Today around 95% of the hydrogen used on the Earth, most of which is used in industry, is obtained by reforming methane or through the gasification of coal, processes that generate substantial emissions of carbon dioxide but which are currently the cheapest methods available. There are, however, other ways to obtain hydrogen, for example through thermochemical processes, and, above all, through the electrolysis of water. This involves systems called electrolyzers that require a certain amount of electrical energy and which therefore, in order to be sustainable, must be powered by renewable sources such as wind power or photovoltaic.
At the moment, plants for producing hydrogen on a large scale are not yet competitive with traditional plants from a cost point of view. However, the expected fall in the cost of electrolyzers, the enormous progress in the efficiency of photovoltaic cells and wind generators and the consequent reduction in costs of kWh from renewable sources, are rapidly changing the scenario.
How do plants like this work? The heart is the electrolyzer or electrolytic cell, where the separation of water into its constituent elements – hydrogen and oxygen – occurs. The water is brought into contact with two electrodes, a positively charged anode and a negatively charged cathode. The electrical current detaches the molecules into H+ hydrogen ions and OH- hydroxide ions. At the cathode the hydrogen ions acquire electrons through a reduction reaction and become gaseous hydrogen. At the anode the hydroxide ions give off electrons through oxidization, leading to the formation of oxygen.
If the electrolytic cell is located in proximity to a renewable energy plant, part of the electricity production (for example, the electricity produced in excess of the capacity that can be fed onto the grid) can be used to power it. In this way the hydrogen produced serves as a chemical energy storage facility that can later provide the energy when required, either as a raw material in the process for steel production or as a fuel to provide high-temperature heat.