|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