Hydrogen (H2) is recognized as a clean energy source for transportation, power generation, aviation, and shipping. In fuel cells, it produces only electricity with only heat and water as by-products, making it a key solution for reducing emissions across various sectors. There are also ongoing efforts to minimize the carbon footprint of H2 production since traditionally, H2 is made from fossil fuels like natural gas and coal through steam methane reforming and coal gasification. These methods are carbon-intensive and less sustainable. Cleaner options include producing H2 from biomass, which can lower emissions depending on the source, or using renewable energy like wind or solar to power electrolyzers for a near-zero carbon process.
H2 Production through Water Electrolysis
H2 produced through water electrolysis is a key component in the green transition toward renewable energy. Because of this, this technology has been experiencing significant growth, particularly in the 21st century. Projections indicate that the total installed capacity of water electrolyzers would be upwards of 134 GW by 2030. That corresponds to a whooping 13300% increase from the 2022 capacity of around 1 GW.
Formally, water electrolysis is the process of splitting water into its constituent gases, H2 and O2, through the application of a direct current. Water splitting can be performed under different reaction conditions. It can occur either in alkaline or acidic conditions and at low (<100 °C) and very high temperatures (>500 °C). Water electrolysis cells can be broadly classified into four categories: alkaline, proton exchange membrane (PEM), alkaline anion exchange membrane (AEM), and solid oxide electrolysis cells (SOEC). Each type involves different electrode reactions, depending on the type of charge transported through the electrolyte.
In this blog article, we will focus on the low-temperature electrolyzer technologies: alkaline, proton exchange membrane (PEM), alkaline anion exchange membrane (AEM) water electrolyzers. SOECs are on an entirely different plane, temperature-wise. We will explore the differences between these types, discussing their operating mechanisms and material compatibility, and highlighting some of the advantages and disadvantages of each.
H2 Production with Alkaline Water Electrolyzers (AWEs)
Alkaline water electrolyzers (AWEs) are the most mature and commercially developed hydrogen production technology. In AWEs, water splitting occurs through the following half-reactions at the anode and cathode:
Anode 2 OH–(aq) → ½ O2(g) + H2O(l) + 2e–
Cathode 2 H2O(l) + 2 e– → H2(g) + OH–(aq)
These reactions take place on electrode surfaces, which are separated by a porous diaphragm that allows ions to pass while preventing the mixing of the product gases. In AWEs, the electrolyte is a liquid caustic solution, typically of NaOH or KOH, pumped through the system. This makes the operating conditions in AWEs basic or alkaline, hence the name.
The first developed AWEs used solid, nonporous nickel-based electrodes. Since AWEs operate in alkaline conditions, nickel and other transition metals are ideal electrode materials due to their excellent corrosion resistance and good electrocatalytic activity for hydrogen and oxygen reactions in alkaline environments. Nickel-based materials, such as Ni foams, Ni wire meshes, sintered Ni powders, and sintered Ni fibers, are also more affordable and abundant than precious metals, making AWEs a cost-effective option for large-scale hydrogen production. To learn more about choosing porous transport layers or electrodes for water electrolyzers, check out our previous blog, Benchmark Porous Transport Layers for Water Electrolyzers.
However, a downside of its construction that in traditional AWEs, the produced H2 and O2 gases are removed from the stack through the electrolyte-filled gap between the electrodes and the porous diaphragm separator. Due to this design, the interelectrode distance in alkaline water electrolysis cells can be several millimeters. As a result, the area-specific resistance (ASR) exceeds 1–2 Ω cm². While I won’t go into the details of ASR here, it is important to note that any resistance that hinders current flow in electrochemical cells is undesirable. Case in point, the high ASR limits the operating current density of AWEs to just 200–400 mA/cm2. This highlights a key disadvantage of AWEs: their low operating current densities, which in turn leads to a limited H₂ output—only around 100–200 N mL of H₂ per cm² per hour. As we’ll see later, this limitation also affects the size of alkaline water electrolyzer stacks.
Another challenge with AWEs is the high concentration of the liquid electrolyte, which can be as corrosive as 20–30% or 5–7 M KOH. While the electrodes can handle this, the porous diaphragm separating them may not. Older AWE stacks used very thick asbestos diaphragms to withstand the harsh conditions, but this increased ASR, further limiting the current density. All these cause the hydrogen production efficiency of industrial AWEs to be low at 62%, 68% would be pushing it. There is still hope, though, as advancements in materials and system designs offer great potential to improve AWE performance.
H2 Production with Proton Exchange Membrane Water Electrolyzers (PEMWEs)
The limitations of AWEs have inspired significant advancements in electrolyzer stack designs. One approach to reduce the high ASR caused by the gas removal design is to decrease the distance between the electrodes. However, to implement this, the water electrolyzer needs to be redesigned so that product gases are removed from the back of the electrodes rather than the gap between the separator and electrodes. This concept led to the zero-gap design of proton exchange membrane water electrolyzers (PEMWEs). PEMWEs operate under highly acidic conditions (pH <2), where water splitting occurs through the following half-reactions:
Anode H2O → 2 H+ + ½ O2 + 2 e–
Cathode 2 H+ + 2 e– → H2
In a stark contrast to alkaline water electrolyzers, PEMWEs use a solid polymer electrolyte membrane instead of a liquid electrolyte. These proton-conducting ion exchange membranes, first developed in the 1960s for fuel cell applications (Check here: proton exchange membranes for fuel cells), were instrumental in early space exploration missions. Over time, their design and functionality were improved to make them suitable for electrolysis. We have previously covered these ion exchange membranes in detail in our article, Choosing the Right Ion Exchange Membrane for Your Water Electrolyzer and Fuel Cell Applications.
