Highly Selective Electrolytic Reduction of CO2 to Ethylene

Abstract

We investigate the reduction of CO₂ to ethylene across buffered anolyte pH values 4 to 14 using a copper-phosphorus (Cu-P) electrocatalyst in a zero-gap membrane electrode assembly. Electrochemical CO₂ reduction using alkaline electrolytes typically shows limited carbon efficiencies and single-pass efficiencies, while acidic conditions typically favor the hydrogen evolution reaction. Results from this work show that weakly phosphate-buffered acidic anolytes (pH 6) maximize ethylene production with a 73% FE at 300 mA cm^-2 and 51% FE at 500 mA cm^-2, including a 51% single-pass CO₂ conversion efficiency for over 400 h of continuous operation. We propose a mechanism based on pH-dependent CO coverage that controls the selectivity at the *HCCOH intermediate. Low CO coverage at pH 6 favors hydroxide elimination to *CCH, yielding ethylene (98% of C₂ products), while high coverage at pH 14 promotes hydrogenation to ethanol (44% of C₂). The HER mechanism transitions from H₂O-mediated at pH 14 to phosphate-mediated (H₂PO₄ ^-/HPO₄ ^2-) at weakly acidic pH, minimizing HER competition at pH 6. This mechanistic understanding enables controlled C₂ product selectivity through manipulation of the CO coverage and local proton activity.

Keywords: CO2 electroreduction; copper-based electrocatalyst; ethylene selectivity; membrane electrode assembly; pH optimization.

1

Morphology and structural characterization. The (A) SEM image of Cu–P showing spherical nanoparticles with average diameters around 100 nm. (B, C) EDS elemental mapping shows a uniform distribution of Cu and P elements. (D) XRD patterns of Cu–P, (E, F) Cu 2p, and P 2p XPS spectra of Cu–P.

2

CO₂ electrolysis to ethylene. (A) Schematic representation of the membrane electrode assembly system studied in this work. (B) Comparison of reported CO₂-to-ethylene electrocatalytic systems.

3

Electrocatalytic CO₂ reduction performance at constant cell potential of 4 V. (A) The effect of phosphorus content on the FE distribution of CO₂ reduction products in KHCO₃ (pH = 8). (B) The influence of pH on the single-pass CO₂ conversion efficiency (SPCE), C₁ product selectivity, and C₂ product selectivity during CO₂ reduction on Cu–P_0.065. (C) The effect of pH on the FE for various CO₂ reduction products. (D) Relative selectivity distribution of C₂ products across the pH range.

4

Membrane, pH, and cation effects on the CO₂ electroreduction performance. (A) FE distribution across pH 2–6 in a CEM system using pure H₃PO₄. (B) FE distribution across different membranes after K⁺ (1 M) addition. (C) Long-term stability test of the AEM system at pH 6. (D) Effect of the K⁺ concentration (0.1–2 M) on the product distribution at pH 6.

5

Effects of cation identity and anion type on CO₂ electroreduction performance. (A) FE distribution across different cation systems (Na⁺, K⁺, and Cs⁺) at pH 14. (B) Relative selectivity of C₂ products showing an ethylene:ethanol ratio at pH 14. (C) Faradaic efficiencies across different anions (PO₄ ^2–, SO₄ ^2–, and NO₃ ^–) at pH 6. (D) Relative selectivity of C₂ products across anions at pH 6.

6

Effect of current density on CO₂ reduction performance in the AEM system. (A) FE distribution for CO₂ reduction products as a function of current density (100–500 mA cm^–2) at pH 6 using Cu–P_0.065 electrocatalyst. (B) Total C₂ product selectivity and relative distribution of C₂ products (ethylene and ethanol) across different current densities (100–500 mA cm^–2) at pH 6 using the Cu–P_0.065 electrocatalyst. (C) FE distribution for CO₂ reduction products as a function of current density (100–500 mA cm^–2) at pH 6 using an unmodified Cu electrocatalyst. (D) Total C₂ product selectivity and relative distribution of C₂ products (ethylene and ethanol) across different current densities (100–500 mA cm^–2) at pH 6 using an unmodified Cu electrocatalyst.

7

Proposed mechanism for the formation of C₂ products from CO₂ electrolysis. As CO coverage increases, selectivity shifts from ethylene to ethanol and then to acetate.

8

Relative rates of the Volmer step for five different proton donors as a function of electrolyte pH at a constant U _RHE. The black line shows the total HER rate for all proton donors in an electrolyte with 1 mol/L total phosphate.