14
$\begingroup$

A maybe (hopefully) simple question about the denotations of "anode" and "cathode". The below image is a schematic of a polymer solar cell (Source (WBM)). (The figure text is quoted as well for the sake of context).

My question is only about the naming of electrodes. Isn't it correct that

  • the cathode is the reducing electrode, which supplies electrons, while
  • the anode is the oxidizing electrode, which accepts electrons?

Why would electrons (black spheres) move towards the negatively charged cathode, while positive holes (white spheres) move towards the positively charged electron-absorbing anode? Wouldn't the opposite make sense?

Is it a mistake on the drawing or should I re-think my understanding of the terms "anode" and "cathode"?

enter image description here

Figure 1. The working principle of the solar cell. Light enters the cell through the transparent anode, and is absorbed in the bulk heterojunction layer through generation of excitons (1). The excitons diffuse in the bulk heterojunction until they either recombine or reach a donor-acceptor interface, where they separate into electrons (black) and holes (white) (2). The electrons and holes will then move to the respective anode and cathode, through the donor and acceptor material phase (3).


Edit

If the figure is correct and I am the one being confused, then there are several other cases that seem to teach false. E.g. the following "PANIC"-remembering-rule to remember, which is cathode and which is anode (from this source, slide 8):

enter image description here

"*Negative is cathode", it says, and we are back to my question in the top: why do the electrons (black spheres) move towards the negative cathode in that schematic?

Many more diagrams agreeing with this latter one are found from a simple google search.

$\endgroup$
  • $\begingroup$ In traditional textbooks, the flow of electricity is shown going the opposite way (than it actually functions) because the guy who came up with the concept couldn't tell which was positive and which was negative- so he guessed. Out of respect for his contributions for science (and the fact that the actual direction has little effect on the math) the scientific community kept it that way, and many people still teach it backwards. $\endgroup$ – Ella Oct 13 '17 at 18:22
  • 1
    $\begingroup$ Negative and positive are not linked to anode and cathode - it depends whether the reaction goes toward equilibrium (voltaic cell) or is forced by an external potential to go against it (electrolytic cell). For details, see my answer. $\endgroup$ – Karsten Theis 2 days ago
  • $\begingroup$ Because there is no oxidation or reduction in a photovoltaic cell, it depends on conventions which are different depending on the type of device: sc.edu/study/colleges_schools/chemistry_and_biochemistry/… $\endgroup$ – Karsten Theis 2 days ago
1
$\begingroup$

There is no completed electronic circuit in an electrochemical cell

In an electrochemical cell, the anode is the source of electrons to the external circuit and the cathode is the sink. The circuit of charge transport gets completed by ions traveling inside the cell. A solar cell is different from an electrochemical cell in that their is no net chemical reaction. In the solar cell, electrons flow in a closed circuit - round and round in the external circuit and through the device.

Designation of anode and cathode

So labeling the anode and the cathode relies on an analogy between a voltaic cell and a photovoltaic cell as a source of electrical work. It makes sense to use the direction of electron flow in the external circuit to define anode and cathode (electrons flow from anode to cathode in the external circuit). In the voltaic cell, there is no electron flow inside the cell (there is ion flow instead to balance charges). In the photovoltaic cell, electrons flow from junction to anode and holes flow from junction to cathode (or you could say electrons flow from cathode to junction).

Unfortunately, anode and cathode are named using different conventions depending on the type of device, see this overview (and beware that the current I sometimes goes in the same direction as the electrons and sometimes not, again depending on conventions).

Negative and positive electrode

The (+) and (-) designation is confusing even just for electrochemical cells. While the designation of anode and cathode is consistent for voltaic and electrolytic cells (i.e. using and charging a battery), the designation of (+) and (-) switches, so it is uncoupled from the direction the electrons flow through the external wire.

Direction of electron flow

For the photovoltaic cell, maybe this picture helps: Before light hits the cell, anode and cathode are neither negative nor positive. Once light hits the cell, the anode becomes negative because electrons are moving toward it from the junction, and the cathode becomes positive because electrons are jumping from it into holes coming from the junction. If you then attach an external consumer of electrical work, you can predict the direction of electron flow through the external circuit.

$\endgroup$
4
$\begingroup$

Regardless of the polarity, the electrode where oxidation takes place is the called the anode and therefore reduction must take place at the cathode. The electron flow from the anode to cathode is as shown in your top picture.

