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Doping cobalt hydroxide nanowires for better supercapacitor performance
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ScienceDirect Acta Materialia 84 (2015) 20–28 www.elsevier.com/locate/actamat
Doping cobalt hydroxide nanowires for better supercapacitor performance ⇑
Huajun Liu, Kuan Hung Ho, Yating Hu, Qingqing Ke, Lu Mao, Yu Zhang and John Wang Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore Received 2 August 2014; accepted 30 September 2014
Abstract—Cobalt hydroxide nanowires were synthesized on Ni foam by a one-pot hydrothermal process. The specific capacitance of pure cobalt hydroxide nanowires can reach 1600 F g1 at a current density of 1 A g1. By suitably doping the redox-active element Mn into cobalt hydroxide nanowires, the specific capacitance is improved to 2800 F g1 at 1 A g1. Doping with the redox-inactive elements La and Sr gives specific capacitances per cobalt mass of 245, 2200 and 3200 F g1 at 1 A g1 for La-doped, Sr-doped and La–Sr-doped cobalt hydroxides, respectively. The effect of doping is clarified in terms of the modifications caused to the morphology, oxidation state of cobalt, electronic conductivity and additional redox reactions of cobalt hydroxide. Our work shows that chemical doping is an effective and simple route to improve the supercapacitor performance of transition metal hydroxides. Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Supercapacitor; Cobalt hydroxide; Chemical doping; Hydrothermal method
1. Introduction Supercapacitors are promising devices for energy storage due to their high power density and long cycle stability [1]. Two general types of supercapacitor exist based on the mechanism underlying their operation: electrochemical double layer capacitors (EDLCs) and pseudocapacitors. The electrode material is the key element that largely determines supercapacitor performance, including energy density, which is the current bottleneck limiting the wider application of supercapacitors [2–4]. For EDLCs, porous carbon materials are widely used as electrodes owing to their high surface area and good electronic conductivity [2,5,6]. For pseudocapacitors involving fast faradaic reactions, conductive polymers and transition metal oxides/hydroxides are selected as electrochemically active electrode materials [1,7–9]. Among the three classes of candidate electrode materials, the transition metal oxides/hydroxides offer particular advantages, as they show higher specific capacitance, and thus higher energy density, than carbon materials, and better electrochemical stability than conductive polymers [10]. A considerable amount of research work has been conducted on transition metal oxides/hydroxides to increase the energy density of supercapacitors, starting from RuO2 [11,12] to MnO2 [13,14], Co3O4 [15], Co(OH)2 [16,17], NiO [18] and Ni(OH)2 [19]. The main strategy to improve the performance is through nanostructure engineering by rational structure design and controllable synthesis of nanostructures of metal oxides/hydroxides [20].
⇑ Corresponding
author. Tel.: +65 6516 3325; fax: +65 6776 3604; e-mail: [email protected]
Co(OH)2-based materials are attractive due to the layered structure and large interlayer spacing, which is beneficial for high surface area and fast ion diffusion [21,22]. This leads to a very good electrochemical performance up to 3500 F g1 for Co(OH)2 nanocomposites [16]. In this work, we further explore the effect of chemical doping on the supercapacitor performance of Co(OH)2-based electrodes, synthesized by a one-pot hydrothermal method. Two types of doping elements were investigated: the redox-active element Mn, and the redox-inactive elements La and Sr. The specific capacitance is improved from 1600 F g1 of pure cobalt hydroxide to 2800 F g1 after doping Mn, tested at a current density of 1 A g1. Sr doping decreases the specific capacitance per cobalt mass to 245 F g1, while La doping increases the capacitance to 2200 F g1 and La–Sr co-doping further enhances the capacitance value to 3200 F g–1 at 1 A g1. The different effects of doping these elements are discussed by considering modifications to the morphology, oxidation state of cobalt, electronic conductivity and additional redox reactions of cobalt hydroxide. Chemical doping is shown to be an effective method and simple route to optimize the supercapacitor performance of transition metal hydroxides. 2. Experimental details 2.1. Synthesis of Co(OH)2-based electrodes The Co(OH)2-based nanowire electrodes were synthesized by a one-pot hydrothermal method. Appropriate
