The vanadium redox-battery: an efficient storage unit for photovoltaic systems

The ‘all vanadium redox flow system’ is a promising candidate for the storage of photovoltaic energy. The reversible cell voltage of 1.3–1.4 V in charged state is well established at various electrode materials in particular carbon based substrate. The kinetics and mechanism were studied for the V2+/V3+ and VO++/VO2+ (V4+/V5+) couples and a one-electron transfer identified as the rate-determining step at smooth surface. The use of activation layers (carbon cloth, felt, etc.) decisively reduced the polarization. Catalysts, which are required for an increase of the reaction rate and the elimination of undesired side reactions, e.g. Ru(O)2 improved the behavior of the positive electrode. The influence of the separator material on mass transfer phenomena (diffusion, migration) and the charge–discharge characteristics were investigated. The requirements to be met as stand alone batteries for the energy supply of users in combination with photovoltaic plants considering the solar irradiation conditions in south Portugal were discussed and the future development goals defined.