Electrochemical pressure impedance spectroscopy for polymer electrolyte membrane fuel cells: Signal interpretation

Electrochemical pressure impedance spectroscopy (EPIS) is an emerging tool for the diagnosis of polymer electrolyte membrane fuel cells (PEMFC). It is based on analyzing the frequency response of the cell voltage with respect to an excitation of the gas-phase pressure. Several experimental studies in the past decade have shown the complexity of EPIS signals, and so far there is no agreement on the interpretation of EPIS features. The present study contributes to shed light into the physicochemical origin of EPIS features, by using a combination of pseudo-two-dimensional modeling and analytical interpretation. Using static simulations, the contributions of cathode equilibrium potential, cathode overpotential, and membrane resistance on the quasi-static EPIS response are quantified. Using model reduction, the EPIS responses of individual dynamic processes are predicted and compared to the response of the full model. We show that the EPIS signal of the PEMFC studied here is dominated by the humidifier. The signal is further analyzed by using transfer functions between various internal cell states and the outlet pressure excitation. We show that the EPIS response of the humidifier is caused by an oscillating oxygen molar fraction due to an oscillating mass flow rate

Rate-dependent morphology of Li2O2 growth in Li-O2 batteries

Compact solid discharge products enable energy storage devices with high gravimetric and volumetric energy densities, but solid deposits on active surfaces can disturb charge transport and induce mechanical stress. In this Letter we develop a nanoscale continuum model for the growth of Li2O2 crystals in lithium-oxygen batteries with organic electrolytes, based on a theory of electrochemical non-equilibrium thermodynamics originally applied to Li-ion batteries. As in the case of lithium insertion in phase-separating LiFePO4 nanoparticles, the theory predicts a transition from complex to uniform morphologies of Li2O2 with increasing current. Discrete particle growth at low discharge rates becomes suppressed at high rates, resulting in a film of electronically insulating Li2O2 that limits cell performance. We predict that the transition between these surface growth modes occurs at current densities close to the exchange current density of the cathode reaction, consistent with experimental observations.Comment: 8 pages, 6 fig

From fuel cells to batteries: Synergies, scales and simulation methods

The recent years have shown a dynamic growth of battery research and development activities both in academia and industry, supported by large governmental funding initiatives throughout the world. A particular focus is being put on lithium-based battery technologies. This situation provides a stimulating environment for the fuel cell modeling community, as there are considerable synergies in the modeling and simulation methods for fuel cells and batteries. At the same time, battery modeling activities have been, and are still today, considerably weaker than fuel cell modeling activities, in particular within Europe. A strong modeling and simulation support is therefore urgently needed to push forward battery technology. This presentation discusses relevant scales and open questions in battery modeling and simulation. The state of the art is reviewed. Synergies and dissimilarities between fuel cell and battery modeling approaches are discussed. For example, while we often model fuel cell performance in terms of stationary operating points, the battery is an inherently instationary system and always requires transient simulations. Examples will be given from both, state-of-the art lithium-ion batteries, and next-generation lithiummetal systems. We furthermore present results of our own activities that are in the fields of multi-scale electrochemical and thermal models of LiFePO4-based lithium-ion batteries as well as detailed kinetic models of lithium-sulfur cells

A multi-timescale modeling methodology for PEMFC performance and durability in a virtual fuel cell car

The durability of polymer electrolyte membrane fuel cells (PEMFC) is governed by a nonlinear cou-pling between system demand, component behavior, and physicochemical degradation mechanisms, occurring on timescales from the sub-second to the thousand-hour. We present a simulation methodol-ogy for assessing performance and durability of a PEMFC under automotive driving cycles. The simu-lation framework consists of (a) a fuel cell car model converting velocity to cell power demand, (b) a 2D multiphysics cell model, (c) a flexible degradation library template that can accommodate physi-cally-based component-wise degradation mechanisms, and (d) a time-upscaling methodology for ex-trapolating degradation during a representative load cycle to multiple cycles. The computational framework describes three different time scales, (1) sub-second timescale of electrochemistry, (2) minute-timescale of driving cycles, and (3) thousand-hour-timescale of cell ageing. We demonstrate an exemplary PEMFC durability analysis due to membrane degradation under a highly transient load-ing of the New European Driving Cycle (NEDC).Peer ReviewedPostprint (author's final draft

