Direct Laser Writing-Assisted Method for Template-Free Fabrication of Biomass-Based Porous Carbon Platelets with Uniform Size and Arbitrarily Designed Shapes.

Because of a wide range of applications of porous carbon platelets (PCPs), a robust method for their facile synthesis/fabrication with controlled porous structure, size, and shape is constantly needed. Herein, we report a simple and scalable method for producing PCPs with uniform size and arbitrarily designed shapes. This approach relies on CO2 laser irradiation to induce carbonization of a biomass composite sheet formed by the infusion of sodium lignosulfonate into a cellulose paper to create porous carbon features with arbitrarily designed shapes. Upon subsequent water immersion treatment, the laser-written carbon features could spontaneously detach to form freestanding PCPs. The PCPs of different shapes were fabricated, characterized, and demonstrated for their potential applications in dye adsorption, as flexible sensors, and as miniaturized supercapacitors. Our method is expected to make great impacts in multiple fields, such as environment, energy storage, sensing, catalysis, and so forth.

Introduction

Porous carbon platelets (PCPs) with tailored size, shape, and porosity structure[1−6] possess a set of highly desired properties, for example, high surface area, thermal and mechanical stability, tailorable electrical/electronic properties, ease for physical and chemical functionalization, and low density. This enables them to be useful for a variety of promising applications, such as electrodes in electrical energy storage,[7−10] active and passive substrates for catalyst support,[11−14] drug delivery or cell transport,[15,16] adsorbent materials for dye and contaminant removal in water treatment,[17] hydrogen storage and CO2 adsorption,[18,19] electromagnetic wave-absorbing materials,[20,21] and being used as the building blocks for preparing novel complex structured metal–organic framework composites.[22]

The traditional methods used for producing porous carbon materials typically include chemical and physical activation of existing carbon materials, catalytic activation of carbon precursors, carbonization of a polymer blend with the presence of both carbonizable and pyrolyzable components, and pyrolysis of organic aerogels, defluorination of fluorine containing polymers, and chlorination of metal carbides.[5,23−30] In terms of precise control of the size, shape, and pore structure of the PCPs, the newly emerging templating method and its variants[1−3] have rivaled the traditional approaches and received great attention in recent years. The currently developed various templating methods, both hard[7,8,10,31−41] and soft templating approaches,[42−45] typically involve tedious multistep processes including ex situ or in situ template creation and then their subsequent removal. To overcome such a disadvantage, a few researchers[12,46−49] discovered some template-free methods for producing PCPs by wisely selecting combinations of carbon precursors, solvents, catalysts, and carbonization methods. These template-free methods greatly simplified the PCP production process, though with some shortcomings such as the lack of versatility and generality in shape-control of the finally formed PCPs.[1] In addition to the aforementioned disadvantages, the previously developed templating and template-free methods face a great challenge. That is, beyond the commonly achieved spherical or hollow spherical forms, these previous methods have difficulties to produce PCPs of complex shapes. Herein, we report a direct laser writing (DLW)-assisted method for template-free producing PCPs with arbitrarily designed shapes. The DLW technique is a recently emerged method for easy creation of patterned porous carbon materials on the surface of a precursor polymer substrate.[50−53] This method has been successfully demonstrated for fabricating a variety of useful devices including a flexible all-solid supercapacitor,[54] piezoresistive sensor,[55−57] force and humidity dual sensor,[58] pH sensor,[59] water quality-monitoring sensor,[60] 3D insulator–conductor composites,[61] heating elements,[62] and so forth. The flexibility and versatility of the DLW method make this technique a promising tool that allows for integrating the design and control of material structures with the device design and its functionality implementation.

