Engineering Geometric Electrodes for Electric Field-Enhanced High-Performance Flexible In-Plane Micro-Supercapacitors

show large enhancement in electrochemical properties of ﬂ exible, in-plane micro-supercapacitor using sharp-edged interdigitated electrode design, which was simply fabricated through direct laser scribing method. The sharp-edged electrodes allowed strong electric ﬁ eld to be induced at the corners of the electrode ﬁ ngers which led to the greater accumulation of ions near the surface of electrode, signi ﬁ cantly enhancing the energy storage performance of micro-supercapacitors. The electric ﬁ eld-enhanced in-plane micro-supercapacitor showed the volumetric energy density of 1.52 Wh L − 1 and the excellent cyclability with capacitive retention of 95.4% after 20 000 cycles. We further showed various practicability of our sharp-edged design in micro-supercapacitors by showing circuit applicability, mechanical stability

DOI: 10.1002/eem2.12581 In plane micro-supercapacitors that are miniaturized energy storage components have attracted significant attention due to their high power densities for various ubiquitous and sustainable device systems as well as their facile integration on various flexible/wearable platform. To implement the micro-supercapacitors in various practical applications that can accompany solid state or gel electrolyte and flexible substrates, ions must be readily transported to electrodes for achieving high power densities. Herein, we show large enhancement in electrochemical properties of flexible, inplane micro-supercapacitor using sharp-edged interdigitated electrode design, which was simply fabricated through direct laser scribing method. The sharpedged electrodes allowed strong electric field to be induced at the corners of the electrode fingers which led to the greater accumulation of ions near the surface of electrode, significantly enhancing the energy storage performance of micro-supercapacitors. The electric field-enhanced in-plane microsupercapacitor showed the volumetric energy density of 1.52 Wh L −1 and the excellent cyclability with capacitive retention of 95.4% after 20 000 cycles. We further showed various practicability of our sharp-edged design in microsupercapacitors by showing circuit applicability, mechanical stability, and air stability. These results present an important pathway for designing electrodes in various energy storage devices.
for the prospects of implementation in practical applications and enhance the ion transport and power density of the EDL-mSCs.
In this work, we propose novel mSC electrode design strategy by demonstrating electric field-enhanced in-plane mSC using sharp-edged interdigitated electrodes which was simply fabricated using the carbonization of the flexible PI film through direct laser scribing method. To verify the field-enhancement through the electrode design, we employed mSCs with various sharp-edged electrode design having different electrode area and gap distance and compared the electrochemical performance with mSCs having conventionally employed rectangular shape electrodes. The sharp-edged electrode design allowed strong electric field to be induced at the sharp-edged corners, supported through finite-difference time-domain (FDTD) simulation. The strong electric field accumulated at the electrode corners led to a greater accumulation of electrolyte ions near the carbon electrodes and enhancement of energy storage performance. Furthermore, we observe the circuit applicability of the mSC design through connecting them in series/parallel as well as prove the practical applicability through monitoring the capacitive retention under multiple mechanical bending and long-term cycle tests. This proof-of-concept mSC device represent an important strategy toward designing flexible mSCs and sustainable device systems.

Results and Discussion
The fabrication of interdigitated carbon-based electrodes is illustrated in Figure 1a. Carbon-based electrodes were fabricated on polyimide (PI) sheets using a stepwise direct laser scribing method as we previously reported. [20] The direct laser scribing allows the any patterns to be created, which is highly suitable for the design of in-plane mSC electrodes. The pulsed laser immediately carbonizes the PI sheets and form carbonbased electrodes. After the laser carbonization process of cathode and anode electrodes, H 3 PO 4 /PVA gel electrolyte composites were drop coated onto the interdigitated carbon-based electrodes using a micropipette to cover the entire electrode areas. The in-plane mSC with carbon-based electrodes and gel electrolyte was then dried overnight to stabilize the gel electrolyte.
