Mass Transport Properties of Silicified Graphite Felt Electrodes - The

Article pubs.acs.org/JPCC

Mass Transport Properties of Silicified Graphite Felt Electrodes Benjamin Le Ouay,† Thibaud Coradin, and Christel Laberty-Robert* UPMC University Paris 06, CNRS, Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP), Collège de France, 11 Place Marcelin Berthelot, F-75005 Paris, France S Supporting Information *

ABSTRACT: Mass transport properties of electrodes prepared from graphite felt, as such and after silicification, have been studied using cyclic voltammetry. Within the graphite felt, the mass transport of a probe changes with decreasing scan rate, from a radial diffusion around fibers to a regime that is analogous to “thin-layer” systems. Furthermore, unlike classical “thin-layer” systems, the volume comprised in the felt is macroscopic (resulting in high current densities), while the time required to consume all diffusive species remains in the 1 min range. Silicification of graphite felt does not impact on the mass transport of the negatively charged molecular probe Fe(CN)63− but significantly slows mass transport of positively charged Ru(NH3)63+. In the latter case, a parallel decrease of peak current intensity reflects limited mobility of the probe due to its strong interaction with the surface of the pore walls. These data provide important information for the optimization of the working conditions of these electrodes for the design of biosensors and biofuel cells.

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INTRODUCTION Sol−gel technology provides a unique opportunity to synthesize materials with tailored structure, porosity, and surface chemistry,1 incorporating organics,2 inorganic species or particles,3 and even biological systems.4,5 They have therefore been developed for applications in almost all fields of materials science.6 Most particular interest in sensing7 and energy-related devices8,9 has led to extensive studies of electrochemical processes occurring in sol−gel materials, as summarized in several reviews.10−13 Although silica is by far the most studied sol−gel system, its very poor conductive properties make it unsuitable for electrochemical applications as such.10 A wide variety of strategies have been explored to address this issue, mainly involving silica doping with conductive organic polymers,14,15 inorganic particles (metal nanoparticles,3 carbon paste,16 carbon nanotubes17), and mixtures of these. Recently, an alternative approach was described based on the silicification of graphite felt, a very promising material for the development of electrochemical systems.18 Being constituted of millimeter long fibers (with a diameter of ca. 10 μm), this material possesses good conduction properties while having a porous volume of typically 95%. It is thus both lightweight and resilient to deformation stress. Its open architecture allows the circulation of fluid and its use as a redox flow electrode.19 Furthermore, its porosity is developed at the micrometer scale, and it is also readily used in the development of microbial fuel cells.20 However, the electrochemical behavior of graphite felt has been only sparingly studied so far.21,22 Moreover, it is expected © 2013 American Chemical Society

that incorporation of a silica gel within the felt porosity can perturb its properties. Especially, as the pore wall surface of silica thin films bears a negative charge over a wide range of pH, positively charged probes such as Ru(bpy)32+ species can be adsorbed and/or accumulated on these surfaces before being electrochemically detected, whereas negatively charged probes such as Fe(CN)63− may be repulsed from the micropores, leading to a signal of very low intensity.23 To address these points, we have here studied diffusive mass transfer of molecular probes in the felt as such and after silicification using cyclic voltammetry. Within the graphite felt, it was possible to identify a transition in the mass transport regime upon decreasing the scan rate, from a radial diffusion around fibers to a regime analogous to “thin layer” systems. As we have a 3D-network of conductive fibers, the electrochemical response will occur in the felt-embedded volume. Providing that the experiment time is long enough, typically in the 1 minrange, all of the probes contained in the felt porosity are consumed, while only a few from outside of the felt react. This approach is interesting because graphite felt is a readily available material and allows working at time-scales that are compatible with conventional detection devices. The presence of the silica network was shown to influence the diffusion kinetics and the intensity of the peak currents for the positively charged probe only. This has significant importance as it affects the electrochemical response of the electrodes. Hence, whereas Received: April 22, 2013 Revised: June 24, 2013 Published: July 12, 2013 15918