By taking advantage of such characteristics of the DLW method and wisely selecting the carbonizable polymeric substrate as well as utilizing hydrophobic/hydrophilic interfacial interactions, we show in our present work a new template-free method for facile fabrication of PCPs with arbitrarily deigned shapes. The DLW carbonizable substrate used in our work is a composite sheet made of cellulose paper and sodium lignosulfonate (CellP/NaLS), both components of which are widely available and biomass-based renewable materials.[63] Upon CO2 laser irradiation of such a composite sheet, the porous carbon feature with a predesigned pattern and its arrays can be easily formed. A subsequent immersion treatment in water would allow the spontaneous detachment of the patterned carbon arrays to give PCPs with a uniform size and arbitrarily designed shapes. As compared to other methods, our new method for PCP production is simple, versatile, environmentally benign, and highly robust. Upon further extension and optimization works, for example, utilizing the laser irradiation source with a shorter wavelength to achieve precisely controlled spatial and temporal light intensity output, we expect this method could lead to a major breakthrough in facile fabrication of PCPs with tailored pore structures, a wide range of uniform sizes, arbitrarily complex shapes, and multiple-functionalities suitable for many different applications.

Results and Discussion

With varying the loadings of NaLS in the CellP/NaLS composite sheet, we identified that a 50 wt % condition was suitable for generating patterned carbon arrays by CO2 laser irradiation at varied laser power. In addition, such formed carbon arrays can also be robustly detached from the parent substrate by water immersion treatment. With this fixed NaLS loading level, we investigated the effect of laser power on the structure and morphology of the line features created by CO2 laser irradiation at a constant scanning speed of 10 mm/s. Figure a shows the scanning electron microscopy (SEM) images of such freestanding line features created at different levels of laser power. Within the investigated range from 0.6 to 1.6 W, the line width increases from 167 to 305 μm. The expansion of the laser-affected zone with increasing laser power not only occurs in the lateral direction of the line feature but also exists in its depth direction. This is evidenced by the cross-sectional images shown in Figure b. The line features created at varied levels of laser power have been examined for their electrical conductive property. With the resistance value R measured for a given line feature of width W and length L, its sheet resistance was estimated according to Rsq = R × W/L. The Rsq results of the line feature produced at different levels of laser power are shown in Figure c. Within the range of the laser power used in our experiments, the sheet resistance of the line feature varied in a range of Rsq = 400–800 Ω/□, which is an indication of a reasonably good electrical conductor. To obtain small-sized PCPs, it is desired to have the basic feature size like the lines as small as possible. With this in mind and guided by the results shown in Figure a,b, we further explored even lower laser power conditions for creating various types of PCPs. As shown by the Raman spectra in Figure d, the PCPs produced at low laser power (0.3–0.7 W) manifest the characteristic D-band (1343 cm–1) and G-band (1581 cm–1) of carbonaceous materials, which indicates that the CellP/NaLS composite sheet can be carbonized at very low laser power irradiation conditions. The relatively strong D-band intensity with respect to the G-band as well as the broad band width all indicate such formed carbon structures are mostly disordered.

Figure 1

(a) SEM surface morphology and (b) cross-sectional optical images of the DLW-created line features at laser power ranging from 0.6 to 1.6 W (from left to right), all scale bars are 200 μm; (c) sheet resistance of the DLW-created line features at laser power ranging from 0.6 to 1.6 W; (d) Raman spectra of the DLW-created line features with laser power ranging from 0.3 to 0.7 W (from the bottom to the top); (e) XRD pattern and (f) infrared absorption spectrum of the PCPs produced at 0.5 W laser power irradiation; high-resolution C 1s (g) and O 1s (h) XPS spectra and the corresponding deconvoluted results for the PCPs produced at 0.5 W laser power irradiation.

