Qualitative behavior of vanadium ions in Nafion membranes using electron spin resonance

Nafion 117 membranes for applications in Vanadium Redox Flow Batteries (VRFBs) are characterized using electron paramagnetic resonance (EPR). EPR can directly detect the V(II) and VO2+ ions, but in the battery environment the membrane is directly exposed to V(II–V). The use of nitroxide spin probes such as 2,2,6,6-tetramethyl-4-piperidone N-oxide (TEMPONE) allows observations of changes in the fluid channels of the membrane as it is exposed to all possible ionic species. These observations can give a clearer picture of the effect of the ions on the ion exchange membrane and serve as a basis of comparison in developing membranes with lower vanadium crossover.

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► The vanadium lineshape is sensitive to water content.
► The probe hyperfine splitting and rotational rate is dependent on vanadium.
► In the membrane, the vanadium rotation rate is roughly ten times that of tempone.
► Vanadium crossover is a problem in VRFBs.
► The VO2+ ion is observed to absorb readily into the membrane.