Post-Mortem Analysis of MNC Catalysts at Different Stages of CO₂ Reduction Reaction

Abstract

For the wellbeing of future generations, the CO2 footprint must be drastically reduced. To address this, CO2 can be used as a starting substance for the fabrication of CO or hydrocarbons as a valuable step in the materials and energy cycles. Metal-nitrogen-carbon catalysts (MNC) are known to be promising materials for the electrochemical CO2 reduction reaction (CO2RR) into various products.1,2 Their activity towards CO2RR has been attributed mainly to molecular structural motifs (MN4 centers) embedded in the carbon network.3,4 The benefit of these single site catalysts is that they offer the possibility of tuning the selectivity and activity by varying the nature of the central metal. However, their suitability for CO2 electrolyzers in the near future depends on the optimization the catalysts for achieving high current densities whilst maintaining the integrity of the active sites. And because high reduction potentials may be required to achieve this, it is important to gain insights on the behavior of MNC under these conditions, as it has been proposed that restructuration of the metal sites may occur.5 Therefore, in this study different post mortem characterization techniques are applied on a group of MNCs for gaining a better understanding of the relation between selectivity and structural changes over time. The preparation of the catalysts was chosen such, that secondary phases in the as-prepared materials were minimized. The synthesis was performed for non-precious and abundant metals such as Fe, Cu, Ni and Sn which also show the best performances for CO2RR.6 The product analysis was done by coupling a gas-fed electrochemical cell with carbon-based gas diffusion electrodes (GDEs) in line with mass spectrometry, subsequently HPLC was used for the liquid product analysis. Techniques such as post mortem XPS, Identic location TEM (IL-TEM) and Raman spectroscopy were used to understand the influence of the applied potential on the changes in product selectivity and morphology of the GDEs.By the comparison of the metal 2p3/2 and N 1s XP-spectra with the onset potentials for the different CO2 reduction products, important changes can be seen to depend on the applied reduction potential. The integrated approach offers unique insights for understanding the role of the metal center in MNC catalysts in pursuit of their adaptation for future applications.(1) Varela, A. S.; Ranjbar Sahraie, N.; Steinberg, J.; Ju, W.; Oh, H.-S.; Strasser, P. Angew. Chemie 2015, 127 (37), 10908–10912.(2) Tripkovic, V.; Vanin, M.; Karamad, M.; Björketun, M. E.; Jacobsen, K. W.; Thygesen, K. S.; Rossmeisl, J. J. Phys. Chem. C 2013, 117 (18), 9187–9195.(3) Leonard, N.; Ju, W.; Sinev, I.; Steinberg, J.; Luo, F.; Varela, A. S.; Roldan Cuenya, B.; Strasser, P. Chem. Sci. 2018, 9 (22), 5064–5073.(4) Karapinar, D.; Tran, N. H.; Giaume, D.; Ranjbar, N.; Jaouen, F.; Mougel, V.; Fontecave, M. Sustain. Energy Fuels 2019, 3 (7), 1833–1840.(5) Karapinar, D.; Huan, N. T.; Ranjbar Sahraie, N.; Li, J.; Wakerley, D.; Touati, N.; Zanna, S.; Taverna, D.; Ga...