Hydrogen production by water electrolysis involves dissociating water molecules into hydrogen and oxygen through an electrochemical process under the influence of direct current, with hydrogen and oxygen being separated at the cathode and anode, respectively. Based on the type of diaphragm used, water electrolysis can be categorized into alkaline water electrolysis, proton exchange membrane (PEM) water electrolysis, and solid oxide water electrolysis.
The industrial application of water electrolysis technology began in the 1920s. Alkaline water electrolysis technology has achieved industrial-scale hydrogen production, meeting the needs of industries such as ammonia production and petroleum refining. After the 1970s, energy shortages, environmental pollution, and the demands of space exploration spurred the development of proton exchange membrane water electrolysis technology. Concurrently, high-pressure and compact alkaline water electrolysis technology was designed to meet the requirements of specific fields.
Alkaline Liquid Electrolyzer For Water Electrolysis
Alkaline liquid water electrolysis technology uses KOH and NaOH aqueous solutions as the electrolyte, with materials like asbestos cloth serving as the diaphragm. Under the action of direct current, water is electrolyzed to produce hydrogen and oxygen. The resulting gases then require treatment to remove alkaline mist. Alkaline liquid water electrolysis was industrialized in the mid-20th century. The structural characteristics of alkaline electrolyzer cells include the presence of liquid electrolytes and porous separators.
Typically, alkaline liquid electrolyte electrolyzers operate at a current density of about 0.25 A/cm², with an energy efficiency of around 60%. In liquid electrolyte systems, the alkaline electrolyte, such as KOH, reacts with CO₂ in the air to form carbonates like K₂CO₃, which are insoluble under alkaline conditions. These insoluble carbonates can clog the porous catalytic layer, hinder the transfer of products and reactants, and significantly reduce the electrolyzer’s performance. Additionally, alkaline liquid electrolyte electrolyzers face challenges in shutting down or starting up quickly. The hydrogen production rate is also difficult to adjust rapidly because the pressure on both sides of the anode and cathode must be balanced at all times to prevent the mixing of hydrogen and oxygen gases, which could lead to an explosion due to the porous asbestos membrane. As a result, alkaline liquid electrolyte electrolyzers are not well-suited to work with renewable energy sources that have fast fluctuation characteristics.
Hydrogen Production From Solid Polymer Water Electrolysis
Due to the many issues that still need improvement in alkaline liquid electrolyte electrolyzers, the rapid development of solid polymer electrolyte (SPE) water electrolysis technology has been encouraged. The first practical application of SPE technology is the proton exchange membrane (PEM), also known as the PEM electrolyzer.
In PEM electrolyzers, the asbestos membrane used in alkaline liquid electrolyte systems is replaced by a proton exchange membrane, which conducts protons and separates the gases on either side of the electrode. This eliminates the disadvantages associated with strong alkaline liquid electrolytes used in traditional alkaline electrolyzers.
Additionally, PEM water electrolysis cells utilize a zero-gap structure, which makes the electrolytic cell more compact and streamlined, reducing ohmic resistance and significantly enhancing overall performance.
The current operating density of PEM electrolyzers typically exceeds 1 A/cm², which is at least four times higher than that of alkaline water electrolyzers. This makes PEM technology one of the most promising options for hydrogen production.
Main Components of a Typical PEM Hydrogen Electrolyzer
The primary components of a typical PEM water electrolysis cell include cathode and anode end plates, cathode and anode gas diffusion layers, cathode and anode catalytic layers, and the proton exchange membrane.
- End Plates: These fix the components of the electrolytic cell, guide the transfer of electricity, and distribute water and gas.
- Diffusion Layer: This layer collects the current and facilitates the transfer of gas and liquid.
- Catalytic Layer: The core of this layer consists of a catalyst, an electron conduction medium, and a proton conduction medium. The three-phase interface formed here is crucial for the electrochemical reaction.
- Proton Exchange Membrane: This is typically a solid electrolyte, commonly made from a perfluorosulfonic acid membrane, which isolates the gases generated at the cathode and anode, prevents electron transfer, and facilitates proton transfer.
The principle of hydrogen production through proton exchange membrane water electrolysis is illustrated in Figure 2. Commonly used proton exchange membranes include Nafion® (DuPont), Dow membrane (Dow Chemical), Flemion® (Asahi Glass), Aciplex®-S (Asahi Chemical Industry), and Neosepta-F® (Tokuyama).
