Investigation into pseudo-capacitance behavior of glycoside-containing hydrogels

Electrochemical pseudocapacitors are an attractive choice for energy storage applications because they offer higher energy densities than electrostatic or electric double layer capacitors. They also offer higher power densities in shorter durations of time, as compared to batteries. Recent efforts on pseudocapacitors include biocompatible hydrogel electrolytes and transition metal electrodes for implantable energy storage applications. Pseudocapacitive behavior in these devices has been attributed to the redox reactions that occur within the electric double layer, which is formed at the electrodeelectrolyte interface. In the present study, we describe a detailed investigation on redox reactions responsible for pseudocapacitive behavior in glycoside-containing hydrogel formulations. Pseudocapacitive behavior was compared among various combinations of biocompatible hydrogel electrolytes, using carbon tape electrodes and transition metal electrodes based on fluorine-doped tin oxide. The hydrogels demonstrated a pseudocapacitive response only in the presence of transition metal electrodes but not in the presence of carbon electrodes. Hydrogels containing amine moieties showed greater energy storage than gels based purely on hydroxyl functional groups. Furthermore, energy storage increased with greater amine content in these hydrogels. We claim that the redox reactions in hydrogels are largely based on Lewis acid-base interactions, facilitated by amine and hydroxyl side groups along the electrolyte chain backbones, as well as hydroxylation of electrode surfaces. Water plays an important role in these reactions, not only in terms of providing ionic radicals but also in assisting ion transport. This understanding of redox reactions will help determine the choice of transition metal electrodes, Lewis acid-base pairs in electrolytes, and medium for ionic transport in future biocompatible pseudocapacitors.