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| Fig. 1: Bar graph of hydrogen generation color percentages. [8] (Image source: P. Dedeler) |
Hydrogen is the first element in the periodic table and the most abundant element in the universe. Yet, naturally occurring geological deposits of pure hydrogen are rare on Earth, because hydrogen is a highly reactive element and tends to be present in bounds for example in methane or water. [1] When we subject hydrogen to the combustion process it reacts with oxygen from the air in which the hydrogen combines with oxygen to form water vapor, and in the process, energy is emitted in the form of heat. This energy is released due to differences in bond energies between breakdown of Hydrogen-Hydrogen (H-H) and Oxygen-Oxygen (O-O) bonds and formation of Hydrogen-Oxygen (H-O) bonds. [2] By burning 1 kg of Hydrogen, we obtain ~1.21 × 108 joules per kilogram of energy under standard conditions. [3]
| 241.8 × 103 J mol-1 0.002 mol kg-1 |
= | 1.209 × 108 J kg-1 |
Performing hydrolysis on water, the only exhaust obtained is water vapor, thus this energy reaction is considered eco-friendly. [4] Nevertheless, hydrogen forms strong bonds that must be broken to produce pure hydrogen, requiring energy. The environmental friendliness of breaking those bonds depends on the hydrogen's source and production method.
Scientists have created a color scale that is used to track hydrogen's production method and environmental impact. [5]
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| Table 1: Colors of Hydrogen. [4,6] |
Today hydrogen is primarily produced via SMR, electrolysis, and gasification. Global hydrogen production in 2022 has almost reached to 95 Mt. Compared to 93 Mt estimation in 2021 there is an increase of 3%. [7] As shown in Fig. 1. production was dominated by the unabated use of fossil fuels. [8]
Looking ahead, as the governments aim to reach net-zero goal utilizing hydrogen as one of the environmentally friendly fuels, ninety-five percent of todays hydrogen is produced is gray hydrogen. According to Majumdar et. al., gray hydrogen has the lowest cost, $1/kg-H2 in the US but produces roughly 10 kg of carbon dioxide (CO2) per kg of hydrogen. [9] Blue hydrogen technologies are emerging as a means of capturing and reducing the adverse environmental effect of gray hydrogen, but on contrast in their 2021 study Howarth and Jacobson concluded that greenhouse gas emissions from blue hydrogen are unacceptably high to justify it on climate bases. In blue hydrogen, though carbon dioxide emissions are lower, fugitive methane emissions surpass those in gray hydrogen due to carbon capture process itself requires increased use of natural gas. Thus the greenhouse gas footprint of blue hydrogen is more than 20% greater than burning natural gas or coal for heat. [10] Meanwhile, low-emission hydrogen production including environmentally friendly hydrogen generation methods, was less than 0.7% of global production. [8]
Single hydrogen atom has a volume of about 6.2 × 10-31 m3 and the ideal gas law states that molar volume for any gas is approximately 22.4 liters. [11] Because hydrogen gas would take a lot of space it needs to be compressed under pressure up to 700 bar (700 times atmospheric pressure). Its mass density and energy density are then:
| 700 × | 0.002 kg mol-1 22.4 L mol-1 |
= | 0.0625 kg L-1 |
| 0.0625 kg L-1 × 1.209 × 108 J kg-1 | = | 7.56 × 106 J L-1 |
Now that we have found the energy content of 1 liters of hydrogen lets compare it to the most used liquid fuel gasoline. Gasoline is known to have an energy content of 32 M J/L. [12]
| EH2 EGasoline |
= | 7.56 × 106 J L-1 3.20 × 107 J L-1 |
= | 0.236 |
This indicates that 1 liter of hydrogen compressed at 700 bar releases approximately 1/4 the energy released by 1 liter of gasoline. This demonstrates that one needs more liters of hydrogen than one would need liters of gasoline to cover the same amount of energy. It is worth noting that liters are a measure of volume. The energy that we get from hydrogen per mass is two times that of gasoline.
Nonetheless, the energy density per volume of hydrogen brings up challenges like energy storage and transportation. This is a limiting factor for hydrogen to be used as a fuel in applications with space constraints.
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| Table 2: Prices of gasoline, gray, blue and green H2 production cost estimations for US in 2019. [13,14] |
Looking at price per kg of different types of H2 shown in Table 2, the prices of green H2 with both lowest assessment (-) and highest assessment (+) comes as one of the more expensive options compared to gray H2 and traditional fuels.
We have explored hydrogen as an energy source and the chemistry underlying the energy obtained from it. It is mentioned that the electrolysis reaction is a promising method to fight climate change and that the only waste product is water. However, as of right now, only 0.7% of the hydrogen created globally comes from eco-friendly, low-emission, technologies, and the majority of hydrogen worldwide is generated through natural gas production. This fact underscores the importance of accurately communicating the process of hydrogen sourcing to prevent potential misconceptions about hydrogen as an entirely eco-friendly energy source. As the laws of economics favor the greatest value for the lowest cost, laws of physics dictates that energy we get from green hydrogen should not be greater than the energy we used to produce it. To conclude, when comparing the prices of each type of hydrogen to traditional fuels, economic principles do not favor green hydrogen as the prominent energy source, suggesting that the cost of green hydrogen and its variations should be reduced to achieve net-zero goals.
© Pelin Dedeler. The author warrants that the work is the author's own, and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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[5] J. Incer-Valverde et al., "Colors of Hydrogen: Definitions and Carbon Intensity," Energy Convers. Manag. 291, 117294 (2023).
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[8] "Global Hydrogen Review 2023," International Energy Agency, 2023.
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[14] "BP Statistical Review of World Energy 2020," British Petroleum, June 2020.