Preparation of platinum catalysts on carbon support for polymer electrolyte membrane fuel cell using strong electrostatic adsorption technique

Abstract

In the polymer electrolyte membrane (PEM) fuel cell development, the catalytic activity requires a small particle size, a high metal dispersion, high conductivity, and long durability. The study was to prepare the platinum on graphene as the catalysts and the strong electrostatic adsorption (SEA) technique was used to adsorb Pt precursors onto supported graphene for better catalytic activity for polymer electrolyte membrane fuel cell (PEMFC). The Pt/graphene using the SEA technique with considering the pH shift and the point of zero charge (PZC) of graphene were acquired at pH of about 5.2. The cationic precursor (i.e., platinum tetra-ammine ([NH3)4 P1]2t or PTA) was chosen due to the mid-to-low PZC. After the graphene surface was treated to be an anionic substrate, the PTA was added and adsorbed onto the graphene by electrostatic force. After the SEA process, the samples were examined the Pt loading by inductively coupled plasma spectroscopy (ICP) to consider Pt percent weight. The crystalline Pt particles onto the supported graphene were made after the reduction in the hydrogen environment. The second adsorption including the reduction was repeated to obtain the high Pt percentage around 20%wt. The different post-treatments were applied to the reducing: (1) heat treatment at 400, 500, 600, 700, and 750 C for 2 hours and (2) Aging time. The average particle sizes (ca. 2.2 nm) and distribution of Pt were inspected using transmission electron microscopy (TEM), where the crystalline structures were verified by X-Ray diffraction (XRD). The oxidation states of platinum on graphene support of the catalyst were determined by X-ray Photoelectron Spectroscopy (XPS). Electrochemical properties were tested using cyclic voltammetry (CV) and the accelerated durability test (ADT) was also carried out. The oxygen reduction reaction (ORR) was also carried out, where the specific activity was examined. It was observed from ADT that the ORR activity loss about 40%. Furthermore, the ORR was performed to verify the first-order reaction, as well as to determine the mechanism pathway for four- electron transfer. Moreover, the kinetic constant of the ORR was also estimated. The optimal post-treatment is 600 C. So, the Pt/graphene catalysts after post-treatment at 600 C for 2h have the best electrochemical performance in term specific activity as 764 HA cm2. Moreover, the polarization for single-cell fuel cell results showed that the catalytic activity and the stability of Pt/graphene can be improved by post-treatment and the performance is better than commercial Pt/C catalyst (around 1.2- fold).