PEM electrolyzer

Introduction of PEM electrolyzer

The Proton Exchange Membrane (PEM) water electrolyzer technology has its roots in the middle of the 20th century, when fuel cells were first created.

A fuel cell is an electrochemical cell that converts the chemical energy from a fuel into electricity through an electrochemical reaction with oxygen or another oxidizing agent. During this time, PEM fuel cells were created. These cells use a solid polymer electrolyte, also known as a proton exchange membrane. They were mainly made for NASA’s space missions in the 1960s. The technology provided a reliable and efficient power source for spacecraft.

The PEM electrolyzer essentially operates as a reverse fuel cell. It doesn’t make electricity; instead, it uses electricity to split water into hydrogen and oxygen. The proton exchange membrane allows protons to pass through it while electrons travel around an external circuit and oxygen is produced at the anode. Hydrogen is then produced at the cathode. This process is known as water electrolysis.

The improvements in membrane technology that happened in fuel cell research had a big impact on the creation of PEM electrolyzers. In the 1950s, sulfonated polystyrene membranes were made, which were the first big step toward PEM electrolyzers. These membranes were not stable enough to be useful, though.

When DuPont made Nafion in the late 1960s, it was a big step forward. Nafion is a fluoropolymer-copolymer that is based on sulfonated tetrafluoroethylene. At first, it was made for military use, but its long life, high chemical stability, and good proton conductivity quickly made it the best material for PEM fuel cells and then for PEM electrolyzers.

In the 1990s and 2000s, PEM electrolyzers started to be sold commercially. This was because of the growing need for clean energy technologies and efficient, scalable ways to make hydrogen. PEM electrolyzers are getting better at what they do, last longer, and cost less thanks to improvements in membrane technology, electrode materials, and system design.

Today, PEM electrolyzers are at the forefront of hydrogen production, especially where there is access to renewable energy sources. They are considered a critical technology for the transition to a hydrogen economy, enabling the storage of renewable energy in the form of hydrogen gas, which can be used in fuel cells for electricity generation, transportation, and various industrial processes.

How PEM electrolyzer produces hydrogen

A PEM water electrolyzer consists of an anode and a cathode, which are separated by a proton exchange membrane (PEM). The anode side is in contact with pure water. Unlike an alkaline electrolyzer, which uses an alkaline electrolyte, the PEM electrolyzer utilizes this solid polymer electrolyte (or PEM) to conduct protons from the anode to the cathode while insulating the electrodes electrically. This not only allows for a more compact design but also enables the electrolyzer to operate at higher pressures and efficiencies.

The diagram below depicts how PEM water electrolysis produces hydrogen and oxygen.

How PEM water electrolysis produces hydrogen.
How PEM water electrolysis produces hydrogen.

At the anode, water is oxidized to produce oxygen gas (O₂), protons (H⁺), and electrons (e⁻), according to the half reaction:

H₂O → 1/2O₂ + 2H⁺ + 2e⁻

The protons are conducted through the membrane to the cathode, where they combine with electrons to produce hydrogen gas (H₂) via the half reaction:

2H⁺ + 2e⁻ → H₂

This process is driven by an external power source, typically from renewable energy sources to ensure the hydrogen produced is green.

PEM electrolyzers are capable of operating at elevated temperatures, ranging from 110 ºC to 150 ºC. Increasing the operating temperature is advantageous. It can improve system efficiency by enhancing electrode kinetics and reducing overpotentials. It can also facilitate heat recovery for secondary applications due to the higher temperature differences.

PEM electrolyzers can operate at higher current densities than alkaline water electrolyzers. This is advantageous for compact systems that produce hydrogen in large quantities. The utilization of high current density operations results in decreased capital costs due to the reduction in electrode area that is necessary. On the other hand, it makes a lot of heat, which can make the temperature inside the cell not spread out evenly. Understanding and managing the internal cell temperature distribution is crucial.

Hydrogen can be produced directly by PEM electrolyzers at pressures of 50 bar (5 MPa) or higher. This eliminates the need for external hydrogen compressors and reduces system complexity.

PEM electrolyzers are capable of producing hydrogen of exceptional purity. The solid membrane ensures that the hydrogen generated is free from contamination, a critical factor for applications in the food industry, electronics manufacturing, and fuel cell vehicles, where even trace amounts of impurities can be detrimental.

PEM electrolyzer cell components

The diagram below illustrates the components of a PEM water electrolyzer.

The components of a PEM electrolyzer cell.
The components of a PEM electrolyzer cell.
  • Flow field plate

Flow field plates have patterned channels which help distribute gasses and liquids. They are typically made from anti-corrosive materials. Titanium is generally considered the most advanced material due to its excellent strength, low resistivity, and high thermal conductivity. However, titanium can corrode at high potentials, so a thin platinum coating may be applied to reduce surface resistance.

  • Membrane electrode assembly (MEA)

The MEA consists of a PEM coated with porous electrocatalyst layers on both the anode and cathode sides. The PEM is thick (above 125 um) and is made of perfluorosulfonic acid (PSFA) polymers, like Nation® or Aquivion®.

There are two main methods for preparing the catalyst layers on the PEM: the catalyst-coating substrate (CCS) method and the catalyst-coating membrane (CCM) method. The CCM method, which directly coats the catalyst active components on the PEM, is the most common due to its higher catalyst utilization rate and reduced proton transfer resistance.

  • Anode

The anode is where water oxidation occurs, producing oxygen, protons, and electrons. Due to the high oxidation potential at the anode, materials that can withstand this environment are required.

