Mechanical interlocking of SWNTs with N-rich macrocycles for efficient ORR electrocatalysis

Abstract

Substitutional N-doping of single-walled carbon nanotubes is a common strategy to enhance their electrocatalytic properties in the oxygen-reduction reaction (ORR). Here, we explore the encapsulation of SWNTs within N-rich macrocycles as an alternative strategy to display electroactive sites on the surface of SWNTs. We design and synthesize four types of mechanically interlocked derivatives of SWNTs (MINTs) by combining two types of macrocycles and two types of SWNT samples. Comprehensive electrochemical characterization of these MINTs and their reference SWNTs allows us to establish structure-activity relationships. First, we show that all MINT samples are superior electrocatalysts compared to pristine SWNTs, which serves as general validation of our strategy. Secondly, we show that macrocycles displaying both N atoms and carbonyl groups perform better than those with N atoms only. Finally, we demonstrate that a tighter fit between macrocycles and SWNTs results in enhanced catalytic activity and stability, most likely due to a more effective charge-transfer between the SWNTs and the macrocycles. These results, focusing on the ORR as a testbed, show the possibility of understanding electrocatalytic performance of SWNTs at the molecular level and thus enable the design of more active and more stable catalysts in the future.

Scheme 1. (a) Synthetic pathway to U-shapes 1 and 2: (i) 11-bromo-1-undecene (1 eq.), tetrabutylammonium bromide (0.5 eq.), NaOH (1 eq.), butanone/water, 85 °C, and 1 h; (ii) 1,4-bis-bromomethyl-benzene (0.4 eq.), K₂CO₃ (1 eq.), KI (cat), DMF, 80 °C, and 20 h; (iii) 4-(Diphenylamino)phenylboronic acid pinacol ester (2.5 eq.), Cs₂CO₃ (2.5 eq.), Pd(PPh₃)₄ (mol 10%), toluene/ethanol/water, 90 °C, and 15 h; an energy-minimized (MM94) model of (b) MINT(6,5)-1 and (c) MINT(7,6)-1. The MINT models are displayed with the alkene in E geometry arbitrarily, as the size of the cavity hardly changes in Z configuration. Note that, experimentally, MINTs are obtained as a mixture of E/Z isomers.

Fig. 1. a) and (b) HR-TEM images of MINT(6,5)-1. Scale bars are 5 nm; (c) and (d) ac-HRTEM images of MINT(6,5)-1. Scale bars are 5 nm for (a) and (b) and 1 nm for (c) and (d).

Fig. 2. Electrochemical characterization of the as-prepared MINT electrocatalysts. (a) Comparison of the ohmic drop corrected ORR polarization curves and (b) Tafel plots for the MINT electrocatalysts with their respective pristine SWNT counterpart. iR-LSV polarization curves of (c) MINT(6,5)-2 and (d) MINT(7,6)-2 initially and after ORR stability tests (5000 cycles).