Electrochemistry Communications 12 (2010) 1509–1512
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Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Iron (III) nanocomposites for enzyme-less biomimetic cathode: A promising material for use in biofuel cells Marccus Victor Almeida Martins a, Clarissa Bonfim a, Welter Cantanhêde da Silva b, Frank Nelson Crespilho a,⁎ a b
Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André 09210-170, Brazil Departamento de Química, CCN, Universidade Federal do Piauı', Teresina 64049-550, Brazil
a r t i c l e
i n f o
Article history: Received 26 July 2010 Received in revised form 16 August 2010 Accepted 17 August 2010 Available online 25 August 2010 Keywords: Biofuel cells Hydrogen peroxide electroreduction Biomimetic Direct electron transfer
a b s t r a c t In this paper, we discuss the synthesis and electrochemical properties of a new material based on iron oxide nanoparticles stabilized with poly(diallyldimethylammonium chloride) (PDAC); this material can be used as a biomimetic cathode material for the reduction of H2O2 in biofuel cells. A metastable phase of iron oxide and iron hydroxide nanoparticles (PDAC–FeOOH/Fe2O3-NPs) was synthesized through a single procedure. On the basis of the Stokes–Einstein equation, colloidal particles (diameter: 20 nm) diffused at a considerably slow rate (D = 0.9 × 10− 11 m s− 1) as compared to conventional molecular redox systems. The quasi-reversible electrochemical process was attributed to the oxidation and reduction of Fe3+/Fe2+ from PDAC–FeOOH/Fe2O3-NPs; in a manner similar to redox enzymes, it acted as a pseudo-prosthetic group. Further, PDAC–FeOOH/Fe2O3-NPs was observed to have high electrocatalytic activity for H2O2 reduction along with a significant overpotential shift, ΔE = 0.68 V from −0.29 to 0.39 V, in the presence and absence of PDAC–FeOOH/Fe2O3-NPs. The abovementioned iron oxide nanoparticles are very promising as candidates for further research on biomimetic biofuel cells, suggesting two applications: the preparation of modified electrodes for direct use as cathodes and use as a supporting electrolyte together with H2O2. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since Galvani's [1] experiments showing the twitching of a frog's leg upon the application of electric current, biofuel cell technology [2] have been one of the most important areas of study in the field of bioelectrochemistry. In the last few years, there has been increasing research on biofuel cells [3–10]; in particular, anodic reactions involving glucose molecules have been considered to be a promising fuel. With regard to cathodic reactions, although O2 is widely used as a fuel because of its high availability in atmospheric air [11,12], other secondary oxidants [13] can be used as well. Among these secondary oxidants, H2O2 is very promising because of its high solubility in an aqueous electrolyte, high thermodynamic potential, and ease of storage. As an alternative to the use of noble metals and/or expensive enzymes in electrocatalytic reactions for the reduction of H2O2 in biofuel cell cathodes, iron oxide-based materials appear interesting. Iron oxides have been the subject of several studies involving the electrocatalytic reduction of H2O2 [14,15]. For example, Compton et al. [14] have recently reported that the catalytic activity of carbon nanotubes for the reduction of H2O2 can be attributed to the impurities of iron oxide and not to the edge effects, as was previously believed.
