Carbon nanofibers in the hydrogen industry

The demand for sus­tain­ably pro­duced hydro­gen is ris­ing due to its role in avoid­ing green­house gas emis­sions. Pro­duc­tion and use of sus­tain­able hydro­gen is made pos­si­ble by two core tech­nolo­gies: Water elec­trol­y­sis, which pro­duces hydro­gen from water using elec­tric­i­ty, and fuel cells, which revers­es the reac­tion to gen­er­ate elec­tric­i­ty. How­ev­er, both tech­nolo­gies use rare and expen­sive cat­a­lyst mate­ri­als such as plat­inum or irid­i­um. Using car­bon nanofibers (CNF) as a cat­a­lyst sup­port can decrease the amount of expen­sive cat­a­lyst mate­r­i­al need­ed. Smoltek nanos­truc­ture fab­ri­ca­tion tech­nol­o­gy can unlock this potential.

The need for hydrogen

Hydro­gen pro­duced by water elec­trol­y­sis can be a solu­tion to sev­er­al prob­lems in the process of reduc­ing green­house gas emis­sions. For exam­ple, inter­mit­tent ener­gy sources such as solar and wind pow­er occa­sion­al­ly pro­duce more elec­tric­i­ty than need­ed, e.g. on very windy or sun­ny days. 

Using this elec­tri­cal pow­er for water elec­trol­y­sis allows the ener­gy to be stored in the form of pro­duced hydro­gen gas, which can lat­er be used con­vert­ed back to elec­tric­i­ty by a fuel cell form­ing only water vapor in the process. Anoth­er key dri­ver is to abate the car­bon diox­ide emis­sion in the pro­duc­tion of steel, where coal and coke can be replaced by hydro­gen gas as reduc­ing agent for the iron ore emit­ting only water vapor and mak­ing a sig­nif­i­cant reduc­tion in glob­al car­bon diox­ide emis­sions pos­si­ble. This enables a fos­sil-free steel production.

Making hydrogen by electrolysis

Water elec­trolyz­ers use two elec­trodes, a pos­i­tive­ly charged anode and a neg­a­tive­ly charged cath­ode, sep­a­rat­ed by an elec­trolyte that allows ions to trav­el between the elec­trodes. The elec­trodes are also elec­tri­cal­ly con­nect­ed to a pow­er source that sup­plies elec­tri­cal pow­er to dri­ve the reac­tion. Oxy­gen gas is pro­duced at the anode through the oxy­gen evo­lu­tion reac­tion, and hydro­gen gas is pro­duced at the cath­ode through the hydro­gen evo­lu­tion reac­tion. Cat­a­lysts are used to pro­mote these elec­tro­chem­i­cal reac­tions and allow them to run at a low­er ener­gy cost. The reac­tions hap­pen at the active cat­a­lyst sur­face, i.e. at sur­faces where the cat­a­lyst is in con­tact with the electrolyte. 

Ben­e­fi­cial con­di­tions for water elec­trol­y­sis depend on the type of elec­trolyte, the oper­at­ing tem­per­a­ture, the pres­sure and the types of cat­a­lysts. His­tor­i­cal­ly the indus­try has most­ly relied on low-tem­per­a­ture elec­trolyz­ers using an alka­line solu­tion with high pH con­tain­ing potas­si­um hydrox­ide in water. They do not require scarce cat­a­lyst mate­r­i­al but cause lim­it­ed cur­rent den­si­ties and there­fore lim­it­ed hydro­gen out­put per cell area. This draw­back can be over­come by poly­mer ionomer mem­branes that con­duct either hydro­gen ions or hydrox­ide ions in direct con­tact with anode and cath­ode, so-called zero-gap design. These ion-con­duct­ing poly­mers are known as pro­ton exchange mem­branes (PEM) or anion exchange mem­branes (AEM) respectively.

Green hydrogen and low-carbon hydrogen

Low-tem­per­a­ture elec­trolyz­ers with PEM elec­trolytes, known as PEM elec­trolyz­ers, are promis­ing not only for their high­er cur­rent den­si­ty but also for their excel­lent match with inter­mit­tent pow­er sources and the longevi­ty of the com­mer­cial­ly offered pro­ton exchange mem­branes. This enables a com­pact and durable elec­trolyz­er design. 

For high-cur­rent den­si­ty oper­a­tion at low over­po­ten­tial¹ PEM elec­trolyz­ers need rare and expen­sive of cat­a­lysts such as plat­inum on the cath­ode side and irid­i­um oxide on the anode side. To real­ize the poten­tial of PEM elec­trolyz­ers, it is cru­cial that the cat­a­lysts are used effi­cient­ly and that the cat­a­lyst load, i.e. the amount of cat­a­lyst per unit area of the elec­trolyz­er cell, is kept to a minimum. 

