There are many reasons that I love working in materials chemistry, and particularly in artificial photosynthesis - splitting water using sunlight to generate hydrogen and oxygen gases. You’ve heard me ramble on about this, my most favourite example of nature-inspired nanotechnology in previous articles to produce cheap, clean and sustainable solar fuel in the form of hydrogen from sunlight, water and some other materials, but while these are all great reasons to love it, what I love the most is how research groups working on this are obliterating the challenges posed by this technology at such a ridiculously fast pace. This is what science and engineering is all about!
Today, scientists at the Technion-Israel Institute of Technology in Israel published a paper in Nano Letters that shattered yet another of these blockades in the path of commercially available water splitting systems. In order to understand what they have done, we need to look at what is going on in this system in slightly greater detail.
An illustration of the nanoparticle photocatalyst, with arrows showing the photosynthesis cycle... [+]
Artificial photosynthesis tries to mimic part of the process through which plants capture sunlight energy and use it to convert water and carbon dioxide into a fuel, glucose sugar. Photosynthesis is divided into two photosystems – Photosystem I, and Photosystem II. It is in photosystem that we find inspiration for this particular area of work.
Liquid water, with chemical formula H2O, cannot split itself into hydrogen gas, H2, and oxygen gas, O2, in the presence of sunlight – thankfully for us, really, as otherwise any glass of water left out in the daylight would soon have a potentially explosive mix of gases evaporating off it. In order for this reaction to take place, we need a middleman that can capture photons of light, discrete packets of certain energy. In a plant, the green dye chlorophyll would capture this solar energy.
If the photon of light is of an appropriate energy, it can excite an electron in our middleman material, which can be conducted away and ultimately reacts with water to reduce it to hydrogen. As we know however, electrons and protons like to hang out together, like two perfectly marked football teams, one with a negative charge (the electron) and one positive (the proton). When the electron is excited by light (also known as photoexcitation), rather than leaving behind a proton, it leaves a vacancy in the valence band of the water that is effectively negatively charged.
We know that opposite charges attract, and so these two charged species would, if held within the same area, attempt to recombine, rather than travel too far into the material. Each time these charged species recombine however, that is one less electron and hole pair being used to split water. This desire to recombine rather than go on to reduce and oxidise water significantly reduces the efficiency of this set up.
In this research, Assistant Professor Lilac Amirav explains how, when looking at one of the half-reactions taking place, the reduction of water to hydrogen using the photoelectron, they were able to obtain 100% conversion from light energy to hydrogen generation! The rate of recombination is reduced and so the efficiency rockets. Mostly thanks to the micro structure, Professor Amirav and her team proved that 100% conversion from light to solar fuel is possible, simply by engineering the catalyst and cocatalyst to exist in the form of nanorods.
While a lot of work still needs to be done on the oxidation half of the equation, this perfect conversion rate for this half of the reaction has smashed previous records, and could well lead to a new method of harnessing solar to power. Unfortunately, this method of water splitting can only be carried out at incredibly high pH (incredibly alkaline) so the next challenge is to reduce this to the kind of pH that would be easy to run, while not corroding the nanorods or disrupting the micro- and nano-structure. With the increased surface area for interface with water and for maximum illumination created by the nanorods, this new discovery could mean that scientist are only a few hurdles away from making this research a reality, and all inspired by plants!
