Stereolithographic 3D Printing-Based Hierarchically Cellular Lattices for High-Performance Quasi-Solid Supercapacitor
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Stereolithographic 3D printing was introduced to fabricate polymeric lattices.
Electroless plating was employed to make the lattices conductive and mechanically robust.
Hydrogen bubbles were served as template for engineering the hierarchically porous graphene.
The supercapacitor device holds great promise in the rational coupling of energy and power density.
Keywords3D printing Lattices Graphene Supercapacitor Porous structure Stereolithography
As potentially promising power candidate to cater for the rapid improvement in the wearable electronics, portable sensors, triboelectric nanogenerators, and solar cell, supercapacitors with a strong coupling of the energy and power density are highly demanded to meet its practical capability [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Building ordered 3D architecture with hierarchical pores is one of the most efficient ways to significantly improve related electrochemical performance for the energy storage, as the spatial configuration of the orderly porous structures can not only shorten the diffusion path of ions penetrating into the electrode materials but also enhance the robust mechanically structural integrity of electrode materials during long-time cycling compared to the conventional stochastic arrangements [11, 12, 13, 14, 15, 16, 17]. The free space within the 3D porous architecture can further serve as a buffer reservoir to allow enough electrolyte infiltration through the continuous channels, bringing high ion transportation. Thusly, self-assembly of using sacrificial templating and additive manufacturing known as 3D printing is generally two typical strategies used to prepare the orderly arranged 3D porous lattices [18, 19]. However, either the high cost for large-scale production or the limited custom-built geometric topology of the templating method restricts the practical application of building supercapacitor. Alternatively, 3D printing technology recently has been demonstrated superior structural manipulation and complex prototype abilities on energy storage and engineering application, which were extensively evidenced by our or other studies [19, 20, 21, 22, 23, 24, 25, 26, 27].
The direct ink writing (DIW)-based 3D printing technology so far was broadly employed for supercapacitor application because of its low-cost, adjustable, and scalable merits [28, 29, 30]. However, its rigid rheological requirement for the preparation of ink materials significantly limits the manufacturing of true 3D lattices in achieving improved energy storage ability. In this contest, many researches recently focused on stereolithography-based 3D printing due to its advantageous low cost, high efficiency, and ease of processability for fabricating almost any complex polymer-based structure [31, 32]. Yet to make the polymeric substrate materials conductive is a challenge which principally determines the fast electronic transportation in the supercapacitor. Carbonization or doping conductive nanoparticles into the photopolymer resin is practically limited because it not only weakens the mechanical integrality of the lattices but also suppresses the high-resolution ability of the stereolithography [31, 32]. Additionally, it is vital to further enhance the capacitive output of the lattices through modifying new materials such as the 2D graphene materials with high electrochemical performance . To fully utilize the macroscopic electrochemical functions of graphene materials with high surface area, a promising way is to prepare large-scale assembly of graphene-based building blocks and graft their inherent properties onto 3D structures. However, an issue to be addressed is the serious stacking of the graphene sheets that significantly lower the ion-accessible surface area, limit the size of the channels, and increase the electronic resistance during the process of macroscale assembly . There is thereby an urgent need, but it is still a significant challenge to rationally build 3D porous graphene electrode materials on the stereolithographic 3D-printed lattices.
Herein, we proposed the stereolithographic 3D printing technology to fabricate the rationally designed stretching-dominated polymeric lattices with octet-truss topology as the basic substrate for supercapacitor. The electroless plating manner was then employed to make the substrate conductive and enhance mechanical behavior. To fabricate the hierarchically porous architecture into the reduced graphene oxide (rGO), we used the hydrogen bubbles as soft templates which were inspired by our earlier study . Through these above-advanced routes, a hierarchically porous graphene-based composite lattice was successfully fabricated, and its great potential application in quasi-solid supercapacitor device with high energy density was carefully investigated and demonstrated. This design concept that combines the 3D printing with hierarchically porous graphene architecture would not only guide the researchers to further improve the energy storage for supercapacitors through a new way but also show a novel concept for engineering other functional electronics.
