A one-step practical strategy to enhance overall supercapacitor performance

Abstract

We introduce a straightforward strategy to simultaneously improve the capacitance, rate capability, and cycle life of a supercapacitor by simply electrodepositing Ni-nanoparticles (Ni-NPs) on an as-prepared electrode. 3D-structured current collectors such as metal foams, metal meshes, and carbon meshes have been widely used in supercapacitors, secondary batteries, glucose sensors, etc. In particular, the 3D-metal foam readily improves device properties due to its unique 3D-nature and high surface area. However, there are practical constraints when applying 3D-current collectors to the industrial world, including high cost. Here, by simply electrodepositing Ni-NPs in a cost-efficient manner, a similar effect to that derived with the use of 3D-metal foam was realized. After deposition, Ni-NPs are preferentially located near the contact area between the active materials and a plate-type current collector, which allows for tight binding between the active materials and the current collector as well as facile charge transfer and high capacitance. The Ni-deposited Ni(OH)₂ electrode pasted on a plate metal substrate showed 350% increased capacitance (1264 F g⁻¹) and stability of 75% and 72% after 10 000 cycles and at a high current density of 20 A g⁻¹, respectively. Given the simplicity and cost-efficiency of this method, it can be readily applied to other energy storage devices with practical applications in the industrial world.

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Introduction

Electrochemical capacitors, also known as supercapacitors or ultracapacitors, offer important advantages as energy storage devices including high energy density, suitable power density, and long lifetimes.^1–3 Supercapacitors have been applied as power sources to a wide variety of electronic equipment such as memory backup devices, military weapons, and electric vehicles.^4–7 Supercapacitors store energy in two ways: by using electrostatic attractive forces (electric double layer capacitors (EDLCs)),^8,9 and using redox reactions occurring near the surface of active materials (pseudocapacitor). Pseudocapacitors have 3–5 times higher capacitance than carbon-based EDLC materials due to the presence of additional redox reactions. Metal oxides such as RuO₂,¹⁰ Co₃O₄,¹¹ NiO,¹² and MnO₂ (ref. 13) are mainly used for pseudocapacitors. Although metal oxides provide their high theoretical capacitances and the capability of relatively good reversible redox reactions, they have several fatal drawbacks caused by the poor conductivity of metal oxide materials impeding their practical use: long delay in the charge transportation rate and thus long charge time, poor capacitance at high scan rates, and poor long-term stability. Recently, direct growth of metal oxides on a 3D-metal foam or other 3D-conducting materials that act as a current collector has been explored in efforts to address these poor pseudocapacitive properties. In fact, most electrodes based on metal oxides have used a 3D-metal foam as a current collector to achieve maximized specific capacitances.^14–17 By growing metal oxides on 3D-current collectors, the metal oxide electrodes obtained increased conductivity as well as shortened diffusion length. As a result, the capacitance of Ni(OH)₂ over 1000 F g⁻¹ has been reported in many studies. However, these capacitance values were mostly obtained utilizing a Ni-nanofoam as a current collector to achieve maximized specific capacitances.^18–21 However, the 3D-current collector is extremely costly and thus the fabricated electrode is still not practically viable.

In this work, we propose a new strategy to address previous problems by achieving similar effects to those derived with the use of Ni-nanofoams such as high capacitance, good kinetic properties, and good cycling stability. By simply depositing cost-efficient Ni particles near the contact region between the active materials and the substrate on the as-prepared Ni(OH)₂ electrode, the contact resistance between active materials and the current collector was decreased. This facilitates rapid transport of ions and charges, leading to improved rate capability and cycle stability. This concept is very simple and easy, it can be extended to other metal oxide electrodes for supercapacitors with real life applicability.

