**3. Mitigation of mass transport losses**

314 Mass Transfer - Advanced Aspects

Uniform current density is an important factor to the lifetime of the membrane. A uniform current density is important to prevent the formation of hotspots, which can further decrease the performance of the electrodes and membranes. If the electrolyzer ratio (flow rate of electrolyzed water divided by the flow rate of the feed water) is less than 10%, the current density distribution in the cell is uniform in the cell (Onda, Murakami et al. 2002). At higher ratios, the current density will increase upstream where there is sufficient water and the current density will decrease downstream where there is insufficient water for the reaction. The performance of PEM electrodes has been found to be sensitive to differential pressures as low as 20 mbar between the cathode and anode sides of the cell (Millet, Andolfatto et al. 1996). If high operating pressures are used, the differential pressure must be controlled. Stainless steel pipes in contact with deionized water can cause a steady decrease in cell voltage. Low concentrations of Fe, Ni and Cr from the stainless steel became concentrated in the membrane, limiting cell performance. On-line deionizers were found to give a more stable performance, but can impose limitations when used with high operating pressures

Demonstration electrolyzer plants, rated at 100kW, have been successfully run for up to 15,000 hours (Stucki, Scherer et al. 1998). One plant was shut down after 15,000 hours due to hydrogen concentrations in the oxygen off-gas of higher than 3 percent. A second demonstration plant, run for only 2300 hours with 50,000 hours of standby operation, was shut down for the same reason. During the standby period, a protective polarization current of 0.34 mA/cm2 was applied in order to prevent corrosion of the current collectors at the cathode. Post mortem analyses of membranes from both plants indicate that stack failure was due to thinning of the Nafion® 117 membrane. The non-uniform membrane thinning coincided with an observed decrease in cell voltage. The ion exchange capacity of the

> **1.2 1.4 1.6 1.8 2.0 Cell Voltage (V)**

Fig. 9. Cell polarization comparison for anode liquid feed and cathode vapor feed PEM electrolyzer cells at 30°C. () Anode liquid feed (U) Cathode vapor feed. Reproduced from

membranes remained consistent throughout operation.

**Cell Temperature**

**Nafion 117**

**30°C**

and temperatures.

**0**

(Greenway, Fox et al. 2009)

**200**

**400**

**Current Density (mA/cm2**

**)**

**600**

Membrane development is of particular interest due to the limitations of current Nafion® membranes such as temperature restrictions due to dehydration and subsequent loss of conductivity. In order to meet these demands researchers have attempted to improve the membrane by doping or by investigating new polymer membranes. These alternate routes may also be used to increase fuel cell performance in the presence of gas impurities such as carbon monoxide.

For example, the effects of carbon monoxide on alternative membranes such as poly(2,5 benzimidazole) have been investigated (Krishnan, Park et al. 2006). These polymers, doped with phosphoric acid, had the ability to be operated at temperatures up to 210°C with 1% CO without performance losses, which are higher temperatures and higher carbon monoxide concentrations than conventional MEA configurations are tolerant. Other investigations involve using alternatives such as glass papers to support organic membranes (Tezuka, Tadanaga et al. 2005). The membranes cast from 3-glycidoxypropyltrimethoxysilane and tetraalkoxysilane would otherwise be too thick and have to high of a resistance for viable fuel cell use. These membranes were able to achieve a maximum power density of 80mW/cm2 at 130°C and 7% relative humidity.

Other methods of development include (Jalani, Dunn et al. 2005) impregnating Nafion® to create more stable composite materials. The authors found that when Nafion® was impregnated with ZrO2, SiO2 and TiO2 the composite membranes has better water retention and thermal stability than Nafion® alone. ZrO2 impregnated Nafion® had the best performance overall and this is believed to be due to the increased acidity and surface area of the membrane. ZrO2 impregnated Nafion® was the only modified membrane that showed increased conductivity over Nafion®. Leading the authors to conclude that the distribution of water between the surface and bulk of a system is as important as the amount of water absorbed.

