The Effect of Electrolyte Composition on the Performance of a Single‐Cell Iron–Chromium Flow Battery

Flow batteries are promising for large‐scale energy storage in intermittent renewable energy technologies. While the iron–chromium redox flow battery (ICRFB) is a low‐cost flow battery, it has a lower storage capacity and a higher capacity decay rate than the all‐vanadium RFB. Herein, the effect of electrolyte composition (active species and supporting electrolyte concentrations), Fe/Cr molar ratio, and supporting electrolyte type (HCl and H2SO4) on the performance (current efficiency (CE), voltage efficiency (VE), energy efficiency, discharge capacity, and capacity decay) of an ICRFB is investigated. The storage capacity of the optimum electrolyte (1.3 m FeCl2, 1.4 m CrCl3, 5.0 mm Bi2O3 in 1.0 m HCl) is 40% higher (from 17.5 to 24.4 Ah L−1), while the capacity decay rate is tenfold lower (from 3.0 to 0.3% h−1) than the performance of the previously used 1.0 m FeCl2, 1.0 m CrCl3 in 3.0 m HCl. At the optimum Fe and Cr concentrations and ratio in 0.5 m HCl, a near constant CE (92.3%), VE (78.7%), and EE (72.6%) are obtained over 50 cycles. The significantly higher capacity decay when using 1.0 m H2SO4 (1.6% h−1) compared to 1.0 m HCl (0.3% h−1) confirms that HCl is the more suitable supporting electrolyte

Molecular modelling of the hydrolysis of tantalum and niobium pentafluoride

Tantalum (Ta) and niobium (Nb) are two metals with similar chemical and physical properties and are found together in nature. One form of Ta is tantalum pentafluoride, which is stable in reducing environments, is corrosive resistant and stable under harsh conditions. Ta is currently used in nuclear reactors with a wide variety of uses and advantages. For these applications, pure Ta is needed to insure high value catalysts, contrary the higher the purity grade the more expensive the production of these high value catalysts. One way of ensuring an economic viable process for the production of high purity Ta, is to find a cost effective way to separate Ta from Nb. Ungerer et al. studied the separation of Ta and Nb, using safer chemicals and techniques for the environment in a solvent extraction (SX) process. During this study, separation was achieved in a sulphuric acid (H2SO4) medium with the extractants diiso-octyl phosphinic acid (PA) and di-(2-ethylhexyl) phosphoric acid (D2EHPA). The main obstacle during this study was the speciation of Ta and Nb, springing the question of why separation occurred with some extractants and not with the others. One method for determining the speciation of the compounds in a reaction mixture is by using computational techniques for molecular modelling. Several molecular modelling programs are available which uses various mathematical equations and approximations. Progress in computational chemistry over the last 20 years has made quantum mechanical calculations on large molecules, chemical systems as well as on macromolecule reactions possible. Calculations based on the density-functional theory (DFT) are now, not only used on light elements and small molecules, but also on metal complexes, heavy metals and especially on metal separation in SX. With these models at hand, SX processes were modelled within realistic margins to fit the experimental setup in a small scale laboratory. It is anticipated that the advances from this work will provide the possibility to determine, with good approximation, the outcome not only of the proposed Ta SX experiments, but also SX in general, before embarking on expensive, time consuming experiments and environmental unfriendly waste generation. In this paper molecular modelling was used to compile a partial energy profile for a proposed reaction mechanism for the reaction of tantalum- and niobium pentafluoride (TaF5, NbF5) with water to form tantalum- and niobium hydroxides. In the process, possible species that may form during the reaction were identified and evaluated

The Time Dependent Solvent Extraction of Ta and Nb

The suitability of Liquid-liquid extraction (LLE) for the selective extraction of chemically similar tantalum (Ta) and niobium (Nb) mixtures was investigated by determining the influence of the acid concentration, extractant to metal mole ratio (E:M) as well as the ageing of the feed on extraction using the extractants, bis (2-ethylhexyl) phosphate (D2EHPA) and di-iso-octylphosphinic acid (PA). The system consisted of a solvent in varying E:M ratio’s, diluted in cyclohexane with 3% (v\v) 1-octanol added as modifier and a feed solution containing sulphuric acid and 100 ppm of the NH4TaF6 and NH4NbF6 complexes. Depending on the acid concentration, extraction percentages (%E) of up to 100% for Ta and 10-20% for Nb were attained. An initial lack of repeatability in experimental results was shown to be caused by variations in the age of the feed solution. The change in extraction trends expressed as a normalized %E for the aged feed solutions were nearly identical for both extractants when using 3, 6, 9 and 14mol/dm3 H2SO4. For the 3 and 6mol/dm3 solutions, the %E decreased significantly within the first 4-5 hours of ageing. At 9mol/dm3 the %E remained stable for feed ages up to 3.5 hours before declining, while the %E remained near constant at 14mol/dm3even after ageing for 24 hours

The time dependent solvent extraction of Ta and Nb

The suitability of Liquid-liquid extraction (LLE) for the selective extraction of chemically similar tantalum (Ta) and niobium (Nb) mixtures was investigated by determining the influence of the acid concentration, extractant to metal mole ratio (E:M) as well as the ageing of the feed on extraction using the extractants, bis (2-ethylhexyl) phosphate (D2EHPA) and di-iso-octylphosphinic acid (PA). The system consisted of a solvent in varying E:M ratio’s, diluted in cyclohexane with 3% (v\v) 1-octanol added as modifier and a feed solution containing sulphuric acid and 100 ppm of the NH4TaF6 and NH4NbF6 complexes. Depending on the acid concentration, extraction percentages (%E) of up to 100% for Ta and 10-20% for Nb were attained. An initial lack of repeatability in experimental results was shown to be caused by variations in the age of the feed solution. The change in extraction trends expressed as a normalized %E for the aged feed solutions were nearly identical for both extractants when using 3, 6, 9 and 14mol/dm3 H2SO4. For the 3 and 6mol/dm3 solutions, the %E decreased significantly within the first 4-5 hours of ageing. At 9mol/dm3 the %E remained stable for feed ages up to 3.5 hours before declining, while the %E remained near constant at 14mol/dm3even after ageing for 24 hours