Iridium (Ir, atomic number 77, CAS 7439-88-5) and platinum are the two platinum group metals that make a proton exchange membrane (PEM) water electrolyzer work: iridium coats the anode, platinum the cathode and several non-catalytic surfaces. Commercial stacks use about 400 kilograms of iridium per gigawatt of installed capacity, the figure at the center of a debate over whether iridium supply can support gigawatt-scale green hydrogen. This page covers each metal's role, current loadings against the DOE's technical targets, the constraint arithmetic and ruthenium's limits as a substitute.
How PEM Electrolyzers Use Iridium and Platinum#
A PEM electrolyzer splits water into hydrogen and oxygen using iridium oxide at the anode for the oxygen evolution reaction and platinum on carbon at the cathode for the hydrogen evolution reaction. The anode half-reaction, 2H2O to O2 + 4H+ + 4e-, and the cathode half-reaction, 2H+ + 2e- to H2, set a thermodynamic minimum voltage of 1.23 V (25°C) and a thermoneutral voltage of 1.48 V; commercial cells run at 1.7 to 2.0 V. Electrolysis is one of the wider industrial applications of platinum group metals, the six-metal family (platinum, palladium, rhodium, iridium, ruthenium, osmium).
Why Iridium: the Anode Oxygen Evolution Reaction#
Iridium is used at the anode because iridium oxide is the only material with a commercially acceptable combination of oxygen-evolution activity and dissolution resistance above 1.6 V. At pH near zero past that potential, most catalysts evolve oxygen too slowly or dissolve outright: nickel- and cobalt-based catalysts, standard in alkaline electrolyzers, dissolve almost immediately. Ruthenium dioxide is intrinsically more active for oxygen evolution, but it over-oxidizes to soluble, volatile ruthenium tetroxide and degrades far faster, a limitation covered further below. Iridium oxide (IrO2, or IrOx amorphous) survives where ruthenium oxide fails, keeping the commercial PEM anode iridium-based despite iridium being rarer and costlier.
Why Platinum: the Cathode and Non-Catalytic Roles#
Platinum's role is not limited to the cathode catalyst: WPIC and Metals Focus name three platinum locations in a PEM electrolyzer, the hydrogen-side electrode, the porous transport layer coating and the bipolar plate coating. WPIC and Metals Focus describe the full picture in Platinum Quarterly Q1 2026: "The electrode on oxygen side (anode) contains iridium oxide while the electrode on hydrogen side (cathode) typically contains platinum. Transport layers are platinum-coated sintered porous titanium, and the bipolar plates would typically have platinum on with other metals." Cathode hydrogen evolution is fast, so its platinum loading is small next to the anode's iridium; the coatings instead stop titanium oxide from raising the cell's internal resistance.
- Cathode catalyst: platinum on carbon, driving the hydrogen evolution reaction.
- Porous transport layer coating: platinum-coated sintered porous titanium.
- Bipolar plate coating: platinum applied together with other metals.
Iridium and Platinum Loadings: Current Figures and DOE Targets#
Today's commercial PEM electrolyzers use about 2 milligrams of iridium per square centimeter at the anode, roughly 400 kilograms of iridium per gigawatt, alongside a smaller platinum cathode loading. These figures come from the DOE's Hydrogen Shot assessment (December 2024) and are repeated by Johnson Matthey and WPIC. The loadings below run from today's stacks through the DOE's technical targets to a range of separately claimed thrifting figures, none a single consensus.
Current Commercial Loadings#
Anode iridium and cathode platinum loadings are quoted on two different DOE datasets that do not fully reconcile, so this page states both and does not average them. The first, from the DOE's Hydrogen Shot assessment (December 2024), gives an anode loading of about 2 mg/cm², equivalent to 0.5 to 0.8 g Ir/kW at 1.9 V and 2 A/cm², or roughly 400 kg Ir/GW. The second, relayed through the JCDREAM and CHARGE white paper on critical materials, splits 2022 loadings into 450 kg Ir/GW-equivalent and 260 kg Pt/GW, an iridium-to-platinum ratio of about 1.7 to 1 by mass. Table 1 lists both series with their own basis.
| Metric | Value | Basis / source |
|---|---|---|
| Anode iridium loading | ~2 mg Ir/cm² | US DOE, Hydrogen Shot, Dec 2024 |
| Anode iridium, per gigawatt | ~400 kg Ir/GW | Johnson Matthey, 16 Nov 2022; WPIC, 17 Aug 2023 |
| Cathode platinum, per gigawatt | 260 kg Pt/GW | US DOE 2022, via JCDREAM/CHARGE white paper |
| Iridium-to-platinum mass ratio | ~1.7:1 | Derived from the JCDREAM/CHARGE dataset |
Table 1: current loadings, a separate 2022 dataset from Table 2's 800 kg/GW status figure below; not summed against it.
