One of the most hotly pursued areas of green energy technology is the search for an economical and practical method of splitting water into oxygen and hydrogen. The main target product, hydrogen, is a clean and energy-rich fuel that could substitute for fossil fuels in many contexts. Water is an obvious source of hydrogen, and it may be possible to produce hydrogen using light energy in a renewable and sustainable fashion. In today's issue of Nature Materials, a team of German, Chinese, and Japanese scientists, led by Xinchen Wang, got one step closer to a fully sustainable method for splitting water.

There is a huge volume of scientific publications on the topic, and it almost seems as if there are endless ways to accomplish the task of splitting water. While quite a few research groups have reported successes, most of them require harsh chemical conditions, expensive metals, and other impracticalities that make commercialization a headache. Wang and his colleagues note that just about everyone's proposed methods require some sort of precious metal—even nature employs metal-based complexes to photocatalyze the decomposition of water.



Polymeric carbon nitride.

Removing the need for precious metals could help keep hydrogen production economical and more environmentally friendly. Previous attempts at creating metal-free systems resulted in modest performance from synthetic polymer semiconductors that only worked with ultraviolet light.

Wang's group synthesized carbon nitride polymers that operate using visible light. Carbon nitrides are some of the oldest known synthetic polymers, as they date back to 1834. They are inexpensive to make and are quite stable in water. These polymers can be manipulated into various shapes and modified in ways that give them different properties.

By adjusting the way the polymers are formed, Wang's research group fine-tuned the electronic and optical properties so that they absorbed light in the visible range, gaining enough energy to catalyze the energetically unfavorable reaction of splitting water. The carbon nitride polymers can operate for days without noticeable degradation. Under optimal conditions, 7.6µmol of the polymer units can generate 4µmol of H 2 every hour.

Although this is an important and significant step forward, there are two main points of concern that must be addressed through further engineering. First, the metal-free method doesn't provide a perfect catalytic cycle, so the authors had to use an electron donor, such as triethanolamine (ethanol, methanol, and EDTA would also work), to complete the reaction. The carbon nitride polymers lack active sites capable of generating O 2 , but thermodynamic calculations show that O 2 evolution should be possible. Thus, some sort of chemical modification will be necessary to improve the catalytic sites so that they handle oxygen and don't need to use outside help to complete a reaction cycle.

The second issue is that the polymer batches vary greatly in performance. Wang's team observed reaction rates between 0.1µmol to 4µmol of H 2 generation per hour, depending on the batch. They need to have better control over the reaction conditions in order to have consistent catalytic surfaces. They have already started to work on overcoming these two deficiencies.

Even if polymeric carbon nitride doesn't end up being commercialized, Wang and his colleagues have opened up a new pathway in the search for ways of producing hydrogen from water. They have achieved the difficult task of generating H 2 using visible light with cheap materials and without the use of any metals.

Nature Materials, 2008. DOI: 10.1038/NMAT2317