X-ray diffraction (XRD) was also used for examining the structures of the PCP formed at a low level of laser power (0.5 W). Figure e shows the result, which is featured by a broad peak centered at 2θ = 22.5° attributed to the diffraction of the (002) plane of turbostratic and disordered carbons. In Figure e, one also notes the presence of residual cellulose as indicated by the sharp peak that appeared at 2θ = 29.4° because of the diffraction of the (200) crystallographic plane of crystalline cellulose.[64] The presence of residual cellulose as well as NaLS in the carbon structures generated by low laser power irradiation was further confirmed by FTIR. The IR spectrum of the PCPs created at a laser power of 0.5 W is shown in Figure f, where we can identify the functional groups associated with cellulose and NaLS, which include the bands around 3450 cm–1 attributed to the vibration of the hydroxyl group, the peak at 1616 cm–1 originated from C–O stretching, the group of bands that appeared in the region of 1000–1400 cm–1 because of symmetric and asymmetric stretching vibration of the SO2 group, as well as the bands around the region of 2855–2926 cm–1 associated with C–H vibrations.[65] X-ray photoelectron spectroscopy (XPS) revealed the atomic contents of the PCP formed at 0.5 W laser power mainly composed of C, O, S, and Na with a percentage of 68.98, 29.53, 1.17, and 0.32%, respectively. In Figure g,h, the high-resolution C 1s and O 1s XPS spectra of the PCP are respectively shown. According to the previous study,[66] we can conclude that, in addition to the dominated graphitic-like sp2-carbon (284.8 eV, 58.6%), there are also abundant various types of oxides existing on the surface of our PCPs. These groups include phenolic, alcoholic, or ether groups (285.3–286.3 eV) as well as carboxyl or ester groups (288.5 eV). The presence of abundant oxides can also be confirmed by the O 1s spectrum, which shows that the oxides mostly appeared as −OH (532.5 eV, 52.7%) as well as C=O (531.7 eV) and C–O–C (533.2 eV).

We have demonstrated that the CellP/NaLS composite sheet can be readily fabricated into electrically conductive carbon patterns with low-power CO2 laser irradiation. Moreover, when immersed in water, we found that such fabricated carbon patterns can be spontaneously detached from their parent CellP/NaLS composite sheet without breakage. In contrast, there was no such behavior being observed for the similar carbon patterns fabricated by DLW on the commonly used polymer substrate—polyimide.[50,57] The flexibility of the DLW process along with the spontaneous detachment behavior of the carbon patterns formed on the CellP/NaLS composite sheet would allow a new process for easily fabricating freestanding PCPs of different shapes. For concept demonstration, in Figure a1, we show a snap-shot of the detachment of a popular Chinese-knot-shaped PCP from its parent CellP/NaLS composite sheet upon immersion in water. The live video of this process can be found in the Supporting Information V1. In Figure a2, the intact and stand-alone Chinese-knot-shaped PCP after the detachment process is shown. Figure a3 shows a snap-shot of the detachment of an array of disc-shaped PCPs upon immersion in water. The live video for this process can be found in the Supporting Information V2. The thickness of the freestanding PCPs fabricated with this new process (laser power of 0.5 W in DLW) was ∼35 μm as determined by SEM (Figure b), which is roughly a third of the original thickness of the parent CellP/NaLS composite sheet. The results shown in Figure a indicate that the simple process developed here, namely, creating arrays of PCPs of different shapes by DLW first and then immersing the parent CellP/NaLS composite sheet in water, can be potentially used for facile fabrication of PCPs of uniform size with arbitrarily designed shapes.

Figure 2

(a1) Snap-shot of the detachment of a popular Chinese-knot-shaped PCP from the parent CellP/NaLS composite sheet upon immersion in water; (a2) photo of a collected Chinese-knot-shaped PCP; (a3) snap-shot of the detachment of an array of disc-shaped PCPs from the parent CellP/NaLS composite sheet upon immersion in water; (b) SEM cross-sectional view of a PCP generated by a laser power of 0.5 W; (c) optical and (d) SEM micrographs of PCPs with various geometric patterns predesigned through the CAD program. All the scale bars are 500 μm. (e) Bar chart of the length, width, and thickness of PCPs with a preset size of 1.5 mm × 1.5 mm (before and after detachment from the substrate).

