Researchers and engineers working on PEM fuel cells usually acknowledge that membrane-electrode assemblies (MEAs) are the “heart” of these devices from an electrochemical standpoint. However, continuing the analogy of the human body, it is actually the other elements (bipolar plates, terminals, current collectors, etc.) and systems (sealing, cooling, gas supply, etc.) that form the “skeleton” the “muscles” and other “vital organs” that ensure the correct operation of the cell.

Work on optimising systems and processes has intensified since 2006 by taking advantage of LIFTEC’s technological know-how. The list includes cell architecture and the cooling, sealing and gas supply systems, all of which exert a strong influence on the final performance of medium-power PEM fuel cells.

Optimisation of membrane-electrode assemblies (MEAs)

Although this is not one of the group’s main objectives, LIFTEC owns automated equipment for highly precise and repetitive spraying of catalyst inks on the membrane or diffusion layer to create three-layer MEA systems. The centre also has the technology needed to create five-layer MEAs using hot pressing.

Experience and improvements in this area’s technology have optimised deposition and pressing parameters to attain uniform catalyst layers and MEAs with a low ohm resistance.

Five-layer MEA developed entirely at the laboratory with a membrane made of Nafion™ and carbon paper

Analysis of the thickness of the catalyst layer using SEM

Robot used to deposit catalyst ink on the polymer membrane

Development of new sealing systems

An efficient sealing system is essential for the correct operation of PEM fuel cells due to the physical characteristics of hydrogen. This is the smallest molecule in the periodic table and, therefore, the one that is most likely to leak out if there is a minimal means of it doing so. That is why special attention needs to be paid to the design of sealing systems and the materials used for them.

Seals and joints are the elements responsible for preventing gas leaks to the exterior of the fuel cell or the exchange of gases from one face to another in the same cell. They can be individual elements or components integrated in the electrode.

The design and optimisation of the sealing system largely depends on the size of the fuel cell. Although silicon sheets are normally used, this option is not financially viable for large MEAs or fuel cells with many cells. That is why we need to develop new sealing systems that can adapt to every specific geometry and size to minimise costs. The LIFTEC Fuel Cell Group has developed its own seal manufacture process in which a programmable robot dispenses a sealing material to seal the membrane-electrode assemblies and then integrate them into the bipolar plates as single components regardless of the size of the plate or the geometry of the MEA. In some cases, this process includes integrating O-rings for the main manifolds of reactant gases.

Typical sealing system using silicon sheets for low-power fuel cells

Depositing liquid joints using a robot dispenser

MEA integrated in the bipolar plate by the sealing process developed at LIFTEC for low-temperature and medium-power fuel cells

Development and optimisation of more efficient cooling systems

The LIFTEC Fuel Cell Group has developed integrated cooling systems based on simple thermal management models that can keep operating temperature parameters within appropriate ranges.

The technologies developed depend on the required power and there are prototypes with closed or open cathodes. For powers under 100 W, the manufactured fuel cells have closed cathode systems with passive cooling based on natural convection with the medium. However, for higher powers (up to 2 kW), we have developed open cathode fuel cells cooled by passive methods (natural convection) or by active systems using forced convection assisted by integrated compact fans or even prototypes for closed cathode fuel cells cooled by water.

Various fuel cells with powers ranging from a few watts to 2 kW developed at LIFTEC with different cooling systems

In this section, the fluid-dynamic characterisation of the air through the cell’s cathode channels is essential to select and optimise the operation of the cooling system. That is why the Fuel Cell Group has concentrated on the experimental characterisation of load losses that occur when the air is forced through cathode geometries and on obtaining reliable correlations to determine friction factors in the cathode channels.

Effect of the depth of the cathode channels on load loss based on the current generated in open cathode fuel cells

An experimental way of determining the efficiency of cooling systems is to measure surface temperature using thermocouples or 2D thermography. The group has several types of thermocouples especially calibrated for the temperature range to be measured and an infrared thermal imaging camera, which can measure surface temperature precisely without coming into contact with it. The latter is an especially useful technique for obtaining temperature maps of the fuel-cell surface during operation to easily identify hot points and assess not only the performance of the cooling systems but also detect possible irregular operation.

The figure shows an image of the deficient temperature distribution of a commercial fuel cell and another with a homogenous temperature distribution manufactured at LIFTEC with an optimised cooling system. The values of the energy the hot cell emits in infrared spectrum wavelengths are shown as temperatures based on a colour table.

Surface temperature map of two low-temperature fuel cells with a power of 2 kW obtained using infrared thermography. The one on the left is a commercial Horizon-2000 and the one on the right was manufactured at LIFTEC.

Optimisation of the assembly and sealing processes of medium-power cells

The cells manufactured so far at the LIFTEC facilities range from 0.5 W to 3 kW of nominal power. Since most of these fuel cells were designed for use in mobile devices, optimising their final weight and size is especially important. Our experience in using materials that weigh little, but still have great mechanical strength, has made it possible to increase the power density in the manufactured fuel cells. By optimising cooling systems and integrating them in the fuel cell, we have managed to obtain compact devices, thus minimising the use of external ancillary equipment.

The group has the instruments it needs to assemble fuel cells with many cells. This means we can gradually improve the final assembly systems of the cell stacks and optimise the designs of terminal plates.

Assembly phases of fuel cells whose electric power is 2–3 kW

Optimised design of a terminal plate that allows for a homogenous distribution of the tightening torque, thus minimising the final stack weight

Evaluation and electrochemical characterisation of fuel cells

The group owns experimental facilities and measurement equipment to evaluate and electrochemically characterise the fuel cells manufactured at LIFTEC. The dual test bench with the ancillary electronic load modules and the potentiostat/galvanostat make it possible to control and measure reagent input conditions (temperature, pressure, flow, moisture) and, therefore, electrochemically characterise the fuel cells in a wide range of operating conditions.

It is worth highlighting that the dual test bench is a unique piece of equipment in Spain. It can condition fuel cells and obtain operating curves (polarisation) from a few watts up to 3 kW. For that purpose, this equipment has two parallel lines for each reagent gas (hydrogen and oxygen/air) with appropriate mass controllers for the range of flow rates to be used depending on fuel cell size.

The membrane-electrode assemblies and the electrochemical performance of the manufactured fuel cells are characterised using complex impedance spectroscopy. This same technique is used to optimise the sealing strength of the fuel cells, thus minimising the global internal resistance of the stack.

Polarisation curves obtained for the two stacks with an electrical power of 1.5 kW and manufactured to be integrated in the hybrid power system of a multipurpose electric vehicle

Complex impedance analysis of a high-temperature MEA

Optimisation of the sealing strength of a small high-temperature stack formed by five cells with an active area of 605 cm² using complex impedance spectroscopy