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Batteries capable of extreme fast-charging (XFC) are a necessity for the deployment of electric vehicles. Material properties of electrodes and electrolytes along with cell parameters such as stack pressure and temperature have coupled, synergistic, and sometimes deleterious effects on fast-charging performance. We develop a new experimental testbed that allows precise and conformal application of electrode stack pressure. We focus on cell capacity degradation using single-layer pouch cells with graphite anodes, LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes, and carbonate-based electrolyte. In the tested range (10–125 psi), cells cycled at higher pressure show higher capacity and less capacity fading. Additionally, Li plating decreases with increasing pressure as observed with scanning electron microscopy (SEM) and optical imaging. While the loss of Li inventory from Li plating is the largest contributor to capacity fade, electrochemical and SEM examination of the NMC cathodes after XFC experiments show increased secondary particle damage at lower pressure. We infer that the better performance at higher pressure is due to more homogeneous reactions of active materials across the electrode and less polarization through the electrode thickness. Our study emphasizes the importance of electrode stack pressure in XFC batteries and highlights its subtle role in cell conditions. [ABSTRACT FROM AUTHOR]

The effect of cathode structures on the chemical stability of Nafion membranes is investigated. Membrane electrode assemblies (MEAs) were prepared by using Nafion 212 membrane and commercially available carbon supported Pt electro-catalysts. The cathode structures were controlled by the use of long side chain (LSC) and short side chain (SSC) perfluorosulfonic acids (PFSAs) as well as three dispersing solvents for the electrode fabrication (a water-isopropanol mixture, N-methyl-2-pyrrolidone, or glycerol). The membrane durability was evaluated by the H2 crossover current density after a 200-hour open circuit voltage accelerated stress test. The MEA with a glycerol-processed SSC PFSA-bonded cathode exhibited a 200-fold less H2 crossover current density than the MEA with a water-isopropanol-processed LSC PFSA-bonded cathodes. The analyzes by electrochemical impedance spectroscopy and microscopy suggest that the structural uniformity of cathodes play the most significant role in the chemical stability of the Nafion membranes. This study emphasizes the importance of cathode structures on the durability of Nafion membranes. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes the stability of Fe species in a heat-treated polyaniline–iron–carbon (PANI–Fe–C) oxygen reduction reaction (ORR) catalyst in an aqueous acidic electrolyte and in a membrane–electrode assembly (MEA) at various potentials. Linear combination fitting of ex situ and in situ X-ray absorption near-edge structure (XANES) spectra to the spectra for a suite of Fe standards was used to determine the catalyst iron speciation at various potentials, after potential cycling in an aqueous electrolyte, and after 200h potentiostatic holds in MEAs. XANES edge-step analysis and inductively-coupled mass spectrometry were used to determine the amount of Fe lost from the catalyst into the aqueous electrolyte and from the MEA cathodes. Results showed that the Fe was lost from the catalyst in the electrochemical environment and the rate and extent of this loss were dependent on potential and on the type of electrolyte. The Fe specie primarily responsible for this loss was iron sulfide. Despite the large overall loss of Fe species from the catalyst that had been subjected to potentiostatic holds in an MEA at either 0.4V or 0.6V for 200h, H2–air polarization curves showed only moderate loss of cathode kinetic performance while the performance in the mass transport region improved. Correlating the performance loss to the XANES speciation, the kinetic losses may be attributed to the oxidation of active site(s) and/or loss of pyrrolic-like and pyridinic-like coordination, as well as the mass transport improvement due to removal of inactive Fe species, predominantly sulfides. Species with porphyrazin-like coordination were stable in both the aqueous and MEA environments. It is speculated that the stability of the porphyrazin is responsible for the durability of this catalyst. [Copyright &y& Elsevier]