheet array precursor.

The Ni(OH)₂ NSAs/NF hybrid was placed in a tube furnace and annealed at 350, 380, 400, 430, and 450 °C for 3 h under NH₃ (50 SCCM), respectively. After reacting for 3 h, the product was collected and denoted as Ni₃N–Y (Y = 350, 380, 400, 430, and 450). To compare, the control catalysts were synthesized using the same technique with slight changes. NF-400 was prepared by annealing the NF at 400 °C. The Ni(OH)₂ precursor was annealed at 400 °C for 3 h under air in a muffle furnace and under an Ar/H₂ (5 Vol % H₂) atmosphere in the tube furnace to obtain nickel oxide (NiO-400) and metal Ni (Ni-400) nanoparticles, respectively.

Transmission electron microscopy (TEM) was conducted on a Tecnai G2 F20. The specimens for TEM were prepared by depositing a drop of a dilute catalyst dispersion onto a mica substrate and a holey carbon-coated copper grid, respectively. X-ray diffraction (XRD) was carried out on a Bruker D8 advance x-ray diffractometer. Scanning electron microscopy (SEM) and EDS were performed on a Hitachi S4800 field-emission microscope. The surface compositions of catalysts were determined by x-ray photoelectron spectroscopy (XPS).

All electrochemical tests were performed using a three-electrode electrochemical system with a prepared Ni₃N NSAs/NF (0.5 × 0.5 cm² ) as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a Pt wire as the counter electrodeon on the CHI 760E electrochemical workstation. Cyclic voltammetry (CV), chronoamperometry (CA), electrochemical active surface area (ECSA), and electrochemical impedance spectroscopy (EIS) were carried out. All tests were conducted in a 1.0 M KOH solution bubbled with N₂ in the absence or presence of methanol.

As shown in Scheme , the highly porous Ni₃N nanosheet arrays are fabricated by a facile two-step approach. Firstly, the Ni(OH)₂ NSAs are grown on well-cleaned, highly porous NF as a conductive template by a hydrothermal process. The Ni(OH)₂ NSAs/NF nitride is annealed at 400 °C under flowing NH₃ to obtain the Ni₃N NSAs/NF composite. The Ni₃N NSAs/NF electrode is evaluated for MOR. X-ray diffraction (XRD) is employed to study the crystal phases and structural components. Figure 1(a) displays the XRD patterns of the as-prepared Ni(OH)₂ and Ni₃N–Y (Y = 350, 380, 400, 430, and 450) NSAs/NF samples. By comparing the XRD patterns of Ni₃N NSAs/NF prepared at different temperatures, Ni₃N-380 and Ni₃N-400 NSAs/NF show the formation of Ni₃N, as the peaks at 38.6°, 42.1°, 44.5°, 58.5°, 70.6°, and 78.4° match the (110), (002), (111), (112), (300), and (113) crystal planes of the hexagonal Ni₃N (PDF#10-0280), respectively. A low temperature (350 °C) leads to partial Ni(OH)₂ un-transformation, while high temperatures (430 and 450 °C) reduce Ni(OH)₂ to metallic nickel. The morphology of the as-prepared samples is characterized by SEM and high resolution transmission electron microscopy (HR-TEM). As shown in Fig. 1(b), the surface of the pure NF is relatively rough after reacting with the nickel salt aqueous solution, revealing the successful preparation of Ni(OH)₂ nanosheet arrays. Figure 1(c) shows that the rough surface is mainly because of uniformly vertically aligned, interconnected nanosheets with large interlayer clearance on the nickel foam. As mentioned above, Ni₃N NSAs/NF is derived from the Ni(OH)₂ precursor through nitridation under NH₃, and different temperatures are adopted in the nitridation process to explore the best conditions. According to the SEM images [Figs. 1(d)–1(f)], the smooth nanosheet arrays of Ni(OH)₂ gradually change to scaly standing porous nanosheets on the nickel foam. As the temperature goes up, the surface of the nanosheets becomes rough and porous, mainly due to the gas release and dehydration of Ni(OH)₂ during the nitridation process.¹⁹ As shown in Fig. 1(e), the SEM image of Ni₃N NSAs/NF-400 clearly shows the formation of highly porous nanosheets with the most suitable size and thickness, which is also in agreement with XRD. This unique porosity and rough surface produce numerous channels for rapid ion transport and provide large surface active sites for facilitating electrocatalysis. The elemental mapping results in Fig. S1 indicate that Ni and N are homogeneously distributed in Ni₃N NSAs/NF-400, revealing that the Ni₃N nanosheets have a uniform composition.

