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Stanford University

Stanford Microfluidics Laboratory

Electroosmotic pumps for water removal

Principal Investigators: J.G. Santiago, Cullen R. Buie, Shawn Litster, Hyuk_Min Kwon, Daniel Strickland, and Matthew Suss

 

Fuel cells offer potential for environmentally benign vehicle propulsion as a high power density alternative to battery powered vehicles.  Water management remains a major technical challenge in proton exchange membrane (PEM) fuel cell systems, however, as liquid water produced at cathode reaction sites can block reactants and reduce system efficiency.  We have developed the world’s first PEM fuel cells with electroosmotic (EO) pumps integrated at the cathode for active water management [1, 2]

In our original work with active water management, we integrated EO pumps directly into the cathode flow field ([2], shown in figure below). Here, liquid water emerging from the cathode GDL is absorbed into hydrophilic porous glass frits forming the channel walls. An electric field is applied across these frits using platinum electrodes, and excess protons in the electric double layers which develop on the glass surface migrate, causing bulk flow and removal of excess product water from the cathode.

 

 

Single channel fuel cell with integrated EO pump. Hydrogen and air flow are into the page.  Water formed due to oxygen reduction reaction at the cathode is forced out of the GDL via hydrophobic forces where it coalesces into droplets.  Liquid water droplets are wicked into the hydrophilic porous glass structure of the EO pump.  Once the EO pump structure is adequately saturated with water, EO pumping actively drives water through the porous glass structure and into integrated water reservoirs in the acrylic top plate.

 

In more recent work we utilize porous, hydrophilic, electrically conductive flow channels and/or wick layers that are hydraulically coupled with an electroosmotic pump.  These provide an active means of removing liquid water directly from reaction sites. We show that electroosmotic pumps can relieve water management issues in PEM fuel cells with less than 0.5% parasitic power. A PEM fuel cell cathode with water management is shown in the figure below. A porous graphite flow field plate, heat treated for hydrophilicity, is inset in a solid graphite base. A wick tab extends from the flow field to outside of the fuel cell. An EO pump is hydraulically coupled to this tab for excess product water removal.

 

plate_schem
Hydrogen fuel cell with wick and coupled electroosmotic pump for active water removal

 


Here we present qualitative evidence of water removal mechanisms in our PEMFC cathode with porous carbon wick.  We used a Phantom V7.3 high speed camera to obtain images for the video (shown at 1/1000 real time).  During these experiments the frame rate of the camera was greater than 30k fps.  The PEMFC operates at a constant current of 600 mA and at an air stoichiometric flow ratio of two.  The cathode channel walls are fabricated from porous carbon that we’ve heat treated to make hydrophilic and the channel width is 1 mm.  As shown, a liquid water droplet on the GDL grows until it is absorbed by the hydrophilic wick.

 

Liquid product water emerges from the GDL. Capillary pressure within the porous carbon transports water from the GDL and flow field channels into the porous flow field. Once the flow field is fully saturated, a pressure gradient driven by the EO pump acts to remove additional product water. Stable fuel cell performance at low air stoichiometries in parallel flow fields has been achieved with negligible EO pump parasitic power (< 0.5%) [1].

 

Polarization curves for the (left) porous carbon cathode flow field with the EO pump off, and (right) with the EO pump activated at 12 V. The porous cathode flow field with active EO pumping shows the largest range of operation and has the best performance. At ? = 1.3, the EO pumped device has a maximum power density of 0.42 W/cm2, compared to the respective values of 0.27 W/cm2 and 0.2 W/cm2 for the solid and passive wick structures.

 


 

Visualization of flooding and recovery in 25cm2 porous carbon cathode flow field with EO pumping.

 

We have performed an in-depth study of water transport in this fuel cell system using a three by three segmented anode flow field and current collector [3].  Here, we were able to measure real time current distributions within the fuel cell, and as a result, local reaction rates. The movie below shows spatially resolved power density measurements during fuel cell flooding and recovery.

 

 

 

 


Power distribution in 25 cm2 fuel cell with active water management. Figure shows orientation offuel cell for power distribution measurements shown in video. EO pump is initiated at 390 sec.

We are currently working to integrate EO pump water management techniques with automotive scale fuel cells and are investigating novel passive water removal methods.

 

[1] S. Litster, C.R. Buie, T. Fabian, J.K. Eaton and J.G. Santiago. Active Water Management for PEM Fuel Cells. Journal of The Electrochemical Society, 2007, 154(10), B1049-B1058.
[2] C.R. Buie, J.D. Posner, T. Fabian, C.A. Suk-Won, D. Kim, F.B. Prinz, J.K. Eaton and J.G. Santiago. Water management in proton exchange membrane fuel cells using integrated electroosmotic pumping. J. Power Sources, 2006, 161(1), 191-202.
[3] D.G. Strickland, S. Litster and J.G. Santiago. Current distribution in polymer electrolyte membrane fuel cell with active water management. J. Power Sources, 2007, 174(1), 272-281.

 

See related publications here