Pulsed Electrodeposition of Tin Electrocatalysts Onto Gas Diffusion Layers for CO2 Reduction to Formate
Authors: Sen Sujat , Skinn Brian , Hall Timothy D , Inman Maria , Taylor E. J. , Brushett Fikile R. 
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, per existing literature, the performance of GDEs in various CO2 electroreduction processes can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. Prior reports have demonstrated electroreduction of CO2 to formate (FA) on commercial tin nanoparticle (150 nm) loaded gas diffusion layers (GDLs) at current densities of 200 mA/cm2, 80% selectivity to FA, and cathodic potentials of -0.8 V vs. RHE.[2, 3] However, these and other studies have also reported that at higher catalyst loadings (thicker catalyst layers), which are desirable for high production rates, conversion efficiencies drop and undesirable side product formation 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. While synthesis methods exist for generating smaller (< 10 nm) particles, these particles must still be deposited on a GDL such that ionic and electronic contact can be maintained with the electrolyte and GDL, respectively. These critical interfaces are key to maximizing electrode performance in terms of product generation rate, selectivity, and catalyst utilization.
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).[4-6] 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.[6]
Here we investigate the electrodeposition of tin (Sn) onto commercially available GDLs through an EC process and benchmark our results against a state-of-the-art Sn nanoparticle catalysts (150 nm) spray-coated on a GDL. Electrolysis experiments are conducted in a three compartment W-Cell setup using the Sn-coated GDEs as cathodes and Pt/H2 counter electrode. We demonstrate that the EC GDE samples can exhibit up to 388 mA/cm2 total current density and 76% selectivity to formate at cathodic potentials of -0.8 V vs. RHE, representing a two-fold improvement in current density over both our benchmark electrode and existing reports using Sn-loaded GDEs prepared by conventional methods.[2, 3] We hypothesize that this enhancement arises from improved catalyst utilization, leading to high electrode activity. Surprisingly, SEM imaging of the EC GDE reveals Sn particles no smaller than the micron scale (~10 μm). Thus, we anticipate further improvement in electrode activity may be realized through suitable tuning of the EC waveform to yield nanoscale Sn particles (< 10 nm). In summary, the EC approach appears promising for fabricating active catalytic layers directly onto GDL substrates.

References

[1] I. Merino-Garcia, E. Alvarez-Guerra, J. Albo, A. Irabien, Chemical Engineering Journal, 305 (2016) 104-120.
[2] D. Kopljar, N. Wagner, E. Klemm, Chemical Engineering & Technology, 39 (2016) 2042-2050.
[3] D. Kopljar, A. Inan, P. Vindayer, N. Wagner, E. Klemm, Journal of Applied Electrochemistry, 44 (2014) 1107-1116.
[4] M. E. Inman, E.J. Taylor, in, U.S. Patent No. 6,080,504, 2000.
[5] N .R.K. Vilambi Reddy, E. B. Anderson, E.J. Taylor, in, U.S. Patent No. 5,084,144, 1992.
[6] E.J. Taylor, E.B. Anderson, N.R.K. Vilambi, Journal of The Electrochemical Society, 139 (1992) L45-L46.