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to Pho Stock lamy A© Taking a leaf out of nature’s book g Learning to mimic natural photosynthesis on an industrial scale might open the door to a fossil fuel-free future. Nina Notman investigates We have created a global climate emergency and our reliance on fossil fuels is largely to blame. To save our climate, our love affair with fossil fuels must end – fast. We can now produce electricity without emitting carbon dioxide, and carbon-neutral electricity production methods are ever more commonplace. But practical and cost issues mean that coal and natural gas continue to dominate in most parts of the world. Renewable energy, such as that from wind turbines, solar panels and hydroelectric power stations, for example, has problems with intermittency, storage and portability. Nuclear power is another lowcarbon method, but suffers from a negative public perception and toxic waste logistics. Biofuels are a step in the right direction. But carbonneutral alternatives with properties indistinguishable to those from fossil fuels are not yet available. One technology is, however, edging ever closer to making these a reality: artificial photosynthesis. Chemical fuels produced this way are ‘really quite analogous’ with those from fossil fuels, explains Erwin Reisner, a renewable energy researcher at the University of Cambridge in the UK. This means that – once produced – they could slip seamlessly into existing infrastructure. ‘In principle, we should be able to replace pretty much everything that is done by fossil fuels with artificial photosynthesis,’ says Reisner. ‘This is a dream scenario, but the underpinning chemistry is there.’ Electricity production is actually the easy bit. It’s much harder to heat homes, power planes, trains and automobiles without using chemicals derived from fossil fuels, not to mention producing the multitude of compounds and materials that the modern world is dependent upon. ‘What we’ve done so far is move towards decarbonising electrical power generation and increased energy efficiency. That’s fantastic, of course, but they are the low hanging fruit,’ explains James Durrant, a photochemist at Imperial College London and Swansea University in the UK. ‘At some point we will have to go beyond that and wean the whole of the chemical industry off fossil fuel-derived materials.’ Harvesting energy from the sun The energy in fossil fuels originated, millions of years ago, from the sun. Plants use solar energy to split water into oxygen gas and hydrogen stored as the coenzyme NADPH (nicotinamide adenine dinucleotide phosphate hydrogen). The hydrogen then reacts with carbon dioxide captured from the air, producing storable chemical energy in the form of carbohydrates. Over millions of years, the heat and pressure of the Earth’s crust has turned these photosynthesis products into fossil fuels. ‘Artificial photosynthesis is inspired by natural photosynthesis,’ Reisner explains. ‘We are trying to harvest 48 JULY 2019
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Taking a leaf out of nature’s book sunlight and use that energy to make chemical fuels from abundant feedstocks such as water and carbon dioxide.’ But the snag is we need to do so much faster and with greater efficiency. Plants only convert about 1% of the solar energy they receive into chemical energy. The idea has been around for over 100 years but the f ield didn’t really take off until the 1970s, when the f irst oil crisis caused oil prices to rocket, Reisner says. ‘When the oil price dropped again, activities slowed down very significantly. Then, around 10–15 years ago, the f ield started to grow very quickly and has now gained substantial momentum.’ ‘Today, it’s a very diverse f ield,’ agrees Durrant. ‘There are almost as many materials studied and device concepts as there are people working in the f ield.’ No devices yet operate on an industrial scale, but those that are closest are all sequential technologies. Ploughing ahead A small number of sequential artificial photosynthesis pilots are already in operation. These all decouple the sunlight capture from the rest of the process, with some pilots using integrated solar cells and others renewable energy from the grid to power water splitting via electrolysis. Critics point out, however, that many of these merely delay the inevitable: the carbon dioxide is from fossil fuels and still released into the atmosphere at the end of the process. The George Olah Renewable Methanol Plant, for example, which opened near Grindavik in Iceland in 2012, uses renewable electricity from the grid (either hydroelectric or geothermal). The hydrogen produced is converted to methanol in a catalytic reaction with carbon dioxide, captured from the f lue gas of a neighbouring geothermal power plant. According to its operator, Carbon Recycling International, the methanol plant now recycles 5500 tonnes of carbon dioxide per year that would otherwise have been released into the atmosphere. The Carbon2Chem pilot plant in Duisburg, owned by German steel manufacturer ThyssenKrupp, launched in 2018. Its electrolysis step is powered by surplus renewable grid energy. The resulting hydrogen is combined with carbon dioxide (separated out from a nearby steel mill’s waste gases) to produce methanol, again with the help of a synthetic catalyst. ThyssenKrupp says that its technology There are almost as many materials and devices studied as there are people in the field will take around 15 years to reach an industrial scale. Ammonia is also produced at the plant, important because the Haber–Bosch process currently produces approximately 1% of global greenhouse gas emissions. Polymers and higher alcohols are longer-term goals. ‘Making carbon–carbon bonds from carbon dioxide looks very simple on f irst sight but it is a tremendous technical challenge,’ explains Matthias Beller, director of the Leibniz Institute for Catalysis and lead of a recent joint position paper on artificial photosynthesis by the German academies of science. It is also a reaction that biology currently performs better than chemistry: www.chemistryworld.com 49

