Dependence of Double Layer Capacitance on Pore Diameter of Carbon Coated Porous Si
Authors: Takiguchi Yutaka , Maki Hideshi , Mizuhata Minoru 
Published: February 2020

Recent growth of using electric vehicles and electronic devices, the increasing demand for energy storage devices. Supercapacitors, also known as ultracapacitors, are the promising energy storage devices with high power density, fast charge discharge properties and long cycle life. 1 Electrical double layer capacitor (EDLC) is a kind of supercapacitors and stores electric energy at solid-electrolyte interfaces by applying a bias voltage, and its specific capacitance mainly depends on specific surface area, therefore there are often utilized graphene based materials, porous materials, pillar like materials, and so on. On the other hand, it is known the nm-sized pore affects the double layer capacity.2 Porous silicon (PSi) is a candidate for supercapacitor for naturally abundant resources of silicon and easy to control porous structure by changing the etching condition and the wafers.3 In this study, to investigate the pore size contribution on specific capacitance we used graphene coated PSi with various pore diameter as a model of porous electrode.
PSi was prepared in a homemade etch cell by anodizing the highly antimony doped Si (100) wafers (~0.02 Ω cm-1) using a HF (~48%)-ethanol mixture as the electrolyte at a various current density (50-150 mA/cm) for 3 min, and then, PSi was rinsed with ethanol and dried under vacuum for 2-3 h.
The carbonization of PSi was carried out in a tube furnace with a quartz tube under a 1 L/min of Ar and 200 mL H2 flow while the furnace was heated to 650 ˚C When the furnace reached 650 ˚C, 10 ml/min of C2H2 was added to the gas mixture, and the temperature was increased to 750 ˚C for 10 minutes, and increased to 850 ˚C for another 10 minutes. The C2H2 was then turned off, and the P-Si was cooled in the presence of Ar and H2 until the sample reached a room temperature. Analysis of PSi and carbonized PSi was performed with a FESEM and Raman spectroscopy. Electrochemical measurement was carried out utilizing PSi materials in a two-electrode homemade electrochemical test cell. Cyclic voltammetry and electrochemical impedance spectroscopy were conducted in a trimethylpropylammonium bis(trifluoromethanesulfonyl)amide (TMPATFSA) ionic liquid. The scan range was -3.0 to 3.0 V. Impedance measurements were performed around zero bias with a signal amplitude of 10 mV.
SEM images of PSi and carbonized PSi indicated that PSi’s was obtained with the pore diameter of 10, 20 and, 80 nm. Also it is confirmed that there is little change during the carbonization of PSi.
Raman spectroscopy was carried out to confirm graphene coating on PSi, and after the carbonization of PSi. We observed Si Raman peak near 520 cm-1 from the pre-coated PSi, and the D (1350 cm-1) and G (1580 cm-1) peaks corresponding to sp3 and sp2-hybridized carbons from the carbonized PSi, respectively. EIS shows drastic change between before or after graphene coating on PSi. This is because the contact resistance decreased by graphene coating and good wettability of graphene. For observation of dependence of double layer capacitance on pore diameter of carbon coated porous Si, we performed cyclic voltammetry of carbonized PSi as shown in Fig.1. The largest electrochemical capacitance was observed from the carbonized PSi with pore diameter of 20 nm. The contact resistance of graphene coated PSi decreased in inverse proportion to the size of the electric potential window. It is suggested that the decrease of the adsorption site of the ion by the decrease in surface area for 10 nm of pore size by hindrance of ionic mobility or decrease of electronic double layer.4

【Reference】
[1] M. Winter et al., Chem. Rev.
104,4245 (2004).
[2] C.-H. Hou et al., J. Colloid Interface Sci.
302,54(2006).
[3] L. Oakes wt al., Sci. Reports,
[4] Y. Kitazumi et al., Electrochim. Acta, 112, 171(2013).

Figure 1