In Situ Construction of a Co2P/CoP Heterojunction Embedded on N-Doped Carbon as an Efficient Electrocatalyst for a Hydrogen Evolution Reaction

Noble metal-free electrocatalysts have received widespread attention in a hydrogen evolution reaction (HER) due to the importance of renewable energy development. Herein, a Co2P/CoP heterojunction embedded on an N-doped carbon (Co2P/CoP/NC) electrocatalyst was prepared via an in situ pyrolysis method. The as-prepared electrocatalyst exhibited efficient electrocatalytic activity for HER in an acidic solution. The Co2P/CoP/NC catalyst displayed an overpotential of 184 mV at 10 mA cm−2 and a low Tafel slope of 82 mV dec−1, which could be attributed to the tight Co2P/CoP heterojunction and the synergetic effect of Co2P/CoP and N-doped carbon. In addition, the electrochemical active surface area of Co2P/CoP/NC was 75.2 m2 g−1, which indicated that more active regions can be applied for the HER process. This report may pave a new way for the design of efficient and low-cost N-doped-carbon-supported 3d transition metal phosphide electrocatalysts.


Introduction
Hydrogen energy is considered as a potential alternative to fossil fuels since it has advantages of high energy density, environmental sustainability, and zero carbon emission [1,2].Electrocatalytic splitting of water is considered as one of the viable methods for continuous large-scale hydrogen production.However, with the most active Pt-based electrocatalysts, it is difficult to achieve large-scale electrolytic water splitting application due to its high cost and low earth reserves [3,4].Thus, the development of efficient electrocatalysts with low cost, high catalytic activity, and stability is the main challenge and key research direction in the field of an electrolytic hydrogen evolution reaction.
As an earth-abundant 3d transition metal, Fe, Co, Ni, Mn, and their compound (such as sulfides, phosphides, nitrides, carbides, and oxides) electrocatalysts have been widely studied as substitutes for noble metals [5][6][7][8].Among them, transition metal phosphides exhibited excellent electrocatalytic hydrogen evolution activity due to the high electronegativity of P atoms [7,9].Particularly, cobalt phosphides, including Co 2 P and CoP, are often used as an electrocatalyst in the HER and OER process [10,11].Previous studies have shown that the synergistic effect of Co 2 P and CoP can effectively improve the electrocatalytic activity [12,13].Nevertheless, the synthesis methods to obtain a Co 2 P/CoP nanojunction are typically complicated as well as using NaH 2 PO 2 as the P source [14][15][16][17], which usually require H 2 as reduction gas or have two steps in the thermal treatment process (pyrolysis followed with the phosphating process).Therefore, one-step synthesis of a Co 2 P/CoP nanojunction using simple methods is a challenging study.In addition, transition metal phosphides tend to aggregate during the synthesis process and are easily oxidized during storage [18].In view of this, to increase the stability of transition metal Materials 2024, 17, 87 2 of 11 phosphides and the conductivity of the catalyst, carbon-supported nanocomposite catalysts have been developed, especially N-doped carbon materials [19].For example, Ma et al. prepared N-doped-carbon-coated CoP nanoparticles supported on N-doped graphene through a subsequent hydrothermal treatment, pyrolysis, and a following phosphating process [18].In another work, Chen et al. reported the preparation of CoP nanoparticles absorbed on defective reduced graphene oxide (CoP@DrGO) using NaH 2 PO 2 •H 2 O as the P source under Ar/H 2 gas flow [11].The synthesis process of carbon-supported cobalt phosphides typically involves multi-step reaction processes and complex control conditions.Therefore, it is imperative to find a method for synthesizing N-doped-C-supported cobalt phosphide nanomaterials.Phytic acid is an organic phosphate that can be extracted from plant seeds and has the characteristics of being green, natural, and environmentally friendly.It has been reported that it can be used as an organic phosphorus source to synthesize phosphide catalysts [7,19,20].Polyaniline is widely used as a supporting conductive base material for catalysts due to its three-dimensional porous structure [21,22].By adding excessive phytic acid in the synthesis process of a polyaniline precursor, the obtained polyaniline-phyticacid can be used as a conductive carbon support material and an organic phosphorus source for the synthesis of N-atom-doped-carbon-supported metal phosphide catalysts, which can reduce catalyst aggregation and oxidation, improve conductivity, and thus improve catalytic performance.In view of these, we developed a synthesis method to prepare a N-doped-carbon-material-supported Co 2 P/CoP nanocomposite catalyst with efficient electrocatalytic performance.
Here, a Co 2 P/CoP heterojunction embedded on N-doped carbon (Co 2 P/CoP/NC) nanocomposite electrocatalysts was prepared through an in situ thermal treatment method.It should be pointed out that the synthesis process does not require additional P sources (NaH 2 PO 2 ) or reducing gases (H 2 ).Moreover, the obtained Co 2 P/CoP/NC catalyst exhibited an exceptional synergistic effect between Co 2 P/CoP and N-doped C, which efficiently improved its electrocatalytic hydrogen evolution performance.This research provides a method for synthesizing high-performance 3d transition metal phosphide nanocomposite electrocatalysts.

