MOF-derived nanostructured cobalt phosphide assemblies for efficient hydrogen evolution reaction

Abstract

The earth-abundant metal phosphides have emerged as new and active alternatives for the precious metal platinum to electrocatalyze the hydrogen evolution reaction (HER) via water splitting. In this study, we report on the straightforward and controllable synthesis of various nanostructured cobalt phosphide assemblies through the direct chemical transformation of a Co-containing metal–organic framework (ZIF-67-Co) under mild phosphorization conditions. The resulting CoP nanoparticle assemblies (NPAs) and nanorod assemblies (NRAs) can be selectively produced by rationally tuning the calcination atmosphere. The CoP NRAs were found to be highly active for electrocatalytic hydrogen generation, which exhibited better performance than that of the CoP NPAs, as evidenced by their relatively low onset overpotential (∼86 mV), small Tafel slope (∼69 mV dec⁻¹) and long-term stability in acidic media.

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Introduction

Electrocatalyzing the hydrogen evolution reaction via water splitting is a promising approach for chemically offering carbon-free energy to advance the sustainable development of human society, which is currently attracting worldwide attention. To efficiently split water to obtain molecular hydrogen, a suitable electrocatalyst for the hydrogen evolution reaction (HER, 2H⁺ + 2e → H₂) is usually needed to afford a high current at a low overpotential. Pt-based materials are the most efficient HER electrocatalysts,^1–3 but their limited abundance and high cost restrict their widespread application. Therefore, there is tremendous interest in developing inexpensive and earth-abundant HER catalysts, exhibiting high activity and robust stability.

As a result, earth-abundant transition metal compounds, including sulfides,^4–10 selenides,¹¹ carbides,^12–16 nitrides,^16–18 phosphides,^19–27 and metal alloys,²⁸ have been explored as highly efficient HER electrocatalysts for replacing precious metal catalysts. Among them, transition metal phosphides (TMPs), formed by the alloying of metals and phosphorus, have attracted special research interest, as they are well-known catalysts for hydrodesulfurization (HDS), which is a similar reaction process to HER. Both HDS and HER rely on reversible binding of catalyst and hydrogen. In a HDS reaction, hydrogen dissociates on the catalyst to react with sulfur, yielding H₂S, whereas in HER, protons bind to the catalyst to form adsorbed H to further produce molecular hydrogen.^29–31 Recent studies have experimentally demonstrated the high electrocatalytic activity and good stability of various TMPs, including MoP,^24,32,33 WP,²⁶ FeP,^23,34,35 CoP,^{20,21,25,36,37} Cu₃P,²² Ni₂P,^19,38–40 NiP₂,⁴¹ and Ni₁₂P₅.⁴² However, their synthetic chemistry poses a number of challenges as most of the available methods typically require extremely high reaction temperatures, long reaction times and harsh conditions.

Metal–organic frameworks (MOFs) are an intriguing class of porous crystalline inorganic–organic hybrid materials built from metal ions or clusters and polyfunctional organic ligands and have attracted increasing attention in recent years, owing to both fundamental scientific interest and attractive applications.^43,44 Recently, Co-based MOFs have been demonstrated to be ideal sacrificial templates for the generation of porous electroactive Co₃O₄ nanostructures by thermal decomposition under controllable atmospheres.^45–47 As an example, porous Co₃O₄-carbon nanohybrids derived from Co-MOFs by thermally treating in nitrogen exhibited excellent performances with respect to electrocatalytically evolving oxygen.^48,49 The zeolitic imidazolate framework-67 (ZIF-67-Co) has an open-framework structure and is composed of tetrahedrally coordinating cobalt ions coordinated by nitrogen atoms in imidazole linkers, forming tetrahedral CoN₄ building units (Fig. 1);⁵⁰ this has been shown as an ideal MOF template for the synthesis of Co₃O₄ micro/nanostructures for application in lithium-ion batteries,⁴⁶ supercapacitors,⁵¹ and gas sensors.⁵²

Fig. 1 Schematic of the formation of CoP NPCs and CoP NRCs derived from ZIF-67-Co.

Similar to commonly used FeP and Ni₂P catalysts, CoP is also known to be active for both HDS and HER, which is also structurally and compositionally distinct from Ni₂P⁵³ and has better stability than that of acid-corrosive FeP.^23,54 In this study, using a Co-based metal–organic framework (ZIF-67-Co) as a suitable precursor, nanostructured CoP assemblies can be directly prepared under mild conditions (Fig. 1), wherein CoP nanoparticle assemblies (NPAs) and nanorod assemblies (NRAs) can be selectively produced by rationally tuning the calcination atmosphere. It is also experimentally demonstrated that MOF-derived CoP NRAs as a potential non-precious HER electrocatalyst display a low overpotential, high current density and good durability.

