It is well known that energy conversion and storage through electrochemical reactions are the most important energy technologies available today. They include low-temperature polymer electrolyte fuel cells, metal-air batteries, and water electrolyzers, which have many advantages over traditional gasoline combustion such as better overall efficiency, high energy density, and the reduction in CO₂ and other emissions [1-3]. These electrochemical energy technologies greatly rely on the oxygen reduction reaction (ORR) and evolution reaction (OER), which is one pair of the most important electrochemical reactions. In particular, one of important applications of bifunctional ORR/OER oxygen catalysts, which is simultaneously active for the ORR and the OER, is for utilized reversible fuel cells. They can significantly reduce the capital cost and enhance energy density. For example, a single device, during the day time, solar generated electricity can be stored via water splitting into H₂ and O₂ through an electrolyzer [4]. During the night time, the stored H₂ and O₂ can be used back in a fuel cell to supply electricity. Thus, a single device is able to be operated in both electrolyzer and fuel cell models. However, the energy conversion efficiency is greatly dependent on the performance of the bifunctional catalysts towards the oxygen evolution for water splitting and oxygen reduction in a fuel cell. The other need of bifunctional catalysts is for rechargeable metal-air batteries such as zinc-air and lithium-air cells. They can provide much higher energy density compared to conventional lithium ion battery [5,6]. This is due to their opening air electrodes, which no longer need to store active species within batteries. Instead of, they can continuously intake O₂ from air. Oxygen reduction and oxygen evolution are associated with the battery discharging and charging processes, respectively.

   Current challenges for developing these highly demanded reversible electrochemical energy conversion and storage is the lack of effective cathode catalysts to simultaneously catalyzing both reactrions [7]. High reaction overpotentials, which mean sluggish kinetics, require catalysts containing large amounts of precious metals such as Pt and Ir to enhance reaction activity and durability. Unfortunately, high cost and limited supply of these precious metals have become a grand challenge for widespread applications of these clean energy technologies [8,9]. Even worse, the highly ORR active Pt is not a good catalyst for the OER. Likewise, the OER active Ir is not an optimal catalyst for the ORR. Thus, development of highly efficient ORR/OER bifunctional catalysts is desperately demanded for such clean energy technologies.

   In this presentation, based on our ongoing researches [4,10-15], I will discuss several bifunctional oxide nanocomposite catalysts. They includes (1) oxygen-deficient perovskite oxides, (2) transition metal oxides, (3) highly stable and active nanocarbons, and (4) their integrated nanocomposites. In addition, perspectives on these catalysts, future approaches, and possible pathways to address current challenges are discussed as well.

 Reference

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[12] C.-F. Chen, G. King, R.M. Dickerson, P.A. Papin, S. Gupta, W.R. Kellogg, G. Wu, Nano Energy, 13 (2015) 423–432.

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[15] Q. Li, H. Pan, D. Higgins, R. Cao, G. Zhang, H. Lv, K. Wu, J. Cho, G. Wu, Small, 11 (2015) 1443–1452.