By way of an example, in an electrochemical cell, suppose that two beakers are connected by a salt bridge. In one beaker is a strip of zinc metal immersed in a $\ce{Zn(NO3)2}$ solution and in the other a strip of silver in $\ce{AgNO3}$ solution. The two metals are then connected by a wire and a current will flow. (The salt bridge supplies a return path so that the solutions remain electrically neutral). The redox of the Zn electrode is $-0.763$ V and that of the Ag $+0.799$ V. This means that electrons will flow from the zinc to the silver electrode. The zinc is oxidised to $\ce{Zn^{2+}}$ and the electrons are released into the metal and flow to the silver electrode through the wire. The zinc electrode is the anode and the silver the cathode.

$\endgroup$
1
$\begingroup$

You are confused about the definition of reduction and oxidation.

  • Reduction is the gain of electrons.
  • Oxidation is the loss of electrons.

Which means, the cathode is gaining electrons, while the anode is losing electrons. The illustration is absolutely correct.

$\endgroup$
  • $\begingroup$ Thank you for the answer. My description of which "gains" and "loses" might not be clear. The actual question is, if the cathode indeed is the negative electrode? So the question is still: Why would electrons move towards the cathode, which is negative? I have added an extra diagram of simple electrolysis to explain this point a bit clearer - (negative) anions move towards the (positive) anode in order to deliver the excess electron. (Positive) cations move towards the (negative) cathode to receive electrons; that is what I meant by the cathode "supplying electrons". $\endgroup$ – Steeven Feb 15 '17 at 14:37
  • $\begingroup$ If this is a galvanic cell, then cathode is positive electrode, hence electrons will move toward cathode. If this is a electrolyser, then yeah, cathode, is negative. But this "negative" is different from the negative in cell case. This "negative" means, the negative end of electricity supply is connected to cathode, and cathode receiving electron from that. $\endgroup$ – Huy Ngo Feb 15 '17 at 14:55
  • $\begingroup$ There is no reduction or oxidation in a photovoltaic cell. The material that loses electrons in one direction gains electrons coming from the other side. $\endgroup$ – Karsten Theis 2 days ago
1
$\begingroup$

The anode is the electrode, where substances are losing electrons and are oxidated. The cathode is the electrode, where substances are gaining electrons and are reduced.

The tricky part for the memorising is, anodes and cathodes flip the position, when the current is reversed, depending on if

  • the cell is in the mode of electrolysis
  • the cell is in the mode of providing current.

The convention for the primary and secondary (rechargable) cells is to take the latter case as the power source.

My mnemotechnic trick is to remember, that

At anode is occuring anabasis (*) of electrons, upwards from the electrode, i.e. oxidation.

It means the anode is

  • the positive electrode at electrolysis,(=there is attached the + wire from the external power source, taking electrons away. It does not necessarily mean it has more positive potential than the cathode),

  • the negative electrode in power cell ( the minus electrode of the cell, providing electrons).

At the cathode is occuring katabasis of electrons, downwards to the electrode, i.e. reduction.

It means the cathode is

  • the negative electrode at electrolysis, accepting electrons from the external power source.
  • the positive electrode for power cells. drawing electrons from the circuit.

(*) Anabasis by Xenophon, some 370 BC, "The journey upwards", about dramatic return of Greek mercenaries from Persia to Greece.

$\endgroup$
0
$\begingroup$

Cathode is the electrode where Reduction takes place and Anode is the electrode where oxidation takes place.

I use the mnemonic OIL RIG

Oxidation -> loss of electrons (or increase in oxidation state)

Reduction - > Gain of electrons (or decrease in oxidation state)

$\endgroup$
  • $\begingroup$ a better one : LEO roars GER : loss of electrons is oxidation and gain of electrons is reduction. $\endgroup$ – glucose 2 days ago
0
$\begingroup$

The above picture depicts the inside of a power device i.e. the solar cell. That can be considered as a battery. In the below figure the positive charges go through the metal and reach the cathode of the battery. Meanwhile internally in the battery the negative charges move towards the cathode to cancel out the positive charges coming from the metal electrodes.

In summary, externally positive charges move to the cathode and internally negative charges move toward the cathode. Therefore all the pictures depicted are correct.

$\endgroup$
-1
$\begingroup$

The Cathode is the positive electrode; the anode is the negative electrode. During discharge, positive ions flow from anode to cathode. This makes the cathode positively charged. Free electrons at the anode flow externally through the load to the cathode. During charge, an external voltage source has its positive electrode connected to the cathode and its negative electrode connected to the anode. The negative electrode of the external voltage source supplies electrons to the anode. Excess positive ions at the cathode are repelled to the anode but move more slowly than electrons move through wires.

$\endgroup$

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.