Experimental

Materials

Sodium hypophosphite (NaH₂PO₂) and 2-methylimidazole (2-mIM) were purchased from Aladdin Ltd (Shanghai, China). Cobalt nitrate (Co(NO₃)₂·6H₂O), methanol, and H₂SO₄ were purchased from Kelong Chemical Reagents Company (Chengdu, China). All chemicals used in this study were of commercially available analytical grade and were used without further purification.

Synthesis of ZIF-67-Co metal–organic framework

Typically, 1.1641 g (4.0 mmol) of Co(NO₃)₂·6H₂O and 1.3136 g (16.0 mmol) of 2-mIM were each dissolved in 50 mL methanol. The latter clear solution was added into the formerly pink solution under stirring with a magnetic bar. Stirring was stopped after combining the component solutions. After 48 hours, a purple solid was collected by centrifugation, washed with methanol three times and dried at room temperature, resulting in so-called ZIF-67-Co MOF.

Synthesis of CoP nanoparticle assemblies (NPAs) and nanorod assemblies (NRAs) derived from ZIF-67-Co metal–organic framework

In a typical procedure, 0.1 g of as-prepared ZIF-67-Co and 0.5 g of NaH₂PO₂ were mixed together and loaded in a ceramic crucible with a cover and then heated at 300 °C for 1 h with a ramp rate of 5 °C in a muffle furnace. After naturally cooling to room temperature, the resulting CoP NPAs product was washed with water and ethanol several times and dried under vacuum at 60 °C for 6 h. The procedures for the CoP NRAs were similar to that for the CoP NPAs product; however the annealing conditions in the tubular furnace were altered to 300 °C for 1 h with a ramp rate of 5 °C under a nitrogen atmosphere.

Characterization

Powder X-ray diffraction (XRD) measurements were obtained on a RigakuDmax/Ultima IV diffractometer with monochromatized Cu Kα radiation (λ = 0.15418 nm). Fourier transform infrared (FTIR) spectroscopy was carried out on a Nicolet 6700 FTIR Spectrometric Analyzer using KBr pellets. The morphology was observed with a Hitachi S4800 field emission scanning electron microscopes (FESEM) and a transmission electron microscope (TEM, FEI Tecnai G20). The elemental compositions of the samples were characterized by energy-dispersive X-ray spectroscopy (EDS, Oxford instruments X-Max). X-ray photoelectron spectroscopy (XPS) measurements were obtained on a Perkin-Elmer PHI 5000C spectrometer using monochromatized Al Kα excitation. All binding energies were calibrated using the contaminant carbon (C_1s = 284.6 eV) as a reference. The specific surface area of the samples was measured by nitrogen adsorption at 77 K using an automated gas sorption analyzer (ASIQ-C, Quantachrome) and calculated according to the Brunauer–Emmett–Teller (BET) method.

Electrochemical measurements

5 mg of cobalt phosphide powder was dispersed in a 1 mL mixture of distilled water and ethanol (3 : 1 v/v). Then, 10 μL of 5 wt% Nafion was added to the abovementioned solution. The mixed solution was sonicated for at least 30 min to form a homogeneous ink. 5 μL of the mixed solution was drop-cast onto a glassy carbon electrode with a diameter of 3 mm for the electrochemical measurements. All the electrochemical measurements were performed on an electrochemical workstation (CHI 660E, CH Instruments Inc., Shanghai) using a typical three-electrode mode with an electrolyte solution of 0.5 M H₂SO₄, a Pt wire counter electrode, an Ag/AgCl (saturated KCl) reference electrode, and a modified glassy carbon working electrode. All potentials measured were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: E_RHE = E_Ag/AgCl + 0.197 + 0.059 pH. The polarization curves were obtained by sweeping the potential from 0.2 to −0.8 V (versus the RHE) at room temperature with a scan rate of 5 mV s⁻¹. Chronoamperometric responses were obtained at −250 mV (versus the RHE) in a 0.5 M H₂SO₄ solution. Chronopotentiometric responses were obtained at a current density of −39.6 mA cm⁻² in a 0.5 M H₂SO₄ solution. AC impedance measurements were carried out at an overpotential of −300 mV in 0.5 M H₂SO₄ from 10⁻¹ to 10⁶ Hz with an AC amplitude of 5 mV. The current density was calculated from the geometric area of the glassy carbon electrode, which is 0.07 cm².

