Thermodynamic and SIMS analyses on the role of proton/water in oxygen reduction process and related degradations in solid oxide fuel cells

Abstract

Roles of Secondary Ion Mass Spectrometry and the local thermodynamic equilibrium approximation are discussed in understanding high temperature electrochemical processes. Fundamental issues are first considered on metal dense electrode (Pt/Au) and oxide electrolyte (YSZ/GDC) systems; the so-called three phase boundary (TPB) mechanism is dominant for YSZ systems, whereas for the GDC systems, a new proton/water mechanism is dominant; protons play an important role in a such way that the cathodic reaction of protons to form water vapors takes place at TPBs, while evaporated water vapors are transported on the GDC surface; oxide ions are transported to the anode and protons are circulated to the TPBs of the cathode; another effect of water vapors is to enhance oxygen isotope exchange reactions over a whole area of surface and a resulting flow of 18O inside GDC masks the charge transfer process in the SIMS analyses. A local proton flow in the proton/water mechanism provides a drastic change in the distribution of chemical potential of water vapors under the dense electrodes, leading to the pore formation or the non-homogenized cation distribution. Concerning industrial issues, two degradation topics are analyzed on drastic changes in chemical potential distribution caused by electrode reactions or by non-uniform flows of oxide ions inside electrolyte.