"Colors" of hydrogen: Definitions and carbon intensity

Hydrogen is expected to contribute towards the decarbonization of the global economy. The high energy content, flexibility, and abundance of this chemical element are reasons why many sectors can benefit from it in the so-called hydrogen economy. However, almost all hydrogen produced today comes from fossil fuels (mostly from natural gas via steam methane reforming) and has a high associated carbon footprint. Hydrogen production from more environmentally friendly sources has been an area of intensive research for the last years. A necessity to categorize the hydrogen production paths became inevitable. It began by assigning green and gray "colors" to hydrogen to distinguish between a "nonpolluting" hydrogen production and one with associated carbon dioxide emissions. The definition of green hydrogen is now widely understood as hydrogen produced from water electrolysis powered by renewable energy sources. However, other energy sources could power electrolysis and produce hydrogen with no carbon dioxide emissions, e.g., nuclear energy. In the need to differentiate between the source of hydrogen and the production process, many different colors to refer to each path started to appear in the literature. There is no universal agreement upon a color code for hydrogen. This paper gives an overview of the commonly assigned colors. It also compares their environmental impact, production cost, and other parameters. Finally, the authors propose a color code with the most common colors gathered from the literature that could be useful in a large-scale hydrogen economy. Green hydrogen is expected to become the most popular production method and economically competitive with natural gas prices by 2050. © 2023 Elsevier Ltd