The scope of electrolyzer maintenance is determined by two dimensions. The first dimension is the depth of maintenance, ranging from least to most extensive: Electrolyzer Overhaul, Electrolyzer Major Overhaul, Electrolyzer Strip-down, and finally Electrolyzer Rebuild. The second dimension is the level of replacement, ranging from the entire unit to the stack to the individual cells, and is categorized as Electrolyzer Replacement, Stack Replacement, and Cell/MEA Replacement. The combination of these two dimensions determines downtime, the spare parts list, technical risks, and the final condition of the asset. Disputes often arise during the bidding phase for many projects, and the root cause is that these two dimensions were not clearly defined in the contract; the supplier's understanding of "overhaul" and the owner's understanding of "overhaul" differ completely in terms of the scope of disassembly, the proportion of components to be replaced, and performance acceptance criteria. A quote for an "Electrolyzer Overhaul" may include only the replacement of seals, whereas a quote for an "Electrolyzer Major Overhaul" may involve the replacement of most membrane electrode assemblies (MEAs), resulting in significant differences in both cost and downtime.
An electrolyzer overhaul is a comprehensive maintenance procedure performed during a planned outage, with the goal of restoring the equipment to operational condition. Typical trigger conditions for alkaline electrolyzers include: a DC power consumption increase of more than 5% above the design value; cell voltage distribution dispersion exceeding ±3%; nickel plating peeling covering more than 20% of the plate surface; or cumulative operating time reaching 30,000 hours. Industrial data indicates that the typical service life of an alkaline electrolyzer stack is approximately 90,000 hours (about 10 years), while auxiliary equipment is typically designed to last 20 to 30 years. This means that, over the system’s lifecycle, the stack will require at least one major overhaul. The standard procedure for an “Electrolyzer Overhaul” follows the principles of reverse disassembly and orderly reassembly, beginning with power disconnection and cell cleaning, followed by nitrogen purging, disassembly and labeling, cleaning and inspection, repair and replacement, and finally reassembly and tightening. Each time a set of electrodes or frames is removed, it must be immediately numbered and photographed to document the distribution of corrosion and deposits. Suspicious corrosion products must be sampled and sent for scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis to determine the source of impurities.
The scope of an Electrolyzer Major Overhaul is broader than that of a standard overhaul. When an inspection reveals that more than approximately one-third of the electrolyzer cells exhibit uniform corrosion, or when electrode coating loss exceeds 20% of the original thickness, it becomes economically viable to undertake a more extensive intervention. An Electrolyzer Major Overhaul typically includes complete disassembly of the stack, replacement of most membrane electrode assemblies (MEAs), inspection and repair of bipolar plates, and, potentially, replacement of end plates and compression hardware. A research team at the German Aerospace Center (DLR) conducted a cell-by-cell component replacement analysis on a commercial PEM electrolyzer stack and found that the catalyst-coated membranes contributed most significantly to performance degradation due to contamination and polymer rearrangement. Traces of calcium were also detected, likely resulting from inadequate water purification during operation. This finding directly impacts the spare parts strategy for an Electrolyzer Major Overhaul: if the water treatment system is not upgraded simultaneously, the replaced membrane electrode assemblies may face the same contamination pathways.
An "Electrolyzer Strip-down" is a diagnostic disassembly, not a repair in itself. The stack is opened, each cell is inspected, gasket compression is measured, membrane condition is recorded, electrode coating thickness is checked, and bipolar plate flatness is verified. The standard operating procedure published in *Frontiers in Energy Research* emphasizes conducting a preliminary visual inspection of battery components prior to any cleaning or treatment, followed by component-specific advanced inspections when initial findings raise concerns—including the identification of signs of degradation such as color changes, differences in texture, electrode delamination, holes, or cracks. The output of an "Electrolyzer Strip-down" is not a repaired stack, but rather a component-by-component status report that distinguishes reusable parts from those requiring replacement or repair. Without this report, overhaul contractors are working blindly, and owners are paying for parts that may not need to be replaced, while also risking component failure due to improper assessment. In commercial contracts, the Electrolyzer Strip-down is typically the first phase of any major maintenance activity and serves as the diagnostic foundation for all subsequent capital decisions.
