Oxygen reduction reaction (ORR) is one of the crucial processes in fuel cells, metal-air batteries and corrosion and, therefore, has been extensively studied. Although Platinum is known to be the best ORR catalyst, intensive studies to develop novel electrocatalysts for ORR with high efficiency and long term stability based on low cost and abundant materials are carried out worldwide due to limited resources, high cost, low stability, and still slow kinetics of Pt-based electrodes. We have predicted theoretically that hexagonal boron nitride (h-BN) would act as ORR electrocatalyst but only to H₂O₂ via 2 electron reduction, although BN is intrinsically an insulator [1-3] and proved experimentally that Au electrodes modified with various types of h-BN indeed act as ORR electrocatalysts [3,4]. Best results were obtained at the Au electrode modified by BN nanosheet (BNNS) where overpotential for ORR is smaller than at an unmodified Au electrode by 0.27 V, although the ORR overpotential is still very high compared with that at a Pt electrode and oxygen is mainly reduced to H₂O₂ via 2-electron process as theoretically predicted [3,4]. We also found that BNNS acts as electrocatalyst for hydrogen evolution reaction (HER) [5]. Substrate dependencies and theoretical evaluation suggest that the interaction between BNNS and Au plays an essential role for improved electrocatalytic activities in both cases [3-5]. We, therefore, have attempted to enhance the electrocatalytic activity for ORR by decorating BNNS deposited on Au with Au nanoparticles (AuNPs) [6]. Not only the overpotential for ORR is reduced further by ca. 50 mV but also 4-electron reduction path way to water is opened by the AuNP decoration of BNNS [6]. Theoretical free energy analysis showed that 4-electron reduction becomes possible because small Au clusters drastically stabilize adsorbed O by 1.6 eV, making the dissociation of adsorbed OOH, which is an uphill process at BNNS/Au electrode, downhill [6]. Considering the importance of BN/Au interface, one would expect further improvement of electrocatalytic activity for ORR by using smaller AuNPs for decoration.

Here, we report that electrocatalytic activity of Au(111) electrode modified with BNNS decorated by small (5 nm) AuNPs with narrow size distribution (denoted as AuNP(5 nm)-BNNS/Au) is very high: the overpotential is reduced further by 100-120 mV in comparison with Au(111) electrode modified with BNNS decorated by random sized (5~30 nm) AuNPs on BNNS/Au (denoted as AuNP(RS)-BNNS/Au) and 80 ~ 90% of oxygen is reduced to water by 4-electron reduction while almost 0% and only 50% of oxygen is reduced to water at BNNS/Au and AuNP(RS)-BNNS/Au electrodes, respectively[7].

[1] A. Lyalin, K. Nakayama, K. Uosaki, and T. Taketsugu, J. Phys. Chem. C, 117 (2013) 21359.

[2] A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, Phys. Chem. Chem. Phys., 15 (2013) 2809.

[3] K. Uosaki, G. Elumalai, H. Noguchi, T. Masuda, A. Lyalin, A. Nakayama, and T. Taketsugu, J. Am. Chem. Soc. 136 (2014) 6542.

[4] G. Elumalai, H. Noguchi, and K. Uosaki, Phys. Chem. Chem. Phys., 16 (2014) 13755.

[5] K. Uosaki, G. Elumalai, H. C. Dinh, A. Lyalin, T. Taketsugu, and H. Noguchi, Sci. Rep., 6 (2016) 32217.

[6] G. Elumalai, H. Noguchi, A. Lyalin, T. Taketsugu, and K. Uosaki, Electrochem. Commun. 66 (2016) 53.

[7] G. Elumalai, H. Noguchi, H. C. Dinh, and K. Uosaki, J. Electroanal. Chem., in press, DOI: 10.1016/j.jelechem.2017.09.033 (2018)