The basic principle of the electrode design and the concept of fieldenhanced electrode are captured in Figure 1b. In order to observe the enhancement of mSC performance via electric field, we have designed two interdigitated electrode structures with 1) conventional rectangular pattern and 2) sharp-edged triangular pattern and compared the performance of mSCs so as to clearly verify the effectiveness of strong electric field in the electrode structure. In the sharp-edged triangular pattern, strong electric field is presented at the corner edges of the interdigitated electrodes (Figure 1b left), arising from the accumulation of charges at the sharp edges. The electric field distribution is rather much uniform in rectangle patterned electrodes (Figure 1b right) compared to the sharp-edged triangular pattern. The optical images of the flexible, inplane mSCs with different electrode design are shown in Figure 1c. Owing to high flexibility of PI sheets and carbon electrodes, the inplane mSCs can sustain their original device structure even under high mechanical stresses and bending without a noticeable damage as shown in right image of Figure 1c.
In order to confirm the effect of sharp-edged electrodes, it is important to fairly compare the electrochemical performance at identical electrode surface area and distance between the electrode fingers. The rectangular interdigitated electrode configuration (mSC-bare, hereafter) has the finger length of 7.5 mm, finger width of 1 mm, gap distance of 0.5 mm, electrode area of 0.83 cm 2 , and eight fingers as shown in Figure 2a. Three sharp-edged interdigitated electrode configuration was designed with 1) eight identical fingers (mSC-8) and~2 times longer gap distance and smaller electrode area of 0.53 cm 2 than mSC-bare; 2) 12 fingers (mSC-12) with identical gap distance and smaller electrode area of 0.68 cm 2 than mSC-bare; and 3) 16 fingers (mSC-16) with identical electrode area of mSC-bare (0.83 cm 2 ). These three sharp-edged interdigitated electrode designs and optical images are shown in Figure 2a,b, respectively. All electrode designs with the carbon electrode showed similar electrical properties as shown in Figure S1, Supporting Information.
To evaluate the carbonization of PI film using the direct laser scribing method, the carbon electrodes were characterized using Raman and XPS as shown in Figure 2c,d. Figure 2c shows Raman spectrum of the carbon electrodes with different interdigitated designs. Three strong peaks were shown at 1347 cm −1 (D-band), 1587 cm −1 (Gband), and 2691 cm −1 (2D-band), and two weak peaks were shown at 2458 cm −1 (D + D″) and 2938 cm −1 (D + D 0 ). [30,31] Typically, the high ratio of intensity of G-band peak and 2D-band peak is a signature of graphite. The intensity ratio I G /I 2D is greater than 1 for all of the interdigitated carbon electrodes. Also, the sharp 2D peak resembles the peak found in typical 2D graphite structure, [33] demonstrating that the PI film was easily converted into graphite composites with layered structures. We also performed X-ray photoelectron spectroscopy (XPS) for the carbon electrodes as shown in Figure 2d. The carbon electrodes exhibited a common graphite characteristic with strong peak shown at 284.02 eV which correspond to C-C peak and is a clear signature of formation of layered graphite structure. Other oxygen-carbon peaks were weakly observed at 289.7 eV (O-C=O) and 285.3 eV (C-O), [32,34,35] demonstrating the high quality of the carbon electrodes fabricated using the direct laser scribing method.
