Where do the "impurities" in hydrogen come from? In-depth investigation: cross-permeation, corrosion, and system contamination

In PEM (Polymer Electrolysis) water electrolysis hydrogen production systems, the purity of hydrogen produced at the cathode is one of the core indicators for evaluating system performance and reliability. We aim for not just "H₂", but "high-purity H₂". However, in reality, the produced hydrogen always contains trace amounts of "impurities," such as oxygen (O₂), nitrogen (N₂), water vapor (H₂O), and metal cations . These impurities not only affect downstream applications of hydrogen (e.g., causing catalyst poisoning in fuel cells) but may also pose safety hazards. This article will delve deeper into the sources of hydrogen impurities, revealing three fundamental origins: inherent cross-permeation in electrochemistry, corrosion and precipitation from system materials, and contamination introduced by external media , and will explore their control logic.

Purity of Hydrogen

I. Inherent source of electrochemistry: cross-permeability and dissolution of membranes

This is the most "stubborn" and essential source of impurities in high-purity hydrogen, directly determined by the working principle of the PEM electrolyzer.

1. Oxygen cross-permeability: The primary threat to safety red lines

Generation mechanism: Oxygen (O₂) generated on the high-pressure side of the anode dissolves and diffuses through the proton exchange membrane (PEM) to the hydrogen side of the cathode, driven by the concentration and pressure differences. This process conforms to Fick's law, and its permeation rate is directly related to the membrane's permeability coefficient , thickness, and the partial pressure difference between the two sides .

Key impacts:

Safety hazard: When hydrogen and oxygen are mixed, the oxygen concentration exceeds 4%, which is when the mixture reaches the explosion limit.

Decreased purity: This directly results in hydrogen purity failing to reach high purity standards such as 99.99% or higher.

Chemical degradation catalysis: Oxygen permeating to the cathode can react with hydrogen at a low potential to generate hydrogen peroxide (H₂O₂) . The hydroxyl radicals (·OH) produced by its decomposition are the main culprits that attack the membrane and ionomers, leading to chemical degradation.

Key factor: Gas permeability is a core material characteristic of membranes . While thin films can reduce resistance, they typically increase gas permeability exponentially . Therefore, commercial systems must balance efficiency with safety/lifespan, often selecting membranes with a certain thickness or composite reinforcement structures.

2. Dissolution and permeation of nitrogen "impurities"

Generation mechanism: Before system startup or during maintenance, the piping and fuel cell stack are filled with air (approximately 78% N₂). Even during operation, if the system is not completely sealed or a nitrogen purging process is used, trace amounts of nitrogen will dissolve in the anode circulating water. Due to the solubility of nitrogen in the membrane and water, it will also slowly permeate to the cathode.

Key impact: Primarily leads to a decrease in hydrogen purity. While it typically doesn't participate in reactions, it accumulates on the high-pressure side of the system and needs to be removed through periodic venting ("venting"). Its concentration is an indirect indicator of system sealing and operational stability.

II. Intrinsic Contamination of the System: Material Corrosion and Interface Release

The PEM electrolytic cell is an extreme environment characterized by high temperature (80℃), strong acidity (anode pH≈2), and high potential (anode>1.5V), which poses a severe challenge to all contact materials.

1. Deposition and migration of metal ions

Main source:

Bipolar plates and flow field: Even titanium bipolar plates plated with precious metals (such as gold and platinum) or with corrosion-resistant coatings (such as TiN) may still slowly dissolve substrate metal ions (such as Ti⁴⁺ and Fe²⁺/³⁺) during long-term operation, especially where there are micro-defects (pinholes) in the coating.

Piping, valves, and fasteners: Any metal component in the system that comes into contact with acidic water, if made of an unsuitable material (such as 316L stainless steel, which will still corrode in an acidic, high-chlorine environment), can become a source of contamination.

Catalyst dissolution: Anode noble metal catalysts (such as Ir and Ru) exhibit extremely slow dissolution at high potentials, forming metal ions such as Ir³⁺.

Migration Pathways and Hazards: Once dissolved, these cations will flow with the circulating water. More seriously, they can undergo ion exchange with sulfonic acid groups (-SO₃H) in the proton exchange membrane , replacing H⁺. Polyvalent metal ions (such as Fe²⁺ and Al³⁺) can "lock" multiple sulfonic acid groups, permanently reducing the membrane's proton conductivity and leading to increased cell voltage. Some ions may migrate to the cathode, depositing on the Pt catalyst surface and poisoning its active sites.

