GWU team uses one-pot process to co-generate H2 and solid carbon from water and CO2; solar fuels
30 December 2014
|One-pot electrolytic process produces H2 and solid carbon from water and CO2. Li et al. Click to enlarge.|
A team at George Washington University led by Professor Stuart Licht has simultaneously co-generated hydrogen and solid carbon fuels from water and CO2 using a mixed hydroxide/carbonate electrolyte in a “single-pot” electrolytic synthesis at temperatures below 650 ˚C. The work is a further development of their work with STEP (solar thermal electrochemical process)—an efficient solar chemical process, based on a synergy of solar thermal and endothermic electrolyses, introduced by Licht and his colleagues in 2009. (Earlier post, earlier post.) (In short, STEP uses solar thermal energy to increase the system temperature to decrease electrolysis potentials.)
Licht and his colleagues over the past few years have delineated the solar, optical, and electronic components of STEP. In this study, they focused on the electrolysis component for STEP fuel, producing hydrogen and graphitic carbon from water and carbon dioxide. A paper on the new work is published in the journal Advanced Energy Materials.
Molten hydroxides are important as conductive, high-current, low-electrolysis-potential electrolytes for water splitting to generate hydrogen, the team notes in the paper. The Coulombic efficiency of electrolytic water splitting, ηH2 (moles H2 generated per 2 Faraday of applied charge), approaches 100% in low melting point, mixed alkali molten hydroxides at temperatures up to 300 ˚C.
|Measured coulombic efficiency of hydrogen generation (ηH2) in various molten hydroxide electrolytes at various temperatures. Data source: Li et al. Click to enlarge.|
In the study, they achieve the synthesis of hydrogen and carbon fuel using a mixed, hydroxide/carbonate electrolyte, nickel anode (generating O2), and nickel or steel cathode (generating graphite and hydrogen). Low hydroxide fractions in the electrolyte ensure efficient carbon formation, while high fractions form only H2 at the cathode; added barium and lithium salts ensure effective nickel anode stability.
The general use of solar thermal energy to lower the potential of useful electrolyses can be applied to liquid, gas, or solid phase electrolyte cells. In general, we have found an energy advantage in applying STEP to liquid, molten electrolyte cells. Such cells can be driven by thermal sunlight to high temperature accommodating both facile kinetics at high current density and a lower endothermic electrolysis potential. Importantly, molten salt cells can often accommodate high reactant concentrations, which lead to a further decrease in the electrolysis potential … We have previously demonstrated molten hydroxide electrolytes for solar water splitting to hydrogen fuel, and molten carbonate electrolytes for solar carbon dioxide splitting to carbon and carbon monoxide fuels.
… In this study, we focus on the electrolysis component for STEP fuel. Specifically, we present the first molten electrolyte sustaining electrolytic co-production of both hydrogen and carbon products in a single cell.
… We demonstrate here the functionality of new lithium–barium–calcium hydroxide carbonate electrolytes to co-generate hydrogen and carbon fuel in a single electrolysis chamber at high current densities of several hundreds of mA/cm2, at low electrolysis potentials, and from water and CO2 starting points, which provides a significant step towards the development of renewable fuels.—Li et al.
The one-pot co-synthesis of hydrogen and carbon and C was carried using a new Li1.6Ba0.3Ca0.1CO3 electrolyte with LiOH as hydroxide component. The synthesis has high coulombic efficiency with ≈62% of the current generating H2 and 20% generating carbon at an applied electrolysis current of 2 A through the 3.75 cm2 planar nickel anode and nickel cathode.
The authors noted that the H2 Coulombic efficiency in the LiOH/Li1.6Ba0.3Ca0.1CO3 electrolyte was higher than that observed at 500 ˚C in a pure barium hydroxide electrolyte, and which had not permitted the co-generation of fuels.
Li, F.-F., Liu, S., Cui, B., Lau, J., Stuart, J., Wang, B. and Licht, S. (2014), “A One-Pot Synthesis of Hydrogen and Carbon Fuels from Water and Carbon Dioxide,” Adv. Energy Mater. doi: 10.1002/aenm.201401791
Carbon can be used in a Direct Carbon Fuel Cell, at 2000C they achieve 80% efficiency. (refer to LBL)
Posted by: SJC | 30 December 2014 at 12:50 PM
Depositing solid carbon on an electrode means the electrode has to be processed outside the cell.
Calcium carbonate isn't on the list of salts used. This is a pity, because it's probably the largest single species of solid carbonate in the world. If the cell could consume CaCO3 and yield CaO (processed to Ca(OH)2 externally) it would be a way to create fuels from electricity and reduce the pH of the oceans.
Posted by: Engineer-Poet | 30 December 2014 at 02:12 PM
Hopefully this will attract interest in the synthesis of graphite. And of course calcium carbonate is a constituent of this eutectic salt (which is different from a non-stoichiometric compound, which this apparently is not.) What is intesting is that lithium can absorb CO2 directly from the air, which was employed on Apollo missions.
Posted by: kalendjay | 30 December 2014 at 02:45 PM
Hurry-up these researchs and start commercialization without subsidies in my area. Im tire of paying high price for gasoline and doing few mpg norwithstanding pollution. I need energy for my home and car and motorcycle.
Posted by: gorr | 30 December 2014 at 06:15 PM
John Cooper at LBL
Posted by: SJC | 30 December 2014 at 09:25 PM
Thanks for the link, SJC.
The number of different possible technologies make it unwise to be too definitive in how things will work out IMO.
I quite fancy shovelling some coal into my car though!;-)
I wonder if they could deliver it by horse and cart as they used to when I was a very small boy, sixty years ago?
Posted by: Davemart | 31 December 2014 at 12:33 AM
Pet coke sells for 5 cents per kilogram, now it is sold to countries that throw it into a furnace boiling water to make electricity. With a carbon fuel cell you get twice the efficiency without the pollution, producing pure CO2 you can sequester to use later.
Posted by: SJC | 31 December 2014 at 09:24 AM
I'm not sure it's worth even that much. 5¢/kg is probably less than the delivered price of coal where I am, and nearby refineries still seem to have trouble getting rid of it.
Posted by: Engineer-Poet | 31 December 2014 at 05:50 PM
Petcoke is essentially 8% of raw petroleum. Oh, now they can get rid of it and produce surplus electricity at refineries. Replacing the Claus process for hydrogen sulfide with a fuel cell based system to scavenge more heat and hydrogen will probably follow quickly. This could be a basis to finally upgrade Canadian tar sands to everyone's satisfaction.
Posted by: kalendjay | 12 January 2015 at 05:37 PM