Electrochemistry Communications 56 (2015) 56–60
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Short communication
Porous cobalt phosphide nanorod bundle arrays as hydrogen-evolving cathodes for electrochemical water splitting Zhiguo Niu, Jing Jiang ⁎, Lunhong Ai ⁎ College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, China
a r t i c l e
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Article history: Received 11 April 2015 Accepted 16 April 2015 Available online 22 April 2015 Keywords: Water splitting Hydrogen evolution Cobalt phosphide Electrocatalyst
a b s t r a c t An advanced electrode composed of porous CoP nanorod bundle arrays (NBAs) on Ti plate was fabricated by using Co3O4 NBAs as precursors and developed as a robust self-supported hydrogen-evolving cathode for electrochemical water splitting. Such electrode exhibited remarkable activity for hydrogen evolution reaction in a wide pH range, affording a low onset overpotential, small Tafel slope, and long-term stability. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is considered as a promising alternative to fossil fuels in the future due to its zero carbon emission. The electrochemical water splitting is an important process to produce hydrogen. However, both half-reactions of water splitting still remain technical challenges. Platinum is most effective for hydrogen evolution reaction (HER), but low reserve and high price limit its practical application. Although nickel and its alloys are potential materials for HER in alkaline media, they experience extensive deactivation during alkaline water electrolysis [1]. In addition to the carbon matrixes-based electrocatalysts [2,3], metal phosphides have also received great attention and emerged as new alternatives for HER. The intense interest is driven by their intrinsically charged natures, where metal and nonmetal components have partial positive and negative charges, respectively, analogous to those of active centers (i.e. proton acceptors and hydride acceptors) in hydrogenase, as demonstrated by both theoretical calculations [4] and experimental investigations [5]. Besides, phosphorus promotes the activity of primary metal through altering its electronic and geometric properties [6,7]. Indeed, metal phosphides including CoP [8–10], Ni2P [5,11], FeP [12], MoP [13], Cu3P [14] and WP2 [15], and their composites [2,16,17] have shown remarkable HER performances. Recently, Sun et al. have demonstrated that metal phosphides can be transformed from the corresponding metals [18], metal oxides [9, 19–21] and hydroxides [11,22,23]. Our group also found that the HERactive Ni2P can be chemically converted from a metal-organic framework [24]. Herein, we report the direct synthesis of porous CoP nanorod bundle arrays (NBAs) on Ti plate from Co3O4 NBAs and utilize them as ⁎ Corresponding authors. Tel./fax: +86 817 2568081. E-mail addresses:
[email protected] (J. Jiang),
[email protected] (L. Ai).
http://dx.doi.org/10.1016/j.elecom.2015.04.010 1388-2481/© 2015 Elsevier B.V. All rights reserved.
robust self-supported hydrogen-evolving cathodes for electrochemical water splitting in a wide pH range, achieving a low onset overpotential, small Tafel slope, and long-term stability. 2. Experimental Typically, Co(NO)3 · 6H2O (4 mmol), NH4F (16 mmol) and urea (20 mmol) were dissolved in 70 mL of distilled water and stirred for 10 min. The solution was transferred into a 100 mL autoclave, and a piece of cleaned Ti plate (3 cm × 4 cm) was immersed into the solution. The autoclave was maintained at 120 °C for 9 h. The obtained product was rinsed with distilled water, dried at 60 °C for 6 h and annealed at 350 °C for 2 h in air. To convert Co3O4/Ti into CoP/Ti, the Co3O4/Ti and NaH2PO2 were placed at two separate positions in a porcelain boat with NaH2PO2 at the upstream side of the quartz tube, and annealed at 300 °C in argon atmosphere for 2 h [17]. The morphology, microstructure and composition of samples were characterized by X-ray diffraction (XRD, Rigaku Dmax/Ultima IV), scanning electron microscopy (SEM, Hitachi S4800 and JEOL JSM-6510LV), transmission electron microscopy (TEM, FEI Tecnai G20) and X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C). All electrochemical measurements were performed in a typical threeelectrode mode using CoP/Ti (loading: 1.96 mg cm−2) as the work electrode, a Pt wire as the counter electrode and an Ag/AgCl as the reference electrode. The 0.5 M H2SO4 (pH 0), 1.0 M phosphate buffer (PBS, pH 7), or 1.0 M KOH (pH 14) were used as electrolytes. Before the electrochemical measurement, the electrolyte was degassed by bubbling argon for 30 min. All potentials measured were converted to the reversible hydrogen electrode (RHE) scale (ERHE = EAg/AgCl + 0.197 + 0.059pH). The mass loading of CoP on Ti plate was calculated from the weight difference between the pristine Ti plate and CoP/Ti.
