Fabrication of C / Co-FeS 2 / CoS 2 with Highly E ﬃ cient Hydrogen Evolution Reaction

: The mainstream strategy for designing hydrogen electrocatalysts is to adjust their surface electronic structure; however, the conductivity of the electrocatalyst and the synergy with its substrate are still challenges to overcome. In this work, we report a carbon-doped Co-FeS 2 / CoS 2 (C / Co-FeS 2 / CoS 2 ) electrode, prepared via a hydrothermal process with carbon cloth (CC) as the substrate and carbon doping. The C / Co-FeS 2 / CoS 2 electrode shows excellent catalytic activity in the hydrogen evolution reaction (HER) with an overpotential of 88 mV at a current density of − 10 mA · cm − 2 in 0.5 M H 2 SO 4 solution. The Tafel slope is 66 mV · dec − 1 . Such superior performance is attributed to the high electrical conductivity of the electrocatalyst and its synergy with the substrate. Our study provides an e ﬃ cient alternative in the ﬁeld of electrocatalysis.

Non-precious metal chalcogenides are considered a class of very promising candidates for the hydrogen evolution reaction (HER) due to their high abundance, low cost, and thermal and mechanical stability [13][14][15][16]. However, practical applications of non-precious metal chalcogenides as HER cathode materials are limited by their low electrical conductivity and lack of active sites. In our previous research, we demonstrated good electrocatalytic properties of Co-doped FeS 2 , and CoS 2 formed heterostructures on Co-FeS 2 petals that can be attributed to the unique three-dimensional hierarchy. For example, the use of carbon materials solely as conductive substrates does not significantly improve the performance of the electrocatalysts [11,24]. If the carbon-doped non-precious metal catalyst is grown on the conductive substrate, its catalytic performance will be greatly improved due to the improvement of the conductivity of Co-FeS 2 /CoS 2 and its interaction with the substrate [25][26][27].
In this study, we used glucose as a carbon source to synthesize carbon doped Co-FeS 2 /CoS 2 (C/Co-FeS 2 /CoS 2 ) on carbon cloth via a one-step hydrothermal method. The resulting electrode shows excellent HER catalytic activity with an overpotential of 88 mV at a current density −10 mA·cm −2 in 0.5 M H 2 SO 4 solution; this is 15 mV lower than the overpotential of our previous research [19]. Because of the incorporation of carbon that results in synergistic catalysis between carbon and Co-aFeS 2 /CoS 2 , the performance of the catalyst is significantly improved due to the improvement in conductivity of C/Co-FeS 2 /CoS 2 and the reduction of resistance between C/Co-FeS 2 /CoS 2 and the substrate [27][28][29][30][31][32]. This study of doping non-metallic elements into non-precious catalysts provides a simple and efficient way to improve performance in the field of electrocatalysis.

Result and Discussion
In this work, we obtained C/Co-FeS 2 /CoS 2 with better performance by incorporating carbon on the basis of our previous research. The typical morphology of C/Co-FeS 2 /CoS 2 was revealed by scanning electron microscopy images, as presented in Figure 1. The micro-spherical structure of C/Co-FeS 2 /CoS 2 shown in Figure 1a,b typically ranged from 500 nm to 1 µm in diameter. Figure 1c,d show the SEM images and energy-dispersive spectrometry (EDS) elemental mapping of C, Fe, Co, and S for C/Co-FeS 2 /CoS 2 , which confirmed the existence of these elements and also suggested that carbon existed in the C/Co-FeS 2 /CoS 2 [11]. Figure S1 (Supplementary Materials) shows a larger-scale element distribution, which further demonstrates the uniformity of the electrocatalyst on the whole carbon fiber. Figure S2 (Supplementary Materials) shows the SEM images of C/CoS 2 , C/FeS 2 , and carbon (C) with particle sizes of 10 to 13 µm, 300 to 400 nm, and 150 to 250 nm, respectively. The addition of carbon led to the micro-spherical morphology of C/FeS 2 and C/CoS 2 . In this study, we used glucose as a carbon source to synthesize carbon doped Co-FeS2/CoS2 (C/Co-FeS2/CoS2) on carbon cloth via a one-step hydrothermal method. The resulting electrode shows excellent HER catalytic activity with an overpotential of 88 mV at a current density −10 mA•cm −2 in 0.5 M H2SO4 solution; this is 15 mV lower than the overpotential of our previous research [19]. Because of the incorporation of carbon that results in synergistic catalysis between carbon and Co-aFeS2/CoS2, the performance of the catalyst is significantly improved due to the improvement in conductivity of C/Co-FeS2/CoS2 and the reduction of resistance between C/Co-FeS2/CoS2 and the substrate [27][28][29][30][31][32]. This study of doping non-metallic elements into non-precious catalysts provides a simple and efficient way to improve performance in the field of electrocatalysis.

