
SGL Carbon: testing open loop-recycling of EoL graphite from the solar value chain in bipolar plates
Internal combustion engines are a major contributor to global warming, as the raw material for all fuels that power cars, trucks, airplanes, and ships is fossil oil. One counter-initiative is the switch to electric drives. Purely battery-powered cars are now well known and are becoming increasingly common in everyday life.
Fuel cell drives have also good prospectives. Fuel cells generate electricity through an electrochemical reaction between hydrogen and oxygen from the ambient air. They have particularly good prospects in long-distance transportation, i.e. large trucks and ships. The stationary applications for generating electricity are also already in use. Major challenges in the switch to electric drives lie in the supply infrastructure location, either electricity grids for purely battery-powered vehicles or gas grids for fuel cell engines.
The schematic structure of a fuel cell can be seen below. The two following components are decisive for the costs of a fuel cell:
- The membrane electrode assembly, which contains the anode, cathode and polymer electrolyte membrane, is responsible for 45-55% of the costs of the cell
- The bipolar plate (Hydrogen and Oxidant Flow Field), responsible for 29-32%.
Looking at these amounts, the bipolar plate must be manufactured cost-effectively.
Schematic structure of a PEM Fuel Cell (Wikipedia)
The requirements for the material properties of a bipolar plate are very wide-ranging. The focus is on electrical and thermal conductivity as well as its impermeability to hydrogen. The durability of graphite as a material is particularly important, and the design and tolerances of a bipolar plate are also very demanding. Previously, machined and impregnated isostatic graphite was the preferred material. However, this method did not allow to produce several tens of millions of plates required for the end of the 2020s.
SGL Carbon provides two production routes for this demanding race that can fulfill these complex and far-reaching requirements. The first prototypes have been produced and received positive feedback from customers. These are bipolar plates based on either graphite foil or graphite compound.
The graphite foil can be processed into a bipolar plate using both a compression molding process and a calendaring process. The graphite compound is processed as a powder or granulate using a compression molding process.
Graphite compounds typically consist mainly of graphite and thermoplastic or thermosetting binder systems. These graphites are predominantly derived from petroleum-based starting products and have to be produced using energy-intensive processes. Since the energy balance and the CO2 balance are therefore rather disadvantageous, it is also interesting to use graphite from recycling processes. Even if the recipe amount is only proportionally, this helps to make the balances less disadvantageous.
These elements drove to test available recycling graphites for use as graphite for the production of bipolar plates.
In order to make short-term assessments, milled recycled graphite powder is being electrically tested. The specific powder resistance under different pressing forces is of particular interest here (cf. schematic drawing of the system).
Schematic drawing of test to determine specific powder resistivity
The ICARUS recycling graphites were ground according to specification and then subjected to this test. Unfortunately, the specific powder resistances were significantly higher than the values shown by the graphite used as standard. This is largely due to the graphite structure. Indeed, recycled graphites are predominantly composed of isotropic carbon raw materials, whereas graphites for the bipolar plates require much more aligned structures. The applied sorting process and the quantities envisaged as part of the project did not lead to a meaningful alternative.
In ICARUS, SGL therefore focusses their recycling activities on closed-loop recycling towards isostatic graphite (cf. article “Presentation of closing the loop of isostatic graphite“). These activities allowed to assess which material could be associated to the project’s recycling process.