Materiais Eletródicos à Base de Carbono para Produção de H2 via Eletrólise da Água em Meio do Líquido Iônico TEA-PS.BF4

High purity hydrogen can be obtained by water electrolysis. Normally, the platinum (Pt) cathode is used as working electrode which is corrosion resistant but is a high-cost material. However, carbon materials are plentiful and capable to making electrodes. Recently, vitreous carbon cathode (VC) was used in water electrolysis system in 3- triethylammonium-propane sulfonic acid tetrafluoroborate ionic liquid (TEA-PS.BF4) aqueous solution, which presented lower activation energy for the hydrogen evolution reaction (HER) when compared to Pt [1]. On the other hand, vitreous carbon is an amorphous material, whereas molded graphite (MG) and pyrolytic carbon (PC) are crystalline and have higher electrical conductivity. In this sense, this work proposes to study the effect of cathodes microstructure on VC, MG, and PC in HER by water electrolysis in TEA-PS.BF4 medium. Raman spectroscopy, X-ray diffraction and scanning electron microscopy (SEM) were used to analyze the carbon materials. The results shown that MG is crystalline and fairly ordered, but presents an irregular surface. The PC is crystalline and ordered with irregular pores, and CV presented a great disorder and an amorphous structure but has a homogeneous surface. The electrodes are stable after 30 cycles of cyclic voltammetry. The reaction mechanism of H2 production is obtained by Tafel parameters. The HER for all electrodes follow the Volmer-Heyrovsky mechanism. In addition, MG presents a lower exchange current density than PC, that present a higher value. Measurements of chronoamperometry at - 2.15 V (EQRPt) are performed to obtain the activation energy (Ea) at temperature range from 20 - 60 oC. The average current density increases with temperature for the VC and the PC but on the MG, due to the presence of adsorbed bubbles on the surface, the current density decreases with increasing of temperature. In addition, VC presents higher current densities for all temperatures. The Ea of the system for PC is 11.8 kJ mol-1, for VC is 9.6 kJ mol-1, while for MG is 8.8 kJ mol-1. Although, MG presents low Ea, the deactivation of the electrode due the presence of bubbles on the surface irregularities, disfavors the use of this material as electrode in the water electrolysis. PC presents a high activation energy, although showing a high crystallinity. VC presents the highest current densities and lower Ea and in addition, the results are better than those of Pt and can be considered a good candidate to a cathode for water electrolysis in TEA-PS.BF4 medium.