- Email: [email protected]
Electrodeposition of nickel–phosphorus nanoparticles film as a Janus electrocatalyst for electro-splitting of water
Journal of Power Sources 299 (2015) 342e346
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Electrodeposition of nickelephosphorus nanoparticles film as a Janus electrocatalyst for electro-splitting of water Qian Liu a, b, Shuang Gu a, b, Chang Ming Li a, b, * a b
Institute for Clean Energy & Advanced Materials and Faculty of Materials and Energy, Southwest University, Chongqing 400715, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, China
h i g h l i g h t s NieP/CF can act as bifunctional HER and OER electrocatalyst in basic electrolytes. NieP/CF exhibits high catalytic activity towards both HER and OER. NieP/CF can achieve 10 mA cm2 water-splitting current at a cell voltage of 1.68 V.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2015 Received in revised form 4 September 2015 Accepted 7 September 2015 Available online xxx
Nickelephosphorus nanoparticles film on copper foam (NieP/CF) was prepared by electrodeposition. This electrocatalyst shows high catalytic activity and durability toward both hydrogen and oxygen evolution reactions in basic electrolytes. The results show that NieP/CF can deliver a current density of 10 mA cm2 at an overpotential of 98 mV for hydrogen production and 325 mV for oxygen generating. A two-electrode water electrolyzer using NieP/CF as cathode and anode produces 10 mA cm2 at a cell voltage of 1.68 V with high stability. © 2015 Published by Elsevier B.V.
Keywords: Nickelephosphorus Electrocatalyst Water splitting Hydrogen evolution reaction Oxygen evolution reaction
1. Introduction Clean, renewable and sustainable energy sources are very necessary for human society due to the depletion of fossil fuels and the increased environmental concerns [1]. Hydrogen, as the best candidate to replace fossil fuels, can be simply produced by water splitting [2]. Electro-splitting of water powered by electric energy has attracted extensive attention because this process can convert electric energy into chemical energy for easier storage and delivery. The water splitting process can be divided into two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Both reactions are crucial for the overall efficiency of water splitting. Although water splitting only needs a theoretical minimum voltage of 1.23 V, commercial electrolyzers typically
* Corresponding author. Institute for Clean Energy & Advanced Materials and Faculty of Materials and Energy, Southwest University, Chongqing 400715, China. E-mail address: [email protected] (C.M. Li). http://dx.doi.org/10.1016/j.jpowsour.2015.09.027 0378-7753/© 2015 Published by Elsevier B.V.
operate at a much higher value of 1.8e2.0 V because it is a strongly uphill reaction with large overpotential [3,4]. Thus, efficient HER and OER electrocatalysts are of vital importance to overcome the large overpotentials. Currently, state-of-the-art HER electrocatalysts are Pt-based materials and OER electrocatalysts are Ru- or Ir-based materials, but the scarcity and high cost of such catalysts limit their mass uses. Therefore, tremendous efforts have been made to develop efficient earth-abundant HER (sulfide [5,6], phosphide [7e10], nitride [11,12], selenide [5,13e15]) and OER (oxide [16e18], hydroxide [19e22], sulfide [23,24] selenide [25]) electrocatalysts. Water splitting should be performed in either strongly acidic or alkaline solution to minimize the overpotentials [26]. Alkaline water splitting has emerged as a strong candidate for commercialization toward mass hydrogen production [27]. Utilizing a same electrocatalyst for both HER and OER has advantages of simplifying the system and lowering the cost. It is thus highly attractive to develop non-precious metal electrocatalyst efficient for both HER
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
and OER in strongly alkaline electrolytes. Although nickel (Ni) has emerged as an interesting non-noble metal for its catalytic power toward HER and OER, their performance needs to be further improved [4]. In this communication, we demonstrate nickelephosphorus nanoparticles film electrochemically deposited on copper foam (NieP/CF) behaves as an efficient HER and OER electrocatalyst with good durability in strongly alkaline solutions. This electrode needs HER overpotential of 98 mV and OER overpotential of 325 mV to achieve current density of 10 mA cm2. A stable two-electrode water electrolyzer made from NieP/CF affords 10 mA cm2 watersplitting current at a cell voltage of 1.68 V.
343
NieP/CF was directly used as work electrode, and a graphite plate as counter electrode and a SCE as reference electrode. In all measurements, the SCE reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). In 1.0 M KOH solution, E (RHE) ¼ 0.242 þ 0.059 pH. For overall water splitting, electrochemical measurements were carried out in a two-electrode setup in 1.0 M KOH solution with using NieP/CF as anode and cathode, respectively. Pt/C and RuO2 ink were prepared by dispersing 10 mg of Pt/C or RuO2 in 480 mL of water/ethanol (v/v ¼ 1:1) with 20 mL of 5 wt % Nafion solution. Then 37.5 mL of the Pt/C or RuO2 ink was loaded onto a piece of CF and air-dried at room temperature for HER or OER measurements.