Materials
Aniline (C 6 H 7 N, 99+%), Ammonium persulphate ((NH 4 ) 2 S 2 O 8 , 98%), and Nafion (5% w/w in water and 1-propanol) were purchased from Alfa Aesar China (Shanghai, China).The CoC 2 O 4 /PANI-PA hybrid precursor was synthesized as follows: Initially, 0.92 mL of aniline and 1.32 mL of phytic acid (70% (w/w) in H 2 O) were dissolved in 4 mL of deionized water with stirring to form a mixture.Then, 1.144 g of ammonium persulfate was dissolved in 2 mL of water to form a uniform solution.In total, 0.5 g of cobalt oxalate dihydrate and an ammonium persulfate aqueous solution were added into the mixture of aniline and phytic acid in turn, and the mixture was stirred for a certain time until the reaction was complete, forming a homogeneous mixture.We then allowed the mixture to undergo a complete polymerization reaction by standing at room temperature for 12 h.Finally, the mixture was washed and filtered with a small amount of deionized water and dried in an oven for about 12 h to obtain the CoC 2 O 4 /PANI-PA hybrid precursor material.

Preparation of Electrocatalysts
The one-step synthesis of the Co 2 P/CoP/NC electrocatalyst was conducted in a tube furnace.In total, 300 mg of the CoC 2 O 4 /PANI-PA hybrid precursor was pyrolyzed at 900 • C under a N 2 atmosphere for 2 h with a ramp rate of 5 • C min −1 .After the thermal reaction, the furnace was cooled down to room temperature and the Co 2 P/CoP embedded on N-doped carbon nanomaterial (Co 2 P/CoP/NC) was obtained.We also synthesized the samples that the CoC 2 O 4 /PANI-PA hybrid precursor pyrolyzed at different temperatures (800, 850, and 950 • C) and they were donated as Co 2 P 2 O 7 /NC, Co 2 P 2 O 7 /Co 2 P/NC, and Co 2 P/NC, respectively.

Structural Characterization
X-ray diffraction (XRD) patterns were examined through an X-ray diffractometer (Rigaku D/MAX-2600, Tokyo, Japan) with Cu-Kα radiation under a scanning rate of 5 • min −1 .The Raman spectrum was collected through a Raman spectrometer (HORIBA HR Evolution, Kyoto, Japan) with a 532 nm laser as the excitation light source.The fine structures and morphologies of the catalyst were characterized by a transmission electron microscope (JEOL JEM-F200, Tokyo, Japan) and scanning electron microscope (ZEISS Gemini 300, Oberkochen, Germany) equipped with energy dispersive X-ray spectra (EDS, Smartedx, Oberkochen, Germany).X-ray photoelectron spectroscopy (XPS) was examined on a Thermo Scientific K-Alpha using a monochromatic Al Kα X-ray source.

