Dinitrogen reduction on a polypyrrole coated Pt electrode under high-pressure conditions: electrochemical impedance spectroscopy studies

Abstract

The electrochemical impedance spectroscopy (EIS) responses of a polypyrrole (PPy)-coated platinum electrode were investigated during N$_{2}$-reduction to ammonia in aqueous medium. Kinetic parameters such as film resistance, pore resistance, and double layer capacitance were analyzed as a function of applied potential and polymer film thickness. The relation between kinetic parameters was discussed by combining electrolysis results. It was found that the optimum film thickness of polypyrrole was 0.73 $\mu $m and optimum potential for ammonia synthesis was -0.150 V under 60 bar N$_{2}$-pressure. The impedance responses under these conditions presented the lowest pore resistance value of ca. 2 $\Omega $ cm$^{2}$. The electrolyte resistance was also 2 $\Omega $ cm$^{2}$ and the film resistance was ca. 5 $\Omega $ cm$^{2}$. Tafel slopes calculated from the Tafel curve and EIS-Tafel diagram gave corresponding results: 0.121 V dec$^{-1}$ and 0.128 V dec$^{-1}$, respectively; $\alpha $-transfer coefficient of 0.49 and an exchange current density with a value of 3.17 10$^{-3}$ A cm$^{-2}$ were characteristic for H$_{ad}$ formation in acidic aqueous medium.