̽��ֱ�� of Cambridge - supercapacitor

Mess is best: disordered structure of battery-like devices improves performance

sc604 — Thu, 18 Apr 2024 18:00:00 +0000

Researchers led by the ̽��ֱ�� of Cambridge used experimental and computer modelling techniques to study the porous carbon electrodes used in supercapacitors. They found that electrodes with a more disordered chemical structure stored far more energy than electrodes with a highly ordered structure.

Supercapacitors are a key technology for the energy transition and could be useful for certain forms of public transport, as well as for managing intermittent solar and wind energy generation, but their adoption has been limited by poor energy density.

̽��ֱ��researchers say their results, reported in the journal Science, represent a breakthrough in the field and could reinvigorate the development of this important net-zero technology.

Like batteries, supercapacitors store energy, but supercapacitors can charge in seconds or a few minutes, while batteries take much longer. Supercapacitors are far more durable than batteries, and can last for millions of charge cycles. However, the low energy density of supercapacitors makes them unsuitable for delivering long-term energy storage or continuous power.

“Supercapacitors are a complementary technology to batteries, rather than a replacement,” said Dr Alex Forse from Cambridge’s Yusuf Hamied Department of Chemistry, who led the research. “Their durability and extremely fast charging capabilities make them useful for a wide range of applications.”

A bus, train or metro powered by supercapacitors, for example, could fully charge in the time it takes to let passengers off and on, providing it with enough power to reach the next stop. This would eliminate the need to install any charging infrastructure along the line. However, before supercapacitors are put into widespread use, their energy storage capacity needs to be improved.

While a battery uses chemical reactions to store and release charge, a supercapacitor relies 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 Forse. “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.”

Because of the inherent messiness of the electrodes, it’s been difficult for scientists to study them and determine which parameters are the most important when attempting to improve performance. This lack of clear consensus has led to the field getting a bit stuck.

Many scientists have thought that the size of the tiny holes, or nanopores, in the carbon electrodes was the key to improved energy capacity. However, the Cambridge team analysed a series of commercially available nanoporous carbon electrodes and found there was no link between pore size and storage capacity.

Forse and his colleagues took a new approach and used nuclear magnetic resonance (NMR) spectroscopy – a sort of ‘MRI’ for batteries – to study the electrode materials. They found that the messiness of the materials – long thought to be a hindrance – 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. ̽��ֱ��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.”

So why is mess good? Forse says that’s the next thing the team is working on. More disordered carbons store ions more efficiently in their nanopores, and the team hope to use these results to design better supercapacitors. ̽��ֱ��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.”

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

Reference:
Xinyu Liu et al. ‘Structural disorder determines capacitance in nanoporous carbons.’ Science (2024). DOI: 10.1126/science.adn6242

For more information on energy-related research in Cambridge, please visit the Energy IRC, which brings together Cambridge’s research knowledge and expertise, in collaboration with global partners, to create solutions for a sustainable and resilient energy landscape for generations to come.

̽��ֱ��energy density of supercapacitors – battery-like devices that can charge in seconds or a few minutes – can be improved by increasing the ‘messiness’ of their internal structure.

This could be a turning point for a field that’s been stuck for a little while.

Alex Forse

Nathan Pitt

Left to right: Clare Grey, Xinyu Liu, Alex Forse

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New technique for ‘seeing’ ions at work in a supercapacitor

sc604 — Mon, 22 Jun 2015 14:32:07 +0000

Researchers from the ̽��ֱ�� of Cambridge, together with French collaborators based in Toulouse, have developed a new method to see inside battery-like devices known as supercapacitors at the atomic level. ̽��ֱ��new method could be used in order to optimise and improve the devices for real-world applications, including electric cars, where they can be used alongside batteries to enhance a vehicle’s performance.

By using a combination of nuclear magnetic resonance (NMR) spectroscopy and tiny scales sensitive enough to detect changes in mass of a millionth of a gram, the researchers were able to visualise how ions move around in a supercapacitor. They found that while charging, different processes are at work in the two identical pieces of carbon ‘sponge’ which function as the electrodes in these devices, in contrast to earlier computer simulations. ̽��ֱ��results are published today (22 June) in the journal Nature Materials.