Electrochemical Characterization
All electrochemical measurements were examined on a three-electrode system using a Gamry Interface 1010E potentiostat (Gamry, Warminster, PA, USA).The electrochemical tests were conducted in 0.5 M H 2 SO 4 and the catalytic electrode, Ag/AgCl electrode, and carbon rod served as a working electrode, reference electrode, and counter electrode, respectively.To prepare the catalyst ink, 4 mg of the catalysts was dispersed in the mixture of 230 µL of H 2 O, 750 µL of ethanol, and 20 µL of the Nafion solution with ultrasonic treatment for 30 min to form a uniform suspension.Then, 15 µL of the catalyst suspension was loaded onto a glassy carbon electrode (5 mm in diameter) and dried naturally in room temperature to obtain the working electrode.The polarization curves were tested using linear sweep voltammetry (LSV) with a scan rate of 5 mV s −1 without IR compensation in electrolytes after saturation with N 2 for 30 min.Cyclic voltammograms (CVs) were examined in the potential range of 0.1 to 0.2 V with the scan rates from 20 to 140 mV s −1 .All potentials in this paper were calibrated to a reversible hydrogen electrode (RHE) based on the equation E(RHE) = E(Ag/AgCl) + 0.059 pH + 0.197 V. EIS measurements were performed at −100 mV with frequency from 0.01 Hz to 100 kHz with an AC voltage of 5 mV.

