High Hydrophilic Mesoporous Carbon As Support of Pd Electrocatalyst for Ethanol Oxidation
Authors: Zhang Guoqing 
Published: February 2020

The new form of carbon materials such as onion-like carbon, flower-like carbon, straw-like carbon and nanorods carbon have been of continuous research interest, since fullerenes in 1985 and carbon nanotube in 1991 were be found. They have a wide range of applications including conductive and high-strength composites, gas/energy storage, nano devices supports of catalytic systems, and adsorbents because of their intrinsic properties involving large BET surface area, low weight, self-sinter, chemical inertia and excellent conduction of electricity and heat and so on. As we know, as supports of electrocatalyst, it is necessary that carbons have an enough conductivity and a large surface area because it is able to improve the catalytic activity through fasting the charge transportation and increasing the gas/liquid interfacial area. In the previous reports, it was found that the material with hollow structure and large micropores as electrocatalyst support can effectively improve the performance of electrocatalyst [1-4].
Several different carbon materials such as spherical carbon capsules [5] carbon hollow nanosphere [6], active fiber carbon (AFC) [7], mesoporous structured carbon materials [8] were used as supports of Pt or Pd electrocatalyst due to their large surface area and high stability, especially functional mesoporous structures. Recently, various approaches have been developed for the chemical modification of carbon surface. The most widely used technique to functionalize the carbon surface involves oxidizing it with acid or ozone, which oxygenated functionalities, such as carboxylic acid, esters is generated. However, the drawbacks of this method lie in the low bonding densities, and damage to the pore structure during oxidative treatment [9]. Though some functional methods retained the carbon structure, it needed two separated synthesis steps, which makes the process quite complex.
We report here on a promising novel and one-step synthesis of highly hydrophilic ordered mesoporous carbon-organic polymer composites with larger specific surface area by a rapid intermittent microwave heating technique (IMH). The resulting composite materials have the bifunctionality of both mesoporous carbon and organic polymer, and retain the ordered pore structure of the carbon. The synthesis strategy can be extended to various compositions of hydrophilic and hydrophobic mesoporous materials-organic polymer. The resultant materials, exhibiting chemical properties of the polymers as well as the electric conductivity of the mesoporous carbon, were used as Pd based electrocatalyst support for ethanol oxidation in alkaline media.
Mesoporous carbon (MC) with high surface area, high hydrophilic and high conductivity has been synthesized by a mixed soft template Poly vinyl alcohol (PVA) and hard template SBA-15 route and investigated as an electrocatalystic support for ethanol oxidation. There are four times higher activity of Pd/MC-PVA for ethanol oxidation than Pd/MC with the same Pd loading in alkaline media. The enhanced activity is attributed to a better Pd dispersion and utilization on mesoporous carbon support because of its high hydrophilic and conductivity. Both cyclic voltammetric and chronoamperametric measurements of the Pd based mesoporous carbon indicate that the performance of this catalyst modified by PVA can be significantly improved for ethanol oxidation.
Cyclic voltammograms of ethanol oxidation on Pd/MC and Pd/MC-PVA were shown in Figure 1. The onset potential, peak potential and peak current density of the ethanol oxidation on Pd/MC and Pd/MC-PVA electrodes are -0.49 V, -0.151 V, 10.0 mA/cm-2 and -0.55 V, -0.185 V, 27.1 mA/cm-2, respectively. The performance of the similar onset potential and peak potential on two electrodes are due to the same structure of carbon supports. However, the peak current density of Pd/MC-PVA electrocatalyst is 2.7 times higher than that of Pd/MC, just due to the better hydrophilic and conductivity on Pd/MC-PVA. According to the insert in Figure 1(a), we can sure that the improvement of catalystic activity was owed to the good hydrophilic and conductivity of MC modified by PVA which was able to give the more active center for the electrocatalysic reaction and species transportation.

References:

[1] F.P. Hu, F. Xiao, J.L. Zhang, X.G. Zhang, Carbon 43 (2005) 2931.
[2] F.P. Hu, F.W. Ding, S.Q. Songm P.K. Shen, J. Power Sources 163 (2006) 415.
[3] Z.Y. Wang, F.P. Hu, P.K. Shen, Electrochim. Commun. 8 (2006) 1764.
[4] F.Y. Xie, Z.Q. Tian, H. Meng, P.K. Shen, J. Power Sources 141 (2005) 211.
[5] S.r Iijima, Nature 354 (6348) (1991) 56.
[6] Y.Y. Song, Y. Li, X.H. Xia, Electrochim. Commun. 9 (2007) 201.
[7] H.T. Zheng, Y.L. Li, S.X. Chen, P.K. Shen, J. power Sources 163 (2006) 371.
[8] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. RyooR. Terasaki, Nature 412 (2001) 169.
[9] J.L. Bahr, J.M. Tour, J. Mater Chem 12 (2002) 1952.

Figure 1