Result and Discussion
In this work, we obtained C/Co-FeS2/CoS2 with better performance by incorporating carbon on the basis of our previous research. The typical morphology of C/Co-FeS2/CoS2 was revealed by scanning electron microscopy images, as presented in Figure 1. The micro-spherical structure of C/Co-FeS2/CoS2 shown in Figure 1a and Figure 1b typically ranged from 500 nm to 1 μm in diameter. Figure 1c and Figure 1d show the SEM images and energy-dispersive spectrometry (EDS) elemental mapping of C, Fe, Co, and S for C/Co-FeS2/CoS2, which confirmed the existence of these elements and also suggested that carbon existed in the C/Co-FeS2/CoS2 [11]. Figure S1 (Supplementary Materials) shows a larger-scale element distribution, which further demonstrates the uniformity of the electrocatalyst on the whole carbon fiber. Figure S2 (Supplementary Materials) shows the SEM images of C/CoS2, C/FeS2, and carbon (C) with particle sizes of 10 to 13 μm, 300 to 400 nm, and 150 to 250 nm, respectively. The addition of carbon led to the micro-spherical morphology of C/FeS2 and C/CoS2. In addition, we further examined the crystal structure of the sample by X-ray diffraction (XRD) and Raman spectroscopy. Figure 2a is the XRD pattern for C/Co-FeS2/CoS2, where the diffraction peaks at 28.4°, 32.9°, 37°, 40.7°, 47.3°, and 56.1° can be precisely indexed to planes of FeS2 (JCPDS#42-1340) at (111), (200), (210), (211), (220), and (311). Only some weaker peaks belong to CoS2 (JCPDS#41-1471) [13,22,23]. The peak at 26.5° corresponds to the bare carbon fiber, which is supplied in the XRD In addition, we further examined the crystal structure of the sample by X-ray diffraction (XRD) and Raman spectroscopy. Figure [13,22,23]. The peak at 26.5 • corresponds to the bare carbon fiber, which is supplied in the XRD text. The XRD results clearly reveal that C/Co-FeS 2 /CoS 2 has a complete nanocrystalline phase. Figure 2b is the Raman spectrum of the C/Co-FeS 2 /CoS 2 . The two peaks at 335 cm −1 and 371 cm −1 exhibited by C/Co-FeS 2 /CoS 2 are due to the incorporation of carbon into Co-FeS 2 /CoS 2 . Two carbon peaks were also observed at the D peak (1351 cm −1 ) and G peak (1490 cm −1 ), where the position of the G peak is determined by the amount of hydrogen from the carbon-hydrogen bond [33]. To further demonstrate the structure of C/Co-FeS 2 /CoS 2 , transmission electron microscopy (TEM) was employed, as shown in Figure 2c,d and Figure S3 (Supplementary Materials) are high-resolution TEM (HRTEM) images, indicating that the carbon element was mainly located at the outer edge of the C/Co-FeS 2 /CoS 2 nanostructure, while the inset shows the selected area electron diffraction (SAED) patterns of C/Co-FeS 2 /CoS 2 , indicating the formation of a good crystal structure. The inter-planar spacing (210) of FeS 2 was 0.24 nm and the spacing (210) of CoS 2 was 0.25 nm. The existence of carbon can be proven from this pattern that had inter-planar spacing of about 0.34 nm. text. The XRD results clearly reveal that C/Co-FeS2/CoS2 has a complete nanocrystalline phase. Figure  2b is the Raman spectrum of the C/Co-FeS2/CoS2. The two peaks at 335 cm −1 and 371 cm −1 exhibited by C/Co-FeS2/CoS2 are due to the incorporation of carbon into Co-FeS2/CoS2. Two carbon peaks were also observed at the D peak (1351 cm −1 ) and G peak (1490 cm −1 ), where the position of the G peak is determined by the amount of hydrogen from the carbon-hydrogen bond [33]. To further demonstrate the structure of C/Co-FeS2/CoS2, transmission electron microscopy (TEM) was employed, as shown in Figure 2c. Figure 2d and Figure   To understand the elemental valence and chemical composition of C/Co-FeS2/CoS2, we conducted X-ray photoelectron spectroscopy (XPS) measurements. Figure 3a shows the full XPS survey spectra of C/Co-FeS2/CoS2. Figure 3b is the XPS spectrum of the C element. In Figure 3c, the two peaks at 708 eV and 720 eV correspond to Fe 2p3/2 and Fe 2p1/2. The other peaks at 712 and 726.6 eV correspond to Fe 2p3/2 and Fe 2p1/2 of C/Co-FeS2/CoS2, which had a positive shift as compared with FeS2, due to the formation of an interface between FeS2 and CoS2 [15]. The XPS spectrum of Co shown in Figure 3d consists of two peaks at 778.8 eV for Co 2p3/2 and 793.5 eV for Co 2p1/2, with two shake-up satellites [16]. The binding energies of S 2p3/2 at 162.6 eV belong to S2 2− of CoS2, whereas the binding energies of S 2p1/2 at around 163.8 eV correspond to S2 2− of Co-FeS2, and the peak at around 168.3 eV is attributed to oxidized S species [16,18,19]. To understand the elemental valence and chemical composition of C/Co-FeS 2 /CoS 2 , we conducted X-ray photoelectron spectroscopy (XPS) measurements. Figure 3a shows the full XPS survey spectra of C/Co-FeS 2 /CoS 2 . Figure 3b is the XPS spectrum of the C element. In Figure 3c, the two peaks at 708 eV and 720 eV correspond to Fe 2p 3/2 and Fe 2p 1/2 . The other peaks at 712 and 726.6 eV correspond to Fe 2p3/2 and Fe 2p1/2 of C/Co-FeS2/CoS2, which had a positive shift as compared with FeS 2 , due to the formation of an interface between FeS 2 and CoS 2 [15]. The XPS spectrum of Co shown in Figure 3d consists of two peaks at 778.8 eV for Co 2p 3/2 and 793.5 eV for Co 2p 1/2 , with two shake-up satellites [16]. The binding energies of S 2p 3/2 at 162.6 eV belong to S 2 2− of CoS 2 , whereas the binding energies of S 2p 1/2 at around 163.8 eV correspond to S 2 2− of Co-FeS 2 , and the peak at around 168.3 eV is attributed to oxidized S species [16,18,19]. HER activity was analyzed by measuring the linear sweep voltammetry (LSV) curve of C/Co-FeS2/CoS2 in a 0.5 M H2SO4 solution. The performances of C/CoS2, C/FeS2, C, and bare