Aqueous batteries have attracted great attentions because they are inexpensive, safe and can be easily fabricated. However, their voltage window is certainly limited by the potential gap of 1.23 V between the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which is much narrower than that of non-aqueous batteries. Therefore, improving the operating voltage is one of the most crucial challenges for developing a promising aqueous battery.  

In a study published in Angewandte Chemie International Edition, the research team led by Prof. WEN Zhenhai from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences reported that a rechargeable alkali-acid Zn-PbO₂ hybrid battery performed high working voltage, high area capacity, and long cyclic stability.  

In the alkali-acid Zn-PbO₂ hybrid battery, the researchers prepared a freestanding PbO₂ on Pb plate as cathode working in H₂SO₄, and applied a commercial Zn plated as anode operating in KOH, between which are the neutral K₂SO₄ solution separated by cation exchange membrane (CEM) in anode chamber and anion exchange membrane (AEM) in cathode chamber, respectively.  

During the discharging process, they oxidized Zn to Zn(OH)₄^2- in alkaline anode, and transferred the producing electrons via external circuit to acidic cathode for reducing PbO₂ into PbSO₄, in which SO₄^2- in catholyte and K⁺ in anolyte pass though AEM and CEM and enter into the in-between neutral solution to complete the circuit. The reaction processes would be reversed during the charging process. 

On account of the high positive and area capacity of acidic PbO₂ cathode (~1.68 V vs. SHE) and pretty negative alkaline Zn anode (~1.21 V vs. SHE), it is highly impressive that the Zn-PbO₂ hybrid battery exhibited high open circuit voltage (V_oc) of 3.09 V and discharge voltage plateau about 2.92 V with a specific capacity of 2.42 mAh cm^-2 at 5 mA cm^-2.  

In addition, the Zn-PbO₂ hybrid battery could be charge-discharge cycled for 250 cycles without no apparent area capacity loss, and the mass energy densities of Zn-PbO2 hybrid battery could reach 252.39 Wh kg^-1 at the power density of 570.53 W kg^-1. 

The researchers also investigated reaction mechanism in Zn-PbO2 hybrid battery including the material change of PbO₂ electrode at different charge/discharge state and the effect of pH variation of catholyte and anolyte on the electrochemical performance of Zn anode and PbO₂ cathode as well as Zn-PbO₂ hybrid battery.  

The high sensitivity to the pH value in electrolytes of alkali-acid Zn-PbO₂ hybrid battery verified the considerable effect of the pH gradient on its electrochemical performance, because the reaction kinetics would be determined by the concentration of H⁺ and OH^- to a certain degree as they serve as the reactants and charge carriers in Zn-PbO₂ hybrid battery. 

This study presents a high-performance aqueous alkali-acid Zn-PbO₂ hybrid battery, which addresses critical issues of traditional aqueous batteries (e.g., lead acid battery) and paves the way for the development of advanced aqueous batteries.