http://dx.doi.org/10.1016/j.actamat.2014.09.055 1359-6462/Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
H. Liu et al. / Acta Materialia 84 (2015) 20–28
amounts of the Co(NO3)26H2O (0.083 g), and urea (0.180 g) were first dissolved in 10 ml H2O by stirring overnight. For doping of Mn, La and Sr, the precursors are 0.036 g Mn(NO3)24H2O, 0.123 g La(NO3)36H2O and 0.060 g Sr(NO3)2, respectively. For co-doping of La and Sr, both La(NO3)36H2O and Sr(NO3)2 were added in the solution, which are referred to hereinafter as La–Sr-doped cobalt hydroxide nanowires. The solution was then transferred to a 45 ml Teflon-lined stainless steel autoclave. One piece of Ni foam (3 cm 4 cm) was vertically inserted into the solution and sealed in an autoclave for thermal treatment at 95 °C for 8 h. The as-synthesized cobalt hydroxides on Ni foam were then taken out, ultrasonically cleaned in deionized water and dried overnight at 80 °C. 2.2. Structural characterization The morphology and microstructure of nanowire samples were studied by scanning electron microscopy (SEM; XL 30 FEG-SEM Philips) and transmission electron microscopy (TEM; JEM2010F). The oxidation states of cobalt were investigated by X-ray photoelectron spectroscopy (XPS; AXIS Ultra). 2.3. Electrochemical characterization The three-electrode configuration was employed for electrochemical characterization. As-synthesized cobalt hydroxides on Ni foam were used as the working electrodes, Pt as the counter-electrode and Ag/AgCl as the reference electrode. The electrolyte was 6 M KOH aqueous solution.
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3. Results and discussion Fig. 1 shows the morphology of the as-synthesized cobalt hydroxide nanowires (Fig. 1a and b) and Mn-doped cobalt hydroxide nanostructures (Fig. 1c and d). Cobalt hydroxide grows radially as nanowires from Ni foam backbone, with a length of 2 lm (see Fig. 1a). The TEM image in Fig. 1b shows the porous structure of the nanowires, which aids ion diffusion. Doping with Mn causes the morphology to change dramatically to a mixture of plates and small nanoparticles, as shown in Fig. 1c. The TEM image in Fig. 1d shows that some porous nanowires exist at the edge of the plates. To identify the chemical composition and oxidation states of cobalt, XPS spectra were measured, as shown in Fig. 2. Due to spin–orbital splitting, Co 2p spectra show two peaks, 2p1/2 and 2p3/2 [23]. From the separation of these two peaks, Co(OH)2 can be distinguished from CoO, Co2O3 and Co3O4 [24,25]. Fig. 2a shows a separation of 15.0 eV, characteristic of a Co(OH)2 phase [26–29]. After doping Mn, the separation changed slightly to 15.1 eV, as shown in Fig. 2b. From the fitting of the high-resolution spectrum of the 2p2/3 peak, the oxidation states of cobalt can be determined [30–32]. For pure cobalt hydroxide sample, a single peak centered 780.5 eV can be fitted to the spectrum, indicating an oxidation state of Co2+, as shown in Fig. 2c. Fig. 2d shows, after doping Mn, another small peak at 781.5 eV, which corresponds to an oxidation state of Co3+. This suggests that Mn doping induces the formation of Co3+. The cyclic voltammetry (CV) curve of pure cobalt hydroxide at 1 mV s1 is shown in Fig. 3a, and displays obvious redox peaks. The mass specific capacitance is calculated by C = Q/DV/m, where Q is the specific charge inte-
Fig. 1. SEM and TEM images of cobalt hydroxide nanowires (a and b), and Mn-doped cobalt hydroxide nanostructures (c and d).
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Fig. 2. XPS spectra of cobalt hydroxide nanowires (a and c), and Mn-doped cobalt hydroxide nanostructure (b and d).
Fig. 3. Electrochemical tests using the three-electrode configuration for a cobalt hydroxide nanowire sample: (a) Cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1. (d) Specific capacitance as a function of current density.
