[ materials ]
Iridium : small but mighty
For PEM electrolysers the key metal is iridium . PEM electrolysers use iridium at the anode and platinum at the cathode , which are typically printed as catalyst-containing ‘ inks ’ in a thin layer applied to the proton-exchange membrane . This forms a catalyst coated membrane ( CCM ), which splits water into oxygen and hydrogen under an electric current , with a semi-permeable membrane that allows proton exchange . Seals are applied and the CCM is then sandwiched between gas diffusion layers to allow the gases to move to and from the active layers , forming a cell . A number of these cells connected in series then forms the heart of the electrolyser stack .
Platinum and iridium are ideal catalysts for PEM electrolysis due to their high activity levels and inherent stability in the electrolyser system – there are no known suitable substitutes that work as effectively in the high‐voltage , acidic conditions of a PEM cell . Platinum is used at a low loading already in CCMs , but current iridium usage for every kilo of hydrogen produced presents a significant opportunity for optimisation .
While platinum is targeted as one of the major products of mining operations , iridium occurs in such small quantities that nobody goes after it in its own right : it gets a free ride to the surface on the back of platinum .
Minor in quantity it may be , but iridium is a very useful industrial metal . Its electrochemical power is harnessed not just in PEM electrolysis , but also in the chlor-alkali industry , in the production of copper foil by electrodeposition , and in other applications . As a chemical catalyst , it is often used to produce acetic acid . In electronics , it is used in OLEDs and as solid iridium crucibles to ‘ grow ’ crystals that are used as filters in our mobile phones . But it is probably most familiar as the ignition tips in long-life spark plugs for our cars .
Currently the iridium market is balanced , in that all metal produced by the mines each year is used by these various applications ( 2021 was unusual : demand was still recovering from Covid , and some of the miners sold additional metal from stocks ). With new applications such as PEM electrolysis expected to see growing demand , demand reduction in other areas will occur in response , through a mixture of technological innovations that allow switching to substitutes , increased efficiency and therefore thrifting , or increased recycling .
Solving the iridium challenge
So , the key question : will there be enough iridium available for PEM electrolysis capacity to grow as
5 %
Other industrial
11 %
Chemicals
38 %
Electrochemical
23 %
Electronics
2021 Iridium Demand 7.2 t
Diagram of the catalyst coated membrane ( CCM ) and PGM use within it Iridium demand by application in 2021
23 %
Spark plugs
Hydrogen Tech World | Issue 7 | December 2022 17