Chemical Vapor Deposition of Hollow Graphitic Spheres for Improved Electrochemical Durability

J Knossalla,K J J Mayrhofer,D Göhl,M Ledendecker,J Mielby,D Jalalpoor,F R Wang,F Schüth,A Hopf

doi:10.1021/acsaem.1c00643

Abstract

The wet-chemical synthesis of hollow graphitic spheres, a highly defined model catalyst support for electrocatalytic processes, is laborious and not scalable, which hampers potential applications. Here, we present insights into the chemical vapor deposition (CVD) of ferrocene as a simple, scalable method to synthesize hollow graphitic spheres (HGScvd). During the CVD process, iron and carbon are embedded in the pores of a mesoporous silica template. In a subsequent annealing step, iron facilitates the synthesis of highly ordered graphite structures. We found that the applied temperature treatment allows for controlling of the degree of graphitization and the textural properties of HGScvd. Further, we demonstrate that platinum loaded on HGScvd is significantly more stable during electrochemical degradation protocols than catalysts based on commercial high surface area carbons. The established CVD process allows the scalable synthesis of highly defined HGS and therefore removes one obstacle for a broader application.

Highlights

Increasing energy demand and depletion of fossil resources lead to an intensified search for alternative and sustainable energy systems
The degradation can be divided into five mechanisms, including Pt dissolution, Ostwald ripening, carbon corrosion, particle detachment, and agglomeration.[9−11] Hollow graphitic spheres (HGS) were reported to significantly suppress degradation mechanisms such as particle detachment and agglomeration, thanks to a confinement of the nanoparticles in the mesopores of the HGS.[10−12] Templating methods are typically utilized for the synthesis of mesoporous materials with tailored textural properties, subdivided into hard and soft templating methods.[13,14]
Ferrocene is heated to 120 °C and transported by a constant Ar flow over the SiO2@mSiO2 silica template, located in the center of a tube furnace heated to 550 °C

Summary

■ INTRODUCTION

Increasing energy demand and depletion of fossil resources lead to an intensified search for alternative and sustainable energy systems. The CVD of ferrocene employing SiO2@mSiO2 templates was reported for the synthesis of hollow graphitic carbon spheres.[33−36]. The catalyst ink consisted of a catalyst powder dispersed in 5 mL of ultrapure water with an ultrasound bath; a 20 μL droplet of the suspension was deposited onto a carbon disk of 0.196 cm[2] geometric surface area, embedded in a Teflon tip. The relative ECSA values were calculated by normalizing the measured ECSA with respect to the ECSA determined after 200 activation cycles More details about this method can be found in ref 11. The electrochemical activation procedure applied consisted of 200 cleaning cycles (0.05−1.4 VRHE, 0.2 V/s) In this phase, the surface carbon impurities are removed. The commercial catalyst Pt@C used for comparison was purchased from Tanaka and had a platinum loading of 46 wt % and a mean platinum particle size of 3 nm.[11]

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES