© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION Catechol-Mediated Reversible Binding of Multivalent Cations in Eumelanin Half-Cells Young Jo Kim, Wei Wu, Sang-Eun Chun, Jay F. Whitacre,* and Christopher J. Bettinger* Prof. C. J. Bettinger Department of Materials Science and Engineering Department of Biomedical Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA E-mail: cbetting@andrew.cmu.edu Prof. C. J. Bettinger McGowan Institute of Regenerative Medicine 450 Technology Drive, Suite 300 Pittsburgh, PA 15219, USA Dr. Y. J. Kim, W. Wu, Prof. J. F. Whitacre Department of Materials Science and Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA E-mail: whitacre@andrew.cmu.edu Dr. S.-E. Chun Department of Chemistry University of Oregon Eugene, OR 97403, USA DOI: 10.1002/adma.201402295 also demonstrated potential utility as a strategy to overcome potential limitations in charge collection. [20,22] Inorganic cathode materials that can overcome the strong polarization of multiva- lent cations are essential to access the advantages of increased volumetric capacity compared to monovalent cations. [17,23] Promi- nent examples include Chevrel phase molybdenum chalcoge- nides (MoS 8-y Se y ), vanadium oxide (V 2 O 5 ) nanowires, nanostruc- tured silicates, TiS 2 nanotubes, and other compounds. [24–26] These materials exhibit charge storage capacities from 25–200 mA h g -1 (at <2 V vs. Mg/Mg 2+ ), coloumbic efficiencies between 85–99.9%, and variable cycling stability. [24] Mo 6 S 8-y Se y cathodes offer enhanced performance compared to oxides when using Mg anodes because sulfides exhibit reduced ionicity. [27] The attenu- ated electrostatic force between Mg 2+ and sulfides also increases cation mobility. [28] Inorganic materials are subject to irreversible multivalent insertion, which reduces both cycle stability and coulombic efficiency. [5,6,24,26,29] Current strategies for improving cathode performance in secondary batteries that use multivalent ions include screening cations with anionic groups and reducing the characteristic length scale of cathode structures. [17,30] Here we demonstrate that organic compounds may serve as cathode materials in electrochemical storage devices that uti- lize multivalent ions. Redox-active catechol-quinone systems represent an attractive chemistry to leverage in electrochemical storage systems. [31] Quinones participate in two-electron two- proton reduction processes to form catechols via intermediates that are stabilized by semiquinones. [32] Catechols and quinones have different affinities for multivalent cations. Therefore we hypothesize that concerted two-electron oxidation of cathodes from catechols into quinones can facilitate the extraction of divalent cations during charging cycles. This mechanism could preserve charge storage capacity and cycling stability in cath- odes of secondary multivalent batteries. Mg 2+ cations are paired with eumelanins cathodes and aqueous electrolytes in half-cells. Mg has a theoretical spe- cific volumetric capacity 3833 mA h cm -3 compared to 2046 mA h cm -3 for Li. [24,33] Mg is environmentally benign, earth abundant, stable in atmospheric conditions, and approxi- mately 24 times less expensive than lithium. [34] Mg 2+ cations deposit on Mg metal anodes without dendrite formation. [35,36] Eumelanins, hereby referred to as melanins for simplicity, rep- resent several classes of naturally occurring pigments found in organisms including Homo sapiens and Sepia officinalis. [37,38] Natural melanin (NatMel) consists of homogenous nanometer- scale textured granules ( Figure 1). Melanin granules are stable in aqueous environments and composed of extended heteroaro- matic networks with a characteristic d-spacing of 3.8 Å and a specific surface area of 19.9 m 2 g -1 . [39–43] Melanin granules contain a high density of redox active polyphenols in the form Rechargeable multivalent ion energy storages can serve as next generation electrochemical energy storage systems for many prospective applications. Devices for grid scale storage will benefit from using abundant multivalent ions such as Cu 2+ , Zn 2+ , Fe 3+ , Al 3+ , and Mg 2+ . [1–6] Electrochemical storage systems that utilize anodes that are compatible with abun- dant multivalent cations materials have prospective advantages of low-cost systems that exhibit increased volumetric charge storage capacities. [4,5,7] Other advantages of multivalent cations include increase stability and facile handling in atmospheric environments. [5] There are countless numbers of cathode materials that can support rapid and reversible insertion/extraction of Li + . [8,9] Con- versely, there are very few cathode materials that can achieve reasonable charge storage capacities and cycling stability in secondary multivalent ion batteries. [1,10,11] There are many prac- tical challenges that limit widespread adoption of secondary multivalent ion batteries. One prominent challenge facing bat- teries based on Mg and Al is the sluggish insertion (extraction) kinetics of multivalent cations into (from) cathodes, which pro- duces a high redox polarization effect and reduces the round- trip voltage efficiency. [12,13] Electrode materials composed of both organic and inorganic materials exhibit potentially advantageous physical properties for use in electrochemical storage systems that use multivalent ions. [14–19] Biopolymers such as lignins can be recovered from waste streams and used as a cost-effective sustainable mate- rial for electrochemical energy storage using monovalent cat- ions. [20,21] Recent advances in using conducting polymers have Adv. Mater. 2014, DOI: 10.1002/adma.201402295 www.advmat.de www.MaterialsViews.com