CoP Nanosheet Arrays Supported on a Ti Plate: An Efficient Cathode

Jul 17, 2014 - chemical fuel with high energy density for the future.1,2. Electrochemical water ... (CoP/Ti). When utilized as a hydrogen evolution ca...
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CoP Nanosheet Arrays Supported on a Ti Plate: An Efficient Cathode for Electrochemical Hydrogen Evolution Zonghua Pu,† Qian Liu,† Ping Jiang,‡ Abdullah M. Asiri,§,∥ Abdullah Y. Obaid,§,∥ and Xuping Sun*,†,§,∥ †

Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical engineering, China West Normal University, Nanchong 637002, Sichuan China ‡ College of Chemistry, Chongqing Normal University, Chongqing 401331, Chongqing, China § Chemistry Department, Faculty of Science and ∥Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia S Supporting Information *

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cloth (CoP/CC) as an integrated hydrogen evolution cathode with superior catalytic performance to CoP/CNT via lowtemperature phosphidation of corresponding CoP/CC precursor.31 However, this work suffers from the use of a timeconsuming hydrothermal process for the precursor preparation. Electrodeposition is a simple, speedy and cheap method for preparing nanostructures film on conductive substrates. Herein, we report the preparation of α-Co(OH)2 nanosheet arrays on a Ti plate by electrodeposition at room temperature. The following low-temperature phosphidation of the α-Co(OH)2/ Ti leads to CoP nanosheet arrays supported on a Ti plate (CoP/Ti). When utilized as a hydrogen evolution cathode in acidic media, the CoP/Ti electrode exhibits high catalytic performance, good stability, and nearly 100% Faradaic efficiency (FE). It needs overpotentials (η) of 90 and 146 mV to afford current densities of 10 and 100 mA cm−2, respectively. Moreover, this electrode also performs well in neutral media. The electrodeposited Co precursor and its phosphided product were scratched down from the Ti plate for X-ray powder diffraction (XRD) analysis. Supporting Information Figure S1 shows the corresponding XRD patterns. The precursor shows four diffraction peaks characteristic of the αCo(OH)2 phase (JCPDS No. 46-0605).32,33 In contrast, the phosphided product only shows peaks at 31.6, 35.3, 36.3, 46.2, 48.1, 52.2, 56.0, and 56.7° corresponding to the (011), (200), (111), (112), (211), (103), (020), and (301) planes of the CoP phase (JCPDS No. 29−0497), respectively. Figure 1A-C show the scanning electron microscope (SEM) images of αCo(OH)2/Ti, indicating the Ti plate is covered by nanosheets with high density and that these nanosheets are solid with smooth surface and lie aslant or perpendicular and interconnected with each other on the Ti plate. After phosphidation, the array format was kept intact and the resulting CoP nanosheets are porous with a rough surface (Figure 1D−F), which could arise from the gas release and dehydration of the precursor during annealing.34 The energy dispersive X-ray (EDX) spectrum verifies that the atomic ratio between Co and P is close to 1:1 (Supporting Information

ydrogen is an ideal renewable and zero-emission chemical fuel with high energy density for the future.1,2 Electrochemical water splitting is a simple way to produce hydrogen on a large scale, but it requires using an efficient electrocatalyst for the hydrogen evolution reaction (HER) to afford high current at low overpotential.3 Although Pt-group metals are currently the most active HER catalysts, the scarcity and high cost of Pt limits its widespread use. The strongly acidic conditions in proton exchange membrane (PEM) technology need acid-stable HER catalysts compatible with PEM-based electrolysis units.4 Although abundant nickel based materials are active for hydrogen evolution,5,6 they suffer from poor corrosion stability in strongly acidic solutions. As such, it is highly attractive to develop acid-stable non-noble metal HER catalysts. Mo-based compounds are a big family of such catalysts with great success, including MoS2, MoSe2, Mo2C, MoB, NiMoNx/C, Co0.6Mo1.4N27−17 etc. Microbial water electrolysis needs catalysts efficiently at neutral pH.18 The development of HER catalysts operating in both acidic and neutral media is thus highly desired but still remains a challenging task. Transition metal phosphides (TMPs) are an important class of compounds with metalloid characteristics and good electrical conductivity19 and have been widely used as catalysts for hydrodesulfurization (HDS) and drodenitrogenation (HDN) reactions and anode materials for Li-ion batteries (LIBs).20,21 Reversible binding of the catalyst and hydrogen is involved in both HDS and HER, with hydrogen dissociation to yield H2S in HDS while with protons bound to the catalyst to promote hydrogen evolution in HER.22,23 This implies that TMPs could function as a HER catalyst. Recent works have indeed shown that Ni2P nanoparticles, FeP nanosheets, and CoP nanoparticles are active HER catalysts, but the catalyst preparation suffers from the involvement of several organic solvents and multiple tedious steps.24−27 We have developed a facile organic sovent-free strategy toward CoP nanocrystals/carbon nanotube (CoP/CNT) nanohybrid as a high-active HER catalyst via lowtemperature phosphidation of its Co3O4/CNT precursor.28 Prior to electrochemical tests, however, the above catalysts must be effectively immobilized on current collectors using a polymer binder. The polymer binder generally increases the series resistance29 and may block active sites and inhibit diffusion leading to reduced catalytic activity.30 We have solved such issues by directly growing CoP nanowire arrays on carbon © 2014 American Chemical Society

