Elaborate Control of Inkjet Printer for Fabrication of Chip-based Supercapacitors

Summary

This paper provides a technique for manufacturing chip-based supercapacitors using an inkjet printer. Methodologies are described in detail to synthesize inks, adjust software parameters, and analyze the electrochemical results of the manufactured supercapacitor.

Abstract

There are tremendous efforts in various fields to apply the inkjet printing method for the fabrication of wearable devices, displays, and energy storage devices. To get high-quality products, however, sophisticated operation skills are required depending on the physical properties of the ink materials. In this regard, optimizing the inkjet printing parameters is as important as developing the physical properties of the ink materials. In this study, optimization of the inkjet printing software parameters is presented for fabricating a supercapacitor. Supercapacitors are attractive energy storage systems because of their high power density, long lifespan, and various applications as power sources. Supercapacitors can be used in the Internet of Things (IoT), smartphones, wearable devices, electrical vehicles (EVs), large energy storage systems, etc. The wide range of applications demands a new method that can fabricate devices in various scales. The inkjet printing method can break through the conventional fixed-size fabrication method.

Introduction

In the past decades, multiple printing methods have been developed for various applications, including wearable devices¹, pharmaceuticals², and aerospace components³. The printing can be easily adapted for various devices by simply changing the materials to be used. Moreover, it prevents the wastage of raw materials. To manufacture electronic devices, several printing methods such as screen printing⁴, push-coating⁵, and lithography⁶ have been developed. Compared to these printing technologies, the inkjet printing method has multiple advantages, including reduced material waste, compatibility with multiple substrates⁷, low cost⁸, flexibility⁹, low-temperature processing¹⁰, and ease of mass production¹¹. However, the application of the inkjet printing method has hardly been suggested for certain sophisticated devices. Here, we present a protocol establishing detailed guidelines to use the inkjet printing method for printing a supercapacitor device.

Supercapacitors, including pseudocapacitors and electrochemical double-layer capacitors (EDLCs), are emerging as energy storage devices that can complement conventional lithium-ion batteries^12,13. Especially, EDLC is a promising energy storage device because of its low cost, high power density, and long cycle life¹⁴. Activated carbon (AC), having high specific surface area and conductivity, is used as electrode material in commercial EDLCs¹⁵. These properties of AC allow EDLCs to have a high electrochemical capacitance¹⁶. EDLCs have the passive volume in devices when the conventional fixed-size fabrication method is used. With inkjet printing, the EDLCs can be fully integrated into the product design. Therefore, the device fabricated using the inkjet printing method is functionally better than that fabricated by existing fixed-size methodologies¹⁷. The fabrication of EDLCs using the efficient inkjet printing method maximizes the stability and longevity of EDLCs and provides a free-form factor¹⁸. The printing patterns were designed by using a PCB CAD program and converted to Gerber files. The designed patterns were printed using an inkjet printer because it has precise software-enabled control, high material throughput, and printing stability.

