Photo-electrochemical characterization of polypyrrol: Application to visible light induced hydrogen production

•The work deals with the hydrogen production on polypyrrole under visible light.•The energy diagram shows the spontaneous hydrogen evolution.•The hydrogen evolution peaks at pH 6.8 with a rate of 660 µmol h-1 (g catalyst)-1.

The semi-conducting properties of polypyrrol (PPy) are investigated by the photo-electrochemical technique. The polymer is stable up to~190 °C, above which a weight loss accounting for~60% is observed. An optical transition of 1.02 eV is determined, directly allowed. Further indirect transition is determined at 0.37 eV. The electrical conduction is characteristic of degenerate behavior with an activationless hopping and a hole mobility of 4.2×10−3 cm2 V−1 s−1. The p-type conductivity is evidenced from the Mott–Schottky plots. In neutral solution, a flat band potential of 0.108 VSCE and a holes density of 4.57×1021 cm−3 are determined. The electrochemical impedance spectroscopy (EIS), measured over a wide frequency range (1 mHz–105 Hz), reveals the bulk contribution with a constant phase element (CPE). The straight line at 43° at low frequencies region is due to the Warburg diffusion. The energy diagram shows the feasibility of H2 photo-evolution with a low over-voltage. The best performance is achieved at pH∼7 in the presence of S2O32− as holes scavenger. A hydrogen liberation rate of 660 μmol h−1 (g catalyst)−1 and a quantum efficiency of 0.21% under visible light (29 mW cm−2) are determined. The system is valuable for the chemical storage through the uphill reaction: (S2O32−+3H2O→2SO32−+2H2+2H+, ΔGf°=24.3 kJ mol−1).