Results and Discussion
The Co 2 P/CoP/NC electrocatalyst was synthesized through one-step pyrolysis of CoC 2 O 4 /PANI-PA precursors.To study the growth mechanism of catalysts and synthesize high-performance catalysts, we conducted the pyrolysis process under 800, 850, 900, and 950 • C. The X-ray diffraction analysis was first performed to characterize the crystalline phase composition of the catalysts.As shown in Figure 1a, the sample obtained at 800  S1 and no other diffraction peaks were found except for Co 2 P and CoP indicating there were not any byproducts in the catalyst.In addition, the diffraction peak shape of Co 2 P became sharper, indicating a higher crystallinity of the sample.It is reported that the intrinsic electrocatalytic activity of CoP is higher than that of pure Co 2 P, while Co 2 P has better conductivity during the electrocatalytic process [23].Thus, the presence of Co 2 P can promote electron transfer performance, and the formation of a Co 2 P/CoP heterojunction can improve the electrocatalytic hydrogen evolution performance [17].However, when the pyrolysis temperature was 950 • C (Figure S2), the diffraction peaks of CoP disappeared, indicating that 900 • C is the optimal temperature for synthesizing the Co 2 P/CoP mixed phase.
Materials 2024, 18, x FOR PEER REVIEW 4 of 11 except for Co2P and CoP indicating there were not any byproducts in the catalyst.In addition, the diffraction peak shape of Co2P became sharper, indicating a higher crystallinity of the sample.It is reported that the intrinsic electrocatalytic activity of CoP is higher than that of pure Co2P, while Co2P has better conductivity during the electrocatalytic process [23].Thus, the presence of Co2P can promote electron transfer performance, and the formation of a Co2P/CoP heterojunction can improve the electrocatalytic hydrogen evolution performance [17].However, when the pyrolysis temperature was 950 °C (Figure S2), the diffraction peaks of CoP disappeared, indicating that 900 °C is the optimal temperature for synthesizing the Co2P/CoP mixed phase.To verify the presence of carbon in the catalyst, we conducted Raman spectroscopy tests on the samples, and the result is shown in Figure 1b.Raman peaks of Co2P/CoP/NC located at 1349 cm −1 and 1593 cm −1 represented the disorder carbon (D-band) and graphitic carbon (G-band), respectively.The higher D band intensity indicated that more defects were generated [11,24].Meanwhile, the intensity ratio (ID/IG) of Co2P2O7/NC, Co2P2O7/Co2P/NC, Co2P/CoP/NC, and Co2P/NC was 1.42, 1.22, 1.24, and 0.96, respectively.The result presented the graphitization of carbon domains formatted as the synthesis temperature increases, which can help the transfer of electrons.As a support material of transition metal phosphide, it can provide more catalytic active sites for the electrocatalytic hydrogen evolution reaction, and it is easier to adsorb hydrogen ions as well as desorb hydrogen, thereby improving the catalytic activity of the material.
The surface elemental composition of Co2P/CoP/NC was analyzed through X-ray photoelectron spectroscopy (XPS).The full XPS survey spectrum in Figure S3 revealed the existence of C, N, O, P, and Co elements in the catalyst.The high-resolution spectrum of O 1s is shown in Figure S4 and it can be deconvoluted into two peaks at 531 and 532.4 eV, which were attributed to the zero valence state and negatively charged oxygen atoms.The presence of the oxygen element is due to the inevitable partial oxidation of the catalyst in the air.The high-resolution spectrum of C 1s resolved into three peaks located at 284.6 eV, 285.5 eV, and 288.8 eV (Figure 2a), which were attributed to C-C, C-P, and C-N, respectively [25].The N 1s high-resolution spectrum in Figure 2b could be deconvoluted into three peaks at 398.8 eV, 400.2 eV, and 401.1 eV, which were ascribed to pyrinic-N, pyrrolic-N, and graphitic-N, respectively [26].It has been reported that the existence of graphitic-N can greatly promote the charge transfer efficiency of a catalyst, thus improving the To verify the presence of carbon in the catalyst, we conducted Raman spectroscopy tests on the samples, and the result is shown in Figure 1b.Raman peaks of Co 2 P/CoP/NC located at 1349 cm −1 and 1593 cm −1 represented the disorder carbon (D-band) and graphitic carbon (G-band), respectively.The higher D band intensity indicated that more defects were generated [11,24].Meanwhile, the intensity ratio (I D /I G ) of Co 2 P 2 O 7 /NC, Co 2 P 2 O 7 /Co 2 P/NC, Co 2 P/CoP/NC, and Co 2 P/NC was 1.42, 1.22, 1.24, and 0.96, respectively.The result presented the graphitization of carbon domains formatted as the synthesis temperature increases, which can help the transfer of electrons.As a support material of transition metal phosphide, it can provide more catalytic active sites for the electrocatalytic hydrogen evolution reaction, and it is easier to adsorb hydrogen ions as well as desorb hydrogen, thereby improving the catalytic activity of the material.
