Electrokinetic energy harvesting over nanometer and sub-nanometer scales

Electrokinetic energy harvesting (EKEH) has emerged as a promising renewable and carbon-neutral energy source for small and large-scale applications, reducing the reliance on conventional fossil fuels and providing innovative solutions for remote, off-grid applications. The underlying mechanism of EKEH relies on the movement of dissolved electrolytes over charged fluid–solid interfaces through confinements resulting in the generation of useful power. The low energy conversion efficiency typically observed in larger (micrometer) confinements can be substantially mitigated by shifting to nanometer and sub-nanometer regimes. This down-scaling unlocks high selectivity and provides unique opportunities to potentially harness Angstrom-scale interactions to maintain and elevate fluid permeability. However, EKEH at sub-nanometric scales remains fraught with considerable challenges in fabrication, economic viability, scaling of power, and maintenance, significantly impeding its advancement. In this review, we detail the electrokinetic processes that drive energy conversion in the presence of pressure, concentration, and temperature gradients. We examine the key factors affecting conversion efficiency and explore the innovative solutions in the recent literature addressing associated challenges. Additionally, we highlight the role of novel nanomaterials and specialized geometries along with new fabrication techniques that enable high permeation without sacrificing selectivity in nanometer and sub-nanometer confinements. Finally, we delve into the major obstacles that EKEH currently faces to reach its full potential of extracting clean and affordable energy and conclude by offering insight into future developmental directions and potential breakthroughs in this rapidly evolving field.