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 ↩︎