Heterogeneous cobalt phosphides nanoparticles anchored on carbon cloth realizing the efficient hydrogen generation reaction

The introduction of excessive reduction of fossil fuels has led to many terrible issues such as global warming and the "energy crisis". Therefore, the urgency of developing environmentally friendly and renewable energy sources attracts a large number of researchers to explore energy-related fields [1e4]. Due to its high energy density and no pollution of combustion products, hydrogen is considered as an ideal energy carrier to replace the traditional exhaustible energy. Future [5e8]. Electrochemical water splitting has been confirmed as the simplest method to collect hydrogen with high purity, while it always requires highly active catalysts to reduce the overpotential and increase the reaction rate towards the hydrogen evolution reaction (HER) [9e11]. Considerable studies have revealed that platinum group metals exhibit excellent HER performance in acidic environments, but their large-scale applications are limited due to their high economic cost and low abundance [12,13]. It has been reported that the abundance of precious platinum is about 3.7 10 6 % in Earth, which is an order of magnitude smaller than other non-precious metals, including cobalt [14,15]. In this regard, the development of non-noble and highly efficient HER electrocatalysts is still pressing and challenging work [16e18]. Attractive transition metal chalcogenides (TMD), which show outstanding HER catalytic activity, have been widely explored in the past years, such as MoS2 [19,20], CoSe2 [21,22], and NiS2 [23,24]. Because of their high conductivity and low cost, nickel-based compounds have already been widely used as HER electrocatalysts [25]. Unfortunately, it has been reported that they are usually unstable in highly acidic solutions [26e28]. Transition metal phosphides (TMPs), as rising stars in transition metal-based HER catalysts, are commonly used in the hydrodesulfurization (HDS) reaction. The similar mechanism between HDS and HER further suggests that HDS catalysts are likely to HER serve [29,30]. A large number of studies reported on TMPs claim outstanding HER activity [31e34]. For example, our group recently reported that the carbon framework of cobalt complexed phosphide (Co2P@C/CC) can act as a flexible and efficient electrocatalyst in the hydrogen evolution process [35]. Cobalt phosphides as one of the common TMPs are considered as promising alternatives for HER catalysts, mainly benefiting from favorable hydrogen adsorption energy (DGH) [36]. It has been reported that the appropriate molar ratio of Co and P is favorable to obtain a higher current density under a lower overpotential, which is more prominent in the catalytic activity [37]. In addition, various components of cobalt phosphides, including nanoparticles (NPs) and nanosheets (NSs) and nanowires (NWs) as well as hollow nanostructures [30,38] have been studied. For example, Sun's group synthesized different CoP nanostructures and showed that CoP nanowires show the best HER catalytic activity [39]. Although several studies have been reported, it is inevitable to accept the related research challenges by adjusting the catalytic active phase of cobalt phosphides. In this work, we are sure to design a nanocatalyst based on heterogeneous cobalt phosphide nanoparticles (CoxP NPs) through A facile step, which takes advantage of the weak reflux process of cobalt acetate, follows the gas-to-solid strategy at low temperature in the presence of PH3 [40,41]. It is interesting to know that CoxP nanoparticles prepared from Co2P and CoP are very fine. nanoparticles is remarkable, when CoxP NPs supported on carbon cloth used as cathode catalyst in 0.5 M H2SO4 only achieve current densities of 100 and 10 mA/cm2 overpotentials of 168, respectively and 90 millivolts. The influence of the active phase on the catalytic activity is also systematically investigated through the provision of electrochemical data of comparative materials (CoP NPs). Our study shows that the modulation of the intrinsic catalytic activity plays an important role in increasing the catalytic efficiency of the experimental part of the materials. All reagents and chemical solvents are of analytical grade and are used as receiving. Co(Ac) 2$4H2O was purchased from Sinopharm Chemical Reagent Co. Ltd. (Ningbo, China). NH3$H2O, NaH2PO2, and anhydrous ethanol were obtained from Aladdin Industrial Co. (Shanghai, China). Carbon fabric (CC) from Wuhan Instrument Surgical Instru Characterizations The phase structures of the catalyst were determined using X-ray diffraction (XRD, Bruker D8 ADVANCE) with Cu-Ka source radiation (l ¼ 1.54178 A) with a scan speed of 2 min 1 . Raman spectra were obtained from Renishaw (UK). using a 532.8 nm laser source. Scanning electron microscopy (SEM) images of the prepared catalyst were performed on a Hitachi S-4800 field emission scanning electron microscope. A JEM-ARM200F high-resolution transmission electron microscope (TEM) to investigate the microstructure and obtain energy-dispersive