Copolymer of Phenylene and Thiophene toward a Visible‐Light‐Driven Photocatalytic Oxygen Reduction to Hydrogen Peroxide

Abstract π‐Conjugated polymers including polythiophenes are emerging as promising electrode materials for (photo)electrochemical reactions, such as water reduction to H2 production and oxygen (O2) reduction to hydrogen peroxide (H2O2) production. In the current work, a copolymer of phenylene and thiophene is designed, where the phenylene ring lowers the highest occupied molecular orbital level of the polymer of visible‐light‐harvesting thiophene entities and works as a robust catalytic site for the O2 reduction to H2O2 production. The very high onset potential of the copolymer for O2 reduction (+1.53 V vs RHE, pH 12) allows a H2O2 production setup with a traditional water‐oxidation catalyst, manganese oxide (MnOx), as the anode. MnOx is deposited on one face of a conducting plate, and visible‐light illumination of the copolymer layer formed on the other face aids steady O2 reduction to H2O2 with no bias assistance and a complete photocatalytic conversion rate of 14 000 mg (H2O2) gphotocat −1 h−1 or ≈0.2 mg (H2O2) cm−2 h−1.

The MALDI spectrum of PPT gave peaks at 721.2-3365.6 (main peak at 1442.4, monomer segment peak at 240.4), indicating the formation of PPT with a high degree of polymerisation.
XPS measurements gave only peaks assignable to C and S, with no peaks ascribable to contaminating metal species (such as Sn, Fe, or Pd) or residual oxidant (i.e. iodine) in the PPT layers (below the detection limit or no additional intensities after background subtraction).
The HOMO and LUMO levels of PPT were calculated from its oxidation potential (+0.79 V vs. Ag/AgCl in a 0.1 M tetrabutylammonium perchlorate acetonitrile solution at a scan rate of 50 mV s −1 ) and maximum absorption edge wavelength (570 nm). The XRD pattern of 1,4di(2-thienyl)benzene gave d 001 ~7° (2), d 100 ~15° (2), and d 101 ~23° (2), suggesting lamellar and fishbone packing among the neighbouring dithienylbenzene molecules. After the formation of PPT, all of the long-range ordering signals from the crystal structure disappeared in the XRD patterns, as has been described in our previous paper. 4
Nitric acid, Mn(CH 3 COO) 2 , NaOH, and H 2 SO 4 , were purchased from FUJIFILM Wako Pure Chemical Corporation. Standard H 2 O 2 solutions were purchased from Millipore and Sigma-Aldrich. Figure S1. a, Normalized UV-Vis absorption spectra of a PPT layer (22 nm thickness), poly(terthiophene) layer (29 nm), and poly(3-hexylthiophene) (P3HT) layer (65 nm) cited from a previous paper [4] . b, Action spectrum of the PPT layer. The UV-vis spectrum of PPT exhibits additional peaks at a relatively high wavelength (520 nm), which suggests an additional π-orbital overlap. [5] No absorption in the range of 600-900 nm supported its undoped state. PPT gave a higher absorbance than those of other typical polythiophenes. The UV-vis spectrum (arbitrary absorption) of dithienylbenzene (10 mg/mL) shows a single absorption peak at 340 nm. Figure S2. a, b, Linear sweep voltammograms recorded in the dark at 1 mV s −1 and different pHs for PPT as a cathode. J is current density calculated from the O 2 reduction reaction.

Electrocatalytic properties of PPT in the dark for H 2 O 2 production
The ability of PPT to electrocatalytically reduce O 2 to H 2 O 2 was also investigated in the dark.
PPT layers formed on glassy carbon were electrochemically tested at pH 2-12. The electrochemical response at pH 12 was clearly different from those at lower pH, which was ascribed to the occurrence of different O 2 reduction reactions ( Figure S2). At pH 12, O 2 reduction to HO 2 − is dominant, which leads to a different electrochemical behaviour. The overpotential of the PPT electrocatalyst decreased with increasing pH and approached zero at pH 12.   This spectrophotometric titration method to chemically quantify highly dilute H 2 O 2 concentrations [7] is based on the simple, selective, and quantitative colour-changing reaction of the copper(I) complex of 2,9-dimethyl-1,10-phenanthroline with H 2 O 2 .  Table S2. Photoelectrochemical H 2 O 2 production in the several cycles. At 0 V vs. Ag/AgCl and pH 12 using an electrochemical cell with a volume of 10 mL. A PPT layer with a thickness of 149 nm was used in these measurements. The experiments were performed at different cycles on the same PPT layer, with the same area employed for each electrolyte. Air (oxygen/nitrogen gas mixture) was bubbled through the cell for 30 min prior to testing.

Supplementary Tables
Illumination was provided by an Asahi Spectra MAX-302 300-W Xe lamp with an equivalent power of 1.0 sun at the distance of the PPT photocathode (5 cm). The glass electrochemical cell window filtered out deep UV light at λ < 320 nm.