In previous work, we found that one-atom thick materials like graphene (a 2D sheet of carbon atoms) are highly permeable to protons. We also found that 2D molybdenum disulphide (MoS2) is completely impermeable to protons even if it is just three atoms thick. This suggested that 2D crystals would be proton-conducting only if they are strictly one-atom-thick.
Our new experiments have revealed that micas, that are ten times thicker than graphene or MoS2, can be 100 times more permeable to protons. From our previous experience, we thought this should be impossible. However, micas can be thought of as aluminosilicate slabs pierced by atomic-scale tubular channels. These channels aren’t empty but filled with hydroxyl groups that are like the proton-conducting 1D chains in water.
We have now found that protons jump along these chains, turning the material into an excellent proton conductor. This is why micas can be used as proton-conducting membranes even if they are rather thick.
Our result also implies that many other 2D crystals with similar nanometer-scale channels that aren’t ionic conductors to begin with, could be turned into such in the 2D limit. This strategy is not limited to protons or micas. Many more 2D crystals with atomic-scale channels similar to those in micas could be explored, hopefully bringing unexpected phenomena and new applications in the field of proton and ionic conductors
The mica membranes also remain proton conducting at temperatures between 100°C and 500°C. For example, their areal conductivity can be higher than 100 S/cm2 at 500°C, which is well above current requirements for the industry roadmap.
There is an acute lack of proton-conducting materials that can reliably operate between 100°C and 500°C. This is unfortunate since this is the ideal operating range for most fuel cells and other hydrogen-related related technologies. Atomically-thin micas work rather well at these temperatures. We believe they merit attention especially from this perspective.
We are now working on building a mica prototype membrane that is big enough for us to test it under industrial conditions where fuel cells and other technologies work.
Beyond this, we believe that our work shows that the field of two-dimensional ionic conductors holds good promise. There is a wealth of other crystals that could be turned into ionic and proton conductors in the 2D limit – each with unique properties based on the specific lattice and chemical composition.