2. Experimental 3. Results and discussion 2.1. Reagents and materials CF was purchased from Kunshan Desco Electronics Co., Ltd. NiSO4$6H2O, KOH and NaOAc were obtained from Beijing Chemical corporation. NaH2PO2 and RuCl3$3H2O were bought from Aladdin Co., Ltd. (Shanghai, China). Pt/C (20 wt% Pt on Vulcan XC-72R) and Nafion (5 wt%) were purchased from SigmaeAldrich Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. 2.2. Synthesis of NieP/CF Electrolyte solution for electrodeposition was prepared by dissolving 1.3142 g of NiSO4$6H2O, 0.4101 g NaOAc, and 2.1997 g NaH2PO2 were dissolved in 50 mL H2O. Prior to electrodeposition, CF was washed with ethanol and water several times to remove the surface impurities. The electrodeposition was carried out in a standard three-electrode setup, with using a piece of CF, a graphite plate and a saturated calomel electrode (SCE) as work, counter and reference electrode, respectively. The CV was cycled 15 times between 1.0 and 0.3 V with a scan rate of 10 mV s1. After electrodeposition, the CF was carefully withdrawn from the electrolyte solution, rinsed with water and ethanol and dried at room temperature. 2.3. Synthesis of RuO2 1.037 g of RuCl3$3H2O was dissolved in 50 mL H2O and heated at 100 C under air atmosphere. After 10 min, 0.5 mL of 1.0 M KOH solution was added and kept stirring for 45 min. Then the precipitates were collected by centrifugation, washed with water and ethanol repeatedly for several times. This product was dried at 80 C for 5 h and then calcined in air at 300 C for 3 h. 2.4. Characterization Scanning electron microscopy measurements were made on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. X-ray powder diffraction data were acquired by a RigakuD/MAX 2550 diffractometer with Cu Ka radiation (l ¼ 1.5418 Å). X-ray photoelectron spectroscopy measurements were performed on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the exciting source. 2.5. Electrochemical measurements The HER and OER electrochemical measurements were performed with a CHI 660E electrochemistry workstation (CH Instruments, Inc., Shanghai) in a standard three-electrode setup. The
The CF changed in color from brick-red to dark grey after electrodeposition (Fig. S1a). The X-ray powder diffraction (XRD) pattern of this product is similar to bare CF (Fig. S1b), indicating NieP nanoparticles film is amorphous. Scanning electron microscopy (SEM) images of the NieP/CF (Fig. 1a and b) reveal that the skeleton of CF is fully covered with smooth-surface NieP nanoparticles film. The corresponding energy dispersive X-ray (EDX) spectrum (Fig. S2) indicates the existence of Ni, P, O, and Cu elements (Cu signal arising from the CF substrate). EDX elemental mapping analysis (Fig. 1c) suggests that Ni and P elements are uniformly distributed in the NieP film. X-ray photoelectron spectroscopy (XPS) survey spectrum of NieP/CF indicates the presence of Ni, P, O, Cu, and C elements (Fig. S3a). The existence of C and O element is attributed to the contamination of the product and oxidized Ni and P species formed at the surface exposed to atmosphere (Fig. S3a and S3b) [28]. The XPS spectrum in Ni 2p region (Fig. 1d) shows two peaks at 852.1 and 869.1 eV, which can be assigned to metallic Ni. The binding energies (BEs) of 856.4 and 874.2 eV correspond to Ni (II). The P 2p region (Fig. 1e) exhibits two peaks at 128.9 and 129.8 eV reflecting the BEs of P 2p3/2 and P 2p1/2, respectively. The BE at 128.9 eV suggests the formation of phosphide [29] and the broad peak at about 133.1 eV is assigned to phosphate [30]. All these results suggest that as-prepared film consisting of metallic Ni and nickel phosphide [31]. The HER activity of NieP/CF (NieP loading: ~5 mg cm2) was measured in a standard three-electrode setup in 1.0 M KOH solution. For reference purposes, bare CF and commercial Pt/C (20 wt% Pt/XC-72) deposited on CF were also tested. Because as-measured reaction currents cannot reflect the intrinsic behavior of electrocatalysts due to the effect of ohmic resistance, resistance tests were made for iR correction of all initial data for further analysis [32]. Fig. 2a presents the polarization curves on the reversible hydrogen electrode (RHE) scale. As can be seen, the commercial Pt/C reveals excellent activity with negligible HER overpotentials. Bare CF almost has no HER activity. In contrast, the hydrogen evolution on NieP/CF electrode begins at a low overpotential about 55 mV, beyond which the cathodic current dramatically increases, and this electrode only needs overpotential of 98 mV to achieve current density of 10 mA cm2. This overpotential compares favorably to behavior of reported Ni-based HER electrocatalysts including Ni(OH)2 on Ni foam (Ni(OH)2/NF, ~250 mV) and NiFe LDH on Ni foam (NiFe LDH/NF, ~210 mV) [4] and HER electrocatalysts like crystalline NiP2 (102 mV) [33] in 1.0 M KOH. The Tafel slope for NieP/CF is 55 mV dec1 (Fig. 2b), which is smaller than that of crystalline NiP2 (64 mV dec1) [33]. Because good durability is an important criterion for electrocatalyst assessment, we measured the durability of NieP/CF. The NieP/CF electrode was continuously cycled between 0.4 V and 0 V versus RHE with a scan rate of 100 mV s1 in 1.0 M KOH solution for 1000 cycles. After cycling,
344
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
Fig. 1. (a) Low and (b) high magnification SEM images of NieP/CF. (c) EDX elemental mapping images of Cu, Ni, and P in NieP/CF. XPS spectra in the (d) Ni 2p and (e) P 2p regions for NieP/CF.
Fig. 2. (a) Polarization curves for NieP/CF, Pt/C, and CF in 1.0 M KOH solution with a scan rate of 2 mV s1. (b) Tafel plots of NieP/CF and Pt/C. (c) Polarization curves for NieP/CF before and after 1000 CV cycles. Inset: SEM image of NieP film after 1000 CV cycles. (d) Time-dependent current density curve for NieP/CF under static overpotential of 118 mV for 15 h.
