For years, proponents of the hydrogen economy have argued that hydrogen will replace traditional hydrocarbon fuels for transportation purposes. But, so far, a lack of new, inexpensive methods for hydrogen production and storage has impeded this goal. Over the last several years, an MIT professor has been pushing cobalt catalysts as a cheap replacement for the expensive metals typically used to split water. A paper in this week'sProceedings of the National Academies of Science describes the latest progress here: integrating the cobalt catalyst with a silicon solar cell to create a device that uses the sun to split water.
Hydrogen is a desirable fuel, because when it is burned or otherwise consumed (as in a fuel cell), it only produces water, although combustion results in small amounts of nitrogen oxides as by-products. However, unlike traditional liquid or gas fuels, hydrogen doesn't exist in its molecular form on Earth, so it must be produced from other sources—it is an energy carrier, rather than an energy source.
The primary industrial method for hydrogen production is steam reforming of hydrocarbons such as oil, coal, and natural gas, where high-temperature steam reacts with the fuel to produce hydrogen and carbon monoxide. But this method is unattractive for a few reasons: the resulting hydrogen is more expensive than the starting fuel, carbon dioxide is still produced (although easier to capture and store at a central location than on a vehicle), and it relies on fossil fuel sources. Due to these limitations, researchers are developing clean and renewable methods of hydrogen production, focusing on solar-based approaches.
Photoelectrochemical water splitting, also known as artificial photosynthesis, essentially combines a photovoltaic solar cell with electrolysis, the process of using electrical current to break water into oxygen and hydrogen. The most efficient devices of this nature, tandem GaInP2/GaAs cells, use platinum catalysts to significantly reduce the energy required to split the water. They can achieve a solar-to-hydrogen conversion efficiency of 16.5 percent. However, both the cell and the catalyst are extremely expensive, and require a high-pH (basic) electrolyte solution to operate, which degrades the materials over time.
Silicon, another semiconductor traditionally employed in photovoltaics, has also been used in less-efficient photoelectrochemical cells (2.5-8 percent so far), but they can be significantly less expensive than the gallium-based cells due to the abundance of silicon. The Si-based devices developed up to this point use the semiconductor surface as a catalyst, but this setup also requires an extremely basic solution—so these suffer the same stability problems over time. To this end, the authors of the current paper integrated a silicon-based photoelectrochemical cell with a cobalt-phosphate (Co-Pi) catalyst that can operate in a neutral pH solution. In addition to avoiding the degrading properties of a high-pH environment, the cobalt-based catalyst is inexpensive compared to a traditional platinum catalyst.
The Co-Pi catalyst acts like—and is structurally similar to—the oxygen-evolving (or water-splitting) complex (OEC), the enzyme used in photosynthesis to break down water. Like the OEC, it also exhibits high activity at room temperature in both seawater and fresh water, and operates under neutral pH conditions. This means that, unlike the previous designs, this device doesn't run into any stability problems over time. When combined with an np-Si junction, the catalyst can increase the efficiency of photoelectrochemical water splitting. We'vecovered this catalyst before being used with zinc oxide, but this is the first demonstration with silicon.
This device in its current configuration looks like a sandwich: a 10 μm photoresist, a 140 nm patterned metal contact (Ti/Pd/Ag), n-type Si, p-type Si, a 1.5 nm SiO2 interface, a 50 nm indium tin oxide (ITO) protective layer, and the Co-Pi catalyst film. The photoresist on the n-side protects the metal contacts and silicon from water, while the ITO layer on the p-side protects the silicon from water that penetrates the catalyst. The sunlight or artificial illumination hits the n-side, passing through the photoresist.
The primary result of this paper (other than demonstration of the new catalyst integrated with a silicon cell) is that most of the generated potential was used towards the water splitting. As a proof-of-concept, this device is promising, but significant effort will still be needed to develop this concept into a functioning photoelectrochemical cell.
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