CC were also analyzed under the same conditions as shown in Figure 4a. The bare CC showed almost no catalytic activity. However, C/Co-FeS2/CoS2 grown on CC substrates showed excellent HER activity, and had a lower overpotential (η = 88 mV) at a current density of −10 mA•cm −2 , much smaller than C/CoS2 (113 mV) and C/FeS2 (177 mV), as shown in Figure 4a. We summarized the opening voltage of the hydrogen evolution reaction of similar catalysts that were published so far, and found that C/Co-FeS2/CoS2 had the best performance, as shown in Figure S4 (Supplementary Materials) [18,19,22,34,35]. The Tafel slope of C/Co-FeS2/CoS2 (66 mV•dec −1 ) was smaller than that of C/CoS2 (77 mV•dec −1 ) and C/FeS2 (119 mV•dec −1 ), as shown in Figure 4b. It is worth noting that the electrocatalyst formed on the carbon cloth after the simple glucose reaction showed significant catalytic activity compared with the blank carbon cloth. The [R(C(RW)] circuit can be obtained by simulating the electrochemical impedance spectroscopy (EIS) data in Figure S6 (Supplementary Materials). As shown in the circuit diagram of Figure S6b (Supplementary Materials), Rs, Rct, C, and W represent bulk solution resistance, charge-transfer resistance, capacitance, and Warburg resistance, respectively. By comparing the simulated data, it can be found that C/Co-FeS2/CoS2 (0.14 Ω) showed a much smaller resistance in the Nyquist diagram than C/CoS2 (0.19 Ω) and C/FeS2 (0.54 Ω). Figure  4c indicates the stability of C/Co-FeS2/CoS2; 8 mV of the overpotential was lost after 500 cycles. In addition, as shown in Figure S7 (Supplementary Materials), C/Co-FeS2/CoS2 hardly changed its microscopic morphology after 500 cycles of cyclic voltammetry (CV). It can be seen from the comparison that there was no significant change in the overall surface of the C/Co-FeS2/CoS2 nanosphere after a 500-cycle durability test in an acidic solution, with only slight corrosion marks. To analyze the activity of C/Co-FeS2/CoS2, we calculated non-faradaic double-layer capacitance (Cdl) by cyclic voltammetry measurements at different scan rates (5, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV•s −1 ) to obtain the electrochemical surface area (ECSA). The catalytic performances of the working electrode were normalized to 1 cm 2 . Data of the cyclic voltammetry measurements at different scan rates are shown in Figure S5 (Supplementary Materials). It was calculated that the Cdl of C/Co-FeS2/CoS2 was 129 mF•cm −2 , which is much larger than the 43 mF•cm −2 of C/FeS2 and 23 mF•cm −2 of C/CoS2. Although the Cdl of C was only 1.2 mF•cm −2 , it can be said that the carbon obtained after the glucose reaction had a certain catalytic activity. Figures S6a,b (Supplementary Materials) show the EIS patterns, whereby C/Co-FeS2/CoS2 had a better conductivity with an improvement in performance.  Figure 4a. The bare CC showed almost no catalytic activity. However, C/Co-FeS 2 /CoS 2 grown on CC substrates showed excellent HER activity, and had a lower overpotential (η = 88 mV) at a current density of −10 mA·cm −2 , much smaller than C/CoS 2 (113 mV) and C/FeS 2 (177 mV), as shown in Figure 4a. We summarized the opening voltage of the hydrogen evolution reaction of similar catalysts that were published so far, and found that C/Co-FeS 2 /CoS 2 had the best performance, as shown in Figure S4 (Supplementary Materials) [18,19,22,34,35]. The Tafel slope of C/Co-FeS 2 /CoS 2 (66 mV·dec −1 ) was smaller than that of C/CoS 2 (77 mV·dec −1 ) and C/FeS 2 (119 mV·dec −1 ), as shown in Figure 4b. It is worth noting that the electrocatalyst formed on the carbon cloth after the simple glucose reaction showed significant catalytic activity compared with the blank carbon cloth. The [R(C(RW)] circuit can be obtained by simulating the electrochemical impedance spectroscopy (EIS) data in Figure S6 (Supplementary Materials). As shown in the circuit diagram of Figure S6b (Supplementary Materials), R s , R ct , C, and W represent bulk solution resistance, charge-transfer resistance, capacitance, and Warburg resistance, respectively. By comparing the simulated data, it can be found that C/Co-FeS 2 /CoS 2 (0.14 Ω) showed a much smaller resistance in the Nyquist diagram than C/CoS 2 (0.19 Ω) and C/FeS 2 (0.54 Ω). Figure 4c indicates the stability of C/Co-FeS 2 /CoS 2 ; 8 mV of the overpotential was lost after 500 cycles. In addition, as shown in Figure S7 (Supplementary Materials), C/Co-FeS 2 /CoS 2 hardly changed its microscopic morphology after 500 cycles of cyclic voltammetry (CV). It can be seen from the comparison that there was no significant change in the overall surface of the C/Co-FeS 2 /CoS 2 nanosphere after a 500-cycle durability test in an acidic solution, with only slight corrosion marks. To analyze the activity of C/Co-FeS 2 /CoS 2 , we calculated non-faradaic double-layer capacitance (C dl ) by cyclic voltammetry measurements at different scan rates (5,10,20,30,40,50,60,70,80,90, and 100 mV·s −1 ) to obtain the electrochemical surface area (ECSA). The catalytic performances of the working electrode were normalized to 1 cm 2 . Data of the cyclic voltammetry measurements at different scan rates are shown in Figure S5 (Supplementary Materials). It was calculated that the C dl of C/Co-FeS 2 /CoS 2 was 129 mF·cm −2 , which is much larger than the 43 mF·cm −2 of C/FeS 2 and 23 mF·cm −2 of C/CoS 2 . Although the C dl of C was only 1.2 mF·cm −2 , it can be said that the carbon obtained after the glucose reaction had a certain catalytic activity. Figure S6a,b (Supplementary Materials) show the EIS patterns, whereby C/Co-FeS 2 /CoS 2 had a better conductivity with an improvement in performance.

