The last crusade for 0.1 mg iridium

Two mil­ligrams. That’s all it takes for con­ven­tion­al PEM elec­trolyz­ers to con­vert water into hydro­gen gas. Two mil­ligrams per square cen­time­ter of electrolyzer’s mem­brane, that is. That doesn’t sound like much. But it costs more than is eco­nom­i­cal­ly jus­ti­fi­able, and it’s more than is avail­able if the promis­es of politi­cians and self-pro­claimed experts about green hydro­gen are to be ful­filled. This is why researchers are pulling out their last gray hairs in pure frus­tra­tion as they fever­ish­ly try to reduce the irid­i­um require­ment by 95% – to 0.1 mg/​cm². Achiev­ing this goal is the holy grail of the hydro­gen indus­try. You could read about it in our arti­cle Unlock­ing the green hydro­gen econ­o­my. 

But how is it going? We’re going to find out. But first, a quick primer on how an elec­trolyz­er works.

Proton exchange membrane (PEM)

PEM elec­trolyz­ers are so named because they have a pro­ton exchange mem­brane (PEM) at the cen­ter of each elec­trol­y­sis cell. As the name implies, the mem­brane allows pro­tons to pass through. As you may remem­ber from high school, a free pro­ton is noth­ing more than a hydro­gen ion. Hydro­gen is made up of a pro­ton and an elec­tron. Remove the elec­tron and you get a hydro­gen ion, which is a pro­ton. The mem­brane allows these to pass freely from one side to the oth­er while block­ing the elec­trons. This is the key to how PEM elec­trolyz­ers work.

On one side of the mem­brane is an elec­trode that is con­nect­ed to the pos­i­tive ter­mi­nal of a pow­er source. This side is called the anode. On the oth­er side of the mem­brane is anoth­er elec­trode con­nect­ed to the neg­a­tive ter­mi­nal of the same pow­er source. This side is called the cath­ode.

Porous transport layer (PTL)

Both the anode and cath­ode are made of a porous, elec­tri­cal­ly con­duc­tive material.

They must be elec­tri­cal­ly con­duc­tive to allow the pow­er source and the elec­trolyz­er to form an elec­tri­cal cir­cuit through which cur­rent can flow.

They must also be porous to allow water to flow past them and the hydro­gen and oxy­gen gas formed to escape.

Both are referred to as gas dif­fu­sion lay­er (GDL) or porous trans­port lay­er (PTL). We pre­fer the lat­ter name, since they not only allow gas to dif­fuse, but also water to flow.

On the cath­ode side, where hydro­gen is formed when hydro­gen ions com­bine with elec­trons from the pow­er source to form hydro­gen gas (H₂), a lay­er made of car­bon fibers serves as the PTL.

On the anode side, where oxy­gen is formed, a pla­tinized tita­ni­um sin­tered lay­er is used. Car­bon can­not be used because it would imme­di­ate­ly react with the oxy­gen to form car­bon diox­ide. And we don’t want car­bon­at­ed water! Tita­ni­um is more resis­tant to the aggres­sive envi­ron­ment, and the plat­inum pro­tects it and pro­vides bet­ter elec­tri­cal conductivity.

Electrolysis cell

The mem­brane is placed between the two trans­port lay­ers. When cur­rent is applied across the two elec­trodes, water flow­ing past the anode is split into hydro­gen ions and oxy­gen. The oxy­gen escapes through the PTL of the anode, while the hydro­gen ions, which are pro­tons, migrate through the mem­brane to the cath­ode side, where they com­bine with elec­trons (cur­rent is free elec­trons) to form hydro­gen gas, which escapes through the PTL of the cathode.

To dis­trib­ute water and elec­tric­i­ty and to evac­u­ate oxy­gen and hydro­gen gas, a bipo­lar plate (BPL) with chan­nels for water and gas trans­port and elec­tri­cal con­nec­tions is mount­ed direct­ly on the out­side of each PTL.

Togeth­er, the mem­brane, both PTLs and both BPLs form an elec­trol­y­sis cell.

Two cells can be con­nect­ed in series by using one bipo­lar plate for both. It serves as the anode side for one and the cath­ode side for the oth­er. That’s why it’s called bipo­lar; it’s the pos­i­tive ter­mi­nal for one side and the neg­a­tive ter­mi­nal for the oth­er. In this way, sev­er­al elec­trolyz­er cells can be con­nect­ed in series to form an elec­trolyz­er stack.

An elec­trolyz­er plant typ­i­cal­ly con­sists of sev­er­al elec­trolyz­er stacks.

Iridium to the rescue

Is that all?

No.

We need a dash of irid­i­um as well. With­out the nobel met­al, the reac­tion is unbear­ably slow. Only small amounts of hydro­gen and oxy­gen gas are produced.

To speed up the process, the water mol­e­cules need irid­i­um, or more specif­i­cal­ly irid­i­um oxide, to ”hold on to” as they split. The mere pres­ence of this pre­cious met­al speeds up the process con­sid­er­ably. It doesn’t con­sume itself, it’s only a cat­a­lyst for the reaction.