Protocol

1. Design of pattern using PCB CAD program

1. Run the CAD program. Click on the File button atop the program window. To form a new project file, click on the New and Project buttons.
2. To generate the board file, click on the File, New, and Board buttons in order. Set the grid size, multiple, and alt values by clicking on the mesh-shaped Grid button at the top left of the created Board File window (or clicking on View and Grid in order at the top of the window).
3. Change both the grid size and alt value from mm to inch so that the inkjet printer can read the PCB CAD pattern. Press Finest to make fine adjustments.
4. Design the pattern of the current collector and EDLC line in an interdigitated form. Design the gel polymer electrolyte (GPE) pattern and current collector pads in a rectangular form (Figure 1).
   NOTE: Pattern width: 43 mm, pattern height: 55 mm, line length: 40 mm, line width: 1.0 mm, line-to-line space: 1.5 mm, and pad size: 15 x 5 mm².
   1. Since the final pattern consists of three types (conductive line, EDLC, and GPE), set the three layers as follows.
      1. Click on View and Layer Settings in order at the top of the window. Create new layers by clicking on the New Layer button at the bottom left of the Visible Layers window.
      2. In the new window (New Layer), set up the name and color for the new layer. For visually distinguishing the layers, set the names of the three layers to Current Collector, EDLC, and GPE, and change the corresponding colors by clicking on the box to the right of Color.
   2. Press Line at the bottom left of the screen, click on the main field (black background), and drag to draw a line. To change the thickness of the line, input the value of Width located at the top center in inch scale (1.0 mm = 0.0393701 inch).
   3. To edit the length of a line, right-click on the line and click on Properties at the bottom. In the From and To fields, input the x and y values of the starting and ending points.
   4. To set the reference point of the pattern, set the upper-left corner of the pattern shown in Figure 1 to (0,0). Draw the rest of the pattern based on the above information.
   5. To set the drawn pattern to the desired layer, right-click on the pattern and click on Properties. Then, click on Layer, and choose the desired layer.
   6. To draw rectangular patterns of the current collector pad and GPE, press Rect at the bottom left of the main window. Click and drag on the screen (main field) where the previously drawn pattern exists.
   7. To edit, right-click on the rectangular surface and click on Properties at the bottom. Input the upper left (x,y) value and the lower right (x,y) value of the rectangle in the From and To fields, respectively. Set the rectangle to the desired layer as mentioned in step 1.4.5.
5. Convert the CAD file of the designed pattern into the Gerber file format that is read by the inkjet printer.
   1. Before converting the designed pattern file, save the Board File in .brd format. To save, click on File, and then on Save (or press ctrl + S on the keyboard).
   2. After saving, click on File at the top of the window and click on CAM Processor. To create a Gerber file of the desired layer, modify the items under Gerber of Output Files on the left side of the window, as follows.
   3. First, delete the sub-lists such as Top Copper and Bottom Copper by pressing the '-' below. Press '+' and click on New Gerber Output to create Gerber output.
   4. On the right side of the screen, set the layer name in Name and Function to Copper by pressing the gear on the right. Set Layer Type to Top and set Gerber Layer Number of the current collector, EDLC and GPE to L1, L2, L3, respectively.
   5. In the Layers window at the bottom of Gerber File, click on Edit Layers at the bottom left, and select each desired layer.
   6. To set the name of the output file to be created, set the Gerber Filename of Output at the bottom of the window to %PREFIX/%NAME.gbr.
   7. Finally, click on Save Job at the top left of the window to save the settings. Click on Process Job at the bottom right to create a Gerber file.

2. Ink synthesis

NOTE: Flexible Ag ink is used as conductive ink for the current collector line and pads.

1. Prepare EDLC ink using terpineol, ethylcellulose, activated carbon (AC), Super-P, polyvinylidene difluoride (PVDF), and Triton-X as follows.
   1. Use 2,951 µL of terpineol with high viscosity as the solvent and 1.56 g of ethyl cellulose as a thickener. Set the ratio of AC to Super-P to PVDF as 7:2:1 with a total weight of 1.8478 g. In addition, use 49 µL of Triton-X as a surfactant for mixing.
   2. Mix all the materials for 30 min using a planetary mixer. Place the well-mixed electrode material in a cartridge for the inkjet printer and centrifuge it at 115 x g for 5 min.
2. Prepare GPE ink using propylene carbonate (PC), PVDF, and lithium perchlorate (LiClO₄) as follows.
   1. Use PC as the solvent, PVDF as the polymer matrix, and LiClO₄ as the salt. Weigh all components of GPE such that the final molar concentration of LiClO₄ is 1 M, and the final weight % of PVDF is 5 wt%.
   2. Stir all the components at 140 °C for 1 h until dissolution. After stirring, cool the GPE ink sufficiently and place it into the ink cartridge.

