Efficient All Solid State Rechargeable Zinc-Air Batteries with a Spinel Type MnCo2O4/Carbon Fiber Bifunctional Electrocatalyst
Authors: Abedi Zahra , Chen Weixing , Ivey Douglas G 
Published: July 2022

Rechargeable zinc-air batteries (ZABs) stand out as promising candidates in the ever increasing search for sustainable energy storage devices. ZABs are inexpensive devices that exhibit relatively high energy density, safe operation with no environmental issues and long shelf life (when sealed).

ZABs are composed of an electrolyte and two electrodes parallel to one another: the air electrode and the zinc electrode. The efficiency and cycle life of rechargeable ZABs are affected by the reactions that take place at the air electrode. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) happen at the air electrode during discharge and charge, respectively.

The poor kinetics for ORR/OER necessitate the use of electrocatalysts at the air electrode. Traditionally, precious metals like Pt and Ru have been used as ORR and OER catalysts, respectively, to lower the energy barrier for these reactions. However, these catalysts are rare and expensive and suffer from poor cycling stability. Transition metal oxides are inexpensive, abundant and safe alternative electrocatalyst options for air electrodes in ZABs. Mn and Co based oxides have been shown to have reasonable activities towards ORR and OER, respectively.

In this work, high performance and efficient all solid state ZABs were prepared. Mn-Co mixed oxide (MnCo2O4) was employed as the bifunctional electrocatalyst. Hydrogel electrolytes, containing polyacrylic acid (PAA), KOH and N,N’-methylenebis (acrylamide) or MBAA as the crosslinker, were used. MnCo2O4 coated carbon fibers (MnCo2O4/CF) were utilized to prepare the air electrodes, with asphaltene based carbon fibers as a conductive substrate for the MnCo2O4 coating. A facile one pot sonication method for coating MnCo2O4 onto the CF, using an ultrasonic bath, was employed. CFs were sonicated in a mixture of 30 ml of reagent alcohol, 100 mg of NaOH, 167 mg of Mn(II) acetate (Mn(Ac)2 or C4H6MnO4) and 333 mg of Co(II) acetate (Co(Ac)2 or C4H6CoO4) for 5 h. A paste, consisting of 90 mg of MnCo2O4/CF, 5 mg of carbon black and 5 mg of PTFE (polytetrafluoroethylene) was used to prepare the air electrodes. Because all the CFs were coated with MnCo2O4 and then used to prepare the air electrodes, the MnCo2O4 bifunctional electrocatalyst was distributed throughout the whole thickness of the air electrode as opposed to only on the surface (which is the case for methods like electrodeposition or spray coating).

Initially, different crosslinker concentrations in the hydrogel electrolytes were tested; a crosslinker concentration of 30 mM (referred to as Hydrogel-30-mM) provided the best ZAB performance. Concentrations higher than 30 mM were too viscous and stiff so the O2/electrocatalyst/electrolyte three phase boundary area was not sufficient for efficient ZAB performance. Concentrations less than 30 mM did not have the rheological performance of a gel polymer electrolyte.

Charge/discharge battery performance at different current densities, cycling behavior (at 10 mA cm-2), polarization curves and power density values for MnCo2O4/CF and the benchmark Pt-RuO2 electrocatalyst in Hydrogel-30-mM were evaluated. MnCo2O4/CF had a very efficient and stable performance, compared with that of Pt-RuO2. The initial and final efficiencies for MnCo2O4/CF were 62.6% and 56.1%, respectively, for 200 cycles (100 hours: 10 min charge, 5 min rest, 10 min discharge) at 10 mA cm-2, while the initial efficiency for Pt-RuO2 was 61.3% and the battery failed after ~100 cycles.

MnCo2O4/CF also had a superior performance to that of Pt-RuO2 in Hydrogel-30-mM, in terms of the maximum power density delivered; i.e., ~ 240 mW cm-2 for MnCo2O4/CF versus 165 mW cm-2 for Pt-RuO2.