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Kinetic evaluation of the viologen-catalyzed carbohydrate oxidation reaction for fuel cell application
Renewable Energy 63 (2014) 370e375
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Kinetic evaluation of the viologen-catalyzed carbohydrate oxidation reaction for fuel cell application Gerald D. Watt Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2013 Accepted 20 September 2013 Available online 17 October 2013
The use of abundant carbohydrates as resources for the production of electrical energy is an important area of research and development. Until recently only limited success has been reported in developing efficient catalysts for use in carbohydrate fuel cells. Viologens are active catalysts in transferring the abundant, low-potential (w1.0 V) electrons stored in carbohydrates (24 electrons/glucose) to O2 or fuel cell electrodes in alkaline solution. To maximize electrical production from an alkaline carbohydrate fuel cell, it is essential to understand the variables determining the rate of electron transfer from the carbohydrate fuel to the viologen catalysts and then to the current collecting electrodes. Electron transfer from viologens to electrodes is a rapid process, so here we report a kinetic investigation evaluating the kinetics of oxidation of various carbohydrates with viologens under a variety of conditions, including viologen type. At a fixed temperature and pH, a first order reaction in both viologen and carbohydrate was observed. In general, carbohydrates with fewer than 5 carbon atoms react rapidly at room temperature and below but those with 5 carbons or more react more slowly and require temperatures of 40 e55 C. The results demonstrate that viologen oxidation of carbohydrates is sufficiently rapid that viable electrical power can be derived from alkaline carbohydrate fuel cells. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Carbohydrate fuel cell Carbohydrate oxidation Kinetics Fuel cell catalyst
1. Introduction Climate change issues and their projected negative effects on the economic and social development of world populations and also the need for developing national energy independence are creating the impetus to search for abundant, alternative energy sources. Solar, nuclear and wind energy sources are being actively utilized and will likely continue making important contributions to the world’s energy needs [1e3]. However, one viable energy resource that has yet to be efficiently developed is production of electricity from the energy present in abundant and bio renewable carbohydrates [4e6]. Glucose has 24 low-potential (w1.0 V) electrons per molecule that could be utilized for the efficient production of electricity via fuel cell technology but until recently inexpensive, functional and readily available catalysts have not been forthcoming to exploit this renewable resource [7e11]. When fully maximized, this approach would bypass the relatively low conversion efficiency of glucose energy into ethanol and directly access the abundant chemical energy of glucose as electricity.
Abbreviations: MV, dimethyl viologen; MMV, monomethyl viologen; BV, benzyl viologen; MHA, monohydroxy acetone; DHA, dihydroxy acetone; GLY, glyceraldehyde. E-mail address: [email protected]. 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.09.025
Recently a class of organic compounds known as viologens (1, 10 dialkyl-4, 40 -bipyridilium salts) were shown to function as catalysts in alkaline solution (pH > 10) and to rapidly remove electrons from carbohydrates and transfer them to metallic electrodes [8e11]. A simple un-optimized fuel cell was constructed which attained greater than 40% conversion of the chemical energy of carbohydrates into electrical energy [8]. Subsequent studies have shown that the viologens initially oxidize the terminal end of the ene-diol form of the carbohydrate and then continue a one-carbon, step-wise oxidation until the carbohydrate was fully oxidized to carbonate and formate [9]. Higher electrical production was observed at pH values > 12, but at these higher pH values the dialkyl viologens that were initially used as fuel cell catalysts were unstable to the slow loss of one of the alkyl substituents, resulting in formation of monoalkyl substituted viologens [10]. In contrast to the dialkyl derivatives, the resulting monoalkyl viologens are stable in strong base and, importantly, were found to also catalyze carbohydrate oxidation [10]. This important discovery demonstrated that the monoalkyl viologens could effectively catalyze carbohydrate oxidation at high pH, where greater electrical energy is produced. However, the rate of carbohydrate oxidation was slower than that of the dialkyl viologens [10]. The rate of electron transfer from the carbohydrate to form the reduced viologen catalyst and the rate from the reduced viologen to the electrode are important reactions in defining power production
G.D. Watt / Renewable Energy 63 (2014) 370e375