Titanium is often used for current collectors at the anode because of its resistance to corrosion. However, titanium can still suffer from corrosion at potentials exceeding 2V, leading to the formation of surface oxides that increase contact resistance and reduce thermal conductivity. To mitigate this, a thin platinum coating can be applied to the titanium to reduce surface resistance.

Iridium oxide (IrO₂) is generally considered the most advanced catalyst for the anode in PEM water electrolysis. It has slightly lower activity than ruthenium oxide (RuO₂) but offers the advantage of higher corrosion resistance, which is crucial for the anode’s durability.

  • Cathode

The cathode is where protons gain electrons and are reduced to form hydrogen gas. Platinum (Pt) and Pt-based materials are recognized as the state-of-the-art electrocatalysts for the cathode side in PEM electrolyzers. They exhibit excellent electrochemical activity for the Hydrogen Evolution Reaction (HER) in acidic electrolytes.

The large-scale deployment of platinum-based catalysts is constrained by their high cost and limited reserves. Therefore, significant research efforts are directed towards reducing the platinum mass loading and exploring alternative Pt-free catalysts.

  • Gas diffusion layer (GDL)

The GDL, also known as the current collector, ensures efficient mass transfer of liquids and gasses between the electrodes and the bipolar plate or flow field plate. The GDL is typically made of materials that provide good electronic conductivity and facilitate the transport of reactants and products to and from the catalyst layers.

The diagram below depicts the working mechanism of a PEM water electrolyzer cell.

The working mechanism of a PEM electrolyzer cell
The working mechanism of a PEM electrolyzer cell

Water is brought to the anode side of the electrolyzer cell, typically in a deionized form to prevent contamination and ensure efficient electrolysis.

At the anode, water molecules are oxidized. The oxidation of water results in the formation of oxygen gas (O₂), protons (H⁺), and electrons (e⁻). The protons produced at the anode are conducted through the proton exchange membrane to the cathode side of the cell.

At the cathode, the protons combine with electrons that have traveled through the external circuit from the anode. This reaction is facilitated by platinum-based catalysts. The combination of protons and electrons at the cathode results in the formation of hydrogen gas (H₂).

PEM electrolyzer stack

A PEM electrolyzer stack is composed of multiple cells, each consisting of a Membrane Electrode Assembly (MEA) sandwiched between bipolar plates, as shown in the diagram below.

PEM electrolyzer stack
PEM electrolyzer stack

Here’s the key components within a PEM electrolyzer stack:

  • Membrane electrode assembly
  • 2. Electrodes
  • 3. Bipolar plates

Serve as separators between individual cells within the stack, facilitating electrical connection and gas distribution. They are often made of corrosion-resistant materials like titanium, sometimes coated with platinum to reduce surface resistance.

  • Gas diffusion layers
  • Current collectors

Made of materials like titanium, these components facilitate the transfer of electrons from the external circuit to the electrodes within the electrolyzer.

  • Gaskets

Ensure the sealing of individual cells within the stack, preventing gas leaks and ensuring that the reactants and products are properly contained and directed.

  • End plates

Provide mechanical support and compression to the stack, ensuring that all components are held tightly together to maintain good electrical contact and efficient operation.

  • Water supply system

Delivers deionized water to the anode side of the electrolyzer.

  • Cooling system

Maintains the optimal operating temperature of the electrolyzer stack, ensuring efficient and stable operation.

PEM electrolyzer operates with renewable energy sources

PEM electrolyzers are a critical technology for producing low-emission hydrogen from renewable or nuclear electricity. They are particularly well-suited for integration with renewable energy sources due to their fast response times and production flexibility, which is ideal for handling the variable nature of renewable power sources like solar and wind.

The electrolyzers can quickly start and stop, making them compatible with the intermittent and fluctuating power supply from renewable energy systems. This capability is essential for maximizing the use of solar and wind energy, which can vary significantly due to changes in solar irradiance and wind speeds.

How PEM electrolyzer produces hydrogen at $1/kg H2

The U.S. Department of Energy (DOE) has set technical targets for PEM electrolysis to achieve the central goal of low-cost hydrogen production of $2/kg H2 by 2026 and $1/kg H2 by 2031. These targets include improvements in stack performance, electrical efficiency, lifetime, and capital cost.

Technical targets for PEM electrolyzer stacks
Technical targets for PEM electrolyzer stacks (from DOE).

According to the DOE, the cost of hydrogen production from PEM electrolysis can be reduced by leveraging low-cost, clean electricity (e.g., ≤$0.03/kWh) with high availability (e.g., >90% capacity factor).

Achieving a production cost of $1/kg H2 for hydrogen from PEM electrolyzers involves several strategies and advancements, as outlined below:

  1. Increased efficiency/current density

Improving the efficiency of the electrolysis process and operating at higher current densities can reduce the amount of electricity required per kilogram of hydrogen produced, thereby lowering the overall production cost.

  1. Decreased platinum loading

Reducing the amount of expensive catalyst materials, such as platinum and iridium, in the electrolyzer can significantly decrease the capital costs associated with the stack.

  1. Scale-up

Manufacturing electrolyzers at larger scales can lead to economies of scale, reducing the per-unit cost of the electrolyzer components and the overall system.

  1. Advanced materials and manufacturing techniques

Utilizing new materials that are less expensive or more abundant and adopting advanced manufacturing techniques can further reduce costs.

  1. Optimized operation with renewable energy

Operating electrolyzers during periods of low-cost electricity, such as when there is excess renewable energy available, can reduce the cost of the electricity input, which is a major component of the hydrogen production cost.

We researched some startups that focus on green hydrogen production technologies. You can become a member and check out our research.

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