The TEM image [Fig. 2(a)] shows that the Ni₃N-400 nanosheet is composed of numerous small nanoparticles. The HR-TEM image of Ni₃N-400 in Fig. 2(b) reveals well-resolved lattice fringes with an interplanar d-spacing of 0.203 and 0.215 nm consistent with the (111) and (002) crystal planes of hexagonal Ni₃N. To identify the chemical state of the surface elements of the catalyst, x-ray photoelectron spectroscopy (XPS) is performed. As shown in Figs. 2(c) and 2(d), the peaks of Ni 2p [Fig. 2(c)] at 852.7, 855.8, and 873.5 eV arise from Ni-N, and the other peaks can be ascribed to Ni²⁺ from surface oxidized Ni species.¹² The satellite peak at 861.4 eV is attributed to the shake-up excitation of the high-spin nickel ions.^18, Figure 2(d) shows the XPS spectra of N 1s deconvoluted into three peaks positioned at 397.8, 399.3, and 403.1 eV, which are assigned to Ni-N in Ni₃N, absorbed N-H_x, and oxidized-N, respectively.²⁰ The intense peak at 397.8 eV confirms the formation of metal nitride by ammonolysis.²¹ 

The electrochemical properties of the as-prepared catalyst (Ni₃N-400) are investigated by CV, but before that, a contrast test is performed to evaluate the MOR characteristics. It has been shown that the oxygen evolution reaction (OER) and MOR may take place under identical potential ranges after the formation of NiOOH (Ni³⁺) on the Ni-based catalysts in an alkaline solution.²² Therefore, to determine the potential range of the catalyst, CV is conducted in 1.0M KOH with/without the methanol solution in the 1.1–1.8 V potential window. Figure 3(a) clearly shows the MOR characteristics of the Ni₃N-400 catalyst between 1.5 and 1.7 V after the oxidation peak of Ni²⁺ appears in the absence of the methanol solution, whereas OER occurs on the catalyst at about 1.75 V vs RHE with a large overpotential, demonstrating the superior performance of the catalyst at a lower MOR potential compared to OER. Therefore, the potential range for evaluating the MOR activity of Ni₃N-400 is determined to be 1.1–1.7 V.

As the formation of NiOOH is very imperative to methanol oxidation, all the samples are subjected to CV tests for 50 cycles at 50 mV s⁻¹ in 1.0M KOH solution [Fig. 3(b)]. The samples are pretreated to promote the formation of Ni(OH)₂/NiOOH on the surface and thicken the electrocatalytic layers.^23,24 As demonstrated in Fig. 3(b), all the catalysts exhibit two redox peaks corresponding to the Ni²⁺/Ni³⁺ redox reaction. Among them, Ni₃N-400 shows the highest oxidation/reduction peak current density, indicating more exposed active sites on the catalyst in 1.0M KOH solution.²⁵ 

Figure S4c shows the linear relationship between the current density of the anodic peak and the square root of the scanning rate, indicating that Ni²⁺/Ni³⁺ is a diffusion-controlled process.²⁸ Therefore, using the above equation, the proton diffusion coefficient (D) is used to characterize the oxidation behavior of the electrocatalysts. D is estimated to be 10.73 × 10⁻⁷ cm² s⁻¹ for Ni₃N-400. It is important to note that this D value is larger than previously reported values (Table I), and moreover, the D value of Ni₃N-400 is more than two times higher than those of other samples (Table S1).

To further study the insight into the better MOR characteristics of Ni₃N-400, the electrochemical double layer capacitance (C_dl) is employed to estimate the ECSA of the as-prepared samples. C_dl is projected by CVs with different scanning rates, as shown in Figs. S5–S10. The high C_dl value implies a larger ECSA (Fig. S7) due to enhanced anion exchange ability between the electrolyte and catalytic active sites. As shown in Fig. S7b, the Ni₃N-400 electrode has the largest C_dl of 10.84 mF cm⁻² and a larger ECSA (271 cm⁻²), which is greater than all the Ni₃N-based catalysts prepared at different temperatures (Table S1). The larger ECSA of Ni₃N-400 signifies that the rational design of the self-supported, uniformly assembled nanosheet arrays can effectively increase the accessibility of active sites.²⁹ 

The kinetics of the samples are investigated by EIS. The diameter of the semi-circle in the Nyquist plot denotes the charge transfer resistance (R_ct). Ni₃N-400 exhibits the smallest semi-circle, as shown in Fig. S11, compared to Ni-400 and pristine NF. The small semi-circle relates to the low mass transfer resistance and high electron transfer rate, contributing to the best MOR kinetics. Therefore, it can be concluded that the highly electrocatalytic performance of Ni₃N-400 is not only due to increased ECSA but also the formation of active interface/sites, rapid reaction kinetics, and lower R_ct value. To further evaluate the reaction kinetics of the MOR, Tafel slopes are calculated. Generally, a small Tafel slope represents fast electrochemical kinetics that favor the high catalytic activity. As shown in Fig. S12, the Ni₃N-400 catalyst exhibits a Tafel value of 84 mV dec⁻¹, which is significantly lower than that of Ni-400 (94.7 mV dec⁻¹), Ni(OH)₂ (113 mV dec⁻¹), NF-400 (126 mV dec⁻¹), and NiO-400 (477 mV dec⁻¹), indicating significantly faster catalytic kinetics.

