Investigation of the catalytic activity of nanostructured electrocatalysts for BH4- ions electro-oxidation in alkaline media

Author(s)

Aniélli Martini Pasqualeti

• Document type: Master's Dissertation
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT)
• Defense date: 2013-02-21

Examining board members

Fabio Henrique Barros de Lima; Fritz Cavalcante Huguenin

• Advisor: Fabio Henrique Barros de Lima

Abstract

The direct borohydride fuel cells (DBFC) display a high theoretical cell voltage (1.64) and a high electron number per borohydride ion. Furthermore, the DBFC also presents the advantages of alkaline fuel cells, in which it is possible to use non-noble metal electrocatalysts and, hence, it is economically feasible for practical applications. However the lack of highly efficient electrocatalysts for the borohydride oxidation reaction (BOR) limits the performance of these devices, since its total oxidation, involving the transfer of eight electrons per BH4- ion, competes with parallel reaction pathways with a lower number of exchanged electrons. Recently, theoretical calculations were applied to determine the elementary steps of the reaction kinetics and also to guide metallic electrocatalyst design for borohydride oxidation. Based on the theoretical results, this work aimed at the BOR electrocatalysis investigation on carbon supported nanoparticles, of pure metals, Au/C, Ag/C, Pt/C and Pd/C, and on bimetallic nanoparticles, composed by the combination of these metals. The electrocatalysts were synthesized by chemical and/or impregnation reduction methods, and X-Ray Diffraction, High Resolution Transmission Electron Microscopy and X-Ray Energy Dispersive Spectroscopy techniques were used for their physical characterization. The electrocatalytic activities were studied by steady state polarization curves using rotating disc electrodes. The borohydride hydrolysis, with the production of H2, was monitored in function of the electrode potential by on-line differential electrochemical mass spectrometry. The electrochemical experiments for the pure metal electrocatalysts showed higher electrocatalytic activity for Pd/C, and this was attributed to its high activity for BH3OH- electro-oxidation. The study of BH4- and BH3OH- concentration effect showed that with the increase of concentration, the onset potential shifted to lower values, indicating that the Faradaic currents of borohydride and hydroxyborane electro-oxidation were much higher than the current for the H2 evolution. The increase in the Pd/C load on the electrode led to an increase of the global current reaction at low potentials, which was associated to an increase in total surface area. Among the investigated bimetallic materials, the Pt2Pd/C electrocatalyst presented the higher Faradaic current. The Ag2Pd/C material showed the lower reaction potential onset. Although theoretical calculations pointed out to an increased activity of the Ag-modified Pd/C electrocatalysts in relation to that of pure Pd/C, the electrochemical and mass spectrometry results of this work indicated higher activity for pure Pd/C, and low synergic effect between the Ag and Pd atoms. This was associated to the low degree of interaction between Ag and Pd for the present investigated atomic composition. (AU)

• FAPESP's process: 11/03672-4 - Investigation of the catalytic activity of nanostructured electrocatalysts for BH4- ions electrooxidation in alkaline media
• Grantee: Aniélli Martini Pasqualeti
• Support Opportunities: Scholarships in Brazil - Master