Electrodeposited behavior of lead on the negative electrode in the neutral lead-carbon hybrid capacitor: Influence and inhibition

Abstract

Because negative electrode of the neutral lead-carbon hybrid capacitor is composed by the active carbon and graphite, it is general believe that only the absorption-desorption process occurs in the negative electrode. However, in this work, we find that there is a second reaction going on the negative electrode: During charging, lead ions diffused from the positive electrode can be reduced to lead crystals when the charging cut-off voltage of the negative electrode is negative to the reduction potential of lead ions. These electrodeposited lead crystals can be converted into lead sulfate crystals during discharging, and these lead sulfate crystals cannot be completely dissolved in the next charging process. When lead sulfate crystals are accumulated on the surface of the carbon material gradually, the performance of the negative electrode is inhibited. Current densities have a great influence on this phenomenon because the charging cut-off voltage of the negative electrode is negatively shifted with the decrease of current density. The anion exchange membrane can completely inhibit this phenomenon by preventing the free diffusion of lead ions. After cycling at 2 A g−1 for 3400 times, the energy retention rate of the neutral lead-carbon hybrid capacitor increases from 34% to 81%.