Fuel cell membranes oxidative stability

Fuel cell membranes are close to optimization with PFSA materials. These materials give excellent performance when fully humidified and have adequate oxidative stability. These materials limit the temperature of operation of the FC to <100°C and have unacceptably high methanol crossover for direct methanol... 

Mitov, S., Delmer, O., Kerres, J., Roduner, E. (2006a) Oxidative and photochemical stability of ionomers for fuel-cell membranes. Helv. Chim. Acta 89, 2354—2370. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Ionomer membranes are used in fuel cells in order to separate the anode and cathode compartment and to allow the transport of protons from the anode to the cathode. The typical membrane is Nation , which consists of a perfluorinated backbone and side chains terminated by sulfonic groups. In the oxidizing environment of fuel cells, Nation , as well as other membranes, is attacked by reactive oxygen radicals, which reduce the membrane stability. Direct ESR was used recently in our laboratory to detect and identify oxygen radicals as well as radical intermediates formed in perfluorinated membranes upon exposure to oxygen radicals [73,74]. The three methods used to produce oxygen radicals in the laboratory and the corresponding main reactions are shown below. 

Gogel et al. [118] compared two CFPs, one untreated and one treated (25 wt% PTFE) as the anode DL (both were TGP-H-120). The fuel cell was operated at a cell temperature of 110°C, and it was observed that the DL without any treatment performed better. However, the difference between both materials was very small and the methanol permeation was actually reduced (increased Faradaic efficiency) with the treated DL. A possible explanation for this is that methanol is oxidized more effectively at the anode due to the formation and stabilization of carbon dioxide bubbles in the active area. As a consequence, the methanol concentration gradient across the membrane is reduced. 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

Figure 29. Conductivity of some intermediate-temperature proton conductors, compared to the conductivity of Nafion and the oxide ion conductivity of YSZ (yttria-stabilized zirconia), the standard electrolyte materials for low- and high-temperature fuel cells, proton exchange membrane fuel cells (PEMFCs), and solid oxide fuel cells (SOFCs).

For the purposes of review. Figure 1 illustrates the basic function of the cathode in a solid oxide fuel cell. Whether acting alone or as part of a stack of cells, each cell consist of a free-standing or supported membrane of an oxygen-ion-conducting electrolyte, often yttria-stabilized zirconia (YSZ). Oxygen, which is fed (usually as air) to one side of the membrane, is reduced by the cathode to oxygen ions via the overall half-cell reaction... 

The electrolyte membrane presents critical materials issues such as high protonic conductivity over a wide relative humidity (RH) range, low electrical conductivity, low gas permeability, particularly for H2 and O2, and good mechanical properties under wet-dry and temperature cycles has stable chemical properties under fuel cell oxidation conditions and quick start-up capability even at subfreezing temperatures and is low cost. Polyperfluorosulfonic acid (PFSA) and derivatives are the current first-choice materials. A key challenge is to produce this material in very thin form to reduce ohmic losses and material cost. PFSA ionomer has low dimensional stability and swells in the presence of water. These properties lead to poor mechanical properties and crack growth. 

The solid polymer electrolyte (SPE) fuel cell makes use of the high stability and the cation selectivity of Nafion, a Teflon-like material that has been modified by the incorporation of sulfonic groups. The membrane is coaled with a porous catalyst on both sides. Hydrogen is oxidized on one side of the membrane and the formed in the process is transported across the membrane to the other side, where it interacts with (OH) ions formed by the reduction of oxygen, to form water. This water is removed from the cell by capillary action with the use of a... 

---