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Article

Facile Synthesis of Metal/Carbide Hybrid toward Overall Water Splitting

by
Junxiang Mo
1,
Nianqing Fu
1,
Songlin Mu
1,
Jihua Peng
1,
Yan Liu
2 and
Guoge Zhang
1,*
1
School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510641, China
2
School of Chemical Engineering and Technology, Sun Yat-Sen University, Zhuhai 519082, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(10), 730; https://doi.org/10.3390/catal14100730
Submission received: 9 September 2024 / Revised: 10 October 2024 / Accepted: 15 October 2024 / Published: 18 October 2024
(This article belongs to the Special Issue Recent Applications of Metal Catalysts in Organic Synthesis, 2nd Edition)
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Abstract

:
The development of cost-effective and high-performance bifunctional catalysts for overall water splitting is crucial for achieving sustainable clean energy. In this study, a metal/carbide hybrid (NiFeMo/NiFeMoCx) was prepared through fast and facile cathodic plasma electrolytic deposition. Due to the synergistic effect between the metal and carbide, NiFeMo/NiFeMoCx exhibited high activity in both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), with overpotentials of 230 mV and 60 mV at 10 mA cm−2, respectively. In addition, robust stability was demonstrated during the overall water splitting (1.52 V at 10 mA cm−2, with little degradation after 18 h of catalysis). This work provides a useful strategy for designing advanced water splitting catalysts for real application.

1. Introduction

Due to the exacerbation of global warming and pollution caused by fossil fuels, environmental issues and energy shortages are severe challenges faced by the scientific community [1,2]. Hydrogen is considered to be a sustainable zero-carbon emission alternative to fossil fuels with a high combustion value [3,4,5]. Electrochemical water splitting is one of the most widely used strategies to produce hydrogen and oxygen with electrical energy [6]. However, the kinetic bottlenecks in the process of water oxidation greatly limit electrolysis’s efficiency and impede the real application of water splitting. Water oxidation involves complex reactions that generate oxygen molecules through a sluggish four electron-proton-transfer process [7]. Therefore, there is an urgent need for the controllable synthesis of advanced electrocatalysts to accelerate reaction rates and effectively reduce overpotential. Accordingly, various strategies have been developed, such as phase engineering [8,9], morphology control [10,11], the design of a single atom catalyst [12,13], and the construction of a heterostructure [14,15]. Currently, precious metals (Pt, Ru, and Ir)-based materials have demonstrated excellent electrocatalytic activity in either the OER (oxygen evolution reaction) or the HER (hydrogen evolution reaction). However, their application is severely hindered by the high cost, scarce resources, and poor durability [16,17]. In addition, there is one problem with using two different electrocatalysts for water electrolysis in an electrolytic cell. Cross-contamination might occur between electrocatalysts with different compositions during electrolysis, leading to decreased activity and inferior stability. Therefore, it is desirable to design a highly efficient bifunctional electrocatalyst based on non-precious metals toward overall water splitting.
In recent years, nickel-based materials have attracted increasing attention thanks to their advantages such as low cost, abundant resources, and high catalytic activity. Various nickel-based catalysts, such as hydroxides [18,19], phosphides [20,21,22], sulfides [23,24,25], nitrides [26,27,28], selenides [29,30], and nickel-molybdenum composites [31,32] have been synthesized as efficient HER catalysts. Among them, although nickel–molybdenum catalysts exhibited the most promising HER activity, their OER activity was far from satisfactory. The incorporation of iron significantly enhanced the OER for nickel-based catalysts [33,34]. For example, a Ni0.36Fe0.64 alloy was synthesized through a hydrothermal process followed by reduction in hydrogen, achieving a η10 (overpotential at 10 mA cm−2) of 298 mV in OER, which surpassed that of pure Ni (430 mV) or Fe (460 mV) [35]. Fe-doped NiS2 was prepared via the sulfurization of NiFe LDH [36]. Due to the rapid charge transfer, abundant active sites, and modulated electronic structure, Fe-doped NiS2 nanowire arrays exhibited an excellent oxygen evolution activity, requiring an overpotential of 278 mV to achieve a current density of 100 mA cm−2. Mesoporous Fe-doped NiO nanosheets were fabricated through the combination of a solvothermal procedure and annealing [37]. Electrons were transferred from nickel to iron, facilitating both the adsorption and desorption of oxygen intermediates at Fe sites. The η10 for the HER and OER was 88 mV and 206 mV, respectively. Although significant progress has been achieved, it is still challenging to develop a fast and facile technique to prepare a bifunctional catalyst with high activity and robust stability.
Cathodic plasma electrolytic deposition (CPED) is a technology combining conventional electrodeposition and plasma processes [38]. When a high voltage is applied, abundant bubbles are formed on the surfaces of both the cathode and anode. Once the electric field on the cathode surface is sufficient to break down the bubble sheath, a plasma is formed and large amounts of heat are generated. Enormous active species, such as free radicals and electrons, are produced and facilitate the rapid deposition of metal ions on the cathode [39,40]. CPED has been applied successfully to prepare highly efficient HER catalysts, such as a carbon-coated Janus metal/oxide hybrid [41] and a metallic cobalt with polymorphism interface (Co-hybrid/CoO_MoC) [42].
Herein, CPED was used to synthesize a metal/carbide hybrid (NiFeMo/NiFeMoCx) as the bifunctional catalyst for water splitting. The OER activity was significantly improved with the doping of iron, while the HER performance was enhanced with the introduction of a heterostructured metal/carbide. The η10 was 230 mV for the OER and 60 mV for the HER. During the overall water splitting, only 1.52 V was required to reach the current density of 10 mA cm−2. There was little degradation in the activity after continuous electrolysis for 18 h. This work provides a promising technique for the preparation of outstanding overall water catalysts based on transition metals.

