Electrochemically Stabilized Porous Nickel Foam as

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Journal of Nanoengineering and Nanomanufacturing Vol. 4, pp. 50–55, 2014 (www.aspbs.com/jnan)

Electrochemically Stabilized Porous Nickel Foam as Current Collector and Counter Electrode in Alkaline Electrolyte for Supercapacitor Kunfeng Chen and Dongfeng Xue∗ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

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ABSTRACT This work systematically studied the chemical properties and electrochemical properties of porous Ni foam in KOH electrolyte within positive potential window. SEM, cyclic voltammetry and galvanostatic charge–discharge cycling measurements were used to characterize the surface morphologies, redox couple, and capacitive properties of porous Ni foam in alkaline electrolyte. The electrochemical properties of Ni foam as counter electrode in three electrode electrochemical cell also were evaluated. The -Ni(OH)2 /-NiOOH and -Ni(OH)2 /-NiOOH redox couples can be selectively obtained. Low concentration electrolyte shows low specific capacitances owing to its low conductivity and little amount of OH− . The specific capacitance of Ni foam is less than 1 F/g calculated from CV and galvanostatic discharge in alkaline electrolyte. Although Ni foam itself does have capacitive properties in alkaline electrolyte, the capacitance contribution from the nickel foam keeps at the lowest level. The present results prove that by subtracting the contribution from the nickel foam, the porous Ni foam is a Delivered by Publishing Technology to: Guest good current collector and counter electrode in alkaline supercapacitors withUser well electrochemical stability. IP: 84.74.127.76 On: Sun, 18 Oct 2015 18:27:04 KEYWORDS: Nickel Foam, Current Collector, Pseudocapacitive, Alkaline Electrolyte, Counter Electrode. Copyright: American Scientific Publishers

1. INTRODUCTION Current collectors are important part in electrochemical energy storage, such as copper foil and aluminum foil for lithium-ion battery anode and cathode, nickel foam, stainless steel, Ti foil for supercapacitors.1–6 In order to improve the electrochemical properties of electrochemical energy storage system, their internal resistance must be kept low. Particular attention must be paid to the contact impedance between the active materials and the current collector.1 2 7 Active materials directly grown on the current collector have already been shown to decrease ohmic drops at the interface between active materials and current collector.1 2 7 8 In this field, self-supported, conductive material-free, binder-free electrodes can be obtained, which is advantageous to design high-energydensity devices.7–9 The design of nanostructured current collectors with an increased contact area is another way to control the interface between current collector and active materials. By growing Cu nano-pillars on a planar Cu foil, a six-fold improvement in the energy density over planar electrodes has been achieved for lithium-ion battery.9 ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: 20 April 2013 Accepted: 30 May 2013

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Recently, extensive research has been launched into the development of nanostructured porous films because of their high surface-to-volume ratio and short path length for ion transportation.1 7 Nickel foam is a low density permeable material that has a very high porosity with typically 75–95% of the volume consisting of void spaces. Nickel foam has a wide variety of applications in heat exchangers, energy absorption, and rechargeable battery applications, such as Ni–Cd and Ni–H rechargeable batteries and hydrogen storage unit.10 Recently, nickel foam has been applied as current collector in the study of electrochemical performance of supercapacitor in most works.7 8 11–13 The proper current collector should satisfy following conditions: (a) high conductivity, (b) chemical stability during electrolyte and the potential window, (c) mechanical strength. Because most works reporting very high capacitance values applied nickel foam as current collector, it is quite indispensable to evaluate the potential contribution of nickel foam to the specific capacitances of active electrode materials.4 7 8 Previously, it is reported the nickel foam possessed pseudocapacitive performance and nickel foam used as the current collector can bring about substantial errors to the specific capacitance values of electrode materials, especially when small amount of electrode active material are used in the measurement.4 The two papers proved that the nickel foam 2157-9326/2014/4/050/006

doi:10.1166/jnan.2014.1168

Chen and Xue

Electrochemically Stabilized Porous Nickel Foam as Current Collector and Counter Electrode