Synthesis of C/Co-FeS2/CoS2
Firstly, the appropriate size of carbon cloth (CC) was prepared, cleaned with deionized water and absolute ethanol, and then dried. Subsequently, FeSO4•7H2O (1.2 mM), Co(NO3)2•6H2O (0.156 mM), SC(NH2)2 (1.8 mM), and C6H12O6 (0.24 mM) were weighed out. The weighed reagent was added to a Teflon-lined autoclave (50 mL), and an appropriate amount of deionized water (25 mL) was added and stirred (15 min). After the first stirring, the sulfur powder (0.72 mM) was weighed and uniformly added into the reaction vessel and then slowly stirred (15 min). After the second stirring was completed, the prepared CC was inserted into the solution vertically. The reaction kettle was heated at 180 °C for 8 h. After the reaction was completed and cooled, the sample was taken out and cleaned with deionized water and absolute ethanol.

Synthesis of C/Co-FeS2/CoS 2
Firstly, the appropriate size of carbon cloth (CC) was prepared, cleaned with deionized water and absolute ethanol, and then dried. Subsequently, FeSO 4 ·7H 2 O (1.2 mM), Co(NO 3 ) 2 ·6H 2 O (0.156 mM), SC(NH 2 ) 2 (1.8 mM), and C 6 H 12 O 6 (0.24 mM) were weighed out. The weighed reagent was added to a Teflon-lined autoclave (50 mL), and an appropriate amount of deionized water (25 mL) was added and stirred (15 min). After the first stirring, the sulfur powder (0.72 mM) was weighed and uniformly added into the reaction vessel and then slowly stirred (15 min). After the second stirring was completed, the prepared CC was inserted into the solution vertically. The reaction kettle was heated at 180 • C for 8 h. After the reaction was completed and cooled, the sample was taken out and cleaned with deionized water and absolute ethanol.

