Ultra-High-Rate CO2 Reduction Reactions to Multicarbon Products with a Current Density of 1.7 a cm-2 in Neutral Electrolyte

Abstract

The electrochemical carbon dioxide reduction reaction (CO2RR) has attracted considerable attention as a promising strategy for the conversion of anthropogenic CO2 into value-added products.1 An advantage of these methods is that this greenhouse gas can be reduced under mild conditions (close to ambient temperature and pressure). However, the operative efficiency, product selectivity and rate must be improved for practical implementation. Among them, it has been reported that the current density for the production of high value-added products is correlated with the capital cost of the electrolyzers employed. Therefore, improving the current density would significantly affect the feasibility of this technology. The use of gas diffusion electrodes (GDEs), which allow the CO2RR to occur at the three-phase interfaces (solid-catalyst/liquid-electrolyte/gaseous-CO2), effectively accelerates the CO2RR by solving the problem of the mass transport limitation due to the inherently low diffusion of CO2 in water. A catalyst layer (CL) composed of metal Cu particles and ionomers on carbon-based GDEs has been widely studied as the standard cathode for gaseous CO2RR to produce C2+ organics, such as C2H4, C2H5OH, n-C3H7OH and CH3COOH (Figure (a)). Recently, a few studies have achieved an ultra-high-rate (UHR-) CO2RR with the partial current density for C2+ organics (j C2+) of over 1 A cm-2 using special catalysts or electrodes in high alkaline solution.2, 3, 4 However, in addition to the toxicity and corrosivity, alkaline solutions quickly absorb CO2 and convert it to electrochemically inert (bi)carbonate. Based on these drawbacks, UHR-CO2RR in neutral electrolytes is thus required from a practical viewpoint. Furthermore, the obtainable maximum j C2+ value is still unknown when optimizing the standard components; CuO nanoparticle catalysts (CuONPs), which are known to serve as oxide-derived Cu catalysts for effective C2+ production, and conventional carbon-based GDEs.In this work, we attempt to pursue a high partial current density for C2+ (j C2+) using neutral electrolytes by optimizing the standard components and properly assembling them as the cathode. In particular, we achieved a record j C2+ of 1.7 A cm-2 in neutral electrolytes using CuONPs and carbon-based GDEs (Figure (b)).5 Moreover, j C2+ reached 1.8 A cm-2 when we used 1 M KOH as catholytes, which is also the record j C2+ value in alkaline electrolytes. When the total current density (J total) was increased, the Faradaic efficiencies for C2+ organics increased monotonically below J total = 2000 mA cm-2. Therefore, the trade-on relationship between the reaction rate and selectivity was shown in the UHR-CO2RR under those J total regions. Although the cathode of this electrochemical system was composed of the conventional materials, we successfully maximized the potential of the catalytic activity by constructing a CL with optimal porosity and thickness. Especially, we revealed that the thickness of CL was directly correlated to j C2+. With a thinner CL (the amount of catalyst loadings is 0.34 mg cm-2), the maximum j C2+ didn’t reach 1 A cm-2. This is because the constructed three-phase interface was too thin. With a thicker CL (the amount of catalyst loadings is 3.1 mg cm-2), the faradaic efficiency for H2 production was higher than optimized CL (the amount of catalyst loadings is 1.7 mg cm-2). This is because the Cu sites fully immersed in the electrolyte cannot serve as CO2RR site but only for H2 evolution site. Therefore, the optimal thickness of CL was required for UHR-CO2RR. The present system is expected to become one of the standards when pursuing UHR-CO2RR because only conventional materials were used in this work.