|Emerging trends in energy storage device technologies for a range of consumer electronics, automotive, and grid-scale applications are targeting thin film materials conducive to flexible substrates. With the objectives of increased energy density, manufacturability, and flexibility, integration strategies for supercapacitor devices include carbon nanostructure composite electrodes with solid-state electrolytes. Utilizing CNT/PANI flexible electrode structures, Meng et. al. investigate the integration of an all-solid-state supercapacitor device structure utilizing a simple two-step approach.|
Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network
- Meng C, Liu C, Fan S. 2008. Flexible carbon nanotube/polyaniline paper-like films and their enhanced electrochemical properties. Electrochemistry Communications 11:186-189. DOI: 10.1016/j.elecom.2008.11.005.
- Meng C, Liu C, Chen L, Hu C, Fan S. 2010. HIghly-flexible All-Solid-State Paperlike Polymer Supercapacitors. Nano Letters 10(10):4025–4031. DOI: 10.1021/nl1019672.
Emerging trends in energy storage device technologies for a range of consumer electronics, automotive, and grid-scale applications are targeting thin film materials conducive to flexible substrates. With the objectives of increased energy density, manufacturability, and flexibility, integration strategies for supercapacitor devices include carbon nanostructure composite electrodes with solid-state electrolytes. This strategy eliminates the utilization of liquid electrolytes thereby alleviating the need for a separator layer between electrodes, along with the need for sophisticated encapsulation packaging to ensure safety during operation. Eliminating these components enhances energy and power density for the device. Furthermore, supercapacitor device designs that do not degrade while bending or flexing enable power sources to be configured in versatile form factors that match the application. These aspects provide a roadmap for the design and manufacturing of next generation energy storage technologies enabled by advanced nanocomposite materials, and large-area, high-rate nanomanufacturing platforms such as roll-to-roll processing.
Carbon nanotube (CNT) networks have received considerable attention as high performance electrodes in electrochemical energy storage applications due to the high surface area and excellent conductivity exhibited by continuous CNT networks formed as a thin film. For supercapacitor applications, pristine CNT networks are dominated by their electric double layer capacitance, with specific capacitance typically on the order of 80 F/g. Additionally, electrodes formed using pristine CNT networks typically exhibit a degradation in conductivity during mechanical deformation and bending due to dissociation of the network. Consequently, approaches incorporating conductive polymers and binders to sustain the electrode properties for a format that experiences bending have been investigated with limited success. Previously Meng et. al.  have reported on the use of CNT/polyaniline (CNT/PANI) nanocomposites to overcome these issues for supercapacitor electrode performance. In their study, the authors synthesized electrodes using CNT/PANI composites formed by a more traditional powder process and compared these to electrodes formed via microporous filtration of a CNT suspension first forming a random, free standing CNT network. The free standing CNT networks form a highly porous film on the order of 25-30 µm thick which is then subjected to aniline polymerization coating the CNTs with ~70-90 nm of polyaniline, thereby forming a nanocomposite film of about 75% PANI by weight. Electrochemical characterization of the PANI polymerized CNT network compared to the powder processed composite and straight PANI films demonstrated superior properties in terms of specific capacitance, cycle degradation, and long term stability. These results were due to the added flexibility and high porosity of the polymerized nanocomposite, along with the continuity of the CNT network that is retained through the charge cycling.
Utilizing the CNT/PANI flexible electrode structures, Meng et. al.  further investigated the integration of an all-solid-state supercapacitor device structure. Utilizing a simple two-step approach, the CNT/PANI electrodes were soaked in a 10% sulfuric acid (H2SO4)-10% polyvinyl alcohol (PVA) aqueous solution for 10 minutes, followed by air-drying for four hours. The two electrodes were then pressed together under a pressure of ~10 MPa for 10 minutes during which the gel electrolyte coatings form a thin, adhesive separating layer. Thus a two-step solution process has been demonstrated to fabricate CNT/PANI electrode structures coated with a H2SO4-PVA gel electrolyte. The benefits of this process include both large specific surface area resulting from the porous electrode layers infiltrated with gel electrolyte, along with the necessary mechanical strength to enable bending and flexing of the integrated structure. Electrochemical testing of the integrated supercapacitor demonstrated a discharge specific capacitance of 332 F/g at 1 A discharge rate, ~8% smaller than that measured using a 0.5 M H2SO4 electrolyte solution. The all-solid-state supercapacitor utilizing the nanocomposite electrode further exhibited only an 11% decrease in capacitance after 1000 charge/discharge cycles. With the thickness of the integrated device only 113µm, the authors reported specific power and energy capabilities using this approach far superior to other supercapacitor technologies. Further research in this area must explore strategies to scale the process and further increase throughput of the process steps.
Images reproduced with permission from Meng C, Liu C, Chen L, Hu C, Fan S. 2010. HIghly-flexible All-Solid-State Paperlike Polymer Supercapacitors. Nano Letters 10(10):4025–4031. DOI: 10.1021/nl1019672. Copyright 2010 American Chemical Society.