Capturing energy from air: The CO2 to fuel converter

Taking inspiration from how plants and microbes convert CO2 into energy may lead to a sustainable fuel of the future that can satisfy our growing energy demands.

Our appetite for energy

Over the past two centuries, our demand for energy has continued to increase dramatically and shows no sign of slowing. Rapid industrialization, urbanization, technological development, and a booming world population are just a few of the drivers responsible for this seemingly insatiable appetite.

Satisfying our growing energy demand cleanly and sustainably is a high priority on the global agenda. And while there is substantial momentum behind expanding renewable and low-carbon infrastructure, we are still heavily reliant on burning the three fossil fuels of oil, coal, and natural gas to meet our energy needs.

The resulting carbon emissions account for the vast majority of greenhouse gases which, as their atmospheric concentrations rise, have an insulating effect. This leads to global warming, and subsequently, climate change — which is the “single biggest threat facing humanity,” according to the World Health Organization [1].  

Did you know?

  • 85%

    of energy is generated from fossil fuels [4]

  • 36.3

    billion metric tons of CO2 emitted in 2021 [7]

  • >90%

    of natural CO2 capture is by photosynthesis

Renewables can’t keep up

Renewable energy infrastructure is growing faster than ever [2]. According to the International Energy Agency, 29% of electricity produced worldwide in 2020 came from renewable sources like wind and solar, up 2% from the previous year [3]. But other statistics are less encouraging. Renewable sources only meet roughly 11% of total energy demand, with nearly 85% being satisfied by fossil fuels [4]. What’s more, rising demand is outpacing new supply available from renewable sources, so the gap is only widening [5].

This is a worrying trend. The use of fossil fuels in energy production is by far the biggest contributor to greenhouse gas emissions worldwide [6], primarily in the form of carbon dioxide (CO2). In 2021, following a rare dip in energy demand as a result of the Covid-19 pandemic, global energy-related CO2 emissions rebounded and rose to 36.3 billion metric tons, the highest level ever recorded [7].

Developing sustainable energy infrastructure and decreasing our reliance on fossil fuels is imperative. While that will passively reduce carbon emissions in the long term, we can also take an active role in bringing atmospheric CO2 concentrations down.

Indeed, this is a key motivator in the growing ‘carbon capture, utilization, and storage’ field. Methods of CO2 capture and conversion developed by heavy industry, startups and researchers are already turning CO2 into everything from chemicals and simple fuels like methanol, to plastics and other polymers, to concrete and similar building materials. Even diamonds and vodka [8].

By far the most important contributor to carbon capture, utilization, and storage, though, is neither a company nor a technology — it’s nature.

Fuel for thought

Since 2019, we've awarded the Future Insight Prize for ambitious dream products in the fields of health, nutrition and energy. Attached to this annual prize is a grant of up to €1 million to support research into groundbreaking science and new technologies that would help make that dream product a reality.

This year, the Future Insight Prize has been awarded to Prof. Dr. Tobias J. Erb, Director of the Department of Biochemistry and Synthetic Metabolism at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany. His research contributes to the dream product concept of the CO2 to fuel converter, which produces high energy-density fuel from atmospheric CO2 with an overall negative carbon footprint.

This dream product already exists — almost. CO2 to fuel converters are everywhere, living in pots around the house, or standing tall in the garden.

A tree, for example, ‘breathes’ CO2 in, converts it into nutritious carbohydrates via photosynthesis, and ‘breathes’ oxygen (O2) out. Cut down that tree for firewood and you are utilizing the energy created by this all-natural CO2 to fuel converter.

And if you plant a tree to grow in the place of the one you’ve felled, then it can effectively be a carbon-neutral transaction, as the CO2 released from the fire becomes food for the new sapling. The same is true for photosynthetic microbes, which also feed on CO2 to grow and multiply. Use them as biofuel, and the CO2 released becomes sustenance for replenishing the community.

Forestation and microbe cultivation is not a complete solution for reducing carbon emissions and meeting global energy demand, of course. But Dr. Erb’s work focuses on the biology and chemistry of naturally occurring CO2 capturing systems — with the ultimate goal of taking inspiration from them to bioengineer novel CO2 converting enzymes and systems that improve upon nature’s designs.

Inspired by nature

It's estimated that more than 90% of CO2 is captured (or ‘fixed’) through the Calvin cycle of photosynthesis and its key enzyme, RuBisCO. It’s the most prevalent carbon-fixing metabolic pathway across plants and microbes, but only one of many that exist in nature. Dr. Erb and colleagues have so far discovered several previously undescribed carbon-fixing microbial pathways and enzymes, including a new class of enoyl-CoA carboxylases/reductases (ECRs) [9, 10]. They are the most efficient CO2 converting enzymes found to date, and up to twenty times more productive than RuBisCO.

Analysis of these ECRs to fully understand their structure and mechanisms in fine detail has given rise to the creation of a new raft of potent and versatile, bioengineered counterparts. But naturally occurring carbon-fixing metabolic pathways are complex, with numerous moving parts. The process of capturing CO2 and converting it into useful carbohydrates is not the work of one single enzyme, but a carefully choreographed dance of many.

