Proton exchange membrane (PEM) electrolysis is a method of producing hydrogen by electrolysis of water where the electrodes are separated by a gas-tight and highly acidic polymer membrane, allowing H+ ions, but no electrons, to pass through. Very reactive, it can be used efficiently with intermittent energies. However, it requires very expensive materials.
Proton exchange membrane (PEM) electrolysis uses as electrolyte an extremely thin (20-300 micrometers) gas-tight polymer membrane with a strongly acidic character, allowing H+ ions (protons) to pass through. [To be more precise: it contains sulfonic functional groups (R-SO3H) which are responsible for the ion exchange mechanism].
PEM technology was born in the early 1950s, with the American space program, mainly with the idea of a fuel cell that could work even in zero gravity. General Electric developed the first PEM electrolyzer in 1966. The concept was then improved by W.T. Grubb, who used a sulfonated polystyrene membrane as electrolyte.
We will study :
- The principle of PEM electrolysis
- The economics of PEM electrolysis
- The advantages and disadvantages of PEM electrolysis
Principle of PEM electrolysis
“The principle of PEM electrolysis differs from that of alkaline electrolysis in that the electrolyte is a solid electrolyte, composed of a proton-conducting membrane; the electrodes are deposited on either side of this polymer material.”
Translated from french, Caroline Rozain (2013)
The chemical reaction
The reaction at the anode is:
H2O(l) → 1/2 O2(g) + 2 H+ (aq)+ 2e−
This is also called the Oxygen Evolution Reaction (OER).
Then, the H+ protons “migrate to the cathode through the membrane under the effect of the electric field and the concentration gradient where they are reduced to molecular hydrogen (with the electrons coming from the negative pole of the generator)” . (Caroline Rozain 2013) Hence the name “proton exchange membrane”.
The reaction at the cathode is
2 H (aq) + 2e− → H2(g)
This is also called the Hydrogen Evolution Reaction (HER).
The materials of PEM electrolysis
The electrodes are “composed of a layer of catalytic material (catalysts + ionomer) and a diffusion layer. The diffusion layer is used to improve current flow and facilitate reactant and product transport.” (Caroline Rozain 2013)
“The choice of catalysts requires the consideration of several factors:
– The catalytic activity towards the reactions involved,
– The chemical stability with respect to the electrolyte and the reaction products,
– The electrochemical stability with respect to the potential of the electrodes,
– The cost,
– The ease to form, on the membranes, porous, homogeneous, adherent deposits with important specific surfaces, – Mechanical stability under strong gas release,
– Electronic conductivity,
– Sensitivity to poisoning, – Morphology (crystal size, crystallinity, networks…).”
Caroline Rozain (2013)
Materials for the cathode
Platinum is the most interesting catalyst, but its price is problematic (> 30 k€ kg-1).
Cobalt and iron could also be good catalysts for the cathode, but they are not stable under PEM electrolysis conditions. One idea being explored is to combine them with organic compounds. (Caroline Rozain 2013)
What is the anode made of
One of the challenges is the high operating potential (1.6V vs. ESH), which cannot be supported by all materials.
Classically the best is to use iridium or ruthenium oxide.
Unfortunately, irridium is rare: 40 times rarer than gold and 10 times rarer than platinum.
For the membrane
The membrane is usually a cation exchange polymer, Nafion being the most commonly used polymer. The company DuPont de Nemours which markets it.
“Nafion® is an ionomer composed of a hydrophobic poly(tetrafluoroethylene) (PTFE) backbone onto which are grafted perfluorinated (perfluorovinyl ether) pendant chains terminated by sulfonic groups.”
(Caroline Rozain 2013, p.38)
Bipolar plates and diffusion layers
Bipolar plates have, in a PEM electrolysis cell stack, the function of supplying current and venting gases. These are the plates with perpendicular trenches (grooves).
They are subjected to difficult constraints: high potential and acidic environment. Titanium (or a titanium coating) is often used.
Profitability of PEM electrolysis
The yields of PEM
In itself, the process does not produce greenhouse gases, neither in the production nor in the use of hydrogen. The problem is in the production of electricity, which is often very carbon intensive, and in the yield of the operation.
Estimates of the efficiency of PEM electrolyzers vary greatly. Here are some perspectives, which I will add to over time:
“Energy yields range from 48% to 65% approximately” Benoit Guenot
The price of PEM electrolysis
For example, here is the cost presentation for Proton brand PEM electrolyzers with a capacity of 13 kg H2/day by Ayer et al. (2010):
Balance of plant prices would decrease significantly with a large-scale installation.
