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 similar to 190 degrees C, above which a weight loss accounting for similar to 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 x 10(-3) cm(2) 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 V-SCE and a holes density of 4.57 x 10(21) cm(-3) are determined. The electrochemical impedance spectroscopy (EIS), measured over a wide frequency range (1 mHz-10(5) Hz), reveals the bulk contribution with a constant phase element (CPE). The straight line at 43 degrees at low frequencies region is due to the Warburg diffusion. The energy diagram shows the feasibility of H-2 photo-evolution with a low over-voltage. The best performance is achieved at pH similar to 7 in the presence of S2O32- as holes scavenger. A hydrogen liberation rate of 660 mu mol 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-+ 3H(2)O -> 2SO(3)(2-)+2H(2)+2H(+), Delta G(f)degrees=24.3 kJ moL(-1)). (C) 2013 Elsevier B.V. All rights reserved.