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Using diffraction to improve material understanding

10th June 2019


Here José Antonio Alonso, Research Professor at the Instituto de Ciencia de Materiales de Madrid, C.S.I.C. outlines the benefits of neutron diffraction in understanding materials better.

Progress towards developing sustainable energy solutions is becoming increasingly critical as concerns grow around the extent of carbon emissions released by burning fossil fuels – principally coal, oil and natural gas. The use of these fuels has led to an increase in greenhouse-gases, including carbon dioxide, which is contributing to human-induced climate change. The gradual increase in global temperatures is predicted to have major consequences – such as wreaking havoc on weather patterns and increasing the risk of floods and droughts, posing a major threat to agriculture. In the hunt for more efficient and sustainable energy solutions in future, researchers are turning to neutrons, neutral subatomic particles, to help unlock our understanding of how greenhouse gases fuels, and renewable sources interact with materials to develop more sustainable alternatives to our current energy cycle.

Revealing the inner workings of fuel cells and batteries

Hydrogen fuel cells have gained much attention in recent years as energy-conversion devices. Hydrogen is considered a more environmentally friendly alternative to fossil fuels. It can be produced in a more sustainable way – for example, by ‘splitting’ water using the sun’s energy – whereas natural gas and oil extraction conversely can be highly damaging to the natural environment. Within a fuel cell, hydrogen can be used as a fuel, taking advantage of the energy released when it is electrochemically combined with oxygen from the air. This process does not release harmful gases, but instead produces only water as a by-product. However, the chemical processes inside need still to be optimised to form the basis of a ‘hydrogen-economy’. Additionally, energy storage is also an important process that is realised in secondary batteries, containing light, diffusible elements like lithium or sodium.

In order to harness fuel cells and batteries to their full potential, we must have a complete understanding of the inner workings of such devices, and the materials used to compose them. As such, in collaboration with the Institut Laue-Langevin (ILL) the world’s flagship neutron science facility, we have conducted research to better understand how we can optimise the properties of the materials within fuel cells and batteries for more efficient use.

Neutron scattering is an ideal tool for exploring the crystal structure of the materials used in energy conversion and storage, as the particles interact directly with the atomic nuclei or with the unpaired electrons of the material’s atoms (in magnetic materials), giving rise to scattering that can be analysed to infer the atomic arrangement, and to establish interesting structure-to-properties relationships.

In particular, important structural factors of fuel-cell and batteries materials are strictly correlated to the position and properties of light elements, such as oxygen, lithium and hydrogen within the structures. Neutrons enable us to visualise these light elements, whereas other analytical tools such as x-ray scattering are unable to pinpoint the location of the atoms in such a precise and reliable manner. Neutron powder diffraction is a particularly powerful tool for this exploratory work. Energy materials for both fuel cells and batteries share some common structural features, such as the presence of defects (vacancy atoms, interstitials) that make the fast diffusion of these elements (such as lithium, hydrogen ions) possible across the solid, in electrodes and electrolytes. Neutrons help to deliver important information about the defective crystal arrangements, to identify the nature of these defects, and to quantify them. Neutron powder diffraction is particularly useful when the properties of interest are associated with the powder state of the materials, which happens to be the case for many materials used in the energy sector (including fuel cells, batteries, thermoelectrics, supercapacitors and hydrogen storage materials).

These techniques are made possible by the state-of-the-art instruments provided for neutron research at ILL, which has helped us to understand and even improve the crystal structures of many materials in fuel cells and batteries, including electrodes and electrolytes. This is guiding more efficient reactions and in turn helping to increase the power density of fuel cells, reduce their operating temperatures, and improving the energy density of batteries. Besides state-of-the-art materials in established technologies (for instance, in lithium secondary batteries with liquid or gel electrolytes), the new chemistries in innovative sodium batteries, or the challenging calcium, magnesium and aluminium-ion batteries have also been tested, for the first time at ILL. In all these materials, the identification of defects, and specific structural features like the anisotropic displacement factors, can yield invaluable hints on the diffusion paths of the atoms playing in the novel chemistries, helping to establish structure-properties correlations. In iterative designs, this can help to improve these devices. Only by making energy storage and conversion devices more reliable can we ensure they can be rolled-out on a large scale, and reduce the extensive carbon-emissions associated with the transport industry and other energy-consuming human activities.

Optimising solar cells

Neutrons are also helping us to optimise how we harness renewable energy sources. In particular, in the development of another device critical to sustainable energy – solar cells. These convert energy from light into electrical energy, and together form modules commonly known as solar panels, which are a popular option across the globe for producing clean, green energy. Particular attention is garnered by the organic−inorganic hybrid perovskites, which have been described to harvest solar energy with a superior efficiency than conventional silicon cells. They constitute the active light-harvesting layer in solar cells. They have emerged as promising, absorptive materials for the next generation of solar cells.

A profound knowledge of the crystal structure detail seems essential to establish structure−property correlations that may drive to further improvements in the application of perovskites in solar cells. The hybrid nature of these perovskites, constituted by an inorganic framework that lodges the active organic molecules (like methyl-ammonium) make neutrons particularly useful for the investigation of these materials in the real life. In particular, the sensitivity of neutrons to the position of hydrogen atoms, contained in the organic part of the perovskites, is invaluable to determine the configuration and dynamics of the organic molecules, which are vibrating and rotating within the inorganic cages. The instruments at ILL are perfectly designed to tackle these structural and dynamical problems, thus helping the search for ideal energy-producing materials that can support the growing use of solar power too.

In summary, during the last 30 years neutron diffraction has become the technique of choice to investigate the above mentioned structural features in a panoply of materials that could form the foundations of future sustainable energy solutions. The flexibility of the experimental environments for neutron instruments means that fuel cells, batteries, thermoelectric devices and solar cells can be tested for functionality at realistic commercial temperatures, degree of irradiation, in reducing (fuel) or oxidising (comburent) atmospheres. This will also help to improve the compatibility of the different constituent materials, decreasing the operation costs. To ensure this, it is essential that energy researchers around the world have access to state-of-the-art facilities like the neutron beams at ILL. In this way, our most pressing and time-critical science is enhanced by the most powerful tools. The progress in these different fronts mean that a future without fossil fuels is one step closer.

 







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