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Mater Renew Sustain Energy (2015) 4:21 DOI 10.1007/s40243-015-0064-7 OR IGINAL PAPER Ternary Ni–Cu–OH and Ni–Co–OH electrodes for electrochemical energy storage 1 1 Nuha A. Alhebshi H. N. Alshareef Received: 15 June 2015 / Accepted: 14 October 2015 / Published online: 28 October 2015 The Author(s) 2015. This article is published with open access at Springerlink.com Abstract In this project, Ni–Cu–OH and Ni–Co–OH Introduction ternary electrodes have been prepared. Different Ni:Cu and Ni:Co ratios were deposited by chemical bath deposition The demand for electrical energy storage devices, batteries (CBD) at room temperature on carbon microfibers. Since and electrochemical supercapacitors is rapidly increasing in Ni(OH) is notorious for poor cycling stability, the goal of many crucial applications such as portable electronics, the work was to determine if doping with Cu or Co could electric transportation, and renewable energy systems [1]. It improve Ni(OH) cycling stability performance and con- is well-known that rechargeable batteries have higher ductivity against reaction with electrolyte. It is observed energy but less power than those of electrochemical that the electrodes with Ni:Cu and Ni:Co composition ratio supercapacitors [2]. Electrochemical supercapacitors can be of 100:10 result in the optimum capacitance and cycling classified into electrical double-layer capacitors (EDLC) stability in both Ni–Cu–OH and Ni–Co–OH electrodes. and Faradic capacitors. In EDLC, charges are held at the This improvement in cycling stability can be attributed to interfacial double layer between a porous material and an the higher redox reversibility as indicated by the smaller electrolyte solution, while charges are stored in Faradic CV redox peak separation. In addition, it is found that capacitors during Faradic oxidation–reduction (redox) decreasing Cu and Co ratios, with fixed CBD time, reactions depending upon the electrode potential [2]. Due to enhances nanoflakes formation, and hence increases elec- the differences in these energy storage mechanisms, Faradic trode capacitance. For the optimum composition capacitors fundamentally exhibit higher capacitance but (Ni:Co = 100:10), composites of the ternary electrodes weaker charge–discharge cycling stability than those of with graphene and carbon nanofibers were also tested, with ELDC. Porous materials with electrical conductivity have resultant improvement in potential window, equivalent been earlier investigated for both fundamental understand- series resistance, areal capacitance and cycling stability. ing and practical implementations in EDLC [3, 4]. For Faradic capacitors and rechargeable batteries, transition Keywords Nickel hydroxide Graphene metal oxides/hydroxides and intrinsically conductive poly- Electrochemical supercapacitors mers have been intensively reported in the literature [5–9]. Nickel hydroxide (Ni(OH) ) nanomaterial is one of the most promising electrode materials for Faradic capacitors and rechargeable alkaline batteries, due to significant Electronic supplementary material The online version of this article (doi:10.1007/s40243-015-0064-7) contains supplementary electrochemical redox reactivity [10], larger specific sur- material, which is available to authorized users. face area than that of Ni(OH) bulk material, natural abundance, environmental friendliness, and low cost. & H. N. Alshareef husam.alshareef@kaust.edu.sa However, the main issue of Ni(OH) electrodes is the capacitance decay during charge–discharge (CD) cycling. Materials Science and Engineering Program, Physical For instance, a high specific capacitance, 2222 F/g at 1 A/g, Science