|Coiled platinum before (left), and after (right), carbon capture @ 750 °C in molten carbonate. Carbon dioxide fed into the electrolysis chamber is converted to solid carbon in a single step. Credit: ACS, Licht et al. Click to enlarge.|
Dr. Stuart Licht at George Washington University and colleagues have published the first experimental evidence of their new solar thermal electrochemical photo (STEP) process, which combines electronic and chemical pathways to convert CO2 to carbon or to carbon monoxide for subsequent use in synthesizing a range of industrially relevant products including hydrocarbon fuels. (Earlier post.) Their paper was published online 14 July in ACS’ Journal of Physical Chemistry Letters.
The STEP process uses a high temperature electrolysis cell powered by sunlight to capture CO2 in a single step. Solar thermal energy decreases the energy required for the endothermic conversion of carbon dioxide and kinetically facilitates electrochemical reduction, while solar visible energy generates electronic charge to drive the electrolysis.
The STEP process is fundamentally capable of converting more solar energy than photovoltaic or solar thermal processes alone, according to the researchers.
As observed experimentally...we split carbon dioxide, fed into a molten lithium carbonate electrolyte, via electrolysis...it is seen via cyclic voltammetry that a solid carbon peak observed at 750 °.C is not evident at 950 °C. At temperatures less than ~900 °C in the molten electrolyte, solid carbon is the preferred CO2 splitting product, while carbon monoxide is the preferred product at higher temperature...the electrolysis potential is <1.2 V at either 0.1 or 0.5 A/cm2, respectively, at 750 or 850 °C. Hence, the electrolysis energy required at these elevated, molten temperatures is less than the minimum energy required to split CO2 to carbon monoxide at 25 °C... As calculated using the available thermochemical enthalpy and entropy of the starting components...molten lithium carbonate (Li2CO3) provides a preferred, low energy route compared to Na2CO3 or K2CO3, for the conversion of CO2, via a Li2 O intermediate.
—Licht et al.
|STEP carbon capture in which three molten carbonate electrolysis in series are driven by a concentrator PV (CPV) system. Credit: ACS, Licht et al. Click to enlarge.|
In their experiment, the team used a concentrator solar cell to generate 2.7 V at a maximum power point, with solar to electrical energy efficiencies of 35% under 50 suns illumination, and 37% under 500 suns illumination. The 2.7 V is used to drive two molten electrolysis cells in series at 750 °C and three in series at 950 °C (depicted in the diagram to right).
At 950 °C at 0.9 V, the electrolysis cells generate carbon monoxide at 1.3-1.5 A, and at 750 °C at 1.35 V generate solid carbon formation at a comparable current.
The open access Supporting Information published along with the paper details the methodology and the materials used in the experiment. It also addresses the question of whether material resources “are sufficient to expand to process to substantially impact (decrease) atmospheric levels of carbon dioxide.”
At 500 suns of 1 kW m-2 sun-1 illumination, 1 m2 of CPV will generate 70 kA at 2.7 V, to drive three series connected molten carbonate electrolysis cells to CO, or two series connected series connected molten carbonate electrolysis cells to form solid carbon. This will capture 7.8 x 103 moles of CO2 day-1, based on 2 moles CO2 per 2 Faraday conversion to solid carbon, and 6 hours of insolation.
The high temperature of emitted CO2 in smokestacks is conducive to STEP carbon capture. In addition, the material resources to decrease atmospheric carbon dioxide concentrations with STEP carbon capture, appear to be reasonable. The buildup of atmospheric CO2 levels from a 280 to 390 ppm occurring over the industrial revolution comprises an increase of 1.9 x 1016 mole (8.2 x 1011 metric tons) of CO2, and will take a comparable effort to remove. It would be preferable if this effort results in useable, rather than sequestered, resources.
From the daily conversion rate of 7.8 x 103 moles of CO2 per square meter of CPV, the STEP capture process, scaled to 700 km2 of CPV operating for 10 years can remove and convert all this CO2 to carbon. A larger current density at the electrolysis electrodes, will increase the required voltage and would increase the required area of CPVs. Alternatively, a greater degree of solar concentration, for example 2000 suns, rather than 500 suns, will proportionally decrease the area of required CPV area to remove anthropogenic carbon dioxide.
—Licht et al., SI
Challenges that remain with STEP include stability and cost of materials, activity of electrocatalysts, effective utilization of excess heat, batch versus continuous process for extracting solid carbon as a cathode product, and the systems engineering of spectrum splitting concentrators to increase the availability of dichroic and dielectric beam splitters, the researchers note in their paper.
Stuart Licht, Baohui Wang, Susanta Ghosh, Hina Ayub, Dianlu Jiang and Jason Ganley (2010) A New Solar Carbon Capture Process: Solar Thermal Electrochemical Photo (STEP) Carbon Capture. J. Phys. Chem. Lett., doi: 10.1021/jz100829s