In a stark contrast to alkaline water electrolyzers (ALK WEs) proton exchange membrane water electrolyzers (PEMWEs) use a solid polymer electrolyte membrane (also known as ion exchange membrane) instead of a liquid electrolyte to split ultra-pure water into hydrogen and oxygen.

The limitations of ALK WEs have driven significant innovation in electrolyzer stack designs; most notably, the development of solid polymer electrolyte membranes, also known as ion exchange membranes. These proton-conducting membranes were originally developed in the 1960s for fuel cell applications (see: proton exchange membranes for fuel cells) and played a key role in early space exploration missions. Over the decades, their design and performance have been refined, making them suitable for electrolysis applications as well.

Advancements in Water Electrolyzer Technology: From Alkaline to PEM Systems

 

Innovations in water electrolyzer technology have emerged to address the limitations of the first-generation alkaline water electrolyzers. A major turning point came in the 1960s with the invention of proton exchange membranes (PEMs), which paved the way for PEM electrolyzers. These newer systems offered improved performance, higher current densities, and more compact designs.

How do PEM Water Electrolyzers Work?

Hydrogen and oxygen gases are produced on either sides of the membrane and are removed from the back of the electrodes through suitable cathode gas diffusion layers and anode porous transport layers. PEMWEs operate under highly acidic conditions (pH <2), where water splitting occurs through the following half-reactions:

• At the anode, water molecules are oxidized, producing oxygen gas, protons (H⁺), and electrons: H₂O (l) → 2 H⁺ (aq) + ½ O₂ (g) + 2 e^–
• At the cathode, protons (H⁺) are reduced, producing hydrogen gas: 2 H⁺ (aq) + 2 e^– → H₂ (g)

PEM Water Electrolyzer Components

PEM water electrolyzers consist of several key components that work together to enable efficient hydrogen production: a proton exchange membrane, catalysts, cathode gas diffusion layer (GDL), and anode porous transport layer (PTL). Each layer is engineered to support water management, gas diffusion, and electrochemical performance under highly acidic and pressurized operating conditions.

In addition to the membrane-electrode assembly (MEA), balance of stack components such as flat gaskets, bipolar plates, and cell frames ensure proper sealing, electrical conductivity, and structural support across the stack.

Proton Exchange Membranes

An ion exchange membrane (IEM) is a thin barrier that allows the selective passage of ions from one electrode to another of electrochemical devices. In PEM water electrolyzers, a proton exchange membrane (also called a proton-conducting membrane) enables the transport of H⁺ ions from the anode—where they are generated—to the cathode, where they are consumed.

More importantly, proton exchange membranes keep the anode and cathode physically separated, preventing the mixing of H₂ and O₂ product gases. This is a crucial safety feature—just 4.6% oxygen in a hydrogen stream or 3.8% hydrogen in an oxygen stream is enough to form an explosive mixture at 80 °C, which is close to the typical operating temperature of water electrolyzers.

Read more about proton exchange membranes for PEM water electrolyzers.

(Blog) Effect of PFAS-ban on Membranes for PEM Water Electrolyzers → (Blog) Choosing the Right Membranes for PEM Water Electrolyzers → (Blog) How Pretreatment Affects Ion Exchange Membranes →

Anode and Cathode Catalyst Layers (Catalyst + Ionomer)

Catalysts speed up reactions without being consumed in the process. In water electrolyzers, catalysts enable reactions that generate H₂ and O₂ gases to happen efficiently at practical input voltage.

In PEM water electrolyzers, the cathode catalyst speeds up the hydrogen evolution reaction (HER). On the other side, the anode catalyst drives the oxygen evolution reaction (OER). These catalysts are typically noble-metal-based due to the harsh, acidic environment of PEM systems. Platinum is the most commonly used catalyst at the cathode because of its excellent HER activity and durability. At the anode, iridium oxide or ruthenium oxide is used for OER.

Catalysts are usually in powder form.  How do we actually use them in electrochemical devices like PEM water electrolyzer stacks? It all starts with an ink.

This catalyst ink is a mixture of the powdered catalyst, a solvent (like water or alcohol), and an ionomer or a polymer that conducts protons and helps bind the catalyst particles to the membrane or gas diffusion layer. The ink is applied as a thin coating, typically through spraying, printing, or brushing, to create what’s called a catalyst layer. Once dried and properly processed, this layer becomes an integral part of the membrane electrode assembly (MEA), where the electrochemical reactions take place.

PEMION® PP1-HNN8-00 Ionomer is a high-performance proton exchange ionomer designed for use in PEM water electrolyzers and fuel cells. It offers excellent proton conductivity, chemical stability in acidic environments, and superior film-forming properties, making it ideal for catalyst ink formulations and membrane electrode assemblies (MEAs).