The electrochemical reduction of carbon dioxide (CO2RR) presents a promising pathway for sustainable carbon chemical feedstock production and renewable energy storage. Copper (Cu) stands out as the most effective electrocatalyst for generating hydrocarbons and oxygenated products from CO2. However, enhancing the selectivity towards multicarbon (C2+) products, which offer higher energy density and wider applicability than single-carbon (C1) products, remains a significant challenge. Current strategies focus on modifying Cu's morphology, oxidation state, and doping, as well as altering the electrolyte composition. Another promising approach involves surface modification with organic molecules to tailor the Cu surface's microenvironment and influence product distribution. Previous research has shown that introducing molecular catalysts or organic compounds can improve CO2RR performance by affecting factors such as electric field strength, intermediate stabilization, hydrophobicity, and local CO2 concentration. However, a comprehensive understanding of how film thickness and porosity impact CO2RR remains limited. This research aims to systematically study the effects of these structural parameters on the CO2RR activity and selectivity on Cu electrodes modified with porous molecular films, paving the way for customized catalysts optimized for C2+ production.

Extensive research has been dedicated to optimizing copper catalysts for CO2RR. Studies have explored modifying Cu morphology (size, grain boundaries, facets), oxidation states, and dopants (alloying). Electrolyte composition, including cations and anions, has also been shown to influence selectivity. The use of molecular catalysts and organic modifiers has gained traction, with mechanisms proposed involving electric field effects, intermediate stabilization, hydrophobicity, and altered local CO2 concentration. For instance, Sargent and co-workers demonstrated the correlation between ethylene selectivity and the ratio of atop-bound to bridge-bound CO on Cu surfaces modified with N-arylpyridinium salts. Züttel and co-workers highlighted the influence of polymer hydrophobicity on H2O and CO2 transport. Grubbs and Goddard III showed that a tricomponent copolymer-modified Cu electrode exhibited excellent performance, attributed to both the polymer-induced electric field and increased film porosity for CO2 diffusion. These findings underscore the importance of film structure in controlling CO2RR activity and selectivity.

The researchers employed a modified electrodimerization procedure to electrodeposit 1,1'-di-p-tolyl-1,1',4,4'-bipyridine (T-bipyridine) films of varying thicknesses onto polished Cu foil electrodes. Different deposition potentials were used to control film thickness. Film thickness was determined using atomic force microscopy (AFM) by measuring the step height between the deposited film and the exposed Cu substrate. Scanning electron microscopy (SEM) was used to characterize film morphology, revealing increasing porosity and roughness with more negative deposition potentials. Film porosity was quantitatively assessed by comparing the film thickness before and after dissolving and reconstructing the film with acetone. Electrochemical CO2RR performance was evaluated using bulk electrolysis experiments at -0.96 V versus RHE in CO2-saturated 0.1 M KHCO3. Gaseous and liquid products were quantified using gas chromatography and 1H NMR spectroscopy, respectively. Electrochemical active surface area (ECSA) was determined using the double-layer capacitance method. Operando Raman spectroscopy was employed to gain insights into the reaction mechanism by monitoring CO adsorption and surface species during CO2RR. A microkinetic model was developed to simulate product distribution based on Langmuir adsorption and reaction rate equations, correlating selectivity changes with CO partial pressure.

The study revealed a remarkable enhancement in CO2RR performance with increasing film thickness and porosity. The Cu electrode modified with the thickest and most porous T-bipyridine film (Cu-5, d = 0.79 ± 0.08 µm) exhibited an almost tenfold increase in the intrinsic current density of ethylene formation (JECSA,C2H4 = 0.52 mA cm⁻²) compared to pristine Cu. The faradaic efficiency (FE) of ethylene reached 46.1%, with a total FE for C2+ products of 61.9%. Interestingly, the total current density remained largely unaffected by the modification when normalized to ECSA. AFM, SEM, and XPS analysis ruled out significant contributions from surface nanostructuring or changes in Cu oxidation state. Operando Raman spectroscopy showed enhanced CO coverage on Cu-5 compared to pristine Cu, indicating that the porous film increases the local CO partial pressure. The microkinetic model supported the hypothesis that the increased CO partial pressure and surface coverage near the Cu surface promoted C-C coupling and C2+ product formation. Experiments with thinner films and a non-porous film (Cu-5block) confirmed the crucial roles of film thickness and porosity in enhancing C2+ selectivity. Cyclic voltammetry with methyl viologen indicated that the porous film allowed sufficient diffusion of small molecules (CO2, H2O) while impeding larger molecules.

The findings demonstrate that tailoring the physical structure of organic modifiers on Cu electrodes significantly enhances CO2RR selectivity towards C2+ products. The increased local CO partial pressure and surface coverage, facilitated by the porous and thick film structure, are key factors in promoting C-C coupling and shifting the reaction pathway towards multicarbon products. The results highlight the importance of considering not only the chemical functionality but also the physical structure of molecular modifiers for catalyst design. The observed independence of total current density from film thickness, once normalized by ECSA, suggests that the active sites density might be influenced by the film, but that the inherent activity of Cu is not modified by the modifier itself. The microkinetic model provides strong support for the proposed mechanism and offers a valuable tool for guiding future catalyst design efforts.

This research successfully demonstrated the control of CO2RR activity and selectivity on Cu electrodes through precise manipulation of the physical structure of a deposited organic film. The significant enhancement of C2+ product formation, especially ethylene, is primarily attributed to the elevated local CO partial pressure and surface coverage within the porous film. This work provides a novel approach for improving CO2-to-C2+ conversion efficiency by customizing the catalyst's microenvironment. Future research should focus on exploring even thicker films, investigating different organic modifiers with tailored porosities, and exploring the influence of electrolyte composition to further optimize C2+ product selectivity and yield.

The study primarily focused on the effects of film thickness and porosity using a specific bipyridine-based modifier. Further investigation with other molecular modifiers is necessary to generalize the findings. The maximum film thickness achieved was 0.79 µm; exploring thicker films could potentially reveal further performance enhancements but may require addressing the film's stability issues. The microkinetic model, while providing valuable insights, is a simplified representation of a complex system. More sophisticated models incorporating adsorbate-adsorbate interactions and detailed surface kinetics would enhance predictive capabilities.