Designing smart membranes for sustainable solutions

Increasing demands for fresh water and clean energy bring a need to develop cost-effective innovations. In the search for new materials for various applications, researchers have been drawn to study and engineer supramolecular materials, made of molecules able to self-assemble into larger constructs, requiring fewer steps to synthesize. They have applications in a wide range of fields, including regenerative medicine, electronic devices and nanotechnology applications. 

Researchers at the Joint Center of Excellence in Integrated Nanosystems (JCIN) are investigating new materials using supramolecular chemistry that could act as functional membranes to either remove salt ions from the seawater or to create green energy. 

The project is part of an ongoing collaboration between KACST and Northwestern University in the USA. A part of the research team focuses on tackling the challenges of water desalination, while the other is working on improving the performance and efficiency of fuel cells. Innovative chemical constructs might be useful for both challenges. 

Tackling today’s problems with supramolecular chemistry 

Often inspired by nature, the field of supramolecular chemistry has already generated a variety of building blocks, with different intriguing shapes and functions, such as molecular lassos, axles, knots, nano-cages, etc. Many of these architectures have been devised by a team led by Professor Sir Fraser Stoddart, a global expert in this field and winner of the 2016 Nobel Prize in Chemistry, who is also JCIN co-director. Nanoscale devices based on supramolecular chemistry are being used in medicine and in the development of new smart materials.

The Northwestern laboratory is currently working on the design and synthesis of highly crystalline polyelectrolytes, otherwise known as ionic polymer single-crystals (IPSCs), which bring together supramolecular chemistry and macromolecular, or polymer, science. The well-defined structure of these ionic materials might offer great benefits and novel applications. JCIN researchers are investigating how such constructs could be part of next- generation water desalination technologies and fuel-cell membranes. 

“Mechanically interlocked architectures offer promising platforms to fulfil Saudi Arabia’s investment in water security and renewable energy,” says Fehaid Alsubaie, JCIN co-director. 

Better water technologies at lower costs

Saudi Arabia is one of the world’s largest producers of desalinated water. Plants remove inorganic and organic substances present in seawater, technically known as total dissolved solids (around 40 000 mg/litre), to reach a quality that is considered acceptable for drinking.  

In one of the steps, seawater is forced through a membrane, via a process called reverse osmosis. Water is allowed to pass though the membrane, while ion salts and other unwanted substances are left. This approach, however, is costly.

“Currently, the most widely used approach to desalinate water is based on reverse osmosis. It requires a large amount of pressure to achieve a high flux of water through the membrane. This translates into a large consumption of energy, and high costs,” explains Radwan Alrasheed, the leader of the water desalination group.  

The team is interested in developing cheaper membrane technologies that are hydrophilic (attracted to and dissolved by water), in order to accelerate water transportation and reduce energy consumption. 

“Our initial aim is to use these novel membranes in reverse osmosis, but there could be further applications in other membrane technologies, such as ultrafiltration and nanofiltration,” continues Alrasheed.

Another challenge faced by the water purification industry is fouling by deposits of organic and inorganic matter. “We intend to improve the water treatment system so that the membranes don’t need to be cleaned as often, thus reducing the maintenance and operation costs,” adds the researcher. 

The project is still at an early stage, and the researchers are studying a wide range of water technologies in order to find the best options. Using supramolecular chemistry, chemical units can be combined in interlocked networks and more complex architectures. These assemblies could be used as coating materials for commercially available polyamide membranes, or as stand-alone products. 

“We are evaluating the strengths and weaknesses of the water treatment options that are currently available. For example, polyamide membranes offer an acceptable water flux and can work at different pH values, but are damaged by disinfectants and oxidising conditions. We could address this problem by coating the existing membranes with new functionalities,” says Alrasheed. 

Another highlight of supramolecular-based membranes is the possibility to control the density and dimension of the pores.

“We are looking for the best way to combine the advantages of both covalent and supramolecular approaches, to obtain membranes that have high numbers of pores which are robust, stable across a wide range of temperatures and pH, and display some flexibility and tuneability with respect to their size. These characteristics are essential for effective water desalination,” he explains. 

The road towards proof-of-concept and system prototyping is still a long one, but the team has clear goals. The researchers expect to extend the membrane lifetime, which is currently five years for commercial membranes. They also aim to beat existing technologies in price by achieving the treatment of 1,000 litres of water for less than %content%.3, that is, less than half of the current costs.  

Fuel cells as alternative energy sources 

The researchers are also working on the development of new materials for fuel cells.  that convert hydrogen and oxygen into electricity, producing water and heat. If used in cars and trucks, fuel cells could have the potential to replace internal combustion engines that utilise fossil fuels.

The heart of the fuel cell is the proton exchange membrane (PEM) that facilitates the easy passage of protons, and acts as a barrier to the transfer of electrons.  

PEMs’ commercialisation, however, is hindered by their high costs, poor thermal stability, and short lifetimes. The team’s goal is to devise more affordable PEMs that can withstand a wider temperature range.     

The researchers have produced a polymer derived from a commercially available and cheap chemical, dibenzo[18]crown-6 (DB18C6). This new polymer is capable of forming extended networks, has high proton conductivity, is accessible in high yields, and exhibits enhanced thermal stability. 

“Proton conductivity is one of the most important features of a PEM and depends on the water content,” explains Dr Alrasheed. “Fuel cells need a certain amount of moisture to work. Absorbed water molecules form hydrogen-bonded networks, which sustain long-range proton conductivity throughout the polymers.” Water adsorption analysis shows that the polymer can retain 23 percent of its own weight in water, even when the relative humidity is lowered to 25 percent.

Although the proton conductivity of this polymer network is still lower than that of Nafion membranes, these materials represent a good basis for further research. “We are hopeful that by tuning this crown ether-based network, we can produce polymers that exhibit proton conductivities that are competitive with Nafion in the near future,” says Dr Alrasheed. 

The team is also exploring the chemistry of other self–assembled supramolecular materials with interesting proton–conduction properties. Characterisations of these materials via microscopic techniques, including atomic force microscopy, transmission electron microscopy and scanning electron microscopy, alongside UV/Vis spectroscopy and powder X-ray diffraction, are ongoing. 

1. Patel, H. A., Selberg, J., Salah, D., Chen, H., Liao, Y. et al. Proton Conduction in Tröger’s Base-Linked Poly(crown ether)s. ACS Applied Materials & Interfaces, 10(30), 25303-25310 (2018).| article