2, Chemistry, University of Bologna, Bologna, , Italy
The development of efficient solar energy conversion/storage technologies is critical to limit the negative effects of the anthropogenic climate change on Earth. Electrochemical technologies have been widely investigated to capture the power generated from intermittent sources, like the Sun. High performance storage solutions, such as batteries and supercapacitors, make use of toxic electrode materials and organic electrolytes with limited environmental sustainability. Nature could be the source of abundant and environmentally benign materials to be used in next generation batteries and supercapacitors, paving a way towards a sustainable use of resources.
Among natural materials, eumelanin is a functional biomacromolecule found in flora and fauna with fascinating properties, such as strong broadband UV-visible absorption, redox activity, metal binding affinity, humidity-dependent electrical response as well as good thermal and photostability . DHI (5,6-dihydroxyindole) and DHICA (5,6-dihydroxyindole-2-carboxylic acid) are the two building blocks of melanin. The synergy between the reduction/oxidation (redox) activity of the building blocks and the capability of several of their functionalities to reversibly bind cations constitutes the foundation for the use of eumelanin in energy storage systems . Photoconductivity of melanin in different form such as pellets and thin films has been reported in the literature  motivating current efforts for using this photoactive bio-pigment in solar assisted electrochemical energy storing systems. . We were capable to overcome the legendary limited processability of melanin by adopting a solid state polymerization strategy for its fabrication. In this way we were capable to keep the environmental friendliness of the melanin controlling its (supra)molecular structure and processability.
In this work, we report about a two symmetric electrode-system based on chemically controlled melanin wrapped with PEDOT: PSS on carbon paper current collectors. PEDOT:PSS enhances electronic transport and intermolecular connectivity between the melanin chains . The electrode materials are processed by simple, solution-based methods. The electrochemical storage properties are evaluated for different chemically controlled eumelanin: polyDHI, polyDHICA and polyDHI:polyDHICA in dark and light condition, using cyclic voltammetry and galvanostatic charge/discharge cycles.
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