The electric field enhancement of the mSCs was firstly evaluated by measuring electrochemical properties of the mSCs with conventional rectangular interdigitated electrodes and sharp-edged interdigitated electrodes. Figure 3a-c show the cyclic voltammetry (CV) curves of mSC-8, mSC-12, and mSC-16, respectively, which are plotted together with CV curve of mSC-bare in order to readily compare the energy storage performance. The CV curves were measured at a scan rate of 100 mV s −1 . All of the CV curves are nearly rectangular in shape, and there are no obvious oxidation/reduction peaks within a voltage window of 1 V, showing EDL type of CV curves for the carbon electrodes. The area surrounded by CV curves was calculated to be 262.28 μF cm −2 for mSC-bare, 233.20 μF cm −2 for mSC-8, 408.26 μF cm −2 for mSC-12, and 518.03 μF cm −2 for mSC-16. It should be noted that even though mSC-8 has longer gap distance and smaller electrode area than mSC-bare, the areal capacitance is similar, and the areal capacitance of mSC-12 is much large than mSC-bare even though the mSC-12 has smaller electrode area and shorter gap distance compared to mSC-bare. These results imply that the electrochemical performance of the mSCs with sharp electrode design is superior than that of the mSC with conventional rectangular design, and such enhancement can be attributed to the enhancement of electric field from the sharp electrode design.
In order to support electric field enhancement in the sharp-edged electrode design, the electric field distribution in interdigitated rectangular and sharp-edged electrodes is evaluated using FDTD simulation. The electrode geometries were chosen for similar electrode surface area as mSC-bare, mSC-8, mSC-12, and mSC-16. To clearly compare the magnitude of electric fields, the electric field distribution was depicted with the identical scale bar. From the steady-state analysis of electric field in Figure 3d, it was clearly observed the stronger electric field at the sharp-edged corners due to edge effect. [36] The sharp-edged corner can collect more electric charges in a tiny space with the same applied voltage, leading to a high electric field. Furthermore, an extremely amplified electric field was generated when the gap distance was shorter and the electrode had larger number of electrode fingers.
The stronger electric field in the mSCs with sharp-edged electrode geometry enhances the energy storage properties through the edge effect. Compared to the rectangular electrodes geometry, sharp-edged electrode geometry has increased electric field intensity or flux, leading to the greater accumulation of electrolyte ions near the carbon electrodes. As the drift of ions under an electric field E can be treated as a classical hydrodynamic reaction (Stokes's law), [37] v E ¼ ze=6πηr i where η is the solvent viscosity, r i is the ion radius, force on ion is zeE, hydrodynamic resistance is 6πηr i v, and v is the ion velocity under electric field E. As the electric field is stronger in mSCs with sharp-edged electrode geometry compared to mSC-bare, the corresponding ion movement is much stronger, and the electrostatic capacitance is enhanced from the greater accumulation of ions near electrodes. Therefore, the edge effect is crucial in enhancing the performance of the mSCs, especially under gel-type or solid electrolyte where ion conductivity is not high.
The galvanic charge/discharge (GCD) curves of mSC-bare and mSC-16 measured at a current of 1 μA is presented in Figure 3e. GCD of mSC-bare and mSC-16 shows that the charge and discharge time of mSC-16 is much longer than mSC-bare. The calculated area capacitance of mSC-16 (1.306 mF cm −2 ) was approximately 1.64 times higher than that of mSC-bare (0.798 mF cm −2 ). In addition, the volumetric capacitance (108.8 F L −1 ) and energy density (1.52 Wh L −1 ) of mSC-16 is higher than those of mSC-bare (66.5 F L −1 for volumetric capacitance and 0.93 Wh L −1 for volumetric energy density) and other reported mSCs (Table S1, Supporting Information). The GCD curves of mSC-16 at different current are shown in Figure 3f. Even at the current of 500 μA, the volumetric capacitance of 10.1 F L −1 for mSC-16 was achieved. In addition, the energy storage performance of mSC-16 was tested for 10 different samples showing reproducible performance of the mSCs with the sharp electrode geometry.
We also evaluated the circuit applicability of the mSCs with sharpedged electrode geometry by assembling four mSC-16 in series and parallel as shown in Figure 4. The operating cell voltage or capacitance is expected to increase proportionally with the series and parallel connection of mSCs. Figure 4a shows the CV curves when multiple mSCs-16 was connected in series. The voltage windows from one to four mSC cells in series were increased from 1 to 4 V. In addition, when the mSCs were connected in parallel (Figure 4b), the CV currents increased proportionally with increasing the number of cells (from 0.18 to 0.70 mA at 1.0 V). The CV results obtained from series and parallel connection of the mSCs with sharp-edged geometry demonstrated great possibility of circuit operation for future energy storing devices.
