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Turning CO₂ to Liquid Fuel

Source: Benjah-bmm27 - Own work, Public Domain

Greenhouse gases are one of the main contributors to climate change. They have increased in concentration over the last century and are a very common air pollutant. To reduce environmental pollution scientists have come up with numerous ideas for reducing greenhouse gases in our environment. Some proposed using lab-grown minerals which suck carbon dioxide out of the air while others favour reactors which are capable of transforming CO2 into molecular oxygen.

Now (2019), scientists at the Rice University lab have developed a catalytic reactor that used carbon dioxide as its feedstock and, in its latest prototype, produced highly purified and concentrated formic acid.

One of the biggest challenges in the generation of formic acid is that when produced by traditional carbon dioxide devices it needs costly and energy-intensive purification steps. This new method of direct production of pure formic acid solutions might help along the way to promote commercial carbon dioxide conversion technologies. In tests, the new electrocatalyst reached an energy conversion efficiency of about 42%. That means nearly half of the electrical energy can be stored in formic acid as liquid fuel.

Scientists have been researching fuel cells with formic acid as energy storage medium for more than a decade. In 2008, scientists at the Leibniz Institute of Catalysis developed a new process for converting formic acid into hydrogen that worked at temperatures of 26°c to 40°c. The researchers mixed formic acid with amines and exposed the mixture to a ruthenium-based catalyst, which broke down the acid into hydrogen and carbon dioxide. The advantage of this process was that it worked at cooler temperatures and did not need a reformer or much energy; it was therefore more suitable for producing hydrogen for smaller fuel cells than powering portable electronic devices.

In 2017, researchers discovered that PtNPs embedded in a nano-chitosan matrix could be used as alternative efficient and stable anode catalyst for direct formic acid fuel cells. They found that chitosan effectively reduced the average size of Pt nanoparticles, improved their dispersion, and prevented the PtNPs sintering under the harsh electrochemical conditions through the strong interaction between the chitosan functionalities and Pt nanoparticles. It also facilitated the FAO charge transfer. Chitosan –OH and –NH2 like functionalities are believed to provide anchoring sites for the Pt nanoparticles facilitating their dispersion and avoiding their aggregation.

In 2018, another team of scientists of the HYFORM-PEMFC project carried out by GRT Group, a company that focuses on energy transition with energy-storage development solutions, developed a new, integrated formic acid-hydrogen fuel cell device. The HYFORM-PEMFC device employed formic acid to store hydrogen and could have domestic and industrial applications. Compared to devices that only use hydrogen, the HYFORM-PEMFC was designed with the aim of ensuring substantial benefits in terms of size (1 litre of formic acid carries 590 litres of hydrogen), ease of transportation, safety, and lower operating costs while being environmentally sustainable.

The advantages of formic acid as a medium for hydrogen production are manifold. Similar to methanol, formic acid is a small organic molecule which is fed directly into the fuel cell, removing the need for complicated catalytic reforming. Storage of formic acid is much easier and safer than that of hydrogen because it does not need to be done at high pressures and low temperatures, as formic acid is a non-flammable liquid at standard temperature and pressure. Formic acid does not cross over the polymer membrane, so its efficiency can be higher than that of methanol. With its current reactor, the scientists at the Rice University lab generated formic acid continuously for 100 hours with negligible degradation of the reactor's components, including the nanoscale catalysts. The scientists believe that the reactor could be easily adapted to produce such higher-value products as acetic acid, ethanol or propanol fuels.

Formic acid fuel cells might play an important role in controlling global warming and enabling green chemical synthesis, as they can help to actively reduce carbon dioxide emissions. The scientists are convinced: "If the electricity comes from renewable sources like the sun or wind, we can create a loop that turns carbon dioxide into something important without emitting more of it."