@article{9447, abstract = {Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) based water-in-salt electrolytes (WiSEs) has recently emerged as a new promising class of electrolytes, primarily owing to their wide electrochemical stability windows (~3–4 V), that by far exceed the thermodynamic stability window of water (1.23 V). Upon increasing the salt concentration towards superconcentration the onset of the oxygen evolution reaction (OER) shifts more significantly than the hydrogen evolution reaction (HER) does. The OER shift has been explained by the accumulation of hydrophobic anions blocking water access to the electrode surface, hence by double layer theory. Here we demonstrate that the processes during oxidation are much more complex, involving OER, carbon and salt decomposition by OER intermediates, and salt precipitation upon local oversaturation. The positive shift in the onset potential of oxidation currents was elucidated by combining several advanced analysis techniques: rotating ring-disk electrode voltammetry, online electrochemical mass spectrometry, and X-ray photoelectron spectroscopy, using both dilute and superconcentrated electrolytes. The results demonstrate the importance of reactive OER intermediates and surface films for electrolyte and electrode stability and motivate further studies of the nature of the electrode.}, author = {Maffre, Marion and Bouchal, Roza and Freunberger, Stefan Alexander and Lindahl, Niklas and Johansson, Patrik and Favier, Frédéric and Fontaine, Olivier and Bélanger, Daniel}, issn = {1945-7111}, journal = {Journal of The Electrochemical Society}, keywords = {Renewable Energy, Sustainability and the Environment, Electrochemistry, Materials Chemistry, Electronic, Optical and Magnetic Materials, Surfaces, Coatings and Films, Condensed Matter Physics}, number = {5}, publisher = {IOP Publishing}, title = {{Investigation of electrochemical and chemical processes occurring at positive potentials in “Water-in-Salt” electrolytes}}, doi = {10.1149/1945-7111/ac0300}, volume = {168}, year = {2021}, } @article{7319, abstract = {In the first paper of this series, an experimental technique for measuring the current-density distribution with a resolution better than the sub-millimeter scale of the channel and rib structures in the flow-field plates of polymer electrolyte fuel cells (PEFCs) was introduced. This method is extended to the determination of local membrane resistance with the same spatial resolution in the present paper. The combined measurement of current and resistance allows for investigating the interaction of mass- and charge-transport processes, which determine the local rate distribution across the domain of channels and ribs. Therewith, the influence of relevant operating parameters such as reactant composition, dew points, and cell compression on local current generation is investigated. The results show that the distribution of water and oxidant across the channel and rib are the main reasons for significant current gradients on a scale smaller than a millimeter. Humidity variation mainly affects the membrane resistance under the channel, while reactant concentration predominantly influences current generation under the rib-covered cell area.}, author = {Reum, Mathias and Freunberger, Stefan Alexander and Wokaun, Alexander and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {3}, publisher = {The Electrochemical Society}, title = {{Measuring the current distribution with sub-millimeter resolution in PEFCs: II. Impact of operating parameters}}, doi = {10.1149/1.3043422}, volume = {156}, year = {2009}, } @article{7321, abstract = {Cell interaction phenomena in polymer electrolyte fuel cell stacks that arise from imbalance between adjacent cells are investigated in detail experimentally and theoretically. A specialized two-cell stack with advanced localized diagnostics was developed and used to analyze the mechanism and effect of cell-to-cell coupling as a result of operationally relevant variations in reactant feed flow. Contributions to overall and local voltage changes with respect to uniformly operated cells are scrutinized. Unequal operation of the cells causes in-plane current in the bipolar plate to redistribute current and result in inhomogeneous polarization. Both increasing and decreasing polarization along the air-flow path reduces cell power as compared to isopotential operation. A two-dimensional model based on a commercial computational fluid dynamics code is used to back and extend the experimental results to more general cases. Furthermore, the experimental setup presented allowed for the first time to perform simultaneous localized electrochemical impedance spectroscopy beyond the single-cell level. The mechanism of mutual cell interaction on local and integral spectra is revealed. Results show that virtually identical operation of the cells is essential to obtain meaningful integral spectra.