As shown in Figure , the recovery efficiency declines significantly when the period between regeneration events increases. For these experiments, cells were cycled with a discharge cutoff voltage of 0.95 V or higher, which is high enough to prevent the formation of the unstable dimer until the regeneration step is initiated. In 2.0 M OH -, whereas the recovery efficiency is over 88% when the regeneration step is performed every one or two days, it drops significantly with increasing period. The same trend is observed for 1.5 M OH -; however, the extent of decline is noticeably lower. Thus, Figure clearly shows that regeneration should be performed frequently to retain a high recovery efficiency over the long term. This may, however, increase the risk of unbalancing the cell. The risk is that the oxygen evolution reaction (OER) may also proceed at the negative electrode during the regeneration step. Given that the OER consumes OH -, the pH of the negolyte gradually declines and, consequently, the fade rate of the cell is accelerated (Figure ). Nevertheless, if the regeneration cutoff voltage is chosen properly (Figure ), the extent of OER can be reduced. Alternatively, after the regeneration step, oxygen (air) can be introduced into the negolyte to counteract the imbalance. This approach, however, requires measuring the pH of both the posolyte and negolyte to prevent overexposure of oxygen.