The deployment of nonaqueous redox flow batteries for grid-scale energy storage has been impeded by a lack of electrolytes that undergo redox events at as low (anolyte) or high (catholyte) potentials as possible while exhibiting the stability and cycling lifetimes necessary for a battery device. Herein, we report a new approach to electrolyte design that uses physical organic tools for the predictive targeting of electrolytes that possess this combination of properties. We apply this approach to the identification of a new pyridinium-based anolyte that undergoes 1e electrochemical charge-discharge cycling at low potential (-1.21 V vs Fc/Fc) to a 95% state-of-charge without detectable capacity loss after 200 cycles.
Recent
efforts have led to the design of new anolytes for nonaqueous
flow batteries that exhibit reversible redox couples at low potentials.
However, these molecules generally cycle through just a single electron-transfer
event, which limits the overall energy density of resulting batteries
on account of the undesirably high equivalent weight (i.e., ratio
of anolyte/supporting electrolyte molecular weight to electrons transferred).
In addition, these anolytes generally require expensive alkylammonium
salts as supporting electrolytes for stable cycling, which further
increases the equivalent weight of the system. The current work describes
the multielectron redox cycling of a low-potential anolyte using alkali
metal salts as supporting electrolytes. These studies reveal that
potassium hexafluorophosphate (KPF6) dramatically lowers
the equivalent weight of the anolyte system while supporting flow
cell cycling through two redox events at low potentials for 150 cycles
with no detectable degradation.
One new and nine explanted zirconia femoral heads were studied using glancing angle X-ray diffraction, scanning electron microscopy, and nanoindentation hardness techniques. All starting zirconia implants consisted only of tetragonal zirconia polycrystals (TZP). For comparison, one explanted alumina femoral head was also studied. Evidence for a surface tetragonal-to-monoclinic zirconia phase transformation was observed in some implants, the extent of which was varied for different in-service conditions. A strong correlation was found between increasing transformation to the monoclinic phase and decreasing surface hardness. Microscopic investigations of some of the explanted femoral heads revealed ultra high molecular weight polyethylene and metallic transfer wear debris.
Cyanopyridines and cyanophenylpyridines were investigated as anolytes for nonaqueous redox flow batteries (RFBs). The three isomers of cyanopyridine are reduced at potentials of À 2.2 V or lower vs. ferrocene + /0 (Fc + /0 ), but the 3-CNPy *À radical anion forms a sigma-dimer that is reoxidized at E � À 1.1 V, which would lead to poor voltaic efficiency in a RFB. Bulk electrochemical charge-discharge cycling of the cyanopyridines in acetonitrile and 0.50 M [NBu 4 ][PF 6 ] shows that 2-CNPy and 4-CNPy lose capacity quickly under these conditions, due to irreversible chemical reaction/decomposition of the radical anions. Density-functional theory (DFT) calculations indicated that adding a phenyl group to the cyanopyridines would, for some isomers, limit dimerization and improve the stability of the radical anions, while shifting their E 1/2 only about + 0.10 V relative to the parent cyanopyridines. Among the cyanophenylpyridines, 3-CN-6-PhPy and 3-CN-4-PhPy are the most promising as anolytes. They exhibit reversible reductions at E 1/2 = À 2.19 and À 2.22 V vs. ferrocene + /0 , respectively, and retain about half of their capacity after 30 bulk charge-discharge cycles. An improved version of 3-CN-6-PhPy with three methyl groups (3-cyano-4-methyl-6-(3,5-dimethylphenyl)pyridine) has an extremely low reduction potential of À 2.50 V vs. Fc + /0 (the lowest reported for a nonaqueous RFB anolyte) and loses only 0.21 % of capacity per cycle during charge-discharge cycling in acetonitrile.
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