Carbon nanofibers in hydrogen electrolysis and fuel cells

Hydro­gen has emerged as a key to store renew­able ener­gy and mak­ing heavy indus­try car­bon-free. Two appli­ca­tion areas of imme­di­ate vital impor­tance. The core tech­nolo­gies that make this pos­si­ble are hydro­gen elec­trol­y­sis and fuel cells. Elec­trol­y­sis con­verts elec­tric­i­ty into hydro­gen, while fuel cells con­vert the hydro­gen back to elec­tric­i­ty. Nat­u­ral­ly, these con­ver­sions are sub­ject to ener­gy loss­es. Unfor­tu­nate­ly, these loss­es can be pret­ty significant—up to eighty per­cent. How­ev­er, there are solu­tions, and car­bon nanofiber (CNF) is part of them.

The need for hydrogen

The inter­mit­tent nature of renew­able ener­gy sources such as solar and wind pow­er cre­ates a demand for solu­tions to store sur­plus elec­tric­i­ty pro­duced on sun­ny or windy days for lat­er use. One method of stor­ing sur­plus elec­tric­i­ty is to con­vert it into hydro­gen by elec­trol­y­sis of water. Fuel cells can then con­vert the hydro­gen back into electricity.

Hydro­gen is expect­ed to become increas­ing­ly impor­tant as a fuel. Hydro­gen-pow­ered vehi­cles are already on the road today. There are even hydro­gen-pow­ered pas­sen­ger cars avail­able for any­one to buy.

Not least, hydro­gen is an increas­ing­ly essen­tial raw mate­r­i­al in heavy indus­try. For exam­ple, hydro­gen is replac­ing coal and coke in the pro­duc­tion of steel, enabling fos­sil-free steel production.

Industrially production of hydrogen

Hydro­gen is pro­duced indus­tri­al­ly in elec­trolyz­ers. Basi­cal­ly, an elec­trolyz­er can be described as a tank of water in which two elec­trodes are immersed. When an elec­tric volt­age is applied across the elec­trodes, a cur­rent flows through the water. The cur­rent breaks down the water into its con­stituent parts: hydro­gen and oxy­gen. Hydro­gen bub­bles up at one of the elec­trodes. Oxy­gen at the other.

Conventional electrolysis

The old­est and most con­ven­tion­al way of pro­duc­ing hydro­gen is alka­line elec­trol­y­sis.

In this process, lye (potas­si­um hydrox­ide or sodi­um hydrox­ide) is added to the water, mak­ing it high­ly cor­ro­sive. The elec­trodes, made of nick­el alloy, are sep­a­rat­ed by a mem­brane which allows hydrox­ide ions (OH^-) to flow through, on their way from the cath­ode to the anode, while sep­a­rat­ing the hydro­gen gas pro­duced at one elec­trode from the oxy­gen gas pro­duced at the other.

This tech­nique has sev­er­al dis­ad­van­tages. Main­ly is the low effi­cien­cy. The ener­gy val­ue of the hydro­gen gen­er­at­ed is only 45–65% of the ener­gy sup­plied. In addi­tion, the method works poor­ly for renew­able elec­tric­i­ty because of its inter­mit­tent nature.

Alternative electrolyzers

There are alter­na­tives to con­ven­tion­al alka­line elec­trol­y­sis. The table below sum­ma­rizes the most com­mon solutions.

Type	Mem­brane	Elec­trolyte	Cat­a­lyst	Cur­rent den­si­ty [A/​cm2]	Oper­a­tional tem­per­a­ture [°C]
Alka­line electrolyzer	Diaphragm	KOH dis­solved in water	Non-noble met­al alloys	0.2–0.7	60–80
High-tem­per­a­ture electrolyzer	Diaphragm	Yttria-sta­bi­lized zirconia	Ni-YSZ alloys or per­ovskite oxides	0.2–1.0	600–700
AEM elec­trolyz­er1	Anion exchange membrane	Poly­mer	Non-noble met­al alloys	0.1–0.5	50–70
PEM elec­trolyz­er2	Pro­ton exchange membrane	Poly­mer	Plat­inum and irid­i­um oxide	1.0–2.2	50–84

PEM elec­trolyz­ers are con­sid­ered the most inter­est­ing going for­ward. So let’s take a clos­er look at them before delv­ing into how car­bon nanofibers (CNF) can improve them.

