What Happens Inside a PEM Water Electrolyzer

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Introduction

My last three blogs have focused on hydrogen proton exchange membrane fuel cells (PEMFCs), in particular, their main components, operation, and physical processes. Based on those tutorials, you can already talk to your friends about how a fuel cell is built and what it does; and you can probably already collect some useful data from your own PEMFC stack. If you have been talking about fuel cells with friends, there is a question that, by now, most of you would have had to answer and for which you might want a bit more information: Where will your hydrogen come from? There are many ways to produce hydrogen, and Prof. Ponce De Leon has already written a blog on the colors of hydrogen. This blog will focus on green hydrogen and the main components and physical processes occurring in a proton exchange membrane water electrolyzer (PEMWE).

Before starting our discussion on PEMWE, it is important to know there are several electrochemical technologies to produce green hydrogen form carbon-neutral electricity sources. These include: a) alkaline electrolyzers (AE); proton exchange membrane water electrolyzers (PEMWE); c) alkaline exchange membrane water electrolyzers (AEMWE); and, solid oxide electrolysis cells (SOEC). Alkaline electrolyzers are the most mature technology and they have been in operation since 1950s; however, due to the use of a thick porous separator between the two electrodes, they are not able to operate at high current densities (because ohmic losses would be too high) and high hydrogen pressures (because hydrogen would permeate through the separator and mix with oxygen), and it is also challenging for them to operate under a dynamic load. These attributes are necessary for coupling the electrolyzer to intermittent renewable energies; therefore, alternative technologies were developed. The essential ingredient to enable the three attributes above is the elimination of the porous separator and the introduction of a solid, ion-conducting membrane. PEMWE, AEMWE and SOEC use a proton-exchange membrane (PEM), an anion-exchange membrane (AEMs), and a ceramic capable of transporting oxygen vacancies (O^2-), respectively. SOECs need to operate at temperatures above 600 ⁰C, as only at these elevated temperatures, are the oxygen vacancies mobile and these limits their ability to operate dynamically. PEMWE and AEMWE operate at temperatures below 100 ⁰C, therefore they can operate under dynamic loads. AEMWEs have the potential to be cheaper, as they can use cheaper catalysts, such as Ni and Ni-alloys, but they usually require the use of an alkaline solution, e.g., 1 M KOH solution, and stability of the membrane and catalyst still need further investigation, therefore PEMWE is currently considered the most mature and commercially viable type of electrolyzer. Considering that most electrolyzer architectures are similar (even though the materials and reactions are different), and that PEMWE is the most widespread technology, we will dedicate the rest of the blog to discuss this technology.

 

Overall physical processes

A proton exchange membrane water electrolyzer (PEMWE) is an electrochemical device that uses deionized water and electricity to produce hydrogen in the cathode and oxygen in the anode. The figure below shows the different physical processes that occur inside the cell. Water is consumed only in the anode, therefore liquid water is usually introduced to the distribution channels in this side of the cell, while the cathode is operated without any inlet streams. A mixture of oxygen bubbles and water leave the anode, with the oxygen being separated from the water in a gas disengagement unit (GDU) – mainly a container with a two-phase mixture where only the gas phase is exhausted, while the water is re-circulated. The cathode exhaust contains mainly hydrogen with a small amount of water. The water appears because of water cross-over through the proton conducting membrane due to electro-osmotic drag and diffusion, as discussed in this blog for PEMFC. Hydrogen and oxygen are then generated at the desired pressure by controlling the GDU pressure. When the cell is to produce oxygen and hydrogen at high pressure, it is sometimes convenient to use liquid water in both compartments simply to minimize pressure gradients and water permeation through the membrane. A schematic of the main physical processes and reactions is given in the figure below.

 

The anode and cathode electrochemical reactions, known as the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), are given by:

Anode: 2 H₂0 -> O₂ + 4H+ + 4 e^-

Cathode: 4 H⁺ + 4 e^- -> 2 H₂

Which results in the following overall reaction:

2 H₂O -> O₂ + 2 H₂

Based on the overall reaction, it can be observed that twice as many moles of hydrogen are generated per mole of oxygen and therefore, at a given temperature and pressure, twice the volume of hydrogen gas will be produced, as shown by ideal gas law (pV = nRT, where n is the number of moles). This observation can easily be proven by collecting all the hydrogen and oxygen in a balloon and observing how the hydrogen balloon fills twice as fast as the oxygen one.

Going back to the reactions, those of you that have already read my previous blogs might be quick to note that these reactions are the same as those that occur in a PEMFC, but in reverse. This observation easily leads to the erroneous conclusion that the materials used in a PEMFC and a PEMWE are the same. Even though this is accurate for the cathode electrode, this is not the case in the anode. The anodic reaction has a high half-cell potential, i.e., 1.23 V with respect to the hydrogen electrode at standard atmospheric temperature and pressure, and it is a sluggish reaction; therefore, to operate the electrolyzer very high anodic potentials, about 1.8 to 2 V, are required. These high potentials would quickly corrode any carbon in the electrode, leading to extremely poor durability. In fact, my research team recently took advantage of this property and used carbon black as a pore former as, during operation the carbon would quickly oxidize to carbon dioxide, thereby increasing the porosity of the electrode [Manas22].

