Fuel cell water management

Practical modes of water management include the supply of water at the anode side by humidification of the anodic feed gas, or the application of a pressure across the membrane that creates the backflow from cathode to anode (anode water removal). Different approaches of effective fuel cell water management are discussed, for example, in Ref. 45. 

Macroscopic models can be classified into two broad categories (i) membrane conductivity models and (ii) mechanistic models, typically for fuel cell water management purposes. The latter usually require the use of a conductivity model, a fit to empirical data, or the assumption of constant conductivity (e.g. fully hydrated membrane at all times), and can be further classified into hydraulic models, in which a water transport is driven by a pressure gradient, and diffusion models, in which transport is driven by a gradient in water content. 

Multi-phase flow, originating from the water production by the ORR, is critical to fuel cell water management Figure 30.9 shows the scanning electron microscopy... 

Dadheech G, Elhamid MHA, Blunk R (2009) Nanostructured and self-assembled superhydrophilic bipolar plate coatings for fuel cell water management. Nanotech Conference Expo 2009, vol 3, Technical Proceedings pp 18-183... 

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. 

Many types of electrolytes have been used in fuel cells. Water solutions of acids, such as phosphoric, sulfuric, and trifluoroacetic acids (acidic electrolytes), and bases such as sodium hydroxide or potassium hydroxide (alkaline electrolytes), can be incorporated into efficient cells. Cells using water solutions as electrolytes have complex problems of water management and electrolyte retention under conditions of severe physical motion. These will probably not be suitable for automobile service. For stationary applications described in Chapter 6 the water based electrolytes may offer advantages. 

Pressure variation also influences water management significantly. The following considerations assume that dry air is fed to the fuel cell. Water saturation at the cathode outlet is achieved only by water produced inside the fuel cell. Figure 4.21 shows the amount ofwater contained per kg of air at saturation (100% relative humidity). It is evident that the amount of water contained in saturated air decreases with increasing pressure. Therefore, less water is carried out of the fuel cell cathode at the same air mass flow. 

An interesting consideration is a possible tradeoff in the design of the fuel cell component and the fuel source. In the case of the direct methanol fuel cell (DMFC), for example, water is required for the reaction of methanol. The discharge product of the fuel cell, water, can be used if the water management or recovery is incorporated in the fuel cell a one-time cost of increased size, weight and complexity of the fuel cell. Or water can be added to the fuel source at the expense of a recurring lower specific energy of the diluted fuel source. 

During operation of the fuel cell, water is formed at the cathode and consumed at the anode. A system for transferring water from the anode to the cathode has been designed and built, but water management on large scale units turned out to be very difficult to maintain. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Heat rejection is only one aspect of thermal management. Thermal integration is vital for optimizing fuel cell system efficiency, cost, volume and weight. Other critical tasks, depending on the fuel cell, are water recovery (from fuel cell stack to fuel processor) and freeze-thaw management. 

This coincides with ongoing research to increase power density, improve water management, operate at ambient conditions, tolerate reformed fuel, and extend stack life. In the descriptions that follow, Ballard Power Systems fuel cells are considered representative of the state-of-the-art. 

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