Hydrogen Environmental Benefits Depend on the Way of Production: An Overview of the Main Processes Production and Challenges by 2050

Hydrogen (H2) is presented as an important alternative for clean energy and raw material in the modern world. However, the environmental benefits are linked to its process of production. Herein, the chemical aspects, advantages/disadvantages, and challenges of the main processes of H2 production from petroleum to water are described. The fossil fuel (FF)-based methods and the state-of-art strategies are outlined to produce hydrogen from water (electrolysis), wastewater, and seawater. In addition, a discussion based on a color code to classify the cleanliness of hydrogen production is introduced. By the end, a summary of the hydrogen value chain addresses topics related to the financial aspects and perspective for 2050: green hydrogen and zero-emission carbon.