grated over the tested potential range, DV is the potential range, and m is the mass of electrode material. The scan rate dependence of specific capacitance is plotted in Fig. 3b, showing a high capacitance of 1890 F g1 at 1 mV s1. The galvanostatic charge–discharge (GC) curves
are shown in Fig. 3c for different current densities. The specific capacitance is calculated by C = I/(dV/dt)/m, where I is the constant current density during the charging and discharging, dV/dt is derived from the slope of the discharge curve, and m is the mass of electrode material. The current
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Fig. 4. Electrochemical tests using the three-electrode configuration for Mn-doped cobalt hydroxide nanostructure sample: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
density dependence of specific capacitance is plotted in Fig. 3(d), showing a capacitance of 1600 F g1 at 1 A g1 and good retention of 1000 F g1 at a high current density of 10 A g1. Similar tests are also conducted on the Mn-doped cobalt hydroxide samples, as shown in Fig. 4. Both CV and GC tests, shows a much enhanced specific capacitance after Mn doping, with a value of 3000 F g1 at a scan rate of 1 mV s1 and 2800 F g1 at a charge density of 1 A g1. This increase of capacitance could well be due to the enhanced activity of Co redox reactions or to the contribution of Mn redox reactions. In order to exclude the effect of additional redox reactions as a result of the doping redox-active element, in the following part of this work the doping of redox-inactive elements, i.e. La and Sr, will be investigated. Both La and Sr have only one oxidation state, and thus would not contribute to the pseudocapacitance. Fig. 5a and b shows the morphology of the Sr-doped cobalt hydroxide nanostructure. Large nanowires 40 lm long are observed to grow radially from Ni foam backbone. As compared to the 2 lm length of pure cobalt hydroxide, this structure would result in a much smaller surface area, and thus is not advantageous for supercapacitor performance. A close look by TEM at these nanowires shows some nanoparticles attached to the surface, as shown in Fig. 5b. The doping of La into cobalt hydroxide changed the morphology to thin plates in Fig. 5c. The plates have a porous structure as seen by the TEM image in Fig. 5d. For the doping of both La and Sr, Fig. 5e and f shows a very porous network of interconnected wall structure. This structure would assist ion diffusion into the electrode and provide more sites for redox reactions during electrochemical tests.
Fig. 6 shows the XPS spectra of the Sr-doped, La-doped and La–Sr-doped cobalt hydroxide nanostructures. The separation distances of 2p1/2 and 2p3/2 peaks are 15.0 eV in Fig. 6a, close to the value of pure cobalt hydroxide in Fig. 2a. The fitting of the 2p3/2 peak for the Sr-doped sample indicates the presence of both Co2+ and Co3+, as shown in Fig. 6b. For La-doped and La–Sr-doped samples, three peaks are resolved in the 2p3/2 peaks. The peak at 785 eV is the satellite peak, which is related to the coupling between unpaired electrons in the atom (multiplet splitting) or a multiple electron excitation (shake-up) [26]. The other two peaks are assigned as Co2+ and Co3+, similar to the case in Mn-doped and Sr-doped samples. This indicates that doping of both redox-active and redox-inactive element introduces the formation of the Co3+ oxidation state in the samples. Figs. 7–9 show the electrochemical performance of Srdoped, La-doped and La–Sr-doped cobalt hydroxide nanostructures, respectively. Although redox peaks still exist for the Sr-doped cobalt hydroxide sample in Fig. 7a, the capacitance shows a large fall to 170 F g1 at a scan rate of 1 mV s1. For La-doped and La–Sr-doped samples, the capacitance value also decreases to 1000 and 1200 F g1, respectively, as shown in Figs. 8b and 9b. However, the specific capacitances plotted in Figs. 7–9 are based on the total mass of electrode materials. As the component from La and Sr is not involved in the redox reactions and will not contribute to the pseudocapacitance, the specific capacitance per Co mass only, plotted in Fig. 10, constitutes a fair comparison with the pure cobalt hydroxide sample. Taking the specific capacitance at a current density of 1 A g1 as the benchmark, the capacitance value of the Sr-doped sample is 245 F g1, lower than
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Fig. 5. SEM and TEM images of the Sr-doped cobalt hydroxide nanostructure (a and b), La-doped cobalt hydroxide (c and d), and La–Sr-doped cobalt hydroxide (e and f).