The surface elemental composition of Co 2 P/CoP/NC was analyzed through X-ray photoelectron spectroscopy (XPS).The full XPS survey spectrum in Figure S3 revealed the existence of C, N, O, P, and Co elements in the catalyst.The high-resolution spectrum of O 1s is shown in Figure S4 and it can be deconvoluted into two peaks at 531 and 532.4 eV, which were attributed to the zero valence state and negatively charged oxygen atoms.The presence of the oxygen element is due to the inevitable partial oxidation of the catalyst in the air.The high-resolution spectrum of C 1s resolved into three peaks located at 284.6 eV, 285.5 eV, and 288.8 eV (Figure 2a), which were attributed to C-C, C-P, and C-N, respectively [25].The N 1s high-resolution spectrum in Figure 2b could be deconvoluted into three peaks at 398.8 eV, 400.2 eV, and 401.1 eV, which were ascribed to pyrinic-N, pyrrolic-N, and graphitic-N, respectively [26].It has been reported that the existence of graphitic-N can greatly promote the charge transfer efficiency of a catalyst, thus improving the conductivity of the catalyst.The C 1s and N 1s spectra fitting results showed that the N element was successfully doped into the carbon matrix to form the N-doped carbon framework.The high-resolution spectrum of Co 2p for Co 2 P/CoP/NC in Figure 2c demonstrated the Co 2p 3/2 binding energies located at 779.5 eV and 781.3 eV with one satellite peak centered at 786 eV, and the Co 2p 1/2 binding energies located at 794.5 eV and 797.3 eV with one satellite peak at 802.7 eV [15,23,27].The peaks located at 779.5 eV and 794.5 eV were attributed to the Co-P bonds in the Co 2 P/CoP/NC catalyst, and the peaks at 781.3 eV and 797.3 eV were distributed to the Co-O bonds, which was caused by the inevitable surface oxidation of Co 2 P/CoP.The high-resolution spectrum of P 2p in Figure 2d displayed three peaks located at 129.8 eV, 132.9 eV, and 133.9 eV that were assigned to the Co-P, P-C, and Co-O-P, respectively [28,29].The P 2p spectrum confirmed the existence of Co-P as well as the close interaction between C and P in the  The morphology and nanostructure of Co2P/CoP/NC were studied through SEM and TEM.The SEM images in Figure 3a,b show a three-dimensional porous interconnected nanostructure with a diameter of about 100 nm.The TEM images in Figures 3c and S10-S12 show the Co2P/CoP nanoparticles were embedded on N-doped carbon.The high-resolution TEM image in Figure 3d shows well-defined lattice fringes of 0.196 nm corresponding to the crystal plane of CoP (112).The layered structure at the edge with the lattice fringes of 0.35 nm corresponded to the (002) lattice planes of graphitic carbon [26].The EDS spectrum of Co2P/CoP/NC is shown in Figure S13, and C, O, Co, and P were The morphology and nanostructure of Co 2 P/CoP/NC were studied through SEM and TEM.The SEM images in Figure 3a,b show a three-dimensional porous interconnected nanostructure with a diameter of about 100 nm.The TEM images in Figure 3c and Figure S10-S12 show the Co 2 P/CoP nanoparticles were embedded on N-doped carbon.The high-resolution TEM image in Figure 3d shows well-defined lattice fringes of 0.196 nm corresponding to the crystal plane of CoP (112).The layered structure at the edge with the lattice fringes of 0.35 nm corresponded to the (002) lattice planes of graphitic carbon [26].The EDS spectrum of Co 2 P/CoP/NC is shown in Figure S13, and C, O, Co, and P were detected.Meanwhile, the atomic ratio of Co to P was calculated to be 1.73:1, indicating the coexistence of Co 2 P and CoP.The EDS element mapping in Figure 3e displays the C, N, Co, and P elements in the catalyst, and the element mappings of Co and P confirmed the formation of Co 2 P/CoP embedded on N-doped carbon, which is consistent with the XPS analysis.
detected.Meanwhile, the atomic ratio of Co to P was calculated to be 1.73:1, indicating the coexistence of Co2P and CoP.The EDS element mapping in Figure 3e displays the C, N, Co, and P elements in the catalyst, and the element mappings of Co and P confirmed the formation of Co2P/CoP embedded on N-doped carbon, which is consistent with the XPS analysis.To explore the electrocatalytic performance of the electrocatalysts, the HER was conducted in a three-electrode system, and commercial Pt/C (20 wt%) was selected as a comparison.Figure 4a  To explore the electrocatalytic performance of the electrocatalysts, the HER was conducted in a three-electrode system, and commercial Pt/C (20 wt%) was selected as a comparison.Figure 4a   O is 0.3 g, the overpotential at 10 mA cm −2 is 332 mV, while when the addition amount is 1 g, the overpotential is 250 mV.The lowest overpotential is found at the addition amount of 0.5 g, indicating that the optimal CoC 2 O 4 •2H 2 O addition amount is 0.5 g, which means that the optimal synthesis condition for the catalyst is at a temperature of 900  To further investigate the mechanism of enhanced electrocatalytic hydrogen evolution performance, the electrochemical double-layer capacitance (C dl ) and electrochemical active surface area (ECSA) of Co 2 P/CoP/NC were derived by testing the CV curves at different scan rates (Figure 5a).The C dl was obtained by linearly fitting ∆j (∆j = j a − j c ) at 0.15 V with the scanning rate in Figure 5b

