(Invited) Linking Ionomer/Electrocatalyst Interactions to Membrane Electrode Assembly Performance and Durability in Proton Exchange Membrane Fuel Cells

Abstract

The proper integration of materials to enable high performing low temperature fuel cell operation has never been more crucial to realizing the isolated (e.g. half-cell or ex-situ) improvements of advanced electrocatalysts, supports and ionomers in an effort to maximize efficiency. The integration pathway for oxygen reduction electrocatalysts, from initial testing in a rotating disk electrode (RDE) to their apex form as porous electrodes in a membrane electrode assembly (MEA) is replete with potential pitfalls. Along the route to improving MEA level electrocatalyst kinetics and electrode transport properties are numerous choices relating to ink formulation (i.e. solvents and solvent ratios), ink processing and deposition methodology, ionomer content and chemistry and electrochemical MEA conditioning. Combined with carbon morphology/porosity, such variables can alter the final electrode morphology, influencing active site accessibility and ORR activity as well as oxygen transport resistance and electrode proton conductivity.[1–5]Here, the development and utilization of in situ diagnostics designed to probe specific phenomena (e.g. catalyst/ionomer interactions, molecular and Knudsen diffusion, electrochemical active site accessibility and catalyst layer proton resistance) will be presented as tools to contrast the ink, processing and fabrication methods necessary to achieve high performance PGM (e.g. Pt/C and Pt alloy/C) and PGM-free (e.g. FeNx/C) electrocatalysts based electrodes. For PGM based MEAs, the dynamic effects of electrochemical conditioning will be examined in an effort to both baseline MEA level performance and decouple the contributions of simultaneous electrocatalyst particle growth and the removal of system impurities.[6] The prerequisite understanding gleaned from the application of electrochemical diagnostics and MEA conditioning procedures will be combined with the development of a novel sulfonated ionic liquid block copolymer ionomer and tailored integration methodology to illustrate a pathway towards improved MEA level performance.[7] Lastly, alterations in ionomer interactions as a function of accelerated stress tests will be discussed, providing insight into the evolving electrode structure as the community targets more robust systems capable of efficient operation for 25,000 hrs.[8]References[1] A. Orfanidi, P.J. Rheinlander, N. Schulte, H.A. Gasteiger, Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC, Journal of The Electrochemical Society. (2018) 11.[2] S. Khandavalli, J.H. Park, N.N. Kariuki, D.J. Myers, J.J. Stickel, K. Hurst, K.C. Neyerlin, M. Ulsh, S.A. Mauger, Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks, ACS Appl. Mater. Interfaces. (2018) 13.[3] D.A. Cullen, R. Koestner, R.S. Kukreja, Z.Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H.M. Meyer, C.M. Parish, K.L. More, Imaging and Microanalysis of Thin Ionomer Layers by Scanning Transmission Electron Microscopy, J. Electrochem. Soc. 161 (2014) F1111–F1117. https://doi.org/10.1149/2.1091410jes.[4] T. Van Cleve, S. Khandavalli, A. Chowdhury, S. Medina, S. Pylypenko, M. Wang, K.L. More, N. Kariuki, D.J. Myers, A.Z. Weber, S.A. Mauger, M. Ulsh, K.C. Neyerlin, Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application, ACS Appl. Mater. Interfaces. 11 (2019) 46953–46964. https://doi.org/10.1021/acsami.9b17614.[5] G. Wang, L. Osmieri, A.G. Star, J. Pfeilsticker, K.C. Neyerlin, Elucidating the Role of Ionomer in the Performance of Platinum Group Metal-free Catalyst Layer via in situ Electrochemical Diagnostics, J. Electrochem. Soc. 167 (2020) 044519. https://doi.org/10.1149/1945-7111/ab7aa1.[6] S. Kabir, D.J. Myers, N. Kariuki, J. Park, G. Wang, A. Baker, N. Macauley, R. Mukundan, K.L. More, K.C. Neyerlin, Elucidating the Dynamic Nature of Fuel Cell Electrodes as a Function of Conditioning: An ex Situ Material Characterization and in Situ Electrochemical Diagnostic Study, ACS Appl. Mater. Interfaces. 11 (2019) 45016–45030. https://doi.org/10.1021/acsami.9b11365.[7] Y. Li, T.V. Cleve, R. Sun, R. Gawas, G. Wang, M. Tang, Y.A. Elabd, J. Snyder, K.C. Neyerlin, Modifying the Electrocatalyst−Ionomer Interface via Sulfonated Poly(ionic liquid) Block Copolymers to Enable High- Performance Polymer Electrolyte Fuel Cells, ACS Energy Letters. (2020) 6.[8] D.A. Cullen, K.C. Neyerlin, R.K. Ahluwalia, R. Mukundan, K.L. More, R.L. Borup, A.Z. Weber, D.J. Myers, A. Kusoglu, New roads and challenges for fuel cells in heavy-duty transportation, Nature Energy. (2021) 1–13. https://doi.org/10.1038/s41560-021-00775-z.