Although Pt based materials are currently the most efficient electrocatalyst for oxygen reduction reaction (ORR), a key process in fuel cell, high cost, less abundance, poor stability, and still sluggish kinetics of Pt inhibit the wide use of fuel cell.  N or B doped carbon materials are found to be effective ORR catalysts and N and B co-doped carbon materials have higher ORR catalytic activity than those doped with single element. It is interesting to see if hexagonal boron nitride (h-BN) monolayer, which is obtained by substituting all carbon atoms in graphene by B and N atoms but is an insulator with wide band gap (3.6 - 7.1 eV depending on experimental methods), acts as as an ORR catalyst.

DFT calculations show that the band gap of free h-BN monolayer is 4.6 eV, which is within the highly dispersed experimental values, the calculated partial DOS (PDOS) of h-BN is slightly modified and a slight protrusion of the unoccupied BN states towards the Fermi level due to the interaction with Au is observed if it is supported on Au(111). Free energy diagrams for ORR at h-BN/Au(111) surface obtained by accounting for entropy contribution, zero-point energy corrections and solvent effects on the adsorption energies of O₂ and all ORR intermediates with the assumption of independent adsorption of ORR intermediates show that h-BN/Au(111) may act as an electrocatalyst for 2-electron reduction of oxygen to H₂O₂.³

Experimental investigations of electrocatalytic activities of various types of BN, i.e., spin coated BN nanotube (BNNT), BN nanosheet (BNNS) and sputter deposited BN, on Au electrodes show that 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. Theoretical calculation shows that the reason why the highest activity was obtained by the BNNS modification is the presence of B-and/or N-edge structures. While the BN 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-substrate interaction.  

Various attempts including Au nanoparticle modification were made to increase the BN-Au interaction so that ORR activity can be improved. The best result obtained so far showed that the overpotential for ORR is only 80 mV more than at Pt and 75 % of current were used for 4-electron reduction.

Further improvements are under way.

References

[1] A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, PCCP, 2013, 15, 2809 – 2820

[2] A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, J. Phys. Chem. C, 2013, 117, 21359-21370.

[3] A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, Topics in Catalysis, 2014, 57, 1032-1041.

[4] K. Uosaki, G. Elumalai, H. Noguchi, T. Masuda, A. Lyalin, A. Nakayama, and T. Taketsugu, J. Amer. Chem. Soc., 2014, 136, 6542-6545.

[5] G. Elumalai, H. Noguchi, and K. Uosaki, PCCP, 2014, 16, 13755-13761.