# Cobalt Phosphide Nanoparticles Synthesized by Solvothermal Phosphidization of Metal-Organic Frameworks for Electrocatalytic Oxygen Evolution

Authors: Jinhao Gao; Shuling Liu; Zhijian Li; Yichuang Xing; Xian Cui; Chao Wang
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Active and stable electrocatalysts for oxygen evolution reaction (OER) are crucial in the widespread application of water electrolyzers. Cobalt phosphide nanoparticles (Co-P) with different degrees of crystallinity are synthesized via solvothermal phosphidization of Co metal-organic frameworks and P4 using different solvents. The Co-P can be converted to active OER electrocatalysts in 1 M KOH, with the most active reaching 10 mA cm-2 OER current densities at 270 mV overpotential. The OER intermediate adsorption energy is tuned by the electron interactions in the crystalline/amorphous heterostructured Co-P, which show the highest intrinsic OER activity. After the long-term OER in base, surface phosphides are converted to (oxy)hydroxides that are the active species towards OER.


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## Body

## 1. Introduction

Water electrolyzer can split water into hydrogen and oxygen using energy from renewable sources and is an essential device in the mass production of hydrogen [1, 2]. The oxygen evolution reaction (OER) at the anode of water electrolyzers is a complicated process involving consecutive four-electron transfer, coupled with proton transfer and with reactant surface adsorption/desorption and diffusion, and requires high overpotentials [3–5]. To enhance the efficiency of the device, active and stable electrocatalysts are required. The high price and scarcity of precious metal catalysts limit their widespread application [6]. Therefore, developing efficient OER catalysts based on earth-abundant elements is crucial to reduce energy consumption and the cost for hydrogen production [7–11].Transition metal phosphides feature high conductivity, rich crystalline structures, and redox chemistry and are earth-abundant. These features render them promising materials for energy storage and electrocatalysis [12, 13]. Cobalt phosphides have recently received intensive studies as they can be converted to active and stable electrocatalysts for OER, owing to their high conductivity (the existence of Co-Co metallic bond) and the in situ generated abundant surface-active sites [14–16]. Different phases of cobalt phosphide (Co-P and Co2P) [17, 18] have been synthesized into various morphologies (nanowires [19, 20], nanoparticles [21, 22], etc.), and their OER activities are evaluated. Compared to the CoP phase, the Co2P phase is favorable for OER as the conductivity is higher, and it offers more electrochemically active sites owing to the higher percentage of Co in the structure [18]. Doped cobalt phosphide nanomaterials are also reported to significantly improve the OER activity by electron structure modulation and by providing more electrochemically active sites [23–25].Despite recent advances on using cobalt phosphides as precatalysts for OER, challenges remain for this type of materials. Among them, the acquisition of high surface area cobalt phosphide nanoparticles (Co-P) under mild conditions and the establishment of structure-activity relationship of the Co-P with different phases and morphologies synthesized under similar condition are crucial. Traditionally, crystalline cobalt phosphides are synthesized under elevated temperature and often involve the generation of poisonous phosphine gas. Also, the elevated temperature inevitably leads to nanoparticle sintering [16]. Electrodeposition offers facile and clean route to synthesize cobalt phosphide films, but the film is often amorphous [26, 27]. Therefore, alternative routes in synthesizing cobalt phosphide nanoparticles with high specific surface area and controlled crystallinity are needed.Metal-organic frameworks (MOFs) are ideal templates/precursors for designing metal phosphides and sulfides owing to their large specific surface area, hierarchical porous structure, and controllable pore size [28–32]. Metal phosphides prepared using MOF precursors under solvothermal condition can largely inherit the original MOF morphology and form nanoparticles with relatively high specific surface area [23, 33]. Moreover, controlled crystallinities are achieved simply by altering the reaction conditions during synthesis, and synthesized nanoparticles with different structural properties can be compared directly to establish the structural-activity relationship. Here, we develop a new method to prepare Co-P from MOF precursors by solvothermal. The solvent of the solvothermal process can significantly affect the nucleation and growth rates of nanocrystals and ultimately influence the structural features of the Co-P. By altering the solvent during solvothermal phosphidization, Co-P with distinct structural features and morphologies are acquired. The electrocatalytic OER activities of the Co-P are further investigated and are correlated to their structural features.


