Characteristics of a Platinum Black Catalyst Layer with Regard to Platinum Dissolution Phenomena in a Membrane Electrode Assembly

Abstract

The nature of platinum dissolution and precipitation in a polymer electrolyte membrane of a membrane electrode assembly ͑MEA͒ for a proton-exchange membrane fuel cell ͑PEMFC͒ was studied using a potential holding experiment at 1.0 V vs a reversible hydrogen electrode and high-resolution transmission electron microscopy. The electrochemically active surface area decreased depending on the holding time, and platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. However, platinum dissolution and deposition out of the catalyst layer were greatly reduced when a platinum black electrode was used. In the experiment using a double-layered catalyst layer, platinum redeposited not on the carbon black surface but rather on the platinum black surface.As promising candidate power sources for electrically powered vehicles, small-scale stationary power generators, and portable electronic devices, proton-exchange membrane fuel cells ͑PEMFCs͒, which electrochemically convert the chemical energy of a fuel directly into electrical energy with high conversion efficiency, have received considerable attention. 1-5 For the commercial success of PEMFC power sources, reliability and life are the most important considerations. However, there is as yet limited information available about failure modes for PEMFCs, and the causes and mechanisms of degradation are not fully understood. 6 A better understanding of the phenomena in a membrane electrode assembly ͑MEA͒ is needed.Carbon-black-supported platinum is used as an electrocatalyst for both electrode reactions in PEMFCs. The kinetics of cathode deterioration by the degradation of cathode platinum is a particularly important problem. Platinum sintering in the catalyst layer during PEMFC operation has been reported to be a factor in the degradation of a cathode. 7-10 Two mechanisms have been proposed to explain the surface area of carbon-supported platinum by a sintering phenomena in aqueous electrolyte: 11,12 dissolution/reprecipitation 13-15 and the migration ͑surface diffusion͒ of platinum particles. 16,17 Platinum dissolution has been studied mainly in sulfuric and phosphoric acid solutions. [18][19][20][21][22][23] The dissolution of platinum is considered to play an important role in the decrease in the surface area of platinum in the case of PEMFCs. 22-24 Some dissolved platinum ion species such as Pt 2+ redeposit on other platinum particles, resulting in platinum particle growth ͑electro-chemical Ostwald ripening͒, 12,25 and other dissolved platinum ion species diffuse out of the electrode and into the electrolyte membrane.In this study, we carried out potential holding experiments and analyzed the conditions in a MEA to investigate the phenomena in which platinum dissolves and diffuses into a polymer electrolyte membrane from the cathode catalyst layer. We applied highresolution transmission electron microscopy ͑TEM͒, which is the most effective technique for investigating the distribution of nanosize platinum particles in MEA, because e...

Instability of Supported Platinum Nanoparticles in Low-Temperature Fuel Cells

“…Similarly, Yasuda et al [28,39] have detected the appearance of numerous Pt single crystals on the order of 10-100 nm in the membrane after potential holds at high voltages such as 0.8 V and 1.0 V in air and nitrogen in PEM fuel cells. In addition, the amount of Pt found in the matrix and thus the amount of Pt dissolution has been shown to increase with increasing cathode potentials [19,28,38].…”

Effect of Reheating Treatment on Oxygen-Reduction Activity and Stability of Zirconium Oxide-Based Electrocatalysts Prepared from Oxy-Zirconium Phthalocyanine for Polymer Electrolyte Fuel Cells

“…[7][8][9][10] The drastic reduction in platium usage, however, would be difficult to achieve because the highly dispersed platinum alloys and coreshell particles generally become unstable and dissolve easily under cathode conditions, resulting in decreasing cell performance during long-term operation. 11 Therefore, the development of low-cost nonprecious metal cathodes as alternative materials for platinum-based electrocatalysts having both high ORR activity and high durability is required for practical applications of PEFCs.…”

New Concepts in the Chemistry and Engineering of Low‐Temperature Fuel Cells

“…Furthermore, the choice of experimental apparatus can significantly impact the reported Pt-M stabilities as authors have observed high stabilities in PEM-based fuel cells and low stabilities in half-cell setups for identical materials [167]. In addition to reducing electrocatalytic activity, leaching transition metal and, to a lesser extent, Pt ions increase ohmic resistances and accelerate MEA degradation in membrane-based fuel cells [128,154,[168][169][170]. The dissolution results in a coreshell configuration consisting of underlying core transition metal atoms (e.g., Co, Fe), which cause stable enhancements to ORR activity of the protective Pt shell via geometric and electronic distortions [160,164,[171][172][173][174].…”