Thermoelectric properties of aqueous electrolyte infiltrated in anodic aluminium oxide (AAO) nanochannels

ABSTRACT: The direct conversion of heat to electricity without any intermediate steps remains one of the most auspicious, yet challenging, methods of power production, especially due to the ongoing global decarbonisation trend. Moreover, due to the wide variety of readily available heat sources, such as the combustion of fossil and other fuels, nuclear reactors, solar heat, or even waste heat recovery from the human body and household appliances, the applications for thermal-to-electric energy converters are broad. The development of a new nanofluidic platform technology based on the ion transfer in nanofluidic membranes would lead to a breakthrough in versatile and sustainable energy harvesting and storage. To achieve this goal, the formation of nanochannels through rational design is required. Fabrication of perfectly engineered nanofluidic membranes is critical to generate a substantial thermovoltage, i.e. high ionic Seebeck coefficient, in the presence of a temperature gradient, by confining ion transport in charged nanochannels. Nanoporous anodic aluminum oxide (AAO) is one of the most popular and cost-effective platforms for various applications: from templates and molecular separation to drug delivery and energy generation. AAO membranes have highly ordered nanochannels and offer the opportunity to precisely engineer the morphology of nanochannels. In this work, a nanofluidic platform based on 25 nm-pore AAO membranes with a thickness of 50 microns was fabricated and infiltrated with Na2SO4 aqueous electrolyte to implement ion transport and energy generation by converting electrokinetic and thermal energy into electricity. The sandwich-type cell was initially designed to test the thermoelectric properties of aqueous electrolyte-infiltrated AAO membranes. Applying a temperature difference to the system revealed an increase in output voltage attributed to thermally driven ion transport in the nanochannels, which tended to decrease with increasing concentration. The dependence of output voltage per Kelvin obtained from ion thermodiffusion inside the nanofluidic membrane on electrolyte concentration could be used to investigate the contribution of electrokinetic effects that occur in nanochannels and are especially noticeable when electrical double layers along the inner walls of the nanopores are completely or partially overlapped.