Electronic double layer supercapacitor based on three-dimensional silicon microchannel plates in organic electrolyte Y. W. Xu1, S. H. Xu**1, M. Li1,2, Y. P. Zhu1, L. J. Zhou1, L. W. Wang*1,2 and P. K. Chu2 Spurred by the fast development and commercial appeal of the energy storage device, supercapacitor with large capacity and wide potential range is being more and more popular. An electronic double layer supercapacitor consisting of three-dimensional silicon microchannel plates and electroplated MnO2 nanoflakes as the active materials of the positive electrode is fabricated and evaluated systematically in an organic electrolyte (1M LiClO4 in propylene carbonate). Its composition, morphology, electrochemical impedance and charge storage mechanism are studied and compared to that of a planar structure. The silicon microchannel plates increase the specific area for loading of active materials. This supercapacitor with a large aspect ratio delivers excellent capacitive performance in limited space and weight. A single device made of the materials can power light emitting diodes, demonstrating its commercial potential. Keywords: Supercapacitor, Silicon microchannel plates, Organic electrolyte, Nanostructured materials, Electrochemical properties
Introduction Supercapacitors, also known as electrochemical capacitors, are attractive by virtue of their fast charge/discharge rate, high energy/power density (103–104 W kg21) and excellent cyclic stability (.106 cycles).1–3 Previous research activities have mostly focused on the methods to improve the energy density of a supercapacitor which is defined as follows4 1 E~ CV 2 (1) 2 where C is the capacitance and V is the actual potential window. There are two approaches to increase the energy density. One way is to enhance the capacitance. Establishment of a three-dimensional substrate is a fundamental prerequisite in order to boost the electrochemical performance of a supercapacitor since a substrate with a large surface to volume ratio and available space can provide more contact with the electrolyte enabling loading of a large amount of active materials to enhance the charge transfer rate and activate the charge/discharge processes.5 Besides, manganese dioxide (MnO2) is an alternative active material due to the large theoretical capacity (y1300 F g21),6–9 low cost, natural abundance,
1
Key Laboratory of Polar Materials and Devices, Ministry of Education and Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China 2 Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China *Corresponding author, email
[email protected] **Corresponding author, email
[email protected]
ß W. S. Maney & Son Ltd 2015 Received 7 September 2014; accepted 19 November 2014 DOI 10.1179/1433075X14Y.0000000264
environmental benignity and convenient fabrication methods.10–13 A nanoscale MnO2 structure has a large contact area with the electrolyte, thus providing a large quantity of sites for ions, and is a suitable electrode material for supercapacitors. However, the effective area and additional mass of the substrate materials must be taken into account from the viewpoint of packaged devices. Another effective strategy according to equation (1), is to widen the working voltage window by using an organic electrolyte instead of conventional mild aqueous electrolytes, since the energy is proportional to the square of the working voltage. Moreover, an organic electrolyte in a supercapacitor is not prone to water decomposition, low solubility and corrosion of the inner framework, since both the solute and organic solvent used are of high purity without any free water. Recently, Chen et al. reported an asymmetrical supercapacitor produced on porous kitchen sponge with a voltage range of up to 2.5 V in 1M LiClO4 in propylene carbonate (PC).14 Nam et al. demonstrated the electrochemical performance of MnO2 and carbon nanotubes (CNTs) composite with 1M LiClO4 in PC as the electrolyte in a three-electrode configuration.15 Previous research on supercapacitors based on three-dimensional silicon microchannel plates has mainly focused on the alkaline aqueous environment,16,17 which has drawbacks such as emission of byproducts and hence, an organic electrolyte which has a larger decomposition voltage than aqueous electrolytes may be preferred.14,18 Among previous works, MnO2 was the pseudocapacitive electrode materials, which used fast, reversible redox reactions at the surface of active materials. MnO2 assembled with carbon based materials formed
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a composite electrode and acted as a pseudocapacitive electrode.19 Carbon based materials enlarged the contact area of the active materials and the electrolyte, thus gaining good electrochemical performance.20,21 In this work, an asymmetrical supercapacitor comprising a three-dimensional substrate of silicon microchannel plates (Si-MCPs) is fabricated and demonstrated, based on the electronic double layer charge storage mechanism. MnO2 and CNTs acted as electrodes respectively. The three-dimensional architecture maximises space utilisation for a fixed footprint and channel depth, and the unique combination of the three-dimensional substrate and stretched MnO2 nanoflakes delivers excellent performance.
