Exploration of Collagen as Binder for Aqueous Zinc Batteries and Symmetric Carbon Supercapacitors

Rechargeable water-based metal batteries attract great attention due to low cost, high safety merits, sustainable cell fabrication and an easier recyclability at the end of life. During cell design, the binder often remains without attention. In this study, aqueous dispersions of native collagen were successfully explored as binders in Na$_{0.44}$MnO$_2$, Na$_3$V$_2$(PO$_4$)$_3$, and V$_2$O$_5$-PEDOT cathodes for zinc batteries and in carbon electrodes for double-layer supercapacitors. The use of collagen is motivated by its environmental friendliness and high availability. Different functional groups in collagen are capable to chelate metal cations and are viable for chemical cross-linking. Collagen shows high water binding capacity, reasonable thermostability, a wide electrochemical stability window between −1.0 and 1.2 V versus Ag/AgCl and water-based processability of electrodes. Intrinsic properties of collagen dispersions like viscosity or pH value can be easily adapted. The pH value of the dispersion has a noticeable impact on the reaction mechanism in electrodes: cycling behavior of V$_2$O$_5$-PEDOT cathodes with acidic collagen dispersion providing 335 mAh g$^{−1}$ capacity in the 300$^{th}$ cycle, is much more stable than that of cathodes with neutral collagen dispersion or CMC binder due to suppression of PEDOT oxidation, which occurs in the neutral medium at the end of charge.