2. Results and Discussion

2.1. Morphological and Structural Characterization

The surface morphologies of the metal/carbide hybrids (NiMo/NiMoCx and NiFeMo/NiFeMoCx) prepared by CPED are shown in Figure 1. The film thickness was 23 μm for both materials. The introduction of Fe resulted in a patterned structure on the surface of the NiFeMo/NiFeMoCx nanoparticles, which enlarged the specific surface area and facilitated the exposure of active sites in the water splitting.
Figure 2a shows the X-ray powder diffraction (XRD) patterns of NiMo/NiMoCx and NiFeMo/NiFeMoCx. Strong peaks near 43.5°, 51°, and 74.5° were observed for both samples and were attributed to the nickel alloy having a face-centered cubic structure. These three peaks were shifted to lower angles compared to the standard patterns of pure nickel, which was caused by lattice expansion that resulted from the doping of larger atoms (Mo and Fe). The diffraction peaks for NiFeMo/NiFeMoCx were wider than those for NiMo/NiMoCx, indicating a smaller crystal size. Other phases (oxides or carbides) were not detected in the XRD for both materials, possibly because of the small quantity or because the crystal size was below the coherence length of the X-ray [43]. According to the Raman pattern (Figure 2b), both samples exhibited TO and LO vibration peaks in the Ni-O bond at 380 and 520 cm−1, respectively. Additionally, a Ni-OH bond vibration peak was observed at 320 cm−1, indicating the presence of NiO and/or Ni(OH)2 species in the film [44,45]. The vibration peak at 906 cm−1 was attributed to symmetric stretching of the Mo-O bond, while those at 820 and 870 cm−1 were assigned to O-Mo-O bond vibrations. The peaks observed at 168, 750, and 780 cm−1 corresponded to Mo-O vibrations in MoO42−, suggesting the existence of molybdate species [46]. The appearance of peaks at 1340 and 1580 cm−1 can be attributed to the carbon’s D band and G band respectively, indicating the presence of carbides in the film. In NiFeMo/NiFeMoCx, significant numbers of vibration peaks were observed at 214, 257, 277, 405, 500 and 612 cm−1, which were assigned to the Fe-O vibrations in Fe2O3 and Fe3O4 [47]. Nickel–iron oxides were detected from the vibration peaks at 460, 540 and 690 cm−1 [48,49]. Furthermore, compared with NiMo/NiMoCx, the vibration peaks of Ni–O bond in NiFeMo/NiFeMoCx were shifted towards higher wave numbers, indicating an increased content of Ni3+ resulting from the oxidation by the solution containing Fe3+ [45,50].
