The effect of solvent type and composition on the synthesis of boron-doped ordered mesoporous carbons

Boron-doped ordered mesoporous carbons (B-OMCs) were synthesized using different solvent types and compositions to study the effect of the solvent medium on the physical properties and oxygen reduction activities of the resultant structures. Materials were synthesized by a one-pot self-assembly method, using resorcinol and formaldehyde as carbon source, boric acid as boron source and triblock copolymer Pluronic F127 as the structure directing agent. Different solvents, namely ethanol, n-propanol, isopropanol and acetone were used as solvent media along with water as the cosolvent. Among different solvents, ethanol and n-propanol gave better-ordered structures and also had higher surface areas. At this stage, the highest boron doping percentage was obtained as 1.38 % with n-propanol. In the second stage, B-OMCs were synthesized with n-propanol in different water/npropanol (W/N) molar ratios varying between 2.5 and 4.75. The orderliness was observed to be better in samples with low W/N ratios of 2.5 and 3.25 and the morphologies shifted from 2D to 3D interconnected type with the increase of water content. The average pore sizes of samples varied between 7.9 and 9.7 nm. On the other hand, the highest surface area (729 m2/g) and the highest boron doping percentage (1.53 %) were obtained with the W/N ratio of 4. XPS analysis showed that 23 % (w/w) of doped boron in this sample was in substitutional state while the remaining boron was found in mixed B-C and B-O states. Cyclic voltammetry measurements for the samples with W/N ratios of 2.5, 3.25 and 4 showed significant oxygen reduction activities at -0.28 and -0.29 V. Results showed that the properties of solvent play an effective role in the orderliness, pore structure and surface composition of B-OMCs, which in turn affect their electrochemical properties. Among different types of solvents, n-propanol can be used as an alternative to ethanol with the W/N ratio of 4 giving the highest surface area and boron doping percentage.