Construction of Ni2P-MoC/Coal-Based Carbon Fiber Self-Supporting Catalysts for Enhanced Hydrogen Evolution

Efficient and inexpensive electrocatalysts play an important role in the hydrogen evolution reaction (HER) of electrolytic water splitting. Herein, Ni2P-MoC/coal-based carbon fiber (Ni2P-MoC/C-CF) self-supporting catalysts were obtained by low-temperature phosphorization and high-temperature carbonization. The Mo source and oxidized coal were uniformly dispersed in the carbon support by electrospinning technology. A precursor of Ni was introduced by the impregnation method. The synergistic effect of MoC and Ni2P may reduce the strong hydrogen adsorption capacity of pure MoC and provide a fast hydrogen release process. In addition, the C-CFs prepared by electrospinning can not only prevent the agglomeration of MoC and Ni2P particles at a high temperature but also provide a self-supporting support for the catalyst. As a result, the catalytic performance of the HER was improved greatly, and a low overpotential of 112 mV at 10 mA cm−2 was exhibited stably by the Ni2P-MoC/C-CFs. This work not only converts coal into coal-based carbon materials but also provides a feasible pathway for the rational design of large-scale molded hydrogen electrocatalysts.


Introduction
The global use of fossil fuels is increasing year by year, and a large amount of greenhouse gas emissions has led to a series of increasingly significant problems such as environmental pollution, energy depletion, and global warming [1].There is an urgent need to develop sustainable clean energy to meet development needs [2].Hydrogen energy, an important alternative energy for carbon-based fuel that is clean, efficient, and environment-friendly, plays a key role in green energy and chemical conversions [3].The reaction involved in the electrolysis of water consists of two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) [4].Among them, hydrogen production by the electrolysis of water is considered the cleanest way to produce hydrogen, and the purity of the hydrogen produced is relatively higher than that of other methods [3,5].At present, the highly expensive noble metal-based materials such as platinum (Pt) and RuO 2 or IrO 2 are considered ideal catalysts for the HER and OER [6], but their limited reserves, high cost, and poor durability limit the large-scale application of highly expensive noble metal-based materials [7].Therefore, it is urgent to develop a highly efficient, low-priced, and stable non-noble metal electrocatalyst for the large-scale production of hydrogen.
Firstly, it has to be mentioned that, in addition to transition metal carbides, transition metal phosphides are often used as catalysts for the HER because P has lone pair electrons on 3p orbitals and vacant 3d orbitals, which can jointly induce surface charge density.Among many transition metal phosphides, nickel phosphide has attracted much attention because of its special electronic structure and excellent chemical stability [32][33][34][35][36].According to previous research, the addition of Ni 2 P can increase the electrochemical active surface area (ECSA) and the inherent HER activity of the catalyst [33,37].The synergistic effect between different phases can achieve goals that cannot be achieved by a single component.Therefore, the performance of the catalyst can be further improved.
Furthermore, to avoid the aggregation of catalyst nanoparticles during high-temperature carbonization, carbon supports such as carbon nanotubes (CNTs) [37], carbon fibers (CFs) [38,39], graphene [40], and carbon black [41] can be introduced.At the same time, these carbon supports can provide a larger specific surface area for the catalyst and expose more catalytic active sites.Furthermore, the precursor of the catalyst encapsulated in CFs prepared by electrospinning technology has limited mobility on the surface of CFs, which can effectively inhibit the aggregation of catalyst nanoparticles [39,42,43].Among different kinds of carbon materials, coal stands out as an important fossil energy [44].Transforming it into coal-based carbon fiber (C-CFs) will improve the utilization rate and the market prospect of coal resources [45].Additionally, the introduction of coal will cause more defects in CFs [46].For example, researchers have developed a method of applying coal to catalysts for fuel cells and battery materials [46,47].Moreover, the self-supporting C-CF electrode not only simplifies the electrode preparation process but also avoids the coating falling off of the electrode and the burying of active sites by the adhesive.At the same time, CFs containing metal carbides can be used as conductive bases to grow other active substances so as to further improve the performance of the HER of the materials.