## 2. Experimental

### 2.1. Synthesis of Co-P

For a typical procedure, 1 mmol (29.4 mM) Co(NO3)2·6H2O was dissolved in 34 mL N, N-dimethylformamide (DMF) to form a pink solution. Then, 1 mmol trimesic acid (29.4 mM) was added to the pink solution and was stirred for 1 h. The solution was transferred to a 40 mL Teflon-lined stainless steel autoclave, and the autoclave was heated to 160°C for 12 h. After cooling naturally, precipitates were collected and were washed and centrifuged (7000 rpm) three times with absolute ethanol. After being dried in a vacuum at 60°C for 12 h, purple powder (Co MOF, Co3(BTC)2·12 H2O) was acquired.The Co MOFs were grounded, and 0.1 g Co MOF (0.12 mmol Co3(BTC)2·12 H2O, 3.53 mM) was transferred to 34 mL (v/v=1:1) DMF and absolute ethanol (EA) mixed solvent. After sonication in a cold water bath for 30 min, the suspension was transferred to an autoclave, with 0.09 g white phosphorus (0.72 mmol, P4, 21.18 mM) added. The autoclave was heated at 160°C for 16 h. After cooling naturally, the black precipitates were washed consecutively with absolute ethanol, benzene, and deionized water, with centrifugation (7000 rpm). Finally, the black powder (labelled as Co2P-1) was obtained after being died under a vacuum at 60°C. Co-P-3 was synthesized using DMF and glycerol (GL) (v/v=1:1) instead of the DMF/EA mixture as the solvent during solvothermal phosphidization, and Co2P-2 was synthesized using pure DMF as the solvent.


### 2.2. Preparation of Co-P/Nickel Foam Electrodes (Co-P/NF)

Appropriate amounts of Co-P were dispersed in 2 mL double-distilled water to prepare the ink, with 10μL of Nafion® solution added. The suspension was sonicated for 30 min. The ink was drop-casted onto 1 cm2 acid-cleaned nickel foam (NF) using a micropipette, and the loading of Co+P on the NF is around 1.5 mg cm-2 (ICP-AES) (Table S2). The prepared Co-P/NF were dried under a vacuum at 60°C.


### 2.3. Electrochemistry

The electrochemical measurements were carried out using three-electrode system, with the Co-P/NF as the working electrode. The counter electrode was a cleaned graphite rod. A saturated calomel electrode (SCE) was used as the reference. Potentials areiR-compensated and are converted to the reversible hydrogen electrode scale and reported. The electrochemical impedance spectroscopy (EIS) was carried out in the frequency ranges from 0.01 to 105 Hz at 1.55 V with 10 mV alternating voltage amplitude. All experiments were repeated at least three times to ensure accuracy and reproducibility. Chemicals used and instrumentation can be found in the supporting information.


## 2.1. Synthesis of Co-P

For a typical procedure, 1 mmol (29.4 mM) Co(NO3)2·6H2O was dissolved in 34 mL N, N-dimethylformamide (DMF) to form a pink solution. Then, 1 mmol trimesic acid (29.4 mM) was added to the pink solution and was stirred for 1 h. The solution was transferred to a 40 mL Teflon-lined stainless steel autoclave, and the autoclave was heated to 160°C for 12 h. After cooling naturally, precipitates were collected and were washed and centrifuged (7000 rpm) three times with absolute ethanol. After being dried in a vacuum at 60°C for 12 h, purple powder (Co MOF, Co3(BTC)2·12 H2O) was acquired.The Co MOFs were grounded, and 0.1 g Co MOF (0.12 mmol Co3(BTC)2·12 H2O, 3.53 mM) was transferred to 34 mL (v/v=1:1) DMF and absolute ethanol (EA) mixed solvent. After sonication in a cold water bath for 30 min, the suspension was transferred to an autoclave, with 0.09 g white phosphorus (0.72 mmol, P4, 21.18 mM) added. The autoclave was heated at 160°C for 16 h. After cooling naturally, the black precipitates were washed consecutively with absolute ethanol, benzene, and deionized water, with centrifugation (7000 rpm). Finally, the black powder (labelled as Co2P-1) was obtained after being died under a vacuum at 60°C. Co-P-3 was synthesized using DMF and glycerol (GL) (v/v=1:1) instead of the DMF/EA mixture as the solvent during solvothermal phosphidization, and Co2P-2 was synthesized using pure DMF as the solvent.