X-ray (EDX) spectra for X-ray photoelectron spectroscopy (XPS) analysis. performed with a Thermal ESCALAB 250 spectrometer Electrochemical measurements Electrochemical experiments were performed on a computer-controlled workstation (CHI660E, China) using a standard Three-Electrode System Typically, as-prepared CoxP NPs on a carbon cloth were used as the working electrode, and a saturated calomel electrode (SCE) was used as the reference electrode for the counter electrode of a neutral platinum foil when testing Short-term activity tests are performed, such as cyclic voltammetry (CV) and linear reciprocating voltammetry (LSV). While a graphite rod was used when performing stability tests, the SCE was first calibrated in 0.5 M H2SO4 saturated with high-purity hydrogen using two platinum sheets as the working electrode and the counter electrode, respectively (Figure S1 in the Supporting Information ). To prepare Pt/C catalyst ink, Pt/C powder Commercial 20% (6 mg) was dissolved in 250 mL of ultrapure water containing 10 mL of Nafion. Then, 50 mL of Pt/C ink was spread on clean CC (1.5 cm2) and dried at room temperature. Before electrochemical measurements, the electrolyte solution (0.5 M H2SO4) was bubbled with pure nitrogen for 30 min to remove dissolved oxygen. Polarization curves were collected using LSV measurements with a scan rate of 2 mV s 1. , and the potential ranged from 0.8 to 0 V vs. SCE at room temperature. Note that all polarization curves in our work were iR compensated. Electrochemical impedance spectroscopy (EIS) tests. In the frequency range from 0.01 Hz to 100 kHz, electrochemical stability was performed using continuous cyclic voltammetry scanning and sweeping the potential range from 0.20 to þ0.10 V vs. RHE at room temperature. In addition, a long-term chronoamperometry was also performed to evaluate the durability of the resulting catalyst. RESULTS AND DISCUSSION Figure 1 shows the preparation process of heterogeneous CoxP NPs/CC. First, a simple reflux process to cobalt acetate was performed under mild conditions, and the subsequent hydrothermal oxidation accelerated the crystallization. Co3O4 NPs. High-quality carbon cloth served as the substrate to support our samples. The Co3O4 NPs/CC precursor was then thermally treated in a low-temperature phosphorization process. The morphology and size of the as-prepared samples were observed by SEM and TEM. The SEM image of the Co3O4 NPs/CC sample is shown in Figure 2B. Compared with no CC (Fig. 2A), it is obvious that a large number of very fine nanoparticles strongly adhere to the CC. Figure 2D shows a typical TEM image of Co3O4 nanoparticles, where it can be seen that the nanoparticles are homogeneously and uniformly dispersed. The statistical analysis (inset in Figure 2D) provides information that the size of the nanoparticles is between 3 and 8 nm. The formation of such fine nanoparticles is further benefited by weak hydrolysis under mild conditions and weaker oxidation of cobalt acetate, which ensures a higher density of HER active sites [34]. In addition, more details of the microstructure of Co3O4 nanoparticles were observed with high resolution TEM (HRTEM) and selected area electron diffraction (SAED). As shown in Figure 2E, the HRTEM image shows a readout lattice spacing of 0.244 nm, indexed to the crystal Co3O4 (311) surface. The SAED pattern (Fig. 2F) also shows a well-identified (311) crystal plane. All the above results show that ultrafine Co3O4 nanoparticles were successfully synthesized by a mild reflux process. Low-temperature energy-saving conversion tactics have been reported to have no effect on the surface appearance [42,43], SEM image of CoxP NPs/CC After heat treatment by gas phase phosphatization (Fig. 2C) it also shows that the ultrafine morphology is not damaged. In addition, the TEM image (Fig. 2G) and the size distribution map (inset in Fig. 2G) also confirm the above result. Figure 2H shows the HRTEM image of CoxP NPs. Two transparent lattice fringes with a distance of 0.376 and 0.330 nm can be attributed to (020) and (101) crystallographic planes of Co2P and CoP, respectively. The corresponding SAED pattern also confirms the heterogeneous structure of the resulting sample, as seen in Fig. 2I, the (020) plane of Co2P and the (101) plane of CoP are well identified. When the amount of phosphorus source and the reaction temperature increased, the pure phase CoP NPs/CC catalyst was obtained. Pictures SEM and TEM of CoP NPs/CC are shown in Figure S2 A and B. Finally, EDX spectra (Figure S3) further confirm the presence of Co and P elements. X-ray Diffraction (XRD) and Raman Measurements to Identify the Composition and Phase of Our Samples Figure 3A shows the XRD pattern of Co3O4 nanoparticles and the three main diffraction peaks at 31.6, 37.2, and 65.7 can be clearly seen that Indexed are the (011), (210) and (031) planes of the cubic phase of Co3O4 (JCPDS No. 65-3103). After the thermal phosphatization method, the XRD pattern of CoP and CoxP NPs as prepared is presented in Figure 3B. For CoP NPs, significant diffraction peaks of 31.6, 