Chapter 2 Nickel Hydroxide Supercapacitor with a Theoretical Capacitance and High Rate
2.2 Experimental methods
2.2.2 Deposition of Ni(OH)
Nickel hydroxide was deposited on the as-prepared current collectors in 0.1M Ni(NO3)2solution at a current density of -11 mA (vs. Ag/AgCl) with platinum as a counter electrode. For the 0.2 mg (Figure 2.11a), 0.3 mg (Figure 2.11c), 0.6 mg (Figure 2.11e) and 1.0 mg (Figure 2.10c) nickel hydroxide samples, the electrodeposition times were set to be 48 s, 55 s, 90 s and 180 s, respectively.
2.2.3 Material characterization
The morphologies of the samples were analyzed by Scanning Electron Microscopy (Hitachi S4800) and HR-TEM (JEOL 2100). The structural information of the materials was characterized by an X-ray diffraction system (Bruker AXS D8 Advance). The electrochemical properties of all the prepared samples were measured by using a three-electrode system in a 1.0M potassium hydroxide aqueous solution. To check the electrochemical performances of the Ni(OH)2/3D-Ni series, CV and galvanostatic charge/discharge measurements were conducted (Bio-logic VMP-3). The CVs and galvanostatic charge/discharge curves of the Ni(OH)2/3D-Ni series were measured at scan rates from 1 to 100 mV/s and at current densities from 1 to 100 A/g (vs. Hg/HgO) with a Pt counter electrode. The full-cell system was evaluated using a test cell jig (Rohtec.). The electrochemical impedance spectroscopy was measured in 1.0M potassium hydroxide solution versus a reference electrode of Hg/HgO with a Pt counter electrode from 100 kHz to 0.1 Hz (VersaSTAT AMTEK).
2.3 Results and discussion
The hollow dendritic three-dimensional-Ni current collector was produced by co-deposition of Cu and Ni ions followed by careful etching of the Cu. First, Cu and Ni were co-deposited to form a mossy dendritic 3D metal structure on the nickel film. High voltage in a deposition solution allows hydrogen gas to excessively grow, and facilitates the deposition of Cu and Ni to form the microporous dendritic 3D structure, with the hydrogen gas acting as a template (in Figure 2.1).26–30Since copper has a higher electrical conductivity and standard reduction potential than Ni, Cu is deposited first, forming a porous dendritic structure, and then Ni covers the core of the copper structure (Figure 2.2).27 The standard reduction potential of hydrogen is known to be -0.197V (vs Ag/AgCl). When the Ni-film is under the applied voltage conditions from 0 to -0.7 V, the hydrogen gas is generated by the reduction of hydrogen ion in the acidic media at around -0.2V as shown in Figure 2.2. Since the standard reduction potential of Cu and Ni is +0.143 V (vs Ag/AgCl) and -0.447 V (vs Ag/AgCl), respectively, Cu in the electrolyte solution is deposited first by using generated hydrogen gas as a 3D- dynamic template near the surface of the Ni-film and then Ni is deposited on the surface of Cu by the reduction of Ni ion forming the core- shell type of 3D-Cu/Ni structures on the Ni-film.
As shown in Figure 2.1a, Cu and Ni were co-electrodeposited on the Ni film, making macroporous dendritic CuNi architecture about 30 micrometer scale pores (3D-CuNi). The 3D-CuNi was further subdivided into few-hundred-nanometer-sized ellipsoidal twigs (Figure 2.1c), making dendritic 3D- architecture.
