Electrolyzer Material Selection: 7 Questions Before You Specify

Most material problems in electrolyzer stacks are not exotic failures — they are specification gaps that showed up six months later as rising cell voltage. The fix is boring and effective: answer seven questions before writing the material specification, in this order.

Question 1 — Which stack type, and which electrode position?

This single answer constrains the metal: acidic and oxygen-evolving (PEM anode) means titanium with Pt coating; alkaline or AEM means nickel. Cathode positions shift the choice toward carbon (PEM) or nickel (ALK/AEM). If you cannot state the local potential window confidently, send the drawing to your supplier before specifying — nickel felt vs titanium felt covers the chemistry in detail.

Question 2 — What electrolyte, at what temperature?

Concentration and temperature set the corrosion margin. 30% KOH at 90 °C is a different duty from 1% KOH at 40 °C; a PEM anode at 80 °C with oxygen evolution is harsher than the same cell idle. State both, not just “alkaline” or “acidic”.

Question 3 — What current density, and how much resistance can you afford?

Current density sets the mass-transport duty (porosity) and the electrical duty (areal resistance). ALK designs at 0.2–0.5 A/cm² live comfortably with nickel mesh collectors; PEM designs at 1–3 A/cm² need the low, uniform areal resistance of Pt-coated Ti felt (≤5 mΩ·cm² in published commercial specs) or a fine-pore sintered plate.

Question 4 — What compression, and what thickness tolerance?

Specify these together. Compression around 0.5–2 MPa is typical; a felt compresses slightly and evenly, a plate does not compress at all, foam collapses if you exceed its limit. Tolerance drives uniformity: on a 0.3 mm felt, ±0.02 mm is 7% — enough to create pressure gradients across a cell.

Question 5 — How big is one sheet, really?

Active area per cell decides whether a single felt works or the sheet must be reinforced. Large single sheets sag and deform during handling and over stack life; mesh-reinforced nickel felt (felt co-sintered onto expanded mesh) solves strength and conductive path together. Know the maximum production sheet size too — 800×1500 mm is a common commercial ceiling, and joining smaller sheets into a large electrode is a quality risk you should not accept by default.

Question 6 — Do you need a coating, and how thick?

For titanium in PEM service: yes, Pt coating, typically 0.2–0.5 μm. The coating stabilizes contact resistance; it is not optional corrosion insurance you can skip to save cost. Nickel in alkaline service needs no coating — and should not be quoted one, because it adds cost for nothing.

Question 7 — What does acceptance look like?

Write the incoming-inspection criteria into the purchase spec, not into a warranty dispute:

Check	Method (typical)	Why it matters
Thickness uniformity	Micrometer grid across sheet	Uniform compression; no hot spots
Porosity / bubble point	Bubble-point or gravimetric	Mass transport and gas crossover behavior
Areal resistance	Four-probe under defined pressure	Direct predictor of cell voltage
Coating presence & thickness	EDX / XRF spot checks	Uncoated Ti areas passivate in service
Edge quality	Visual + magnification on cut edge	Fibre burrs initiate membrane pinholes
Flatness (plates)	Surface plate / dial gauge	Rigid plates cannot absorb partner flatness errors

The eight mistakes we see most often

• Nickel specified in a PEM anode because the datasheet said “corrosion resistant”.
• Uncoated titanium felt purchased to save coating cost — then voltage drift in month two.
• Foam in a high-compression zero-gap design.
• Thickness tolerance copied from the steel spec instead of the stack compression budget.
• Sheet size chosen without checking production limits, forcing welded seams.
• Porosity specified “as high as possible” — mass transport up, areal resistance up too.
• Coating thickness specified without a measurement method, so it cannot be verified.
• No incoming inspection plan, so a batch problem surfaces as a stack recall.

Ready-to-send spec sheet

Field	Example entry
Stack type / position	PEM, anode PTL
Electrolyte / temperature	PFSA membrane, 80 °C, deionized water
Current density target	2 A/cm²
Material & form	Ti fiber felt, Grade 1
Thickness / tolerance	0.4 mm ±0.02
Porosity / pore size	60% / 20–40 μm
Coating	Pt 0.3 μm both sides, XRF-verified
Sheet size	500×800 mm, edge-cut
Annual volume	2,000 sheets

Send that table to any qualified supplier — including us — and you will get comparable, checkable quotations instead of brochures. For the structural context behind these choices, see electrolyzer stack structure and water electrolysis routes compared.

Frequently asked

What does a supplier need for a quote?

Stack type and electrode position, electrolyte chemistry and temperature, active area, target thickness and porosity, compression range, coating requirement and annual volume. That is enough to confirm a specification within one business day.

What thickness tolerance is realistic?

Commercial sintered felt production commonly holds ±0.02 mm across a sheet. The tolerance should be chosen together with the stack compression budget, not independently.

Why does porosity have a sweet spot?

Porosity trades mass transport against contact area and areal resistance. Felts typically land between 50% and 80%, tuned to the target current density.

What should incoming inspection check?

Thickness uniformity, porosity or bubble point, areal resistance, coating presence and thickness, edge quality, and flatness for rigid plates — on a defined per-batch sample plan.

How are Pt / Ir coatings applied and verified? See the dedicated guide: Platinum or Iridium? Coating Processes for Titanium PTLs.