Results and discussion

The Fig. 1a and b present cross-sectional scanning electron microscopy (SEM) images of conventionally prepared electrodes composed of Ni(OH)₂ powders, carbon black, and poly vinyl difluoride (PVDF) with a ratio of 85 : 5 : 10. The thickness of the pasted active materials is ∼50 μm and the 3D nature of Ni(OH)₂ structures was retained after the process of pasting the materials on the current collector (Fig. S1†). The Fig. 1c and d show the electrode after Ni-deposition on the as-prepared electrode. Distinct differences in the samples before and after Ni-deposition were observed near the contact area, which ranges from the bottom to a height of ∼4 μm, as shown in Fig. 1b and d. The magnified SEM images reveal that Ni(OH)₂ petals became thicker due to Ni-particles deposited on the surface of the active materials. It should be noted that the Ni-particles were deposited from the bottom to only a certain extent of height (∼4 μm) at mild electrodeposition conditions, likely due to the low conductivity of Ni(OH)₂; Ni(OH)₂ petals Ni located near the Ni-electrode showed thicker and rough surfaces, and the ones near the top remained thin without any deposited Ni-particles, as shown in Fig. S2.† The insets in Fig. 1b and d show the fringe spacing of Ni(OH)₂ and deposited Ni, measured from transmission electron microscopy (TEM). The spacing of 0.26 nm corresponds to the (012) plane of α-Ni(OH)₂ and the lattice fringe with the spacing of 0.2 nm corresponds to the (111) plane of Ni. The superior performance of the Ni-deposited electrode can be explained from the morphological difference in two-type of samples. As can be seen in Scheme 1, only a small fraction of the Ni(OH)₂ surface contacted the Ni plate before Ni-deposition due to the unique 3D-nature of the Ni(OH)₂ powder, which causes loose contact with the current collector. On the other hand, Ni(OH)₂ could be more tightly adjoined to the substrate after deposition of Ni because Ni-particles were preferentially deposited near the substrate by covering the area of loose contact. As the deposited Ni-particles tightly link the active materials with the substrates without acting as active materials we designate Ni-particles as supporting current collectors (SCCs). As creating SCCs on the electrode, the deposited Ni-particles were tightly connected to each other, forming a conducting network, which is expected to promote fast electron transfer during the cycling process.²²

Fig. 1 Cross-sectional SEM images of devices based on Ni(OH)₂ electrode before (a and b) and after Ni-electrodeposition (c and d). The insets in (b) and (d) show the fringe spacing of the corresponding samples.

Scheme 1 Morphology of the Ni(OH)₂ electrode before and after Ni-electrodeposition.

The improved electrochemical properties by the presence of SCCs were confirmed by the potentiostatic and galvanostatic methods. Fig. 2a shows the cyclic voltammograms of the Ni(OH)₂ electrode before and after forming Ni supporting current collectors (NiSCCs). In order to remove the possibility of overestimation in the capacitance value due to the presence of Ni, we performed the electrochemical test on Ni, which was prepared by Ni-electrodeposition on the current collector without active materials with the same conditions as used for the preparation of the NiSCC electrode (Fig. S7†). We also confirmed that the amount of loaded Ni (0.54 mg) is equal to that used in the NiSCC electrode. The capacitance of the electrode prepared solely with Ni was below 20 F g⁻¹, which is negligible compared to that of active materials. The calculated specific capacitance obtained from the CV curves of pure Ni(OH)₂ and NiSCC–Ni(OH)₂ at 1 mV s⁻¹ is 648 F g⁻¹ and 1363 F g⁻¹, respectively (Fig. S5 and S6†), which presents an increase of 210%. This is ascribed to the creation of firm contact between the current collector and Ni(OH)₂ by the deposited Ni, which facilitates effective transfer of charges.

Fig. 2 Cyclic voltammograms of (a) Ni(OH)₂ and NiSCC–Ni(OH)₂ at 50 mV s⁻¹ and (b) NiSCC–Ni(OH)₂ at various scan rate. (c) Specific capacitance of Ni(OH)₂ and NiSCC–Ni(OH)₂ at different current densities from 1 to 20 A g⁻¹. (d) Capacitance retention plots during 10 000 cycles.

The exact capacitance value of the NiSCC–Ni(OH)₂ electrode after 2 min of Ni-deposition was calculated from the discharge curve which shows an increase from 363 F g⁻¹ to 1264 F g⁻¹ at a current density of 1 A g⁻¹ (Fig. S5 and S6†). It is well known that the poor conductivity of Ni(OH)₂ increases the “dead volume” of electrodes and hampers ion transportation between particles, and thus severely decreases the capacitances of Ni(OH)₂ at a high scan rate and long cycling number.^23,24 However, the dead volume that did not serve as surface storage becomes available due to the improved conductivity in the NiSCC–Ni(OH)₂ electrode. As a result, NiSCC–Ni(OH)₂ presented dramatically improved capacitance values with a maximum increase of 350% for the optimized sample (2 min deposition). This result is really interesting and meaningful. In the previous reports, the Ni(OH)₂ based supercapacitors fabricated on a metal plate showed a lower capacitance²⁵ (about 400 F g⁻¹) than their theoretical values due to the poor conductivity of Ni(OH)₂. Therefore, most of the reports utilized the 3D Ni-nanofoam^18–20,26 rather than the metal-plate as a current collector to overcome these problems although the metal-form is relatively expensive. However, by applying the NiSCC instead of the 3D Ni-foam to the electrode, the cost pressure is relieved without losing the original properties by forming the 3D-conductive networks and direct contact in Ni-SCC based electrodes; the price of Ni-foam in a piece (300 mm × 300 mm) and that of Ni-foil in a roll (600 mm × 2000 mm) are about $482 and $687, respectively. When fabricating the supercapacitor with a dimension of 10 cm × 10 cm, $53.5 and $6 are needed for the current collector of the Ni-foam based and NiSCC-based supercapacitors, respectively, which is a nine times lower cost.