Experimental methods and materials Preparation of the asymmetrical supercapacitor The Si-MCPs were fabricated on p-type ,100. silicon by a series of MEMS processes as described in Refs. 22 and 23. The square array with channels about 250 mm deep had 565 mm pores and 1 mm thick wall with an intrinsic resistance on the order of kV. The chemical reagents were analytical (AR) grade and used without further purification. The aqueous solutions were prepared with 18 MV deionised water and all the experiments were performed in a clean room at 297 K. Nickel layer and MnO2 were electrodeposited on SiMCPs successively with the details described in Ref. 24. The adhesive nickel layer was conducted to improve the conductivity, serving as the current collector, while MnO2 was synthesised as the active material of the supercapacitor. Afterwards, the as prepared MnO2/Ni/ Si-MCPs electrode was fixed leaving approximately an exposed area of 0.64 cm2. The experiments were repeated on a planar silicon substrate with the same footprint area for comparison. To study the properties and potential of the MnO2/Ni/ Si-MCPs and MnO2/Ni/Si electrodes, asymmetrical supercapacitor devices with a hierarchical structure were fabricated with MnO2/Ni/Si-MCPs or MnO2/Ni/ Si as the positive electrodes. CNT (80 wt-%) powders, acetylene black (Alfa Aesar, .99.9%, surface area5 80 m2 g21, 15 wt-% conducting agent) and polytetrafluoroethylene (a 5 wt-% binder) latex were coated on the porous nickel foam forming the negative electrode to work synergistically with the positive electrode.25,26 The high resistivity of MnO2 might give rise to a restriction in the capacitive performance,27 whereas the association of MnO2 and CNTs combined the advantages of both conductivity and capacity. The electrodes and separator were impregnated with an organic electrolyte consisting of 1M LiClO4 in PC and packaged in a CR2025 cell.
Characterisation The crystal structures of the nanoscale electrodes were determined by X-ray diffraction (XRD, RINT2000; Rigaku, Japan) and the morphology were examined by field emission scanning electron microscopy (S-4800; Hitachi, Japan). A three-electrode electrochemical working station (CHI660D; Chenhua, Shanghai, China) was used in electrochemical characterisation with the as fabricated supercapacitors acting as the working electrodes. Impedance spectroscopy was conducted in the frequency range from 0.01 to 100 000 Hz with a 5 mV perturbation signal.
Electronic double layer supercapacitor
Results and discussion Structure characterisation Figure 1a displays the XRD patterns of MnO2/Ni/SiMCPs and MnO2/Ni/Si. The prominent characteristic peak (311) at about 37u is indexed to amorphous birnessite type MnO2 in the nanocrystals (PDF# 421317)28 in MnO2/Ni/Si-MCPs. The narrow peak near 12u is attributed to MnO2 as an auxiliary judgment. In comparison, the XRD pattern of MnO2/Ni/Si does not show obvious peaks attributable to MnO2 except the characteristic peaks of Si and Ni because the planar Si substrate offers poor space utilisation and loading of active materials. The micromorphology of MnO2/Ni/Si and MnO2/Ni/ Si-MCPs are studied. As shown in Fig. 1b, the rough surface on MnO2/Ni/Si has many bumps and shallow and disordered pores forming triangles to only store a limited amount of active materials. Figure 1c depicts the square array of the Si-MCPs and an overall petal-like structure is observed from the uniform MnO2 nanoflakes. The interwoven and tightly packed MnO2 lamellas exhibit anisotropic morphological characteristics among the cross-linked grids and are fully exposed to the electrolyte. They are firmly anchored on the channels giving rise to a nanoporous thin film facilitating electrolyte infiltration and ion diffusion, thereby yielding excellent electrochemical characteristics. Figure 1d presents the scattered distribution of MnO2 on the sidewalls from a cross-sectional perspective.