To make this point, we applied this new process and fabricated the PCPs with a variety of different shapes. Figure c,d respectively show the optical and SEM images of such prepared freestanding PCPs with different levels of complexity in shape design. Limited by the laser-processing equipment used in our experiments, the smallest carbon feature we produced was a disc of 490 μm in diameter (Figure c6,d6). The accuracy of our new process in size control for fabricating the freestanding PCPs was examined by fabricating multiple PCPs with a predesigned square-shaped pattern of size 1500 × 1500 μm. The results are shown in Figure e. After the DLW process (0.6 W laser power), the as-prepared PCP patterns were measured to have an averaged lateral size of 1550 ± 15 μm × 1466 ± 19 μm, which slightly deviate (1–5%) from the targeted value. Upon water immersion treatment, the finally formed freestanding PCPs showed no significant dimension change, which had an averaged lateral dimension of 1490 ± 26 μm × 1463 ± 14 μm and thickness of 53 ± 6 μm.

The spontaneous detachment of the as-prepared PCP upon water immersion treatment left a fractured surface at both the PCP side and the side of its parent CellP/NaLS composite sheet. The morphology of the fractured surfaces was examined by SEM; and the results are shown in Figure a–d. The low magnification images reveal the flat and smooth morphologies of the fractured surface at both the PCP side (Figure a) and the parent composite sheet side (Figure c), which suggests that the breakage/fracture at the PCP/cellulose fiber interface occurs quite uniformly across the entire surface of the PCP. As shown by the high-magnification images, the fractured surface at the PCP side (Figure b) is dominated by a hierarchical porous structure. In contrast, the fractured surface left on the parent composite sheet mainly shows a solid texture occasionally decorated by a few imprinted porous fiber-like structures (Figure d). The formation of porous structures observed on the PCP side is because of pyrolysis/carbonization of the CellP/NaLS composite upon laser irradiation treatment, which has not occurred at the side of the CellP/NaLS composite sheet, to leave a solid texture. The morphological results shown in Figure a–d indicate that, as compared to the strength of bulk PCP and CellP/NaLS composite, the interfacial strength between these two parts is much weaker. As such, when the entire system is strained, the mechanical breakage/fracture occurs at the interface between PCP and the underlying CellP/NaLS composite substrate. This is a necessity for forming intact and freestanding PCPs upon subsequent water immersion treatment. Because of the different affinities of cellulose/NaLS and carbon to water, during such a treatment, there would be an unbalanced interfacial tension created at the interfaces of carbon/water (or PCP and water interface), cellulose/water (or CellP/NaLS and water interface), and carbon/cellulose (or PCP and CellP/NaLS interface). This then drives the interface between PCP and the underlying CellP/NaLS to be strained, causing its spontaneous detachment. To corroborate this hypothesis, we investigated the PCP detachment behavior in ethanol. The reduced capability for the hydrogen bonding formation between ethanol and cellulose as compared to water and cellulose would expect to decrease the tendency for the formation of the cellulose/ethanol interface. As such, the spontaneous detachment would be impeded. This is indeed the case. When immersed in ethanol, the PCP did not detach at all. Further experiments were performed to observe the PCP detachment behavior in a mixture of ethanol and water. Figure e shows that, as the ethanol concentration increases, the time for the PCP to detach correspondingly increases. By increasing the concentration of ethanol from 0 to 80 wt %, the time for the completion of the detachment gradually increases from ∼4 to ∼80 s. This observation suggests that, in addition to the abovementioned thermodynamic reason, the kinetic factors such as dissolution of NaLS and swelling of the cellulose fiber may also play some roles responsible for the spontaneous detachment of PCPs. With inclusion of both the thermodynamic and kinetic factors mentioned above, we proposed a mechanism schematically shown in Figure f to explain the spontaneous detachment of PCP from its parent CellP/NaLS composite sheet.

Figure 3

Morphologies of the interface revealed by SEM images between a PCP and its parent cellulose/NaLS composite sheet upon a spontaneous detachment process by water immersion treatment: (a,b) fractured surface on the side of the PCP; (c,d) fractured surface on the side of the parent cellulose/NaLS composite sheet; (e) time-dependent spontaneous detachment behavior of a PCP from its parent cellulose/NaLS composite sheet when immersing in a mixture of ethanol and water; (f) proposed mechanism(s) for the spontaneous detachment of the PCP from the CellP/NaLS composite paper upon water immersion treatment.