and Engineering Division, King Abdullah University has been achieved using Ni(OH) electrode prepared by of Science and Technology (KAUST), Thuwal 23955-6900, chemical bath deposition (CBD) on nickel foam at room Saudi Arabia 123 21 Page 2 of 9 Mater Renew Sustain Energy (2015) 4:21 temperature, but with a capacitance retention of 62 % after room temperature on Nickel foam, but with a capacitance only 2000 CD cycles at 1 A/g [11]. It has been reported in retention of 69 % after only 1600 CD cycles at 6 A/g [21]. the same study that the major contributor to such capaci- Reduced graphene oxide (rGO) nanosheets have been tance decay is the phase transformation from a-Ni(OH) or also utilized for electrochemical supercapacitors with much c-NiOOH to b-Ni(OH) or b-NiOOH phases, at relatively better cycling stability than that of metal oxide and low discharge current densities [11]. hydroxide electrodes [3]. One of the best results for Such problem of low charge–discharge cycling stability reduced graphene oxide supercapacitors was reported by has long been recognized for rechargeable Ni/Cd and Ni/ Bai et al. [22] using a modified Hummer’s method and MH alkaline batteries, and has been tackled by means of hydrothermal reduction, and obtaining a full-cell specific using metal additives such as Co, Ca, Zn and Al [12–16]. capacitance of 230 F/g at 1 A/g and a capacitance retention Co-additive results in a good electrode material for elec- of *89 % after 10,000 cycles. However, there are other trochemical supercapacitors because its oxides and simpler methods which can be used to prepare graphene, hydroxides exhibit high capacitance, while Cu offers and it is useful to study their performance. One such simple excellent electrical conductivity. Recently, metal additives, method is the chemical exfoliation of graphite [23]. binary and ternary oxides/hydroxides have been used in Ni- Chemical exfoliation is an environmentally friendly and based electrochemical supercapacitors to enhance the cost-effective method as it does not require strong acids (as cycling stability, electrical conductivity and capacitance. in Hummer’s method), or high temperature and high Even though enhanced performances of Ni-based electro- pressure (as in the hydrothermal method). In addition, it chemical supercapacitors have been reported in the litera- can produce large quantity of electrically conductive gra- ture, many of the synthetic processes used in electrode phene since it is exfoliated directly from graphite without materials preparation are energy consuming, environmen- forming graphene oxide. An et al. [23] have reported a full- tally unfriendly, and costly. For instance, a cell-capacitance cell specific capacitance of 120 F/g at 2 mA for chemically retention of 94.5 % after 4000 CD cycles at 5 A/g and a exfoliated graphene-based supercapacitors. cell capacitance retention of 86 % after 47,000 CD cycles In this project, Ni–Cu–OH and Ni–Co–OH electrodes at 25 A/g have been reported for (Ni–Co–Cu)(OH) –CuO// have been prepared using different Ni:Cu and Ni:Co ratios AC asymmetric supercapacitor. However, its cell capaci- by CBD at room temperature on carbon microfibers. tance of *58 F/g at 5 A/g and *54 F/g at 25 A/g was Effects of changing Ni:Cu and Ni:Co ratios on electrode relatively low, and it was prepared by cathodic deposition morphology and electrochemical performance are investi- which requires external energy source [17]. gated. For the optimum composition (Ni:Co = 100:10), In contrast to electrodeposition and hydrothermal syn- composites of the ternary electrodes with graphene and thesis methods, chemical bath deposition (CBD) is a highly carbon nanofibers were also tested, with