Hydrogen production by electrolysis of water is to dissociate water molecules into hydrogen and oxygen through an electrochemical process under the action of direct current, which are separated at the cathode and anode, respectively. According to the different diaphragms, it can be divided into alkaline water electrolysis, proton exchange membrane water electrolysis(PEM), and solid oxide water electrolysis.
The industrial application of industrialized water electrolysis technology began in the 1920s. The water electrolysis technology in alkaline liquid electrolyzers has achieved industrial-scale hydrogen production for industrial needs such as ammonia production and petroleum refining. After the 1970s, energy shortages, environmental pollution and the lack of space exploration led to the development of proton exchange membrane water electrolysis technology. At the same time, the high-pressure and compact alkaline electrolyzed water technology required for developing particular fields has also been designed accordingly.
Alkaline Liquid Electrolyzer For Water Electrolysis
Alkaline liquid water electrolysis technology uses KOH and NaOH aqueous solution as the electrolyte, such as using asbestos cloth as the diaphragm. Under the action of direct current, the water is electrolyzed to generate hydrogen and oxygen. Then, the produced gas needs to be treated with a dealkalizing mist. Alkaline liquid water electrolysis was industrialized in the mid-20th century. Alkaline electrolyzer cells are structurally characterized by containing liquid electrolytes and porous separators.
Typically, alkaline liquid electrolyte electrolyzers operate at a current density of about 0.25 A/cm2, and their energy efficiency is typically around 60 %. In liquid electrolyte systems, the alkaline electrolyte used, such as KOH, reacts with CO2 in the air to form carbonates such as K2CO3, which are insoluble under alkaline conditions. These insoluble carbonates can block the porous catalytic layer, hinder the transfer of products and reactants, and significantly reduce the performance of the electrolyzer. On the other hand, the alkaline liquid electrolyte electrolyzer is also challenging to shut down or start up quickly. The hydrogen production rate is also difficult to adjust quickly because the pressure on both sides of the anode and cathode of the electrolytic cell must be balanced at all times to prevent the hydrogen-oxygen gas from passing through.
As a result, the porous asbestos membrane mixes, causing an explosion. As such, it is difficult for alkaline liquid electrolyte electrolyzers to cooperate with renewable energy sources with fast fluctuation characteristics.
Hydrogen Production From Solid Polymer Water Electrolysis
Since there are still many problems in alkaline liquid electrolyte electrolyzers that need to be improved, the rapid development of solid polymer electrolyte (SPE) water electrolysis technology has been promoted. The first practical SPE is a proton exchange membrane (PEM), also called PEM electrolyzer.
The asbestos membrane is replaced by a proton exchange membrane, which conducts protons and isolates the gas on both sides of the electrode. This avoids the disadvantages of using vital alkaline liquid electrolytes in alkaline liquid electrolyte electrolyzers.
At the same time, the PEM water electrolysis cell adopts a zero-gap structure. As a result, the volume of the electrolytic cell is more compact and streamlined, reducing the electrolytic cell’s ohmic resistance and dramatically improving the electrolytic cell’s overall performance.
The current operating density of PEM electrolytic cells is usually higher than 1 A/cm2, which is at least four times that of alkaline water electrolytic cells. It is recognized as one of the most promising electrolytic hydrogen production technologies in the field of hydrogen production.
The main components of a typical PEM hydrogen electrolyzer
The main 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 proton exchange membranes.
Among them, the end plate plays the role of fixing the electrolytic cell components, guiding the transfer of electricity and the distribution of water and gas; the diffusion layer plays the role of collecting current and promoting the transfer of gas and liquid; the core of the catalytic layer is composed of catalyst, electron conduction medium, and proton conduction. The three-phase interface formed by the medium is the core place for the electrochemical reaction; the proton exchange membrane is generally used as a solid electrolyte, and a perfluorosulfonic acid membrane is usually used to isolate the gas generated by the cathode and anode, prevent the transfer of electrons, and transfer protons at the same time.
