Comparison of parameters between planar and interdigital electrode supercapacitor design.
Power MEMS can be defined as microelectromechanical systems for power generation and energy conversion. Energy harvesting has become an increasingly popular option for powering electronic devices as a long-lasting power source. Energy scavenging is defined as the process by which the energy is derived such as vibration, solar, wind, and thermal. Energy harvesting from the environment can prolong the life cycle and reduce the maintenance costs of electronic devices. Among the various sources of energy storage, Among the various of energy storage, supercapacitor has recently gained much interest in fields such as bioMEMS, biomedical implants and power electronic devices due to its advantages such as high power density, rapid charge and discharge and unlimited number of recharge cycles. In biomedical and bioMEMS systems, an energy storage device is needed to power other active biomedical devices within the system. For implantable devices such as a heart pacemaker, the power requirement is in the range of 30–100 μW. Microsupercapacitors play an important role in energy harvesting system, such as collecting energy from ambient energy sources. Human body is very resourceful in generating micropower in the form of heat dissipation, deformation of elastic tissue, and motion. Due to the advantages of MEMS energy harvesting system, the system can be use widely for biomedical implant devices, such as heart pacemakers and hearing aids, and can be used for a long time and without the need for battery replacement. In this work, planar and double-stacked interdigital electrode supercapacitor designs were modeled using Coventorware software. From simulation, it is observed that for planar structure, the specific capacitance is 0.22 mF/cm−2, and for double-stacked structure specific capacitance can be increased to 0.48 mF/cm−2. In terms of specific power, the planar structure produces 0.99 mW/cm−2, and the double-stacked structure produces 2.18 mW/cm−2. These results highlight the superiority of the double-stacked MEMS interdigital supercapacitor design compared with its planar counterpart in terms of charging capacity and electrical performance, thus making it favorable for powering heart pacemaker.
- energy scavenging
- interdigital electrode microsupercapacitor
- heart pacemaker
Biomedical implantable devices usually required micro power sources with smaller dimensions and higher power density. The output power generated is in the range of nanowatt to microwatt. Power requirement for implantable devices for heart pacemaker is 30-100 μW [1, 2]. An energy generation system producing power within that range would qualify as a probable candidate to replace the conventional battery system. Typically, a Lithium-ion battery is widely used in biomedical implants such as pacemakers. However, batteries have many disadvantages such as short lifetime and contain a finite amount of depletable chemical energy . Thus, patients using biomedical implants devices such as cardiac pacemakers have to replace the battery once every 5 years to 10 years. To avoid frequent battery replacement, a renewable energy generation and storage system could become a vital solution to infinitely empower biomedical devices, without the need for any powering unit replacement [4, 5].
MEMS is one of the fundamental area that consists of mechanical, electrical, chemical, optical, and fluids engineering and integrated to the MEMS such as micropump,microvalve,microneedles and micronozzles [6–8]. Supercapacitor has high potential to replace conventional battery usage due to its high power density, rapid charge and discharge, long cycle life, high energy densities and unlimited number of recharge cycles. Miniaturized electrochemical capacitor or micro supercapacitor has promising capability to power small electronic devices. [9–11]. Supercapacitors have the capability to store up to 10,000 times more energy compared with its conventional counterparts [12, 13]. For electrode structure design of a supercapacitor, it can be classified into interdigital, sandwich, and roll structure . Interdigital electrodes structure has their advantages such as low charging current, reduced solution resistance effects, and diffusion controllable current .
Conducting polymers have high capacitance and high conductivity with low cost compared to carbon as the electrode material for supercapacitor [16, 17]. Conducting polymers such as Polypyrrole (PPY), Polyaniline (PANI), and Polythiophene are mostly used as the electrode material in supercapacitors due to its advantages such as easy fabrication, low cost, high charge density, good conductivity, and more flexible [18–21]. In terms of electrolyte, Polyvinyl Alcohol (PVA) has been chosen due to its high charging density and low cost .
Planar interdigital electrode supercapacitor with polypyrrole (Ppy) – polyvinyl alcohol (PVA) as electrodes coating and electrolyte material respectively was first introduced by Wei Sun and Xuyuan Chen, having the advantage of high charging capacity due to its interdigital structure. In this structure, the supercapacitor consists of two Ppy coated nickel electrodes as current collectors and PVA as a solid-state electrolyte (Figure 1) .
