Development of Novel Tin Nanostructures Using Pulse Plating Methods for the Electroreduction of Carbon Dioxide to Formic Acid

Abstract

The development of energy efficient carbon dioxide (CO2) electroreduction processes would simultaneously curb anthropogenic CO2 emissions and provide sustainable pathways for generation of fuels and chemicals. While significant efforts have focused on heterogeneous CO2 electroreduction to various products, to date, no process has demonstrated both high energetic efficiencies and high current densities. A key challenge in the development of active, selective, and stable electrocatalysts in scaling performance nanomaterials to appropriate electrode structures which augment catalytic activity by maximizing utilization, facilitating species transfer, and minimizing undesirable side reactions. The electroreduction of CO2 to formic acid (FA) is attractive due to the low charge requirement (e.g., 2 e- per FA), liquid state product, and high selectivity on a number of low cost catalytic materials. Furthermore, FA has a range of contemporary commercial uses including leather tanning, latex processing, and animal feeds. FA may also find use as a fuel in direct liquid fuel cells. Tin (Sn) is of particular interest due to its high current efficiency for electroreduction of CO2 to FA as well as low cost and toxicity. Prior reports have demonstrated electroreduction of CO2 to FA on Sn catalysts at high faradaic efficiencies (10-95%) on disk electrodes, metal meshes, and gas diffusion electrodes (GDEs) which operate at a range of currents (10-200 mA/cm2) depending on modes of CO2transport.[1-4] Recent work has also indicated that product selectivity decreases with increasing catalyst loading likely due to mass transport losses within a thick catalyst layer.[4] Prior GDE studies have used Sn catalyst particles (150 nm to 150 µm) incorporated into an ionomer which is then applied to a microporous carbon layer (MPL). The MPL is “supported” by a gas diffusion layer (GDL) consisting of hydrophobic (i.e., TEFLON) carbon paper or carbon cloth (Figure 1a). As depicted in Figure 1b this approach limits the electroreduction of CO2 due to 1) low tin catalyst specific surface area due to the relatively large Sn particle size (>150 nm), and 2) unutilized Sn catalyst particles within the ionomer but not in electrical contact with the carbon in the MPL. Previous work directed towards platinum (Pt) catalyst utilization in PEM fuel cell GDEs demonstrated a novel “electrocatalyzation” approach to obtain highly dispersed (~5 nm) Pt catalyst particles using pulse and pulse reverse electrodeposition.[5-7] Additionally, since the Pt catalyst was electrodeposited through an ionomer applied to an uncatalyzed carbon MPL, the catalyst was inherently in electronic and ion contact within the GDE and consequently catalyst utilization was enhanced. Specifically, for oxygen reduction, the electrodeposited catalyst exhibited equivalent performance at 0.05 mg/cm2 loading compared to a conventionally prepared GDE with a catalyst loading of 0.5 mg/cm2.[5] Herein we investigate the electrocatalytic performance of novel Sn nanostructures electrodeposited directly onto the MPL after the application of ionomer devoid of Sn catalyst particles. Using pulse and reverse pulse waveforms to electrodeposit the Sn catalyst particles, as shown in Figure 1c, our goal is to improve the electrochemical reduction of CO2 by 1) increasing the Sn catalyst specific surface area by decreasing the Sn catalyst particle size (<<150 nm), and 2) improve the Sn catalyst utilization by eliminating Sn catalyst particles not in electronic contact with the carbon in the MPL. This approach enables the deposition of robust and uniform catalytic Sn layers on the surface of highly structured GDL/MPL substrates. State of the art Sn catalysts (150 nm) mixed within the ionomer and applied to the MPL are used as a benchmark for comparison with the electrodeposited Sn catalysts. Electrolysis experiments are conducted in a continuous mode lab-scale reactor with flowing gaseous CO2 delivered across a GDE using advanced flow fields. FA production is measured as at cell outlets and used to determine the activity, efficiency, and stability of these novel catalytic structures for CO2to FA conversions.