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, η_H2 (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 Li_1.6Ba_0.3Ca_0.1CO₃ 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/Li_1.6Ba_0.3Ca_0.1CO₃ 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.

Resources

• 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