Metabolic retrosynthesis is the next step beyond optimizing individual enzymes. It’s the science of engineering entirely new synthetic pathways of CO2 conversion, building them from scratch using known enzymes from across the natural world, and filling in the gaps with novel proteins.

The discovery of ECRs allowed Dr. Erb’s lab, for instance, to successfully create the ‘CETCH cycle’ — a pathway comprising 17 enzymes, 14 of which were sourced from nine different organisms, with three engineered proteins completing the design [11]. The rates of carbon fixation of the CETCH cycle are 20 times higher than that of natural photosynthesis, and it uses 20% less energy per CO2 fixed.

The ‘TaCo pathway’ is another synthetic system, which includes three bioengineered enzymes, including a completely novel carbon-fixing enzyme. It has been developed to fix the inefficiencies of photorespiration, an undesirable step in natural photosynthesis that sees roughly 25% of fixed carbon re-released as CO2. Rather than simply plugging this leak, the TaCo pathway reduces the energy demands of photorespiration and increases carbon efficiency by up to 150%, turning it into a carbon-capturing process [12, 13].

Supercharging photosynthesis

Building on this progress, Dr. Erb and colleagues from multiple disciplines have managed to construct artificial chloroplasts. Just as chloroplasts within plant cells house the machinery of natural photosynthesis, these artificial equivalents use light as their energy source and employ the more efficient and productive CETCH cycle to convert CO2 into multi-carbon compounds [14].

Improving carbon-fixing productivity to supernatural levels is only one piece of the converter puzzle, though. If you were to take this synthetic metabolism and design an over-achieving microorganism that uses it, you could create a source of fast-growing biomass fuel. But at best, that’s a carbon-neutral outcome. Hence Dr. Erb is also working on expanding the steps of the CETCH cycle to tailor it to chemical production, including high energy-density fuel.

Early results from this are promising. By combining the CETCH cycle with additional natural and synthetic pathways, coordinating up to 50 enzymes in a singular process, his lab has already demonstrated the direct conversion of CO2 into various desirable carbon-based compounds [15] — including pentadecane, an important component of petrol and diesel fuel [16].

This line of investigation could ultimately realize a carbon-negative fuel source of the future, where a ‘designer’ microbe or plant uses CO2 to both sustain itself and produce biofuel to meet our rising energy demands.

But there are still many interesting research avenues to explore. Making synthetic carbon-fixing pathways even more efficient and productive, for instance. Synthetic pathways engineered in the lab are not subject to the same evolutionary selection pressures that have fine-tuned the metabolism of living organisms over billions of years.  

And so, Dr. Erb is using a combination of machine-learning, high-throughput screening, and lab automation techniques to induce artificial evolution, as well as invent entirely new pathways. Already, these strategies have optimized elements of the original CETCH cycle to improve its productivity ten-fold.  

From lab to leaf

An artificial chloroplast is an impressive proof of concept, but chloroplasts are just the engine of photosynthesis. That is one part of the complex carbon capture, utilization, and storage machinery of living photosynthetic organisms. The next milestone is to develop a multicellular ‘artificial leaf’ to mimic this machinery in greater detail.

This would take Dr. Erb’s work ever closer to the ambitious breakthrough of translating these synthetic processes from the lab into living systems.

It’s a huge challenge. Namely, encoding the synthetic pathway into the genes of an organism such that all the individual enzymes are expressed and function as intended. What’s more, it’s paramount that this organism relies exclusively on that metabolism, creating the evolutionary pressure to preserve and improve that pathway.

Should this reprogramming eventually graduate to higher organisms like plants, however, it’s estimated that the pairing of the existing CETCH cycle and TaCo pathway could increase the carbon fixation rate of these bioengineered plants by 20% to 200%.

In search of

Dr. Erb’s work complements diverse efforts across industry and academia, where advances in industrial chemistry and bioengineering are already enabling the recycling of CO2 into useful products.

The dream product of a carbon-negative CO2 to fuel converter is just one of the countless possibilities. Dr. Erb’s research, in its simplest form, is about unlocking the full potential of carbon-fixation systems, using nature as inspiration. That could result in ‘designer’ organisms capable of producing value-adding complex carbon products such as fuel, pharmaceuticals, food additives, and fragrances.

The research may also lead to the development of next-generation food crops with improved metabolism, creating significantly higher yields capable of feeding our growing population. There is no one path of progress on which success or failure hangs. As Dr. Erb puts it: “Our technology is not providing a single product or process. We rather believe that our synthetic biological approaches have a transformative character that will allow us to design different tailor-made solutions for a sustainable world of tomorrow.”

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In 2012, the United Nations set out 17 Sustainable Development Goals (SDGs) that meet the urgent environmental, political, and economic challenges facing our world. Three years later, these were adopted by all member states. We are committed that our work will help to achieve these ambitious targets. Supporting research towards the creation of a dream CO2 to fuel converter fits under ‘Goal 7 — Affordable and clean energy; Target 7.b — Facilitate clean energy research and technology.’ A negative emissions fuel converter capable of creating a sustainable energy source could help us meet our growing energy.

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