Effects Of Microstructures And Surface Properties Of Carbon Nanofibers On Their Electrocatalytic Activity For Oxygen Reduction

Carbon nanofibers (CNFs) have attracted considerable attention as catalyst and catalyst support for ORR in recent years because of their special electrical and structural properties. The oxygen reduction reaction (ORR) at the cathode of fuel cells plays a key role in controlling the performance of a fuel cell, and efficient ORR electrocatalysts are essential for practical applications of the fuel cells.The purpose of this paper is to study the microstructure and surface properties of CNFs towards oxygen reduction reaction (ORR) using CNFs as catalysts or catalyst supports. The main results obtained up to now are as follows:(1) Oxygen- and nitrogen-containing functional groups were successfully introduced onto the t-CNF and f-CNF surface by sonochemical treatment in mixed acids (concentrated sulfuric acid and nitric acid) and ammonia, respectively. The ORR activity of the CNF catalyst was measured in an oxygen-saturated 0.1 M KOH electrolyte solution by rotating disk electrode technique. The RDE results show that the electrocatalytic activities of the two types of CNFs increase in the same sequence untreated CNFs< oxygen-containing CNFs< nitrogen-containing CNFs, while the f-CNFs-based catalysts have higher electrocatalytic activities for ORR than their t-CNFs-based counterparts. The above results indicate that the surface properties and the microstructures of CNFs both have effects on the electrocatalytic activity of CNFs for ORR, although the former may have a dominant effect.(2) The oxygen-and nitrogen-containing functional groups were introduced onto the f-CNF surface by sonochemical treatment in mixed acids (concentrated sulfuric acid and nitric acid) and ammonia, respectively. The Pt/CNF-ON/GC electrode exhibited a better electrocatalytic activity toward ORR compared with Pt/CNF-O/GC electrode. It is believed that the enhanced catalytic activity exhibited by the Pt/CNF-ON/GC electrode can mainly be attributed to the smaller Pt particle size and more uniform particle size distribution. The synergistic effect, the enhanced Pt-CNF-ON interaction, and the unique structural and electronic properties of CNF-ON may also contribute to the enhanced catalytic activity.(3) Pt-Pd alloy catalysts with different ratios of Pt and Pd were prepared by glycol method. RDE results show that the catalytic activity toward ORR of the alloy catalysts increased with the increasing platinum content, except for Pt3Pd1/CNF. Among these alloy catalysts, Pt2Pd1/CNF shows the highest ORR catalytic activity. The enhanced catalytic activity can be attributed to the appropriate Pt-Pd atomic ratio, a favorable degree of atomic spacing and the proper alloy extent.