If CO2 emissions do not fall fast enough, then CO2 will have to be removed from the atmosphere to limit global warming. New technologies for artificial photosynthesis could contribute to negative emissions of CO2, say researchers from Helmholtz-Zentrum Berlin (HZB) and the University of Heidelberg. Materials systems currently being researched for artificial photosynthesis have the potential to bind CO2 with considerably greater efficiency than natural photosynthesis occurring through large-scale forestation. Today, on a lab scale, photo-electrochemical systems made of semiconductor materials and oxides can utilize about 19 percent of the light to split water, for example, and thus realize part of the photosynthesis process. The system envisioned by Matthias May at the HZB Institute for Solar Fuels and Kira Rehfeld at the University of Heidelberg is not about producing hydrogen with sunlight, but instead about binding CO2 molecules and converting them into stable chemical compounds. “However, this is a relatively similar problem from the point of view of physical chemistry,” May said. Negative emissions via artificial photosynthesis would require the development by 2050 of large-scale, durable modules that use solar energy to convert atmospheric CO2 into other compounds. Assuming efficiency of 19 percent and 50 percent system losses, around 30,000 square kilometers of modules could be sufficient to extract 10 gigatons of CO2 from the atmosphere annually, said the researchers. The atmosphere can be compared to a bathtub that can be filled only to its rim if global warming is to be limited to a certain level. We could create another small outward flow with negative emissions. However, there is no way around turning off the tap. Courtesy of M. May/HZB. “These kinds of modules could be placed in nonagricultural regions — in deserts, for example. In contrast to plants, they require hardly any water to operate, and their efficiency does not suffer when exposed to intense solar radiation,” May said. The extracted CO2 could be converted to formic acid, alcohol, or oxalate and combined with other compounds to form solid minerals that could be stored or even used in the form of plastic as a building material. However, such systems still only function at the smallest scale, are expensive, and are not stable in the long term, said the researchers, who also noted that making such systems more practical would require large investments in research and development. The research was published in Earth System Dynamics (https://doi.org/10.5194/esd-10-1-2019).