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. ↩︎