The sea contains multitudes. Even the word itself. For some, it is home; for others, it is work. The sea inspires us and terrifies us in equal measure. It is a refuge and it is a threat — something we must fear, but also protect, and conserve.
For some, it is a memory that transports them back to a moment in their childhood. For others, it is something completely unknown that they have never seen with their own eyes. The sea is all that and much more.
However, in the context of ASTERISK, a multidisciplinary research project, we must remain practical. The sea is a solution of compounds, consisting of approximately 96,5% water and 3,5% mineral salts: this is very, very unpoetic, but at least it is simple and it is the truth.
To this, we must add other stuff present in seawater, such as microorganisms, a vast amount of organic matter (like phytoplankton), heavy metals, and other suspended solids, and finally and unfortunately, increasing amounts of microplastics. How do we know all this? Well… we are scientists! We use seawater to produce hydrogen and, furthermore, we want to turn our weak point into our strong point, which involves valorising seawater.
This is complicated, but at ASTERISK we’re really trying. If you want to know how, keep reading this story!
Closing the circle
Given that we live on a finite planet with finite resources, us scientists always try to develop technologies that are increasingly environmentally friendly, have a longer lifespan, and use less toxic, more abundant materials. This inevitably leads us to approach our scientific objectives with a focus on circularity, as we want our ideas to work in real life.
We can understand circularity as a way of thinking and designing our products that challenges the traditional linear economy system based on “take, make, and dispose”. In circularity, what we seek is to “reduce, reuse, and recycle” the materials we employ, trying to create a system that works in a more natural and durable way.
And how can we apply this to ASTERISK?
Well, there is no single way to seek circularity in a project like ours, and we must always keep in mind that our ideas must work in an industrial environment: there is no point in using a material that is environmentally friendly if its price is so high that no company would consider implementing it into production.
That is why, in ASTERISK, we are trying to design an electrolyser that uses more abundant and affordable materials:
- For the catalysts, which traditionally use scarce platinum group metals (PGMs), we are exploring options based on cobalt, nickel, and iron. Since seem stable in saline conditions, they look like a safe bet. Remember? Our electrolyser will run on seawater.
- For our membrane, we want to replace the use of fluorinated “forever chemicals”, compounds that are toxic, harmful, and practically indestructible. Our main alternative lies in block polymers – sequences of different chemical compounds, mostly hydrocarbons.
However, if there is one thing that characterises and sets us apart in terms of circularity and sustainability, it is our water treatment… another reality contained in the sea.
The sea is the beginning and the end
To produce green hydrogen, ASTERISK will use seawater, pre-filtered to remove impurities and compounds that could interfere with electrolysis or damage the device.
However, since we are pursuing circularity, we have decided to focus part of our efforts on recovering certain minerals and metals present in seawater. In this way, we transform a waste product into a resource of potential interest to industry.
The recovery of mineral salts is an attractive idea, but a complex one: it usually requires high energy consumption, as most of these resources are available in the sea in very low concentrations. To revalue the brine, we need highly selective techniques to extract, separate, and purify the resources we are interested in.
In ASTERISK, we selected the membrane distillation crystallisation (or MDC), a technology to filter used seawater to obtain clean water on one hand, and minerals such as magnesium (Mg), calcium(Ca), lithium(Li), or strontium (Sr) on the other. MDC technology uses a hydrophobic membrane, and this means that it repels water.
How can we separate the salts from the water?
The answer is temperature. After the electrolysis of seawater, we obtain a concentrated brine, which is just slightly saltier seawater – enriched with an extra amount of salt.
After that, we place the brine on one side of our membrane and a stream of cold water on the other. If we apply heat to the brine, we create a contrast on both sides of the membrane, and a pressure difference that will evaporate the water in the brine. The water waltzes across the membrane, and condenses on the other side as completely clean cold H₂O.
On the brine side of our membrane, the many minerals will increasingly concentrate and collect crystals – until a point when mineral salts solidify.
MDC has many advantages, such as high selectivity and a reduced environmental impact. However, it requires large amounts of thermal energy to heat the water. Once again, at ASTERISK we make the most of the adversity, and turn troubple into opportunity. Remember circularity? We seek to “reduce, reuse, and recycle”. And in this case, we repurpose the heat produced by our electrolysers to create the extra energy needed by the MDC.
Thanks to the technology in ASTERISK, we will not only revalorise the seawater salts, but also obtain clean, high-purity water – a resource that is as essential as it is scarce.