## 2.2. Preparation of Co-P/Nickel Foam Electrodes (Co-P/NF)

Appropriate amounts of Co-P were dispersed in 2 mL double-distilled water to prepare the ink, with 10μL of Nafion® solution added. The suspension was sonicated for 30 min. The ink was drop-casted onto 1 cm2 acid-cleaned nickel foam (NF) using a micropipette, and the loading of Co+P on the NF is around 1.5 mg cm-2 (ICP-AES) (Table S2). The prepared Co-P/NF were dried under a vacuum at 60°C.


## 2.3. Electrochemistry

The electrochemical measurements were carried out using three-electrode system, with the Co-P/NF as the working electrode. The counter electrode was a cleaned graphite rod. A saturated calomel electrode (SCE) was used as the reference. Potentials areiR-compensated and are converted to the reversible hydrogen electrode scale and reported. The electrochemical impedance spectroscopy (EIS) was carried out in the frequency ranges from 0.01 to 105 Hz at 1.55 V with 10 mV alternating voltage amplitude. All experiments were repeated at least three times to ensure accuracy and reproducibility. Chemicals used and instrumentation can be found in the supporting information.


## 3. Results and Discussion

Scheme1 illustrates the preparation process of Co-P. The Co MOFs were firstly prepared in DMF by the solvothermal reaction between Co2+ and the trimesic acid. The XRD pattern of the Co MOF is shown in Figure S5, and the strong crystalline peak at 2θ=10.2° corresponds to a lattice spacing of 0.87 nm. The XRD pattern of Co MOF is consistent with previous reports of the Co3(BTC)2·12 H2O [34, 35]. The Co MOF then reacted with excess P4 solvothermally in different solvents to form Co-P. Various solvothermal reaction durations were explored, and the degree of crystallinity of Co2P phase did not increase for solvothermal reactions over 16 h. We propose that the solvothermal synthesis of Co-P follows dissolve-crystallization mechanism [36], where the dissolved cobalt ions/complexes reduced by P4 and Co-P are formed. When using DMF/EA or DMF/GL as the solvent, PH3 is formed by the reaction of P4 with the alcohols and can reduce the cobalt ions to form phosphides immediately. The Co MOFs are converted completely to phosphides, with the organic ligands serving as the surface-active agents to stabilize nanoparticles. Besides the different formation mechanisms of the cobalt phosphides using different solvents, solute solubility and solute-solvent interactions would affect the supersaturation and nucleation rates. Consequently, by adopting different solvents during solvothermal phosphidization, Co-P with different morphologies and crystallinities are obtained.Scheme 1
Procedure to synthesize Co-P.Figure1(a) displays the scanning electron microscopy (SEM) image of the Co MOF. The surface of the Co MOF is composed of irregular columnar particles with diameters at ~50 nm, and the particle size is uniform. The Co2P-1 is composed of large numbers of nanoparticles with ~20 nm in diameter and is agglomerated into 100 nm assembly (Figure 1(b)). The surface of the nanoparticles is smooth. Larger nanoparticles are also observed for the Co2P-2, and the particle size varies (Figure 1(c)). The Co-P-3 is composed of nanoparticles with ~30 nm in diameter (Figure 1(d)). From the SEM image, obvious changes in morphology of Co-P are observed using different solvents during solvothermal phosphidization. Using pure DMF as the solvent (Co2P-2), the nanoparticles tend to grow larger with a smooth surface, while adding ethanol or glycerol would significantly decrease the size of nanoparticles. This is due to the different formation mechanisms of cobalt phosphide in different system. When using DMF as the solvent, the Co2P-2 is formed solely by the reaction of P4 liquid with the Co ion/complexes. In case of mixed solvents system, the generated PH3 would react with the