Saturday, November 14, 2009

Using photosynthesis to power hydrogen production

Using photosynthesis to power hydrogen production

The processes we use to obtain fuel, from pumping fossil fuels up from beneath the ocean to harvesting crops to turn into ethanol, create many environmental and practical concerns. These types of fuel work fine with the current generation of cars, but hydrogen has sometimes been touted as the fuel of the future. A publication in Nature Nanotechnology describes how researchers have found a way to use the photosynthetic machinery of a bacteria to produce the hydrogen equivalent of up to 79 gallons of gas per-acre, per-day. Their technique involved capturing the electrons produced during photosynthesis and binding them to some strategically placed protons.

The production of fuel has accelerated lately, from waiting millions of years for fossil fuels to waiting a few days or weeks for biomass-derived fuels such as ethanol. However, biomass fuels still present some difficulties: the fuel produced relative to the land area required is pretty small (the equivalent of a little more than a gallon of gas per acre), the conversion to ethanol requires a distilling period, and all the materials for making the fuel must be harvested, handled, and transported, all of which requires a significant energy expenditure.

The problem can be boiled down to one relationship: the more directly solar energy can be used, the more efficient the fuel production will be. In the case of ethanol, plants process the solar energy through photosynthesis, but we lose a good deal of that when we process the plants. Researchers have been trying to catch the energy earlier in the photosynthetic process, hoping to divert it into a fuel with a high energy density.

In order to manipulate photosynthesis, the researchers worked with a thermophilic cynobacterium, T. elongatus. Scientists isolated its photosystem I (PSI), which is one of two reaction centers that the bacteria use to conduct oxygenic photosynthesis. At the most basic level, the PSI helps to transfer electrons from water to systems that use them to produce sugar and oxygen.

The researchers found that they could intercept these electrons if they coupled nanoclusters of platinum or convalently linked hydrogenase to the acceptor end of PSI complexes. The electrons would then bond with extra protons, or H+, that were adsorbed onto the platinum, resulting in hydrogen (the process is outlined in the picture above).

The experiments were also performed on the PSI obtained from both spinach and another variety of bacteria, the mesophilic cynobacterium Synechocystis. They found the T. elongatus version remained stable at higher temperatures than the one from Synechocystis; its chlorophyll became less efficient at temperatures above 130°F. The T. elongatus version remained sufficiently functional up to 194°F, and in fact worked better at temperatures above 130°F.

The spinach's PSI's ability to absorb light reached an early plateau; researchers found that the productivity of the PSI from T. elongatus could continue to scale with light four times brighter than the PSI of the spinach. For this method of hydrogen harvesting, the T. elongatus proved to be the best choice of the three.

Given the functionality of the PSI of T. elongatus, the researchers calculated that a one-acre solar collector that is 10 centimeters deep and operated at a temperature of at least 130°F could produce the energy equivalent of 79 gallons of gas per day. The collector could operate continuously for about three months before the PSIs would be exhausted and need to be replaced, but during that time, it would pump out the hydrogen equivalent of over 7000 gallons of gas. Unlike the PSI, the platinum catalyst could be reused for many more cycles.

The scientists point out that this method has many advantages over the production of biofuels. Notably, it doesn't involve a single-use crop that has to be planted, harvested, fermented, and distilled to obtain more fuel.

However, there's still the challenge of delivery. Since the proteins work best in the warmest parts of the planet, it would be hard for most people to produce hydrogen in their backyard. Nor does the paper consider the cost of isolating the protein or the use of precious metals in hydrogen production. There is also the small issue of hardly anyone currently owning anything that runs on hydrogen, aside from the occasional concept car. Still, the 79-gallon yield per day of the collectors is impressive, and further modification of the basic approach may make the hydrogen economy look relatively practical.

Nature Nanotechnology, 2009. DOI: 10.1038/nnano.2009.315

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