Results and discussion

The ZIF-67-Co precursor is facilely synthesized by a solution-based method in methanol at room temperature. Fig. 2a shows a typical powder XRD pattern of the ZIF-67-Co precursor. All the diffraction peaks are identical to its corresponding simulated pattern as well as the reported experimental patterns.^46,50,55 The strong and sharp diffraction peaks reveal the high crystallinity of the ZIF-67-Co precursor. The FTIR spectrum further confirms the molecular structure of ZIF-67-Co (Fig. 2b). The primary absorption bands match well with previously reported results,^56,57 wherein the absorption band at 434 cm⁻¹ is attributed to Co–N stretching and the intense bands at 1350–1500 cm⁻¹ are associated with the entire ring stretching. The typical scanning electron microscopy (SEM) images of the ZIF-67-Co (Fig. 2c and d) reveal that the MOF precursor is composed of a large quantity of regular particles with uniform and perfect rhombic dodecahedral morphology. The size of the rhombic dodecahedral structure is on the micrometer scale with a diameter in the range of 0.5–1.5 μm.

Fig. 2 XRD pattern (a), FTIR spectrum (b) and SEM images (c and d) of ZIF-67-Co.

The subsequent phosphorization of the ZIF-67-Co precursor via solid chemical transformation under heat treatment leads to its color change from purple to dark. Fig. 3 shows a typical XRD pattern of the ZIF-67-Co-derived CoP NPAs calcined at 300 °C for 1 h in a ceramic crucible with a cover. The diffraction peaks at 31.7°, 36.5°, 46.2°, 48.3°, 52.3°, and 56.8° can be indexed to the (011), (111), (112), (211), (103), and (013) planes of orthorhombic CoP (JCPDS file no. 65-2593), indicating the effective phase transformation from ZIF-67-Co to orthorhombic CoP. Fig. 4a shows a typical low-magnification SEM image of the CoP NPAs, which indicates that the CoP product is composed of a large quantity of irregular polyhedron-like particles. These polyhedral particles have sizes from several hundreds of nanometers to one micrometer. The magnified SEM image (Fig. 4b) of an individual polyhedral particle suggests that these particles present as a hierarchical architecture built up from many single CoP crystallites with sizes of 40–80 nm. Details of the hierarchical structures can be obtained from the transmission electron microscopy (TEM) images. As shown in Fig. 4c, the hierarchical architectures consist of many loosely aggregated crystallites, some of which become discrete during ultrasonication for TEM observation. The secondary structure of these CoP crystallites can be observed more clearly in a high-magnification TEM image. Fig. 4d displays the nanoparticulate assembly structure of the CoP crystallites, which are composed of numerous nanocrystals with a size of about 5–10 nm. The high-resolution TEM (HRTEM) image shown in Fig. 4e shows that the distances between the lattice fringes are 0.257 and 0.247 nm (see the corresponding line profiles in Fig. 4f), agreeing with the d-spacings of the (200) and (111) planes of the CoP. The well-defined lattice fringes in the CoP NPAs reflect their good crystallinity. The corresponding fast Fourier transform (FFT) pattern (inset in Fig. 4e) shows that the diffraction spots are projected by the (200) and (111) planes, further revealing the orthorhombic phase of the CoP NPAs.

Fig. 3 XRD patterns of CoP NPAs and CoP NRAs.

Fig. 4 SEM images (a and b), TEM images (c and d), HRTEM image (e) and corresponding FFT pattern (inset in panel e) and line profiles (f) of the CoP NPAs.

When calcination treatment of the ZIF-67-Co precursor is carried out in a tubular furnace under an N₂ atmosphere at 300 °C for 1 h, the CoP NRAs can be directly obtained. A typical XRD pattern (Fig. 3) of the ZIF-67-Co-derived CoP NRAs shows that all diffraction peaks can be indexed as the orthorhombic CoP (JCPDS file no. 65-2593). A general scanning SEM image (Fig. 5a) of the CoP NRAs reveals that the product is composed of plenty of the hierarchical flower-like architectures with rough surfaces, each of which are formed by the accumulation of several short rod-shaped petals (Fig. 5b). The compositional analysis determined by energy-dispersive X-ray spectroscopy (EDS) confirms the presence of Co and P in the CoP NRAs (Fig. S1a, ESI†) and indicates that the CoP NRAs have an atomic ratio of Co : P = 1.008 : 1, which is quite consistent with that of stoichiometric CoP (1 : 1). The corresponding SEM-EDS mapping images further verify that both Co and P are homogeneously distributed in the CoP NRAs (Fig. S1b, ESI†). Details of the hierarchical structures can be clearly observed in the transmission electron microscopy (TEM) images. Fig. 5c confirms the hierarchical structures of the CoP NRAs. These hierarchical structures are not uniform and their sizes are in the range of 200–500 nm, which are actually made of short-rod-shaped petals with a certain orientation, identical to the SEM images. These round-tipped rods have a width of 5–10 nm and a length of 30–50 nm and connect side-by-side to form the flower-shaped clusters (Fig. 5d). A HRTEM observation on an individual nanorod (Fig. 5e) shows fringe spacings of 0.285 and 0.280 nm, matching well with the (011) and (002) planes of orthorhombic CoP. The diffraction points of the corresponding FFT pattern (Fig. 5f) can be indexed to the (011) and (002) planes and display the highly crystalline nature of the CoP NRAs.

