Access to carbon nanofiber composite hydrated cobalt phosphate nanostructure as an efficient catalyst for the hydrogen evolution reaction

Attractive technology for producing sustainable hydrogen with water electrolyzers was foreseen as one of the most promising ways to meet the increasing demands of renewable resources and electricity storage. Mainly used for the efficient generation of H2, water electrolysis involving hydrogen evolution reactions (HERs) depends on efficient and affordable electrocatalysts. Hydrogen is an effective fuel that can be produced by splitting water. Hence, the search for highly efficient HER catalysts is a major challenge as efficient hydrogen evolution catalysts are sought to replace catalysts such as platinum. Here, we describe a low-cost and highly effective electrocatalyst for the proper incorporation of the HER electrocatalyst with low overpotential, effective charge transfer kinetics, low Tafel slope, and good durability. By using a simple hydrothermal approach to produce Co3(PO4)2.8H2O/CNF, it is possible to attach Co3(PO4)2.8H2O to the surface of carbon nanofibers (CNFs), which also exhibit remarkable HER activity at an overpotential of 133 mV and produce a current density of 10 mA/cm2 and a 48 mV/decade for the Tafel slope. Large electrochemical surface areas and easy charge transfer from Co3(PO4)2.8H2O to the electrode through conductive Co3(PO4)2.8H2O/CNF composites are the reasons for the improved performance of Co3(PO4)2.8H2O/CNF.


Introduction
The energy crisis, environmental degradation, and global warming-all driven by the extensive use of fossil fuels-have motivated the production of clean and renewable energy. Currently, due to the accelerating spread of the global energy crisis and the ongoing decay of traditional fossil fuels, a clean, high-energy-density hydrogen economy derived from renewable sources is being extensively studied and developed (Zou and Zhang, 2015). The H 2 evolution reaction (HER) critically depends on appropriate electrocatalysts such as platinum (Pt) and its alloys, which catalyze the conversion of pairs of protons and electrons to H 2 at high reaction rates with low overpotentials. However, the high price and relative scarcity of Pt severely limits its widespread use. Therefore, finding reliable and effective alternative catalysts that are geologically abundant is imperative for the future of the hydrogen economy (Gao et al., 2015). The development of an inexpensive HER electrocatalyst, using base elements with superior activity and high stability as a replacement for expensive platinum, has been one of the most pressing goals in recent years. In recent decades, several studies have been conducted to replace noble-metal-based electrocatalysts (Bui et al., 2020).
Because of its ability to catalyze the splitting of water, cobalt (Co) has become an intriguing base metal. The preparation of Co-based composites and complexes using homogeneous molecular catalysts has received much research attention (Anjum et al., 2018;Ahmed et al., 2021;Hu et al., 2021a;Singh et al., 2022a;Singh et al., 2022b). In addition, cobalt-based electrocatalysts (CoP, Co 3 O 4 , CoOOH, CoSe 2 , etc.) have attracted significant attention for a variety of applications, including sensors (Hu et al., 2021b;Sari et al., 2022), supercapacitors (Rovetta et al., 2017;Li et al., 2018), lithium-ion batteries (Khan et al., 2016), and OER (Sun et al., 2021;Kubba et al., 2022). Because of their strong electrochemical activity, cobalt phosphates have attracted much attention in recent decades and have been used extensively in electrochemical energy storage and as electrocatalysts for water splitting (Shu et al., 2018;Majhi and Yadav, 2021a). An inductive influence due to the presence of (PO 4 ) 3 groups means that the redox coupling of the transition metal is significantly higher than that of the comparable oxide. In addition, the excellent ionic conductivity of the large (PO 4 ) 3 units creates open pathways that can facilitate rapid ionic migration (Samal et al., 2016). Carbon-based materials (carbon nanofibers and carbon nanotubes) have attracted significant interest in an attempt to improve the long-term stability of the catalysts, while transition metal oxides (Zhao et al., 2018;Zhu et al., 2019;Ahmed et al., 2021;Majhi and Yadav, 2021b), sulfides Majhi and Yadav, 2021c;Sun et al., 2021), and phosphates Majhi and Yadav, 2022a;Majhi and Yadav, 2022b) of other transition metals have been used as binding and support materials due to their excellent corrosion resistance, good conductivity, and adjustable chemical surface properties. Carbonbased materials have received significant attention due to their adaptable surface chemistry, strong conductivity, and excellent corrosion resistance. Therefore, to achieve highly efficient overall water splitting to produce clean H 2 , a rational design of electrocatalysts is required. This must account for processing costs, catalytic activity, and long-term stability (Woo et al., 2020).
In this work, we synthesized a hydrated phosphate-based carbon-nanofiber-supported material (Co 3 (PO 4 ) 2 .8H 2 O/CNFs) to study the HER performance of the catalyst in acidic media. Hydrated cobalt phosphate with carbon nanofibers has shown excellent stability over 24 h, implying its superior stability during the HER reaction. The composite helps increase the highly electroactive surface area, high conductivity, and vertical growth relative conducting CNFs, exposing a high density of edge phosphate. Other factors that increase the activity of Co 3 (PO 4 ) 2 .8H 2 O/CNFs are high current density, low Tafel slope, and low charge transfer resistance, which are 133 mV, 48 dec/cm 1 , and 43.04, respectively. The developed catalyst showed electrocatalytic performance comparable to commercial Pt/C in the acidic HER medium.