Synthesis of C/CoS 2 , C/FeS 2 , and C
Co(NO 3 ) 2 ·6H 2 O (1.2 mM), SC(NH 2 ) 2 (1.8 mM), and C 6 H 12 O 6 (0.24 mM) were prepared for C/CoS 2 . FeSO 4 ·7H 2 O (1.2 mM), SC(NH 2 ) 2 (1.8 mM), and C 6 H 12 O 6 (0.24 mM) were prepared for C/FeS 2 . C 6 H 12 O 6 (1.2 mM) and SC(NH 2 ) 2 (1.8 mM) were added to synthesis C. They were added to a 50-mL Teflon-lined autoclave; then, deionized water (25 mL) was added and stirred (15 min). Sulfur powder (0.72 mM) was then added and stirred (15 min) slowly for C/CoS 2 and C/FeS 2 . Stirring was stopped and the magnetic stirrer was removed. The cleaned and dried carbon fiber paper was inserted into the reactor solution vertically, and the reactor was heated at 180 • C for 8 h. After the reaction was completed and cooled, the sample was washed repeatedly with deionized water and ethanol.

Electrochemical Measurements
The CHI760E electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China) was used to analyze the performance of samples. The test uses a three-electrode system with the sample, a platinum electrode, and a saturated calomel electrode as the working electrode, counter electrode, and reference electrode, respectively. The measured potentials were converted to a reversible hydrogen electrode (RHE) (E(RHE) = EHg/Hg 2 Cl 2 + 0.241 + 0.0591 pH). The electrolyte solution used H 2 SO 4 solution (0.5 M). Prior to testing, nitrogen needed to be bubbled into H 2 SO 4 solution to remove oxygen from the solution. LSV was measured from −0.8 to 0 V at 2 mV·s −1 . The Tafel slope was obtained by computing the LSV data. The CV was tested at different scan rates with a potential range of 0 to 0.20 V vs. RHE for HER, and the resulting data were used to calculate the ECSA. EIS measurements were carried on in a frequency range from 10 5 to 0.01 Hz with an alternating current (AC) voltage of 5 mV.

Conclusions
In summary, C/Co-FeS 2 /CoS 2 with superior performance was prepared successfully by doping carbon in a one-step hydrothermal method. The synergy between non-metallic elemental carbon and C/Co-FeS 2 /CoS 2 , as well as the optimization of conductivity, further enhanced the catalytic efficiency. We believe that the doping of non-metallic elements in the catalyst provides a simple, feasible, and effective direction for the preparation of highly efficient non-precious metal catalysts.

Author Contributions:
The experiments and characterizations were carried out by Y.G., K.W., with the assistance of H.W., H.S. and X.X., under the guidance of S.Y. and Y.S., Y.G. and S.Y. wrote the manuscript and prepared all figures. Y.S. and S.Y. supervised and coordinated all the work.