Humans have relied on fossil fuels for concentrated energy for the past two centuries; hundreds of millions of years of photosynthesis packed into a convenient, energy-dense substance. However, that supply is limited, and fossil fuel consumption has a significant negative impact on the Earth's climate.
"The biggest challenge that many people don't realize is that even nature has no solution for the amount of energy we use," said Wenbin Lin, a chemist at the University of Chicago. "We will have to do better than nature, and that's scary," he said, referring to photosynthesis.
Scientists are looking into "artificial photosynthesis," which involves reworking a plant's system to produce our own types of fuel. However, the chemical equipment in a single leaf is extremely complex and difficult to adapt to our needs.
A study published in Nature Catalysis by six chemists at the University of Chicago demonstrates an innovative new system for artificial photosynthesis that is orders of magnitude more productive than previous artificial systems. In contrast to natural photosynthesis, which generates carbohydrates from carbon dioxide and water, artificial photosynthesis could generate ethanol, methane, or other fuels.
Though the method has a long way to go before it can be used to fuel your car every day, it provides scientists with a new avenue to investigate—and may be useful in the short term for the production of other chemicals.
"This is a huge improvement over existing systems, but more importantly, we were able to lay out a very clear understanding of how this artificial system works at the molecular level," said Lin, the James Franck Professor of Chemistry at the University of Chicago and the study's senior author.
'We'll require something else.'
"We would not exist without natural photosynthesis. It created the oxygen we breathe on Earth and the food we eat "Lin stated. "However, it will never be efficient enough to supply fuel for us to drive cars, so something else will be required."
The problem is that photosynthesis is designed to produce carbohydrates, which are excellent for fueling humans but not automobiles, which require much more concentrated energy. As a result, researchers seeking alternatives to fossil fuels must re-engineer the process to produce more energy-dense fuels such as ethanol or methane.
In nature, photosynthesis is carried out by a number of highly complex protein and pigment assemblies. They absorb water and carbon dioxide, disassemble the molecules, and rearrange the atoms to form carbohydrates, which are a long string of hydrogen-oxygen-carbon compounds. However, scientists must rework the reactions to produce a different arrangement with only hydrogen surrounding a juicy carbon core—CH4, also known as methane.
This re-engineering is far more difficult than it appears; people have been tinkering with it for decades, attempting to mimic nature's efficiency.
Lin and his research team thought they'd try something that artificial photosynthesis systems haven't yet: amino acids.
The researchers began with a metal-organic framework, or MOF, which is a class of compounds made up of metal ions held together by organic linking molecules. The MOFs were then designed as a single layer to provide the most surface area for chemical reactions, and everything was immersed in a solution containing a cobalt compound to ferry electrons around. Finally, they added amino acids to the MOFs and tested which ones worked best.
They were able to improve both halves of the reaction: the one that splits water and the one that adds electrons and protons to carbon dioxide. In both cases, the amino acids aid in the efficiency of the reaction.
Even with the significantly improved performance, artificial photosynthesis still has a long way to go before it can produce enough fuel to be useful in widespread applications. "From where we are now, it would need to scale up by many orders of magnitude to produce enough methane for our consumption," Lin explained.
The breakthrough could also be widely applied to other chemical reactions; it requires a large amount of fuel to have an impact, but much smaller quantities of some molecules, such as the starting materials for pharmaceutical drugs and nylons, among others, could be very useful.
"So many of these basic processes are the same," Lin said. "Good chemistries can be plugged into many systems if they are developed."
More information: Guangxu Lan et al., Nature Catalysis, Biomimetic active sites on monolayered metal-organic frameworks for artificial photosynthesis (2022). DOI: 10.1038/s41929-022-00865-5
Journal information: Nature Catalysis