Pulsed Electrodeposition of Gas Diffusion Electrocatalysts for CO2 Reduction to Value-Added Products

Abstract

There have been many attempts to find efficient approaches to reduce CO2 to various organic compounds due to the industrial need for a carbon source and the large amounts of CO2 generated by human activities. One of the promising approaches for CO2 conversion is the use of electrocatalytic reduction reactions (eCO2RRs), which can be achieved on various cathode materials. Depending on the catalyst choice, CO2 can be selectively reduced to carbon monoxide, hydrocarbons (methane, ethylene), alcohols (methanol, ethanol), aldehydes, or carboxylic acids (formic, oxalic acids) [1,2]. A variety of reactor configurations have been explored in the literature that can be broadly classified as based on either liquid- or gas-phase reactant delivery for eCO2RR. These configurations utilize a range of electrode types including metal plates, meshes, packed granules, and gas diffusion electrodes (GDEs) [3]. Amongst these methods, the use of gas-phase reactor designs employing GDEs enables a dramatic increase in current density (typically an order of magnitude or larger) over liquid-phase reactor designs, where the low solubility and aqueous diffusivity of CO2 result in severe mass transport limitations. Efficient electrocatalytic conversion requires three key features: (1) facile transport in/out of gaseous reactants/products; (2) electrical continuity with the solid substrate; and (3) ionic continuity with the (typically) liquid electrolyte. The GDE form factor itself facilitates rapid gas-phase transport, but traditional methods of catalyst application provide suboptimal electrical & ionic continuity. This talk surveys recent work toward development of methods for preparation of selective gas-diffusion electrode electrocatalysts by electrodeposition which alleviates both of these latter inefficiencies, as electrodeposition can only occur at sites also generally suitable for electrocatalysis. Previous work directed towards platinum catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated an “electrocatalyzation” (EC) approach that used pulse/pulse-reverse electrodeposition to obtain highly dispersed and uniform platinum catalyst nanoparticles (~5 nm) [4-6]. Moreover, since the catalyst was electroplated through an ionomer layer onto the gas diffusion layer (GDL), the formed nanoparticles were inherently in both electronic and ionic contact within the resulting GDE and, consequently, utilization was enhanced. Specifically, for the oxygen reduction reaction, the electrodeposited catalyst exhibited performance at 0.05 mg/cm2 loading comparable to a conventionally prepared GDE with a ten-fold greater loading of 0.5 mg/cm2 [6]. Results will be presented from application of the above EC GDE preparation technique to two eCO2RR electrocatalyst systems, hydrocarbon-selective copper and formic acid-selective tin. These data illustrate the capability of the EC technique for fabrication of GDEs with substantially enhanced performance characteristics as compared to GDEs prepared by conventional techniques. The GDEs were tested in custom electrochemical cells and electrocatalysis performance characteristics such as total current density and selectivity for desired products (ethylene and formic acid for Cu and Sn catalysts, respectively) were measured as a function of various GDE fabrication parameters (e.g., electrodeposition waveform amplitudes/timings, substrate pretreatment conditions, and electrodeposition bath composition). For instance, effect of pre/post-treatment methods of GDLs such as ionomer coating and air-plasma in controlling the metallic character of electrodeposited copper/copper oxide micro/nano-particles were explored. Notably, preliminary data indicates that the post treatment of ionomer treated, Cu plated GDEs tend to improve the catalytic activity towards selective CO2 conversion to ethylene and durability of GDEs. Such an EC approach has also been applied to tin-based GDEs for the electroreduction of CO2 to formate. This represents nearly two-fold improvement in total current density up to 388mA/cm2 with 76% faradaic efficiency at about half the catalyst loading, compared to the existing reports of Sn-loaded GDEs prepared by various conventional methods [7, 8].