The 3D-CuNi was soaked in a 0.1M ammonium persulfate solution for 1 or 5 min to selectively etching copper from inner structures. The 3D-Ni networks created by removing the 3D-CuNi for 1 min or 5 min are assigned to 3D-Ni-1 or 3D-Ni-5. Once the copper in the inside structure was exposed by
Ni(OH)2/3D-Ni-5. The 2 theta peaks of 3D-CuNi (Figure 2.3a) at 43.3 o, 50.5 oand 74.2 o are from copper and at 44.4 o, 51.8 o and 76.4 oare from nickel. 3D-Ni-1 and 3D-Ni-5 were prepared by the etching of the 3D-CuNi. During the initial etching process, a small amount of nickel of the outside was removed and copper was exposed. After that, the copper was removed. Due to the removal of copper, the XRD patterns of the copper were dramatically decreased (Figure 2.3b and 2.3c). The peaks at 43.3
o, 50.5 o and 74.2 o have almost disappeared with increasing etching time up to 5 min (Figure 2.3c).
After the deposition of nickel hydroxide on 3D-Ni-5, peaks at 21o, 33oand 60ooriginating from the α- Ni(OH)2 are newly formed in the XRD patterns of Ni(OH)2/3D-Ni-5(Figure 2.3d). However, after 5 min of immersing, the 3D-Ni-5 exhibited large hollow pores inside a thin hollow structure, attribute to the nearly complete etching of copper inside the 3DCuNi structure (Figure 2.1e and Figure 2.3 c).
Finally, the surface area of the 3D-Ni-5 was expanded two-fold as compared to the 3D-CuNi (2.1 m2g-
1vs. 0.9 m2g-1, Figure 2.4). Next, Ni(OH)2was deposited on the 3D-Ni-5 (Figures 2.1 f–g, 2.3d and 2.4 a–c). The amount of Ni(OH)2-electrode materials was controlled by adjusting the electrodeposition time.
After the deposition process, the morphology of the 3D-Ni retained, meanwhile the macro void of the 3D-Ni-5 became smaller. The 3D-Ni-5 current collector offers many effective sites in three-dimensional conductive networks, bringing short electron and ion pathways which are helpful for easy and fast transport.31–34
Figure 2.2. Hydrogen evolution reaction on the Ni film in the aqueous electrolyte.
Figure 2.3. The normalized XRD (X-ray diffraction) of the (a) 3D-CuNi, (b) 3D-Ni-1 and (c) 3D-Ni- 5, and (d) Ni(OH)2/3D-Ni-5. The insets are the enlarged patterns of each of the XRD peaks.
The electrochemical properties of the Ni(OH)2-electrode materials on 3D-CuNi, 3D-Ni-1 and 3D-Ni- 5 was tested at a scan rate of 50 mV/s with a potential range of 0.1 - 0.6 V (Figure 2.5a). All the electrodes displayed a pair of redox peaks, indicating reversible reduction/oxidation reactions of the Ni2+/Ni3+ions. However, the 3D-CuNi and 3D-Ni-1, 3D-Ni-5 showed a much larger area in the cyclic voltammetry(CV) curves, implying that an improved electrochemical performance was achieved by the presence of generated pores in the complete hollow-type 3D-Ni-5. Considering that amount of pure nickel was 10.724 mg, 96% of specific capacitance attained from the redox reaction of 0.2 mg Ni(OH)2. Therefore, the specific capacitance of the current collector was negligible (Figure 2.6). The capacitance value induced by pure 3D-Ni-5 without Ni(OH)2at the scan rate of 50 mV s-1is negligible (<3.2 F g-1 vs. 3,637 F g-1), and specific capacitance from the current collector is ignored. This improvement originated from the expanded surface area and a thin nickel hydroxide electrode material, which allows shortened electron pathways from the electrolyte to the current collector (Figures 2.4 and 2.7). Since the amount of active materials is the same, the thickness of the active materials depends on the surface area of the current collector. The 3D-Ni-5 supports the thinnest Ni(OH)2layer, because it has the largest surface area. That allows for the double-sided deposition of active materials. It also provides open pores as an electrolyte reservoir, which facilitates the electrolyte transport and reduces the diffusion resistance.
