posted on 2020-03-31, 18:05authored byMohammad Tavakkoli, Emmanuel Flahaut, Pekka Peljo, Jani Sainio, Fatemeh Davodi, Egor V. Lobiak, Kimmo Mustonen, Esko I. Kauppinen
Mesoporous heteroatom-doped
carbon-based nanomaterials are very
promising as catalysts for electrochemical energy conversion and storage.
We have developed a one-step catalytic chemical vapor deposition method
to grow a highly graphitized graphene nanoflake (GF)–carbon
nanotube (CNT) hybrid material doped simultaneously with single atoms
of N, Co, and Mo (N–Co–Mo–GF/CNT). This high-surface-area
material has a mesoporous structure, which facilitates oxygen mass
transfer within the catalyst film, and exhibits a high electrocatalytic
activity and stability in oxygen reduction and evolution reactions
(ORR and OER) in alkaline media. We have shown that in this metal
(M)–N–C catalyst, M (Co, Mo)–C centers are the
main sites responsible for OER, while, for ORR, both M and N–C
centers synergistically serve as the active sites. We systematically
investigated tuning of the ORR and OER activity of the porous catalyst
depending on the choice of the underlying substrate. The ORR kinetic
current and OER activity for N–Co–Mo–GF/CNT were
significantly enhanced when the catalyst was deposited onto a Ni substrate,
resulting in an advanced electrocatalytic performance compared to
the best bifunctional ORR/OER catalysts reported so far. Using a developed
scanning electrochemical microscopy analysis method, we demonstrated
that the higher OER reactivity on Ni was attributable to the formation
of underlying catalyst/Ni interfacial sites, which are accessible
due to the porous, electrolyte-permeable structure of the catalyst.