Received: April 7, 2014 Revised: July 17, 2014 Published: July 17, 2014 4326

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Figure 2. (A) Polarization curves for bare Ti, CoP/Ti, and Pt/C on the Ti plate in 0.5 M H2SO4 with a sweep rate of 2 mV s−1. Inset a: onset overpotential histograms for CoP/Ti and Pt/C. Inset b: current density histograms at 75 mV for CoP/Ti and Pt/C. (B) Tafel plots for CoP/Ti and Pt/C. Inset: Tafel value histograms for CoP/Ti and Pt/C. Error bars represent the standard deviations of three CoP/Ti samples and three measurements for each sample.

Figure 1. SEM images (A−C) of α-Co(OH)2/Ti and (D−F) of CoP/ Ti. (G) HRTEM image and (H) SAED pattern taken from the CoP nanosheet.

Figure S2). The high-resolution TEM (HRTEM) image taken from one CoP nanosheet (Figure 1G) reveals clear lattice fringes with interplanar spacings of 0.19 and 0.28 nm corresponding to the (211) and (011) planes of CoP, respectively.35 The selective area electron diffraction (SAED) pattern (Figure 1H) shows several bright rings made up of discrete spots, which can be indexed to the (011), (111), (211), and (301) planes of an orthorhombic CoP structure.36 We applied the CoP/Ti as a hydrogen evolution cathode to highlight the merits of the unique architecture. The linear scan voltammogram (LSV) measurements were performed using a three-electrode system in 0.5 M H2SO4 with a scan rate of 2 mV s−1. Commercial Pt/C on the Ti plate and bare Ti plate were also tested for comparison. A resistance test was made, and iR compensation was applied for all the electrochemical measurements. Figure 2A shows the polarization curves. The Pt/C exhibits the expected HER performance while the bare Ti plate shows little HER activity. In sharp contrast, the CoP/Ti electrode exhibits a small onset overpotential of 40 mV for the HER, and further negative potential leads to rapid rise of cathodic current density; thus, it needs overpotentials of 90 and 146 mV to attain current densities of 10 and 100 mA cm−2, respectively. These overpotentials compare favorably to the behavior of most reported non-noble metal HER catalysts in

acidic solutions (Supporting Information Table S1). It suggests that CoP/Ti functions as an efficient hydrogen evolution cathode. Note that the HER performance of this CoP/Ti electrode is also influenced by CoP loading, and the optimal loading was determined to be about 2.0 mg cm−2 (Supporting Information Figure S3). The electrode with catalyst loading of 2.0 mg cm−2 was measured to have the maximal electrochemically active surface area (EASA) according to the reported method37,38 (Supporting Information Table S2), suggesting the trend in performance tracks with surface area rather than mass loading of CoP. Figure 2B shows Tafel plots for CoP/Ti and Pt/C on Ti plate. Tafel plots were recorded with the linear regions fitted into the Tafel equation (η = a + b log j, where b is the Tafel slope and j is the current density).39 The Tafel slope of 30 mV dec−1 for Pt/C is consistent with the reported value.24 CoP/Ti exhibits a Tafel slope of 43 mV dec−1 in the region of η = 40− 120 mV. This value is smaller than those of most reported nonnoble metal HER catalysts listed in Supporting Information Table S1. The strong durability of catalyst is another important criterion by which to evaluate a catalyst. We thus probed the 4327

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durability of the CoP/Ti electrode by conducting continuous cyclic voltammetry (CV) scanning between +0.18 V and −0.22 V vs RHE at a scan rate of 100 mV s−1 in 0.5 M H2SO4. After the 1000th cycle, this electrode still performs as well as the first cycle with negligible loss of cathodic current density (Figure 3).

Figure 3. Polarization curves for CoP/Ti before and after continuous CV scanning of 1000 cycles from +0.18 V to −0.22 V vs RHE at a scan rate of 100 mV s−1 in 0.5 M H2SO4 (inset: time-dependent current density curve for the CoP/Ti at an overpotential of 150 mV for 10000 s).

