Electrochemically converting carbon dioxide into useful chemicals or fuels is a promising CO2-utilization strategy, but scaling up the process is a challenge. One reason is that the gas-diffusion electrodes (GDEs) used to facilitate contact between gaseous CO2, solid catalyst and liquid electrolyte need to have gas-diffusion layers that are simultaneously hydrophobic and electrically conductive, leading to a tradeoff. Now, researchers at the Massachusetts Institute of Technology (MIT; Cambridge, Mass.; www.mit.edu) have developed a way to get around this tradeoff when designing larger electrodes for scaled-up systems.
Within a GDE, the gas-diffusion layer (GDL) must fulfill three functions, the researchers say: (1) physically support the catalyst, yet be sufficiently porous for gas transport; (2) have adequate electronic conductivity to facilitate electron transport to the catalyst layer with minimal ohmic [resistance] loss; and (3) maintain robust hydrophobicity to ensure that the triple-phase contact is sustained close to the catalyst.
This sets up a tradeoff because highly conductive materials are generally hydrophilic, and the most hydrophobic materials are non-conductive. The MIT team, led by Kripa Varanasi, developed a GDL that achieves both conductivity and hydrophobicity by introducing micro-scale copper conductors that span the hydrophobic membrane (which is made from expanded polytetrafluoroethylene (ePTFE)). Weaving the copper wire through the ePTFE layer provides a highway for electrons to pass with minimal resistance.
The design of the new electrode, which the researchers call a hierarchically conductive GDE platform (photo), eliminates resistance losses that affect conventional ePTFE electrodes, and enable low voltages and high charge-transfer efficiencies for CO2-to-ethylene conversion at pilot scale, the researchers say.
“The improvement of conductivity is achieved with a minimal footprint (<2% area) and without sacrificing the bulk hydrophobicity of the ePTFE,” the MIT team writes in a recent issue of Nature Communications. The hierarchically conductive GDE approach decouples the electron conduction and hydrophobicity requirements of the GDL, the researchers say, which “unlocks electrode architecture design without placing additional constraints on catalyst or electrolyzer design,” and allows the scaleup of GDE-based electrochemical reduction of CO2.