the 1600 F g1 of pure cobalt hydroxide sample, while the capacitance of the La-doped and La–Sr-doped samples show much better enhanced values of 2200 and 3200 F g1, respectively. Impedance spectroscopy is used to investigate the effect of chemical doping on the electrical conductivity of the samples, as shown in Fig. 11. The impedance data of all five samples are analyzed using Nyquist plot, with imaginary component of impedance (Z00 ) vs. the real component (Z0 ) spectroscopy, as shown in Fig. 11a. Equivalent series resistance (ESR) can be derived from the intersection of the curves at the X axis [33]. The ESR, shown in Fig. 11b, indicates that resistance does not change much after doping Mn and Sr. The La-doped sample gives the highest resistance. For the La–Sr-doped sample, the resistance drops back to be close to that of the undoped sample. Thus, after chemical doping, the electronic conductivity is either not changed as in the Sr-doped sample, or becomes worse as in the other three samples. Fig. 12 shows the cyclic performance of
Mn-doped sample tested at scan rate of 5 mV s1 for 5000 cycles. Interestingly, for the first 50 cycles, the capacitance increased up to 140% of the first cycle. This may be related to the activation process of the redox reactions, which takes a certain number of cycles to reach the maximum activity. After 2000 cycles, the performance stabilizes and remains at 95% until 5000 cycles. Based on the above experimental results, the modification of cobalt hydroxide after chemical doping can be understood in the following terms. Firstly, the nanowire structure of pure cobalt hydroxide was completely changed after doping. This is probably due to the modification of thermodynamic equilibrium in the solution after adding additional salts of doping elements. The favored morphology for supercapacitor application should be highly porous so as to provide a large number of surface sites for redox reactions to take place. Of the five samples, the La– Sr-doped sample shows the best morphology in this respect. Secondly, the oxidation state of cobalt changed after
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Fig. 6. (a) XPS spectra of the Sr-doped, La-doped and La–Sr-doped cobalt hydroxide nanostructures. Fittings of XPS spectra of the Co 2p2/3 peak are shown for Sr-doped (b), La-doped (c) and La–Sr-doped cobalt hydroxide (d).
Fig. 7. Electrochemical tests using the three-electrode configuration for Sr-doped cobalt hydroxide sample: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
doping with a foreign element. Most importantly, Co3+ is introduced, as shown by XPS. The possible redox reactions for Co(OH)2 during electrochemical tests in base solution are [16,17,34]: CoðOHÞ2 þ OH $ CoOOH þ H2 O þ e
CoOOH þ OH $ CoO2 þ H2 O þ e It is beneficial to have Co3+ around to facilitate the redox reactions. Thirdly, the electrical conductivity changed after doping. However, the highest conductivity of the Sr-doped sample does not give the best capacitance
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Fig. 8. Electrochemical tests using the three-electrode configuration for La-doped cobalt hydroxide nanostructure sample: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
Fig. 9. Electrochemical tests using the three-electrode configuration for La–Sr-doped cobalt hydroxide nanostructures: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
performance, probably owing to the low surface area of the large nanowire structure. Finally, additional redox reactions are advantageous to enhance the capacitance performance. Although the Mn-doped sample does not show the best morphology in terms of high surface area,
the capacitance per total electrode mass is the best of all the samples. This is due to the additional redox reactions from Mn element during the electrochemical tests, which provide an additional source for pseudocapacitance.
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Fig. 10. Specific capacitance in terms of Co mass as a function of current density (a) and scan rate (b) for Sr-doped, La-doped and La–Sr-doped cobalt hydroxide.
Fig. 11. (a) Impedance spectroscopy of all five samples, and (b) equivalent series resistance derived from impedance spectroscopy.
morphology, the oxidation state of cobalt, the electronic conductivity and additional redox reactions of cobalt hydroxide. They show that chemical doping is an effective and simple route to improve the supercapacitor performance of transition metal hydroxides. The amount of doping has not been optimized, leaving room for future investigation of this chemical doping approach. To achieve even better performance, it would be of interest to investigate whether the morphology of the highly porous nanostructure of La–Sr-doped samples can be designed to work in combination with the additional redox reactions of Mn-doped samples. Acknowledgement Fig. 12. Cyclic nanostructure.
performance
of
Mn-doped
cobalt
hydroxide
4. Conclusion In summary, we have demonstrated an effective way to improve supercapacitor performance of cobalt hydroxide nanowires by chemical doping in a one-pot hydrothermal synthesis method. Two types of doping were investigated. By doping the redox-active element Mn the specific capacitance is improved from 1600 to 2800 F g1 at 1 A g1. Doping the redox-inactive elements La and Sr gives specific capacitances per cobalt mass of 245, 2200 and 3200 F g1 at 1 A g1, for La-doped, Sr-doped and La– Sr-doped cobalt hydroxides, respectively. The effect of doping has been clarified in terms of the modifications to the
This work is supported by the Agency for Science, Technology and Research (A*star, Singapore), Grant No. 1121202013, conducted at the National University of Singapore.