Polyclonal B-cell activation by a synthetic analogue of bacterial lipoprotein is functionally different from activation by bacterial lipopolysaccharide

The reactivity of 38 murine strains to a synthetic analogue of bacterial lipoprotein, tripalmitoyl-pentapeptide (TPP), was tested and compared with the reactivity to lipopolysaccharide (LPS). These strains include common laboratory mice and H-2 recombinant inbred lines, as well as some newly bred lines originating from animals recently captured in different regions of Europe. All animals analysed were reactive to TPP and polyclonally activated to proliferation and immunoglobulin synthesis. Large differences in mitogen reactivities of various H-2 recombinant inbred strains suggest that MHC or closely linked gene products influence the reactivity to the LPS and TPP mitogens. By analysing the frequencies of precursor cells reactive to TPP or LPS and the isotype patterns obtained after stimulation, we demonstrated that both mitogens activate individual B cells in different ways.Peer reviewe

Integration of a lithium-ion battery in a micro-photovoltaic system: Passive versus active coupling architectures

A balcony photovoltaic (PV) system, also known as a micro-PV system, is a small PV system consisting of one or two solar modules with an output of 100–600 Wp and a corresponding inverter that uses standard plugs to feed the renewable energy into the house grid. In the present study we demonstrate the integration of a commercial lithium-ion battery into a commercial micro-PV system. We firstly show simulations over one year with one second time resolution which we use to assess the influence of battery and PV size on self-consumption, self-sufficiency and the annual cost savings. We then develop and operate experimental setups using two different architectures for integrating the battery into the micro-PV system. In the passive hybrid architecture, the battery is in parallel electrical connection to the PV module. In the active hybrid architecture, an additional DC-DC converter is used. Both architectures include measures to avoid maximum power point tracking of the battery by the module inverter. Resulting PV/battery/inverter systems with 300 Wp PV and 555 Wh battery were tested in continuous operation over three days under real solar irradiance conditions. Both architectures were able to maintain stable operation and demonstrate the shift of PV energy from the day into the night. System efficiencies were observed comparable to a reference system without battery. This study therefore demonstrates the feasibility of both active and passive coupling architectures

Nonstationary Collisional Dynamics in Determining Nitric Oxide Laser-Induced Flourescence Spectra

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77237/1/AIAA-8783-947.pd

Performance and degradation of Proton Exchange Membrane Fuel Cells: State of the art in modeling from atomistic to system scale

Jahnke, T. et al.Proton Exchange Membrane Fuel Cells (PEMFC) are energy efficient and environmentally friendly alternatives to conventional energy conversion systems in many yet emerging applications. In order to enable prediction of their performance and durability, it is crucial to gain a deeper understanding of the relevant operation phenomena, e.g., electrochemistry, transport phenomena, thermodynamics as well as the mechanisms leading to the degradation of cell components. Achieving the goal of providing predictive tools to model PEMFC performance, durability and degradation is a challenging task requiring the development of detailed and realistic models reaching from the atomic/molecular scale over the meso scale of structures and materials up to components, stack and system level. In addition an appropriate way of coupling the different scales is required. This review provides a comprehensive overview of the state of the art in modeling of PEMFC, covering all relevant scales from atomistic up to system level as well as the coupling between these scales. Furthermore, it focuses on the modeling of PEMFC degradation mechanisms and on the coupling between performance and degradation models.The research leading to this review has been partially supported by the European Union's Seventh Framework Program for the Fuel Cells and Hydrogen Joint Technology Initiative under the project PUMA MIND (grant agreement no 303419).Peer Reviewe