Nitrogen-doped hierarchical porous carbons derived from biomass for oxygen reduction reaction

Abstract

Nowadays biomass has become important sources for the synthesis of different carbon nanomaterials due to their low cost, easy accessibility, large quantity, and rapid regeneration properties. Although researchers have made great effort to convert different biomass into carbons for oxygen reduction reaction (ORR), few of these materials demonstrated good electrocatalytical performance in acidic medium. In this work, fresh daikon was selected as the precursor to synthesize three dimensional (3D) nitrogen doped carbons with hierarchical porous architecture by simple annealing treatment and NH₃ activation. The daikon-derived material Daikon-NH₃-900 exhibits excellent electrocatalytical performance towards oxygen reduction reaction in both alkaline and acidic medium. Besides, it also shows good durability, CO and methanol tolerance in different electrolytes. Daikon-NH₃-900 was further applied as the cathode catalyst for proton exchange membrane (PEM) fuel cell and shows promising performance with a peak power density up to 245 W/g.

Keywords: acidic medium; biomass; fuel cell; hierarchical porous carbons; oxygen reduction.

FIGURE 1

SEM images of (A) as-synthesized Daikon-NH₃-900 without HCl washing treatment and (B) Daikon-NH₃-900 after HCl washing treatment. (C,D) TEM images of Daikon-NH₃-900 in different magnifications showing the holy nanostructure under different magnifications. (E) Pore size distribution of Daikon-NH₃-900.

FIGURE 2

Structure characterizations of Daikon-NH₃-900 and Daikon-Ar-900. (A) Raman spectra; (B) XRD patterns; (C) Nitrogen sorption isotherms and (D) Thermogravimetric analysis (TGA) in air condition.

FIGURE 3

XPS analysis of Daikon-NH₃-900. (A) XPS full spectrum of Daikon-NH₃-900 and Daikon-Ar-900; (B) High resolution XPS C1s deconvoluted spectrum of Daikon-NH₃-900; (C) High resolution XPS N1s deconvoluted spectrum of Daikon-NH₃-900 and (D) Proposed chemical structure of Daikon-NH₃-900.

FIGURE 4

Catalytic activity towards electrochemical reduction of oxygen in acidic electrolyte at room temperature. (A) Cyclic voltammetry (CVs) of Daikon-NH₃-900 in O₂-saturated and N₂-saturated 0.5 M H₂SO₄ obtained at a sweep rate of 50 mV s⁻¹; (B) Linear sweep voltammograms (LSVs) of Daikon-NH₃-900 on the RRDE at 1,600 rpm in 0.5 M O₂-saturated H₂SO₄, the insert: electron transfer number of Daikon-NH₃-900 estimated from the ring and disk currents; (C) LSVs of Daikon-Ar-900, Daikon-NH₃-800, Daikon-NH₃-900, Daikon-NH₃-1000 and Pt/C electrodes in 0.5 M O₂-saturated H₂SO₄ obtained at a sweep rate of 5 mV s⁻¹ at 1,600 rpm; (D) Durability curves (i–t) of Daikon-NH₃-900 and Pt/C obtained in at 0.3 V versus Ag/AgCl at a rotation rate of 1,000 rpm; (E) The current-time (i–t) chronoamperometric responses for ORR at the Daikon-NH₃-900 and Pt/C electrodes in 0.5 M O₂-saturated H₂SO₄ aqueous solution at 0.3 V versus Ag/AgCl, CO was added at around 200 s; (F) Polarization curve and power density of the MEA fabricated with of Daikon-NH₃-900 (3.0 mg/cm²) as cathode electrode for H₂/O₂ at 80°C, DuPont Nafion 211 membrane, 30/30 psi anode and cathode back pressure. Anode electrode was Pt coated electrode with loading amount of 1.0 mg/cm².

FIGURE 5

(A) CVs of Daikon-NH₃-900 obtained at a sweep rate of 50 mV s⁻¹ in O₂- and N₂-saturated 0.1 M KOH aqueous solution; (B) LSVs of Daikon-NH₃-900 on the RRDE at 1600rpm in O₂ saturated 0.1 M KOH, the insert: electron transfer number of Daikon-NH₃-900 estimated from the ring and disk currents; (C) LSVs of Daikon-Ar-900, Daikon-NH₃-900 and Pt/C obtained at a rotation rate of 1600rpm obtained at a sweep rate of 5 mV s⁻¹ in 0.1 M O₂-saturated KOH; (D) The current-time (i–t) chronoamperometric responses for ORR at the Daikon-NH₃-900 and Pt/C electrodes in 0.1 M O₂-saturated KOH aqueous solution at −0.3 V versus SCE, and CO was added at around 200 s.