Electrolysis/Galvanic cell is basically redox reaction split in half-reactions, running on different electrodes. For example, hydrogen fuel cell has two electrodes. On one hydrogen donates electrons to electrode:
$H_2 = 2H^+ + 2e^-$
on another, oxygen drains electrons from electrode
$O_2 + 4 e^- + 4 H^+ = 2 H_2O $
The electrons travel from one electron to another via external electricity consumer. If electrons are forced to travel by external force, the reaction may be reversed. This ideas are utilized in electric chemical batteries, accumulators, fuel cells, industrial electrolyzers and so on.
This way, half-reactions are separated between different electrode. Running half-reaction by itself without other half elsewhere is problematic at best, because of electroneitrality violation. You'll need extreme voltage source and even then it will stop quickly.