Electrode Perforation Induced Performance Increase in Vanadium Redox Flow Batteries

Abstract

In the pursuit of providing reliable energy to an increasingly energy dependent society, the development of electrochemical devices that can meet various demand output conditions is not only important, but necessary. The Redox Flow Battery (RFB) is one option that can provide for this demand by offering the ability to modularize energy and power at a high standard of safety over a long product lifetime. While the RFB has already been successfully implemented at the commercial scale, there exists a need to research further methods towards cost/kWh reduction and power density increase. A conventional cell stack for a RFB consists of end plates that sandwich a set of current collectors, bipolar plates, electrodes, and an exchange membrane arranged in a symmetrical cell.[1] In this study we explore the potential of electrode perforation towards increases in battery performance using cell reactive resistivity as a performance indicator. The perforation method in this study has been chosen for its commercial feasibility and ease of implementation; it has been achieved through the use of a scalable punch press to bore a matrix of holes with variable diameter dimensions of 1-2 mm and a range of hole densities from ~3 holes/cm2 to ~6 holes/cm2. Three sets of serpentine bipolar plates were also prepared with varying turn densities of 4, 7, and 11. These bipolar plates were created keeping effective channel diameter consistent while varying depth and width. The channel to ridge ratios in all three plates were kept at 1:1 and the effect of electrode perforation was explored. The test cells were assembled with the aforementioned bipolar plates and SGL 39AA carbon paper electrodes activated at 600°C for 30 min. Nafion NR211 was used as the membrane material. The cell was assembled with 0.5 mm silicon spacers and 3 sheets of activated SGL 39AA for both the positive and negative electrodes. The electrolyte for the experiment was a 1.5M Vanadium solution prepared with VOSO4-nH2O, 20%w/w H2SO4, and DI water. An averaged improvement in reactive resistivity of 20.8% from 0.262 Ohms*cm2 to 0.207 Ohms*cm2 was demonstrated for a set of three serpentine flow fields via the mechanical perforation of 1 mm diameter holes at a density of ~3 holes/cm2. It is noted that reactive resistivity was separated out of ASR resistivity using a LCR meter to measure frequency independent bulk effects which is referred to as conductive resistance.[2] Reactive resistivity was further broken down into mass transport, charge transfer, and diffusion resistivities through the analysis of relaxation times during cell charge and discharge. Through this, the effect of perforated electrodes for charge transfer resistivity and mass transfer resistivity are shown. (Figure 1) It is realized that electrode perforation via 1 mm bores improves all subsections of reactive resistivity, and it is hypothesized that electrode perforation allows for increased reactant mass transport to the membrane side electrode partition, which has been shown to exhibit lower electrode volumetric utilization than the bipolar plate side electrode partition.[3] Is it also observed that the difference in performances of the 4, 7, 11 turn bipolar plates, which were significant and proportional to channel depth when evaluated with non-perforated electrodes showed a significant decrease in variance with the introduction of perforated electrodes. It can thus be extrapolated that the electrode modification method yields a higher apparent consequence to cell performance than the variation of channel density for the same active electrode area. In conclusion, it is shown that the introduction of 1 mm diameter perforations at ~3 holes/cm2 in carbon paper electrodes is effective in improving performance in a vanadium redox flow cell with serpentine flow channels. The over perforation of electrodes at ~6 holes/cm2 was noted to have the decreased performance associated with an extreme reduction in useable electrode surface area. Pressure drop analysis also reveals that the introduction of perforated electrodes reduces the pump pressure needed to operate at the same nominal flow rate. It is thus realized that the introduction of perforations in electrode material can improve both system efficiency and cell efficiency in the VRFB. [1] N. Tokuda et al.: “Development of a Redox Flow Battery System”, SEI Technical Review, No. 151 (2000) [2] J. Ross Madonald.: “Simplified impedance/frequency response results for intrinsically conducting solids and liquids”, The Journal of Chemical Physics 61, 3977 (1974); doi: 10.1063/1.1681691 [3] Q. Liu et al.: “In Situ potential disctribution measurement in an all-vanadium flow battery”, Chem. Commun. 49, 6292-6294 (2013); doi: 10.1039/C3CC42092B Figure 1