The main cost of the stacks is based on the membrane electrode assembly (MEA) and the flow gaps and separators, which together account for 72%.
Advantages and disadvantages of PEM electrolysis
Advantages of PEM electrolysis
“Proton Exchange Membrane (PEM) electrolyzers are very promising for hydrogen production: they are compact, electrically efficient (1 to 4 A cm-2), produce very pure hydrogen (no or little pollution from the electrolyte), and have a very high efficiency. (1 to 4 A cm-2), produce very pure hydrogen (no or little pollution from the electrolyte), require little maintenance and can be powered by electricity from RE.”
Caroline Rozain (2013)
“PEM electrolyzers can operate at much higher densities, capable of reaching values above 2A/cm², which reduces operational costs and, potentially, the overall cost of electrolysis. […] PEM allows for a thinner electrolyte than alkaline electrolysis.”
Carmo et al. A comprehensive review on PEM water electrolysis, Volume 38, Issue 12, 22 April 2013, Pages 4901-4934
The main advantages of PEM over alkaline electrolysis are that they are
- more compact (the electrolyte takes up less space) ;
- more portable (the absence of liquid is convenient);
- more reactive (they can be combined with an intermittent energy source, allowing “power to gas”);
- there is less contamination between the gases produced;
- can operate at higher current densities, of the order of 2.0 A/cm². the hydrogen produced is purer.
The challenges of PEM electrolysis
“However, this technology needs to be improved, particularly in terms of the costs of its components, which are still very high. The membranes (about 550 € m-2 for a Nafion® 115 membrane – Sigma Aldrich), the catalysts (noble metals of several k€ kg-1) and the bipolar plates used are expensive, and it is important to optimize their use while looking for alternatives because there is little hope of reducing costs, even when considering mass production.”
Caroline Rozain (2013)
The main problems of PEM electrolysis are
- the need for noble metals (platinum, irridium, rhutenium) for the electrodes and titanium for the separation plates
- the price of the membrane
- a shorter lifetime than alkaline electrolyzers
Carmo et al. (2013) present several avenues they consider interesting to overcome these difficulties:
- “Core-shell catalysts”
- “Bulk mettalic glasses”
- “Nanostructured thin films”
I do not detail, it seems to me too complex for the framework of this article.
There is research to develop alloys that can reduce the use of these noble metals. We are talking about DSA (Dimensionally Stable Anode) type electrodes. However, they would not be viable for PEM electrolyzers, as their only interest would be in improving the life span of ruthenium-based electrodes (Caroline Rozain 2013, p.33).
The PEM technology today and tomorrow
One way to improve the process would be to increase the pressure.
High pressure PEM electrolysis, advantages and disadvantages
PEM electrolyzers can operate at very high pressures, some models even claiming to reach 350 bars. This has several advantages, but also defects. I will take here the article by M.Carmo, D.L.Fritz, J.Mergel and D.Stolten, A comprehensive review on PEM water electrolysis, (Volume 38, Issue 12, 22 April 2013, Pages 4901-4934).
Benefits of high pressure PEM electrolysis
The hydrogen produced is itself at high pressure, which limits the need for energy to compress it.
This reduces the gaseous phase at the electrodes, significantly improving gas ejection.
In a differentiated pressure configuration, only the cathode (hydrogen) is under pressure, which limits the risks associated with handling pressurized oxygen.
Pressure minimizes membrane expansion and dehydration
The increase in pressure results in higher thermodynamic voltages.
Defects of high pressure PEM electrolysis
There is a greater risk of gas penetration through the membrane. Therefore, a thicker membrane is needed from 100 bars.
Areva H2Gen
In 2014, Ademe, Areva and Smart Energies joined forces in a joint venture, Areva H2Gen, which aims to produce PEM electrolyzers with a power output ranging from 100 kilowatts (kW) to 1 megawatt (MW). The first plant was inaugurated in 2016 in the 91. The project also reportedly presented a concept for a PEM electrolysis plant with a production capacity of 60MW. They had presented a 1MW “stack” at the Hannover Fair in 2017. The joint venture was acquired in 2020 by ENGIE subsidiary GTT.
Air Liquide and the world’s largest PEM electrolyzer
In January 2021, Air Liquide inaugurated in Quebec the world’s largest proton exchange membrane electrolysis unit (according to the company): 20MW, or 8.2 tons of low-carbon H2 per day. The energy would be supplied by Hydro-Québec. Siemens would have joined Air Liquide to develop the process on an industrial scale through a memorandum of understanding announced on February 8, 2021.