As revealed above, our PCPs possess a multitude of characteristics like self-supporting, freestanding, electrically conductive, porous, and easy fabrication with predesigned size and shape. This would allow them to be valuable in many different applications. Here, we demonstrate their use in dye adsorption, force sensing, and electrical energy storage. Because of their high specific surface area, high porosity, adsorption capacity, and excellent thermal/chemical stability, porous carbon materials were often used for pollutant removal.[67] As compared to the typically used powder-form or granule-form carbon-based adsorbents, our PCPs possess self-supporting and freestanding characteristics, which would be expected to simplify the assembly of pollutant removal equipment as well as the recycling process of adsorbents. With this in mind, we examined our PCPs for their potential application in dye (methylene blue—MB) adsorption. The corresponding real-time UV–vis spectra during MB adsorption are shown in Figure a. Figure b shows a representative MB adsorption kinetics of a single piece of PCP. As shown in Figure b, a two-stage adsorption kinetics can be identified, which is presumably caused by the hierarchical pore structures of our PCPs. The macrospores inherited from the porous structure of paper are responsible for the rapid MB uptake during the first stage. The meso- and micropores originated from pyrolysis/carbonization of cellulose/NaLS responsible for the slow MB adsorption occurred in the second stage. The total MB adsorption capacity of our PCP is ∼0.21 g/g, which is in the same range as that of other types of high MB-binding materials—activated carbon, monocrystalline cellulose, and porous carbon monoliths.[67] The high dye-binding capacity of our PCPs along with their controllable uniform size and shape could be used for developing novel adsorption assembly device/equipment that cannot be realized by other types of dye adsorption materials.

Figure 4

(a) Time evolution of the UV-vis absorption spectrum of the MB aqueous solution with a piece of PCP immersed; (b) mass adsorption kinetics of MB by unit mass of PCP.

In addition to dye adsorption, our PCP can also be used as the sensing element for making flexible electronics if bonded with a compliant matrix, such as polydimethylsiloxane (PDMS). Figure a shows a prototypical device and Figure b shows its relative resistance change when subjected to a cyclic tensile deformation up to a maximum strain of 10%. The gauge factor (GF = (R/R0 – 1)/ε) of the PDMS/PCP flexible strain sensor was estimated to be about 21, which is comparable to the flexible sensors made of other types of carbon materials—graphene,[68−71] carbon nanotubes,[72−75] graphite,[76] and so forth.

Figure 5

(a) Photo of a flexible sensor made by bonding a piece of PCP on the PDMS substrate; (b) relative resistance change of the PCP/PDMS sensor (bottom) upon cyclic tensile deformation (top).

Porous carbons are the most commonly used materials as supercapacitor electrodes because of their large surface area, high electrical conductivity, tailorable pore structure, and extraordinary chemical stability.[77] Here, we demonstrated the potential application of our PCPs as binder-free supercapacitor electrodes. Because of its self-supporting and free-standing nature, the supercapacitor assembly process facilitated by our PCPs is very simple. Two supercapacitor assemblies, a solid square (Super-A) and a square-shape circle arrays (Super-B), were prepared and tested for their long-term (1000 times) charging–discharging performance. The mass of a single PCP electrode for Super-A and Super-B is 0.12 and 0.05 mg, respectively. With a constant current (I = 8 × 10–4 A) charging–discharging protocol up to a potential window of ±0.3 V, we examined the charge storage performance of Super-A and Super-B. The device pictures and testing results of Super-A and Super-B are shown in Figure a,b. On the basis of the charging–discharging testing results, according to C = (itd)/(mΔV), where i, td, m, and ΔV are respectively the constant current, charging or discharging time, the PCP mass, and the potential window for charging or discharging, we estimated that the specific capacitance for a single PCP electrode in Super-A and Super-B was 22.2 and 53.4 F·g–1, respectively. The self-supporting and freestanding nature, their controllable uniform size and shape, as well as their excellent stability and reversibility make our PCPs a promising candidate for facile fabrication of miniaturized supercapacitor devices with ease of assembly and consistent charge storage performance, which can be hardly surpassed by other types of carbon materials.