3. RESULTS AND DISCUSSION Figure 1 shows the surface morphologies of porous Ni foam before and after electrochemical measurements. Before CV, the Ni foam shows typical smooth surface (Fig. 1(a)). Optical image shows the porous character of Ni foam and silvery metallic luster (Fig. 1(b)). By contrast, a small amount of particles were formed on the surface of Ni foam after electrochemical measurement, which indicated the occurrence of redox reaction during electrochemical process. To investigate the change of Ni foam during electrochemical process, CV experiments were performed. The CVs were firstly measured in 2 M KOH electrolyte at rate of 5 mV/s with three-electrode cell from 0.1 V to 0.45 V. Figure 2(b) shows the 1st and 50th CVs, which shows that both of anodic and cathodic peaks are shifted to negative potential and their peak current intensity are increased. The results proved that the capacitances of nickel foam were firstly increased, then kept stable. During the electrochemical processes, the following chemical reactions can be occurred: Ni + 2OH− → NiOH2 + 2e− −

(3) −

NiOH2 + OH ↔ NiOOH + H2 O + e

2. EXPERIMENTAL DETAILS

(4)

KOH are purchased from Aldrich and nickel foams are In higher concentration of KOH electrolyte, Ni(OH)2 can purchased from Changsha Liyuan Technology Develop- Technology Delivered by Publishing Userof metal Ni foam. However, the be formedto: at Guest the surface IP: foam 84.74.127.76 On:ofSun, 18 Oct 2015 18:27:04 ment Co. Ltd. Clean porous nickel with a size reaction 3Publishers is irreversible. The redox current peaks in CV Copyright: American Scientific electrochemical 1 × 1 cm2 was used in this work. The can be mainly related to Faradaic reaction of Ni(OH)2 measurements were performed in a three electrode elec(reaction 4), which is reversible. The chemical reaction is trochemical cell with KOH aqueous solution (0.1 M, 1 M, also the reaction mechanism of Ni–H rechargeable battery 2 M) as electrolyte, nickel foam as working electrode, and Ni(OH)2 supercapacitor. The results prove that the Ni saturated calomel electrode (SCE) as reference electrode foam can indeed contribute to the specific capacitance of and a Pt wire as counter-electrode. CV measurements the electrode. However, the highest peak current is less and Chronopotentiometry (CP) measurements were carried than 0.5 mA at scan rate of 5 mV/s, which indicated little out on a CHI660D electrochemical workstation. The morcontribution of the nickel foam to electrochemical capaciphology of nickel foam was observed by both a digital tance. The specific capacitance of Ni foam calculated from camera and Field-emission scanning electron microscopy CV is shown in Table I, which is smaller than the reported (FESEM, Hitachi-S4800). specific capacitance value of Ni(OH)2 , 500–1000 F/g and The specific capacitance of Ni foam can be calculated theoretical value of 2082 F/g for 0.5 V. from CV according following equation: Figure 2(a) shows CVs with different scan rates. With C = Q/mV

(1)

where C (F · g−1 is the specific capacitance, m (g) is the mass of Ni foam without active material, Q (C) is an average charge during the charging and discharging process, and V (V) is the potential window. The specific capacitance can be calculated based on discharge/charge profiles of Ni foams according to the following equation: C = It/mV

(2)

where C (F · g−1 is the specific capacitance, m (g) is the mass of Ni foam without active material, I (A) is discharge current, t (s) represents discharge time, and V (V) is the potential window. J. Nanoeng. Nanomanuf., 4, 50–55, 2014

increase of scan rate from 5 mV/s to 50 mV/s, redox peak currents are increased and the change of peak potential is little, which indicates better electrochemical activity. The specific capacitances of Ni foams at different scan rate are shown in Table I, which indicates the little contribution to specific capacitance during electrochemical measurement. It is interesting that the number of reduction peak is different at Figures 2(a) and (b). At the initial electrochemical process, only one reduction peak formed. After 100 cycles, two reduction peaks present in the CV. The present of new reduction peak proves the formation of new phase during the electrochemical reaction. It has been reported that,17 Ni(OH)2 can generally exist in two different crystallographic forms designated as -Ni(OH)2 and -Ni(OH)2 . In addition, Ni(OH)2 can oxidized into two 51

