(301b) Oxidative Chemical Vapor Deposition of Polyaniline: Influence of Process Conditions on Film Chemistry and Electrochemical Performance

The use of conducting polymers as
the active material for supercapacitors is a promising approach to improve the
performance of supercapacitors. 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 ultrathin (5-10 nm) 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. To conveniently bypass these challenges, polyaniline
(PANI) is synthesized via oxidative chemical vapor deposition (oCVD) in a
single-step, solvent-free process using aniline monomer and antimony
pentachloride oxidant, to deposit ultrathin films into Mo₂C
carbide-derived-carbon (CDC) electrodes (Figure 1).¹ we are realizing a strategy proposed by Simon and Gogosti² to improve both the energy and power
densities of electrochemical capacitors by integrating conducting polymers into
nanostructures electrodes.

By understanding the oCVD PANI
synthesis parameters (reagent flow rates, reactor pressure, substrate
temperature), the reaction and diffusion rates can be carefully controlled to
optimally coat the porous carbide derived carbon (CDC) electrode. The oxidant
flowrate, substrate temperature, and reactor pressure were varied, and their
influence on film chemistry and supercapacitor performance was explored. The
study reveals that a higher substrate temperature, pressure, and oxidant
flowrate are critical for depositing emeraldine PANI for optimal
electrochemical performance. Spectroscopic results,³ such as FTIR,
SEM and XPS, indicate that a substrate temperature of 90 °C is needed to
minimize the formation of oligomers during polymerization. Increasing the
oxidant flowrate to nearly match the monomer flowrate favors the deposition of
PANI in the emeraldine state. Lower substrate temperatures (Low-T), such as 25
°C, lower pressures (Low-P) and lower oxidant flow (Low-O) generally lead to poorer
quality PANI films that results in more oligomers. By optimizing the oCVD
processing conditions, CDC electrodes integrated with oCVD PANI exhibits more
than double the gravimetric capacitance (115 F/g) vs bare CDC electrodes (52
F/g) and a 79% capacity retention after over 10,000 cycles.⁴
Interestingly, the optimally performing PANI-CDC devices have a porous PANI
morphology as determined by SEM, which may facilitate ion transport, improve
scan rate performance, and impart electric double layer capacitance in addition
to the intrinsic faradaic pseudocapacitance.

1.         Smolin,
Y. Y.; Van Aken, K. L.; Boota, M.; Soroush, M.; Gogotsi, Y.; Lau, K. K. S.,
Engineering Ultrathin Polyaniline in Micro/Mesoporous Carbon Supercapacitor Electrodes
Using Oxidative Chemical Vapor Deposition. Adv.
Mater. Interfaces 2017,
DOI: 10.1002/admi.201601201

2.         Simon,
P.; Gogotsi, Y., Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845-854.

3.          Smolin, Y.Y.; Soroush, M.; Lau, K. K.
S., Oxidative Chemical Vapor Deposition of Polyaniline Thin Films. Beilstein J. Nanotech. 2017 (under review).

4.          Smolin, Y. Y.; Soroush, M.; Lau K. K. S.,
Influence of oCVD Polayanilnie Film Chemistry in Carbo-Based Supercapacitors. Ind. Eng. Chem. Res. 2017 (under review).

Figure 1. Left: oCVD process where
the oxidant (SbCl₅, orange) and monomer (aniline, green) vapors
enter the reaction chamber and surface polymerize onto the porous electrode. Right:
A comparison of the cyclic voltammograms and cycle life for both bare (black)
CDC and oCVD PANI-CDC (colored) at various oCVD process conditions (at 20 mV/s).