Highly dispersed platinum nanoparticles on graphitic carbon nitride: A highly active and durable electrocatalyst for oxidation of methanol, formic acid and formaldehyde
Graphical abstract
Introduction
The use of fossil fuel as well as environmental pollution is rising. Therefore, there is a need of green alternate and sustainable energy source. The fuel cells are considered as alternative green energy sources, transforms the chemical energy of fuel to electrical energy due to electrochemical oxidations of fuel molecule. High efficiency, low cost and low pollutant emission are the main advantages of fuel cells [1]. The electrochemical oxidation of small organic molecules such as methanol (CH3OH), formic acid (HCOOH) and formaldehyde (HCHO) is important for the development of direct fuel cell. The methanol is the most preferred alcohol among all alcohols for fuel cell applications. High solubility in aqueous electrolytes, easily handled, low cost, easily transportable and good storage property of methanol create to use as an energy source in the fuel cell application [2]. From natural sources methanol can be produced and it contains huge amount of energy (6000 W h kg−1) that can be used as future power needs [2]. The Direct Methanol Fuel Cell (DMFC) is also better than hydrogen gas fuel cell. Storage of hydrogen gas and transportation problems are the main issues in hydrogen gas fuel cell [3]. Methanol is used as fuel in DMFCs, without converting methanol to hydrogen. The advantages of DMFCs over hydrogen fuel cell are the high energy conversion efficiency, low operating temperature, low pollutant emissions. Thus, electrocatalytic oxidation of methanol with better efficiency is important for its application in DMFCs. The main obstacles for commercialization of DMFCs are the carbon monoxide (CO) poisoning, high methanol cross over from anode to cathode through the proton exchange membrane and slow kinetics of methanol electrooxidation due to its higher theoretical energy density [4]. The high methanol cross over problem can be overcome by improving the membrane or by developing a new membrane and the methanol oxidation kinetics can be improved by developing a new anode catalyst. Formic acid can be used as an alternative to the methanol fuel since formic acid is a strong electrolyte and is also electron and proton transporter. The Direct Formic Acid Fuel Cells (DFAFCs) has many advantages such as higher power density, low fuel crossover, higher energy efficiency and higher electromotive force (theoretical open circuit potential 1.48 V vs. saturated hydrogen electrode) [5]. The main drawback in DMFC is the methanol crossover which is not the limitation for DFAFC [6]. The low fuel crossover in DFAFC is due to dissociation of formic acid to formate ions (HCOO−) and their repulsive interaction [7]. The efficient formic acid oxidation is thus important for polymer electrolyte fuel cell applications. The main problem of Pt, usually used formic acid oxidation catalyst in DFAFCs, is the CO poisoning, since the electrooxidation of formic acid on Pt proceeds mostly through hydration (indirect) pathway forming large amount of poisonous CO species. In addition, the formaldehyde electrooxidation study is important for development of DMFCs since formaldehyde is the intermediate product of methanol electrooxidation [8]. There are several reports available in literature on Pt-based electrocatalysts for the oxidation of formaldehyde [9], [10]. Fuel cells with small organic molecules have advantage of high current density, low operating temperature whereas the hydrogen fuel cells have the disadvantage of expensive, highly flammable storage and transportation problem of hydrogen fuel.
The platinum is known to be the best catalyst for the oxidation of methanol [11], [12], [13], [14], formic acid [15], [16], [17] and formaldehyde in fuel cells. The electrooxidation of methanol, formic acid and formaldehyde on Pt surface yields CO-based byproducts, and decreases the performance of the electrocatalyst. The slow oxidation kinetics and CO poisoning are the main issues of Pt catalyst. In addition, the cost of platinum is also one of the main obstacles for the commercialization of fuel cells. Therefore, the utilization of the Pt metals with proper way is thus important for their application in fuel cells. The PtNPs of few nanometers size, narrow size distribution, stability and their high dispersion can provide proper implementation of the noble metals by increasing the electrocatalytic activity. Several groups demonstrated that the different size and shape of PtNPs can affect the rate of electrooxidation [18], [19]. Another important issue is the long term durability of carbon supported Pt-catalysts used for methanol, formic acid oxidation at anode. The normally used carbon support can easily electrochemically oxidized to carbon dioxide under fuel cell operating condition and this lead to structural degradation of support and PtNPs are also electrically detached from the support. Migration, aggregation and Oswald ripening [20] of PtNPs can decrease the electrochemical properties. Usually, there are two common strategies available to improve catalytic activity and durability of Pt-catalysts: (1) by forming alloy of Pt with other metals and (2) by using good support materials. Liu et al. demonstrated the design and preparation of Ag–Pt bimetallic catalyst which have good electrocatalytic activity towards methanol oxidation by decreasing the affinity of CO chemisorptions [21]. It is well known that incorporation of ruthenium (Ru) into the Pt based electrocatalyst helps to do the oxidation of CO which is produced during oxidation of methanol. Such a way the Pt–Ru electrocatalyst removes the CO poisoning effect on the catalyst during methanol oxidation [22]. The bimetallic catalyst also exhibited increased electrooxidation of formic acid by suppressing the formation of adsorbed CO. The electrocatalytic