Zwitterionic Membrane Desalinates Water and Generates Electricity

Stable energy supplies and sustainable clean water resources are a growing need in society due to population growth, industrialisation, and the increasing effects of climate change. To reduce environmental degradation beyond the levels that we are seeing today, there needs to be plans in place that can ensure and balance long-term social development with sustainable development. 

Solar energy, as one of the biggest generators of renewable energy today, are a key sustainable and clean energy source. They will continue to play a key role in mitigating climate change, reducing air pollution, and generally helping to create a cleaner society. 

There are currently many different solar driven energy conversion processes in use today. Solar-driven desalination is one such process that has the potential to address the scarcity of freshwater in many locations in a clean and sustainable manner. Out of all the process available, photothermal conversion technology is the most popular for desalination and treating wastewater due to being more cost-effective. 

Interfacial solar steam generation (ISSG) is an approach that utilises photothermal conversion materials and confines the thermal energy (heat) at the air-liquid interface. This approach enables the continuous transfer of water to the evaporating surface, leading to stable evaporation. There are many different photothermal materials available, many of which are integrated with other substrate materials to form a functional membrane that can facilitate the evaporation process.  

While solar-driven desalination has a lot of potential for addressing the challenges of freshwater in society, there are still some challenges. Solar desalination membranes can be made highly efficient by optimising the functional groups within photothermal materials when they are integrated into the membrane substrate. However, no matter how efficient the membrane is, the accumulation of salt on the membrane after prolonged operation is still a challenge that can affect the performance of these devices over time. 

Fibrous Membranes have Potential, But Challenges Persist 

While there are many membrane materials that can house photothermal materials, fibrous membranes have a lot of promise due to their high surface area and porosity. These two properties enable the device to have highly efficient water transport and evaporation mechanisms. The porous nature of fibrous membranes means that they can easily incorporate a high concentration of photothermal materials to improve the photothermal conversion efficiency of the device. They are also easier to produce at scale than some other membrane materials and are self-buoyant, so they can act as efficient photothermal evaporators. 

However, despite all the benefits, these membranes have limited water channels, which leads to the accumulation of salt on the evaporatiion surface. This reduces the solar absorption potential of the device, blocks the water transport pathways, and reduces the steam generation efficiency of the device. 

Current strategies to reduce salt accumulation on these membranes include: 

• Introducing hydrophilic and hydrophobic structures 
• Modulation of the porous network 
• Incorporation of specific ionic groups 
• Integration of polyelectrolytes  
• Integration of zwitterionic polyionic liquids (PILs) 

Using PILs in Desalination Devices 

PILs can attract both sodium and chloride ions in salt water through electrostatic interactions. This can help to reduce the aggregation of salts at the evaporation interface. It has also been thought that PILs can regulate water and ions using intermolecular electrostatic interactions to ensure that the flow of water is not affected by salt ions—even at high salt concentrations.  

PILs also exhibit anti-biofouling properties that prevent the build-up of algae, bacteria and other microorganisms on the evaporation surface, as the build-up of microorganisms can stop the water from becoming potable. For the long-term use of ISSG evaporators, this is critical as it prolongs the useful life of the device and prevents it from degrading easily. It also prevents the need for regular cleaning and maintenance. 

Zwitterionic Fibrous Membrane Developed for Generating Electricity and Water Desalination 

Researchers in a new study have created a photothermal zwitterionic fibrous membrane for an ISSG using a combination of electrospinning and in situ oxidative polymerisation methods.  The porous zwitterionic fibrous membrane could selectively transport water across the membrane and reject both sodium and chloride ions. This enabled the solar-driven desalination device to achieve an efficient evaporation performance alongside a high salt resistance to prevent salt buildup during operation. 

According to the Nature Communications study, the design of the zwitterionic fibrous membrane was bioinspired, drawing on natural mechanisms that maintain ionic balance in living organisms. This bioinspired approach helped improve ion transport selectivity and contributed to the membrane’s ability to reject salts without sacrificing water permeability.

The membrane substrate was made of polyacrylonitrile (PAN). The membrane uses porphyrin-based conjugated microporous polymers as the photothermal material, which helped to improve the photothermal efficiency of the device. These polymers also enhance the antibacterial properties of the membrane to ensure that bacteria don’t attach to it and risk the quality of the water output. 

Porphyrin-Based Polymers and Antibacterial Function

The research team demonstrated that the porphyrin-based polymers not only enhanced solar-to-thermal conversion but also exhibited intrinsic antibacterial activity. This reduced the likelihood of biofilm growth, which is a major operational challenge for long-term desalination membranes.

Zwitterionic PILs were used as the anti-salt component of the membrane. These PILs contain hydrophilic groups that redistribute salt concentration, disrupt salinity gradients, and reduce the evaporation enthalpy via ion interactions, preventing salt deposition. The ability to prevent salt build up led to a rapid water transport across the membrane. 

The zwitterionic PILs used in the membrane showed durable anti-scaling behaviour under repeated cycles of operation. Results from the study indicated that the material maintained stable performance even when exposed to high salinity levels that typically cause fouling in conventional systems.

Integration of Thermoelectric Energy Conversion

The device was also equipped with a thermoelectric module (that converts heat into electricity) to store electricity. When there was no sunlight to power the device, the thermoelectric module released electricity to ensure that the device continued operating in all conditions. 

This hybrid integration of solar evaporation with thermoelectric generation represents a dual-function system. The researchers highlighted that by recycling otherwise wasted thermal energy, the device could contribute to decentralised clean energy generation while simultaneously addressing water scarcity.

In terms of device performance, the membrane achieved an evaporation rate of 2.64 kg m-2 h-1 and a photothermal efficiency of 97.6% under 1 kW m-2 solar irradiation. The device was also able to maintain a high evaporation rate in highly concentrated brine water. When the device was coupled to the thermoelectric module, it achieved a stable electric output with a power density of 1.5 W m-2—enough to power small electronic devices. 

Beyond the reported power density, the system demonstrated operational stability across multiple test cycles. This reliability is significant for off-grid and coastal applications where both potable water and small-scale electricity supplies are needed in a single compact platform.

Operational Stability and Future Potential

Overall, this approach provided a synergistic approach for purifying water, generating electricity, and preventing the buildup of salts on the desalination membrane. Such an approach offers a way to potentially advance the design of integrated solar-driven systems. 

Experts note that combining desalination and energy harvesting within one device could reduce system costs and land use compared to deploying separate units. While still at the research stage, the findings mark a step toward scalable solutions that respond to both clean water and renewable energy demands.

Reference: 

Liao Y. et al, Bioinspired photothermal zwitterionic fibrous membrane for high-efficiency solar desalination and electricity generation, Nature Communications, 16, (2025), 6373.