resultant recommended synthesis method for industrial and com- improvement in potential window, equivalent series resis- mercial implementations because it is simple, scalable, tance, areal capacitance and cycling stability. fast, cost effective, does not require external energy source, and can be done at room temperature. Ni(OH) and NiO thin films were prepared by CBD for the first time by Experiments Pramanik and Bhattacharya [18]. Since then, Ni(OH) nanomaterials prepared by CBD have been investigated Electrode preparation and utilized as electrodes for electrochemical supercapac- itors. For example, a specific capacitance of 1416 F/g at 1 Ni–Cu–OH electrodes were prepared by CBD with Ni:Cu A/g has been reported for conformal coating of Ni(OH) ratios of 100:3, 100:10 and 100:25. The chemical bath nanoflakes electrode, prepared by CBD at room tempera- consisted of 1 M of nickel(II) sulfate hexahydrate ture on carbon microfibers, with a capacitance retention of (NiSO 6H O), different molarities (0.03, 0.10 and 4 2 66 % after 10,000 CD cycles at 20 A/g [19]. In addition, 0.25 M) of copper(II) sulfate pentahydrate (CuSO 5H O) 4 2 Ni-based electrodes with metal additives, binary, and and deionized (DI) water (H O) in Pyrex beakers at room ternary hydroxides have been prepared by CBD and used in temperature. Then, 1.56 mL of ammonium hydroxide electrochemical supercapacitors. A specific capacitance of solution (30–33 % NH in H O) and 0.15 M of potassium 3 2 1030 F/g at 3 A/g has been reported for Ni–Co binary persulfate (K S O ) were added subsequently to the mix- 2 2 8 hydroxides electrode, prepared by CBD at 80 C on nickel ture. Several pieces of commercial carbon microfibers foam, but its cycling stability was studied for only 1000 CD substrates from Fuel Cell Store (carbon cloth 7302003, cycles at 15 A/g [20]. Specific capacitances of 1970 F/g at 99 % carbon content and 11.5 mg/cm ) were immersed in 5 mV/s and 859 F/g at 6 A/g have been reported for Ni–Cu the chemical bath by clamps at room temperature. The spherical double hydroxide electrode, prepared by CBD at immersed parts of all carbon substrates have the same area 123 Mater Renew Sustain Energy (2015) 4:21 Page 3 of 9 21 of 1.00 cm . After 1 h, the coated substrates were taken resistances, charge transfer resistances and diffusion control out, washed several times with DI H O and dried in air at were studied by electrochemical impedance spectroscopy room temperature overnight. Ni–Co–OH electrodes with (EIS). All experiments were carried out in standard three Ni:Co ratios of 100:3, 100:10 and 100:25 were prepared by electrode configuration using a multi-channel Potentiostat/ the same previous procedures but with cobalt(II) chloride Galvanostat/EIS from BioLogic Science Instruments hexahydrate (CoCl 6H O) of different molarities (0.03, (VMP3). 2 2 0.10 and 0.25 M). All the chemicals used in this project are of analytical grade (SIGMA-ALDRICH) and were used without further purification. Results and discussion Graphene and carbon nanofibers (G-CNF) were pre- pared using a modified chemical exfoliation process [23]. Elemental and morphological properties A mixture of 100 mg of graphite and 16.5 mg of 1-pyrenecarboxylic acid (PCA) in 50 mL of methanol were EDX elemental mapping images for Ni–Cu–OH electrode sonicated for 45 min (BRANSON Ultrasonic Cleaner and Ni–Co–OH electrode with Ni:Cu and Ni:Co ratios of 2510). Then, 200 mL of DI H O was added to the mixture 100:10 are shown in Fig. 1a, b, respectively. Figure 1a with continuous sonication for several hours. G-CNF were confirms the presence of carbon, nickel, copper and oxygen collected by vacuum filtration using nanoporous mem- in Ni–Cu–OH electrode. In