Proton exchange membrane methanol crossover

Therefore, one main drawback of the PEMFC configuration with a standard proton exchange membrane (such as Nafion) and a standard platinum gas diffusion cathode is the cathode depolarization caused by a mixed potential resulting from the methanol crossover through the mem-... 

An additional problem arises from methanol crossover through the proton-exchange membrane. It results that the platinum cathode experiences a mixed potential, since both oxygen reduction and methanol oxidation take place at the same electrode. The cathode potential is thus lowered, leading to a smaller cell voltage and thus to a decrease of the voltage efficiency. 

N. Jia, M.C. Lefebvre, J. Halfyard, Z. Qi and P.G. Pickup, Modification of Nafion proton exchange membranes to reduce methanol crossover in PEM fuel cells, Electrochem. Solid-State Lett., 2000, 3, 529-531. 

Zhong S, Cui X, Fu T, Na H (2008) Modification of sulfonated poly(ether ether ketone) proton exchange membrane for reducing methanol crossover. J Power Sources 180 23-28... 

DMFCs are PEMFCs fed with methanol as fuel. The technologies required by DMFCs are similar to those of PEMFCs. What differ between DMFCs and PEMFCs are in the following two aspects The proton exchange membrane used for DMFCs must possess low methanol permeability or crossover, and the anode catalyst must possess high activity toward the oxidation of methanol and high tolerance to CO and other intermediates from methanol oxidation. In this section, the applications of NMR techniques for the development of DMCFs as well as essential materials are going to be briefly reviewed. 

Due to methanol crossover, Reaction 7.4 accounts for methanol oxidation at both the anode and the cathode, while Reaction 7.5 accounts for oxygen reduction at the cathode. The protons generated at the anode by Reaction 7.4 diffuse across the proton exchange membrane, while the electrons pass as current through the external circuit to reach the cathode, where oxygen is reduced by the protons and the electrons to form water, according to Reaction 7.5. Figure 7.3 shows the difference at the anode between H2/O2 (or air) fuel cell and DMFC. 

Tseng C-Y, Ye Y-S, Cheng M-Y et al (2011) Sulfonated polyimide proton exchange membranes with graphene oxide show improved proton conductivity, methanol crossover impedance, and mechanical properties. Adv Energy Mater 1 1220-1224... 

The second major problem is that of fuel crossover. This was discussed briefly in Section 3.5. It is particularly acute in the DMFC because the electrolyte used is usually a proton exchange membrane (PEM), as described in Chapter 4. These readily absorb methanol, which mixes well with water, and so quickly reaches the cathode. This shows itself as a reduced open circuit voltage but affects the performance of the fuel cell at all currents. DMFC electrolytes and this fuel crossover problem are further discussed in Section 6.3. 

Proton exchange membranes (PEMs) in DMFC should transport protons as an electrolyte and prevent fuel and oxidant mixing as a separator. Proton transport capacity affects the resistance and performance of fuel cells. The ability to separate influences the long-term stabiUty and fuel efficiency. The insufficiency of function in separation, which is called methanol crossover, leads to deterioration of cathode catalysts, and thus generates mixed potential and decreases the perfoimance and fuel efficiency of DMFC. 

Kuver, A., and Potje-Kamloth, K., 1998, Comparative study of methanol crossover across electro-polymerized and commercial proton exchange membrane electrolytes for the acid direct methanol fuel ceU , Electrochem. Acta 43 (16-17) 2527-2535. 

V. S. SUva, A. Mendes, L. M. Maderia, and S. R Nunes, Proton exchange membranes for direct methanol fuel cells Properties critical study concerning methanol crossover and proton conductivities, J. Membr Sci. 276(1-2), 126-134 (2006). 

In order to achieve more control of methanol crossover, composite membranes are synthesized. Organic-inorganic composite membranes comprising Nafion with inorganic materials silica, mesoporous zirconium phosphate (MZP) and mesoporous titanium phosphate (MTP) are made as proton-exchange-membrane electrolytes for direct methanol fuel cells (DMFCs) [206] with increase in proton conductivity and low methanol crossover. Composite membranes with mordenite incorporated in polyvinyl alcohol-polystyrene sulfonic acid blend tailored with varying degree of sulfonation also retards the methanol release kinetics considerably [199]. 

The most widely studied fuel-ceU membrane is DuPont s Nafion , a copolymer of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octene-l-sulfonic acid). Nafion is the membrane material of choice for most proton-exchange membrane fuel cells that operate at a temperature <80 °C. While Nafion offers high conductivity combined with exceptional chemical and mechanical stability [3], it suffers from several critical drawbacks. When used in a direct methanol fuel cell, Nafion shows significant methanol leakage (crossover from the anode to the cathode) with the resultant reduction in fuel-ceU performance. To overcome this shortcoming the methanol concentration in the anode feed is usuaUy reduced to 0.5-2.0 M, which necessitates... 

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