The Anode Is Not Always Positive

What students typically write

“The anode is the positive electrode.” Full stop. Students memorise this from electrolysis lessons and apply it universally, including to electrochemical cells and fuel cells where it is incorrect.

Why it is wrong

The defining feature of an anode is not its charge. It is the electrode where oxidation occurs. In electrolysis and in electrochemical cells, oxidation happens at the anode, but the electrical sign of that electrode is different in each case.

In electrolysis: An external power supply drives the reaction. The anode is connected to the positive terminal of the battery, so it is the positive electrode. Negative ions (anions) are attracted to it and lose electrons (oxidation).

In an electrochemical cell (battery): The reaction is spontaneous. The more reactive metal oxidises (loses electrons) at the anode, making it the source of electrons. Since electrons flow out of it into the external circuit, the anode is the negative terminal of the cell.

The confusion arises because most IGCSE teaching focuses heavily on electrolysis, where the anode is positive. When the syllabus introduces fuel cells or simple cells, students carry over the “anode = positive” rule and get it wrong.

What the mark scheme actually wants

For electrolysis questions, “anode is the positive electrode” is accepted. But for electrochemical cell questions, the mark scheme expects:

A typical question: “In a zinc-copper cell, identify the anode and state its polarity.” [2]

Marking points:

1. Zinc is the anode (1 mark)
2. The anode is the negative electrode / zinc is more reactive and is oxidised (1 mark)

An answer that says “the anode is positive” in this context scores 0 for the polarity mark.

Worked example: wrong vs right

Question: In a hydrogen-oxygen fuel cell, state which electrode is the anode and whether it is positive or negative. [2]

Wrong answer: “The anode is the positive electrode, where oxygen enters.” This scores 0/2. The anode is where hydrogen is oxidised (not oxygen), and in a fuel cell the anode is the negative terminal.

Right answer: “The anode is where hydrogen is oxidised / loses electrons (1). It is the negative electrode of the fuel cell (1).” This scores 2/2.

How to avoid this mistake

Use the process, not the sign, to identify the anode:

• Anode = oxidation (always, in every context)
• Cathode = reduction (always, in every context)

Then determine the sign from the context:

Context	Anode sign	Cathode sign	Why
Electrolysis	Positive	Negative	Battery forces current direction
Electrochemical cell / fuel cell	Negative	Positive	Spontaneous reaction determines current

The mnemonic AN OX, RED CAT (Anode-Oxidation, Reduction-Cathode) works universally. The sign of the electrode changes; the process does not.

For electrolysis where the anode is positive, see electrolysis. For fuel cells where the anode is negative, see hydrogen-oxygen fuel cells.