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
NieP/CF electrode reveals a polarization curve similar to the initial one (Fig. 2c). The SEM image of the inset in Fig. 2c shows that NieP nanoparticles film maintains the morphology before 1000 CV cycles. We also tested the durability of this electrode by electrolysis at a fixed overpotential of 118 mV and this electrode maintains its catalytic activity for at least 15 h (Fig. 2d). These results reveal the good long-term stability of this NieP/CF HER electrode in 1.0 M KOH. Next, the OER activity of NieP/CF, bare CF, and RuO2 immobilized on CF were also assessed in the same electrolyte. All the polarization curves in Fig. 3 were also iR-corrected. As shown in Fig. 3a, NieP/CF reveals a sharp onset of OER current at overpotential approximately 294 mV. And it requires an overpotential of 325 mV to achieve a current density of 10 mA cm2. This overpotential compares favorably to other reported Ni-based OER electrocatalysts like Ni(OH)2/NF (~372 mV) [4], NiCo2O4 (~391 mV) [16], Ni0.9Fe0.1Ox (336 mV) [34]. and Ni@NC (390 mV) [35]. Note that bare CF shows much inferior OER activity to NieP/CF. Although RuO2 exhibits smaller onset overpotential (235 mV) and needs smaller overpotential (286 mV) to obtain 10 mA cm2, its performance is exceeded by that of NieP/CF beyond potential of 1.670 V vs. RHE. The Tafel slopes for NieP/CF, bare CF, and RuO2 are 120, 159, and 88 mV dec1, respectively (Fig. 3b). Similarly, continuous CV cycles and long-term electrolysis experiment were also carried out to assess the durability of this NieP/CF OER electrocatalyst. As shown in Fig. 3c and d, NieP/CF electrode still has good stability as OER eletrocatalyst. After 1000 CV cycles, the NieP film still keeps the nanoparticles film morphology (the inset in Fig. 3c), albeit with a rougher surface. Generally, the most active non-noble metal (Fe, Co, Ni) based electrocatalysts for the OER are metal oxyhydroxides. The Raman spectra analysis of NieP/CF (Fig. S4) after 1000 continuous CV scans reveals the formation of NiOOH [36]. The XPS analysis for the post-OER NieP/CF (Fig. S5) further confirms the formation of Ni(III) and an increase in intensity of O 1s peak. Zhou
345
et al. reported Ni3S2 nanorods can catalyze water oxidation but NiOOH species in situ formed on Ni3S2 surface also act as active site [37]. It is thus reasonable to conclude that the formed NiOOH species may also serve as the active OER site in our present study. Based on the good catalytic performance of NieP/CF towards both HER and OER, we expect that NieP/CF can be used as an active electrocatalyst for overall water splitting. Thus, we made a twoelectrode water electrolyzer using NieP/CF as both cathode and anode. For comparison, we also made another electrolyzer using Pt/ C on CF as cathode and RuO2 on CF as anode (Pt/CkRuO2). As shown in Fig. 4a, the NieP/CFkNieP/CF system affords current density of 10 mA cm2 at a cell voltage of 1.68 V with vigorous gas evolution on both electrodes (Fig. 4b and Movie S1). Although this voltage is larger than that of Pt/CkRuO2 system (1.56 V), it is smaller than that for NiFe LDH/NFkNiFe LDH/NF (1.70 V) [4] and Ni(OH)2/ NFkNi(OH)2/NF (~1.82 V) [4]. The long-term electrochemical durability was probed by electrolysis under a constant current density of 10 mA cm2 in 1.0 M KOH solution. This NieP/CFkNieP/CF water electrolysis system can maintain excellent stability at least for 15 h (inset of Fig. 4a). These results suggest that our eletrocatalyst can drive full water splitting. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.09.027. 4. Conclusions In summary, NieP nanoparticles film has been electrochemically deposited on CF as an efficient and stable elecrtocatalyst for both HER and OER in alkaline solutions. This NieP/CF electrode needs overpotential of 98 mV for HER and 325 mV for OER to achieve current density of 10 mA cm2. Furthermore, a durable alkaline water electrolyzer using NieP/CF as cathode and anode is demonstrated to afford current density of 10 mA cm2 at a cell voltage of 1.68 V. Our work would open up a new avenue to
Fig. 3. (a) Polarization curves for NieP/CF, RuO2, and CF in 1.0 M KOH solution with a scan rate of 2 mV s1. (b) Tafel plots of NieP/CF, RuO2, and CF. (c) Polarization curves for NieP/ CF before and after 1000 CV cycles. Inset: SEM image of NieP film after 1000 CV cycles. (d) Time-dependent current density curve for NieP/CF under static overpotential of 357 mV for 15 h.