Plat­inum is used on the cath­ode side for the same reason.

Triple phase boundary

For each irid­i­um atom to per­form its mag­ic trick, it must be in con­tact with the anode, the water, and the mem­brane. All at the same time. This is called hav­ing a triple phase bound­ary or three phase bound­ary. And achiev­ing this is one of the biggest chal­lenges in design­ing elec­trode struc­tures for PEM electrolyzers.

The con­ven­tion­al solu­tion is to mix irid­i­um oxide with a sol­vent to form a slur­ry, which is then used to ”paint” the anode side of the membrane.

The irid­i­um atoms on the out­er­most sur­face of the paint lay­er are active as cat­a­lysts. But many more irid­i­um atoms are inac­tive; they don’t get a triple phase bound­ary because they are inside the paint lay­er. They are pure waste of the extreme­ly rare and expen­sive metal.

But you can’t paint too thin. The paint will crack and form islands of irid­i­um atoms that don’t have elec­tri­cal con­tact. The lay­er of ”irid­i­um paint” must be 5–10 microns thick to work, and then it typ­i­cal­ly requires 2 mil­ligrams of irid­i­um per square cen­time­ter of membrane.

The big challenge for the hydrogen industry

But, as those of you who have read our arti­cle Unlock­ing the green hydro­gen econ­o­my know, 2 mg/​cm² is not sus­tain­able either eco­nom­i­cal­ly or in terms of resources. A sus­tain­able lev­el is 0.1 mg/​cm² – a 95 % reduc­tion from today’s lev­el. Get­ting there is the big chal­lenge for the hydro­gen industry.

A chal­lenge that Smoltek has tak­en on.

But before we take a clos­er look at our solu­tion, let’s exam­ine oth­er avenues that have been explored.

Composite anode

The com­pos­ite anode tech­nique is basi­cal­ly the same idea as the one already used. How­ev­er, most of the irid­i­um is replaced with plat­inum black – a fine pow­der of plat­inum. It doesn’t act as a cat­a­lyst, but as a con­duc­tor between the islands of few irid­i­um atoms on the surface.

This method has shown the abil­i­ty to reduce irid­i­um use by up to 80 %, but the fun­da­men­tal struc­ture makes it dif­fi­cult to go below 0.5 mg/​cm² with­out los­ing per­for­mance. In addi­tion, the increased use of oth­er pre­cious met­als increas­es the over­all cost, mak­ing it dif­fi­cult to achieve an eco­nom­i­cal­ly sus­tain­able solution.

Core-shell structures

Core-shell struc­tures are based on cre­at­ing nanopar­ti­cles with a core of a cheap­er mate­r­i­al, such as ruthe­ni­um or nick­el, and a thin shell of irid­i­um oxide. This design allows the amount of irid­i­um to be dras­ti­cal­ly reduced, because only the shell needs to be made of the expen­sive metal.

A triple phase bound­ary is cre­at­ed because the irid­i­um oxide in the shell is in direct con­tact with both the elec­trolyte and the elec­tri­cal­ly con­duc­tive core mate­r­i­al. The most promis­ing attempts have achieved an 85% reduc­tion in irid­i­um use.

Unfor­tu­nate­ly, core-shell struc­tures have proven dif­fi­cult to man­u­fac­ture on an indus­tri­al scale. The process is com­plex and requires care­ful con­trol of the syn­the­sis to ensure uni­form dis­tri­b­u­tion of the shell. In addi­tion, there are sta­bil­i­ty issues where the core mate­r­i­al can leak out over time in the acidic envi­ron­ment. This means that despite promis­ing results in the lab, the tech­nol­o­gy is unlike­ly to reach the 0.1 mg/​cm² goal in com­mer­cial production.

Manganese-iridium composites

Man­ganese-irid­i­um com­pos­ites are some­what like core-shell struc­tures, but now man­ganese oxide is used as the core, and there is no shell of irid­i­um, but indi­vid­ual atoms scat­tered over the entire sur­face, max­i­miz­ing the uti­liza­tion of each irid­i­um atom while tak­ing advan­tage of the sta­bil­i­ty of man­ganese oxide in acidic environments.

A triple phase bound­ary is cre­at­ed because the irid­i­um atoms are per­fect­ly exposed on the man­ganese oxide sur­face, pro­vid­ing direct con­tact with both the elec­trolyte and the con­duc­tive substrate.

Although the tech­nol­o­gy has shown impres­sive results with up to 95% reduc­tion in irid­i­um con­sump­tion in lab­o­ra­to­ry set­tings, there are sig­nif­i­cant chal­lenges to indus­tri­al imple­men­ta­tion. The com­plex syn­the­sis process is dif­fi­cult to scale up, and there are ques­tions about the long-term sta­bil­i­ty of these atom­i­cal­ly dis­persed cat­a­lysts under real­is­tic oper­at­ing conditions.

