Since the properties of Fe(Bhmbpy)3 in two-chamber standard ARFB at near neutral starting conditions have been tested in a previous study, here we evaluate the properties of Fe(Bhmbpy)3 in a three-chamber two-membrane cell (5 cm 2 working area) with pH gradient with various initial conditions. The negolyte was composed of 0.1 M (50 mL) potassium ferricyanide/ferrocyanide (4:1 ferricyanide:ferrocyanide) at pH 13 (adjusted by NaOH) as non-capacity limiting side, with 1 M NaCl as supporting salt. A cation exchange membrane (CEM, Fumasep E620 K) was used to separate the negolyte from the center chamber. The center chamber contained 1.3 M NaCl that was circulated through the cell from a 50 mL tank. The posolyte was prepared by adding 0.35 M Bhmbpy into 0.1 M FeCl2 while adjusting initial pH to 2 using HCl. An anion exchange membrane (AEM, Selemion DSV-N) was used to separate the posolyte side from the center chamber. The electrolyte flow rate for posolyte and negolyte was 30 mL min -1 . The center chamber electrolyte flow rate was about 10 mL min -1 . The cell was cycled with CCCV protocol at 20 mA cm -2 with voltage cut-off values of 0.9 V and -0.4 V during charge and discharge, respectively. The cutoff voltage was held until current density dropped lower than 1 mA cm -2 (charging) and 0.2 mA cm -2 (discharging). Discharging current cutoff is smaller to fully recover the dimerized iron complex. The discharging voltage has to be at least 0.7 V lower than cell OCV to realize 'deep discharge' of Fe(Bhmbpy)3 to avoid accumulation of dimers. The capacity fading of this cell is shown in Fig. . With low pH operation (< 1), the cycling showed lower Coulombic efficiency and rapid capacity fading (Fig. ) and Fe(Bhmbpy)3 crossover became severe (Fig. ). After cycling, the pH of Fe(Bhmbpy)3 is still about 1, and center chamber salt solution became pink, indicating fast crossover. Crossover of Fe(Bhmbpy)3 at different pH was further investigated by standard UV-Vis crossover tests (Fig. ). A stirred H-cell was assembled with Selemion DSV-N (pre-soaked overnight in 1 M NaCl) clamped between 72 mM Fe(Bhmbpy)3Cl2 in 1 M NaCl (donating side, dark color) and 1.1 M NaCl (receiving side), both sides initially at pH 2.5, and the diffusion of the iron complex across the membrane was monitored with UV-vis spectrophotometry using a 1 cm length cuvette . Over two days, the permeability of Fe(Bhmbpy)3 2+ was determined to be 2 × 10 -11 cm 2 /s, similar to our previous report, and the receiving chamber became light pink in color (see digital photographs in a, days 1 and 2). Then, HCl was added dropwise to both sides of the H-cell to bring the pH to 1.0, and the pink color of the receiving chamber disappeared (digital photographs, days 3 and 4). From UV-vis of samples from the receiving chamber diluted 100x, a sharp absorbance peak was clear at 300 nm and grew over time between days 3 and 4 of the experiment, while the peak indicating the Fe(Bhmbpy)3 2+ was no longer present. Fig. shows that the peak at 300 nm matches the shape of the peak for the Bhmbpy ligand (see spectra normalized to maximum absorbance), indicating that at low pH, Bhmbpy ligands are removed from the iron and can cross the membrane. Results show that Fe(Bhmbpy)3 will disassociate into iron ions and protonated ligands when pH reaches or lower than 1. This may explain the capacity fade shown in Fig. . Thus, for optimized operation, the lower bound of the pH for Fe(Bhmbpy)3 is 2.