Industrial-Scale PEM Electrolysers: Principles, System Architecture, and Applications

Introduction

As the global energy transition accelerates, green hydrogen has emerged as a pivotal energy carrier for decarbonising sectors that are inherently difficult to electrify. Among the principal water electrolysis technologies, the Proton Exchange Membrane (PEM) electrolyser has gained considerable traction for industrial-scale deployment owing to its rapid dynamic response, compact footprint, and capacity to produce hydrogen at elevated purity. Under the European Union's regulatory framework — specifically the Delegated Acts supplementing Directive (EU) 2018/2001 (the Renewable Energy Directive, or RED II) — green hydrogen must be produced exclusively from renewable electricity sources, satisfying stringent additionality, temporal correlation, and geographical correlation criteria. Industrial PEM electrolysis is well positioned to meet these requirements.

Working Principles

A PEM electrolyser operates on the principle of electrochemical water splitting, utilising a solid polymer electrolyte — typically a perfluorosulphonic acid (PFSA) membrane such as Nafion™ — to conduct protons whilst simultaneously serving as a gas separator between the anodic and cathodic half-cells.

At the anode, deionised water is oxidised in the oxygen evolution reaction (OER):

$2H_{2}O\to O_2+4H^{+}+4e^{-}$

2H₂O → O₂+4H⁺+4e^-

The protons thus generated migrate through the proton exchange membrane under the influence of the applied electric field. Simultaneously, the electrons travel through the external circuit to the cathode. At the cathode, the hydrogen evolution reaction (HER) takes place:

$4H^{+}+4e^{-}\to 2H_2$

4H⁺+4e^- → 2H₂

The overall cell reaction is:

$2H_{2}O\to 2H_2+O_2$

2H₂O → 2H₂+O₂

The thermodynamic minimum voltage required for water splitting at standard conditions is 1.23 V (the reversible cell voltage). In practice, industrial PEM cells operate at voltages between 1.8 V and 2.2 V, with the overpotential attributable to activation losses at the electrode catalysts, ohmic resistance across the membrane and interfacial contacts, and mass transport limitations. The anode catalyst is conventionally iridium oxide (IrO₂) or iridium-ruthenium mixed oxides, necessitated by the highly acidic and oxidising anodic environment. The cathode catalyst is typically platinum supported on carbon (Pt/C). Current densities in industrial PEM electrolysers commonly range from 1.0 to 3.0 A/cm², substantially higher than those achievable in alkaline electrolysers.

Complete System Process Flow

The industrial PEM electrolysis system comprises an integrated process chain, described sequentially below:

Stage 1: Raw Water Intake and Pre-treatment

Raw water — sourced from municipal supplies, surface water, or industrial sources — first undergoes pre-treatment to remove suspended solids, colloids, and organic matter. This stage typically comprises multimedia filtration, activated carbon adsorption, and microfiltration or ultrafiltration membranes.

Stage 2: Primary Purification via Reverse Osmosis

The pre-treated water passes through a reverse osmosis (RO) system operating at pressures of 10–15 bar. The RO membranes reject dissolved salts, silica, and residual organic compounds, yielding permeate with conductivity below 10 µS/cm. A second-pass RO stage may be incorporated for enhanced purity.

Stage 3: Electrodeionisation and Polishing

Following reverse osmosis, the water enters an electrodeionisation (EDI) module, which employs ion-exchange resins regenerated continuously by an applied electric field. The EDI effluent is subsequently polished through mixed-bed ion exchange columns, producing ultrapure deionised water with resistivity exceeding 18.2 MΩ·cm and total organic carbon (TOC) below 5 ppb — essential for preventing membrane degradation and catalyst poisoning.

Stage 4: Deionised Water Storage and Conditioning

The ultrapure water is stored in nitrogen-blanketed polypropylene tanks to prevent recontamination. Prior to entering the stack, the water is degassed to remove dissolved oxygen and nitrogen, then heated to the optimal operating temperature of 50–80 °C via plate heat exchangers.

Stage 5: Electrochemical Reaction in the Stack

The conditioned deionised water is pumped to the anode compartments of the electrolyser stack. Upon application of direct current from the rectifier system, water molecules undergo oxidation at the anode, releasing oxygen gas, protons, and electrons. Protons migrate through the membrane to the cathode, where they combine with electrons to form hydrogen gas. The bipolar plate configuration ensures uniform current distribution and efficient gas-liquid separation.

Stage 6: Gas-Liquid Separation

The two-phase mixtures exiting the anode and cathode compartments enter dedicated gas-liquid separators. Unreacted water is recirculated to the feed system, whilst the separated oxygen and hydrogen gases proceed to their respective processing trains.

Stage 7: Hydrogen Purification and Drying

The raw hydrogen stream, saturated with water vapour, passes through a coalescing separator followed by a temperature swing adsorption (TSA) or pressure swing adsorption (PSA) dryer. This reduces the moisture content to below 5 ppm. Catalytic deoxidisers remove trace oxygen to achieve hydrogen purity exceeding 99.999% (5.0 grade), compliant with ISO 14687 fuel cell grade specifications.

Stage 8: Compression, Storage, and Dispatch

The purified hydrogen is compressed via multi-stage diaphragm or ionic compressors to pressures of 30–50 bar for pipeline injection or 350–700 bar for mobile storage applications. The hydrogen is then stored in composite pressure vessels or tube trailers pending dispatch.

Stage 9: Control, Monitoring, and Safety Systems

A distributed control system (DCS) or programmable logic controller (PLC) orchestrates all subsystems, monitoring parameters including stack voltage, current density, temperatures, pressures, water quality, and gas compositions. Safety instrumentation encompasses hydrogen leak detection, emergency shutdown sequences, and ventilation interlocks in accordance with ATEX directives.

Applications

Industrial PEM electrolysers are deployed across diverse applications aligned with EU green hydrogen objectives:

• Industrial Feedstock Decarbonisation: Substituting grey hydrogen in ammonia synthesis, petroleum refining, and methanol production.

• Long-Duration Energy Storage: Converting surplus renewable electricity into hydrogen for subsequent reconversion via fuel cells or gas turbines.

• Sustainable Mobility: Supplying green hydrogen to fuel cell electric vehicles, particularly heavy-duty transport and maritime applications.

• Power-to-X Pathways: Synthesising e-fuels and green chemicals through catalytic hydrogenation of captured carbon dioxide.

Conclusion

Industrial PEM electrolysis constitutes a technologically mature pathway for producing green hydrogen compliant with EU regulatory standards. The complete system — from raw water purification through electrochemical conversion to hydrogen dispatch — exemplifies the integration of chemical engineering, electrochemistry, and process control essential for the hydrogen economy.