PEM Electrolyzer: Complete Guide to PEM Water Electrolysis and Hydrogen Production

Introduction

As the demand for low-carbon hydrogen continues to grow, PEM electrolyzers are becoming an increasingly important technology for hydrogen production.

A proton exchange membrane (PEM) electrolyzer uses electricity to split purified water into hydrogen and oxygen. Unlike alkaline electrolysis, PEM electrolysis uses a solid polymer electrolyte membrane rather than a liquid alkaline electrolyte. This enables a compact cell architecture, high current density, rapid response to changing electrical loads, and high-purity hydrogen production.

A complete PEM electrolysis system is more than an electrolyzer stack. It combines the PEM membrane, catalyst-coated membrane (CCM), porous transport layers (PTL/GDL), bipolar plates, gaskets, end plates, water circulation, cooling, gas-liquid separation, power electronics and control systems.

For laboratory equipment, gas calibration, fuel-cell applications and hydrogen-related equipment, a single PEM electrolyzer cell may be sufficient. For industrial hydrogen production, multiple cells are connected into a stack and integrated with a balance-of-plant (BOP) system.

This guide explains how a PEM electrolyzer works, its main components, important performance parameters, applications, advantages and limitations, and what buyers should consider when selecting a PEM electrolyzer manufacturer.

1. What Is a PEM Electrolyzer?

A PEM electrolyzer, or proton exchange membrane electrolyzer, is an electrochemical device that converts electrical energy and water into hydrogen and oxygen.

The overall reaction is:

2H₂O → 2H₂ + O₂

During electrolysis, water is supplied to the anode side of the cell. At the anode, water is oxidized to oxygen, protons and electrons. The protons pass through the proton exchange membrane, while electrons travel through the external electrical circuit.

At the cathode, the protons combine with electrons to form hydrogen.

The basic half-reactions are:

Anode:

H₂O → ½O₂ + 2H⁺ + 2e⁻

Cathode:

2H⁺ + 2e⁻ → H₂

This electrochemical architecture allows the membrane to conduct protons while separating hydrogen and oxygen.

2. How Does a PEM Electrolyzer Work?

The operating principle can be understood in five basic steps.

Step 1: Purified water enters the anode

High-quality water is supplied to the anode side of the electrolyzer.

The water must be sufficiently pure because ionic contaminants can affect membrane performance, catalyst stability and long-term operation.

Step 2: Water is oxidized at the anode

When electrical energy is applied, the oxygen evolution reaction (OER) occurs at the anode catalyst layer.

Water is separated into:

• Oxygen

• Protons

• Electrons

Step 3: Protons travel through the PEM

The proton exchange membrane allows protons to move from the anode to the cathode.

At the same time, the membrane acts as an electrical insulator and helps prevent direct mixing of hydrogen and oxygen.

Step 4: Electrons travel through the external circuit

Electrons cannot pass directly through the membrane.

Instead, they travel through the external electrical circuit from the anode to the cathode.

Step 5: Hydrogen is generated at the cathode

At the cathode, hydrogen evolution occurs:

2H⁺ + 2e⁻ → H₂

The generated hydrogen is then collected and sent to downstream equipment for drying, purification, compression, storage or direct use.

The membrane, catalyst layers, PTLs and bipolar plates therefore work together as one integrated electrochemical system. 

3. Main Components of a PEM Electrolyzer

A PEM electrolyzer cell consists of several highly engineered components.

These components are not independent. Their interaction strongly affects voltage, efficiency, gas purity, durability and stack lifetime. 

4. Proton Exchange Membrane

The PEM is one of the most important components of the electrolyzer.

The membrane performs two critical functions:

1. Conducting protons from the anode to the cathode

2. Separating the hydrogen and oxygen reaction environments
   
   
   

PEM electrolyzers commonly use perfluorosulfonic acid (PFSA) membranes. Nafion is one well-known example.