346
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
Fig. 4. (a) Polarization curves for NieP/CFkNieP/CF and Pt/CkRuO2 in a two-electrode setup for overall water splitting with a scan rate of 2 mV s1. Inset: Chronopotentiometric curve of NieP/CF water splitting system with constant current density of 10 mA cm2. (b) NieP/CFkNieP/CF water-splitting system driven by direct-current power supply at 1.68 V.
fabricate Ni-based non-precious metal electrocatalysts for water splitting applications. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.027. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J. Chow, R.J. Kopp, P.R. Portney, Science 302 (2003) 1528e1531. N. Armaroli, V. Balzani, ChemSusChem 4 (2011) 21e36. S.M. Barnett, K.I. Goldberg, J.M. Mayer, Nat. Chem. 4 (2012) 498e502. J. Luo, J.-H. Im, M.T. Mayer, M. Schreier, M.K. Nazeeruddin, N.-G. Park, D. Tilley, H. Fan, M. Gr€ atzel, Science 345 (2014) 1593e1596. D. Kong, J.J. Cha, H. Wang, H.R. Lee, Y. Cui, Energy Environ. Sci. 6 (2013) 3553e3558. D. Merki, S. Fierro, H. Vrubel, X. Hu, Chem. Sci. 2 (2011) 1262e1267. Q. Liu, J. Tian, W. Cui, P. Jiang, N. Cheng, A.M. Asiri, X. Sun, Angew. Chem. Int. Ed. 53 (2014) 6710e6714. E.J. Popczun, C.G. Read, C.W. Roske, N.S. Lewis, R.E. Schaak, Angew. Chem. Int. Ed. 53 (2014) 5427e5430. E.J. Popczun, J.R. McKone, C.G. Read, A.J. Biacchi, A.M. Wiltrout, N.S. Lewis, R.E. Schaak, J. Am. Chem. Soc. 135 (2013) 9267e9270. Z. Xing, Q. Liu, A.M. Asiri, X. Sun, Adv. Mater. 26 (2014) 5702e5707. W. Chen, J.T. Muckerman, E. Fujita, Chem. Commun. 49 (2013) 8896e8909. B. Cao, G.M. Veith, J.C. Neuefeind, R.R. Adzic, P.G. Khalifah, J. Am. Chem. Soc. 135 (2013) 19186e19192. Y. Xu, M. Gao, Y. Zheng, J. Jiang, S. Yu, Angew. Chem. Int. Ed. 52 (2013) 1e6. D. Kong, H. Wang, Z. Lu, Y. Cui, J. Am. Chem. Soc. 136 (2014) 4897e4900. Q. Liu, J. Shi, J. Hu, A.M. Asiri, Y. Luo, X. Sun, ACS Appl. Mater. Interfaces 7 (2015) 3877e3881. Z. Peng, D. Jia, A.M. Al-Enizi, A.A. Elzatahry, G. Zheng, Adv. Energy Mater. 10. 1002/aenm.201402031.
[17] Y. Meng, W. Song, H. Huang, Z. Ren, S. Chen, S.L. Suib, J. Am. Chem. Soc. 136 (2014) 11452e11464. €blinger, [18] K. Fominykh, P. Chernev, I. Zaharieva, J. Sicklinger, G. Stefanic, M. Do €cklein, C. Scheu, T. Bein, D. Fattakhova-Rohlfing, A. Müller, A. Pokharel, S. Bo ACS Nano 9 (2015) 5180e5188. [19] X. Lu, C. Zhao, Nat. Commun. 6 (2015) 6616. [20] B.Y. Jeong, D.Y. Shin, J.K. Lee, D.H. Kim, Y.D. Kim, J.Y. Lee, Phys. Chem. Chem. Phys. 16 (2014) 13807e13813. [21] F. Song, X. Hu, J. Am. Chem. Soc. 136 (2014) 16481e16484. [22] X. Long, J. Li, S. Xiao, K. Yan, Z. Wang, H. Chen, S. Yang, Angew. Chem. Int. Ed. 53 (2014) 7584e7588. [23] W. Zhou, X. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, J. Wang, H. Zhang, Energy Environ. Sci. 6 (2013) 2921e2924. [24] Z. Zhang, X. Wang, G. Cui, A. Zhang, X. Zhou, H. Xu, L. Gu, Nanoscale 6 (2014) 3540e3544. [25] M. Gao, X. Cao, Q. Gao, Y. Xu, Y. Zheng, J. Jiang, S. Yu, ACS Nano 8 (2014) 3970e3978. [26] E.A. Hern andez-Pag an, N.M. Vargas-Barbosa, T.H. Wang, Y. Zhao, E.S. Smotkin, T.E. Mallouk, Energy Environ. Sci. 5 (2012) 7582e7589. [27] K. Zeng, D. Zhang, Prog. Energy Combust. Sci. 36 (2010) 307e326. [28] A. Panneerselvam, M.A. Malik, M. Afzaal, P. O'Brien, M. Helliwell, J. Am. Chem. Soc. 130 (2008) 2420e2421. [29] A.P. Grosvenor, S.D. Wik, R.G. Cavell, A. Mar, Inorg. Chem. 44 (2005) 8988e8998. [30] Q. Yu, Z. Zeng, W. Zhao, Y. Ma, X. Wu, Q. Xue, ACS Appl. Mater. Interfaces 6 (2014) 1053e1060. [31] J. Xiang, J. Tu, X. Wang, X. Huang, Y. Yuan, X. Xia, Z. Zeng, J. Power Sources 185 (2008) 519e525. [32] Z. Xing, Q. Liu, A.M. Asiri, X. Sun, Adv. Mater. 26 (2014) 5702e5707. [33] P. Jiang, Q. Liu, X. Sun, Nanoscale 6 (2014) 13440e13445. [34] L. Trotochaud, J.K. Ranney, K.N. Williams, S.W. Boettcher, J. Am. Chem. Soc. 134 (2012) 17253e17261. [35] J. Ren, M. Antonietti, T.P. Fellinger, Adv. Energy Mater. 5 (2015) 1401660. [36] K.S. Joya, X. Salac, 17 (2015) 21094e21103. [37] W. Zhou, X. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, L. Wang, H. Zhang, Energy Environ. Sci. 6 (2013) 2921e2924.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Electrodeposition of nickelephosphorus nanoparticles film as a Janus electrocatalyst for electro-splitting of water Qian Liu a, b, Shuang Gu a, b, Chang Ming Li a, b, * a b
Institute for Clean Energy & Advanced Materials and Faculty of Materials and Energy, Southwest University, Chongqing 400715, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, China
h i g h l i g h t s NieP/CF can act as bifunctional HER and OER electrocatalyst in basic electrolytes. NieP/CF exhibits high catalytic activity towards both HER and OER. NieP/CF can achieve 10 mA cm2 water-splitting current at a cell voltage of 1.68 V.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2015 Received in revised form 4 September 2015 Accepted 7 September 2015 Available online xxx
Nickelephosphorus nanoparticles film on copper foam (NieP/CF) was prepared by electrodeposition. This electrocatalyst shows high catalytic activity and durability toward both hydrogen and oxygen evolution reactions in basic electrolytes. The results show that NieP/CF can deliver a current density of 10 mA cm2 at an overpotential of 98 mV for hydrogen production and 325 mV for oxygen generating. A two-electrode water electrolyzer using NieP/CF as cathode and anode produces 10 mA cm2 at a cell voltage of 1.68 V with high stability. © 2015 Published by Elsevier B.V.
