## JCAP researchers propose artificial photosynthetic system for high-yield production of ethanol

##### 09 November 2015

A team at the Joint Center for Artificial Photosynthesis (JCAP) at Lawrence Berkeley National Laboratory and UC Berkeley is proposing an artificial photosynthesis scheme for direct synthesis and separation to almost pure ethanol with minimum product crossover using saturated salt electrolytes.

In a paper in the RSC journal Energy & Environmental Science, Professor Alexis Bell and postdoc Meenesh Singh describe the novel design of an integrated artificial photosynthetic system that continuously produces >90 wt% pure ethanol using a polycrystalline copper cathode and an IrO2 anode at a current density of 0.85 mA cm-2. The annual production rate of > 90 wt% ethanol using such a photosynthesis system operating at 10 mA cm-2 (12% solar-to-fuel (STF) efficiency) can be 15.27 million gallons per year per square kilometer, corresponding to 7% of the industrial ethanol production capacity of California, they suggest.

Alcohols are particularly attractive products for artificial photosynthesis of liquid fuels because of their high energy density and market value per amount of energy input, they note. (Other work at JCAP has investigated the electrochemical reduction of CO2 to methanol.)

The major challenges in the photo/electrochemical synthesis of alcohols from sunlight, water and CO2 are low product selectivity, high membrane fuel-crossover losses, and the high cost of product separation from the electrolyte.

In the proposed system, the ethanol produced in the saturated salt electrolytes can be readily phase separated into a microemulsion, which can be collected as pure products in a liquid-liquid extractor.

JCAP was established by the US Department of Energy in 2010 as an Energy Innovation Hub, with the goal of developing an integrated solar energy-to-chemical fuel conversion system and moving this system from the bench-top discovery phase to a scale where it can be commercialized. (Earlier post.)

The program’s first phase focused on solar H2 generation, which was completed in September 2015. In April 2015, the DOE refunded JCAP for another five years with $75 million. (Earlier post.) Led by the California Institute of Technology, JCAP has an integral partnership with the Lawrence Berkeley National Laboratory. Additionally, JCAP draws on the expertise and capabilities of key collaborators from the University of California campuses at Irvine (UCI) and San Diego (UCSD), and the SLAC National Accelerator Laboratory. Resources • Meenesh R. Singh and Alexis T. Bell (2015a) “Design of an Artificial Photosynthetic System for Production of Alcohols in High Concentration from CO2Energy & Environmental Science doi: 10.1039/C5EE02783G • Meenesh R. Singh, Ezra L. Clark, and Alexis T. Bell (2015b) “Thermodynamic and achievable efficiencies for solar-driven electrochemical reduction of carbon dioxide to transportation fuels” PNAS doi: 10.1073/pnas.1519212112 ### Comments At 15.27 million gallons per year per square kilometer, or 15.27 gallons per sq. meter, and 22.2 kWh/gallon, the annual energy output is 339 kWh/sq. meter. This implies an efficiency greater of about 20%, given a solar isolation of 1500 to 2000 kWh per square meter, not 12%. So, there must be another unquantified loss in the process. 50,000 gallons per acre compared with maybe 500 for fermentation, so the next question is capital cost. Obviously they are thinking of using this ethanol as a vehicle fuel but I had a passing thought: Ethanol is stored energy so what's efficient? PV to battery to grid or this to fuel cell to grid? Offhand I'd say PV/battery/grid but a simple tank farm storage system for ethanol might cost so much less compared to batteries to tip the scales? If we had 20 million EVs I would say go for it. This is a really big deal if it scales and has manageable capital costs. It has approximately 140x better conversion ratio than corn ethanol / sqKm / year. Thus, you could make your plants quite small and place them in optimal locations. Now, I wonder could they do butanol ? @northernPiker, just use a higher insolation number (!) http://www.greenrhinoenergy.com/solar/radiation/empiricalevidence.php It is way above 2000 in the sunny parts of the world. Because the system is so compact, you can put them in sunny places (with access to water). Reform this ethanol for a FCHEV. The U.S. has 20 million FFV E85 capable vehicles, the problem is pumps, oil companies will not let them in THEIR stations, whether franchised or owned. Iridium is a platinum-group metal and not at all cheap, but if the capital cost can be brought down far enough this could be a worthwhile dump load for any electric generating system. That could be solar, wind or nuclear. If you've got 60% electricity-to-fuel efficiency, a gallon of EtOH (76,000 BTU) requires 37 kWh of electricity (plus whatever is needed to collect the CO2 feedstock). At 5¢/kWh that is just$1.85/gallon for the juice.

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