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was a good current collector in alkaline supercapacitors as long as we subtract the contribution from the nickel foam to get correct specific capacitances of the active material.8 11 The specific capacitance of this Ni foam calculated from CV curves were 4.3, 4.2, 4.0, 3.4 and 2.7 F · g−1 at 1, 2, 5, 10 and 20 mV · s−1 , respectively.11 Herein, we systematically studied the surface chemical properties and electrochemical properties of nickel foam in different concentrations of KOH electrolyte within positive potential window. The surface morphologies, capacitive properties, and redox couple of porous nickel foam before and after electrochemical measurement were characterized by SEM, cyclic voltammetry (CV) and galvanostatic charge–discharge cycling measurements. The electrochemical measurements were performed in different concentrations of KOH electrolyte, different scan rates, and different cycling ways, such as CV and galvanostatic charge–discharge. The electrochemical properties of Ni foam as counter electrode in three electrode electrochemical cell also were evaluated. The present results prove that the nickel foam is a good current collector and counter electrode in alkaline supercapacitors.


(a)

Chen and Xue

(b)

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1 mm (c)

(d)

(e)

(f)

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(g)

(h)

Fig. 1. SEM images of pristine porous nickel foam (a) and porous nickel foams at different electrochemical measured conditions (c)–(h). The porous nickel foams were obtained after 100 discharge/charge cycles in KOH electrolyte (c)–(h). Optical image of fresh porous nickel foam (b).

NiOOH varieties, -NiOOH and -NiOOH.18 The existence of different redox couples, -Ni(OH)2 /-NiOOH redox couple and -Ni(OH)2 /-NiOOH or -NiOOH, can explain the presence of the two reduction peaks during the backward sweep. 52

We further investigate the chemical and electrochemical properties of Ni foam in different concentrations of KOH electrolyte. The CV curves and discharge/charge profiles of Ni foams measured with different concentrations of KOH electrolyte are given in Figure 3. In 0.1 M KOH J. Nanoeng. Nanomanuf., 4, 50–55, 2014

Chen and Xue


(a) 0.003

(a) 0.0005 5 mV/s 10 mV/s 50 mV/s

0.0003

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0.000 C1 –0.001

0.1 M 1M 2M

0.0004

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0.0002 0.0001 0.0000 –0.0001 –0.0002

–0.002 0.10

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0.1 M 1M 2M

0.4 50th cycle

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0.0002 0.0001 0.0000 –0.0001

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–0.0002

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–0.0003 0.1

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IP: (V) 84.74.127.76 Potential

Fig. 2. (a) CVs with different scan rates in 2 M KOH electrolyte. (b) Variation of CV curves before and after 50 discharge/charge cycles, in 2 M KOH electrolyte at the rate of 5 mV/s.

electrolyte, the potential of redox peaks are higher than that in high KOH concentration, while the current of redox peaks is smaller. Ni foam in higher KOH concentration exhibits larger specific capacitance value than that in low concentration of electrolyte. Specific capacitances of Ni foams calculated from CV and galvanostatic discharge is shown in Table II. Low concentration electrolyte shows low conductivity and can provide little amount of OH− for the electrochemical reaction (Eq. (4)), which are responsible for the low specific capacitances. It is indicated that the capacitance contribution from the nickel foam is kept in low level, which can reduce the substrate effect by subtracting the capacitance of nickel foam. During the electrochemical measurement of Ni foam, we found different measurement method can control the formation of different Ni(OH)2 and NiOOH phases, which Table I. Specific capacitance of Ni foam at different scan rates in 2 M KOH. Scan rate (mV/s)

Specific capacitance from CV (F/g)

5 10 50

J. Nanoeng. Nanomanuf., 4, 50–55, 2014

0.53 0.55 0.45

1.5

2.0

2.5

3.0

3.5

Time (s)

Fig. 3. (a) CVs with different concentrations of KOH electrolyte at scan rate of 5 mV/s. (b) Discharge–charge curves with different concentrations of KOH electrolyte at current rate of 0.1 A/g.

