Water vapor from fuel cells

An entirely new level of sophistication—not only in experiments but also in modeling—will be required for particles, aerosols, and the associated radiation field sets. New mid-IR laser-based instrumentation and use of long-duration balloons have helped make major advances in observations. The balloons can sit in the upper stratosphere and then be lowered to the lower stratosphere with power from fuel cells and solar panels. The modeling elements are equally important it is necessary to test the model and its validity, and the model must link the measurements. The observations must be linked to trajectories, the trajectories must be initialized, and sources of specific chemicals must be identified along with the positions of those sources. Considerable progress has been made on observations and refinement of models to help explain low ozone loss at the mid-altitudes, the increase in UV dosage, the appearance of water vapor in the stratosphere, and possibly, of climate changes 50 million years ago. 

The amount of heat needed to ensure that all the water at the fuel cell entrance is in vapor form can be calculated from the energy balance (the numbers correspond to those in Figure 9-3) ... 

The presence of a small amount of water vapor (up to pH20/pH2 = -0.03) in fuel reduces anode overpotential. For anode-supported cells, the use of pore formers is important to tailor the shrinkage during cofiring and to create adequate porosity for better performance. The difference in cell power output could differ by as much as 100% for cells as porosity changes from -30 to -50%. 

Later, Hinatsu et al. studied the uptake of water, from the liquid and vapor states at various temperatures, in acid form Nafion 117 and 125, and Aciplex and Flemion membranes, although the latter two similar products will not be discussed here. These studies were motivated by a concern over the deleterious effects, involving either overly dry or overly wet membranes, on electrical conductivity within the context of polymer electrolyte fuel cells and polymer electrolyte water electrolyzers. 

The authors also mention the interesting result that uptake from water vapor at 80 °C was less than that at lower temperatures, as reported by others, and that this difference was not due to the predrying procedure. It was suggested that water would condense on the membrane surface with more difficulty at the higher temperature and that this would retard sorption. This situation is of obvious significance with regard to humidified membranes in fuel cells. Also, as seen in other studies, the water uptake increases with decreasing EW. 

Reactants in this cell need not be pure. Hydrogen may be extracted from fuel mixtures and oxygen from air. Since product moisture is formed in an acid cell on the cathode, the air depleted in oxygen can be used Ibr water removal if Ihe cell is operated at a sufficiently high temperature to vaporize the water as it is formed. 

Knowing that 1 mg/cm2 of product water is a threshold, how much water can be stored at maximum within each component of the fuel cell, and how much can be removed to the outside For the cathode catalyst layer (CCL) with typical thickness of 10 p,m and 50% pore volume fraction, the CCL water storage capacity is approximately 0.5 mg/cm2. A 30- un-thick membrane can store 1.5 mg/cm2 of water, but its actual water storage capacity depends on the initial water content, A., and therefore is proportional to (ks.where A.sa, denotes the water content of a fully hydrated membrane. The escape of water into the GDL is unlikely due to the very low vapor pressure at cold-start temperatures (Pv>sa, = 40 Pa at —30°C). For reference, the GDL with 300 pm thickness and 50% porosity would store about 15 mg/cm2 of water, if it could be fully utilized. This capacity is too large to be used for cold start. From this simple estimation we can conclude that the CCL water storage capacity alone is not sufficient for successful cold start and that a successful strategy is to store water in the membrane. 

The species distribution within a PEM fuel cell is critical to fully characterize the local performance and accurately quantify the various modes of water transport. The most commonly used analytic technique for measuring the gas composition within a fuel cell is gas chromatography (GC). Mench et al.13 demonstrated the measurement of water vapor, hydrogen, and oxygen concentration distributions at steady state. A micro gas chromatograph was utilized to measure the samples, which were extracted from eight... 

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