⁎ Corresponding author. Tel.: + 55 11 4437 8438; fax: + 55 11 4996 3166. E-mail address:
[email protected] (F.N. Crespilho). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.08.020
Therefore, in this paper, we discuss the synthesis of a new material based on iron oxide; this material can be used as a biomimetic cathodic material for the reduction of H2O2 in biofuel cells. In this study, hybrid iron oxide and iron hydroxide nanoparticles (FeOOH/Fe2O3-NPs) were easily synthesized, when the nanoparticles were conjugated with an organic polyelectrolyte poly(diallyldimethylammonium chloride) (PDAC). This new material was used as a solution-phase electrolyte in half-cell experiments. The experimental results suggest that this material is an excellent biomimetic catalyst for H2O2 reduction. 2. Experimental PDAC–FeOOH/Fe2O3-NPs were synthesized using a co-precipitation [16] method with significant new modifications. In this method, a solution containing Fe3+ ions (20 mL of 25-mmol L− 1 FeCl3) was magnetically stirred (10 min at 60 °C) into 20 mL of PDAC solution (1 mol L− 1). Then, NH4OH (1 mol L− 1) was added to the reaction solution in order to obtain a PDAC–FeOOH/Fe2O3-NP suspension. The nanoparticles were then characterized using an electronic spectroscope (UV–VIS Varian) and a transmission electron microscope (TEM) (JEOL JEM 2011). Voltammetry experiments were carried out using a potentiostat/ galvanostat μAutolab with the GPES software. We used a conventional system of three electrodes: indium tin oxide (ITO)-coated glass as the working electrode (Aldrich), platinum sheet as the
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auxiliary electrode, and Ag/AgClsat as the reference. The supporting electrolyte was H2SO4 (0.1 mol L− 1). For solution-phase voltammetry, 100 μL of PDAC–FeOOH/Fe2O3-NP suspension was added to 30 mL of the supporting electrolyte (0.1-mol L− 1 H2 SO4 ). All experiments were conducted in thoroughly nitrogen deaerated solutions at a temperature of 24 °C.
3. Results and Discussion 3.1. Synthesis and characterization of PDAC–FOOH/Fe2O3-NPs The synthesis of FeOOH/Fe2O3 in the presence of PDAC molecules exhibited good stability; a change in the colour of the solution and the different stages of the stabilization of the colloidal suspension were evident (see inset of Fig. 1a). We obtained the spectra of a solution containing Fe3+ (green line) and the PDAC–FeOOH/Fe2O3-NP suspension (blue line) (Fig. 1). The absorption band at 300 nm (green line) is characteristic of the electronic transitions (d-orbitals) of the Fe3+/Cl− aqua-complex present in an aqueous solution. After the addition of OH− ions to the Fe3+ solution (by magnetic stirring and in the presence of oxygen from air), we observed a decrease in the peak at 300 nm and an increase in the baseline; these changes are attributed to the formation of the dark-orange PDAC–FeOOH/Fe2O3NP suspension. The increase in the baseline is attributable to the scattering effects (αext = αsca). According to Mie's theory [17–19], the
increase is also attributable to radiation absorbed by the nanoparticles (αabs) and the scattered radiation (αsca), where the extinction (or total extinction) is given by Eq. (1). αext = αabs + αsca
ð1Þ
PDAC–FeOOH/Fe2O3-NPs have no defined plasmon band in the UV–Vis region (Fig. 1). In this case, the delocalized movement of free electrons on the surface particle is impossible because the iron atoms are surrounded by oxygen atoms. Therefore, the increase in the baseline is attributed to the nanoparticle scattering effects (αext = αsca). Eq. (2) summarizes the reaction. 3+
Fe
O2 ;60-C
− PDAC
pH N 8:0
3OH ⇄ FeðOHÞ3 ⇄ FeOOH−NP ⇄ Fe2 O3 −NP
ð2Þ
The most striking difference between our work and the others [20–23] is the presence of the polymer PDAC, which contains a highly stable suspension of PDAC–FeOOH/Fe2O3-NPs. Interestingly, TEM images (Fig. 1b) suggest that FeOOH/Fe2O3-NPs are structured as needle-like nanostructures having an average length of approximately 20 nm, and these conjugate with PDAC molecules in the PDAC–FeOOH/Fe2O3-NP system to form spherical colloids. Furthermore, as we will show below, there actually exists a dynamic equilibrium (metastable phase) between the different species containing an iron structure. 3.2. Solution-phase voltammetry Fig. 2a shows the electrochemical response of the PDAC–FeOOH/ Fe2O3-NP (0.89 and 0.29 V) suspension. By varying the scan rate, ν, the charge transfer is seen to be diffusion-limited [25] up to 300 mVs− 1 in the I versus ν1/2 plot (inset Fig. 2).The electrochemical processes are attributed to the oxidation and reduction of Fe3+/Fe2+ ions from the PDAC–FeOOH/Fe2O3-NPs, as described in Eq. (3).
