Blog posts

Designing sponges to deliver clean water

According to the UN, 2.4 billion people do not currently have access to basic clean water and sanitation, and each day, nearly 1,000 children die due to preventable water and sanitation-related diseases. Meanwhile, pollution from fertilizers, oil spills and human waste contaminate rivers, lakes and oceans.  More than 80 percent of wastewater resulting from human activities is discharged into rivers or seas without any treatment to remove hazardous contaminants (Figure 1).

Untreated sewage being discharged
Figure 1. Untreated sewage being discharged into the environment

Given the UN Sustainable Development Goal 6 of delivering access to water and sanitation for all, how can new materials be deployed to help? Pavani Cherukupally is working on developing low-cost sponges which can remove pollutants from water.

Read more about her work

Molecular mental health 3 – Diagnostic tools  

For most physical illnesses, there are objective tests to determine what a patient’s issue is. Currently, diagnosis of mental health conditions is more subjective, as it relies on patient’s descriptions of their own symptoms. What if digital tools could identify biomarkers which were clearly linked to specific mental illnesses?

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Molecular mental health 1 – Depression and anxiety

Depression and anxiety are the most common mental health illnesses, affecting 264 million and 284 million people worldwide, respectively – equivalent to 3.4% and 3.8% of the global population. However, it’s thought that many cases are unreported – the real figures are expected to be double what is recorded. What’s going on at a molecular level in the brain during depression and anxiety? How does medication change this? 

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How can molecular science help to understand mental health?

Mental health is the sum of our psychological, emotional, and social wellbeing. Combined, these help us cope with life’s difficulties. Yet a worryingly substantial proportion of the population will suffer from poor mental health at some point in their lives. This is the first in a series of blogs exploring the molecular basis of mental health, and how a molecular perspective can help develop new treatments.

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A simple guide to solar fuels 3 – Finding the best material for the job

In this blog series, we’ve investigated why solar fuels are needed, what properties are needed by solar materials such as Fe2O3 (haematite), and how novel materials with more suitable properties could be found. In this, our final post, we will see how computers can help find the perfect material for the production of solar fuels. Finally, we will introduce the key steps that are usually done in a research lab to build a tangible device, a device that can be used to produce a “solar fuel”.

By Dr Miriam Regue and Dr Minsu Park, Research Associates in the Department of Chemical Engineering.

Predicting the properties of potential light absorber materials is an excellent starting point to evaluate their suitability as candidates for solar fuels devices. For example, the Computational Materials Science group focuses on modelling and simulating materials for solar energy conversion applications. Computational modelling allows us to predict the absorption properties as well as understand the inherent features of existing or new material candidates before building a tangible device.

For instance, oxide perovskites are a class of light absorbing materials that are gaining significant interest in the field of solar fuels.  Some examples are lanthanum ferrite (LaFeO3), praseodymium ferrite (PrFeO3) or gadolinium ferrite (GdFeO3). They are made of abundant elements and offer good visible light absorption but their understanding for solar fuels production is still scarce. 

Perovskite found in Hot Spring County, Arkansas. Each black crystal is approx. 6-7mm. From https://www.mindat.org/photo-155026.html.

Computational approaches

Computers are an excellent tool to predict material properties. They are so powerful that we can use them to run calculations and predict some of these unknown material properties. The results of these calculations help scientists to design a more efficient experimental approach to improve the solar fuel performance. They can acquire knowledge on how thick the material needs to be for boosting light absorption, how chemical modifications can affect the performance and how different structures of the same material can alter the optoelectronic properties.

Once the theoretical knowledge is gathered, the race on designing an efficient device begins. From a practical point of view, the successful material candidate must be stable in water conditions for at least 2,000 hours and provide a solar to fuel efficiency of at least 10%.

But how can we make a device that can be used to produce a ‘solar fuel’?

The first stage of the experimental design is to test the light absorber material at a lab scale. This is usually done following two different experimental approaches: (1) By depositing the light absorber material on top of a flat conductive surface to form a photoelectrode. This photoelectrode generates electric current upon solar light irradiation (photoelectrochemistry) and the electric current is used to produce the solar fuel. (2) Or alternatively, by directly immersing a powdered material in a reactor (photocatalysis) that is illuminated with solar light. Using this approach we can also measure and quantify the amount of solar fuel (i.e H2 (and O2), CO, CH4 …) produced (see the below diagram). Although both approaches are feasible for solar fuels production, thephotoelectrochemical approach is often the preferred one mainly for safety reasons, but also due to better reproducibility in results.

As simple as it seems though, there are numerous challenges that experimentalists face, such as low efficiencies and performances, and poor product selectivity and scaling-up reproducibility. Different experimental approaches are currently under investigation to address some of these issues. In our research group, we follow various novel approaches to improve the response of these materials and to understand and describe their behaviour. For instance, we recently discovered that (1) mimicking the shape of natural desert roses on TiO2, (2) using a polymer as a template for the preparation of LaFeO3 and Fe2O3, and (3) loading copper and graphene oxide on novel halide perovskite materials improves the performance and efficiency of the water splitting process and solar CO2 conversion to methane and carbon monoxide upon solar light irradiation (below diagram).

Schematic diagram of the different materials and approaches employed in the ‘Eslava Group: Applied Energy Materials’ group.

So, which technology readiness does solar fuel production have right now? Will we ever power our home using a stand-alone H2 device? Is our society ready to produce commodity chemicals (plastic, fertilizers, clothes…) from waste CO2 using solar light?

