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8. Conclusion: What would be the ideal RFB? The primary properties of the ideal RFB chemistry are high energy- and power-density, long- time stability and low capital costs. Secondary features are high efficiencies and safety, and low toxicity of the chemistries. However, we will not go into detail in these secondary features, because efficiencies are very much related to the issues power density and stability, and the ideal toxicity level is easily assessed: The ideal RFB chemistry is not harmful to health of humans or the environment, such as claimed by the nanoFlowcell Holdings Ltd. for their undisclosed nanoFlowcell technology [154], which, due to its unverified claims, is regarded very sceptical by the scientific community. Energy density: The (volumetric) energy density E for a RFB is given by the combination of equations 3 and 6. There are three parameters that can be adjusted to increase E: Number of transferred electrons per molecule n, concentration c and cell voltage . For most molecules discussed in this paper n = 1, with some exceptions such as the AQDS and Br2/Br- systems which transfer n = 2 electrons [43,129,140] and POM systems with n [120,127]. In the latter case, this property is achieved only by employing heavy molecules (molar mass larger than 2000 g mol-1). Due to the high solubility of POMs, this can increase the volumetric energy density, but hardly the gravimetric energy density. Highly charged species are often not stable, therefore multi-electron transfer is usually accompanied by proton transfer [122,123,155]. While this prevents the formation of radicals, balancing of protons adds an additional challenge to the system, if it does not operate at very low pH. As shown in section 4, the question of maximum cell voltage is intricately connected to the question of aqueous or non-aqueous chemistry. While the limit of in water is roughly 1.6 V, this value can be significantly higher in organic electrolytes. However, as the use of non- aqueous electrolytes comes with a serious penalty in terms or costs (as we will show later) and ease of operation, the cell voltage has to be increased at least to 3 V in order to warrant drastic step [156]. Concentration c is probably the most freely adjustable parameter that determines the energy density. Employing slurries of Lithium ion battery cathode materials concentrations as high as 12 M for the active material are reported [60]. Classical RFBs such as the VRFB reach concentrations of 1-3 M, often with the use of stabilisers or in concentrated acids. Looking at the discussion of the three parameters above, we can define something like the Page 43 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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