3. Inkjet printer software parameter set-up

1. Run the printer program. Click on the Print button, select Simple, and then select Flexible Conductive Ink in order as shown in Figure 2.
2. Upload the Gerber file of the designed pattern by following the 1 arrow in Figure 3. Choose and open the Gerber file of the conductive line (see 2 and 3 arrows in Figure 3). Click on the NEXT button as indicated by the 4 arrow.
3. Fix the PCB board as shown in Figure 4A, and mount the probe as shown in Figure 4B.
4. Adjust the zero point of the PCB printer through the probe by clicking on the OUTLINE button (see the 1,4 red arrow in Figure 5).
   NOTE: The probe moves over the PCB board while showing the outline of the pattern (see the bottom right of Figure 5).
5. Move the pattern image on the screen by dragging (see the yellow dashed arrow in Figure 5). Click on the OUTLINE button once more to check whether the probe moves through the desired path. Click on NEXT (indicated by the 5 arrow in Figure 5).
6. Click on PROBE to measure the height of the substrate for checking whether the substrate is flat (Figure 6).
   NOTE: The probing region on the substrate is automatically selected by the program built into the printer.
7. Remove the probe once the height measurement is completed. Insert the ink cartridge into the ink dispenser and connect the nozzle (inner diameter: 230 µm) to prepare the dispenser.
8. Mount each ink (conductive line, EDLC, GPE) dispenser, and print a sample pattern by pressing the CALIBRATE button, while adjusting the parameters of each ink (Figure 7).
9. Visually check the printing result and record the parameter values for each ink. See Representative Results for details.

4. Printing the conductive line

NOTE: Since steps 4.1. to 4.7. overlap with section 3, they are only briefly summarized below.

1. Run the inkjet printer program and click on Print in the start menu and select Simple (Figure 1).
2. Click on the Choose File button next to Ink to load the designed pattern file and click on NEXT (Figure 3).
3. Fix the PCB board onto the printer and install the probe (Figure 4).
4. Check the position of the pattern on the substrate and measure the height of the substrate (Figure 5 and Figure 6).
5. Remove the probe, and then mount the conductive ink (flexible Ag ink) dispenser.
6. Change the software parameters of conductive ink by clicking on the Settings button (see Figure 7 and Table 1).
7. Print a sample pattern to check whether the setting from step 4.6 is successful.
8. Erase the sample printing pattern with a cleaning wipe moistened with ethanol.
9. Print the designed pattern of the conductive line by pressing the START button.
10. After printing, cure the conductive line at 180 °C for 30 min. Then, measure the combined weight of the substrate and the conductive line.

5. Printing the EDLC line

1. Select the Aligned option on the start screen of the printer program. Load the EDLC line pattern file and click on NEXT (see step 3.2).
2. Ensure the position of the conductive line is detected through two alignment points to align the pattern positions of the EDLC line and the conductive line. Then, move to a random point and check whether the location is correct.
3. Measure the overall height of the conductive line to check the height of the dispenser nozzle above the conductive line by clicking on the PROBE button (see Figure 6).
4. Change the software parameter values of EDLC inks (Figure 7 and Table 1).
5. Print a sample pattern to check whether the software parameter values are appropriate. Erase the sample printing pattern with a cleaning wipe moistened with ethanol. Print the EDLC line by pressing the START button.
6. Dry the printed EDLC line overnight at room temperature to evaporate the solvent.
7. To calculate the weight of the dried EDLC line, measure the combined weight of the substrate, conductive line, and EDLC line.

6. Printing the GPE pattern

1. Select the Aligned option on the start screen of the printer program. Load the Gerber file of the GPE pattern and click on NEXT (see step 3.2).
2. Check the alignment points and move to any point to check whether the position is correct.
3. Measure the height of the EDLC line to set the default height for the nozzle.
4. Change the software parameter values of GPE inks (Figure 7 and Table 1).
5. Print a sample pattern to check whether the software parameter values are appropriate.
6. Erase the sample printing pattern with a cleaning wipe moistened with ethanol. Print the GPE pattern.
7. To have a stabilization process and evaporate the residual solvent, dry the GPE pattern at room temperature for 24 h.