Keywords: Nickelephosphorus Electrocatalyst Water splitting Hydrogen evolution reaction Oxygen evolution reaction
1. Introduction Clean, renewable and sustainable energy sources are very necessary for human society due to the depletion of fossil fuels and the increased environmental concerns [1]. Hydrogen, as the best candidate to replace fossil fuels, can be simply produced by water splitting [2]. Electro-splitting of water powered by electric energy has attracted extensive attention because this process can convert electric energy into chemical energy for easier storage and delivery. The water splitting process can be divided into two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Both reactions are crucial for the overall efficiency of water splitting. Although water splitting only needs a theoretical minimum voltage of 1.23 V, commercial electrolyzers typically
* Corresponding author. Institute for Clean Energy & Advanced Materials and Faculty of Materials and Energy, Southwest University, Chongqing 400715, China. E-mail address: [email protected] (C.M. Li). http://dx.doi.org/10.1016/j.jpowsour.2015.09.027 0378-7753/© 2015 Published by Elsevier B.V.
operate at a much higher value of 1.8e2.0 V because it is a strongly uphill reaction with large overpotential [3,4]. Thus, efficient HER and OER electrocatalysts are of vital importance to overcome the large overpotentials. Currently, state-of-the-art HER electrocatalysts are Pt-based materials and OER electrocatalysts are Ru- or Ir-based materials, but the scarcity and high cost of such catalysts limit their mass uses. Therefore, tremendous efforts have been made to develop efficient earth-abundant HER (sulfide [5,6], phosphide [7e10], nitride [11,12], selenide [5,13e15]) and OER (oxide [16e18], hydroxide [19e22], sulfide [23,24] selenide [25]) electrocatalysts. Water splitting should be performed in either strongly acidic or alkaline solution to minimize the overpotentials [26]. Alkaline water splitting has emerged as a strong candidate for commercialization toward mass hydrogen production [27]. Utilizing a same electrocatalyst for both HER and OER has advantages of simplifying the system and lowering the cost. It is thus highly attractive to develop non-precious metal electrocatalyst efficient for both HER
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
and OER in strongly alkaline electrolytes. Although nickel (Ni) has emerged as an interesting non-noble metal for its catalytic power toward HER and OER, their performance needs to be further improved [4]. In this communication, we demonstrate nickelephosphorus nanoparticles film electrochemically deposited on copper foam (NieP/CF) behaves as an efficient HER and OER electrocatalyst with good durability in strongly alkaline solutions. This electrode needs HER overpotential of 98 mV and OER overpotential of 325 mV to achieve current density of 10 mA cm2. A stable two-electrode water electrolyzer made from NieP/CF affords 10 mA cm2 watersplitting current at a cell voltage of 1.68 V.
343
NieP/CF was directly used as work electrode, and a graphite plate as counter electrode and a SCE as reference electrode. In all measurements, the SCE reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). In 1.0 M KOH solution, E (RHE) ¼ 0.242 þ 0.059 pH. For overall water splitting, electrochemical measurements were carried out in a two-electrode setup in 1.0 M KOH solution with using NieP/CF as anode and cathode, respectively. Pt/C and RuO2 ink were prepared by dispersing 10 mg of Pt/C or RuO2 in 480 mL of water/ethanol (v/v ¼ 1:1) with 20 mL of 5 wt % Nafion solution. Then 37.5 mL of the Pt/C or RuO2 ink was loaded onto a piece of CF and air-dried at room temperature for HER or OER measurements.