The long-term stability of this electrode was also evaluated by electrolysis at a fixed overpotential of 150 mV, suggesting the current density remains at about 118 mA cm−2 over 10000 s with negligible degradation (Figure 3 inset). These results suggest superior stability of this CoP/Ti electrode in a longterm electrochemical process to amorphous MoS2.40 It is of importance to mention that the CoP/Ti electrode also performs well for the HER in neutral media. Figure 4A shows the polarization curves for CoP/Ti before and after continuous CV scanning of 1000 cycles from +0.27 V to −0.43 V vs RHE at a scan rate of 100 mV s−1 in 0.2 M PBS (pH 7). As observed, this electrode exhibits an onset overpotential of 100 mV and good durability. Furthermore, it needs overpotentials of 102 and 149 V to afford current densities of 2 and 10 mA cm−2, respectively. These overpotentials still compare favorably to the behavior of most reported Pt-free HER catalysts in neutral media (Supporting Information Table S3). The Tafel slope for this electrode is 58 mV dec−1 (Figure 4B), which is smallest among all HER catalysts listed in Supporting Information Table S3. We also determined the FE of the CoP/Ti electrode for hydrogen evolution using the reported method.28,31 The agreement of the amount of experimentally quantified hydrogen with theoretically calculated hydrogen (assuming 100% FE) shows that the FE is nearly 100% in both acidic and neutral solutions, as shown in Supporting Information Figure S4 and S5. Supporting Information Figure S6 shows the X-ray photoelectron spectroscopy (XPS) 2p spectra in the Co(2p) and P(2p) regions for CoP nanosheets. The Co(2p3/2) region shows two peaks at 781.2 and 778.9 eV, and the Co(2p1/2) region shows one peak at 794.0 eV. The high-resolution P(2p) region shows two peaks at 130.5 and 129.8 eV reflecting the binding energy (BE) of P 2p1/2 and P 2p3/2, respectively,41 along with one peak at 133.9 eV. The peaks at 778.9 and 129.8

Figure 4. (A) Polarization curves for CoP/Ti and Pt/C in 0.2 M PBS with a scan rate of 2 mV s−1. (B) Tafel plot for CoP/Ti.

eV are close to the BEs for Co and P in CoP.41 The peaks at 781.2 and 133.9 eV can be attributed to oxidized Co and P species due to surface oxidation of CoP.42,43 The Co 2p BE of 778.9 eV exhibits a positive shift from that of Co metal (778.1− 778.2 eV) while the P 2p BE of 129.8 eV exhibits a negative shift from that of elemental P (130.2 eV).44 Such observation suggests that the Co and P have a partial positive and negative charge, respectively, in CoP because of electron transfer from Co to P.43,45 Indeed, calculations and electron density maps have also suggested covalency for Co−P bonds with charge separation due to charge transfer from Co to P.43,46 Hydrogenase features pendant bases proximate to the metal centers as active sites47 and metal complex HER catalyst incorporates proton relays from pendant acid−base groups close to the metal center where hydrogen evolution occurs.48,49 Because CoP also has pendant base P close proximity to the metal center Co, it is reasonable to conclude that CoP adopts a similar catalytic mechanism with hydrogenase and metal complex catalyst toward the HER.28,31 In summary, we have shown CoP/Ti can be chemically converted from its α-Co(OH)2/Ti precursor via a lowtemperature phosphidation reaction. As an integrated hydrogen evolution cathode, such CoP/Ti electrode exhibits high catalytic performance in acidic electrolytes with good durability and nearly 100% FE. It is also highly active in neutral solutions. Our present study would open up new avenues to explore the design of self-supported electrode of TMPs toward applications 4328

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as catalysts for hydrogen evolution, anode materials for LIBs,21 and counter electrodes for dye-sensitized solar cells.50

