Boron Nitride - Gold As a Novel Electrocatalyst for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) is one of the most important fundamental electrochemical reactions and is the key process in fuel cell. Pt based electrocatalyst is most efficient for ORR with low overpotential but have various problems such as high cost, less abundance, poor stability in electrochemical environment, and still sluggish kinetics. Thus, there are worldwide research efforts to find non-precious metal catalysts. N- and B-doped carbon materials have been demonstrated to be effective metal free ORR catalysts. 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, can be 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 various substrates and BN on appropriate substrates can be used as an ORR catalyst.^1-3 In the present study, ORR activity of various types of BN on Au(111) is predicted theoretically and proved experimentally.^4,5

DFT calculations show that the band gap of free h-BN monolayer is 4.6 eV but a slight protrusion of the unoccupied BN states towards the Fermi level is observed if BN is supported on Au(111) due to the BN-Au interaction. Theoretically predicted metastable configuration of O₂ on h-BN/Au(111), which can serve as precursors for ORR, and free energy diagrams for ORR on h-BN/Au(111) via two- and four-electron pathways show that ORR to H₂O₂ is possible at this electrode. Electrocatalytic activities of various types of h-BN, i.e., spin coated BN nanotube (BNNT) and BN nanosheet (BNNS) and sputter deposited BN, on Au electrodes as well as those of BNNS modified glassy carbon (GC) and Pt electrodes for oxygen reduction reaction (ORR) were examined in O₂ saturated 0.5M H₂SO₄solution. 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, proving the theoretical prediction.  The reason why the highest activity was obtained by the BNNS modification is attributed to the presence of B-and/or N-edge structures. While the BNNS modification was very effective to improve ORR activity at Au electrode, it has no and negative effects at GC and Pt electrodes, respectively, confirming the important role of BN-Au interaction for ORR activity enhancement.

XPS measurements were carried out to show the modulation of electronic structure and several attempts are made to increase the BN-Au interaction so that electrocatalytic activity can be enhanced.

References:

[1] A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, PCCP, 15(8), 2809 (2013).

[2] A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, J. Phys. Chem. C, 117(41), 21359 (2013).

[3] A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, Topics in Catalysis, 57(5), 1032 (2014).

[4] K. Uosaki, G. Elumalai, H. Noguchi, T. Masuda, A. Lyalin, A. Nakayama, and T. Taketsugu, J. Amer. Chem. Soc., 136(18), 6542 (2014).

[5] G. Elumalai, H. Noguchi, and K. Uosaki, PCCP, 16(27), 13755 (2014).