Pulsed Electrodeposition of Gas-Diffusion Electrocatalysts for CO2 Reduction
Authors: Skinn Brian , Sen Sujat , Leonard McLain , Radhakrishnan Rajeswaran , Wang Dan , Forner-Cuenca Antoni , Hall Timothy D , Snyder Stephen , Brushett Fikile R. , Taylor E. J. 
Published: February 2020

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 (see Figure - Schematic comparison of catalyst particle size and location distributions in conventional spray-painting versus electrodeposition application methods). 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.

Figure 1