Programme

P.18 -- Poster, NCTP-39

Date: Tuesday, 29-07-2014

Time: 10h30 - 12h00

Surface Oxide Formation on Pt Cathode of Proton Exchange Membrane Fuel Cells: Effects of Reaction Intermediates

D. N. Son (1), Nguyen Thi Gam (2), Kaito Takahashi (3), Viorel Chihaia (4)

(1) Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet Street, Dist. 10, Ho Chi Minh City, Vietnam; (2) College of Natural Sciences, Can Tho University, Campus II, 3/2 Street, Ninh Kieu District, Can Tho City, Vietnam; (3) Institute of Atomic and Molecular Sciences, Academia Sinica, No. 1, Roosevelt Road, Section 4, P.O. Box 23-166, Taipei, 10617, Taiwan, ROC; (4) Institute of Physical Chemistry “Ilie Murgulescu” of the Roumanian Academy, Splaiul Independentei 202, Sector 6, 060021 Bucharest, Roumania.

One of great challenges of using Pt in designing proton exchange membrane fuel cells (PEMFCs) for automotive applications is the degradation of Pt cathode electrocatalysts due to Pt dissolution under operating conditions over the time. Pt surface oxides are believed to be a crucial cause for the Pt dissolution in the oxygen reduction reaction (ORR) environment. Although gaseous O2 is always present during the ORR, ex situ techniques in studies of the Pt surface oxide formation have been conducted in the absence of O2 owing to experimental limitations. In the absence of O2, the surface oxides form at high potentials of more than 1.1 V at which surface reconstructions often occur simultaneously. In fact the PEMFC cathode operates over a potential range of 0.7 – 1.0 V for majority of the time during highway cruising conditions where the ORR are taking place. In situ techniques were conducted to resolve the ex situ experimental limitations. It was found that the Pt oxide formation initiated at potentials as low as 0.75 V under O2-sparging, and gaseous O2 significantly enhances the Pt dissolution during potential cycling. Many researches suggested that atomic oxygen in the subsurface, which resulted from oxygen diffusion from on-surface to subsurface of adsorbed oxygen atoms, should play as an initial driving force for the Pt dissolution in the ORR environment at low potentials. Furthermore, our computational studies have suggested that the ORR is a multistep reaction with many possible reaction intermediates such as OH, OOH, and HOOH. Using density functional theory calculations, Balbuena et al. has studied the diffusion of adsorbed oxygen to Pt subsurface in the presence of another atomic oxygen atom on Pt surface. In the presence of gaseous O2, it is, however, possible to find adsorbed O2 rather than only atomic oxygen on Pt surface. Regarding to adsorption strength, O2 is much weaker than atomic oxygen that may cause different effects on the oxygen diffusion. It means that the diffusion can happen in the presence of adsorbed O or/and adsorbed O2. The overall aforementioned context has led to a question that how adsorbed O2 and other reaction intermediates affect the Pt surface oxide formation. By answering this question, we can intimately elucidate the experimental data that at which reaction step the enhanced-dissolution effect is really performed by gaseous O2, and hence, can introduce the way to solve the deterioration of Pt catalyst. Acknowledgement. This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.01-2013.74.

Presenter: Do Son