Thermoelectric response of liquid electrolyte in graphene nanochannel membranes

Recent theoretical studies have suggested that the liquid electrolytes confined in nanofluidic channels exhibit a much higher ionic Seebeck coefficient than those in bulk. However, little experimental investigation has been conducted on this phenomenon, especially for electrolytes in sub-nanometer nanochannels reconstructed by the two-dimensional (2D) layered materials. In this study, we experimentally investigated the thermoelectric response of simple binary electrolytes, specifically KCl solution, in graphene oxide (GO) nanochannel membranes. The results showed that the ionic Seebeck coefficient with increasing the KCl concentration, and the largest negative ionic Seebeck coefficient of -0.4 mV/K was obtained at a KCl concentration of 0.1 mM. It was suggested that ionic thermoelectric response is dominated by the classical Soret-type thermodiffusion in the high KCl concentration regime, resulting in a small value of positive ionic Seebeck coefficient. However, in the low KCl concentration regime, as the electrical double layer (EDL) becomes thicker, ion diffusion driven by the temperature-dependent ion electrophoretic mobility (TDEIM) becomes a dominant, resulting in a large negative ionic Seebeck coefficient. The experimental results were shown to be qualitatively in agreement with numerical results based on the continuum model. In other words, the trend in the variation of ionic Seebeck coefficient with the electrolyte concentration ar consistent with the thermoelectricity theory of nanoconfined electrolytes proposed in recent literature. These findings may have implications for the development of nanofluidic-based thermoelectric generators using 2D materials in the future.