Posner Research Group
 

Micro/Nanofluidics Laboratory

Transport in PEM fuel cells

We are developing polymer electrolyte membrane fuel cells (PEMFCs) as high power density alternatives to portable power batteries and automobile internal combustion engines. Recent reports from automobile manufacturers (Gasteiger et al. Applied Catalysis B-Environmental, 2005) and the DOE suggest a five-fold reduction of the specific Pt-catalyst loadings (to 0.2 g/kW) must be made for large scale deployment of fuel cell based automobiles. There are two main strategies for decreasing Pt-specific catalyst loadings for PEM fuel cells: 1) reduce the Pt loading of MEAs from current 0.4 mg/cm² to 0.15 mg/cm² and 2) increase MEA power density from 0.7 W to 0.8-0.9 W by reducing mass transport associated losses by 50%. Hence, reducing mass transport related losses and managing water remains one of key challenges in PEM fuel cells.

In-situ image of liquid water of GDL in flooded conditions.

The majority of PEMFCs use polyperfluorosulfonic acid membranes (e.g., Nafion) as the electrolyte (or ion conducting membrane). Ion transport occurs along pathways established by anionic (sulfonic acid anion) groups within the hydrated polymer therefore require proper hydration levels. Several mass transport and reaction mechanisms affect PEM hydration including diffusion, electroosmotic drag, and evaporation and condensation in reactant gas streams. Maintaining adequate hydration is difficult as such mechanisms are strongly coupled, vary spatially, depend on the cell history and are influenced by membrane thickness and formulation, cell temperature, reactant composition and humidity, pressure, cell voltage, and channel design/topology.

Perhaps the most critical aspect of fuel cell water management is the delicate balance between membrane hydration (favoring high water content) and avoiding cathode flooding. Cathode flooding occurs when water production at the oxygen reduction reaction and electroosmotic drag of water to the cathode exceed the water removal rate resulting from air based advection, evaporation, and back diffusion. Liquid water that builds up at a fuel cell cathode decreases performance and inhibits robust operation. Flooding in the cathode reduces oxygen transport to reaction sites and decreases the effective catalyst area. Cathode flooding can result in a catastrophic decrease of performance and has been observed over a wide range of operating conditions.

Our lab focuses on three main areas of research related to PEM fuel cells:
• Novel fuel cell stack water management strategies²-³
• In-situ optical diagnostics detailing fuel cell performance
• Role of ambient conditions on the performance of air-breathing PEMFC.


Novel Fuel Cell Stack Water Management Strategies

 We use a novel water management technique whereby liquid water in hydrogen-air fuel cell cathodes is actively pumped out of the fuel cell using an integrated porous electroosmotic (EO) pumping layer. EO pumps are compact, have no moving parts, and scale favorably with fuel cell design. We integrate EO pumps to effectively remove liquid water from the cathode and enable air flow rates of just two to three times stoichiometric requirements. Theoretically, this active pumping architecture should consume less than 1% of fuel cell power (to date this number is closer to 4%, see Buie et al.). These strategies improve performance by extending the Ohmic operating range and increasing cell stability.

Schematic of the cross-sectional of PEMFC with integrated electroosmotic pumping structures.

Although various water management strategies have been proposed, membrane hydration and flooding mitigation is still typically resolved by fully humidifying reactant gas streams and pumping air into serpentine cathode channels at flow rates significantly higher than required by fuel cell stoichiometry. This strategy is not effective in managing water, is thermodynamically unfavorable, constrains cathode flow channel design, and requires auxiliary humidification equipment.

Schematic of the cross-sectional of PEMFC with integrated electroosmotic pumping structures.

Integrated EO pumps can increase fuel cell power density and stability as well as reduce mass transport and Ohmic losses by:

  • Removing liquid flooding water from channels and gas diffusion layers
  • Directly hydrating polymer electrolyte membranes.

Electroosmotic pumps are fabricated from planar porous materials. These pumps can generate high pressures (more than 340 atm at 12 kV applied potentials) and high flow rates (e.g., 40 ml min¯¹ at 100 V in a pumping structure less than 1 cm³ in volume), and their low voltage (10 V or less) performance is very well suited for fuel cell applications.

Galvanostatic (constant current) measure-ment versus time. The pump is activated at 240 s and the fuel cell voltage quickly (within about 15 s) increases (to 390 mV) when the flooding cathode water is removed.
Polarization curves at three stoichiometric air flow ratios, α (flowrate of air / stiochiometric flow rate of air). Open symbols indicate an activated EO pump and closed symbols a deactivated pump. The data shows that fuel cell performance increases with α as expected, and with the activation of the EO pump.


Air-Breathing Fuel Cells for Portable Power:

 Diffusion and free convection are the primary transport mechanisms for delivering oxygen to the cathode of air-breathing fuel cells. Air-breathing cells are typically characterized by low output power densities compared to forced-convection fuel cells. They are nevertheless attractive for the portable-power applications where the simplicity of free-convection oxidant delivery can outweigh the cost, limited lifetime, reliability, complexity, noise, volume, weight, and parasitic power consumption of an auxiliary fan or compressor. Balancing water in free-convection cells is challenging due the lack of control of ambient air stream conditions (flow stoichiometry, temperature, and humidity). We are interested in the performance of air-breathing fuel cells as a function of ambient conditions.

Figure 12 a) Increase in GDL surface temperature as a function of heat generated by the cell. b) High frequency resistance (indictative of membrane resistance and, therefore, membrane humidification) as a function of GDL surface temperature, at 80% relative humidity.



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Thermal plume shadowgraph visualizations of air breathing fuel cell. The left video is a horizontal fuel cell cathode surface (gravity points down). Buoyant plumes of hot, humid air convect upwards. The right video shows the case where the fuel cell surface is vertical.



This fuel cell project has been developed as a close collaboration with Stanford investigators Juan Santiago, John Eaton, Fritz Prinz.

Selected Publications:

 1. C.R. BUIE, J.D. POSNER, T. FABIAN, S-W. CHA, D. KIM, F.B. PRINZ, J.K. EATON, AND J.G. SANTIAGO. 2006. Water Management in Proton Exchange Membrane Fuel Cells using Integrated Electroosmotic Pumping, Journal of Power Sources, V161, 191-202


2. Buie, C., J.D. Posner, T. Fabian, S.-K. Cha, J.K. Eaton, F.B. Prinz, and J.G. Santiago. Active Water Management for Proton Exchange Membrane Fuel Cells using an Integrated Electroosmotic Pump ASME International Mechanical Engineering Congress and Exposition. 2005. Nov. 5-11, 2005, Orlando, FL.

3. Santiago, J., J. Posner, F.B. Prinz, T. Fabian, J. Eaton, S.-W. Cha, C. Buie, D. Kim, H. Tsuru, J. Sasahara, T. Kubota, and Y. Saito, US Patent: Fuel cell with electroosmotic pump (2006).

4. T. FABIAN, J.D. POSNER, R. O'HAYRE, S-W. CHA, J.K. EATON, F.B. PRINZ, AND J.G. SANTIAGO. 2006. The Role of Ambient Conditions on the Performance of Planar, Air-Breathing Hydrogen PEM Fuel Cell, Journal of Power Sources, V161, 168-182.