Figure 2.5b displays the capacitance retention of the three different electrodes with the same amount of nickel hydroxide, compare with the commercial nickel-foam. 3D-CuNi, 3D-Ni-1 and 3D-Ni-5 current collectors had an enhanced rate capability (>90%) as compared to the nickel foam at a current density of 100 Ag-1, due to their reduced size, and the decreased dead volume. 3D-Ni-5, showed the highest capacitance with a good rate stability. The Ni foam has pores with 300 micrometer, whereas the 3D- Ni-5 (Figure 2.8 c and d) had much small-sized pores and was further decorated with mossy dendritic branches (Figure 2.8). Compared to the nickel-foam, 3D-Ni-5 has one order down scaled structures with many more active sites.
Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the transport behaviors of the supercapacitors. The Nyquist plots are demonstrated in Figure 2.5c. The point on the real axis shows the series resistance (Rs), which includes the resistance of the electrode material
and electrolyte and the contact resistance at the interface between the electrode material and the current collector. The Rsof the Ni foam, 3D-CuNi, 3D-Ni-1 and 3D-Ni-5 were measured to be 2.40, 1.79, 1.56 and 1.32 Ω. There is a reduction in Rswith the increase in etching duration, which is consistent with the generation of active sites, which allows for faster ion access through the generated pores. The semicircle in the high-frequency regions (Rct) indicates excellent charge-transfer properties between the electrolyte and the solid electrode at the interface.8, 9 Compared to the large semicircle of the nickel- foam (Rct=9.13 Ω), the Rct of the 3D current collectors was much reduced, implying lower charge transfer resistance (0.36 Ω for 3D-CuNi , 0.35 Ω for 3D-Ni-1 and 0.31 Ω for 3DNi-5). The 3D-Ni-5 had the least Rct, which had the greatest kinetic performances as a current collector at a high scan rate, along with fast charge and discharge rate. This can be owing to the deposition of double-sided nickel hydroxide layers with reduced electron paths open-pore structure, which facilitates faster electron transfer than the others.
In the low-frequency region, perpendicular to the Z’ axis, indicates efficient diffusion in the electrode due to the presence of pores in the networks.9 The vertical slope indicates the highest capacitive behavior, which makes the 3D-Ni-5 electrode better. This high electrochemical performance comes from hollow 3D-Ni networks act as electrolyte reservoirs with reduced diffusion resistance (Figure 2.7).35
Cycle stability for nickel hydroxide electrodes were measured(Figure 2.5d). Because the Cu was encapsulated by nickel, the cycle stability of 3D-CuNi was relatively higher on the than 3D-Ni-1.
Partially etched 3D-Ni-1 showed the lowest stability, due to the instability of the Cu-exposed sites.
However, after almost all Cu was etched on 3D-Ni-5, Ni(OH)2 exhibited much higher cycle stability (~90%) than the others.
Figure 2.5. Electrochemical performance of Ni(OH)2 on different current collectors; (a) Cyclic voltammogram of Ni(OH)2 on 3D-CuNi, 3D-Ni-1 and 3D-Ni-5 at a scan rate of 50 mVs-1. (b) Capacitance retention with an increase in current density. (c) EIS curve of Ni(OH)2 on the Ni-foam, 3D-CuNi, 3D-Ni-1 and 3D-Ni-5. The inset is a close-up image with the fitted model. (d) The cyclic stability of Ni(OH)2on 3D-CuNi, 3D-Ni-1 and 3D-Ni-5.
Figure 2.6. Cyclic voltammogram of 3D-Ni-5 and Ni(OH)2/3D-Ni-5.
Figure 2.7. A Simple diagram providing a comparison of the Ni(OH)2/Ni film supercapacitor and Ni(OH)2/3D-CuNi, Ni(OH)2/3D-Ni-5.
