Background: Electrochemical CO2 reduction (CO2R) to value-added carbon-based products can provide a pathway towards decarbonizing sectors such as transportation and manufacturing. When driven by renewable energy such as solar, the process can be a promising method of producing sustainable fuels and chemicals. However, challenges with the implementation of CO2R technology include improving catalyst selectivity and performance to produce higher-order carbon products as well as catalyst durability.
Using a Co-design Approach to Upgrading CO2: We utilized a two-step solar-driven electrocatalytic and photothermocatalytic process for ethylene hydroformylation to produce C6 products. To obtain the desired product distribution from the electrolyzer step (an equimolar ratio of H2, CO, and C2H4), we designed a Cu/Ag tandem electrode, where CO2 is first reduced to CO on Ag, and CO is then reduced further on Cu to C2H4. CO2 electrolysis experiments in a 5 cm2 zero-gap membrane electrode assembly (MEA) were first conducted in the dark to test varying Ag and Cu ratios and operating parameters such as membrane thickness, CO2 inlet flow rate, and cell compression. The catalyst used to obtain the desired equimolar product ratio with ~7 vol % ethylene was a Cu thin film deposited onto a gas diffusion electrode with a 10 nm thin film of Ag covering a 6 mm x 2.23 cm area near the inlet of the MEA flow field. The MEA was scaled up from 5 cm2 to 25 cm2 in order to increase the concentration of gas products from the electrolyzer into the photothermal reactor. In the scaled-up MEA, possible electrochemical-mechanical degradation of the membrane and the anode contributed to fluctuating product distribution over time. This study provides insights into potential challenges with MEA scaleup within the context of a co-design approach to couple electrocatalytic and thermocatalytic processes.
Investigating Catalyst Durability: We developed an experimental framework to detect and quantify trace amounts of catalyst degradation, in the forms of both nanoparticle detachment and dissolution into metal ions, using on-line inductively coupled plasma mass spectrometry (ICP-MS). By coupling an ICP-MS to a potentiostat, one can probe catalyst degradation while varying applied potential and microenvironment. To verify the use of ICP-MS for quantifying nanoparticle detachment, sizes of Au nanoparticle standards were measured by ex-situ ICP-MS, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The size distributions exhibited significant overlap when comparing each method. For in-situ on-line ICP-MS experiments, a custom flow cell was utilized to compare degradation behavior of Au foil and Cu foil catalysts under CO2R and hydrogen evolution reaction (HER) operating conditions. Both foils were found to degrade via nanoparticle detachment at the femtogram scale when applying potential steps at -0.1 V and -1 V vs. the reversible hydrogen electrode (RHE). However, Au degraded ~4x more under CO2R than under HER operating conditions, while Cu was found to degrade in similar quantities under both conditions. Under CO2R operating conditions, Au lost ∼1.8× more mass and ~7.5x more nanoparticles than Cu. On-line ICP-MS was also used to determine the onset potential of nanoparticle detachment for Au and Cu in varying microenvironments. It was determined that Cu nanoparticle detachment appears to be more dependent on electrolyte environment than current density, while Au nanoparticle detachment appears to depend on both electrochemical conditions and electrolyte environment. The developed experimental framework demonstrated in this study can be expanded to survey degradation of other electrocatalysts and to determine the effects of real-world feedstocks for CO2R on electrocatalyst degradation.