Future generations will inherit the burden of achieving negative greenhouse gas emissions. Solar-driven carbon sinks offer hope of a sustainable and scalable solution.

How to change climate change

According to the International Panel on Climate Change (IPCC), 3.5 billion people are already highly vulnerable to climate change and continued global warming. It warns that an increase in global temperature of just 1.5°C in the near term would present serious risks to ecosystems and humans [1].

Under the United Nations Paris Agreement, 196 nations have committed to halting the increase in global average temperature to well below 2°C, preferably to 1.5°C at the most [2]. The agreement requires that “anthropogenic greenhouse gas emission sources and sinks are balanced by the second half of this century.” This means we must not only reduce emissions as far as possible, but increase total negative emissions significantly – that is, find technical ways of removing CO2 from the atmosphere – to mitigate the climate crisis.

In 2021, together with Manager Magazin, we awarded Dr. Matthias May from the Institute of Physical and Theoretical Chemistry at the University of Tübingen our Curious Mind Researcher Award for his research into solar hydrogen production and photoelectrochemical CO2 reduction – that is, using sunlight as an energy source to convert CO2 into products that can be stored easily, safely, and sustainably.

Did you know?

  • 3.5

    billion people already vulnerable to climate change [1]

  • 1

    billion land hectares required to achieve negative emissions from natural photosynthesis [5]

  • 3

    million land hectares required to achieve negative emissions using photoelectrochemical cells [5]

From feasible to scalable

To understand the implications of the Paris Agreement, analysts have converted desired temperature limits into carbon budgets, specifying how much CO2 can be emitted across the remainder of the century before those limits are breached [3]. Common to each scenario is an assumption that the rollout of negative emissions technologies is technically, economically, and socially viable. Yet most of these technologies currently exist at early stages of development and few have moved beyond theoretical studies or small-scale demonstration [3].

To remove the required amount of CO2 from the atmosphere – in the order of 10 gigatons per year from 2050 onward – negative emission technologies will have to be scaled up rapidly. But it remains to be seen which approaches can be implemented quickly enough and provide substantial contributions at minimal financial or environmental cost.

“Any viable approach for negative emissions must rely on a scalable and sustainable energy source and result in a safely storable product,” says May. “It also needs to be highly efficient in terms of water and energy use and be feasible and affordable to roll out large-scale.”  

CO2 removal: how to move on?

The most advanced negative emissions technology is bio-energy combined with carbon capture and storage (BECCS), where energy is extracted from plant biomass and the carbon is captured and stored in the earth [4].

However, although carbon capture occurs in nature through the process of photosynthesis (a process that ultimately creates safely stored fossil fuels), the efficiency of natural photosynthesis drops at high light conditions and a significant proportion of the energy produced is used for plant metabolism. This makes afforestation or ocean fertilization an inefficient way to achieve negative emissions, because the land required to achieve anticipated carbon removal targets would be around one billion hectares [5]  – about ten times the arable land available in China alone [6]. Moreover, a recent analysis found that widespread deployment of BECCS could result in unacceptable trade-offs between biodiversity loss, global freshwater consumption, and temperature rise [7].

Other mainstream negative emissions technologies are direct capture of CO2 from the atmosphere and enhanced weathering, but these are energy intensive, expensive, and have not demonstrated scalability [5,8].

Will we be able to reduce CO2
 in the atmosphere?


The art of artificial photosynthesis

An alternative to existing negative emission technologies is artificial photosynthesis using photoelectrochemical solar cells. This approach has already been shown to deliver ten-fold higher efficiencies than natural photosynthesis, either producing energy through CO2 reduction or splitting water [8]. But using the products of electrochemical CO2 reduction (even if generated by renewable solar energy) as energy carriers still releases captured greenhouse gases back into the atmosphere. Only solar cells driving water-splitting and producing hydrogen would eliminate carbon completely from the energy system but this approach is at an early stage of development and would require completely new infrastructure to that used for fossil fuels [8].

In a similar approach but targeting different products, May is combining the concept of artificial photosynthesis using solar cells with a method of producing carbon products that can be safely stored – a concept called photoelectrochemical CO2 reduction.

Like natural photosynthesis, a photoelectrochemical cell (an artificial photosynthesis machine) can convert the energy from photons into fuels or other products. These cells comprise an electrolyte and a semi-conductor or dye that absorbs light. Atmospheric CO2 is captured into the electrolyte and converted to carbon-rich products using directly absorbed solar radiation. The conversion takes place via co-catalysts located at the interface between the light-absorbing material and the electrolyte.

Photoelectrochemical cells can convert CO2 into the classical fuels like methanol, ethanol, methane, and carbon monoxide, but May’s team are more interested in which products are best for storing carbon.

“The principle is very similar to solar cells,” says May, “but we are not splitting water anymore, we are using the energy from the sun to power an electrochemical reaction that transforms carbon dioxide into something else. And the something else is a big question mark, actually, because we are in this iterative process where we are looking for the most promising sink products.”

