Bimodal boron-doped ordered mesoporous carbons as oxygen reduction electrocatalysts synthesized in water/acetic acid medium

Abstract

Boron-doped ordered mesoporous carbons (B-OMCs) were synthesized by a one-pot self-assembly technique in a water/acetic acid solvent system, using resorcinol and formaldehyde as carbon source, boric acid as boron source and triblock copolymer (Pluronic F127) as the structure directing agent. The effects of the water/acetic acid (W/A) molar ratio, polymerization temperature (T_p), and carbonization temperature (T_c) on the resultant structural properties were examined in detail. Synthesized B-OMCs displayed bimodal pore size distributions, consisting of identically small-sized mesopores (D_p = 3.8 nm) and larger mesopores which varied in size and texture depending on the solvent composition and temperature. In general, lower W/A ratios and high polymerization temperature favored the formation of smaller-sized mesopores, leading to higher surface areas as high as 764 m²/g (W/A: 0.75, T_p: 80 °C and T_c: 900 °C). On the other hand, the average pore size increased with the W/A ratio, up to 10.5 nm (W/A: 3.25, T_p: 50 °C and T_c: 750 °C). According to TEM analysis, low polymerization temperatures led to predominantly two-dimensional hexagonal mesostructures, which shifted to three-dimensional interconnected type with the increasing temperature. Boron-doping was higher in the samples with lower W/A ratios, with 0.79% (wt) doping obtained from B-OMC-3.25-50/750. The boron doping content decreased with the polymerization temperature, down to 0.62% (wt). B-doped carbons were evaluated for their catalytic properties towards oxygen reduction reaction (ORR). Cyclic voltammetry measurements showed significant OR activities around 0.29 V which increased with the doped-boron content.