For the Fe(Bhmbpy)3 -BHPC test, a cell composed of ~ 4.0 mL 0.1 M Fe(Bhmbpy)3 (10.7 mAh theoretical capacity) in 1 M NaCl at pH 3 (HCl was added to adjust pH) paired with 5 mL of 0.1 M BHPC (26.8 mAh theoretical capacity) in 1 M NaCl at pH 13 (NaOH was added to adjust pH).