Basically this experiment shows you how to construct and test a rechargeable battery. It uses platinum for both electrodes in a solution of sodium chlorine (table salt) in water.
As it is first constructed, NO electrical voltage is measurable. But if you connect a DC supply (the external battery) to it, you can cause chemical reactions at the cathode and anode which create free gases - hydrogen and chlorine - that accumulate on the electrode surfaces as visible bubbles. This is an electrolysis reaction. NOTE that the instructions clearly say do NOT move the equipment because this will cause the bubbles to detach and float to the surface, and then your stored "fuel" is gone!
Having stored electrical energy in this battery in the form of two chemical elements, if you remove the external battery but leave the voltmeter attached, you can find that the unit now operates as a charged battery, consuming the two stored gases (one at each electrode) and generating a small current at a particular voltage. This is just the reverse of the electrolysis reaction. Note that the only load placed on the battery (to allow current to flow) is a very high-impedance voltmeter that allows only a small current to flow through it. Under those conditions the voltage remains above zero for some time, but the write-up clearly says it will diminish over time and you will have to switch your voltmeter to lower ranges to be able to measure it. Eventually the supply of stored hydrogen and chlorine gas bubbles on the electrodes will be so reduced you can't get any current out of the battery.
The experimental write-up does not make any statement about what current you might expect if the load were something other than the voltmeter -that is, something that actually allows significant current to flow if there's sufficient voltage. I expect this unit would provide very small currents only for a short period before the charge stored in the battery is depleted.
So, whether you can make this work for your project depends on how much electrical power you need for your load device(s). If you are hoping to get "free electrical energy" from it, forget it. This device is simply a battery you can charge from some external source and then get some of that energy back. My guess is it is NOT very efficient, and what you get back is a lot less than you put in to charge it. Moreover, the output voltage changes over time somewhat rapidly (depending on your load and the current it draws).
By the way, the platinum for electrodes is important. That metal does not actually participate at all in the reactions at its surfaces, precisely because it is a very inert metal. Using lesser materials for the electrodes will change the battery design significantly so that one of the electrodes may be consumed in different electrochemical reactions.
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