Cotton nanofibers strengthen MXene membranes and boost ion selectivity, enabling efficient and durable conversion of saltwater and freshwater mixing into usable electricity at commercially viable levels.
(Nanowerk Spotlight) Every time a river meets the sea, energy is quietly being wasted. Billions of gallons of freshwater pour into salty oceans every day, creating invisible gradients in chemical potential. These differences contain extractable energy, but unlike the roar of a hydroelectric dam or the spinning blades of a wind turbine, this process is silent and untapped. The force that drives it is subtle—ions moving from high to low concentration—but the scale is enormous. This is the basis of what scientists call blue energy.
Blue energy refers to the energy released when saltwater and freshwater mix. It is a form of osmotic energy, generated from the entropy increase that occurs when two liquids with different salt concentrations come into contact. If controlled through the right materials, this mixing process can be converted into electricity using membrane-based systems.
The global potential is considerable. According to some estimates, the natural mixing of river and seawater could generate up to 2 terawatts, which is roughly the output of 2,000 large power plants, if harvested efficiently at every river mouth on Earth.
Yet despite its promise, blue energy remains largely underutilized. The challenge lies not in the physics, which are well understood, but in the engineering. Specifically, the membranes used to extract energy from salinity gradients remain a critical weak point. Most are made from polymers that were not originally designed for this purpose. These membranes have limited ion selectivity, degrade over time in saline environments, and cannot maintain high output without significant losses. As a result, many experimental systems have failed to produce the efficiency and power density required for commercial deployment.
Advances in nanomaterials science, particularly the development of two-dimensional, layered materials, have shifted the research landscape. Among these, MXenes have shown strong potential for next-generation membranes. Still, their practical limitations have kept them from fulfilling that promise.
A team led by researchers from Tsinghua University, Leibniz University Hannover, and the SINOPEC Research Institute has addressed these limitations using a simple but effective strategy. They introduced cotton-derived nanofibers into the MXene structure to form a composite membrane. These cotton nanofibers (CNFs) are small, negatively charged, and readily form hydrogen bonds with MXene surfaces. By inserting them between the nanosheets, the team created what they call an MCM—MXene/CNF membrane—with wider and more regular channels, increased surface charge, and improved physical strength.
The MXene nanosheets were produced by selectively removing aluminum from the parent compound Ti₃AlC₂. The resulting sheets, about 4.6 micrometers wide and 1.5 nanometers thick, were mixed with CNFs in water and filtered into membranes using a vacuum-assisted process. The fibers were uniformly distributed, and the final composite membrane was smooth, freestanding, and easy to handle.
Schematic of MXene/CNF membrane (MCM) fabrication. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
By incorporating CNFs, the dense, lamellar structure of the original MXene membrane was transformed. The fibers introduced controlled disorder into the stacking of the sheets, increasing the spacing between them and reducing resistance to ion flow. Structural and spectroscopic analysis confirmed strong hydrogen bonding between MXene and CNFs, and tests showed an increase in negative surface charge from -48 millivolts to -57 millivolts. The surface chemistry was stable, and the membrane maintained its structure even after prolonged immersion in salt solutions.
The mechanical improvement was striking. The composite membrane reached a tensile strength of 145 megapascals—seven times greater than the pristine MXene membrane. Nanoindentation showed a nearly threefold increase in modulus, confirming the material’s improved resistance to deformation. Unlike the pure MXene membrane, which disintegrated under sonication, the MCM remained intact, pointing to a significant boost in robustness for real-world applications.
To evaluate performance in energy harvesting, the membrane was placed between two chambers with different salt concentrations, simulating seawater (0.5 molar sodium chloride) and river water (0.01 molar). The movement of cations through the membrane generated a measurable electrical current. The key metric, known as energy conversion efficiency, reached 28.6%. This number reflects how much of the potential energy from the concentration difference was converted into electricity. Another important figure, the power density, peaked at 9.7 watts per square meter, surpassing the commonly cited 5 W/m² benchmark for commercial viability.
These results compare favorably to other state-of-the-art 2D membranes. For example, membranes made from graphene oxide or clays often reach power densities between 2 and 6 W/m² under similar test conditions. The MCM’s superior performance arises from the way CNFs help balance competing needs: increasing channel size for easier ion flow while maintaining enough surface charge to selectively attract cations and block anions.
Tests showed that performance remained stable over ten days, and the membrane retained 85% of its strength after soaking in high-salinity solutions. The team also evaluated the MCM with real seawater and river water, recording a power output of 6.5 W/m². This is especially significant given the complexity of natural water, which contains a mix of ions and organic matter. The membrane’s structure, with its combination of flexibility, strength, and precise ion pathways, held up under these less controlled conditions.
Further experiments explored how different types of ions affect performance. Membranes performed best with monovalent ions like sodium and potassium. When tested with divalent ions like calcium or magnesium, power output dropped. These larger, more strongly hydrated ions move more slowly and interact more strongly with the membrane surface, making transport less efficient.
These observations are consistent with other studies of ion transport in nanochannels and reinforce the importance of fine control over membrane chemistry and structure.
The researchers also noted that too much CNF loading reduced performance. Excessive fiber content increased membrane porosity beyond optimal levels and disrupted the regularity of the channels, making ion separation less effective. This underscores the need to balance structural enhancement with transport efficiency.
The work shows a practical path to address key limitations of MXene membranes for osmotic energy generation. By reinforcing the structure with inexpensive, biodegradable cotton nanofibers, the researchers achieved higher ion selectivity, lower resistance, and significantly improved mechanical performance. These improvements could allow osmotic energy systems to operate at higher efficiencies, with membranes that are easier to fabricate and more stable in saline environments.
Rather than relying on synthetic polymers or complex chemical modifications, the approach here uses naturally derived fibers to enhance a known 2D material. The simplicity of the method, combined with the strength of the results, suggests that similar composite strategies could be applied to other membrane systems.
The findings also provide a clear benchmark for future development of osmotic energy technologies. With a power density well above commercial thresholds and proven durability, the MCM represents a step toward making salinity gradient energy more viable as a renewable power source.
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