Degradation and Stabilization of Porphyrin-Based Metal Organic Framework Electrocatalysts for the Oxygen Reduction Reaction

Abstract

One limiting factor in the efficiency of hydrogen fuel cells is poor stability and activity of non-platinum group catalysts for the oxygen reduction reaction (ORR). Porphyrin metal organic frameworks (PMOFs) are an interesting class of materials that have seen a large increase in interest as electrocatalysts for the ORR due to the potential for confinement-based activity enhancement.1 Characterizing the fundamental or intrinsic activity of these catalysts is convoluted due to the high activity of the porphyrin precursor, their low electrical conductivity, and the unclear active site accessibility within the 3D structure. We have characterized the stabilization and degradation of Co-based PMOFs (CoPMOFs) compared to their metal porphyrin precursor as a function of pH and electrolyte composition. CoPMOFs were synthesized via a clean, non-pyrolysis technique, in which the cobalt-tetrakis(4-carboxyphenyl)porphyrin (CoTCPP) precursor was coordinated in a face-to-face orientation with aluminum-based nodes.2,3 As deposited structures were characterized using x-ray diffraction. Using coupled transmission electron microscopy and quantitative elemental analysis (using an inductively coupled plasma) we show that the framework structure can degrade while the catalyst retains activity. This framework degradation was monitored by measuring the relative Al and Co dissolution (see attached figure). Degradation of the framework, as seen by loss of ~ 80 % of the Al, but only ~ 15% of the Co at pH 1, indicates that the Co-N4 porphyrin rings are more important in activity than the framework-structure. This suggests that the electrochemical evaluation of CoPMOFs in pH 1 electrolyte is largely a measurement of the precursor performance. We further demonstrate the stabilization of the 3D structure of CoPMOF nanoparticles in near-neutral, phosphate buffered conditions, however, there are no clear activity enhancements due to the framework. We hypothesize the leaching of the metal-porphyrin site and not the destruction of the 3D framework is responsible for loss of activity in these types of catalysts. While no structure-based enhancement was evident, it is important that we were able to find the optimal electrochemical testing conditions to evaluate these structure-activity relationships. This work highlights the importance of rigorous post-test or operando structural-electrochemical analyses for the validation of complicated activity mechanisms and provides simple engineering controls to facilitate the improvement of these analyses. References (1) Kulkarni, A.; Siahrostami, S.; Patel, A.; Nørskov, J. K. Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction. 2018, 118, 2302–2312. (2) Fateeva, A.; Chater, P. A.; Ireland, C. P.; Tahir, A. A.; Khimyak, Y. Z.; Wiper, P. V.; Darwent, J. R.; Rosseinsky, M. J. A Water-Stable Porphyrin-Based Metal-Organic Framework Active for Visible-Light Photocatalysis. Angew. Chemie - Int. Ed. 2012, 51, 7440–7444. (3) Lions, M.; Tommasino, J.-B.; Chattot, R.; Abeykoon, B.; Guillou, N.; Devic, T.; Demessence, A.; Cardenas, L.; Maillard, F.; Fateeva, A. Insights into the Mechanism of Electrocatalysis of the Oxygen Reduction Reaction by a Porphyrinic Metal Organic Framework. Chem. Commun. 2017, 53, 6496–6499. Figure 1