In addition, carbon nano tube (CNT) can also be used as electrodes for the supercapacitor. In 2009, Jiang et al.  used carbon nano tube as supercapacitor electrodes and produced a specific capacitance of 428 μF/cm2 and a power density of 0.28 mW/cm2. In 2011, Shen et al. have used an activated carbon as part of a supercapacitor material to obtain a larger surface area. Beidaghi and Wang 2011  also used carbon as supercapacitor electrodes to increase the surface area to be more effective but had to be improved in terms of ion diffusion through the thickness of the dielectric film for the electrodes.
Microsupercapacitors play an important role in the energy harvesting system such ascollecting energy from ambient energy sources. In energy harvesting systems, it does not require any chemicals. In fact, the human body is very flexible in generating power from heat dissipation, deformation of elastic tissue, and the others. Due to its advantages as a energy harvesting system, it can be used widely for biomedical implant devices such as pacemakers and hearing aids and can be used for a long time and recharge without any replacement. Figures 2 and 3 show examples of supercapacitor applications in MEMS devices. For a current pacemaker as depicted in Figure 2, the batteries are frequently changed and required a high cost for surgical operations compared with a pacemaker using energy harvesting that is more reliable and comfortable to the patients. For a normal pacemaker as shown in Figure 2(a), it consists of two elements which are first, a pulse generator, placed under the skin in the chest, a battery, and the impulse control system, and second, a lead inserted directly in the heart through a vein, delivering the impulses . Figure 3 shows a small chip in the middle-ear cavity. The main components are the surgical implant, which is placed underneath the skin, and externally the audio processor that converts sound into electrical signals .
2. Design of planar interdigital electrode MEMS supercapacitor
In this research, planar interdigital electrode supercapacitor design was modeled using Coventorware ver.2008 via its process simulator tool. The software uses finite element analysis . The planar supercapacitor structure consists of silicon (Si), silicon oxide (SiO2), nickel (Ni), Polypyrrole (Ppy), and Polyvinyl Alcohol (PVA) layers. The structure was initiated by setting silicon as substrate 200 nm SiO2 that was deposited on the silicon layer. Interdigital electrode structure was patterned using Ni on the SiO2 layer. After that, the seed nickel layer was electroplated to construct high aspect ratio of nickel [28, 29]. The Ni electrodes were coated with Ppy to create the dielectric layer. PVA layer was fill deposited between the fingers to function as solid-state electrolyte. The process flow for modeling the structure of planar interdigital electrode supercapacitor is as shown in Figure 4 and three-dimension structure of planar interdigital electrode supercapacitor is as shown in Figure 5.
3. Design of double-stacked interdigital electrode MEMS supercapacitor
Double-stacked supercapacitor design was modeled using Coventorware ver.2008 via its process simulator tool. The double-stacked supercapacitor structure consists of Si, SiO2, Ni, Ppy, and PVA layers. For a double-stacked structure, the mirrored cell from initial cell was developed by similar process steps. The electrodes would lie next to and atop of each other when the two cells are sandwiched together. The structure was initiated by setting silicon as substrate. 200 nm SiO2 was stack deposited on the silicon layer. Interdigital electrode structure was patterned using Ni on the SiO2 layer. The Ni electrodes were coated with Ppy to create the dielectric layer. PVA layer was deposited between the fingers to function as a solid-state electrolyte (Figures 6–9).
4. Electrical characteristics for planar interdigital electrode supercapacitor
Comsol Multiphysics ver.4.2 simulation tool was used to simulate the electrical characteristic of planar interdigital electrode design. Secondary current distribution and transport of diluted are chosen as the application modules . Two dimensional time dependent cyclic voltammetry model was developed for the interdigital structure. Cyclic voltammetry measures current response when potential is applied to the working electrodes. For interdigital structure, the boundary conditions define the interface between the electrode and the electrolyte. The applied voltage range is set to −0.5 V to 0.5 V at the working electrode, and the current response at the working electrode can be measured. In the structure, Ppy-coated nickel is the current collector and PVA is the solid-state electrolyte. For the interdigital electrode, it consists of two electrode which is working electrode and counter electrode. The boundary conditions can be identify the interface between electrode and electrolyte. We set the boundaries at the counter electrode as ground and the electrode potential was applied at the working electrode boundaries based on 1[V]/Scan (t/1[s])-0.5[V] function [31, 32].