Figure 6

Photo of a supercapacitor assembly and its 1000 cycles of constant current charging–discharging performance; (a) electrodes of the supercapacitor are made of PCPs with a solid-square pattern; (b) electrodes of the supercapacitor are made of PCPs with square-shaped circle arrays.

The experimental results on the application of PCPs for dye adsorption, strain sensing, and as supercapacitors demonstrated their good mechanical and electrochemical stability. Given that the chemical composition of our PCPs is mainly composed of carbon, we also expect that they should possess good thermal and chemical stabilities. All these characteristics are important for PCPs to have long service time in practical applications.

Conclusions

In summary, we rely on laser irradiation-induced carbonization of a biomass-based composite, which is formed by infusion of sodium lignosulfonate into cellulose paper (CellP/NaLS), and take advantage of the strong affinity between water and cellulose to develop a facile method for readily fabricating freestanding porous carbon platelets (PCPs) with uniform size and arbitrarily designed shapes. The versatile applications of such prepared PCPs in dye adsorption, as a flexible sensor, and as miniaturized supercapacitors are demonstrated. The advantages of their uniform size and arbitrarily designed shapes would make the assembly/fabrication process of our PCP-based functional devices much simpler, easy to control, and highly consistent. It is believed that the method reported here will open a door for facile fabrication of PCPs in a highly controlled manner and for exploration of their more exotic applications in the fields of environment, energy storage, sensing, catalysis, and so forth.

Experimental Section

Materials and Methods

Commercial NaLS (Aladdin, uncertain molecular weight) and cellulose filter paper (89 g/m2, 0.1 mm in thickness, pore size of 30–50 μm, Hangzhou Double Circle Filter Paper Co., Ltd.) were used in this work. The raw materials were dried at 60 °C for 24 h in a vacuum oven before use. The PCPs were fabricated by a three-step process. In the first step, the CellP/NaLS composite sheet was prepared by soaking a piece of filter paper (mpaper = 2 g) into an aqueous solution of NaLS (10 wt %) for 20 min. The cellulose paper soaked with NaLS solution was then dried at 80 °C to give the CellP/NaLS composite sheet. This soaking/drying process was repeated multiple times to achieve the CellP/NaLS composite sheet with different loading levels of NaLS. To create porous carbon features, a laser engraving and cutting system (SCE4030, Wuhan Sunic Photoelectricity Equipment Manufacture Co., Ltd.; laser wavelength 10.64 μm) was used for irradiating the CellP/NaLS composite sheet with a predesigned pattern. The beam scanning speed was set at 10 mm/s and the laser power used for fabricating the PCP array was varied in a range from 0.3 to 1.6 W. Subsequent to the laser irradiation treatment, the CellP/NaLS composite sheet was then immersed in deionized water. Upon immersion, the carbonized region with a predesigned shape and array pattern would spontaneously detach from the parent sheet to form plenty of freestanding PCPs, which were then collected and dried for later use in structure characterization and property/performance evaluation. A real-time video to show the spontaneous detachment of the PCPs can be found in the Supporting Information—V1 and V2.

Characterization

The shape, size, and morphology of the selected PCPs were examined by a scanning electron microscope (Hitachi SU8010) and an optical microscope (MP41, Guangzhou Mingmei Optoelectronic Technology Co., Ltd.). The carbonaceous structure of the PCPs was characterized by Raman scattering spectroscopy. The spectra were collected by a confocal Raman microscope in backscattering geometry with a 633 nm excitation laser (LabRAM HR800, 50× objective). With grinding the PCPs into fine powders and mixing with potassium bromide for tablet sample preparation, the PCP chemical composition was examined by a Nicolet IS50 Fourier transform infrared spectrometer. The surface chemical composition of PCPs was measured by an ESCALAB 250Xi X-ray photoelectron spectrometer. The XRD pattern of the samples in powder form was recorded on an X’Pert-Pro MPD diffractometer system (PANalytical) operating at 40 kV and 40 mA with Cu radiation (λ Cu Kα = 0.15406 nm) over an angular range of 5°–60° at a step size of 0.026°. To examine the effect of laser power on the electrical property of the laser irradiation-carbonized CellP/NaLS composite, we fabricated a series of line features 3 mm long by varying the laser power from 0.6 to 1.6 W with an interval of 0.2 W, the resistance of which was measured by a Keithley 2182A Nanovoltmeter and Keithley 6221 current source at room temperature using a two-probe method. For this purpose, the electrodes were prepared by adhering two copper wires respectively to the two ends of the line feature with conductive silver adhesive.

