Pulsed Electrodeposition of Tin Gas-Diffusion Electrocatalysts for CO2 Reduction to Formate

Abstract

The performance of electrocatalysts for the electrochemical carbon dioxide (CO2) reduction reaction (eCO2RR) is largely dependent on the ability to efficiently deliver CO2 to the active sites. To date, the best-identified reactor designs for CO2 conversion are based upon gas diffusion electrodes (GDEs) to supply CO2 to cathodic electrocatalytic sites, as gas-phase mass transport is significantly more rapid than that of CO2 dissolved in an aqueous electrolyte. As such, GDE-based electroreactors typically provide current densities that are significantly greater than those that convert solvated CO2, often by an order of magnitude or more [1]. However, the performance of GDEs for eCO2RR can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. Higher catalyst loadings, generally achieved via application of thicker catalyst layers, can yield higher total reaction rates, but often at the cost of reduced conversion efficiency arising from accelerated formation of undesired side products (e.g., hydrogen evolution from water splitting or diversion of carbon to alternative reaction pathways) due to reactant starvation. Reducing catalyst particle size typically enhances both utilization and activity per unit mass. This in turn may allow use of thinner catalyst layers, mitigating or avoiding such decreases in product selectivity. While synthetic methods exist for generating smaller (< 10 nm) particles of many catalytic metals, these particles must still be applied to a gas-diffusion layer (GDL) substrate in a fashion that maintains ionic and electronic contact with the electrolyte and GDL, respectively, in order to obtain both high current density and high selectivity.Previous work directed towards platinum (Pt) catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated an “electrocatalyzation” approach that used pulse and pulse-reverse electrodeposition to obtain highly dispersed and uniform Pt catalyst nanoparticles (~5 nm) [2-4]. Moreover, since the catalyst was electroplated through an ionomer layer onto the bare GDL, the formed nanoparticles were inherently in both electronic and ionic contact within the GDE and, consequently, utilization was enhanced. Specifically, for the oxygen reduction reaction, the electrodeposited catalyst exhibited equivalent performance at 0.05 mg/cm2 loading compared to a conventionally prepared GDE with a loading of 0.5 mg/cm2 [4].This talk will present recent and ongoing results from the application of this electrocatalyzation approach to fabrication of tin-loaded GDEs for reduction of CO2 to formic acid. The effect of GDL pre-treatment, pulse/pulse-reverse waveform parameters and duration of electrocatalyzation on the size distribution and uniformity of deposited tin catalyst particles will be discussed, and the performance of these catalysts in a flow electroreactor will be reported in terms of total current density and faradaic efficiency for formic acid. The electroreactor form factor includes an anion-exchange membrane at the GDE cathode and ion-exchange media within the single flow channel to facilitate ionic transport across the cell, similar to a form factor reported in literature to provide high performance [5]. This configuration provides at least two significant benefits: the anion-exchange membrane avoids the excessive wetting of and electrolyte permeation through the GDE in membraneless configurations; and, the ion-exchange media allows use of deionized or reverse-osmosis purity water as the liquid feed, potentially eliminating the need for a costly salt/formic acid separation step in a production context.