Probing the characteristics of casein as green binder for non-aqueous electrochemical double layer capacitors' electrodes Alberto Varzi a, b, * , Rinaldo Raccichini a, b, c , Mario Marinaro d , Margret Wohlfahrt-Mehrens a, d , Stefano Passerini a, b a Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany b Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany c Institute of Physical Chemistry, University of Muenster, Corrensstrasse 28/30, 48149 Muenster, Germany d Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Helmholtzstrasse 8, 89081 Ulm, Germany highlights  Casein is as water-soluble binder easily obtainable from waste food.  Casein provides good mechanical, thermal and electrochemical stability.  Optimized electrodes enable excellent performance in conventional non-aqueous electrolytes. article info Article history: Received 4 February 2016 Received in revised form 17 March 2016 Accepted 20 March 2016 Available online xxx Keywords: Electrochemical double layer capacitors Binder Casein Electrodes manufacturing Water processing abstract Casein from bovine milk is evaluated in this work as binding agent for electrochemical double layer capacitors (EDLCs) electrodes. It is demonstrated that casein provides excellent adhesion strength to the current collector (1187 kPa compared to 51 kPa achieved with PVdF), thus leading to mechanically stable electrodes. At the same time, it offers high thermal stability (above 200  C) and electrochemical stability in organic electrolytes. Apparently though, the casein-based electrodes offer lower electronic conduc- tivity than those based on other state-of-the-art binders, which can limit the rate performance of the resulting EDLC. In the attempt of improving the electrochemical performance, it is found that the application of a pressing step can solve this issue, leading to excellent rate capability (up to 84% capacitance retention at 50 mA cm 2 ) and cycling stability (96.8% after 10,000 cycles at 10 mA cm 2 ) in both PC- and ACN-based electrolytes. Although the adhesive power casein is known since ancient times, this report presents the first proof of concept of its employment in electrochemical power sources. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The role of the binder is often underrated in electrochemical energy storage devices. With the only function of keeping the electroactive materials together, it does not take an active role into charge storage processes and, therefore, it is considered as “inert” component. However, in spite of its role, the choice of the binder is crucial for the development of high performance as well as envi- ronmentally friendly devices. Actual electrode fabrication tech- nologies involve the casting (or spraying) of slurries constituted by a mixture of the electroactive components dispersed in a binder solution/dispersion. Hence, the binder determines the solvent needed for the electrode production and, in turn, the overall pro- cess' sustainability. Polyvinylidene fluoride (PVdF) has represented for decades the state-of-the-art binder for Li-ion batteries elec- trodes [1]. With regards to electrochemical double layer capacitors (EDLC), both PVdF and Polytetrafluorethylene (PTFE) have been extensively used for organic- [2e4], aqueous- [5], as well as ionic liquid (IL)-based systems [6e8]. Nowadays, however, with the growing market of such devices, the employment of fluoropol- ymers is being questioned for several reasons. Besides containing fluorine, which makes them difficult to dispose at the end-of-life, PVdF, for example, requires the use of toxic solvents (i.e., N- methyl-2- pyrrolidone) that need to be properly handled to avoid health hazards, thus increasing the production costs. Water soluble * Corresponding author. Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany. E-mail address: alberto.varzi@kit.edu (A. Varzi). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2016.03.072 0378-7753/© 2016 Elsevier B.V. All rights reserved. Journal of Power Sources xxx (2016) 1e8 Please cite this article inpress as: A. Varzi, et al., Journal of Power Sources (2016), http://dx.doi.org/10.1016/j.jpowsour.2016.03.072