Co ion/complexes. The higher rate of reaction between the PH3 and Co ion/complex would generate higher amount of nucleation at a shorter time, therefore leading to the formation of nanoparticles with relatively homogeneous and smaller sizes. In comparison, the diffusion of P4 liquid and its reaction with the Co ion/complexes are slower, leading to more sluggish nucleation and lower growth rates of the Co-P. Consequently, larger nanoparticles with dispersed sizes and higher crystallinities are formed. Also, the viscosity and polarity of the solvents can affect the reaction and nucleation rates and impose variations in the particles sizes and crystallinity [37].Figure 1
SEM images of the (a) Co MOF, (b) Co2P-1, (c) Co2P-2, and (d) Co-P-3.
(a)(b)(c)(d)Figure2(a) shows the transmission electron micrograph (TEM) of Co2P-1, as the Co2P-1 exhibits the highest electrocatalytic OER activity. The Co2P-1 consists of aggregated nanoparticles with ~20 nm in diameters, consistent with the SEM results in Figure 1(b). Figure 2(b) exhibits the high-resolution TEM (HRTEM) of Co2P-1. Lattice stripes are observed, with the interplanar distance at 0.186 nm, which corresponds to the (120) plane of the crystalline Co2P phase. Besides, amorphous region also exists, covering the surface of the crystalline region, and is composed of phosphates formed by surface oxidation. Figures 2(c)–2(f) show the high-angle annular dark field (HAADF) image and the elemental mapping image of the corresponding region (EDS spectra in Figure S7). The Co and P are homogeneously distributed, and the existence of O is caused by the surface oxidation of the Co2P.Figure 2
(a) TEM image of the Co2P-1; (b) HRTEM image and (c) HAADF image of the Co2P-1; (d–f) elemental mapping of the corresponding regions.
(a)(b)(c)(d)(e)(f)Figure3(a) displays the X-ray diffraction (XRD) patterns of Co-P. The Co-P-3 shows no crystalline peak in the XRD, suggesting their amorphous nature. The XRD patterns for Co2P-1 and Co2P-2 both show peaks at 2θ=40.8°, 44.8°, and 48.3°, which match the (111), (021), and (120) planes of the hexagonal Co2P phase (JCPDS #54-0413) [38, 39], respectively. As reported in the literature, the Co2P nanoparticles generally require high temperature to reach high crystallinities. The solvothermal phosphidization temperature adopted here for Co-P is only 160°C, much lower than the temperature commonly reported for the highly crystalline Co2P [40, 41]. Therefore, relatively poor crystallinities are observed. Figure 3(b) shows the deconvoluted high-resolution XPS spectra of the Co 2p region of the Co-P (survey spectra in Figure S6). The two main peaks at 781.5 eV and 797.4 eV are the Co 2p3/2 and Co 2p1/2 peaks in Co2P-1, respectively, with the two satellite peaks at 787.6 eV and 803.1 eV. These binding energies correspond to the Co2+ [41]. In addition, peaks at 778.2 eV and 794.1 eV can be assigned to the Co-P bond [42, 43]. In the P 2p region spectra of Co-P (Figure 3(c)), two peaks appear at the binding energies of 129.1 eV and 133.6 eV, corresponding to the Co-P bond in cobalt phosphide, and the P-O bond in phosphate (P-O bond) which is formed due to the surface oxidation by oxygen in air, respectively [41]. The Co 2p peaks of the Co-P-3 appear at lower binding energies than those for the Co2P-1 and Co2P-2. This shows that the crystallinity would perturb the electron density of cobalt in the Co-P, as amorphous cobalt phosphide would contain higher amounts of defects or vacancies [44]. The XPS surface ratios of Co to P are 0.39, 1.03, and 1.10 for the Co2P-1, Co2P-2, and Co-P-3, respectively, all lower than their corresponding bulk ratios (Table S1). This is commonly observed for the thermally prepared cobalt phosphides [45]. XRD and XPS analyses show that Co-P with different sizes and crystallinities are prepared by the solvothermal phosphidization of Co MOF.Figure 3
