When it comes to the available sources of renewable energy, most of the focus is on the Sun, since all the other sources, like wind and biomass, are ultimately derived from it. But solar presents its own challenges, since it's most easily converted into heat or electricity, and we can't store either of these at a high enough density for uses like transportation. This explains why there's a lot of effort going into things like biofuels and using electricity to produce hydrogen. Each additional step, however, involves a potential inefficiency.

These problems are what makes a system described in today's issue of Science very appealing. The authors demonstrate a device that is capable of taking solar energy and using it directly to split water, releasing oxygen and hydrogen. It can also perform a similar conversion on carbon dioxide, converting it to carbon monoxide and oxygen. Better yet, it doesn't need an exotic catalyst; instead, its catalyst is based on cerium, an element that's about as abundant as copper, and is stable for hundreds of cycles.

The structural part of the device is remarkably simple. Most of it acts simply as a focusing lens, which directs sunlight through a transparent quartz window and into a reaction chamber. That chamber is designed for internal reflection, and is efficient enough that most of the photons get captured. "The selected dimensions ensure multiple internal reflections and efficient capture of incoming solar energy; the apparent absorptivity exceeds 0.94, approaching the ideal blackbody limit," the authors claim.

Once absorbed, those photons are converted to heat. Temperatures rise at a rate of 140°C a minute until they clear over 1250°C, before stabilizing between 1400 and 1600°C. Those temperatures are hot enough to cause a chemical change in the catalyst, a cylinder of porous cerium dioxide.

At the high temperatures present in this phase of the reaction cycle, the cerium dioxide loses one of its two oxygens. By flowing some inert gas over the porous cylinder, the authors were able to detect a steady flow of oxygen off the device, which lasted for more than an hour before falling off (the peak rate was 34ml of oxygen per minute from the 325mg sample of cerium dioxide).

Once oxygen production tailed off, the device could be dropped to a lower temperature (900°C) and a reactant pumped into the chamber. When water vapor was used, the catalyst would strip out its oxygen to reform cerium dioxide. This releases hydrogen quickly and efficiently; this portion of the reaction was typically complete in less than 10 minutes. Alternately, carbon dioxide could be pumped in, in which case carbon monoxide was produced.

The devices produced by the authors would tend to have an erratic drop in performance over the first hundred cycles, which they found was associated with a rearrangement of the cerium oxide structure through the repeated heatings. Once the material formed somewhat larger particles, performance stabilized and remained stable out to 400 cycles.

The authors use a complex formula to calculate the efficiency of the device, one that accounts for things like the solar input, the flow rate of the inert gas, and the energy required to purify the outputs. According to their calculations, the results are pretty impressive. "The solar-to-fuel energy conversion efficiency obtained in this work for CO 2 dissociation is about two orders of magnitude greater than that observed with state-of-the-art photocatalytic approaches," they state. "The gravimetric hydrogen production rate exceeds that of other solar-driven thermochemical processes by more than an order of magnitude."

There are some drawbacks to this system, of course. A steady supply of inert gas is needed, and the water and carbon dioxide that are used as inputs have to be kept pure to keep other chemicals from building up on the porous material, and pure water is often a fairly rare commodity that requires significant energy to produce. But the system also produces significant amounts of waste heat that could be harvested and put to use (the primary inefficiency right now is heat loss).

The ability to switch the system between carbon monoxide and hydrogen production is also intriguing. We already use these two ingredients to produce methanol, which can be transported in bulk and used in fuel cells, and it may be possible to combine them into more complex hydrocarbons. It might also be possible to use this as a part of a carbon sequestration system.

In any case, the researchers involved specifically designed the hardware to be easy to manufacture in bulk and incorporate into a industrial-size facility, so it seems to be a serious attempt at getting something that could be tested in a real-world deployment.

Science, 2010. DOI: 10.1126/science.1197834 (About DOIs).