A paper recently published in the journal ACS Applied Energy Materials demonstrated the feasibility of using a covalent organic framework (COF)-based nanofluidic hybrid membranes (NHMs) to attain enhanced interfacial ion transport for the generation of osmotic energy.

Importance of Osmotic Energy

Osmotic energy exists in the form of a salinity gradient between seawater and freshwater and represents a critical blue energy source. This form of energy is used extensively to meet the needs of sustainable development owing to its easy accessibility and rich reserves.

Limitations of the Existing NHMs for Osmotic Energy Generation

Reverse electrodialysis is often used to capture osmotic energy through permeable membranes. Specifically, versatile NHMs have gained considerable attention due to their delicate structural properties.

These membranes are fabricated by integrating two functional membranes and feature asymmetrical geometric configuration, chemical composition, and surface charge, which contribute to the membrane’s unique ionic transport behavior.

Thus, several osmotic energy conversion systems were developed based on NHMs, such as ionomer-based nanofluidic diode membranes and polyethylene terephthalate (PET)/block copolymer heterogeneous membranes.

However, the low power densities/extremely limited output power of the NHMs due to the low efficiency/inefficient interfacial ion transport made them unviable for commercial applications.

The inefficient interfacial ion transport is caused by the pore alignment mismatch between the interface of two functional layers and a limited number of pores. These limitations necessitated the development of enhanced osmotic energy conversion devices based on NHMs with sufficient ion transport and abundant selective pores.

Novel Method to Develop an Effective NHM

COFs, a new type of porous crystalline polymers, are formed by the covalent linking of organic building blocks through covalent bonds. These crystalline polymers possess nanospaces that can be functionalized, well-ordered channels, and ultrahigh porosity. Overall, this makes them a suitable platform for efficient interfacial ion transport.

Additionally, these modified nanospaces of COFs can be utilized to control ion selectivity intelligently. Several studies have demonstrated novel ion transport behavior in the COF membrane.

For instance, polyethylene glycol-modified COFs provide a fast ion transport pathway, which is suitable for designing high-performance ion conductors. Thus, the COF membrane can be a promising candidate for achieving efficient interfacial ion transport in NHM.

Synthesis and Evaluation of COF-based NHM

In this study, researchers synthesized an NHM containing ultrahigh pore density COF- Lan Zhou University-1 (COF-LZU1) layer and cellulose nanofibers/carbon nanotubes (CNF-CNT) membrane to achieve efficient conversion of the ionic gradient to electricity. The synthesized membrane was designated as COF-LZU1@CNF-CNT.

1,4-dioxane (Diox), 1,3,5-benzene-tri carboxaldehyde (TFB), tetrahydrofuran, 1,4-diaminobenzene (PDA), acetic acid (HAc), acetone, CNT suspension, and CNF gel were used as the starting materials for the study. A Millipore direct-Q system was used to produce ultrapure water for experiments.

Synthesis of COF-LZU1 Nanoseeds

TFB and Diox were mixed and stirred for 15 minutes and then PDA was added to the as-prepared solution. The resultant mixture was again stirred for 20 minutes. Subsequently, a yellow suspension was observed after HAc was added to the as-obtained solution. The suspension was left undisturbed for 24 hours at room temperature.

The obtained COF-LZU1 nanoseeds were centrifuged, cleaned with Diox, tetrahydrofuran, and acetone, and activated by methanol in sequence. Eventually, the nanoseeds were vacuum dried at 80 degrees Celsius for 24 hours.

Synthesis of CNF-CNT Membrane

The CNT suspension and CNF gel were initially mixed with water and stirred vigorously. CNF-mixed CNT membrane was then obtained by vacuum filtration of a part of the mixed suspension.

Fabrication of COF-LZU1@CNF-CNT NHM

Initially, the synthesized COF-LZU1 nanoseeds were spin-coated on the CNF-CNT film, and the coated side of the film was then submerged in a mixture of PDA, TFB, and Diox. The temperature of the resultant growth solution was maintained at 50 degrees Celsius for six hours to obtain COF-LZU1@CNF-CNT NHM.

Evaluation of the Synthesized Samples

The osmotic energy conversion and ion transport behavior of the synthesized membranes were investigated using the Keithley 6487 picoammeter. A pair of silver/silver chloride electrodes were used to apply the transmembrane potential. During the ion transport measurement, the working electrode was fixed on the CNF-CNT side.

Researchers also systematically studied the influence of several external factors, such as electrolyte species, temperature, and pH, on salinity gradient energy conversion performance.

Significance of the Study

COF-LZU1@CNF-CNT with typical NHM characteristics was synthesized successfully by the hybridization of the CNF-CNT membrane and functional COF-LZU1 layer. The hybridization of two layers substantially increased the efficiency of interfacial ion transport and promoted osmotic energy conversion.

The CNF-CNT membrane with numerous carboxylic acid groups interacted with the COF-LZU1 amino groups to form an effective hybrid membrane. The well-organized and abundant pores of the COF-LZU1 layer ensured sufficient ion transport, while the interlaced CNF-CNT membrane provided a three-dimensional (3D) charged space to modulate the ion transport.

Moreover, the delicate design of the synthesized membrane greatly facilitated the interfacial ion transport across the membrane by constraining the ion polarization effect and contributing to ion diffusion.

A significantly high power density of 4.26 watts per square meter was attained when the synthesized membrane was used in an energy conversion device to capture the osmotic energy stored between river water and natural seawater.

Taken together, the findings of this study demonstrated the effectiveness of COF-LZU1@CNF-CNT NHM in achieving high-efficiency interfacial ion transport for osmotic power generation and bolstered the application of COF membranes in this field.