dx.doi.org/10.1021/jp403990m | J. Phys. Chem. C 2013, 117, 15918−15923

The Journal of Physical Chemistry C

Article

(K3Fe(CN)6) at a C0 = 32 μM concentration, supplemented with NaCl (0.129 M) as a supporting electrolyte. Voltammograms recorded at scan rate v comprised between 1000 and 0.5 mV s−1 are shown in Figure 2, together with additional curves

the silicification of a macroporous conductive material is a promising strategy for designing immobilization hosts, future developments of electrochemical devices based on these modified electrodes should rely on controlling the interface between the electroactive species and the silica network, especially pH, nature of the electrolyte, and its concentration.

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MATERIALS AND METHODS The raw graphite felt (Figure 1A) used in this study (99.9%, 1 cm thick, Morgan Carbon, Luxemburg) consists of loosely

Figure 1. Scanning electron microscopy views of raw (A) and gelified (B) graphite felt.

assembled carbon fibers, 12 μm in diameter, defining an open framework with cavities of ca. 100 μm. Its conductivity is ca. 0.04 S cm−1. To favor water penetration, the carbon surface was first coated with a hydrophilic polymer, alginic acid, following a methodology previously described.18 Silica gel was prepared by mixing 1 mL of sodium silicate solution (0.35 M) (Aldrich) with 1 mL of LUDOX HS-40 (Aldrich) followed by neutralization with 90 μL of HCl (4 M). Thirty seconds after the addition of acid, 1 mL of an aqueous solution of either Ru(NH3)6Cl3 or K3Fe(CN)6 was added. The alginate-treated felt was then immersed in the final sol, which became a gel in ca. 10 min. The gel is thus formed in the felt porosity and occupies the volume between the fibers (Figure 1B). These gels are constituted of nanoparticles with an average diameter of 12 nm that delineate a disordered mesoporous network with pore size of ca. 10 nm.24 Cyclic voltammograms were recorded using a VSP Bio-Logic potentiostat. Unmineralized felt pieces were immersed in solutions of probe ions supplemented with NaCl at the same concentration as in the gel (0.129 M). For both bare and silicified felt, connection was made using a platinum wire. Measurement on silicified felt was performed in a solution contaning the same concentration of probe and electrolyte, to avoid leaching or accumulation of these over the experiments. Counter-electrode was a steel grid, and Ag/AgCl (saturating KCl) (E = 0.197 V/NHE) was used as a reference.

Figure 2. Cyclovoltammograms recorded using the graphite felt electrode in the presence (−) or absence (− − −) of K3Fe(CN)6. Scan rate and probe concentration are indicated above each diagram. Potentials are given in V vs Ag/AgCl (saturating KCl) reference electrode.

obtained at a higher probe concentration (320 μM). Voltammograms were recorded on several cycles and did not show significant variations over consecutive cycles, indicating the absence of a leaching of the species out of the felt. The system presents three different behaviors, depending on the scan rate (and thus on the associated characteristic time = ΔV/ v). At high scan rates (v ≥ 200 mV s−1), the cyclovoltammograms do not exhibit any distinct peak but indicate an increase of the current with the increasing potential. At low probe concentration (32 μM), faradaic current is weak, and the electrochemical response remains close to the probe-free system, where only capacitive current exists. At a higher concentration (320 μM), the faradaic current becomes predominant, and the cyclovoltammograms present an elongated shape. At intermediate rates (5 mV s−1 < v < 200 mV s−1), the faradaic current reaches a maximum, and then decreases to a value close to 0. The remaining current is then due to capacitive phenomena. The faradaic peaks show an asymmetric bell shape with a shift between the oxidation and the reduction peaks. Both the asymmetry and the shift are

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RESULTS AND DISCUSSION Graphite Felt. Mass transport in the graphite felt was studied using an aqueous solution of potassium ferricyanide 15919



