End Plate Material for Electrochemical Cell

[skarf2]

Hello all,

I've been working on a design of an analytical electrochemical cell. The basic design involves a stack of 4 plates, 2 end plates, a graphite counter-electrode plate and a garolite electrode plate. In the center of these plates resides a thin teflon-gasketed catalyst membrane layer. It is imperative that I achieve very good uniformity in compression across this gasket, and therefore across the entire cell. I have specified good tight tolerances on flatness and parallelism of the individual plates, and have taken bolt count and type into consideration.

The final design issue that I face is this: what material to use for the end plates. I am torn between stainless steel and aluminum. Stainless offers far superior stiffness and corrosion resistance, but Aluminum excels in every other parameter. My question is this: based upon a plate thickness of 1/2" (which is the minimum required for certain integral features to be included), will an Aluminum plate provide sufficient stiffness for uniform compression?

The two other intermediate plates are 5/8" thick, and all the plates are 10"x10". I am using 16 grade 8 5/16"-24 UNF bolts/nuts on a 4.25" circle. The center membrane will not see pressures above 100psi, most likely not even above 50.

I have attached a rendering of the stack for reference.

Thanks in advance.

J

 

[skarf2]

 

[unclesyd]

Are the holes in the middle of your top plate shown drilled all the way through?

 

[skarf2]

Yes. They are bored through because there are electrode connectors in there that require wire leads, which will run out of the cell.

 

[dogtop]

From the picture it seems that if you use a metallic endplate you are going to get a lot of stray electrical currents? Are the endplates isolated and insulated from the electrochemical current paths?

HTH

 

[desertfox]

Hi skarf2

Acouple of questions firstly you give 2 good points for stainless steel ie stiffness and corrosion resistance but then say aluminium is better in all other aspects, what aspects are these? secondly the pressure of the center membrane will not see more than 100psi I am presuming that pressure is generated by the bolts?

desertfox

 

[skarf2]

Dogtop,

Yes. The endplates are isolated, so that is not of a concern.

Desertfox,

The aluminum offers us lower cost or material and machining, lower weight, and far superior thermal conductivity. This is important because heating elements will be applied to the two end plates to bring the cell up to the prescribed temperature.
Also, the pressure is from the bolts.

 

[CoryPad]

What is the temperature? You may have problems with differential thermal expansion/contraction, especially with the bolted joints. From a first glance, I think Al could be made to work, though you made need a special alloy + temper. Depending on exposure to corrosive fluids, you probably should coat the bolts with an Al-friendly coating like Magni 565 or a PTFE-style like Xylan.

 

[skarf2]

We'll be operating at a steady stat temperature between 60-80C. The cell will be very well insulated.

I'm not familiar with the bolt coatings you mentioned. Where can I find more information on this? (Vendor, requirements, properties etc)

 

[desertfox]

hi skarf2

Well I don't know if this will influence your decision but the Aluminium plate will have double the expansion to that of the steel bolts and further as the alumium plate expands as well as trying to expand the bolts it could also increase the pressure on the inner plates depending on how stiff the inner plates are and also the coeff of expansion of the latter.

desertfox

 

[CoryPad]

Magni:

http://www.themagnigroup.com

Xylan:

http://whitfordww.com