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WIREs Energy Environ.

Hydrogen production via solid electrolytic routes

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Hydrogen is considered as a sustainable fuel, especially for transport applications, if it is generated using renewable energy sources. Currently, most of the hydrogen is generated from fossil fuels for use in ammonia and fertilizer production, and petrochemical and metallurgical industries with very little use as an energy carrier. In future, the use of hydrogen as a transport fuel in fuel cell or internal combustion engine vehicles is likely to increase stemming from concern over potential oil shortage and greenhouse gas and other pollutant emissions. Electrolysis of water is one of the key technologies to generate hydrogen both at small (distributed productions at end‐use sites) and large scale. Solid electrolytic routes have been under consideration for many years with low‐temperature systems mainly based around polymer electrolyte membranes now reaching commercialization status. High‐temperature solid electrolyte systems can take advantage of utilizing the waste heat from nuclear or coal‐fired power plants or other chemical processes to reduce the electrical energy required, thus boosting the overall system efficiency for steam electrolysis. An overview of various solid electrolytic routes for hydrogen production, materials used, performance, and technology status is given.

Most commonly proposed solid electrolytic routes for hydrogen production and typical coupling technologies that can provide energy input.
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The basic operating principles of three main types of solid electrolyte systems differentiated by their operating temperature range.
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Process diagram for the production of hydrogen from various hydrocarbon fuels showing breakdown of process steps and chemical reactions. LT, low temperature; HT, high temperature.
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Ionic conductivity of electrolytes for potential use in solid electrolyte hydrogen generation systems. (Data taken from Refs 37, 45, 46, and 48.)
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CSIRO developed stand‐alone electrolyzer for direct coupling to a renewable energy source such as solar photovoltaic or wind generator2 with 1 Nm3/h hydrogen generation capacity.
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The energy requirements per liter of H2 produced by low‐ (25°C) and high‐temperature (1000°C) electrolysis, and energy output components by power generation in a fuel cell. The calculations assume that the high‐temperature electrolysis produce similar voltage losses compared to normal electrolysis, and the power generation in a fuel cell can achieve 55% electric efficiency, 25% heat output, and 20% energy lost in the process.
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Potential primary energy sources for solid electrolytic hydrogen production. Thermochemical processes involve a series of chemical reactions leading to the net decomposition of water for hydrogen and oxygen generation with heat provided from an external source. Some most common cycles are sulfur/iodine, calcium/bromine, copper/chlorine, and metal/oxide. LT, low temperature; HT, high temperature.
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Energy balance between total energy, heat, and electrical input for solid‐state electrolysis systems as a function of the operating temperature. (Redrawn based on a figure from Ref 12. Copyright 1980, Elsevier.)
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The voltage degradation for a 929 cm2 active area polymer electrolyte membrane electrolysis cell with time showing a continuous drop in the efficiency. (Figure redrawn from Ref 33.)
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