Green hydrogen, also known as renewable hydrogen, is an important piece of the puzzle in the transition to a sustainable energy future. Hydrogen has the potential to decarbonize a variety of industries, from transportation to power generation. However, the production of hydrogen must also be sustainable and low-carbon. In this article, we will compare several green hydrogen production technologies.
Gasification is a mature technology that can use coal or biomass as a feedstock. Air or pure oxygen is used to initiate chemical reactions with the feedstock to create ‘syngas’, a mixture of carbon monoxide and hydrogen, and slag mineral residue. This syngas can be further refined through additional steam and catalysts to yield pure hydrogen. However, this process also yields several greenhouse gases such as CO2 and CO.
Steam reforming is another mature technology that uses natural gas or biogas as a feedstock. The gas feedstock is heated to over 700°C in the presence of a catalyst, producing syngas, which can be further refined to yield pure hydrogen gas. However, this process also yields several greenhouse gases such as CO2 and CO.
Electrolysis is a promising technology that uses electricity and water to produce hydrogen. There are several types of electrolysis, including proton-exchange membrane (PEM) and alkaline electrolysis. In PEM electrolysis, a proton-exchange membrane is used to split water in the presence of electricity. This process occurs at relatively low temperatures (70°–90°C). Alkaline electrolysis uses a liquid alkaline solution of sodium or potassium hydroxide as the solution to generate hydrogen in the presence of electricity. This process occurs at relatively low temperatures (100°–150°C). However, the efficiency of electrolysis is dependent on the source of electricity used to power the process.
Solid oxide electrolysis is a research and development technology that uses a solid ceramic material as the electrolyte. This process must occur at relatively high temperatures (700°–800°C) but operates at a higher electrical efficiency.
Photoelectrochemical splitting is another research and development technology that uses solar energy plus water to produce hydrogen. This process uses semiconductor materials, similar to those found in solar photovoltaic panels, to directly harness the energy from light to split water molecules. This process can occur in a panel-based reactor, in which the semiconductor material is submerged in water and generates electricity when exposed to light, which is used to generate hydrogen. Alternatively, semiconductor photocatalyst particles can be dispersed throughout a volume of water, which will generate hydrogen gas when exposed to sunlight.
Thermochemical splitting is also a research and development technology that uses high heat plus water to drive chemical reactions in a closed loop to split water into hydrogen and oxygen. This process can be either direct (only using temperatures ~2,000°C) or hybrid (using lower temperatures, ~500°C, and electricity).
Once produced, hydrogen can be stored and used later for electricity generation. Hydrogen can be stored as either a gas or liquid, or on the porous surfaces of nanostructures of certain materials. When stored as a gas, hydrogen must be stored in large, pressurized containers or at very low temperatures, which can complicate transportation and increase associated storage costs. Similar to natural gas and CAES, large underground caverns, such as retired salt mines, could be used to store high volumes of hydrogen gas. When stored as a liquid, the overall storage volume required decreases, but storage costs may increase, and efficiency may decrease. Finally, hydrogen atoms can be stored within the spaces inside metal or alloy lattices or on the surface of carbon structures, such as carbon nanotubes.
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