can be proved by the presence of different reduction peaks in CVs. In addition to supercapacitor based on the electrochemical double-layer capacitance (EDLC), another class of capacitor is based on pseudocapacitance, and thus associated with electrosorption and surface redox processes at high surface area electrode materials such as metal oxides and conducting polymers.1 12 14 19 The nickel foam shows a pair of redox peaks with low current intensities, which shows the reaction mechanism of the pseudocapacitance. This redox couple is attributed to the reversible reaction of Ni(II)/Ni(III) formed on the nickel surface (Eq. (4)). Pure -Ni(OH)2 is unstable in the alkaline electrolyte, which is easily transformed to the -Ni(OH)2 . Large Table II. Specific capacitance of Ni foam in different concentrations of KOH electrolyte. Specific capacitance F/g

Concentration of KOH electrolyte (M)

CV

Galvanostatic discharge

0.1 1 2

0.19 0.35 0.53

01 034 045

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Current (A)

0.0003


discharge capacity can be obtained for -Ni(OH)2 /NiOOH redox couple because the oxidation state of Ni in the -NiOOH reaches 3.3–3.7.20 -NiOH2 ↔ -NiOOH theoretical capacity = 289 mA h/g

(5)

-NiOH2 ↔ -NiOOH theoretical capacity = 480 mA h/g

(6)

(a)

0.0006 A

0.0005

different reduction peaks by using different measurement method. During potential-sweeping cycles in 1 M KOH electrolyte, the main anodic and cathodic peaks shift to higher potentials, which is due to un-stoichiometric property of electrochemically active nickel hydroxide and nickel, and the number of cathodic peak was changed from one (-Ni(OH)2 /-NiOOH redox couple) to two (C1 and C2 Fig. 4(a)). The development of new cathodic peak (C2 ) is due to the formation of -Ni(OH)2 /-NiOOH redox couple. However, there only exists one anodic peak. It is because that -Ni(OH)2 is unstable in the alkaline electrolyte, and when -Ni(OH)2 is formed at the initial stage of electro-oxidation of the Ni electrode, it is further slowly converted to the -Ni(OH)2 .21 Figures 4(b) and 2(a) show that the relative intensity of two cathodic peaks is different, and this can be controlled by using different measurement methods. With consecutive potential-sweeping cycles, the current intensity of C2 cathodic peak is larger than that of C1 cathodic peak (curve II and III in Fig. 4(b)). Curve I in Figure 4(b) shows higher current intensity of C1 cathodic peak, where the measured Ni foam is not pretreated with pressing step. Curve III

0.0004

(a)

0.0002 0.0001 0.0000 –0.0001 –0.0002

C2

–0.0003 C1

–0.0004 0.10

0.003

Pt counter electrode

Ni counter electrode Delivered by Publishing Technology to: Guest User 0.002 IP: 84.74.127.76 On: Sun, 18 Oct 2015 18:27:04 Copyright: American Scientific Publishers

Current (A)

Current (A)

0.0003

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0.001 0.000

–0.001 0.25

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–0.002

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–0.003

(b) 0.0000

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I –0.0001

(b)

II

Pt counter electrode 0.4

Ni counter electrode

–0.0002 III C1 –0.0003

C2

–0.0004

–0.0005 0.10

Potential (V)

Current (A)

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In the -Ni(OH)2 /-NiOOH redox couple, the number of the transferred electron is 1, while the number of the transferred electron can reach 1.67 in the -Ni(OH)2 / -NiOOH redox couple. Therefore, the synthesis of -Ni(OH)2 has received much attention as active material for the Ni-MH battery and supercapacitor. The present results prove that -Ni(OH)2 and -Ni(OH)2 can be selectively obtained during electrochemical measurement. Figure 4 shows the CV with

Chen and Xue

0.15

0.20

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0.40

0.3

0.2

0.1

Potential (V) Fig. 4. (a) CV curves show the presence of two reduction peaks after 100 cycles in 1 M KOH electrolyte at the rate of 5 mV/s. A represents anodic (oxidation) peak and C represents cathodic (reduction) peak. (b) Variation of two reduction peaks in CV curves after different electrochemical measurements. The CVs were measured in 2 M KOH electrolyte at the rate of 5 mV/s. I–III represent different measurement methods.

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0.0 0

5

10

15

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35

Time (s) Fig. 5. CVs (a) and galvanostatic discharge–charge curves (b) with Ni foam and Pt wire as counter electrodes in three electrode electrochemical cell. J. Nanoeng. Nanomanuf., 4, 50–55, 2014

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In summary, we systematically studied the chemical and electrochemical properties of porous Ni foam in different concentrations of KOH electrolyte within positive potential window. SEM, cyclic voltammetry and galvanostatic charge–discharge cycling measurements were used to characterize the surface morphologies, redox couple, and capacitive properties of porous Ni foam in alkaline electrolyte. It is interesting that Ni foam can serve as a counter electrode in three electrode electrochemical cell, and can show well electrochemical stability. The present results prove that -Ni(OH)2 and -Ni(OH)2 , and their oxidation products, -NiOOH and -NiOOH, can be selectively obtained during electrochemical measurement. -Ni(OH)2 /-NiOOH redox couple and -Ni(OH)2 / -NiOOH redox couple can be reflected in CV with different anodic and cathodic peaks. Low concentration electrolyte shows low conductivity and can provide little amount of OH− for the electrochemical reaction, which are responsible for the low specific capacitances. The specific capacitance of Ni foam is less than 1 F/g calculated from

J. Nanoeng. Nanomanuf., 4, 50–55, 2014

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in Figure 4(b) shows higher overall current intensity than CV and galvanostatic discharge in alkaline electrolyte. It is that of curves I and II, where the measured Ni foam is proved that the nickel foam itself does have capacitive initially electro-oxidized at 40 C by potential-sweeping properties in KOH electrolyte, however, the capacitance cycles. High temperature help to produce more Ni(OH)2 on contribution from the nickel foam keeps at the lowest level. the surface of Ni foam. Although different Ni(OH)2 phases The present results prove that by subtracting the contrican be formed, the contribution of Ni foam to electrochembution from the nickel foam, the porous Ni foam is a ical property of active materials is kept at the lowest level. good current collector in alkaline supercapacitors with well In order to further evaluate the electrochemical stability stability. of Ni foam, we used Ni foam as counter electrode in a three electrode electrochemical cell. Figure 5 shows the Acknowledgments: Financial support from the NationCVs and galvanostatic discharge–charge curves with Ni al Natural Science Foundation of China (grant nos. foam and Pt wire as counter electrodes. Figure 5(a) shows 50872016, 20973033 and 51125009) and National Natuthe potential and current of anodic and cathodic peaks with ral Science Foundation for Creative Research Group (grant Ni foam as counter electrode are similar to that with Pt no. 21221061), and Hundred Talents Program of Chinese wire as counter electrode. Figure 5(b) also shows their Academy of Science is acknowledged. discharge–charge curves have similar contour. However, the specific capacitance shows little difference in value. References and Notes The specific capacitance with Ni foam as counter electrode 1. P. Simon and Y. Gogotsi, Nat. Mater. 7, 845 (2008). is 33.6 F/g, while the value is 44 F/g with Pt wire as 2. K. Chen, S. Song, and D. Xue, CrystEngComm 15, 144 (2013). counter electrode. When measured active materials with 3. F. Liu, S. Song, D. Xue, and H. Zhang, Adv. Mater. 24, 1089 (2012). 4. W. Xing, S. Qiao, X. Wu, X. Gao, J. Zhou, S. Zhuo, S. B. Hartono, high specific capacitance, several hundred F/g, the little and D. Hulicova-Jurcakova, J. Power Sources 196, 4123 (2011). difference in value can not affect the evaluation for the 5. K. Chen and D. Xue, CrystEngComm 15, 1739 (2013). active materials. Measured as counter electrode, Ni foam 6. J. Liu, H. Xia, D. Xue, and L. Lu, J. Am. Chem. Soc. 131, 12086 shows electrochemical stability during the electrochemical (2009). measurement. It is significant to using Ni foam as counter 7. Y. F. Yuan, X. H. Xia, J. B. Wu, J. L. Yang, Y. B. Chen, and S. Y. electrode instead of Pt wire due to low-price of Ni. Guo, Electrochim. Acta 56, 2627 (2011). Delivered by Publishing Technology to: Guest User 8. X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X. Zhao, and H. Fan, ACS IP: 84.74.127.76 On: Sun, 18 Nano Oct 2015 18:27:04 6, 5531 (2012). Copyright: American Scientific Publishers 4. CONCLUSIONS 9. P. L. Taberna, S. Mitra, P. Poizot, P. Simon, and J. M. Tarascon,