36.3, 46.2, and 48.1 were observed as well as standard orthorhombic CoP (JCPDS No. 29-0497). While for CoxP NPs, other strong peaks at 40.7, 40.9 and 43.3 can be easily assigned to (121), (201) and (211). Co2P aircraft (JCPDS No. 32-0306). This unusual phenomenon may be caused by the higher molar ratio of Co and the lower reaction temperature (300 °C) in the progress of the phosphorization, which mostly leads to the generation of the heterogeneous phase of phosphides. There are no other distinct peaks observed in these XRD patterns, which indicates the successful synthesis of Co3O4 and CoxP nanoparticles. Moreover, the Raman spectrum shown in Figure S4 also confirms the above phenomenon. The Raman peaks of Co3O4 nanoparticles were observed at 188, 471, 510, 600 and 665 cm 1 corresponding to the five characteristic Raman vibration modes of Co3O4. There is no clear peak for the prepared CoxP nanoparticles, which is consistent with the data of a previous study [44]. XRD data and Raman analysis are a favorable evidence to show the high crystallinity of Co3O4 and CoxP NPs in our work. In order to determine the elemental constitution and capacity status of the constituent elements in the prepared CoP and CoxP NPs, XPS measurements were performed in this study. According to the full scan spectral data of CoxP nanoparticles displayed in Figure S5, the peaks indexed to O, C, P and Co elements are easily identified. In addition, a detailed XPS survey has also been carried out. Figure 3Ceb shows the high-resolution XPS spectrum of the Co 2p3/2 core surface for CoxP NPs. which is easily placed in the three peaks. The peak located at 778.3 eV is attributed to Co 2p3/2 Co species in CoxP NPs, consistent with XRD analysis [45]. Another peak at 782.1 eV is assigned to Co2þ and Co3þ, and it is clear that the satellite peak is related to the excitation of the shaking of high Co ions in CoxP catalysis [46,47]. Figure 3 shows the high resolution XPS spectrum of P 2p for CoxP NPs. The peaks with binding energy at 129.5 and 130.2 eV can be attributed to P2p3/2 and P2p1/2 P species in CoxP, while the peak at 133.6 eV is assigned to PeO bonding [48]. The oxide appears as the final product, which can be attributed to a partial surface oxidation of CoxP in the atmosphere [49]. It is noteworthy that the binding energy of Co 2p3/2 (778.3 eV) is very close to metallic Co (777.9 eV), and the value of P 2p3/2 is slightly lower than elemental P (130.0 eV). The above data show that the reduced Co species are partially positively charged, and show the presence of negatively charged P species, indicating the presence of electron density conversion from Co to P, which is completely with the previous study [37,45]. In addition, the XPS data of as-obtained CoP NPs have also been investigated (Figure 3Cea and Figure 3Dea). Compared with the XPS data of CoxP NPs, the appropriate Co 2p3/2 peak (778.7 eV) for CoP NPs was positively shifted, implying a smaller cobalt valence state in the targeted CoxP NPs. The electrocatalytic performance of CoP NPs and CoxP nanoparticles attached to CC was evaluated in 0.5 M H2SO4 solution through a typical three-electrode configuration. For comparison, the catalytic activity of commercial Pt/C was also investigated. The polarization curves of the different materials above are presented in Figure 4A. It is obvious that the Pt/C adsorber shows the lowest overpotential, which means that its activity is excellent under the same test conditions, pure phase CoP NPs/CC shows an overpotential higher than 150 and 223 millivolts for a current density of 10 and 100 milliamperes per square centimeter. Remarkably, the overpotential required to drive current densities of 10 and 100 mA cm2 on the as-prepared CoxP NPs/CC heterogeneous catalysts is only 90 and 168 mV. Respectively. The value of the overpotential is lower than that of the pure phase of CoP NPs/CC and most of the non-noble metals reported as HER catalysts (Table S1), which indicates its outstanding catalytic performance relative to HER. In addition, in order to investigate the possible kinetics of the HER plots, TOEFL was also estimated by plotting the overpotential (h) against the logarithm of the current density (j). Figure 4B shows the TOEFL domains of Pt/C, CoxP NPs/CC, and CoP NPs/CC. By fitting the linear part of the Tefel plot, a slope of 37.9 mV dec 1 is observed for Pt/C, which is in agreement with the value reported [50]. The pure phase Tofel slope of CoP NPs/CC is 92.3 mV Dec 1, while the heterogeneous CoP NPs/CC shows a lower Tofel slope of 67.9 mV Dec 1. , indicating an easier catalytic process towards HER. The Tofel slope values ​​for all catalysts are in the range of 40e120 mV December 1, inducing the Volmer-Heyrovsky HER mechanism in this study [51]. Furthermore, by applying the extrapolation method to the Tofel diagram, the exchange current density of CoxP NPs/CC is calculated to be 0.58 mA cm2 (Figure S6), which is relatively large among some non-noble metal HER catalysts listed in Table S1. The above phenomena indicate the improvement of the catalytic activity of the