Pyrolysis of Conductive 2D Coordination Polymers for Use as Solid Acid Fuel Cell Electrode Materials

The U.S. Department of Defense has shown great interest in the development of next-generation hydrogen-based energy systems that have long-term power-output, higher energy density, lower production and maintenance costs, and greater reliability [1]. In pursuit of those goals, solid acid fuel cells (SAFCs), which use solid acid electrolytic membranes (<i>e.g. </i>CsH₂PO₄) and operate at higher temperatures (150 – 300 C) compared to polymer electrolyte membranes (PEMs, &lt;100 C), have shown great promise since their initial discovery two decades ago [2,3]. Yet, due to corrosive and reductive chemistries as well as the higher temperatures required, current electrode paradigms are ill-suited for direct translation from PEM fuel cells. SAFC electrodes need to be both thermally stable and chemically resistant to the corrosive environment of the solid acid electrolyte and fuel streams, porous to facilitate mass transport of fuels to the triple phase boundary, electrically conductive, and catalytic towards hydrogen oxidation or oxygen reduction. Metal-organic coordination polymers [4] are well suited for investigation as precursor compounds for SAFC electrode materials based on the requisite properties for SAFC electrodes. We have investigated several compounds as porous scaffold-like architectures suitable for pyrolysis in inert atmospheres to create metal-doped carbonized materials. These are highly electrically conductive and porous to facilitate nano-Pt catalyst impregnation and gas transport. Materials were characterized by powder X-ray diffraction and thermogravimetric analysis for bulk structure and stability and electrochemical impedance spectroscopy for electrical conductivity while pyrolysis was studied with TEM by <i>in situ</i> heating under vacuum. Pyrolysis of carbon-based coordination polymers generates carbonized materials with greater chemical stability and <i>in situ</i> TEM pyrolysis characterization was utilized to monitor thermally induced changes in the carbonized materials.<br/><br/>[1] Hydrogen fuel cells for Unmanned Systems, Briefing to: DOE Hydrogen and Fuel Cell Technical Advisory Committee, Washington DC https://www.hydrogen.energy.gov/pdfs/htac_mar19_06_swiderlyons.pdf (accessed October 10, 2023)<br/>[2] S. M. Haile, C. R. I. Chisholm, K. Sasaki, D. A. Boysen,T. Uda, <i>Faraday Discussions</i>, 134, (2007<b>)</b> 17-39.<br/>[3] D. A. Boysen, T. Uda, C. R. I. Chisholm, S. M. Haile, <i>Science</i>, 303, (2004) 68-70.<br/>[4] L. S. Xie, G. Skorupskii, M. Dinca, <i>Chem. Rev.</i>, 120, 16, (2020) 8536-8580.