“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.”Marcelo Lozada-Hidalgo, University of Manchester

Researchers from the University of Warwick and the University of Manchester, both in the UK, have finally solved the long-standing puzzle of why graphene is so much more permeable to protons than suggested by theory.

A decade ago, scientists at the University of Manchester demonstrated that graphene is permeable to protons, the nuclei of hydrogen atoms. That unexpected finding started a debate in the community because theory predicted that it should take billions of years for a proton to permeate through graphene’s dense crystalline structure. This had led to suggestions that protons permeate not through the crystal lattice itself, but through pinholes in its structure.

Now, in a paper in Nature, researchers at the University of Warwick, led by Patrick Unwin, and the University of Manchester, led by Marcelo Lozada-Hidalgo and Andre Geim, report ultra-high spatial resolution measurements of proton transport through graphene and prove that perfect graphene crystals are permeable to protons. Unexpectedly, protons are strongly accelerated around nanoscale wrinkles and ripples in the crystal.

This discovery has the potential to accelerate the hydrogen economy. The expensive catalysts and membranes, sometimes with significant environmental footprints, currently used to generate and utilize hydrogen could be replaced with more sustainable 2D crystals, reducing carbon emissions and contributing to net zero through the generation of green hydrogen.

The researchers used a technique known as scanning electrochemical cell microscopy (SECCM) to measure minute proton currents collected from nanometer-sized areas. This allowed them to visualize the spatial distribution of proton currents through graphene membranes.

If proton transport took place through holes, as some scientists have speculated, 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.

“We were surprised to see absolutely no defects in the graphene crystals,” said Segun Wahab and Enrico Daviddi from the University of Warwick, who are leading authors of the paper. “Our results provide microscopic proof that graphene is intrinsically permeable to protons.”

Unexpectedly, the proton currents were found to be accelerated around nanometer-sized wrinkles in the crystals. The researchers 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. This observation now reconciles the experiment and theory.

“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,” explained Lozada-Hidalgo.

“These results showcase SECCM, developed in our lab, as a powerful technique to obtain microscopic insights into electrochemical interfaces, which opens up exciting possibilities for the design of next-generation membranes and separators involving protons,” said Unwin.

The authors are excited about the potential of this discovery to spur new hydrogen-based technologies. "Exploiting the catalytic activity of ripples and wrinkles in 2D crystals is a fundamentally new way to accelerate ion transport and chemical reactions,” said Lozada-Hidalgo. “This could lead to the development of low-cost catalysts for hydrogen-related technologies.”

This story is adapted from material from the University of Warwick, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.