An electrolyzer rebuild goes a step further than a major overhaul, with the goal of restoring performance to near-original specifications. During an electrolyzer rebuild, the stack is completely disassembled; all membranes and electrodes are replaced; bipolar plates are either replaced or recoated; the porous transport layer is renewed; and the stack is reassembled using new compression hardware and seals. INEOS Electrochemical Solutions has over 35 years of experience in electrode coating development, and its refurbishment services cover module repairs and recoating for a variety of electrolyzer designs, demonstrating that Electrolyzer Rebuild has already established a mature commercial service chain in the industry. Denora also provides maintenance and refurbishment services for a variety of electrolyzer technologies, including electrode reactivation. The core principle of Electrolyzer Rebuild is to precisely identify the source of degradation and then replace only the affected components, rather than blindly replacing the entire unit. If degradation is confined solely to the anode catalyst layer, while the cathode and bipolar plates remain in acceptable condition, a complete replacement strategy would result in unnecessary material waste and increased costs.
Stack Replacement falls between a major overhaul and a complete replacement. The stack is removed and a new or rebuilt stack is installed, but the system infrastructure remains in use. This is the preferred approach when the stack has reached the end of its service life but the auxiliary equipment is still within its design life. The alkaline electrolyzer stack replacement project at BTU Cottbus provides a real-world example: the old stacks were dismantled step by step, with each connection documented; technical drawings were shared with the replacement stack supplier to ensure the new stacks could be installed on the existing pressure vessels without modification. The most common cause of failure in stack replacement projects is not defects in the new stacks, but rather incomplete interface documentation for the old stacks. Before the old stacks are removed from the site, photographs must be taken of each connection, each flange face must be measured, and the dimensions of each gasket must be recorded. If the original stack model is no longer in production and no equivalent replacement is available, the new design may require modification and integration work, which significantly increases project complexity and duration. Force Technology’s failure analysis of the Eurowind project also indicates that the complexity of replacement decisions rises sharply when equivalent replacements are unavailable.
Electrolyzer Replacement refers to the decision to decommission existing units and install new ones; it is not an overhaul, a rebuild, or a stack replacement. Triggering factors include a repair-to-replacement ratio exceeding 50% of the replacement value, unavailability of spare parts for obsolete stack architectures, or technological advancements that render continued maintenance uneconomical. When an Electrolyzer Replacement is performed, the removed unit becomes a recycled asset. The decommissioning process involves strict procedural requirements. Prior to physical dismantling, all electrical, gas, and cooling connections must undergo lockout-tagout procedures, and the unit must be purged and isolated with blind flanges. H2Tools' best practices recommend that maintenance personnel verify the stack is discharged by measuring the stack voltage before removing safety guards and electrical connections. Decommissioning documentation for an electrolyzer replacement must include updates to the asset registry, closure of maintenance records, and a spare parts inventory reconciliation. Failing to properly close these records is why "ghost assets" continue to appear in maintenance plans years after physical dismantling.
The bipolar plates and porous transport layers in PEM electrolyzer stacks contain strategically important raw materials, primarily titanium and platinum group metals. The ReCycle project at the University of Leoben has established a step-by-step recovery strategy that begins with manual disassembly to preserve component integrity and minimize contamination, followed by the application of hydrometallurgical leaching technology to selectively extract titanium and platinum group metals. Manual disassembly has been successfully completed, achieving a clean separation of the bipolar plates and porous transport layers. The commercial reality is that manual disassembly is labor-intensive, limiting scalability, and the chemical leaching process has not yet been optimized for industrial throughput. For operators planning an electrolyzer overhaul or rebuild, the practical approach is to sort titanium components by type and entrust them to specialized titanium recyclers with a track record of handling coated materials. The average recovery rate for titanium recycling processing is approximately 40%, with embedded carbon emissions of about 12,500 kilograms of CO₂ equivalent per electrolyzer stack. Titanium that has come into contact with iridium or ruthenium oxide coatings requires coating removal before entering the recycling stream. Sulfuric acid electrolysis has been shown to effectively strip ruthenium-containing coatings from titanium anodes, resulting in less corrosion of the titanium substrate and higher precious metal recovery rates. The CircuPEM project at Fraunhofer UMSICHT is exploring a complete circular pathway for recovering titanium, iridium, and platinum from membrane electrode assemblies, with the goal of achieving recovery rates approaching 100 percent.