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At intermediate scan rate, τDiff ≈ τ. The experiment is slow enough to allow the diffusion layer to reach the center of the pores. Hence, all species that were initially filling the felt porosity can have access to the graphite surface and react until the faradaic current falls to zero. However, the diffusion time is not negligible, and causes asymmetry and shifts in the observed peaks. This phenomenon is conceptually similar to the case of irreversible systems studied with thin layer electrodes. In each case, the limited quantity of active species at the vicinity of the electrode causes the signal to have a bell shape.27 Limitations in the electron transfer cause these peaks to have an asymmetric shape and to be shifted. However, in the present case, the peak asymmetry is not due to a slow charge transfer at the electrode, but to a mass transfer in solution that is not instantaneous. At slower scan rate, τ ≫ τDiff. The delay due to the diffusion of probes to felt is negligible. The system is thus close to the ideal thin layer case. The oxidation and reduction peaks are detected at the same potential and have quasi-symmetrical Gaussian shape. Hence, even if the system is of centimetric size, the graphite felt system does not differ conceptually from the traditional thin layer system.28 In both cases, there is a limited amount of species at the vicinity of the electrode surface, causing the current to drop down once they all have been consumed. The intensity of peaks in thin layer systems is given by the following formula that contains two terms for capacitive and faradaic current,25 with C the capacity of the electrode, n the number of exchanged electrons, - the Faraday constant, V the volume of the thin layer, C0 the concentration in reagent, R the ideal gas constant, T the temperature, and v the scan rate:

reduced when the scan rate decreases. Furthermore, if the scan rate is low enough to observe a complete peak (i.e., when the faradaic current reaches 0), the ratio between the peak height and the capacitive current remains constant. At lower scan rate (v < 2 mV s−1), both the oxidation and the reduction peaks possess nearly symmetrical bell shapes, and the shift between the peaks is very small. The existence of these three different regimes can be explained by the unique architecture and typical length scale of the graphite felt. The characteristic distance l between fibers is ca. 100 μm (Figure 1A), corresponding to a diffusion time τDiff = l2/D, with D the diffusion coefficient of the species. In the present case (with DFe(CN)3− = 7.6 × 10−10 m2 s−1),25 this 6 corresponds to a diffusion time τDiff = 10 s. This value is interesting as a standard potentiostat can perform experiments on a range of potential ΔV with an experimental characteristic time τ = ΔV/v that can be either under or over this diffusion time. The observed three regimes can therefore be distinguished on the basis of the comparison between τDiff and τ, as described in Figure 3.

Figure 3. Chart of the different regimes of a graphite felt electrode.

Ipeak = ICapa + IFarad n2 - 2VC0v 4RT ⎛ n2 - 2VC0 ⎞ ⎟v = ⎜C + 4RT ⎠ ⎝

At high scan rate, τ ≪ τDiff. During the experiment, ions react at the graphite surface, and the medium is depleted of reagent at the vicinity of the fibers. This creates a concentration gradient that causes fresh reagent to diffuse from the pores interior toward the surface of the fibers. In this regime, the diffusion layer grows radially around the fibers, but the experiment is too short to allow that it reaches the center of the felt pores. Because there is always fresh reagent that can diffuse toward the electrode, the faradaic current increases with the overpotential. An elongated shape of the cyclic voltammogram, as obtained at 200 mV/s and 320 μM in Figure 2, is usually attributed to a very slow electron transfer.25 In our case, the chosen system (ferricyanide reduction on carbon) is wellknown to have a fast electron transfer. We also studied the impact of the alginic acid deposit on the fibers, which was required to make the felt hydrophilic. To do so, we compared the voltammograms with ones that were obtained in the absence of coating, in an organic solvent that could wet the felt surface easily. Ferrocene in acetonitrile was selected, as it is a model system with a fast electron transfer. Although the transition between the different regimes occurred at higher speeds, due to higher diffusion coefficients of the probe in this solvent,26 the behavior of the system was the same as the aqueous one (see the Supporting Information). Thus, the elongated shape of the voltammograms was attributed mostly to a diffusion effect rather than to an electron transfer limitation. This diffusion creates a delay in the electrochemical reaction that has a similar impact on the cyclovoltammograms as a slow charge transfer would, although both phenomena differ in their origins.

