Researchers in Spain have developed hydrogen production without contact electrodes via water electrolysis mediated by the microwave-triggered redox activation of solid-state ionic materials at low temperatures (< 250 °C). A paper on their work is published in Nature Energy.

Schematic illustration of the microwave-induced redox cycle. Microwave irradiation of doped ceria materials induce its reduction and triggers the release of gaseous oxygen. The input of a suitable sweep gas on microwave switching-off leads to the material reoxidation via gas deoxygenation and the formation of valuable molecular energy carriers. Serra et al.

H₂ produced from water and green power through solar thermochemical or photocatalytic water splitting and water electrolysis has become a sustainable alternative with negligible on-site greenhouse gas contribution. In thermochemical cycles, the highly energy-demanding splitting of water molecules (ΔH_H2O=285kJmol⁻¹) is often realized by using regenerable energy carriers (molecular or solid agents as metals or ceramics) that reduce water to yield H₂. The redox activation of water is usually carried out by very high-temperature heating or galvanic methods to enable this non-spontaneous equilibrium-limited reaction, that is, with a large positive Gibbs free-energy change (ΔG_H2O).

Electromagnetic processes such as microwave heatinig hold promise for smart manufacturing and activating chemical reactions and can enable electrochemical operation without contact electrodes and the restrictions of conventional electrolysis cells, those being constrained operation conditions and equipment complexity. Here we report the contactless H₂ production from water, mediated by microwave-triggered redox activation of solid-state ionic materials. Water splitting is realized by the sole application of microwave radiation, which enables the chemical redox cycling of ceramic oxides at very low temperatures (<250 °C).

—Serra et al.

A Sankey energy diagram of the energy distribution of the complete microwave-assisted redox cycle for hydrogen production. The heat excess from CGO reoxidation is reused to supply the energy demand of the reduction step. Serra et al.

Water was reduced via reaction with non-equilibrium gadolinium-doped CeO₂ that was previously in situ electrochemically deoxygenated by the sole application of microwaves. The microwave-driven reduction was identified by an instantaneous electrical conductivity rise and O₂ release.

First, microwaves interact with the crystalline oxide, leading to an instantaneous rise in electrical conductivity that is accompanied by the material reduction (deoxygenation).

The second step involves splitting water through a spontaneous reaction with the activated material, which leads to direct H₂ formation and reoxygenation of the material.

This process was cyclable; H₂ yield and energy efficiency were material- and power-dependent. Deoxygenation of low-energy molecules (H₂O or CO₂) led to the formation of energy carriers and enabled CH₄ production when integrated with a Sabatier reactor.

The researchers suggested that this method could be extended to other reactions such as intensified hydrocarbons synthesis or oxidation.

Resources

• Serra, J.M., Borrás-Morell, J.F., García-Baños, B. et al. (2020) “Hydrogen production via microwave-induced water splitting at low temperature.” Nat Energy doi: 10.1038/s41560-020-00720-6