Atom-thick solution to energy storage conundrum

The transition from fossil fuels to renewable energy, combined with the increasing power of today’s portable devices, calls for cheap materials that can store electricity on an unprecedented scale. Researchers are exploring the extraordinary characteristics of two-dimensional nanomaterials to achieve this goal.

According to Professor Xinliang Feng, at the Technical University Dresden, Germany, confining the thickness of materials to nanoscopic dimensions can endow them with exciting physical and chemical properties.

This is because, at small scales, electrons obey the exotic laws of quantum mechanics. They spread like waves, exist in multiple places at the same time, and engage in all manner of activities that contradict our experience in the macroscopic world.

Prof. Feng is trying to put quantum effects to work in cheaper and more powerful energy storage technologies. Already atomically thin layers of carbon, known as graphene, are helping researchers squeeze more electricity out of conventional lithium-ion batteries.

The negative electrode of these batteries is commonly made of bulk carbon. Each lithium ion stores energy by fixing itself to this electrode, typically by bonding with six carbon atoms on its surface. Because graphene is so thin, the lithium ions only need three carbon atoms to hold them in place. This doubles the amount of energy that can be stored in a given size of battery.

Quantum scale

Denser electrodes could be just the tip of the iceberg. Emerging storage technologies rely on electronic processes that naturally take place on a quantum scale. While 2D materials hold great promise for boosting their performance, they also face two great challenges.

The first is that modern electronics run on semiconductors like silicon, in which the flow of electrons can be controlled and interrupted using an electric field. In contrast, graphene behaves like a metal, meaning electrons shuttle through its structure at close to the speed of light and little can be done to interact with them.

With support from a research grant from the EU’s European Research Council (ERC), Prof. Feng has developed a new generation of 2D materials to get around this problem. By blending carbon with a range of metals and other heavy elements, Prof. Feng can tune the properties of the materials, causing them to behave like semiconductors.

A similar approach is being investigated by Professor Aurelio Mateo-Alonso at the POLYMAT Institute of the University of the Basque Country in Spain. He coordinates the 2D-INK project, funded under the EU's Future and Emerging Technologies programme, in which a consortium of research centres and small- and medium-sized enterprises (SMEs) are developing layers of graphene with nanoscopic holes inside them.

By fixing different atoms and molecules to the edge of the holes, 2D-INK partners can alter the properties of the graphene, thereby turning it into a semiconductor.

Prof. Mateo-Alonso said that porous graphene could provide key building blocks for a range of new energy technologies. In principle, its high surface area makes it an ideal electrode for more efficient supercapacitors, and its plasticity could be exploited to make flexible solar cells.

Incorporating nitrogen into porous graphene also creates highly reactive chemical traps that help split water into hydrogen and oxygen. This reaction lies at the heart of hydrogen fuel-cell technologies. It usually requires the use of expensive metals to take place but porous graphene could perform the same task with affordable and environmentally friendly materials.

‘The prospects of 2D materials for energy storage are very exciting,’ said Prof. Feng. But before they can be turned into reality, the second technical hurdle must be addressed. So far, no one knows how to build 2D materials on an industrial scale. 

Atomic layers

Graphene is conventionally obtained by peeling atomic layers off high-quality coal. According to Prof. Feng, the approach is slow and expensive. If 2D materials are to find applications in the energy sector, new fabrication methods will need to roll them out over larger areas and at higher rates.

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‘The prospects of 2D materials for energy storage are very exciting.’

As part of his ERC-funded research, Prof. Feng has patented an electrochemical process that dissolves solid carbon and redeposits its atoms as graphene over scalable electrodes. In 2015, mining company Talga Resources launched a pilot plant to field-test the technology in Rudolstadt, Germany. Talga expects to turn a profit in less than two years by converting its highest purity ores into two-dimensional nanomaterials.

The 2D-INK project plans to synthesise large areas of porous graphene by stitching together small cut-outs through chemical processes similar to those used when making plastic.

‘We are investigating how chemically modifying the pores affects the solubility of the 2D materials,’ said Prof. Mateo-Alonso. ‘The objective is to formulate the layers in ink, so as to spray coat or print them directly into electronic devices.’

Tweaking, assembling and handling materials on such small scales is testing current limits of technical feasibility. Prof. Mateo-Alonso remains positive. Each step forward reveals new opportunities, and beckons a welcome breakthrough in energy-storage technology, as well as applications that could use ultra-thin electronic devices.

‘It certainly makes matters harder to work with 2D materials,’ he said. ‘But if you think of the potential applications, it is surely worth it.’