Structure and Electrochemical Properties of Pyrolyzed Magnesium Citrate/SU-8 Nanocomposites for Supercapacitators
Authors: Li Gang , Li Dawei , Gao Xiaozhuan , Li Tingyu , Hu Jie , Wang Yingge , Chen Xuyuan , Wang Kaiying 
Published: February 2020

Supercapacitors have attracted significant research interest due to their fast cycling and high power densities, charging and discharging time for millions of cycle [1]. Highly porous carbon materials are commonly used as supercapacitor electrodes due to their high specific surface area and good conductivity. To realize MEMS supercapacitors, a scalable technique for on-chip integrated high-performance microelectrodes are in urgent required. Pyrolyzed patterned photoresist is one attractive way to deposit porous carbon electrode for electrochemical applications. However, current research on pore structure control of photoresist-derived carbon electrodes with large specific capacitance and good cycling performance is rather limited including the problem of uneven porosity [2, 3].
For porous structure control of pyrolyzed photoresist-derived carbon, 200 mg, 300 mg and 400 mg of magnesium citrate nanoparticles were mixed in 10 ml SU-8 photoresist during pyrolysis process respectively.The ultra-thick SU-8 photoresist layer mixed with magnesium citrate was deposited on silicon wafer with multiple spin-coating methods at the rotation speed of 1500 rpm for 35s. Then, the doped SU-8 photoresist was pyrolyzed in a horizontal tube furnace under inert forming gas (95% N2, 5% H2) atmosphere with flow rate of 2000 sccm, which the temperature was programmed from room temperature to 950 oC. Fig. 1 shows the SEM images of pyrolyzed SU-8 photoresist before and after magnesium citrate doping. Fig. 2 presents the Current-Voltage (C-V) curves of carbon microelectrodes at different scan rate in 1M Na2SO4 electrolyte. Clearly, the C-V curves of the carbon electrodes with magnesium citrate doping is more close to symmetric rectangular shape than that without doping, which optimized capacitance is 205mF/cm-2capacitance for the sample (30 mg/ml). Fig. 3 plots the galvanostatic charging and discharging curves of carbon microelectrodes at different current densities. The charging and discharging curve exhibits a near isosceles triangle, which indicates a better irreversible discharging and discharging performance. However, the charging/discharging time of the carbon electrodes with magnesium citrate doping is much longer than that without doping, which indicates the specific surface area of the carbon electrodes was greatly increased after magnesium citrate doping.
In summary, carbon microelectrodes derived from pyrolyzed SU-8 photoresist with different doping concentration have been prepared and characterized by SEM and electrochemical technique. The optimized concentration of magnesium citrate nanoparticles in SU-8 photoresist is 30 mg/ml, and optimized capacitance is 205mF/cm-2correspondingly. The high capacitance could be associated with the highly porous structure.

References:

[1] Jiang S, Shi T, Liu D, et al. Integration of MnO2 thin film and carbon nanotubes to three-dimensional carbon microelectrodes for electrochemical microcapacitors. Journal of Power Sources
, 262 (2014): 494-500.
[2] Wang S, Hsia B, Carraro C, et al. High-performance all solid-state micro-supercapacitor based on patterned photoresist-derived porous carbon electrodes and an ionogel electrolyte. Journal of Materials Chemistry A 2.21 (2014): 7997-8002.
[3] Jin Z, Xun Y, Wei X, et al. Mesoporous carbons derived from citrates for use in electrochemical capacitors. New Carbon Materials, 25.5 (2010): 370-375.

Figure 1