Conclusions
In this study, we successfully synthesized a Co2P/CoP heterojunction embedded on N-doped carbon nanocomposites through a pyrolysis method.The formation of Co2P/CoP heterojunctions contributes to the improvement in electrocatalytic performance, and the presence of N-doped-defective-C-supported materials helps to boost the conductivity of the catalyst and the adsorption/desorption of H + and H2, while the close contact between Co2P/CoP and N-doped C Co2P/CoP/NC to possess efficient electrocatalytic performance.Consequently, the Co2P/CoP/NC electrocatalyst exhibits efficient electrocatalytic hydrogen evolution ability, with an overpotential of 184 mV at 10 mA cm −2 , a Tafel slope of 82 mV dec −1 , and remarkable long-term stability.This work provides a method for synthesizing N-doped-carbon-supported iron, cobalt, and nickel phosphide nanocomposite electrocatalysts.

Supplementary Materials:
The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1

Conclusions
In this study, we successfully synthesized a Co 2 P/CoP heterojunction embedded on N-doped carbon nanocomposites through a pyrolysis method.The formation of Co 2 P/CoP heterojunctions contributes to the improvement in electrocatalytic performance, and the presence of N-doped-defective-C-supported materials helps to boost the conductivity of the catalyst and the adsorption/desorption of H + and H 2 , while the close contact between Co 2 P/CoP and N-doped C enables Co 2 P/CoP/NC to possess efficient electrocatalytic performance.Consequently, the Co 2 P/CoP/NC electrocatalyst exhibits efficient electrocatalytic hydrogen evolution ability, with an overpotential of 184 mV at 10 mA cm −2 , a Tafel slope of 82 mV dec −1 , and remarkable long-term stability.This work provides a method for synthesizing N-doped-carbon-supported iron, cobalt, and nickel phosphide nanocomposite electrocatalysts.

Figure 1 .
Figure 1.(a) XRD patterns of the as-prepared samples; (b) Raman spectra of the samples.

Figure 1 .
Figure 1.(a) XRD patterns of the as-prepared samples; (b) Raman spectra of the samples.
Co 2 P/CoP/NC catalyst.The XPS spectra of Co 2 P 2 O 7 /NC, Co 2 P 2 O 7 /Co 2 P/NC, and Co 2 P/NC were also characterized and are illustrated in Figures S5-S9, which can further confirm the formation of Co 2 P 2 O 7 /NC, Co 2 P 2 O 7 /Co 2 P/NC, and Co 2 P/NC at different temperatures.Materials 2024, 18, x FOR PEER REVIEW 5 of 11 conductivity of the catalyst.The C 1s and N 1s spectra fitting results showed that the N element was successfully doped into the carbon matrix to form the N-doped carbon framework.The high-resolution spectrum of Co 2p for Co2P/CoP/NC in Figure 2c demonstrated the Co 2p3/2 binding energies located at 779.5 eV and 781.3 eV with one satellite peak centered at 786 eV, and the Co 2p1/2 binding energies located at 794.5 eV and 797.3 eV with one satellite peak at 802.7 eV [15,23,27].The peaks located at 779.5 eV and 794.5 eV were attributed to the Co-P bonds in the Co2P/CoP/NC catalyst, and the peaks at 781.3 eV and 797.3 eV were distributed to the Co-O bonds, which was caused by the inevitable surface oxidation of Co2P/CoP.The high-resolution spectrum of P 2p in Figure 2d displayed three peaks located at 129.8 eV, 132.9 eV, and 133.9 eV that were assigned to the Co-P, P-C, and Co-O-P, respectively [28,29].The P 2p spectrum confirmed the existence of Co-P as well as the close interaction between C and P in the Co2P/CoP/NC catalyst.The XPS spectra of Co2P2O7/NC, Co2P2O7/Co2P/NC, and Co2P/NC were also characterized and are illustrated in Figures S5-S9, which can further confirm the formation of Co2P2O7/NC, Co2P2O7/Co2P/NC, and Co2P/NC at different temperatures.

