Electrode Messiness leads to Better Supercapacitors

Supercapacitors have the potential to be a key component in the transition to renewable energy. They could easily be used more often in transportation or renewable energy to provide energy, in tandem with, or instead of batteries. Like batteries, they have been designed to store energy, but they offer advantages in that they can charge a lot faster and can charge and discharge for millions of cycles. However, they also have a couple of major drawbacks in that they have much lower energy density, around ten times less, and they tend to leak current, meaning they are currently incapable of delivering long-term energy storage or continuous power. Even with those drawbacks, it wouldn’t be hard to imagine a vehicle like a bus that recharges at every stop as passengers get on and off. Now, applications like these could be a bit closer, as scientists have discovered a way to improve that density by changing their internal structure.

The team from the University of Cambridge used experimental and computer modelling techniques to study the porous carbon electrodes found in supercapacitors. The study found that electrodes with a more disordered chemical structure stored far more energy than electrodes with a highly ordered structure.

Supercapacitors rely on the movement of charged molecules between porous carbon electrodes, which have a highly disordered structure. “Think of a sheet of graphene, which has a highly ordered chemical structure,” said Dr Alex Forse from Cambridge’s Yusuf Hamied Department of Chemistry, who led the research. “If you scrunch up that sheet of graphene into a ball, you have a disordered mess, which is sort of like the electrode in a supercapacitor.”

That inherent messiness of the electrode makes it difficult for scientists to study supercapacitors and determine which parameters are the most important to improve in terms of performance. Many scientists have thought that the size of the nanopores in the carbon electrodes was the key to improved energy capacity. However, the Cambridge team has found there is no link between pore size and storage capacity.

To investigate the electrodes, the research team used nuclear magnetic resonance (NMR) spectroscopy. This showed that the messiness of the materials was the key to their success.

“Using NMR spectroscopy, we found that energy storage capacity correlates with how disordered the materials are – the more disordered materials can store more energy,” said first author Xinyu Liu, a PhD candidate co-supervised by Forse and Professor Dame Clare Grey. “Messiness is hard to measure – it’s only possible thanks to new NMR and simulation techniques, which is why messiness is a characteristic that’s been overlooked in this field.”

When analysing the electrode materials with NMR spectroscopy, a spectrum with different peaks and valleys is produced. The position of the peak indicates how ordered or disordered the carbon is. “It wasn’t our plan to look for this, it was a big surprise,” said Forse. “When we plotted the position of the peak against energy capacity, a striking correlation came through – the most disordered materials had a capacity almost double that of the most ordered materials.”

Forse and his team are now trying to discover why that messiness is useful. More disordered carbons store ions more efficiently in their nanopores, and the team hope to use these results to design better supercapacitors. The messiness of the materials is determined at the point they are synthesised.

“We want to look at new ways of making these materials, to see how far messiness can take you in terms of improving energy storage,” said Forse. “It could be a turning point for a field that’s been stuck for a little while. Clare and I started working on this topic over a decade ago, and it’s exciting to see a lot of our previous fundamental work now having a clear application.”

The research was supported in part by the Cambridge Trusts, the European Research Council, and UK Research and Innovation (UKRI).

https://www.cam.ac.uk/