The hydrogen you are familiar with, found in the air we breathe, is commonly referred to as "white" hydrogen. However, the hydrogen you are about to learn about is categorized based on its production method, each represented by a color:

• "Brown" hydrogen: Obtained from coal gasification, it is highly polluting, generating 25 kg of CO2 for every kg of hydrogen produced.

• "Grey" hydrogen: Produced from natural gas through the steam methane reforming process, it has an environmental impact of 11 kg CO2/kg H2.

• "Blue" hydrogen: Involves capturing and storing carbon dioxide, but still emits approximately 3-6 kg CO2/kg H2 due to technological limitations.

• "Pink" hydrogen: Generated if the energy comes from a nuclear power plant without CO2 emissions but with radioactive waste.

• "Green" hydrogen: Produced from renewable sources, it has no CO2 emissions.

Hydrogen does not serve as a direct source of energy but rather acts as a vector enabling the storage of clean energy for use as needed. 

This approach helps create a flexible energy source that supports the current electrical system, thereby aiding decarbonization in sectors where electrification is challenging, such as heavy transportation.

 Furthermore, it enables the reduction of carbon footprint in industrial sectors like fertilizer production, which utilize hydrogen in their processes 

To harness energy from hydrogen, a process called "fuel cell" or "electrolyzers" is utilized. In this device, hydrogen and oxygen undergo an electrochemical reaction to produce electricity, water, and heat as byproducts. 

The basic chemical reaction is: 

2H2+O2→2H2O + electrical energy. 

This process is clean and efficient, as it does not produce polluting emissions, with water being the only byproduct. 

Fuel cells are employed in hydrogen-powered electric vehicles (ex: Toyota Mirai in Japan) and stationary energy systems for electricity generation (ex: Plug Power in the US).