For cyclic voltammetry simulation, the bidirectional reaction can be formulated as in Ref. :
According to Eq. (2), for mass flow density of forward reaction Nf, it can be expressed as the rate constant kf multiplied by concentration species during oxidation process CO. For backward reaction, according to Eq. (3), for mass flow density of backward reaction Nb, it can be expressed as the rate constant kb multiplied by concentration species during reduction process CR.
The rate constants of forward and backward reaction at the electrodes are described by the Butler-Volmer reaction kinetics
where ks is the standard rate constant and α is the transfer coefficient. In symmetrical reactions, α has a value 0.5. E is the applied voltage on the electrode interface and E0 has a value 0 V. For F, R, and T, it is a Faraday constant (96485 C/mol), molar gas constant (8.3144 J mol−1 K−1), and temperature (298 K), respectively.
Additionally, the relationship for current, potential, and concentration can be summarized as follows:
where θ = nF(E − E0)/RT.
Initially, the Comsol simulation is simplified and performed on planar interdigital electrode structure as depicted in Figure 8(a). It is observed that surface charge density is evenly distributed throughout the interdigital structures. As applied voltage between anode and cathode is slowly increased from 0 to 3 V, the charge density increases as more and more ions accumulate between the electrodes. A closer observation into the structure reveals that maximum charge density often occurs at the intersection of electrodes and its anchor, as depicted in Figure 8(b). This is due to the proximity of the intersection point with the adjacent electrode, causing the charge to concentrate at the sharp edged points. The maximum charge density is as shown in Figure 10(b).
Cyclic voltammetry portrays current response when potential is applied to the electrodes. In our case, the potential is applied at working electrode is between -0.5 V and 0.5 V, with the other electrode set as ground. The increasing voltage increases oxidation and reduction processes between the electrodes, driving current flow between working and counter electrodes. Figure 11 shows that the oxidation and reductions occur within a voltage range of −0.5 and 0.5 V. For our design, ion flux reaches a peak within that region, resulting in maximum peak current (ip) of 6.0 A/m.
For a charge discharge of a microsupercapacitor, there are two techniques. One is the charging and discharging at a constant voltage to record current response with time, and the other is the charging discharging at a constant current to record the voltage response with time. In Figure 12, it is shown that when time is zero, the value of voltage is also zero. Discharging process for planar interdigital electrode occurred when the value of voltage decreased and was back to zero at 1 s.
For the purposes of studying effects of design parameter changes, Comsol simulations were performed on the planar design with varying electrode length, width, and gap between the electrodes. Length of the interdigital electrodes are varied between 500 µm and 700 µm, with 700 µm being the maximum allowable length due to design limitation. From Figure 13, we can see the longer the electrode, the higher the current response. The maximum current response achieved is 1.6 A/m corresponding to electrode length of 700 µm. Increasing electrode length increases active surface area, which in turn enable a higher redox activity between the electrodes. Thus, increasing electrode length increases current generating mass transport activity on the cyclic voltammetry. Adversely, increasing electrode width from 50 µm to 250 µm produces the opposite effect as shown in Figure 14. From the figure, we can see that the smaller the width, the higher the current response. The maximum value of current response is 0.9 A/m, which occurs at electrode width of 50 µm. The reactions occurs at working electrode per depth of electrode data because the modelling is done in 2-D. The increasing value of gap increases space between the electrodes. This allows higher amount of active species to be present at the active interface. At the working electrode, the reduction and oxidation occurs more and more rapidly as more electrolyte are available with the increasing gap (Figure 15).
5. Electrical characteristics for double-stacked interdigital electrode supercapacitor
Cyclic voltammetry for the determination of a plane design double-digit supercapacitor between MEMS is the same as determining the design for cycle voltammetry for a coplanar. But there is a slight difference compared with the maximum current value for the design of the plane. Figure 16 shows that the maximum flow for the cycle voltammetry multiple planar design is 13.2 A/m, which is more than twice the maximum current value relative to the plane designs. In the process of oxidation and reduction, equilibrium is reached and is limited by the voltage on the electrode surface. In the redox cycle, a decrease in current flow is from the effects of depletion. To complete the cycle, the voltage is not only directed to the front but also, on the other hand, within the range specified above. The curved shape also depends on the scan rate. Scan rate can be described as the speed at which the potential is varied. In addition, Figure 17 shows the voltage response to the time indicated at 0.5 s, and the voltage reaches 1 V.
