Water purity is a critical factor in electrolysis, as impurities can degrade performance, reduce efficiency, and damage equipment. Different electrolysis technologies—alkaline, proton exchange membrane (PEM), and solid oxide electrolysis cells (SOEC)—have distinct purity requirements and pretreatment methods to ensure optimal operation.
### Water Purity Standards for Electrolysis Technologies
#### Alkaline Electrolysis
Alkaline electrolyzers are less sensitive to water impurities compared to PEM or SOEC systems due to their robust construction and liquid electrolyte (typically potassium hydroxide, KOH). However, high purity is still necessary to prevent scaling, corrosion, and electrode poisoning.
Key water quality parameters for alkaline electrolysis:
- Conductivity: < 10 µS/cm
- Total dissolved solids (TDS): < 1 ppm
- Chlorides (Cl⁻): < 0.1 ppm
- Sulfates (SO₄²⁻): < 0.1 ppm
- Heavy metals (Fe, Cu, Ni): < 0.05 ppm
- Silica (SiO₂): < 0.1 ppm
Alkaline systems can tolerate slightly higher impurity levels because the electrolyte neutralizes some contaminants. However, excessive impurities lead to precipitation on electrodes, reducing efficiency over time.
#### Proton Exchange Membrane (PEM) Electrolysis
PEM electrolyzers require ultra-high-purity water due to their sensitive membrane electrode assembly (MEA). Contaminants can irreversibly degrade the Nafion membrane and catalyst layers.
Key water quality parameters for PEM electrolysis:
- Conductivity: < 1 µS/cm
- TDS: < 0.1 ppm
- Chlorides (Cl⁻): < 0.01 ppm
- Sulfates (SO₄²⁻): < 0.01 ppm
- Heavy metals (Fe, Cu, Ni): < 0.001 ppm
- Silica (SiO₂): < 0.01 ppm
- Organic compounds: Undetectable
PEM systems are highly sensitive to chloride ions, which accelerate membrane degradation. Even trace amounts of iron or copper can poison platinum catalysts, reducing hydrogen output.
#### Solid Oxide Electrolysis Cells (SOEC)
SOEC operates at high temperatures (700–900°C), reducing sensitivity to some impurities but requiring strict control of others. Steam is the feedstock rather than liquid water, but feedwater must still meet stringent standards to avoid fouling or electrode degradation.
Key water quality parameters for SOEC:
- Conductivity: < 5 µS/cm
- TDS: < 0.5 ppm
- Chlorides (Cl⁻): < 0.05 ppm
- Sulfates (SO₄²⁻): < 0.05 ppm
- Heavy metals (Fe, Ni, Cr): < 0.01 ppm
- Silica (SiO₂): < 0.05 ppm
High temperatures can volatilize some impurities, but silica and heavy metals deposit on electrodes, reducing performance.
### Pretreatment Methods for Electrolysis Feedwater
To meet these purity standards, feedwater undergoes multiple pretreatment stages. The exact sequence depends on the source water quality and electrolyzer type.
#### Deionization (DI)
Deionization removes ions via ion exchange resins, producing high-purity water. Two primary methods are used:
1. **Mixed-Bed Deionization**
- Combines cation and anion exchange resins in a single vessel.
- Produces water with conductivity < 1 µS/cm.
- Suitable for PEM and SOEC systems.
2. **Two-Bed Deionization**
- Separate cation and anion exchange columns.
- Less efficient than mixed-bed but easier to regenerate.
- Typically achieves conductivity of 1–10 µS/cm, suitable for alkaline systems.
Limitations:
- Does not remove non-ionic contaminants (e.g., organics, silica).
- Resins require periodic regeneration with acids and bases.
#### Reverse Osmosis (RO)
RO uses semipermeable membranes to remove up to 99% of dissolved salts and organics. It is often the first pretreatment step.
Performance metrics:
- Rejection rates: 95–99% for most ions.
- Produces water with TDS < 10 ppm.
- Often paired with DI for final polishing.
Advantages:
- Effective for bulk impurity removal.
- Low energy compared to distillation.
Disadvantages:
- Membrane fouling from silica or organics.
- Does not achieve ultra-high purity alone.
#### Electrodeionization (EDI)
EDI combines ion exchange resins with electrically driven ion migration, continuously regenerating resins without chemicals.
Performance:
- Produces water with conductivity < 0.1 µS/cm.
- Used in PEM systems after RO pretreatment.
Advantages:
- No chemical regeneration needed.
- Consistent high-purity output.
Disadvantages:
- High capital cost.
- Sensitive to feedwater fluctuations.
#### Additional Pretreatment Steps
1. **Filtration**
- Multimedia filters remove suspended solids.
- Activated carbon filters adsorb organics and chlorine.
2. **Ultraviolet (UV) Oxidation**
- Destroys bacteria and organic compounds.
- Prevents biofouling in storage tanks.
3. **Degasification**
- Removes dissolved oxygen and CO₂, which can corrode components.
### System-Specific Pretreatment Configurations
#### Alkaline Electrolysis Pretreatment
Typical sequence:
1. Multimedia filtration → Activated carbon → RO → Mixed-bed DI.
2. Conductivity monitored to ensure < 10 µS/cm.
#### PEM Electrolysis Pretreatment
Stricter requirements demand:
1. Multimedia filtration → Activated carbon → RO → EDI or mixed-bed DI → UV oxidation.
2. Final conductivity < 1 µS/cm, with undetectable organics.
#### SOEC Pretreatment
Since SOEC uses steam, feedwater is pretreated similarly to PEM but with less stringent ion limits:
1. RO → Mixed-bed DI → Degasification.
2. Silica removal is critical to prevent deposition at high temperatures.
### Operational Considerations
- **Real-Time Monitoring**
- Conductivity sensors ensure consistent purity.
- Ion chromatography detects trace contaminants.
- **Maintenance**
- Regular resin replacement in DI systems.
- RO membrane cleaning to prevent fouling.
- **Cost Trade-offs**
- PEM systems require the most extensive pretreatment, increasing capital and operational costs.
- Alkaline systems offer lower purity tolerances, reducing pretreatment expenses.
### Conclusion
Water purity standards and pretreatment methods vary significantly across electrolysis technologies. Alkaline systems tolerate moderate impurity levels, while PEM and SOEC demand ultra-high purity with multi-stage pretreatment. Deionization, reverse osmosis, and advanced methods like EDI are critical to meeting these requirements. Proper pretreatment ensures efficient, long-lasting electrolyzer operation, minimizing downtime and maintenance costs.