In order to achieve reliable data regarding high temperature polymer electrolyte fuel cell (HT-PEFC) electrode degradation novel diagnostic techniques for the determination of the electrochemically active surface area (ECSA) were developed. Cyclovoltammetric CO-monolayer oxidation (CO stripping) charge measurements were combined with real-time CO 2 exhaust gas analysis. Different evaluating methods were developed to overcome the problem of side reactions during CO stripping. Furthermore, a calibration curve for absolute ECSA determination via CO stripping was established, eliminating the temperature dependency of CO adsorption. In addition, these methods were successfully extended and implemented to locally resolved ECSA measurements. In summary, this paper introduces novel fundamental HT-PEFC electrode diagnostics for improved understanding of degradation phenomena.

In this study the cause and effect of fuel electrode carbon corrosion in high temperature polymer electrolyte fuel cells are highlighted for the first time. Here, measurements of the CO2 concentration in the fuel electrode effluent and spatially resolved current mapping suggest that the reverse‐current decay mechanism, which is responsible for the well‐known air electrode carbon corrosion, is not responsible for the observed fuel electrode carbon corrosion. We propose a chemical carbon oxidation mechanism, caused by the alternating exposure of the fuel electrode to hydrogen and air during start‐up/shut‐down cycling. Furthermore, a degradation study was performed which indicates a severe performance decrease, up to 40 %, due to fuel electrode carbon corrosion after 600 start‐up/shut‐down events under reformate conditions.

In a recent study we demonstrated that CO has a mitigating effect on carbon corrosion in HT-PEFCs during simulated start/stop cycling. In this study we extend our investigations regarding this phenomenon. At first, a parameter study was carried out in which the temperature, the water partial pressure, the gas flow rate and the CO partial pressure were varied and their individual influence on carbon corrosion examined. Subsequently, a detailed comparison between start/stop cycling with and without CO in the fuel gas was performed (rapid aging study). This comparison includes real-time carbon corrosion detection via a CO 2 sensor and current mapping in 100 segments. In addition, the electrochemically active surface area (ECSA) was measured in spatially resolved manner alongside with polarization curves, characterizing the fuel cell prior and after the simulated start/stop cycling. The results show how CO is mitigating the degradation of the HT-PEFC cathode on a local level. Moreover it was demonstrated that CO in the anodic fuel gas can increase the life-time of HT-PEFCs. The HT-PEFCs top argument in the ongoing global quest for establishing alternative and renewable energy supplies is its ability to tolerate diluted or impure hydrogen as fuel gas. Depending on operating temperature, CO hardly affects fuel cell performance up to a concentration of approximately 3%.1,2 LT-PEFCs, in comparison, are only capable to tolerate a significantly smaller CO concentration of around 10 ppm.3 Nevertheless, notwithstanding other positive aspects, such as the possibility to use dry reactants, PEFCs (LT as well as HT) suffer from start/stop induced carbon corrosion. During start-up or shutdown of a PEFC a fuel/air gas front propagates through the fuel electrode (anode) compartment. This transient gas front causes high potentials (up to 1.5 V) 4,5 on the air electrode (cathode). Carbon is already beyond its thermodynamic stability (cf. Equation 1, E • = 0.207 V versus SHE) on the air electrode under steady-state conditions. But due to the sluggish reaction rate at 1 V the corrosion rate is rather slow. The high potential peaks during start/stop, in contrast, are very much relevant and lead, besides catalyst particle growth and platinum dissolution, to rapid and irreversible carbon corrosion. For a detailed introduction to the start/stop-or reverse current decay-mechanism the reader is referred to previous literature. 4,[6][7][8][9][10] Carbon corrosion drastically reduces fuel cell performance and life-time as a result of catalyst particle detachment and particularly loss in mechanical electrode integrity. Therefore, in order to reach the designated life-time of 60'000 to 80'000 hours until 2020 11 it is indispensable to find reliable, efficient and effective methods to suppress carbon corrosion caused by fuel cell start-ups or shut-downs. There are a vast number of publications and patents, for HT-as well as for LT-PEFCs, which tackle this very problem. In general, the approaches can be categorized into material and operation bas...

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The influence of CO in the fuel gas on cathode carbon corrosion during start/stop cycles in high temperature polymer electrolyte fuel cells (HT-PEFC) was investigated. The fuel cell underwent simulated start/stop cycles with a constant time interval, reactant gas flow rate and temperature by switching between H 2 /CO mixtures and O 2 at the fuel electrode. The results reveal that increasing amounts of CO in the fuel gas reduce the amount of carbon corrosion at the air electrode. Therefore, HT-PEFC operation with CO containing fuel has the benefit of mitigating start/stop induced degradation effects. One of the most beneficial and unique features of a hightemperature polymer electrolyte fuel cell (HT-PEFC) is the capability to tolerate orders of magnitude lager amounts of CO (up to 3%) 1,2 compared to a LT-PEFC (approx. 10 ppm).3 Therefore, the system can be used to generate electricity from hydrogen-rich reformate gases without complex and energy intensive CO-cleanup.Nevertheless, the HT-PEFC can suffer from numerous degradation effects in different locations of the membrane electrode assembly (MEA), 4 e.g. pinhole formation in the membrane, acid evaporation and membrane thinning. 5,6 Electrodes can also represent a major area of degradation. Interestingly, specific degradation mechanisms, such as carbon corrosion, catalyst detachment and catalyst particle growth (Ostwald-ripening) can be found in the LT-PEFC as well. Additionally, structural changes and acid flooding of the porous layers of the cell can occur. All degradation mechanisms are complex functions of the specific operation conditions, making their individual identification challenging. Hence, it is necessary to gain insight into limitations of MEA lifetime, which is the key to successful development of mitigation strategies to reach the desired lifetimes, i.e. >40.000 hours 6 for stationary applications.One of the specific degradation triggers is known as "reversecurrent decay" or "start/stop"-mechanism, respectively. 7-9 This degradation mode is induced during fuel cell start-up or shut-down, when air or pure oxygen and fuel gas co-exist in the fuel electrode compartment (see Figure S1 and its description in the supplemental material). This mechanism can be one of the main reasons for carbon corrosion at the air electrode. The number of start/stop cycles is closely related to the overall lifetime of the fuel cell if the induced degradation effects are not mitigated. If, for instance, a HT-PEFC is used in a combined heat and power (CHP) 10-12 system, it is conceivable that several hundred cycles will be accumulated, which can lead to a rapid and irreversible damage of the stack.13 It is necessary, therefore, to improve the understanding of the start/stop mechanism in order to deduce appropriate mitigation strategies. Mitigation strategies can be divided into two categories. One is focused on developing new materials, such as carbon free electrodes [14][15][16] or selective hydrogen oxidation catalysts. 17-19The second category consists of system st...