Insulating Boron Nitride Nanosheet on Inert Gold Electrode As a Novel Efficient Electrocatalyst for Oxygen Reduction Reaction
Authors: Uosaki Kohei , Elumalai Ganesan , Noguchi Hidenori , Dinh Hung Cuong , Lyalin Andrey , Taketsugu Tetsuya 
Published: February 2020

Large overpotential for oxygen reduction reaction (ORR) is one of the most serious problems in the development of fuel cells. Although Pt based electrocatalysts are known to be most efficient, they have several problems such as high cost, less abundance, poor stability, and still sluggish kinetics. Many efforts have been made to find alternative catalysts and N- and B-doped carbon materials have been demonstrated to be effective metal free ORR catalysts and their ORR activity is further increased by co-doping of B and N atoms. If all carbon atoms in graphene are substituted by B and N atoms, hexagonal boron nitride (h-BN) monolayer, which has geometric structure similar to the graphene, as an extreme case, is obtained. Although BN is an insulator with a wide band gap (5.8eV), our recent theoretical studies showed that the band gap of h-BN monolayer can be considerably reduced by B- and N- vacancy and impurity defects as well as by interaction with metal substrate and BN on appropriate substrates can be used as an ORR catalyst.1-3 In the present study, electrocatalytic activity of various forms of BN on Au(111) for ORR is investigated both theoretically and experimentally. DFT calculations for BN/Au(111) predicted the possible ORR activity of BN/Au(111)4 and electrocatalytic activities of various types of BN, i.e., spin coated BN nanotube (BNNT), BN nanosheet (BNNS) and sputter deposited BN, on Au electrodes as well as those of BNNS modified glassy carbon (GC) and Pt electrodes for ORR were examined.4
,5 The overpotential for ORR at Au electrode was reduced by ca. 100, ca. 270, and ca.150 mV by spin coating of the dispersion of BNNT and liquid exfoliated BNNS, and sputter deposition of BN, respectively. Oxygen is reduced only to H2O2 by 2-electron process as at Au electrode in all cases. The reason why the highest activity was obtained by the BNNS modification is attributed to the presence of B-and/or N-edge structures. If the edge plays important role for higher activity, one would expect higher ORR activity by increasing the BN – Au interaction. We have attempted to increase the interaction by depositing gold nanoparticles (AuNP) on BNNS, which is then placed on a Au electrode (Au-BNNS/Au). The decoration of BNNS with AuNP not only reduces the overpotential for ORR further by ca. 50 mV, but also opens a 4-electron reduction route to water.6 Both rotating disk electrode experiments with Koutecky-Levich analysis and rotating ring disk electrode measurements show that more than 50% of oxygen is reduced to water via 4-electron process at Au-BNNS/Au electrode while less than 20 and 10 % of oxygen are reduced to water at the BNNS/Au and bare Au electrodes, respectively. Theoretical analysis of free energy profiles for ORR at the BN monolayer with and without Au8 cluster placed on Au(111) show significant stabilization of adsorbed oxygen atom by the Au8 cluster, opening a 4-electron reduction pathway.6

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