= Cv +

This intensity is therefore a linear function of the scan rate. Figure 4 represents the plot of oxidation peak intensity (Ipeak) versus v for aqueous Fe(CN)63− in the felt system. At slower speed, the intensity of the peak is described by a linear function, confirming the similarity between classical thin layer and graphite felt systems. As speed increases, the experiment time becomes insufficient to allow the full consumption of the

Figure 4. Evolution of Ipeak for different values of v (■). Dotted line is the linear function used to fit the experimental data at low scan rate. The inset represents the low scan-rate region, plotted with a linear scale. 15920



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Figure 5. (A,B) Cyclic voltammograms recorded on felt electrode using Fe(CN)63− (A) and Ru(NH3)63+ (B) as a redox probe (−, probe in solution; − − −, probe in silica gel). Scan-speed was 10 mV/s, and probe concentration was 320 μM in both cases. (C,D) Charge transferred to the electrode, plotted as a function of time spent at a reducing potential. (D, inset) squares, Fe(CN)63−; circles, Ru(NH3)63+; closed symbols, aqueous solutions; empty symbols, gels.

of the redox probes in the felt. The amount of reduced species at long time is slightly superior to the one initially present in the felt, due to a contribution of species in the bulk solution.30 For Fe(CN)63− in silicified felt, both the initial slope and the threshold are similar to the pure felt. In contrast, in the case of Ru(NH3)63+, the slope of the linearity domain is smaller in the composite material, and no clear threshold is reached in the conditions of the measurements. These data can be interpreted considering a radial diffusion around a cylindrical electrode. In this situation, the diffusion layer forms a coaxial cylinder of radius δ = (Dt)1/2 around the fiber.31 The amount of molecular probe that reacts while the potential of the electrode is in the reducing domain can be expressed as a fraction α of the total amount of redox probes in this cylinder. In the case of a linear concentration profile approximation, α value is 0.5. This leads to the following equations, where l is the total length of carbon fiber in the electrode:

probes, resulting in a deviation from linearity. The slope of the Ipeak versus v line is 0.041 A s V−1 (R2 = 0.9976, calculated for v < 5 mV s−1), and C = 0.006 A s V−1, based on the probe-free system. Silicified Felt. After silicification, felt porosity was filled with a mesoporous gel containing either Fe(CN)63− or Ru(NH3)63+ at a 320 μM concentration. Volumetric measurement indicates that at least 95 vol % of the free volume in the felt is filled with hydrated silica gel. The silica network is itself mesoporous, with an average pore diameter of ca. 10 nm and a specific surface of 160 m2/g.24 This porosity allows a circulation of molecular species within the gel. Cyclic voltammograms recorded in these materials are shown in Figure 5A and B, and compared to the silica-free felt systems. At a scan rate of 10 mV s−1, Fe(CN)63− ions have similar behavior in both situations, suggesting that these probes can diffuse freely in the mesoporous network and from silica to the graphite surface. In contrast, Ru(NH3)63+ ions present much weaker peak intensities in the silicified felt, indicating that the probe diffusion is hindered in the silica gel. This effect was previously observed by Kanungo and Collinson29 and attributed to the attractive electrostatic interactions between the cationic probe and the negatively charged silica surface. This hypothesis was confirmed here using a planar electrode conformation, showing a decrease of the diffusion coefficient of Ru(NH3)63+ by 2 orders of magnitude when trapped in the silica gels of our composition. In the same conditions, diffusion of Fe(CN)63− remained almost unaffected (see the Supporting Information). The charge Q circulating through the electrode during reduction waves at different speeds is represented in Figure 5C. For Fe(CN)63− and Ru(NH3)63+ in the felt pores, these plots present two domains. The amount of species consumed over short experiment times (ca. less than 10 s) rises quickly and linearly as a function of time (Figure 5D). This behavior was attributed to a radial growth of the diffusion layer around the fibers. For sweep time over 30 s, Q grows much more slowly and reaches a plateau corresponding to the total consumption

nReaction(t ) =

Q (t ) -

= α · C 0 · V (t ) = α ·C0·π ·l·(δ(t )2 − R fiber 2)

