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Fuel cells are advanced CO2 emission-free generators that are currently used for automobiles. Since the cost of platinum group metal (PGM) catalysts for both hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) is too high, switching to alkaline fuel cells (AFCs) is necessary. However, the alkaline media is much more challenging for HOR since extra energy is required to generate proton from hydroxide anion [1]. In addition, the alkaline durability of PGM/C catalysts is not granted, because Pt assists the local carbon corrosion, by promoting oxidation of “CO-like species”, leading to CO2 and carbonates formation, and finally particles detachment [2]. Lower degradation rate for Pd/C was measured compared to Pt/C in neutral (Ar) or reducing (H2) environment [3-5]. This work is an in-depth investigation on the differences between Pt/C and Pd/C in alkaline during AST in H2 (0-0.5 VRHE at 100 mV/s, for 1000 CVs). Figure 1 shows that prolonged cycling in this low potential region in reducing atmosphere induces “poisoning” of the PGM NPs surface by “CO-like species”; one cleaning method to recover the ECSA of Pt and Pd, is to simply strip these “CO-like species” by cycling the potential at higher values, as shown by Castanheira, et al. on Pt/C [6]. Comparing electrochemistry results and IL-TEM images suggests that stripping the “CO-like species” is a reversible process for the remaining particles that are still intact on the carbon substrates. Another cleaning method is to apply 500 CVs, from 0-1 VRHE, at 100 mV/s in Ar inert gas (Ar cleaning method). This method gives higher recovering percentage comparing to the “CO-like species” stripping one and is considered to be a better cleaning method. The reversible ECSA percentage for the catalysts from the Ar cleaning methods were calculated to be 11 % wrt. the initial ECSA of Pt/C and 14% wrt. the initial ECSA of Pd/C. Finally, the overall ECSA loss of Pt/C after cleaning was 46% (wrt. initial ECSA) while Pd/C’s case was only 8%: so, Pt assists the C corrosion better than Pd. Moreover, Pt is easy to be poisoned and is not as reversible as Pd. References [1] Danilovic, N., Subbaraman, R., Strmcnik, D., Chang, K. C., Paulikas, A. P., Stamenkovic, V. R., Markovic, N. M. Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts. Angew. Chemie - Int. Ed. 2012, 51 (50), 12495–12498. [2] Zadick, A., Dubau, L., Sergent, N., Berthome, G., Chatenet, M. Huge instability of Pt/C catalysts in alkaline medium. Acs Catalysis, 2015, 5(8), 4819-4824. [3] Zadick, A., Dubau, L., Chatenet, M., Demirci, U., Serov, A., Atanassov, P. Instability of commercial Pt/C and Pd/C electrocatalysts in alkaline media. ECS Transactions, 2015, 69(17), 553-558. [4] Lafforgue, C., Zadick, A., Dubau, L., Maillard, F., Chatenet, M. Selected Review of the Degradation of Pt and Pd‐based Carbon‐supported Electrocatalysts for Alkaline Fuel Cells: Towards Mechanisms of Degradation. Fuel Cells, 2018, 18(3), 229-238. [5] Lafforgue, C., Maillard, F., Martin, V., Dubau, L., & Chatenet, M. Degradation of Carbon-Supported Platinum-Group-Metal Electrocatalysts in Alkaline Media Studied by in Situ Fourier Transform Infrared Spectroscopy and Identical-Location Transmission Electron Microscopy. ACS Catalysis, 2019, 9(6), 5613-5622. [6] Castanheira, L., Dubau, L., Mermoux, M., Berthomé, G., Caqué, N., Rossinot, E., ... & Maillard, F. Carbon corrosion in proton-exchange membrane fuel cells: from model experiments to real-life operation in membrane electrode assemblies. ACS Catalysis, 2014, 4(7), 2258-2267. Figure 1

The Direct Borohydride Fuel Cell (DBFC) is investigated since several years as a potential power generator technology for portable and mobile applications. It takes advantage of the high intrinsic energy density of the borohydride fuel: the Borohydride Oxidation Reaction (BOR), that occurs at the anode can theoretically provide 8 electrons and has a really low theoretical onset potential of -0.42 V vs RHE. Although Pt-group metal (PGMs) catalysts are the benchmarks for this reaction, they are severely limited by the competition of the desired BOR with the unwanted hydrogen evolution reaction (HER) at potential below 0 V vs RHE. In order to limit this competition, one should consider a catalyst with a poor HER activity. In this regard, Ni showed impressive performance at low potentials but more limited at higher potentials [1]. Such performance has been obtained by precisely controlling the state of the nickel surface as described by Oshchepkov et al. [1–4]. This catalyst has been used in a single cell configuration and promising results were obtained [1] (NiED/GDL, figure 1.A). In order to increase even more the performance of the cell, Ni-foam structures have been employed to obtain highly porous and totally metallic (nickel) anodes. This new electrode morphology allowed to reach performance competing with that of Pt electrodes will be shown (NiED/Ni Felt, figure 