addition, it can be seen that branes (Celgard 3501). The collected G-CNF were dis- nickel, copper and oxygen are homogenously distributed solved in ethanol and casted drop by drop on carbon around carbon microfibers. Figure 1b confirms the pres- microfibers substrate at 60 C followed by washing with DI ence of carbon, nickel, cobalt and oxygen on Ni–Co–OH H O several times and drying in air overnight. Finally, electrode with homogenous distribution around carbon G-CNF on carbon microfibers were used as substrates in microfibers. EDX spectra of all Ni–Cu–OH and Ni–Co–OH CBD of the same previous procedure of Ni–Co–OH with electrodes of composition 100:3, 100:10 and 100:25 are Ni:Co ratio of 100:10 and deposition time of 1 h–2 h to shown in Figure 1S (of the Supplementary Information). prepare Ni–Co–OH/G–CNF electrodes. SEM images in Fig. 2a–c show the morphology changes for Ni–Cu–OH electrodes of 100:3, 100:10 and 100:25, Electrode characterizations respectively. Nanoflakes are clearly shown at Ni:Cu ratio of 100:3 in Fig. 2a, while increasing the Cu content to The mass loading of each electrode was calculated by 100:10 and 100:25 leads to suppression of nanoflake for- weighing the carbon substrate before and after the CBD mation and to growth of flat morphology, as shown in using a sensitive microbalance from METTLER TOLEDO Fig. 2b, c. SEM images Ni–Co–OH electrode of 100:3, (XP26, 0.001 mg resolution). The morphology of Ni–Cu– 100:10 and 100:25 in Fig. 2d–f, respectively, show nano- OH, Ni–Co–OH and Ni–Co–OH/G-CNF electrodes were flakes morphology at different stages of growth. It seems observed by scanning electron microscopy (SEM) at dif- that nanoflakes in Fig. 2f have grown incompletely. It can ferent magnifications. The elemental spectra and mapping be concluded that increasing Co ratio to 100:10 and 100:25 distribution in Ni–Cu–OH and Ni–Co–OH electrodes are slows nanoflake formation and growth, even though the confirmed by energy dispersive X-ray spectroscopy (EDX). CBD time was fixed for all studied electrode compositions. SEM was equipped with EDX from FEI Company (Nova Nanoflake morphology is considered as one of the most Nano SEM 630). G-CNF was characterized using X-ray preferred morphologies for electrochemical supercapaci- diffraction (XRD) from Bruker Corporation (A D8 Advance tors due to the large possible interface between nanoflake System) and Raman spectroscopy from HORIBA Scientific. surface and electrolyte. Therefore, it is expected that the nanoflake morphology, such as in Fig. 2a, d and e, leads to Electrode performance higher capacitance than the flat morphology, such as in Fig. 2b, c and f. Small magnification SEM images of all Ni–Cu–OH, Ni–Co–OH and Ni–Co–OH/G-CNF were used Ni–Cu–OH and Ni–Co–OH electrodes of 100:3, 100:10 as working electrodes. Saturated calomel electrode (SCE) and 100:25 are shown in Figure 2S (a)–(f) in the Supple- and a platinum (Pt) wire were used as reference electrode mentary Information which shows that the carbon micro- and counter-electrode, respectively. The aqueous elec- fibers substrates have been conformally coated. Graphene trolyte solution was 1 M of potassium hydroxide (KOH). and carbon nanofibers (G-CNF) can be seen clearly in Electrochemical redox reactions were studied by cyclic Fig. 2g. The diameter of an individual CNF is around voltammetry (CV). Electrode capacitances and cycling 200 nm as shown in Figure 2S (g) in the Supplementary stability were calculated by chronopotentiometric (gal- Information. Figure 2h shows Ni–Co–OH coated on vanostatic) charge–discharge (CD). Equivalent series G-CNF using Ni:Co ratio of 100:10 and a deposition time 123 21 Page 4 of 9 Mater Renew Sustain Energy (2015) 