Fe-, S-, and N-Doped Carbon Nanotube Networks as Electrocatalysts for the Oxygen Reduction Reaction

Abstract

It remains a challenge to develop an efficient and facile method to synthesize nonprecious metal electrocatalysts with controlled structure and desired properties. This study reports a two-step pyrolysis method plus acid leaching between the two pyrolysis stages to synthesize Fe-, S-, and N-doped carbon nanotubes (CNTs) with a controlled size as efficient electrocatalysts for the oxygen reduction reaction (ORR). The synthesis involves a first-step pyrolysis at 700 °C to form small-diameter CNTs, a subsequent acid-etching to remove the majority of the metal nanoparticles, and a second-step pyrolysis at 900 °C to improve the crystallinity but retain the small diameter of the CNTs. The method is proved to be effective in achieving a large specific surface area and stabilizing the S species of the catalyst. Owing to the synergistic effect between the S functionalities and Fe–N x sites, the high specific surface area, and the 3D interconnect network of small-diameter CNTs, the catalyst exhibits superior activity toward the ORR with a half-wave potential of 0.854 V and a limiting current density of 5.62 mA cm–2 in alkaline medium, better than those prepared by one-step pyrolysis and the commercial Pt/C. The present simple synthetic strategy for small-diameter CNTs may have potentials for diverse applications.

Composite of Double Transition Metals (Fe, Ni) and N-Doped Carbon Nanotubes as Cathode Catalysts for Zinc–Air Batteries

“…The interconnected network formed by CNT contributed to the presence of mesopores within the material which is favorable for proton transport within the catalytic layer. 39 The EDS mapping results of the Fe 0.5 Ni 0.5 -N-CNT catalyst (Figure 2e) revealed the uniform distribution of C, N, Fe, and Ni across the catalyst, thus confirming its successful preparation. The presence of carbon can be attributed to the CNT, while the use of phenanthroline during the subsequent high-temperature treatment can explain the presence of nitrogen.…”

“…Notably, the high-temperature pyrolysis of the doped metal did not change the morphology of the CNT. The interconnected network formed by CNT contributed to the presence of mesopores within the material which is favorable for proton transport within the catalytic layer …”

Atomically Dispersed Cu Nanostructures with pH-Universal Electrocatalytic Oxygen Reduction Activity for Zn–Air Batteries

“…In addition, the good linear relationship of the Koutecky− Levich (K−L) diagram illustrates the first-order reaction kinetic characteristics in the high current density region and a nearly four-electron ORR selectivity of CuNSC-3 (Figure 4c inset). 39,40 The polarization curve was collected by rotating the ring disk electrode (RRDE) to count the yield of hydrogen peroxide (%H 2 O 2 ) and the number of electron transfer (n) (Figure 4d). As expected, in the potential range of 0.09 to 0.8 V, the n of CuNSC-3 is very close to 3.72, while the H 2 O 2 yield of CuNSC-3 (<15%) is still lower than that of Pt/C (∼23%), confirming an ideal four-electron ORR pathway and an excellent catalytic activity for the CuNSC-3 reaction mechanism, consistent with the K−L equation fitting values.…”

Protruding N-doped carbon nanotubes on elongated hexagonal Co–N–C nanoplates as bifunctional oxygen electrocatalysts for Zn–air batteries

“…38 Nevertheless, what matters to high-performance ZABs is the design of bifunctional oxygen catalysts with the preferred architecture, enhanced activity and excellent stability, needing to be further investigated. 39–41…”

“…38 Nevertheless, what matters to high-performance ZABs is the design of bifunctional oxygen catalysts with the preferred architecture, enhanced activity and excellent stability, needing to be further investigated. [39][40][41] In this work, high density NCNTs in situ grown on Co-N-C hexagonal elongated nanoplates have been designed and synthesized via a ZIF-phase-conversion derived strategy, which can be employed as a bifunctional oxygen electrocatalyst for rechargeable ZABs. Specifically, the pre-synthesized bimetallic CoZn-ZIF-L elongated nanoplates, an unstable crystalline phase, were first converted to phase-stable Zn-doped ZIF-67 nanoplates with a sword-like shape through a topochemical phase-conversion process, which were then rationally pyrolyzed to form Co-N-C@ NCNTs with a perfect morphological maintenance.…”