We then turned to optimizing the nickel hydroxide on the 3D-Ni-5. Figure 2.9a exhibits the cyclic voltammogram curves of Ni(OH)2/3D-Ni-5 at a scan rates (1 mVs-1to 50 mVs-1). Clear reduction and oxidation peaks of the nickel hydroxide were observed, implying that the battery-like behavior of the Ni(OH)2-based electrode. When the scan rate was increased, the symmetry of the I–E curves of Ni(OH)2/3D-Ni-5 was well-retained, implying that the electron conduction and mass transport with battery-like behavior are stable and reversible, with good rate capability.
Galvanostatic charge/discharge curves of the Ni(OH)2/3D-Ni-5 were measured (Figure 2.9b). The specific capacitance of 3,637 F/g was achieved at a 1 A/g, this value is comparable to previously reported studies (Table 2.1), and 97% of the capacitance was maintained at 100 A/g (Figure 2.5b). The stable rate capability of the Ni(OH)2/3D-Ni-5 can be attributed to the short electron path of the 3DNi-5 and large surface area, as described in Figure 2.7.
The specific capacitance was optimized by adjusting the amount of the nickel hydroxide. As the amount of Ni(OH)2was increased, the generated pores were obstructed, and the active surface area was drastically reduced (Figure 2.10). Furthermore, the electron pathways from the surface of the 3D-Ni-5 to the electrolyte became longer through the thicker Ni(OH)2layer, resulting in higher charge-transfer resistance (Figure 2.7).
Figure 2.9c displays the Nyquist plot for Ni(OH)2/3D-Ni-5 with various loading amounts of Ni(OH)2. The Rcthad the minimum value (0.15 Ω) when 0.2 mg were loaded, because the path was the shortest.
The cycle stability was confirmed at a current density of 100 A/g, as shown in Figure 2.9d. The capacitances of all the samples deteriorated for 1,000 cycles due to the fast charge/discharge rate and detachment of Ni(OH)2. For the 0.2 mg sample, the capacitance retained almost the same up to 10,000 cycles after reaching a steady state. The low performance with more nickel hydroxide was likely a result of the blocked of micro- or mesopores and longer electron path (lower Rct). Furthermore, detachment of the nickel hydroxide was observed due to poor adhesion, as the thickness of the electrode materials was raised (Figure 2.11).
Asymmetric supercapacitor performance was measured using activated carbon and Ni(OH)2/3D-Ni-5 as the negative and positive electrodes, respectively. The electrochemical performance of the
AC//Ni(OH)2/3D-Ni-5 asymmetric supercapacitor is demonstrated in Figure 2.12. Figure 2.12a shows large redox peaks, which are characteristic of an asymmetric supercapacitor containing EDLC, and the Faradaic contribution in the potential window of 0.6-1.4 V. When the scan rate was increased from 1 to 100mV s-1, the cyclic voltammogram was not much changed, still exhibiting the good rate capacitive performance of the two electrode system. In Figure 2.12b, the galvanostatic charge-discharge curves measured by varying current densities are quasi-triangular shaped, indicating a reduced charge transfer resistance in a mixed EDLC and Faradaic system. The specific capacitance of the AC//Ni(OH)2/3D-Ni- 5 asymmetric supercapacitor was 332 F g-1at 1A g-1and 220 F g-1of the capacitance still remained at 20 A g-1(Figure 2.12c). The energy density of 22.4 Wh kg-1was achieved at a power density of 696.6 W kg-1. This is comparable to many previously reported two electrode systems based on carbonaceous material//nickel hydroxide, as shown in the Ragone plot (Figure 2.13). As shown in Figure 2.12d, the AC//Ni(OH)2/3D-Ni-5 asymmetric supercapacitor still had 67% of its initial capacitance after 12,000 cycles, indicating that the asymmetric supercapacitor still exhibited good reversible electrochemical performance.
