Institute for Molecular Science and Engineering

Last time we saw the potential solar fuels have in producing 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 how!

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

Solar fuels have the potential to not only help us reach net zero carbon emissions, but play an important role in the making of new carbon-based products following a sustainable circular economy approach (Fig. 1).

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

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.

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 %. To absorb the complete range of the solar spectrum is still a challenging process within the research community. Unfortunately, the infrared radiation – the largest band in the solar spectrum – is not very energetic which makes it very difficult for activating the light absorber material and trigger the production of solar fuels. Therefore, light absorber materials can mainly be activated by either visible or ultraviolet radiation.But, considering the largest portion of visible radiation in the solar spectrum, 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 highest the absorption wavelength, the lowest the band gap energy. It turns out that the optimal band gap values that correspond to the visible wavelengths of the solar spectrum range from 1.9 to 3.1 eV.

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!

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 absorbtion), 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? How can we find novel materials with more suitable properties? We will find out in our next outing.

Read A simple guide to solar fuels 2 – Bringing to market in full

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: energy in that sunlight can be sued to convert CO₂ and methane, potent greenhouse gasses, into high-value products for the production of fertilizers, plastics or even pharmaceuticals.

In our first post we find out how solar fuels have the potential to produce clean green hydrogen.

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

“Climate change is one of the biggest threats that we are facing today” – is a phrase we have probably heard countless times at this stage. It may have lost some of their shock factor with repetition, or have made some of us too disheartened by their meaning that we choose to ignore it, but its message gets more urgent year by year.

As most of us already know, human-made CO₂ emissions are one the main triggers of climate change, contributing directly to global warming. However, the evidence for just how rapid and irreversible this change is happening is stark. Over the last 20 years alone, the concentration of CO₂ in the atmosphere has increased sharply, reaching a record value of 416 ppm of atmospheric CO₂ (June 2020), a ⁓13 % increase since 2000 [1]. That is according to the Mauna Launa Observatory in Hawaii, the institution with the longest record of direct CO₂ measurements in the atmosphere. Indeed these concentrations are almost double the amount of atmospheric CO₂ that has ever been on Earth at any time over the past 400,000 years. Data shows that this continuous rise in CO₂ is directly related to the combustion of fossil fuels such as oil, natural gas or coal that, unfortunately, our society still widely uses on a daily basis. Failing to reduce our reliance on fossil fuels to heat our homes, power our transport systems, and produce our goods, will cause unprecedented change, and damage, to both our way of life and the natural world alike.

The sharp increase in CO₂ over the last 20 years clearly shows that scientists, engineers and policy makers must work together, and quickly, to ensure that the energy we produce and the products that we make are not as a result of releasing CO₂ into the atmosphere. Indeed scientists and engineers are currently devoting considerable efforts to find efficient and scalable approaches for the production of alternative clean fuels. One revolutionary and promising alternative to conventional fossil fuels are what are known as Solar Fuels. As the name might suggest, these are fuels produced by capturing the abundant solar energy that reaches the Earth’s surface. But as our title asks, how can we turn sunlight into a fuel?

Certain materials can absorb the energy from sunlight and transform it into another form of energy, including electric current – the same principle used in a solar panel. The electric current generated can then be used to split water (H₂O) into its components, hydrogen (H₂) and oxygen (O₂). Currently, the main industrial method for mass hydrogen production is done using a process known as steam methane reforming – which emits CO₂. But hydrogen gas produced from solar energy is emission free and among the most promising solar fuels currently being investigated. One huge potential application of solar hydrogen is as an emission free fuel to power the hydrogen vehicles of the future! 

In fact, the European Union has recently released a green hydrogen strategy as part of the European Green Deal in which it aims to deploy green hydrogen at a large scale to ensure decarbonization of industries, transport, buildings and power by 2030. Therefore, it is now the perfect time to boost the potential of solar hydrogen to ensure it can be part of such a fascinating but challenging transition to a fossil-free economy.

In the next blog we will discover that in spite of the great promise and potential of solar fuels, they are still far from being commercialised.

Bibliography

1. NASA, relentless rise carbon dioxide. https://climate.nasa.gov/climate_resources/24/graphic-the-relentless-rise-of-carbon-dioxide/

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

The final part of our blog looks to the future, including introducing electrocatylists, which could give the potential game-changer of both generating electricity and producing fuels like petrol and  diesel using naturally abundant substances. We also see how predicting a specific catalyst for any reaction will soon be within reach. It looks like the best days of the alchemist might still be ahead.

By Aditya Sengar, Research Associate in the Department of Bioengineering.

