After Gutenberg

“Chemists at the University of California, San Diego, have demonstrated a prototype device that can capture energy from the sun, convert it to electrical energy and “split” carbon dioxide into carbon monoxide (CO) and oxygen. The device is not yet optimized — it still needs external energy for the process to work. However, the researchers hope that their results will encourage interest in the device.”

Technology Review puts forth the case that pyrolysis is the most climate-friendly way to consume biomass. After converting the biomass to carbon-rich char in an oxygen-starved environment and extracting methane, hydrogen, and other byproducts for combustion, the remainder goes toward soil augmentation.

This report reminded me of an Engineer Poet scheme. A component of his Switchgrass to Syngas proposal was carbon capture and sequestration, specifically with algae. However, as part of research supported by the U.S. Department of Energy, a group of scientists at the University of California, San Diego have constructed a prototype of a “splitter” that converts solar energy to electrical energy and then separates an oxygen from carbon dioxide. According to a Treehugger story, the lead researchers, Kubiak and Sathrum, use a semiconductor and two thin layers of catalysts to split the carbon dioxide.

[The device] splits carbon dioxide to generate carbon monoxide and oxygen in a three-step process. The first step is the capture of solar energy photons by the semiconductor. The second step is the conversion of optical energy into electrical energy by the semiconductor. The third step is the deployment of electrical energy to the catalysts. The catalysts convert carbon dioxide to carbon monoxide on one side of the device and to oxygen on the other side.

Since in the EP scheme carbon monoxide is a main product of the first step, pyrolysis of biomass, it would seem possible to loop the carbon monoxide that is split from the carbon dioxide in the exhaust back into the fuel cell input. However, from what I could discern, such a loop would work better with MCFC (Molten Carbonate Fuel Cell) rather with a SOFC (Solid Oxide Fuel Cell) because, according to DoE info, molten carbonate fuel cells can “use carbon oxides as fuel” and “are more resistant to impurities than other fuel cell types.”

The AG-modified EP scheme requires a MCFC since they can use carbon monoxide as a fuel. Molten carbonate fuel cells can reach efficiencies approaching 60 percent. When waste heat is captured and used, overall fuel efficiencies could be as high as 85 percent. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance.

This makes MCFCs “more attractive for fueling with gases made from coal… Scientists believe that they could even be capable of internal reforming of coal, assuming they can be made resistant to impurities such as sulfur and particulates.”

Although carbon monoxide is poisonous, it is highly sought after. Millions of pounds of it are used each year to manufacture chemicals including detergents and plastics. It can also be converted into liquid fuel.

“The technology to convert carbon monoxide into liquid fuel has been around a long time,” said Kubiak. “It was invented in Germany in the 1920s. The U.S. was very interested in the technology during the 1970s energy crisis, but when the energy crisis ended people lost interest. Now things have come full circle because rising fuel prices make it economically competitive to convert CO into fuel.”

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Because electrons are passed around in these reactions, a special type of catalyst that can convert electrical energy to chemical energy is required Researchers in Kubiak’s laboratory have created a large molecule with three nickel atoms at its heart that has proven to be an effective catalyst for this process.

Choosing the right semiconductor is also critical to making carbon dioxide splitting practical say the researchers. Semiconductors have bands of energy to which electrons are confined. Sunlight causes the electrons to leap from one band to the next creating an electrical energy potential The energy difference between the bands — the band gap — determines how much solar energy will be absorbed and how much electrical energy is generated.

Kubiak and Sathrum initially used a silicon semiconductor to test the merits of their device because silicon is well-studied. However, silicon absorbs in the infrared range and the researchers say it is “too wimpy” to supply enough energy. The conversion of sunlight by silicon supplied about half of the energy needed to split carbon dioxide, and the reaction worked if the researchers supplied the other half of the energy needed.

They are now building the device using a gallium-phosphide semiconductor. It has twice the band gap of silicon and absorbs more energetic visible light. Therefore, they predict that it will absorb the optimal amount of energy from the sun to drive the catalytic splitting of carbon dioxide.

“This project brings together many scientific puzzle pieces,” said Sathrum. “Quite a bit of work has been done on each piece, but it takes more science to mesh them all together. Bringing all the pieces together is the part of the problem we are focused on.” Tech News Watch