}, author = {Freunberger, Stefan Alexander and Schneider, Ingo A. and Sui, Pang-Chieh and Wokaun, Alexander and Djilali, Nedjib and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {7}, publisher = {The Electrochemical Society}, title = {{Cell interaction phenomena in polymer electrolyte fuel cell stacks}}, doi = {10.1149/1.2913095}, volume = {155}, year = {2008}, } @article{7325, abstract = {Our experimental results shown here disprove that finite diffusion can generally be assumed in ac impedance models for H2/air-polymer electrolyte fuel cells (PEFCs) to account for the diffusive transport of oxygen through the gas diffusion layer (GDL) toward the air electrode. It is shown that the amplitude of the oxygen concentration oscillation created as a consequence of superimposed ac current at the air electrode is not zero at the channel/GDL interface but extends into the gas channels, at least below modulation frequencies of fmod=10 Hz . By this, sinusoidal oxygen-concentration oscillations within the cathode gas channels are excited locally along the flow field. Due to the forced air convection in the cathode flow-field channels, a coupling via the gas phase occurs downstream of the flow field. The coupling strongly affects the local and by this the overall impedance response of the cell and evokes the formation of a low-frequency arc in H2/air-PEFC impedance spectra. Based on the experimental results, a qualitative model is presented explaining the local impedance response of a segmented 200cm2H2/air PEFC.}, author = {Schneider, I. A. and Freunberger, Stefan Alexander and Kramer, D. and Wokaun, A. and Scherer, G. G.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {4}, publisher = {The Electrochemical Society}, title = {{Oscillations in gas channels: Part I. The forgotten player in impedance spectroscopy in PEFCs}}, doi = {10.1149/1.2435706}, volume = {154}, year = {2007}, } @article{7328, abstract = {An experimental technique for measuring the current density distribution with a resolution smaller than the channel/rib scale of the flow field in polymer electrolyte fuel cells (PEFCs) is presented. The electron conductors in a plane perpendicular to the channel direction are considered as two-dimensional resistors. Hence, the current density is obtained from the solution of Laplace's equation with the potentials at current collector and reaction layer as boundary conditions. Using ohmic drop for calculating the local current, detailed knowledge of all resistances involved is of prime importance. In particular, the contact resistance between the gas diffusion layer (GDL) and flow field rib, as well as GDL bulk conductivity, are strongly dependent on clamping pressure. They represent a substantial amount of the total ohmic drop and therefore require careful consideration. The detailed experimental setup as well as the concise procedure for quantitative data evaluation is described. Finally, the method is applied successfully to a cell operated on pure oxygen and air up to high current densities. The results show that electrical and ionic resistances seem to govern the current distribution at low current regimes, whereas mass transport limitations locally hamper the current production at high loads.}, author = {Freunberger, Stefan Alexander and Reum, Mathias and Evertz, Jörg and Wokaun, Alexander and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {11}, publisher = {The Electrochemical Society}, title = {{Measuring the current distribution in PEFCs with sub-millimeter resolution}}, doi = {10.1149/1.2345591}, volume = {153}, year = {2006}, } @article{7331, abstract = {A previously developed mathematical model for water management and current density distribution in a polymer electrolyte fuel cell (PEFCs) is employed to investigate the effects of cooling strategies on cell performance. The model describes a two-dimensional slice through the cell along the channels and through the entire cell sandwich including the coolant channels and the bipolar plate. Arbitrary flow arrangements of fuel, oxidant, and coolant stream directions can be described. Due to the serious impact of temperature on all processes in the PEFC, both the relative direction of the coolant stream to the gas streams and its mass flow turns out to significantly affect the cell performance. Besides influencing the electrochemical reaction and all kinds of mass transfer temperature, variations predominantly alter the local membrane hydration distribution and subseqently its conductivity.}, author = {Freunberger, Stefan Alexander and Wokaun, Alexander and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {5}, publisher = {The Electrochemical Society}, title = {{In-plane effects in large-scale PEFCs: II. The influence of cooling strategy on cell performance}}, doi = {10.1149/1.2185282}, volume = {153}, year = {2006}, } @article{7332, abstract = {A quasi-two-dimensional, along-the-channel mass and heat-transfer model for a proton exchange membrane fuel cell (PEFC) is described and validated against experimental current distribution data. The model is formulated in a dimensional manner, i.e., local transport phenomena are treated one-dimensional in through-plane direction and coupled in-plane by convective transport in the gas and coolant channels. Thus, a two-dimensional slice running through the repetitive unit of a cell from the anode channel via membrane-electrode assembly (MEA) and cathode channel to the coolant channel and from inlet to outlet is modeled. The aim of the work is to elucidate the influence of operating conditions such as feed gas humidities and stoichiometric ratios on the along-the-channel current density distribution and to identify the distinct underlying voltage loss mechanisms. Furthermore, a complicated technical flow field is modeled by a combination of co- and counterflow subdomains and compared with experimental current densities.}, author = {Freunberger, Stefan Alexander and Santis, Marco and Schneider, Ingo A. and Wokaun, Alexander and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {2}, publisher = {The Electrochemical Society}, title = {{In-plane effects in large-scale PEMFCs}}, doi = {10.1149/1.2150150}, volume = {153}, year = {2006}, }