1.A). Another aspect limiting the performance of the DBFC is the use of inappropriate membranes such as commercial Nafion 212 (in Na+ form) in figure 1.A. The group of Ramani developed a bipolar interface allowing to maintain highly different pH at the anode and the cathode, which improves drastically the performance of the system [5]. The fully-nickel-based anodes presented earlier have been used with such interfaces and performance greatly surpassing noble-based electrodes has been obtained (figure 1.B). Fully non-noble cells using Fe-N-C catalysts (from the group of Jaouen) at the cathode have also been tested and presented interesting results, when used in conjunction with an anion-exchange membrane. Acknowledgments This work has been performed in the frame of the MobiDiC project, funded by the French National Research Agency (ANR, grant # ANR-16-CE05-0009-01) and the DGA. The authors also thank the Russian Science Foundation (RSF 18-73-00143) and the US Office of Naval Research (ONR) under grant number N00014-16-1-2833 for their financial support. References [1] A.G. Oshchepkov, G. Braesch, S. Ould-Amara, G. Rostamikia, G. Maranzana, A. Bonnefont, V. Papaefthimiou, M.J. Janik, M. Chatenet, E.R. Savinova, ACS Catal. (2019) 8520–8528. [2] A.G. Oshchepkov, A. Bonnefont, E.R. Savinova, Electrocatalysis. (2020) 133–142. [3] A.G. Oshchepkov, A. Bonnefont, V.N. Parmon, E.R. Savinova, Electrochim. Acta. 269 (2018) 111–118. [4] A.G. Oshchepkov, A. Bonnefont, V.A. Saveleva, V. Papaefthimiou, S. Zafeiratos, S.N. Pronkin, V.N. Parmon, E.R. Savinova, Top. Catal. 59 (2016) 1319–1331. [5] Z. Wang, J. Parrondo, C. He, S. Sankarasubramanian, V. Ramani, Nat. Energy. 4 (2019) 281–289. Figure 1

The estimation and increase of the lifetime of PEM fuel cell under dynamic conditions is one of a major challenge. Increasing the durability of the fuel cell must be treated by both the development of new material and design but also by optimal strategies and management of the operating conditions of the fuel cell. The startup and shut down phase are important to optimize to reduce the carbon support corrosion [1].x Based on previous development of a multi-physics modeling framework, combining complex transport such as multicomponent transport in porous media and electrochemistry with the use of local conditions for MEA and channel design optimization [2], two 2D multi-physics models (2D model along the channel and 2D model at the rib/channel scale) are updated. The different reactions (hydrogen oxidation reaction, oxygen reduction reaction and the oxidation of the carbon support) are written in the general form [3-4]. The linking of the two models allows to simulate the transient potentials during startup and shutdown phase in two direction (along the channel and in the section of the MEA). In particular, during the injection of hydrogen in the channel, the reverse current mechanisms that accelerate the carbon support corrosion, is directly simulated without hypothesis. The validated models provide in-silico characterization to better explain the reverse current mechanisms and the interactions between the operating conditions of the cell and the local conditions in the catalyst layer. The CO2 concentration at the outlet of the channel is used as an observer to quantify the degradation of the carbon support. In a second step, different mitigation strategies are proposed. In particular, some strategies are studied to limit the high potential during the startup and shutdown phase (influence of the catalyst loading in the anode, external electrical resistance). Other strategies decrease the time during the reverse current mechanism (sensitivity of the hydrogen flow rate during the startup (Fig. sensitivity study of the hydrogen flow rate during startup on the cathodic potential and the CO2 production), design optimization of the rib/channel patern). These different strategies are explained and compared. An improvement of the carbon support corrosion is quantified and can be decreased by 50%. [1] Qiang Shen, Ming Hou, Dong Liang, Zhimin Zhou, Xiaojin Li, Zhigang Shao, and Baolian Yi. Study on the processes of start-up and shutdown in proton exchange membrane fuel cells. Journal of Power Sources, 189(2):1114–1119, 2009. [2] Bolahaga Randrianarizafy, Pascal Schott, Marion Chandesris, Mathias Gerard, and Yann Bultel. Design optimization of rib/channel patterns in a pemfc through performance heterogeneities modelling. International Journal of Hydrogen Energy, 43(18):8907 – 8926, 2018. [3] G. Maranzana, A. Lamibrac, J. Dillet, S. Abbou, S. Didierjean, and O. Lottin. Startup (and shutdown) model for polymer electrolyte membrane fuel cells. Journal of the Electrochemical Society, 162(7):F694–F706, 2015. [4] B. Randrianarizafy. Multi-physics modeling of startup and shutdown of a PEM fuel cell and study of the carbon support degradation: mitigation strategies and design optimization. PhD thesis, Communauté Université Grenoble Alpes, 2018. Figure 1