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ScienceDirect Acta Materialia 84 (2015) 20–28 www.elsevier.com/locate/actamat
Doping cobalt hydroxide nanowires for better supercapacitor performance ⇑
Huajun Liu, Kuan Hung Ho, Yating Hu, Qingqing Ke, Lu Mao, Yu Zhang and John Wang Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore Received 2 August 2014; accepted 30 September 2014
Abstract—Cobalt hydroxide nanowires were synthesized on Ni foam by a one-pot hydrothermal process. The specific capacitance of pure cobalt hydroxide nanowires can reach 1600 F g1 at a current density of 1 A g1. By suitably doping the redox-active element Mn into cobalt hydroxide nanowires, the specific capacitance is improved to 2800 F g1 at 1 A g1. Doping with the redox-inactive elements La and Sr gives specific capacitances per cobalt mass of 245, 2200 and 3200 F g1 at 1 A g1 for La-doped, Sr-doped and La–Sr-doped cobalt hydroxides, respectively. The effect of doping is clarified in terms of the modifications caused to the morphology, oxidation state of cobalt, electronic conductivity and additional redox reactions of cobalt hydroxide. Our work shows that chemical doping is an effective and simple route to improve the supercapacitor performance of transition metal hydroxides. Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Supercapacitor; Cobalt hydroxide; Chemical doping; Hydrothermal method
1. Introduction Supercapacitors are promising devices for energy storage due to their high power density and long cycle stability [1]. Two general types of supercapacitor exist based on the mechanism underlying their operation: electrochemical double layer capacitors (EDLCs) and pseudocapacitors. The electrode material is the key element that largely determines supercapacitor performance, including energy density, which is the current bottleneck limiting the wider application of supercapacitors [2–4]. For EDLCs, porous carbon materials are widely used as electrodes owing to their high surface area and good electronic conductivity [2,5,6]. For pseudocapacitors involving fast faradaic reactions, conductive polymers and transition metal oxides/hydroxides are selected as electrochemically active electrode materials [1,7–9]. Among the three classes of candidate electrode materials, the transition metal oxides/hydroxides offer particular advantages, as they show higher specific capacitance, and thus higher energy density, than carbon materials, and better electrochemical stability than conductive polymers [10]. A considerable amount of research work has been conducted on transition metal oxides/hydroxides to increase the energy density of supercapacitors, starting from RuO2 [11,12] to MnO2 [13,14], Co3O4 [15], Co(OH)2 [16,17], NiO [18] and Ni(OH)2 [19]. The main strategy to improve the performance is through nanostructure engineering by rational structure design and controllable synthesis of nanostructures of metal oxides/hydroxides [20].
⇑ Corresponding
author. Tel.: +65 6516 3325; fax: +65 6776 3604; e-mail: [email protected]
Co(OH)2-based materials are attractive due to the layered structure and large interlayer spacing, which is beneficial for high surface area and fast ion diffusion [21,22]. This leads to a very good electrochemical performance up to 3500 F g1 for Co(OH)2 nanocomposites [16]. In this work, we further explore the effect of chemical doping on the supercapacitor performance of Co(OH)2-based electrodes, synthesized by a one-pot hydrothermal method. Two types of doping elements were investigated: the redox-active element Mn, and the redox-inactive elements La and Sr. The specific capacitance is improved from 1600 F g1 of pure cobalt hydroxide to 2800 F g1 after doping Mn, tested at a current density of 1 A g1. Sr doping decreases the specific capacitance per cobalt mass to 245 F g1, while La doping increases the capacitance to 2200 F g1 and La–Sr co-doping further enhances the capacitance value to 3200 F g–1 at 1 A g1. The different effects of doping these elements are discussed by considering modifications to the morphology, oxidation state of cobalt, electronic conductivity and additional redox reactions of cobalt hydroxide. Chemical doping is shown to be an effective method and simple route to optimize the supercapacitor performance of transition metal hydroxides. 2. Experimental details 2.1. Synthesis of Co(OH)2-based electrodes The Co(OH)2-based nanowire electrodes were synthesized by a one-pot hydrothermal method. Appropriate
http://dx.doi.org/10.1016/j.actamat.2014.09.055 1359-6462/Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
H. Liu et al. / Acta Materialia 84 (2015) 20–28
amounts of the Co(NO3)26H2O (0.083 g), and urea (0.180 g) were first dissolved in 10 ml H2O by stirring overnight. For doping of Mn, La and Sr, the precursors are 0.036 g Mn(NO3)24H2O, 0.123 g La(NO3)36H2O and 0.060 g Sr(NO3)2, respectively. For co-doping of La and Sr, both La(NO3)36H2O and Sr(NO3)2 were added in the solution, which are referred to hereinafter as La–Sr-doped cobalt hydroxide nanowires. The solution was then transferred to a 45 ml Teflon-lined stainless steel autoclave. One piece of Ni foam (3 cm 4 cm) was vertically inserted into the solution and sealed in an autoclave for thermal treatment at 95 °C for 8 h. The as-synthesized cobalt hydroxides on Ni foam were then taken out, ultrasonically cleaned in deionized water and dried overnight at 80 °C. 2.2. Structural characterization The morphology and microstructure of nanowire samples were studied by scanning electron microscopy (SEM; XL 30 FEG-SEM Philips) and transmission electron microscopy (TEM; JEM2010F). The oxidation states of cobalt were investigated by X-ray photoelectron spectroscopy (XPS; AXIS Ultra). 2.3. Electrochemical characterization The three-electrode configuration was employed for electrochemical characterization. As-synthesized cobalt hydroxides on Ni foam were used as the working electrodes, Pt as the counter-electrode and Ag/AgCl as the reference electrode. The electrolyte was 6 M KOH aqueous solution.