PEM electrolyzer

From the out­side and inwards, a PEM cat­a­lyst con­sists of the fol­low­ing parts:

• An elec­trode that is con­nect­ed to a pow­er source. The elec­trode con­nect­ed to the pos­i­tive pole of the volt­age source is called the anode. The oth­er is called the cath­ode.
• A porous mate­r­i­al that makes it eas­i­er for the gas form­ing on the cat­a­lysts to be released and facil­i­tates trans­porta­tion away from the cat­a­lysts to an outlet.
• A cat­alyt­ic lay­er facil­i­tates the chem­i­cal reac­tions. It is in close con­tact with the poly­mer elec­trolyte mem­brane. Usu­al­ly, irid­i­um oxide is used on the anode side and plat­inum on the cath­ode side.
• A mem­brane of an elec­trolyt­ic poly­mer (usu­al­ly Nafion) that allows pro­tons but not elec­trons to pass through.

Water flows through the porous mate­r­i­al at the anode side. Oxy­gen is formed at the anode and hydro­gen at the cathode.

The reactions within a PEM electrolyzer

At the anode-side, an oxy­gen evo­lu­tion reac­tion (OER) takes place:

1. Water flows in. Some of the water comes into con­tact with the cat­a­lyst. A water mol­e­cule that comes into con­tact with the cat­a­lyst splits into one oxy­gen atom, two elec­trons, and two hydro­gen ions (pro­tons).
2. The elec­trons are drawn to the anode and moves towards the pos­i­tive pole of the pow­er source.
3. The hydro­gen ions are drawn towards the cath­ode and must pass through the mem­brane on the way.
4. Oxy­gen atoms com­bine in pairs to form oxy­gen gas.

At the cath­ode-side a hydro­gen evo­lu­tion reac­tion (HER) takes place:

1. Elec­trons have been sup­plied to the catalyst.
2. Hydro­gen ions pass through the mem­brane and come into con­tact with the catalyst.
3. Hydro­gen ions and elec­trons com­bine to form hydro­gen atoms.
4. Hydro­gen atoms com­bine in pairs to form hydro­gen gas.

Each indi­vid­ual reac­tion occurs with­in a micro­scop­ic region called the triple-phase bound­ary (TPB). You can think of TPB as a point where the reac­tion hap­pens because all nec­es­sary con­di­tions are met simul­ta­ne­ous­ly. For OER, this means that a water mol­e­cule comes into con­tact with the cat­a­lyst and the poly­mer elec­trolyte mem­brane. For HER, it means that a hydro­gen ion comes into con­tact with the cat­a­lyst and the poly­mer elec­trolyte membrane.

Major advantages

One of the most sig­nif­i­cant advan­tages of PEM elec­trol­y­sis is its high effi­cien­cy. Cur­rent­ly, the hydro­gen pro­duced has an ener­gy val­ue of up to 80% of the elec­tri­cal ener­gy sup­plied. The effi­cien­cy is expect­ed to be even high­er in the com­ing years—up to 86% by 2030.

Anoth­er advan­tage to PEM elec­trol­y­sis is its abil­i­ty to cope with rapid changes in the cur­rent sup­ply, which is a chal­lenge with some renew­able ener­gy sources such as solar and wind.

But PEM elec­trolyz­ers also have their share of problems.

Need for rare and precious metals

PEM elec­trolyz­ers require plat­inum and irid­i­um oxide as cat­a­lysts. They are scarce and pre­cious met­als. For com­par­i­son, gold is 40 times more abun­dant in the Earth’s crust than iridium.

So, to reduce the amount need­ed, a sup­port­ing struc­ture of a cheap­er mate­r­i­al is coat­ed with a thin lay­er of met­als. Alter­na­tive­ly, par­ti­cles of the met­als are dis­persed onto a porous and elec­tri­cal­ly con­duct­ing sup­port­ing material.

How­ev­er, since the total sur­face area of a cat­a­lyst in an indus­tri­al appli­ca­tion is quite large, the cost of cat­a­lysts is still sig­nif­i­cant. And worse, most of it is nev­er used in the reac­tions because they are not at the triple-phase boundaries.

