Unravelling graphene's proton permeability

Researchers from the University of Warwick and the University of Manchester have determined why graphene is more permeable to protons than expected by theory. A decade ago, scientists at the University of Manchester demonstrated that graphene is permeable to protons, nuclei of hydrogen atoms. The result started a debate in the community because theory predicted that it would take billions of years for a proton to permeate through graphene’s dense crystalline structure. This led to suggestions that protons permeate not through the crystal lattice itself, but through the pinholes in its structure.

Now, a collaboration between the University of Warwick, led by Professor Patrick Unwin, and the University of Manchester, led by Dr Marcelo Lozada-Hidalgo and Professor Andre Geim, has reported ultra-high spatial resolution measurements of proton transport through graphene and proven that perfect graphene crystals are permeable to protons.

The discovery has the potential to accelerate the hydrogen economy. Expensive catalysts and membranes, sometimes with a significant environmental footprint, currently used to generate and utilise hydrogen could be replaced with more sustainable 2D crystals, reducing carbon emissions. The researchers used a technique known as scanning electrochemical cell microscopy (SECCM) to measure minute proton currents collected from nanometre-sized areas. This allowed the researchers to visualise the spatial distribution of proton currents through graphene membranes.

If proton transport took place through holes, the currents would be concentrated in a few isolated spots. No such isolated spots were found, which ruled out the presence of holes in the graphene membranes. Dr Segun Wahab, leading author of the paper, said the results provide microscopic proof that graphene is intrinsically permeable to protons. “We were surprised to see absolutely no defects in the graphene crystals,” Wahab said.

Unexpectedly, the proton currents were found to be accelerated around nanometre-sized wrinkles in the crystals. The scientists found that this arises because the wrinkles effectively ‘stretch’ the graphene lattice, thus providing a larger space for protons to permeate through the pristine crystal lattice. “We are effectively stretching an atomic scale mesh and observing a higher current through the stretched interatomic spaces in this mesh — this is truly mind-boggling,” Lozada-Hidalgo said.

Unwin said the results showcase SECCM as a powerful technique to obtain microscopic insights into electrochemical interfaces, which opens up new possibilities for the design of next-generation separators involving protons. “Exploiting the catalytic activity of ripples and wrinkles in 2D crystals is a fundamentally new way to accelerate ion transport and chemical reactions. This could lead to the development of low-cost catalysts for hydrogen-related technologies,” Lozada-Hidalgo said.

Image credit: University of Manchester