Electrochemical characterisation Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are common methods to gauge the capacitive behaviour of a supercapacitor. The CV curves of Ni/Si-MCPs and MnO2/Ni/Si-MCPs at a scanning rate of 100 mV s21 are presented in Fig. 2. Obviously, compared with the enclosed area of the CV curve of Ni/Si-MCPs, the MnO2/Ni/Si-MCPs electrode possesses considerable capacity which strongly proves the charge storage potential of MnO2 and successfully subtracts the effect of Ni layer on capacitance. Figure 3a displays the CV features of the positive and negative electrodes at the same scanning rate of 25 mV s21. The quasirectangular shape of the MnO2/ Ni/Si-MCPs electrode indicates a major restriction in the capacitive performance from the theoretical value due to the high resistivity of MnO2, whereas the combination of MnO2 and CNTs overcomes the drawback finally forming a balanced and complementary configuration. Their working ranges overlap resulting in a stable 1.2 V potential window for the supercapacitor. CV tests are conducted on the as assembled MnO2/Ni/ Si-MCPs asymmetric cell in the potential window of 1.2 V. The uniform and rectangular shaped areas enclosed by the curves obtained at different scanning rates indicate stable performance of the supercapacitor, as shown in Fig. 3b. As the scanning rate increases, the curves gradually change shape caused by the kinetics of electron transportation in the electrode materials and limited adsorption–desorption in the organic electrolyte.29,30 The capacitance can be determined from the CV curves31,32 by equation (2)
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1 a XRD patterns acquired from MnO2/Ni/Si-MCPs and MnO2/Ni/Si; SEM images: b top view of MnO2/Ni/Si and c top view and d cross-sectional view of MnO2/Ni/Si-MCPs
Ð DV C~
i(u)du
o AvDV
(2)
where i(u) is the voltammetric current changing with potential (A), DV is the potential window (V), v is the scanning rate (mV s21) and A is the effective contact area of the electrode immersed in the electrolyte (cm2). The specific capacitance calculated from the CV curve (Fig. 4a) of the MnO2/Ni/Si-MCPs supercapacitor is
2 CV curves of Ni/Si-MCPs and scanning rate of 100 mV s21
MnO2/Ni/Si-MCPs
at
y0.745 F cm22. The basic performance of the MnO2/ Ni/Si supercapacitor can also be extracted from the CV curves, showing that it possesses negligible capacitance at a scanning rate of 100 mV s21 (Fig. 4a). The planar substrate has poor space utilisation and hinders the effectiveness of the nanostructured MnO2. As a result, its effective potential range is only 0.5 V, while the stable cell voltage of MnO2/Ni/Si-MCPs supercapacitor in an organic electrolyte reaches 1.2 V, which exceeds the previous result in aqueous electrolyte.33 Both the CV curves without any reversible redox peaks prove the supercapacitor an electronic double layer capacitor. From the perspective of the charge storage interface, the adsorption and desorption of ions form the basic charge–discharge processes. Because of the viscosity and molecular characteristics of the organic electrolyte, it is not prone to decomposition compared with the previous aqueous electrolyte, although it may impede the movement of the charges. The organic electrolyte to a large extent helps extend the operation voltage range of the supercapacitor and realise the package of the whole energy storage device, which is unattainable by using aqueous electrolyte. To investigate the charge storage processes of the two supercapacitors, electrochemical impedance spectra are taken and fitted with equivalent circuits as shown in Fig. 4b in which Z9 and Z0 are the real and imaginary parts of the impedance. The Nyquist plot shows a spike in the low frequency region that is controlled by mass transfer and a semicircle in the high frequency region
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3 a CV curves of positive and negative electrodes of MnO2/Ni/Si-MCPs supercapacitor at scanning rate of 25 mV s21; b CV curves of MnO2/Ni/Si-MCPs supercapacitor measured at various scanning rates from 5 to 200 mV s21
dictated by the electrode reaction kinetics.34–36 The supercapacitor oscillates between two states: resistive at high frequencies and capacitive at low frequencies.17,35 The resistance of the MnO2/Ni/Si-MCPs supercapacitor
is nearly 4 V as derived from the x-intercept entitled bulk solution resistance (Rs), indicating that the supercapacitor is highly conductive. Comparatively, a resistance of more than 30 V is obtained from the MnO2/Ni/
