One of the alternatives--in fact the only alternative, but one emphasized differently by different people--is hydrogen. In point of fact, our economy has always been based on burning hydrogen. Before the Industrial Revolution, we burned hydrogen in trees. This was carbon-neutral because the carbon released by burning was still in an active part of the carbon cycle. During the industrial revolution, new fuel sources derived from fossils were introduced. Now we were releasing carbon that had been sequestered from the carbon cycle, and the balance was slowly upset. However, the point remains: our energy source was and continues to be burning hydrogen. So why don’t we just cap the wells and start using hydrogen directly? Well, for one, hydrogen gas is a nasty thing to store and transport. Secondly, the most efficient way to use hydrogen is in a fuelcell, and they have a way to go before they are cheap enough to replace combustion.

Recent research published in PLoS One shows that imitating nature may solve the transportation and storage problems. The easiest way to solve the infrastructure problems associated with the direct use of hydrogen is to transport hydrogen as hydrocarbons, such as sugar, starch, gasoline, or ethanol. Then we require either a fuelcell that can use these products directly, or an efficient method for extracting the hydrogen from the hydrocarbon. Hydrogen production can be performed by anaerobic digestion, but the efficiency is very low and it has little prospect for improvement.

In principle, sugar and water can be broken down into hydrogen gas and carbon dioxide, producing a carbon-neutral energy source. At peak efficiency, this reaction forms a closed cycle with photosynthesis, and no excess carbon dioxide is released. Furthermore, the energy extraction has the potential to be better than all fossil fuels (as measured by energy per kg of fuel). The problem is getting that reaction to run efficiently. Here, we can turn to the natural world for help, as there are lots of examples in life where reactions that don’t want to happen are performed very efficiently and at low pressure and temperature. This is achieved through enzymes, which are essentially catalysts. Enzymes have a specific area that grabs hold of the reactants. The surrounding functional groups act to stretch and distort the bonds of the reactants, causing them to break and fall out of the enzyme.

Unfortunately, there are no enzymes that convert sugar directly to hydrogen gas, so the researchers looked for a sequence of steps--each of which performed by a naturally occurring enzyme--that would lead to hydrogen gas. They constructed a reaction pathway by combining multiple enzymes, taken from rabbit muscle, S. cerevisiae, spinach, E. coli, and P. furiosus. This soup of 13 enzymes produces hydrogen gas directly from sugar and water at 30¡C and atmospheric pressure.

To be sure, this research hasn’t got all the bits and pieces together to make this commercially viable. For one, the yield is still too low to be useful. Secondly, they will need to hit nearly one hundred percent of the theoretical hydrogen yield before it is actually carbon-neutral--at the moment nearly all the carbon is released but only half the hydrogen. As the researchers point out, this research is still in its infancy, and they have already doubled the efficiency of previous work. Furthermore, transporting and storing starch already meets the department of energy efficiency goals for hydrogen storage and transport. That leaves improving the reaction pathway efficiency and making a decent fuelcell.

PIN/ ARSTECHNICA.COM