Splitting Water and Controlling Gases

Breaking apart molecules of water is an essential part of the photosynthetic process. It provides a source of charged particles which drive the photosynthetic machinery to create energy and structure. A central feature of our artificial plant will be to show how energy from the sun can be used to split water, analogous to the process that occurs in plants algae and bacteria.

Although our system is far less complex than nature’s, at the molecular scale the fundamental process is effectively the same. By applying energy, the forces which hold the hydrogen and oxygen atoms together can be disrupted, splitting H2O molecules into constituent parts; hydrogen ions (H+), gaseous oxygen, and electrons (e-).

Water splitting

Oxygen has no further use in the photosynthetic process and is released as a waste product. The hydrogen ions and electrons are however of vital importance. These tiny charged particles join complex enzyme facilitated pathways, which eventually lead to the generation of energy storage molecules.

In photosynthetic organisms the energy used for splitting water is obtained from photons radiated from the sun. These photons are absorbed by chlorophyll pigments (molecules that absorb light of a particular colour), which pass the photon’s energy to a reaction centre where water is split. This is a process termed photolysis; photo- light, lysis -separation.

A parallel can be drawn between nature and the system we intend to incorporate into our plant; photons will be captured by leaves and the energy transported to a reaction centre where water is split. The key difference (other than the disparity in complexity!) is that the action of water splitting will be through electrolysis rather than photolysis. The light energy captured by the photovoltaic panels on our artificial leaves will generate an electrical current. The energy provided by the current will then power an electrolyser to split water into its constituents.

olysis

In our artificial plant there are no special enzymes to capture the electrons produced from electrolysis. Instead they combine with hydrogen ions (opposites attract) to create hydrogen gas. The inability to use the electrons and protons from electrolysis to create fuels, as nature does, will allow us to highlight the limits of current research into artificial photosynthesis.

One of our goals is to highlight the importance of photosynthesis to our lives. An aspect of this is the production of oxygen by photosynthetic organisms and our need for it to survive. It is essential that our plant can demonstrate the production of oxygen. However, designing a demonstration to show the importance of oxygen for life is a challenging task without endangering life. Some leftfield thinking will be needed here!

A big challenge that we face is being able to control the gases in our plant. We have ambitious aims for our design, but are well aware of the safety considerations required to store and manipulate potentially dangerous gases. On their own hydrogen and oxygen are flammable gases. Combine enough of them together, introduce a spark, and you can get sufficient energy to propel a rocket into space! (1:20)

Once we have obtained the key pieces of equipment for our artificially photosynthesising plant, the following weeks should see us creating a working prototype. After this we can take our system to the design team we’re collaborating with, who will hopefully create us a beautiful looking plant to take out into the world.

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