Electrochemical Performance of MnO₂‐based Air Cathodes for Zinc‐air Batteries

Abstract

This paper compares the oxygen reduction on four MnO2‐based air cathodes assembled in home‐made electrochemical cells, with some particular observations on α‐MnO2 cathode. The results show that the catalytic activity decreases in the following order: electrolytic MnO2 (EMD) > natural MnO2 (NMD) > β‐MnO2 > α‐MnO2. The maximum power density of the zinc‐air battery with EMD as the catalyst reaches up to 141.8 mW cm−2 at the current density of 222.5 mA cm−2, which is about 60%, 20% and 10% higher than that of α‐MnO2 (90.0 mW cm−2 at 120.3 mA cm−2), β‐MnO2 (121.5 mW cm−2 at 150.4 mA cm−2) and NMD (128.2 mW cm−2 at 207.8 mA cm−2), respectively. It is believed that its unique crystal structure and biggest BET surface area make EMD have the smallest charge transfer resistance (Rct), thus EMD has the highest activity.

Electrolyte Study with in Operando pH Tracking Providing Insight into the Reaction Mechanism of Aqueous Acidic Zn//MnO2 Batteries

“…By this, the above‐mentioned phenomena (e. g., s. Figure 5a+b, marker 6*) are disappearing or getting weaker with proceeding cycling through the decrease of overpotentials, and the accumulation of the afore‐mentioned Zn 2+ /Mn 2+ hexaaquo complexes. For the 0.9 mM H 2 SO 4 (a), the presence of the ORR during the initial discharge cycle could explain the excess discharge capacities (s. Figure 4), the strong pH increase (s. Figure 5a, marker 2) and the reduction current peak at low potentials during only the initial CV cycle (s. Figure 6a, marker 1). According to the sluggish reaction kinetics of the ORR, this reaction is only enabled for this electrolyte due to the high overpotentials and discharge potential shifts (s. literature of metal‐air batteries, e. g.) [52–56] . For the 0.5 M MnSO 4 (b) electrolyte, the CV measurement (s. Figure 6b) showed the same characteristic of the current peaks of a MnO 2 deposition/dissolution process as seen in other publications [35,61–64] …”

“…Therefore, the Mn 2+ /MnO 2 deposition/dissolution mechanism could be taken as the main explanatory approach for the generation of capacity. Furthermore, at least for the initial discharge cycle, the presence of the ORR (s. metal‐air batteries including MnO 2 as catalyst for the ORR at the cathode) [52–56] in consequence of the high overpotential could contribute to the discharge capacity. Interestingly, the cycling of symmetrical MnO 2 //MnO 2 cells (for details, s. Figure S1, Supporting Information) still showed a comparable behaviour, underlining the above‐mentioned interpretations and the major influence of Mn 2+ /MnO 2 deposition/dissolution mechanisms.…”

Chitosan‐Derived NiO‐Mn2O3/C Nanocomposites as Non‐Precious Catalysts for Enhanced Oxygen Reduction Reaction

“…For decades, transitional metal oxides have been extensively used as a promising electrocatalyst for ORR. Manganese oxides (MnO x ) have attracted much attention due to its high electrocatalytic activity, availability, low cost, and benign nature . Despite its poor stability in acidic medium, MnO x can be employed as a favorable electrocatalyst for cathodic reduction of oxygen in both AFCs and metal air batteries.…”

Advanced Architectures and Relatives of Air Electrodes in Zn–Air Batteries

“…Plentiful materials have appeared in the area of ORR electrocatalysts, including noble metals and their alloys, transitional metals, metal oxides/chalcogenides/carbides/nitrides, carbon nanomaterials, and their composites . However, different reaction mechanisms may happen on the surface of different catalysts . The 2‐electron reduction path is favored on most of nanocarbons and transitional metal based composites, while the direct 4‐electron reduction is favored on noble metal‐based catalysts .…”