Electrolyzers are critical components in hydrogen production, and their long-term performance depends heavily on the durability of materials such as bipolar plates, membranes, and other structural elements. Degradation mechanisms vary across alkaline, proton exchange membrane (PEM), and solid oxide electrolyzer cell (SOEC) systems due to differences in operating conditions, material compositions, and electrochemical environments. Understanding these mechanisms is essential for improving system longevity and efficiency.
Corrosion is a primary degradation pathway in electrolyzer materials. In alkaline electrolyzers, the highly caustic environment accelerates the corrosion of metallic components, particularly bipolar plates and electrodes. Nickel and nickel-plated steel are commonly used due to their resistance to alkaline corrosion, but prolonged exposure can still lead to surface oxidation and thinning. The formation of passive oxide layers may initially protect the material, but localized pitting corrosion can occur due to inhomogeneities in the electrolyte or impurities.
PEM electrolyzers operate under acidic conditions, requiring materials that resist corrosion while maintaining electrical conductivity. Titanium is often used for bipolar plates due to its stability, but it can still suffer from passivation, leading to increased interfacial resistance. Thin coatings of gold or platinum are sometimes applied to mitigate this, but cost remains a challenge. The membrane itself, typically a perfluorosulfonic acid polymer like Nafion, degrades through chemical attack by reactive oxygen species generated during operation. This results in membrane thinning, loss of mechanical integrity, and eventual failure.
SOEC systems face high-temperature corrosion, where materials must withstand temperatures exceeding 700°C. Interconnects and bipolar plates are often made from chromium-based alloys or ceramic materials to resist oxidation. However, chromium evaporation can poison electrodes, while thermal cycling induces mechanical stresses that lead to cracking or delamination. The oxygen-ion-conducting electrolyte, usually yttria-stabilized zirconia, is relatively stable but can degrade if exposed to reducing atmospheres or contaminants.
Fouling is another significant issue, particularly in alkaline and PEM systems. In alkaline electrolyzers, impurities in the water feed, such as metal ions or silica, can deposit on electrode surfaces, reducing active sites and increasing overpotentials. Gas bubbles trapped at the electrode surface can also hinder ion transport, leading to uneven current distribution and localized heating. PEM systems are sensitive to cationic contaminants, which displace protons in the membrane, reducing conductivity. Even trace amounts of iron or copper from upstream components can accelerate membrane degradation.
SOEC systems are less prone to traditional fouling but face challenges with electrode poisoning. Sulfur and carbon-containing species in the feed gas can adsorb onto nickel-based electrodes, blocking active sites and reducing efficiency. Additionally, dust or particulate matter in the air stream can deposit on porous electrodes, impeding gas diffusion.
Performance loss over time is inevitable due to cumulative degradation. In alkaline electrolyzers, gradual corrosion of bipolar plates increases electrical resistance, while membrane degradation in PEM systems leads to higher gas crossover and reduced efficiency. SOEC systems experience electrode delamination and electrolyte cracking, which degrade ionic conductivity. Each system exhibits distinct failure modes:
- Alkaline: Gradual loss of electrode activity due to corrosion and fouling.
- PEM: Membrane thinning and catalyst layer detachment.
- SOEC: Interconnect oxidation and thermal stress-induced fractures.
Mitigation strategies differ by system. Alkaline electrolyzers benefit from regular electrolyte purification to remove contaminants and periodic replacement of nickel components. PEM systems require ultra-pure water feeds and protective coatings on bipolar plates to slow corrosion. SOEC systems rely on material improvements to reduce chromium evaporation and enhance thermal cycling tolerance.
Material selection plays a crucial role in extending electrolyzer life. For bipolar plates, alternatives like stainless steel with protective coatings are being explored for alkaline systems, while composite materials may offer better durability in PEM environments. SOEC researchers are investigating perovskite-based ceramics for improved stability at high temperatures.
Operational conditions also influence degradation rates. Higher current densities accelerate corrosion and mechanical wear in all systems. Temperature fluctuations in SOECs exacerbate thermal stresses, while voltage reversals in PEM systems can cause irreversible membrane damage. Monitoring and control systems are essential to detect early signs of degradation, such as increased cell voltage or decreased gas purity.
In summary, electrolyzer material degradation is a complex interplay of corrosion, fouling, and mechanical failure, with distinct challenges for alkaline, PEM, and SOEC technologies. Addressing these issues requires advances in material science, improved system design, and careful operational management to ensure long-term reliability and efficiency.