Herein, the Mo source and oxidized coal were uniformly dispersed in the support of the C-CFs by electrospinning, the Ni source was introduced by the impregnation method, and the self-supporting catalysts of Ni 2 P-MoC/C-CFs were obtained by low-temperature phosphorization and high-temperature carbonization.The catalytic performance for the HER of as-prepared Ni 2 P-MoC/C-CFs was improved greatly, and a low overpotential of 112 mV at 10 mA cm −2 was exhibited stably.The above results show that it is a promising catalyst for the HER.

Physical Characterization
The synthetic route for Ni 2 P-MoC/C-CFs is presented in Scheme 1.In this work, using oxidized coal and PAN as carbon sources, molybdenyl acetylacetonate as the Mo source was added to the spinning solution, and a precursor containing MoO x was obtained through electrospinning and pre-oxidation.Then nickel nitrate hexahydrate as the Ni source was introduced into the precursor through impregnation.Using NaPO 2 H 2 •H 2 O as the P source, Ni 2 P was obtained by phosphorization at 300 • C for 2 h, followed by carbonization at 800 • C for 2 h to obtain MoC and C-CFs, ultimately resulting in Ni 2 P-MoC/C-CF composite materials.Figure 1a   Raman spectroscopy was adopted to study the surface structure of different C terials. Figure 1b compares the Raman test results of the Ni2P-MoC/C-CF sample, a as the MoC/C-CF, Ni2P/C-CF, and C-CF comparison samples.The two strong pea tween 1330 and 1580 cm −1 correspond to defect characteristics (D band) and graphitiz degrees (G band), respectively.In general, the D band reflects defects in the crystal ture, and the G band represents graphitelike carbon [48].The larger the intensity from the D peak to the G peak (ID/IG) are, the more defects and exposed edges there the carbon material, which is conducive to electron transfer in the electrochemical re process [49].The ID/IG ratios of Ni2P-MoC/C-CFs, MoC/C-CFs, Ni2P/C-CFs, and C-C calculated to be 1.09, 1.07, 1.06, and 1.01, respectively.The Ni2P-MoC/C-CFs hav  Raman spectroscopy was adopted to study the surface structure of different CF terials.Figure 1b compares the Raman test results of the Ni2P-MoC/C-CF sample, as as the MoC/C-CF, Ni2P/C-CF, and C-CF comparison samples.The two strong peak tween 1330 and 1580 cm −1 correspond to defect characteristics (D band) and graphitiza degrees (G band), respectively.In general, the D band reflects defects in the crystal s ture, and the G band represents graphitelike carbon [48].The larger the intensity r from the D peak to the G peak (ID/IG) are, the more defects and exposed edges there a the carbon material, which is conducive to electron transfer in the electrochemical reac process [49].The ID/IG ratios of Ni2P-MoC/C-CFs, MoC/C-CFs, Ni2P/C-CFs, and C-CF calculated to be 1.09, 1.07, 1.06, and 1.01, respectively.The Ni2P-MoC/C-CFs have Raman spectroscopy was adopted to study the surface structure of different CF materials.Figure 1b compares the Raman test results of the Ni 2 P-MoC/C-CF sample, as well as the MoC/C-CF, Ni 2 P/C-CF, and C-CF comparison samples.The two strong peaks between 1330 and 1580 cm −1 correspond to defect characteristics (D band) and graphitization degrees (G band), respectively.In general, the D band reflects defects in the crystal structure, and the G band represents graphitelike carbon [48].The larger the intensity ratios from the D peak to the G peak (I D /I G ) are, the more defects and exposed edges there are in the carbon material, which is conducive to electron transfer in the electrochemical reaction process [49] The results suggest that transition metal elements play a prominent role in facilitating carbon graphitization and higher electrical conductivity in carbon materials, leading to better efficiency.Furthermore, Figure S1 shows that the I D /I G ratio of Ni 2 P-MoC/C-CFs (1.09) is higher than that of Ni 2 P-MoC-CFs (1.05).This may be attributed to the fact that the addition of oxidized coal reduces the ordered arrangement of PAN chains in CFs, which leads to more structural defects.
