New insights into the charge storage chemistry of polymer cathodes in aqueous Zn batteries

Redox polymers for aqueous rechargeable Zn-ion batteries have a large variety of types, and can be exploited to build air-rechargeable batteries due to their spontaneous oxidation. Yet, understanding of the amorphous polymer cathode is insufficient. Here, the dominant role of H⁺ in the charge storage process of aqueous Zn-organic batteries and factors that influence the fraction of H⁺ uptake in a polymer cathode are unveiled both experimentally and theoretically. Four different polymers containing amino or carbonyl groups, or both, are used to make cathodes, and their percentages of H⁺ uptake are experimentally determined. The percentage of H⁺ uptake is 61-66 at% in the four polymer cathodes. A higher H⁺ uptake fraction in the polymer also corresponds to a higher contribution fraction of the diffusion-controlled charge storage process and higher activation energies in both the redox reaction and ion diffusion. The Zn²⁺-insertion leads to greater change of entropy than the H⁺-insertion. The polymer with a higher H⁺ uptake fraction (i.e., less Zn²⁺ uptake) is subject to a smaller change in entropy. The H⁺-involved redox reaction is facilitated by lower activation energy and more negative change in Gibbs energy, accounting for more H⁺ than Zn²⁺ in the polymer cathode.