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

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).