Voltage is still in negative values until at t = 0.25 s. After that, the voltage increases until it reaches a maximum of 1.0 V at a value that is twice the value of the design of the plane at the time, t = 0.5 s, where the charging process occurs so as to achieve the maximum voltage. After reaching the maximum voltage, the discharge process occurs, whereby the voltage drop causes the charging rate to reduce back to a negative value of −1.0 V at a time t = 1 s.
For the purpose of studying the effects of changes in response to the current design of the multiple-plane and the plane, the value for the electrode length, width, and the gap between the electrodes was varied as the design of the plane. The length of the electrode between the digits are 500 and 900 m, where the length of the electrode between the digit at 900 m is the maximum length allowed for the design limits. According to Figure 18, it can be seen that the more the length of the electrode, the higher the reaction flow. The maximum current response was obtained at 37.4 A/m to 900 m-long electrode. Increasing the length of the electrode leads to an increase in active surface area, which in turn allows a higher redox activity to occur between the electrodes. Therefore, increasing the length of the electrodes improves the mass transport activity generated during the cycle voltammetry. High flow shows the life cycle of a higher supercapacitor.
According to Figure 19, we can see that the smaller the width of the electrode, the higher is the current response. The situation is similar to the design of the plane, and there is a difference in the maximum response current that is 13.2 A/m compared with 6.0 A/m for the design of the plane, which took place on the electrode width of 50 m. This phenomenon occurs because there is a balance between the surface area of the electrodes and the electrolyte. When the electrode width is increased, it will reduce the surface area of the electrolyte and also the amount of active species produced also decreasing.
For the gap between the electrodes to design multiple planes, the same value is used as in the design of the plane that is between 50 and 250 μm. In response to the voltage, resulting flows are plotted in Figure 20. Based on these figures, we can see that the greater the gap between the electrodes, the higher the current response. On the sidelines of the electrode 250 μm, the reaction flow reaches a maximum value of 26.4 A/m,which is doubled in value compared the design of the plane. This is because the number of cells for multiple planar design is twice that of the coplanar design that has 40 pairs of cells, so, with the increase in the number of cells, allowing for a number of active species of a higher existence on the active surface Table 1 shows the comparison of parameters between planar and interdigital electrode supercapacitor design.
|Number of fingers||20||Total 40 (20 top, 20 bottom)|
|Layout size (mm2)||8.8||8.8|
|Thickness of cell (μm)||925||1950|
|Maximum charge (voltage input 3 V) (pC)||6.77||15.50|
|Specific capacitance (mF/cm−2)||0.22||0.48|
|Specific power (mW/cm−2)||0.99||2.18|
Energy harvesting system has a high potential as an alternative power especially in biomedical implant devices such as pacemakers. Due to the disadvantages such as short lifetime and containing a finite amount of depletable chemical energy of lithium-ion batteries, the patients using heart pacemaker have to replace the battery once every 5–10 years. To avoid any powering unit replacement, a renewable energy generation and storage system could become a vital solution. From this research, it can be concluded that double-stacked MEMS interdigital supercapacitor has same layout, only slightly thicker compared with the planar structure due to double stacking, but with much superior charging capacity. For both planar and double-stacked MEMS interdigital supercapacitor designs, the electrodes width, length, and gap between electrode fingers were fixed at 50, 500, and 50 μm, respectively. Furthermore, the planar and double-stacked MEMS interdigital supercapacitor designs were simulated using COMSOL ver.4.2a for electrical performance verification such as cyclic voltammetry and charge discharge performance. For cyclic voltammetry performance, applied voltage range is set to −0.5 to 0.5 V. For capacitance performance, it is observed that capacitance increases linearly with increasing number of cell, length of fingers, and width of fingers due to charge interactions among adjacent cells. The simulation results show that the planar structure has a charging capacity of 6.77 pC and the double-stacked structure has a charging capacity of 15.5 pC. Furthermore, for specific capacitance, it is observed that for planar structure is 0.22 mF/cm−2 and for double-stacked structure is 0.48 mF/cm−2, while for specific power, the planar structure is 0.99 mW/cm−2 and for double-stacked structure is 2.18 mW/cm−2. For charge discharge curve, it is observed that the curves are almost linear in the potential range. These results highlight the superiority of the double-stacked MEMS interdigital supercapacitor design compared with its planar counterpart in terms of charging capacity and electrical performance, thus making it favorable for powering heart pacemakers.