4 a CV curves of MnO2/Ni/Si-MCPs and MnO2/Ni/Si supercapacitors at scanning rate of 100 mV s21 (inset amplifies CV curve of MnO2/Ni/Si supercapacitor); b Nyquist plot of both supercapacitors (equivalent circuit fits impedance curve, while inset amplifies impedance curve in high frequency region of MnO2/Ni/Si-MCPs supercapacitor); normalised reactive power |Q|/|S| and active power |P|/|S| versus frequency plots of c MnO2/Ni/Si-MCPs supercapacitor and d MnO2/Ni/ Si supercapacitor
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5 Single supercapacitor or series of supercapacitors is demonstrated to power LEDs
Si supercapacitor implying more difficult charge transfer. The diameter of the semicircle is equivalent to the charge transfer resistance (Rct) caused by the double layer capacitance (Cdl) at the electrolyte/electrode interface. A constant phase element (CPE) is proposed to evaluate the diffusion rate and capacitance. Its equivalent impedance can be determined by equation (3) Z~
1 T(l w)P
(3)
where T and P refer to CPE-T and CPE-P respectively. The CPE is defined by two values, CPE-T and CPE-P. Since CPE-P functions as the index of the denominator in the equation, it is usually deemed as the key parameter of CPE. Often, a CPE is used in a model in place of a capacitor to compensate for non-homogeneity in the system, and a CPE with CPE-P value of 0.5 can be used to produce an Infinite Length Warburg element. A Warburg element occurs when charge carrier diffuses through a material. A CPE-P value larger than 0.5 corresponds to the spike in the low frequency range with a large slope in the plot, thus reflecting pronounced capacitive characteristics and small diffusion resistance.37–39 Table 1 lists the values of the elements mentioned above to fit the actual impedance curves. The MnO2/Ni/Si supercapacitor with a smaller Rct value indicates its easier ion traverse path as a planar
substrate. However, the MnO2/Ni/Si-MCPs supercapacitor with smaller contact resistance, larger interface capacitance and more ideal slope in the low frequency region is better than the MnO2/Ni/Si supercapacitor, which is not too promising as an energy storage device. A brief control experiment has been conducted to explore the effects of the elements of two supercapacitors through a series of fitting. The changes of the values of these elements may lead to fitting errors and serious deformation in EIS curves. The deviation between the original EIS curve and fitting curve is all dependent upon the values of these elements. Results illustrate that the key element for MnO2/Ni/Si-MCPs supercapacitor is Cdl, while that for MnO2/Ni/Si supercapacitor is CPE-P. The value of Cdl largely affects the diameter of the semicircle of the fitting curve of MnO2/ Ni/Si-MCPs supercapacitor in the high frequency region, while the value of CPE-P determines the slope of the spike of MnO2/Ni/Si supercapacitor in the low frequency region. The larger value of Cdl of MnO2/Ni/ Si-MCPs supercapacitor indicates the larger double layer capacity. By contrast, the fitting curve of MnO2/ Ni/Si supercapacitor without an arc in high frequency range proves its inferior charge storage capability. The EIS pattern also reveals the form of the complex power (S) based on the resistance related to power dissipation. The real and imaginary parts of S are P
Table 1 Fitted elements in equivalent circuit
Rs/V Cdl/mF Rct/V CPE-P
MnO2/Ni/Si-MCPs supercapacitor
MnO2/Ni/Si supercapacitor
4.101 3.8311 3.752 0.75084
32.93 0.18306 0.20641 0.50809
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(active power (watt) and Q (reactive power, volt ampere reactive) respectively.40 The electrochemical impedance of a supercapacitor is defined as ZðvÞ~
1 jv|C ðvÞ
(4)
Or in its complex form Z ðvÞ~Z’ðvÞzjZ’’ðvÞ
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about 0.2 cm thick and 2 cm in diameter. The weight of the device with Si-MCPs is 1.980 g and that with planar silicon is 2.832 g. The fact that the device with planar silicon is heavier is due to the silicon substrate which only provides support. Even one device with the SiMCPs can power a 3 mm red LED (1.5 V, 10 mA) for 2 min after charging by a constant current source for 8 s, and this performance exceeds that reported in the literature.44 In contrast, the planar device fails to power the LED.