2 Experimental Section
2.1 Preparation of the Composite Lattices
Firstly, the rationally designed computer-aided design (CAD) file of the stretching-dominated octet-truss architecture was directly imported into the customized 3D software Creation Workshop with slicing distance of 25 µm for the 3D printing with an exposure time of 20 and 8 s for bottom and left parts of the sample, respectively. The 3D geometry structure of the polymeric lattices was chosen to be 5 unit cells wide by 5 unit cells long by 1 unit cell tall. The overall size of the fabricated lattices was 20.00 × 20.00 × 4.00 mm3, with a strut diameter of 500 µm. The polymer lattices were synthesized acrylate-based UV photosensitive resin (L101, Nova 3D), mainly composed of non-toxic acrylic polyester and curable using a 405-nm light. The sample was then carefully taken away from the substrate and immersed into the alcohol solution and deionized (DI) water with ultrasonic for 1 min, respectively, to sufficiently remove the uncured photopolymer. After drying it enough, the printed lattice prototype was electroless deposited with nickel–phosphorus (NiP) metal layer to make it conductive. This process is similar to our earlier reports . Briefly, the dried sample was thoroughly immersed into 10 g L−1 of SnCl2 for 20 min, washed gently with DI water and further dried at room temperature. Upon drying, the treated polymeric lattices were again placed into an activation solution containing 0.25 g L−1 PbCl2 and 10 mL L−1 hydrochloric acid for 20 min. Similarly, after being washed with DI water and dried, the activated lattices were carefully immersed into the water bath and kept at 90 °C for 15 min to obtain the metallic composites. For the NiP plating solution, 40 g L−1 of NiSO4∙5H2O, 20 g L−1 of sodium citrate, 10 g L−1 of lactic acid, and 1 g L−1 of dimethylamine borane were mixed homogeneously with further dilution of fivefold. With regard to the copper plating solution, its composition is similar to the reports elsewhere with a slight modification . Then, 14 g L−1 of CuSO4 5H2O, 12 g L−1 of NaOH, 16 g L−1 of potassium sodium tartrate, 20 g L−1 of EDTA·2Na, 26 mL L−1 of HCHO, 20 mg L−1 of 2,2′-dipyridyl (− 1), and 10 mg L−1 of potassium ferrocyanide were accordingly prepared. Note that the prepared temperature is kept at 45 °C during the copper plating procedure. The final composite lattices were obtained after DI water washing and drying. More detailed information is found in supporting information.
2.2 Preparation of the Porous rGO
The rGO layer acting as the electrode materials was synthesized via the electroplating way. To obtain the 3D porous rGO, the synthesized NiP-coated lattices, namely NiP/polymer that serves as the scaffold (or current collector), were directly immersed into the 3 mg mL−1 of GO solution (Hengqiu Tech. Inc., Suzhou) mixed with 0.1 mL L−1 HCl for electrodepositing active materials. After electrodeposited for 5 min under the voltage of 30 V (the Pt plate was regarded as the counter electrode), the as-synthesized porous rGO hydrogel (rGO-1) was washed with abundant DI water and further reduced with a 0.1 M ascorbic acid at 90 °C for 6 h (rGO-2) . Finally, the as-synthesized sample was freeze-dried at − 80 °C for 24 h and dried in a vacuum oven and the mass loading of sample was to be ~ 2.2 mg cm−2.
2.3 Preparation of the Quasi-Solid Supercapacitor
The gel-type electrolyte was prepared according to our previous procedure , or similar methods reported elsewhere . Polyvinyl alcohol (PVA, 6 g) was dissolved in DI water (50 mL) under vigorous magnetic stirring at 100 °C until the solution became transparent enough, and 20 mL of KOH solution (0.3 g mL−1) was added slowly. The prepared rGO lattices were immersed into the above gel electrolyte for 10 min, and it was repeated three times. Then, two lattices were carefully compressed together with the commercial polypropylene (PP) membrane with a thickness of 0.15 mm as separator into the 3D-printed sealing shell. Finally, it was carefully sealed by the 3D-printed shell.
3.1 Structural Characterization
The structural and morphological information was characterized by field emission scanning electron microscope (FESEM, Quanta 450, 20 kV) equipped with energy-dispersive X-ray (EDX) and the transmission electron microscope (TEM, JEM 2100F). The phase and crystal structures were identified by X-ray diffraction (XRD, Rigaku SmartLab, Cu Kα, 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was conducted on VG ESCALAB 250 spectrometer with Al Kα X-ray radiation.
3.2 Mechanical and Electrochemical Characterizations
The mechanical performance of the lattice was tested in Material Test System (Alliance RT 30kN, USA). At least three samples were tested to obtain reliable values.
The electrochemical tests were associated with the cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) curves as well as the electrochemical impedance spectroscopy (EIS) measurements. (Frequency ranged from 10−1 to 105 Hz.)
4 Results and Discussion
In summary, the hierarchically cellular lattices were rationally constructed by combining the stereolithographic 3D printing technology with the hierarchically porous graphene electrode materials. Thanks to the unique architecture and good intrinsic conductivity of the composite lattices, fast transport for electrons and ionic kinetics is well improved. Thusly, the as-synthesized supercapacitor device achieved superior areal capacitance (57.75 mF cm−2) and rate capability (70% retention, 2–40 mA cm−2) as well as long lifespan (96% after 5000 cycles), which are comparable to the state-of-the-art carbon-based supercapacitor device. Moreover, the admirable maximum energy density (0.008 mWh cm−2) and power density (12.56 mW cm−2) evidenced their future in practical application and the smart design concept. This study opens a new door in manufacturing the energy storage device in addition to the conventional DIW method and provides a novel concept in rationally building low tortuosity and fast transportation of ordered composite lattices through the synergetic combination of the 3D printing with self-assembly.
The authors wish to thank the Research Grants Council of the Hong Kong Special Administrative Region of China (GRF No. CityU11216515) and City University of Hong Kong (Nos. 7005070 and 9667153), as well as Shenzhen Science and Technology Innovation Committee under the grant JCYJ20170818103206501. Weidong Wang greatly thanks the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2017JM5003).
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