Evaporation-driven electrokinetic energy conversion: Critical review, parametric analysis and perspectives
Energy harvesting from evaporation has become a ‘hot’ topic in recent years. Researchers have speculated on several possible mechanisms, with electrokinetic energy conversion being the least hypothetical.
The basics of pressure-driven electrokinetic phenomena, such as streaming current and streaming potential, have long been established. Similarly, the regularities of evaporation from porous media are well known. However, the ‘coupling’ of these two classes of phenomena has not yet been thoroughly explored.
In this critical review, we aim to consolidate the available knowledge from these two fields to present a coherent picture of electrokinetic electricity generation during evaporation from (nano)porous materials.
To illustrate, we will consider several configurations, including single nanopores, arrays of nanopores, systems with a reduced area of electrokinetic-conversion elements, and devices with side evaporation from thin nanoporous films.
For the latter (which is practically the only configuration studied experimentally), we will formulate a simple model describing correlations of system performance with principal parameters such as nanoporous-layer length, width, and thickness, as well as pore size, pore-surface hydrophilicity, effective zeta-potential, and electric conductivity in nanopores.
These correlations will be qualitatively compared with experimental data available in the literature. We will show that experimental data do not always agree with the model predictions, which may be due to simplifying model assumptions but also because the mechanisms are different from classical electrokinetic energy conversion.
This is particularly relevant to the mechanisms of converting evaporation-driven ion streaming currents into electron currents in external circuits.
Finally, we will propose directions for future experimental and theoretical studies that could help clarify these issues.
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