Plasmon-Assisted Electrochemical CO2 Reduction on Copper Nanocubes
Authors: Al-Zubeidi Alexander , Bourgeois Briley , Dai Alan , Herndon Liam , Yuan Lin , McKeown-Green Amy , Dionne Jennifer 
Published: December 2024

Optical excitation of plasmons on metal nanoparticles presents a promising opportunity to enable electrochemical CO2 conversion into hydrocarbons at low overpotentials on stable electrodes. Electrochemical CO2 conversion typically requires high overpotentials, resulting in low energy efficiency and catalyst reshaping. By using light to overcome reaction barriers, plasmon excitation has been shown to lower the required overpotential for electrochemical reactions. Plasmon excitation also facilitates C-C coupling on gold and silver, metals that usually produce no multicarbon products when used as electrocatalysts. While previous studies indicate clear potential that plasmon-based photo-electrocatalysts could enable new electrochemical pathways for CO2 reduction at low overpotentials and stable catalysts, yields and catalytic turnover remained low. To develop industrially viable photo-electrochemical CO2 reduction catalysts, the effect of plasmon excitation at high current density and high mass flux needs to be understood and optimized.
Here we investigate the photo-electrochemical CO2 reduction on gold and copper nanoparticles on a gas diffusion electrode. We choose copper nanoparticles because they are the most promising electrocatalysts for C-C coupling and exhibit a strong plasmon resonance around 600 nm, yet have not been explored for plasmonic CO2 reduction. Gold nanoparticles are chosen due to their exceptional stability and because gold has been shown to perform C-C coupling under plasmon excitation, which does not happen electrochemically. Using a gas diffusion electrode allows efficient gas phase mass transport to and from the electrode. We synthesize 50 nm (100) faceted copper nanocubes using copper bromide and oleylamine, and gold nanorods using hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL). In both cases, the nanoparticles have a plasmon resonance of 600 nm. We spray coat these nanoparticles onto a carbon paper gas diffusion membrane and incorporate them into our home-built gas reactor. First, we use real-time GC to investigate electrocatalysis in the dark and find high selectivity for CO on gold and high selectivity for ethylene and CH4 on copper. Then, we vary the optical illumination wavelength from 480 nm to 700 nm and the applied potential, investigating how these affect Faradaic efficiencies for the production of ethylene, CO, H2, and CH4 as well as photothermal contributions using an infrared camera. Finally, we use in-situ liquid cell transmission electron microscopy (TEM) to monitor nanoparticle stability and correlate morphological evolution with CO2 reaction rate. Our work provides the fundamental understanding necessary for building photo-electrocatalysts that can convert CO2 into value-added products under industrially relevant conditions.
Figure caption: Schematic of our photo-electrochemical plasmonic gas diffusion electrode converting CO2 into hydrocarbons. Electrochemical CO2 conversion on gold typically leads to CO, under plasmon excitation gold can form hydrocarbons.

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