2. Pollution from small-molecule organic compounds such as organosilicon.

Sources: Primarily from the degradation and leaching of sealing materials (such as silicone rubber gaskets and silicone sealants) in high-temperature acidic environments, or residues of certain lubricants and release agents.

Hazards: These small organic molecules can adsorb onto the catalyst surface, covering active sites and causing temporary or permanent poisoning of the catalyst, manifested as an increase in activation overpotential. They may also accumulate inside the membrane, affecting proton transport.

III. External Pollution: Water, Air, and Operational Introducement

This is the most controllable but also the easiest source of impurities to overlook.

1. The Challenge of Feed Water Purity: The quality of the feed water is the cornerstone determining the purity of hydrogen and the lifespan of the system. Even deionized water (DI water) or reverse osmosis (RO) water may contain:

Trace ions: Ca²⁺, Mg²⁺, Na⁺, Cl⁻, etc. As mentioned earlier, they can poison membranes and catalysts.

Dissolved gases: O₂, N₂, and CO₂ from the air dissolve in water and are carried into the system. CO₂ dissolves in water to form carbonate ions (CO₃²⁻), which may precipitate under the high pH environment of the cathode.

Non-metallic elements such as silicon and boron mainly come from water sources or resin beds, and their hazards are similar to those of metal ions.

Particulate matter and bacteria: can clog flow channels, create localized dead zones, and even form biofilms.

2. Introduction of System Assembly and Maintenance

Particles and fibers: Insufficient cleanliness of the assembly environment can introduce dust, fibers, etc., which may puncture the membrane or block the micropores.

Chemical residues: If solvents, alkaline solutions, etc., used to clean pipelines are not thoroughly rinsed off, they will directly contaminate the fuel cell stack.

IV. System-wide strategy for impurity control

Based on the above tracing, controlling hydrogen impurities requires a systematic engineering approach that spans the entire design, manufacturing, and operation cycle.

1. Materials and component levels:

Membrane: A reinforced composite membrane with low gas permeability and high selectivity is selected to balance efficiency and purity.

Bipolar plates: ensure the density, defect-free nature, and long-term stability of corrosion-resistant coatings (such as precious metal coatings and conductive ceramic coatings) .

Sealing and contact materials: Avoid using silicone rubber and choose high-performance materials such as perfluoroelastomer (FFKM).

Piping and valves: High-purity polymer piping (such as PFA) or fully passivated metal piping are used.

2. System design and operation levels:

Water treatment system: Equipped with a "deep purification" module, typically a combination of "RO/EDI + terminal mixed bed ion exchange ," ensuring influent resistivity ≥18.2 MΩ·cm . An online conductivity meter is installed for real-time monitoring.

Gas management: Optimize the cathode-side purging strategy to regularly remove accumulated N₂ and permeated O₂. Install hydrogen purification equipment (such as a catalytic deaerator, dryer, and precision filter) at the outlet.

Operation monitoring: Online monitoring of oxygen concentration in hydrogen (usually required to be <2 ppm), hydrogen purity , and fluoride ion concentration (membrane chemical degradation indicator) and metal ion concentration in system wastewater .

3. Assembly and maintenance procedures:

The fuel cell stack is assembled in a cleanroom environment.

Establish a rigorous system flushing and activation process to ensure that assembly residues are removed before startup.

Regular offline laboratory analysis of circulating water quality is conducted as a supplement to online monitoring.

Impurities in hydrogen are far more than simply "impure"; they profoundly reflect the complex physical, chemical, and material interactions within the PEM water electrolysis system. Like a mirror, they reveal the integrity of the membrane, the corrosion resistance of the materials, the purity of the water, and the comprehensiveness of the engineering design. From the physical nature of cross-osmosis to the material challenges of corrosion precipitation , and then to the engineering control of system contamination , each step demands meticulous attention from researchers and engineers. Only through this comprehensive, multi-dimensional, and in-depth source tracing and systemic control can we ensure the "truly high purity" of the produced hydrogen from the source, laying a solid and reliable foundation for the large-scale and high-end application of hydrogen energy.