05-09-2020 | | By Robin Mitchell
Recently, a team from the University of Cambridge developed an artificial photosynthesis device to produce formate fuel from water and CO2. What powers this device, what fuel does it produce, and how may this help further fuel-cell technology?
Photosynthesis is the process in which plants combine carbon dioxide and water into sugar and oxygen, using light energy from the sun to initiate the reaction. It is one of the most important processes on the planet and is the reason why oxygen exists in the atmosphere. Billions of years ago, the Earth’s atmosphere had a very different chemical composition, with free oxygen almost being non-existent. Hundreds of millions of years of evolution saw the rise of organisms that could take advantage of the high concentrations of CO2 to produce sugars useful for later usage. After millions of years, the O2 released by this process began to replace CO2 in the air, and plant matter that was buried helped to remove CO2 from the air.
Interesting fact – plants remove CO2 from the atmosphere by acting as a carbon sink. Some of the sugars created by photosynthesis go into building plant matter such as trunks, bark, leaves, and stems. It is this physical matter that holds carbon, and plants do not only take in CO2, they also take in O2 to combine with sugar to release CO2.
Fuel cells are devices that typically generate electricity by combining two different reactants. The most common type of fuel cell currently in commercial use are HHO cells which combine hydrogen with oxygen to produce electricity (note, the combination is not ignition based). It is worth noting that many get the term fuel cell confused with electrolysis devices that produce hydrogen and oxygen when a current is passed through an electrolytic fluid (such as saltwater).
Formic acid fuel cells are similar to HHO fuel cells except that they work with formic acid instead of hydrogen and oxygen. Such fuel cells combine formic acid with oxygen to produce carbon dioxide and water, and in the process, produce electricity. One significant advantage of formic acid over hydrogen is that it can easily be stored as a liquid, and thus does not require pressurised containers. While formic acid is flammable, hydrogen has an explosive tendency which can cause complications when incorporated into automotive applications. Therefore formic acid would be a better alternative to hydrogen. However, formic acid is an acid, and as such is mostly a danger when coming into contact with skin and eyes.
While formic acid fuel cells can produce electricity from easily stored formic acid, the by-product of the reaction is carbon dioxide, something that the environment already has too much of. Typical production methods for formic acid can produce harmful by-products, thus making a formic acid fuel cell environmentally unfriendly.
However, this may change thanks to a team of researchers from the University of Cambridge. They have developed an artificial photosynthesis device that uses sunlight, CO2, and water to produce formic acid similarly to photosynthesis. The first and most important factor to note in this development is that the device requires no electrical power, and is entirely self-standing (as any tree or plant would be). This means that the formic acid produced by the device is a result of sunlight and specialised catalysts that produce no additional CO2. If formic acid fuel cells use the formic acid produced by this device, the resulting generated energy is carbon neutral. As such, artificial photosynthesis could be the holy grail of renewables.
One of the biggest challenges faced by researchers is the difficulty in creating highly-efficient artificial photosynthesis processes that selectively produce only the needed by-products. Another issue faced by researchers in the past was the separation of gaseous fuels from by-products, which can be a costly process. The formic acid-producing devices developed by the University of Cambridge allow for liquid fuel to be created that is mostly free from unwanted by-products, and the cost of the device is much less than that of other artificial photosynthesis systems. The device utilises lanthanum and rhodium-doped SrTiO3 (SrTiO3:La,Rh) and molybdenum-doped BiVO4 (BiVO4:Mo) light absorbers modified by phosphonated Co(II) bis(terpyridine) and RuO2 catalysts onto a gold layer. The device's efficiency at converting solar to formate is around 0.08% and has a selectivity of 97%.
However, it should be worth noting that artificial photosynthesis machines are not magic, the source of energy to produce formic acid is sunlight, and the produced formic acid is chemical energy whose energy derives from sunlight. Thus, per square unit of area, when comparing artificial photosynthesis vs solar panels, solar power would produce significantly more energy than these faux-tosynthesis (if you will). But the energy from solar panels is in the form of electrical energy, and thus must be used immediately.