Hydrogen mainly exists in water and fossil fuels in the form of compounds on earth, and hydrogen energy, as a secondary energy, needs to be extracted by hydrogen production technology. At present, most of the existing hydrogen production technologies rely on fossil energy and cannot avoid carbon emissions. According to the source of hydrogen energy production and the emission in the production process, people call hydrogen energy gray hydrogen, blue hydrogen and green hydrogen respectively.

Grey hydrogen is hydrogen produced by fossil fuel combustion, which will emit carbon dioxide in the production process. At present, the vast majority of hydrogen on the market is gray hydrogen, accounting for about 95% of today's global hydrogen production.

Green hydrogen refers to the hydrogen obtained by decomposing water with renewable energy. When it is burned, it only produces water, realizing zero carbon dioxide emission from the source. It is a pure green new energy and plays an important role in the global energy transformation.

Blue hydrogen can be prepared from fossil fuels such as coal or natural gas. In the preparation process of blue hydrogen, carbon dioxide by-products can be captured, utilized and stored (CCUs), so as to realize carbon neutralization. Although natural gas is also a fossil fuel and will produce greenhouse gases when producing blue hydrogen, due to the use of advanced technologies such as CCUs, greenhouse gases are captured, reducing the impact on the earth's environment and realizing low emission production.

"Green hydrogen is the original intention of developing hydrogen energy." Mao Zengqiang, vice president of the international hydrogen energy society and professor of Tsinghua University, once stressed that the development of hydrogen energy is for the "decarbonization" of energy. This goal can be achieved only by producing "green hydrogen" through carbon-free energy.

It can be seen that although all "color" hydrogen will play a role in the future, in the final analysis, green hydrogen is the most sustainable and truly carbon free, so it is also becoming the focus of global hydrogen energy development.