Unfortunately, photoelectrochemical approaches are still in lab-scale studies, but tremendous improvement has been achieved since their discovery in 1972 by Fujishima and Honda.2 Researchers have achieved solar-conversion efficiencies of 10 % for ca. 40 h and it is expected that similar or even higher efficiencies will be obtained by 2025 on square metre devices with the final goal to achieve a decentralised and local production of solar fuel (H2) at a household level in the near future. Direct CO2 capture and solar conversion is on a similar technological readiness as H2 production. However, the success of this technology relies on the capability of coupling direct CO2 capture technologies with photoelectrochemical set-ups.

But undoubtedly, what will mark the deployment of these disruptive technologies and ensure a proper share in the current market is the ability to compete economically with conventional methods of hydrogen and carbon-based products production. Currently, steam methane reforming produces H2 at an average cost of ~ 1.40 $/ kg (it depends on natural gas price), is significantly lower than what is currently estimated for photoelectrochemical devices ( 9 – 11 $ kg-1).[1] However, if the research community manages to overcome the current technical barriers, the cost of hydrogen via photoelectrochemical methods could reach 2 – 4 $ kg-1.[2] This will be an extraordinary breakthrough that will pave the way for solar hydrogen to be included in the future energy mix.

Bibliography

  1. R. J. Detz, J. N. H. Reek and B. C. C. Van Der Zwaan, Energy Environ. Sci., 2018, 11, 1653–1669. DOI: https://doi.org/10.1039/C8EE00111A
  2. B. A. Pinaud et al, Energy Environ. Sci., 2013, 6, 1983–2002. DOI: https://doi.org/10.1039/C3EE40831K

A simple guide to solar fuels 2 – Making solar materials

Last time we saw the potential of solar fuels to produce clean green hydrogen. But it turns out solar energy can also be used to convert CO2 and methane, potent greenhouse gasses, into high-value products for the production of fertilizers, plastics or even pharmaceuticals. In this post we find out about the materials needed to do that!

By Dr Miriam Regue and Dr Minsu Park, Research Associates in the Department of Chemical Engineering.

Solar fuels have the potential to help us reach net zero carbon emissions. But they can also play an important role in the making of new carbon-based products following a sustainable circular economy approach (Fig. 1).

Infographic showing direct air capture of CO2 and its conversion into commodity chemicals.
Direct CO2 capture from air and solar CO2 conversion following a circular economy approach.

However, despite great promise, solar fuels are still far from being commercialised. But why? What is hindering their deployment, and what limitations are we facing?

The ideal solar material

It is both hard and challenging to give just one simple answer to this question. Remember from our first post that we said certain materials can absorb the energy from sunlight and transform it into electrical current? Well at the moment, the ideal light absorber materials that would lead the transition to a solar fuel-based society are still being researched. In fact, the “ideal” light absorber must fulfil several requirements. For one, it must absorb the widest possible range of the solar spectrum, so we can make the most of the solar energy to use it for the production of solar fuels. Secondly it must be made of elements with a plentiful supply on the Earth’s crust.

Solar materials and the radiation spectrum

To understand the first requirement, we must consider the solar spectrum. It is composed of three different bands; infrared, visible and ultraviolet radiation. Infrared and visible radiation are the majority bands, accounting for 52 – 55 % and 42 – 43 % of the spectrum respectively, whereas ultraviolet radiation represents only 3 to 5 %. Infrared radiation, the largest band in the solar spectrum, is not very energetic. This makes it very difficult for activating the light absorber material and triggering the production of solar fuels. Therefore, light absorber materials can mainly be activated by either visible or ultraviolet radiation. The “optimum” light absorber material should be particularly good at absorbing light from the visible range (400 – 700 nm).

Each material has a specific and unique absorption range – called band gap energy. The band gap energy and the absorption energy are indirectly related. The higher the absorption wavelength, the lower the band gap energy. It turns out that the band gap values that correspond to the visible wavelengths of the solar spectrum are 1.9 to 3.1 eV.

Solar materials and elemental abundance

The second requirement, that our light absorber material must be made of elements with a plentiful supply on the Earth’s crust, can be solved by choosing a green-coloured elements of The Element scarcity periodic table (Fig. 2). Haematite (α-Fe2O3), which is one of the renowned materials for solar water splitting, is a promising light absorber candidate: It has a band gap energy of 2.1 eV, meaning that it can absorb near the range of 590 nm and it is made of large abundant elements, iron (Fe) and oxygen (O). This makes it fulfil both our requirements!

Relative abundance of elements in the periodic table: only abundant elements should be used to make solar materials
Elements Scarcity – EuChemS Periodic table

Ultimately, the efficiency of the process also depends on several inherent features of the light absorber material, such as their energetic properties and their ability to convert the absorbed solar energy to the desired solar fuel. In fact, these are the most challenging features to control and as mostly agreed by the research community, probably the limiting step of the solar fuel production. Finding the proper material able to meet all these requirements at once is a cumbersome process and requires the collaboration of scientists and engineers from different fields of expertise such as theoreticians and experimentalists. In fact, although Fe2O3 (haematite) fulfils the first and second requirements (made of abundant elements and suitable light absorption), its inherent properties are still not good enough to produce solar fuels in an efficient manner.

But if existing materials such as Fe2O3 (haematite) or TiO2 are not good enough for solar fuels, how can we improve them? Read our next blog about how can we find novel materials with more suitable properties.

A simple guide to solar fuels 1 – Turning sunlight into a fuel

Turning sunlight into a liquid fuel that is an abundant, sustainable, storable, and portable source of energy might sound like the fantasy machinations of a sci-fi novel. However, the reality is possibly even more exciting. Solar fuels could use energy in that sunlight to convert COand methane, potent greenhouse gasses, into high-value products for the production of fertilizers, plastics or even pharmaceuticals.

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