Self-assembly of graphene-based planar micro-supercapacitor with selective laser etching-induced superhydrophobic/superhydrophilic pattern
Facile production of graphene-based in-plane interdigital electrode is critical for microscale supercapacitors but remains a great challenge (time consuming, expensive equipment needed, etc). The preparation of fine interdigital electrode planar micro-supercapacitors (MSCs) by selective laser etching of ceramic substrate is proposed, which provides an alternative method for rapid fabrication of planar MSCs. The formation of self-assembled electrode of graphene ink via the laser etching-induced heterogeneous surface wettability was firstly investigated, and then an all-solid-state MSCs microdevice was obtained by adding a polyvinyl alcohol–sulfuric acid gel electrolyte to the interdigital electrode region. The results of cyclic voltammetry test showed that the micro-supercapacitor device could offer an area capacitance of 5.5 mF cm−2 at a potential scan rate of 5 mV s−1 and the capacitance retention of 88.5% after 4500 cycles.
KeywordsSupercapacitors Interdigital electrode Self-assembly Surface wettability
With the widespread use of portable electronic components, demands for multifunctional, small-scale electrochemical energy storage devices are growing in modern society . In recent years, micro-supercapacitors (MSCs) are gradually becoming one of the emerging and cutting-edge research directions in the study of on-chip energy storage devices . MSCs, as a micropower source, is compatible with various microelectronic components, which has broad application prospects, such as powering for microsensor , biomedical implants , and microrobots . The graphene-based two-dimensional materials can be used as electrode materials for MSCs due to its unique structure and physical behavior, such as excellent surface area and high in-plane conductivity . Liu et al.  adopted inkjet printing technology to prepare planar MSCs on different substrates using electrochemically stripped graphene as electrode material. The area capacitance of paper-based MSCs reached 5.4 mF cm−2, which provides a new idea for the development of new flexible MSCs.
Compared with traditional sandwich supercapacitors, graphene-based planar interdigital MSCs have a greater advantage in that they own features of both graphene and planar device geometry. Yoo et al. reported two kinds of graphene-based planar MSCs, i.e., MSCs based either on the single-layer graphene prepared by chemical vapor deposition or on the multilayer graphene films prepared by reduced graphene oxide. It was found that the area capacitance of planar MSCs were 80 and 394 μF cm−2, respectively, which were much higher than that of traditional sandwich supercapacitors . Combining the photolithography and electrophoretic deposition method, Niu et al.  used PVA/H3PO4 gel as electrolyte to produce ultrathin graphene interdigitated patterned electrode on PET substrate, thus flexible and ultrathin solid-state graphene-based MCSs were obtained. Because the electrolyte ions are limited in the narrow gap between the planar interdigital electrodes, the ion diffusion distance is very short and they can be easily transmitted to provide better area capacitance.
However, there are still some problems in the design and production of planar MSCs. For instance, the large distance between the interdigital electrodes will result in poor specific energy and specific power [5, 10]. So, it is still a great challenge to further enhance the electrochemical performance of MSCs by accurately controlling the number and width of interdigital electrodes. Moreover, traditional micro–nanofabrication technologies are usually costly and complex. Wu et al.  manufactured a planar interdigitated graphene-based MSCs on silicon wafer by combining methane-plasma-assisted reduction with photolithography microfabrication. These methods require complex equipment and multiple steps, and the treatment process of the generated waste is expensive and time consuming. Therefore, we need to exploit an advanced fine pattern-processing technology during the preparation process of patterned electrode.
Laser etching has the advantage of facile fabrication of micropatterned electrodes, which allows the gaps and electrode fingers formed in the micropattern and can be easily adjusted and finely integrated on the same substrate . Moreover, laser etching can also change the substrate surface property, which will affect the wetting properties. For example, the patterned superhydrophobic (SH) and superhydrophilic (SHL) structure can be prepared on the surface of various materials by laser selective etching. Sun et al.  provided a facile method for preparing SH/SHL hybrid patterns on aluminum alloy substrate by using a selective picosecond laser etching. He et al.  realized patterned deposition of nanoparticles on SH and SHL composite surfaces fabricated by laser etching on titanium substrates. According to the surface wettability, it is feasible to let the graphene-based aqueous self-assemble into the micro-SHL channel form the hybrid SH/SHL pattern.
In this paper, we employed laser micro–nanomanufacturing technology to fabricate an all-solid graphene-based planar interdigital MSCs. During the fabrication, the graphene-based aqueous can self-assemble alone the patterned channels to form ultra-fine conductive interdigital electrodes. The electrochemical behavior of MSCs demonstrates the attractive application prospects of these self-assembled MSCs.
2 Materials and methods
Alumina ceramic plates (96 wt% of Al2O3, Sigma-Aldrich) with dimensions of 40 mm × 40 mm × 2 mm were used as the rigid substrates. Aqueous graphene ink (8.9 wt% graphene nanopowder, Sixth Element Material Co., Ltd., China) performed as the conductive ink materials for preparing electrodes. Concentrated sulfuric acid (H2SO4) and polyvinyl alcohol (PVA) were purchased from Sigma-Aldrich Company to be used as gel electrolyte.