Figure 3 .
Figure 3. (a,b) SEM images of Co 2 P/CoP/NC; (c,d) TEM images of Co 2 P/CoP/NC at different magnifications; (e) EDS element mapping of Co 2 P/CoP/NC.
shows the LSV curves of Pt/C, Co 2 P/NC, Co 2 P/CoP/NC, Co 2 P 2 O 7 /Co 2 P/NC, and Co 2 P 2 O 7 /NC at a scan rate of 5 mV/s without iR correction.Obviously, the Pt/C catalyst demonstrated the supreme HER activity with the lowest overpotential of 41 mV at a current density of 10 mA cm −2 .Moreover, Co 2 P/CoP/NC displayed an overpotential of 184 mV at 10 mA cm −2 , which was lower than that of Co 2 P/NC (261 mV), Co 2 P 2 O 7 /Co 2 P/NC (241 mV), and Co 2 P 2 O 7 /NC (395 mV).Through comparison, it was found that the electrocatalysts with Co 2 P and CoP coexisting had the highest HER performance, indicating the efficient catalytic activity of Co 2 P/CoP/NC.In order to further verify the optimal amount of CoC 2 O 4 •2H 2 O added for the synthesis of catalysts, we compared the HER activity of the catalysts when 0.3 g, 0.5 g, and 1 g of CoC 2 O 4 •2H 2 O were added to synthesize Co 2 P/CoP/NC at 900 • C, respectively.As shown in Figure S14, when the addition amount of CoC 2 O 4 •2H
, and the C dl of Co 2 P/CoP/NC was 13.76 mF cm −2 , which is comparable compared to other cobalt phosphorus compound catalysts, such as Co 2 P/CoP (0.434 mF cm −2 ) [15], Co 2 P/CoMoP x /NF (11.1 mF•cm −2 ) [27], and CoP/NiCoP/NC (19.3 mF cm −2 ) [6].The ECSA of Co 2 P/CoP/NC was calculated in Figure S16 according to the C dl , and the ECSA was 75.2 m 2 /g by assuming a standard value of 60 µF/cm 2 .The large ECSA of Co 2 P/CoP/NC indicates more active regions for the electrocatalytic hydrogen evolution process.

Figure 5 .
Figure 5. (a) CV curves of Co2P/CoP/NC under different scan rates, (b) extraction of electrochemical double-layer capacitance (Cdl) through the relationship between the current density variation (Δj) and scan rate at 0.15 V versus RHE for Co2P/CoP/NC.

Figure 5 .
Figure 5. (a) CV curves of Co 2 P/CoP/NC under different scan rates, (b) extraction of electrochemical double-layer capacitance (C dl ) through the relationship between the current density variation (∆j) and scan rate at 0.15 V versus RHE for Co 2 P/CoP/NC.The EIS was performed to investigate the catalytic kinetics for HER, and Nyquist plots are shown in Figure6.Co 2 P/CoP/NC presents a smaller semicircle of the Nyquist plot, which means a smaller R ct and a faster electron transfer ability of the catalyst.According to the characterization results and the electrochemical studies, the electrocatalytic performance of the Co 2 P/CoP/NC catalyst mainly originated from the following aspects: (1) The Co 2 P/CoP heterojunction formed through the combination of CoP with high intrinsic electrocatalytic activity and Co 2 P with good conductivity, endowing the catalyst with high electrocatalytic activity.(2) The presence of defective carbon supporting materials was beneficial to the adsorption and desorption of H + and H 2 , thereby improving the electrocatalytic performance.Meanwhile, the formation of N-doped C could provide a better charge transfer efficiency, resulting in high conductivity for the catalyst.(3) The high ECSA indicates that the catalyst can provide more reactive active regions and synergistic effect between Co 2 P, CoP, and NC, significantly promoting the electrocatalytic hydrogen evolution performance of the catalyst.