Main-chain engineering of polymer photocatalysts with hydrophilic non-conjugated segments for visible-light-driven hydrogen evolution

Abstract

Photocatalytic water splitting is attracting considerable interest because it enables the conversion of solar energy into hydrogen for use as a zero-emission fuel or chemical feedstock. Herein, we present a universal approach for inserting hydrophilic non-conjugated segments into the main-chain of conjugated polymers to produce a series of discontinuously conjugated polymer photocatalysts. Water can effectively be brought into the interior through these hydrophilic non-conjugated segments, resulting in effective water/polymer interfaces inside the bulk discontinuously conjugated polymers in both thin-film and solution. Discontinuously conjugated polymer with 10 mol% hexaethylene glycol-based hydrophilic segments achieves an apparent quantum yield of 17.82% under 460 nm monochromatic light irradiation in solution and a hydrogen evolution rate of 16.8 mmol m^-2 h^-1 in thin-film. Molecular dynamics simulations show a trend similar to that in experiments, corroborating that main-chain engineering increases the possibility of a water/polymer interaction. By introducing non-conjugated hydrophilic segments, the effective conjugation length is not altered, allowing discontinuously conjugated polymers to remain efficient photocatalysis.

Fig. 1. Schematic illustration of design strategy and polymer structures.

a Schematic illustration of the polymer photocatalysts containing hydrophilic non-conjugated segments. b The molecular structures of hydrophilic segments and the corresponding DCPs.

Fig. 2. Optical properties of polymers.

a Solid-state UV-vis diffuse reflectance spectra, b Tauc plots, and c energy level diagram of all polymers as measured by PESA in conjunction with Tauc plot. The dashed lines correspond to the proton reduction potential (H⁺/H₂). All energy levels and electrochemical potentials are expressed relative to vacuum (using −4.44 V versus vacuum as equivalent to 0 V versus SHE). Energy levels measured by cyclic voltammetry (Supplementary Fig. 7) in conjunction with Tauc plot indicate slightly different energy levels. However, in both cases the energy levels are suitable for proton reduction.

Fig. 3. Optical and electrochemical properties of polymers.

a Photoluminescence spectra were obtained in suspensions (5 mg photocatalyst in 10 mL solution mixture consisting of equal volumes of H₂O, MeOH, and TEA). b Time-resolved photoluminescence spectra were obtained in suspensions (5 mg photocatalyst in 10 mL solution mixture consisting of equal volumes of H₂O, MeOH, and TEA). Samples were excited with a 405 nm laser and emission was measured at 455 nm. c EPR spectra were collected in the solid state under additional light illumination. d Electrochemical impedance spectra of polymers were carried out in dark with an AC potential frequency ranging from 0.1 Hz to 100 kHz. In the equivalent circuit, R_s represents the circuit series-resistance, R_ct is the charge transfer resistance across the interface, and C_dl is the capacitance phase element of the semiconductor-electrolyte interface. e Photocurrent were generated upon light on-off switching. f Wide-angle X-ray scattering profiles of PFBPO and P-HEG-10 were measured in a mixture solution consisting of equal volumes of H₂O, MeOH, and TEA.

Fig. 4. Photocatalytic hydrogen evolution experiments in solution.

a Time-dependent HER of DCP and PFBPO photocatalysts under 380–780 nm irradiation (5 mg of polymer powder and 10 mL of a mixed solution consisting of 33.3 vol.% H₂O, 33.3 vol.% MeOH, and 33.3 vol.% TEA). b Concentration dependence of the HER over PFBPO, P-HEG-10, and P-HEG-20. c Linear relationship between the HER and the quantity of photocatalytic solution (10, 15, and 20 mL of solution with a constant P-HEG-10 concentration). d Correlation between the apparent quantum yield (AQY) and the UV-vis absorption spectra of P-HEG-10. e HER values in the absence of MeOH (i.e., in an 80 vol.% H₂O/20 vol.% TEA solution) over PFBPO, P-HEG-10, and P-EA-5 (L columns) compared to the HER values in presence of MeOH (i.e., in a 33.3 vol.% H₂O/33.3 vol.% MeOH/33.3 vol.% TEA solution) (R columns). f Comparison of the time-dependent HER over P-HEG-10, PFBPO, and PFTEGBPO using the same photocatalytic conditions as used in a.

Fig. 5. Photocatalytic hydrogen evolution experiments in film.

a Schematic illustration of the photocatalytic hydrogen evolution in film systems. b Images of the 0.6 × 0.6 cm² film over PFBPO and P-HEG-10. c Time-dependent HER of PFBPO and P-HEG-10 film immersed in 10 mL of a H₂O/MeOH/TEA solution under 380–780 nm irradiation. d Correlation between the time and the water contact angles of Si-wafer, PFBPO, and P-HEG-10. e Time-dependence water contact angles without light illumination of PFBPO and P-HEG-10 films.

Fig. 6. Molecular dynamics simulations.

a The workflow of molecular dynamics study. The conjugate polymer models were built according to the overall mass and different repetition units. The system was filled with water in an amorphous cell to evaluate the hydro-bonding. b The statistics of hydrogen bounds and c the possibility of hydrogen bond formation of polymer photocatalysts.