• The Importance of Cell Compression Pressure for Flow Battery Performance

      Davies, Trevor J.; Gunn, Natasha; Ward, David; University of Chester (The International Flow Battery Forum, 2016-06-30)
      Compared to fuel cells, which possess similar cell architecture, flow batteries have poor performance. For example, conventional fuel cells can easily achieve current densities of 1.5 A cm-2 whereas the corresponding figure for the all vanadium flow battery (VFB) is an order of magnitude less, often less than 0.2 A cm 2 [1]. Consequently, relatively large flow battery cells are required for a given power, increasing the cost of the technology. There are a few noticeable exceptions to the relatively poor performance of flow batteries, including the work of Zawodzinski et al. who achieved current densities in excess of 0.8 A cm 2 with a VFB [2]. Most impressively, Weber and co-workers achieved current densities as high as 4 A cm 2 with a H2-Br2 flow battery [3]. In both cases, the researchers used fuel cell components and fuel cell assembly techniques to minimize the cell ohmic resistance, particularly the contact resistance between the cell parts (electrodes, bipolar plates and current collectors). Typically, fuel cells are assembled using compression pressures of above 8 bar to minimize contact resistance. In comparison, flow batteries use compression pressures less than 1 bar during cell assembly with carbon fibre felt electrodes; hence contact resistance values are relatively high. A number of studies have measured the effect of felt compression on battery performance [4-5], where the felt compression is increased from 0 to 30%, resulting in a decrease in cell resistance and a noticeable improvement in performance. This study builds on previous felt compression work by exploring a much wider range of electrode compression pressures in a VFB system.