PEMFC systems

Even if we restrict the definition of a system to a specific technology, numerous variants of architectures can be envisaged, as well as numerous control solutions. However, it is possible to give a few points which are more specific to PEMFCs [CAN 07 PER 08]. 

PEMFCs, in most apphcations, work at atmospheric pressure. This means that the fuel and oxidant are injected at a pressure that is slightly higher than atmospheric pressure, so as to compensate for the pressure drop of the cell, which is no more than a few bars (typically 0.5-2 bar). The polymer membrane can withstand a moderate difference in pressure between the anode and the cathode - typically less than 1 bar. Beyond this, there is a danger of mechanical fatigue, leading to the breakdown of the membrane. Also, it is useful to balance the pressure of the fuel and the oxidant at input to the cell, whilst it is operational. 

This mode of operation is the most favorable for the performances of the stack. However, too high a Fsa decreases the net efficiency of the system, because some of the fuel is ejected back into the atmosphere. 

The fuel cell stack produces water at the cathode. The simplest system is to let diffusion balance the gradient of water concentratioa However, this solution may lead to cases of severe dehydration when the flowrate at the cathode is high enough and drains out too much water as it passes the cathode. 

In order to control the amoimt of water entering the cell, we can inject water into the flow of gases into the cell. This injection may be done at the cathode (which is most common), the anode or possibly on both sides. Many solutions are possible  

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For the practical use of this CO removal reactor, the microchannel reactor should be operated carefully to maintain operating temperature ranges because the reaction temperature is critical for the microchannel reactor performance such as CO conversion, selectivity and methanation as disclosed in the above results. It also seems that the present microchannel reactor is promising as a compact and high efficient CO remover for PEMFC systems. 

A microchannel reactor for CO preferential oxidation was developed. The reactor was consisted of microchannel patterned stainless steel plates which were coated by R11/AI2O3 catalyst. The reactor completely removed 1% CO contained in the Ha-rich reformed gas and controlled CO outlet concentration less than Ippm at 130 200°C and 50,000h. However, CH4 was produced from 180"C and CO selectivity was about 50%. For high performance of present PrOx reactor, reaction temperature should be carefully and uniformly controlled to reach high CO conversion and selectivity, and low CH4 production. It seems that the present microchaimel reactor is promising as a CO removal reactor for PEMFC systems. 

Schematic diagram of a hydrogen-fueled, PEMFC system for automotive applications. (Reproduced with permission from Elsevier Ahluwalia, R.K., and Wang, X., /. Power Sources, 139(1-2), 152-164, 2005.)...

A more detailed picture of a PEMFC system, including the auxiliary and control equipments, is shown in Figure 1.9. 

Figure 1.9 Detailed scheme of a PEMFC system with its auxilia and control equipments.

In PEMFC systems, water is transported in both transversal and lateral direction in the cells. A polymer electrolyte membrane (PEM) separates the anode and the cathode compartments, however water is inherently transported between these two electrodes by absorption, desorption and diffusion of water in the membrane.5,6 In operational fuel cells, water is also transported by an electro-osmotic effect and thus transversal water content distribution in the membrane is determined as a result of coupled water transport processes including diffusion, electro-osmosis, pressure-driven convection and interfacial mass transfer. To establish water management method in PEMFCs, it is strongly needed to obtain fundamental understandings on water transport in the cells. 

Beil, A. and Seume, J. (2006) Unsteady performance of a PEMFC system including autothermal methane reforming, in Proceedings of the 4th International ASME Conference on Fuel Cell Science, Engineering and Technology, June 19-21, Irvine, CA. 

Carbon meets many of these requirements and has been used by fuel cell makers over many years. Nevertheless, the development of low-cost, nonporous, carbon materials continues to be a challenge and the bipolar plate remains one of the most costly components in a PEMFC system. 

This cost structure could very well be valid for PEMFC systems, although for high-temperature stacks, insulation can be an important factor, especially for low nominal power outputs (US DOE, 2002). 

Given these requirements, hybrid and nonhybrid PEMFC systems are the leading contenders for automotive fuel cell power, with additional attention focusing on the direct-methanol fuel cell (DMFC) version of the technology and the possibility of using solid oxide fuel cell (SOFC) systems as auxiliary power units for cars and trucks. 

To achieve a steady-state environment, some actions must be taken before starting each impedance measurement for a PEMFC system. For example, Wagner [23] prepolarized the cell for at least 15 min at the measuring potential. The current densities before and after measurement were taken to prove the stability of the cell during measurement times. Guo et al. [38] operated a fuel cell at 0.6 V for 20 h to reach its steady-state operating current. Pickup et al. [34, 39] ran a H2/02 fuel cell for 30 min at 0.5 V before the impedance measurements were performed. In Gode et al. s work [40], the cell was mn galvanostatically for 1 h prior to the impedance measurement. 

Figure 5.26. Two typical equivalent circuits for a two-arc PEMFC system...

Figure 5.5 Operating scheme of a commercial NaBH4-PEMFC system (With permission from Millennium Cell.)...

Chapters 7-10 cover the syngas purification and separation. When reforming and water-gas shift are applied to PEMFC systems, trace amounts of CO in the gas that poisons anode catalyst must be removed. This is achieved by preferential CO oxidation, which is covered in Chapter 7 by Marco J. Castaldi of Columbia... 

Figure 8.3. Block diagram of a PEMFC system with (a) internal fuel-reforming module and (b) externally supplied merchant-grade hydrogen.