4:21 (a) Ni Cu C 20µ O K L L (b) C 20µ O Ni Co K K L L Fig. 1 a, b EDX elemental mapping images for Ni–Cu–OH and Ni–Co–OH electrodes, respectively Fig. 2 SEM images of a–c Ni–Cu–OH electrodes of 100:3, 100:10 and 100:25, respectively. d–f Ni–Co–OH electrodes of 100:3, 100:10 and 100:25, respectively. g G-CNF electrode. h Ni–Co–OH/G–CNF electrode. All electrodes are coated on carbon microfibers substrates of 1 h. Using the same ratio but with a deposition time of than that of graphite indicating that the interplanar distance 2 h leads to increased nanoflake growth as shown in Fig- between graphene nanosheets is increased and the particles ure 2S (h). size is decreased. D band and G band of Raman spectrum XRD pattern of G-CNF is compared with that of gra- are shown in Figure 3S (b) confirming graphene formation phite in Figure 3S (a) in the Supplementary Information. It in our G-CNF sample. Figure 3S (c) shows a photograph of can be seen that (002) peak of G-CNF at 26.5 is broader G-CNF dissolved homogenously in water and methanol. 123 Mater Renew Sustain Energy (2015) 4:21 Page 5 of 9 21 Electrochemical performance previous changes in the potential plateau, number of pla- teaus and shape of charging–discharging curves can be due Figure 3a and b shows the CD behavior of Ni–Cu–OH and to structural re-arrangement in the electrode materials as Ni–Co–OH electrodes of 100:10, respectively. Before confirmed by CV curves in Fig. 3c and d. CD curves at 10,000 cycles, there are two separate charging plateaus. different current densities of all Ni–Cu–OH and Ni–Co– The first plateau spans a potential of 0.348 to 0.450 V, OH electrodes of 100:3, 100:10 and 100:25 are shown in while the second plateau spans from 0.450 to 0.532 V. Figure 4S of the Supplementary Information. After 10,000 cycles, the first charging plateau is shifted Figure 3c and d illustrates CV behavior of Ni–Cu–OH toward higher potential from 0.419 to 0.480 V, while the and Ni–Co–OH electrodes of 100:10, respectively. Before second plateau covers from 0.480 V to the pre-set maxi- 10,000 cycles, it can be seen in Fig. 3c that there are two mum charging potential (0.532 V) which is fixed during all partially overlapping oxidation peaks that can be attributed 10,000 cycles. This shift in the range of charging plateaus to Ni(OH) /NiOOH and Cu(OH) /CuOOH based on the 2 2 toward higher potentials can be one of the reasons behind following well-known reactions in which electrons can be the capacitance decay during long CD cycles as confirmed stored and then released, reversely: and explained in the following CV results section. In - - 1. Ni(OH) ? OH $ NiOOH ? H O ? e 2 2 addition, there are two separated discharging plateaus after - - 2. Cu(OH) ? OH $ CuOOH ? H O ? e 2 2 10,000 cycles instead of one discharging plateau. Similar CD behavior is also observed for Ni–Co–OH electrode as After 10,000 cycles, the oxidation peak of Ni–Cu–OH seen in Fig. 3b, but with one broad plateau in the discharge electrode is shifted towards higher potential than the initial curve consisting of two or more overlapping plateaus. As one of the same electrode. In addition, there are two sep- expected, both Ni–Cu–OH and Ni–Cu–OH electrodes have arated reduction peaks after 10,000 cycles instead of the shorter charging–discharging time after 10,000 cycles, initial overlapping one. These two observations are in indicating a decrease in capacitance as shown in Fig. 4. All agreement with a similar reported case for Ni(OH) (a) (c) (e) 0.7 15 10 A/g Before 10000 cycles Ni:Cu of 100:10 0.6 After 10000 cycles 10 Before 10000 cycles Ni:Cu of 100:25 Ni:Cu of 100:10 After 10000 cycles 0.5 Ni:Cu of 100:3 0.4 0.3 4 -5 0.2 -5 0.1 -10 -10 10 mV/s -2 10 mV/s Ni:Cu of 100:10 0.0 0.2 0.4 0.6 0.0 Potenial vs. SCE [V] -15 -15 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 20 40 60 80 100 120 140 160 Time [s] Potenial vs. SCE [V] Potenial vs. SCE [V] (b) (d) (f) 15 15 0.7 10 A/g Before 10000 cycles Ni:Co of 100:10 After 10000 cycles 0.6 10 Before 10000 cycles 10 Ni:Co of 100:25 After 10000 cycles Ni:Co of 100:10 0.5 Ni:Co of 100:3 5 5 0.4 0 0 0.3 -5 1 -5 0.2 0.1 -10 -10 10 mV/s -1 0.0 0.2 0.4 0.6 Ni:Cu of 100:10 10 mV/s Potential vs. SCE [V] 0.0 -15 -15 0 20 40 60 80 100 120 140 160 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 Potential vs. SCE [V] Time [s] Potential vs. SCE [V] Fig. 3 a, b CD curves of Ni–Cu–OH and Ni-Co–OH electrodes of cycles are shown in the insets. e, f CV curves of Ni–Cu–OH and Ni– 100:10, respectively. c, d CV curves of Ni–Cu–OH and Ni–Co–OH Co–OH electrodes, respectively, of 100:25, 100:10 and 100:3 electrodes of 100:10, respectively. Enlarged CVs curves after 10000 Potential vs. SCE [V] Potential vs. SCE [V] Current [mA] Current [mA] Current [mA] Current [mA] Current [mA] Current [mA] 21 Page 6 of 9 Mater Renew Sustain Energy (2015) 4:21 (a) (c) (e) 100 1000 10 A/g 10 A/g 10 A/g Before 10000 cycles Before 10000 cycles Ni:Cu of 100:25 After 10000 cycles After 10000 cycles 80 800 769 Ni:Cu of 100:10 Ni:Cu of 100:3 71%C 71%C 69%C Drop s Drop Drop 40 400 53%C 53%C 372 219 226 s s 69%C 400 20 200 265 Drop 155 Drop Drop 73 68 0 0 0 2000 4000 6000 8000 10000 100:25 100:10 100:3 100:25 100:10 100:3 Cycle Number Ni:Cu Ratio Ni:Cu Ratio (b) (d) (f) 1400 2000 10 A/g 10 A/g 10 A/g Before 10000 cycles Before 10000 cycles Ni:Co of 100:25 After 10000 cycles After 10000 cycles Ni:Co of 100:10 80 1600 Ni:Co of 100:3 60 1200 45%C 62%C 80%C Drop 600 Drop 80%C 40 Drop Drop 400 62%C 45%C Drop 252 20 226 Drop 400 200 156 0 0 0 100:25 100:10 100:3 0 2000 4000 6000 8000 10000 100:25 100:10 100:3 Ni:Co Ratio Cycles Number Ni:Co Ratio Fig. 4 a, b The cycling stability curves of Ni–Cu–OH and Ni–Co–OH electrodes, respectively. c, d Areal capacitances of Ni–Cu–OH and Ni– Co–OH electrodes, respectively. e, f Specific capacitances of Ni–Cu–OH and Ni–Co–OH electrodes, respectively electrode [11] and can be explained based on Bode’s dia- electrode of 100:25 has better redox reversibility than Ni– gram of the electrochemical phase transformation of nickel Co–OH electrodes of 100:10 and 100:3 as indicated in hydroxide in alkaline solution [24]. One of the Bode’s Fig. 3f. This redox reversibility seems to lead to better diagram findings is that the redox potential of b-Ni(OH) / cycling stability of the devices. CVs at different scan rates b-NiOOH is higher than that of a-Ni(OH) /c-NiOOH. of all Ni–Cu–OH and Ni–Co–OH electrodes of 100:3, Analogous to Bode’s diagram, our Ni–Cu–OH electrode 100:10 and 100:25 are shown in Figure 5S of the Supple- may have a-Ni(OH) /c-NiOOH and a-Cu(OH) /c- mentary Information. 2 2 CuOOH-coupled phases in the initial cycles and then they Figure 4a and b represents the cycling stability curves of may be transformed to b-Ni(OH) /b-NiOOH and b- Ni–Cu–OH and Ni–Co–OH electrodes, respectively, of Cu(OH) /b-CuOOH-coupled phases due to prolonged 100:3, 100:10 and 100:25. It can be concluded that using cycling in alkaline electrolyte (KOH). Similar CV behavior Ni–Cu–OH and Ni–Co–OH electrodes of 100:25 leads to is also observed for Ni–Co–OH electrode as seen in higher capacitance retention during 10,000 CD cycles at 10 Fig. 3d. However, three separated reduction peaks are A/g, than that of other electrodes of 100:10 and 100:3. This observed after 10,000 cycles, which supports our prediction improvement in cycling stability can be ascribed to the of the phase transformations from