Membrane degradation, electrocatalyst

This volume of Modern Aspects of Electrochemistry is intended to provide an overview of advancements in experimental diagnostics and modeling of polymer electrolyte fuel cells. Chapters by Huang and Reifsnider and Gu et al. provide an in-depth review of the durability issues in PEFCs as well as recent developments in understanding and mitigation of degradation in the polymer membrane and electrocatalyst. 

Other technical hurdles must be overcome to make fuel cells more appealing to automakers and consumers. Durability is a key issue and performance degradation is usually traceable to the proton exchange membrane component of the device. Depending on the application, 5,000 40,000 h of fuel cell lifetime is needed. Chemical attack of the membrane and electrocatalyst deactivation (due to gradual poisoning by impurities such as CO in the feed gases) are critical roadblocks that must be over come. 

In the acid medium of PEM fuel cells that may also contain some levels of fluoride derived from membrane degradation, Pt cannot be replaced with a non-noble metal or a metallic oxide, as both will corrode in such environment. This chapter describes some of the efforts that have been made over 40 years to obtain other non-precious metal electrocatalysts for ORR in acidic medium. They aU started with the discovery in 1964 that C0N4 phthalocyanine was capable of oxygen reduction in an alkaline solution. ... 

Key words proton exchange membrane fuel cells, durability, durability testing protocols, PEM degradation, electrocatalyst degradation, carbon support... 

High-temperature proton exchange membrane fuel cells (HT-PEM fuel cells), which use modified perfluorosulfonic acid (PFSA) polymers [1—3] or acid-base polymers as membranes [4—8], usually operate at temperatures from 90 to 200 °C with low or no humidity. The development of HT-PEM fuel cells has been pursued worldwide to solve some of the problems associated with current low-temperature PEM fuel cells (LT-PEM fuel cells, usually operated at <90 °C) these include sluggish electrode kinetics, low tolerance for contaminants (e.g. carbon monoxide (CO)), and complicated water and heat management [4,5]. However, operating a PEM fuel cell at >90 °C also accelerates degradation of the fuel cell components, especially the membranes and electrocatalysts [8]. 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Platinum was the initial choice for both the anodic and cathodic electrocatalysts for the following reasons (1) platinum shows minimum degradation or corrosion in acid or when used as an anodic or cathodic electrocatalyst (2) recent technical advances have been made in forming efficient high surface area porous platinum electrocatalyst structures at the surface of PEM membranes and (3) preliminary work... 

The use of Pt-Ru and Pt-Sn electrocatalysts has been found to introduce a stability problem during extended period of fuel cell operation, the catalyst surface becomes depleted of Ru and Sn [70]. It was reported that Ru can nucleate in other parts of the cell and facilitate degradation of the membrane. Ordered intermetallic systems, in general, provide predictable control over structural, geometric, and electronic effects and high structure stability. DiSalvo et al. [70] studied a wide range of ordered intermetallic phases, and the PtBi, Ptin, and PtPb ordered intermetallic phases appeared to be very promising fuel cell anode electrocatalysts. 

In situ NMR Investigation of electrocatalytic mechanism and degradation mechanism of proton exchange membranes, development of electrocatalysts h, H, Pt... 

Bae, S.J., Kim, S.-J., Park, J.I., Lee, J.-H., Cho, H., and Park, J.-Y. (2010) Lifetime prediction through accelerated degradation testing of membrane electrode assemblies in direct methanol fuel cells. Int.J. Hydrogen Energy, 35, 9166-9176. Shao, M. (2011) Palladium-based electrocatalysts for hydrogen oxidation and oxygen reduction reactions. J. Power Sources, 196, 2433-2444. 

The impact of CO on the PEMFC anode performance has been widely studied experimentally and by modelling, and maity mitigation methods have been proposed. In recent years, the impacts of H2S and NHj have become an important subject of research. It is well accepted that the major impact of CO and H2S contaminants on the hydrogen-fed PEMFC anode is a kinetic effect due to poisoning of the electrocatalyst, while NHj mainly affects the ionomeric membrane by reducing ionomer conductivity.In both cases, significant performance degradation can be induced. In order to enhance CO tolerance, bimetallic catalysts such as Pt-Ru, Pt-Mo and Pt-Sn have been proposed however, the... 

Swider, K. E. and Rolison, D. R. (2000) Reduced poisoning of platinum fuel-cell electrocatalysts supported on desulfurized carbon. Electrochem. Solid-State Lett. 3,4-6 Tandon, R. and Pintauro, P. N. (1997) Divalent/monovalent cation uptake selectivity in a Nafion cation-exchange membrane experimental and modeling studies. J. Membr. Sci. 136, 207-219 Tan, J., Chao, Y. J., Van Zee, J. W. and Lee, W.-K. (2007) Degradation of elastomeric gasket materials in PEM fuel cells. Mater. Sci. Eng. A, 445-446, 669-675 Tawfik, H., Hung, Y. and Mahajan, D. (2(X)7) Meted bipolar plates for PEM fuel cell - A review. J. Power Sources 163, 755-767... 

---