Every single leaf holds a clue to a cleaner, greener future. And we have been ignoring this fact.
The Sun is the primary source of energy for nearly all life on Earth. The process which converts the Sun’s light into chemical energy is known as Photosyntheis. Plants, algae and some type of bacteria are capable of photosynthesis, with the help of “photo systems”.
The photosynthesis system, in which the energy conversion takes places in biological systems, has two parts — Photosystem-I (PS-I) and Photosystem-II (PS-II).
In PS-II, water gets converted to hydrogen and oxygen using Sunlight. Apart from the energetic conversion, photosynthetic water splitting (or oxygen evolution) is one of the most important reactions on the planet, since it is the source of nearly all the atmosphere’s oxygen. If mankind manages to mimic this natural process, sunlight can be used directly for making fuel out of water. Imagine the possibilities.
The overall process of photosynthetic water splitting consists of two half-reactions — oxidation of water to molecular oxygen. These require the extraction of protons and the electrons from water and the reduction of protons by electrons or the evolution of hydrogen.
The protons and electrons extracted in the PS-II are delivered to PS-I for the reduction of CO2 to carbohydrates. This reduction of protons by electrons is thermodynamically favorable and is less energy-demanding. On the other hand, the molecular mechanism of water oxidation is still not fully elucidated. But we know many details about this half-reaction. The oxidation of water to O2 is a multi-step process, demanding loss of 4e- and 4H+ from two water molecules, accompanied by the formation of the O-O bond, occurring thermodynamically at 1.23V at pH=1. For this step to take place, an over potential is present. To minimise this over potential and achieve the formation of oxygen at a good rate, a water oxidation catalyst (WOC) is required. Ever since it was reported that an inorganic Mn4O5Ca cluster in PS-II is responsible for the oxidation of water into molecular oxygen, there have been a number of WOC mimics.
The challenges in water oxidation are complex. An ideal WOC should have a variable range of oxidation state, it must oxidise water at a low over potential and most importantly, should have a high turnover number. It should also be economically viable.
It will not be easy to find a single material that has all these properties. This area of research has stayed in the limelight for decades but some of it remains a mystery. The next few years could uncover significant findings because yes, if plants are able to do it, we should be too. After all, this fantastic process could save us all.
The author is a scientist with the department of science and technology