Other than membrane development, an alternative method of improving the MEA is through catalyst development. A current area of interest is the use of non-precious metals or new binary catalysts for the oxygen reduction reaction at the cathode. Presently Pt and Pt alloys are widely used as anode and cathode materials in Proton Exchange Membrane

Mass Transport Limitations in Proton Exchange Membrane Fuel Cells and Electrolyzers 317

Greenway, S. D., E. B. Fox, et al. (2009). "Proton exchange membrane (PEM) electrolyzer

Halseid, R., P. J. S. Vie, et al. (2006). "Effect of ammonia on the performance of polymer electrolyte membrane fuel cells." Journal of Power Sources 154(2): 343-350. ISO (under development). Hydrogen fuel- product specification- Part 2: proton exchange

Jalani, N. H., K. Dunn, et al. (2005). "Synthesis and characterization of Nafion-MO2

Knights, S., N. Jia, et al. Fuel cell reactant supply- Effects of reactant contaminants. Fuel Cell

Krishnan, P., J.-S. Park, et al. (2006). "Performance of a poly(,5-benzimidazole) membrane

Li, B., J. Qiao, et al. (2009). "Carbon-supported Ir-V nanoparticle as novel platinum-free

Majsztrik, P., A. Bocarsly, et al. (2008). "Water permeation through Nafion membranes: the

Martínez-Rodríguez, M. J., E. B. Fox, et al. (2011). "The effect of low concentrations of

Martinez-Rodriguez, M. J., E. B. Fox, et al. (2011). "The effect of low concentrations of

Millet, P., F. Andolfatto, et al. (1996). "Design and performance of a solid polymer electrolyte

Mohtadi, R., W. K. Lee, et al. (2005). "The effect of temperature on the adsorption rate of

Onda, K., T. Murakami, et al. (2002). "Performance analysis of polymer-electrolyte water

Passalacqua, E., F. Lufrano, et al. (2001). "Nafion content in the catalyst layer of polymer

Pisani, L., G. Murgia, et al. (2002). "A new semi-empirical approach to performance curves of polymer electrolyte fuel cells." Journal of Power Sources 108(1-2): 192-203. Soto, H. J., W.-K. Lee, et al. (2003). "Effect of transient ammonia concentration on proton

Springer, T. E., T. Rockward, et al. (2001). "Model for polymer electrolyte fuel cell operation

cell." Journal of the Electrochemical Society 149(1): A1069-A1078.

role of water activity." J. Phys. Chem. B 112: 16280-16289.

water electrolyzer." Int. J. Hydrogen Energy 21(2): 87-93.

Journal of Hydrogen Energy 34: 6603-6608.

Electrochimica Acta 51: 553-560.

J. Power Sources 159(2): 817-823.

Hydrogen Energy 34: 5144-5151.

Seminar, Palm Springs, CA.

197 WG 12.

accepted.

preparation.

56: 37-42.

46(6): 799-805.

J. Electrochem. Soc. 148(1): A11-A23.

A133.

operation under anode liquid and cathode vapor feed configurations." International

membrane (PEM) fuel cell applications for road vehicles. ISO/CD 14687-2. ISO TC

(M=Zr,Si,Ti) nanocomposite membranes for higher temperature PEM fuel cells."

based high temperature PEM fuel cell in the presence of carbon monoxide."

anodic catalysts in proton exchange membrane fuel cell." International Journal of

tetrachloroethylene on the performance of PEM fuel cells." J. Electrochem. Soc.

ammonia on the performance of PEM fuel cells." J. Electorchem. Soc. under

hydrogen sulfide on Pt anodes in a PEMFC." Applied Catalysis B: Environmental

electrolysis cell at a small-unit test cell and performance prediction of large stacked

electrolyte fuel cells: effects on structure and performance." Electrochimica Acta

exchange membrane fuel cell performance." Electrochem. Solid-State Lett. 6(7):

on reformate feed: Effects of CO, H2 dilution and high fuel utilization."

(PEM) Fuel Cells. Despite a cathodic over potential loss of 20%, Pt and Pt alloys are still preferred for their resistance towards corrosion in acidic media. Pt however, being an expensive metal of low abundance, it is of interest for researchers to develop a corrosion resistant non noble metal substitutes. These non-noble metal catalysts can range from metalloporphyins and bimetallic transition metals to heat treated metal catalyst (Wang 2005; Colón-Mercado and Popov 2006; Li, Qiao et al. 2009). The main advantage of the use of nonnoble metal catalysts is the reduction in cost and ease of availability, although the precious metal based catalysts consistently have higher activity for the reaction, the results are promising.
