PH-Controlled Electrolysis of Electrolytic Manganese Dioxide (EMD) for Improved Rechargeable MnO2/Zn Batteries

Abstract

Alkaline batteries with the MnO2/Zn chemistry offer a number of advantages over other aqueous batteries (e.g., lead acid) including: a) high energy density, b) lower cost, and c) longer shelf life. The main active material used in the cathode electrode of these batteries is manganese dioxide, which has a low production cost, low toxicity, high specific capacity, low self-drain rate, and is abundant in nature. However, alkaline batteries cannot be easily recharged due to the rapid capacity loss and low energy efficiency of the MnO2 product used. An approach to improving the rechargeability of alkaline batteries may be synthesizing higher quality and/or modified manganese dioxide material. Manganese dioxide is a highly versatile substance that can grow into various crystal structures with different synthesis methods and be used in applications such as lithium-ion batteries, water oxidation electrocatalysts, etc. The preferred manganese dioxide used in alkaline batteries is electrolytic manganese dioxide (EMD) with a crystal structure which is an intergrowth of two different phases, pyrolusite and ramsdellite, which consist of arrays of MnO6 octahedra in 1 x 1 and 1 x 2 tunnels, respectively. The tunnels in the structure allow for the intercalation/deintercalation of protons during the discharge and charge processes. The EMD structure also contains protons (Ruetschi and Coleman protons) in the form of OH- complexes (also referred to as structural water) which provide a proton bridge, thus increasing the rate of proton diffusion into the lattice structure. The structural water content of EMD is an important parameter for the overall performance and has not been thoroughly reported before. Electrodeposition of EMD relies on the oxidation of Mn2+ to Mn4+ on a stable, conductive metal anode (e.g. titanium, nickel, etc.) in an acidic environment (typically H2SO4) which determines the water content. The EMD samples were electrodeposited by electrolysis of MnSO4 onto a titanium anode (4 cm × 14 cm × 2 mm) in an electrolyte solution containing 1.25 M MnSO4 and H2SO4 ranging from 0.5-5 M of H2SO4 at 95 oC using a current density of 115 A m-2. Electrosynthesis was performed for a period of 15 hours, thereafter, the deposits were chipped, grounded, washed, and filtered. The samples were then characterized using Thermogravemetric Analysis (TGA), X-Ray Diffraction (XRD), and Brunauer–Emmett–Teller (BET) surface area analysis techniques. Electrochemical properties of the EMD sample were examined with flat-plate cells using a zinc anode and a 9M KOH electrolyte solution and were cycled at a rate of C/10. Electrochemical impedance spectroscopy analysis was performed using a half-cell set-up using a nickel mesh as the counter electrode and an Hg/HgO reference electrode. The EMD samples synthesized with a manganese salt to acid ratio of 1.25:2 show a structural water content and surface area which are nearly twice that of the commercially available EMD products. Additionally, the ramsdellite fraction of the EMD samples increases by about 40%. EMD samples prepared in 2M acid solution exhibit improved cycling performance (see Figure) and energy efficiency when compared to commercial EMD products. The increase in water content is one possible explanation for the enhancement of the protonic conductivity of the in-house EMD as well as its improved physical characteristics. Synthesis and characterization of EMD samples and their electrochemical results will be presented Figure 1