12
$\begingroup$

If you have two metals i.e. iron / copper and an electrolyte solution you can make a battery. Typically all of the designs have several of these connected in series to boost the output. But I have seen some instances where people claim you can connect several nails sharing the same lemon as an electrolyte, connect the metals in sequence and get that boost in output.

Example: https://www.sciencebuddies.org/stem-activities/lemon-battery

If that is the case could you do the same with several nails (iron/copper) connected in sequence but all sharing the same saltwater electrolyte and still get the boost in power output?

$\endgroup$
8
  • $\begingroup$ Can you provide a reference for the several nails sharing the same lemon? $\endgroup$
    – Ed V
    Jul 14, 2021 at 0:08
  • 1
    $\begingroup$ Thanks! So what you have is two series cells: lemons have membranes such that you do not have a simple contiguous fluid beneath the rind. So you could slice the lemon into pieces and make a lemon cell from each piece with its two electrodes. Then two or more gives a battery proper. $\endgroup$
    – Ed V
    Jul 14, 2021 at 0:17
  • 1
    $\begingroup$ Say you had a single cell consisting of a large (plastic) drum of salt water. And on separate sides of this drum (say the circumference is 2 feet) you had the electrode/ anode pairs and then you connected them in sequence via a copper wire. Would the size of the cell potentially provide enough distance to allow for a boost or would all the power likely flow between the electrode/anodes via the electrolyte and not via the series. $\endgroup$
    – Joseph U.
    Jul 14, 2021 at 0:27
  • 1
    $\begingroup$ I think your second scenario is the likely one. But the nice thing about your question and proposed scenario is that they are fairly easy to test. You could use a big plastic bowl, clothespins to hold electrodes to the side, wire and metal paper clips to make connections, and a cheap DMM to measure the voltage. $\endgroup$
    – Ed V
    Jul 14, 2021 at 0:32
  • 1
    $\begingroup$ @CaiusJard I agree. I upvoted all three answers because they are all informative and address the question and they supplement each other. The accepted answer includes the important recognition that the shared electrolyte effectively puts parallel resistances (“leakages”) across each cell. So compartmentalizing the electrolyte, as in using the different lemon segments, is beneficial. This also shows that a balance exists: if the electrolyte is too conductive, the battery is shorted quite a lot. But if the electrolyte is not sufficiently conductive, then the battery will be very low powered. $\endgroup$
    – Ed V
    Jul 17, 2021 at 11:29

3 Answers 3

18
$\begingroup$

Yes, you could have multiple electrodes in the same electrolyte, but to some extent, that would short-circuit the battery. For example, if you stack copper and silver coins with blotting paper (bp) between them, in the order: Cu bp Ag Cu bp Ag ... Cu bp Ag and immerse the whole in an electrolyte, rather than just wetting each piece of blotting paper, some of the electricity will "sneak around" the edges, using up some current from the battery and reducing voltage because of internal resistance voltage drop, as well as shortening the life of the battery.

The same applies to iron and copper tacks, stuck into a lemon and connected as above. That said, if the non-connected electrodes are closely spaced, and the wired-together tacks are widely separated, not too much juice would be lost from the lemon battery for a brief demo, e.g., lighting a 3 volt blue LED.

DrMP Lemon Battery

$\endgroup$
7
  • $\begingroup$ As EdV and the OP (who appears to be from electrical engineering side) postulate in the comments, the (-) and (+) poles will be oblivious of the other electrodes. You have to use separate lemons. $\endgroup$
    – AChem
    Jul 14, 2021 at 3:02
  • 8
    $\begingroup$ @M.Farooq DrMoishePippik appears to claim that because the lemon is not a very good conductor, the smaller resistances within each cell and greater resistance between cells should be enough to create a greater voltage, at least for a short time. S/he's not claiming it's efficient or that 98% of the available energy isn't wasted in self-heating. $\endgroup$
    – user253751
    Jul 14, 2021 at 8:31
  • 3
    $\begingroup$ I've done this many years ago with some success. The separation ratio I recall was rather larger than shown, and we used a voltmeter so the current drawn would be tiny. I think an experiment is in order but won't have time tonight $\endgroup$
    – Chris H
    Jul 14, 2021 at 9:05
  • 1
    $\begingroup$ @ChrisH, Worth trying. The distances have to be larger, I believe. $\endgroup$
    – AChem
    Jul 14, 2021 at 14:22
  • 1
    $\begingroup$ I'd double the distance at least, but the theory behind it is correct. $\endgroup$
    – Mast
    Jul 16, 2021 at 6:29
14
$\begingroup$