An ideal membrane should provide:

• High proton conductivity

• Low gas crossover

• Good chemical stability

• Good mechanical strength

• Stable performance under pressure

• Compatibility with catalyst layers and PTLs
  
  
  

Membrane selection also influences hydrogen purity, efficiency and durability. 

For this reason, simply choosing a membrane based on thickness or price is not enough. The membrane needs to be evaluated together with the catalyst layer, PTL, operating pressure, current density and cell compression.

5. Catalyst-Coated Membrane (CCM)

The catalyst-coated membrane, or CCM, is at the heart of a PEM electrolyzer.

A CCM consists essentially of:

PEM + Anode Catalyst Layer + Cathode Catalyst Layer

The catalyst layers accelerate the electrochemical reactions.

Anode catalyst

The anode operates under highly oxidizing conditions. Iridium-based catalysts, including iridium oxide, are widely used because of their activity and stability under PEM electrolysis conditions.

Cathode catalyst

Platinum-based catalysts are commonly used for the hydrogen evolution reaction.

The challenge for PEM electrolyzer manufacturers is to achieve a combination of:

• High catalytic activity

• Low precious-metal loading

• Uniform catalyst distribution

• Strong catalyst/membrane interface

• Good durability

• Consistent large-area coating
  
  

Research and industrial development increasingly focus on reducing platinum-group metal loading while maintaining performance and durability. The U.S. Department of Energy, for example, has established targets for reducing platinum-group-metal content while increasing current density and improving efficiency. 

6. Porous Transport Layer: PTL and GDL

The porous transport layer (PTL) is another critical part of the PEM electrolyzer.

PTLs perform several functions simultaneously:

• Transport water toward the catalyst layer

• Remove generated gases

• Conduct electrical current

• Facilitate heat transfer

• Provide mechanical support

• Help maintain a controlled interface between the CCM and bipolar plate
  
  

Titanium-based PTLs are commonly used on the anode side because of the highly oxidizing environment.

Depending on the cell design, the cathode side may use a carbon-based GDL or a titanium-based structure.

PTL properties such as:

• Porosity

• Thickness

• Pore-size distribution

• Surface roughness

• Electrical resistance

• Compression behavior

• Surface coating
  
  

can significantly influence electrolyzer performance. 

7. Bipolar Plates and Flow Fields

The bipolar plate, sometimes called a flow plate or separator plate, has several important responsibilities.

It must:

• Distribute water

• Remove hydrogen and oxygen

• Conduct electrical current

• Transfer heat

• Separate the gas chambers

• Provide mechanical support

The flow-field design is particularly important because water and gases need to be distributed uniformly across the active area.

Poor flow distribution can result in:

• Local dry-out

• Gas accumulation

• Uneven current distribution

• Local hot spots

• Increased cell voltage

• Accelerated degradation

Titanium and coated metallic materials are commonly considered for PEM electrolyzer bipolar plates because the anode environment is strongly oxidizing and acidic. 

8. From a PEM Cell to a PEM Electrolyzer Stack

A single PEM electrolyzer cell produces a relatively limited amount of hydrogen.

To increase production capacity, multiple cells are connected electrically in series to form a PEM electrolyzer stack.

A simplified structure is:

End Plate → Bipolar Plate → PTL → CCM → PTL → Bipolar Plate → Cell → Cell → Cell → End Plate

The number of cells, active area and operating current determine the overall stack output.

For example, increasing the active area can increase hydrogen production at a given current density. Increasing the number of cells can also increase total hydrogen production while allowing the stack architecture to remain modular.

This makes stack engineering one of the most important steps between laboratory PEM cells and industrial hydrogen production systems.

9. Key PEM Electrolyzer Performance Parameters

When comparing PEM electrolyzers, buyers should look beyond hydrogen production rate.

The following parameters should be evaluated together.

The DOE's PEM electrolysis targets illustrate why performance cannot be reduced to a single number: current density, voltage, efficiency, degradation, lifetime and capital cost all need to be considered together.