Ever since the industrial revolution the world has been in a crisis mode. We need more efficient and cleaner energy over the dirty fossil fuels we currently have. The science of catalysis has been challenging the energy sector by constantly developing processes that either reduce or abandon the usage of fossil fuels. It is unfortunate that the biggest bottlenecks for adaptation of such processes are political. The USA signing out of the Paris Agreement on climate change, and China’s increasing reliance on coal, do not help the cause. We have seen historically that man-made devastating events can be tackled when global powers work together in a timely manner. I remember as a child reading about the depleting ozone layer. The problem started in 1980s when it was realised that typical refrigerants, called chlorofluorocarbons (CFCs), used in our air-conditioners and refrigerators cause breakdown of ozone molecules making us vulnerable to harmful ultraviolet (UV) light from the sun. Global events like Montreal Protocol in 1987 and Kyoto Protocol in 1997 started a wave of cooperation among politicians. Scientists unleashed the power of catalysis by experimenting with different catalysts to find ways to produce variations of carbon, hydrogen, and chlorine that are ozone friendly. The plan worked and the usage of new refrigerants (hydrofluorocarbons- HFCs) have already brought the size of ozone hole at its minimum since 1982. Unfortunately, it turns out that these HFCs contribute to greenhouse gas emissions and countries are now working to replace HFCs with more environmentally friendly natural refrigerants.

The ozone story can be used to ask a bigger question: Wouldn’t it be remarkable to be able to generate electricity or produce useful chemicals like petrol, diesel, and ammonia using naturally abundant substances like water, carbon dioxide, and nitrogen rather than fossils? This is the exact question chemists are trying to solve with electrocatalysis. Let us dig a bit deeper into this.

Electrocatalysis: Existential crisis to the fossil fuel economy

Electrolysis uses an electric current to force a reaction to occur over a catalytic electrode. Under normal circumstances, this reaction would be extremely unlikely to occur. Consider hydrogen fuel cells. In the previous blog, we discussed how the hydrogen fuel cell economy is challenging the current electricity generation and distribution landscape. The only downside of hydrogen fuel cells is the usage of coal and natural gas to produce hydrogen. Using electrocatalysis, a water molecule is split using electricity, generally produced from a renewable source (wind, solar), to produce hydrogen and oxygen. In another instance, carbon dioxide from the atmosphere is reduced catalytically to form important products like methane (called green methane), hydrocarbons, carbon monoxide etc. The present research challenges mainly surround the choice of optimum catalyst and the ability to scale up a successful experiment on a lab scale to a commercial scale.

The challenge of finding the optimum catalyst

Certain metals and compounds have a natural tendency to increase reaction rates. Ironically, gold and silver, the metals that alchemists wanted to synthesize, have excellent catalytic properties. Finding the perfect catalyst, however, remains a challenge. Major research labs rely on spectroscopy methods like X-ray photoelectron spectroscopy (XPS) or Nuclear magnetic resonance spectroscopy (NMR) to determine catalyst dynamics during an ongoing reaction. The data is used to compare the catalyst dynamics from other samples to find the optimum catalyst. In a positive turn of events, the advent of supercomputers has been like a holy grail to catalyst science. With more processing power, scientists can model the catalyst dynamics on molecular levels and transform the information to something reaction engineers can understand. Computational techniques like Density Functional Theory (DFT), microkinetic modelling and kinetic Monte Carlo (kMC) are used together to predict catalyst properties and reaction dynamics a priori.

As computational power progresses, the dream of predicting a specific catalyst for any reaction will soon be within reach and will provide scientists to engage in public undertakings of world crisis at much earlier stages.

Conclusion

Catalysis as a science is almost 200 years old. The 20th century has seen an explosion of industrial use of catalysts spurred by major historic events like the world war, and the need to find energy replacement by countries. In 2020, the global catalyst market stands at a net worth of USD 35 billion and is expected to reach USD 48billion by 2027 with a 4.4% annual growth rate. It looks small but estimates show that catalysts are used in 90 percent of U.S. chemical manufacturing processes and to make more than 20 percent of all industrial products with a direct or indirect contribution of 30-40% on the world GDP. The industry produces 20% of the greenhouse gas emissions with cleaner options, like blue hydrogen (produced via natural gas), green hydrogen (produced via electrolysis) and green methane, already starting to penetrate the global markets. Processes like Fischer-Tropsch that still take coal as a feedstock to produce synthetic fuel are an active field of research with hopes to reduce the coal dependence by employing biomass as feedstocks. Catalyst industries have also grown hand in hand with the market for production of sustainable energy solutions. Biofuels, from modern day catalysts, are already available for retail consumption in the western world with hopes to enter the markets for consumers in the rest of the world soon. Electrocatalysis is also a budding research field that hopes to completely stop the usage of fossil fuels for energy generation.

Although, countries with natural resources have been self-sufficient with their energy production (Norway with hydroelectricity, Denmark with solar and wind energy), for the rest of the countries relying on oil or coal, catalysts might just be the alchemist’s gold that saves them from an impending energy crisis whilst addressing climate change concerns.

Read Catalysis through the ages 3 – Looking to the future in full