Abstract
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.
Highlights
The need for low-cost electrocatalysts based on earth-abundant materials, rather than critical noble metals, such as Pt, Ru, and Ir, for catalyzing oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has motivated a large body of research
N−Co−Mo−graphene nanoflakes (GFs)/ carbon nanotubes (CNTs) was synthesized by modifying a catalytic chemical vapor deposition (CCVD) method[50] that was initially developed for the scalable synthesis of CNTs
The CCVD synthesis of the CNTs is a chemical vapor deposition (CVD) process in which the carbon precursor molecules are catalytically decomposed at high temperatures on the surface of a metallic support that acts as the catalyst for the growth of CNTs
Summary
The need for low-cost electrocatalysts based on earth-abundant materials, rather than critical noble metals, such as Pt, Ru, and Ir, for catalyzing oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has motivated a large body of research. Owing to the high surface energy of single atoms, immobilization of atomic metal centers on the support for the fabrication of SACs has been challenging.[47] Here, in contrast to other synthetic strategies for SACs,[47−49] the SAC is produced during the synthesis of the carbon support (GF/CNT), providing a facile one-step synthesis process for the fabrication of high-performance SACs without any extra cost arising from the immobilization process of the single-atom metals on the support This highsurface-area mesoporous catalyst shows high ORR and OER activities in alkaline media. Our study of substrate effects on the OER and ORR catalytic activity of porous materials establishes a basis for the rational design of electrodes with optimized activity and provides guidelines for future studies
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