activity towards oxidation of methanol and formic acid is dependent on composition of bimetallic catalysts which is demonstrated in different literatures [23], [24], [25], [26]. The shape of bimetallic nanocatalyst can also play important role in strengthen the electrocatalytic activity of formic acid oxidation [27], [28]. The main drawback is in the synthesis of bimetallic catalyst, since it is important to maintain specific ratio of two metals and its shape in order to achieve effective catalytic activities. Graphene, a two dimensional support material, due to its unique physical and chemical properties [29] such as high surface area, high electrical conductivity, high thermal stability has proven to be a very good supporting material for dispersion of nanoparticles for their high electrocatalytic activity [30], [31], [32]. However, the low polarity and high hydrophobicity of the graphene carbon materials are the limitation for its electrocatalytic activity. The aggregation of graphene in aqueous solution and difficulties in uniform loading of metal nanoparticles on graphene surface prevent its further improvement in the electrocatalytic activities [33]. In acidic media, corrosion of the carbon support typically occurs at the interface with the novel metal nanoparticles lead to agglomeration of nanoparticles leading to decrease the performance of the supported catalysts. In order to increase the stability and dispersion of the nanoparticles on the graphene surface, functionalization of graphene sheets is needed. Functionalization of graphene improves their solubility as well as their ability to disperse of nanoparticles on its surface. The attachment of graphene with polymer or surfactant molecules and oxidation of graphene to form graphene oxide, decrease the aggregation of graphene and high dispersion of metal nanoparticles improves for their electrocatalytic activities [34], [35]. Besides, doping of graphene with heteroatom such as boron, nitrogen etc can be applied to modify the properties of graphene. It is well known that nitrogen doped graphene can be used as support for the metal catalysts and enhanced electrocatalytic activity as well as durability was observed resulting from strong catalyst-support interactions [23], [36], [37]. g-C3N4, two dimensional material has similar structure like graphene can be used as a support material due to the presence of Lewis acid/base sites into the moiety [38], [39], [40], [41], [42], [43], [44]. The g-C3N4 supported metal/semiconductor nanoparticles composite have confirmed to be effective composites for the application in photo-catalysis [45], organo-catalysis [46], biosensors [47], electrochemical applications [48] etc. In recent years considerable efforts has been given to synthesize monometallic Pt catalysts supported on nitrogen doped graphene [37] and on porous graphene composites [49], [50], [51] for electrochemical oxidation of methanol. In addition, there are only few reports available in literature for monometallic Pt catalysts on electrochemical oxidation of formic acid and formaldehyde [9], [52]. The oxidation of formic acid on monometallic [9], [52] Pt mainly proceeds through indirect (less preferred) pathways forming carbon monoxide, although several bimetallic catalysts [26], [28], [53], [54] reported for direct pathway oxidation of formic acid. Thus, there is a need of new Pt-electrocatalyst for oxidation of methanol, formic acid and formaldehyde for direct fuel cell applications.
In this work, Pt/CNx composite was synthesized by sodium borohydride reduction using ultrasound method. The Pt/CNx composite has large electrochemical surface area due to the high dispersion of PtNPs on CNx sheets. XPS and FT-IR studies confirm the strong metal-support interactions exist in the composite. Pt/CNx composite is highly active catalyst towards electrochemical oxidation of small molecules (CH3OH, HCOOH, HCHO). At 0.3 V (vs. NHE), a typical working voltage in DFAFC, the mass activity of Pt/CNx catalyst for formic acid oxidation is 25 times higher than that of commercial Pt/C catalyst. The superior catalytic activity and exceptional long term stability of the composite towards electrooxidation of methanol, formic acid and formaldehyde makes it promising catalyst for direct liquid fuel cell.
Section snippets
Synthesis of g-C3N4
Formamide (HCONH2) has been used as only starting material to synthesize the g-C3N4 compound by microwave mediated method [55]. Briefly, at 180 °C temperature 10.0 ml of HCONH2 was heated by microwave synthesizer. Then the brownish black colored solution was vacuum evaporated at 180 °C to get solid product. Distilled water is used to wash this black colored solid product and dried under vacuum to get solid g-C3N4.
Synthesis of Pt/CNx composite
The ultrasound assisted sodium borohydride reduction method is used to prepare the
Characterization of Pt/CNx catalyst
In this work, Pt/CNx composite was prepared by sodium borohydride and ultrasound reduction of H2PtCl6 acid solution in presence of g-C3N4. The Powder X-ray diffraction (p-XRD) patterns of Pt/CNx composite and g-C3N4 are displayed in Fig. 1. In both the p-XRD patterns, the peak at 2θ value of 27.2° arise from graphitic (002) plane of g-C3N4. Five additional diffraction peaks has been observed in the p-XRD pattern of Pt/CNx composite positioned at 2θ values of 40.02°, 46.4°, 67.8°, 81.5° and
Conclusion
In conclusion, we have demonstrated a facile procedure to synthesize of highly dispersed PtNPs supported on graphitic carbon nitride sheet. The Pt/CNx catalyst exhibited superior catalytic activity for the electrochemical oxidation of methanol, formic acid and formaldehyde in acid media. In compared to commercial Pt/C, this Pt/CNx catalyst showed a significantly enhanced mass activity, long term stability and CO poisoning tolerance for methanol, formic acid and formaldehyde electrooxidation. At
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