# Fuel cells: Science and limits

Why hot ceramic cells beat cool platinum ones on efficiency

Source page: https://www.materialinsights.org/energy/e_fuel_cells/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.

A fuel cell lets hydrogen (or a hydrogen-rich fuel) combine with oxygen from air without a flame. Electrons are sent round an outside circuit as direct current, leaving water and heat. There is no heat-engine step, so the Carnot limit does not apply; the ceiling is set by the chemistry.

#### The conversion, step by step
**Ideal reaction**
Chemical (H2) → Electricity plus unavoidable heat · At most about 83% of the higher heating value (HHV) at 25 C (calculated: Gibbs energy over enthalpy)^[15](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=1)**PEM stack**
Chemical (H2) → DC electricity plus heat · Peak about 64.5% of lower heating value (LHV), 2024 truck system^[1](https://www.energy.gov/documents/record-26002-heavy-duty-fuel-cell-system-cost-2024)**Balance of plant (air compressor, pumps, cooling) and ageing**
Gross DC → Net DC · System 56% LHV when new, 50% LHV at end of life (heavy-duty PEM)^[1](https://www.energy.gov/documents/record-26002-heavy-duty-fuel-cell-system-cost-2024)**Stationary solid-oxide unit on natural gas**
Chemical (natural gas) → AC electricity · 65% falling to 53% LHV over the life of the cells (maker's figure)^[2](https://www.bloomenergy.com/wp-content/uploads/bloom-energy-server-datasheet-2025.pdf)**Heat recovery (combined heat and power)**
Waste heat → Useful heat · About 60% electrical, about 90% total, for a hydrogen solid-oxide unit (maker's claim)^[4](https://www.businesswire.com/news/home/20240805342400/en/Bloom-Energy-Announces-Hydrogen-Solid-Oxide-Fuel-Cell-with-60-Electrical-Efficiency-and-90-High-Temperature-Combined-Heat-and-Power-Efficiency)

#### The science

This is electrochemistry. In a proton-exchange membrane (PEM) cell a platinum catalyst splits hydrogen into protons and electrons; the protons cross a thin, damp polymer membrane, the electrons go round the circuit, and both meet oxygen at the cathode to form water. A solid-oxide cell (SOFC) does it the other way round: a ceramic carries oxide ions (O2-) from the air side to the fuel side, which it can only do when hot (about 500 to 1,000 C). The work available is the Gibbs free energy, 237 kJ per mole of H2 at 25 C; divided by the charge on 2 electrons this gives an ideal voltage of 1.23 V (calculated). Real cells run lower, losing voltage to the slow oxygen reaction, to resistance in the electrolyte and to gas transport. Against the 1.48 V that matches the full heat of reaction (HHV), every 0.1 V lost costs about 7 percentage points of efficiency (calculated). Losses rise with current, so more cell area buys efficiency at the price of more catalyst.

#### Limits set by science

| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| **Gibbs energy limit** | About 83% of HHV at 25 C; about 78% of LHV near 730 C (calculated)^[1](https://www.energy.gov/documents/record-26002-heavy-duty-fuel-cell-system-cost-2024)^[2](https://www.bloomenergy.com/wp-content/uploads/bloom-energy-server-datasheet-2025.pdf) | Part of the reaction energy must leave as heat. The ratio falls as temperature rises, but hot cells lose far less voltage, so they end up more efficient. | PEM truck system 64.5% LHV peak (DOE 2030 target 68%); solid-oxide units up to 65% LHV on gas |
| **Platinum catalyst** | 0.35 mg platinum per cm2 in the 2024 truck design (0.45 in 2023)^[1](https://www.energy.gov/documents/record-26002-heavy-duty-fuel-cell-system-cost-2024) | Low-temperature cells need platinum to speed up the oxygen reaction. | 53% of stack cost in the DOE 2024 truck design |
| **Durability** | 25,000 hours design life for a truck^[1](https://www.energy.gov/documents/record-26002-heavy-duty-fuel-cell-system-cost-2024) | Catalyst particles dissolve and grow, and membranes thin; the DOE design makes the stack 67% bigger than needed when new to cope. | Efficiency falls from 56% to 50% LHV over life |

#### Sources

1. [Hydrogen Program Record 26002: Heavy-Duty Fuel Cell System Cost - 2024](https://www.energy.gov/documents/record-26002-heavy-duty-fuel-cell-system-cost-2024) US Department of Energy, Hydrogen and Fuel Cell Technologies Office, 2026
2. [Bloom Energy Server 6.5 datasheet](https://www.bloomenergy.com/wp-content/uploads/bloom-energy-server-datasheet-2025.pdf) Bloom Energy (company data), 2025
3. [Bloom Energy announces hydrogen solid oxide fuel cell with 60% electrical efficiency and 90% combined heat and power efficiency (company press release)](https://www.businesswire.com/news/home/20240805342400/en/Bloom-Energy-Announces-Hydrogen-Solid-Oxide-Fuel-Cell-with-60-Electrical-Efficiency-and-90-High-Temperature-Combined-Heat-and-Power-Efficiency) Bloom Energy via Business Wire, 2024
4. [NIST Chemistry WebBook: Water (standard enthalpy and Gibbs energy of formation)](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=1) US National Institute of Standards and Technology, 2023

## Other sections

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