7. Electrochemical test

1. Perform the electrochemical measurements for the inkjet-printed supercapacitor device following the below steps. Turn on the potentiostat device and run the program to measure cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS).
   1. Connect the potentiostat to the supercapacitor device printed earlier.
      NOTE: Four connection lines are used in the potentiostat: the working electrode (WE), working sensor (WS), counter electrode (CE), and reference electrode (RE).
   2. Connect the WS line to the WE line and the RE line to the CE line as the fabricated device is a symmetric supercapacitor.
   3. Connect the WE\WS line and CE\RE line to the opposite current collector pads on the supercapacitor device.
2. Generate a sequence of CVs and run it to get the result.
   1. Run the program to generate the sequence file.
   2. Click on the New Sequence button.
   3. Click on the Add button to generate step 1.
   4. Check whether the potential displayed by the potentiostat is 0 V or not. If the potential is not 0 V, do as follows.
      1. Set Control as CONSTANT and for Configuration, set Type as PSTAT, Mode as NORMAL, and Range as AUTO. For Voltage (V), set Ref. as Eref, and Value as 0.
      2. For Condition-1 of Cut Off Condition, set Item as Step Time, OP as >=, DeltaValue as 1:00 and Go Next as Next. For Misc. setting push the Sampling button and set Item as Time(s), OP as >= and DeltaValue as 30.
   5. Click on the Add button to create the next step.
      1. Set Control as SWEEP and for Configuration, set Type as PSTAT, Mode as CYCLIC and Range as AUTO. For Initial (V) and Middle (V), set Ref. as Eref, Value as 0. For Final (V), set Ref. as Eref and Value as 800.00e-3.
      2. Use voltage scan rates of 5, 10, 20, 50, and 100 mV/s. Therefore, according to each scan rate, set Scanrate (V/s) as 5.0000e-3, 10.000e-3, 20.000e-3, 50.000e-3, and 100.00e-3, respectively.
      3. For all scan rates, set Quiet time(s) as 0 and Segments as 21. For Condition-1 of Cut Off Condition, set Item as Step End and Go Next as Next.
      4. For Misc. setting, push the Sampling button and set Item as Time(s) and OP as >=. For each scan rate, set DeltaValue as 0.9375, 0.5, 0.25, 0.125, and 0.0625.
   6. Click on the Save As button to save the sequence file of the CV test.
   7. Click on Apply to CH and run the sequence file of the CV test to obtain the result.
3. Generate a sequence of GCD and run it to get the result.
   1. Run the program to generate the sequence file.
   2. Click on the New Sequence button.
   3. Click on the Add button to generate step 1.
   4. Check whether the potential displayed by the potentiostat is 0 V or not. If the potential is not 0 V, do as follows.
      1. Set Control as CONSTANT and for Configuration, set Type as PSTAT, Mode as NORMAL and Range as AUTO. For Voltage (V), set Ref. as Eref, Value as 0.
      2. For Condition-1 of Cut Off Condition, set Item as Step Time, OP as >=, DeltaValue as 1:00 and Go Next as Next. For Misc. setting, push the Sampling button and set Item as Time(s), OP as >=, and DeltaValue as 30.
   5. Click on the Add button to create the next step (Charge step).
      1. Set Control as CONSTANT and for Configuration, set Type as GSTAT, Mode as NORMAL, and Range as AUTO. For Current (A), set Ref. as ZERO.
      2. Current density varies between 0.01 A/g and 0.02 A/g. Therefore, set the Value of Current (A) for each current density to 310.26e-6 and 620.52e-6.
      3. For Condition-1 of Cut Off Condition set Item as Voltage, OP as >=, DeltaValue as 800.00e-3, and Go Next as Next. For Misc. setting, set Item as Time(s), OP as >= and DeltaValue as 1.
   6. Click on the Add button to create the next step (Discharge step).
      NOTE: This step is set the same as the Charge step.
      1. Set Value of Current (A) for each current density to -310.26e-6 and -620.52e-6.
      2. For Condition-1 of Cut Off Condition set Item as Voltage, OP as <=, DeltaValue as 0.0000e+0 and Go Next as Next. For Misc. setting, set Item as Time(s), OP as >= and DeltaValue as 1.
   7. Click on the Add button to create the next step (Loop step).
      1. Set Control as LOOP and for Configuration set Type as Cycle and Iteration as 21.
      2. For Condition-1 of Cut Off Condition set Item at List 1 as Loop Next. For each current density, set Go Next as STEP-2 for 0.01 A/g and STEP-5 for 0.02 A/g.
   8. Click on the Save As button to save the sequence file of the GCD test.
   9. Click on Apply to CH and run the sequence file of the GCD test to obtain the result.
4. Generate a sequence of EIS and run it to get the result.
   1. Run the program that can generate the sequence file.
   2. Click on the New Sequence button.
   3. Click on the Add button to generate step 1.
      1. Set Control as CONSTANT and for Configuration, set Type as PSTAT, Mode as TIMER STOP, and Range as AUTO.
      2. As the operating potential window in this study is set as 0.0 to 0.8 V, for Voltage, set Value at 400.00e-3, which is the average value of the operating potential window. Set Ref. as Eref.
   4. Click on the Add button to generate the next step.
      1. Set Control as EIS and for Configuration, set Type as PSTAT, Mode as LOG and Range as AUTO.
      2. Set the frequency range as 0.1 Hz to 1 MHz. Therefore, set Initial (Hz) and Middle (Hz) to 100.00e+6, and Final (Hz) to 100.00e-3.
      3. As mentioned in section 7.4.3.2, set Value of Bias (V) to 400.00e-3, and set Ref. to Eref.
      4. To maintain a linear response, set the amplitude (Vrms) to 10.000e-3.
      5. Set Density as 10 and Iteration as 1 for this experiment.
   5. Click on the Save As button to save the sequence file of the GCD test.
   6. Click on Apply to CH and run the sequence file of the EIS test to get the result.