As shown in Figure 2, the microstructures and morphologies of the samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).Figure 2a-c shows SEM images of Ni 2 P-MoC/C-CFs, MoC/C-CFs, and Ni 2 P/C-CFs, respectively.It can be seen from Figure 2a and Figure S2 that the nanoparticles are uniformly grown on C-CFs, and the average diameter of the nanoparticles is about ~19.3 nm (inset in Figure 2d).In Figure 2b and Figure S3, the rough surfaces of MoC/C-CFs endow the self-supporting electrode with a large specific surface area and provide more active sites.In comparison with Ni 2 P-MoC/C-CFs, the nanoparticle size of Ni 2 P/C-CFs is obviously increased, and the growth was uneven and agglomerated (Figure 2c and Figure S4).This shows that the addition of the Mo source has a certain dispersion and anchoring effect on Ni 2 P nanoparticles grown on the C-CFs surface.maximum ID/IG ratio, which shows that the introduction of Ni2P and MoC increases the number of structural defects in the carbon lattice and reduces the order degree of C-CFs.The results suggest that transition metal elements play a prominent role in facilitating carbon graphitization and higher electrical conductivity in carbon materials, leading to better efficiency.Furthermore, Figure S1 shows that the ID/IG ratio of Ni2P-MoC/C-CFs (1.09) is higher than that of Ni2P-MoC-CFs (1.05).This may be attributed to the fact that the addition of oxidized coal reduces the ordered arrangement of PAN chains in CFs, which leads to more structural defects.
As shown in Figure 2, the microstructures and morphologies of the samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).Figure 2a-c shows SEM images of Ni2P-MoC/C-CFs, MoC/C-CFs, and Ni2P/C-CFs, respectively.It can be seen from Figures 2a and S2 that the nanoparticles are uniformly grown on C-CFs, and the average diameter of the nanoparticles is about ~19.3 nm (inset in Figure 2d).In Figures 2b and S3, the rough surfaces of MoC/C-CFs endow the self-supporting electrode with a large specific surface area and provide more active sites.In comparison with Ni2P-MoC/C-CFs, the nanoparticle size of Ni2P/C-CFs is obviously increased, and the growth was uneven and agglomerated (Figures 2c and S4).This shows that the addition of the Mo source has a certain dispersion and anchoring effect on Ni2P nanoparticles grown on the C-CFs surface.Figure 2f,g shows that the lattice spacing of the nanoparticles on the C-CFs at 0.221 and 0.244 nm is consistent with the (111) crystal planes of Ni2P and (006) crystal planes of MoC, respectively, which further shows that Ni2P and MoC nanoparticles were successfully grown on C-CFs.In addition, the EDS-mapping image (Figure 2h  X-ray photoelectron spectroscopy (XPS) analysis was carried out to understand the surface electronic conditions and chemical states of the samples before and after the introduction of Ni2P.In Figure 3a    bond and are generally considered to be the electrocatalytic active centers [28].Herein, the existence of the Mo 4+ and Mo 6+ species can be attributed to the surface oxides formed during the pre-oxidation process [50,51].Compared with MoC/C-CFs, these two peaks about Mo 2+  were negatively shifted by approximately 0.5 and 0.8 eV, respectively (Figure 3d), suggesting enriched electrons around the Mo species [50].In addition, Mo 4+ and Mo 6+ showed a similar phenomenon.Generally, an increase in the valence electron charge results and a reduction in binding energy were observed [52].Therefore, the negative binding energy shifts of the Mo 3d can be attributed to the transfer of valence electrons from Ni to MoC across their interfaces.This efficient electron transfer contributes to a synergistic effect on Ni 2 P-MoC/C-CFs, which diminishes the hydrogen binding energy and accordingly contributes to the desorption of hydrogen adsorbate (H ads ) and H 2 generation [52,53].The characteristic peaks of Ni 2p 3/2 , Ni 2p 1/2 , and satellite peaks can be observed in the high-resolution Ni 2p spectrum (Figure 3f).The Ni 2p 3/2 and Ni 2p 1/2 peaks can be decomposed into two parts: the peaks at 854.6 and 871.88 eV are attributed to metal Ni, and the peaks at 856.46 and 874.6 eV are attributed to Ni 2+ .The peaks at 861.14 and 879.32 eV are satellite peaks.In addition, the peak positions of P 2p 3/2 , P 2p 1/2 , and oxidized P species displayed in the P 2p (Figure 3e) spectrum are 130.05, 130.9, and 133.5 eV, respectively.The P-O peak at 133.5 eV is due to the inevitable surface oxidation of the sample exposed to air, which can be attributed to the high oxidation state of P species (PO x ) [32,54,55].
