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A variety of techniques for management of carbon dioxide emissions from power generation facilities and other industrial sites have been under active investigation for decades, in an effort to mitigate the environmental impacts of these releases. Once such approach is electrochemical reduction, which treats the waste CO2 as a source material for the production of value-added materials. Currently, the most promising form factor for this electrocatalytic application appears to be a stack-based system, where catalyst is immobilized on porous media that is interfaced with a liquid or solid electrolyte, and the reactant carbon dioxide is delivered to the active region by gaseous diffusion, which is orders of magnitude faster that diffusion though aqueous media. Various carbonaceous reduction products can be created depending on the composition, size, and microstructure of the catalyst particles, including formic acid/formate, carbon monoxide, alcohols, and hydrocarbons. In general, smaller catalyst particles, ideally in the nanoparticulate (<< 1 μm) range, tend to yield superior catalytic performance, due to a combination of factors such as a higher density of exposed grain boundaries and a higher fraction of exposed crystalline facets that are uncommon in particles of micron size or larger.This talk will survey recent work illustrating the ability of pulse/pulse-reverse electrodeposition processes to tune the size of particles applied to gas-diffusion electrode substrates, with a primary focus on two single-metal catalyst materials relevant to carbon dioxide electroreduction: tin and copper. The former is a catalyst primarily for formic acid production, while the latter is unique among single-metal catalysts as the only element known to date to produce significant amounts of hydrocarbons and/or alcohols. Particle morphology and representative particle size will be discussed as a function of pulsed electrodeposition waveform parameters, with the goal of highlighting overarching trends across the waveform space.

Space programs around the globe have begun to consider the logistical demands of missions beyond the orbital neighborhood of Earth. Unlike local installations such as the International Space Station, long-range missions will not have the option to resupply critical materials from Earth. Thus, the development of capabilities for what is often termed “In-Situ Resource Utilization” (ISRU) have been a continuing focus of research through NASA and other agencies. One particular long-range mission of interest is to place human astronauts on Mars; the major component of the thin Martian atmosphere is carbon dioxide, making CO2 a natural input to ISRU technologies for production of carbon-containing materials. Production of mission consumables from in-situ Mars resources will be critical to enabling human exploration of Mars, in part by minimizing the number and size of descent/ascent vehicles. Potential ISRU products from CO2 include that seem likely to provide significant mission benefits with minimal infrastructure required are propellants (e.g., hydrocarbons), fuel cell reactants (e.g., formic acid, methanol, carbon monoxide), and life support consumables (e.g., oxygen). The first portion of this talk will comprise a high-level overview of the chemical transformations that can be imparted to CO2 via electrocatalysis on gas-diffusion electrodes (GDEs), in the form of a summary of literature reports on the catalytic performance of a wide variety of single-metallic and metal-alloy systems. The remainder will encompass an exposition of the electrocatalytic performance of tin and copper single-metal GDE electrocatalysts prepared by pulsed electrodeposition. These metals are well known for their ability to reduce carbon dioxide to formic acid and hydrocarbons/carbon monoxide, respectively, and are under active development in numerous academic research groups and industrial entities to this end. These experimental results clearly demonstrate the power and flexibility of the pulse/pulse-reverse electrodeposition approach to catalyst fabrication, as evidenced by the appreciable effects of the pulsed-waveform electrodeposition parameters on CO2 electroreduction product distribution and total current density.

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. 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. These configurations utilize a range of electrode types including metal plates, meshes, packed granules, and gas diffusion electrodes (GDEs) [1]. 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.However, the performance of GDEs in various CO2 electroreduction processes can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. At higher catalyst loadings (thicker catalyst layers), which are desirable for high production rates, conversion efficiencies drop and undesirable side product formation (both from hydrogen evolution and diversion of carbon to alternative reaction pathways) increases due to reactant starvation. Reducing particle size typically enhances both catalyst utilization and activity per unit mass. This, in turn, may enable thinner catalyst layers, mitigating or avoiding such decreases in product selectivity. While synthesis methods exist for generating smaller (< 10 nm) particles, these particles must still be deposited on a gas-diffusion layer (GDL) substrate such that ionic and electronic contact can be maintained with the electrolyte and GDL, respectively.Previous work directed towards platinum (Pt) catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated an “electrocatalyzation” (EC) 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 discuss the electrodeposition of tin (Sn) and copper (Cu) onto both commercially-available and custom-fabricated GDLs through an EC process, and the electrocatalysis performance of these catalysts as compared to state-of-the-art Sn and Cu nanoparticle catalysts (75-150 nm) prepared by spray-coating. Testing in a custom flow-cell electroreactor has demonstrated that the EC GDEs exhibit electrocatalytic performance comparable or superior to both literature reports and the spray-painted catalysts. Further, clear effects of the pulsed-waveform EC parameters on product distribution and total current density will be highlighted. Preliminary work toward development of GDLs robust against electrolyte saturation/penetration over many hours of operation will also be discussed. In summary, the highly scalable EC approach appears promising for fabricating active catalytic layers directly onto GDL substrates for carbon dioxide reduction applications.References[1] I. Merino-Garcia, E. Alvarez-Guerra, J. Albo, A. Irabien, Chemical Engineering Journal, 305 (2016) 104-120.[2] M. E. Inman, E.J. Taylor, in, U.S. Patent No. 6,080,504, 2000.[3] N .R.K. Vilambi Reddy, E. B. Anderson, E.J. Taylor, in, U.S. Patent No. 5,084,144, 1992.[4] E.J. Taylor, E.B. Anderson, N.R.K. Vilambi, Journal of The Electrochemical Society, 139 (1992) L45-L46.