Iridium is a bottleneck constraining the economic viability of PEM electrolyzers. Iridium is not mined as a primary material but is recovered solely as a byproduct of platinum mining. For operators planning an Electrolyzer Major Overhaul or Electrolyzer Replacement, the iridium content in the catalyst-coated membranes represents a recoverable asset that should be valued before the old stack is scrapped. The EU-funded IRION project is developing processes to recover iridium and depolymerized materials from end-of-life catalyst-coated membranes. These processes utilize ionic liquids, eutectic solvents, and other green solvents, along with electrodeposition methods, to avoid the harmful emissions associated with traditional calcination methods. The recovered iridium and depolymerized materials are purified and used to manufacture new catalyst-coated membranes. Johnson & Marsie’s patented method describes a process involving oxidative acid leaching to recover platinum, palladium, and/or ruthenium, followed by reductive acid leaching to recover iridium, with platinum group metal recovery rates reaching 90% to 95%. Hydrometallurgical recovery of iridium from electrolyzer waste faces challenges posed by extremely low concentrations, stable oxide forms, and the demand for high-purity recovery; however, optimized iridium-targeted hydrometallurgy can achieve recovery rates exceeding 90%. Membranes account for 20% to 30% of the stack cost; therefore, retaining the membranes during the catalyst recovery process has direct economic value.
The sequence for minimizing total cost of ownership follows a clear ladder. It begins with a condition assessment. If degradation is localized, perform a Planned Electrolyzer Overhaul, involving targeted component replacement. If degradation is widespread but the stack architecture remains intact, perform an Electrolyzer Rebuild, involving full membrane and electrode replacement. If the stack itself has reached the end of its service life but the system infrastructure remains viable, perform a Stack Replacement. If the entire unit is beyond the scope of cost-effective repair, perform an Electrolyzer Replacement and recover the value of titanium and precious metals from the decommissioned unit. An Electrolyzer Strip-down precedes every step above the overhaul level mentioned above and serves as the diagnostic foundation for each subsequent capital decision. An Electrolyzer Major Overhaul is only justified once the condition report generated by the Electrolyzer Strip-down confirms the scope of replacement.
Before requesting a quote, the following must be clearly specified: whether the scope is the electrolyzer, stack, cell, or membrane electrode assembly; whether the scope is an Electrolyzer Overhaul, Electrolyzer Major Overhaul, Electrolyzer Strip-down, Electrolyzer Rebuild, or Electrolyzer Replacement; and which components are to be replaced, including membranes, electrodes, bipolar plates, seals, end plates, and fasteners; Acceptance criteria include leak rate, hydrogen production, energy consumption, purity, and current density; for the recovery of used parts, clearly specify the testing methods, settlement formulas, and ownership for titanium and precious metal recovery; commercial terms should include asset numbers, warranty, ownership of used parts, and downtime windows.
An "Electrolyzer Overhaul" restores functionality. An "Electrolyzer Major Overhaul" replaces most active components. Electrolyzer Strip-down generates accurate condition data across the entire scope. Electrolyzer Rebuild restores the stack to near-new performance. Stack Replacement retains the system while updating its core components. Electrolyzer Replacement decommissions the asset and initiates the titanium and precious metal recovery process. Used parts are not waste; titanium components must be sorted for recycling, precious metals must be tested and refined, and precious metal recycling proceeds must be specified in the contract.
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