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 electrolysis, proton exchange membrane electrolysis(PEM), and solid oxide 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 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.
How to deal with hydrogen from electrolyzer oxygen not included
How much oxygen and hydrogen separate from the electrolyzer
It depends on the size of the electrolyzer and the current passing through it. Generally, an electrolyzer uses a direct current (DC) electric current to split water into its component parts, oxygen and hydrogen. This process, called electrolysis, involves passing an electric current through a water-based electrolyte solution, which causes the water molecules to break apart into oxygen and hydrogen atoms.
