PEM electrolysis is a technology for producing hydrogen. The article explains how it works, its advantages over other electrolysis technologies, technical challenges and how system simulation can be used to optimize the process and the plant.
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PEM electrolysis is a technology for producing hydrogen from water and electricity by electrolysis. The physical concept of electrolysis was discovered by Alessandro Volta in 1800 and has been used in the chemical and metal industries for many decades. In recent years, hydrogen (H2) has increasingly become the focus of research and industry as a possible energy carrier in a decarbonized energy system. Possible applications include use in a fuel cell or as a basic chemical for the synthesis of more complex molecules (known as PtX process).
Currently, hydrogen is mainly produced from natural gas using steam reformation. Electrolysis is a technology that does not require natural gas. This means that no CO2 is emitted during the production of hydrogen if the electricity requirement is covered by renewable energies.
There are various technologies for implementing electrolysis. The best known are AEL, SOEC and PEM electrolysis. The abbreviation PEM stands for either Polymer Electrolyte Membrane or Proton Exchange Membrane. In this technology, water molecules (H2O) are separated at the anode of an electrochemical cell using an electric current. This produces oxygen and H+ ions (protons). These ions cross the membrane and react at the cathode to form hydrogen. The process is shown graphically in the following diagram.
In industry, this technology is used in PEM electrolyzers. The heart of such an electrolyzer is the electrolysis stack, which consists of several interconnected electrolysis cells. One such cell is shown in Figure 1. This design with many different cells makes it possible to easily scale the electrical output of the system.
In the electrolysis cell, water is separated into oxygen and H+ ions at the anode as described above and this reaction follows the following reaction equation: $${H_{2}}O \rightarrow 0.5 O_{2} + 2 {H^{+}} + 2 {e^{-}}$$ At the cathode, the H+ ions and the electrons react to form hydrogen: $$2 {H^{+}} + 2 {e^{-}} \rightarrow H_{2}$$ The overall reaction balance is therefore: $${H_{2}}O \rightarrow 0.5 O_{2} + {H_{2}}$$ The reactions are made possible by the different electrochemical potentials of the fluids at the anode and cathode. These different potentials are only possible because the membrane separates the two fluids.
From the reaction balances at the anode and cathode, it can be seen that 2 electrons must flow from the anode to the cathode for the reaction to take place. This flow of electrons is only possible if the difference in electrochemical potentials is balanced out by applying an electrical voltage: the cell voltage. The cell voltage required to operate the electrolysis in the ideal case (=no losses) is called the reversible cell voltage (URev). It is described by the following equation: $${U_{Rev}}= \frac{\Delta G }{ z\cdot F}$$ Where ∆G is the difference in Gibbs free enthalpy between reactants and products, z is the number of electrons transferred and F is Faraday's constant.
In practice, however, losses occur at various points in the electrolysis cell. These losses are referred to as voltage losses. The cell voltage can be calculated in the non-ideal case using the following equation: $${U_{cell}}=U_{Rev}+ \sum \Delta {U_{losses}}$$
A distinction is usually made between 3 different types of voltage losses:
These 3 types of losses depend on the design of the electrolysis cell and the operating conditions.
A common method of illustrating the electrical properties of a cell is the polarization curve, which is shown in the graph below. It shows the cell voltage as a function of the current. It can be seen that the cell voltage increases as the current increases.
The reversible voltage can be read off the y-axis and the efficiency of the electrolysis cell is calculated using the following formula: $$η = U / {U_{Rev}} $$ A higher cell voltage leads to a correspondingly poorer efficiency.
If the efficiency of a PEM stack is less than 100%, heat is generated there. Due to these heat losses, the electrolyzer must be cooled. Each manufacturer uses different concepts for this. The cooling of the stack has a direct influence on the achievable efficiency, as the cell voltage is also related to the cell temperature. Furthermore, many components are designed for operation in certain temperature ranges. Operation outside this range generally shortens the service life of the components.
When the electrolyzer is operated, water is pumped through the anode. The electrolysis creates oxygen there, so that a two-phase water/oxygen mixture finally leaves the anode. In order to be able to pump the unused water through the electrolyzer again, the oxygen must first be removed from it. This is done in a suitable gas separator.
When H+ ions cross the PE membrane, their charge draws water molecules with them to the cathode side of the electrolysis cell.
Often several water molecules per ion!!
This also produces a two-phase mixture of water and hydrogen at the cathode, which must be purified in gas separators. On the one hand, the aim is to produce hydrogen that is as dry as possible for most applications; on the other hand, the unused water should also be pumped through the cell again. As a small proportion of hydrogen is absorbed by the water, several separation stages are often used in practice to guarantee a high level of water purity. The aim is to prevent hydrogen dissolved in the water from being released again at the anode, and create the explosive oxygen/hydrogen mixture.
For certain applications, such as PEM fuel cells, high-purity hydrogen must be produced. This particularly high purity cannot be achieved with normal phase separators. In this case, adsorption systems are often used. Depending on the manufacturer, either temperature or pressure swing adsorption is used. We have explained the physics behind adsorption in another blog article: Simulation of Adsorption Processes – Modeling Basics.
In addition to these fluid-related components, various components are also provided in the electrolyzer for coupling to the electrical grid. These are, for example, the rectifier and the transformer.
PEM electrolysis is not yet very widespread, as it is more expensive than AEL electrolysis. This is mainly due to the fact that iridium- and platinum-based catalysts are used for the electrodes. Accordingly, active research is being carried out into how these catalysts can be replaced or at least how their consumption in a PEM electrolyser can be reduced. PEM electrolysis is an interesting technology above all because it enables faster start-up and shut-down times compared to other technologies and thus enables more synergies when coupled with volatile renewable power generation systems such as wind or photovoltaic systems.
In order to design PEM electrolyzers correctly, a good understanding of the dynamic behavior of the system is required. A simple tool for this is system simulation, which enables engineers to analyze the interaction of the different components under variable framework conditions.
At TLK Energy, we have developed the PSL (Process Systems Library) model library for this purpose. This library is implemented in the Modelica modeling language and enables the user-friendly simulation of complex Power-to-X systems.
The models created can also be used to develop a suitable control strategy, e.g. using HiL technology. At the International Modelica Conference 2023, we presented our work on the modeling and control of PEM electrolyzers. The model used for this is shown in Figure 3.
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