(Invited) Why and How Should the PEMFCs be Recycled? – Focus on the MEA Case

Abstract

Proton-Exchange Membrane Fuel Cells (PEMFCs) are electrochemical devices with potential applications in mobility and stationary energy storage. Last decades have seen a growing interest in the research, the development and the small-scale industrialization of PEMFC. However, PEMFC requires Pt as catalyst for both electrochemical reactions.From the geological point of view, Pt is a scarce element, while between 7 to 12 t of mineral must be extracted to obtain 31.1 g of Pt. Moreover, Pt is considered as a critical raw material by the E.U. with a high supply shortage/breakdown risk.1 The recycling might be a feasible way how to face the current and future supply issues and to secure the access to resources.Although there are currently no legal obligation regarding the PEMFCs recycling, it is highly probable that regulations close to those already existing for batteries will be soon applied, implementing mandatory collection and recycling targets for spent PEMFCs. Finally, this might also be a way to decrease the costs and the environmental impacts of the PEMFCs’ sector as the use of Pt affects both its cost and is considered as a burden from the environmental point of view.2,3 Recycling it might be a feasible option to counteract these issues in a comparable manner to (i) an increase of the Pt catalyst durability, (ii) a decrease of its loading and (iii) its substitution by less noble metals. While the three latter pathways are already widely studied by different research teams, the recycling option has not attracted, up to now, significant attention and will be the scope of this presentation.Two different recycling approaches might be used to recover Pt. Pyrometallurgy is based on thermal treatment of metals, namely their reduction and smelting, while hydrometallurgy is a chemical approach consisting in metals dissolution and further purification. Each of them has its own pros and cons, whereas both approaches can be combined. Both have also been exploited for MEA recycling.Within our research group hydrometallurgical approach has been applied to MEA recycling. This approach is traditionally composed of several successive steps: pretreatment (including shredding and grinding), leaching, separation and purification followed by the final recovery. We have progressively exploited several manners to dissolve platinum. The Pt/C catalyst might be dissolved at room temperature by combining appropriate concentrations of oxidizing and complexing agents. We have namely achieved very high leaching yield when using either H2O2 (3%) and HNO3 (5%) as oxidizer diluted in concentrated HCl playing the role of the complexing agent.4 We have further demonstrated that electro-assisted leaching of Pt might also be feasible if an appropriate electrolyte is combined to the appropriate choice of oxidation and reduction potential that are applied alternatively.Once the Pt is dissolved its purification might be carried out via ion exchange or solvent extraction.4 These operations might also be feasible for its separation from Co in case of Pt3Co alloys presence in the spent MEAs.5,6 Finally, the recovery of Pt might be carried out by precipitation or Pt/C catalyst might be synthetized in a closed-loop manner.5 Two different approaches have been attempted within our research team and both of them have provided a catalyst which exhibited similar performance to a reference Pt/C catalyst in a single-cell PEMFC.5 Moreover, the environmental benefits of the recycling process have been assessed using the LCA methodology.5,7 The results show that primary platinum production remains the most impacting stage, even when MEA recycling is considered. Moreover, the assessment reveals that the MEA life-cycle impacts can be reduced significantly if electrodes recycling is carried out, while the main impact categories decrease is proportional to the platinum recycling rate.References1 European Commission, Study on the Critical Raw Materials for the EU 2023 - Final Report, 2023.2 M. Miotti, J. Hofer and C. Bauer, Int. J. Life Cycle Assess., 2017, 22, 94–110.3 A. Simons and C. Bauer, Appl. Energy, 2015, 157, 884–896.4 L. Duclos, L. Svecova, V. Laforest, G. Mandil and P.-X. Thivel, Hydrometallurgy, 2016, 160, 79–89.5 L. Duclos, R. Chattot, L. Dubau, P. X. Thivel, G. Mandil, V. Laforest, M. Bolloli, R. Vincent and L. Svecova, Green Chem., 2020, 22, 1919–1933.6 M. Gras, L. Duclos, N. Schaeffer, V. Mogilireddy, L. Svecova, Eric Chaînet, I. Billard and N. Papaiconomou, ACS Sustain. Chem. Eng., 2020, 8, 15865–15874.7 L. Duclos, M. Lupsea, G. Mandil, L. Svecova, P.-X. Thivel and V. Laforest, J. Clean. Prod., 2017, 142, 2618–2628.