Previous works have demonstrated that the super-hydrophilic surface of the electrocatalyst is beneficial for increasing the contact area of the catalyst with the electrolyte and H 2 bubbles released from the electrode.The compatibility with water and the charge transfer has thus been remarkably enhanced [56].Figure 4  After the impregnation treatment, the content of the pyridine-N species significantly increases, and the more pyridine-N there is on the surface, the more conducive it is to improve the surface wettability of the material [56,57].This is consistent with the XPS results.
Herein, the existence of the Mo 4+ and Mo 6+ species can be attributed to the surface oxides formed during the pre-oxidation process [50,51].Compared with MoC/C-CFs, these two peaks about Mo 2+ were negatively shifted by approximately 0.5 and 0.8 eV, respectively (Figure 3d), suggesting enriched electrons around the Mo species [50].In addition, Mo 4+ and Mo 6+ showed a similar phenomenon.Generally, an increase in the valence electron charge results and a reduction in binding energy were observed [52].Therefore, the negative binding energy shifts of the Mo 3d can be attributed to the transfer of valence electrons from Ni to MoC across their interfaces.This efficient electron transfer contributes to a synergistic effect on Ni2P-MoC/C-CFs, which diminishes the hydrogen binding energy and accordingly contributes to the desorption of hydrogen adsorbate (Hads) and H2 generation [52,53].The characteristic peaks of Ni 2p3/2, Ni 2p1/2, and satellite peaks can be observed in the high-resolution Ni 2p spectrum (Figure 3f).The Ni 2p3/2 and Ni 2p1/2 peaks can be decomposed into two parts: the peaks at 854.6 and 871.88 eV are attributed to metal Ni, and the peaks at 856.46 and 874.6 eV are attributed to Ni 2+ .The peaks at 861.14 and 879.32 eV are satellite peaks.In addition, the peak positions of P 2p3/2, P 2p1/2, and oxidized P species displayed in the P 2p (Figure 3e) spectrum are 130.05, 130.9, and 133.5 eV, respectively.The P-O peak at 133.5 eV is due to the inevitable surface oxidation of the sample exposed to air, which can be attributed to the high oxidation state of P species (POx) [32,54,55].
Previous works have demonstrated that the super-hydrophilic surface of the electrocatalyst is beneficial for increasing the contact area of the catalyst with the electrolyte and H2 bubbles released from the electrode.The compatibility with water and the charge transfer has thus been remarkably enhanced [56].Figure 4 shows that the water contact angles of Ni2P-MoC/C-CFs, MoC/C-CFs, Ni2P/C-CFs, and C-CFs at the same time interval are 61°, 114°, 92°, and 140°, respectively.In comparison with Ni2P-MoC/C-CFs and Ni2P/C-CFs, the hydrophilic effect of MoC/C-CFs and C-CFs is decreased significantly, which is attributed to the introduction of the Ni source by the impregnation treatment.After the impregnation treatment, the content of the pyridine-N species significantly increases, and the more pyridine-N there is on the surface, the more conducive it is to improve the surface wettability of the material [56,57].This is consistent with the XPS results.

Electrochemical Properties
A typical three-electrode system was used to test the performance of the HER of a series of catalysts in an N 2 -saturated acid electrolyte (0.5 M H 2 SO 4 with a pH of 0.3).The effects of the Ni source impregnation amount and the introduction of oxidized coal improve the performance of the HER of the materials that were studied.
