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Das P.R.,EWE Research Center for Energy Technology | Komsiyska L.,EWE Research Center for Energy Technology | Osters O.,EWE Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
ECS Transactions | Year: 2015

The electrodes in Li-ion batteries consist of multiple components such as active materials, conductive additives and polymeric binder. The polymeric binder influences significantly the properties and stability of the composite electrode and the overall battery performance. We proposed the application of poly-3,4-ethylendioxythiophen: polystyrene sulfonic acid (PEDOT:PSS) poly-ion complex as a conductive binder material for cathodes in lithium ion battery. In this paper we report the electrochemical behavior and stability of PEDOT:PSS in battery electrolyte in terms of cyclic voltammetry and electrochemical impedance spectroscopy. The impedance behavior of PEDOT:PSS has been studied at different ambient temperatures and the ionic and electronic conductivities of PEDOT:PSS has been evaluated using a modified transmission line model. PEDOT:PSS shows stable behavior during multiple cycling in the operated potential range up to 4.2 V and no change in the impedance was visible after 200 cycles. © The Electrochemical Society.


Diedrichs A.,EWE Research Center for Energy Technology | Wagner P.,EWE Research Center for Energy Technology
ECS Transactions | Year: 2012

This work explores how the performance of a high-temperature polymer electrolyte membrane fuel cell is affected by the degree of compression. Contact pressure measurements in the range of 2 to 25 bars have been conducted on commercial membrane-electrode-assemblies (MEAs). When increasing the contact pressure, the MEA performance continuously decreases for lower current densities and increases or goes through a small maximum for higher current densities. The electrochemical characterization reveals a decrease in membrane as well as contact resistance and shows an increase in mass transport restriction, hydrogen crossover as well as electrical short circuits. The electrochemical active surface area is not affected by contact pressure rising. A comparison of two flow field types illustrates that the MEA performance not only depends on the geometry of the analyzed flow fields, but it is also influenced by the real contact area between flow field and MEA surface. © The Electrochemical Society.


Grosse Austing J.,EWE Research Center for Energy Technology | Nunes Kirchner C.,EWE Research Center for Energy Technology | Komsiyska L.,EWE Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
Journal of Power Sources | Year: 2016

In this paper the losses in coulombic efficiency are investigated for a vanadium/air redox flow battery (VARFB) comprising a two-layered positive electrode. Ultraviolet/visible (UV/Vis) spectroscopy is used to monitor the concentrations cV2+ and cV3+ during operation. The most likely cause for the largest part of the coulombic losses is the permeation of oxygen from the positive to the negative electrode followed by an oxidation of V2+ to V3+. The total vanadium crossover is followed by inductively coupled plasma mass spectroscopy (ICP-MS) analysis of the positive electrolyte after one VARFB cycle. During one cycle 6% of the vanadium species initially present in the negative electrolyte are transferred to the positive electrolyte, which can account at most for 20% of the coulombic losses. The diffusion coefficients of V2+ and V3+ through Nafion® 117 are determined as DV2+,N117=9.05·10-6 cm2 min-1 and DV3+,N117=4.35·10-6 cm2 min-1 and are used to calculate vanadium crossover due to diffusion which allows differentiation between vanadium crossover due to diffusion and migration/electroosmotic convection. In order to optimize coulombic efficiency of VARFB, membranes need to be designed with reduced oxygen permeation and vanadium crossover. © 2015 Elsevier B.V. All rights reserved.


Bohn P.,Audi AG | Bohn P.,Carl von Ossietzky University | Liebig G.,EWE Research Center for Energy Technology | Komsiyska L.,EWE Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
Journal of Power Sources | Year: 2016

In this paper a 3D model based on the thermal material characteristics of an automotive prismatic Li-NiMnCoO2 (NMC) cell was created in COMSOL Multiphysics® in order to simulate the temperature propagation in the cell during short term thermal stress. The thermal characteristics of the battery components were experimentally determined via laser flash analysis (LFA) and differential scanning calorimetry (DSC) and used as an input parameter for the models. In order to validate the modelling approach, an experimental setup was built to measure the temperature propagation during thermal stresses within a dummy cell, equipped with temperature sensors. After validating, the model is used to describe the temperature propagation after a short-term temperature stress on automotive prismatic lithium-ion cells, simulating welding of the contact leads. © 2016 Elsevier B.V.


grosse Austing J.,EWE Research Center for Energy Technology | Nunes Kirchner C.,EWE Research Center for Energy Technology | Komsiyska L.,EWE Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
Journal of Membrane Science | Year: 2016

Vanadium/air redox flow batteries (VARFB) promise higher energy densities compared to all-vanadium redox flow batteries (VRFB). However, VARFB suffer from crossover processes through the membrane, i.e. vanadium crossover and oxygen permeation. The vanadium crossover causes ongoing capacity losses and therefore reduces the lifetime of the battery. Additionally, the coulombic efficiency is reduced due to vanadium crossover and oxygen permeation. In this contribution we propose a straightforward routine for Nafion 117 (N117) membrane modification to reduce both vanadium crossover and oxygen permeation. Layer-by-layer (LbL) deposited films of polyethylenimine (PEI) and Nafion ionomer are build up on the membrane by dipping the membrane alternatingly in solutions of the polyelectrolytes. The modification of the membranes is characterized with infrared (IR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and film thickness measurements. The properties of the modified membranes are investigated by determining the proton conductivity, vanadium crossover and oxygen permeation. By the application of a LbL film of PEI/Nafion obtained after 10 LbL deposition repetitions, the selectivity (σH+/PV 2+) of the membrane towards protons is increased by factor 21. Using this membrane in a VARFB reveals a strongly reduced vanadium crossover (approx. -70%) during a cycle as determined with inductively coupled plasma mass spectroscopy (ICP-MS) analysis of the positive electrolyte. The coulombic efficiency increases from 81% to 93% and the energy efficiency from 41.5% to 45.2%. TGA and IR measurements of the membrane after VARFB operation indicated a vanadium ion uptake into the membrane and the stability of the LbL film under conditions of VARFB operation. © 2016 Elsevier B.V.


Grosse Austing J.,EWE Research Center for Energy Technology | Nunes Kirchner C.,EWE Research Center for Energy Technology | Hammer E.-M.,EWE Research Center for Energy Technology | Komsiyska L.,EWE Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
Journal of Power Sources | Year: 2015

The performance of a unitised bidirectional vanadium/air redox flow battery (VARFB) is described. It contains a two-layered cathode consisting of a gas diffusion electrode (GDE) with Pt/C catalyst for discharging and of an IrO2 modified graphite felt for charging. A simple routine is shown for the modification of a graphite felt with IrO2. A maximum energy efficiency of 41.7% at a current density of 20 mA cm-2 as well as an average discharge power density of 34.6 mW cm-2 at 40 mA cm-2 were obtained for VARFB operation at room temperature with the novel cathode setup. A dynamic hydrogen electrode was used to monitor half cell potentials during operation allowing to quantify the contribution of the cathode to the overall performance of the VARFB. Four consecutive cycles revealed that crossover of vanadium ions took place and irreversible degradation processes within the reaction unit lead to a performance decrease. © 2014 Elsevier Ltd. All rights reserved.

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