Meaning and mechanism
To create one molecule of hydrogen in electrolysis of water, the reduction requires two electrons. The theoretical molar ratio between hydrogen and oxygen is 2:1.
Even when the same amount of charge is discharged, the number of moles generated and the electrical energy produced are different. Electrical energy depends not only on the charge, but also on the operating voltage.
Relationships and conditions
The current efficiency is n(H₂)=It/(2F), n(O₂)=It/(4F). We calculate the round-trip efficiency by comparing the electrical energy recovered from the fuel cell with the electrical energy inputted.
In real systems, voltage loss, gas processing, and the energy of storage and auxiliary devices are included. While hydrogen is not a source that creates energy, it can serve as a storage and delivery medium.
What to observe in the simulation
Electrolysis produces hydrogen and oxygen in a 2:1 ratio. Reversing the reaction does not recover all electrical energy.
The model assumes 100% Faradaic efficiency, 1.8 V electrolysis, a 0.7 V fuel cell and hydrogen HHV of 285830 J/mol. Compression, leakage and kinetics are omitted. Do not reproduce this with real gas, flames or high voltage.
Key relationships and symbols
n(H₂) = It/(2F) · n(O₂) = It/(4F) · F = 96485.33212 C/mol
| Symbol | Meaning | Unit |
|---|---|---|
| F | Faraday’s constant | C/mol |
| It | Charge transferred | C |
| n | Amount of gas produced | mol |
Worked example
For equal charge, what fraction of electrolysis input returns as electricity?
38.9 % · Electrolysis produces hydrogen and oxygen in a 2:1 ratio. Reversing the reaction does not recover all electrical energy.
Common misconceptions
- It is not possible to obtain a greater amount of electrical energy by repeating the process of creating and then using hydrogen, compared to the initial process.