Yes, you can have same the electrolyte and a pair of two different metals, but the key point is that if you wish to increase voltage difference, you need to connect them in series and use separate containers for each pair, and of course each pair must be connected. Your postulate in the comment is correct. If we use a large bucket, only the pair connected to the terminals of the battery will be responsible for the voltage. The rest of the pairs sitting in the same bucket are sort of "short circuited".

Now, there is a problem with same electrolyte batteries. The Nernst equation cannot predict the potential. This is why for theoretical purposes, it is beneficial to match the ions in solution and the metallic electrode, i.e., Zn must dip in zinc ions (zinc sulfate). Just by chance, I was reading a translation of a century old book (from German): Quantitative analysis by electrolysis. It is amazing how much people knew in the 1890s. There is a list of batteries, whose names we have never heard of including a "gravity battery". The two solutions of copper sulfate and zinc sulfate are separated not by membranes but by their densities and this battery was quite popular in telegraphic days.

Of course real battery science is more an art. This is the only branch of modern electrochemistry where fast progress is needed. If someone can beat the electron transfer kinetics he/she will deserve a Nobel Prize. You see lead acid batteries, despite all the bad things about lead, are still used worldwide, because its electron kinetics is fast and electrochemists do not have a viable alternative.

I do recall a series potato battery, which could light a small bulb here (https://www.livescience.com/62570-potato-battery-conduct-electricity.html). Replace potatoes with salt water beakers or any inert salt.

Battery

$\endgroup$
3
  • $\begingroup$ Ok extreme example. Say you have a Electrode/Anode pair that will react well with sea water. The ocean is reasonably big so its possible to have enough separation that the pairs of metal won't short each other. Once you have achieved this minimum distance (no idea what it might be but pretty sure if they are far enough away from each other the single cell could service both reactions without them sharing in the reaction with each other) is it likely that the minimum distance will be too great to productively attach the items in serial? $\endgroup$
    – Joseph U.
    Jul 14, 2021 at 5:00
  • 2
    $\begingroup$ I upvoted: good answer. As for the idea by @JosephU., a simple test might be to carefully open up (with needle nose pliers) a standard 9 V battery, spread the 6 individual cells apart while keeping their electrical connections, and then placing that exposed battery in a bowl of saltwater. Will it give the original (nominal) 9 V output? I may try this for fun: I have seen 9 V batteries with blue cylindrical internal cells, like wikipedia shows in its 9 V battery article. $\endgroup$
    – Ed V
    Jul 14, 2021 at 12:37
  • $\begingroup$ @EdV Worth trying. $\endgroup$
    – AChem
    Jul 14, 2021 at 14:06
6
$\begingroup$

I've only basic training in chemistry, since my competences are in Electrical Engineering, but I'd like to tackle your question from another angle.

I'm assuming your question is sort of an X-Y problem and you are not really interested in increasing the efficiency of an actual lemon-based cell.

From an engineering POV, what you want from a power source is, simply speaking, to extract the maximum amount of energy without losing much in the process.

There are 3 basic quantities involved in this: energy, power and efficiency. Voltage and/or current are of much lesser importance, since there are electronic methods that can boost one of those at the expense of the other.

For AC generators, a transformer can increase/decrease the voltage while decreasing/increasing current (with a given load) at the same time.

For DC generators such as batteries, you can add a switching DC/DC converter that can step the voltage up or down as needed with high efficiency.

