Equilibrium and Discharge Characteristics of a Single Cell of Hydrogen–Vanadium Battery with Electrolyte of Different Acidity

A single-unit of rechargeable power source, the hydrogen–vanadium battery (Pt‒C)H₂|Nafion|\({\text{VO}}_{2}^{ + }\)(C), is studied for various sulfuric-acid contents in the vanadium electrolyte (catholyte) over the 3–6 M range of the total amount of sulfuric-acid residues and a total concentration of vanadium compounds of 1 M. For this composition range, the dependences of the cell voltage and the half-cell potentials on the vanadyl-to-vanadate ratio in the electrolyte is determined for the open-circuit regime while the voltage and potentials shifts are measured for the current passing through the cell in both directions. The contributions to the cell voltage from both half-cell potentials as well as their polarizations are found separately by means of an external reference electrode branched to the vanadium flow electrode via a film-shaped Luggin capillary. The vanadium electrolyte conductivity is measured in the course of charge–discharge cycling and its dependence on the vanadyl-to-vanadate ratio for the series of electrolyte compositions is determined. For the high-current region, the cell maximal specific discharge power is found to decrease from 0.68 to 0.45 W/cm² with increase of the catholyte acidity, as a consequence of the concentration polarizations of both the positive and the negative half-cells, with a much higher relative contribution from the latter one. For the low-current region (±0.25 A/cm²), the current–voltage curves of both half-cells are linear. With growing electrolyte acidity, the slope (i.e., the polarization resistance) increased in the hydrogen half-cell; decreased, in the vanadium one. As a result, their sum (i.e., the total cell resistance) increased from 0.34 to 0.39 Ω cm² over the studied acidity range.