The greatly improved capacitance properties by the presence of SCCs are clearly seen at severe conditions such as high scan rates and long cycling times. Fig. 2c show the variation of specific capacitance of Ni(OH)₂ and NiSCC–Ni(OH)₂ with different scan rates from 1 A g⁻¹ to 20 A g⁻¹. The Ni(OH)₂ electrodes without SCCs at 20 A g⁻¹ showed a dramatic decrease of 62% in the specific capacitance value from 363 F g⁻¹ at 1 A g⁻¹ to 136 F g⁻¹ at 20 A g⁻¹.

On the other hand, the NiSCC–Ni(OH)₂ electrode maintained specific capacitance of 906 F g⁻¹ from 1264 F g⁻¹ with only a 28% decrease at the same condition of 20 A g⁻¹. When high current is applied to the electrodes, only some superficial areas that directly contact the electrolyte are active, and thus the specific capacitance rapidly decreased. However, the NiSCC noticeably facilitates rapid charge transport due to increased conductivity of Ni(OH)₂ with low contact resistance, and thus the specific capacitance is not greatly affected even at high scan rates. Furthermore, the SCC improved the stability of the capacitance as can be seen in the capacitance retention curves in Fig. 2d. During the early cycling period, the capacitance of the pristine Ni(OH)₂ electrode dramatically decreased by 50%, reaching only 38% of the initial capacitance at 10 000 cycles. On the other hand, ∼75% of the specific capacitance was retained in the case of the NiSCC–Ni(OH)₂ electrode at 10 000 cycles. The outstanding stability of NiSCC–Ni(OH)₂ upon long cycling is attributed to the protection of Ni(OH)₂ by Ni-NPs from decomposition of the active materials, thereby sustaining the original structure. When the Ni(OH)₂ electrode is charged/discharged, the electrode is under an equilibrium of a reversible reaction between α-Ni(OH)₂ and γ-NiOOH accompanied by the structural change. The repeated volume expansion/shrinkage due to the interlayer spacing difference between α-Ni(OH)₂ (7 Å) and γ-NiOOH (8 Å) causes Ni(OH)₂ to be detached from the current collector at a high scan rate or during the long cycling process.^27,28 On the other hand, the decomposition of active material might be alleviated in the NiSCC–Ni(OH)₂ electrode because the tightly deposited Ni NPs may buffer the volume change. Therefore, the NiSCC–Ni(OH)₂ electrode readily retained the original specific capacitance value. The advantages achieved by this simple Ni-deposition process can be summarized as follows: first, Ni-particles deposited on both the 3D-Ni(OH)₂ electrode and a current collector plate provide firm binding between active materials and the current collector, thereby decreasing the contact resistance; second, Ni-particles build “conductive networks” similar to 3D-metal foams. This facilitates rapid transport of ions and charges through the shortened pathways, leading to improved rate capability and cycle stability; third, the deposited Ni-particles increase the active sites in electrodes and lead to an enhanced catalytic effect, and thus the specific capacitance is further increased by the synergetic effect; fourth, more importantly, considering the price of Ni-foam and Ni-foil, the Ni-SCC could reduce the cost by about nine times compared to the Ni-foam based one for the fabrication of the supercapacitors with a similar capacitance value of 1300 F g⁻¹.

