White Yeti
National Hazard
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Registered: 20-7-2011
Location: Asperger's spectrum
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On the subject of salt bridges.
So, I'm designing a battery that has one half cell with an unstable oxidising agent, and a very good reducing agent on the other side. I'm using
aluminium as a reducing agent, but I hit a stumbling block. The oxidising agent reacts with electrons to form an inert compound, water, and a bunch of
irrelevant side products. But it also produces sodium and potassium hydroxides, which if allowed to react with aluminium will destroy the anode and
waste the anode material.
So inevitably, I need to separate the half cells and connect them with a salt bridge. I still need the aluminium to react with hydroxide anions
produced at the cathode:
Al + 3OH- ----> Al(OH)3 + 3e-
But I absolutely need to keep the sodium and potassium ions out of the reduction half cell (we all know what happens when you mix aluminium and NaOH).
I also don't want to compromise the diffusion rate of OH- ions from one cell to the other, I need the diffusion rate to be as high as possible to
enable the battery to churn out the maximum amount of power it can provide.
So what do I use for a salt bridge? I can't use sodium hydroxide, or else I'm defeating the point of the salt bridge. All the other hydroxides are
pretty much insoluble in water.
What do you guys think?
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bbartlog
International Hazard
Posts: 1139
Registered: 27-8-2009
Location: Unmoored in time
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I think what you need is a battery separator, not really a salt bridge. See e.g. http://www.tinhoahoc.com/Battery/cr020738u.pdf for something resembling an overview (there are probably better overviews but no open access ones I
could find).
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smaerd
International Hazard
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Registered: 23-1-2010
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Mood: hmm...
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I'm not sure if this is even relevant but I have recently heard of people using shopping bags as semi-permeable membranes.
http://sites.google.com/site/sed695b/projects/longitudinal-r...
I'm not sure if they would hold up with strong base or not or allow the ions you want to pass through or whatever, but it's a pretty cheap thing to
test.
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