Fuel cells are often described as a promising clean-energy technology because they can produce electricity from hydrogen without directly releasing carbon dioxide at the point of use. But making fuel cells powerful, durable and practical at the same time remains a major engineering challenge.
Now, researchers led by Zhenguo Zhang and his team have developed an unusually thin membrane that could help solve one of the biggest problems in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). The new membrane is only 20 micrometres thick, yet it is mechanically strong, can repair itself, and helps the fuel cell operate efficiently at temperatures as high as 200°C.
Even more impressive, a fuel cell using this membrane reached a peak power density of 3.08 watts per square centimetre at 200°C and showed almost no performance degradation during a 503-hour test.
Why high-temperature fuel cells matter
Polymer electrolyte membrane fuel cells generate electricity through an electrochemical reaction between hydrogen and oxygen.
Hydrogen is supplied to one side of the fuel cell, while oxygen is supplied to the other. The membrane between them allows protons to move through while helping prevent hydrogen from crossing directly to the other side.
In conventional systems, water management and temperature control can become complicated. High-temperature fuel cells offer an important advantage because they can operate at much higher temperatures.
At elevated temperatures, these fuel cells can tolerate certain impurities better and have simpler thermal and water management. This could make them attractive for applications where reliable operation under demanding conditions is important.
However, high-temperature operation creates another challenge: the membrane must remain stable while containing phosphoric acid.
The problem with phosphoric acid membranes
Many HT-PEM fuel cells use polymer membranes containing phosphoric acid (H₃PO₄).
Phosphoric acid is important because it helps transport protons through the membrane, allowing the fuel cell to generate electricity.
But there is a major trade-off.
Phosphoric acid can weaken the polymer membrane. To prevent mechanical damage, conventional membranes are generally made relatively thick, often more than 50 micrometres.
That extra thickness provides mechanical strength, but it also increases the distance that protons must travel through the membrane.
This creates greater ohmic resistance, which means more electrical energy is lost inside the fuel cell.
In simple terms, the membrane needs to be strong enough to survive, but making it thicker can reduce the amount of useful electricity the fuel cell produces.
The researchers wanted to break this trade-off.
A membrane only 20 micrometres thick
The team developed a new phosphoric-acid-doped membrane that is just 20 micrometres thick.
That is less than half the thickness commonly used in conventional systems.
Making such a thin membrane is difficult because reducing its thickness can make it mechanically weaker. A damaged membrane can also allow hydrogen to cross through it, reducing efficiency and potentially affecting fuel-cell durability.
The researchers addressed this problem by introducing copper ions (Cu ions) into the polymer.
These copper ions do more than simply sit inside the membrane. They interact with the polymer chains and create what the researchers describe as dynamic coordination networks.
Think of the polymer chains as long strands. Instead of having them loosely connected, the copper ions act like tiny, reversible connectors between the strands.
This creates a flexible internal network that strengthens the membrane without making it excessively rigid.
A membrane that can heal itself
One of the most interesting features of the new material is its self-healing ability.
The connections formed between copper ions and polymer chains are dynamic. In other words, they can break and reform.
If the membrane experiences mechanical damage, these reversible interactions can reorganize and reconnect.
This gives the material a form of spontaneous self-healing.
The result is a membrane that combines strength, toughness and extensibility while remaining extremely thin.
This is particularly important for fuel cells because membranes experience mechanical and chemical stresses during long-term operation.
A membrane that can tolerate damage and recover some of its structure could potentially last longer than a conventional thin membrane.
Copper also helps phosphoric acid stay inside
The copper ions provide another important advantage.
For the membrane to work efficiently, phosphoric acid needs to remain inside the polymer structure.
If too much phosphoric acid is lost, proton conductivity can decrease and fuel-cell performance can suffer.
The copper-containing network improves phosphoric acid retention through electrostatic interactions.
The researchers also found that the copper ions influence the phosphoric acid molecules themselves.
They can polarize the O–H bonds in phosphoric acid, making it easier for the acid to release protons.
This is important because protons are the charge carriers that move through the membrane during fuel-cell operation.
So the copper ions perform several jobs at once: they strengthen the polymer, help it heal, retain phosphoric acid and improve proton transport.
Very low resistance
The thin membrane achieved an ohmic resistance of only 0.06 Ω cm².
This is a significant result because membrane resistance is one of the factors that can limit fuel-cell performance.
A thinner membrane generally gives protons a shorter path to travel.
Imagine trying to move through a tunnel. A shorter tunnel requires less travel distance. Similarly, reducing membrane thickness can reduce the resistance experienced by protons moving through the fuel cell.
But the challenge is ensuring that the membrane does not become so thin that hydrogen begins crossing through it.
The new membrane managed to maintain a low hydrogen crossover current density of just 0.95 mA cm⁻².
That means the researchers were able to make the membrane very thin without creating excessive hydrogen leakage.
More power from a smaller membrane
The performance of the complete fuel cell was particularly impressive.
Using hydrogen and oxygen, the fuel cell reached a peak power density of 3.08 W cm⁻² at 200°C.
Power density describes how much power a fuel cell can generate from a given active area.
A higher power density means that more electricity can potentially be produced from a smaller fuel-cell stack.
This could be important for applications where size and weight matter.
The ability to operate at 200°C also demonstrates the membrane's ability to function under demanding high-temperature conditions.
Long-term durability
High power is useful, but a fuel cell also needs to maintain its performance over time.
The researchers therefore tested the fuel cell for 503 hours at 160°C while operating at a current density of 1.0 A cm⁻².
The membrane showed negligible degradation during this test.
That result is important because fuel-cell technologies must survive thousands of hours of operation in many real-world applications.
Although a 503-hour test does not prove that the membrane will operate reliably for years, it provides encouraging evidence that the dynamic copper-polymer structure can remain stable under demanding conditions.
What could this mean for future fuel cells?
The research demonstrates a clever approach to solving a long-standing problem.
Instead of simply making the phosphoric-acid membrane thicker to improve its mechanical strength, the researchers strengthened the polymer at the molecular level.
The copper ions essentially create a dynamic internal support system while also improving the chemical environment needed for proton transport.
This allows the membrane to remain extremely thin without sacrificing mechanical robustness.
If the approach can be scaled up and proven over much longer operating periods, it could help improve the power density and efficiency of high-temperature fuel cells.
The technology could eventually contribute to more compact and powerful fuel-cell systems for transportation, stationary electricity generation and other applications.
A small membrane with a big impact
The most important lesson from this research is that thinner does not always have to mean weaker.
The new 20-micrometre membrane combines several desirable properties in one material: high mechanical toughness, self-healing capability, improved phosphoric acid retention, efficient proton transport and low hydrogen crossover.
Its 0.06 Ω cm² resistance, 3.08 W cm⁻² peak power density at 200°C, and stable operation for 503 hours demonstrate the potential of this strategy.
There is still work to be done before such membranes can become a widespread commercial technology. Long-term durability, large-scale manufacturing and performance under real operating conditions will all need further investigation.
Nevertheless, this research offers a promising new direction for high-temperature fuel cells.
By engineering the membrane at the molecular level, researchers may have found a way to make fuel cells thinner, tougher and more powerful at the same time.
Reference: Zhang, Z., Zhang, Q., Li, W. et al. Thin membranes with Cu-ion crosslinking for high temperature polymer electrolyte membrane fuel cells. Nat Energy 11, 1043–1054 (2026). https://doi.org/10.1038/s41560-026-02049-y
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