One way of reduc­ing the cat­a­lyst load is to deposit the cat­a­lyst mate­r­i­al on anoth­er mate­r­i­al known as a cat­a­lyst sup­port, either in the form of a thin film or as par­ti­cles with a diam­e­ter of a few nanome­ters. The cat­a­lyst sup­ports act as a scaf­fold­ing, allow­ing the cat­a­lyst to be spread over a larg­er area. An ide­al cat­a­lyst sup­port should have a large sur­face area, an open struc­ture that lets water and gas­es flow to and from the cat­a­lyst, excel­lent con­tact with the pro­ton exchange mem­brane and good and sta­ble elec­tri­cal con­duc­tiv­i­ty to enable the elec­tro­chem­i­cal reac­tions. Car­bon black is often used as a cat­a­lyst sup­port on the cath­ode side in PEM elec­trolyz­ers, but the irid­i­um cat­a­lyst on the anode side is gen­er­al­ly used with­out sup­port due to the harsh acidic con­di­tions at the anode.

Better catalysts with carbon nanofibers (CNF)

Car­bon nanofibers (CNF) are car­bon struc­tures with a diam­e­ter that is typ­i­cal­ly below 100 nm and a length between 1 and 100 µm. Like many car­bon nano­ma­te­ri­als, CNF are elec­tri­cal­ly con­duc­tive and mechan­i­cal­ly strong. 

CNF are grown by chem­i­cal vapor depo­si­tion (CVD) and have the poten­tial to improve on exist­ing cat­a­lyst sup­ports. The CVD growth method makes it pos­si­ble to con­trol the ori­en­ta­tion of the CNF so that they are ver­ti­cal­ly aligned with a well-defined aver­age spac­ing, width, and height. This means that the struc­ture of a CNF cat­a­lyst sup­port can be adjust­ed to achieve the large sur­face area and degree of poros­i­ty that is needed.

The struc­ture of a CNF cat­a­lyst sup­port also makes it pos­si­ble to con­trol the posi­tion of the cat­a­lyst that is deposit­ed on it, which in turn opens pos­si­bil­i­ties for opti­miz­ing the active sur­face area of the cat­a­lyst and reduc­ing the cat­a­lyst load. For exam­ple, the cat­a­lyst can be placed in direct con­tact or even embed­ded into the mem­brane. The CNF can be con­for­mal­ly coat­ed and pro­tect­ed for use on the anode side of the electrolyzer. 

Although reduc­ing cat­a­lyst load is most impor­tant in PEM elec­trolyz­ers, CNF cat­a­lyst sup­ports may also be used in AEM elec­trolyz­ers and in PEM fuel cells. There are clear advan­tages to using CNF grown by CVD as a cat­a­lyst sup­port, such as increas­ing the active cat­a­lyst sur­face area and decreas­ing the need­ed cat­a­lyst load. The CVD meth­ods used for CNF pro­duc­tion by Smoltek can be used to real­ize the poten­tial of CNF in elec­trol­y­sis and fuel cells.

Radically reducing the price for hydrogen production

For future needs of the PEM elec­trolyz­er mar­ket, when the capac­i­ty is scaled to pro­duce Gigawatts of water elec­trol­y­sis year­ly it will be cru­cial to man­age a low irid­i­um cat­a­lyst load on PEM anodes to enable cost-effi­cient hydro­gen production.

Smoltek’s nanofiber-based cell mate­ri­als cre­ates an opti­mal anode struc­ture that allows irid­i­um cat­a­lyst nanopar­ti­cles to form a high­ly active and acces­si­ble sur­face. In prin­ci­ple, all of the nanopar­ti­cles come into con­tact with the pro­ton exchange mem­brane of the cell poten­tial­ly reduc­ing the need­ed amount of irid­i­um by 80% – or more. 

Anoth­er ben­e­fit is that the cells can be opti­mized for high cur­rent den­si­ty, thus the capac­i­ty to pro­duce hydro­gen per cell area increas­es. This is achieved by a cor­re­spond­ing increase of the irid­i­um load. These design choic­es can cre­ate a 2–3 times low­er invest­ment cost for the elec­trolyz­er in a hydro­gen plant.

Are you interested in partnering with us?

Our next step is to indus­tri­al­ize our solu­tion for the elec­trolyz­er cell mate­r­i­al (CNF-ECM). We are there­fore look­ing for indus­tri­al partner(s) that, in close col­lab­o­ra­tion with us;

• build and test pro­to­types with dif­fer­ent com­bi­na­tions of sub­strates, nanos­truc­ture mor­phol­o­gy, anti-cor­ro­sion pro­tec­tion and cat­a­lyst deposition,
• estab­lish per­for­mance and long-term durability,
• devel­op a high-vol­ume man­u­fac­tur­ing concept,
• pro­duces a test series of CNF-ECM to be used in actu­al production.

Is your com­pa­ny a poten­tial part­ner in tak­ing advan­tage of our dis­rup­tive tech­nol­o­gy?
Con­tact us today, and let’s arrange a meet­ing to dis­cuss it further. 

1. The over­po­ten­tial describes the volt­age dif­fer­ence, from the min­i­mum pos­si­ble volt­age to trig­ger the reac­tion to the volt­age that needs to be reached for a suf­fi­cient cur­rent ↩︎