Characterization of Biopolymer-Based Porous Carbon as Electrode Material in Battery Applications

Abstract

The quick characterization of materials plays an important role in both the development of new compounds as well as in quality control purposes. Therefore, it can be critical to be able to evaluate materials on the base level way before they have even been electrochemically tested. Electrical conductivity is the most obvious property that needs to be monitored e.g. via powder resistivity measurements. Properties on the molecular scale include functionalities and order which can be evaluated quantitatively using Raman spectroscopy. Ultimately, physisorption techniques enable detailed insights into the pore structure by providing a specific surface area (SBET), pore volume as well as pore size distribution (PSD). Herein, these methods have been applied to a chitosan-based aerogel carbonized for 1 h at 800 °C in a nitrogen atmosphere. Electrical conductivity can be measured very quickly in the form of a powder, monolith or as a thin layer as occurring in electrode coatings. In powder form, the resistivity can be measured by filling the sample into a cylinder and pressing on a four-pin setup with defined pressures. By monitoring the starting point of compression, the density of the compressed material can be calculated automatically. This allows different kinds of data representation and therefore delivers information about conductivity behaviour as a function of pressure or density. The expenditure of time amounts to 20 to 30 minutes per sample, depending on user experience. Raman spectroscopy can provide information about functionalities and order of the observed material. Data evaluation using mathematical fits (e.g. Gaussian) allows the separation of overlapping signals and therefore provides quantitative results concerning the graphitic (AG) and disordered contribution (AD) represented by the surface area below the fit. The AD/AG ratio is commonly used to allow a comparison of various materials. The expenditure of time amounts to 20 to 30 minutes per sample, depending on user experience and used setup. Herein, a Raman microscope was used. The limiting factor in the characterisation of porous materials is most frequently the physisorption technique. However, depending on what kind of information is required, analysis time can be cut down dramatically. While the collection of a whole isotherm for a carbonised chitosan aerogel can last about 9 hours, the investigation of nothing but the micropore size distribution takes about 1.5 hours. The collection of data points in the BET range which is typically between relative pressures of 0.05 and 0.3 takes only 20 minutes. These net times signify only the collection of data points and do not include evacuation routines the analytical instrument performs beforehand and free space measurement. The latter is particularly time-consuming with sometimes about 2 h, but can be avoided if the skeletal density of the material and the free space of the empty measurement cell is known.