Lili Liab,
Xingyue Lia,
Lunhong Ai*a and
Jing Jiang*a
aChemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, 1 Shida Road, Nanchong 637002, P. R. China. E-mail: ah_aihong@163.com; 0826zjjh@163.com; Fax: +86-817-2568081; Tel: +86-817-2568081
bi-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS), 398 Ruoshui Road, Suzhou, 215123, P. R. China
First published on 5th October 2015
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.
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
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.
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.
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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.
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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
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 = blog
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 (Cdl) for the CoP NPAs and NRAs can be extracted by plotting the difference in current density between the anodic and cathodic scans (Δj = ja − jc) 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 Coδ+Pδ− 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 2p3/2 are ascribed to Co species in CoP and oxidized Co species of Co 2p3/2, 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.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17427a |
This journal is © The Royal Society of Chemistry 2015 |