  Carbon(C) doped hexagonal boron nitride(hBN) has been experimentally reported
in recent years to be a possible catalytic host to oxygen reduction
reaction(ORR), as well as a possible ferromagnet at room temperature.
Substitution by C in hBN has been also reported to form islands of graphene. In
this work, we explore from first principles, the connection between these
different aspects of C doped hBN. We find formation of graphene islands
covering unequal number of B and N sites in hBN to be energetically plausible.
They possess a net non-zero magnetic moment and are also found to be
substantially more chemically active than their non-magnetic counterparts
covering equal number of B and N sites. On-site Coulomb repulsion between
electrons, known to be responsible for magnetism in bipartite lattices like
graphene and hBN, is also found to play a central role in chemical activation
of not only the C atoms at the zigzag interface of magnetic graphene islands
and hBN, but also of boron(B) sites in the immediate hBN neighborhood. However,
such activated B or C due to substitution at B site, which is energetically
more favorable than at N site, has been reported to be unfavorable for ORR.
Advantageously, we find that the activation of C at B sites moderates
systematically with increasing size of graphene islands, paving the way for
abundance of efficient catalytic sites at the edges of magnetic graphene
islands covering more B sites than N sites. Accordingly, as an alternate to
precious metals for electrodes, we propose a class of graphene-hBN hybrids with
lattices of magnetic graphene islands embedded in hBN, which can be metallic.