Compared to alkaline water electrolysis, the PEM water electrolysis system does not require dealkalization and offers a more significant pressure regulation margin. Initially, the cost of PEM technology is primarily concentrated in the PEM cell itself. In PEM water electrolysis cells, the membrane electrode assembly, comprising the diffusion layer, catalytic layer, and proton exchange membrane, is where the electrolysis reaction occurs, making it the core component of the electrolyzer. Increasing the current operating density can reduce the equipment investment in electrolysis, and the ability to operate over a wide range of current densities makes PEM systems more compatible with the variability of renewable energy sources.
Polarization
Due to the presence of polarization, the actual electrolysis voltage of the electrolyzer exceeds the theoretical electrolysis voltage ErevE_{rev}Erev predicted by thermodynamics.
The polarization of the electrolyzer includes activation polarization, ohmic polarization, and concentration polarization. In the PEM water electrolysis electrode reaction, the polarization of the anodic oxygen evolution reaction is significantly higher than that of the cathodic hydrogen evolution reaction, which is a crucial factor affecting the efficiency of electrolysis.
Electrochemical Polarization
Electrochemical polarization is primarily related to the activity of electrocatalysts. Selecting highly active catalysts and improving the three-phase interface of the electrode reaction can help reduce electrochemical polarization. Additionally, during the water electrolysis reaction, the evolution of hydrogen and oxygen—especially the precipitated atomic oxygen—exhibits strong oxidizing properties. This imposes high demands on the oxidation and corrosion resistance of the catalyst carrier on the anode side and the materials used in the electrolyzer.
Why PEM is expensive
ThThe ideal oxygen evolution electrocatalyst should possess a high specific surface area and porosity, high electronic conductivity, excellent electrocatalytic performance, long-term mechanical and electrochemical stability, minimal bubble effects, high selectivity, low cost, and non-toxicity.
Catalysts meeting these criteria are mainly noble metals/oxides such as iridium (Ir) and ruthenium (Ru), as well as binary and ternary alloys/mixed oxides based on these metals. However, Ir and Ru are expensive and scarce resources, and the current Ir loading in PEM electrolyzers often exceeds 2 mg/cm². Therefore, it is urgent to reduce the IrO₂ dosage in PEM water electrolyzers. Commercialized platinum (Pt)-based catalysts are already in use for the hydrogen evolution reaction at the cathodes of PEM water electrolyzers. At present, the Pt loading at the cathode of PEM water electrolysis cells is around 0.4-0.6 mg/cm².
Lowering ohmic polarization
The primary source of ohmic polarization in PEM water electrolysis is the ohmic resistance of the electrodes, membranes, and current collectors. Membrane resistance is the main contributor to ohmic polarization loss, and it increases with membrane thickness.
To reduce ohmic polarization and membrane resistance, thinner membranes can be used. However, it is essential to consider factors such as gas permeation and membrane degradation. Gas permeation through the membrane increases with electrolysis time and temperature and is inversely proportional to membrane thickness. Selecting materials with excellent electrical conductivity for electrodes and current collectors, improving proton conductivity in the catalytic layer and membrane, reducing contact resistance between components, and minimizing the thickness of the catalytic layer all contribute to lowering ohmic polarization.
Concentration Polarization
Concentration polarization is directly related to the water supply and the removal of produced gases. It is influenced by the hydrophilic and hydrophobic properties of the diffusion layer and the design of the flow field. The diffusion layer in PEM water electrolysis is typically made of titanium-based materials with corrosion-resistant surface treatments to withstand the corrosive conditions during hydrogen and oxygen evolution.
The diffusion layer material is also involved in both ohmic polarization and diffusion processes, necessitating careful consideration. The cost of the titanium substrate and the surface treatment materials represent a relatively high proportion of the total cost in PEM stacks. Due to the high costs of catalysts and electrolyzer materials, the price of PEM water electrolysis technology is currently higher than that of traditional alkaline water electrolysis technology. The primary approach to reducing costs is to improve the efficiency of the electrolyzer, which involves advancing the technology of catalysts, membrane materials, and diffusion layer materials.
For a simple comparison of parameters of alkaline electrolyzers VS PEM electrolyzers, please click here.
