(Invited) Membrane Development and Transport Study for Redox Flow Batteries

Abstract

This presentation describes the membrane development and transport study at PNNL for the redox flow batteries. Several topics will be covered in the talk including the development of the PTFE/SiO2 porous separator and the research on the correlation between the Nafion membrane microstructure and the vanadium redox flow battery performance. The impact of the equivalent weight on the microstructure of the membrane and thus the performance of VRBs is systematically studied. By tailoring the equivalent weight and membrane thickness, the new membrane contains optimal pore geometry with extremely low vanadium-ion permeability was developed. In addition, its area resistance is comparable to, and its cost is significantly less than, the widely used Nafion 115 membrane. Excellent VRB single-cell performance (89.3% energy efficiency at 50mA∙cm-2) was achieved along with a stable cyclical capacity over prolonged cycling (>200 cycles). The chemical composition (Figure 1), pore geometry, transport properties and flow cell performance will be reported. In addition, a detailed study on the ions transport behavior across the membrane and its impact on the redox flow battery performance will be presented, using vanadium redox flow battery as an example. In operando study on the vanadium ion crossover unveiled the mechanisms contributing to the battery capacity fading, as well as the state-of-charge changes along cycling. The studies were carried out on various membranes, such as cation exchange membrane and porous separators. Figure 1. 19F MAS-NMR spectra measured at an 11.7 T magnetic field with a spinning speed of 13.5 kHz for NDM142, NDM220, and NDM221 membranes. The circles and red line represents the experimental spectra and the fitted curve with deconvolution analysis, respectively (see text for details). The asterisk (*) indicates MAS spinning side bands. Figure 1