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A key centerpiece of renewable energy hydrogen generation is the highly efficient electrolytic hydrogen production technology. Hydrogen production by electrolysis is the electrochemical dissociation of water into hydrogen and oxygen gas, which are precipitated at the cathode and anode, respectively, under the action of direct current.
Anode: H2O→1/2O2+2H++2e-(1)
Cathode: 2H+2e-→H2(2)
Total reaction: H2O→H2+1/2O2(3)
Depending on the electrolyte system, hydrogen production by electrolysis can be divided into three types: alkaline electrolysis, mass exchange membrane (PEM) electrolysis and solid oxide electrolysis.The basic principle of all three is the same, i.e.,during the redox reaction, the free exchange of electrons is prevented, and the charge transfer process is broken down into the transfer of electrons in the external circuit and the transfer of ions in the internal circuit, so that hydrogen gas can be produced and utilized.However, the electrode materials and electrolysis reaction conditions are different, and the technical comparison between the three is shown below.
The alkaline liquid electrolysis technology uses KOH and NaOH water solutions as the electrolyte and quartz cloth as the diaphragm. The purity of the produced hydrogen gas is about 99% and needs to be treated with de-alkaline mist.The main structural features of an alkaline electrolyzer are a liquid electrolyte and a porous partition.The maximum working current density of an alkaline electrolyzer is less than 400mA/cm2 and the efficiency is usually around 60%.Alkaline liquid electrolysis was industrialized in the mid-20th century. The technology is mature and has a lifetime of 15 a.The main drawbacks are as follows:
1).In the liquid electrolyte system, the alkaline electrolyte used (e.g. KOH) will react with CO2 in the air to form carbonates (e.g. K2CO3) that are insoluble under alkaline conditions, leading to blockage of the porous catalytic layer, thereby impeding the transfer of products and reactants and greatly reducing the performance of the electrolyzer;
2).Alkaline liquid electrolyte cells have long start-up lead times, slow load response, and must always maintain pressure equalization on both sides of the anode and cathode of the electrolytic cell to prevent hydrogen and oxygen gas from mixing through the porous quartz membrane and causing an explosion.Therefore, alkaline liquid electrolyte electrolyzers are more difficult to use with renewable energy sources that have fast fluctuating characteristics.
PEM electrolysis of water, called solidpolymerelectrolyte (SPE) electrolysis of water, works on the following principle.Water (2H2O) creates a hydrolysis reaction at the anode and splits into a proton (4H+), an electron (4e-), and gas oxygen with the help of an electric field and a catalyst; the 4H+ proton passes through a proton exchange membrane to the cathode with the help of an electric potential difference; the 4e- electron is conducted through an external circuit to create the 4H+ +4e- reaction at the cathode, resulting in the precipitation of hydrogen (gas).The PEM electrolyzer operates at a current density of more than 1A/cm2 , which is at least four times that of an alkaline electrolyzer, and has high efficiency, high gas purity, adjustable current density, low energy consumption, small size, no alkaline solution, and low energy consumption, It has the advantages of high efficiency, high gas purity, adjustable current density, low energy consumption, small size, no alkaline solution, green environment, safety and reliability, and higher gas pressure, etc. 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 hydroelectric cell include the cathode and anode plates, the cathode and anode gas diffusion layer, the cathode and anode catalyst layer, and the mass exchange membrane.The cathode terminal plate is used to hold the electrolytic cell components and guide the transfer of electricity and the distribution of water and gas; the cathode gas diffusion layer is used to collect the flow and facilitate the transfer of gas; the cathode catalytic layer is a three-phase interface consisting of catalyst, electric conduction medium, and mass conduction medium, and is the central site for electrochemical reactions; the mass exchange membrane is used as a solid electrolyte, generally using a perfluorosulfonic acid membrane.As a solid electrolyte, a perfluorosulfonic acid membrane is generally used to isolate the cathode from the anode and to prevent the transfer of electricity while transferring the substrate.PEM electrolytic water requires a high catalyst carrier.The ideal catalyst should have high surface area and porosity, high electrical conductivity, and good electrocatalytic properties.The ideal catalyst should have high surface area and porosity, high electrical conductivity, good electrocatalytic properties, long-term mechanical and electrochemical stability, small gas bubble effect, high selectivity, low cost and no toxicity.The catalysts that satisfy the above conditions are mainly noble metals/oxides such as Ir and Ru, and their two- and three-dimensional metals/mixed oxides. Because Ir and Ru are expensive and scarce, the catalysts for the current PEM electrolyzers are not available.
There is an urgent need to reduce the amount of IrO2 used in the PEM water electrolyzer, as the amount of Ir used in PEM electrolyzers often exceeds 2 mg/cm2.Commercially available Pt-based catalysts can be used directly in PEM water cathodes. At this stage, the Pt loading of PEM electrolytic cathodes is 0.4~0.6 mg/cm2.Despite the obvious advantages of coupling PEM electrolytic hydrogen production technology with renewable energy sources, further development is needed to better meet the needs of renewable energy applications in the following areas To better meet the demand for renewable energy applications, further developments are needed in the following areas:
(1).Increase the power of PEM hydrogen production to match the demand for large-scale renewable energy consumption;
(2).To improve the current density and the ability to work with wide load variations to reduce system costs and achieve efficient renewable energy consumption, as well as to facilitate auxiliary grid peaking, reduce the burden on the grid, and improve energy use efficiency;
(3).Increase the output pressure of gas, facilitate gas storage and transportation, reduce the need for subsequent pressurization equipment, and reduce overall energy consumption.
Hydrogen production by solid oxide electrolysis
A high-temperature solid oxide electrolysis cell (SOEC) is the inverse reaction of a solid oxide fuel cell (SOFC).The cathode material is generally Ni/YSZ porous metal ceramics, the anode material is mainly calcium titanate oxide material, and the intermediate electrolyte is YSZ oxygen ion conductor.A small amount of hydrogen gas mixed with water vapor enters from the cathode (the purpose of mixing hydrogen is to ensure a reducing atmosphere at the cathode and to prevent oxidation of the cathode material Ni), and an electrolytic reaction occurs at the cathode, decomposing into H2 and O2-,which passes through the electrolyte layer at high temperature to the anode, where it loses its charge and becomes O2.Due to the good thermal and chemical stability of the solid oxide, the whole system is electrolyzed at a low voltage at high temperature, resulting in a low energy consumption and a system efficiency of up to 90% for hydrogen production. However, the stability of the anode and cathode materials under high temperature and humidity conditions and the rapid degradation of the stack system over a long period of time still need to be solved.As a result, SOEC technology is still in the technology development stage, with some small demonstration projects in Karlsruhe, Germany, supported by projects such as HELMETH.
