Electrochemical performance of poly(arylene piperidinium) membranes and ionomers in anion exchange membrane fuel cells

Awakening interest in anion exchange membrane fuel cells (AEMFC) for low temperature applications has led to an increased demand for high-performing polymers stable under alkaline conditions. In this study a poly(p-terphenylene piperidinium)-based (PAP) membrane and ionomer was synthesised and applied in membrane electrode assemblies (MEAs), with porous gas-diffusion electrodes based on Pt catalysts supported by VULCAN® and high surface area carbon, respectively. The MEAs were evaluated in AEMFC single-cell tests. In order to identify specific beneficial characteristics of the polymer, the results were compared to reference tests using a commercial Aemion™-polymer. Steady-state polarisation performance measurements were carried out as well as electrode characterisations via cyclic voltammetry and electrochemical impedance spectroscopy, in addition to ex-situ characterisation of the polymer and the membrane electrode assemblies. PAP-based membranes showed great potential with an in-situ measured average ohmic resistance of 0.09 Ω cm2. Mass transport limitations at higher current densities were observed for high surface area carbon electrodes, leading to an overall higher performance with the use of VULCAN®. Properties of the ionomer related to water uptake capabilities were observed to inhibit performance as well. The higher water uptake of PAP-based ionomers appears to be a key property for increasing electrode performance

Fuel cell evaluation of anion exchange membranes based on poly(phenylene oxide) with different cationic group placement

Four novel poly(phenylene oxide)-based anion exchange membranes were investigated for electrochemical performance, ionic conductivity and water transport properties in an operating anion exchange membrane fuel cell (AEMFC) , using Pt/C gas diffusion electrodes with Tokuyama ionomer. The poly(phenylene oxide)-membranes have a 1- or 5-carbon alkyl spacer between the backbone and a trimethylalkylammonium (TMA) or piperidinium (Pip) cationic group, and ion-exchange capacities (IECs) between 1.5 and 1.9 mequiv g-1. The polymer with a 5-carbon alkyl spacer, a TMA cationic group, and a higher IEC showed the highest ion conductivity and performance in the AEMFC. The results also show that introducing a 5-carbon alkyl spacer does not improve performance unless the IEC is increased and that exchanging the TMA with a Pip cationic group results in lower fuel cell performance despite a higher IEC. A discrepancy in ion conductivity between fuel cell and ex-situ test was observed for the 5-carbon spacer polymers and is attributed to a higher sensitivity for dehydration. Similar water flux under load, from the anode to the cathode with increased water content at both electrodes, was observed for all membranes and only varied with membrane thickness. The deviation in fuel cell performance observed between the membranes could not be explained by differences in water flux or ionic conduction, suggesting that the electrodes – membrane interaction plays a major role. Nevertheless, the study emphasizes that high membrane conductivity (for the λ-range in a fuel cell) and an efficient water transport (obtained by lower membrane thickness) promote higher electrochemical performance

Fast-charging effects on ageing for energy-optimized automotive LiNi1/3Mn1/3Co1/3O2/graphite prismatic lithium-ion cells

The reactions in energy-optimized 25 Ah prismatic NMC/graphite lithium-ion cell, as a function of fast charging (1C-4C), are more complex than earlier described. There are no clear charging rate dependent trends but rather different mechanisms dominating at the different charging rates. Ageing processes are faster at 3 and 4C charging. Cycling with 3C-charging results in accelerated lithium plating but the 4C-charging results in extensive gas evolution that contribute significantly to the large cell impedance rise. Graphite exfoliation and accelerated lithium inventory loss point to the graphite electrode as the source of the gas evolution. The results are based on careful post-mortem analyses of electrodes using: scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). SEM results show particle cracking independent of the charging rate used for the cycling. XPS and EIS generally indicate thicker surface film and larger impedance, respectively, towards the edge of the jellyrolls. For the intended application of a battery electric inner-city bus using this type of cell, charging rates of 3C and above are not feasible, considering battery lifetime. However, charging rates of 2C and below are too slow from the point of view of practical charging time