The film composition was detected using energy dispersive X-ray spectroscopy (EDS) and is shown in Table S1. More oxygen was found in NiFeMo/NiFeMoCx than in NiMo/NiMoCx, confirming the presence of more high-valence ions such as Ni3+. This result was in line with the Raman spectra shown above. X-ray photoelectron spectroscopy (XPS) was carried out to study the electronic state as well as the surface chemical environment. The peaks at 283.96, 284.80, 286.21, and 288.51 eV in the C 1s spectrum can be attributed to carbon bonded with the metals, C-C/C=C in adsorbed organic compounds, C-O bonds, and C=O bonds, respectively (Figure 2c). In the O 1s spectrum (Figure 2d), peaks at 829.67, 831.07, and 833.04 eV corresponded to lattice oxygen species, oxygen vacancies, and adsorbed hydroxyl groups, respectively [51,52]. For NiMo/NiMoCx, the Ni 2p spectra exhibited a prominent peak pair around 852.08/869.18 eV (Figure 2e), which was assigned to the metallic nickel (Ni0). Two sets of peaks were observed at 855.38/872.89 eV and 857.48/875 eV, corresponding to Ni2+ and Ni3+ species. In addition, a weak peak pair was detected at 853.74/870.85 eV and was ascribed to Ni with a partially positive charge (Niδ+) [53]. The four sets of peaks centered around 227.44/230.49 eV, 228.97/232.32 eV, 230.94/233.99 eV, and 232.04/235.22 eV in the Mo 3d spectrum were assigned to Mo0, Mo4+, Mo5+, and Mo6+, respectively (Figure 2f). During the synthesis of NiFeMo/NiFeMoCx, the strong oxidizing power of Fe3+ under acidic conditions led to the oxidation of the exposed surface layer of metals and alloys. This resulted in a significant increase in the proportion of ions with a high valence and the absence of Ni0 on the surface. The percentage of Ni3+ was increased from 22.13% to 51.47%, and the percentage of Mo6+ was increased from 45.72% to 98.81%. In terms of the Fe 2p spectrum (Figure S1) of NiFeMo/NiFeMoCx, three distinct sets of peaks were observed at 707.58/721.22, 710.28/723.08, and 712.57/725.37 eV, which can be attributed to Fe(0), Fe(II), and Fe(III), respectively. Notably, the proportion of Fe(III) was as high as 62.89%. As a potent Lewis acid, Fe(III) extracts electrons from adjacent metal sites, producing more Ni3+ and Mo6+ within the catalyst. Ni3+, Fe3+, and Mo6+ favored the adsorption of oxygen-containing intermediates, facilitating water dissociation and leading to better HER/OER activity.
In order to gain a more comprehensive understanding of NiFeMo/NiFeMoCx, a detailed characterization was performed using transmission electron microscopy (TEM). In the high-resolution images, distinct lattice fringes were revealed with interplanar spacings of 0.223 nm, 0.245 nm, and 0.315 nm, corresponding to the (021) plane of MoNi4, the (006) plane of MoC, and the (031) plane of Fe2O3, respectively (Figure 3b,c). Selected area electron diffraction patterns (SAED) are presented in Figure 3d, illustrating well-defined diffraction rings for MoNi4, MoC, and Fe2O3. These results confirmed the existence of nickel alloy and metal carbide in NiFeMo/NiFeMoCx, along with the small amounts of oxide that resulted from the natural oxidation of metal/carbide in the atmosphere.

