How to produce hydrogen - 1

The expression "sustainable society" sounds so great, but it has faced with many challenges. To achieve the sustainability, the society must stop relying on fossil fuels and utilize exclusively carbon-neutral energy such as renewables (see The amount of energy we consume). But the timing and locations of renewable energy supply does not match with those of energy demand (see Issues associated with renewable energy utilization). To fill the gap, great efforts have been devoted to energy carrier (see the seires "How to store energy" -1, -2, -3, -4, -5).

Being the most representative energy carrier, hydrogen has been of great focues nowadays. A seires of posts, starting with this one, shall focus on the production of hydrogen; especially this post summarizes how the hydrogen has currently been produced.

Methods of producing hydrogen and their share

The society actually has produced and used hydrogen in various manners. The figure below shows the methods of producing hydrogen and their share in the market with price. It was drawn based on several reprots,^[1-4] and the market share is in 2014.^[1]

*Sorry that the figure is in Japanese; but will be translated into English.
*電気分解 = electrolysis, 水素価格 = hydrogen price, 化石資源由来 = derived from fossil-fuels, 石炭 = coal, 石油 = oil, 天然ガス = natural gas

Hydrogen derived from fossil-fuels
The largest portion of 95% is accounted for by fossil-fuel-derived hydrogen. For instance, a reaction between methane and steam can produce hydrogen.
CH₄ + H₂O --> 3H₂ + CO ...(1)
This reaction is called "steam reforming" of methane. The hydrogen produced by this reaction further reacts with steam, whereby the hydrogen is additionally produced.
CO + H₂O --> H₂ + CO₂ ...(2)
This second reaction is called "water-gas shift".

Other than this route, the gasification of coal yields hydrogen. Likewise, naphtha derived from oil can undergo steam reforiming to produce hydrogen. The hydrogen produed in those manners are pretty cheap; roughtly 1 USD per 1 [kg]

Hydrogen derived from non-fossil-fuels
In a sustainable society, hydrogen must be produced from sourses other than fossil fuels, such as biomass or water. Herein, this post takes a look at electrolysis that has been industrially operational even now. Water electrolysis can be driven by using the renewable electric power, therefore regarded as the suitabe method of producing green hydorgen. However, the most of currently available electrolyzers are mainly developed not for the water electrolysis but for chlor-alkali process. The smallest portion of 5% in the figure is accounted for by the electrolysis. The thus produced hydrogen is quite costly; roughly 5 USD per 1 [kg]!! The exact price varies with the statistics report, but roughly four (4) to six (6) times more expensive than the fossil-fuel-derived one.

Without radical insentive, the cost-inefficient hydrogen would not be used.

Just as a side node, the hydrogen produced by electrolyzer is characterized by its high purity. In a occasions where high-pure hydrogen is needed, it would best-fit for now.

Ultimately,

• using renewable energy
• electrolyzing water into hydrogen

would be ideal from the standpoint of carbon-neutral.

Just for your information; nowadays hydrogen is colored in text depending on how it is produced. Grey hydrogen originates from fossil-fuels, green hydrogen is produced via electrolyzer using renewable electricity, and blue hydrogen originates from fossil-fuels but additional carbon-capture technology.

References

1. Shell, Shell Hydrogen study Energy of the Future?, 2017.
2. P. De Luna et al., Science 2019, 364, eaav3506.
3. S.E. Hosseini et al., Renewable Sustainable Energy Rev. 2016, 57, 850.
4. M. David et al., J. Energy Storage 2019, 23, 392.