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Taking a leaf out of nature’s book sunlight and use that energy to make chemical fuels from abundant feedstocks such as water and carbon dioxide.’ But the snag is we need to do so much faster and with greater efficiency. Plants only convert about 1% of the solar energy they receive into chemical energy.

The idea has been around for over 100 years but the f ield didn’t really take off until the 1970s, when the f irst oil crisis caused oil prices to rocket, Reisner says. ‘When the oil price dropped again, activities slowed down very significantly. Then, around 10–15 years ago, the f ield started to grow very quickly and has now gained substantial momentum.’

‘Today, it’s a very diverse f ield,’ agrees Durrant. ‘There are almost as many materials studied and device concepts as there are people working in the f ield.’ No devices yet operate on an industrial scale, but those that are closest are all sequential technologies.

Ploughing ahead A small number of sequential artificial photosynthesis pilots are already in operation. These all decouple the sunlight capture from the rest of the process, with some pilots using integrated solar cells and others renewable energy from the grid to power water splitting via electrolysis. Critics point out, however, that many of these merely delay the inevitable: the carbon dioxide is from fossil fuels and still released into the atmosphere at the end of the process.

The George Olah Renewable Methanol Plant, for example, which opened near Grindavik in Iceland in 2012, uses renewable electricity from the grid (either hydroelectric or geothermal). The hydrogen produced is converted to methanol in a catalytic reaction with carbon dioxide,

captured from the f lue gas of a neighbouring geothermal power plant. According to its operator, Carbon Recycling International, the methanol plant now recycles 5500 tonnes of carbon dioxide per year that would otherwise have been released into the atmosphere.

The Carbon2Chem pilot plant in Duisburg, owned by German steel manufacturer ThyssenKrupp, launched in 2018. Its electrolysis step is powered by surplus renewable grid energy. The resulting hydrogen is combined with carbon dioxide (separated out from a nearby steel mill’s waste gases) to produce methanol, again with the help of a synthetic catalyst. ThyssenKrupp says that its technology

There are almost as many materials and devices studied as there are people in the field will take around 15 years to reach an industrial scale. Ammonia is also produced at the plant, important because the Haber–Bosch process currently produces approximately 1% of global greenhouse gas emissions. Polymers and higher alcohols are longer-term goals.

‘Making carbon–carbon bonds from carbon dioxide looks very simple on f irst sight but it is a tremendous technical challenge,’ explains Matthias Beller, director of the Leibniz Institute for Catalysis and lead of a recent joint position paper on artificial photosynthesis by the German academies of science. It is also a reaction that biology currently performs better than chemistry:

www.chemistryworld.com 49

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