Gas diffusion electrodes (GDEs) intertwine an ionically conducting liquid and a gas with an electrically conducting solid, supporting electrochemical reactions involving constituents linked to the three phases (i.e., chemical species, electrons). GDEs are broadly used in electrochemical energy-conversion devices, such as fuel cells and metal-air batteries, as well as in electrolyzers aiming at chemical synthesis, like in the chlor-alkali industry, hydrogen peroxide production, CO₂ conversions to fuels and fine chemicals, or N₂ reduction to ammonia. Recently, the use of GDEs was pioneered for metal recovery and the synthesis of nanostructures, in a process named gas-diffusion electrocrystallization (GDEx).^[1–4]

A liquid solution containing dissolved metal or metalloid ions (e.g., Cu²⁺, Fe³⁺, As³⁺, PtCl₂⁻⁶) flows through an electrochemical cell equipped with a GDE, filling in its porosity. The gas (e.g., O₂, O₂ in air, CO₂, etc.) percolates through a hydrophobic backing (e.g., PTFE) on the GDE. After the gas diffuses to the electrically conducting layer acting as an electrocatalyst (e.g., hydrophilic porous activated carbon), the gas is electrochemically reduced. For instance, by imposing specific cathodic polarization conditions (e.g., at −0.145 V_SHE O₂ is reduced producing H₂O₂, H₂O and OH^–). As the highly abundant hydroxyl ions accompanied by redox reactive species spread to the bulk electrolyte, a reaction front develops throughout the hydrodynamic boundary layer. This creates local saturation conditions at the electrochemical interface, where metal ions precipitate in metastable or stable phases, depending on the operational variables. When O₂ is the oxidizing gas, GDEx has been explained with an oxidation-assisted alkaline precipitation mechanism.^[4] Conversely, when CO₂ is used, the reaction front, rich in reducing species, yields elemental nanoparticles.

This centennial celebration talk will explain the underlying principles of GDEx, portray reflections on its design and scale-up, and substantiate some of the experimental merits achieved. It will include the GDEx: (a) synthesis of iron oxide nanoparticles with high control of their magnetic susceptibility^[1]; (b) recovery and immobilization of arsenic into crystalline scorodite^[2]; (c) synthesis of nanoparticles with novel magnetic ground states (e.g., spin liquids and spin glasses)^[3]; (d) synthesis of libraries of electrochemically-active materials^[5] and (e) formation of elemental nanoparticles of platinum group metals (PGMs).

References:

[1] Prato et al. (2019) Sci Rep https://doi.org/10.1038/s41598-019-51185-x

[2] Pozo et al. (2020) React Chem Eng https://doi.org/10.1039/D0RE00054J

[3] Pozo et al. (2020) Nanoscale https://doi.org/10.1039/C9NR09884D

[4] Eggermont et al. (2021) React Chem Eng https://doi.org/10.1039/D0RE00463D

[5] Prato et al. (2020) J Mat Chem A https://doi.org/ 10.1039/D0TA00633E

Acknowledgements:

This research has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreements No 958302 (PEACOC project), No 730224 (Platirus project), No 796320 (MAGDEx project), and No 654100 (CHPM2030 project). Support from the Flemish SIM MaRes programme, under grant agreement No 150626 (Get-A-Met project) is also acknowledged. The author thanks Rafael Prato, Sam Eggermont, Guillermo Pozo, Luis Fernando Leon Fernandez, Omar Martinez Mora, Ramin Rabani, Kudakwashe Chayambuka, Elisabet Andres Garcia, Katrijn Gijbels, Yolanda Alvarez Gallego, and Jan Fransaer for their valuable contributions to the development of the GDEx process.