Quantifying the local pH of a gas diffusion electrode undergoing CO2 reduction is a complicated problem owing to a multitude of competing processes, both electrochemical- and transport-related, possibly affecting the pH at the surface. Here, we present surface-enhanced Raman spectroscopy (SERS) and electrochemical data evaluating the local pH of Cu in an alkaline flow electrolyzer for CO2 reduction. The local pH is evaluated by using the ratio of the SERS signals for HCO3 – and CO3 2–. We find that the local pH is both substantially lower than expected from the bulk electrolyte pH and exhibits dependence on applied potential. Analysis of SERS data reveals that the decrease in pH is associated with the formation of malachite [Cu2(OH)2CO3, malachite] due to the presence of soluble Cu­(II) species from the initially oxidized electrode surface. After this initial layer of malachite is depleted, the local pH maintains a value >11 even at currents exceeding −20 mA/cm2.

Electrodeposition of Cu, Cu/Ag, and Cu/Sn alloy films by using 3,5-diamino-1,2,4-triazole (DAT) as an electrodeposition inhibitor yields a high surface area Cu-based catalyst. All three Cu-based electrodes exhibit high Faradaic efficiency (FE) of CO2 reduction toward C2H4 production. The CuSn-DAT electrode exhibits the highest FE for CO (∼90% at −0.4 V) and C2H4 (∼60% at −0.8 V) production and high current density (∼−225 mA/cm2 at −0.8 V). In situ surface enhanced Raman spectroscopy (SERS) studies in a flow cell obtained from the three Cu-based samples show a correlation between the decreased oxide content on the Cu surface, increased presence of CO, and increased activity for CO and C2 production. The CuSn-DAT electrode has the lowest amount of Cu2O and exhibits the highest activity, whereas the Cu-DAT electrode has an increasing Cu2O content and exhibits lower activity as the potential is made negative. These results demonstrate that incorporation of different well-mixed alloy materials provides a way to tune CO2 reduction speciation.

While the use of flow electrolyzers has enabled high selectivity (>80%) and activity (>200 mA cm −2 ) in the reduction of CO 2 to value-added chemicals, the durability of these systems is still insufficient for feasibility at scale. A key component of flow electrolyzers, the gas diffusion electrode, must be hydrophobic and stable to maintain the triple phase boundary at the catalyst layer. The catalyst layer consists of an active catalyst and a binder to augment hydrophobicity and stability. Many CO 2 electrolysis systems utilize Nafion as the binder, yet, these cathodes are prone to carbonate formation and are often not stable beyond 20 h. Inspired by knowledge from other electrocatalysis applications, this paper explores alternatives to Nafion in the catalyst layer as well as different methods of catalyst layer preparation. Cathodes with a poly(tetrafluoroethylene) (PTFE) binder elude carbonate formation, although their performance still decreases over time. However, the addition of PTFE to Nafion (mixed binders) limited carbonate formation. Furthermore, we found that coating cathodes with a Sustainion ionomer over layer extends lifetimes, presumably by hindering carbonate formation. The characteristics of cathodes with these binders are further explored using surface-enhanced Raman spectroscopy to help explain their effect on the electroreduction of CO 2 .

Water-in-salt electrolytes (WiSE) are concentrated aqueous electrolytes recently developed that are of great interest because of their possible relevance for batteries. The origin for their promising application has been ascribed to the formation of percolating nanodomains in the bulk. However, the interfacial structure of WiSE still remains to be understood. In this paper, we characterize the potential-dependent double layer of a LiTFSIbased electrolyte on a charged electrode surface. Ultramicroelectrode (UME) measurements reveal a surface-confinement effect for a ferricyanide redox species at the electrode/WiSE interface. Potential-dependent atomic force microscopy (AFM) shows the presence of layers, the structure of which changes with the applied potential. Thicker layers (6.4 and 6.7 Å) are observed at positive potentials, associated with [Li(H 2 O) x ] + ([TFSI] − ) y ion pairs, while thinner layers (2.8 and 3.3 Å) are found at negative potentials and associated with [Li(H 2 O) x ] + alone. Vibrational spectroscopy shows that the composition of the double layer also changes with potential, where [TFSI] − is enriched at positive and [Li(H 2 O) x ] + enriched at negative potentials.