Sciencemadness Discussion Board

Really need feedback on a battery design.

White Yeti - 1-10-2011 at 09:25

Hello everyone.

I've been designing an electrochemical cell that uses two energy dense materials as oxidizing and reducing agents. Here's the theory:
At the anode, aluminium reacts with hydroxide anions:
Al + 3OH- ----> Al(OH)3 +3e- +1.66V

At the cathode, electrons come from the load and react with perchlorate anions and hydrogen ions in solution:
ClO4− + 2 H+ + 2 e− <-----> ClO3− +  H2O +1.2V

Then once all the perchlorate ions are used up, another reaction kicks in:
ClO3− + 2 H+ +  e− <-----> ClO2(g) +  H2O +1.18V

Another reaction takes care of the chlorine dioxide to form hypochlorite:
ClO2(g) +  H+ +  e− <-----> HClO2(aq) +1.19V

Last but not least ,three final reactions reduce chlorine to +1, 0 and finally to -1:
HClO2(aq) + 2 H+ + 2 e− <----> HClO(aq) +  H2O +1.67V
2 HClO(aq) + 2 H+ + 2 e− Cl2(g) + 2 H2O +1.63V
Cl2(g) +H2O(l) <-----> HCl(aq) + HClO(aq)

The theory looks simple, but making such a cell almost looks like a failed venture. What kind of separator do I use? Do I need to use a salt bridge? Should I decrease the pH at the cathode to increase reaction rate? What kind of cathode do I use, graphite? Or copper wool or mesh?

I'm mainly throwing this idea out into the open. I don't have any perchlorate right now, and I'll have to wait quite some time before I can get any. So please, feel free to try this out if you think this idea might work.

Thanks for reading.

[Edited on 10-2-2011 by White Yeti]

hissingnoise - 2-10-2011 at 01:50

The theory looks simple, but making such a cell almost looks like a failed venture.

And looks totally bonkers since there are simpler routes to chlorine-water . . .

White Yeti - 2-10-2011 at 04:53

Quote: Originally posted by hissingnoise  

And looks totally bonkers since there are simpler routes to chlorine-water . . .

I'm not looking for a simple route. Explain yourself.
This is a BATTERY DESIGN, not a chlorine generator. I thought I made that perfectly clear.

I originally thought of this design because both perchlorate and aluminium are substances that require much energy to be manufactured. Thus, reacting them both in a battery would release tremendous amounts of stored energy. A simple parallel to this is flash powder (whether it uses chlorate or perchlorate it doesn't matter).

hissingnoise - 2-10-2011 at 05:07

Sorry WY, I skimmed through your post while still only half-awake . . .

Chordate - 2-10-2011 at 05:28

According to my table that "anode" reaction has a reduction potential of +2.31 volts, and the cathodes there are all smaller values. I think you need to go check your math, because I don't think that cell is going to do what you want it to.

blogfast25 - 2-10-2011 at 06:20

Potassium perchlorate is very poorly soluble in cold water, potassium chlorate slightly better. Why not attempt to put together a cell with an Al anode and a graphite cathode. Try a lightly alkaline electrolyte for the anode, a saturated, slightly acidic KClO3 solution for cathode electrolyte and connect the electrodes over a voltmeter and a salt bridge? Just to see if you get any potential?

[Edited on 2-10-2011 by blogfast25]

AndersHoveland - 2-10-2011 at 07:13

Perchlorate will not will not funtion as an effective oxidizer in such a cell (unless perhaps if heated to boiling).

The reaction of perchloric acid (at least under 70% concentration) with aluminum does not reduce any portion of the perchlorate. In this regard, perchlorate is surprisingly resistant to reduction, as even nitrate is partially reduced in the same situation. Similarly, neither will Zn+HCl reduce KClO4, although it reacts KNO3.

For this reason, it may be best to just use chlorate in the cell, rather than perchlorate. Sodium chlorate is also more soluble than potassium chlorate.

White Yeti - 2-10-2011 at 08:01

Thanks for the feedback.
Sounds like I need to use chlorate instead of perchlorate. I was actually planning on using sodium perchlorate, which is much more soluble than its potassium counterpart.

Blogfast25: I see what you are saying, but what should I use for the salt bridge? It seems like the trickiest part of building such a cell is controlling pH and the diffusion of OH- ions from one half cell to the other. The way I looked at it, was that water would react with (per)chlorate and electrons at the cathode to form more water and OH- ions that would migrate to the anode and react with aluminium. So, do I build a salt bridge made out of a hydroxide or will a weak base such as NaCO3 suffice?

AndersHoveland - 2-10-2011 at 08:14

Using a base in the salt bridge may prove rather difficult, considering that alkali chorate will not react with aluminum on its own. The solution will likely need to contain some dilute acid. Ideally, dilute perchloric acid should be used, but nitric acid likely would also work, although it might complicate the reactions. HCl should not be used since it is oxidized by chlorate. H2SO4 does not seem to readily dissolve aluminum metal.

blogfast25 - 2-10-2011 at 08:54

Quote: Originally posted by White Yeti  
Blogfast25: I see what you are saying, but what should I use for the salt bridge?

I just wouldn't worry about that so much just yet. If you can close the circuit with a charge carrying solution and your redox reactions work then you should get a significant cell potential. That may not last very long because the actual amount of energy a cell can deliver depends enormously on its construction. But it's perhaps better to try and produce 'proof of concept' before thinking about the patent office yet...:D

[Edited on 2-10-2011 by blogfast25]

White Yeti - 2-10-2011 at 09:19

OK. I extracted some potassium chlorate from matches this morning. I'm going to try this out very soon. I'll share my results as soon as I have the chance:)
Wish me luck!


White Yeti - 2-10-2011 at 10:00

The results are in for those who are interested:
Short circuit current:
Dry charge: 0mA
Wet charge: <1mA
Salt water charge: ~3mA
Salt water and potassium chlorate: ~8mA decreasing down to 5mA after 2-3 minutes.
Salt water, potassium chlorate and 3% hydrogen peroxide (just for the hell of it): Rises from 5-18mA and stabilises
Salt water, potassium chlorate H2O2 and iron(II) acetate, (to make dillute fenton's reagent): rises from 18-20mA

Conclusion, the crude design of the cell showed that adding chlorate marginally increases the performance of an aluminium battery. A more sophisticated cell design is required to benefit from the oxidising power of chlorate to any extent.