from an alkaline carbohydrate fuel cell. To create an efficient alkaline carbohydrate fuel cell, the kinetics of these two component reactions must be rapid and their reaction rates maximized. The electrochemical reaction between viologens and electrodes has been extensively studied [12e15] and is rapid so electron transfer from carbohydrate to viologen will likely be a factor in controlling current flow in an alkaline carbohydrate fuel cell. Kinetic information to maximize the fuel cell design and to project power production from such a fuel cell is essential. Here, we report the rate of carbohydrate oxidation by various viologen catalysts as a function of a number of variables, including temperature, variation of reactant concentrations, pH and viologen type. The results establish that both monoalkyl and dialkyl viologens react rapidly with carbohydrates near room temperature and demonstrate that an efficient alkaline carbohydrate fuel cell (pH 12e14) is feasible to utilize the abundant and bio renewable carbohydrate resources obtainable from biomass. 2. Materials and methods The rate of reaction between dimethyl viologen (methyl viologen, MV, Sigma), and glucose (6), fructose (6), ribose (5), xylose (4), dihydroxy acetone (DHA, 3), monohydroxy acetone (MHA, 3) and glyceraldehyde (GLY, 3) was determined by measuring the increase in absorbance of reduced MV at 730 nm (ε ¼ 2.17 103 cm1 M1) using an HP 5483 spectrophotometer to collect the absorbance-time data sets. The number of carbon atoms in each carbohydrate type is given in parentheses. Reaction with glycerol was also examined. Initial rates were measured in triplicate during the first 200e500 s of the reaction and used to determine the reaction order and evaluate rate constants. The uncertainty of the triplicate measurements was 5e9%. Reactions were run from 25 to 55 C and at pH values from 9 to 12 (0.50 M phosphate buffer) and at >pH 12 (0.10e1.0 M KOH). Pseudo first order conditions in both viologen and carbohydrate were used at viologen/carbohydrate and carbohydrate/viologen ratios of 20/1 to 5/1. Variations of initial rates with variation of carbohydrate (viologen) concentration while holding the viologen (carbohydrate) concentration constant provided the information necessary to determine the reaction order in both reagents. Some reactions displayed lag phases (time periods during which no reaction was observed) of 20e500 s. After this lag phase, the reactions proceeded normally and the straight-line portions of the progress curves were used to evaluate the rate of the reaction. Because viologens are reduced by one electron to form stable free radicals and the carbohydrates studied yield 12 (three-carbon carbohydrates) or 24 (six-carbon carbohydrates) electrons/carbohydrate upon oxidation, the electron/viologen ratios are larger by factors of 12 or 24 than the molar ratios used. The reaction between monomethyl viologen (MMV) and carbohydrates was conducted as described above for MV at the same pH and temperatures values. For both reactions types, anaerobic solutions were prepared by several evacuationeflush cycles of argon of the carbohydrate solution in the desired buffer and then equilibrated at the selected temperature. When thermal conditions were established, viologen was added to initiate the reaction. The results presented next first consider reactions of MV and MMV with three-carbon carbohydrates followed by reactions of MV and MMV with six-carbon carbohydrates. 3. Results 3.1. MV reaction with three-carbon carbohydrates
Fig. 1. The reaction of 12.5 mM MV with MHA at pH 12 and 25 C. MV was held constant at 0.0125 M and at DHA concentration of: 2.5 mM (purple, ), 5.0 (red, -), 6.25 (blue, A), 10.0 (green, :) and 12.5 (blue, H). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
were too rapid to conveniently measure so the reaction of MHA was used to represent the oxidation reaction of all three-carbon carbohydrates. Fig. 1 shows kinetic traces for MV oxidation of MHA at a fixed MV concentration but at various MV/carbohydrate molar ratios at 25 C. The reactions show several important features. At low concentrations, the rate for some reactions does not begin immediately upon addition of MV but instead a lag period of up to 40 s is required before the reaction begins. This lag phase varies inversely with the MHA concentration, so at concentrations of MHA at > 6.25 mM, it is eliminated. Once the lag phase is complete the reaction accelerates and the kinetic traces form straight-line sections before the reaction slows as MV is completely reduced. Rate constants were evaluated from the straight line sections of the progress curves, giving an overall rate law that was first order in both MHA and MV. The curving portion of the progress curve as the reaction goes to completion was also analyzed and confirmed the first order behavior in each reactant. Equation (1) summarizes the rate equation at a fixed pH observed for the general reaction of carbohydrates with viologens that were studied and Table 1 summarizes the measured rate constants for 3 as well as 5- and 6-carbon carbohydrates discussed below.
Rate ¼ ½carbohydrate1 ½viologen1
(1)
The second feature of interest is that after the lag phase, the final absorbance change generally increases slightly with increasing MHA concentration at a fixed MV concentration. This effect results from the redox equilibrium between the MV oxidant and the MHA reductant, which shifts the equilibrium to more reduced MV at higher MHA concentrations. Conducting identical reactions with DHA or GLY in place of MHA produced rapid kinetic behavior similar to that for MHA but the rates of reaction were too rapid to analyze. Table 1 Second order rate constants for the reaction of MV and MMV with the carbohydrates listed in the first column. All reactions were run in 0.50 M potassium phosphate buffer pH 12. The rate constants for the reaction of glucose with MV and MMV at 55 C were 0.31 and 0.085 M1 s1, respectively.