2.2. HER Performance

In alkaline conditions, both materials exhibited good HER activity. The η10 was 72 and 60 mV for NiMo/NiMoCx and NiFeMo/NiFeMoCx, respectively (Figure 4a). The control sample of NiFeMo was prepared via a conventional electrodeposition (without the introduction of plasma) at 2 V for 10 min in the same solution as that used for the synthesis of NiFeMo/NiFeMoCx. No active carbon species were produced during the conventional electrodeposition because of the much lower potential and/or much lower temperature (the local temperature in the solution during CPED could be >3000 K) [38]. Therefore, no carbide was formed and only NiFeMo was deposited. The η10 was 172 mV for NiFeMo (Figure S2), much higher than that of NiFeMo/NiFeMoCx (60 mV). The greatly enhanced activity of NiFeMo/NiFeMoCx might have resulted from the synergistic effect between metal and carbide, which facilitated the charge redistribution on the interface and promoted the tandem electron transfer process [54,55]. The Tafel slope provides valuable information regarding the reaction mechanism and is utilized to assess the kinetics of catalysis. The doping of iron increased the concentration of Ni3+, which exhibited a strong affinity towards water molecules and oxygen-containing intermediates. This facilitated the dissociation process of water and mitigated its hindrance of the HER. Consequently, the Tafel slope of NiFeMo/NiFeMoCx (59.1 mV dec−1) was lower than that of NiMo/NiMoCx (72.8 mV dec−1), as shown in Figure 4b. The accelerated dissociation of water and appropriate hydrogen binding strength enhanced the alkaline HER kinetics for NiFeMo/NiFeMoCx, leading to a higher intrinsic catalytic activity.
The catalytic activity of electrode materials is significantly influenced by the specific surface area. The electrochemical surface area (ECSA) was estimated based on the CV curves collected in a non-Faradaic potential window (Figure S3a,b). The double layer capacitance of NiFeMo/NiFeMoCx was slightly higher than that of NiMo/NiMoCx (Figure S3c, 5.22 mF cm−2 vs. 4.73 mF cm−2). Assuming an ideal planar material with a specific capacitance of 40 μF cm−2, the calculated electrochemical surface areas were 118.25 and 130.50 cm2 for NiMo/NiMoCx and NiFeMo/NiFeMoCx, respectively. As revealed in the SEM image (Figure 1b), the superficial nanoflakes provided NiFeMo/NiFeMoCx with a larger specific surface area and facilitated the exposure of active sites. Electrochemical impedance spectroscopy (EIS) was measured at an overpotential of 60 mV for each sample (Figure 4c). Experimental data were fitted using an equivalent circuit composed of series resistance (Rs), H adsorption resistance (RHER), and charge transfer resistance (Rct) (Figure S4) [56]. The fitting results are presented in Table S2. Due to its larger electrochemical surface area, NiFeMo/NiFeMoCx exhibited improved contact with the electrolyte, resulting in a slight decrease in series resistance. The presence of abundant Ni3+ and Fe3+ ions accelerated the dissociation of water molecules into hydrogen atoms, while an appropriate hydrogen binding strength sped up the renewal process at active sites. Therefore, both hydrogen adsorption resistance (RHER) and charge transfer resistance (Rct) were reduced in comparison with NiMo/NiMoCx [57].
After 12 h of HER at the current of 10 mA cm−2, both NiMo/NiMoCx and NiFeMo/NiFeMoCx exhibited little decrease in their performance (Figure 4d). A previous study has confirmed the formation of a thin carbon coating through CPED in organic solvents [41]. The formation of nickel carbide requires high energy [58], and the active carbon atoms as well as methyl radicals are more likely to be adsorbed between deposited particles to form a protective carbon layer. Therefore, both electrocatalysts demonstrated a high HER stability.

