The commercial viability of alkaline fuel cells and metal-air batteries as future energy applications hinges to a large extent on the development of highly efficient catalysts for the oxygen reduction reaction (ORR). Currently, platinum nanoparticles are the de facto cathode electrode materials to catalyze the sluggish ORR [1]. However, the scarcity of Pt metal coupled with their ever-growing demand are key obstacles for the broad deployment of this precious metal in all commercial technologies [2]. The logical solution to this persistent problem is an alternate catalyst with good ORR capabilities and durability at a reduced cost.

Several metals, metal-oxides, and non-metal based catalyst are being studied as alternate ORR catalysts with limited success [3]. Among metal-oxides, manganese oxide (MnO₂) has gathered enormous interest due to their ability to catalyze the disproportionation reaction of HO₂^- to H₂O and catalyze 4-electron reduction of oxygen in combination with another material (usually carbon) active for 2-electron reduction of oxygen to peroxide [4]. Another method to improve ORR capabilities is to incorporate a small amount of noble metals in to the metal-oxide catalyst structure. It is expected that modifying catalysts with an inexpensive noble-metal such as silver would result in an improvement in the electrochemical performance of the cathode without significantly increasing catalyst cost. Additionally, the high activity of silver for oxygen reduction [5] and their superior electrical conductivity probably have additional synergy effect on the catalysts as shown for other bi-functional catalyst [6].

The objectives of this study are as follows: (i) synthesis and evaluation of MnO₂ nanoparticles structure/morphology with and without silver, (ii) characterization of the synthesized catalyst materials using RDE to determine the effect of silver addition on the ORR kinetics, and, (iii) long-term durability testing of the cathodes prepared with different catalysts at higher current density to ensure their feasibility as ORR cathodes in alkaline media.

Fig. 1. shows the TEM and XRD of the MnO₂ catalyst synthesized using the hydrothermal method. The XRD peaks of the catalyst powder as shown in Fig. 1a were identified as MnO₂, while the TEM image shows a flake-like structure. Long-term durability testing of the MnO₂-based cathode was performed using the three electrode cell design.

Fig. 2 shows the voltage measured at the cathode maintained at a constant current density of -150 mA.cm^-2in 6 M KOH. Clearly, no noticeable drop in voltage was observed for 200 hr indicating good stability of the cathode for ORR over extended periods.

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