2. Experimental 3. Results and discussion 2.1. Reagents and materials CF was purchased from Kunshan Desco Electronics Co., Ltd. NiSO4$6H2O, KOH and NaOAc were obtained from Beijing Chemical corporation. NaH2PO2 and RuCl3$3H2O were bought from Aladdin Co., Ltd. (Shanghai, China). Pt/C (20 wt% Pt on Vulcan XC-72R) and Nafion (5 wt%) were purchased from SigmaeAldrich Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. 2.2. Synthesis of NieP/CF Electrolyte solution for electrodeposition was prepared by dissolving 1.3142 g of NiSO4$6H2O, 0.4101 g NaOAc, and 2.1997 g NaH2PO2 were dissolved in 50 mL H2O. Prior to electrodeposition, CF was washed with ethanol and water several times to remove the surface impurities. The electrodeposition was carried out in a standard three-electrode setup, with using a piece of CF, a graphite plate and a saturated calomel electrode (SCE) as work, counter and reference electrode, respectively. The CV was cycled 15 times between 1.0 and 0.3 V with a scan rate of 10 mV s1. After electrodeposition, the CF was carefully withdrawn from the electrolyte solution, rinsed with water and ethanol and dried at room temperature. 2.3. Synthesis of RuO2 1.037 g of RuCl3$3H2O was dissolved in 50 mL H2O and heated at 100 C under air atmosphere. After 10 min, 0.5 mL of 1.0 M KOH solution was added and kept stirring for 45 min. Then the precipitates were collected by centrifugation, washed with water and ethanol repeatedly for several times. This product was dried at 80 C for 5 h and then calcined in air at 300 C for 3 h. 2.4. Characterization Scanning electron microscopy measurements were made on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. X-ray powder diffraction data were acquired by a RigakuD/MAX 2550 diffractometer with Cu Ka radiation (l ¼ 1.5418 Å). X-ray photoelectron spectroscopy measurements were performed on an ESCALABMK II X-ray photoelectron spectrometer using Mg as the exciting source. 2.5. Electrochemical measurements The HER and OER electrochemical measurements were performed with a CHI 660E electrochemistry workstation (CH Instruments, Inc., Shanghai) in a standard three-electrode setup. The
The CF changed in color from brick-red to dark grey after electrodeposition (Fig. S1a). The X-ray powder diffraction (XRD) pattern of this product is similar to bare CF (Fig. S1b), indicating NieP nanoparticles film is amorphous. Scanning electron microscopy (SEM) images of the NieP/CF (Fig. 1a and b) reveal that the skeleton of CF is fully covered with smooth-surface NieP nanoparticles film. The corresponding energy dispersive X-ray (EDX) spectrum (Fig. S2) indicates the existence of Ni, P, O, and Cu elements (Cu signal arising from the CF substrate). EDX elemental mapping analysis (Fig. 1c) suggests that Ni and P elements are uniformly distributed in the NieP film. X-ray photoelectron spectroscopy (XPS) survey spectrum of NieP/CF indicates the presence of Ni, P, O, Cu, and C elements (Fig. S3a). The existence of C and O element is attributed to the contamination of the product and oxidized Ni and P species formed at the surface exposed to atmosphere (Fig. S3a and S3b) [28]. The XPS spectrum in Ni 2p region (Fig. 1d) shows two peaks at 852.1 and 869.1 eV, which can be assigned to metallic Ni. The binding energies (BEs) of 856.4 and 874.2 eV correspond to Ni (II). The P 2p region (Fig. 1e) exhibits two peaks at 128.9 and 129.8 eV reflecting the BEs of P 2p3/2 and P 2p1/2, respectively. The BE at 128.9 eV suggests the formation of phosphide [29] and the broad peak at about 133.1 eV is assigned to phosphate [30]. All these results suggest that as-prepared film consisting of metallic Ni and nickel phosphide [31]. The HER activity of NieP/CF (NieP loading: ~5 mg cm2) was measured in a standard three-electrode setup in 1.0 M KOH solution. For reference purposes, bare CF and commercial Pt/C (20 wt% Pt/XC-72) deposited on CF were also tested. Because as-measured reaction currents cannot reflect the intrinsic behavior of electrocatalysts due to the effect of ohmic resistance, resistance tests were made for iR correction of all initial data for further analysis [32]. Fig. 2a presents the polarization curves on the reversible hydrogen electrode (RHE) scale. As can be seen, the commercial Pt/C reveals excellent activity with negligible HER overpotentials. Bare CF almost has no HER activity. In contrast, the hydrogen evolution on NieP/CF electrode begins at a low overpotential about 55 mV, beyond which the cathodic current dramatically increases, and this electrode only needs overpotential of 98 mV to achieve current density of 10 mA cm2. This overpotential compares favorably to behavior of reported Ni-based HER electrocatalysts including Ni(OH)2 on Ni foam (Ni(OH)2/NF, ~250 mV) and NiFe LDH on Ni foam (NiFe LDH/NF, ~210 mV) [4] and HER electrocatalysts like crystalline NiP2 (102 mV) [33] in 1.0 M KOH. The Tafel slope for NieP/CF is 55 mV dec1 (Fig. 2b), which is smaller than that of crystalline NiP2 (64 mV dec1) [33]. Because good durability is an important criterion for electrocatalyst assessment, we measured the durability of NieP/CF. The NieP/CF electrode was continuously cycled between 0.4 V and 0 V versus RHE with a scan rate of 100 mV s1 in 1.0 M KOH solution for 1000 cycles. After cycling,
344
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
Fig. 1. (a) Low and (b) high magnification SEM images of NieP/CF. (c) EDX elemental mapping images of Cu, Ni, and P in NieP/CF. XPS spectra in the (d) Ni 2p and (e) P 2p regions for NieP/CF.
Fig. 2. (a) Polarization curves for NieP/CF, Pt/C, and CF in 1.0 M KOH solution with a scan rate of 2 mV s1. (b) Tafel plots of NieP/CF and Pt/C. (c) Polarization curves for NieP/CF before and after 1000 CV cycles. Inset: SEM image of NieP film after 1000 CV cycles. (d) Time-dependent current density curve for NieP/CF under static overpotential of 118 mV for 15 h.
