H2 PEFCs

An alternative to the use of H2 as fuel is methanol, which is a liquid fuel and easy to handle. This can be directly transformed to electrical current in a DMFC (direct methanol fuel cell). The DMFC allows a simple system design. However, presently achieved performance data of DMFC is not satisfactory and material costs are too high. As another alternative, methanol or hydrocarbons (e.g. natural gas, biogas) can be transformed to hydrogen on board the electric vehicle by a reformation reaction. This allows use of the H2-PEFC cell, which has a higher level of development. The reformate feed gas may contain up to 2.5% carbon monoxide (CO) by volume, which can be reduced to about 50ppm CO using a selective oxidizer (Wilkinson et al. [1997]). 

Direct Methanol Fuel Cell The liquid-fed direct methanol fuel cell (DMFC) is generally seen as the most viable alternative to lithium ion batteries in portable applications because DMFC systems require less ancillary equipment and can therefore potentially be more simphfied compared to an H2 PEFC. Additionally, the use of a liquid fuel simphfies storage. The DMFCs can potentially compete favorably with advanced Li ion batteries (which currently power many wireless portable apphcations) in terms of gravimetric energy density of 120-160 Wh/kg and volumetric energy density of 230-270 Wh/L. While both H2 PEFCs and DMFCs are strictly PEFCs (both use the same flexible polymer electrolyte), the DMFC feeds a liquid solution of methanol and water to the anode as fuel. The additional complexities of the low-temperature methanol oxidation reaction prevent the DMFC from... 

List the relative advantages and disadvantages of the PAFC, SOFC, MCFC, AFC, H2 PEFC, and DMFC. List one potential power application weU suited for each type of fuel cell. 

Example 4.12 Calculating Crossover Losses In ref. [9], the authors noted a hydrogen crossover loss of 3.3 mA/cm for their automotive H2 PEFC applications. Calculate the mass crossover rate of hydrogen through the membrane. Also, calculate and plot the cathode activation overpotential loss at open circuit and 1 A/cm as a function of cathodic exchange current density. Assume the cathodic charge transfer coefficient at the cathode is 1.5 at a temperature of 353 K, and the fuel cell has a 50 cm geometric area. 

High relative performance H2 PEFCs can reach >1.3 kW/L fuel cell power density, >0.6 kW/L system power density, and >0.6 kW/kg mass specific power density. 

Low-temperature operation H2 PEFCs can start and operate in subfreezing temperature, although normal operating temperatures are 20-90°C. 

Facile anode kinetics for the HOR As a result, H2 PEFCs have the lowest precious metal loadings of all the PEFC types (typically 0.2-0.8 mg/cm total active area i.e. anode plus cathode). 

However, the H2 PEFC has many complex technical issues that have no simple solution. Besides issues of manufacturing, ancillary system components, control, cost, and market acceptance not treated in this text, the main technical design issues for the H2 PEFC system include the following ... 

Solid Polymer Electrolyte The most common solid polymer electrolytes consist of a hydrophobic and inert polymer backbone which is sulfonated with hydrophilic acid clusters to provide adequate conductivity as discussed in Chapter 5. In order to ensure adequate performance, some membrane hydration is required. However, excess water in the electrodes can result in electrode flooding, so that a precarious balance must be achieved. Modern perflourosulfonated ionomer electrolytes for H2 PEFCs are 18-25 xm thick with a practical operating temperature limit of 120°C, although PEFC operation is rarely greater than 90°C due to excessive humidity requirements and operational low lifetimes. 

Flooding Condition Adjustment When the water balance is accumulating liquid water mass, a periodic ejection of droplets can maintain the water balance in the fuel cell since the liquid droplets are so dense compared to gas phase ejection. An illustration of the process of water buildup and ejection from the DM is shown in Figure 6.19. In the steady state, the water from generation must be exactly balanced by that removed. Although water droplet ejection is a periodic process, a H2 PEFC can be operated in a net flooding condition and still achieve relatively stable performance with periodic ejection. If the liquid water accumulation restricts gas-phase flow to the catalyst surface, performance instability will occur, however, until a new equilibrium is achieved. 

Increased cost per kilowatt due to much higher catalyst loadings, on the order of 1-8 mg/cm total (anode plus cathode) precious metal loading, compared to 0.2-1.0 mg/cm total loading for the H2 PEFC... 

A mixed potential on the cathode and oxidation of methanol at this location both poison the cathode catalyst, consume oxygen, and greatly reduce the OCV, even more so than hydrogen crossover in H2 PEFC systems. Typical OCVs of DMFCs are significantly below 0.8 V. 

For DAFCs using a liquid feed, like the DMFC, the water balance and fuel crossover problem are more acute than the hydrogen fuel cell. Dilute Uquid solutions, thicker membranes, and capillary pressure management are used to control these two issues. As a result of the high methanol and water crossover in the DMFC, the open-circuit potential is very low, and performance is also low compared to the H2 PEFC. However, the use of diffusion barriers in the anode and capillary pressure management eliminates the need for highly dilute methanol solutions, and these systems may ultimately be more appropriate than their H2 PEFC counterparts for portable applications. 

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