The enhanced electrical conductivity of electrodes and the ion transfer properties between electrodes and electrolytes were proven by electrochemical impedance spectroscopy (EIS). EIS spectra can be analyzed in three regions based on the measured frequency.^4,29 The intercept value on the real axis at high frequency in the Nyquist plots is called R_s, and represents the ohmic resistance of the electrolyte and the internal resistance of the electrode. The contact resistance of the NiSCC–Ni(OH)₂ with an optimized deposition time (2 min) is 2.9 Ω (4.6 Ω for pristine Ni(OH)₂), which implies the presence of tight binding between the active materials and the current collectors, as shown in Fig. 3. The information about the interfacial charge transfer resistances is obtained from the diameter of the semicircle, R_ct, at the middle frequency region. The diameter of the semicircle of pristine Ni(OH)₂ and NiSCC–Ni(OH)₂ is 1.6 Ω and 0.55 Ω, respectively, a three times decrease after applying NiSCCs on the electrode. The smaller diameter of the semicircles at the middle frequency regions represents fast charge transfer properties of ions in the electrolyte, which explains the reason for the better kinetic properties of the NiSCC electrode at a high scan rate materials. A long tail at the low frequency region in Nyquist spectra provides information on the diffusion of ions in the electrode. The slope of the NiSCC–Ni(OH)₂ electrodes with the vertical feature is higher than the pristine electrode, which implies the NiSCCs make the electrode of supercapacitors more ideal.

Fig. 3 (a) Nyquist plots of Ni(OH)₂ (in black) and NiSCC–Ni(OH)₂ (in red). The insets are an equivalent circuit used for fitting data and a close-up EIS image at the high frequency region.

Conclusions

We have introduced a new concept of a supporting current collector to greatly improve the properties of pseudocapacitors as well as the value of capacitance by simple deposition of Ni-particles on as-prepared Ni(OH)₂ electrodes. NiSCC reduces the contact resistance by tight binding of active materials onto the current collector plate and facilitates fast charge transportation and ion diffusion by increasing the conductivity of the electrodes. In addition, NiSCC plays key roles in preventing α-Ni(OH)₂ from volume. The aforementioned advantages of NiSCC lead to synergetic effects, resulting in a 350% increase in the capacitance value (1264 F g⁻¹) relative to the pristine sample (363 F g⁻¹) and greatly improved stability values of 72% and 75%, at a high scan rate of 20 A g⁻¹ and a long cycle time of 10 000, respectively.

Experimental section

Preparation of the electrodes and NiSCC

The SCC was fabricated by electrodeposition of nickel on the as-prepared electrodes. In order to fabricate the control electrode, synthesized powders (Ni(OH)₂, 85 wt%) were mixed with acetylene black (5 wt%) and polyvinylidene difluoride (PVDF, 10 wt%). 1.2 mg of the mixture was then pasted onto an etched Ni plate and dried at 150 °C in an air atmosphere for 12 hours. The precursor solution for Ni-deposition was prepared in a mixture of 66 g L⁻¹ NiSO₄, 9 g L⁻¹ NiCl₂, and 7 g L⁻¹ H₃BO₃ in D.I water. The as-prepared electrode was dipped into the Ni-precursor solution and electrodeposited at a constant current (−10 mA) condition for various times from 1 min to 5 min to form NiSCC. The carbon grid and the Ag/AgCl reference electrode were used as a counter electrode and reference electrode, respectively. After Ni-deposition, the Ni-deposited electrode was fully washed to remove remaining precursor solutions, and then the samples were dried at 120 °C for 12 hours under vacuum.

Electrochemical measurements

The electrochemical performance of the Ni(OH)₂ electrode with SCC was evaluated using Pt foil as a counter electrode and a KOH solution (1 M) as an electrolyte. The electrochemical properties were conducted by galvanostatic charge/discharge and cyclic voltammetry using a computer controlled electrochemical interface (VMP3 biologic) from 0.1 V to 0.65 V at room temperature. EIS data were obtained at a frequency range from 100 kHz to 0.1 Hz using a potentiostat (Versa STAT 3, AMETEK).

Characterization

The structures of synthesized Ni(OH)₂ and electrodes were characterized using FE-SEM (SEM, FEI/USA Nanonova 230) and high-resolution TEM (FETEM, JEOL TEM 2100) at an accelerating voltage of 200 kV. The crystallinity of the electrodes was checked using a X-ray diffraction system (Bruker D8 Advance system) with Cu Kα radiation (λ = 1.5406 Å) at a scan rate of 2° min⁻¹ the 2θ range from 10° to 80°. The specific surface area, pore size, and pore volume were analyzed using the Brunauer–Emmett–Teller (BET) method with a Belsorp max system (Bel Japan).

Acknowledgements

This work is supported by NRF with the contract no. NRF-2010-0019408 and 2014-M2B2A4030415 (National nuclear R&D program, MSIP) and by the development program of local science park funded by the Ulsan Metropolitan City and the MSIP.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization, SEM (S1 and S2), BET (S3), XRD (S4), CV curve (S5–S7), and Table S1 and S2. See DOI: 10.1039/c4ra12886a

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