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Researchers Demonstrate New Solar Carbon Capture Process; STEP Converts CO2 to Solid Carbon or CO for Use in Fuels and Chemicals Synthesis

Step2
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
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.

Resources

  • 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

Comments

HarveyD

Who really wants to perpetuate our addiction to liquid fuel, specially when electrified vehicles will soon be more efficient, more durable with less maintenance and much cleaner.

Peter_XX

HarveyD, in your dreams...

HarveyD

Peter:

Please read DOE's new report on EVs future.

Henry Gibson

HarveyD is correct if it is remembered that most automobile trips are under forty miles. People can be required to have an electric version of the TATA Nano for short cheap trips in addition to a long range automobile, and any battery, including lead ones as proved by CALCARS and ACPropulsion, can be used for such trips along with an emergency range extender charger engine. Practically any city road speed trip of any length can be done at general traffic speeds with a one kilowatt range extender in a series hybrid design with a 20 mile electric range battery. ..HG..

Henry Gibson

Someone figure out how many square miles of solar energy collection area is needed to capture enough solar energy to convert all of the CO2 back into CO or solid carbon for all of the CO2 produced in the US including that done by people breathing.

Large Trees are cheaper.

Nuclear power plants can eliminate much release of CO2 as the French have proved, and they can also provide heat and electricity for CO2 recycling if necessary in addition.

Some people have proposed solar heat melted salts for steam locomotives, and the same system should be proposed for automobiles in Arizona. Steam Locomotives can have a life of over fifty years. Electric and fuel heating can be used in emergencies. The Philips light bulb company of the Netherlands designed and built a Stirling engined prototype of such an automobile for a US auto company. ..HG..

HarveyD

HG:

Owners of all electric residences can reduce their e-energy consumption enough for 2 or 3 electrified vehicles. By using existing technologies, we managed to reduce our electricity consumption from 65+ Kwh/day (avg) to an average of only 20 Kwh/day. (NB: Warmer temps +3.5C also helped). The 45 Kwh/day saved could supply all the energy required for at least 3 mid-size electrified vehicles.

If all home owners in USA and Canada would take similar measures, very few extra Kwh would be required for 2 or 3 electrified vehicles per family. The total e-energy used could even be less in many places.

Henry Gibson

HarveyD is again correct about the fact that sufficient energy can be saved from ordinary use to power most electric automobile needs. CALCARS estimates 200 watt hours for mile traveled in a Prius even.

The dry hot Southwestern states of the US could use hybrid airconditioning systems for all homes and businesses where water evaporation cools the condenser coils of compressor systems to provide lower use of electricity. Such systems were long used in commercial buildings.

Small cogeneration systems, as demonstrated by Capstone turbines, can also save much energy and be used to charge electric cars whilst providing heat AND cooling. Perhaps 100,000 Honda home cogeneration systems have been sold in Japan, but a simpler chaeper one could be built for automobile charging whilst heating water and the house. ..HG..

Mannstein

@ Henry Gibson

"People can be required to have an electric version of the TATA Nano..."

And who is going to mandate that? A Communist Apparatchik no doubt.

HealthyBreeze

Photosynthesis is cheap because of self-assembling devices (plants) but it is not efficient. Look at how much biodiesel you get from an acre of crops vs. how many equivalent KwHr you get from an acre of photovoltaic cells...the difference is orders of magnitidue in favor of PV.

So, maybe it's possible this could someday be a viable on a per-acre basis, but with Platinum catalysts, and CO2 having to be in a liquid carbonate first...how could it ever be more cost effective?

HealthyBreeze

@ Henry Gibson,

People don't always need the waste heat from turbines for their house or office. In my temperate climate, I turn off my furnace in April, and don't turn it on again until October. I don't use AC either.

HarveyD

HB:
An ultra high efficiency dual heat pump can supply hot water for your home at 1/3 to 1/4 the cost while cooling the house free in summer time. The same unit can supply most of the heat required in winter time. Hot water + cooling + heating with the same unit while saving 60% to 75% in e-energy consumption is possible. Installation is not cheap but where energy saving is an important factor, it is viable.

Donough Shanahan

@ healthybreeze the advantage there is that liquid biofuels have inbuilt storage whereas wind and solar electricity require significant infrastructural modifications i.e. increase of substations to account for diverse nature of supply; storage systems etc. When you take that into account figures closer to parity become apparent.

Henry Gibson is correct and I will go one further; without first being more efficient it will be almost impossible to supply our current electricity demands as well as electrical transport at the same time. For the first time there is a large drive to increase efficiency so that this may be accomplished.

Reel$$

Healthy...

sounds like you elect to no use air cooling/conditioning as a personal choice. In most temperate zones heat pump cooling from CHP waste heat would be welcome since it would arrive at a cost lower than the day grid rates. Turbine CHP is not best for residences and the Honda units run NG in an ICE. But low temp SOFCs are coming ala Bloom Box and they will meet most requirements for efficient CCHP.

An interesting aftermarket device might be Henry's suggestion for a small NG fueled genset that kicks in when you plug in your EV - avoiding high grid rates in certain regions.

Engineer-Poet

As others have noted, the idea of using solar power to reduce CO2 in combustion effluent is silly. If the goal is emissions reduction, it makes far more sense to run the process directly with the solar energy source and skip the combustion.

On the other hand, the short paper mentions that the process does not need a separate supply of heat. Capturing CO2 from e.g. fermentation of crop wastes (from production of EtOH or CH4) would allow it to be turned into another product stream instead of just being dumped. Excess wind power over immediate needs might feed such a process in places like Iowa, where so much corn ethanol is made with little thought as to higher-value co-products.

Donough Shanahan

@ Reel$$

Bloom box is not a low temperature SOFC; not by any standard.

richard schumacher

Aircraft at least will need liquid hydrocarbon fuels for the foreseeable future, until some energy storage breakthrough is made or we build a fleet of electric aircraft remotely powered by a network of microwave or laser power transmitters. Fuel manufactured from atmospheric CO2 and any non-fossil energy source will be needed to make air transport carbon neutral.

SJC

I like the idea of turning CO2 into CO for SOFCs. The Bloom Box is a lower cost version of an SOFC, but it still runs at high temperatures. I think using CO2 from ethanol plants makes more sense. I would rather see IGCC for coal plants and then you get the CO2 before you have combustion.

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