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Consider the following hydrogen fuel cell fuel cell

http://butane.chem.uiuc.edu/pshapley/Enlist/Labs/FuelCellLab/FuelCell.html

At the anode, hydrogen is oxidised (losing electrons). My first question is this: why is the anode negative if it is attracting electrons?

My second question refers to the fact that once the hydrogen atoms are iodised, the H+ ions move through the electrolyte towards the oxygen ions at the cathode, reacting to form water. If the anode is negative, why would positive H+ ions move away from it?

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  • $\begingroup$ Here is a better picture: phys.org/news/… The anode is labeled negative because that is where the electrons enter the wire. The cathode is labeled positive because that is where the electrons leave the wire (so it is from the perspective of the consumer of electricity). $\endgroup$
    – Karsten ♦
    Sep 24, 2019 at 16:53

4 Answers 4

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At the anode, hydrogen is oxidised (losing electrons). My first question is this: why is the anode negative if it is attracting electrons?

It is not attracting electrons, it is attracting hydrogen atoms. The electrode binds the hydrogen and releases the electrons. The electrons are then able to flow out through the wire.

My second question refers to the fact that once the hydrogen atoms are iodised, the H+ ions move through the electrolyte towards the oxygen ions at the cathode, reacting to form water. If the anode is negative, why would positive H+ ions move away from it?

Once the electrons have returned to the cathode, the cathode is more negative than the anode. The positive ions move to the cathode and the negative ions move to the anode.

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In a galvanic cell, the anode is positively charged. The cathode is negatively charged. A good mnemonic I learned a while ago is that anions go to the anode, and cations go to the cathode. This is because opposite electric charges attract. So negatively charged anions are attracted to the positively charged anode. And positive cations are attracted to the negative cathode.

The electron itself was originally discovered as the component particles of cathode rays from cathode ray tubes. That electrons come out of the cathode is a hint about cathode charge: the buildup of electrons on the cathode eventually results in the emission of an electron beam. If electrons are building up, the cathode must be negative.

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  • $\begingroup$ in the diagram in my question, the anode is negative. $\endgroup$
    – lagrange103
    Feb 19, 2015 at 6:42
  • $\begingroup$ Can you add a link or reference to where you got your diagram from in the question? $\endgroup$
    – Curt F.
    Feb 19, 2015 at 6:44
  • $\begingroup$ The writeup at wikipedia is pretty good. Check it out: en.wikipedia.org/wiki/Anode . Reading it made me realize that I need to add a few qualifiers to my answer which I will do now. $\endgroup$
    – Curt F.
    Feb 19, 2015 at 6:44
  • $\begingroup$ Reference added. I still dont get why the anode is negative in the diagram. $\endgroup$
    – lagrange103
    Feb 19, 2015 at 6:51
  • $\begingroup$ Is the diagram could be right if it were a "reverse" fuel cell where electrical energy and water is converted to hydrogen. This is a an electrolytic cell (the reverse of a galvanic cell). In this mode the anode is negative. $\endgroup$
    – Curt F.
    Feb 19, 2015 at 7:07
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You should check the standard reduction potentials of each reaction in the cathode and anode. In a fuel cell (which is a type of galvanic cell), the hydrogen oxidation reaction occurs on the anode at 0V vs RHE while the oxygen reduction reaction on the cathode occurs at around 1.23V vs RHE. As you can see, the anode reaction occurs at a negative potential compared to the cathode reaction. That is why the anode is marked negative and the cathode is positive.

At the anode, hydrogen is oxidised (losing electrons). My first question is this: why is the anode negative if it is attracting electrons?

The anode is negative because it is accumulating electrons from the oxidation of hydrogen atoms.

My second question refers to the fact that once the hydrogen atoms are iodised, the H+ ions move through the electrolyte towards the oxygen ions at the cathode, reacting to form water. If the anode is negative, why would positive H+ ions move away from it?

As there is an accumulation of electrons at the anode, electrons repel each other and are attracted towards the cathode where there is a deficiency of electrons. At the same time, the cathode is becoming the "meeting place" for the incoming electrons and provided oxygen atoms which attract the hydrogen ions to drive the oxygen reduction reaction that forms water. So H+ ions are attracted to the cathode due to the increasing number of electrons in the cathode.

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Anode as such is positive and cathode is negative.Hydrogen gets oxidized at the anode by giving an electron,so the anode becomes rich in electrons. The flow of electrons is from anode to cathode. Hence anode which is rich in electrons, compared to cathode, is represented as negative. Cathode being negative attracts the hydrogen ions, which gets reduced when it receives electrons from anode via an external circuit.

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  • $\begingroup$ "Hence anode which is rich in electrons, compared to cathode, is represented as negative." - does this not contradict your first line? sigh I am so confused $\endgroup$
    – lagrange103
    Feb 19, 2015 at 6:27

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