The enhanced electrochemical properties of the three-dimensional dendritic current collector with a Ni(OH)2-based supercapacitor owes to the following: (i) The dendritically interconnected structure offers a better electrically conductive pathway for electrons, which enables rapid charge transfer while retaining the desirable rate performance. (ii) The abundance of open pores and the large surface area in the mesoporous hollow dendritic 3Dnetworks provides a great number of active sites, which promotes the redox reactions by providing easy access to the active surfaces. (iii) Compared to other nickel hydroxide supercapacitors, this work is a binder/conducting agent free system.
As a whole, these results show the superior potential of easily produced hollow dendritic 3D-Ni
Figure 2.9. (a) Cyclic voltammetry (CV) and (b) galvanostatic charge–discharge curves of the Ni(OH)2/3D-Ni-5 versus an Hg/HgO reference electrode with a scan rate of 1 to 50 mVs-1and a current density of 1 to 100 Ag-1. (c) EIS of Ni(OH)2 of 0.2, 0.3 and 0.6 mg deposited on 3D-Ni-5 and (d) capacitance retention of samples at the current density of 100 Ag-1.
Active mateiral Current collector
Potential range
in three electrode Electrolyte Specific
capacitance Ref No.
NiCo2O4nanosheet Ni foam -0.1 to 0.4 V
(vs SCE) 3M KOH 2010 F g-1
at 2 A g-1 16 NiCo(OH)2nanosheet Ni foam 0 to 0.5 V
(vs SCE) 1M KOH 2682 F g-1
at 3 A g-1 36 Amorhpous Ni(OH)2
Ni core shell on Ti foil
0 to 0.5 V
(vs SCE) 1M KOH 2848 F g-1
at 1 mV s-1 37 Nanoporous Ni(OH)2
thin film
Ultra thin graphitic foam
0 to 0.5 V
(vs Ag/AgCl) 6M KOH 1560 F g-1
at 0.5 A g-1 38 Ultrathin porous
Ni(OH)2-MnO2
Ni foam 0 to 0.5 V
(vs SCE) 1M KOH 2628 F g-1
at 3 A g-1 39 MnCo2O4@Ni(OH)2
belt-based nanoflowers Ni foam 0 to 0.5 V
(vs SCE) 2M KOH 2154 F g-1
at 5 A g-1 40
Ni(OH)2thin layer 3D-Ni-5
0.1 to 0.6 V
(vs Hg/HgO) 1M KOH 3637 F g-1
at 1 A g-1 This work
Table 2.1. Comparison study of Ni(OH)2 based supercapacitor in this work and previously reported studies.
Figure 2.11. SEM images of (a and b) 0.2 mg, (c and d) 0.3 mg and (e and f) 0.6 mg Ni(OH)2deposited 3D-Ni-5 after 10000 cycles. Locations where Ni(OH)2has detached are marked with red circles.
Figure 2.12. (a) Electrochemical performance of the asymmetric AC//Ni(OH)2/3D-Ni-5 supercapacitor with a scan rate of 1-100 mV s-1and (b) current density of 1-20 A g-1. (c) Capacitance retention as a function of discharge currents. (d) Capacitance retention of the asymmetric AC//Ni(OH)2/3D-Ni-5 supercapacitor as a function of cycle number.
Figure 2.13. Ragone plots of the asymmetric AC//Ni(OH)2/3D-Ni-5supercapacitor.
2.4 Conclusion
In brief, we have fabricated a nickel hydroxide-based supercapacitor on a hollow dendritic 3D-Ni network current collector. The hollow three-dimensional nickel current collector offers a high surface area for the nickel hydroxide, resulting in a thin layer of nickel hydroxide, which offers lower charge- transfer resistance and reduced electron pathways, resulting in improved electrical performance. The specific capacitance was recorded up to 3,637 F/g. About stability rate capability were maintained after 10,000 cycles and ~90% of the cycle, and up to ~97% at a current density of 100 A/g, respectively.
These results clearly suggest that Ni(OH)2/3D-Ni-5 is a promising energy material for high- performance supercapacitors.
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