So the point is: is yours an efficient method to improve either the power or the energy output of your cell?

As already pointed-out, your "multi-cell" arrangement can be modeled as a series connection of cells with every cell having a resistor in parallel. This latter models the fact that the electrolyte of the various cells is shared among them.

Those parallel resistances represent an additional load to the cells. This wastes power and energy. Are these losses negligible or not? To answer this you should consider other losses in the system, e.g. the internal resistance of the electrolyte, the contact resistance of the connection wires and the resistance of the wires themselves.

To do a meaningful comparison, you have to know the value of those parallel resistances, and this is probably tricky business.

But remember, the energy stored in a cell depends on the volume of the electrolyte (well, probably on its mass, but assuming a more or less constant density...), so you should ask if there is a better method to suck out the energy out of a given volume of electrolyte. Well this is what is already done when you need more energy: you build a bigger cell. And if you need also more power, you increase its current rating (since it's voltage can't be changed) by increasing the surface of the electrodes.

If you need higher voltage, for a given power, you simply put a DC/DC boost converter between the cell and the load. Nowadays, this is what is done in lots of products that can be powered with a single AA or AAA cell, for example.

In this way you avoid entirely those extra losses represented by those parallel resistances.

BOTTOM LINE

As already pointed out in other answers and comments, your idea could work, but it's inefficient. To increase the efficiency you should increase those parallel resistances and this means more distance between pairs of electrodes. This implies a need for a bigger volume of electrolyte.

This is also (probably) inefficient because there are already other well-tested means to extract the energy from the same amount of electrolyte and provide to the end-user (the load) the voltage level it needs to work properly.

Moreover your system is much more complicated to set-up, even if you could make the losses negligible, since the output voltage can only be set by multiples of the cell voltage, and you would need a DC/DC converter (or an DC/AC converter if your load needs AC) anyway to produce the voltage the load needs.

$\endgroup$
5
  • 3
    $\begingroup$ (+1) Excellent answer, covering a number of important issues, and welcome to the chemistry SE! One thing, though: the electrolyte is not the energy source in a simple voltaic cell, even for the lemon cell. The cohesive energy is the main source of energy. So in the present case, with zinc and iron electrodes in seawater, the zinc is the main energy source. There is little iron ion content, per se, in seawater. $\endgroup$
    – Ed V
    Jul 16, 2021 at 11:58
  • 2
    $\begingroup$ Reference for the cohesive energy statement in my other comment: K. Schmidt-Rohr, "How Batteries Store and Release Energy: Explaining Basic Electrochemistry", J. Chem. Ed., 95 (10) (2018) 1801-1810. The Zn and Cu Daniell cell is addressed in great detail. TL; DR Cohesive energy differences are the major factor in explaining the behavior of this famous galvanic cell. $\endgroup$
    – Ed V
    Jul 16, 2021 at 12:01
  • $\begingroup$ @EdV Thanks! And also thanks for the clarification. As I said, I'm not a chemistry specialist and my electrochemistry basics dates back to ~30yrs ago. I never heard the term "cohesive energy" (unless it is called in a completely different way in Italian). I hope I can get a bit of spare time to give a quick refresher to my rusty electrochemistry notions. Thanks for the pointer. $\endgroup$ Jul 16, 2021 at 21:25
  • 2
    $\begingroup$ Glad to have EEs here! I had a grad minor in EE, but having real pros around keeps thing more grounded (pardon the pun)! Klaus’s paper is nice because it is way too easy to jump to a simplistic explanation like electronegativity differences, etc. And, for the Daniell cell, zinc and copper are adjacent elements in the periodic table, so he addresses why zinc gets oxidized and copper ions get reduced. $\endgroup$
    – Ed V
    Jul 16, 2021 at 22:15
  • $\begingroup$ @EdV. I appreciate puns. They help engineers keep their feet on solid ground. ;-) $\endgroup$ Jul 17, 2021 at 9:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.