A hydrogel-ionic polymer blend for humidity-insensitive ion gradient driven electricity generation†

Abstract

Sustainable means of providing electricity are of paramount importance for next-generation electronic devices. Recently, exploiting ion concentration gradients has been suggested a viable method to generate electricity. When a cationic polymer and an anionic polymer come into contact, the freely mobile anions and cations in the charged polymers migrate down the concentration gradient producing electrical potential. In this work, we show that by blending the charged polymers in a hydrogel, the electrical potential can be sustained for an extended period of time providing opportunities for reliable energy harvesting. Agarose blocks blended with poly(diallyldimethyl ammonium chloride) (PDDA) and with anionic poly(sodium 4-styrenesulfonate) (PSS) are stacked together to allow the diffusion of Na⁺ and Cl⁻ ions down the concentration gradients and produce electricity. The high-moisture absorption and retainment capability of agarose facilitate the diffusion of the ions, and accordingly, a peak open-circuit voltage (V_OC) and peak short-circuit current (I_SC) of ∼120 mV and ∼48 μA are produced, respectively. These values can be increased to 426 mV and 89.2 μA when connected serially or in parallel. Both ionic polymer concentration and gel thickness are found to strongly influence the V_OC and I_SC while the interfacial contact area and relative humidity do not affect the energy harvesting performance. Enabled by the high-moisture absorption capability and retainment of the agarose hydrogel, the V_OC can be sustained for over 100 h with no change.