(a) XRD patterns of Co-P; XPS spectra of (b) Co 2p and (c) P 2p regions of the Co-P.
(a)(b)(c)Figure4(a) shows the cyclic voltammograms (CV) of the Co-P/NF in 1 M KOH. All the Co-P/NF exhibit broad redox peaks prior to OER. For the Co2P-1/NF, the first oxidation peak at 1.15 V is originated from the oxidation of Co2+ to Co3+, while the second redox peaks centered at 1.37 V is the Co3+/4+ redox couple [46]. The lack of reduction peak at around 1.15 V suggests that the oxidation of Co2+ is irreversible. In comparison, the Co2P-2/NF and Co-P-3/NF show only the redox peaks of the Co3+/4+. The oxidation of Co2+ process is reported as a necessary step in generating Co-OOH active sites for OER [46, 47]. The existence of Co2+ oxidation peak in the Co2P-1/NF implies that this electrode is more active towards OER. Figure 4(b) shows the anodic linear sweep voltammetry (LSV) of the Co-P/NF. As expected, the Co2P-1/NF shows the highest OER activity, with 270 mV overpotential to reach 10 mA cm-2. The overpotential to reach 10 mA cm-2 OER current densities for Co2P-2/NF and Co-P-3/NF are 292 mV and 301 mV, respectively. In comparison, the bare Ni foam exhibits negligible current density towards OER. The Co2P-1/NF exhibits the lowest Tafel slope value at 111.6 mV dec-1 (Figure 4(c)), which indicates its most facile kinetics towards OER. The Tafel slope values for the Co2P-2/NF and Co-P-3/NF are 128 mV dec-1 and 142 mV dec-1, respectively. All the Tafel slope values are around 120 mV dec-1, which indicates that the first electron transfer process (formation of peroxyl intermediate at the oxide surface) is the turnover limiting step for OER [48–50]. All electrodes are stable towards the 500 s galvanostatic test at 10 mA·cm-2 (Figure 4(d)), with the Co2P-1/NF exhibiting the lowest overpotential (274 mV). The overpotential values at 10 mA cm-2 acquired by the galvanostatic measurement and by LSV are consistent. CV at various sweep rates in the potential region where no Faradaic process happens are acquired (Figure S1), and the Cdl values of the Co-P/NF are obtained (Figure 4(e)). The Cdl value for the Co-P-3/NF (46.1 mF cm-2) is the highest. The Cdl values for the Co2P-1/NF and Co2P-2/NF are 38.4 mF cm-2 and 37.1 mF cm-2, respectively. The Cdl value is proportional to the electrochemically active surface area (ECSA); however, the conversion factor is not defined [51]. This indicates that the amorphous Co-P-3/NF has the highest ECSA. The LSV is further normalized to the Cdl value to compare the intrinsic OER activities (Figure 4(f)). At the OER potential, the Co2P-1/NF exhibits higher Cdl-normalized current densities than Co2P-2/NF and Co-P-3/NF. This shows that the intrinsic OER activity is the highest for the Co2P-1/NF. In comparison, amorphous Co-P-3/NF exhibits the lowest intrinsic OER activity.Figure 4
(a) CV of the Co-P/NF at 5 mV·s-1; (b) anodic LSV of the Co-P/NF at 1 mV s-1; (c) Tafel plots of these electrodes; (d) the 10 mA cm-2 galvanostatic test results of the Co-P/NF for 500 s; (e) The Cdl values of Co-P/NF; (f) plots of the OER current densities normalized to Cdl. The solution is 1 M KOH.
(a)(b)(c)(d)(e)(f)Figure5(a) shows the EIS Nyquist plots of the Co-P/NF at 1.55 V. The equivalent circuit used for fitting is shown in Figure S4, and the fitting results are shown in Table S1. The charge transfer resistance (Rct) for the Co2P-1/NF is 0.28 Ω, lower than the Rct of 0.33 Ω for Co2P-2/NF and 0.3 Ω for Co-P-3/NF. This indicates that the OER kinetics is the most facile for the Co2P-1/NF, consistent with the Tafel analysis. We further carried out the distribution of relaxation time constant (DRT) analysis based on the EIS spectra [52]. The results are shown in Figure 5(b). Each peak with different time constants (τ) in the DRT plot represents a specific electrochemical