US DOE PEM Electrolysis Technical Targets#
The DOE's PEM electrolysis technical targets cut total PGM content from 3.0 mg/cm² (0.8 g/kW) today to 0.125 mg/cm² (0.03 g/kW) at the ultimate target, with longer stack life and lower degradation alongside it. As of 6 August 2026, the 2026 interim targets are not met at commercial scale; anodes remain around 2 mg Ir/cm² alone. DOE publishes no separate iridium-only target, and its Table 2 combines both electrodes, so the 0.8 g/kW status figure is not added to the 450 + 260 kg/GW split in Table 1; the two are different DOE-linked datasets for the same year.
| Parameter | 2022 status | 2026 interim target | Ultimate target |
|---|---|---|---|
| Total PGM content (both electrodes) | 3.0 mg/cm² (0.8 g/kW) | 0.5 mg/cm² (0.1 g/kW) | 0.125 mg/cm² (0.03 g/kW) |
| Stack lifetime | 40,000 h | 80,000 h | 80,000 h |
| Average degradation rate | 4.8 mV/kh | 2.3 mV/kh | 2.0 mV/kh |
Table 2: US DOE, Hydrogen Shot: Water Electrolysis Technology Assessment, Table 2, December 2024.
The Iridium Thrifting Roadmap: Who Has Claimed What#
The iridium loading roadmap runs through at least six separately claimed figures from 400 down to 15 kilograms per gigawatt, each from a named party and none a single consensus. Johnson Matthey and WPIC cite 400 kg Ir/GW as today's baseline; Heraeus reports a commercialized catalyst at 300 kg/GW as of 2023. WPIC's 60 Seconds in Platinum (17 August 2023) reports OEMs pre-marketing catalysts under 100 kg/GW near term, and the same note projects 80 kg/GW feasible by 2030 and 30 kg/GW by 2050. Heraeus's Dr. Christian Gebauer, head of R&D hydrogen systems, separately named 15 kg/GW as the company's long-term outlook on 14 November 2023. No single party has published all six as one roadmap; each is its own claim, and none is the industry's agreed path.
| Loading (kg Ir/GW) | Timing | Claimant | Date |
|---|---|---|---|
| 400 | Baseline, today | Johnson Matthey; WPIC | 2022 / 2023 |
| 300 | Commercialized catalyst | Heraeus | 2023 |
| Under 100 | Near-term, OEM pre-marketing | WPIC | 17 Aug 2023 |
| 80 | Feasible by 2030 | WPIC | 17 Aug 2023 |
| 30 | Feasible by 2050 | WPIC | 17 Aug 2023 |
| 15 | Long-term outlook | Heraeus (Dr Christian Gebauer) | 14 Nov 2023 |
Is Iridium a Bottleneck for Green Hydrogen?#
Whether iridium constrains PEM electrolyzer growth depends on the deployment rate, not just the loading: the constraint is not binding today but becomes binding at higher gigawatt-per-year build rates. Global electrolyzer deployment across all technologies was under 1 GW/yr as recently as 2024, well below the level at which iridium supply would bind even at today's loadings.
The Constraint Arithmetic: GW of Electrolyzers per Year of Iridium Supply#
At today's loading of 400 kg Ir/GW, one year of global primary iridium supply (229 koz, 7.1 t) could build about 17.8 GW of PEM electrolyzers if all of it went to hydrogen, a derived ceiling, not a forecast, using Johnson Matthey's 2026f supply figure. At the same loading, that ceiling falls to 4.4 GW/yr at a quarter of supply and 1.8 GW/yr at a tenth; thrifted to 100 kg/GW, the three scenarios rise to 71.0, 17.8 and 7.1 GW/yr. Every ounce bought for PEM electrolysis in 2026 already competes with the chemical, electrical and other electrochemical demand tracked on iridium price today.
| Ir loading (kg/GW) | GW/yr at 100% of supply | GW/yr at 25% of supply | GW/yr at 10% of supply |
|---|---|---|---|
| 400 (today) | 17.8 | 4.4 | 1.8 |
| 100 (near-term OEM) | 71.0 | 17.8 | 7.1 |
Table 4: derived from Johnson Matthey's 2026f iridium supply of 229 koz (7.1 t). A labelled ceiling, not a forecast.
The Counter-Case: Why WPIC and Johnson Matthey Say It Is Not Yet Binding#
Both major PGM market houses argue publicly that iridium availability is not a bottleneck to PEM electrolyzer ramp-up, citing thrifting, substitution and recycling. WPIC wrote on 17 August 2023 that "iridium availability is not a bottleneck to PEM electrolyser ramp-up," citing thrifting toward 80 to 100 kg/GW and substitution freeing about 20% of iridium demand by 2030 (roughly 45 koz/yr) and 30% by 2040 (roughly 67 koz/yr). Johnson Matthey's M. Ryan wrote on 16 November 2022 that "through thrifting and recycling, there will be sufficient iridium supply to support the required ramp-up in hydrogen demand." Both arguments lean on recycling rates that remain aspirational, not proven at scale.
