Electrochemical oxidation of ordered mesoporous carbons and the influence of graphitization
Introduction
Ordered mesoporous carbon (OMC) materials obtained by the template carbonization pathway from silica templates (hard-templating or sacrificial methodology) exhibit tunable nanostructures and developed mesoporosity with widely open interconnected pores, making them promising materials for many applications, such as sensors and biosensors [1], electrochemistry [2], adsorption and catalysis [3], or electrochemical energy storage [[4], [5], [6], [7]] and conversion [8,9]. Among them, OMC have attracted special attention for their application at the catalytic layers of low temperature fuel cells (e.g. polymer electrolyte membrane fuel cells, PEMFC and direct alcohol fuel cells, DAFC). Their large surface area and ordered structure facilitate the dispersion of the active phase for OMC-supported Pt-based nanoparticles, thus favoring the performance of these electrochemical devices [[10], [11], [12], [13], [14], [15], [16], [17], [18]]. Open-frame porous carbons obtained by the sacrificial methodology from silica templates have also been recently investigated as a hosting structure for metal-nitrogen-carbon coordinates to be applied as platinum-free catalysts [[19], [20], [21], [22], [23], [24], [25], [26]].
One drawback of OMC is their low electrical conductivity due to their amorphous character, and consequently high ohmic resistances can be found in electrochemical applications, leading to a decrease of the performance of the device (fuel cells, capacitors, etc.). Furthermore, carbon materials tend to undergo electrooxidation at highly positive potentials [27], which is considered to be one of the main problems facing the lifetime of positive electrodes in electrochemical devices. In order to address these limitations, different graphitization methodologies of templated mesoporous carbons have been proposed, such as heat treatments at high temperature [[27], [28], [29], [30], [31]] or catalytic approaches [[32], [33], [34]]. Catalytic approaches involve the use of metals (e.g. Fe or Ni), which need to be removed in a final step using oxidizing agents and increasing the cost of the final catalyst. Whereas, heat treatments at high temperature have been used to increase the graphitization degree of OMC supports and successfully deposit metal nanoparticles on the surface despite the decrease of the support oxygen content upon annealing [27,35].
Carbon electrooxidation is a complex multi-step process involving both the formation of oxygen species and the complete oxidation to CO2 at elevated potentials through the reaction in equation (1) [36]:C + 2H2O → CO2 + 4H+ + 4e− (E0 = 0.207 V vs. RHE)
Although many works have been devoted to study carbon electrooxidation using different nanostructured carbon materials [[37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]], the exact mechanism is not completely clear. Additionally, the influence of OMC graphitization on carbon electrooxidation needs to be further studied.
In this work, two different OMC with adjustable textural and morphological properties have been synthesized and heat treated at 1500 °C to increase their crystallinity and their electrical conductivity. The influence of graphitization on the electrochemical oxidation of carbon was evaluated by potential holding at 1.4 V vs RHE in aqueous acid medium (0.5 M H2SO4). Additionally, cyclic voltammetry and electrochemical impedance spectroscopy have been used to study the effect of heat treatment on the electrochemical behavior, together with chemical analysis to elucidate changes in the electrode composition.
Section snippets
Synthesis of ordered mesoporous carbons
OMC were obtained from SBA-15 silica (template) following the hard-templating methodology using different mass ratio (R) of the silica precursor (Tetraethyl Orthosilicate, TEOS, 98 wt %, Aldrich) and the surfactant (P123) (R = TEOS/P123). A furan resin/acetone mixture was used as carbon precursor. Further details can be found in Ref. [50]. Following this procedure, two different materials were obtained: OMC-R2 and OMC-R5, with R = 2 and 5, respectively. Previous works of our group showed that
Physicochemical characterization of ordered mesoporous carbons
TEM images evidence the typical ordered honeycomb structure of OMC, consisting of carbon nanorods with uniform mesopores (Fig. 1). Comparing the bare (or untreated) materials (OMC-R2 and OMC-R5), an increase of the TEOS/P123 mass ratio from 2 to 5 led to a decrease of the ordering degree of mesoporous carbons, due to a decrease of the copolymer amount (P123). In acid media, P123 surfactant forms structures of interconnected liquid crystals, which drive the silica structuration and generate the
Conclusions
Two different ordered mesoporous carbons have been synthesized and thermally treated at 1500 °C to induce graphitization. These materials have been investigated for the electrooxidation of carbon at room temperature and 1.4 V vs. RHE in 0.5 M H2SO4. The main physicochemical properties, including surface area, pore volume and chemical composition have been evaluated in order to establish a correlation with the electrochemical behavior. Graphitization resulted in an important decrease of the
Acknowledgments
The authors gratefully acknowledge financial support given by Spanish Ministry of Science, Innovation and Universities MICINN (ENE2017-83976-C2-1-R) and to the Aragón Government to the Fuel Conversion Group (T06_17R). DS acknowledges also MICINN for his Ramón y Cajal contract (RyC-2016-20944). Furthermore, the authors wish to thank Dra. Ana Beatriz García (INCAR-CSIC) for the graphitization treatment of OMC.
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