Chemicals used Materials
Cobalt (II) chloride (CoCl 2 ), diammonium hydrogen phosphate (NH 4 ) 2 HPO 4 , commercially available CNF, ethanol (99.9% AR grade), and potassium hydroxide (KOH) were purchased from Loba Chemie, and platinum carbon (Pt/C) and 5% Nafion ™ 117 solution were purchased from Sigma-Aldrich. All chemicals were stored in a dry place and used without any further purification.

Experimental section
Preparation of Co 3 (PO 4 ) 2 .8H 2 O/CNF Co 3 (PO 4 ) 2 .8H 2 O/CNFs were prepared using a facile hydrothermal method. In a typical synthesis, 259.6 mg of CoCl 2 was dispersed in DIH 2 O and then 45 mg of CNF (commercial) was added, followed by stirring for 10 min. The reaction mixture was transferred to a Teflon-lined autoclave for hydrothermal treatment at 180°C for 24 h. The sample was collected by centrifugation and washed several times with both distilled water and ethanol to remove undesired species, and the product was dried in an oven at 70°C for 12 h.
Furthermore, for comprehensive conclusiveness, we performed the Rietveld refinement of the PXRD pattern of Co 3 (PO 4 ) 2 .8H 2 O/ CNF ( Figure 2) by FullProf Suite software. The crystal structure was generated using a refined CIF file in VESTA software. As shown by the atomic positions given in Supplementary Table S2, we clearly identified the presence of hydrogen atoms along with the copper, phosphorus, and oxygen atoms. These hydrogen atoms came from the H 2 O molecules conjugated with the Co 3 (PO 4 ) 2 lattice. Now, the Co 3 (PO 4 ) 2 .8H 2 O lattice crystallizes into a 2D monoclinic structure with the space group C12/m 1. It is composed of two Co 3 P 2 (OH) 16 sheets with preferential orientation in the (0 1 0) direction. Among the two inequivalent Co 2 ⁺ sites, the first site (corresponding to the Co1 atom at the 2a Wyckoff site) is bonded to six O 2 ⁻ atoms in the CoO₆ octahedron, which shares corners with the two equivalent PO₄ tetrahedra. There are two shorter (2.0624 Å) and four longer (2.1646 Å) Co-O bond lengths. At the second Co 2 ⁺ site (corresponding to the Co 2 atom at the 4g Wyckoff site), Co 2 ⁺ is bonded to six O 2 ⁻ atoms to form a CoO₆ octahedron that shares corners with four equivalent PO₄ tetrahedra and an edge with one CoO₆ octahedron. The Co-O bond distances range between    At higher temperatures and treatment in an inert atmosphere, the cobalt cation and the phosphate anion reinstall a crystal structure. The removal of coordinated water molecules is also confirmed by thermal derivative thermogravimetric (DTG) and gravimetric analysis (TGA), as shown in Figure 3B. The thermogram of the Co 3 (PO 4 ) 2 .8H 2 O/CNF material exhibits two distinct loss stages. The first step shows the initial 4.8% weight loss in the temperature range of 105°C; in the second stage, a 19.3% weight loss is observed in the temperature range of 106°C-153 C due to the loss of water. The total weight loss of the sample was found to be 46.2% when the temperature was up to 700°C.

