The Origins of the High Performance of Pd Catalysts Supported on Carbon Black-Embedded Carbon Nanofiber for Formic Acid Oxidation

Norraihanah Mohamed Aslam,Akio Kodama,Takuya Tsujiguchi,Yugo Osaka

doi:10.3390/app9245542

Norraihanah Mohamed Aslam, Akio Kodama + Show 2 more

Open Access

https://doi.org/10.3390/app9245542

Copy DOI

Journal: Applied sciences	Publication Date: Dec 16, 2019
Citations: 3	License type: CC BY 4.0

Affiliation: Kanazawa University

Abstract

In this study, we developed a carbon black (CB)-embedded carbon nanofiber (CNF) as a Pd support, which showed a high level of formic acid oxidation reaction (FAOR) activity. For the support preparation, heat treatment involving calcination at 1000 °C in a nitrogen atmosphere (carbonization) followed by calcination at 850 °C in water vapor (steam activation) was conducted to form a CB, which contained carbon nanofibers made from a polyacrolynitrile (PAN) fiber prepared by electrospinning. This catalyst showed a high level of FAOR activity. In this situation, the CB was also heat-treated, therefore, it was unclear whether the origin of the high FAOR activity of the CB-embedded CNF was caused by the CNF itself or the heat treatment of the CB. In order to establish the cause of the high FAOR activity of the CB-embedded CNF, the CBs underwent several heat treatments; i.e., stabilization, carbonization, and steam activation. Two types of carbon black with different pore structures, i.e., Ketjen black and Vulcan XC-72, were used to investigate the FAOR activity. The appropriate heat treatment of the CB promotes the improved FAOR activity; however, excessive heat treatment caused a deterioration in the FAOR activity, especially for Ketjen due to the presence of numerous micropores. However, by embedding the CB into the CNF, the FAOR activity improved, especially in the case of Ketjen, even though the embedded CB underwent several heat treatments. The optimum ratio of CB/PAN in the CB-embedded CNF was also investigated. The highest FAOR activity was observed at 0.25 CB/PAN for both the Vulcan and Ketjen. The electronic state of Pd3d in which the binding energy of the metallic Pd shifted to a lower binding energy suggested that the metal–support interaction is strong at the CB/PAN ratio of 0.25. On the basis of these results, it was found that heat treatment of the CB by embedding it in the CNF is a promising way to achieve a metal–support interaction without destroying its structure.

Highlights

Direct formic acid fuel cells (DFAFCs) are a promising type of direct liquid fuel cell due to their favorable oxidation kinetics which enable low temperature operation (50–80 ◦ C), a high theoretical open-circuit voltage (1.45 V), and a relatively low fuel crossover through the Nafion membrane [1,2].Platinum (Pt) and palladium (Pd) have been widely studied as anode catalysts for DFAFCs because of their interactions with formic acid, in which Pd tends to break the O–H bonds across the entire potential window, and Pt discriminates between the cleavage of O–H bonds at a high overpotentialAppl
We demonstrated that Pd supported by TiO2, SiO2, and carbon black (CB) (Vulcan) embedded into the carbon nanofiber (CNF) showed higher formic acid oxidation reaction (FAOR) activities than unembedded CNFs [13]
The effects of the heat treatment of the CBs, Vulcan XC-72 and Ketjen black, on their morphologies, pore volume distributions, and FAOR activity were investigated with the aim of clarifying the high

Summary

Introduction

Direct formic acid fuel cells (DFAFCs) are a promising type of direct liquid fuel cell due to their favorable oxidation kinetics which enable low temperature operation (50–80 ◦ C), a high theoretical open-circuit voltage (1.45 V), and a relatively low fuel crossover through the Nafion membrane [1,2]. By embedding the non-carbon material into the CNFs, metal–support interactions were obtained as a result of the high electrical conductivity of the CNFs. In our previous studies, we demonstrated that Pd supported by TiO2 , SiO2 , and carbon black (CB) (Vulcan) embedded into the CNFs showed higher formic acid oxidation reaction (FAOR) activities than unembedded CNFs [13]. Carbonization and steam activation, which were conducted to prepare the CB-embedded CNF in a previous study [13], were selected as the heat treatments to clarify the factors governing the performance enhancement of the CB catalyst supports. Different weights of the CB-embedded CNFs for Vulcan XC-72 and Ketjen black were synthesized to compare the effect of the heat treatment of CB, and the quantity of CB in the CNF was studied to find the optimum weight ratio of CB in the CNF

Catalyst Preparation

Material Characterization

Electrochemical Measurements

Results and Discussion

Formic Acid Oxidation Reaction on Carbon Black Embedded in Carbon Nanofibers

Conclusions