As shown in Figure S6, the crossing points of Ni 2 P-MoC/C-CFs(1:1), Ni 2 P-MoC/C-CFs(1:2), and Ni 2 P-MoC/C-CFs(1:3) at 10 mA cm −2 current density are 165, 112, and 142 mV, respectively.It shows that the best molar ratio between the Mo and Ni source is 1:2.Correspondingly, Figure S7 compares the properties of the samples with (Ni 2 P-MoC/C-CFs) or without oxidized coal (Ni 2 P-MoC-CFs) and proves that the introduction of oxidized coal makes the material contain more oxygen-containing functional groups, which makes the material have better performance for the HER. Figure 5a  Accordingly, the Tafel plots (Figure 5b) derived from the polarization curves imply that the Ni 2 P-MoC/C-CF electrode has a relatively small Tafel slope value of 140.0 mV dec −1 , which is only greater than Pt/C among all electrolytic samples (149.6 mV dec −1 for MoC/C-CFs, 167.4 mV dec −1 for Ni 2 P/C-CFs, and 254.3 mV dec −1 for C-CFs).This indicates that Ni 2 P-MoC/C-CFs have the most favorable kinetics for the HER.Generally, the semicircle in the low-frequency range of the Nyquist plots is related to the charge-transfer resistance R ct , while the solution resistance R s is implicit in the high-frequency range [58].As shown in Figure 5c, Ni 2 P-MoC/C-CFs revealed the R ct of 6.26 Ω, smaller than the R ct of MoC/C-CFs (6.91 Ω), Ni 2 P/C-CFs (9.95 Ω), and C-CFs (13.58 Ω), which indicated a significantly faster reaction rate for the HER.In addition, the electrochemical specific surface area (ECSA) of the electrocatalyst was measured to understand the source of the HER activity enhancement, which was proportional to the double-layer capacitance (C dl ).In this work, the C dl was determined by cyclic voltammetry (CV) at diverse scan rates (2-6 mV s −1 ) in the nonfaradaic potential range (Figure 5d).Figures S8-S10 is a typical cyclic voltammogram of different samples at different scanning rates.The C dl value of Ni 2 P-MoC/C-CFs is estimated to be 1342.2mF cm −2 by fitting the capacitive currents with the scan rates, greatly larger than that of MoC/C-CFs (962.2 mF cm −2 ), Ni 2 P/C-CFs (506.7 mF cm −2 ), and C-CFs (84.3 mF cm −2 ), suggesting that the higher catalytic activity for the HER of Ni 2 P-MoC/C-CFs is relevant to its larger ECSA.The HER stability of Ni 2 P-MoC/C-CF in 0.5 M H 2 SO 4 was tested.In Figure 5e, after 3000 cycles of CV test, the LSV curve of Ni 2 P-MoC/C-CFs is almost the same as the initial curve.Furthermore, the long-term stability of the catalyst investigated by a potentiostatic method showed that the current density remains almost unchanged within 24 h (Figure 5e inset).These results indicated that Ni 2 P-MoC/C-CFs had good stability.It is worth noting that, as shown in Figure 5f and Table S1, the catalytic activity of the Ni 2 P-MoC/C-CF electrode is equivalent to or even better than that of the non-noble catalysts reported recently.In addition, the XRD, SEM, and XPS of the Ni 2 P-MoC/C-CF catalyst were also tested after the long-term stability test.The XRD results in Figure S11 show that after the long-term stability test, the XRD peak of Ni 2 P in the Ni 2 P-MoC/C-CF catalyst has been reduced to a certain extent, and the crystallinity of Ni 2 P has decreased, but the corresponding peak of Ni 2 P can still be observed.After long-term stability testing, the MoC species were least affected and the XRD peaks remained almost unchanged.The SEM images show that after 24 h i-t testing, the nanoparticles on the surface of Ni 2 P-MoC/C-CFs tend to expand, which may be attributed to the aggregation phenomenon of the catalyst after the HER.In addition, comparing XPS before and after 24 h i-t testing (Figure S13), the Mo 6+ peaks at 232.4 and 235.5 eV decreased, while the peaks of Ni 2+ at 856.46 and 874.6 eV increased, indicating that some of the zero-valent nickel transformed into high-valent nickel and that Ni electrons transferred to Mo.In conclusion, the coupling of Ni 2 P and MoC makes the Ni 2 P-MoC/C-CF self-supporting electrode obtain excellent activity and kinetics for the HER.
the surface of Ni2P-MoC/C-CFs tend to expand, which may be attributed to the aggregation phenomenon of the catalyst after the HER.In addition, comparing XPS before and after 24 h i-t testing (Figure S13), the Mo 6+ peaks at 232.4 and 235.5 eV decreased, while the peaks of Ni 2+ at 856.46 and 874.6 eV increased, indicating that some of the zero-valent nickel transformed into high-valent nickel and that Ni electrons transferred to Mo.In conclusion, the coupling of Ni2P and MoC makes the Ni2P-MoC/C-CF self-supporting electrode obtain excellent activity and kinetics for the HER.

