Electrochemistry

[Anon_LG]

Hello all at Electronicspoint. I have not posted for a while, I have been busy with GCSE's, electronics and other things.

A few months ago I made an accidental discovery involving a liquid (shall remain nameless due to intellectual property rights, mine) and aluminium, finding that a voltage was produced between them. At the time I did not know about aluminium based electrochemical cells, I googled and it was obvious to me that aluminium had been used before in cells. Upon reading the list of electrolytes used I read over the contents of the liquid carefully. None of them matched, see here. I initiated my test with water, this produced approximately 0.15 volts, wildly off the third of a volt I managed to produce. I started trying other liquids with known chemical ingredients. Upon trying another mixture I found a far more successful 0.5 volts from a single cell. The one thing the liquids both shared was water, this however may not be held responsible as the finding of 0.15 volts shows. One of the mixtures contains alcohol, the other does not, one mixture contains a base and an acid, the other does not. There are no correlating ingredients. Other than water there are no obvious oxidisers. After building several revisions due to electrolyte leakage plaguing the first revisions, I have a stable construction, four cells in series, producing a total of 2 volts.

Compare this to other chemistries and this may seem insignificant, however my mixtures were completely safe. I have been in prolonged skin contact with no effects, I have worn no eye protection either. With the obvious exceptions of ingestion and contact with eyes, my battery has no harmful effect whatsoever.

The most exciting part happened today, upon gathering all the information I could on the electro chemistry I tried to find whether the battery is rechargeable. The battery was connected to an LED for an extended period of time, the voltage dropped from 2.05 volts to 1.97 volts. I applied a reverse voltage across the battery for about thirty seconds, upon testing again with my voltmeter, the voltage had risen to 2.00 volts. I will not jump to conclusions, can anyone give me advice as to how to proceed?

The mixtures are not ones that may be manufactured at home, I only have access to them under certain circumstances which I shall not name for risk of revealing clues.

I would like to find out:
Which chemicals are responsible for reacting with the aluminium?
Is the chemistry rechargeable?
Are there any books that would aid me in the field of aluminium electrochemistry?

Any pointers are appreciated, I have worked hard on this.

 

[wingnut]

Hi Lavaguava

We all remember from school how the teacher made electricity by taking a lemon and poking a piece of zinc and copper into the lemon and attaching these electrodes to a galvanometer. Here the electrolyte is completely safe, and all kinds of fruits and vegetables can serve as the electrolyte, and all kinds of metals as the electrodes producing all kinds of voltages.

The next step was to go to the redox table, a list of half reactions, with oxidising agents at the top and reducing agents at the bottom. From what I remember, Al was quite good at losing electrons therefore a good reducing agent and therefore near the bottom, and therefore produced a better voltage than say Fe.

I have made batteries in the past by taking copper coins and interspersing them with paper and coin-sized bits of Al and soaking the whole lot in vinegar. The more alternating Al/paper/ Cu/paper/Al/paper/Cu... that one uses the bigger the voltage. Current produced is pretty pathetic.

Also I have taken normal non-rechargeable AA batteries or AAA batteries and recharged them, and they indeed do recharge somewhat (there probably is a hazard to doing this, such as they may pop), but in my poorer days I recharged my old non-rechargeable batteries.

All I can do is encourage you to keep experimenting. Get a good redox table and I am sure you will be able eventually to find the two half reactions which when added together, will explain your voltage.

 

[Anon_LG]

OK, thankyou for the advice, I will look into the redox tables. So, I need to locate on a redox table a chemical located within my mixture that corresponds with my half a volt? For example with the vinegar, I would need to locate ethanoic acid?

You say that with the vinegar the current was poor, what voltage did you achieve per cell?

Your help is very much appreciated,

 

[wingnut]

I made a battery using 3 cells or sandwiches of Al and Ni and got 0.6 V or 0.2 per cell.
With Al and Cu (coins the South African 5c piece) I got 0.53 V or 0.18 V per cell.
By poking a piece of Zn (salvaged from the shell of a AA battery) and a piece of Cu (from flex) into a lemon, I get from this one cell 0.93 V.

Lets look at the lemon, Zn, Cu cell.
From the Standard Reduction Potentials table, the Cu half reaction is Cu2+ + 2e- --- Cu = + 0.34 V
The Zn half reaction is Zn2+ + 2e- ---> Zn = -0.76 V

Normally one ignores the electrolytes except that they should have ions of the substance oxidised and reduced and they should have a concentration of 1 mole/litre.

Then one finds the half reaction with largest V irrespective of sign, and one makes this positive.
Thus we find that the Zn ---> Zn2+ + 2e- = + 0.76 V. Zn is being oxidised.
Add this to the Cu half reaction which stays unchanged since Cu is being reduced, we expect
a final V of +0.76 V + 0.34 V = 1.1 V
But for our lemon we only get 0.93 V.
Normally this means that the concentration of ions in our lemon is less than 1 mole/litre (dm cubed).

At school we ALWAYS used a salt which contained Cu and Zn so we were assured that there were ions of our anode and cathode (Zn and Cu). I frankly cannot help you further if you choose to use vinegar or lemon juice as your electrolytes because these contain unknown ions, and there may be other smaller reactions between these ions.

Obviously one can experiment and by luck find the perfect fruit juice for your electrolyte.
But, in general one only uses the table for the two half reactions of cathode and anode, not the vinegar.
That is as far as I can help.