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
NieP/CF electrode reveals a polarization curve similar to the initial one (Fig. 2c). The SEM image of the inset in Fig. 2c shows that NieP nanoparticles film maintains the morphology before 1000 CV cycles. We also tested the durability of this electrode by electrolysis at a fixed overpotential of 118 mV and this electrode maintains its catalytic activity for at least 15 h (Fig. 2d). These results reveal the good long-term stability of this NieP/CF HER electrode in 1.0 M KOH. Next, the OER activity of NieP/CF, bare CF, and RuO2 immobilized on CF were also assessed in the same electrolyte. All the polarization curves in Fig. 3 were also iR-corrected. As shown in Fig. 3a, NieP/CF reveals a sharp onset of OER current at overpotential approximately 294 mV. And it requires an overpotential of 325 mV to achieve a current density of 10 mA cm2. This overpotential compares favorably to other reported Ni-based OER electrocatalysts like Ni(OH)2/NF (~372 mV) [4], NiCo2O4 (~391 mV) [16], Ni0.9Fe0.1Ox (336 mV) [34]. and Ni@NC (390 mV) [35]. Note that bare CF shows much inferior OER activity to NieP/CF. Although RuO2 exhibits smaller onset overpotential (235 mV) and needs smaller overpotential (286 mV) to obtain 10 mA cm2, its performance is exceeded by that of NieP/CF beyond potential of 1.670 V vs. RHE. The Tafel slopes for NieP/CF, bare CF, and RuO2 are 120, 159, and 88 mV dec1, respectively (Fig. 3b). Similarly, continuous CV cycles and long-term electrolysis experiment were also carried out to assess the durability of this NieP/CF OER electrocatalyst. As shown in Fig. 3c and d, NieP/CF electrode still has good stability as OER eletrocatalyst. After 1000 CV cycles, the NieP film still keeps the nanoparticles film morphology (the inset in Fig. 3c), albeit with a rougher surface. Generally, the most active non-noble metal (Fe, Co, Ni) based electrocatalysts for the OER are metal oxyhydroxides. The Raman spectra analysis of NieP/CF (Fig. S4) after 1000 continuous CV scans reveals the formation of NiOOH [36]. The XPS analysis for the post-OER NieP/CF (Fig. S5) further confirms the formation of Ni(III) and an increase in intensity of O 1s peak. Zhou
345
et al. reported Ni3S2 nanorods can catalyze water oxidation but NiOOH species in situ formed on Ni3S2 surface also act as active site [37]. It is thus reasonable to conclude that the formed NiOOH species may also serve as the active OER site in our present study. Based on the good catalytic performance of NieP/CF towards both HER and OER, we expect that NieP/CF can be used as an active electrocatalyst for overall water splitting. Thus, we made a twoelectrode water electrolyzer using NieP/CF as both cathode and anode. For comparison, we also made another electrolyzer using Pt/ C on CF as cathode and RuO2 on CF as anode (Pt/CkRuO2). As shown in Fig. 4a, the NieP/CFkNieP/CF system affords current density of 10 mA cm2 at a cell voltage of 1.68 V with vigorous gas evolution on both electrodes (Fig. 4b and Movie S1). Although this voltage is larger than that of Pt/CkRuO2 system (1.56 V), it is smaller than that for NiFe LDH/NFkNiFe LDH/NF (1.70 V) [4] and Ni(OH)2/ NFkNi(OH)2/NF (~1.82 V) [4]. The long-term electrochemical durability was probed by electrolysis under a constant current density of 10 mA cm2 in 1.0 M KOH solution. This NieP/CFkNieP/CF water electrolysis system can maintain excellent stability at least for 15 h (inset of Fig. 4a). These results suggest that our eletrocatalyst can drive full water splitting. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.09.027. 4. Conclusions In summary, NieP nanoparticles film has been electrochemically deposited on CF as an efficient and stable elecrtocatalyst for both HER and OER in alkaline solutions. This NieP/CF electrode needs overpotential of 98 mV for HER and 325 mV for OER to achieve current density of 10 mA cm2. Furthermore, a durable alkaline water electrolyzer using NieP/CF as cathode and anode is demonstrated to afford current density of 10 mA cm2 at a cell voltage of 1.68 V. Our work would open up a new avenue to
Fig. 3. (a) Polarization curves for NieP/CF, RuO2, and CF in 1.0 M KOH solution with a scan rate of 2 mV s1. (b) Tafel plots of NieP/CF, RuO2, and CF. (c) Polarization curves for NieP/ CF before and after 1000 CV cycles. Inset: SEM image of NieP film after 1000 CV cycles. (d) Time-dependent current density curve for NieP/CF under static overpotential of 357 mV for 15 h.
346
Q. Liu et al. / Journal of Power Sources 299 (2015) 342e346
Fig. 4. (a) Polarization curves for NieP/CFkNieP/CF and Pt/CkRuO2 in a two-electrode setup for overall water splitting with a scan rate of 2 mV s1. Inset: Chronopotentiometric curve of NieP/CF water splitting system with constant current density of 10 mA cm2. (b) NieP/CFkNieP/CF water-splitting system driven by direct-current power supply at 1.68 V.