Acidification Treatment of Coal
Firstly, we sieved the raw coal with a 200-mesh sieve, took 10 g of the screened coal powder, and placed it in a 5 L beaker.We mixed 180 mL of concentrated sulfuric acid (98%) and 60 mL of concentrated nitric acid (68%), cooled to room temperature, and slowly poured them into a beaker bathed in ice water.After the mixture reached room temperature, we added DI water and let it settle.We rinsed the lower sediment of the turbid solution twice with 5 L of DI water.The oxidized coal obtained after filtration was dried in an air-blast drying furnace at 80 • C for 12 h.

Synthesis of Precursor
We added 0.5 mmol of Mo(acac) 2 to 10 mL DMF, stirred evenly, and then added 0.8 g of PAN and 0.8 g of oxidized coal and stirred overnight.We poured the spinning solution into a 10 mL needle tube and connected the No. 20 stainless steel needle.The voltage and distance between the needle tip and the collector were 18 kV and 15 cm, respectively, and the propulsion speed was 0.8 mL h −1 .We collected the initially spun fibers on a drum wrapped in aluminum foil.The obtained initial spun fibers were heated in a muffle furnace to 280 • C and maintained for 2 h (heating rate: 1 • C min −1 ) to obtain the precursors.

Synthesis of Independent Self-Supporting Catalyst
We set the dimensions to 2 × 4 cm, and the precursor was immersed in 20 mL DI water containing 1, 2, and 3 mmol Ni(NO 3 ) 2 •6H 2 O for 12 h.After removal, the precursor was rinsed with DI water and ethanol three times.Vacuum drying was performed at 60 • C for 10 h.The precursor was placed downstream of the porcelain boat, and NaPO 2 H 2 •H 2 O was placed upstream of the porcelain boat as the phosphorus source.We carefully removed the air by pumping three times (using Ar).Annealing was performed at 300 • C for 2 h (heating rate: 1 • C min −1 ) and carbonizing at 800 • C for 2 h (heating rate: 5 • C min −1 ) with Ar.The synthesized products were named Ni 2 P-MoC/C-CFs (1:1), (1:2), and (1:3), according to the content of the impregnated Ni source.

Synthesis Method of Comparison Samples
The precursor synthesized in Section 3.3 was carbonized under an Ar atmosphere at 800 • C for 2 h to obtain the MoC/C-CF comparison sample.
We added 0.8 g PAN and 0.8 g oxidized coal to 10 mL DMF and stirred evenly.We poured the spinning solution into a 10 mL syringe and connected the No. 20 stainless steel needle.The voltage and distance between the needle tip and the collector were 18 kV and 15 cm, respectively, and the electrospinning was carried out at a speed of 0.8 mL h −1 .The obtained precursor was pre-oxidized in an atmosphere of 280 • C. Subsequently, carbonization was carried out in an Ar atmosphere at 800 • C for 2 h to obtain C-CF comparison samples.
With the dimensions of 2 × 4 cm, C-CFs were immersed in 20 mL DI water containing 1 mmol Ni(NO 3 ) 2 •6H 2 O for 12 h.After removal, the sample were rinsed with DI water and ethanol three times.Vacuum drying was performed at 60 • C for 10 h.The precursor was placed downstream of the porcelain boat, and NaH 2 PO 2 was placed upstream of the porcelain boat as the phosphorus source.Carefully, we removed the air by pumping three times (using argon).Then annealing was performed at 300 • C for 2 h (heating rate: 1 • C min −1 ) and carbonization was performed at 800 • C for 2 h (heating rate: 5 • C min −1 ) to obtain the Ni 2 P/C-CF comparison sample.

Characterizations
We determined the phase and composition structure of the sample through X-ray diffraction (XRD, German, BRUKER D8 Advanced); Raman spectra were taken on a Raman spectrometer (Raman, France, HORIBA Lab RAM HR Evolution).The morphology and structure of the samples were characterized by field emission scanning electron microscopy (SEM, Japan, Hitachi, SU8010) and field emission high-power transmission electron mi-croscopy (HRTEM, Japan, JEM-2100F).The surface elemental composition and valence states of the samples were studied by X-ray photoelectron spectroscopy (XPS, Waltham, MA, USA, Escalable 250Xi from Thermo Fisher Scientific).The hydrophilicity and hydrophobicity of the samples were determined by an optical contact angle tester (Shanghai, China, Powereach, JJ200B2).

