Asymmetric Nanoporous Alumina Membranes for Nanofluidic Osmotic Energy Conversion

Abstract

The potential of harnessing osmotic energy from the interaction between seawater and river water has been recognized as a promising, eco‐friendly, renewable, and sustainable source of power. The reverse electrodialysis (RED) technology has gained significant interest for its ability to generate electricity by combining concentrated and diluted streams with different levels of salinity. Nanofluidic membranes with tailored ion transport dynamics enable efficient harvesting of renewable osmotic energy. In this regard, anodic aluminum oxide (AAO) membranes with abundant nanochannels provide a cost‐effective nanofluidic platform to obtain structures with a high density of ordered pores. AAO can be utilized in constructing asymmetric composite membranes with enhanced ion flux and selectivity to improve output power generation. In this review, we first present the fundamental structure and properties of AAO, followed by summarizing the fabrication techniques for asymmetric membranes using AAO and other nanostructured materials. Subsequently, we discuss the materials employed in constructing asymmetric structures incorporating AAO while emphasizing how material selection and design can resist and promote efficient energy conversion. Finally, we provide an outlook on future applications and address the challenges that need to be overcome for successful osmotic energy conversion.

Charge-Gradient Sulfonated Poly(ether ether ketone) Membrane with Enhanced Ion Selectivity for Osmotic Energy Conversion

“…4 The passage of ions through the membrane leads to the accumulation of counterions on the side with a low ion concentration, causing an ion concentration polarization effect and diminishing ion selectivity. 5,6 To address these issues, researchers have turned to the development of heterogeneous ion-selective membranes with asymmetric structures, chemical compositions, and surface charge densities to enhance the efficiency of osmotic energy conversion. These approaches leverage the creation of an ionic diode effect, facilitating oneway ion diffusion while preventing the generation of reverse current and mitigating the ion polarization phenomenon.…”

“…Among technologies for harnessing osmotic energy, reverse electrodialysis (RED) stands out as a promising method that utilizes ion-selective membranes to extract osmotic power directly. − However, the performance of ion-selective membranes, a pivotal component of RED systems, is often hindered by low ion selectivity and low ionic conductivity, resulting in poor energy generation efficiency . The passage of ions through the membrane leads to the accumulation of counterions on the side with a low ion concentration, causing an ion concentration polarization effect and diminishing ion selectivity. , To address these issues, researchers have turned to the development of heterogeneous ion-selective membranes with asymmetric structures, chemical compositions, and surface charge densities to enhance the efficiency of osmotic energy conversion. These approaches leverage the creation of an ionic diode effect, facilitating one-way ion diffusion while preventing the generation of reverse current and mitigating the ion polarization phenomenon. − For instance, researchers have developed an asymmetric heterostructured membrane composed of ordered mesoporous silica (∼8.3 nm) and macroporous alumina (∼80 nm), achieving an output power density of up to 4.5 W m –2 under mixing seawater and river water, attributed to its ionic diode effect.…”