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Membranes 2022, 12, 1228 300 350 400 450 500 550 600 l, nm Figure 5. Discharging curve of a H2/BrO3− flow battery (a) and evolution of the corresponding spec- trum of the catholyte (b) in the course of galvanostatic polarization by −0.075 A/cm2, starting com- 10 of 16 position of the positive-electrode solution: product of stage (Figure 4a), reactant of the negative elec- trode: hydrogen, gas flow rate: 10 mL/min. 2.0 curve 1 1.6 curve 2 1.2 0.8 1.0 0.8 0.6 0.4 0.2 0.0 (a) (b) 2C(Br2)/Ctot U, V 0 2 4 6 8 10 t, h Figure 6. Charge—discharge (curve 1) and polarization (curve 2) curves of the positive electrode Figure 6. Charge—discharge (curve 1) and polarization (curve 2) curves of the positive electrode of of a hydrogen-bromate flow battery (a), and a graph representing the variation in the fraction of a hydrogen-bromate flow battery (a), and a graph representing the variation in the fraction of Br Br atoms which are inside Br2 molecules at a time moment, t, which is equal to the ratio of the atoms which are inside Br2 molecules at a time moment, t, which is equal to the ratio of the number number of bromine atoms inside Br2 molecules to the initial content of bromine atoms in the system, of bromine atoms inside Br2 molecules to the initial content of bromine atoms in the system, i.e., 2 i.e.,02 C−(Br )/C0(Br−) where C(i) is the molar concentration of species i; (b). Cha−rging stage (Br− C(Br2)/C (Br ) w2here C(i) is the molar concentration of species i; (b). Charging stage (Br oxidation): 2 galvaonxoisdtatiocnm):ogdael,vcaunrorsetnatidcemnosidtye:,c0u.0r7r5enAt/dcmens,iitnyi:ti0a.0l7c5omAp/ocmsiti,oiniotifatlhceomcapthoosliytitoensoufptphleiecdathoolyte 2 thepsousiptpivleieedletocttrhoedep:o0s.i3tivMeHeleBcrtr+o3deM:0H.32SMOH4,Bpru+m3piMngHraSteO:3,0pmumL/pmining,rtahte:n3e0gamtiLv/emelienc,ttrhoedneeigsative −− open to the atmosphere. Discharge stage (BrO3 reduction): galvanostatic mode, current density dur- electrode is open to the atmosphere. Discharge stage (BrO3 reduction): galvanostatic mode, current ing discharge: −0.075 A/cm2, initial composit2ion of the catholyte supplied to the positive electrode: density during discharge: −0.075 A/cm , initial composition of the catholyte supplied to the positive solution at the end of the charge stage, hydrogen is supplied to the negative electrode at a rate of 10 mL/min. electrode: solution at the end of the charge stage, hydrogen is supplied to the negative electrode at a rate of 10 mL/min. Figure 6 represents the results of a hydrogen bromate flow battery under cyclic testing with an extended duration over a voltage range of 0.4–1.9 V. In addition to the time dependence of the voltage (Figure 6a, curve 1), the potential of the positive electrode relative to the silver chloride reference electrode was registered (Figure 6a, curve 2). The processing of catholyte spectra in these tests was carried out according to the procedure described in the work of Petrov et al. [68] to isolate the molecular bromine fraction from the overall absorption spectra. The result is demonstrated in Figure 6b as a dependence of the relative amount of bromine atoms in the form of Br2 molecules with regards to time. While collating the curves for cycles 2–4 with the previously discussed data for the first cycle a noticeable difference was revealed: as the cycle number increases, its duration decreases, mainly due to the diminishing of the charging half-cycle stage, responsible for the rise of the bromine oxidation state from +1 to +5. As a result, the catholyte contains a bromate ion concentration decreasing from cycle to cycle by the moment the system reaches the current reversal voltage (1.9 V). At the same time, the proportion of bromine atoms in the form of HBrO increases. This corresponds to an increase in UV absorption (Figure 7a) compared to an acidic solution of sodium bromate, caused by intermediates with bromine oxidation states +1, +3 for cycles 2–3. A noticeable contribution of molecular bromine manifests in the fourth cycle. In this way, the average oxidation degree of bromine atoms in the catholyte by the end of the charge process decreases significantly from cycle to cycle: from ~+5 after the first charge to less than +1 after the fourth charge. 24PDF Image | Hydrogen-Bromate Flow Battery
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