Effects of heat transfer and the membrane thermal conductivity on the thermally nanofluidic salinity gradient energy conversion

Abstract

Originating from advances in nanofabrication, functional materials have been developed to construct bioinspired or mimic nanochannels for efficient nanofluidic energy conversion. In previous studies regarding nanofluidic energy conversion, the material thermal conductivity has been never taken into consideration, however, which shall be addressed with transmembrane temperature difference applied. Thermal conductivities of widely used bulk materials for fabricating nanopores vary from 0 to 120 W/(m·K). Heat transfer occurs both in the liquid solution and solid membrane, impacting trans-channel solution temperature distribution and ion transportation, and yielding counterintuitive phenomena. Under positive temperature differences, the electric power shifts from inhibition to promotion as the membrane thermal conductivity increases. Larger membrane thermal conductivity evens temperature distribution in the nanochannel, weakening ion aggregations or depletions, which result in degraded electric power improvement under a negative temperature difference and upgraded power enhancement under a positive one. At a temperature difference of 30 K, the electric power is enhanced by 56.22% for a thin PET membrane (length = 50 nm) under the negative temperature difference. If the membrane thermal conductivity can be well tuned to nearly thermal insulation, the electric power can be enhanced by 120.07%. To step further, we proposed a criterion for membrane selection based on thermal conductivities in the thermally nanofluidic energy conversion: For thick membranes, materials of large thermal conductivity with a positive temperature difference are preferred for obvious power and energy efficiency enhancement. For thin membranes, materials of small thermal conductivity with a negative temperature difference are appealing. These findings reveal the importance of a long-overlooked factor, membrane thermal conductivity, in nanofluidic energy harvesting and can serve as a guidance for selecting appropriate membrane materials and developing high-performance nanofluidic power devices.