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Energy and water are two essential factors for the continued development of humanity. These two interlinked and fundamental aspects of our survival represent ongoing challenges and opportunities for a range of researchers and inventors. Current water treatment processes require tremendous amounts of energy, while energy regeneration itself can lead to further environmental contamination. Researchers at the UNSW Materials and Manufacturing Futures Institute (MMFI) partnered with scientists from the Australian Nuclear Science and Technology Organisation (ANSTO) to study and find alternative approaches with an eye towards a more sustainable future. 

Background

Since time began humans have been driven by the need for clean, reliable and sustainable water sources, and the less energy involved in collecting and enjoying that water, the better. Most societies today have this access thanks to a local water treatment plant, a complex infrastructure that typically begins with collection from natural water sources through to advanced electrochemical disinfection processes. However, all of this requires a lot of energy and some disinfection processes also produce by-products that are harmful to us and the environment. 

Ongoing inefficient water and energy management places further stress on the sustainability of our society and environment and so there continues to be a need to refine and improve our systems of generating these two essentials. This complex interdependence has resulted in an ongoing search for materials that can be used to collect potable water with low energy usage, or more ideally, generate energy whilst collecting potable water. A recent paper published in Energy & Environmental Science has demonstrated that the layer structured Magnèli titanium oxides possess co-existing functionalities, which can be applied to both energy harvesting and water treatment using one material. 

In other words, scientists today are studying and developing sustainable and commercially viable multifunctional materials that can capture energy and treat water in a single system, therefore paving the way for a diverse field of sustainable development. The challenge has been to identify the right materials—cheap and cost-effective, safe and earth-abundant, and durable enough to withstand high energy densities in harsh conditions—and to grasp their underlying physical and chemical structure as the evidence foundation to develop systems for a range of environments.

Taking advantage of our multidisciplinary collaboration platform through the UNSW Materials and Manufacturing Futures Institute, academics from three different schools and scientists from the Australian Nuclear Science and Technology Organisation have demonstrated that the arrangement of layered defects in the Magnèli titanium oxides is correlated with their physical and chemical properties, which are the key factors to enable its functional abilities for both energy harvesting and water treatment. This correspondence between molecular scale properties and application is explored here through an interdisciplinary investigation that spans the fields of materials science and materials engineering, physics, chemistry, chemical engineering and civil engineering.

Key Innovations and Significances 

As multifunctional materials, titanium oxides have created strong interest not only in the area of thermoelectrics but also for other sustainable energy applications, such as for use in photovoltaics and optoelectronics, as well as photocatalysts. The general idea is to develop a material that can be used as the anode in a hybrid system that receives heat and solar energy to power a photo-/thermal-catalysts to oxidize organic contaminants in the water, whilst simultaneously generating electrical energies from solar radiated heat and light.

Researchers at MMFI and ANSTO have found that control of layered oxygen defects in the TinO2n-1 system can be achieved via structural and compositional engineering. By introducing controlled defects to the titanium oxide system, it is possible to create specific layered structures that can be developed for diverse multifunctional systems, such as energy collection and water treatment.

Yichen Liu is a PhD candidate with MMFI and he believes other material scientists should take note of this concept of manipulating the multifunctionalities in a single material to maximise its societal benefits.

“The modification of the intrinsic structure of the titanium oxides has different impacts on different physical properties, electrical, thermal, and electrochemical. Through structural manipulation of the material, we can make it fit different applications,” he says.

Scientia Professor David Waite from the UNSW Water Research Centre and School of Civil and Environmental Engineering agrees, noting the simultaneous challenges and opportunities of working with researchers from diverse fields.

“The unique properties of these “titanium suboxides” render them suitable for a range of applications—in my case, for electrochemically-mediated water treatment. The improved understanding that will be disseminated through this article will assist other researchers—and maybe practitioners—to both design better products and recognise new applications,” he says.

“While challenging, cross-boundary insights were invaluable.”

Collaboration between MMFI and ANSTO

As a part of the collaborative work between MMFI and ANSTO, researchers experimentally demonstrated that the introduction of defects and the existence of a Ti3O5 secondary phase significantly enhances phonon scattering, resulting in excellent room temperature thermoelectric performance through optimising the compositional ratio of Ti2O3, Ti3O5, and TixO.

“The biggest challenge was understanding the actual nature of the generated defects and how this ‘nature’ can influence the particular property (and in turn performance) of the material,” says Associate Professor Jason Scott, Senior Research Fellow in the Particle and Catalysis Research Group.

“Often defects and imperfections within, or on the surface of, a crystal are key in defining material performance. There are often multiple variations of a defect within a single material whereby identifying the key defect and understanding how it influences performance adds an entirely new level of complexity to the field,” he says.

“Sometimes just having the capability to actually observe defects in a material can be challenging, let alone identifying variations in defects.”

Professor Garry McIntyre, Research Leader at the Australian Centre for Neutron Scattering at ANSTO, reflects along similar lines.

“Disordered materials offer greater possibilities for tuning of particular properties but can be difficult to manufacture and characterise reproducibly. Another challenge was the interpretation of the experimental phonon densities of state which reflect directly the lattice vibrations and the thermal conductivity but varied little with temperature and across the material series,” he says.

Importantly, the team found that sub-stoichiometric Ti2O3 performs differently in regard to solar absorption, oxygen evolution, and photocatalysis depending on its composition. By manipulating the planar oxygen defects, the solar-thermal, electrocatalytic, and photocatalytic properties of these materials can be optimised, opening an avenue to develop particular material systems that possess the multifunctionality required for energy conversion and water treatment. 

“It is early days, but a practical demonstration of a multifunctional catalytic reactor powered by both solar heat and/or light would raise the impact further,” says Professor Garry McIntyre.

“Especially with small and mid-size enterprises who have the agility to transform the scientific results into commercial items.”

Professor Garry McIntyre also notes the cross-disciplinary benefits of such collaborative projects.

“The most rewarding aspect of this project to me was seeing how the expertise of researchers from quite different disciplines within materials science and engineering amongst others came together to give detailed understanding of a hybrid material system that has high potential for technological application in the energy and environmental fields,” he says.

“This project is an excellent example of the benefits of a multidisciplinary research team with the final output being greater than the sum of the parts. Guiding such research is a challenge but with the right leadership, the results are outstanding.”

This sentiment was shared across the team, with Scientia Professor David Waite urging other researchers to take on collaborative work that stretches beyond their disciplinary boundaries.

“We should undertake more of this “cross-boundary” research as it enables insights obtained by one group to be shared with others who may, in turn, either recognise the possibility for new applications or understand better how to optimise the materials for their own application,” he says.

The full paper is titled ‘’ and can be accessed online in the Energy and Environmental Science journal (DOI: 10.1039/D0EE02550J).