Solid acid fuel cells operate at intermediate temperatures utilizing a solid electrolyte (CsH2PO4, CDP). However, relatively little is known about the degradation mechanism and the topic is rarely addressed. Phosphate...
Understanding the reaction pathways for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) is the key to design electrodes for solid acid fuel cells (SAFCs). In general, electrochemical reactions of a fuel cell are considered to occur at the triple-phase boundary where an electrocatalyst, electrolyte and gas phase are in contact. In this concept, diffusion processes of reaction intermediates from the catalyst to the electrolyte remain unconsidered. Here, we unravel the reaction pathways for open-structured Pt electrodes with various electrode thicknesses from 15 to 240 nm. These electrodes are characterized by a triple-phase boundary length and a thickness-depending double-phase boundary area. We reveal that the double-phase boundary is the active catalytic interface for the HOR. For Pt layers ≤ 60 nm, the HOR rate is rate-limited by the processes at the gas/catalyst and/or the catalyst/electrolyte interface while the hydrogen surface diffusion step is fast. For thicker layers (>60 nm), the diffusion of reaction intermediates on the surface of Pt becomes the limiting process. For the ORR, the predominant reaction pathway is via the triple-phase boundary. The double-phase boundary contributes additionally with a diffusion length of a few nanometers. Based on our results, we propose that the molecular reaction mechanism at the electrode interfaces based upon the triple-phase boundary concept may need to be extended to an effective area near the triple-phase boundary length to include all catalytically relevant diffusion processes of the reaction intermediates.
Reliable, stable, and long-term performance is one of the most important requirements for fuel cells in general. Widespread application of intermediate temperature solid acid fuel cells is still hindered by...
With the ongoing effort to decarbonize and decentralize the electricity infrastructure by shifting to renewable energy sources, fuel cells and in particular intermediate temperature fuel cells receive a growing interest as a promising technology for a highly efficient, on demand transformation of chemical into electrical energy. Since the 2000s, fuel cells have cycled from high expectations to disillusion, caused by the high production cost an insufficient stability. High temperature polymer electrolyte membrane fuel cells (HT PEM FC) are a prominent and well investigated candidate for the intermediate temperature range. However, the usage of concentrated, highly corrosive phosphorous acid as membrane dopant required expensive production materials and causes deterioration of the catalyst over time. Changing from a liquid to a solid electrolyte can simultaneously improve the long term stability and reduce the construction price. A suitable solid acid is CsH2PO4. It is a non-toxic and less corrosive electrolyte with a proton conductivity of 2*10-2 S/cm at 240°C.[1] Since first demonstrated for fuel cell fabrication in 2003,[2] the platinum utilization and power densities have been improved ever since.[3,4] Solid acid fuel cells (SAFC) are already used in industry in a small scale, but the development has been hampered by the poor stability of the cathode electrode. The rate limiting reaction step is the complex oxygen reduction reaction (ORR) at the cathode side. During operation, it is the largest source of overpotential and hence waste heat generation.[5] For an active site, the current collector, electrolyte and gas phase need to be in direct contact with each other to provide all necessary reactants, thereby the interface resistance between the current collector and the catalyst is crucial for the performance. In the work we are going to present, we investigated electrodes based on a platinum thin film catalyst layer.[6,7] These electrodes enable us to investigate degradation effects on a reasonable time scale. We will show that only a few areas close to the current collectors meet the requirements for active sites and consequently generating most of the measured current, leading to a localized heating. We observed a current density depended morphology chance of CsH2PO4 during operation at these active sites, located close to the current collector. While a variety of degradation processes can result from this process, we determined a phosphate adsorption on the catalyst as the most pronounced one. Similar to HT PEM FC, phosphate species from the electrolyte can adsorb at the platinum surface and act as catalyst poison.[8–10] We will present, that these poisoning effects can be reversed but not prevented by cyclic voltammetry (CV) measurements. After reactivating the cell by CV measurements, the cell performance increased to the initial value before degrading again. The reversibility of the process was shown for five reactivations over a period of 50 h. Overall, we identified the cause and nature of the main degradation mechanism in solid acid fuel cells with platinum thin film electrodes and will present different design optimizations for a stable, high performance fuel cell which arise from our findings. References [1] Haile, S. M.; Boysen, D. A.; Chisholm, C. R.; Merle, R. B., Nature, (2001) 410, 910. [2] Boysen, D. A.; Uda, T.; Chisholm, C. R. I.; Haile, S. M., Science (New York, N.Y.), (2004) 303, 68. [3] Lohmann, F. P.; Schulze, P. S. C.; Wagner, M.; Naumov, O.; Lotnyk, A.; Abel, B.; Varga, Á., J. Mater. Chem. A, (2017) 5, 15021. [4] Papandrew, A. B.; St. John, S.; Elgammal, R. A.; Wilson, D. L.; Atkinson, R. W.; Lawton, J. S.; Arruda, T. M.; Zawodzinski, T. A., J. Electrochem. Soc., (2016) 163, F464-F469. [5] Unnikrishnan, A.; Rajalakshmi, N.; Janardhanan, V. M., Electrochimica Acta, (2018) 261, 436. [6] Wagner, M.; Dreßler, C.; Lohmann-Richters, F. P.; Hanus, K.; Sebastiani, D.; Varga, A.; Abel, B., J. Mater. Chem. A, (2019) 7, 27367. [7] Louie, M. W.; Haile, S. M., Energy Environ. Sci., (2011) 4, 4230. [8] Kaserer, S.; Caldwell, K. M.; Ramaker, D. E.; Roth, C., Journal of Physical Chemistry C, (2013) 117, 6210. [9] Prokop, M.; Kodym, R.; Bystron, T.; Drakselova, M.; Paidar, M.; Bouzek, K., Electrochimica Acta, (2020) 333. [10] Doh, W. H.; Gregoratti, L.; Amati, M.; Zafeiratos, S.; Law, Y. T.; Neophytides, S. G.; Orfanidi, A.; Kiskinova, M.; Savinova, E. R., Chemelectrochem, (2014) 1, 180.
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