Three technical routes for hydrogen production from electrolytic water - alkaline, PEM and solid oxide electrolyzers
In principle, electrolysis of water for hydrogen production is an electrochemical process in which water molecules are dissociated into hydrogen and oxygen at the cathode and anode, respectively, under the action of a direct current. Depending on the reaction principle, there are three main options: alkaline water electrolysis (ALK), pure water electrolysis with proton exchange membranes (PEM) and solid oxide water electrolysis (SOEC). Alkaline aqueous electrolysis (ALK) and proton exchange membrane (PEM) electrolysis for hydrogen production have been commercially launched, while solid oxide electrolysis is in the laboratory development stage.
Alkaline electrolysis (ALK): Alkaline electrolysis uses an alkaline aqueous solution such as KOH as the electrolyte and a non-woven cloth (fluorine or fluorine chlorine polymer) as the diaphragm to electrolyse water to produce hydrogen and oxygen under direct current. The gas yield is proportional to the current and the electricity consumption per unit of gas yield is related to the electrolysis voltage and reaction temperature. The theoretical decomposition voltage of water is 1.23V and the theoretical power consumption is 2.95kWh/m3, while the actual power consumption of alkaline water electrolysis is about 5.5kWh/m3 and the conversion efficiency of the electrolyzer is around 60%.
ALK has been commercially available for nearly 100 years and the technology is relatively mature, with a lifespan of 15-20 years, and the cost is only one fifth of that of a PEM electrolyser of the same size.
Disadvantages: large size, low efficiency and slow dynamic response. 1) The size of an alkaline electrolyser is much larger than a PEM electrolyser for the same scale of hydrogen production because of the slow reaction rate and low current density due to the use of non-precious metal catalysts. 2) The alkaline solution is very maintenance intensive and therefore requires frequent maintenance. 3) The cold start-up time of an ALK electrolyser is 1-2 hours because of the power consumption required to heat the electrolyte. 4) the dynamics of the alkaline electrolyser are slow and do not allow for good tracking of the fluctuating renewable energy generation. In addition, to ensure the purity of hydrogen production, the alkaline electrolyser must maintain a power level of more than 20% of its rated power, which can lead to waste when renewable energy generation is suddenly reduced.
Proton exchange membrane (PEM) electrolysers: PEM electrolysis of water for hydrogen production and PEM fuel cell workflows are inverse processes to each other. The main components of a typical PEM cell include membrane electrodes (proton exchange membrane, catalytic layer, diffusion layer), bipolar plates, epoxy resin plates and end plates. The catalytic layer is a three-phase interface consisting of a catalyst, an electron transfer medium and a proton transfer medium, which is the core of the electrochemical reaction, The proton exchange membrane is used as a solid electrolyte, usually a perfluorosulphonic acid membrane, to isolate the cathode from generating gas, to prevent the transfer of electrons and to transfer protons.
Advantages: high efficiency, no alkaline solution, small size, safety and reliability, good dynamic response, etc. The corresponding power consumption of PEM electrolysis technology is approximately 5.0kWh/m3 and the efficiency is approximately 70%. Compared to ALK, PEM water electrolysis systems do not require de-alkalisation. At the same time, PEM electrolysis cells are more compact and dynamic, making them ideal for use in series with fluctuating renewable energy sources.
Disadvantage: high cost due to the need to use precious metals. Currently only precious metals such as iridium and ruthenium can be used as catalysts. To reduce the material cost of the catalyst and the electrolyzer, especially the precious metal loading of the cathode and anode electrocatalysts, and to improve the efficiency and lifetime of the electrolyzer, is a key research priority for the development of PEM water electrolysis for hydrogen production.
Solid Oxide Electrolyzer (SOEC): operating at around 800°C, this is a very promising water electrolysis technology compared to alkaline electrolysis and PEM electrolysis, which operate at around 80°C. It is still in the laboratory stage of development. The cathode material for high temperature SOEC is generally Ni/YSZ (yttrium doped zirconia) porous cermet and the anode material is mainly chalcogenide oxide, with the possibility of LSCF (lanthanum strontium cobalt iron) in the future. The intermediate electrolyte is a YSZ oxygen ion conductor. Water vapour mixed with a small amount of hydrogen enters from the cathode (the purpose of mixing hydrogen is to ensure a reducing atmosphere at the cathode and to prevent the oxidation of the cathode material Ni), where the electrolysis reaction takes place to form H2 and O2-, which passes through the electrolyte layer to the anode, where it loses electrons to form O2. SOEC is also the reverse operation of SOEF.
(1) Unlike alkaline water electrolysis and PEM water electrolysis, high temperature solid oxide water electrolysis uses solid oxide as the electrolyte material and works at 800-1000°C. The electrochemical performance of the hydrogen production process is significantly improved and the energy utilisation efficiency is higher, reaching ≥90%; (2) The electrolyzer can use non-precious metal catalysts and is made of all ceramic materials, reducing the problem of equipment corrosion. The corrosion problem of the equipment is reduced.
Disadvantages: poor durability. The high temperature and humidity environment limits the choice of materials for the electrolyser that are stable, long-lasting and resistant to decay, limiting the choice of application scenarios for SOEC hydrogen production technology and its widespread use.