Alcohol crossover

The flow of protons and the crossover of alcohol (ROH) from the anode to the cathode are also indicated in Fig. 1.5. Alcohol crossover decreases the current through the cell because part of the fuel permeates through the membrane without reacting in the anode. Thus, a fuel efficiency (or fuel utilization) can be defined as ... 

In practice is not simple to determine whether the current reduction in the cell is due to alcohol crossover, incomplete oxidation, or both. A way to quantify the contribution of both effects is to measure the methanol concentration in the anode exhaust (for determining the amount of methanol oxidized and permeated), and the concentration of methanol and CO2 in the cathode exit. Part of the CO2 at the cathode exhaust is due to parasitic oxidation of methanol at the cathode, while the rest is a consequence of CO2 crossover from the anode. The last can be determined by a half-ceU experiment by flowing hydrogen through the cathode, to avoid methanol oxidation, in such a way that all the CO2 measured in the cathode outlet stream must have crossed the membrane from the anode [29]. 

Nafion, a sulfonated perfluorinated polymer developed by Dupont, which triggered the development of PEM fuel cells fed with hydrogen was also the PEM used in all the first generation of DMFC and DEFC. Very early in the development of DAFC it was recognized that alcohol crossover due to the relatively large permeability of Nafion to alcohols was a severe limitation to be overcome. In the case of methanol the high permeability can be understood by the preference of Nafion for sorbing methanol instead of water in the binary mixtures. 

The decrease of alcohol permeability and, consequently, of alcohol crossover, even if accompanied by a reduction of the proton conductivity, open the possibility to new strategies of MEAs preparation by choosing the optimal membrane thickness and alcohol concentration, among several parameters, in order to increase DAFC performance. Other beneficial effect of incorporating fillers in Nafion-based membranes, is the chance of increasing the operation temperature of the fuel cell, due to the retention of water, avoiding the dramatic drop of proton conductivity taking place in Nafion above 100 °C. 

Anion-exchange membranes are also discussed in Chap. 6 because of the facile electro-oxidation of alcohols in alkaline media and because of the minimization of alcohol crossover in alkaline direct alcohol fuel cells. 

As mentioned above, the alcohol crossover from the anode to the cathode is a important problems to be overcome to improve the DAFC performance. This is due to the fact that the commonly used Pt-based cathode electrocatalysts are also active for the adsorption and oxidation of methanol [1]. So, in addition to the resulting mixed potential at the cathode, there is a decrease in the fuel utilization. Therefore, considering the above exposed reactions for the alcohol electrooxidation, and the features that govern the ORR electrocatalytic activity, as discussed in the Sect. 5.2, it is ready to conclude the importance of the modification of the active ORR electrocatalyst surfaces in order to inhibit the methanol or ethanol oxidative adsorption steps. In the next sections, some recent materials being developed to overcome the problems caused by the alcohol crossover will be presented. 

The vast catalogue of polymeric materials reviewed here included Nafion composite with inorganic and organic fillers, and non-fluorinated proton conducting membranes such as sulfonated polyimides, poly(arylene ether)s, polysulfones, poly (vinyl alcohol), polystyrenes, and acid-doped polybenzimidazoles. Anion-exchange membranes are also discussed because of the facile electro-oxidation of alcohols in alkaline media and because of the minimizatirHi of alcohol crossover in alkaline direct alcohol fuel cells. 

Alcohol crossover and cell resistance are the relevant properties determining the DAFC performance, which are closely related to the membrane used in the preparation of the membrane-electrode assembly (MEA). Mechanical properties, as well as the chemical and thermal stability, of the membrane could also be important when durability is considered. 

Membrane selectivity is probably the most important parameter to compare potential polymeric membranes for DAFC because low ohmic resistance and low alcohol crossover are desired. Kim and Pivovar [4] have noted that both of these quantities are independent of membrane thickness, which affect resistance and alcohol crossover in opposite ways. The thinner is a membrane in a DAFC, the lower the resistive losses, but the higher the crossover losses. Thus, a minimum conductivity is required in membranes for DAFC, regardless of how high its... 