For (Dt)1/2 ≫ Rfiber (Rfiber = 6 μm and thus tfiber = 0.06 s): Q (t ) = α · - · C 0 · π · l · D · t

Hence, in the case of radial diffusion, Q grows linearly with t. Experimentally, the slope of the line is ca. 5.5 × 10−8 mol s−1 in the case of Fe(CN)63− in both unsilicified and silicified gels. For Ru(NH3)63+, it decreases from 3.5 × 10−8 to 5.8 × 10−9 mol s−1, indicating that the gel induces a 6-fold decrease of the diffusion coefficient. Noticeably, calculated values for probes in the graphite felt are 2.1 × 10−7 mol s−1 for Fe(CN)63− and 1.5 × 10−7 mol s−1 for Ru(NH3)63+. The most probable cause for this overestimation is that the calculation is made under the hypothesis that radial diffusion is occurring freely around each 15921



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fiber of the felt. However, it is expected that diffusion layers of two adjacent fibers overlap at cross-linking points, leading to a decrease in the effective surface of felt.

(6) Sanchez, C.; Belleville, P.; Popall, M.; Nicole, L. Applications of Advanced Hybrid Organic-Inorganic Nanomaterials: from Laboratory to Market. Chem. Soc. Rev. 2011, 40, 696−753. (7) Lev, O.; Tsionsky, M.; Rabinovich, L.; Glezer, V.; Sampath, S.; Pankratov, I.; Gun, J. Organically Modified Sol-Gel Sensors. Anal. Chem. 1995, 67, 22A−30A. (8) Long, J. W.; Dunn, B.; Rolison, D. R.; White, H. S. ThreeDimensional Battery Architectures. Chem. Rev. 2004, 104, 4463−4492. (9) Reddy, A. L. M.; Gowda, S. R.; Shaijumon, M. M.; Ajayan, P. M. Hybrid Nanostructures for Energy Storage Applications. Adv. Mater. 2012, 24, 5045−5064. (10) Lev, O.; Wu, Z.; Bharathi, S.; Glezer, V.; Modestov, A.; Gun, J.; Rabinovich, L.; Sampath, S. Sol−Gel Materials in Electrochemistry. Chem. Mater. 1997, 9, 2354−2375. (11) Walcarius, A.; Mandler, D.; Cox, J. A.; Collinson, M.; Lev, O. Exciting New Directions in the Intersection of Functionalized Sol-Gel Materials with Electrochemistry. J. Mater. Chem. 2005, 15, 3663−3689. (12) Walcarius, A.; Collinson, M. M. Analytical Chemistry with Silica Sol-Gels: Traditional Routes to New Materials for Chemical Analysis. Annu. Rev. Anal. Chem. 2009, 2, 121−143. (13) Etienne, M.; Guillemin, Y.; Grosso, D.; Walcarius, A. Electrochemical Approaches for the Fabrication and/or Characterization of Pure and Hybrid Templated Mesoporous Oxide Thin Films: a Review. Anal. Bioanal. Chem. 2012, 405, 1497−1512. (14) Wei, Y.; Yeh, J.-M.; Jin, D.; Jia, X.; Wang, J.; Jang, G.-W.; Chen, C.; Gumbs, R. W. Composites of Electronically Conductive Polyaniline with Polyacrylate-Silica Hybrid Sol-Gel Materials. Chem. Mater. 1995, 7, 969−974. (15) Gangopadhyay, R.; De, A. Conducting Polymer Nanocomposites: A Brief Overview. Chem. Mater. 2000, 12, 608−622. (16) Tsionsky, M.; Gun, G.; Glezer, V.; Lev, O. Sol-Gel-Derived Ceramic-Carbon Composite Electrodes: Introduction and Scope of Applications. Anal. Chem. 1994, 66, 1747−1753. (17) Gong, K.; Zhang, M.; Yan, Y.; Su, L.; Mao, L.; Xiong, S.; Chen, Y. Sol−Gel-Derived Ceramic−Carbon Nanotube Nanocomposite Electrodes: Tunable Electrode Dimension and Potential Electrochemical Applications. Anal. Chem. 2004, 76, 6500−6505. (18) Le Ouay, B.; Coradin, T.; Laberty-Robert, C. Silica-Carbon Hydrogels as Cytocompatible Bioelectrodes. J. Mater. Chem. B 2013, 1, 606−609. (19) Leung, P.; Li, X.; de Leon, C. P.; Berlouis, L.; Low, C. T. J.; Walsh, F. C. Progress in Redox Flow Batteries, Remaining Challenges and their Applications in Energy Storage. RSC Adv. 2012, 2, 10125− 10156. (20) Zhou, M.; Chi, M.; Luo, J.; He, H.; Jin, T. An Overview of Electrode Materials in Microbial Fuel Cells. J. Power Sources 2011, 196, 4427−4435. (21) Golub, D.; Oren, Y. Graphite Felt as an Electrode for ThinLayer Electrochemistry. J. Appl. Electrochem. 1990, 20, 877−879. (22) Delanghe, B.; Tellier, S.; Astruc, M. Mass Transfer to a Carbon or Graphite Felt Electrode. Electrochim. Acta 1990, 35, 1369−1376. (23) Etienne, M.; Quach, A.; Grosso, D.; Nicole, L.; Sanchez, C.; Walcarius, A. Molecular Transport into Mesostructured Silica Thin Films: Electrochemical Monitoring and Comparison between p6m, P63/mmc, and Pm3n Structures. Chem. Mater. 2007, 19, 844−856. (24) Nassif, N.; Roux, C.; Coradin, T.; Rager, M.-N.; Bouvet, O. M. M.; Livage, J. A Sol−Gel Matrix to Preserve the Viability of Encapsulated Bacteria. J. Mater. Chem. 2003, 13, 203−208. (25) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (26) Wang, Y.; Rogers, E. I.; Compton, R. G. The Measurement of the Diffusion Coefficients of Ferrocene and Ferrocenium and their Temperature Dependence in Acetonitrile Using Double Potential Step Microdisk Electrode Chronoamperometry. J. Electroanal. Chem. 2010, 648, 15−19. (27) Hubbard, A. T. Study of the Kinetics of Electrochemical Reactions by Thin-Layer Voltammetry: I. Theory. J. Electroanal. Chem. 1969, 22, 165−174.