process. Five peaks are observed that are assigned to the process related to electrode-electrolyte interface (P1), the intermediate adsorption (P2–P4), and the charge transfer (P5). The P5 of Co2P-1/NF has the smallest area compared to others, suggesting that the Rct is the lowest. Moreover, the area of the peaks related to intermediate adsorption is the lowest for Co2P-1/NF, which also indicates the most facile intermediate adsorption process at Co2P-1/NF. It is interesting to note that the Co2P-1/NF lacks P4, which suggests that the process related to intermediate adsorption is different, likely the OER on the Co2P-1/NF happens through a different mechanistic pathway.Figure 5
(a) Nyquist plots of the Co-P/NF in 1 M KOH; (b) DRT plots of the Co-P/NF.
(a)(b)To further probe the intermediate adsorption process on the Co-P/NF, electrocatalytic methanol oxidation using these electrodes are carried out to probe the methanol oxidation onset potential, which is related to the OH∗ adsorption energy (ΔGOH∗) [53–55]. The methanol oxidation onset potentials are 1.317 V, 1.297 V, and 1.288 V for the Co2P-1, Co2P-2, and Co-P-3/NF, respectively, and the exchange current densities are provided in Figure S2. These onset potential values correspond to the ΔGOH∗ at 1.317 eV, 1.297 eV, and 1.288 eV for the Co2P-1, Co2P-2, and Co-P-3/NF, respectively. Following the kinetic-thermodynamic model for OER [55], ΔGOH∗ for the ideal electrocatalysts should be 1.371 eV. The Co site is located at the left side of the volcano plot, which implies that the OH∗ binds too strong to the Co sites and that the formation of peroxyl intermediate is rate limiting [55]. Among the Co-P/NF, the Co2P-1/NF shows the closest to the apex ΔGOH∗. This is consistent with the observed best OER activity on the Co2P-1/NF. Therefore, the enhanced intrinsic OER activity of the Co2P-1 is mainly originated from the optimized ΔGOH∗, which is tuned by the electron interactions induced by the crystalline/amorphous heterostructure.Figure6(a) shows the 10 h 10 mA cm-2 galvanostatic test result of Co2P-1/NF in 1 M KOH. After 10 h, the overpotential of the Co2P-1/NF stabilized at 290 mV. The Co2P-1/NF can stably catalyze OER at 10 mA cm-2 for 24 h and longer (Figure S3), and no significant morphological change is observed by SEM after the long-term OER (Figure S8). These indicate that the Co2P-1 can form active and stable electrocatalysts for OER. The LSV before and after the 10 h 10 mA cm-2 galvanostatic test in Figure 6(b) almost overlaps, which indicates that the catalysts are robust and the OER activity is not degraded. The Nyquist plots before and after the 10 h 10 mA cm-2 galvanostatic test remain similar in shape, with the Rct rising slightly (Figure 6(c)). This may be due to the conversion of the surface phosphide layers into (oxy)hydroxides during OER, which hinders the electron conduction, or the thickened hydroxide layer hinders the diffusion of OH- towards the center of the active site, resulting in an increase in Rct. Figure 6(d) shows DRT plots of Co2P-1/NF before and after the 10 h galvanostatic test. The peak related to the double-layer structure (P1) decreases, which suggests changes in the double-layer structure. Also, peaks associated with intermediate adsorption all shift to higher time constants and increase in area. Both of which indicate hindered intermediate adsorption process. A new P4 peak emerges, which implies that the intermediate adsorption process is different from the as-prepared Co2P-1/NF. Therefore, based on DRT analysis, the surface of Co2P-1/NF has undergone a restructuring that affects the double-layer and intermediate adsorption process of OER.Figure 6
(a) The 10 mA cm-2 galvanostatic test results of the Co2P-1/NF in 1 M KOH for 10 h; (b) LSV before and after the galvanostatic test of the Co2P-1/NF; (c) Nyquist plots of Co2P-1/NF before and after the 10 h galvanostatic test; (d) DRT plots of the Co2P-1/NF before and after the 10 h galvanostatic test.
