Effect of phosphoric acid purity on the electrochemically active surface area of Pt-based electrodes

Abstract

• H 3 PO 4 should be purified before ORR measurements to remove impurities which can be oxidized. • H 3 PO 3 was suggested as one of the impurities in the unpurified H 3 PO 4 solution. • ESCA HUPD analysis is more accurate than ESCA CO in the presence of H 3 PO 3. • H 3 PO 3 and CO compete for Pt surface active sites. • Commercial “crystalline H 3 PO 4 ” contains low amount of reduced phosphorus impurities. In this work, the effect of H 3 PO 4 purity on the activity of Pt/C thin film catalysts towards the oxygen reduction reaction (ORR) was investigated. H 3 PO 4 is routinely introduced in the electrolyte during rotating disk electrode (RDE) measurements to simulate the existing environment within high-temperature proton exchange membrane fuel cells (HT-PEMFC). Three different purity grades were tested: crystalline (99.99% purity), commercial H 3 PO 4 solution (85 wt%), hereafter, designated as non-purified H 3 PO 4 , and commercial H 3 PO 4 solution (85 wt%) purified with H 2 O 2 . H 3 PO 4 and/or its anions are known to strongly adsorb and interact with Pt surfaces. The presence of H 3 PO 4 negatively affected the electrochemically active surface area (ECSA) measured by H upd (ECSA H ), and by CO stripping (ECSA CO ), kinetic parameters in the high current density region and the limiting current density ( j lim ) of ORR. One major finding was that the crystalline and purified H 3 PO 4 solutions have similar effects on the Pt/C catalyst activity while the non–purified H 3 PO 4 showed a significantly more negative effect on the ECSA as well as on the ORR measurements. This was found to be due to the presence of H 3 PO 3 in the non-purified H 3 PO 4 solution. Adsorption isotherms of H 3 PO 3 were also measured using H upd and CO stripping in order to evaluate its adsorption on the catalyst surface. From these investigations, the purity level of H 3 PO 4 was shown to be an important factor in reliable ORR testing.