Effect of adding silica on ORR catalysts of HT-PEMFC: Prevention of the phosphate poisoning

Abstract

High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) operating at 120-180 ° C can be used under dry conditions and reduce CO poisoning, so overall system size can be smaller than low temperature PEMFCs. At the HT-PEMFC, the phosphoric acid distributes evenly through the membrane and the electrode, moving the protons from the anode to the cathode and then allowing the fuel cell to operate. However, the Pt used as the catalyst of the electrode is poisoned by the phosphate ion so the catalytic activity drops. Phosphate poisoning occurs in most sections of the fuel cell operating voltage and is one of the causes of declining of fuel cell efficiency. [1]. Therefore, a catalytic study of high temperature PEMFC has been studied to prevent poisoning by phosphoric acid [2]. When ordered mesoporous carbons (OMCs) are made using the ordered mesoporous silica (OMS) as a hard template, the phosphoric acid is added. The phosphate ions attach to the silica wall, allowing the carbon to be impregnated well in the OMS pores [3]. This study is effect of preventing phosphate poisoning by adding silica on the oxygen reduction reaction (ORR) catalysts. Since silica can increase the electrode resistance due to non-conductive materials, we have found a suitable composition ratio through a half-cell test. The catalysts were 50% PtCo / C of TKK (Tanaka co.) and 40% of LUDOX silica of Sigma Aldrich, and adjusted to 4wt%, 8wt%, and 12wt% silica, respectively. In order to investigate the effect of phosphate ions, 0.05M H3PO4, 0.15M H3PO4 and 0.3M H3PO4 solutions were added to 0.1M HClO4 solution to increase the amount of phosphate ions in the solution and the test was also conducted in only 0.5M H3PO4 solution. Comparing cyclic voltammetry, peaks between 0.2V and 0.4V caused by phosphoric acid poisoning decreased as the amount of silica increased [4]. As the amount of silica increased, the half-wave potential gap of 0.1M HClO4 and 0.5M H3PO4 was reduced from 52mV to 4mV. The highest ORR activity was obtained when 8wt% silica was added. This shows that silica has an effect of preventing the phosphate poisoning of ORR catalysts, and when too much silica is used, the performance of the electrode is decreased by covering the catalyst. We will apply it to the electrode and test it to see if it can be applied to actual application through the membrane electrode assembly (MEA) test. We will study the addition method of silica to improve durability.