The impact of in-situ hydrogen evolution on the flow resistance of electrolyte flowing through the carbon felt electrode in a redox flow battery

The parasitic hydrogen evolution reaction (HER) leads to capacity fade of aqueous redox flow batteries. In addition, the evolved hydrogen gas bubbles stagnating inside the porous electrode may block the flow of electrolyte, increase the flow resistance, and reduce the battery performance. By precisely controlling the HER and electrolyte flow rates, we explore the impact of the mA cm⁻² scale HER on the relative permeability of HCl-based aqueous electrolyte flowing through a carbon felt electrode. Experimental results show that the HER with a current density of 2 mA cm⁻² reduces the quasi-steady two-phase flow relative permeability in the negative half-cell from 1.0 to 0.84, even at a high electrolyte flow velocity of 16 mm s⁻¹. With an ultrasonic gas sensor, the delay of gas release out of the cell from the beginning of HER has been measured. After the halt of HER, the recovery of the liquid permeability lasts for more than an hour if the electrolyte velocity is maintained at 8 mm s⁻¹. Moreover, significant instability of pressure drop is observed at a low electrolyte velocity of 2 mm s⁻¹.