Fig. 1

Schematic diagram of the development of electrochemical testing methods for AOFB"

Fig. 2

Schematics of the two in situ NMR setups (a) operando detection setup; (b) On-line detection setup"

Fig. 3

(a) Experimentally determined fraction of DHAQ?3-radicals; (b) Insitupseudo-2D 1H NMR spectra of anolyte; (c) In situ pseudo-2D 1H NMR spectra of catholyte[44]; (d) and (e) In situ NMR spectra; (f) Proton labels of the several anthraquinone anions[46]; (g) Insitupseudo-2D 1H NMR spectra of MB; (h) Current and potential profiles; (i) Molecular structures of oxidation (red), radical (yellow) and reduction (blue)[26]"

Fig. 4

(a) Schematic diagram of operando reflected IR in the three-electrode system; (b) Schematic diagram of flow battery in-situ infrared detection; (c) On-line transmission infrared; (d) On-line internal reflection exfrared; (e) On-line internal reflection infrared"

Fig. 5

(a) Operando FTIR spectra of 2, 6-DHAQ during reduction and oxidation processes in the first cycle of CV scan; (b) Operando FTIR spectra of 1, 5-DHAQ during reduction and oxidation processes in the first cycle of CV scan; (c) The hydrogen bond-mediated degradation process of 2, 6-DHAQ and 1, 5-DHAQ [15]; (d) On-line ATR-FTIR spectra of DHPS electrolyte recorded duringcycling[51]"

Fig. 6

(a) Representative flow battery; (b) Charge/discharge profiles (left) of PSPR cell and in situ UV-vis spectra of PSPR during charge/discharge process (right)[53]"

Fig. 7

Application of in-situ fluorescence microscope (a) Operando advection-reduction profiles contrasting with mean field theory (MFT) map; (b) Fluorescent particle flow imaging within a porous carbon electrode; (c) Spatial flow field-porous electrode flow profiles [58]"

Fig. 8

(a) Proposedmechanism of fluorenone; (b) EPRdetectionforradical; (c) Radical versus t plot[27]; (d) EPR spectraofdifferentSOCelectrolyte; (e) Spectra of EPR intensity comparisons among 0 h, 16 h and 24 h; (f) The experiment and simulation EPR spectra; (g) Reaction mechanism of MB[26]"

Fig. 9

(a) and (d) Voltage of a 10 mmol/L DHAQ versus 15 mmol/L K4[Fe(CN)6] and 3.75 mmol/L K3[Fe(CN)6] full cell as a function of time[61]; (b) NMR spectra of the anolyte in the aromatic region; (c) EPR spectra of the anolyte; (e), (f) Concentrations of DHAQ3–? radical anions as a function of time, estimated from the NMR spectra and EPR experiments, respectively; (g) Fractions of radicals as a function of state of charge for total concentrations of 10 mmol/L, 100 mmol/L, and 200 mmol/L DHAQ"

Fig. 10

Prospect of in situ characterization technique in FB"