What if we could use just the sun and water to generate energy, like plants do with their photosynthesis? Perhaps by splitting water molecules using sunlight-producing green hydrogen fuel and oxygen?
Sunlight and water are two of the most abundant natural resources on Earth. They could offer one of the most promising ways to carbon neutrality. However, attempts so far to actually do this cost-effectively have run into problems. First of all, only highly energetic photons can cut the bonds between the hydrogen and oxygen atoms of water, i.e., the short-wave photons of ultraviolet and visible light. The sun’s infrared photons — solar heat — that make up half of those that reach Earth aren’t energetic enough.
As reported in Science magazine, solar water splitters have tried to address the challenge of splitting water in two ways. The first and most efficient employs a photoelectrochemical cell. Somewhat like a battery, it has two electrodes submerged in a liquid electrolyte. One of these, a semiconductor, absorbs sunlight and uses the energy to generate electrical charges that are fed to catalysts on the surface of the electrodes, where they split water molecules, generating hydrogen gas at one electrode and oxygen gas at the other.
Photoelectrochemical cells can convert nearly 25% of the sunlight’s energy into hydrogen fuel. Unfortunately, the electrolytes required are corrosive, which limits their performance stability and environmental sustainability by gradually destroying the light-absorbing semiconductor.
A second strategy abandons the battery-like setup and simply immerses a light-absorbing semiconductor into water. The semiconductor absorbs sunlight and generates electrical charges fed to catalytic metals on the semiconductor’s surface, splitting water molecules.
The trouble is that the resulting hydrogen and oxygen are generated right next to each other where they can readily react with one another, reforming water. This has limited the efficiency of these photocatalytic water splitters to only 3% of the solar energy converted to hydrogen. Enlarging the semiconductor to conventional solar panel size could help by dispersing the catalyst more widely. However, water-splitting-capable semiconductors are far more expensive than solar PV panels, making this option too expensive.
Recently, as reported in the journal Nature, University of Michigan Ann Arbor chemist Zetian Mi and colleagues have developed a strategy to achieve a high solar to hydrogen efficiency of 9.2% , using pure water, concentrated solar light and an indium gallium nitride photocatalyst. The success of this strategy derives from simultaneously promoting hydrogen-oxygen production and inhibiting water recombination, by operating at an optimal reaction temperature of 70ºC , which is achieved by harvesting the otherwise wasted infrared component of solar radiation. This temperature-dependent strategy also leads to an solar to hydrogen efficiency of 7 % from widely available ordinary tap water and 6.2% from sea water in a large-scale photocatalytic water-splitting system energized by natural solar light.
Mi’s team achieved these results by placing a lens about the size of a typical house window above their photocatalytic equipment. The lens focuses the incident sunlight onto an area 99% smaller, allowing the team to reduce the size and cost of the water-splitting semiconductor. The electrical charges generated in the semiconductor by the more intense sunlight are passed to nano-sized metal catalysts spread out on top, which carry out the water-splitting reactions. The latest modifications to the Mi team’s device permit the use of not only the visible and ultraviolet photons to split water. The less energetic infrared photons produce and maintain the operating temperature.
The Mi team has developed a practical approach to produce hydrogen fuel efficiently from natural solar light and water. Overcoming the efficiency bottleneck of solar hydrogen production “is quite an accomplishment,” according to University of California Berkeley chemist Peidong Yang, whose team helped pioneer photocatalytic water splitting 20 years ago but who was not involved with the current work.
Todd Deutsch, a water-splitting expert at the National Renewable Energy Laboratory, adds that the solar to hydrogen efficiency is now within striking distance of the 10% target likely needed to make solar water splitting to produce hydrogen fuel commercially viable. Still, there are remaining challenges. For example, a commercial system would have to prevent potentially explosive mixing of hydrogen and oxygen gases, adding to the cost.
For the commercial market, engineers would likely need to create massive solar water-splitting farms to generate enough hydrogen to power vehicle fleets, industrial furnaces, and commercial fuel cells that could convert the hydrogen into electricity to be fed to the grid. That day remains distant, but photocatalytic water-splitting cells are relatively simple to design, which should make them fairly easy to mass produce.
Furthermore, being able to convert seawater cheaply into carbon-free fuel would truly be the ultimate green energy achievement.
(Paul Kando is a co-founder of the Midcoast Green Collaborative, which promotes environmental protection and economic development via energy conservation. For more information, go to midcoastgreencollaborative.org.)