So the main chal­lenge of the PEM elec­trolyz­er is to fur­ther reduce the use of plat­inum and irid­i­um oxide in rela­tion to the gen­er­at­ed current.

Gas bubbles block the reaction

The oxy­gen evo­lu­tion reac­tion and the hydro­gen evo­lu­tion reac­tion result in gas that needs to be quick­ly trans­port­ed away from the cat­a­lyst. Oth­er­wise, the gas mol­e­cules may block the reactions.

The porous mate­r­i­al between the elec­trodes and the poly­mer elec­trolyte mem­brane dis­si­pates the gas­es. Yet, on the anode side, prob­lems can arise with oxy­gen gas not being removed effi­cient­ly enough, thus form­ing gas bub­bles in the water, instead.

So anoth­er chal­lenge of the PEM elec­trolyz­er is to fur­ther improve gas trans­porta­tion away from the cat­alyt­ic surfaces.

Carbon nanofibers (CNFs) to rescue

A car­bon nanofiber (CNF) is a car­bon-made mate­r­i­al so thin that its diam­e­ter is mea­sured in nanome­tres (1 nm = 0.001 µm). Its length is tens of thou­sands of times longer than its diam­e­ter. Typ­i­cal­ly a CNF has a diam­e­ter of 1–100 nm and a length of 1–100 μm.

Smoltek has devel­oped and patent­ed a tech­nol­o­gy to pro­duce CNFs with extreme pre­ci­sion by chem­i­cal vapor depo­si­tion (CVD). The tech­nique allows the cre­ation of straight rows and columns of ver­ti­cal CNFs.

In addi­tion, the tech­nique allows nano-par­ti­cles, such as a few atoms of plat­inum or irid­i­um oxide, to be placed on each indi­vid­ual fiber. In con­junc­tion with CNFs elec­tri­cal con­duc­tiv­i­ty with low resis­tance, that can be used to solve both issues of the PEM electrolyzer.

The solu­tion is sim­ple. Start from the porous mate­r­i­al that will dif­fuse the gas that will be formed. On its sur­face towards the mem­brane, place a large num­ber of CNFs arranged in rows and columns. Attach to each of them nano-par­ti­cles of the cat­alyt­ic met­al. Embed the pre­pared CNFs in the poly­mer elec­trolyte mem­brane. Each CNF becomes a “cru­cible” where the reac­tion takes place.

The advan­tage of this is that the amount of plat­inum or irid­i­um oxide need­ed is sig­nif­i­cant­ly reduced, as very lit­tle of these rare and expen­sive met­als are used and only where they are instrumental.

More­over, the array of CNFs improves the gas trans­port, par­tic­u­lar­ly on the anode side where the prob­lem of gas bub­bles in water block­ing the trans­port may occur.

This tech­nol­o­gy pro­duces two to three times more hydro­gen com­pared to exist­ing tech­nol­o­gy. This is because two to three times more cat­a­lyst par­ti­cles can be in con­tact with the mem­brane simul­ta­ne­ous­ly. This, in turn, can lead to sav­ings of up to 30 per­cent for hydro­gen pro­duc­tion plants.

Fuel cells and other applications

It’s not only the PEM elec­trolyz­er that uses an ion exchange mem­brane (IEM) such as a poly­mer elec­trolyte mem­brane. They are also used in fuel cells. Thus, Smoltek’s CNF solu­tion applies to those as well.

Read more

Read more about how car­bon nanofibers can improve PEM elec­trolyz­ers in our whitepa­per Intro­duc­ing Smoltek Elec­trolyz­er Tech­nol­o­gy.

1. AEM is short for anion exchange mem­brane, which describes the func­tion of the mem­brane. ↩︎
2. PEM can read in two ways. A com­mon read is poly­mer elec­trolyte mem­brane, which describes the struc­ture of the mem­brane; it con­sists of a mem­brane coat­ed with a poly­mer that con­ducts cur­rent with ions but not elec­trons. But since the mem­brane in AEM has strict­ly the same struc­ture, oth­ers choose to read PEM as a pro­ton exchange mem­brane which describes the func­tion of the mem­brane. ↩︎