3.2. AQDS-Br RFB Single-Cell Tests

The performance of both AQDS negolytes was compared in the AQDS/Br RFB single-cell using std. RFB lab-cell (Pinflow Energy Storage, s.r.o.). The cell consisted of copper current collectors, PPG86 composite plates (Eisenhuth), PVC-based distribution frames, polyacrylonitrile-based graphite felt electrodes (thermally activated according the procedure described in [25]). The felts were mutually separated by cation-exchange membrane F930-RFD (30 µm thick, Fumatech) with an active geometric area of 4 cm². Negative electrolyte comprised of 10 mL of AQDS-based electrolyte and positive electrolyte contained 20 mL of 3.0 M HBr dissolved in deionized water. Experimental procedure consisted of EIS and load curve characterization in 50% SoC of negolyte. EIS was measured under OCV conditions with 5 mV amplitude in the frequency range of 10 kHz–20 mHz. Load curve was measured using current scan from 0 to 500 mA cm⁻² with 5 mA cm⁻² s⁻¹ scan rate. The following parameters were evaluated from EIS and load curve measurements at various SoC: Ohmic resistance (R_Ω) and charge transfer resistance (R_CT) were evaluated from EIS spectra by fitting to the corresponding equivalent circuit (shown in Figure S3) and related to the geometric active area. Discharging resistance (R_dis) was evaluated as a slope of discharging load curve in linear part region (0–200 mA cm⁻²) and related to the geometric active area. Finally, series of 25 galvanostatic charge-discharge cycles at 125 mA cm⁻² was performed. Coulombic, voltage and energy efficiencies (CE, UE, EE) of the battery were calculated using standard equations from the charge-discharge cycles and averaged over 11th–25th cycle. Relative capacity decay per cycle (CD) was evaluated as a slope of dependency of discharge capacity (Q_dis) on cycle number for 11th–25th cycle and related to the theoretical capacity (Q_theor.) according to Faraday’s law. For all experiments an electrolyte flow rate of 40 mL min⁻¹ corresponding to a linear flow velocity of 54 cm min⁻¹ through felt electrodes using a Watson Marlow peristaltic pump. Both electrolytes were initially purged with nitrogen (for 1 h) and nitrogen atmosphere was used within the whole experiment to prevent oxidation of reduced AQDS by atmospheric oxygen. The testing was carried out at room temperature.