(603d) Integration of Ultrathin Polyaniline Films into Carbide Derived Carbon Supercapacitors Via Oxidative Chemical Vapor Deposition

Authors: 
Smolin, Y. Y., Drexel University
Van Aken, K. L., Drexel University
Boota, M., Drexel University
Soroush, M., Drexel University
Gogotsi, Y., Drexel University
Lau, K. K. S., Drexel University

The use of
conducting polymers as the active materials for supercapacitors is a promising
approach to improve the performance of supercapacitors1, 2.
Conducting polymers enhance charge storage capacity through Faradaic redox
reactions, and have the ability to be p- or n-doped. They 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 via wet chemistry routes, 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 pores and polymerize (Figure 1). By
understanding the oCVD polyaniline (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 carbide derived carbon (CDC) electrode. By
flowing vapors of aniline and antimony pentachloride (a strong oxidizer), a
solid thin PANI film can be directly polymerized on a substrate. FTIR, UV-vis,
and XPS confirm that oCVD produces an
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 are realizing a strategy proposed by Simon and Gogosti3 to
improve both the energy and power densities of electrochemical capacitors by
integrating conducting polymers into nanostructures electrodes. In particular,
we demonstrate that charge storage capacity can be improved significantly
through the integration of ultrathin (<30 nm) PANI films into 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
4
. 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, which decreases only by 10% 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.         Laforgue, A.;
Simon, P.; Sarrazin, C.; Fauvarque,
J.-F. Journal of Power Sources 1999, 80, (1–2),   142-148.

2.         Liu, D. Y.; Reynolds, J. R. ACS Applied
Materials & Interfaces 2010, 2, (12), 3586-3593.

3.         Simon, P.; Gogotsi,
Y. Nature Materials 2008, 7, (11), 845-854.

4.         Snook, G. A.; Kao, P.; Best, A. S.
Journal of Power Sources 2011, 196, (1), 1-12.