Supercapacitors are used in applications where quick charging and power delivery are important, such as regenerative braking in trains and buses, elevators and cranes. They are also used in flashes in mobile phones and as a complementary technology to batteries in order to boost performance. For example, when placed alongside a battery in an electric car, a supercapacitor is useful when a short burst of power is required, such as when overtaking another car, with the battery providing the steady power for highway driving.

“Supercapacitors perform a similar function to batteries but at a much higher power – they charge and discharge very quickly,” said Dr John Griffin, a postdoctoral researcher in the Department of Chemistry, and the paper’s lead author. “They’re much better at absorbing charge than batteries, but since they have much lower density, they hold far less of that charge, so they’re not yet a viable alternative for many applications. Being able to see what’s going on inside these devices will help us to control their properties, which could help to make them smaller and cheaper, and that might make them a high-power alternative to batteries.”

At its most basic level, a battery is made of two metal electrodes (an anode and a cathode) with some sort of solution between them (electrolyte). When the battery is charged, electrolyte ions are stored in the anode. As the battery discharges, electrolyte ions leave the anode and move across the battery to chemically react with the cathode. ̽��ֱ��electrons necessary for this reaction travel through the external circuit, generating an electric current.

A supercapacitor is similar to a battery in that it can generate and store electric current, but unlike a battery, the storage and release of energy does not involve chemical reactions: instead, positive and negative electrolyte ions simply ‘stick’ to the surfaces of the electrodes when the supercapacitor is being charged. When a supercapacitor is being discharged to power a device, the ions can easily ‘hop’ off the surface and move back into the electrolyte.

̽��ֱ��reason why supercapacitors charge and discharge so much faster is that the ‘sticking’ and ‘hopping’ processes happen much faster than the chemical reactions at work in a battery.

“To increase the area for ions to stick to, we fill the carbon electrode with tiny holes, like a carbon sponge,” said Griffin. “But it’s hard to know what the ions are doing inside the holes within the electrode – we don’t know exactly what happens when they interact with the surface.”

In the new study, the researchers used NMR to look inside functioning supercapacitor devices to see how they charge and store energy. They also used a type of tiny weighing scale called an electrochemical quartz crystal microbalance (EQCM) to measure changes in mass as little as a millionth of a gram.

By taking the two sets of information and putting them together, the researchers were able to build a precise picture of what happens inside a supercapacitor while it charges.

“In a battery, the two electrodes are different materials, so different processes are at work,” said Griffin. “In a supercapacitor, the two electrodes are made of the same porous carbon sponge, so you’d think the same process would take place at both – but it turns out the charge storage process in real devices is more complicated than we previously thought. Previous theories had been made by computer simulations – no-one’s observed this in ‘real life’ before.”

What the experiments showed is that the two electrodes behave differently. In the negative electrode, there is the expected ‘sticking’ process and the positive ions are attracted to the surface as the supercapacitor charges. But in the positive electrode, an ion ‘exchange’ happens, as negative ions are attracted to the surface, while at the same time, positive ions are repelled away from the surface.

Additionally, the EQCM was used to detect tiny changes in the weight of the electrode as ions enter and leave. This enabled the researchers to show that solvent molecules also accompany the ions into the electrode as it charges.

“We can now accurately count the number of ions involved in the charge storage process and see in detail exactly how the energy is stored,” said Griffin. “In the future we can look at how changing the size of the holes in the electrode and the ion properties changes the charging mechanism. This way, we can tailor the properties of both components to maximise the amount of energy that is stored.”

̽��ֱ��next step, said Professor Clare P. Grey, the senior author on the paper, “is to use this new approach to understand why different ions behave differently on charging, an ultimately design systems with much higher capacitances.”

Funding for the project was provided by the UK Engineering and Physical Sciences Research Council and the European Research Council.

A new technique which enables researchers to visualise the activity of individual ions inside battery-like devices called supercapacitors, could enable greater control over their properties and improve their performance in high-power applications.

Being able to see what’s going on inside these devices will help us to control their properties, which could help to make them smaller and cheaper

John Griffin

Supercapacitors store charge by adsorbing ions on a porous carbon surface

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