1. Why a Coated Titanium PTL Exists

Plain titanium is the workhorse substrate for PEM porous transport layers. It survives the anode-side oxidative environment, takes the differential pressure of pressurized electrolysis, and ships as a controlled porous body. The problem is that plain titanium is a poor catalyst for the hydrogen evolution reaction. At high current density the cathode-side hydrogen evolution overpotential climbs, two-phase flow across the PTL becomes unstable, and the membrane sees localized hot spots that shorten stack life.

Coating the titanium PTL with a thin layer of platinum or iridium changes the cathode kinetics and, indirectly, the mass-transfer behavior of the layer. The catalytic coating drops the HER overpotential by 150––300 mV at 1 mV, which means a coated titanium PTL can sustain 3––4 A/cm² where an uncoated one stalls at 1.5––2 A/cm². For stack designers targeting a 70% LHV efficiency at 3 A/cm², that is the difference between a viable product and a research project.

The substrate is a sintered titanium fiber felt (typically 0.3––0.6 mm thick, 50––70% porosity). After vacuum sintering, the surface is rough, mechanically interlocked, and electrically continuous. That is what makes it a good base for a thin noble-metal coating — the coating does not have to provide mechanical strength, only electrocatalytic activity.

2. The Substrate and the Coating Layer

The coated titanium PTL is a two-layer system. Engineers that confuse the two end up specifying a coating on a substrate that is too dense, too thin, or too rough, and the cell underperforms in unpredictable ways.

Substrate: Sintered titanium felt, expanded mesh, or multilayer wire mesh, 0.3––1.0 mm thick, 30––60% porosity, mean pore size 10––50 µm. Pure titanium (GR1) is the standard; Ti-0.2Pd (GR7) is used when chloride contamination is expected. For the full substrate specification, see our titanium PTL design guide.

Coating layer: Platinum group metals deposited by one of three industrial routes:

• Electroplating. A chloroplatinic acid bath deposits Pt at controlled current density. Low cost, good thickness control (0.5––3 µm), but adhesion depends on substrate surface prep. Risk of non-uniform coverage in deep pores.
• Magnetron sputtering. Vacuum deposition of Pt or Ir with sub-micron uniformity. Best for thin (≤200 0.5 µm) catalytic layers where every milligram of Pt counts. Higher equipment cost.
• Thermal decomposition of chloride salts. Painted or dip-coated precursor, then decomposed at 350––450 °C in air. Standard for IrO₂ / RuO₂ coatings on DSA anodes; also used for mixed Pt-Ir oxide layers on PEM PTLs.

The loading that makes a difference is small. 0.5––2 mg/cm² of Pt (roughly 0.5––2 µm thickness on a smooth surface) is enough to drop the HER overpotential by 200 mV at 1 A/cm². Beyond ~3 mg/cm² the marginal benefit shrinks and the platinum cost takes over.

3. Pt versus Ir: Where Each One Wins

Both platinum and iridium belong to the platinum group, both lower HER overpotential, and both are expensive. The choice between them is set by three factors: the side of the cell, the operating voltage window, and the duty cycle.

For a single-purpose PEM electrolyzer running steady at 2––3 A/cm², sputtered Pt on sintered titanium felt is the most common commercial choice. Ir earns its premium on stacks that cycle rapidly between idle and full load, where the brief excursions above 1.4 V risk Pt dissolution — Ir survives those transients with less mass loss.

4. Coating Specification: Loading, Thickness, Uniformity

Specifying a coated titanium PTL is more than specifying a metal. Three numbers control whether the coating does its job in service:

1. Loading. Total mass per unit area, usually expressed as mg/cm² of Pt or Ir. Typical commercial range: 0.5––2.0 mg/cm². Below 0.3 mg/cm² the coating is patchy and durability drops. Above 3.0 mg/cm² the marginal electrocatalytic benefit shrinks and the cost climbs.
2. Thickness. Equivalent coating thickness on a smooth substrate, typically 0.3––2 µm. Rough substrates (sintered felt) will hold more Pt at the same thickness because of their higher BET surface area. Always specify loading, not just thickness, when you want a meaningful comparison between vendors.
3. Uniformity. Loading variation across the active area. Acceptable for a 10 × 10 cm PTL is ±15% by XRF; anything looser indicates poor bath control and you will see hot spots in operation.

Two PTLs at the same nominal loading can give very different cell voltages because of how the coating sits inside the porous structure. A coating that buries itself deep in the pores sees less electrolyte and contributes less to the reaction. The best coatings sit on the outer 30––40% of the pore depth where protons, electrons and evolved gas can all reach them. The image shows the open porous architecture a well-deposited coating has to live in.

For substrate geometry, the porosity, thickness and pore size selection guide walks through the trade trade-offs in more detail. For a finished coated PTL ready for a 10 × 10 cm MEA, see the 0.5 µm platinum-coated titanium felt PTL in the catalog.

5. Operating Window and Lifetime

A coated titanium PTL is rated for current density, not just pressure drop. Three operating limits matter for service life:

• Voltage ceiling. Pt dissolves at potentials above 1.6 V vs RHE in PEM conditions; IrO₂ forms above 1.4 V. For pure Pt-coated PTLs running on the cathode side (HER), this is rarely a problem — the cathode potential sits below 0 V vs RHE in operation. The risk appears in bipolar or unitized regenerative stacks where the same PTL can see the OER side during reverse-current events.
• Temperature. PEM electrolyzers run at 60––80 °C. Below 60 °C the catalyst kinetics slow, above 80 °C membrane lifetime shortens. Coatings survive the standard window without measurable degradation for thousands of hours.
• Pressure cycling. Differential pressure between the anode and cathode sides stresses the coating adhesion. Stack builders that pressure-cycle the cell daily should qualify the PTL with at least 1,000 thermal and 500 pressure cycles before signing off.

Typical accelerated stress test results on a 1.5 mg/cm² Pt-coated sintered Ti felt PTL at 3 A/cm² and 80 °C show 10––15 µV/h decay over 1,000 hours — equivalent to roughly 5––8 µV/h at 2 A/cm² steady state. Ir coatings last about 30% longer under the same protocol but cost roughly four times more per gram.

6. Specifying and Sourcing a Coated Titanium PTL

When you write the spec for a coated titanium PTL, separate the substrate specification from the coating specification. The substrate belongs in a materials table (thickness, porosity, pore size, GR number). The coating belongs in a process table (metal, loading, deposition route, uniformity tolerance, accelerated stress test result). Bundling them into one line item hides the real engineering choices and locks the buyer into a one one-stop quote.

For a pilot or first article, ask the supplier for three numbers per sample: loading by XRF in mg/cm², coating thickness by cross-section SEM, and an ASTM B265 trace for the substrate. Combine that with a polarization curve at 1 A/cm² in 1 mV H₂SO₄ at 80 °C — a good coated PTL should sit below −100 mV vs RHE on the HER side at that current density.

Production volumes start at the 100-piece level for the standard 10 × 10 cm size; custom sizes, custom loadings, and Ir-only or mixed Pt-Ir coatings scale from there. The full titanium PTL and porous plate family is listed in the catalog for reference, and the cleaning regime that keeps a coated PTL serviceable across cycles is covered in our sintered metal filter cleaning guide.

7. FAQ

8. References & Data Sources