Electrochemical Properties of Polyoxometalate (H3PMo12O40)-Functionalized Graphitic Carbon Nitride (g-C3N4)
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A functionalized polyoxometalate/graphitic carbon nitride (PMo12/g-C3N4) composite has been constructed to promote direct electron and ion exchange based on a facile, rapid wetness incipient method. The PMo12 displayed a natural affinity towards the carbon support which facilitated enhanced reversible redox processes with surface-controlled electron diffusion. The g-C3N4-functionalized PMo12 composite promoted effective oxidation and reduction of the Keggin molecule addenda atoms with reduced overpotential without the mediation of a polymeric linker to promote their exposure and interaction. The g-C3N4 offered increased surface area for anchoring PMo12, structural stability at increased temperatures, and repeated cycling as well as control of the density and position of PMo12 as probed by scanning electron microscopy and nuclear magnetic resonance spectroscopy, respectively. These results demonstrate that PMo12 clusters are sensitive to their local environment, including the interaction with the support, which stimulated enhanced current mobility.
KeywordsElectrochemistry Composite Electron diffusion g-C3N4 Polyoxometalates
Over the last decade, polyoxometalates (H3PMo12O40) have enthused many research activities over a broad spectrum of science, such as catalysis, materials, and medicine, because of their chemical properties such as redox potential, acidity, and solubility in various media [1, 2]. The PMo12 belongs to a large family of heteropolyanions with the so-called Keggin structure, where a tetrahedral XO4 core (X = P in this case) serves as a template for the aggregation of 12 MoO6 octahedral sharing corners and edges. Keggin-type polyoxometalates are capable of multiple redox reactions which make them particularly useful for electrochemistry . Their hierarchical crystal structure however greatly influences the accessibility of their active sites and the location of the protons [4, 5]. Owing to this unique cluster structure, PMo12 has very low surface area with negligible conductivity. Therefore, in order to maximize electron transfer kinetics, ideally each PMo12 cluster molecule should be electrically linked to a conductive substrate [6, 7]. According to the literature, PMo12 anchored to carbon-based materials in particular promotes multielectron reversible redox reactions. This allows each cluster anion to engage in electrochemistry and as such provide the greatest active surface for interaction with the electrolyte. Graphene in particular has been reported as a good support material due to its large surface area and high electrical conductivity. The difficulties related to the integration between PMo12 and graphene as well as effective control of the density and position of PMo12, however, are alleviated through an organic linker used to bridge the two components enabling contact and charge transfer at the interface [8, 9]. As a novel function material, graphitic carbon nitride is regarded as an ideal support matrix because of its exceptionally high surface area and the prospect of monolayer coverages even at high PMo12 loading. Concomitantly, g-C3N4 exhibits a unique stability, heat endurance, and chemical resistance to intimately bond PMo12 [10, 11]. In addition, the presence of nitrogen in g-C3N4 itself plays an important role in improving the wettability of the electrode with the electrolytes and concomitantly promotes electrochemical interactions [12, 13].
In this study, the aptitude of the mesoporous g-C3N4 to firmly anchor bulky PMo12 electroactive inorganic anions to facilitate enhanced redox activity and stability under catalytic conditions in the absence of an organic linker was explored. The morphology, electronic structure, and synergy of the composite materials towards exposing PMo12 active sites for improved interactions were probed by advanced spectroscopic, microscopic, and electrochemical techniques. The positive stimulus of the PMo12/g-C3N4 was validated by enhanced redox properties. The area under the current-voltage curve for PMo12/g-C3N4 was indicative of enhanced capacitive performance and increased electron density on the PMo12 anion and reducibility.
The mesoporous graphitic carbon nitride, g-C3N4, was synthesized using a method previously developed with minor adjustments [14, 15]. In a typical procedure, 3.0 g dicyandiamide was added to 50 mL of an aqueous solution containing Triton X-100 at 60 °C under magnetic stirring. After evaporation, the resultant solid was dried in an air oven at 60 °C overnight, followed by heating to 550 °C at a heating rate of 5 °C min−1 in N2 and kept there for 4 h. Phosphomolybdic acid hydrate (PMo12O40·xH2O) was obtained from Fischer Scientific. The synthesis of the PMo12/g-C3N4 composite was accomplished by a simple wetness incipient method . In a typical procedure, the concentration of the PMo12 solution was adjusted to obtain 24% of the theoretical maximum surface coverage, corresponding to 10 wt%. The required volume of the solutions was slowly added to the g-C3N4 support (under vigorous magnetic stirring). After water evaporation, the resultant solid was dried at 100 °C, and then calcined at 350 °C for 2 h with a heating rate of 5 °C min−1 to obtain a desired composite. The obtained sample was denoted PMo12/g-C3N4. The morphology, composition, and particle size were probed by scanning electron microscopy (SEM, Zeiss Neon 40EsB FIBSEM). The PMo12/g-C3N4 local environment and stability with increased temperature were probed by 31P NMR analysis (Bruker, Advance 400 WB, DSX-400). The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were recorded on a multichannel potentiostat (Bio-Logic VMP2) in order to further probe any changes in the electronic structure of PMo12 clusters that resulted from their dispersion and electronic interaction with g-C3N4. X-ray photoelectron spectroscopy (XPS) was performed to measure the elemental composition, chemical state, and electronic state of the elements that exist within a material. This was done on an ultrahigh vacuum VG ESCALAB 210 electron spectrometer with Mg Kα radiation (hν = 1253.6 eV). The binding energy was referenced to the C1s peak at 283.8 eV of the surface-exposed adventitious carbon.
Results and Discussion
We have accomplished the chemical synthesis of a hybrid material based on PMo12 successfully attached to g-C3N4 as support; the POMs functioned as electron storage sites, as well as electron transfer mediators. The high surface area of the g-C3N4 allowed a high dispersion of PMo12 down to the molecular level. Spectroscopic and electrochemical investigations revealed that the structure and the electronic properties of the PMo12 are improved as a result of the synergy with the graphitic carbon nitride (g-C3N4). The preparation of PMo12/g-C3N4 was simple to perform, leading to reproducible and stable electrode material. The results show how the local chemical environment influences the electron transfer activity of PMo12. Disseminated PMo12, anchored on g-C3N4 as electrocatalytically active sites, contributes towards higher interfacial charge transfer and proves feasible as active electrode material in technological electrochemical devices.
This paper was financially supported by South Africa’s National Research Foundation (NRF).
Compliance with Ethical Standards
The authors declare that they have no competing interests.
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