Modified Carbon Cryogel As Tunable Electrocatalyst for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

Abstract

Pt-free catalysts for Oxygen Reduction Reaction (ORR) are broadly studied for Proton Exchange Membrane Fuel Cells (PEMFC) in order to widely develop hydrogen as a fuel for individual transportation. To reduce the performance gap between noble and non-noble catalysts, kinetics and thermodynamics are to be considered. The kinetics of the reaction is influenced by both the turn over frequency of the catalytic site and the density of said sites in the materials. The diffusion of gaseous reactants and liquid water produced is equally important as it modifies the thermodynamic of the reaction. For a long time now, Fe-C-N materials have been identified as the most promising Pt-free electrocatalysts for ORR despite the limited knowledge on the actual structure of the involved sites. Several approaches are currently studied to produce materials with high active site density and high porosity. Different carbon structures are modified to include active sites. The most reported and promising routes are Metal Organic Frameworks, polymers on carbon nanoparticles and silica templated organic structures[1], [2]. With the recent works on the structure and activity of Fe-N4 active sites[3], the carbon plane surrounding active sites seem to have a great impact on the sites’ Turn-Over Frequency (TOF). Cheaper routes are explored to produce tunable carbon structure adapted to the requirement of ORR catalysis. The sol-gel polymerization of organic polymer is a newly investigated route[4], [5]. In this approach, carbon cryogels are chosen for the tenability of their porous structure and the versatility of precursors that can be tested. Porous Fe-C-N catalysts are synthesized in one-step by a modified resorcinol-formaldehyde aerogel method. Gelation, drying and pyrolysis steps are carefully studied and optimized to obtain a high density of accessible catalytic sites. As iron is both a precursor of the active site and a catalyst for the graphitization of carbon during the pyrolysis, its impact on the final structure is studied. Different types of precursors are used in several loadings. The family of materials therefor obtained presents various degrees of graphitization and TOF of the active sites. Structure-Performance relationships are investigated for this family of materials. Electrochemical performances of the materials are assessed in RDE-setup in acidic electrolyte as well as in differential cell of PEMFC. The porosity of the formed materials is measured with nitrogen adsorption. The electrochemical performances in acidic electrolyte are comparable with state of the art materials.[1] A. Serov, K. Artyushkova, et P. Atanassov, « Fe-N-C Oxygen Reduction Fuel Cell Catalyst Derived from Carbendazim: Synthesis, Structure, and Reactivity », Adv. Energy Mater., vol. 4, no 10, p. 1301735, 2014, doi: 10.1002/aenm.201301735.[2] E. Proietti et al., « Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells », Nat. Commun., vol. 2, p. 416, 2011, doi: 10.1038/ncomms1427.[3] T. Asset et P. Atanassov, « Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells », Joule, vol. 4, no 1, p. 33–44, janv. 2020, doi: 10.1016/j.joule.2019.12.002.[4] A. Sarapuu, L. Samolberg, K. Kreek, M. Koel, L. Matisen, et K. Tammeveski, « Cobalt- and iron-containing nitrogen-doped carbon aerogels as non-precious metal catalysts for electrochemical reduction of oxygen », J. Electroanal. Chem., vol. 746, p. 9–17, 2015, doi: 10.1016/j.jelechem.2015.03.021.[5] Y. Wang et S. Berthon-Fabry, « One-Pot Synthesis of Fe-N-Containing Carbon Aerogel for Oxygen Reduction Reaction », Electrocatalysis, nov. 2020, doi: 10.1007/s12678-020-00633-8.