A sunny solution to car pollution

07 September 2018

Combining the energy of the sun with the reaction-accelerating power of an enzyme, scientists have successfully split water into hydrogen and oxygen with 85% efficiency. This breakthrough, they say, could help us make more environmentally fuels in the future...

Car exhausts release more than two billion tonnes of carbon dioxide per year. This is a major contributor to global warming and climate change, resulting in dangerous levels of pollution in our cities. Cleaner fuels like hydrogen, on the other hand, which can be converted to electricity leaving behind only water, would be an ideal alternative. The sticking point, however, is that hydrogen production by the methods available at the moment is inefficient and requires expensive metals.

In nature though, the process of photosynthesis achieves much the same outcome, using solar energy to link carbon dioxide and water to produce sugars, but without the high cost. It does this by using protein catalysts called enzymes to speed up the process. And now a team led by Cambridge University's Erwin Reisner have found a way to harness this natural system in the lab and make it produce hydrogen rather than sugars.

In their setup, two different enzymes are attached to separate conductive supports and immersed in water. On one of the supports the common photosynthesis enzyme component "photosystem II" is combined with a titanium dioxide semiconductor. When this material is activated by sunlight, the enzyme drives the formation of oxygen gas, yielding electrons, which are tapped off by the circuit. These are sent to the other support, which is coated with a second enzyme called hydrogenase. This enzyme force-feeds the negatively-charged electrons arriving through the circuit to positively-charged hydrogen atoms in the water. The result is molecular hydrogen gas.

This hydrogen can be used to generate clean electricity in a device called a "fuel cell" that could, for example, be used to power a car. The only byproduct is water, which can be recycled by the cell to produce more hydrogen.

Making hydrogen from water with sunlight is called a solar fuel. The overall concept is to collect solar fuels on sunny days and store them for use later, either over night or when it's cloudy. The problem with hydrogen-powered technologies is that hydrogen is explosive, and squeezing sufficient amounts of the gas into small, high-pressure tanks uses a lot of energy.

At the moment, we also lack the infrastructure, such as hydrogen refueling stations, that would be required for the world to embrace hydrogen technologies on an industrial scale. Nevertheless, a small fleet of hydrogen-powered buses are currently being road-tested in central London to assess the feasibility of strategies like this.

If industry can be pursuaded to come on board, we could be looking at a much cleaner street-scene in future...

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