Thermal-Driven Ion Transport in Porous Materials for Thermoelectricity Applications

Ion Steric Effect Induces Giant Enhancement of Thermoelectric Conversion in Electrolyte-Filled Nanochannels

Ionic thermoelectricity in nanochannels has received increasing attention because of its advantages, such as high Seebeck coefficient and low cost. However, most studies have focused on dilute simple electrolytes that neglect the effects of finite ion sizes and short-range electrostatic correlation. Here, we reveal a new thermoelectric mechanism arising from the coupling of the ion steric effect due to finite ion sizes and ion thermodiffusion in electric double layers, using both theoretical and numerical methods. We show that this mechanism can significantly enhance the thermoelectric response in nanoconfined electrolytes depending on the properties of electrolytes and nanochannels. Compared to the previously known mechanisms, the new mechanism can increase the Seebeck coefficient by 100% or even 1 order of magnitude enhancement under optimal conditions. Moreover, we demonstrate that the short-range electrostatic correlation can help preserve the Seebeck coefficient enhancement in a weaker confinement or in more concentrated electrolytes.

Blue energy generation by the temperature-dependent properties in funnel-shaped soft nanochannels

Salinity energy generation (SEG) studies have only been done under isothermal conditions at ambient temperature. The production of salinity energy can be improved under non-isothermal conditions, albeit preserving the energy efficiency. In the current study, the effects of gradients of temperature and concentration on the salinity energy generation process were examined simultaneously. Based on the temperature-dependent properties resulting from both temperature and concentration gradients, a numerical study was carried out to determine the maximum efficiency of salinity energy generation in funnel-shaped soft nanochannels. It was presumed that a dense layer of negative charge, called a polyelectrolyte layer (PEL), is coated on the walls of the nanochannels. Co-current and counter-current modes were used to obtain temperature and concentration gradients. Under steady-state conditions, the Poisson-Nernst-Planck, Stokes-Brinkman, and energy equations were numerically solved using equivalent approaches. The results revealed that by increasing the temperature and concentration ratios in both co-current and counter-current modes of operation, the salinity energy generation increased appreciably. The salinity energy generation increased from 30 to 80 pW upon increasing the temperature ratio from 1 to 8 at a constant concentration ratio of 1000 in counter-current mode. As verified from this analysis, low-grade heat sources (<100 °C) provide considerable energy conversion in PEL grafted nanofluidic confinement when placed between electrolyte solutions of different temperatures.

Simultaneous thermoosmotic and thermoelectric responses in nanoconfined electrolyte solutions: Effects of nanopore structures and membrane properties

HYPOTHESIS

Nanofluidic systems provide an emerging and efficient platform for thermoelectric conversion and fluid pumping with low-grade heat energy. As a basis of their performance enhancement, the effects of the structures and properties of the nanofluidic systems on the thermoelectric response (TER) and the thermoosmotic response (TOR) are yet to be explored.

METHODS

The simultaneous TER and TOR of electrolyte solutions in nanofluidic membrane pores on which an axial temperature gradient is exerted are investigated numerically and semi-analytically. A semi-analytical model is developed with the consideration of finite membrane thermal conductivity and the reservoir/entrance effect.

FINDINGS

The increase in the access resistance due to the nanopore-reservoir interfaces accounts for the decrease of short circuit current at the low concentration regime. The decrease in the thermal conductivity ratio can enhance the TER and TOR. The maximum power density occurring at the nanopore radius twice the Debye length ranges from several to dozens of mW K^-2 m^-2 and is an order of magnitude higher than typical thermo-supercapacitors. The surface charge polarity can heavily affect the sign and magnitude of the short-circuit current, the Seebeck coefficient and the open-circuit thermoosmotic coefficient, but has less effect on the short-circuit thermoosmotic coefficient. Furthermore, the membrane thickness makes different impacts on TER and TOR for zero and finite membrane thermal conductivity.