Understanding the fundamental principles of electron flow and voltage summation is critical for optimizing the design, efficiency, and scalability of Proton Exchange Membrane (PEM) water electrolyzers for green hydrogen production. This article provides a clear, systematic breakdown of the electrolytic current path and electrical behavior, contrasting the operation of a single membrane electrode assembly (MEA) with that of an industrial-scale, series-connected stack. We will delineate how electrons, liberated at the anode during the oxygen evolution reaction (OER), are conducted through an external circuit and bipolar plates-not the ion-conductive PEM itself-while protons migrate to the cathode for the hydrogen evolution reaction (HER). The analysis will explicitly show how individual cell voltages, comprising the thermodynamic reversible voltage, kinetic overpotentials, and ohmic losses, add linearly in a stack configuration, whereas the current density remains constant. Mastering these core concepts of charge transport and series circuitry is essential for engineers and developers aiming to enhance stack performance, reduce specific energy consumption, and achieve cost-effective, high-purity hydrogen generation.
How electrons move in a single PEM electrolyzer cell
- In a single PEM cell:
Anode (oxygen side):
Electrons are released.
Cathode (hydrogen side):
Electron path:
Anode → external circuit → cathode
This is straightforward.
How electrons move when multiple PEM cells are connected in series (a stack)
- The key component is the bipolar plate.
A bipolar plate has two faces:
One side is the cathode of Cell N
The other side is the anode of Cell N+1
And it conducts electrons between them.
✔ So the electron path in a stack becomes:
Electron from Cell 1 anode
→ flows through external circuit / bipolar plate
→ enters Cell 2 anode
→ flows to Cell 2 cathode
→ next bipolar plate
→ Cell 3 anode
→ … and so on
Electrons do NOT go through the membrane (the PEM only transports H⁺, not e⁻).
So electrons move cell-to-cell along the bipolar plates, not through the electrolyte.
Does the voltage of each cell simply add up?
- Yes.
In a series connection, the voltages add linearly:
If each cell operates at 0.7 V, then a 3-cell stack is:
In real systems you add losses (ohmic, overpotential, contact resistance), but the principle remains:
Total stack voltage ≈ (number of cells) × (single-cell voltage)
Does the current remain the same for all cells?
- Yes.
In a series circuit:
The current is identical through every cell, and it is determined by the cell with the weakest performance.
So:
- Voltage adds
- Current stays the same
Just like batteries connected in series.
Summary
In a PEM electrolyzer stack, electrons flow from the cathode of one cell to the anode of the next cell through the bipolar plates. The voltages of all cells add up, while the stack current is the same as the current in each individual cell.