Nanostructured thin films

Nanos­truc­tured Thin Films (NSTF) use a tech­nique in which irid­i­um oxide is deposit­ed in extreme­ly thin lay­ers on crys­talline organ­ic whiskers. These whiskers, made of a spe­cial organ­ic pig­ment, cre­ate sub­mi­cron sup­port struc­tures that allow for a very effi­cient cat­a­lyst arrange­ment in which almost all of the irid­i­um atoms can par­tic­i­pate in the reaction.

The triple phase bound­ary is opti­mized by the large avail­able sur­face area of the whisker struc­tures and the direct con­tact between cat­a­lyst, elec­trolyte and con­duc­tive sub­strate, while the crys­talline nature of the whiskers pro­vides excep­tion­al cor­ro­sion resistance.

While NSTF tech­nol­o­gy is ele­gant in the­o­ry, its prac­ti­cal imple­men­ta­tion has proven prob­lem­at­ic. The man­u­fac­tur­ing process is del­i­cate and dif­fi­cult to scale up. In addi­tion, the films have proven to be sen­si­tive to mechan­i­cal stress and can delam­i­nate dur­ing oper­a­tion. This means that despite its poten­tial, the tech­nol­o­gy prob­a­bly won’t be able to deliv­er sta­ble per­for­mance at 0.1 mg/​cm² on an indus­tri­al scale.

Nanoprinting technology

Nanoprint­ing uses an advanced print­ing process to cre­ate extreme­ly small (2 nm) irid­i­um par­ti­cles that are print­ed direct­ly onto the mem­brane sur­face. The tech­nol­o­gy allows for opti­mal irid­i­um uti­liza­tion by ensur­ing that each par­ti­cle is avail­able for catal­y­sis while main­tain­ing direct con­tact with the pro­ton-con­duct­ing membrane.

The triple phase bound­ary is opti­mized by the pre­cise place­ment of the par­ti­cles on the mem­brane and their min­i­mal size, ensur­ing each par­ti­cle has both elec­tri­cal con­tact and access to water.

While the tech­nol­o­gy has shown promis­ing results with a fac­tor of 10 reduc­tion in irid­i­um usage, there are still chal­lenges with scal­a­bil­i­ty and cost. The advanced equip­ment and pre­cise process para­me­ters make it dif­fi­cult to achieve cost-effec­tive mass pro­duc­tion of PEM elec­trolyz­ers at 0.1 mg/​cm².

Smoltek’s solution

Smoltek’s tech­nol­o­gy uses ver­ti­cal­ly ori­ent­ed car­bon nanofibers coat­ed with an ultra-thin lay­er of plat­inum on which irid­i­um atoms are placed. This cre­ates a unique struc­ture where each nanofiber acts as a ”stick” to which the cat­a­lyst can attach.

The triple phase bound­ary is opti­mized because the irid­i­um is on the sur­face of the elec­tri­cal­ly con­duc­tive nanofibers, which are per­fect­ly inte­grat­ed into the membrane.

This approach has already demon­strat­ed sta­ble oper­a­tion with as lit­tle as 0.2 mg/​cm² of irid­i­um and shows great poten­tial to reach the tar­get of 0.1 mg/​cm². The sim­ple but ele­gant solu­tion is also well suit­ed for indus­tri­al pro­duc­tion. The nanofibers can be pro­duced using proven CVD tech­nol­o­gy, and the sub­se­quent met­al coat­ing is a well-known process. Smoltek’s suc­cess­ful 1,000 hour test shows that the tech­nol­o­gy is ready for scale-up to indus­tri­al production.

The winning technology emerges

Look­ing at the dif­fer­ent approach­es to reduc­ing irid­i­um con­tent in PEM elec­trolyz­ers, a clear pat­tern emerges. While sev­er­al tech­nolo­gies show promise in lab­o­ra­to­ry set­tings, most face sig­nif­i­cant hur­dles in scal­ing up to indus­tri­al pro­duc­tion. The key to suc­cess lies not just in achiev­ing the 0.1 mg/​cm² tar­get, but in doing so with a tech­nol­o­gy that can be reli­ably man­u­fac­tured at scale.

Smoltek’s solu­tion rep­re­sents one promis­ing path for­ward. Our approach using car­bon nanofibers has already demon­strat­ed sta­ble oper­a­tion at 0.2 mg/​cm² in demand­ing 1,000-hour tests. By bas­ing our inno­va­tion on estab­lished man­u­fac­tur­ing meth­ods like PECVD, we’ve pri­or­i­tized indus­tri­al scal­a­bil­i­ty from the start – because a break­through tech­nol­o­gy only mat­ters if it can reach the market.

And now, after two decades of research and devel­op­ment, this cru­sade for the holy grail of green hydro­gen is draw­ing to a close. The 0.1 mg/​cm² mile­stone is with­in reach.