Synthesis and Integration of Ultrathin Polyaniline Films into Carbide Derived Carbon Supercapacitors

Abstract

Utilizing conducting polymers as the active materials for supercapacitors is a novel and promising approach to improve performance [1-4]. Conducting polymers have been shown to have high specific capacity, high energy density, and promising stability. Furthermore, they are less costly than oxide-based materials and have the ability to be p and n-doped. Moreover, conducting polymers enhance charge capacity through Faradaic redox reactions. As reported by Simon and Gogotsi [5], the integration of ultrathin pseudocapacitive materials within porous carbon nanostructures can result in significant improvement in energy and power densities, allowing for having both battery-like (high capacity) and capacitor-like (high rate) behavior in a single electrode. Conducting polymers are typically deposited via solvent-based techniques such as chemical bath deposition, electrodeposition, and casting from suspension. Nevertheless, challenges exist with making uniform conformal coatings, especially with poor solubility of many conducting polymers and poor accessibility of the highly tortuous pore space within nanostructured electrodes. Oxidative chemical vapor deposition (oCVD) allows one to conveniently bypass these challenges as it is a solvent-free method where the reagents for oxidative polymerization, the monomer and oxidizer, are heated to vapor form that can easily penetrate into mesoscale pores and polymerize (Figure 1). By understanding the oCVD PANI synthesis parameters (reagent flow rates, reactor pressure, substrate temperature, saturation pressure), the reaction and diffusion rates can be carefully controlled to optimally coat the CDC electrode. By flowing vapors of aniline and antimony pentachloride (a strong oxidizer), a solid thin PANI film can be polymerized on a substrate. FTIR, UV-vis, and XPS confirm that oCVD produces the electrochemically active emeraldine state of PANI. The pore distribution of the CDC electrode, as determined by N2 sorption, remains mostly unchanged before and after coating because the fine oCVD control over film thickness allows the pores to be coated without filling them, thereby maintaining the electrochemical double layer of the intrinsic carbon electrode while imparting additional energy density through Faradaic redox reactions from PANI. In this work, we demonstrate that charge storage capacity can be improved significantly through the integration of ultrathin (<30 nm) polyaniline (PANI) films into bimodal mesoporous Mo2C-derived carbide derived carbon (CDC, 800 °C synthesis temperature) electrodes using a single-step oCVD process. The ultrathin coating preserves to a large extent the native electrode surface area and pore distribution while simultaneously improving capacitance. The oCVD process allows PANI to be integrated into pores as small as 1.7 nm, and the oCVD PANI integrated supercapacitors have a specific capacitance more than 100% higher than that of bare CDC ones (136 F/g for 11 wt% of PANI in the CDC electrode vs. 60 F/g for bare CDC at 10 mV/s). This yields a PANI-only capacitance of ~690 F/g, which is close to the theoretical value of 750 F/g [6]. Even after high scan rates of over 100 mV/s, the added pseudocapacitance from PANI remains evident (Figure 1). The composite device exhibits excellent cyclability, lowering to only 90% of the initially stabilized value (~100 F/g) after 10,000 cycles. To our knowledge, this work is the first reported synthesis of PANI via oCVD and demonstrates oCVD’s advantages compared to other approaches in the integration of ultrathin PANI films into carbon supercapacitors. Figure 1. Left: oCVD process where the oxidant (SbCl5, orange) and monomer (aniline, green) vapors enter the reaction chamber and surface polymerize onto the electrode. Middle: A comparison of the cyclic voltammograms for both bare (red) and polyaniline (PANI) (green) electrodes at 20 mV/s. Inset shows SEMS of bare and PANI-coated CDC electrodes. Right: Rate performance of both devices based on cyclic voltammetry. [1] A. Laforgue, P. Simon, C. Sarrazin, J.-F. Fauvarque, Journal of Power Sources, 80 (1999) 142-148. [2] D.Y. Liu, J.R. Reynolds, ACS Applied Materials & Interfaces, 2 (2010) 3586-3593. [3] A. Rudge, I. Raistrick, S. Gottesfeld, J.P. Ferraris, Electrochimica Acta, 39 (1994) 273-287. [4] S. Nejati, T.E. Minford, Y.Y. Smolin, K.K.S. Lau, ACS Nano, 8 (2014) 5413-5422. [5] P. Simon, Y. Gogotsi, Nature Materials, 7 (2008) 845-854. [6] G.A. Snook, P. Kao, A.S. Best, Journal of Power Sources, 196 (2011) 1-12. Figure 1