2.3. OER Performance

During OER, the NiMo/NiMoCx exhibited a high overpotential of 321 mV to deliver the current of 10 mA cm−2. In comparison, with the doping of Fe3+, the η10 was largely decreased to 230 mV for NiFeMo/NiFeMoCx. The anodic peak at 1.4 V vs. RHE in the polarization curve originated from the oxidation of Ni (Figure 5a). There was a positive shift in the oxidation peak for NiFeMo/NiFeMoCx, indicating that the incorporation of Fe affected the electronic structure of the nickel. High-valence Ni and Fe exhibited strong adsorption of OH− and oxygen-containing intermediates, facilitating the oxygen evolution reaction. The high Tafel slope of NiMo/NiMoCx (Figure 5b, 125 mV dec−1) indicated the hindrance caused by OH− adsorption and the sluggish formation of an oxygen-containing intermediate. As for NiFeMo/NiFeMoCx, the Tafel slope was significantly reduced to 50 mV dec−1 because of the lower energy barrier that resulted from Ni3+ and Fe3+ [59].
EIS was carried out for both materials at an overpotential of 250 mV. The Nyquist plots of OER also exhibited two semicircles at low and high frequencies (Figure 5c). The experimental data were fitted with the equivalent circuit (Figure S5) and the result is presented in Table S3. Rs represented contact resistance, ROER stood for the resistance related to the adsorption of oxygen intermediate, and Rct represented the charge transfer resistance. Upon the doping of Fe3+, ROER was decreased significantly from 1.12 to 0.185 Ω, indicating a substantial reduction in the intermediate adsorption resistance. This led to diminished kinetic hindrance and enhanced intrinsic catalytic activity. Additionally, Rct decreased from 9.11 to 0.565 Ω, confirming that NiFeMo/NiFeMoCx possessed a superior charge transfer capability during OER. The stability of these two samples was evaluated through a chronopotentiometric test at 10 mA cm−2. No detachment of the catalyst was observed throughout the test. The overpotential of NiMo/NiMoCx was increased by 14 mV after 12 h of catalysis. As for NiFeMo/NiFeMoCx, there was only a 2 mV change in the overpotential under the same conditions, indicating excellent stability in OER (Figure 5d).

2.4. Overall Water Splitting Performance

The overall water splitting voltage of NiFeMo/NiFeMoCx was only 1.52 V at 10 mA cm−2, and outperformed that of NiMo/NiMoCx (1.67 V) (Figure 6a). The overpotential for overall water electrolysis on NiFeMo/NifeMoCx (0.29 V) was essentially equal to the sum of its HER overpotential (0.06 V) and OER overpotential (0.23 V), suggesting no cross-contamination between the cathode and anode materials. Cross-contamination leads to catalyst poisoning and is a common issue in the overall water splitting system using different electrocatalysts for the HER and OER. Subsequently, the stability was tested for these two samples (Figure 6b). There was only a 14 mV change in the overpotential for NiFeMo/NiFeMoCx after 18 h. It is, therefore, a promising transition-metal-based electrocatalyst for overall water splitting. The overall water splitting activity of NiFeMo/NiFeMoCx (1.52 V at 10 mA cm−2) was comparable or superior to that of recently reported advanced catalysts, such as Mn–W–CoP/NF (1.57 V), FeN0.023/Mo2C/C (1.55 V), FeS2/Fe–Ni3S2 (1.5 V), and RuO2-Co3O4 (1.54 V, Table S4) [60,61,62,63].