a
pHb8:0
Fe2 O3 −NP ⇄ Fe Metastable
−
e
= FeOOH−NP ⇄ Fe −
3+
e
2+
= FeOOH−NP
ð3Þ
The reactions proposed here are in agreement with the results obtained using a Pourbaix diagram [24], when Fe3+ species are thermodynamically stable at a pH of less than 4.0 and with the potential range shown in the voltammograms. Because of the intrinsic colloidal characteristics of the system presented here, the rate of diffusion can be estimated on the basis of the Stokes–Einstein [11] equation (Eq. 4), where the diffusion coefficient D of a nanoparticle can be calculated on the basis of the temperature T, viscosity η = 10−3 Pa s (at 24 °C), and radius r of the particle.
b
D = kT = 6πηr
20 nm
Fig. 1. a) Electronic spectra for FeCl3 solution (green line) and PDAC–FeOOH/Fe2O3-NP (blue line) suspension. Inset shows the change in colour of the solution containing Fe3+ ions (yellow solution) after the addition of NH4OH (precipitate, middle image) and in the presence of PDAC (stable suspension). b) Images of Transmission Electron Microscopy (TEM) of PDAC–FeOOH/Fe2O3-NPs. Inset shows spherical colloids of PDAC–(FOOH/Fe2O3)-NPs having a mean diameter of 20 nm.
ð4Þ
In the TEM images, we can clearly observe spherical colloids of PDAC–(FeOOH/Fe2O3)-NPs having a mean diameter of 20 nm. Therefore, we used the obtained data to calculate D, whose mean value is 0.9 × 10− 11 m2 s− 1. This value suggests that the particles diffuse at a considerably slow rate as compared to that in conventional molecular redox systems [6,26]. Thus, we have to verify if the electrochemical responses are from the nanomaterials rather than from dissolved molecular iron species equilibrated with the nanoparticles. For this propose, we determine the amount of dissolved iron in solution. Using successive additions of ferric chloride in the electrolyte and compared with voltammograms obtained for the nanoparticles (data not shown), it was possible to estimate that 10% of iron from PDAC–(FeOOH/Fe2O3)-NPs are dissolved. Using Randles–Sevcik equation [25] to interpret the voltammograms, we obtained a value of 1.4 × 10− 11 m2 s− 1 for the
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mobility, studies indicated that they had high catalytic activity for the electroreduction of H2O2. This confirms that this new material is promising for studies on half cells applied in biomimetic biofuel cells, as shown below.
