Synthesis and functionalization of green carbon as a Pt catalyst support for the oxygen reduction reaction

Highlights

• We synthesized and characterized a green carbon catalysts support.

• The carbon supported catalyst was prepared by a sublimation of natural camphor.

• Improved ORR catalytic activity of the carbon supported Pt NP was obtained after functionalization.

• Results compared with carbon Vulcan supported Pt NP under identical conditions.

Abstract

This study investigates the synthesis and characterization of a carbon derived by a simple combustion process of natural camphor. Resulted carbon was subjected to a chemical (HNO₃ and KOH) and physical treatment (thermal treatment –TT, liquid Nitrogen treatment – LN2) for surface functionalization. Synthesized carbon materials were used as Pt catalyst support for the oxygen reduction reaction (ORR) in acid media. Physicochemical characterization of the carbon support was performed by multiple techniques such as, SEM, EDAX, FTIR, and Raman spectroscopy. Results conducted to spherical particles of camphor carbon (CC) with 50–60 nm in size. The chemical composition revealed a CC without sulfur content. This could be an advantage for avoiding the sulfur poisoning on the Pt surface. FTIR results shows that the oxidative chemical treatment (HNO₃ and KOH) incorporate functional group of CO to CC. However, the TT eliminates the functional surface group on the CC. Apparently CC and CC-LN2 samples have the same behaviour, consequently it can be inferred that the cryogenic treatment with liquid nitrogen does not significantly modify the functional chemistry of carbon surface. Raman spectroscopy reveals the disorder in the CC (I_D/I_G = 1.01) which is lesser than the Vulcan Carbon (I_D/I_G = 1.16) defect. Furthermore, the chemically treated CC and LN2 shows fewer defects than the thermally treated CC. Electrochemical results show that materials with enhanced specific catalytic activity toward the ORR follow the tendency: Pt/CC-KOH > Pt/CC-TT > Pt/CC > Pt/CC-LN2 > Pt/Vulcan > Pt/C Etek > Pt/CC-HNO₃. Our finding results open a new avenue for carbon from natural source as an effective ORR catalyst support.

Introduction

The oxygen reduction reaction (ORR) is of relevant importance in the performance of the energy generation in batteries and proton exchange membrane fuel cells (PEMFCs), because the cathodic sluggish kinetics reaction is the determining step in the overall process and limiting the efficiency of the system [1]. Platinum and its alloys supported on carbon black are extensively used as cathode electrodes because they catalyze the four-electron transfer oxygen reduction reaction to form H₂O in low and medium temperature PEMFCs [1], [2]. Generally Pt nanoparticles are dispersed on a conductive support material of higher surface area such as carbonaceous matrices. The catalytic supports based on carbonaceous matrices reduce the effects of particles agglomeration and promote a good homogeneous distribution of the nanocatalyst with great influence on the stability and improvement of the activity [3]. Many materials have been investigated in order to obtain a better support for metal catalysts. Aerogels, xerogels and other mesoporous carbon supports, provide the high surface area for the metal catalyst [4], [5], [6], [7], [8]. Carbon nanotubes used as 3D structural catalytic support confers high stability and high catalytic properties of materials [9], [10]. In recent years graphene and WO₃ had significant interest as catalytic support. Graphene sheets whose high number of edges and small pores allows to the formation of catalyst active sites [11], [12]. Tungsten trioxide shows a higher catalytic activity than Pt/C and excellent tolerance to CO [13]. On the other hand, the significant improvement of catalyst support is achieved after functionalization process. This involves a surface modification of the black carbon supports conducted through chemical agents or heat treatments. This process has a favourable effect on the performance and stability of the catalyst by improving the metal-support interaction through anchoring sites, which leads to a high distribution of the metal nanoparticles on the carbon matrix and reduces the probability of agglomeration during the synthesis process [3], [4]. Derbyshire et al. [14] found that the surface chemistry of carbon is associated with the surface functional groups, which are formed during chemical pre-treatments given to the carbon. Many functional groups can be incorporated into carbon surface, which are based on elements such as oxygen, hydrogen, carbon, nitrogen, sulfur, phosphorus and some halogens [15]. The most investigated are those groups containing C–O bonds. It is difficult to establish the precise nature of surface oxygen groups on the carbonaceous material; however, the most common types of such groups are carboxylic, phenolic, lactones, ethers, quinones, carbonyl, anhydrides and others groups, which are the result of oxidative processes given to carbon mainly with HNO₃, H₂SO₄, KOH, H₂O₂ or with O₂ and O₃ in gas phase [4], [15], [16], [17]. The presence of such groups reduces the hydrophobic character of the carbon material, thereby making the surface more accessible to the metal precursors during the impregnation process with aqueous solutions and serve as anchoring sites for metal nanoparticles and preventing agglomeration during the reduction of metal nanoparticle process [3], [4], [16], [17]. Among all the catalyst supports mentioned above, pre-treated Vulcan Carbon XC-72R with nitric acid is the most widely material used as catalyst supports for PEM fuel cell applications [18]. However its origin derived from the pyrolysis of hydrocarbons from the petroleum fractionation and natural gas sources (non-renewable energy sources) makes it unattractive for future applications due to the environmental pollution, furthermore the sulfur content and other impurities degrade and decrease the lifetime of the metal catalysts [4], [17], [18]. This could open a wide research around the world, whose objective is the search for new materials that could replace the conventional carbon source, and focus the natural sources for possible environmental friendly process [19], [20].

The aim of this work is to synthesize and characterize a carbon derived from natural camphor obtained by a simple combustion process. The produced carbon is subjected to chemical (HNO₃ and KOH) and physical treatments (thermal treatment –TT, liquid Nitrogen treatment – LN2) for surface functionalization. The functionalized carbon materials are used as Pt catalysis support for the study of the oxygen reduction reaction (ORR) in acid media.