Ri Xu1 Carmela Tania Prontera2 Eduardo Di Mauro1 Alessandro Pezzella2 Francesca Soavi3 Clara Santato1

1, Polytechnique Montreal, Montreal, Quebec, Canada
2, Università di Napoli Federico II, Napoli, , Italy
3, Università di Bologna, Bologna, , Italy

Eumelanin, a brown-black pigment ubiquitous in fauna and flora, can be synthesized by the oxidative polymerization of 5,6-dihydroxyindole (DHI) and/or 5,6-dihydroxyindole-2 carboxylic acid (DHICA). This redox-active pigment features interesting functional properties, such as photoprotection, antioxidant behavior, hydration dependent conductivity, metal binding affinity and free radical scavenging [1][2]. Furthermore, eumelanin features broadband visible light absorption.
Photoconductivity experiments of melanin are available in the literature [3]. Melanin-based electrodes for supercapacitors and batteries have been demonstrated by our and other groups [4][5]. Here we report on the possibility to enhance the storage properties of the biopigment by photoinduced charging, in parallel to electrical charging. In our study, we used chemically controlled melanin, obtained by solid-state polymerization of the DHI and DHICA building blocks (to give respectively polyDHI and polyDHICA). Controlling the (supra)molecular structure of eumelanin is imperative to gain insight on its (photo)storage properties and to fully exploit its technological potential. Indeed, due to the limited processability of eumelanin, fundamental aspects related to the effect of the (supra)molecular structure on the optical and redox properties of the pigment are largely undiscovered.
We studied the electrochemical behavior of DHI- and DHICA-melanin as well as Sepia melanin (extracted from sac of cuttlefish) loaded on carbon paper as electrodes, under dark and light (1 sun) conditions. The samples were investigated by cyclic voltammetry in aqueous acetate electrolytes pH=5. Higher (photo)currents were observed for DHI-melanin, likely due to a better pi-pi stacking with respect to DHICA-melanin [2].
Considering the optical properties of eumelanin, our work contributes to advance the development of biologically derived all organic solar supercapacitors batteries, integrating the solar conversion and the energy storage functions.

[1] E. Di Mauro, R. Xu, et al., MRS Communications (2017).
[2] R. Xu, E. Di Mauro, et al., APL Materials (2017) (under revision).
[3] a) Mostert, A. B., Powell, B. J., et al. Proceedings of the National Academy of Sciences 109((2012). b) Abbas, M., D’Amico, F., et al. Eur. Phys. J. E. 28.3 (2009).
[4] P. Kumar, E. Di Mauro, S. Zhang, A. Pezzella, F. Soavi, C. Santato, F. Cicoira, J. Mater. Chem. C. 4, 9516 (2016).
[5] Y.J. Kim, W. Wu, S.-E. Chun, J.F. Whitacre, C.J. Bettinger, Adv. Mater. 26, 6572 (2014).