Fig. 5 SEM images (a and b), TEM image (c and d), HRTEM image (e) and corresponding FFT pattern (f) of the CoP NRAs.

The electrocatalytic activities of the CoP NPAs and NRAs towards HER were evaluated in a standard three-electrode system in an acidic medium. The working electrode was prepared by loading the catalyst onto a surface (0.07 cm²) of a glassy carbon electrode (GCE) (mass loading: ∼0.35 mg cm⁻²). Fig. 6a shows the typical linear scan voltammogram (LSV) profiles of the CoP NPA and NRA-modified GCE in 0.5 M H₂SO₄ solution. For comparison, the ZIF-67-Co, the commercial Pt/C catalyst (20 wt%) and the bare GCE were also measured under identical conditions. As expected, the Pt/C catalyst presents good HER activity with a near zero overpotential. The bare GCE is ineffective toward evolving molecular hydrogen, while the ZIF-67-Co shows negligible electrocatalytic activity as well. Under the measured potential window, the CoP NPAs are effective toward electrocatalytically producing molecular hydrogen, but a relatively low HER activity is reflected by a high onset overpotential and a low cathodic current density at applied potentials. In a sharp contrast, the CoP NRAs are highly efficient toward HER, which afford much greater catalytic current density and lower onset potential (∼86 mV) compared with CoP NPAs. The potential required for a current density of 10 mA cm⁻², as a commonly used HER reference and a metric reference to solar fuel synthesis, is used to evaluate HER activity. The CoP NRAs at 10 mA cm⁻² achieve a small overpotential (η) of ∼181 mV, which is superior to that of CoP NPAs (∼393 mV) and compares favorably to those of reported nanostructured CoP/GCEs such as CoP nanoparticles (∼226 mV,²¹ ∼221 mV,³⁶ and ∼297 mV (ref. 58)), nanotubes (∼129 mV),⁵⁸ nanowires (∼110 mV)³⁶ and nanosheets (∼164 mV).³⁶

Fig. 6 (a) Polarization curves of the CoP NPAs, CoP NRAs, ZIF-67-Co, commercial Pt/C catalysts and bare GCE with a scan rate of 5 mV s⁻¹ in 0.5 M H₂SO₄ solution. (b) Tafel plots of the CoP NPAs, CoP NRAs, and commercial Pt/C catalysts. (c) Nyquist plots of the CoP NPAs and CoP NRAs. (d) Capacitive currents at 0.377 V as a function of scan rate for the CoP NPAs and CoP NRAs (Δj₀ = j_a − j_c). (e) Durability test for the CoP NPAs and CoP NRAs by CV scanning after 1000th cycles in 0.5 M H₂SO₄ solution. (f) Chronopotentiometric durability test of the CoP NRAs at a constant current density of ∼39.6 mA cm⁻² and chronoamperometric durability test of the CoP NRAs at an overpotential of ∼250 mV.

