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Electroreduction of CO 2 (eCO 2 RR) is a potentially sustainable approach for carbon-based chemical production. Despite significant progress, performing eCO 2 RR economically at scale is challenging. Here we report meeting key technoeconomic benchmarks simultaneously through electrolyte engineering and process optimization. A systematic flow electrolysis studyperforming eCO 2 RR to CO on Ag nanoparticles as a function of electrolyte composition (cations, anions), electrolyte concentration, electrolyte flow rate, cathode catalyst loading, and CO 2 flow rate -resulted in partial current densities of 417 and 866 mA/cm 2 with faradaic efficiencies of 100 and 98 % at cell potentials of À 2.5 and À 3.0 V with full cell energy efficiencies of 53 and 43 %, and a conversion per pass of 17 and 36 %, respectively, when using a CsOH-based electrolyte. The cumulative insights of this study led to the formulation of system design rules for high rate, highly selective, and highly energy efficient eCO 2 RR to CO.[a] S.

The electrochemical reduction of CO 2 (ECO 2 R) is a promising method for reducing CO 2 emissions and producing carbonneutral fuels if long-term durability of electrodes can be achieved by identifying and addressing electrode degradation mechanisms. This work investigates the degradation of gas diffusion electrodes (GDEs) in a flowing, alkaline CO 2 electrolyzer via the formation of carbonate deposits on the GDE surface. These carbonate deposits were found to impede electrode performance after only 6 h of operation at current densities ranging from −50 to −200 mA cm −2 . The rate of carbonate deposit formation on the GDE surface was determined to increase with increasing electrolyte molarity and became more prevalent in K + -containing as opposed to Cs + -containing electrolytes. Electrolyte composition and concentration also had significant effects on the morphology, distribution, and surface coverage of the carbonate deposits. For example, carbonates formed in K + -containing electrolytes formed concentrated deposit regions of varying morphology on the GDE surface, while those formed in Cs + -containing electrolytes appeared as small crystals, well dispersed across the electrode surface. Both deposits occluding the catalyst layer surface and those found within the microporous layer and carbon fiber substrate of the electrode were found to diminish performance in ECO 2 R, leading to rapid loss of CO production after ∼50% of the catalyst layer surface was occluded. Additionally, carbonate deposits reduced GDE hydrophobicity, leading to increased flooding and internal deposits within the GDE substrate. Electrolyte engineering-based solutions are suggested for improved GDE durability in future work.

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.

In this work, we examine the direct influence of ligand coverage on a catalytically active surface for the hydrogen evolution reaction (HER). We tested the electrochemical and electrocatalytic properties of colloidally synthesized WSe2 with dodecylamine ligands before and after treatment with Meerwein’s reagent, which was shown to reduce the coverage of amine and lead to an improvement in the overpotential for HER by as much as 180 mV on the same electrode (reversible upon re-ligation with amine). The underlying mechanism of the improvement in HER catalysis following treatment with Meerwein’s reagent was investigated using a combination of Tafel slope analysis, electrochemically active surface area measurements, and impedance spectroscopy. These electrochemical measurements are rationalized with Fermi level and d-band center analysis from XPS. Together, these measurements suggest that, while the surface area of the WSe2 increases upon ligand stripping, the intrinsic catalytic capability of the WSe2 also changes. This is likely due to a decrease in ΔG H in the Meerwein’s treated WSe2, improving the HER kinetics.

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 .

In the search for nonprecious metal catalysts for the hydrogen evolution reaction (HER), transition metal dichalcogenides (TMDCs) have been proposed as promising candidates. Here, we present a facile method for significantly decreasing the overpotential required for catalyzing the HER with colloidally synthesized WSe. Solution phase deposition of 2H WSe nanoflowers (NFs) onto carbon fiber electrodes results in low catalytic activity in 0.5 M HSO with an overpotential at -10 mA/cm of greater than 600 mV. However, two postdeposition electrode processing steps significantly reduce the overpotential. First, a room-temperature treatment of the prepared electrodes with a dilute solution of the alkylating agent Meerwein's salt ([EtO][BF]) results in a reduction in overpotential by approximately 130 mV at -10 mA/cm. Second, we observe a decrease in overpotential of approximately 200-300 mV when the TMDC electrode is exposed to H, Li, Na, or K ions under a reducing potential. The combined effect of ligand removal and electrochemical activation results in an improvement in overpotential by as much as 400 mV. Notably, the Li activated WSe NF deposited carbon fiber electrode requires an overpotential of only 243 mV to generate a current density of -10 mA/cm. Measurement of changes in the material work function and charge transfer resistance ultimately provide rationale for the catalytic improvement.