(330a) Synthesis and Application of the Spherical Nanoporous Carbons for Energy Conversion and Energy Harvesting

Nanoporous carbons that have nanopores as pore size of nano dimensions have been widely adopted in energy-related applications such as supercapacitors, supports and catalysts for fuel cells due to the high surface areas, tunable average pores size, and particle shapes [1-4]. Among the many shapes of the nanoporous carbon, spherical nanoporous carbons (SNCs) prepared via hard-templated methods have several unique features such as tunable particle size distribution, regular morphology, and controllable surface areas [1,4]. In this presentation, the SNCs with Fe and N doping for oxygen reduction reaction (ORR) as a usual application is discussed at first. And the SNCs with different average sizes for the triboelectric nanogenerator (TENG) for energy harvesting of wasted mechanical energy as a new application area are presented [5,6].

As hard-template materials, mesoporous spherical silicas (MSSs) with different particle sizes were prepared by the Stöber process using high concentrations of tetraethyl orthosilicate (TEOS) and n -hexadecylamine as a nonionic surfactant. To control the particle size of MSS, the concentration of the ammonia is adjusted [5,6]. The Fe-SNC catalysts were prepared using the incipient wetness impregnation method with Iron(III) chloride hexahydrate and 1,10-phenanthroline in a mixture of ethanol and sulfuric acid. The four Fe-SNC catalysts are prepared using four different MSS with controlled particles sizes, which are designated as Fe-SNC_151, _337,_723, and _1173. The number at the end of the sample name is the average size of the catalyst in nm, respectively [5]. For the energy harvesting application, three SNC samples with sizes of 1000, 700, and 300 nm were prepared with phenanthrene, p-toluenesulfonic acid, and MSS as a hard template. The electrode for the TENG with SNC samples was prepared with PVDF nanofiber and gradient distribution of the different sizes of SNC samples [6].

As a result, among the Fe-SNC catalysts, the Fe-SNC_337 showed the highest half-wave potential and the second highest limiting current values; 0.918 V and 5.881 mA/cm², respectively, which is a higher activity than that of Pt/C in the alkaline conditions. The ORR activity was dependent on the catalyst particle size. This size effect can be explained by the fact that the smaller catalyst particles allow for improved electrolyte accessibility; this provides access to a larger number of active sites within the same reaction time. One exception to this was Fe-SNC_151, which has agglomerated particles, unnecessary active sites and a different limiting current because of the small protrusions on the catalyst surface due to undesired iron species. This resulted in low ORR performance for Fe-SNC_151, despite it having the smallest catalyst particle size.

In addition, the SNC samples with different three sizes of 1000, 700, and 300 nm, respectively, was adopted in the positive and negative electrode with gradient structure and displayed the enhanced the charge confinement in the smaller SNC sample by fast transferring the surface charge from the larger SNC sample. Via this process, the output voltage is changed from 15.2 V to 600 V after high voltage charge injection, thus representing an increase of about 40 times, which leads to turning on the 300 LEDs. Further, to amplify the low output current, which is a disadvantage of triboelectric energy, two types of electrical energy (triboelectric and electromagnetic energy) are produced in a single mechanical device. The output current produced by the cylindrical TENG and electromagnetic generator is recorded as being 1300 times higher, increasing from 12.8 μA to 17.5 mA.

To sum up, the hard-templated SNC samples can enhance the electrochemical reduction of oxygen if Fe and N are introduced into the carbon matrix. A new application of SNC samples with different particles sizes is proposed in the energy harvesting of mechanical energy.

[1] J. Liu, N. P. Wickramaratne, S. Z. Qiao, M. Jaroniec, Nat. Mater., 14, 763 (2015).

[2] H. Shao, Y.-C. Wu, Z. Lin, P.-L. Taberna, P. Simon, Chem. Soc. Rev. 49, 3005 (2020).

[3] V. Yarlagadda, N. Ramaswamy, R. S. Kukreja, S. Kumaraguru, J. Power Sources, 532, 231349 (2022).

[4] H. Chang, S. H. Joo, C. Pak, J. Mater. Chem. 17, 3078 (2007).

[5] J. Lee, J. G. Kim, C. Pak, J. Energy Chem. 52, 326 (2021).

[6] S. Cha, Y. Cho, J. G. Kim, H. Choi, D. Ahn, K. Sun, D.-S. Kang, C. Pak, J.-J. Park, Small Methods, 2101545 (2022).