Electrochemical Measurements
Electrochemical measurements were performed on a CHI 760E electrochemical workstation with a three-electrode system.The 0.5 × 1 cm self-supporting catalyst was used as the working electrode, carbon rod as the counter electrode, and Ag/AgCl as the reference electrode.The electrolyte is a 0.5M H 2 SO 4 solution with a pH of 0.3.Before each electrochemical measurement, we ventilated with high-purity N 2 for 30 min to remove dissolved O 2 from the electrolyte.During the test, we stopped stirring and continued to apply N 2 .We performed cyclic voltammetry (CV) curve testing at a voltage window of −0.2-−0.1 V and a scanning rate of 100 mV s −1 .We recorded the linear sweep voltammetry (LSV) curve at a scanning rate of 2 mV s −1 and a potential range of −0.80-−0.10V. We converted all potentials to reversible hydrogen electrodes (RHE) using the equation E (RHE) = E (Ag/AgCl) + 0.059 pH + 0.2046 and corrected with 90% iR compensation.The Tafel plot was calculated from the corresponding LSV curve.The double-layer capacitance (C dl ) was estimated from the CV curves in the unlawfully charged potential region (0.022-0.122V) at different scanning rates (2-6 mV s −1 ).

Conclusions
In summary, the Mo source and oxidized coal were uniformly dispersed in the carbon support by electrospinning technology.The Ni source was introduced by the impregnation method.Highly efficient non-noble-metal self-supporting catalyst Ni 2 P-MoC/C-CFs were obtained after low-temperature phosphorization and high-temperature carbonization.It is worth noting that due to the uniform distribution of the Mo species in C-CFs, Ni 2 P nanoparticles grow evenly on the surface of C-CFs.As a result, the catalytic performance of the HER was improved greatly, and a low overpotential of 112 mV at 10 mA cm −2 was exhibited stably by the Ni 2 P-MoC/C-CF material in an acidic electrolyte.In addition, the results demonstrate that the excellent electrochemical performances of the self-supporting electrodes are attributed to the electron interaction between Ni 2 P and MoC as well as the unique electrode structure without the usage of any binder.Therefore, this research has made a breakthrough in the development of efficient and low-cost self-supporting catalysts for the HER, brightening the prospects for its application.
. The I D /I G ratios of Ni 2 P-MoC/C-CFs, MoC/C-CFs, Ni 2 P/C-CFs, and C-CFs are calculated to be 1.09, 1.07, 1.06, and 1.01, respectively.The Ni 2 P-MoC/C-CFs have the maximum I D /I G ratio, which shows that the introduction of Ni 2 P and MoC increases the number of structural defects in the carbon lattice and reduces the order degree of C-CFs.
Figure 2f,g shows that the lattice spacing of the nanoparticles on the C-CFs at 0.221 and 0.244 nm is consistent with the (111) crystal planes of Ni 2 P and (006) crystal planes of MoC, respectively, which further shows that Ni 2 P and MoC nanoparticles were successfully grown on C-CFs.In addition, the EDS-mapping image (Figure 2h) shows the uniform distribution of C, O, N, P, Ni, and Mo elements in Ni 2 P-MoC/C-CFs.
) shows the uniform distribution of C, O, N, P, Ni, and Mo elements in Ni2P-MoC/C-CFs.
, the full spectra of as-prepared catalysts exhibit the chemical signals of Mo, C, N, O, Ni, and P elements in the Ni2P-MoC/C-CF sample and Mo, C, N, and O elements in the MoC/C-CF comparison sample.In the C 1s XPS spectrum (Figure 3b), six peaks at 284.0, 284.8, 285.6, 286.6, 288.2, and 291.0 eV are assigned to Mo-C, C-C,

Figure 2 .
