Rechargeable lithium-oxygen battery is an attractive candidate for the next generation energy storage device. However, there are various critical issues to overcome to make them practical. It is necessary to develop a suitable bifunctional catalyst that catalyzes oxygen redution reaction(ORR) as well as oxygen evolution reaction(OER) that takes place in Li-O2 system.
In this regard, Ba0.5Sr0.5Co0.8Fe0.2O3(BSCF) is a well known OER catalyst though it shows a relatively poor catalytic activity for ORR. In order to improve ORR activity, palladium was made to grow on the surface of BSCF. The synthesized Pd/BSCF showed good ORR and OER activity when tested in aqueous medium (ORR onset potential : -0.072 V vs Hg/HgO, OER onset potential : 0.700 V vs Hg/HgO). When tested in non-aqueous lithium air battery, it delivered a two-fold increase in specific capacity (4170 mAh/g) with an overpotential of 0.8 V. When compared to BSCF, Pd/BSCF delivered good cyclability (50 cycle) at 500 mAh/g limit with low overpotential (0.8 V).
To further improve the activity of BSCF, the ball-milling was performed over the synthesized BSCF. Ball-milling process increased the overall specific surface area, which plays a major role in the increase of the active reaction sites, resulting a great influence on the catalytic activity. Ball-milling for 24 hours showed better results in terms of onset potential (ORR onset potential : -0.126 V vs Hg/HgO, OER onset potential : 0.665 V vs Hg/HgO) and current density. BM-BSCF showed lower catalytic activity than Pd/BSCF.
To enhance both ORR and OER activity, palladium were grown on 24hours ball-milled BSCF. The Pd/BM-BSCF delivered excellent electrocatalytic activity when compared to only Pd/BSCF electrode. Especially, 20 wt% Pd on BM-BSCF was found to be the best catalyst among all other synthesized catalysts. The ORR onset potential of 20 wt% Pd/BM-BSCF was ?0.053 V and OER onset potential was 0.700 V.
Consequently, it was found that the increased surface area and the appropriate amount of palladium decorated on BSCF greatly improved the electrocatalytic activity. It is concluded that 20 wt% Pd on BM-BSCF is found to be the better suitable bifunctional electrocatalysts.