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I understand that if you have a fuel cell or flow battery with a single cell that electrons will go from the electrode and through an external circuit across the load which the fuel cell / flow battery is attached to.

But, if you have multiple cells in series, how are the electrons in the middle cells able to travel anywhere? Each cell is separated by a non-electrically conductive membrane. There are only external connections on the first and last cells. Where are the electrons traveling in the middle cells?

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    $\begingroup$ Internally, within any cell, electrons are not traveling. Ions are. Externally, between 2 electrodes of the same or different cells, electrons travel via wires as in any circuit, regardless of batteries. $\endgroup$
    – Poutnik
    Commented Jan 27 at 21:12
  • $\begingroup$ Right, I understand electrons are not traveling internally. But the cells of a fuel cell, or at least of a flow battery, are not connected externally. The first and last plate are connected to an external circuit. The middle cells are not. Where's the connection to the electrode of each cell? Look at lead acid batteries or any battery that has multiple cells in series. The electrodes are welded together so there's a path for electrons. Where is this connection in flow batteries and fuel cells? $\endgroup$
    – DavePiae867
    Commented Jan 27 at 21:16
  • $\begingroup$ There is always a connection otherwise current could not flow. Just their geometry may create false impression there is none. $\endgroup$
    – Poutnik
    Commented Jan 27 at 21:40
  • $\begingroup$ Linking two examples below. The only external connection to each of the electrodes is where voltage is monitored (as you can see with the connections on the side of each battery). So there has to be some internal connection from plate to plate where electrons pass. But the membrane covers the active area of each plate so it's totally unclear how this is done. pubs.acs.org/cms/10.1021/acssuschemeng.2c03148/asset/images/… researchgate.net/publication/307877246/figure/fig8/… $\endgroup$
    – DavePiae867
    Commented Jan 27 at 21:46
  • $\begingroup$ Wire-A-C-wire-A-C-wire-A-C-wire-A-C-wire / Between A and C, ions are passing according their charge sign. $\endgroup$
    – Poutnik
    Commented Jan 27 at 22:08

2 Answers 2

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Suppose three cells are coupled in series. First a cell Zn/MnO2, then a cell Cu/Ag, and then a Daniell cell Zn/Cu. The middle cell Cu/Ag has a negative pole made of Cu, and a positive pole made of Ag. In the middle cell the electrons are coming out of the negative Cu electrode (anode). They are immediately sent through a copper wire to the positive pole (MnO2) of the first cell. In this first cell, the electrons are getting out of the anode (zinc) electrode. But these electrons are not sent to its own positive pole (MnO2). No. They are sent to the positive pole (cathode) of the 3rd cell, which is a copper plate from the Daniell cell. The cycle is closed when the electrons produced in the zinc anode (negative pole) of the Daniell cell (3rd cell) are sent through a copper wire to the silver cathode (positive pole) of the middle cell

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  • $\begingroup$ Can you point to a diagram, paper, video, etc. that shows the electrode of each cell in a flow battery or fuel cell being connected to the next cell with an external wire? I have never seen this hence my confusion. I have seen these connections in conventional batteries (e.g. lead acid), but not in flow batteries or fuel cells. I understand in principle what should be happening. $\endgroup$
    – DavePiae867
    Commented Jan 27 at 21:23
  • $\begingroup$ @DavePiae867, Wires are not magic. If the anode of one cell touches the cathode of the next, electrons flow from one to the other. The anode and cathode are simple conductors, just as is the wire. $\endgroup$
    – DrMoishe Pippik
    Commented Jan 28 at 0:36
  • $\begingroup$ @DavePiae867 Direct contact of electrodes of adjacent cells (like stacking "button" cells or columns of AA or AAA cells or the original Volta's Cell column) is fully equivalent to connecting them by wires (if we neglect wire resistance). $\endgroup$
    – Poutnik
    Commented Jan 28 at 9:13
  • $\begingroup$ @DRMoishePippik I have not seen any fuel cell or flow battery design in which the electrode of one cell directly touches the electrode of the next cell. There's an ion selective membrane that covers the entirety of the active area on each cell. Each cell/plate is non-conductive outside of this active area. Which is why I'm asking if you can provide any sort of literature, diagram, picture, etc. that would clarify this. $\endgroup$
    – DavePiae867
    Commented Jan 28 at 15:48
  • $\begingroup$ @Poutnik I would appreciate seeing anything that describes how/where this internal contact is made when there's a membrane between each cell that covers the conductive active area. $\endgroup$
    – DavePiae867
    Commented Jan 28 at 15:51
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Every electrochemical cell is circuit-wise like a black box with two galvanic contacts, containing particular cell design geometry and technology, that is irrelevant with respect to the circuit schema.

A battery of such cells is serialisation of such 2-contact black boxes, galvanically connecting respective contacts of adjacent cell and connecting the end contacts of the whole cell battery to external circuit.

Being a fuel or flow cell is irrelevant – the schema is the same.

Below you can see the vanadium flow cell schema from ResearchGate. A battery of such cells is assembled exactly as for any other cells, galvanically connecting external cell contacts with the opposite sign contacts of adjacent cells. The porous arrangement of electrodes is intended to maximize the contaxt area between the active electrolyte and the conductor, to have minimal internal resistance and maximal specific power.

The vanadium flow cell schema from ResearchGate

Note that the schema

$\mathrm{...(E_1|El_1||El_2|E_2)-(E_1|El_1||El_2|E_2)-(E_1|El_1||El_2|E_2)}...$

where $\mathrm{E_1}$, $\mathrm{E_2}$ are electrodes, $\mathrm{El_1}$ and $\mathrm{El_2}$ respective electrolytes and $\text{||}$ is a selective ion permeable membrane.

the schema can be simplified as

$\mathrm{...(E|El_1||El_2|)(E|El_1||El_2|)(E|El_1||El_2|)}$

what can be confused with wrong one:

$\mathrm{...||(El_2|E|El_1)||(El_2|E|El_1)||(El_2|E|El_1)....}$

(...) means formal interpretation of a single cell in a battery.

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    $\begingroup$ Please describe the EXACT path of electrons in a fuel cell or flow battery. I can do this for batteries. If I place several Li ion batteries in series, I am forming a contact between the electrodes in each battery. In each battery, the cap is welded to the positive electrode. The negative electrode is in contact with the casing. When you connect the cap (+) of one battery to the casing (-) of another battery, you form an electrically conductive circuit. Or take a battery which is composed of several cells, such as a lead acid battery, I can point to the welded joint between each electrode. $\endgroup$
    – DavePiae867
    Commented Jan 30 at 3:39
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    $\begingroup$ @DavePiae867 but it is exactly the same for fuel cell batteries and flow cell batteries as for classical cell batteries. Just technology and geometric arrangement is different. $\endgroup$
    – Poutnik
    Commented Jan 30 at 8:15

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