Hydrogen from water and sunlight; is this the answer to our energy needs?
Research carried out by the UCL Solar Energy and Advanced Materials Group (SEAM) aims to produce hydrogen by photocatalytic water splitting.
By Flavia Bernabo
Research in the field of green technology is driven by society’s need for energy. Fossil fuels currently provide over 80% of it, but these are finite, non-renewable resources that will run out. A renewable energy source is needed. An obvious provider is the sun; it has been producing energy for the last few billion years and is scheduled to continue doing so. Although we already get significant useful energy from it via photovoltaic cells and thermal installations, these routes suffer the problems of availability; they do not always provide the energy when and where we want it. A solution is to use the sun’s energy to provide us with an intermediate energy source which can be collected, stored, transported and used at will, and preferably without producing harmful by-products. It would be a satisfying cycle if the sun, powered by the fusion of hydrogen, provided the means of separating out the same hydrogen for use by man! This is where hydrogen technology comes in.
Dan Kong, a PhD student in the SEAM department, is currently conducting research on hydrogen technology. Kong tells ENGins, “the wide availability of hydrogen and its high energy density (142 MJ/kg) mean it has the potential to store all the energy needed to sustain life”. Energy from hydrogen can be released by combustion, for which the by-product is water, or by fuel cell, a device that converts chemical energy from a fuel into electricity. The infrastructure exists to store and transport such energy because it is already produced by a number of means, including thermo-chemical reduction of fossil fuels and biomass feedstocks. Kong is focusing on “the clean and renewable process of using sunlight to split water into its components of hydrogen and oxygen, termed photocatalytic water splitting” (because the process requires the use of a catalyst). Photocatalytic water splitting is an artificial photosynthesis process, which converts water to hydrogen gas in the presence of a photocatalyst and solar energy. It is akin to the natural process of photosynthesis, in which atmospheric carbon dioxide is split into carbon and oxygen using sunlight and a biological catalyst.
The catalyst is the critical component in the process. In response to excitation by the photons of sunlight, the catalyst releases electrons and holes in the conduction band and valence band, respectively. The excited electrons and holes become separated from each and migrate to the catalyst’s surface where, with their respective reductive and oxidative properties, they react with water molecules to split off hydrogen or oxygen. “This may sound easy, but challenging technical issues remain”, Kong tells ENGINS. “Currently reported catalysts suffer from a low quantum efficiency (the ratio of incident photons that are converted to electrons) in the visible light range, which constitutes a major part of the incoming solar energy. Many photocatalysts are made of rare and expensive materials, catalysts can show poor stability and degradation with use, and sacrificial reagents can be required to facilitate the reaction process”, Kong explains. She is attempting to circumvent such problems, however, with a different and less-explored base catalyst material: carbon nitride. She describes the latter as “low-cost and earth-abundant so that any degraded material is inexpensive to replace”. She proceeds to explain that “its valence and conduction band positions allow it to work effectively in the visible light range, with quantum efficiencies of 16% at a wavelength of 420nm and 6.3% at 580nm”. It is also capable of generating both hydrogen and oxygen in absence of additional catalytic metals, and is easy to fabricate.
This research is very much still a work in progress. The secret will be (and is!) in the synthesis, modification and fabrication of the catalyst. However, Kong is prepared to disclose a measure of efficiency—that is, the energy produced by combustion of a given mass of hydrogen as a percentage of the light energy needed to produce it—which is unusual in this field. “Currently this is in the order of 2%”, Kong tells ENGins. This may seem low, but it has to be remembered that the input energy (from the sun) is free and continuous, the by-products of energy release are harmless and all efforts are directed at increasing the number. Watch this space!