2.5. Mechanism of Stability Enhancement

After a stability test of overall water splitting, the surface of NiFeMo/NiFeMoCx maintained a patterned structure. The nanoflakes were well-decorated over the particles. No agglomeration or detachment of the particles was observed. The film thickness after the HER/OER was very close to that after the film was prepared, and was constantly around 23 μm as shown in the cross-section image (Figure S6).
To gain deeper insight into the improved stability of NiFeMo/NiFeMoCx, Raman spectroscopy was conducted on the as-prepared samples as well as those subjected to 18 h of HER/OER testing (Figure 7a). Although a slight shift towards a lower wave number was noticed in the vibration peak of NiFeO, there was little change in the vibration peaks of Ni-O and Ni-OH after the HER. During hydrogen evolution, nickel–iron oxides were more easily reduced than NiO or Ni(OH)2 [64]. Therefore, the stability of NiO/Ni(OH)2 was improved due to the sacrificing effect of NiFeO. Additionally, molybdates and molybdenum oxide were gradually dissolved under alkaline conditions, leading to a decrease in the peak intensity near 906 cm−1, which was associated with Mo-O vibration. The D peak and G peak of carbon were well reserved, suggesting the good stability of metal carbide even under the strong reductive environment rich in hydrogen atoms. After OER, the peaks in the NiFe-O bond vibrations at 460 and 546 cm−1 exhibited a significant increase and were redshifted to 471 and 549 cm−1, respectively. Meanwhile, the intensities of both the D peak and G peak of carbon were decreased. This was attributed to the oxidation of the NiFe alloy and carbide by (Ni, Fe)OOH during the prolonged OER process. It is worth noting that (Ni, Fe)OOH not only exhibits excellent stability under oxidative conditions, but also demonstrates an exceptional intrinsic catalytic activity as an OER catalyst [37,65]. Therefore, the OER activity of NiFeMo/NiFeMoCx was maintained during the stability test.
The element content of NiFeMo/NiFeMoCx after HER was essentially the same as the as-prepared sample (Table S5), confirming the robust stability of a metal/carbide hybrid under a reductive environment. After the OER, the C content was decreased, and the O content was increased slightly. This suggested that only the outmost carbide was transformed into the oxide, which prevented the further oxidation of the catalyst. XPS was conducted to investigate the change in surface state. After the HER, metal-O bonds as well as Mo 3d and Fe 2p peaks experienced a slight shift towards a lower binding energy by 0.2~0.4 eV. The proportions of lattice oxygen, Mo6+, and Fe3+ were all decreased, from 38.7, 98.81, and 62.89 to 32.17, 97.42, and 57.83%, respectively (Figure 7c–e). In the Ni 2p spectrum (Figure 7f), the Ni3+ peak was shifted towards lower binding energy by 0.14 eV, while its percentage was decreased from 51.47 to 39.48%. In comparison, the peaks’ position of metal-C bonds, Ni-C bonds, and Ni(II) remained unchanged with only a slight increase in the peak area, suggesting the high stability of carbide under reductive conditions. After the OER, metal–C and Ni-C peaks were shifted towards higher binding energies with reduced percentage. At the same time, metal–O, Ni3+, and Fe3+ peaks were shifted towards higher binding energies with increased percentage (Figure 7b–f). This further confirmed that the carbide was transformed into oxide or hydroxide during the OER, producing a highly active and stable catalyst.

3. Materials and Methods

3.1. Materials

Copper foil with the thickness of 0.1 mm was bought from Taizhou Biling Hardware Products Co., Ltd. (Xinghua, China); NiSO4·6H2O and Fe2(SO4)3·xH2O were purchased from Fuchen Chemical Reagent Co., Ltd. (Tianjin, China); Na2MoO4·2H2O was purchased from Tianjin Chemical Reagent Kaida Chemical Factory (Tianjin, China); and C6H5Na3O7·2H2O and C3H8O3 were purchased from Shanghai Runjie Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Catalyst Preparation

The copper foils were firstly cut into pieces with the dimension of 1 × 5 cm and then ultrasonically cleaned in acetone and ethanol for 30 min and 10 min, respectively, followed by rinsing with deionised water and drying in hot air.
Synthesis of NiMo/NiMoCx: Cathodic plasma electrolytic deposition (CPED) was carried out using a DC power supply (Agilent N5772A) in the 250 mL solution containing 50 mmol NiSO4·6H2O, 25 mmol C6H5Na3O7·2H2O, 7.5 mmol Na2MoO4·2H2O, and 75 mL C3H8O3. The deposition potential was 130 V and the time was 6 min.
Synthesis of NiFeMo/NiFeMoCx: The synthesis of NiFeMo/NiFeMoCx was similar to that of NiMo/NiMoCx, except that an extra 2.5 mmol of Fe2(SO4)3·xH2O was added in the solution.

3.3. Material Characterization

Powder X-ray diffraction (XRD, Bruker-D8 ADVANCE, Cu Kα) was performed on the material mechanically scratched from the deposited film to investigate the crystalline structure. X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) was carried out to study the surface electronic state. The electron binding energies of the XPS spectra were calibrated assuming the C 1s peak at 284.6 eV. Raman spectra were collected on Horiba HR800 with a blue light (488 nm) as the excitation source. The morphology of the samples was examined using a field emission scanning electron microscope (SEM, Axioskop 40 POL, ZEISS, Oberkochen, Germany) and a transmission electron microscope (TEM, JEM-2010, JEOL, Tokyo, Japan).