fabricate Ni-based non-precious metal electrocatalysts for water splitting applications. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.027. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
J. Chow, R.J. Kopp, P.R. Portney, Science 302 (2003) 1528e1531. N. Armaroli, V. Balzani, ChemSusChem 4 (2011) 21e36. S.M. Barnett, K.I. Goldberg, J.M. Mayer, Nat. Chem. 4 (2012) 498e502. J. Luo, J.-H. Im, M.T. Mayer, M. Schreier, M.K. Nazeeruddin, N.-G. Park, D. Tilley, H. Fan, M. Gr€ atzel, Science 345 (2014) 1593e1596. D. Kong, J.J. Cha, H. Wang, H.R. Lee, Y. Cui, Energy Environ. Sci. 6 (2013) 3553e3558. D. Merki, S. Fierro, H. Vrubel, X. Hu, Chem. Sci. 2 (2011) 1262e1267. Q. Liu, J. Tian, W. Cui, P. Jiang, N. Cheng, A.M. Asiri, X. Sun, Angew. Chem. Int. Ed. 53 (2014) 6710e6714. E.J. Popczun, C.G. Read, C.W. Roske, N.S. Lewis, R.E. Schaak, Angew. Chem. Int. Ed. 53 (2014) 5427e5430. E.J. Popczun, J.R. McKone, C.G. Read, A.J. Biacchi, A.M. Wiltrout, N.S. Lewis, R.E. Schaak, J. Am. Chem. Soc. 135 (2013) 9267e9270. Z. Xing, Q. Liu, A.M. Asiri, X. Sun, Adv. Mater. 26 (2014) 5702e5707. W. Chen, J.T. Muckerman, E. Fujita, Chem. Commun. 49 (2013) 8896e8909. B. Cao, G.M. Veith, J.C. Neuefeind, R.R. Adzic, P.G. Khalifah, J. Am. Chem. Soc. 135 (2013) 19186e19192. Y. Xu, M. Gao, Y. Zheng, J. Jiang, S. Yu, Angew. Chem. Int. Ed. 52 (2013) 1e6. D. Kong, H. Wang, Z. Lu, Y. Cui, J. Am. Chem. Soc. 136 (2014) 4897e4900. Q. Liu, J. Shi, J. Hu, A.M. Asiri, Y. Luo, X. Sun, ACS Appl. Mater. Interfaces 7 (2015) 3877e3881. Z. Peng, D. Jia, A.M. Al-Enizi, A.A. Elzatahry, G. Zheng, Adv. Energy Mater. 10. 1002/aenm.201402031.
[17] Y. Meng, W. Song, H. Huang, Z. Ren, S. Chen, S.L. Suib, J. Am. Chem. Soc. 136 (2014) 11452e11464. €blinger, [18] K. Fominykh, P. Chernev, I. Zaharieva, J. Sicklinger, G. Stefanic, M. Do €cklein, C. Scheu, T. Bein, D. Fattakhova-Rohlfing, A. Müller, A. Pokharel, S. Bo ACS Nano 9 (2015) 5180e5188. [19] X. Lu, C. Zhao, Nat. Commun. 6 (2015) 6616. [20] B.Y. Jeong, D.Y. Shin, J.K. Lee, D.H. Kim, Y.D. Kim, J.Y. Lee, Phys. Chem. Chem. Phys. 16 (2014) 13807e13813. [21] F. Song, X. Hu, J. Am. Chem. Soc. 136 (2014) 16481e16484. [22] X. Long, J. Li, S. Xiao, K. Yan, Z. Wang, H. Chen, S. Yang, Angew. Chem. Int. Ed. 53 (2014) 7584e7588. [23] W. Zhou, X. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, J. Wang, H. Zhang, Energy Environ. Sci. 6 (2013) 2921e2924. [24] Z. Zhang, X. Wang, G. Cui, A. Zhang, X. Zhou, H. Xu, L. Gu, Nanoscale 6 (2014) 3540e3544. [25] M. Gao, X. Cao, Q. Gao, Y. Xu, Y. Zheng, J. Jiang, S. Yu, ACS Nano 8 (2014) 3970e3978. [26] E.A. Hern andez-Pag an, N.M. Vargas-Barbosa, T.H. Wang, Y. Zhao, E.S. Smotkin, T.E. Mallouk, Energy Environ. Sci. 5 (2012) 7582e7589. [27] K. Zeng, D. Zhang, Prog. Energy Combust. Sci. 36 (2010) 307e326. [28] A. Panneerselvam, M.A. Malik, M. Afzaal, P. O'Brien, M. Helliwell, J. Am. Chem. Soc. 130 (2008) 2420e2421. [29] A.P. Grosvenor, S.D. Wik, R.G. Cavell, A. Mar, Inorg. Chem. 44 (2005) 8988e8998. [30] Q. Yu, Z. Zeng, W. Zhao, Y. Ma, X. Wu, Q. Xue, ACS Appl. Mater. Interfaces 6 (2014) 1053e1060. [31] J. Xiang, J. Tu, X. Wang, X. Huang, Y. Yuan, X. Xia, Z. Zeng, J. Power Sources 185 (2008) 519e525. [32] Z. Xing, Q. Liu, A.M. Asiri, X. Sun, Adv. Mater. 26 (2014) 5702e5707. [33] P. Jiang, Q. Liu, X. Sun, Nanoscale 6 (2014) 13440e13445. [34] L. Trotochaud, J.K. Ranney, K.N. Williams, S.W. Boettcher, J. Am. Chem. Soc. 134 (2012) 17253e17261. [35] J. Ren, M. Antonietti, T.P. Fellinger, Adv. Energy Mater. 5 (2015) 1401660. [36] K.S. Joya, X. Salac, 17 (2015) 21094e21103. [37] W. Zhou, X. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, L. Wang, H. Zhang, Energy Environ. Sci. 6 (2013) 2921e2924.