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(27) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Angew. Chem., Int. Ed. 2014, 53, 5427. (28) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Angew. Chem., Int. Ed. 2014, 53, 6710. (29) Luo, Y.; Jiang, J.; Zhou, W.; Yang, H.; Luo, J.; Qi, X.; Zhang, H.; Denis, Y.; Li, C. M.; Yu, T. J. Mater. Chem. 2012, 22, 8634. (30) Roy-Mayhew, J. D.; Boschloo, G.; Hagfeldt, A.; Aksay, I. A. ACS Appl. Mater. Interfaces 2012, 4, 2794. (31) Tian, J.; Liu, Q.; Asiri, A. N.; Sun, X. J. Am. Chem. Soc. 2014, 136, 7587. (32) Gupta, V.; Kusahara, T.; Toyama, H.; Gupta, S.; Miura, N. Electrochem. Commun. 2007, 9, 2315. (33) Zhang, J.; Kong, L. B.; Cai, J.; Luo, Y.; Kang, L. J. Solid State Electron. 2010, 14, 2065. (34) Li, L.; Seng, K. H.; Chen, Z.; Guo, Z.; Liu, H. Nanoscale 2013, 5, 1922. (35) Maneeprakorn, W.; Malik, M. A.; O’Brien, P. J. Mater. Chem. 2010, 20, 2329. (36) Li, Y.; Malik, M. A.; O’Brien, P. J. Am. Chem. Soc. 2005, 127, 16020. (37) Liu, W. C.; Lin, H. K.; Chen, Y. L.; Lee, C. Y.; Chiu, H. T. ACS Nano 2010, 4, 4149. (38) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nat. Commun. 2013, 4, 1. (39) Trasatti, S., Ed. Electrodes of Conductive Metallic Oxides; Elsevier: New York, 1981. (40) Benck, J. D.; Chen, Z.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. ACS Catal. 2012, 2, 1916. (41) Grosvenor, A. P.; Wik, S. D.; Cavell, R. G.; Mar, A. Inorg. Chem. 2005, 44, 8988. (42) Korányi, T. Appl. Catal., A 2003, 239, 253. (43) Wang, J.; Yang, Q.; Zhang, Z.; Sun, S. Chem.Eur. J. 2010, 16, 7916. (44) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; John Wiley & Sons: New York, 1983. (45) Burns, A. W.; Layman, K. A.; Bale, D. H.; Bussell, M. E. Appl. Catal., A 2008, 343, 68. (46) Yang, Z.; Liu, L.; Wang, X.; Yang, S.; Su, X. J. Alloys Compd. 2011, 509, 165. (47) Nicolet, Y.; Lacey, A. L.; Vérnede, X.; Fernandez, V. M.; Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123, 1596. (48) Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.; Dubois, M. R.; Dubois, D. L. J. Am. Chem. Soc. 2006, 128, 358. (49) Wilson, A. D.; Shoemaker, R. K.; Miedaner, A.; Muckerman, J. T.; Dubois, D. L.; Dubois, M. R. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6951. (50) Faber, M. S.; Park, K.; Cabán-Acevedo, M.; Santra, P. K.; Jin, S. J. Phys. Chem. Lett. 2013, 4, 1843.

ASSOCIATED CONTENT

S Supporting Information *

Experimental details; XRD patterns; EDX and XPS spectra; Tables S1−S3; polarization curves; FE determination. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414, 33. (2) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729. (3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110, 6446. (4) Goff, A. Le; Artero, V.; Jousselme, B.; Tran, P. D.; Guillet, N.; Métayé, R.; Fihri, A.; Palacin, S.; Fontecave, M. Science 2009, 326, 1384. (5) Brown, D. E.; Mahmood, M. N.; Man, M. C. M.; Turner, A. K. Electrochim. Acta 1984, 29, 1551. (6) Raj, I. A.; Vasu, K. I. J. Appl. Electrochem. 1990, 20, 32. (7) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Science 2007, 317, 100. (8) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Nat. Mater. 2012, 11, 963. (9) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. J. Am. Chem. Soc. 2013, 135, 10274. (10) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X.; Xie, Y. Adv. Mater. 2013, 25, 5807. (11) Bonde, J.; Moses, P. G.; Jaramillo, T. F.; Norskov, J. K.; Chorkendorff, I. Faraday Discuss. 2009, 140, 219. (12) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. J. Am. Chem. Soc. 2011, 133, 7296. (13) Chen, Z.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T. F. Nano Lett. 2011, 11, 4168. (14) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Nano Lett. 2013, 13, 1341. (15) Vrubel, H.; Hu, X. Angew. Chem., Int. Ed. 2012, 51, 12703. (16) Chen, W.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Angew. Chem., Int. Ed. 2012, 51, 6131. (17) Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G. J. Am. Chem. Soc. 2013, 135, 19186. (18) Kundu, A.; Sahu, J. N.; Redzwan, G.; Hashim, M. A. Int. J. Hydrogen Energy 2013, 38, 1745. (19) Oyama, S. T.; Gott, T.; Zhao, H.; Lee, Y. K. Catal. Today 2009, 143, 94. (20) Prins, R.; Bussell, M. E. Catal. Lett. 2012, 142, 1413. (21) Carenco, S.; Portehault, D.; Boissière, C.; Mèzailles, N.; Sanchez, C. Chem. Rev. 2013, 113, 7981. (22) Liu, P.; Rodriguez, J. A.; Asakura, T.; Gomes, J.; Nakamura, K. J. Phys. Chem. B 2005, 109, 4575. (23) Liu, P.; Rodriguez, J. A. J. Am. Chem. Soc. 2005, 127, 14871. (24) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. J. Am. Chem. Soc. 2013, 135, 9267. (25) Feng, L.; Vrubel, H.; Bensimon, M.; Hu, X. Phys. Chem. Chem. Phys. 2014, 16, 5917. (26) Xu, Y.; Wu, R.; Zhang, J.; Shi, Y.; Zhang, B. Chem. Commun. 2013, 49, 6656. 4329

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