(232g) Pulse/Pulse-Reverse Electrodeposition of Gas-Diffusion Electrocatalysts for CO2 Reduction

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 [2, 3], the performance of GDEs in various CO₂ electroreduction processes can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. These studies have 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 such that ionic and electronic contact can be maintained with the electrolyte and gas-diffusion layer (GDL) substrate, 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 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 platinum catalyst nanoparticles (~5 nm) [4-6]. Moreover, since the catalyst was electroplated through an ionomer layer onto the 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].

In this talk, results will be presented from application of the above EC GDE preparation technique to two eCO₂RR electrocatalyst systems, formic acid-selective tin and hydrocarbon-selective copper. 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. EC GDEs were prepared by electrodeposition of pretreated GDLs in patented flow cells, which provide repeatable, controlled fluid dynamics, using pulsed electrical waveforms broadly similar to those described above [4-6]. The GDEs were tested in custom electrochemical cells and electrocatalysis performance characteristics such as total current density and selectivity for desired products (formic acid and ethylene for Sn and Cu 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). The available data for both catalytic systems will be summarized and compared to state-of-the-art catalysts as reported in the academic and/or patent literature.

Preliminary data for the Sn/formic acid system indicates the EC GDE samples can exhibit up to 388 mA/cm² total current density with 76% faradaic efficiency for formate at a cathodic half-cell potential of -0.8 V vs. RHE. This level of performance represents a two-fold improvement in current density at comparable formate efficiency as compared both to our benchmark spray-painted electrode and to existing reports of Sn-loaded GDEs prepared by conventional methods [2, 3]. Surprisingly, SEM imaging of the EC GDE revealed 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).

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.