Application

MB Adsorption

The dye adsorption behavior for the PCPs was determined by submerging the platelets in an aqueous solution of MB (RG, MW = 319.85, 98%, Adamas-beta) at an initial concentration of 7.43 μM for 1 day at room temperature, ∼30 °C. During the submersion process, the time-dependent absorption spectra of the dye solution were collected using a vis–NIR high-resolution fiber-optic spectrometer (Aurora4000, Changchun New Industries Optoelectronics Technology Co., Ltd.).

Sensor Fabrication and Its Piezoresistive Performance Evaluation

With the previously described procedure, an array of square-shaped porous carbons with individual size of 3 mm × 3 mm was fabricated. One of such freestanding carbon squares was randomly selected and used for preparing a flexible strain sensor with PDMS rubber as the elastic substrate. In brief, an appropriate amount of room temperature-vulcanized PDMS formulation provided by Fluorochem Ltd., vinyl-terminated PDMS (DMS41, MW = 62 700), mixed with a hydride functional siloxane crosslinker (HMS-151, cSt: 25–36) at a mass ratio of 1:0.0249 and ∼150 ppm platinum complexes (C8H18OPtSi2, MW = 381.41, J&K Scientific) was poured upon a glass slide at room temperature. After waiting for the pre-curing process to proceed for ∼15 min, the carbon square was manually placed on top of the gelled PDMS, followed by waiting for another 24 h for the system to completely cure.

Subsequent to the curing process, the electrodes of the sensor were prepared by adhering four copper wires separately to the four corners of the carbon square with silver adhesive and curing at 150 °C for 5 min. This mentioned procedure allowed for tightly bonding of the carbon square with the PDMS substrate and it showed good piezoresistive responses when the PDMS substrate was under mechanical deformation. The carbon square/PDMS flexible sensor of size 25 mm × 6 mm × 0.1 mm (length × width × thickness) was tested at 30 °C for its piezoresistive response under a cyclic tensile deformation by a coupled electrical–mechanical test. In such a test, a Q800 dynamic mechanical analyzer (TA Instruments) was used to ramp the tensile force from 0 to 0.075 N at a rate of 0.075 N/min and the corresponding maximum strain is 10%. Whereas the sample was subjected to tension, the resistance of the carbon square was simultaneously recorded by a Keithley 3706A and a 3721 dual 1/20 multiplexer card system switch/multimeter at a sampling rate of 1 s–1.

Supercapacitor Fabrication and Its Electrochemical Performance Evaluation

With the previously described procedure, the square-shaped PCP of size 3 mm × 3 mm as well as the square-shaped circle arrays of size 3 mm × 3 mm with their interior formed by a 5 × 5 array of ring feature were fabricated. One piece of these two different porous carbon patterns was weighted respectively as 0.12 and 0.05 mg. The supercapacitor was constructed by sandwiching two pieces of the previously prepared porous carbon along with a piece of nylon filter paper (0.45 μm aperture, Suzhou Taislai Scientific Instrument Co., Ltd.) as the separator between two nickel foil current collectors (purity 99.96%, Yixing Xuneng Electronic Technology Co., Ltd.). The entire assembly had a thickness of 0.6 mm. A 3 M aqueous solution of H2SO4 (AR, 98 wt %, Chinasun Specialty Products Co., Ltd.) was used as the electrolyte for the electrochemical performance evaluation of the supercapacitor assembly, which was performed by an electrochemical workstation (RST5260F, Suzhou Risetest Electronic Co., Ltd.) with the galvanostatic charge–discharge testing method (voltage range setting: ±0.3 V; current setting: 8 × 10–4 A).