Proton Drive

At the American Chemical Society, meeting in Washington[1] researchers presented solar cells mimicking photosynthesis, generating electricity, protons and hydrogen. In nature carbon is the proton partner in reducing energy from the sun into plant hydrocarbons. Remarkably, this water splitting process cuts out the carbon cycle; it leaves out the middleman.

The team, including Charles Dismukes, from Rutgers University, Leone Spiccia from Monash University in Australia and Gerhard Swingers from the University of Wollongong in Australia created what amounts to synthetic photosynthesis. Most of us recognize chlorophyll as the plant catalyst for capturing sunlight. The natural process involves calcium-manganese-oxide and complex organic molecules.

The synthetic structure, on the other hand, resembles a fuel cell containing the same manganese-oxide, while adding ruthenium dye and nano particles of titanium dioxide to capture the photons. Sunlight striking the cell generates both electricity and protons that migrate to a plastic membrane. The cathode then supplies electrons converting the protons into H2, Hydrogen.

To its credit, this report adds to the science by open presentation rather than hiding behind secrete patent applications. Furthermore, the device meets the goal of capturing solar energy as hydrogen. You can store hydrogen and transport it, unlike a traditional solar cell that produces electrons only while the sun is shining.

The problem with H2 is that it is a gas, which liquefies only under very low temperature or very high pressure. Both high enough pressure and cold enough temperature is expensive and requires further energy. Nothing has as much energy as hydrogen by weight, but the molecule of two atoms of hydrogen is small and light and they are thinly distributed as a gas, so hydrogen has even less energy per cubic foot than propane. And, there still remains the problem of practical storage and transportation.

Let us back up a little bit with this solar cell that generates hydrogen. Before the protons cross the membrane and receive electrons, the protons accumulate along the membrane, elemental hydrogen with a positive charge. This is precisely what human intermediary metabolism does with protons. We get our protons from all kinds of ingested hydrocarbons and we strip the protons, storing them in the mitochondria as ATP, adenosine triphosphate. From there on metabolism is clean with no CO2 and no waste. An enormous number of protons are stored in a microscopic space. If we stored these protons as H2, it would require a gas tank as big as a house to hold them.

I think the focus is in the wrong place but not by much. If we focus on the proton, the mitochondrial storage device and the final oxidative process of the proton, we might pack large amounts of energy into a very small space without much weight.

Looking more closely at the mitochondria, we face a completely new challenge of mimicking nature, this time on the oxidative side. From Wikipedia: “ATP is made from adenosine diphosphate (ADP) or adenosine monophosphate (AMP) and its use in metabolism converts it back into these precursors. ATP is therefore continuously recycled in organisms, with the human body turning over its own weight in ATP each day.[3]”

“Cytochrome oxidase is the last enzyme in the respiratory electron transport chain of mitochondria in the mitochondrial membrane. It receives an electron from each of four cytochrome c molecules, and transfers them to one oxygen molecule, converting molecular oxygen to two molecules of water. In the process, it binds four protons from the inner aqueous phase to make water, and in addition translocates four protons across the membrane, helping to establish a transmembrane difference of proton electrochemical potential that the ATP synthase then uses to synthesize ATP.” The energy that is stored in the mitochondria as ATP comes from the proton. The proton swings its way from hand-to-hand like a single at a contra dance seeking to go outside with oxygen its favored partner.

Proton power: When I learned my biochemistry, the professor[2] placed the emphasis on the proton transport chain rather than the electron chain. This proton view was a departure from conventional thinking that emphasizes the electron as the purveyor of energy through these complex metabolic steps.

Find a way to mimic the proton in the mitochondria and you have the solution to both the transport and storage of hydrogen power, thus the Proton Drive.

[1] Science 325, 4September2009, p1200 Robert F Service New Tricks for Splitting Water
[2] Halvor N Christensen PhD, U of Michigan almost 50 years ago.