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H2

Carbon nanofibers in hydrogen electrolysis and fuel cells

Carbon nanofibers in hydrogen electrolysis & 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.

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.

Schem­at­ic of how alkaline hydro­lys­is works.

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.

TypeMem­braneElec­tro­lyteCata­lystCur­rent dens­ity [A/​cm2]Oper­a­tion­al tem­per­at­ure [°C]
Alkaline elec­tro­lyz­erDia­phragmKOH dis­solved in waterNon-noble met­al alloys0.2–0.760–80
High-tem­per­at­ure electrolyzerDia­phragmYttria-sta­bil­ized zirconiaNi-YSZ alloys or per­ovskite oxides0.2–1.0600–700
AEM elec­tro­lyz­er1Anion exchange membranePoly­merNon-noble met­al alloys0.1–0.550–70
PEM elec­tro­lyz­er2Pro­ton exchange membranePoly­merPlat­in­um and iridi­um oxide1.0–2.250–84
Char­ac­ter­ist­ics of com­mon electrolyzers.

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.

Schem­at­ic of how PEM hydro­lys­is works.

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.

An array of car­bon nan­ofibers (CNFs) pre­cisely placed in rows and columns.

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.

Schem­at­ic present­a­tion of a mem­brane with embed­ded car­bon nan­ofibers with nano-particles of cata­lyt­ic metal.

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. 

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