| Summary: | The local pH values in the close vicinity of the cathode/anode were experimentally determined to be strongly acidic/weakly basic, respectively, during the operation of a high-voltage aqueous sodium-ion battery with Prussian blue-type electrodes and a concentrated aqueous electrolyte. The observed pH gradient was ascribed to O2/H2 evolution at the cathode/anode in aqueous cells, which should contribute to the expansion of the electrochemical window if the pH gradient is maintained. An increase in the distance between the cathode and anode proved to be one of possible solution to suppress the undesired pH neutralization. On the other hand, the reduction of water as well as the dissolved O2 in the electrolyte may diminish the capacity of the anode by a self-discharge. This became more pronounced if the full cell was left to stand for a longer time after charging.
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