Other authors [7] consider the properties of the fuel cell instead of the freestanding membrane to define a MEA-selectivity parameter, a, as the reciprocal of the high frequency resistance times the alcohol crossover limiting current. In this... 

It should bear in mind that the requirements for active DAFC could be significantly different than the passive DAFC systems used for lower power, portable applications [4]. Water balance and alcohol crossover become more critical in passive cell systems, where information on the performance of advanced ionomer membranes is limited by the cmifidential nature of the research and development. 

The significant contribution of Nafion or perfluorosulfonic membranes to the cost of the fuel cells stacks and the high alcohol crossover levels that affect the fuel efficiency, prompted the development of radiation grafted proton exchange membranes based on poly(ethylene-tetrafluoroethylene) (ETFE) [172-178], PVdF [175], andPTFE [179]. The peroxy radicals produced on the base polymer by y-ray, electron- or proton-beam, react with styrene to form a co-polymer that is then sulphonated. 

Kim and Pivovar [4] remarked that, at constant current density, 5 is the parameter that determines the relative contribution of diffusion and electroosmotic to alcohol crossover because diffusion flux is depending of the membrane thickness, while electro-osmotic drag is not. Therefore, the thinner the membrane, the lower the electro-osmotic contribution, and the current density for having roughly equal diffusion and convection contribution to crossover is 4 times higher in Nalion 112 (8 = 50 pm) as compared to Nafion 1 (5 = 178 pm). 

Open circuit voltage (OCV) has been used as an indication of cell performance, although it is mainly determined by the electrochemical reversibility of the catalyst and it does not account for ohmic loss in the membrane and alcohol crossover. Thus, is not surprising to find a poor correlation between OCV of DMFC and membrane relative selectivity [4]. Other criteria used to compare DMFC performance is to compare current density at a fixed voltage [17], which is only practical if comparison is performed at the same temperature and feed methanol concentration. 

One of the reasons to use AEM in PEM fuel cells is to reduce alcohol crossover due to the elimination of electrosmotic drag by protons moving from anode to cathode. In AD AFC the current through the AEM is conducted by HO ions, which are transported in the opposite direction minimizing the electrosmotic flux and, in addition, alcohol oxidation kinetics can be facilitated in alkaline media. 

The development of new membranes for DAFC has grown exponentially during the last decade, reaching a maximum in 2009 (Fig. 6.1). The main objective was focused on reducing the alcohol crossover in relation to Nafion while maintaining high ionic conductivity. 

Three approaches to reach those purpose have been reviewed and discussed in this Chapter the improvement of Nafion with fillers that reduce alcohol permeability, the synthesis of new polymers and blends with better alcohol selectivity, and the development of AEM with good conductivities for use in alkaline fuel cells where the alcohol crossover is not important. 

Fig. 8.5 Internal short circuit created by alcohol crossover (Source [19] reproduced with permission of the royal society of chemistry)...

Among the many applications of NMR techniques, the most essential application of NMR is the characterization of molecular structures of polymeric proton-conducting materials, in terms of one-dimensional (ID) and two-dimensional (2D) NMR techniques. The second type of application is to characterize the dynamic properties of protons, water, and alcohol molecules. These dynamic properties are directly related to the mobility of protons, the macroscopic transport of water in terms of difilision and the electroosmosis, and the alcohol crossover or permeability, respectively. The third type of application is related to the identification of the water types soaked in the proton exchange membranes and the visuafization of water distribution in the proton exchange membranes. 

Considering (Mily the thermodynamics of the DMFC (used here as a representative of direct alcohol fuel cells), methanol should be oxidized spraitaneously when the potential of the anode is above 0.05 V/SHE. Similarly, oxygen should be reduced spontaneously when the cathode potential is below 1.23 V/SHE, identical to a H2-O2 fuel cell. However, kinetic losses due to side reactions cause a deviation of ideal thermodynamic values and decrease the efficiency of the DMFC. This is presented in Fig. lb, which includes various limiting effects as kinetics, ohmic resistance, alcohol crossover, and mass transport. The anode and cathode overpotentials for alcohol oxidation and oxygen reduction reduce the cell potential and together are responsible for the decay in efficiency of approximately 50 % in DMFCs [13, 27]. 

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