## 6 Water Transfer Studies  
Preferential water transfer is an important issue to be considered while selecting a suitable membrane. Water transfer between two half cells can lead to precipitation of vanadium salts in the cell due to the concentration of one half cell electrolyte and dilution of the other. It can also cause flooding of the solution reservoir thus causing operational difficulties in commercial systems [PERSON] et al. (2012). The water transfer behaviour of several commercial ion exchange membranes was initially studied by [PERSON], [PERSON] and [PERSON] (2006, 2007) who found that there is a preferential volumetric transfer of the solution across the membrane during charge/discharge cycling in redox flow batteries, including VRB. It was observed that for a cell that employs an anion exchange membrane or a non-ionic separator, the net volumetric water transfer is towards the negative half-cell, whereas for a cation exchange membrane, the net volumetric water transfer is towards the positive half-cell (2006, 2007). The net amount of water transfer across a membrane is the summation of the water transferred by the hydration shells and the water transferred by osmosis. In the case of cation exchange membranes, a significant amount of water transferred across membranes is caused by the hydration shells of V\({}^{2+}\) and V\({}^{3+}\) ions which carry large amount of water and can easily permeate through cation exchange membranes due to their relatively high charge numbers (2006). By changing the number and type of co-ions and counter ions in the membrane pores, different rates of transfer will be observed for each of the species in solution, so that the net water transfer direction would be expected to change, as would the net transfer of vanadium and sulphate/bisulphate ions across the membrane (2007). Further water transfer study of cation exchange membranes has been investigated in terms of different state of charge (SOC) (2007). Interestingly, the direction of preferential water transfer is dependent on the SOC of the vanadium electrolytes. When the SOC is between 100% and 50%, the direction of water transfer is towards the positive half-cell, which is consistent with the study by [PERSON] _et al._(2006). Further, when the electrolyte discharges from 50% to 0% SOC, the direction of water transfer reverses towards the negative half-cell. These studies are very important, because in long term VRB operation, huge water transfer can cause flooding of the electrolyte reservoir. Figure 4 depicts the movement of water and ions for a 2M Vanadium solution in 5M total sulphate at 50% SOC (2007). [PERSON] and [PERSON] measured the transport number for water during charge-discharge cycling of the VRB using AMV, DMV, CMV, Nafion 117, and Flemion and treated Daramic and first observed a disparity in solution fluxes for anion and cation exchange membranes. A net accumulation of solution was observed in the negative half-cell electrolyte for anion exchange membranes, whilst cation exchange membranes showed the opposite trend. They proposed the use of alternate anion and cation exchange membranes in a cell stack to counter these processes and reduce if not eliminate the net solution transfer from one reservoir to the other [65]. In an effort to improve the water transport properties of commercial anion exchange membranes, they have either been sulfonated (the incorporation of some cation exchange capacity) or subjected to the addition of a cationic polyelectrolyte (poly (sodium-4-styrene sulfonate) [59]. The results show that sulfonation of an anion exchange membrane can incorporate some cation exchange capacity, which results in reduction of water transfer. The same mechanism is also applicable to the incorporation of cation polyelectrolyte into an anion exchange membrane. In a different study by [PERSON] and [PERSON] [15], different types of polyelectrolytes were used to modify cation exchange membranes by soaking them in a solution of the polyelectrolyte.