 

Main components

Based on the schematic above, a PEMWE cell is mainly composed of two compartments, the anode and the cathode, separated by a proton conducting membrane. As discussed above, while the cathode of a PEMWE usually mimics the design of a PEMFC anode, i.e., it uses a hydrophobic carbon-based gas diffusion layer and a platinum-on-carbon catalyst in the catalyst layer, the PEMWE anode is remarkably different and therefore, it is the focus of this section. The membrane used in PEMWE cells is usually made of Nafion, as it is the case in PEMFCs; however, since the membrane must support larger pressure and concentration gradients, a thicker membrane is usually used to prevent mechanical damage and minimize hydrogen cross-over. While PEMFCs usually use 5-20 µm membranes, it is not uncommon for electrolyzers to use 50 – 150 µm membranes, e.g., Nafion N112 and N115. The thicker membrane results in larger ohmic losses due to proton transport, as discussed in our second blog.

The anode compartment of a PEMWE cell needs to be completely re-designed to eliminate any materials that might corrode or oxide at high potentials. This essentially includes all components, i.e., flow field plates, the distribution layer, and the catalyst layer. While most flow fields in PEMFC and the cathode of a PEMWE can be manufactured using a carbon composite, the PEMWE anode flow field is usually made of titanium, as it is corrosion resistant. Unfortunately, the titanium corrosion resistance stems from the formation of a titanium oxide passivation layer which is not very conductive; therefore, in some cases, the titanium plate might be coated with platinum to decrease contact resistances.

The conductive porous layer responsible for transporting the reactants and products from the channels to the reaction sites also needs to be corrosion resistant. Therefore, a porous layer made of platinum coated titanium is usually used. In this case, the layer transports liquid water while allowing for oxygen bubbles to escape the catalyst layer; therefore, the layer is commonly known as porous transport layer (PTL), since it transports both gases and liquids by convection, instead of diffusion. The Pt-coated titanium layer is hydrophilic, therefore it can easily wet and transport water. The fact that the PTL is made of titanium has some disadvantages, such as very high cost, but also one advantage, it allows for flexibility in the manufacturing process. As a results PTLs can be made of titanium either meshes/felts, where titanium fibers are intertwined to form a porous paper, or sinters, where titanium particles are sintered together by heating them either in the oven or using a laser beam. The latter process allows for functionally graded layers to be formed which can improve the contact resistance between layers [Schuler19].

PEMWE CLs are usually made of unsupported iridium oxide (IrO_x) as it provides a good balance between catalytic activity and resistance to dissolution. A catalyst ink, made of IrOx catalyst and 5-15% wt ionomer solution, is usually deposited, using film applicators, spray coating or inkjet printing, onto the PEM to make the porous electrode. Even though iridium is a very scarce material, usually obtained as a by-product of platinum mining [Clapp23], its use in PEMWE is not capricious. It turns out it is one of the only metals that provides high activity towards the OER while providing reasonable resistance to dissolution [McCrory13]. Platinum for example is not very active towards the OER as it will quickly oxidize and passivate. Even though one might think that there might not be a large selection OER catalyst, considering only one metal can be used, iridium oxide can form in several oxidation states and varying shapes, therefore catalyst selection is still of paramount importance with iridium oxyhydroxide exhibiting high activity but accelerated dissolution and iridium dioxide exhibiting poor activity [Tan19]. Finally, you might be wondering, if iridium is so expensive, why it is not supported on another materials, like the case of platinum on carbon. Again, due to the high potentials necessary for the OER, most materials would corrode, and the few materials left, such as tin oxide, have poor conductivity; therefore, the quest for novel catalyst and catalyst supports continues to be an active area of research.

 

Conclusion

In summary, in a net-zero energy system, green hydrogen will be produced using electricity from wind and solar power plants using an electrolyzer. There are several types of electrolyzer that could be used, but today PEMWE appears to be the most popular due to its ability to operate at high current density, efficiency and hydrogen pressures, and its ability to handle dynamic loads. The main physical processes occurring in a PEMWE are proton transport through a gas impermeable membrane, hydrogen evolution reactions in the cathode, and oxygen evolution in the anode. This understanding of the physical processes inside the cell allowed us to identify its main components and to understand that even though the cathode and the membrane would be similar to a PEMFC, the electrolyzer anode needed to be re-designed. We then showed that the anode of a PEMWE is usually made of unsupported Iridium oxide catalyst, as it is both active to the OER and resistant to dissolution. Furthermore, to transport water effectively to the reaction sites and prevent either corrosion or oxidation, the anode porous transport layer is made of platinum coated titanium.

 

References:

[Clapp23] M. Clapp, C. Zalitis, M. Ryan, Perspectives on current and future iridium demand and iridium oxide catalysts for PEM water electrolysis, Catalysis Today, 420, 2023.

[Mandal22] M. Mandal, M. Secanell, Improved polymer electrolyte membrane water electrolyzer performance by using carbon black as a pore former in the anode catalyst layer, Journal of Power Sources 541, 231629, 2022.

[McCrory13] C. McCrory, S. Jung, J. Peters and T. Jaramillo, Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction, Journal of the American Chemical Society 2013 135 (45), 16977-16987, 2013.

[Schuler19] T. Schuler et al., Hierarchically Structured Porous Transport Layers for Polymer Electrolyte Water Electrolysis, Advanced Energy Materials, 10(2):1903216, 2019.

[Tan19] X. Tan, J. Shen, N. Semagina, M. Secanell, Decoupling structure-sensitive deactivation mechanisms of Ir/IrOx electrocatalysts toward oxygen evolution reaction, Journal of Catalysis 371, 57-70, 2019.