Life-cycle analysis of zinc-cerium redox flow batteries

Abstract

The life-cycle of a zinc-cerium redox flow battery (RFB) is investigated in detail by in situ monitoring of the half-cell electrode potentials and measurement of the Ce(IV) and H+ concentrations on the positive and negative side, respectively, by titrimetric analysis over its entire life. At a current density of 25 mA cm−2, the charge efficiency of the battery is initially limited by the zinc redox reaction, which leads to the incomplete reduction of Ce(IV) to Ce(III) during discharge and the accumulation of Ce(IV) on the positive side. Titration experiments confirm the accumulation of Ce(IV) on the positive side of the battery and reveal that the proton concentration on the negative half-cell of the battery increases over the cycles. This increases the rate of hydrogen evolution on the negative side and contributes to Ce(IV) precipitation on the positive side. These effects combine to cause capacity fade and ultimate failure of the battery. In order to mitigate these effects, the battery life-cycle is evaluated when the Nafion 117 cation exchange membrane is replaced with an anion exchange membrane (AEM). Among the AEMs tested, Fap-375-PP is found to be the most chemically stable in the presence of Ce(IV). Full life-cycle experiments show that Fap-375-PP can reduce the rate of enhancement in the proton concentration on the negative side and increases the life of the system relative to that possible using Nafion 117, although a very gradual proton enhancement on the negative side is still observed. A future focus should be to fabricate AEMs with extremely low proton leakage and high stability in presence of Ce(IV) in order to further enhance the life-cycle of zinc-cerium RFBs.