In order to determine the durability and stability of the anodic catalyst in DMFC, chronoamperometry (CA) is performed in 1.0M KOH/CH₃OH at 0.5 V for 10 000 s (Fig. S13a). The Ni₃N-400 catalyst shows the highest current density among the catalysts even after 10 000 s. To further demonstrate the high stability of Ni₃N-400, the i–t test is continuously repeated for 20 h under the same conditions, while the electrolyte is changed every 5 h [Fig. 4(a)]. After 20 h, the retention rate of Ni₃N-400 is still 72.58%, which is the highest among the other Ni-based catalysts, indicating a greater tolerance for the intermediates formed during methanol oxidation (Table I). However, the 27% decrease in stability may stem from that some of the NiOOH species during MOR are changed to Ni(OH)₂ and from the poisoning of the active sites.^24,30 This is further confirmed by the CV results (Fig. S13b) that the peak current densities of the Ni²⁺/Ni³⁺ redox after the 20-h stability test decrease compared to the initial sample. In addition, the stability of the catalyst is studied by continuous CV scanning. As shown in Fig. S13c, the retention rate of all the samples is 70%, while Ni₃N-400 shows a 74.25% retention rate after 1000 cycles. The SEM and TEM images shown in Fig. 4(b) and 4(c) demonstrate that electrocatalysis has no discernible effects on the size and morphology of Ni₃N-400 and that no growth or agglomeration occurs. XRD also shows no changes in the catalyst crystallinity, confirming the outstanding durability of Ni₃N-400 during MOR (Fig. S14). It is noted that an amorphous layer of about 3 nm is formed at the edges site after the long-time test [Fig. 4(c)]. This layer is composed of NiOOH active species,^31,32 and XPS confirms that the NiOOH species are formed on the surface of the sample. Comparing the Ni 2p spectra of Ni₃N-400, the Ni–N peak at 852.7 eV vanishes. In addition, all the peaks of Ni₃N-400 in the N 1s spectrum disappear due to the absence of Ni₃N as it reacts to form new active species of NiOOH.³¹ These results suggest that Ni₃N-400 has a good long-term stability as an anode electrocatalyst in DMFCs due to the excellent structural stability of the mesoporous Ni₃N and surface-active species NiOOH.

A self-supporting catalyst composed of hierarchical Ni₃N NSAs on Ni foam is demonstrated for enhanced MOR via ammonification. The Ni₃N catalyst delivers the best MOR performance in alkaline media as manifested by a current density of 186.15 mA cm⁻² and high durability of up to 20 h at a retention rate of 72.58%. The good characteristics are attributed to the following reasons: (1) the highly porous nanosheet arrays comprised of small nanoparticles expose more active sites and facilitate fast charge/mass transfer; (2) Ni foam acts as a conductive support to provide abundant catalytic active sites by dispersing the active centers, resulting in a high loading of Ni₃N NSAs; and (3) the stability is significantly improved due to the seamless contact between Ni₃N nanosheet arrays and Ni foam, thus preventing leaching and triggering rapid charge transfer and producing synergistic effects between Ni₃N NSAs and Ni foam. The findings reveal a significant step forward in the development of highly efficient and economically viable electrocatalysts in the pursuit of efficient DMFCs.

See the supplementary material for Figures S1–S14 and Table S1.

This work was financially supported by the 2022 Special Fund Project for Science and Technology Innovation Strategy of Guangdong Province (Grant Nos. STKJ202209083 and STKJ202209077), the 2020 Li Ka Shing Foundation Cross-Disciplinary Research Grant (Grant Nos. 2020LKSFG01A and 2020LKSFG09A), Jieyang Science and Technology Project (Grant No. skjcx039), City University of Hong Kong Strategic Research Grant (SRG) (Grant No. 7005505), and City University of Hong Kong Donation Research Grant (Grant No. DON-RMG No. 9229021).

The authors have no conflicts to disclose.

Wanying Zhang and Madiha Rafiq contributed equally to this work.

Wanying Zhang: Conceptualization (equal). Madiha Rafiq: Writing – original draft (equal). Jingcheng Lu: Validation (supporting). Abebe Reda Woldu: Writing – review & editing (supporting). Jianhong Zhou: Formal analysis (supporting). Hong Xia: Methodology (supporting). Paul K. Chu: Writing – review & editing (equal). Liangsheng Hu: Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). Fushen Lu: Project administration (equal); Resources (equal); Supervision (equal).

The data supporting the results of this study are available upon reasonable request from the corresponding author.