3.3. Half-cell experiments In order to evaluate the thermodynamic parameters related to the development of a cathode for the reduction of H2O2, we focus on only the cathodic reaction. Fig. 3b shows voltammograms of the FeOOH/Fe2O3-NPs suspension in the presence of H2O2. FeOOH/Fe2O3-NPs exhibited high electrocatalytic activity for H2O2 reduction. From the electrochemical data (Fig. 3c), we concluded that there was a significant increase in the electroreduction current with an increase in the H2O2 concentration at a micromolar level (from 1.16 to 11.67 μmol L− 1). When we compare the reduction potential for H2O2 onto an ITO substrate in the presence and absence of PDAC–FeOOH/Fe2O3-NPs, a high overpotential shift, ΔE = 0.68 V from − 0.29 to 0.39 V, is evident. This leads to two main questions: (1) How does the transient current on the pathway of the catalytic reduction of H2O2 shift near the Nernstian process? (2) How does a nanoparticle act in the reactions of the outer-sphere electron transfer? For instance, we propose that the overall reaction rate of PDAC–FeOOH/Fe2O3-NPs is controlled by an enzyme-like system. First, the diffusion coefficient has a low value because of the low mobility of the nanoparticles. Moreover, Fe3+ can be strongly adsorbed onto the surface of PDAC–FeOOH/Fe2O3-NPs and it acts as an electron mediator in order to participate in the electrochemical reactions, as shown in Fig. 3. The Fe3+ species adsorbed onto the FeOOH/Fe2O3-NPs retained their nanostructure because the diffusion coefficient was very low. A possible explanation for this is that the polymer maintains the redox couple Fe3+/Fe2+ structured at the electrode PDAC–FeOOH/Fe2O3-NPs solution interface. This system is an enzyme-like system that has iron complexes in its protein structure [27–30]. Further, the particle size, low mobility, and high catalytic activity of PDAC–FeOOH/Fe2O3-NPs with respect to H2O2 reduction are characteristics very similar to those of enzymatic systems. The pathway of a molecular interaction between a ‘pseudoprosthetic’ group (Fe3+) from PDAC–FeOOH/Fe2O3-NPs and the H2O2 is another interesting point that corroborates our hypothesis that at certain sites, Fe3+, in a manner similar to redox enzymes, can promote a direct electron transfer in a reversible manner. It is worth mentioning that this system is essentially an electrochemical variant of Fenton's reagent [31], with the iron-salt-dependent decomposition of dihydrogen peroxide, generating the highly reactive hydroxyl radical, possibly via an oxoiron(IV) intermediate.
4. Conclusions
Fig. 2. a) Cyclic voltammograms of ITO (0.1-mol L− 1 H2SO4) in the presence of PDAC– FeOOH/Fe2O3-NPs at different scan rates. Inset shows the anodic and cathodic peak currents versus scan rate for the oxidation and reduction peak currents. b) Cyclic voltammograms of ITO (black line) (0.1-mol L− 1 H2SO4 + 11.6-μmol L− 1 H2O2) in the presence (red line) and absence (blue line) of PDAC–FeOOH/Fe2O3-NPs. c) Solutionphase voltammetry of PDAC–Fe2O3-FeOOH-NPs at different concentrations of H2O2 (a = 0.0, b = 1.16, c = 2.32, d = 3.48, e = 5.80, f = 9.12 and g = 11.67 mol L− 1).
diffusion coefficient; very close to that obtained previously. Based on this, we suggest that dissolved molecular iron species are equilibrated with the nanoparticles. Although these particles have low
PDAC–FeOOH/Fe2O3-NPs are a new class of candidate materials for application to biomimetic systems; these nanoparticles are highly stable water. PDAC–FeOOH/Fe2O3-NPs diffuse at a slow rate as compared to conventional molecular redox systems. Further, the metastable structures contain a pseudo-prosthetic group of Fe3+ that is responsible for the high electrocatalytic activity for H2O2 reduction. The synthesized material is very promising as a candidate for a biomimetic cathodic material for the reduction of H2O2 in biofuel cells.
Acknowledgements We would like to gratefully acknowledge the financial support from FAPESP, CAPES, CNPq, Instituto Nacional de Eletrônica Orgânica (INEO), and Rede NanoBioMed-Brasil (CAPES).