The laser scanning process was carried out once again on the fabricated SH surface to fabricate the SHL interdigital pattern. In the laser scanning process, the grid interval of laser scanning was 20 μm, and the scanning speed, repeat frequency, and scanning numbers were 1000 mm/s, 20 kHz and 10 times, respectively. Then, a micropipette was used to drop the graphene ink material into the interdigital pattern to form the electrodes, and the self-assembly behavior according to the different surface wettability is shown in Fig. 8.
The surface morphologies of the SH and SHL were observed by a 3D measuring laser microscope (Olympus, OSL4100) and a scanning electron microscope (SEM, Zeiss, SUPRA55) before and after the laser treatment. Electrochemical characterization of the prepared MSCs was done by means of cyclic voltammetry (CV) test and galvanostatic charge and discharge (GCD) test with an electrochemical workstation.
3 Results and discussion
Surface of the prepared ordered micropillars sample exhibited a “petal effect,” and the water droplets presented a mixed wetting state of Wenzel and Cassie–Baxter on this surface . The surface of this wetting state provided a large solid–liquid contact region to get high adhesion, so excess aqueous droplets will adhere to the SH surface and cannot easily roll to SHL area to contaminate the electrodes. As the scanning pitch increased to 100 μm, the surface of the sample showed a protrusion similar to the “Lotus effect,” indicating that the surface was changed to the Cassie–Baxter wetting state. In this wetting state, it is assumed that the rough plate cannot be completely wetted by the liquid. The average length, height, and width of the microcolumn are 87.94 μm, 63.88 μm, 75.67 μm, respectively, and the average width of the microgroove is 13.75 μm. The microgrooves exhibited the intermittent melting and resolidification effect, which is characterized by a large number of blind holes (Fig. 3e).
The specific parameters of the MSCs designed with 12 and 18 interdigital fingers
Number of the fingers in MSCs
Length, l (mm)
Width, w (μm)
Interspace, i (μm)
GCD curves were tested at different current densities of 5–15 μA cm−2, as shown in Fig. 6c, which reveals the pseudo-linear response feature of a typical MSCs. The presence of a small amount of nonlinear phenomena in the profile means the occurrence of charge transfer, which is due to the surface electrochemically active reaction formed during the laser etching process. At the beginning of each discharge cycle, a slight decrease in IR voltage drop is obvious. We can calculate the ESR of a microdevice by dividing the magnitude of the voltage reduction by twice the specific current . For the decrease in capacitance in CV curve at high discharge rate, we can explicate it with high ESR. As the scanning speed increases, the resistance of the dozens or even hundreds of layers of graphene material restricts the full contact between the electrode and the electrolyte interface, thus restricting the increase in capacitance. In addition, different charging and discharging currents will lead to different charging and discharging time. The larger the current, the shorter the discharging time. Furthermore, the cycling stability of the MSCs(18) was tested up to 4,500 cycles at a scan rate of 80 mV s−1, as shown in Fig. 6d. The CV curves shapes still retained basically unchanged after thousands of cycles and 88.5% capacitance was remained, demonstrating expected stable capacitive behavior.
In fact, we can effectively reduce the average ion diffusion path between the adjacent fingers by narrowing the finger width and increasing the number of interdigitated fingers of MSCs. Consequently, the electrolyte resistance is reduced under the low ion transport constraints . The results highlight the significant part of the MSCs configuration in determining the electrochemical performance of a device.
Since the wettability characteristic will disappear after the aqueous graphene ink solidified in the SHL region, we did not cover the graphene interdigital pattern with current collector by SH/SHL property in this experiment, which prevented our devices from operating at a higher scanning rate.
However, there is a problem to be solved during the procedure. For example, the aqueous graphene ink will diffuse out to the SH area as a result from the wide interface area between the SH and SHL regions, so that the actual width of the interdigital electrode is large than the patterned width formed by the laser direct writing. This makes it necessary to etch a pattern with a narrower interdigitated width to obtain the electrodes satisfy our desired width. We will study this issue in future experiments, such as using a picosecond or femtosecond laser with nonthermal effect to fabricate patterned interdigital electrodes.
A simple and fast method for manufacturing graphene-based planar micro-supercapacitors without mask and photolithographic micropatterning was proposed. The planar MSCs were fabricated via selective laser etching-induced SH and SHL patterns on the alumina ceramic substrate, which allows the aqueous graphene ink rapidly covers the patterned interdigital electrodes area according to the wettability difference. This approach is distinct from the earlier report that the graphene ink can self-assemble on the hydrophilic interdigital pattern area. The resultant microdevices demonstrated relatively good electrochemical characteristics, the area capacitance of MSCs can reach 5.5 mF cm−2 and with cycling stability of ~ 88.5% after 4500 cycles. It is believed that these graphene-based MSCs, as a micropower supply, will have promising applications in miniaturized electronics and other chips.
This work is financially supported by the National Natural Science Foundation of China (Grant Nos. U1609209, 11704285), Zhejiang Provincial Natural Science Foundation (Grant Nos. LZ20E050003, LY17F050006), and the Wenzhou Science and Technology Plan Projects (G20180023, G20170001).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 21.Ding Q, Li WL, Zhao WL, Wang JY, Xing YP, Li X, Xue T, Qi W, Zhang KL, Yang ZC, Zhao JS (2017 ) Plasma assisted fabrication of multi-layer graphene/nickel hybrid film as enhanced micro-supercapacitor electrodes. In: IOP conference series: materials science and engineering, vol 182, no 1, p 012014. IOP PublishingCrossRefGoogle Scholar