Cathode Platinum Degradation in Membrane Electrode Assembly Studied Using a Solid-State Electrochemical Cell

Abstract

The electrochemical dissolution of the Pt cathode catalyst, which determines the lifetime of polymer electrolyte fuel cells (PEFCs), has been studied. A newly developed solid-state electrochemical cell, which consisted of a Nafion membrane sandwiched by Pt/C-based catalyst layers, was employed for the electrochemical measurement. When humidified O 2 and H 2 gases were supplied to working and counter electrodes, respectively, the Pt electrochemical surface area (ESA) decreased during the potential cycle for 20 h. The highest rate of ESA decrease was recorded for an alternating potential cycle between (i) a positive-potential region of 0.2−0.65 V vs Ag/Ag 2 SO 4 and (ii) a negative-potential region of −0.73 to −0.4 V vs Ag/Ag 2 SO 4 . From an SEM−EDS observation of the cross section of a membrane sample after the potential cycle, Pt particles are found in the Nafion membrane, indicating that the decrease in ESA caused by Pt dissolution brings about the Pt deposition in the membrane. Under the assumption that O 2 reduction at the negative-potential region generates a large amount of H 2 O 2, EQCM measurement was conducted in an acid solution, which revealed the fact that H 2 O 2 accelerates the Pt dissolution. According to this demonstration, the Pt dissolution in the solid state was carried out by adding H 2 O 2 . As a result, the potential range for the Pt dissolution in the presence of H 2 O 2 was determined to be an alternating potential cycle between (i) a positive-potential region of 0.4−0.65 V vs Ag/Ag 2 SO 4 and (ii) a negative-potential region of −0.2−0.15 V vs Ag/Ag 2 SO 4 . It is considered that the mechanism for the Pt degradation in the solid state is the generation of H 2 O 2, which can reduce the Pt-oxide that protects Pt from dissolution.