Why surface hydrophobicity promotes CO2 electroreduction: a case study of hydrophobic polymer N-heterocyclic carbenes

Abstract

We report the use of polymer N-heterocyclic carbenes (NHCs) to control the microenvironment surrounding metal nanocatalysts, thereby enhancing their catalytic performance in CO₂ electroreduction. Three polymer NHC ligands were designed with different hydrophobicity: hydrophilic poly(ethylene oxide) (PEO-NHC), hydrophobic polystyrene (PS-NHC), and amphiphilic block copolymer (BCP) (PEO-b-PS-NHC). All three polymer NHCs exhibited enhanced reactivity of gold nanoparticles (AuNPs) during CO₂ electroreduction by suppressing proton reduction. Notably, the incorporation of hydrophobic PS segments in both PS-NHC and PEO-b-PS-NHC led to a twofold increase in the partial current density for CO formation, as compared to the hydrophilic PEO-NHC. While polymer ligands did not hinder ion diffusion, their hydrophobicity altered the localized hydrogen bonding structures of water. This was confirmed experimentally and theoretically through attenuated total reflectance surface-enhanced infrared absorption spectroscopy and molecular dynamics simulation, demonstrating improved CO₂ diffusion and subsequent reduction in the presence of hydrophobic polymers. Furthermore, NHCs exhibited reasonable stability under reductive conditions, preserving the structural integrity of AuNPs, unlike thiol-ended polymers. The combination of NHC binding motifs with hydrophobic polymers provides valuable insights into controlling the microenvironment of metal nanocatalysts, offering a bioinspired strategy for the design of artificial metalloenzymes.

Scheme 1. Synthesis of three Im-terminated polymer ligands, including PEO₄₄–Im (P1), PS₁₀₀–Im (P2), PEO₄₄-b-PS₂₃₀–Im (P3), and molecular BMB–Im.

Fig. 1. (a) CVs of Au/C with different polymer ligands measured in N₂-saturated 0.5 M H₂SO₄ with a scan rate of 100 mV s⁻¹. (b) ECSAs of AuNPs modified with various ligands. (c) and (d) CV curves of AuNPs (c) and Au–P3 (d) under N₂ (black) and CO₂ (red). (e) and (f) LSV curves measured in CO₂-saturated 0.1 M KHCO₃ at a scan rate of 10 mV s⁻¹ normalized to (e) geometric area of electrodes and (f) ECSA of AuNPs.

Fig. 2. Electrocatalytic performance of CO₂ reduction on different catalysts: FE, (partial) current density and product ratio (ordered vertically) using pure Au (a)–(c), Au–P1 (d)–(f), Au–P2 (g)–(i) and Au–P3 (j)–(l). All tests were carried out in CO₂ saturated 0.1 M KHCO₃.

Fig. 3. (a) Scheme of BMB–Im and PEO–NHC modified AuNPs through Au–C binding. Catalytic selectivity to CO and H₂ using Au–BMB–Im (b) and Au–NHC–PEO (c). (d) Product ratio and (e) current density analysis of Au–BMB–Im and Au–NHC–PEO.

Fig. 4. Infrared spectroscopical study of Au electrodes with polymer NHC ligands. (a) CV scans in N₂ saturated 0.1 M KHCO₃ and (b) in situ ATR-SEIRAS spectra obtained at −0.3 V_RHE of AuNPs and Au–NHC–PS-b-PEO, respectively. In situ ATR-SEIRAS spectra and vibrational peak intensity study for Au–NHC–PS-b-PEO (c) and (d) and pure AuNPs (e) and (f) in CO₂-purged 0.1 M KHCO₃ under various reductive potentials. The spectra in (c) and (e) are background subtracted at 0 V.

Fig. 5. Interfacial diffusive properties of all catalysts. (a) CV scans of AuNPs and (b) Au–P3 measured with 50 mM K₃Fe(CN)₆ at various scan rates. (c) Linear relationship between peak current (I_p) and the square root of scan rates for pure Au and Au–P3. (d) Summary of diffusion coefficient (D) using various probe molecules including Fe(CN)₆³⁻, Ru(NH₃)₆³⁺, Fc–COOH, and CO₂.

Fig. 6. (a) Illustration of the simulation setup, with the Au, C, N, O, H atoms represented in yellow, grey, blue, red, and white, respectively; (b) density profiles as a function of z coordinates, with water and CO₂ shown in blue and red; (c) O_w–O_w radial distribution function, analyzed for water molecules above z = 11.8 nm (bulk region, blue) and those below z = 11.8 nm (polymer layer, red).

Fig. 7. Stability measurement using i–t curves (a), current retention (b) and ECSA (c). The experiments were carried out using constant-potential electrolysis at −1.0 V for 1 h. TEM images of pure Au (d) and Au–P3 (e) loaded on carbon before (top) and after (bottom) CO₂ electroreduction for 1 h. All scale bars are 40 nm.

Fig. 8. In situ ATR-SEIRAS spectra of surface polymer modified electrode demonstrating the chemical stability of NHC ligands to counterpart thiol ligands. In situ ATR-SEIRAS spectra of (a) Au–NHC–PS-b-PEO and (b) Au–SH–PS₂₆₇-b-PEO₄₄ at −0.8 V on Au film electrode in CO₂ saturated 0.1 M KHCO₃ electrolyte. (c) The desorption vibrational peak intensity at 2924 cm⁻¹ against reaction time. (d) The adsorption peak intensity of different electrodes after electrocatalysis under −0.8 V. Blue: Au–P3, red: Au–SH; dark: Au film.