(5)
Conclusion Equations (4) and (5) lead to equation (6) 1 {½Z’’ðvÞzjZ’ðvÞ C ðvÞ~ ~ (6) v|½jZ’ðvÞ{Z’’ðvÞ vjZ ðvÞj2 Or define C ðvÞ~C’ðvÞ{jC’’ðvÞ
(7)
Leading to C’ðvÞ~
{z’’ðvÞ
(8)
vjzðvÞj2
C’’ðvÞ~
z’ðvÞ
(9)
vjzðvÞj2
and the following expressions PðvÞ~vC’’ðvÞjDVrms j2
(10) 2
QðvÞ~{vC’ðvÞjDVrms j
(11) 1=2
where v is the pulsation, DVrms ~DVmax =2 , and DVmax is the signal amplitude. Z9(v) and Z0(v) are the real part and imaginary part of the impedance respectively, defined as Z’ðvÞ2 zZ’’ðvÞ2 ~jZ ðvÞj2 .41 The variation in the normalised imaginary part |Q|/|S| and real part |P|/|S| versus frequencies of the two supercapacitors are presented in Fig. 4c and d respectively. The supercapacitor behaves like a pure resistance originally and so all the power is dissipated at high frequencies. However, it becomes more capacitive as the frequencies diminish.41 In fact, |Q|/|S| varies oppositely from |P|/|S| versus frequency. The only crossover point of the two variables from 0.1 to 100 000 Hz observed from the MnO2/Ni/Si-MCPs supercapacitor appears at the frequency (fo) of y11 Hz. It leads to a dielectric relaxation time constant (to, being the minimum time needed to discharge all the energy with an efficiency of greater than 50%) of y91 ms, revealing fast ion penetrating rate and short diffusion routes in the inner interface of the supercapacitor.42,43 It also represents the transition from a resistive phase at frequencies higher than 1/to to a capacitive phase at lower frequencies. In contrast, the two quasiparallel lines of |Q|/|S| and |P|/|S| signify an infinite relaxation time of the MnO2/Ni/Si supercapacitor, and its EIS pattern without a semicircle in the high frequency region is consistent with its inferior capacitance. The supercapacitor devices composed of MnO2/Ni/SiMCPs and MnO2/Ni/Si are evaluated by powering light emitting diodes (LEDs) as shown in Fig. 5. The device is
In summary, an asymmetrical supercapacitor comprising a positive electrode of MnO2/Ni/Si-MCPs is produced by a template free method. The electrode has a large effective surface area and the nanoflake morphology increases the contact area with the organic electrolyte and promotes the charge/discharge processes. The MnO2/Ni/Si-MCPs supercapacitor has a shorter relaxation time, more extended potential window, smaller contact resistance, and larger specific capacitance than the MnO2/Ni/Si supercapacitor primarily due to the three-dimensional skeleton and the significant application of MnO2, CNTs and the organic electrolyte. A fully packaged supercapacitor with MnO2/Ni/Si-MCPs is lighter than that with planar silicon substrate with the same footprint. One supercapacitor device with the Si-MCPs can efficiently power a 3 mm LED thus demonstrating its commercial promise.
Acknowledgements The authors acknowledge the financial support provided jointly by Shanghai Pujiang Program No. 14PJ1403600, Shanghai Natural Sciences Foundation No. 11ZR14 11000, Shanghai Fundamental Key Project No. 11JC 1403700, China NSFC Grant No. 61176108, PCSIRT, Research Innovation Foundation of ECNU No. 7821 0245, City University of Hong Kong Applied Research Grant (ARG) No. 9667085 and Guangdong–Hong Kong Technology Cooperation Funding Scheme (TCFS) GHP/ 015/12SZ.
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