Photovoltaic Conversion of CO2 and Hydrogen to BioFuels

Split Water, Add Carbon Dioxide, Get Energy

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Alternative energy.   We all know that means non-petroleum based sources.  We’ve looked at solar panels, tidal power, algae, and wind.  But, those are not really our only choices.

Because we know that photosynthesis – the conversion of solar energy to chemical energy- is not just limited to algae.  And, now scientists in the Chemistry and Chemical Biology departments at Harvard- and Systems Biology unit at Harvard Medical School- may have developed a “bionic leaf”.

Drs. Chong Liu (postdoc), Daniel Nocera (co-primary researcher) and Brendan Colon (grad student) at Harvard, along with Drs. Marika Ziesack, and Pamela Silver (co-primary researcher) at Harvard Med published their findings in Science.  The title of their paper, “Water splitting-biosynthetic system with CO2 reduction efficiencies exceeding photosynthesis” informs us that this new development may be more efficient than that of the natural process.

Their research goal was to both store solar energy (not just convert it for current use) and find a process to employ atmospheric carbon dioxide in a positive way.

Photovoltaic Conversion of CO2 and Hydrogen to BioFuels

The microbe they chose to use, Ralstonia eutropha, is found in both soil and water.  Interestingly, it has utility in bioremediation, since it degrades chlorinated aromatic compounds. It can also use hydrogen as the sole energy source (as well as carbon dioxide and the chlorinated aromatics.)

The microbe population is placed in a jar with two electrodes and water.  Electric current is used to split the water, producing hydrogen, which is the “food source”, along with carbon dioxide, by the microbes.  These microbes then produce alcohol and plastics (ok, the precursors- the one Tony Sinskey of MIT first demonstrated).  The other trick the researchers employed?   The anodes and cathodes, instead of nickel-molybdenum-zinc composition, are based upon cobalt (it’s a cobalt-phosphorus alloy), which require lower voltages to effect hydrolysis and also don’t leach heavy metals that would  decay the activity of the microbes.  And, there are no reactive oxygen species produced that would kill the microbes (by destroying their DNA).

The above is Dr. Daniel Nocera explaining the process.

Dr. Pamela Silver has a longer (18 minute) discussion in this podcast.

One of the next steps for the researchers is to connect the jar to photovoltaic cells, so the power of the sun (and not an electric power plant) can be used to drive the water hydrolysis reaction and have the microbes produce biofuels and plastics.  (This is the process by which they can store energy for use when there is no incident solar energy.)  If the scaled-up biofuel concept works, the carbon dioxide reduction would reach 50%, removing some 180 grams (4 moles) of carbon dioxide per kw-h of electrical power.

In the past, these sorts of processes barely achieved 1% energy conversion efficiency.  The generally recognized threshold is 8% for a process like this to be useful in real life.  The Harvard system? With that photovoltaic cell, this process would be operating at a 10% efficiency- which exceeds that of natural photosynthetic systems!

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