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3. Results and discussion Fig. 1 shows the morphology of the as-synthesized cobalt hydroxide nanowires (Fig. 1a and b) and Mn-doped cobalt hydroxide nanostructures (Fig. 1c and d). Cobalt hydroxide grows radially as nanowires from Ni foam backbone, with a length of 2 lm (see Fig. 1a). The TEM image in Fig. 1b shows the porous structure of the nanowires, which aids ion diffusion. Doping with Mn causes the morphology to change dramatically to a mixture of plates and small nanoparticles, as shown in Fig. 1c. The TEM image in Fig. 1d shows that some porous nanowires exist at the edge of the plates. To identify the chemical composition and oxidation states of cobalt, XPS spectra were measured, as shown in Fig. 2. Due to spin–orbital splitting, Co 2p spectra show two peaks, 2p1/2 and 2p3/2 [23]. From the separation of these two peaks, Co(OH)2 can be distinguished from CoO, Co2O3 and Co3O4 [24,25]. Fig. 2a shows a separation of 15.0 eV, characteristic of a Co(OH)2 phase [26–29]. After doping Mn, the separation changed slightly to 15.1 eV, as shown in Fig. 2b. From the fitting of the high-resolution spectrum of the 2p2/3 peak, the oxidation states of cobalt can be determined [30–32]. For pure cobalt hydroxide sample, a single peak centered 780.5 eV can be fitted to the spectrum, indicating an oxidation state of Co2+, as shown in Fig. 2c. Fig. 2d shows, after doping Mn, another small peak at 781.5 eV, which corresponds to an oxidation state of Co3+. This suggests that Mn doping induces the formation of Co3+. The cyclic voltammetry (CV) curve of pure cobalt hydroxide at 1 mV s1 is shown in Fig. 3a, and displays obvious redox peaks. The mass specific capacitance is calculated by C = Q/DV/m, where Q is the specific charge inte-
Fig. 1. SEM and TEM images of cobalt hydroxide nanowires (a and b), and Mn-doped cobalt hydroxide nanostructures (c and d).
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Fig. 2. XPS spectra of cobalt hydroxide nanowires (a and c), and Mn-doped cobalt hydroxide nanostructure (b and d).
Fig. 3. Electrochemical tests using the three-electrode configuration for a cobalt hydroxide nanowire sample: (a) Cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1. (d) Specific capacitance as a function of current density.