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3. Results and discussion A general SEM image (Fig. 1a) of Co3O4 NBAs/Ti confirms that the Ti plate is covered by numerous nanorod arrays with high density. The magnified image (Fig. 1b) presents that the structure actually consists of bundles of smaller nanorod subunits. These nanorods have diameters of 50–110 nm (inset). SEM image (Fig. 1c) of CoP NBAs reveals that they effectively preserve the morphological profile of Co3O4 NBAs. Close observation (Fig. 1d) reveals the porous morphology of CoP NBAs, consisting of several nanoparticles and pores on rough surfaces. TEM image (Fig. 1e and f) of CoP NBAs further confirms their nanorod bundle structures. The bundles consist of aligned porous nanorods with about 90 nm in diameter. The magnified image (Fig. 1g) suggests that nanorods are constructed from numerous aggregated nanoparticles with sizes of 10–20 nm. HRTEM image (Fig. 1h) displays the distance between lattice fringes is 0.19 nm, corresponding to the (211) plane of CoP. EDX spectrum confirms the presence of Co, P and Ti elements in CoP NBAs/Ti (not shown). The estimated atomic ratio of Co to P is consistent with stoichiometric CoP. SEM-EDX elemental mapping images (not shown) suggest that both Co and P elements are uniformly distributed in CoP NBAs. XRD patterns (Fig. 1i) clearly reveal that Co3O4 NBAs change completely into CoP phase. Before polarization curve measurements, cyclic voltammetry (CV) curves were performed to stabilize CoP NBAs in electrolytes (Fig. 2a). Fig. 2b shows the polarization curves of CoP NBAs/Ti, Co3O4 NBAs/Ti, commercial Pt/C and bare Ti plate in 0.5 M H2SO4. Pt/C presents the expected HER activity. Bare Ti plate is totally inactive for HER and Co3O4 NBAs/Ti shows negligible activity. In contrast, CoP NBAs/Ti delivers a small onset potential (conventional calculation from extrapolation of polarization curve [25]) of ∼ 155 mV and achieves a sharp increase in
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cathodic current at more negative potentials, reflecting its excellent HER activity. Furthermore, CoP NBAs/Ti at 10 mA cm− 2 affords an overpotential of ∼203 mV, which is lower than that of Co3O4-derived CoP nanoparticles (∼ 212 mV [2], ∼ 226 mV [9], ∼ 221 mV [19]) and CoP/graphene [16], and compares favorably to other reported metal phosphides [15,24,26]. Also, CoP NBAs exhibit a Tafel slope of 40 mV dec−1 (Fig. 2c), which is much smaller than that of the best reported CoP [8–10] and Co3O4-derived CoP catalysts [2,16], implying more favorable HER kinetics. To further explore the electrocatalytic activity, turnover frequency (TOF) is estimated by calculating the number of active sites from the CV sweep [8,27]. CoP NBAs/Ti shows a TOF of 4.36 s−1 at an overpotential of 250 mV, which is close to reported values for CoP nanostructures [8,27]. The durability was tested by cycling CoP NBAs/Ti continuously at a scan rate of 100 mV s−1 in 0.5 M H2SO4 (Fig. 2d). After 3000 cycles, the overpotential to attain a current density of 10 mA cm−2 increases by less than 10 mV. The chronoamperometric measurement on CoP NBAs/Ti further confirms its long-term stability (inset), showing cathodic current attenuation less than 10% within 10 h. We further examined the HER activity of CoP NBAs/Ti in alkaline media (Fig. 2e). In 1.0 M KOH (pH 14), it affords an onset overpotential of ∼96 mV and a Tafel slope of ∼115 mV dec−1. The HER current generated by CoP NBAs/Ti at 250 mV sustains about 78% after 1000 cycles. We also studied the HER activity of CoP NBAs/Ti under neutral condition (Fig. 2f). In 1.0 M PBS (pH 7), it achieves an onset overpotential of ∼126 mV, a Tafel slope of ∼116 mV dec−1 as well as a good long-term stability (∼12% of current decay after 3000 cycles at 250 mV). Notably, CoP NBAs/Ti delivers lower onset overpotential in alkaline and neutral conditions but larger Tafel slope compared with those in acidic condition. This result is consistent with the Ni2P/Ni electrode [28]. Additionally, SEM and XRD (Fig. 1i) results indicate that CoP NBAs/Ti preserves original morphology and crystalline structure after cycling test in acidic
Fig. 1. SEM images of Co3O4 (a, b) and CoP NBAs (c, d). TEM (e–g) and HRTEM (h) images of CoP NBAs. XRD patterns (i) of Co3O4, and CoP NBAs before and after cycling test under different conditions.
