How Catalyst Dispersion Solvents Affect CO2 Electrolyzer Gas Diffusion Electrodes

How Catalyst Dispersion Solvents Affect CO2 Electrolyzer Gas Diffusion Electrodes

The carbon dioxide reduction reaction (CO2RR) technology has the potential to produce carbon-based products at costs competitive with existing industrial processes if the CO2RR can be performed at high current densities, high faradaic efficiencies (FE) and low full cell voltages (Ecell). Typically, a “zero-gap” CO2 electrolyzer with a gaseous CO2 feedstock has been shown to selectively mediate the CO2RR at high current densities and low voltages. The cathode gas diffusion electrode (GDE) is a key component of the CO2 electrolyzer because the cathode GDE manages reagent transport and facilitates the CO2RR. Therefore, it is important to fabricate GDEs that enable sustained CO2 electrolysis with high performance. However, the design of high-performing GDEs for CO2RR electrolyzers is challenged by an absence of relationships between fabrication methods, cathode properties, and device performance. Motivated by the situation, Curtis P. Berlinguette and co-workers studied the influence of the catalyst ink solvent on the properties and performance of the spray-coated cathode GDEs for the CO2RR. Their research work was published in Energy Fuels in 2021 (“How Catalyst Dispersion Solvents Affect CO2 Electrolyzer Gas Diffusion Electrodeshttps://doi.org/10.1021/acs.energyfuels.1c01731).

In their work, GDEs were prepared from catalyst inks containing a fixed amount of Ag catalyst and anion-exchange ionomers dispersed in different solvent systems, MeOH, IPA, EtOH, 1:1 (v/v) IPA/H2O, and 7:3 (v/v) IPA/H2O. The catalyst inks were spray-coated onto the gas diffusion layers. Characterization of GDEs includes electrochemical surface area (ECSA), hydrophobicity, and capillarity. The GDEs were then tested for CO2 conversion to CO in a CO2 electrolyzer consisted of an anode, an anion-exchange membrane, and the prepared cathode GDE. Ionomer aggregation in catalyst inks was characterized by dynamic light scattering (DLS) experiments on ionomer dispersions.

The obtained results revealed that higher FECO and lower Ecell were facilitated by GDEs with high surface area, hydrophobicity, and capillarity. On the basis of the data, the authors identified solvent-mediated ionomer aggregation in inks as a key factor for preparing GDE catalyst layers, where moderate aggregation yields the highest performing GDEs. Specifically, the results showed that the choice of catalyst ink solvent modulates FECO by up to 52%. The effect was less apparent at lower current densities (25-100 mA cm-2), where all GDEs except for MeOH-derived GDEs achieved FECO of >80%. At 200 mA cm-2, however, GDEs prepared using EtOH achieved a significantly higher FECO (76 ± 5%) than the other four GDEs. GDEs prepared using 1:1 (v/v) IPA/H2O also achieved moderately higher FECO (61 ± 3%). Electrolyzers containing GDEs prepared using EtOH, 1:1 (v/v) IPA/H2O, and 7:3 (v/v) IPA/H2O required the joint lowest voltage at 200 mA cm-2 (4.2V), while GDEs from an IPA ink required an additional ~600 mV. The stability of electrolyzers was also impacted by the choice of the ink solvent. Electrolyzers containing GDEs prepared using EtOH showed ~90% FECO for nearly 8 h at 100 mA cm-2, while GDEs prepared using MeOH only maintained ~50% FECO for 2.5 h. The study clearly shows that the choice of solvent used when depositing catalyst layers on GDEs significantly affects FECO and Ecell achieved by a gas-fed CO2RR electrolyzer. Therefore, the catalyst ink solvent can be used to modulate the properties of GDE catalyst layers to achieve efficient CO2 electrolysis.

This work, for the first time, explored the influence of ink properties in the preparation of GDEs for CO2RR electrolyzers and defined the relationships between GDE properties and CO2RR electrolyzer performance. The revealed strategy in this work is very promising and feasible in preparing high-performance GDEs for CO2RR electrolyzers and other related fields.