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CONCLUSION Silicification of graphite felt provides a simple method to combine the immobilization properties of sol−gel materials with the large reactive volume in conductive carbon networks. In the pure graphite felt, the characteristic time of molecular probes diffusion in the porosity is about 10 s, resulting in three observable behaviors, depending on the experiment duration. At long times, all of the included redox probes can diffuse to the electrode, leading to a “thin layer”-like behavior for the graphite felt. At short times, the diffusion layer is radial around the graphite fibers. The mesoporous silica network does not perturb these regimes when a negatively charged probe is used, whereas a significant slowing of the diffusion of a positively charged ion is observed due to its electrostatic adsorption on the gel surface. Indeed, modification of gel structure and pH conditions should provide a simple way to overcome this issue, making these composite materials promising hosts for the design of electrochemical devices, including biosensors and bioreactors.

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ASSOCIATED CONTENT

S Supporting Information *

Cyclovoltammograms of ferrocene in acetonitrile on untreated graphite felt electrodes. Cyclovoltammograms of Fe(CN)63− and Ru(NH3)63+ realized on flat electrodes, in solution, and in gelified systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

EPFL STI IMX SuNMIL, Ecole Polytech Fed Lausanne, Inst Mat, CH-1015 Lausanne, Switzerland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.L.O. thanks the Direction Générale de l’Armement (DGA) for its funding. This work was supported by the Direction Générale de l’Armement - Mission pour la Recherche et l’Innovation Scientifique (DGA-MRIS).

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REFERENCES

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Mass Transport Properties of Silicified Graphite Felt Electrodes - The

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