It’s in the water: how to solve our planet’s biggest challenges
Many of our planet’s biggest challenges are related to water and ice, and I’m fascinated by our quest to fundamentally understand it, in all its forms. There is more than enough water on Earth, for example, but not enough of it is drinkable, or in the right place to become drinkable at low cost. If we knew more about water, perhaps we could help with these global challenges.
At the ICE (Interfaces: Catalytic and Environmental) Group, we are still discovering new and fundamental aspects of water – such as a new form of ice. Recently, our collaborators at UCL, led by Professor Christoph Salzmann, carried out an experiment using a technique known as ball-milling – grinding crystalline ice into small particles using metal balls in a steel jar. Ball-milling is often used to make amorphous materials, but it had never been applied to ice. His team found that the ice had turned into a white, powdery substance that hadn’t been seen before – and asked our lab to investigate.
When our PhD student, Dr Michael Davies, simulated the process on a supercomputer, he discovered that a new kind of ice with a different structure to any existing forms had been created. When liquid freezes, it can freeze into a crystalline solid where all the H2O molecules are arranged on a lattice – or it can freeze into a disordered solid, where molecules aren’t arranged on a lattice. This second ice – amorphous ice – is what we found. It’s characterised by its density, as it doesn’t have the conventional symmetry by which we describe most solids. We currently know of two families of amorphous ice structures: one has a density greater than liquid water, and one has a density lower than liquid water. This new structure sits in the gap between the two – hence ‘medium density amorphous ice’ or MDA.
This discovery is hugely significant: it changes our understanding of liquid water. The structure of liquid water is itself still debated. One hypothesis is that liquid water, under certain conditions, is not a uniform, homogeneous substance but a mixture of two different types of liquid – one that resembles high-density amorphous ice and another that resembles low-density amorphous ice. Throwing a new form of amorphous ice into the mix muddies the waters – quite literally – and presents a new conundrum for our understanding of liquid water itself. But MDA is also of interest in itself – because when it crystallises, it releases energy. It is essentially a high-energy material, and the energy stored in it could be harnessed.
That’s one of the big challenges right now – using water for energy; splitting H2O into oxygen and hydrogen and using hydrogen as a fuel. If you can find a cost-effective way to get hydrogen out of water, then this has enormous potential to solve one of the world’s biggest energy challenges.
Last year, we produced a paper that predicted that water under nanoconfinement would relatively easily produce another form of ice – though this hasn’t yet been experimentally verified under nanoconfinement. That would produce a phase of ice known as superionic water – a lattice of oxygen atoms with a gas of hydrogen atoms. We want to understand how we can make this material – and characterise it – experimentally. What are the precise conditions under which it will form? And then, how can it be exploited?
And this discovery doesn’t just give us insights into water on Earth. When we talk about discovering water in space, we usually mean liquid water because, as we already know, there is an abundance of frozen water in space, and that ice is mostly amorphous ice. We concluded that events that randomly shear crystalline ice – which ball-milling mimics – are relevant to the production of MDA. And there are similar types of events, like giant tidal waves, inside the giant planets in the solar system. There is a chance that there could be a lot of undiscovered MDA out there in the universe, too – raising yet more questions about this extraordinary, ubiquitous but mysterious substance.