Using Carbon for Good

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Carbon dioxide (CO₂) emissions are one of the largest culprits for climate change. CO₂, a major greenhouse gas, is responsible for trapping heat within the atmosphere and fueling rising temperatures across the globe. But what if we could effectively turn CO₂ into fuel?

It turns out that light and water can react with CO₂ to produce methanol—a safe, less flammable, renewable energy source—in a process called photoelectrochemical CO₂ reduction. Photoelectrochemical CO₂ reduction could open the possibility of converting common greenhouse gases into liquid fuels. However, the practical applications of this process are currently limited because the reaction is slow and inefficient.

A team of researchers from Yale and Emory is working towards finding novel, environmentally-friendly ways to produce fuel. Recently, they engineered a faster pathway for the photoelectrochemical reduction of CO₂. Their reaction setup includes both a catalyst, which speeds up the rate of the reaction, and a semiconductor. The catalyst consists of carbon nanotubes loaded with cobalt-based molecules, while the silicon semiconductor is coated with a layer of fluorinated carbon, which stops unhelpful side reactions from occurring. When photons from the sun hit the surface of the semiconductor, CO₂ reacts with protons in water with the help of the catalyst, reducing it to methanol.

Thanks to their innovative strategy, the researchers have found unprecedented success in the efficiency of their methanol production. Electrochemists often measure the efficiency of a reduction reaction by calculating its faradaic efficiency, which is a measure of the percentage of electricity directed toward methanol production. This experiment marked the first time that a faradaic efficiency of over twenty percent was achieved for this line of photoelectrochemical methanol production.

Bo Shang, the first author of the paper documenting this discovery, attributes the team’s success to the unique structural design of their semiconductor. Their silicon semiconductor consists of arrays of microscopic columns, which greatly increase the surface area of their reaction environment and allow for more frequent collisions between the catalyst and the CO₂.

The researchers hope to continue pushing the limits of environmentally friendly reactions. For instance, in this experiment, the researchers added a low voltage to improve their reaction yield. Now, Shang is trying to see if their team can achieve similar results without any external source of electricity. “[We are] creating a perovskite photovoltaic self-powered photoelectrochemical CO₂ reduction device for methanol production, wherein solar energy serves as the sole energy input,” Shang said. In addition, the researchers’ current semiconductor breaks down relatively quickly, so they hope to find ways to improve the longevity of their system.

Although the efficiency of methanol production is still not sufficient for this reaction to become widely used, this research brings us one step closer to generating large amounts of energy from just sunlight and CO₂—both abundantly available resources. Sustainable fuel production using photochemical CO₂ reduction would reduce carbon emissions rather than add to them, creating a greener future for our planet.