Hydrogen technology - adhesive foil for PEM fuel cells

Alternative "self-adhesive coating" for short cycle times

However, in order to achieve particularly high cycle rates, the heating step for melting the adhesive can be troublesome. For this reason, CMC Klebetechnik also offers a version that sticks at room temperature (CMC 61145). In this case, the foil coated with the catalyst layer can be connected to the subgasket foil by simple "placement". No heating is necessary, especially heating that could reach the load limits of the CCM.

Investigations have shown that the acrylic adhesive used by CMC Klebetechnik still increases slightly in its adhesive strength even under the typical cell loads (temperature and hydrolysis).

Due to the considerable pressure load within fuel cell stacks, mechanical deformation of the subgasket foil can occur, depending on the geometries of the seals and bipolar plates used. Therefore it is always possible to use thicker foils (50µm, 75µm - depending on availability). Kapton® foils are a possible alternative, especially due to their high stiffness even at increased temperatures.

Customer-specific changes - in the simplest case modifications of the material thicknesses - are possible in a joint development, as are adhesive systems adapted to special requirements (e.g. adhesion to repellent surfaces, SPEEK, Nafion® from DuPont, Caltec® from BASF, for alkaline fuel cells (AFC) or phosphoric acid fuel cells (PAFC) and other special forms).

In addition to adhesive foils for the production of the frame foil (sub-gasket), there is also the possibility of bonding moistening fleeces in a sealing manner. These fleeces improve the service life and efficiency of larger fuel cells by actively influencing the moisture content of the PEM membrane. Air humidification is an important component of many PEM fuel cells for optimum membrane humidification. Durable and reliable sealing and bonding are typical tasks for acrylic adhesive tapes.

High temperature PEM cells

Heat management on fuel cell stacks

As with any conversion process, waste heat is also generated during "cold combustion" in the fuel cell. This must be discharged to prevent overheating. One of the heat management techniques is to connect the fuel cell to the coolant circuit of a hybrid vehicle. Alternatively, a separate coolant circuit with heat exchanger and fan can be used.

The thermal connection of elements of the heat path requires in many cases a mediating layer. Dimensional tolerances, warping and differences in thermal expansion can lead to air gaps that considerably impede heat transfer.

Heat exchangers, air coolers, condensers, cooling plates or heating elements for preheating cryogenic NT-PEM stacks can be thermally bonded with heat conductive silicone foils from CMC Klebetechnik. The flexible, elastic foils are highly thermally conductive and compensate for manufacturing tolerances and other unevenness. Due to the 100% wetting of the surfaces, the maximum possible cross-sectional area is available to the heat flow. The products can be used at continuous temperatures of up to 200°C.

Fuel cells for hybrid drives

It is still a dream of the future that regenerative energy could also be massively used in road traffic. As with lithium-ion battery-powered fully electric or hybrid vehicles, a satisfactory infrastructure is still not in place at present. Isolated solutions have been in place for well over a decade. City buses have been running on hydrogen propulsion for a considerable time. This hydrogen can also be used for fuel cells. But there are still not enough filling stations for H2.

Nevertheless, the fuel cell has decisive advantages for range extender vehicles (hybridisation). Unlike current solutions, drive technologies from two worlds are not required (combustion engine and electric drive). The relatively simple system of a fuel cell, which in continuous operation simply recharges the battery permanently, significantly simplifies the design of hybrid vehicles. For heavy vehicles and vehicles with a long range, the fuel cell will even be able to be used as the main drive in the future.

But there is still a lack of market drivers such as TESLA, which has an extensive network of charging stations, forcing the often much larger co-suppliers to follow suit. Nevertheless, many experts predict that fuel cell technology will finally find its way into the mainstream in the next decade.

Sealing foils for subgasket assembly in FC and RFB stacks

Fuel cells, redox-flow batteries and also e.g. lithium-ion batteries have one thing in common: They use an inner separation layer as ion exchanger. This separation layer is also called separator or membrane.

Adapted to the respective technology, manufacturers use very different materials for this separation layer, whose main task is to separate the two electrolytes that give off or take up electrons. In addition to chemical resistance, ion conductivity is therefore an important criterion. In addition, there are tasks that are more likely to serve safety purposes, for example: Highly temperature-resistant separators in lithium-ion batteries prevent metal fires even with faulty operation. In the stacks of fuel cells, the membrane foil in the form of a frame not only serves as an assembly aid, but also enables cost-effective production in a roll-to-roll process (or immediately roll-to-punch). In the stack of the redox-flow battery, it makes a significant contribution to keeping the two electrolytes safely separated. It chemically seals the two half-cells and thus prevents direct mixing.

For fuel cells and redox-flow batteries, CMC Klebetechnik offers products that allow a combination with the actual ion exchange membrane. Depending on the operating temperature (NT-PEMFC, HT-FC), process-typical media (e.g. vanadium-based, methanol, water) or mounting method (gluing, heat-sealing), different foils can be offered. Even membrane foils that are difficult to bond, such as Nafion® from DuPont, can thus be clamped in a frame that functions as a seal on the one hand and also allows the active area to be fixed mechanically (subgasket frame).

Possible structure of an adhesive tape for complete bonding of the MEA including subgasket foil and associated bipolar plates

By double-sided coating with different adhesives, the PEM functional membrane can be provided with a subgasket frame in a first step. For this purpose, a pressure-sensitive adhesive (adhesive coating B) is used on the PEM membrane side, which does not "poison" the functional coating of the anode and cathode side. After this manufacturing step, either the Gas Diffusion Layer (GDL) or the associated bipolar plates can be finished bonded to the Membrane Electrode Assembly (MEA) under pressure and temperature with a heat activatable adhesive (adhesive coating A). Stacking this assembly in a fuel cell stack is greatly simplified due to the greater mechanical strength of the unit.