# Electrolysers: Science and limits

Why every kilogram of hydrogen needs a fixed electric charge

Source page: https://www.materialinsights.org/energy/e_electrolysers/science/

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

An electrolyser pushes current through a substance to drive a reaction uphill: water into hydrogen and oxygen, brine into chlorine, caustic soda and hydrogen, alumina into aluminium. Faraday's law fixes how much product each amp-hour gives; the cell voltage sets the energy cost.

#### The conversion, step by step
**Transformer, rectifier and balance of plant**
AC electricity → DC at the stack · About 93% (calculated: 51 kWh/kg at a PEM stack against 55 kWh/kg for the system, 2022)^[1](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis)**Water splitting in the stack**
DC electricity plus water → H2 and O2 plus heat · 65% LHV at the stack (51 kWh/kg); 61% for the whole 2022 PEM system (55 kWh/kg), target 72%^[1](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis)**Hot (solid-oxide) electrolysis**
Electricity plus steam and heat → Hydrogen · About 90% LHV on the electricity, because heat supplies part of the energy^[18](https://www.iea.org/reports/the-role-of-e-fuels-in-decarbonising-transport)**Aluminium smelting (Hall-Heroult)**
AC electricity plus carbon anodes → Molten aluminium · 14.1 MWh per tonne world average, about 2.3 times the theoretical 6.24 MWh/t (calculated)^[12](https://giz.de/en/downloads/giz2025-en-factsheet-aluminium-smelting-with-use-of-low-carbon-electricity.pdf)^[13](https://blog.sintef.com/energy/aluminium-electrolysis-using-inert-anodes/)

#### The science

Each hydrogen molecule needs 2 electrons, so 1 kg of hydrogen needs about 26,600 amp-hours whatever the design (Faraday's law, calculated). Energy is that charge times the cell voltage: at 1.9 V, a typical 2022 PEM cell at 2 A/cm2, it is about 50.5 kWh/kg (calculated), matching the DOE's 51. Two voltages matter. The reversible voltage, 1.23 V at 25 C, is the minimum electrical work (the Gibbs energy). The thermoneutral voltage, 1.48 V, also covers the heat the reaction absorbs, so the cell neither warms nor cools; at 1.48 V hydrogen costs 39.4 kWh/kg, its higher heating value (calculated). Anything above 1.48 V becomes waste heat, caused by activation (the slow oxygen reaction), resistance and gas bubbles. Higher current density cuts capital cost per kW but raises voltage: the central design trade-off. The same law gives about 756 kAh per tonne of chlorine (2 electrons per Cl2) and 2,980 Ah per kg of aluminium (3 electrons per atom) (calculated).

#### Limits set by science

| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| **Thermoneutral voltage** | 1.48 V, 39.4 kWh per kg H2 (calculated)^[1](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis) | A low-temperature cell running below this would need heat from outside. | PEM stack at 1.9 V, about 78% of HHV (calculated); DOE ultimate target 1.6 V at 3 A/cm2 |
| **Reversible (Gibbs) voltage** | 1.23 V at 25 C, about 32.7 kWh/kg (calculated); lower at high temperature^[18](https://www.iea.org/reports/the-role-of-e-fuels-in-decarbonising-transport)^[19](https://www.sec.gov/Archives/edgar/data/1664703/000162828026006516/be-20251231.htm) | At 700-900 C more of the energy can come as heat, so solid-oxide cells need less electricity. | Solid-oxide systems near 90% LHV on electricity |
| **Iridium supply for PEM** | About 7.5 tonnes mined a year, 90-95% in South Africa^[8](https://arxiv.org/html/2509.05357v1) | At about 0.75 g/kW, 1 GW needs about 750 kg, a tenth of a year's supply (calculated). | A 90% cut in iridium per kW is thought technically feasible |
| **Aluminium minimum energy** | 6.24 kWh per kg with carbon anodes; 9.16 kWh/kg with inert (oxygen-evolving) anodes^[13](https://blog.sintef.com/energy/aluminium-electrolysis-using-inert-anodes/)^[16](https://www.ap-technology.com/wp-content/uploads/2019/03/AP_Factsheet_AP60-APXe.pdf)^[17](https://icsoba.org/assets/files/publications/2017/KN06S - History and Recent Developments in Aluminum Smelting in China.pdf) | The burning carbon anode supplies part of the energy; an oxygen-evolving inert anode needs more electricity. | Chinese 600 kA cells about 12.1 DC kWh/kg; Rio Tinto AP60 about 13.1 |
| **Chlor-alkali cell energy** | About 2.95 V average implied by best membrane cells (calculated)^[10](https://www.thyssenkrupp-nucera.com/newsroom/blog/the-right-chlor-alkali-electrolysis-technology-for-your-needs) | 1,979 kWh per tonne NaOH at 6 kA/m2 is about 2,230 kWh per tonne of chlorine (calculated). | Oxygen-depolarised cathodes cut power use by up to 25% but give up the hydrogen |

#### Sources

1. [Technical Targets for Proton Exchange Membrane Electrolysis](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis) US Department of Energy, Hydrogen and Fuel Cell Technologies Office, 2023
2. [Critical Iridium Demands arising from future Expansion of Proton Exchange Membrane Electrolysis (Wortmann, Stolten, Heinrichs), preprint](https://arxiv.org/html/2509.05357v1) arXiv 2509.05357, 2025
3. [The right chlor-alkali electrolysis technology for your needs](https://www.thyssenkrupp-nucera.com/newsroom/blog/the-right-chlor-alkali-electrolysis-technology-for-your-needs) thyssenkrupp nucera, 2024
4. [Factsheet: Aluminium smelting with use of low-carbon electricity](https://giz.de/en/downloads/giz2025-en-factsheet-aluminium-smelting-with-use-of-low-carbon-electricity.pdf) GIZ, 2025
5. [Aluminium electrolysis using inert anodes (A. Solheim)](https://blog.sintef.com/energy/aluminium-electrolysis-using-inert-anodes/) SINTEF, 2025
6. [AP60 and APXe smelting technology factsheet (company data)](https://www.ap-technology.com/wp-content/uploads/2019/03/AP_Factsheet_AP60-APXe.pdf) Rio Tinto AP Technology, 2015
7. [History and Recent Developments in Aluminum Smelting in China (Gao, Wang, Shi, Hu), 35th ICSOBA Conference](https://icsoba.org/assets/files/publications/2017/KN06S - History and Recent Developments in Aluminum Smelting in China.pdf) ICSOBA, 2017
8. [The Role of E-fuels in Decarbonising Transport (revised January 2024)](https://www.iea.org/reports/the-role-of-e-fuels-in-decarbonising-transport) International Energy Agency, 2024
9. [Bloom Energy Corp Form 10-K for fiscal year 2025](https://www.sec.gov/Archives/edgar/data/1664703/000162828026006516/be-20251231.htm) Bloom Energy / US SEC, 2026

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_electrolysers/index.md)
- [Overview](https://www.materialinsights.org/energy/e_electrolysers/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_electrolysers/correlation/index.md)
- [Value chain](https://www.materialinsights.org/energy/e_electrolysers/valuechain/index.md)
- [Unit economics](https://www.materialinsights.org/energy/e_electrolysers/economics/index.md)
- [Engineering methods](https://www.materialinsights.org/energy/e_electrolysers/methods/index.md)
- [Equipment and suppliers](https://www.materialinsights.org/energy/e_electrolysers/equipment/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_electrolysers/links/index.md)