3.4. Electrochemical Measurement

All electrochemical tests were carried out using a three-electrode system on a CHI 660B electrochemical workstation (CH Instrument, Bee Cave, TX, USA). The test solution was 1 M KOH. A standard Hg/HgO and a graphite rod were used as the reference and counter electrode, respectively. The linear sweep voltammetry (LSV) curves were recorded at a sweep rate of 5 mV s−1 and were 95% iR-corrected for Ohmic losses. Electrochemical impedance spectroscopy (EIS) was measured with an amplitude of 5 mV in the frequency range from 0.01 to 100 kHz. Cyclic voltammetry (CV) tests at different scan rates were carried out to evaluate the electrochemical surface area (ECSA). The ECSA was calculated as Cdl/Cs, where Cdl is the double layer capacitance and Cs (40 µF cm−2) is the specific capacitance of a planar surface. All the potentials were converted to those versus the reversible hydrogen electrode (RHE) following the Nernst equation: E vs. RHE = E vs. Hg/HgO + 0.098 V + 0.059 V × pH.

4. Conclusions

In conclusion, a transition metal/carbide hybrid (NiFeMo/NiFeMoCx) was synthesized via a facile and fast cathodic plasma electrolytic deposition. Under alkaline conditions, the as-prepared NiFeMo/NiFeMoCx exhibited a good electrocatalytic performance and high stability for water electrolysis, highlighting its promising potential as a binder-free electrode for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The enhanced electrocatalytic performance of NiFeMo/NiFeMoCx was attributed to the iron doping that optimized the surface electronic structure of the electrode, thereby enhancing its catalytic activity and durability. NiFeMo/NiFeMoCx was applied as an overall water splitting catalyst, requiring only 1.52 V to deliver 10 mA cm−2. This work provides valuable guidance for designing efficient transition metal-based electrocatalysts.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14100730/s1: Table S1. Composition (at. %) of NiMo/NiMoCx and NiFeMo/NiFeMoCx; Figure S1. XPS spectra of Fe 2p of NiFeMo/NiFeMoCx; Figure S2. The iR-corrected linear sweep voltammetry curves measured at a scan rate of 5 mV s−1 in 1M KOH; Figure S3. CV curves recorded at different scan rates for (a) NiMo/NiMoCx and (b) NiFeMo/NiFeMoCx. (c) Linear plots of capacitive current density vs. scan rate; Figure S4. Equivalent circuit used to fit the EIS data of HER; Table S2. Electrical elements fitted by the equivalent circuit in Figure S4; Figure S5. Equivalent circuit used to fit the EIS data of OER; Table S3. Electrical elements fitted by the equivalent circuit in Figure S5; Table S4. Performance comparison between NiFeMo/NiFeMoCx and recently reported non-precious water splitting catalysts; Figure S6. SEM images of NiFeMo/NiFeMoCx, (a) before stability test, (b) after OER and (c) after HER. Inset: The corresponding cross-section image; Table S5. Element content (at. %) of NiFeMo/NiFeMoCx before and after stability test [66,67,68,69,70,71].