Field emission scanning electron microscope (FE-SEM)
The surface morphology of the Co 3 (PO 4 ) 2 .8H 2 O/CNF material was observed by FE-SEM. Typical FE-SEM images of a synthesized sample are shown in Figure 4. The FE-SEM image of Co 3 (PO 4 ) 2 .8H 2 O shows a rod-shaped flower ( Figure 4A) and resembles CNF with a thread-like morphology ( Figure 4B). The Co 3 (PO 4 ) 2 .8H 2 O/CNF composite shows that cobalt phosphate flowers are stacked with a uniform and regular structure on the surface of carbon nanofibers; images of the composite are shown in Figures 4C, D. It can be observed that the surface morphology of cobalt phosphate is a tightly packed flower, which may suggest support for effective electron transport. In addition, the elemental color map of the Co 3 (PO 4 ) 2 .8H 2 O/CNF composite is shown in Figures 5A-E, where it can be observed that the distribution of Co, O, P, and C is suboptimal, resulting in the exposed substrate observed in these pictures. Energy-dispersive X-ray spectroscopy (EDX) was performed to confirm the elemental composition and is shown in Figure 5F, where all elements are presented while their ratios in weight percentage and atomic percentage are included in Figure 5F. The FE-SEM image of Co 3 (PO 4 ) 2 .8H 2 O distribution over CNF, where a large percentage of cobalt phosphate is distributed  Frontiers in Chemistry frontiersin.org 05 over CNF, could indicate the increase in conductivity essential for enhancing the electrocatalytic water-splitting reaction.

X-ray photoelectron spectroscopy (XPS)
XPS measurements were performed to validate the chemical composition and oxidation state of Co 3 (PO 4 ) 2 .8H 2 O/CNF. The XPS survey spectra of the composite are presented in Supplementary Figure S4, and the high-resolution XPS spectra of Co 2p, P 2p, O 1s, and C 1s are shown in Figure 6. The highresolution Co 2p orbital consists of two spin-orbit components of 2p 3/2 and 2p 1/2 for the Co 2+ and Co 3+ states. The two peaks at the binding energies 780.87 and 796.82 eV are assigned to the Co 3+ state, and 782.71 and 798.34 eV are assigned to the Co 2+ state for the Co 2p 3/2 and 2p 1/2 nuclear levels, respectively. Co 2p shows two satellite peaks of Co 2p 3/2 and Co 2p 1/2 core levels ( Figure 6A) (Song et al., 2020). The P 2p region of Co 3 (PO 4 ) 2 .8H 2 O/CNF shows two characteristic peaks at a binding energy of 132.97 eV and 133.95 eV, corresponding to the 2p 3/2 nuclear levels and 2p 1/2 , which can be assigned to the phosphate group ( Figure 4B). O1s signals are centered at a binding energy of 530.78, and 531.68 eV corresponds to the phosphate oxygen and OH group of H 2 O molecules present in the lattice ( Figure 6C). In the C1s spectrum ( Figure 6D), the peaks centered at about 284.30, 285.63, and 287.77 eV are indicated on sp 2 -hybridized C-C, C-N, and C-O, respectively (Yuan et al., 2016).

