Q&A: how osmotic membranes are unlocking the potential of blue energy

Yoana Cholteeva 28 April 2020 (Last Updated September 8th, 2020 21:50)

A group of scientists from the US and Australia have sourced inspiration from living organisms to develop an osmotic system to generate electricity. We speak with Nicholas Kotov, lead researcher of the research project and professor of chemical engineering at the University of Michigan, US to find out more.

Q&A: how osmotic membranes are unlocking the potential of blue energy
“It serves as a universal source of energy for all living organisms,” said Professor Nicholas Kotov of the University of Michigan. Credit: Andrew Home

Yoana Cholteeva (YC): Could you tell me a bit more about how your osmotic research is aiming to generate energy?

Nicholas Kotov (NK): Osmosis is a ubiquitous process of ion diffusion from the region with high salinity to the region with low salinity, which happens virtually everywhere in the world. It is particularly important for biology because osmosis creates transmembrane potential in cells, which drives many biochemical processes. So, it serves as a universal source of energy for all living organisms.

The challenge for chemists, materials scientists, and engineers is to extract and concentrate this energy because osmosis is a low power source, the density of this energy is not as high as solar rays, for example. The biological membranes can provide the blueprint for harvesting this ‘blue’ energy. The mechanisms of osmotic energy use in cells are all based on membranes with nanometre scale channels which enable the selective passing of some ions.

We decided to replicate the structure of the nanoporous membranes from a material that would be robust and technologically accessible, by turning to a past experience of engineering biomimetic nanostructures. During this process, aramid nanofibers, which are made from well-known Kevlar fabric, served as our starting point. Aramid nanofibers provide excellent mechanical properties inherited from Kevlar and have high thermal and chemical resilience. They also form high density fibrous networks with nanometre scale channels required for selective ion transport.

In collaboration with professor Weiwei Lei from Deakin University in Australia, we further augmented their properties by making composite membranes combining aramid nanofibers and nanoflakes of boron nitride.

YC: What are the main advantages of the composite membranes?

NK: The creation of membranes with nanoscale channels is a long-standing technological challenge. Besides osmotic energy generation, they are also used in desalination to supply fresh water in many parts of the world. They can be made from both organic and inorganic materials and both types have desirable and undesirable properties. When one asks what kind of properties the membranes need in order to actually help out the transition to renewables in the energy industry, the answer will be all the desirable ones from both classes of membranes and none of the undesirable ones. It turns out that the combination of organic aramid nanofibers and inorganic boron nitride nanosheets leads to membranes that approach this case. Compared to other types of membranes, [the aramid ones] stand out with robustness to temperature, pH, salinity, and contamination. The ability of both components to spontaneously self-assemble into the sheets with nanoscale channels also make them scalable.

There are already a number of pilot plants which generate osmotic energy, particularly in Scandinavian regions and the United States, but the lifecycle of those membranes, along with their viability and efficiency, remain key problems. The cost of energy is also still not adequate, it needs to be lower and I think our biomimetic membranes can help with that.

YC: What about their disadvantages?

NK: With any new technology there’s always this significant effort embedded in the early stages of development. It’s not a disadvantage of the material, but a disadvantage of the stage this technology is in. We are currently working to find out if there are any disadvantages tothe materials. For now, there is some inhomogeneity in the membranes which we need to figure out and also make the whole membrane meter by meter more uniform.

YC: Do you believe this discovery could change the way water is used to generate power?

NK: Yes, absolutely, it can. As the world is turning to new, environmentally conscious methods to generate energy, their spectrum expands. Osmosis will be part of this spectrum, of course not in every location, but in many, especially in coastal areas and some waste streams with higher salinity. There are a lot of ways these membranes can contribute to the generation of large amounts of power. The likely rise of sea levels should also be taken into account. Dams protecting the lands from floods are expensive to build but their construction can be coupled with osmotic energy plants as it was in the Netherlands at the Afsluitdijk dam.

YC: Do you think these membranes will attract commercial interest in the near future?

NK: Self-assembled biomimetic composites are already being produced in large quantities from graphene, clay, and other materials. This provides a good stepping stone for osmotic membrane production. The unique combination of their properties, simplicity of production methods, and low-cost materials make them commercially attractive. My impression is that interest from companies will soon follow. You might be interested that these aramid nanofibers are made from old bullet proof vests, recycling the Kevlar plastic, which adds another dimension to their commercialisation prospects. Potentially, other plastics can be recycled into nanofibers and this opportunity inspires us to develop the technology further.