Some of the carbon-rich products might require further processing before they can be stored, whereas others can be directly extracted for long-term storage. “We are exploring oxalate, for instance, which is the material also found in kidney stones. It can be post-processed by adding a cation into an inorganic mineral so that you can store it easily in open pit storage near the land surface,” says May. “This makes the storage process much easier and safer than pumping CO2 underground into where it could potentially leak or acidify the soil.

“It’s also possible to directly form carbon flakes on your catalyst surface, which you can then shake off and store in open pits. This really is reversing fossil fuel consumption – where we have removed carbon from coal to burn it into the atmosphere, now we have to basically reverse the process.”

There are two main advantages to this approach. The first is that there are many potential carbon sink products, so these can be adapted to different storage locations and safely and sustainably stored for thousands of years.

The second advantage is that the higher solar-to-carbon efficiencies that can be achieved with photoelectrochemical CO2 reduction require far less land use for carbon removal – using only 3 million hectares if the carbon product is oxalate [5], which is just 0.3% of the land required if relying on natural photosynthesis.

May has already developed customized photoelectrochemical cells where the light absorption material is more closely integrated with the electrolyte, a technically challenging development but one that promises to advance the field toward commercial development by reducing costs [9].

“We primarily focus on tightly integrated photoelectrochemical systems, where the light absorber is immersed into the electrolyte,” says May. “Although this imposes restrictions on the design of the light absorber, the tight integration promises cost benefits that are essential if these technologies are to be used at scale.”

Increase collaboration, reduce CO2

To develop the technology further, May recently launched the NETPEC project, a collaboration with five other German universities. The project will take a highly interdisciplinary approach, bringing in expertise in computational chemistry, spectroelectrochemistry, electrocatalysis, photovoltaics, climate modelling, geological reservoir investigations, and sustainability analysis.

There is no time to waste. Currently proposed methods for negative emissions are estimated to cost between $89 and $535 trillion this century, if these methods even work [10].

“Continued fossil fuel emissions place a huge burden on young people to move beyond mitigation strategies and undertake vast efforts to remove CO2 from the atmosphere to mitigate a climate disaster,” says May. “While a rapid defossilization of human activities has to be the first priority, we urgently need to develop highly efficient, sustainable and safe technologies that can remove greenhouse gases and are affordable and feasible for widespread use.”

Chair of the Executive Board and CEO of Merck KGaA, Darmstadt, Germany Chair of the Executive Board and CEO of Merck KGaA, Darmstadt, Germany

No challenge of our time will be met without innovative ideas from young researchers. Whether it's sustainable technologies, the biotechnology of the future, or research into AI-driven solutions – what's needed are curious minds. With the Curious Mind Researcher Award, we want to honor this pioneering work.

Belén Garijo

Chair of the Executive Board and CEO of Merck KGaA, Darmstadt, Germany

A prize for curious minds

For his work in advancing solar fuel cell concepts, May was awarded the Curious Mind Researcher Award in the "Mobility and Energy" category. The award recognizes young scientists under the age of 40 in Germany whose work not only demonstrates the highest level of academic excellence, but also indicates that it will provide impetus for the future of the economy.

“The Curious Mind Award has opened the door for us to potential investors and collaborators in the biotech industry who could help realize the commercialization of these negative emission technologies,” says May. “It’s really motivating to us that people are interested in our research and in helping us to take it further. We are already collaborating with another Curious Mind Researcher Awardee, Professor Michael Saliba, who works on solar cells, and other people who read about the award also got in touch with us so hopefully this will lead the way for further collaborations.”

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. Dr. Matthias May’s prize-winning work on photoelectrochemical cells will help meet ‘Goal 7 - Ensure access to affordable, reliable, sustainable, and modern energy for all’, and ‘Goal 13 – Take urgent action to combat climate change and its impacts’ by advancing urgently needed negative emissions technologies required to meet international commitments to reduce global warming.

Learn more about SDGs


[1] Intergovernmental Panel on Climate Change, 2022. Climate Change 2022: Impacts, Adaptation and Vulnerability. [online] Available:
[2] United Nations Climate Change Paris Agreement. [online] Available:
[3] Anderson K, Peters G. The trouble with negative emissions. Science. 2016;354(6309):182-183. doi:10.1126/science.aah4567
[4] Sanchez, D., Nelson, J., Johnston, J. et al. Biomass enables the transition to a carbon-negative power system across western North America. Nature Clim Change 5, 230–234 (2015).
[5] May MM and Rehfeld K. Negative Emissions as the New Frontier of Photoelectrochemical CO2 Reduction. Adv. Energy Mater. 2022, 2103801.
[7] Heck, V., Gerten, D., Lucht, W. et al. Biomass-based negative emissions difficult to reconcile with planetary boundaries. Nature Clim Change 8, 151–155 (2018).
[8] May MM and Rehfeld K. ESD Ideas: Photoelectrochemical carbon removal as negative emission technology. Earth Syst. Dynam., 10, 1–7 (2019).
[9] May, M., Lewerenz, HJ., Lackner, D. et al. Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure. Nat Commun 6, 8286 (2015).
[10] Hansen J, Sato M, Kharecha P, et al. Young people's burden: requirement of negative CO2 emissions. Earth Syst. Dynam., 8, 577–616, 2017.

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