The HER kinetics of the CoP NPAs and NRAs were accessed using corresponding Tafel plots (Fig. 6b) derived from polarization curves. The recorded linear regions of the Tafel plots are fitted to the Tafel equation (η = b log j + a, where η is overpotential, j is the current density, and b is the Tafel slope). The Tafel slope of the Pt/C catalyst is ca. 30 mV dec⁻¹, consistent with the reported value.¹⁹ The resulting Tafel slope for the CoP NRAs is ∼69 mV dec⁻¹, which is much smaller than that of the CoP NPAs (∼105 mV dec⁻¹), suggesting more favorable kinetics towards the electrocatalytic HER. Furthermore, electrochemical impedance spectroscopy (EIS) was employed to investigate the electrode kinetics under the catalytic HER operating conditions. The Nyquist plots (Fig. 6c) reveal the dramatically reduced charge transfer resistance of the CoP NRAs relative to the CoP NPAs, suggesting a much faster electron transfer process and facile kinetics toward hydrogen evolution. We also estimated the effective electrode surface area of the CoP NPAs and NRAs using cyclic voltammetry (CV) at a region without electrochemical reactions by means of determining the electrochemical double layer. The double-layer capacitances (C_dl) for the CoP NPAs and NRAs can be extracted by plotting the difference in current density between the anodic and cathodic scans (Δj = j_a − j_c) at a certain overpotential against the scan rate (Fig. 6d), which should be directly proportional to the effective electrode surface area. The calculated capacitance of the CoP NRAs is 672 μF cm⁻², which is much larger than that of the CoP NPAs (242 μF cm⁻²). The increased active sites reflected by the effective electrode surface area could benefit the catalytic HER performance. In addition, the CoP NRAs also have a larger BET surface area (33.3 m² g⁻¹) than that of CoP NPs (25.4 m² g⁻¹), which would provide more active sites for the electrocatalytic HER.

We further investigated the long-term durability of the CoP NPAs and NRAs toward HER by cycling tests in 0.5 M H₂SO₄ solution, which can determine the practicability of electrocatalysts. Fig. 6e shows the polarization curves measured before and after 1000th CV cycle at a scan rate of 50 mV s⁻¹. After the 1000th cycle, both the CoP NPAs and NRAs show a slight loss of current density compared with the initial cycle. The chronoamperometric response (Fig. 6f) shows that 97.9% of the original catalytic current can be maintained after 10 h of testing. The chronopotentiometric response (Fig. 6f) also demonstrates the good durability of the CoP NRAs. They can produce a steady overpotential under a constant operating cathodic current density of ∼39.6 mA cm⁻² after a 10 h test. Moreover, the CoP NRAs also preserve their original morphology and crystalline structure after electrolysis (Fig. S2, ESI†). These results indicate that the CoP NRAs are highly stable during a long-term electrochemical HER process.

As previously reported, the covalent nature of the cobalt–phosphorus bond dominates the metal phosphides, which is reasonably represented as Co^δ+P^δ− due to charge transfer from Co to P.⁵⁹ Furthermore, Sun's recent studies have demonstrated that the charged natures of Co and P in CoP are analogous to those of proton acceptors and hydride acceptors in hydrogenase,^20,21,36,58 which should be the main contributor to the HER activity of CoP. To clarify the origin of the present HER activity, the charged characteristic of the CoP NRAs was further analysed by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 7a, the binding energies at 778.5 and 784.1 eV in the energy level of Co 2p_3/2 are ascribed to Co species in CoP and oxidized Co species of Co 2p_3/2, respectively.^20,21 Correspondingly, the binding energies at 129.1 and 133.2 eV of the P 2p spectrum (Fig. 7b) are attributed to P species in CoP and oxidized P species, respectively.^20,21 The presence of surface oxygen species in the CoP NRAs is also confirmed by the SEM-EDS spectrum (Fig. S1a, ESI†) because of superficial oxidation of CoP, consistent with previously reported CoP.^20,21,36,58 Furthermore, the binding energy (778.5 eV) of Co species in CoP shows a positive shift from metallic Co(0) (778.1–778.2 eV), whereas the binding energy (129.1 eV) of P species in CoP is negatively shifted from elemental P (130.0 eV),⁶⁰ suggesting that the related Co and P species in CoP NRAs have a partial positive charge (δ⁺) and a negative charge (δ⁻), respectively. This result implies that the intrinsic HER activity of the CoP NRAs might be correlated with the charged natures of Co and P.

Fig. 7 XPS spectra of the CoP NRAs: (a) Co 2p and (b) P 2p.

Conclusions

In summary, we have demonstrated the straightforward synthesis of CoP NPAs and NRAs using ZIF-67-Co as an effective precursor under mild phosphorization conditions. Such MOF-derived nanostructured CoP assemblies display remarkable activity for the electrocatalytic HER with a low onset overpotential, large cathodic current density and good durability, exhibiting great potential as a low cost alternative to the precious metal Pt catalyst in practical applications. Moreover, our synthesis method can be extended for the general preparation of other multinary TMPs from transition metal-based MOFs. However, the employed heat treatment in our case could induce a certain degree of particle agglomeration. We believe that introducing active supports would be a suitable strategy to stabilize the particles to improve the surface area, which needs to be investigated further.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (51572227, 21207108), the Sichuan Youth Science and Technology Foundation (2013JQ0012), and the Research Foundation of CWNU (12B018, 14E016).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17427a

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