Moreover, PEMFC systems fed by pure hydrogen show the highest relative performance in terms of system dynamics, costs of fuel cells (the precious metal loading of anode is minimum), and in terms of stack and system power densities, which result 1.3 and 0.6 kW/1, respectively [2, 3]. 

Fig. 4.1 General scheme of a direct Hi PEMFC system (HiFCS) for vehicles...

A compressor should also match the other typical requirements of a road vehicle, in particular noise, cost reduction, and compactness. A further consideration derives from the quality of oxidant required by the stack. The air must be very clean since the presence of few small oil droplets or traces of chemical contaminants could damage dramatically the fuel cells, reducing their efficiency and durability. Therefore, PEMFC systems require oil-free compressors or air filtration for removing particulate and contaminants (sulfurs, salts, CO, and hydrocarbons). 

McGrath, J.E., Hickner, M., Kim, Y.S. etal. (2003) Advances in Materials for PEMFC systems, Asilomar Conference Grounds, February 23-26, Pacific Grove, California, p. 22. 

Fabricate and evaluate 1-kg H2 and 5-kg H2 capacity hydrogen supply systems in conjunction with proton exchange membrane fuel cell (PEMFC) systems. 

In the subsequent four years, annually update the baseline cost model and system scenarios based on assessments of developments in PEMFC system technologies and manufacturing processes. 

An MEA performance of -600 mV at 400 mA/ cm under realistic system operating conditions has been demonstrated, representing a major milestone towards developing a viable atmospheric PEMFC system. To further enhance cell performance (to desired operating conditions of 0.8 V at 400 mA/ cm ), optimization of cathode structure and formulation to enhance proton conduction and cathode catalyst utilization is required. 

PA. Adcock, S. Pacheco, E. Brosha, T. Zawodzinski, and F. Uribe, "Maximization of CO Tolerance of PEMFC Systems Using Reconfigured Anodes". To be presented at the Electrochemical Society Meeting, Salt Lake City, UT (Fall 2002). 

SD is routinely used to deposit thin films and has proven benefits from economies of scale in the metallization of plastics. The technique has already been used to create enhanced and unique MEAs for H2 -air proton exchange membrane fuel cell (PEMFC) systems. In this project, JPL is pursuing the use of SD to create DMFC membrane electrode assembly structures with highly electro-active catalyst layers that will reduce the amount and cost of the Pt-alloy catalyst at the fuel cell anode. 

Carbon aerogels and xerogels have been used as supports for Pt and Pt-based electrocatalysts for proton-exchange membrane fuel cells (PEMFCs), also known as polymer-electrolyte fuel cells [56,58,83-90], These fuel cells are convenient and environmentally acceptable power sources for portable and stationary devices and electric vehicle applications [91], These PEMFC systems can use H2 or methanol as fuel. This last type of fuel cell is sometimes called a DMFC (direct methanol fuel cell). 

Pt-doped carbon aerogels have been used successfully in the preparation of cathode catalyst layers for oxygen reduction reaction (ORR) in PEMFC systems [83-86]. Thus, different Pt-doped carbon aerogels with a Pt content of around 20 wt% were prepared by impregnation [83]. Results obtained with these Pt catalysts were compared with others supported on carbon blacks Vulcan XC-72 and BP2000, which are commonly used as electrocatalysts. The accessibility of the electrolyte to Pt surface atoms was lower than expected for high-surface-area... 

In summary, AFCs based on pure oxygen have been proved to be both powerful and successful, whereas those operating on air appear not to have a commercial future at present. In fact, it is generally considered that the sensitivity of alkaline electrolyte solutions to carbon dioxide is the principal reason why PAFCs have made such an inroad into fuel-cell technology since the 1970s and have been followed by PEMFC systems in recent years. If it were not for this poisoning problem, it is likely that air-based AFCs would also be serious competitors. 

A third way to build up pFCs based on MEMS-polymers such as poly-dimethylsiloxane (PDMS) or polymethyl methacrylate (PMMA) or PCB-materials such as polyimid (PI) or FR4. These polymers can be micro-machined by molding or by laser ablation. Shah et al. [22,23] have developed a complete PEMFC system consisting of a PDMS substrate with micro-flow channels upon which the MEA was vertically stacked. PDMS micro-reactors were fabricated by employing micro-molding with a dry etched silicon master. The PDMS spin coated on micro-machined Si was then cured and peeled off from the master. The MEA employed consisted in a Nafion - 12 membrane where they have sputtered Pt through a Mylar mask. Despite an interesting method, this FC gave poor results, a power density of 0.8 mW cm was achieved. 

The earliest PEMFC system models [1,2] were for single cells at steady state, assuming isothermal and isobar conditions. Performance is averaged over the cross-channel direction, and transport in gas channels is decoupled from transport through the Membrane Electrode Assembly (MEA). The power of... 

Nevertheless, the full commercialization of PEMFC systems needs a stable supply of hydrogen, which must be characterized by high purity for avoiding the CO poisoning of the PEMFC anodic catalyst [26]. 

Due to the low operation temperature of PEMFC an extra reactor is needed to convert the carbon monoxide into hydrogen by the water gas shift reaction. The extra reactor needs water or steam supply for the water gas shift reaction and the operation temperature of the shift catalyst has to maintain. So the shift reactor has to be part of the thermal management of PEMFC system. So the thermal management of a PEMFC system becomes more complex in comparison to a SOFC system. 

Su Zhou and Fengxiang Chen, PEMFC System Modeling and Control... 

De Falco M (2011), Ethanol membrane reformer and PEMFC system for automotive applications . Fuel, 90,739-747. 

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