In addition to circuit applicability of the mSCs, mechanical flexibility and stability of the mSCs with sharp-edged electrode geometry were evaluated under different bending radius and compressive/tensile strain. Figure 5a,b show schematic and optical image of applying external strain to the mSCs and the corresponding bending radius from 10 to 5 mm. Figure 5c presents the CV curves measured at a scan rate of 500 mV s −1 under different bending radius (5, 6, 7.5, and 10 mm). The CV curves did not show any noticeable degradation in the energy storage performance even though the bending radius reached 5 mm. Furthermore, the CV curves measured at a scan rate of 500 mV s −1 did not show any degradation upon the application of tensile and compressive strain with identical bending radius of 5 mm as shown in Figure 5d,e. Note that the tensile and compressive strain was applied depending on the position of the mSCs respective to the location of the neutral axis. To further test the mechanical stability of the energy storage performance, the CV curves were measured after multiple bending cycles (tensile strain) up to 1000. Interestingly the stability was reached 97.83% of its original value even up to bending cycles of 1000, showing outstanding mechanical stability of our mSCs with sharp-edged electrode configuration. After the application of diverse external strain, it can be clearly seen that the mSCs can be successfully applied to diverse flexible and wearable applications sowing the high flexibility of carbon and PI substrate.
We further demonstrated air stability and cyclic stability of the mSCs with sharp-edged electrode geometry by monitoring the capacitive retention behavior when the mSCs are measured after multiple weeks and multiple cycles. Figure 5g shows the capacitive stability of the mSCs in air. Due to the structural and material stability of carbon electrodes fabricated using the direct laser scribing method, the capacitive retention was maintained 95.4% from the original value even after the mSC was measured after 10 weeks. Furthermore, the capacitive stability of the mSCs was measured for multiple cycles up to 20 000, which exhibit a promising capacitive retention of 95.4% after 20 000 cycles. Moreover, after the capacitive stability evaluation, it was confirmed that the carbon electrode of mSC maintains high crystallinity of graphite with layered structure, although some surface oxidation was present ( Figure S2, Supporting Information). These results suggest that the mSCs with carbon electrodes have suitable energy storage performance that can be employed for various sustainable device systems.

Conclusion
To conclude, we demonstrated the electric field enhancement of microsupercapacitors based on carbon electrodes fabricated using laser scribing of PI film. The strong electric field was induced by employing sharp-edged interdigitated electrode instead of conventional rectangular electrode geometry, which was confirmed through FDTD simulation. The electric field-enhanced in-plane mSC showed the volumetric energy density of 1.52 Wh L −1 and the outstanding cyclability with capacitive retention of 95.4% after 20 000 cycles. The effect of sharp electrode design was demonstrated through enhanced capacitive behaviors achieved through enhanced ion transport to the electrode surface. Furthermore, the feasibility of the sharp electrode applications was demonstrated using series/parallel circuit connection and verifying the high mechanical and air stability of the mSCs. The results demonstrated in this work pave an important pathway toward designing mSCs and sustainable device systems. under different bending radius, ranging from 10 to 5 mm. c) CV curves measured under different bending radius (5, 6, 7.5, and 10 mm). The CV curves did not show any noticeable degradation in the energy storage performance even though the bending radius reached 5 mm. d) Application of tensile and compressive strain with identical bending radius of 5 mm. e) CV curves measured under tensile and compressive strain. f) Capacitance retention test after multiple bending cycles (tensile strain) up to 1000, reaching 97.83% of its original value even up to bending cycles of 1000. g) Air stability and h) cyclic stability of the mSCs with sharp-edged electrode geometry by monitoring the capacitive retention behavior when the mSCs are measured after multiple weeks and multiple cycles, showing high stability under both air and multiple cycles.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.