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A1.10 - Structure-Property Relationship in Biologically-Derived Eumelanin Cathodes Electrochemical Energy Storage 
December 1, 2014   11:30am - 11:45am

Organic compounds represent promising alternatives to inorganic materials for renewable and sustainable energy storage devices. Organic molecules can be processed into non-conventional form factors, are cost-effective, and exhibit reduced toxicity compared to other exotic inorganic materials. Carbonyls, carboxylates, amines are the attractive redox-active chemical signatures of organic compounds that could potentially be utilized as electrode materials by reversible binding of cations. While these redox-active organic compounds enable the novel pathway as charge storage devices, there are many persistent challenges that may limit the prospective utility of organic electrodes. Major challenges include low charge collection abilities and high solubility in electrolytes.

Here we introduce the use of biologically-derived eumelanin pigments as cathode materials in aqueous sodium-ion electrochemical storage devices. Eulemanin, a sub-class of melanin, exhibits redox-active signatures including pendant carboxylates, aromatic amines, and catechols that can support reversible cation binding. Eulemanins are ideal electrode materials because they are stable in aqueous electrolytes and are composed of nanostructured granules. Homogeneous microstructures overcome kinetic limitations of organic electrodes. Current collection in semiconducting melanins can be improved by incorporating conducting silver nanowire (AgNW) networks. Full cells are composed of sodium titanium phosphate (NaTi2(PO4)3) anodes and eumelanin cathodes. Galvanostatic full cell discharge exhibits a stable working potential of 0.5 V. Specific capacities of full cells discharged at 0.05 Ag-1 contain 49 and 78 mAhg-1 for natural (NatMel) and synthetic melanin (SynMel) respectively. FT-IR and Raman spectra corroborate that Na+ associates with pendant carboxylates of eumelanins during discharge. Natural melanins exhibit unique potential plateaus during discharge that are not present in synthetic melanins. These data support the presence of porphyrin structures in natural melanins, which are largely absent in synthetic melanins. The details regarding the structure-property relationships will be discussed.

Taken together, eumelanin-based cathodes represent a material that is compatible with broader strategies of sustainable and renewable energy storage. Melanins are a class of organic molecules that exhibit suitable performance, cost-effectiveness, and limited processing to produce electrochemical storage devices.

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