a-Ni(OH) c-NiOOH and characteristic difference in the redox reversibility as indi- a-Co(OH) /c-CoOOH in the initial cycles to b-Ni(OH) /b- cated by previous CV curves. In addition, Ni–Co–OH 2 2 NiOOH and b-Co(OH) /b-CoOOH in the final cycles. electrode of 100:25 exhibits higher capacitance retention of Figure 3e and f compares the CVs of Ni–Cu–OH and 55 % than 47 % exhibited by Ni–Cu–OH of 100:25, after Ni–Co–OH electrodes, respectively, at the same scan rate 10,000 CD cycles at 10 A/g. Areal capacitances of Ni–Cu– of 10 mV/s. Based on the well-generalized relationship OH and Ni–Co–OH electrodes are compared in Fig. 4c and stating that smaller potential separation between CV redox d, respectively, while specific capacitances are compared in peaks indicates higher redox reversibility, it can be con- Fig. 4e and f for Ni–Cu–OH and Ni–Co–OH electrodes, cluded that our Ni–Cu–OH electrode of 100:25 has better respectively. As Ni–Cu–OH and Ni–Co–OH electrodes can redox reversibility than Ni–Cu–OH electrodes of 100:10 be used in rechargeable alkaline batteries, areal and and 100:3 as indicated by Fig. 3e. Similarly, Ni–Co–OH specific capacities are calculated and compared in Capacitance Retention [%] Capacitance Retention [%] Areal Capacitance [mF/cm ] Areal Capacitance [mF/cm ] Specific Capacitance [F/g] Specific Capacitance [F/g] Mater Renew Sustain Energy (2015) 4:21 Page 7 of 9 21 (a) (b) 12000 800 After 10000 cycles Before 10000 cycles Ni:Cu of 100:10 700 Ni:Cu of 100:10 Ni:Co of 100:10 Ni:Co of 100:10 50 50 0.6 0.6 0.4 0.4 40 500 40 0.2 0.2 0.0 0.0 30 30 11.8 12.0 12.2 12.4 11.6 11.8 12.0 12.2 Z [Ω] Z [Ω] 4000 20 20 10 10 0 0 0 1020304050 0 1020304050 Z [Ω] Z [Ω] 0 2000 4000 6000 8000 10000 12000 0 100 200 300 400 500 600 700 800 Z [Ω] Z [Ω] Fig. 5 a, b Complex impedance (Nyquist) plots of Ni–Cu–OH and Ni–Co–OH electrodes of 100:10 before and after 10,000 cycles, respectively. Enlarged Nyquist plots are shown in the insets (a) (c) (b) 0.8 1800 Ni-Co-OH 10 A/g 10 mV/s 0.7 Ni-Co-OH/G-CNF Ni-Co-OH Ni-Co-OH Ni-Co-OH/G-CNF 1400 Ni-Co-OH/G-CNF 0.6 0.5 7 0.4 0 5 0.3 -10 0.2 0.1 -20 789 10 11 12 13 14 Z [Ω] 0.0 0 200 400 600 800 1000 1200 1400 1600 1800 0 20 40 60 80 100 120 140 160 0.0 0.2 0.4 0.6 0.8 Z [Ω] Time [s] Potential vs. SCE [V] (e) (d) (f) 100 700 10 A/g 10 A/g 10 A/g 1400 Before 10000 cycles Before 10000 cycles Ni-Co-OH After 10000 cycles After 10000 cycles Ni-Co-OH/G-CNF 1200 62%C 41%C 409 s Drop Drop 62%C 40 600 Drop 516 41%C 200 Drop 20 249 0 0 0 0 2000 4000 6000 8000 10000 Ni-Co-OH Ni-Co-OH/G-CNF Ni-Co-OH Ni-Co-OH/G-CNF Cycles Number Sample Sample Fig. 6 a CD of Ni–Co–OH and Ni–Co-OH/G-CNF electrodes. b CV plots are shown in the insets. d Cycling stability curves of Ni–Co–OH of Ni–Co–OH and Ni–Co-OH/G-CNF electrodes. c Nyquist plot of and Ni–Co–OH/G-CNF electrodes. e, f Area and specific capacitances Ni–Co–OH and Ni–Co–OH/G-CNF electrodes. Enlarged Nyquist of Ni–Co–OH and Ni–Co–OH/G-CNF electrodes, respectively Figure 6S of the Supplementary Information. Both areal OH and Ni–Co–OH electrodes 100:3 leads to better areal and specific capacitances have been calculated from CD capacitance than that of other electrodes of 100:10 and curves of Fig. 4a and b. It is concluded that using Ni–Cu– 100:25. Such differences in capacitances are expected due P o te n tia l v s . S C E [V ] C a p a c ita n c e R e te n tio n [% ] -Z'' [ Ω ] -Z'' [ Ω ] -Z'' [ Ω ] A re a l C a p a c ita n c e [m F /c m ] C u rre n t [m A ] -Z'' [ Ω ] -Z'' [ Ω ] -Z'' [ Ω ] S p e c ific C a p a c ita n c e [F /g ] -Z'' [ Ω ] -Z'' [ Ω ] 21 Page 8 of 9 Mater Renew Sustain Energy (2015) 4:21 to the different morphologies as shown in previous SEM Conclusion images. Ni–Co–OH electrode of 100:10 