PEMFC stack

The most promising fuel cell for transportation purposes was initially developed in the 1960s and is called the proton-exchange membrane fuel cell (PEMFC). Compared with the PAFC, it has much greater power density state-of-the-art PEMFC stacks can produce in excess of 1 kWA. It is also potentially less expensive and, because it uses a thin solid polymer electrolyte sheet, it has relatively few sealing and corrosion issues and no problems associated tvith electrolyte dilution by the product water. 

A fuel cell system for automobile application is shown in Figure 1.5 [41]. At the rated power, the PEMFC stack operates at 2.5 atm. and 80°C to yield an overall system efficiency of 50% (based on lower heating value of hydrogen). Compressed hydrogen and air are humidified to 90% relative humidity at the stack temperature using process water and heat from the stack coolant. A lower system pressure is at part load and is determined by the operating map of the compressor-expander module. Process water is recovered from spent air in an inertial separator just downstream of the stack in a condenser and a demister at the turbine exhaust. 

The costs of a PEMFC stack are composed of the costs of the membrane, electrode, bipolar plates, platinum catalysts, peripheral materials and the costs of assembly. For the fuel-cell vehicle, the costs of the electric drive (converter, electric motor, inverter, hydrogen and air pressurisation, control electronics, cooling systems, etc.) and the hydrogen storage system have to be added. Current costs of PEM fuel-cell stacks are around 2000/kW, largely dominated by the costs of the bipolar plates and... 

Figure 14.16 the shows fuel cell stack performance of a 1 kWe atmospheric PEMFC stack using PtRu anodes, operating on various gas compositions. As can be clearly seen, already small concentrations of CO lead to a large decrease of fuel cell performance. An air-bleed of 1.5% air in hydrogen is able to mitigate this ef-... 

Fig. 14.16 Averaged current density-voltage characteristics of 30 cells for an atmospheric PEMFC stack on various anode feed compositions.

High-temperature PEMFC Stack with Methanol Reforming (AMFC) (Volvo (S), Technical University of Denmark (DK), Statoil (N), NTNU (N), University of Newscastle-Upon-Tyne (UK), Proton Motor Fuel Cell (D)). 

Fig. 5.19 Potential distribution across the six cell cross-flow PEMFC stack at 4A discharge (after Liu et al., 2006). (Reprinted from Journal of Power Sources, Vol. 160, Liu, Z., Mao, Z., Wang, C., Zhuge, W., and Zhang, Y., Numerical simulation of min PEMLC stack , pp. 1111-1122, Copyright 2006, with permission from Elsevier.)...

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. 

Figure 6.34. A Connections used for the EIS study of each element of a fuel cell stack running on a load. WE working electrode CE counter electrode RE reference electrode. B Two-electrode arrangement used for the measurement of Zw [37], (Reproduced by permission of ECS—The Electrochemical Society, and the authors, from Diard JP, Glandut N, Le-Gorrec B, Montella C. Impedance measurement of each cell of a 10 W PEMFC stack under load.)...

Consequently, the impedance of the individual cell of a PEMFC stack and the total impedance of the stack have been determined separately. The results are shown in Figure 6.36. 

Diard JP, Glandut N, Le-Gorrec B, Montella C (2004) Impedance measurement of each cell of a 10W PEMFC stack under load. J Electrochem Soc 151(12) A2193-7... 

D Small series cost is reflected the current 85-kW PEMFC stack cost is about 10000 euro (with -2500 euro projected for 2025) (Sorensen, 1998 Tsuchiya and Kobayashi, 2004). 

The main operative characteristics of PEMFC stacks can be summarized in the following points ... 

FCS water management is the key factor for an efficient and reliable operation of a PEMFC stack. Membrane hydration control and water balance for a durable operation of FCS are the main objectives of this sub-system, whose design and control issues, strictly connected to thermal management but also to reactant subsystem components, are discussed in Sect. 4.5. The possibility of interactions between the wet and warm cathode outlet stream and the components of thermal and water management sub-systems is also discussed. 

Finally the possibility to utilize electrolysers to elevate hydrogen pressure and recirculate it inside anode flow fields in H2FCS power sources has been recently considered [10]. This electro-chemical approach for hydrogen pumping could be effected with an external additional smaller PEMFC component, connected to the stack devoted to electric power generation, or alternatively using in electrolysis mode a group of cells of the main PEMFC stack. 

A discussion on manufactured costs of fuel ceU vehicles, with particular emphasis to PEMFC stacks and BOP components, is useful to assess the present limits for the commercialization with high production volumes. Recent evaluations evidenced that the estimated costs related to PEMFC and integrated FCSs results too high, at least four to five times higher than conventional engines [15, 53, 54]. At this regard PEMFC stack materials and the relative complexity of FCS play an important role in determining the overaU expected cost. A drastic reduction of the costs is then crucial for a realistic application in automotive field. 

Kandlikar SG, Lu Z (2009) Thermal management issues in a PEMFC stack-A brief review of current status. Appl Thermal Eng 29 1276-1280... 

The above projects have served to demonstrate the practical operation of fuel cell vehicles using PEMFC stacks. There is now little doubt that technically successful buses, cars and other vehicles can be manufactured and that the results from showcase in-service programmes have been encouraging. The remaining challenges relate to ... 

If activation losses of the electrodes, ohmic losses and concentration losses of the DBFC were on the same level as the PEMFC with an operation voltage of 0.75 V, the DBFC would obtain 1V of operation voltage under the similar operation conditions, as shown in Fig. 8.20. This high operation voltage will benefit the stack and the system design for fuel ceU users because the number of cells can be reduced by 25 % compared with the PEMFC stacks. 

Beside the MEA the bipolar plates are the key components in a PEMFC stack in terms of their contribution to weight, volume, and costs. Bipolar plates contain a fine mesh of gas channels called the flow-field, to ensure a uniform distribution of the process gasses (hydrogen and oxygen) of fuel and air across both sides of the MEA and the removal of the reaction products. Furthermore, the bipolar plates in PEM fuel cells separate the individual cells from each other and guarantee an electrical connection between them in series. Substantial requirements for the bipolar plates are a high electrical conductivity and corrosion resistance. 

---