grated over the tested potential range, DV is the potential range, and m is the mass of electrode material. The scan rate dependence of specific capacitance is plotted in Fig. 3b, showing a high capacitance of 1890 F g1 at 1 mV s1. The galvanostatic charge–discharge (GC) curves
are shown in Fig. 3c for different current densities. The specific capacitance is calculated by C = I/(dV/dt)/m, where I is the constant current density during the charging and discharging, dV/dt is derived from the slope of the discharge curve, and m is the mass of electrode material. The current
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Fig. 4. Electrochemical tests using the three-electrode configuration for Mn-doped cobalt hydroxide nanostructure sample: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
density dependence of specific capacitance is plotted in Fig. 3(d), showing a capacitance of 1600 F g1 at 1 A g1 and good retention of 1000 F g1 at a high current density of 10 A g1. Similar tests are also conducted on the Mn-doped cobalt hydroxide samples, as shown in Fig. 4. Both CV and GC tests, shows a much enhanced specific capacitance after Mn doping, with a value of 3000 F g1 at a scan rate of 1 mV s1 and 2800 F g1 at a charge density of 1 A g1. This increase of capacitance could well be due to the enhanced activity of Co redox reactions or to the contribution of Mn redox reactions. In order to exclude the effect of additional redox reactions as a result of the doping redox-active element, in the following part of this work the doping of redox-inactive elements, i.e. La and Sr, will be investigated. Both La and Sr have only one oxidation state, and thus would not contribute to the pseudocapacitance. Fig. 5a and b shows the morphology of the Sr-doped cobalt hydroxide nanostructure. Large nanowires 40 lm long are observed to grow radially from Ni foam backbone. As compared to the 2 lm length of pure cobalt hydroxide, this structure would result in a much smaller surface area, and thus is not advantageous for supercapacitor performance. A close look by TEM at these nanowires shows some nanoparticles attached to the surface, as shown in Fig. 5b. The doping of La into cobalt hydroxide changed the morphology to thin plates in Fig. 5c. The plates have a porous structure as seen by the TEM image in Fig. 5d. For the doping of both La and Sr, Fig. 5e and f shows a very porous network of interconnected wall structure. This structure would assist ion diffusion into the electrode and provide more sites for redox reactions during electrochemical tests.
Fig. 6 shows the XPS spectra of the Sr-doped, La-doped and La–Sr-doped cobalt hydroxide nanostructures. The separation distances of 2p1/2 and 2p3/2 peaks are 15.0 eV in Fig. 6a, close to the value of pure cobalt hydroxide in Fig. 2a. The fitting of the 2p3/2 peak for the Sr-doped sample indicates the presence of both Co2+ and Co3+, as shown in Fig. 6b. For La-doped and La–Sr-doped samples, three peaks are resolved in the 2p3/2 peaks. The peak at 785 eV is the satellite peak, which is related to the coupling between unpaired electrons in the atom (multiplet splitting) or a multiple electron excitation (shake-up) [26]. The other two peaks are assigned as Co2+ and Co3+, similar to the case in Mn-doped and Sr-doped samples. This indicates that doping of both redox-active and redox-inactive element introduces the formation of the Co3+ oxidation state in the samples. Figs. 7–9 show the electrochemical performance of Srdoped, La-doped and La–Sr-doped cobalt hydroxide nanostructures, respectively. Although redox peaks still exist for the Sr-doped cobalt hydroxide sample in Fig. 7a, the capacitance shows a large fall to 170 F g1 at a scan rate of 1 mV s1. For La-doped and La–Sr-doped samples, the capacitance value also decreases to 1000 and 1200 F g1, respectively, as shown in Figs. 8b and 9b. However, the specific capacitances plotted in Figs. 7–9 are based on the total mass of electrode materials. As the component from La and Sr is not involved in the redox reactions and will not contribute to the pseudocapacitance, the specific capacitance per Co mass only, plotted in Fig. 10, constitutes a fair comparison with the pure cobalt hydroxide sample. Taking the specific capacitance at a current density of 1 A g1 as the benchmark, the capacitance value of the Sr-doped sample is 245 F g1, lower than
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Fig. 5. SEM and TEM images of the Sr-doped cobalt hydroxide nanostructure (a and b), La-doped cobalt hydroxide (c and d), and La–Sr-doped cobalt hydroxide (e and f).