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Fig. 2. (a) CV curve of CoP NBAs/Ti in 0.5 M H2SO4, scan rate: 100 mV s−1. (b) Polarization curves of CoP NBAs/Ti, Co3O4 NBAs/Ti, Pt/C and Ti plate in 0.5 M H2SO4, scan rate: 5 mV s−1. (c) Tafel plots of CoP NBAs/Ti and Pt/C. (d) Polarization curves of CoP NBAs/Ti before and after 3000 cycles and I–t curve of CoP NBAs/Ti for 10 h in 0.5 M H2SO4 (overpotential: 253 mV). Polarization curves and Tafel plots of CoP NBAs/Ti before and after 1000 CV cycles in 1.0 M KOH (e) and after 3000 cycles in 1.0 M PBS (f).
(Fig. 3a) and neutral (Fig. 3b) conditions, but morphology is slightly changed in alkaline condition (Fig. 3c). We further performed the XPS to clarify electronic states of Co and P in CoP NBAs. As shown in Fig. 3d, binding energies at 778.4 and 781.4 eV in Co2p3/2 correspond to Co in CoP and oxidized Co species, respectively [8,9]. P2p XPS spectrum (Fig. 3e) displays three peaks at 129.5, 130.3 and 133.5 eV, assigned to P2p3/2, P2p1/2 in CoP and oxidized P species due to superficial oxidation of CoP [8,9]. Noticeably, binding energies of Co (778.4 eV) and P (129.5 eV) in CoP have a positive shift from metallic Co(0) (778.1–778.2 eV) and a negative shift from elemental P (130.0 eV), respectively [29], indicating that Co and P species in CoP NBAs present partial positive charges (δ+) and negative charges (δ−), respectively.
Based on structural characteristics and intrinsic properties of CoP NBAs, we think CoP NBAs/Ti electrode offers several advantages for HER. The first advantage in technique and catalytic performance is the characteristic of self-supported electrode, compared with the reported CoP-related composite powders [2,16]. The growth of CoP NBAs on Ti plate ensures intimate contact and excellent electrical connection between them, promoting the electron transport and structural stability. Secondly, the binder-free feature of our electrode avoids the use of polymer binder, providing an efficient pathway for electron transport through the entire electrode. The electrochemical impedance spectroscopy (Fig. 3f) reveals that CoP NBAs/Ti electrode exhibits a low impedance of 10.6 Ω, which is much smaller than that of CoP powder by dropcasting on the glassy carbon electrode [2]. Thirdly, CoP NBAs have a
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Fig. 3. SEM images (a–c) of CoP NBAs/Ti after cycling test in different conditions. XPS spectra of CoP NBAs: (d) Co 2p and (e) P 2p. (f) Nyquist plots of CoP NBAs/Ti at an overpotential of 282 mV in 0.5 M H2SO4.
larger BET specific surface area (11.7 m2 g−1), compared with CoP nanoparticles (6.0 m2 g−1) [19]. This allows for increased utilization of more active sites, leading to better HER activity than that of CoP nanoparticles [2,16,19]. Such surface-area effect has been demonstrated in MoP nanoparticles for HER [13]. Additionally, well-developed porosity would facilitate diffusion of electrolytes and efficient transfer of reactants to the catalyst's active sites [30], resulting in their small Tafel slope. These advantages in CoP NBAs/Ti combined with its intrinsically charged natures are responsible for their excellent HER activity. 4. Conclusions In summary, we have presented the synthesis of CoP NBAs grown on Ti plate by using Co3O4 NBAs precursor and demonstrated the potential application in electrochemical water splitting. The resulting CoP NBAs/ Ti was electrocatalytically active for HER in a wide pH range, affording a low onset overpotential, small Tafel slope, and long-term stability. Acknowledgements This work was supported by the National Natural Science Foundation of China (21207108) and the Sichuan Youth Science and Technology Foundation (2013JQ0012). References [1] F. Rosalbino, S. Delsante, G. Borzone, E. Angelini, Electrocatalytic behaviour of Co– Ni–R (R = rare earth metal) crystalline alloys as electrode materials for hydrogen evolution reaction in alkaline medium, Int. J. Hydrog. Energy 33 (2008) 6696–6703. [2] M. Li, X. Liu, Y. Xiong, X. Bo, Y. Zhang, C. Han, L. Guo, Facile synthesis of various highly dispersive CoP nanocrystal embedded carbon matrices as efficient electrocatalysts for the hydrogen evolution reaction, J. Mater. Chem. A 3 (2015) 4255–4265. [3] M. Li, X. Bo, Y. Zhang, C. Han, L. Guo, Comparative study on the oxygen reduction reaction electrocatalytic activities of iron phthalocyanines supported on reduced graphene oxide, mesoporous carbon vesicle, and ordered mesoporous carbon, J. Power Sources 264 (2014) 114–122. [4] P. Liu, J.A. Rodriguez, Catalysts for hydrogen evolution from the [NiFe] hydrogenase to the Ni2P(001) surface: the importance of ensemble effect, J. Am. Chem. Soc. 127 (2005) 14871–14878.
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