An Investigation into Capacitive Performances and Impedance Contributions Due to Structural Differences of Biochar Thin Film and Monolith Supercapacitor Electrodes

Abstract

Supercapacitors are an upcoming, high power density energy storage technology. Unlike in batteries, energy is stored physically through the adsorption of oppositely charged ions to a surface. Electrode materials that facilitate this process have high conductivity, extensive porosity, and high specific surface area. Porous carbon powders are used commercially, being pressed into thin films and held together with a binder material [1]. While carbon nanotubes and graphene are the focus of many researchers in this field, our group is investigating a unique alternative: biochar. Biochar is pyrolyzed biomass, and our group uses different types of wood as precursor materials. Through a controlled pyrolysis process, it is possible to preserve the internal macrostructures of wood, creating pathways throughout the carbon structure that should facilitate ion transport. By creating large, continuous monolithic pieces of carbon, devices can be constructed differently from the status quo. Using monolithic slices as electrodes simplifies the manufacturing process, reduces the number of ancillary components required per device, eliminates the need for a non-conductive binder material, and enables the construction of larger electrodes. Currently there is a lack of understanding regarding the relationships between biochar macrostructures and capacitive performance. While monolithic biochar electrodes were found to show similar capacitive performance to their thin film counterparts in [2], the study did not include an evaluation from a structural point of view. Additionally, although it was found that increasing electrode thickness of thin films resulted in an increase in device resistance [3], this relationship has not been explored for monolithic electrodes nor at a relevant scale, as it becomes possible to make electrodes hundreds of times thicker than those currently used [1]. Competitive capacitive performance of powdered biochar thin film electrodes compared to thin film alternatives has been demonstrated in [4] and [5]. The overall goal of this project is to determine if monolithic biochar electrodes can compete with the capacitive performance of powdered biochar thin film electrodes. By constructing monolithic slices and powdered thin films from the same biochar, electrochemical influences of the macrostructure are investigated. Capacitive performance metrics such as charge/discharge rate capability and self-discharge rates are explored for the two electrode structures, as well as for different electrode thicknesses. Frequency-dependent resistances and their respective contributions to total device resistance are analyzed through Electrical Impedance Spectroscopy (EIS). Mass transfer and diffusional resistances, which are believed to be highly dependent on both electrode geometry and structure will be reported. Characterization of the electrode materials using N2 and CO2 physisorption; helium pycnometry; and SEM imaging coincides with the electrochemical testing methods to help resolve the causes of performance differences and evaluate their significance. Preliminary results show that monolithic biochar electrodes up to 1mm thick have comparable specific capacitance performance to the thin films at 100mA/g, which was the highest current density employed thus far. Upcoming work will apply larger current densities to determine if the channels of the macrostructures contribute to high power performance or reduce volumetric capacitance. Monolithic electrodes up to 5mm thick are able to achieve similar specific capacitances to the thin films at a low current density (5mA/g), but their performance degrades significantly with charge rate. The experiments mentioned in the preceding paragraphs were all conducted on biochar from sugar maple wood. Research direction for early 2017 will involve analysis on biochar from different types of precursor wood. Soft and hard woods, and the variety of species within these categories have vastly different types and sizes of internal structures (Figure) [6]. The effects of these macrostructures on capacitive performance and ion transport will be assessed, and these results will be available for presentation.