the 1600 F g1 of pure cobalt hydroxide sample, while the capacitance of the La-doped and La–Sr-doped samples show much better enhanced values of 2200 and 3200 F g1, respectively. Impedance spectroscopy is used to investigate the effect of chemical doping on the electrical conductivity of the samples, as shown in Fig. 11. The impedance data of all five samples are analyzed using Nyquist plot, with imaginary component of impedance (Z00 ) vs. the real component (Z0 ) spectroscopy, as shown in Fig. 11a. Equivalent series resistance (ESR) can be derived from the intersection of the curves at the X axis [33]. The ESR, shown in Fig. 11b, indicates that resistance does not change much after doping Mn and Sr. The La-doped sample gives the highest resistance. For the La–Sr-doped sample, the resistance drops back to be close to that of the undoped sample. Thus, after chemical doping, the electronic conductivity is either not changed as in the Sr-doped sample, or becomes worse as in the other three samples. Fig. 12 shows the cyclic performance of
Mn-doped sample tested at scan rate of 5 mV s1 for 5000 cycles. Interestingly, for the first 50 cycles, the capacitance increased up to 140% of the first cycle. This may be related to the activation process of the redox reactions, which takes a certain number of cycles to reach the maximum activity. After 2000 cycles, the performance stabilizes and remains at 95% until 5000 cycles. Based on the above experimental results, the modification of cobalt hydroxide after chemical doping can be understood in the following terms. Firstly, the nanowire structure of pure cobalt hydroxide was completely changed after doping. This is probably due to the modification of thermodynamic equilibrium in the solution after adding additional salts of doping elements. The favored morphology for supercapacitor application should be highly porous so as to provide a large number of surface sites for redox reactions to take place. Of the five samples, the La– Sr-doped sample shows the best morphology in this respect. Secondly, the oxidation state of cobalt changed after
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Fig. 6. (a) XPS spectra of the Sr-doped, La-doped and La–Sr-doped cobalt hydroxide nanostructures. Fittings of XPS spectra of the Co 2p2/3 peak are shown for Sr-doped (b), La-doped (c) and La–Sr-doped cobalt hydroxide (d).
Fig. 7. Electrochemical tests using the three-electrode configuration for Sr-doped cobalt hydroxide sample: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
doping with a foreign element. Most importantly, Co3+ is introduced, as shown by XPS. The possible redox reactions for Co(OH)2 during electrochemical tests in base solution are [16,17,34]: CoðOHÞ2 þ OH $ CoOOH þ H2 O þ e
CoOOH þ OH $ CoO2 þ H2 O þ e It is beneficial to have Co3+ around to facilitate the redox reactions. Thirdly, the electrical conductivity changed after doping. However, the highest conductivity of the Sr-doped sample does not give the best capacitance
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Fig. 8. Electrochemical tests using the three-electrode configuration for La-doped cobalt hydroxide nanostructure sample: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
Fig. 9. Electrochemical tests using the three-electrode configuration for La–Sr-doped cobalt hydroxide nanostructures: (a) cyclic voltammetry test at a scan rate of 1 mV s1; (b) specific capacitance as a function of scan rate; (c) galvanostatic charge–discharge test at current densities of 1, 2, 5 and 10 A g1; (d) specific capacitance as a function of current density.
performance, probably owing to the low surface area of the large nanowire structure. Finally, additional redox reactions are advantageous to enhance the capacitance performance. Although the Mn-doped sample does not show the best morphology in terms of high surface area,
the capacitance per total electrode mass is the best of all the samples. This is due to the additional redox reactions from Mn element during the electrochemical tests, which provide an additional source for pseudocapacitance.
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Fig. 10. Specific capacitance in terms of Co mass as a function of current density (a) and scan rate (b) for Sr-doped, La-doped and La–Sr-doped cobalt hydroxide.
Fig. 11. (a) Impedance spectroscopy of all five samples, and (b) equivalent series resistance derived from impedance spectroscopy.
morphology, the oxidation state of cobalt, the electronic conductivity and additional redox reactions of cobalt hydroxide. They show that chemical doping is an effective and simple route to improve the supercapacitor performance of transition metal hydroxides. The amount of doping has not been optimized, leaving room for future investigation of this chemical doping approach. To achieve even better performance, it would be of interest to investigate whether the morphology of the highly porous nanostructure of La–Sr-doped samples can be designed to work in combination with the additional redox reactions of Mn-doped samples. Acknowledgement Fig. 12. Cyclic nanostructure.
performance
of
Mn-doped
cobalt
hydroxide
4. Conclusion In summary, we have demonstrated an effective way to improve supercapacitor performance of cobalt hydroxide nanowires by chemical doping in a one-pot hydrothermal synthesis method. Two types of doping were investigated. By doping the redox-active element Mn the specific capacitance is improved from 1600 to 2800 F g1 at 1 A g1. Doping the redox-inactive elements La and Sr gives specific capacitances per cobalt mass of 245, 2200 and 3200 F g1 at 1 A g1, for La-doped, Sr-doped and La– Sr-doped cobalt hydroxides, respectively. The effect of doping has been clarified in terms of the modifications to the
This work is supported by the Agency for Science, Technology and Research (A*star, Singapore), Grant No. 1121202013, conducted at the National University of Singapore.
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