Water impurities : Generally, improper water circulation and quality cause cell and stack degradation, which affects the lifetime of the plant. So, water quality is an important factor in water electrolysis operation. In AWE, diaphragm, catalysts, and other components (like bipolar plate and end plate) can be affected by water impurities such as iron, copper, chromium, silica, aluminium, and boron, which reduces the stack lifetime. In PEM electrolysers, membrane, ionomer in the catalyst layer, catalysts, and porous transport layers (PTLs) can be affected by the partial circulation of water and its impurities. PEM is more sensitive to water impurities. Generally, water will get contaminated by ions, which conduct electricity. Hence these contaminants should be removed from the circulation water by using mixed bed resins and carbon filters, increasing the resistivity of the water. The leading OEM demands minimum water resistivity of 1 MΩ-cm per American Society for Testing and Materials (ASTM) Type II (>1 MΩ-cm). However, the recommended water quality for operation of water electrolysers is minimum water resistivity of >10 MΩ-cm (for example, resistivity at 18.2 MΩ-cm for ultra- pure water) per ASTM Type I or ISO 3696. Other properties of total silica, total organics, and total carbon need to be monitored, and water quality maintained as per the OEM guidelines. Dynamic/variable load: Typically, grid electricity (constant power source) has been used in the production of hydrogen via electrolysis. Solar photovoltaic and wind energy are intermittent or more variable sources of electricity, resulting in voltage fluctuations. This voltage fluctuation can potentially trigger additional corrosion of stack components, thereby reducing durability and stability. OEMs should provide a suitable system to maintain a consistent DC supply to the electrolyser. The durability of water electrolysis is typically assessed by various methods like (a) measuring the cell voltage over time at a constant current density, (b) accelerated stress test (AST), (c) accelerated degradation test (ADT) using degradation accelerating parameters such as higher operating temperatures and high current density, and (d) empirical equations. ADT will
help to investigate the durability of the electrode against the repeated fluctuations of the renewable power sources (Ashraf, et al. , 2021). The ADT procedure comprises the steady-state operation of the alkaline water electrolyser under a DC current of 0.6 A/cm 2 in addition to other potential control steps most likely to mimic fluctuations in the renewable electricity supply. This simulates the actual conditions and degradation mechanisms on a shorter timescale and is of great importance in the development of sustainable electrocatalysts. Electrode separator membrane/diaphragm : AWE electrolysers use a diaphragm or porous separator as the electrode separator. This can be damaged by various conditions such as an alkali environment, gas permeation, and local hot spots created by the deposition of impurities (that is, electrode coatings material). A surface crack or pin-hole failure in the separator will increase in size over time, which leads to gas crossover or contamination. If the hydrogen concentration reaches 2% on the oxygen side, the stack will be damaged. Some OEMs use polyphenylene sulphide fabric diaphragms, which improves durability as it limits gas permeation but affects hydrogen production efficiency. The most common cationic membrane (Nafion) used in a PEM electrolyser demonstrates reasonable thermal stability and tensile strength over earlier membrane materials. The membrane may be exposed to a large differential pressure that negatively affects the mechanical stability of the membrane. The criteria for selecting the diaphragm or membrane depends on the material’s ability to resist corrosion, chemical and mechanical degradation. Summary This article reviews current development in water electrolysis for hydrogen production and shows the potential to increase efficiency and hydrogen production rates over longer lifetimes. These developments will be important for increasing the scale and decreasing the cost of green hydrogen production.
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Dr Sakthivel. S ssakthivel@tce.co.in
www.decarbonisationtechnology.com
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