Rechargeable zinc–air batteries
(ZABs) require
bifunctional
electrocatalysts presenting high activity in oxygen reduction/evolution
reactions (ORR/OER), but the single-site metal–N–C catalysts
suffer from their low OER activity. Herein, we designed a series of
single-site Fe–N–C catalysts, which present high surface
area and good conductivity by incorporating into mesoporous carbon
supported on carbon nanotubes, to study the doping effect of N and
P on the bifunctional activity. The additional P-doping dramatically
increased the content of active pyridine-N and introduced P–N/C/O
sites, which not only act as extra active sites but also regulate
the electron density of Fe centers to optimize the absorption of oxygenated
intermediates, thereby ultimately improving the bifunctional activity
of Fe–N–C sites. The optimized catalyst displayed a
half-wave potential of 0.882 V for ORR and a low overpotential of
365 mV at 10 mA cm–2 for OER, which significantly
outperforms the counterpart without P, as well as noble-metal-based
catalysts. The ZABs with air cathodes containing the N,P-co-doped catalysts exhibited a high peak power density of 201 mW cm–2 and a long cycling stability beyond 600 h. Doping
has shown to be an effective way to optimize the performance of single-site
catalysts in bifunctional oxygen electrocatalysis, which can be extended
to other catalyst systems.
Possessing excellent electronic and mechanical properties and great stability, single-walled carbon nanotubes (SWCNTs) are exceptionally attractive in fabricating flexible transparent conductive films. Doping is a key step to further enhance the conductivity of the SWCNT films and the reliable doping is highly needed. We developed a feasible strategy that uses solid acids such as phosphotungstic acid (PTA) to dope the SWCNT films stably relying on the nonvolatility of the dopants. The sheet resistance of the films was reduced to around a half of the original value meanwhile with no obvious change in transmittance. The doping effect maintained during a 700 days' observation. The excellent flexibility of the PTA-doped films was demonstrated by a bending test of 1000 cycles, during which the sheet resistance and transmittance was basically unaffected. The blue shifts of G band in the Raman spectra and the increase of work function measured by the Kelvin probe force microscopy both reveal the p-type doping of the films by PTA. The strong acidity of PTA plays a key role in the doping effect by increasing the redox potential of the ambient O 2 and thus the Fermi level of the SWCNTs is brought down. The great feasibility and robustness of our doping strategy are desirable in the practical application of SWCNT-based flexible transparent conductive films. This strategy can be extended to the p-type doping of various CNT-based assemblies (such as sponges and forests) as well as other material families, expanding the application spectrum of polyacids.
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