Ruthenium as a Partial Iridium Substitute#
Ruthenium dioxide is more catalytically active for oxygen evolution than iridium oxide but dissolves faster in the electrolyzer's acidic environment, which is why iridium, not ruthenium, is the commercial anode catalyst. Above 1.6 V in acidic media, ruthenium dioxide over-oxidizes to soluble, volatile ruthenium tetroxide and degrades far faster than iridium oxide under the same conditions. Catalyst developers, including a Heraeus and Sibanye-Stillwater program announced 14 November 2023, blend ruthenium into iridium oxide anodes to cut iridium content, not to replace it outright.
Why Ruthenium Cannot Simply Replace Iridium#
Substituting ruthenium for iridium moves the constraint onto a metal that is itself in forecast deficit and mined from the same ore bodies as iridium. A full swap moves demand from a 229 koz iridium market to a 977 koz ruthenium market, nearly 4.3 times larger, but Johnson Matthey's May 2026 report forecasts a 216 koz ruthenium deficit for 2026 against depleted South African producer stocks. Ruthenium and iridium are co-products of the same South African and Russian ore bodies, so a large substitution re-weights demand across two products of one mine rather than diversifying supply.
- Market size mismatch: ruthenium demand (977 koz/yr) runs about 4.3 times larger than iridium demand (229 koz/yr).
- Ruthenium's own deficit: Johnson Matthey forecasts a 216 koz shortfall for 2026, against already-depleted stocks.
- Shared ore bodies: both metals come from the same South African and Russian deposits, so substitution does not diversify supply.
Related PGM Applications and Pages#
PEM is one of four electrolyzer technologies, and the only one that requires iridium; alkaline, anion-exchange-membrane and solid-oxide electrolyzers use non-PGM or much smaller PGM loadings. Platinum, palladium, rhodium, iridium, ruthenium and osmium are tracked together on the platinum group metals market page.
PEM Electrolyzers vs Alkaline, AEM and SOEC Technology#
Electrolyzer technologies differ most in electrolyte, operating temperature and PGM catalyst use, compared below.
| Technology | Electrolyte | Operating temperature | PGM catalyst use |
|---|---|---|---|
| PEM | Perfluorosulfonic acid solid polymer | 50-80°C | Iridium anode, platinum cathode and coatings |
| Alkaline (LA/AWE) | 20-30% KOH liquid | 60-90°C | Essentially none; nickel and nickel alloys |
| AEM | Anion-exchange solid polymer | 40-60°C | Essentially none; nickel/nickel alloys, stainless steel targeted |
| SOEC | Solid oxide ceramic (YSZ) | 700-850°C | None; nickel-YSZ and perovskite electrodes |
Table 5: US DOE, Hydrogen Shot: Water Electrolysis Technology Assessment, Table 2, December 2024.
PEM Electrolyzer FAQ#
How much iridium does a PEM electrolyzer use per gigawatt?#
Today's commercial PEM electrolyzers use about 400 kilograms of iridium per gigawatt of installed capacity, per Johnson Matthey and WPIC. That figure sits alongside a smaller cathode platinum loading, and thrifting roadmaps from Heraeus and WPIC project it falling toward 80 to 100 kilograms per gigawatt in the near term.
Is iridium a bottleneck for green hydrogen?#
Iridium is not a binding constraint at today's low PEM deployment rate, but the constraint arithmetic shows it becomes binding at roughly 15 to 25 gigawatts of PEM built per year at current loadings. Global electrolyzer deployment across all technologies was under 1 gigawatt a year as recently as 2024, well short of that range.
Can ruthenium replace iridium in electrolyzers?#
Ruthenium cannot simply replace iridium because ruthenium dioxide dissolves faster under electrolyzer conditions, and ruthenium is itself forecast to run a 216,000-troy-ounce deficit in 2026. Catalyst developers blend the two metals to cut iridium content rather than substitute ruthenium outright.
What is the difference between a PEM fuel cell and a PEM electrolyzer?#
A PEM fuel cell converts hydrogen and oxygen into electricity, while a PEM electrolyzer runs the same chemistry in reverse, using electricity to split water into hydrogen. Both use platinum group metal catalysts, and platinum in fuel cells covers the loadings and thrifting curve on the fuel cell side.
What are iridium's and platinum's symbol, atomic number and CAS number?#
Iridium has the symbol Ir, atomic number 77 and CAS number 7439-88-5; platinum has the symbol Pt, atomic number 78 and CAS number 7440-06-4. Full property pages for both metals are at iridium (Ir), element 77 and platinum (Pt), element 78.