Electrochemical activity HER performance
The linear sweep voltammetry (LSV) curves were measured in the potential window range from 0 V to -1 V versus Ag/AgCl for the HER process, with a sampling rate of 10 mV s −1 and 0.5 M H 2 SO 4 solution as the electrolyte. The long-term HER stability test of the catalyst was performed using an Ag/AgCl electrode in the acidic medium. Electrochemical impedance spectroscopy (EIS) was performed in 0.5 M H 2 SO 4 solution over the frequency range of 100 kHz to 0.1 Hz, at an overpotential of 400 mV. The HER electrocatalytic activity of Co 3 (PO 4 ) 2 .8H 2 O/CNF, Co 3 (PO 4 ) 2 .8H 2 O, and carbon nanofiber (CNF) was assessed by linear sweep voltammetry (LSV) and compared to data obtained for commercial Pt/C as a reference (Figure 7). Co 3 (PO 4 ) 2 .8H 2 O/ CNF had the lowest overpotential among all the catalysts, indicating its superior HER activity. Overpotentials of the catalysts are given in Table 1, with the composites Co 3 (PO 4 ) 2 .8H 2 O/CNF, Co 3 (PO 4 ) 2 .8H 2 O, CNF, and Pt/C having overpotential values of 133 mV, 188 mV, 275 mV, and 32 mV, respectively. Record a current density of 10 mA/cm 2 ( Figure 7A). Using the Volmer and Heyrovsky equation (η = a + b log j, where η is over potential, j is current density, b is Tafel slope and a is constant), we calculated the Tafel slopes of the linear domains to determine the kinetics of the catalysts. We found the Tafel slope values for composites Co 3 (PO 4 ) 2 .8H 2 O/CNF, Co 3 (PO 4 ) 2 .8H 2 O, CNF, and Pt/C to be 48 mV/dec 1 , 87 mV/dec 1 , 106 mV/dec 1 , and 36 mV/dec 1 where, the composite Co 3 (PO 4 ) 2 .8H 2 O/CNF showed the fastest HER kinetics among all the catalysts prepared ( Figure 7B). The long-term cyclic stability of the Co 3 (PO 4 ) 2 .8H 2 O/CNF electrocatalyst toward HER activity was also tested, using 0.5 M H 2 SO 4 solution as the electrolyte for 3000 continuous LSV cycles ( Figure 7C). After the stability test, it was found that the overpotential rise was only 8 mV at a current density of 10 mA/ cm 2 , proving the superior long-term stability of the Co 3 (PO 4 ) 2 .8H 2 O/CNF catalyst to HER activity in the acidic medium. Furthermore, long-term stability was tested by chronoamperometry using 0.5 M H 2 SO 4 solution as the electrolyte at a constant overvoltage of 133 mV for 24 h ( Figure 7D), and the result shows a very high durability with negligible loss in current density of the catalyst. Figure 8A Figure 8B) at an overvoltage of 133 mV. Co 3 (PO 4 ) 2 .8H 2 O/CNF has the highest mass activity of 169.30 A g −1 , compared to its counterparts Co 3 (PO 4 ) 2 .8H 2 O (119.34 A g −1 ) and CNF (42.00 A g-1 ). Mass activity was calculated using the following equation: Mass activity A g −1 j Acm −2 m gcm −2 .
( 1 ) The HER process proceeds through three principal steps, called the Volmer, Heyrovsky, and Tafel steps, in the acidic medium (Pentland et al., 1957;Conway and Tilak, 2002;Xie et al., 2013;Basu et al., 2017). The Volmer reaction is associated with proton absorption, which is a primary discharge step (Step 1). The Heyrovsky step is the electrochemical desorption stage (i.e., the combination of a second proton with an absorbed H atom of H 2 gas) (Step 2). The Tafel step is a recombination step (i.e., the combination of two nearby absorbed H atoms to produce H 2 gas) (Step 3 where H ads represents a hydrogen atom chemically adsorbed on an active site of the catalyst surface (M). If the Volmer reaction is the rate-determining step, then the Tafel slope should be 120 mV dec −1 , and for the Heyrovsky process and Tafel process, Tafel slopes of 40 and 30 mV dec −1 should be obtained, respectively (Xie et al., 2013;Hu et al., 2016). Therefore, combinations of steps (i.e., the Volmer-Heyrovsky or Volmer-Tafel pathways) are required to produce molecular hydrogen in a complete HER process.
In addition, the electrochemical double layer capacitance (C dl ) and the electrochemically active surface area (ECSA) were investigated by cyclic voltammetry (CV) performed at different sampling rates, from 10-50 mV s −1 ( Figure 9A). C dl was estimated by measuring voltammograms in a non-Faradic region, and C dl was measured to determine the origin of the high HER activity of Co 3 (PO 4 ) 2 .8H 2 O/CNF  Frontiers in Chemistry frontiersin.org composite nanostructures. Both the anodic and cathodic double-layer charging currents (Ja and Jc, respectively) were calculated, and the values were plotted against the corresponding sample rates. Thus, the calculated C dl for the Co 3 (PO 4 ) 2 .8H 2 O/CNF composite is shown in Figure 9B and is 3.072 mF cm −2 ; the corresponding ECSA is 76.97 cm 2 , and 1.66/41.7 and 0.83/20.96 for the Co 3 (PO 4 ) 2 .8H 2 O and CNF catalysts C dl /ECSA, respectively (Supplementary Figures S1C, D and Table 1). The Brunauer-Emmett-Teller (BET) study shows that Co 3 (PO 4 ) 2 .8H 2 O/ CNF has the highest surface area of 37.6 m 2 g −1 compared to other constituents Co 3 (PO 4 ) 2 .8H 2 O (28.2 m 2 g −1 ) and CNF (21.4 m 2 g −1 ); the results for these constituents are displayed in Supplementary Figure S2. The catalyst possesses excellent durability and stability after 20 h, without apparent chemical or structural deformation; XRD and FE-SEM after stability measurements are presented (Supplementary Figure S3).

Conclusion
Co 3 (PO 4 ) 2 .8H 2 O/CNF and Co 3 (PO 4 ) 2 .8H 2 O were synthesized by a simple hydrothermal procedure. The structural characterizations confirmed the formation of Co 3 (PO 4 ) 2 .8H 2 O/CNF with a flower-like structure attached over carbon nanofibers. The synthesized catalyst Co 3 (PO 4 ) 2 .8H 2 O/CNF shows excellent performance for HER with the low overpotential (133 mV) required to generate current densities of 10 mA cm −2 , a small Tafel slope (48 mV decade −1 ), and good stability at 24 h. The composite helps increase the high electroactive surface area, high conductivity, and vertical growth over conductive CNFs, exposing a high density of edge phosphate. This newly developed [Co 3 (PO 4 ) 2 .8H 2 O/CNF] can be considered a promising electrocatalyst for HER in acidic media because of its straightforward synthetic procedure and low cost.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.