Author Contributions

Conceptualization, G.Z.; methodology, J.M., S.M. and J.P.; formal analysis, N.F.; investigation, Y.L.; writing—original draft preparation, J.M.; writing—review and editing, G.Z.; project administration, G.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22002192 and 62374059), and the Guangdong Basic and Applied Basic Research Foundation (2023A1515030006 and 2023A1515010533).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM images of (a) NiMo/NiMoCx and (b) NiFeMo/NiFeMoCx. Inset: a cross-section image.
Figure 1. SEM images of (a) NiMo/NiMoCx and (b) NiFeMo/NiFeMoCx. Inset: a cross-section image.
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Figure 2. (a) XRD patterns, (b) Raman spectra, and XPS spectra of (c) C 1s, (d) O 1s, (e) Ni 2p, and (f) Mo 3d of NiMo/NiMoCx and NiFeMo/NiFeMoCx.
Figure 2. (a) XRD patterns, (b) Raman spectra, and XPS spectra of (c) C 1s, (d) O 1s, (e) Ni 2p, and (f) Mo 3d of NiMo/NiMoCx and NiFeMo/NiFeMoCx.
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Figure 3. Characterization of NiFeMo/NiFeMoCx, (a) low magnification TEM, (b,c) high-resolution TEM (HRTEM), and (d) SAED image.
Figure 3. Characterization of NiFeMo/NiFeMoCx, (a) low magnification TEM, (b,c) high-resolution TEM (HRTEM), and (d) SAED image.
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Figure 4. Electrochemical characterization of metal/carbide hybrid in HER. (a) The iR-corrected linear sweep voltammetry curves measured at a scan rate of 5mV s−1. (b) Tafel plots. (c) Electrochemical impedance spectra at 60 mV versus RHE. (d) Chronopotentiometric test at 10 mA cm−2. All tests were carried out in 1 M KOH.
Figure 4. Electrochemical characterization of metal/carbide hybrid in HER. (a) The iR-corrected linear sweep voltammetry curves measured at a scan rate of 5mV s−1. (b) Tafel plots. (c) Electrochemical impedance spectra at 60 mV versus RHE. (d) Chronopotentiometric test at 10 mA cm−2. All tests were carried out in 1 M KOH.
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Figure 5. Electrochemical characterization of metal/carbide hybrid in OER. (a) LSV curves at a scan rate of 5 mV s−1 and (b) the Tafel plots. (c) Electrochemical impedance spectra at 250 mV versus RHE. (d) Chronopotentiometric test at 10 mA cm−2. All tests were carried out in 1 M KOH.
Figure 5. Electrochemical characterization of metal/carbide hybrid in OER. (a) LSV curves at a scan rate of 5 mV s−1 and (b) the Tafel plots. (c) Electrochemical impedance spectra at 250 mV versus RHE. (d) Chronopotentiometric test at 10 mA cm−2. All tests were carried out in 1 M KOH.
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Figure 6. (a) LSV curves at a scan rate of 5 mV s−1. (b) Stability test for overall water splitting at 10 mA cm−2. All tests were carried out in 1 M KOH.
Figure 6. (a) LSV curves at a scan rate of 5 mV s−1. (b) Stability test for overall water splitting at 10 mA cm−2. All tests were carried out in 1 M KOH.
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Figure 7. (a) Raman spectra. XPS spectra of (b) C 1s, (c) O 1s, (d) Fe 2p, (e) Mo 3d, and (f) Ni 2p of NiFeMo/NiFeMoCx before and after stability test.
Figure 7. (a) Raman spectra. XPS spectra of (b) C 1s, (c) O 1s, (d) Fe 2p, (e) Mo 3d, and (f) Ni 2p of NiFeMo/NiFeMoCx before and after stability test.
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Mo, J.; Fu, N.; Mu, S.; Peng, J.; Liu, Y.; Zhang, G. Facile Synthesis of Metal/Carbide Hybrid toward Overall Water Splitting. Catalysts 2024, 14, 730. https://doi.org/10.3390/catal14100730

AMA Style

Mo J, Fu N, Mu S, Peng J, Liu Y, Zhang G. Facile Synthesis of Metal/Carbide Hybrid toward Overall Water Splitting. Catalysts. 2024; 14(10):730. https://doi.org/10.3390/catal14100730

Chicago/Turabian Style

Mo, Junxiang, Nianqing Fu, Songlin Mu, Jihua Peng, Yan Liu, and Guoge Zhang. 2024. "Facile Synthesis of Metal/Carbide Hybrid toward Overall Water Splitting" Catalysts 14, no. 10: 730. https://doi.org/10.3390/catal14100730

APA Style

Mo, J., Fu, N., Mu, S., Peng, J., Liu, Y., & Zhang, G. (2024). Facile Synthesis of Metal/Carbide Hybrid toward Overall Water Splitting. Catalysts, 14(10), 730. https://doi.org/10.3390/catal14100730

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