PEMFC operation

For last few years, extensive studies have been carried out on proton conducting inorganic/organic hybrid membranes prepared by sol-gel process for PEMFC operating with either hydrogen or methanol as a fuel [23]. A major motivation for this intense interest on hybrid membranes is high cost, limitation in cell operation temperature, and methanol cross-... 

Kraemer, S. V., Lindbergh, G., Lafitte, B., Puchner, M., andJannasch, P. Substitution of Nafion with sulfonated polysulfone in membrane-electrode assembly components for 60-120°C PEMFC operation. Journal of the Electrochemical Society 2008 155 B1001-B1007. 

From these examples, it can be seen that water content has a strong effect upon proton conductivity. Thus, it is clear that water management is an important factor for efficient PEMFC operation. It will be discussed in Section 3.2.3. 

The EOD coefficient, is the ratio of the water flux through the membrane to the proton flux in the absence of a water concentration gradient. As r/d,3g increases with increasing current density during PEMFC operation, the level of dehydration increases at the anode and normally exceeds the ability of the PEM to use back diffusion to the anode to achieve balanced water content in the membrane. In addition, accumulation of water at the cathode leads to flooding and concomitant mass transport losses in the PEMFC due to the reduced diffusion rate of O2 reaching the cathode. 

Adjemian, K. T., Srinivasan, S., Benzieger, J. and Bocarsly, A. B. 2002. Investigation of PEMFC operation above 100°C employing perfluorosulfonic acid silicon oxide composite membranes. Journal of Power Sources 109 356-364. 

N. Wan, C. Wang, and Z. Mao. Titanium substrate based micro-PEMFC operating under ambient conditions. Electrochemistry Communications 9 (2007) 511-516. 

For higher temperature operation, a polybenzimid-azole-based polymer electrolyte may be preferred. The PEMFC structures have good mechanical integrity under compression and expansion from differential temperature and pressure gradients that occur during operation. This system has minimal materials problems, except for the cost and operation characteristics of the membrane. The PEMFC operates at 1 A/cm at 0.7 V. The electrode reactions in acidic media have been discussed above. 

The MEA (Figure 3.3.4) is the heart of every individual PEMFC. Operated on hydrogen and oxygen, the maximum individual cell voltage is approximately 1.23 V. In order to obtain higher cell voltages, individual MEAs are put in series in so-called PEMFC stacks as shown in Figure 3.3.6. In between each individual MEA, there are electrically conductive flow plates that provide flow paths for the fuel and oxidant. 

The ability to operate the PAFC stack at 190 °C has in large part enabled a reduction in Pt loading to 0.25 mg Pt/cm2 electrode area. In addition, the efficacy for increasing the temperature removes the need for producing more advanced alloy electrocatalysts to overcome the carbon monoxide poisoning problem. That is not the case for PEMFCs operating at lower temperatures below 100 °C. 

The membrane and ionomer humidification requirements are of paramount importance for PEMFC operation since the proton conductivity is a fundamental necessity in the membrane as well as in the electrode for the fuel cell to function. The operating conditions of current PEMFCs are dictated by the properties of the membranes/ionomers. Now, the most important membrane type (e.g., Nafion membranes from DuPont) is based on PFSA ionomers that are used in the membrane... 

PEMEAS, a 2004 spin-off from Celanese AG, has developed a membrane made from the heat-resistant polymer PBI. The PBI membrane marketed by PEMEAS under the brand name Celtec enables a fuel cell to operate at temperatures of up to 200°C (392°F), while more conventional technologies allow PEMFC-operating temperatures of up to 100°C (212°F). Due... 

Higher operating temperature PEMFCs operate with smaller cooling elements. This is especially good for automotive applications. 

Fig. 11 Current density-voltage curves for PEMFC operating with...

Gotz, M. Wendt, H. Binary and ternary anode catalyst formulations including the elements W, Sn and Mo for PEMFCs operated on methanol or reformate gas. Electrochim. Acta 1998, 43 (24), 3637-3644. 

The kinetics of the HOR on polyctyslallinc Ru and carbon-supported nanoparticles is about two orders-of-magnitude smaller than that on Pt or Pt-Ru alloys, and it usually is assumed that the Ru contribution to the H2 oxidation current of fuel cell anodes is negligible. However, at temperatures at which PEMFCs operate (60-80 °C) the kinetics of HOR on Ru is considerably faster than at room temperature, so that the effect of Ru surfaces may be of importance in PEMFC catalysis. 

During the course of membrane electrode assembly (MEA), manufacture and PEMFC operation, the membranes are exposed to the impacts of temperature, humidity, and pressure. Consequently, it is important that the membranes possess a good mechanical stability, and in particular a high mechanical strength and minimal swelling. 

Zhou, T. and Liu, H.T., Simulation of Performance of PEMFC Operated by Reformed Gas, accepted for the 2002 Fuel Cell Seminar, Palm Springs, California, November 18-21, 2002. 

In order to estimate the input of the cathode catalyst layer on the MEA cell performance, each cell before testing in methanol solution has been evaluated in PEMFC operation conditions, namely with hydrogen on the anode side and oxygen or air on the cathode. Fig. 6 shows the dependencies of cell voltage and cell voltage compensated for membrane resistance vs. current density. 

G. M. Anilkiimar, S. Nakazawa, T. Okubo, and T. Yamaguchi. Proton conducting phosphated zirconia-sulfonated polyether sulfone nanohybrid electrolyte for low humidity, wide-temperature PEMFC operation. Elec-trochein. Commun., 8(1) 133-136, January 2006. 

M. Li, H. Zhang, and Z.-G. Shao. Quatemized poly(phthalazinone ether sulfone ketone) membrane doped with H3PO4 for high-temperature PEMFC operation. Electrochem. Solid-State Lett., 9 A60-A63, 2006. 

Basically, a fuel cell electrode can, thus, be seen as a highly dispersed interface between Pt and electrolyte (ionoiner or water). Due to the random composition, complex spatial distributions of electrode potential, reaction rates, and concentrations of reactants and water evolve under PEMFC operation. A subtle electrode theory has to establish the links between these distributions. 

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