(La,Sr)CoO3-Rgo Hybrid Oxygen Reduction Reaction/Oxygen Evolution Reaction Bifunctional Catalyst

A bifunctional oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER) catalyst is essential for rechargeable metal-air batteries and regenerative fuel cells. Platinum (Pt) and iridium oxide (IrO₂) are the state-of-the-art ORR and OER catalysts, respectively. However the high price and scarcity of these platinum group metals (PGMs) has been an obstacle for wide spread application of these catalysts. Recently, in alkaline media, carbon based ORR and perovskite OER catalysts have demonstrated similar or even better catalytic activities compared to the counterpart PGM catalysts [1, 2]. Therefore, if we combine these two non-PGM catalysts, a non-PGM bifunctional ORR/OER catalyst can be obtained. A hindrance in this approach is the vulnerability of carbon-based ORR catalysts to oxidation in the OER potential range, i.e., potentials > 1.5 V vs. RHE. Thus development of robust ORR catalysts under practical OER conditions is a key to realize this kind of bifunctional catalysts.

 The carbon support used in our ORR catalysts was black pearl (BP) 2000 [1]. In preliminary tests, however, we found that BP 2000 undergoes oxidization at potentials around ca. 1.2 V vs. RHE and above (data not shown). In this work, we used reduced graphene oxide (rGO) as an alternative support to synthesize oxidation resistant ORR catalysts. The OER catalyst we chose was a perovskite (La_1-xSr_x)CoO_3-δ (LSC). Pre-synthesized LSC was added into the initial solution of the rGO based ORR catalyst synthesis process, and after drying and heat-treatment, bifunctional (LSC + rGO) catalysts were obtained. In measuring the OER activity of the LSC catalyst, acetylene black (AB) carbon was added to the LSC (LSC + AB) to increase the electrical conductivity. Fig. 1 shows the comparison of ORR/OER activities between (LSC + AB) and (LSC + rGO). As expected, the ORR activity of (LSC + rGO) is greatly improved by ca. 200 mV in terms of E_½, in comparison to that of (LSC + AB). Interestingly even the OER activity of (LSC + rGO) becomes higher than that of (LSC + AB). Thanks to the enhancement of both ORR and OER activities with (LSC + rGO), highly active bifunctional catalysts are obtained. In this talk, material analysis results and diverse electrochemical performances of the (LSC + rGO) catalysts will be presented.

Acknowledgements

Support from the Directed Research of the Los Alamos National Laboratory’s Laboratory Directed Research & Development (LDRD-DR) is greatly acknowledged.

References

1. Chung et al., Nat. Commun. 4, 1922 (2013).
2. Suntivich et al., Science, 334, 1383 (2011).