Evaluation of the electrocatalyst performance data includes an electrode preparation step. Herein, we compare the structural composition of Fe−N−C materials, used to electrocatalyze the oxygen reduction reaction in proton-exchange membrane fuel cells, before and after catalyst layer preparation. The effects of this step on the electronic structure and local coordination of Fe were investigated by X-ray absorption (XAS) and emission spectroscopies (XES), for Fe−N−C materials prepared via different synthetic routes. This work underlines the importance of determining the Fe−N−C catalyst structure in the prepared electrode for further studies of the structure−activity−stability correlations.
The membrane electrode assembly (MEA) is the core component of the fuel cell stack. In it, the conversion of chemical energy into electrical energy takes place at the catalyst layers. The functional capability of the MEA is an indispensable prerequisite for the operation of the fuel cell stack and thus plays a decisive role in defining its stability. In addition to the selection of suitable materials, the mode of operation plays a major role in the durability of the MEA. It is known from the literature that electrochemical degradation reactions generally depend on operating parameters such as temperature, potential and dynamics. [1] Therefore, in order to derive a durability promoting mode of operation for the PEM Fuel Cell as well as to make lifetime predictions, it is necessary to identify and understand the processes occurring during the aging of the MEA. This work deals with the investigation of the influence of the operating parameters in the drive cycle as an important operating mode on the aging of the cathode catalyst layer in the MEA. According to the state of the art, nanoparticulate platinum supported on carbon (Pt/C) is usually used as the cathode catalyst. The degradation of Pt/C has been intensively studied in recent years. Platinum dissolution and Pt agglomeration are discussed as the main causes of Pt/C degradation during PEMFC operation. [2, 3] Studies have shown that the dissolution rate generally increases with potential and can accelerate under potentiodynamic conditions. [1] Dissolution of Pt from the catalyst, transport of Pt ions through the electrode, and precipitation of Pt in the membrane, possibly due to reduction of Pt ions by the H2 crossover from the anode can be associated with the loss of cathode electrochemical surface area (ECSA) and thus irreversible performance loss. [4] The primary objective of this work is to investigate the influence of different operating parameters such as temperature and potential range on the degradation rate of the cathode using an accelerated stress test (AST) which was developed to simulate an analogous to real drive cycle operation of a PEM fuel cell in a vehicle. A secondary objective is to evaluate the AST itself in order to determinate its capability of fuel cell lifetime prediction. For that the transferability of the observed degradation rates are to be evaluated on different integration levels such as single-cell PEM test bench and short-stack multi-cell test bench. As a measure to evaluate the transferability of the accelerated stress tests, the loss of electrochemical active surface area (ECSA) serves as an indirect measure of catalyst degradation and the resulting Pt particle size distribution which is determined by TEM serves as a direct measure. The derivation of acceleration factors for the lifetime tests of the different integration levels would allow an early estimation of the lifetime, this way test time and cost could be significantly saved in the evaluation of new materials. References [1] Borup, R. L., Davey, J. R., Garzon, F. H., Wood, D. L., and Inbody, M. A. 2006. PEM fuel cell electrocatalyst durability measurements. Journal of Power Sources 163, 1, 76–81. [2] Mahlon S. Wilson, Fernando H. Garzon, Kurt E. Sickafus, and Shimshon Gottesfeld. Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells. In J. Electrochem. Soc., 2872–2877. [3] Yu, X. and Ye, S. 2007. Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC Part II: Degradation mechanism and durability enhancement of carbon supported platinum catalyst. Journal of Power Sources 172, 1, 145–154. [4] Ahluwalia, R. K., Arisetty, S., Wang, X., Wang, X., Subbaraman, R., Ball, S. C., DeCrane, S., and Myers, D. J. 2013. Thermodynamics and Kinetics of Platinum Dissolution from Carbon-Supported Electrocatalysts in Aqueous Media under Potentiostatic and Potentiodynamic Conditions. J. Electrochem. Soc. 160, 4, F447-F455.
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...
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