Results

The ink was synthesized according to step 2, and the characteristics of the ink could be confirmed according to reference¹⁸. Figure 8 shows the structural properties of conductive ink and EDLC ink, as well as the rheological properties of EDLC ink reported in the previous research¹⁸. The conductive ink is well sintered to form continuous conducting paths, and the nanoscale roughness is expected to increase the contact area with the EDLC ink (

Discussion

The critical steps in this protocol are involved in the software parameter setup to print the designed pattern by finely adjusting the parameter values. Customized printing can lead to structural optimization and obtaining new mechanical properties¹⁹. The inkjet printing method with software parameter control can be used for sophisticated printing in various industries by selecting the optimized material for the printing process.

In the fabrication of supercapacitors us...

Materials

Name	Company	Catalog Number	Comments
2” x 3” FR­4 board	Voltera	SKU: 1000066	PCB substrate
Activated carbon	MTI	Np-Ag-0530HT
Eagle CAD	Autodesk		PCB CAD program
Ethyl cellulose	Sigma Aldrich	46070	48.0-49.5% (w/w) ethoxyl basis
Flex 2 conductive ink	Voltera	SKU: 1000333	Flexible Ag ink
Lithium perchlorate	Sigma Aldrich	634565
Propylene carbonate	Sigma Aldrich	310328
PVDF	Sigma Aldrich	182702	average Mw ~534,000 by GPC
Smart Manager	ZIVE LAB	ver : 6. 6. 8. 9	Electrochemical analysis program
Super-P	Hyundai
Terpineol	Sigma Aldrich	432628
Thinky mixer	Thinky	ARE-310	Planetary mixer
Triton-X	Sigma Aldrich	X100
V-One printer	Voltera	SKU: 1000329	PCB printer
ZIVE SP1	Wonatech		Potentiostat device