Synthetic Photosynthesis — Turning Carbon Dioxide into Raw Materials

There is probably no other chemical reaction that is as productive as photosynthesis —a biological process that uses light energy and water to convert CO2 into energy-rich substances such as sugars inside plants.

Scientists estimate that plants produce around 150 billion metric tons of energy-rich biomass worldwide every year. In view of this, researchers are investigating ways of replicating the biomechanisms involved in photosynthesis.

Unfortunately, they haven’t been very successful so far. Photosynthetic processes involve many closely interconnected and extremely complex protein structures based on precisely-defined atomic arrangement that cannot be easily replicated in the laboratory.

As a result, scientists have so far failed to achieve their dream of using sunlight to operate an efficient biochemical “factory.”

However, developers at Siemens Corporate Technology (CT) in Munich have now come a big step closer to making the vision of synthetic photosynthesis a reality. They did this by creating shoebox-sized modules in which carbon dioxide is energetically stimulated in the same way as in plant cells.

Depending on the testing conditions, the activated CO2 reacts to create a variety of other molecules such as ethylene, which the chemical industry needs for the production of plastics. CO2 can also be converted into the energy-rich gas methane, the main component of natural gas, or carbon monoxide, which can be used to produce fuels such as ethanol, for example.

Exploiting Carbon Dioxide

Plants exploit carbon dioxide by absorbing light energy using pigments such as green chlorophyll. This process releases energy-rich electrons in the chlorophyll. Enzymes then transfer these electrons to CO2, which becomes chemically active and reacts with other compounds. “A number of teams are trying to completely replicate photosynthesis, especially in the United States and Japan,” says Prof. Maximilian Fleischer, who manages synthetic photosynthesis research at CT as part of a project known as “CO2toValue.” “But this is currently almost impossible to achieve, due to its complexity. That’s why we are taking a more pragmatic approach, in which we are gradually getting closer to achieving photosynthesis in a number of steps. Such an approach is necessary if you want to quickly launch a product on the market.”

As a result, Fleischer and chemists Günther Schmid and Kerstin Wiesner, as well as about ten others, are not yet trying to capture light. Instead, they are focusing on activating CO2 and converting it into products. To do this, they are using electricity generated from renewable sources.

Working with Universities

The key elements of the CO2toValue project are chemical catalysts that charge the inert CO2 with energy-rich electrons. The challenge is to charge only the carbon dioxide with electrons and not surrounding water molecules, because the latter would merely result in the production of conventional hydrogen. Specialists at the University of Lausanne in Switzerland and materials scientists at the University of Bayreuth are working with Fleischer’s team to develop catalysts on behalf of Siemens. This work has already led to the creation of a variety of catalysts, some of which contain copper, which have high yields of products such as carbon monoxide.

The development of such catalysts is challenging work because their behavior can only be partially predicted. As a result each new catalyst must be examined in a long series of tests and under a variety of conditions. Another factor to be considered is that a catalyst’s effectiveness is partly determined by its surface structure. That’s why its manufacturing process must be carefully controlled to create a highly reactive surface resembling a miniature craggy mountain range with a large surface . The catalysts that Schmid developed in cooperation with university partners are already very effective at converting a large part of CO2 into the desired products.

Fleischer looks through two Plexiglas windows at a bubbling reaction that is taking place inside a small photosynthesis module. The module is basically an electrolysis cell, in which a current is conducted through electrodes into highly-carbonated water, that functions as an electrical conductor.

The trick now is to manufacture the cathode, the negative pole of the special catalyst, in such a way that it is able to transfer electrons directly to the CO2 in order to produce the desired product. The water in the module is separated into hydrogen and oxygen at the other pole. The hydrogen is needed to create hydrocarbons, and the oxygen released during this process can also be used, depending on which product is desired.

The carbon dioxide that is contained in the water is initially blown into the electrolysis cell from a gas cylinder at the lab. “This process already works very well for the production of carbon monoxide, during which 95 percent of the electricity is used to produce the carbon monoxide,” explains Fleischer. By selecting appropriate catalysts and changing the current density or the salts that are dissolved in the water, the researchers can precisely control the reaction and make it convert the carbon dioxide into ethylene or carbon monoxide, for example.

Fleischer primarily focuses on such creating high-quality substances that are needed by the chemical industry. What makes these substances particularly interesting is that today the chemical industry is still almost wholly dependent on raw materials derived from petroleum.

“Of course we could also produce methane gas, but it wouldn’t be a profitable business model because you can obtain it much more cheaply from natural gas,” says Fleischer. However, a manufacturing facility would pay off if it produced sought-after chemicals such as carbon monoxide, ethylene or alcohols. These currently cost between €650 and €1,200 per metric ton, and many millions of tons of them are needed every year. A large-scale demonstration facility is scheduled to go into operation in Fleischer’s lab as early as 2015. Unlike the current facility, the new one’s output won’t be measured in watts, but in kilowatts.

By then at the latest, Fleischer wants to capture the power of the sun. He is thinking of having the photosynthesis occur in glass modules similar to photovoltaic cells. Light would stream in from the top, while carbon dioxide would flow into the system from the bottom. Fleischer has also determined how this “light trap” would work.

Instead of trying to imitate complex chlorophyll molecules, Fleischer would use “light-collecting grains” based on semiconductors. These grains would be enveloped by catalysts. If everything goes as planned, the semiconductor would supply energy-rich electrons, which the catalyst would then transfer to CO2 in fractions of a second. The entire process would be driven by light.

The system is expected to be ready in about two years. Depending on the application, the facility of the future will initially use CO2 from the exhaust produced by power stations, factories, and chemical plants. Subsequently, however, it will use CO2 from the atmosphere. With a view to accomplishing this, researchers are developing materials capable of absorbing CO2 like a sponge – and thus concentrating it. This would allow production of methanol, a valuable biofuel. Fleischer considers these prospects very tempting. “The modules could cover building façades, where they would extract CO2 from the air and from exhaust — and turn it into fuel,” he says.

Synthetic photosynthesis is a fascinating concept — even in the current initial stage, when it lacks any light-collecting ability. Fleischer thinks it could be used to store energy from renewable sources. “On windy and sunny days, Germany already has more electricity generated from renewable sources than it needs. What it lacks is sufficient energy storage capacity,” he says. “However, if the electricity were fed into photosynthesis modules, it could be used to produce valuable chemicals. This would help to reduce demand for petroleum and thus cut greenhouse gas emissions. What’s more, human beings will have incidentally managed to imitate the most productive chemical process on Earth. The dream of operating biochemical factories efficiently with sunlight could become a reality.”
Tim Schröder