prepared NPs/CC CoxP compared to the pure one. The CoP NPs/CC phase catalyst that can benefit from modifying the simple intrinsic activity of electrocatalysts, moreover, such outstanding catalytic performance may be explained as follows. In the first step, the cooperative effect of Co2P and CoP in heterogeneous CoxP species increases the intrinsic catalytic property and electron transfer rate of cobalt phosphides. According to previous calculations and experiments, both intermediate metal atoms and P in TMPs play an important role in strengthening It has HER activity [45]. In this work, CoxP nanoparticles with different Co species were found to successfully modify the intrinsic catalytic activity at some levels. Secondly, the ultrafine morphology of the prepared CoxP NPs/CC not only enables greater exposure of the catalytic active sites but also ensures maximum utilization of the reactants. Finally, CoxP NPs in tight contact with CC aid stable chemical and electronic coupling and further support the mobility of electrons between CC and CoxP nanoparticles during the hydrogen production reaction. Stability in strong acidic medium is extremely important for catalysts used in obtaining hydrogen from water. Therefore, we performed a series of accelerated aging tests to assess durability and recyclability, such as continuous CV scanning and long-term chronoamperometry. Notably, recently, an increasing number of researchers have reported that the use of noble platinum foil as a counter electrode is not able to determine the catalysts are exactly stable, because platinum metal can dissolve and precipitate on the catalysts along the continuum. Electrochemical experiments [52]. Therefore, a graphite rod was used as the counter electrode when performing the stability study. Figure 4C shows the CoxP NPs/CC polarization curves before and after 5000 CV scan cycles in the potential range from 0.20 to þ0.10 V vs. RHE. Unlike the initial curve, an imperceptible difference can be observed for the curve obtained after 5000 cycles. In addition, for a long time (at least 12 hours) electrolysis was also performed at a controlled overpotential of 150 mV. The time-dependent current density curve is shown in Fig. 4D and acceptable damping with a graphite rod used as the counter electrode. The corresponding current density is about 50 mA cm2 (Figure 4A) overpotential of 150 mV, it is worth noting that the current density of 50 mA cm2 is a relatively large value in strength tests among most of the studies reported [53,54 ]. All the above results show that the as-prepared CoxP NPs/CC has good stability in acidic solution and the potential possibility of practical application. As the improved catalytic activity is mainly attributed to the smaller size CoxP NPs and supported CC. With a high surface area, it is necessary to evaluate the electrochemically active surface areas (ECSA). The result of electrocatalysts [55,56]. Hence we used CV scanning to measure electrochemical double layer capacitances (Cdl), where a potential region without faradic current was selected [57]. CV curves were recorded at different scan rates ranging from þ0.05 to þ0.25 V (Fig. S7 AeC). Figure 5A shows the capacitive current density at þ0.15 V as a function of scan rate for CoxP NPs/CC and CoP NPs/CC (Dj ¼ ja jc). The prepared CoP NPs/CC catalyst shows a Cdl value of 17.7 mF cm2, which is approximately 6.7 times higher than that of the pure phase CoP NPs/CC (2.63 mF cm2 ), showing a relatively larger ECSA of the resulting catalyst. In addition, the electrochemical impedance was studied at an overpotential of 100 mV and the obtained Nyquist plot was fitted. The equivalent circuit is shown in Figure 5B. All Nyquist maps of different samples show only one readable semicircle in the high-frequency region, which is closely related to HER charge transfer kinetics [43,58]. EIS data show that the CoxP NPs/CC catalyst has a much lower charge transfer resistance (Rct) than CoP NPs, indicating a higher level of electron transfer rate between the electrocatalyst and the electrolyte. Conclusion In summary, we have successfully developed a HER We have designed an efficient electrocatalyst based on heterogeneous CoxP nanoparticles, i.e. both orthorhombic CoP and Co2P present as a result of the catalyst. Thanks to the weak reflux process and following the low-temperature phosphatization method, the resulting CoxP nanoparticles with very small size expose more active sites. In addition, the synergistic effect of Co2P and CoP in heterogeneous CoxP nanoparticles enhances the intrinsic catalyst property and charge transfer rate in the HER process. In sulfuric acid medium, CoxP NPs/CC HER shows excellent catalytic performance, with a small overpotential (90 mV at 10 mA cm2, low Tafel slope (67.9 mV dec. 1, large exchange current density ( 0.58 mA cm 2 ) and good durability. This shows that the intrinsic activity of the catalysts is changing and plays an important role in increasing the catalytic activity of HER refrece:

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editor:M.Golmohammadnejad