(a)(b)(c)(d)The Co2P-1/NF was sonicated, and the powders were collected after the 10 h 10 mA cm-2 galvanostatic test. Figure 7(a) shows the XRD pattern of the collected powder. Weak crystalline peaks assigned to crystalline Co2P phase still exit. This indicates that Co2P-1/NF remains stable during the OER. However, the diffraction peaks decrease in intensities compared to the as-synthesized Co2P-1. This indicates that the crystallinity of the phosphide phase decreases during OER. XPS spectra of the powder after the OER are displayed in Figures 7(b)–7(d). In the P 2p region (Figure 7(b)), only P-O bonds in phosphate exist after OER. The P-M bond disappears. The O 1 s peak (Figure 7(c)) can be deconvoluted into peaks assigned to metal oxides (M-O), hydroxides (M-OH), phosphates (P-O), and adsorbed water. The Co exists in 2+ oxidation states, with the peak assigned to Co-P completely disappears (Figure 7(d)). In addition, after the OER test, the atomic ratio of Co: P at the surface increased to 2.97. This suggests that phosphate is leached into the electrolyte. These indicate that the cobalt phosphide surface is converted to (oxy)hydroxides and phosphates during OER. Conversion of the cobalt phosphide surface layer to (oxy)hydroxide has been widely observed in the literature [41, 47, 51, 56]. This conversion happens progressively, and the formed (oxy)hydroxide shell can protect inner phosphide core [57]. We also observed partial conversion of phosphides to (oxy)hydroxides after OER for 10 h, as the XRD pattern still shows weak peaks assigned to cobalt phosphide. This conversion is reported to facilitate the generation of electrochemically active sites during OER, which explains the advantages of using phosphides rather than hydroxides for OER [41, 48]. Phosphates are also frequently identified at the surface of phosphides after OER and are proposed to act as the proton transport mediator [58]. Based on the reported literature on cobalt phosphides and our results, we conclude that the electrocatalytic active center is cobalt, and real active specie at the OER potential is the cobalt (oxy)hydroxide.Figure 7
(a) XRD patterns of the Co2P-1 powder after the long-term galvanostatic OER test; high-resolution XPS spectra (b) P 2p, (c) O 1 s, and (d) Co 2p regions of the Co2P-1/NF after the long-term galvanostatic test.
(a)(b)(c)(d)


## 4. Conclusion

Co-P were constructed via changing the solvent during the solvothermal phosphidization, and differences in crystallinities and morphologies are originated from the different reaction mechanisms of phosphide formation. When loaded onto Ni foam, the crystalline/amorphous heterostructured Co2P-1 as the precatalysts show the best OER activity in 1 M KOH, that achieves 10 mA cm-2 current densities with only 270 mV overpotential, and are stable towards long-term galvanostatic test. The OER activity is also comparable to the reported cobalt phosphide electrocatalysts in the literature (Table S3). The electron interaction in the crystalline/amorphous heterostructured Co2P optimizes the ΔGOH∗ and leads to accelerated OER kinetics. Conversion of the surface cobalt phosphides to (oxy)hydroxides is observed after OER, and the Co2P phase still exists but with decreased degree of crystallinity. This study offers an effective route in modulating the morphology and crystallinity of cobalt phosphide nanoparticles by solvothermal phosphidization. Identifying the key role of solvent during phosphide nanoparticle growth and understanding the in situ conversion mechanism of surface phosphide layers would aid further design of highly active electrocatalysts for OER.



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*Source: 3893229-2023-02-07.xml*