Role of Humidification in PEM Fuel Cell

Proton Exchange Membrane (PEM) fuel cells are at the forefront of clean energy technology due to their high efficiency and low emissions, and one critical factor influencing their performance and longevity is humidification. This article covers the role of humidification in PEM fuel cells and compares self-humidified and externally humidified systems to determine which approach suits different applications.

Importance of Humidification in PEM Fuel Cells

PEM fuel cells rely on a hydrated membrane to conduct protons from the anode to the cathode. If the membrane dries out, its conductivity decreases, leading to reduced performance and potential damage. Excessive water, on the other hand, can flood the electrodes, obstructing gas flow and decreasing efficiency. Maintaining an optimal hydration level is therefore crucial for effective PEM fuel cell operation.

Comparing Self-Humidified and Externally Humidified Fuel Cells

Self-Humidified Fuel Cells

Self-humidified fuel cells utilize the water produced during the electrochemical reaction to maintain membrane hydration, eliminating the need for external humidification equipment entirely.

Advantages include simplified system design (lower overall complexity and cost by eliminating external humidification hardware), compactness (beneficial for space-constrained applications like portable electronics, drones and passenger vehicles), and energy efficiency (no additional energy needed to run external humidifiers).

Challenges include limited control over hydration level, which can cause performance fluctuations under varying operating conditions, and a real risk of membrane dehydration in low-humidity environments or during transient operation.

Design strategies for self-humidification include hydrophobic cathode designs that use capillary action to draw water back into the membrane, integrated humidification channels that distribute produced water evenly across the membrane surface, and recycled water from cathode exhaust, where exhaust water vapor is recirculated to humidify incoming reactant gases. Performance depends heavily on operating conditions and how evenly water distributes across the membrane.

Externally Humidified Fuel Cells

Externally humidified fuel cells use dedicated external devices to supply water vapor to reactant gases before they enter the fuel cell, allowing precise control over humidity levels.

Advantages include controlled humidification (precise humidity control ensures optimal membrane hydration and consistent performance across operating conditions), adaptability to varying environmental conditions and loads, and enhanced durability by preventing both dehydration and flooding.

Challenges include added complexity, weight and cost from the external humidifier hardware, and additional energy consumption to operate it, which can reduce overall system efficiency.

Common techniques include steam injectors that directly inject steam into the gas stream, membrane humidifiers that use a semi-permeable membrane to transfer water vapor from a humidified stream to dry reactant gases, and bubble humidifiers that pass reactant gases through a water bath to pick up moisture. These methods ensure consistent, controlled humidity regardless of external conditions, which is why they remain common in test station and laboratory setups where precise, repeatable test conditions matter more than compactness.

Real World Examples

Toyota Mirai is the most well-known real-world example of self-humidification done at commercial scale. Toyota's own published fuel cell teardown describes the Mirai's system as "humidifier-less," a world-first achieved by recirculating water generated at the cathode back to the anode side to keep the membrane hydrated, eliminating the external humidifier entirely and reducing system size, weight, and cost compared to Toyota's earlier FCHV-adv system.

Toshiba's stationary fuel cell near the Fukushima Hydrogen Energy Research Field (FH2R) is another documented self-humidification example, not an externally humidified one. Toshiba's own materials describe this solid polymer fuel cell stack as internally humidified and dehumidified automatically, explicitly eliminating the need for an external humidifier, and crediting this design for the system's durability and stability.

For genuine examples of externally humidified systems, look to typical laboratory and test station equipment, where dedicated membrane or bubble humidifiers are standard precisely because researchers need precise, repeatable control over test conditions rather than the compactness self-humidification offers.

Conclusion

The choice between self-humidified and externally humidified PEM fuel cells depends on specific application requirements. Self-humidified systems, exemplified by Toyota's Mirai, offer a simpler, more compact solution with cost and efficiency benefits well suited to vehicles and portable devices. Externally humidified systems, common in laboratory and stationary power applications, provide the precise control and consistency needed where performance repeatability matters more than compactness. Understanding this trade-off, and correctly identifying which real-world systems use which approach, is essential for specifying the right fuel cell setup for a given application.