This ion exchange membrane is sandwiched between the cathode and anode porous transport layers (PTLs), which help deliver reactants, transport electrons, and remove product gases. The benchmark cathode PTLs for PEMWEs are typically carbon-based, such as graphitized carbon fiber panels or plates, or carbon cloths. The anode PTLs, on the other hand, are made of titanium (either sintered titanium fibers or titanium powders) because they need to withstand the acidic and highly oxidizing conditions present during operation. The PTLs are tightly held to the membrane, creating a “zero-gap configuration,” which helps minimize ASR. This design also allows the product gases to be removed through the backside of the PTLs, unlike in alkaline water electrolyzers, where gases are removed from the space between the separator and electrode.
Effects of the Zero-Gap Configuration in Proton Exchange Membrane Water Electrolyzers
The “zero-gap configuration” of PEMWEs has two significant implications:
Higher Operating Current Densities in PEMWE and More Compact Stacks
First, the area specific resistance in PEMWE is significantly minimized in comparison to that of AWE. What this means is that PEMWE can operate and achieve higher current densities. For reference, the commercial operating current density of PEMWE ranges from 2–3 A/cm2, with some reporting as high as 20 A/cm2. AWE’s current density, as mentioned above, is just 0.2–0.4 A/cm2. What this further implies is that to produce the same amount of hydrogen, for example, at 2 A, assuming similar efficiencies, an AWE stack would need to be 10 times larger in area than a PEMWE stack. This highlights how PEMWE can achieve the same results with much more compact systems.
Need for Catalysts, Specifically Platinum Group Metals, for PEMWE Stacks
In alkaline water electrolyzers, reactions typically occur on the surface of metallic electrodes. In PEMWEs, there is no gap between the electrodes and the membrane. To match the high operating current density of PEMWEs, there must be enough contact surface between the electrodes and the membrane. Unfortunately, the contact surface is often insufficient. To address this, catalyst layers are applied either on the membrane or the porous transport layers. These active layers are typically composed of catalyst powder and an ion-exchange binder.
This is where a particular disadvantage of PEMWE becomes apparent. Since PEMWE operates under highly acidic conditions, the catalysts that can be used are often limited to platinum group metals (PGMs), such as iridium, ruthenium, and platinum. This significantly increases both the capital expenditures (CAPEX) and operational expenditures (OPEX) of PEMWEs. Not only are PGM catalysts expensive, but they are also rare. In stark contrast, alkaline water electrolyzers (AWEs) can use more affordable and abundant nickel catalysts and electrodes, making them a less costly option.
This has got scientists and engineers thinking. We should have a technology that combines the advantages of AWE and PEMWE. That is, a technology that has a compact configuration and does not require the use of PGM metal catalysts. This explains the emergence of a new electrolyzer technology: the alkaline anion exchange membrane water electrolyzer.
H2 Production with Alkaline Anion Exchange Water Electrolyzers (AEMWEs)
Alkaline anion exchange membrane water electrolysis (AEMWE) is an emerging technology that uses an anion exchange membrane. The white paper, “Hydrogen Production Cost by AEMION+®,” compares the investment costs of AEMWE systems with those of AWE and PEMWE systems. At the 1 MW scale, AEMWE systems using AEMION+® are much cheaper, costing $931/kW without PGM catalysts and $926/kW with PGM catalysts, compared to AWE ($1279/kW) and PEMWE ($1168/kW). Even at the 5 MW scale, AEMWE remains the most cost-effective, with costs of $444/kW and $459/kW.
Central to AEMWE technology is the anion exchange membrane. In this respect, Ionomr’s AEMION+® membranes, AEMION+™-AF3-HWC9-70 and AEMION+™ – AF3-HWK9-75, show great promise, offering exceptional stability even under harsh conditions (90 °C in 2 M KOH). Unlike proton exchange membranes, which conduct H+ ions, AEMs transport OH– ions. As a result, AEMWE operates under the same reactions as alkaline water electrolyzers, but with a theoretically more compact stack due to its zero-gap structure. Talk about the best of both worlds, right?
Anode 2 OH–(aq) → ½ O2(g) + H2O(l) + 2e–
Cathode 2 H2O(l) + 2 e– → H2(g) + OH–(aq)
However, unlike PEMWE and AWE, AEMWEs are generally considered to be at a Technology Readiness Level (TRL) between 4 and 6. This means they are still in the prototype development stage, with lab-scale demonstrations and validation ongoing, but are not yet fully commercialized at a large scale.
Cheat Sheet: Comparing Alkaline, Proton Exchange Membrane, and Anion Exchange Membrane Water electrolyzers
Comparing Water Electrolysis Technologies: PEM, Alkaline, and AEM
TLDR: The cheat sheet below compares alkaline, proton exchange membrane, and anion exchange membrane water electrolyzers, highlighting key differences in technology, materials, and efficiency.
Alkaline vs Proton Exchange Membrane vs Anion Exchange Membrane Water Electrolyzers
CAPLINQ Solutions for Water Electrolyzers
At CAPLINQ, we understand that each type of water electrolyzer—whether it is alkaline, proton exchange membrane, or anion exchange membrane—comes with its own set of challenges. While no single technology is perfect, advancements in materials and scalability are bringing us closer to bridging those gaps. We offer a wide range of solutions, including carbon and metal porous transport layers (nickel, titanium, stainless steel), cation and anion exchange membranes, flat gaskets for sealing, ion exchange binders for catalyst layers, and even hydrogen detection tapes for safety. Contact us at CAPLINQ today and let us work together to find the right materials for your needs.