seems to be the optimum composition of our electrode as it has higher In this project, Ni–Cu–OH and Ni–Co–OH electrodes were capacitance retention than Ni–Cu–OH and Ni–Co–OH prepared using different Ni:Cu and Ni:Co ratios by CBD at electrodes of 100:3 in addition to higher areal and specific room temperature on carbon microfibers. It is observed that capacitances than Ni–Cu–OH and Ni–Co–OH electrodes incorporation of Co at lower concentration (100:10) of 100:25. enhances nanoflake formation and hence increases capac- Electrochemical impedance spectroscopy results of Ni– itance and cycling stability. The improvement in cycling Cu–OH and Ni–Co–OH electrodes of 100:10 are repre- stability can be ascribed to the characteristic difference in sented by the complex impedance (Nyquist) plots in the redox reversibility as indicated by CV curves. Phase Fig. 5a and b, respectively. At low frequencies, the 45 transformations of Ni–Cu–OH and Ni–Co–OH electrodes phase angle of Ni–Co–OH electrode’s Nyquist plot repre- are indicated by the shifting and the splitting of CD pla- sents the presence of Warburg impedance (W) that is due to teaus and the shifting and the splitting of CV redox peaks. diffusion of electrolyte ions to the electrode. At high fre- Composites based on the ternary hydroxides, graphene and quencies, Ni–Cu–OH electrode has smaller equivalent carbon nanofibers on carbon microfibers increase potential series resistance (ESR) than Ni–Co–OH electrode, as can window, decrease equivalent series resistance (ESR), areal be seen from the x-axis intercept. After 10,000 cycles, the capacitance and enhance cycling stability. semicircles became more obvious and larger than the initial Acknowledgments Research reported in this publication was sup- semicircles. Such increase in charge transfer resistance ported by King Abdullah University of Science and Technology (R ) during prolonged cycling is one of the factors behind CT (KAUST). the decrease in capacitance. Figure 6 shows the electrochemical performance of Ni– Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// Co–OH/G-CNF electrode compared with the previous Ni– creativecommons.org/licenses/by/4.0/), which permits unrestricted Co–OH electrode of Ni:Co ratio of 100:10. Adding G-CNF use, distribution, and reproduction in any medium, provided you give to Ni–Co–OH electrode leads to increase potential window appropriate credit to the original author(s) and the source, provide a as seen in CD and CV curves in Fig. 6a and b, respectively, link to the Creative Commons license, and indicate if changes were made. as expected. Figure 6c of Nyquist plot shows that the ESR of Ni–Co–OH/G-CNF is smaller than that of Ni–Co–OH due to the electrical conductivity of graphene. Moreover, adding G-CNF to Ni–Co–OH electrode leads to enhanced capacitance retention over 10,000 cycles as shown in References Fig. 6d. This improvement in the cycling stability can be explained by the fact that there is no phase transformation 1. BCC Research. Supercapacitors: Technology Developments and Global Markets. Report No. EGY068B. BCC Research involved during the charge–discharge process of G-CNF (2015). ISBN: 1-62296-035-1 electrode. Figure 6e and f represents areal capacitance and 2. Conway, B.E.: Electrochemical Supercapacitors: Scientific Fun- specific capacitance, respectively. Ni–Co–OH/G-CNF damentals and Technological Applications. Springer, New York electrode has higher areal capacitance (507 mF/cm ) than (1999). doi:10.1007/978-1-4757-3058-6 3. 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Materials for Renewable and Sustainable Energy – Springer Journals
Published: Oct 28, 2015
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