Figure 2. SEM image of (a) Ni 2 P-MoC/C-CFs, (b) MoC/C-CFs, and (c) Ni 2 P/C-CFs; (d-g) TEM and HRTEM images of Ni 2 P-MoC/C-CFs and (h) the corresponding element mapping images of C, O, N, P, Ni, and Mo.X-ray photoelectron spectroscopy (XPS) analysis was carried out to understand the surface electronic conditions and chemical states of the samples before and after the introduction of Ni 2 P. In Figure 3a, the full spectra of as-prepared catalysts exhibit the chemical signals of Mo, C, N, O, Ni, and P elements in the Ni 2 P-MoC/C-CF sample and Mo, C, N, and O elements in the MoC/C-CF comparison sample.In the C 1s XPS spectrum (Figure 3b), six peaks at 284.0, 284.8, 285.6, 286.6, 288.2, and 291.0 eV are assigned to Mo-C, C-C, C-N, C-O-H, C-O-C, and O=C-O, respectively, revealing that impregnation treatment

Figure 3 .
Figure 3. (a-d) XPS Survey spectrum and high-resolution XPS C 1s, N 1s, and Mo 3d spectra of MoC/C-CFs and MoC/C-CFs; (e,f) high-resolution XPS Ni 2p and P 2p spectra of Ni2P-MoC/C The high-resolution N 1s spectrum (Figure 3c) of Ni2P-MoC/C-CFs and MoC/C can be deconvoluted into four peaks, corresponding to Mo-N bond (~396.5 eV), pyri N (~398.65 eV), pyrrole-N (~401.1 eV), and graphite-N (~403.7 eV), respectively.The lecular model structures of these three types of N are shown in Figure S5.The dopi N atoms in C-CFs not only affects the electronic structure of MoC but also improve chemical stability.The peaks of Mo 3d in Ni2P-MoC/C-CFs and MoC/C-CFs could b convoluted into three doublets, corresponding to the three valence states (+2, +4, an of Mo on the surface.The Mo 2+ peaks at 228.6 and 231.6 eV, respectively, are attribut the Mo-C bond and are generally considered to be the electrocatalytic active centers

Figure 3 .
Figure 3. (a-d) XPS Survey spectrum and high-resolution XPS C 1s, N 1s, and Mo 3d spectra of Ni 2 P-MoC/C-CFs and MoC/C-CFs; (e,f) high-resolution XPS Ni 2p and P 2p spectra of Ni 2 P-MoC/C-CFs.The high-resolution N 1s spectrum (Figure 3c) of Ni 2 P-MoC/C-CFs and MoC/C-CFs can be deconvoluted into four peaks, corresponding to Mo-N bond (~396.5 eV), pyridine-N (~398.65 eV), pyrrole-N (~401.1 eV), and graphite-N (~403.7 eV), respectively.The molecular model structures of these three types of N are shown in Figure S5.The doping of N atoms in C-CFs not only affects the electronic structure of MoC but also improves the chemical stability.The peaks of Mo 3d in Ni 2 P-MoC/C-CFs and MoC/C-CFs could be deconvoluted into three doublets, corresponding to the three valence states (+2, +4, and +6) of Mo on the surface.The Mo 2+ peaks at 228.6 and 231.6 eV, respectively, are attributed to the Mo-Cbond and are generally considered to be the electrocatalytic active centers[28].Herein, the existence of the Mo 4+ and Mo 6+ species can be attributed to the surface oxides formed during the pre-oxidation process[50,51].Compared with MoC/C-CFs, these two peaks about Mo2+ shows that the water contact angles of Ni 2 P-MoC/C-CFs, MoC/C-CFs, Ni 2 P/C-CFs, and C-CFs at the same time interval are 61 • , 114 • , 92 • , and 140 • , respectively.In comparison with Ni 2 P-MoC/C-CFs and Ni 2 P/C-CFs, the hydrophilic effect of MoC/C-CFs and C-CFs is decreased significantly, which is attributed to the introduction of the Ni source by the impregnation treatment.
compares the LSV curves of 20% Pt/C, Ni 2 P-MoC/C-CFs, MoC/C-CFs, Ni 2 P/C-CFs, and C-CFs at a scanning rate of 2 mV s −1 .The overpotential of Ni 2 P-MoC/C-CFs at a current density of 10 mA cm −2 is only 112 mV, which is superior to the 155 mV of MoC/C-CFs, 210 mV of Ni 2 P/C-CFs, and 301 mV of C-CFs and close to 27 mV of commercial Pt/C.After the combination of MoC with Ni 2 P, the overpotentials are decreased significantly, indicating that the synergistic effect between MoC and Ni 2 P can improve the activity for the HER of the Ni 2 P-MoC/C-CF self-supporting electrode.Then, the Tafel slope is used to further analyze the process.