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a)
Fe2O3(solid) + 2 e– + 6H+(aq)
b)
2 Fe2+
c)
H2O2 + 2H+
2 Fe2+(ads) + 3H2O
2 Fe3+ +2 e– 2e-
2H2O
H+ H2O
E=-0.29 V (H2O2) Fe3+ 3+ NP Fe Fe3+
ΔE
Fe3+ Fe3+ Fe3+ eEF
FeOOH/Fe2O3-NP
e-
EF kBT
E=0.39 V
(η = 0.39 V vs. Ag/AgCl)
ΔE
kBT
(η = 0.39 V vs. Ag/AgC)
E=0.39V
Fig. 3. Proposed electrochemical mechanism for H2O2 reduction (a, b, and c), and schematic representation of the electrode process involving PDAC–FeOOH/Fe2O3-NPs.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M. Piccolino, Trends Neurosci. 20 (1997) 443. J.A. Cracknell, K.A. Vincent, F.A. Armstrong, Chem. Rev. 108 (2008) 2439. Y. Ayato, N. Matsuda, Chem. Lett. 38 (2009) 504. M.J. Moehlenbrock, S.D. Minteer, Chem. Soc. Rev. 37 (2008) 1188. S. Kerzenmacher, J. Ducrée, R. Zengerle, F. Von Stetten, J. Power Sources 182 (2008) 1. Y. Liu, S. Dong, Electrochem. Commun. 9 (2007) 1423. K. Stolarczyk, E. Nazaruk, J. Rogalski, R. Bilewicz, Electrochim. Acta 53 (2008) 3983. J. Kim, H. Jia, P. Wang, Biotechnol. Adv. 24 (2006) 296. F.N. Crespilho, M. Emilia Ghica, M. Florescu, F.C. Nart, O.N. Oliveira Jr., C. Brett, Electrochem. Commun. 8 (2006) 1665. D.H. Fan, J.Y. Sun, K.J. Huang, Colloids Surf. B Biointerfaces 76 (2010) 44. K.J. McKenzie, F. Marken, Pure Appl. Chem. 73 (2001) 1885. J. Zhang, M. Oyama, Electrochim. Acta 50 (2004) 85. Z. Zhang, S. Chouchane, R.S. Magliozzo, J.F. Rusling, Anal. Chem. 74 (2002) 163. B. Šljukić, C.E. Banks, R.G. Compton, Nano Lett. 6 (2006) 1556. K.J. McKenzie, F. Marken, M. Hyde, R.G. Compton, New J. Chem. 26 (2002) 625. M. Rajendran, R.C. Pullar, A.K. Bhattacharya, D. Das, S.N. Chintalapudi, C.K. Majumdar, J. Magn. Magn. Mater. 232 (2001) 71.
[17] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 4212. [18] G. Mie, Ann. Phys. 25 (1908) 377. [19] G. Compagnini, E. Messina, O. Puglisi, R.S. Cataliotti, V. Nicolosi, Chem. Phys. Lett. 457 (2008) 386. [20] G. Cepria, A. Uson, J. Perez-Arantegui, J.R. Castillo, Anal. Chim. Acta 477 (2003) 157. [21] Y. Hu, K. Chen, J. Cryst. Growth 308 (2007) 185. [22] J. Cai, J. Liu, Z. Gao, A. Navrotsky, S.L. Suib, Chem. Mater. 13 (2001) 4595. [23] B.L. Cushing, V.L. Kolesnichenko, C.J. O'Connor, Chem. Rev. 104 (2004) 3893. [24] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. [25] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 2001. [26] F. Marken, D. Patel, C.E. Madden, R.C. Millward, S. Fletcher, New J. Chem. 26 (2002) 259. [27] K. Ariga, J.P. Hill, M.V. Lee, A. Vinu, R. Charvet, S. Acharya, Sci. Technol. Adv. Mater. 9 (2008) 014109. [28] E. Ando, Y. Gotoh, K. Morimoto, K. Ariga, Y. Okahata, Thin Solid Films 180 (1989) 287. [29] Y. Okahata, T. Tsuruta, K. Ijiro, K. Ariga, Langmuir 4 (1988) 1375. [30] M. Onda, K. Ariga, T. Kunitake, J. Biosci. Bioeng. 87 (1999) 69. [31] A.D. Mcnaught, A. Wilkinson, IUPAC Compendium of Chemical Terminology, second edBlackwell Science, 1997.