The principle of proton exchange membrane water electrolysis for hydrogen production is shown in Figure 2. At present, the 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 with alkaline water electrolysis, the PEM water electrolysis system does not require dealkalization. It has a more significant pressure regulation margin. At the beginning of commercialization, the cost of PEM is mainly concentrated in the PEM cell itself. In the PEM water electrolysis cell, the membrane electrode composed of the diffusion layer, the catalytic layer and the proton exchange membrane is where the electrolysis reaction occurs and is the core component of the electrolysis cell. Therefore, increasing the current density of operation can reduce the equipment investment in electrolysis. Moreover, a wide range of current operating densities is more beneficial to match the volatility of renewable energy sources.
Due to the existence of polarization, the actual electrolysis voltage of the electrolysis cell exceeds the theoretical electrolysis voltage Erev obtained by thermodynamics.
The polarization of the electrolytic cell includes activation polarization, ohmic polarization and concentration polarization. In the PEM water electrolysis electrode reaction, the polarization of the anodic oxygen evolution reaction is much higher than that of the cathodic hydrogen evolution reaction, which is an essential factor affecting the electrolysis efficiency.
Electrochemical polarization is mainly related to the activity of electrocatalysts. Selecting highly active catalysts and improving the three-phase interface of electrode reaction is beneficial in reducing electrochemical polarization. In addition, the hydrogen/oxygen evolution in the electrolyzed water reaction, especially the precipitated atomic oxygen, has solid oxidizing properties and has high requirements on the oxidation resistance and corrosion resistance of the catalyst carrier on the anode side and the electrolytic cell material.
The reason why PEM is expensive
The ideal oxygen evolution electrocatalyst should have a high specific surface area and porosity, high electronic conductivity, good electrocatalytic performance, long-term mechanical and electrochemical stability, small bubble effect, high selectivity, cheap and available and no toxicity, etc.
The oxygen evolution catalysts satisfying the above conditions are mainly noble metals/oxides such as Ir and Ru and binary and ternary alloys/mixed oxides based on them. Because Ir and Ru are expensive and scarce resources, and the current Ir dosage in PEM electrolyzers often exceeds 2 mg/cm2, it is urgent to reduce the dosage of IrO2 in PEM water electrolyzers . Commercialized Pt-based catalysts can be directly used in the hydrogen evolution reaction of PEM water electrolysis cathodes. At this stage, the Pt loading of PEM water electrolysis cathodes is about 0.4-0.6 mg/cm.
Lowering ohmic polarization
The primary source of ohmic polarization in PEM water electrolysis is the ohmic resistance of electrodes, membranes and current collectors. Membrane resistance is the primary source of ohmic polarization loss, and membrane resistance increases with the increase of membrane thickness.
A thinner membrane can be selected to reduce the ohmic polarization and membrane resistance. At the same time, the gas permeation and membrane degradation factors need to be comprehensively considered. The permeation of the generated gas in the membrane increases with the increase of electrolysis time and temperature. It is Inversely proportional to the thickness of the film. Selecting materials with excellent electrical conductivity to prepare electrodes and current collectors, improving the proton conductivity in the catalytic layer and membrane, reducing the contact resistance of each component, and reducing the catalytic layer thickness is conducive to lowering ohmic polarization.
The concentration polarization is directly related to the water supply and the discharge of produced gas. It is affected by the diffusion layer’s hydrophilic and hydrophobic properties and the flow field’s design. The diffusion layer of PEM water electrolysis is mainly made of Ti-based materials. It has a corrosion-resistant surface treatment to resist the corrosion problem under the conditions of hydrogen evolution and oxygen evolution.
The diffusion layer material itself involves both ohmic polarization and diffusion layer structure. Careful consideration is required. The cost of Ti substrate itself and the cost of surface treatment materials account for a relatively high proportion in PEM stacks. Due to the high cost of catalysts and electrolytic cell materials, the price of PEM water electrolysis technology at this stage is higher than that of traditional alkaline water electrolysis technology. The primary way is to improve the efficiency of electrolytic cells, that is, to improve the technical level of catalysts, membrane materials and diffusion layer materials.
For a simple comparison of parameters of alkaline electrolyzers VS PEM electrolyzers, please click here.