Hydrogen production by electrolysis (green hydrogen)
By the electrolysis of water (electrochemical decomposition of water) pure H2 gas can be produced with the formation of O2 as the by-product, and without the emission of harmful substances. Oxidation (anodic reaction) and reduction (cathodic reaction) during water decomposition take place in spatially separated half-cells, to avoid the formation of an explosive product mixture (H2 + O2). The electrodes are separated by an electrolyte solution and a solid separator (membrane or diaphragm), or a solid electrolyte that replaces both the electrolyte and the separator. The electrolyte must provide the proper ionic conductivity required for electrochemical processes.
Alkaline water electrolysis is the most mature technology, which has been used over the past 100 years to produce H2 on the industrial scale. In this process, an alkaline electrolyte solution (typically aqueous KOH solution) is electrolyzed by means of the cathode and the anode. A porous diaphragm is used to separate the two half-cells, so that mixing of O2 with H2 is prevented. Electrolysis cells separated by a proton exchange membrane (PEM) have been developed to reduce the electrical resistance (and thus the cell voltage) caused by the solution interlayer. In these, the anode and the cathode are pressed directly onto each other through a membrane (with a technique used in fuel cells). On the anode side, the oxidation of pure water takes place, while at the cathode reduction of H+ proceeds. Hydrogen ions migrating through the membrane to form molecular hydrogen takes place at the same rate as oxygen formation.
Other novel hydrogen production methods.
Several other technologies that produce less mature green hydrogen are either available, or under development. Examples are direct water decomposition, using solar energy (e.g., thermal, photo- or combined photothermal catalysis).
Biological hydrogen production (e.g., with algae or bacteria), in addition to hydrogen production, also allows the manufacturing of additional CCU products (e.g., methane or alcohols). In addition to the exploratory research, there is a domestic applied research capacity in the field within University of Szeged, University of Pannonia, Budapest University of Technology and Economics (BME), and Power-to-Gas Hungary Kft.
Hydrogen can be used for energy production in fuel cells, gas turbines, and internal combustion engines. It is currently used in gas turbines only after being mixed with natural gas, because of the high temperature of the combustion chamber. Attempts are also made to make possible to use pure hydrogen. An essentially similar scenario applies for gas engines. In terms of both energy production and transport use, fuel cells represent the main conversion equipment. Therefore, we will discuss them in more detail below.
Balancing the energy system
Energy produced by wind and solar energy fluctuates significantly. In addition, wind farms are often far away from the consumers. Therefore, this power must either be transmitted to the consumption centers via cable or stored and then fed into the energy system as required. Hydrogen technology is a means to accomplish this task. Here, electricity generated from renewable sources is used to feed an electrolysis process that electrochemically splits water into hydrogen and oxygen, which represent a means in which this energy can be stored and transported. Hydrogen produced in this way can then be converted into electricity. This green hydrogen serves to increase security of supply and network resilience.
Ways of storing and using hydrogen
Hydrogen thus produced can be used directly. For example, it can be converted into electricity by means of fuel cells or utilized as a raw material for chemical industry. The first question is how to make hydrogen available for such purposes.