Elucidating the Roles of Nafion/Solvent Formulations in Copper-Catalyzed CO2 Electrolysis

Abstract

Nafion ionomer, composed of hydrophobic perfluorocarbon backbones and hydrophilic sulfonic acid side chains, is the most widely used additive for preparing catalyst layers (CLs) for electrochemical CO₂ reduction, but its impact on the performance of CO₂ electrolysis remains poorly understood. Here, we systematically investigate the role of the catalyst ink formulation on CO₂ electrolysis using commercial CuO nanoparticles as the model pre-catalyst. We find that the presence of Nafion is essential for achieving stable product distributions due to its ability to stabilize the catalyst morphology under reaction conditions. Moreover, the Nafion content and solvent composition (water/alcohol fraction) regulate the internal structure of Nafion coatings, as well as the catalyst morphology, thereby significantly impacting CO₂ electrolysis performance, resulting in variations of C₂₊ product Faradaic efficiency (FE) by >3×, with C₂₊ FE ranging from 17 to 54% on carbon paper substrates. Using a combination of ellipsometry and in situ Raman spectroscopy during CO₂ reduction, we find that such selectivity differences stem from changes to the local reaction microenvironment. In particular, the combination of high water/alcohol ratios and low Nafion fractions in the catalyst ink results in stable and favorable microenvironments, increasing the local CO₂/H₂O concentration ratio and promoting high CO surface coverage to facilitate C₂₊ production in long-term CO₂ electrolysis. Therefore, this work provides insights into the critical role of Nafion binders and underlines the importance of optimizing Nafion/solvent formulations as a means of enhancing the performance of electrochemical CO₂ reduction systems.

Figure 1

Structural and morphological analysis of Nafion films and catalyst layers. (a) Schematic illustration of the effects of Nafion/solvent formulations on the resulting Nafion films. (b, c) AFM images of Nafion spin-coated films on Si substrates produced from 0.5 wt % Nafion in solutions with 0 and 50 vol % water, respectively. (d) Horizontal line cuts of the 2D GISAXS data of Nafion films prepared with 0.5 wt % Nafion solutions with different water contents. (e) Thicknesses of spin-coated Nafion films on Si as a function of Nafion content and solvent composition, measured by spectroscopic ellipsometry. (f–h) SEM images of CuO nanoparticles on Si produced from solutions with 0 vol % water containing 0, 0.005, and 0.5 wt % Nafion, respectively. (i) Electric double-layer capacitance of CLs deposited on glassy carbon as a function of Nafion weight ratio and solvent composition, measured in Ar-purged 0.1 M K₂SO₄ aqueous electrolyte.

Figure 2

Effect of Nafion/solvent formulations on potential-dependent CO₂ electrolysis. Catalytic performance of electrodes prepared by different Nafion/solvent formulations evaluated in terms of total current density (a), as well as Faradaic efficiency of H₂ (b), CO (c), and C₂H₄ (d).

Figure 3

Product distributions from CLs during chronopotentiometric CO₂ electrolysis at a total current density of −50.0 mA cm^–2. The electrodes were prepared by varied Nafion/solvent formulations containing seven different Nafion weight ratios (0, 0.005, 0.025, 0.05, 0.125, 0.25, and 0.5 wt %) and four different water contents, namely, (a) 0, (b) 25, (c) 50, and (d) 75 vol %.

Figure 4

Effect of Nafion/solvent formulations on the selectivity of chronopotentiometric CO₂ electrolysis at a total current density of −50.0 mA cm^–2 on carbon paper. Heatmaps of Faradaic efficiency for H₂ (a), CO (b), formate (c), C₂H₄ (d), ethanol (e), and C₂₊ products (f).

Figure 5

Effect of solvent composition on water uptake and reaction microenvironment during CO₂ electrolysis. (a) Plots of calculated water concentration in Nafion films from in situ ellipsometry tests vs water content in Nafion solutions. (b) Estimation of molar ratio of CO₂/H₂O (gray bars) on the basis of measured water concentration within Nafion coatings in (a) and the corresponding partial current densities for CO₂R (red squares) in chronopotentiometry test of −50 mA cm^–2.

Figure 6

Effect of Nafion weight ratio on morphological and compositional changes of CLs, as well as local reaction environments, during long-term CO₂ electrolysis. (a–c) SEM images of three carbon paper-based electrodes after 5 h of CO₂ 5 h of chronopotentiometry test at −50 mA cm^–2: 50 vol % water-0 wt % Nafion (a), 50 vol % water-0.005 wt % Nafion (b), and 50 vol % water-0.5 wt % Nafion (c). Changes of Faradaic efficiencies of H₂ (c), CO (d), and C₂H₄ (e) within 5 h of CO₂ electrolysis on three electrodes. (g–k) In situ Raman spectra recorded in cyclic voltammetry tests (g–i) and chronoamperometry tests (j, k) on three electrodes.