Nanostructuring Electrode Surfaces and Hydrogels for Enhanced Thermocapacitance

Thermogalvanic cells have the ability to convert low-temperature waste heat energy (<200 °C) into electrical energy. However, these systems cannot store this electrical energy. Thermocapacitors use analogous mechanisms to convert heat into electrical energy, but crucially can store this electrical energy. Previous work has used polyelectrolyte gels to frustrate the mobility of a redox couple, causing an accumulation of charge imbalance at the two electode:gelled electrolyte interfaces, resulting in stored thermocapacitance. Here we report an investigation into two methods of utilizing nanostructuring to increase the thermocapacitance performance of a [Fe(CN)6]3–/4– containing polyacrylate hydrogel; specifically, nanostructuring the bulk of the hydrogel and modifying the electrode with carbon nanotubes and liquid crystalline graphene oxide. Nanomaterial-enhanced carbon cloth electrodes significantly increased the available electroactive surface area and charging–discharging rates of these hydrogels, but only resulted in a modest increase in thermocapacitance. Conversely nanostructuring of the hydrogels by changing the degree of neutralization of the hydrogels resulted in significant improvement in thermocapacitance, but only when high concentrations (1 M) of ferricyanide/ferrocyanide electrolyte are available. This study indicates how two different aspects of the system can be nanostructured to enhance device performance.