High performance nanofluidic thermoelectricity using smooth polarized surfaces

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Waste heat energy discharged into the atmosphere is one of the largest sources of clean, fuel-free and inexpensive energies available. The most commonly used technologies to convert waste heat to electric energy are solid-state thermoelectric generators and liquid-based thermoelectrochemical cells. Despite the continuous improvement in their figures-of-merit, they are still inadequate for the conversion of low-grade waste heat due to material limitations, cost and poor efficiencies. A promising alternative method to convert thermal into electric energy is based on electrolyte-filled nanochannels: when a temperature gradient is applied along the channel, an electric field is created that drives an electric current. The efficiency of such nanofluidic devices could reach that of the best solid-state thermoelectric materials, when boosted by liquid-solid slip arising on low-friction surfaces. However, high performance also requires high surface charge, which is detrimental to slip. In that context, we have recently shown that polarized graphene surfaces display a very favorable charge-slip coupling; consequently, these surfaces could provide excellent thermoelectric conversion. Therefore, the first objective of the project is to identify especially promising configurations with high conversion efficiency by studying thermoelectric energy conversion in graphene nanochannels. Another result of our preliminary studies is that efficient energy conversion is achieved especially when the variation of liquid properties with temperature is very pronounced. This is the case close to the critical point of the fluid (transcritical conditions), and the second objective of the project is therefore to explore the potential of thermoelectric energy conversion under such transcritical conditions. To develop a comprehensive picture of thermoelectricity in nanofluidic channels, we will use a multiscale approach, combining atomistic simulations, to capture molecular effects in the nanometric vicinity of the interface, and continuum-mechanical simulations, to describe the entire system at a larger scale. Specifically, atomistic simulations will provide boundary conditions or surface equations for the continuum models. The results of this approach will finally be tested against molecular simulations in situations of extreme confinement. Our goal is to identify configurations with high thermoelectric energy conversion efficiency, especially channels with polarized graphene walls and operation regimes close to the critical point, providing theoretical guidelines for the development of future thermoelectric energy converters.