Carbohydrate MHA (3)b Ribose (5) Glucose (6) Fructose (6) a
Three carbon carbohydrates react rapidly with MV at 25 C and pH 10e12 in the order DHA > GLY > MHA. The first two reactions
371
b c
MV
MV
MMV
MMV
ka (25 C) 0.056 0.031 NDc 0.041
ka (40 C) 1.31 0.57 0.043 0.044
ka (25 C) 0.034 0.0012 NDc NDc
ka (40 C) NDc NDc 0.013 0.18
The units of the second order rate constant k are M1 s1. ( ), indicates the number of carbon atoms in the carbohydrate. The reactions are either too slow or to rapid to measure.
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The variation in rate of MV with MHA was also conducted at pH 9e12. The reaction at pH 9 occurs at a rate that is just measurable but the rate increases rapidly with pH and attains maximum velocity at pH 12. This pH dependent behavior was previously explained as due to the carbohydrate undergoing proton ionization with a pKa of w10 to form the reactive ene-diol form of the carbohydrate [8]. For MV, the reaction conditions were limited to pH < 12 because dimethyl-substituted viologens begin to slowly hydrolyze and lose an alkyl group above this pH to form monoalkyl viologens [10]. The reaction of MHA with MV as a function of temperature was not studied above 25 C because the reactions were essentially complete upon mixing. However, the reaction proceeds easily at 5 C indicating small activation energy for MHA oxidation by MV. 3.2. MMV reaction with three-carbon carbohydrates Fig. 2 shows the reaction of MHA with MMV at pH 12 and 25 C as a function of MHA concentration. The results show that a significant lag phase occurs at low MHA concentrations but it is shortened by an increase in MHA concentration. At MHA concentrations >50 mM, the lag phase is eliminated and the reactions begin immediately. The lag phase can also be eliminated by increasing the temperature. Once the reaction starts, a linear section of the curve results from which first order behavior in both MHA and MMV was determined. The rate constant for this reaction is given in Table 1 and shows that the MMV reacts w3-fold slower than MV, which is consistent with previous electrochemical oxidation measurements [10]. The slower rate probably arises because MMV is a mono cation whereas MV is a dication and interaction with the negatively charged ene-diol of MHA with MMV is decreased. Another factor is that the reduction potential of MV (0.46 V) is more positive than that of MMV (0.81 V) so that the carbohydrate oxidation driving force is less for MMV, resulting in a slower rate. However, during fuel cell operation, electrons are rapidly transferred from reduced MMV to the anodic electrode and the equilibrium rapidly shifts in favor of product formation, so that complete oxidation of the carbohydrate occurs. In addition, fuel cell operation would be conducted at reagent concentrations 0.1 M or greater, so that the lag phase behavior would be completely eliminated and the reactions would proceed rapidly in accordance with the rate constants in Table 1. 3.3. MV reaction with six-carbon carbohydrates
concentration at 40 C and pH 12. Several important features are observed. First, at the concentrations of MV and glucose examined, some reactions only begin after 100e500 s, even though all reagents are present. The reaction begins after a time delay in an inverse glucose concentration dependent manner. At higher glucose concentrations, the lag phase is eliminated and the reaction begins immediately. For those reactions with a lag phase, raising the concentrations of both MV and glucose while maintaining the same ratio eliminates the lag phase. Temperature increase also eliminates the lag phase. This unusual rate behavior is likely a consequence of the low concentrations required by the optical method used to follow the rate because of the high extinction coefficients of reduced MV. Under the much higher concentrations (0.10 M or more) of fuel cell operation, the lag phase is completely eliminated and reactions proceed immediately. From the straight-line sections of the uninhibited reactions in Fig. 3 and the linear dependence after the lag phase in the other reactions, a first order dependence on glucose and MV was determined as outlined above with rate constants shown in Table 1. The rate of oxidation of glucose by MV was also examined as a function of pH between 10 and 12. The reaction does not precede below pH 10 but increases rapidly at pH 11 and reaches a maximum rate at pH 12. The reaction was run under identical conditions at 40 and 45 C at pH 11 and 12 using phosphate and carbonate buffers. The reaction was 1.3 times faster in carbonate than in phosphate buffer. This is an important result because carbonate would be a product during fuel cell operation but the results show that there is no inhibition relative to phosphate even in 0.50 M carbonate buffer. Fig. 4 shows the variation in rate for MV oxidation of glucose at pH 12 as a function of temperature. At temperatures below 40 C the rate is very slow. However, the rate increases rapidly between 40 and 50 C. Surprisingly, there is only slight enhancement in rate at 55 C. The results suggest that at pH 12, 50e55 C is the optimum temperature for the MV-glucose reaction for fuel cell use Fig. 5. 3.4. MMV reaction with six-carbon carbohydrates The oxidation of six-carbon glucose using MMV (þ1) is shown in Fig. 6 and was shown also to be first order in glucose and MMV. This reaction can be compared to the same reaction with MV (2þ) in Fig. 3. The reaction of MMV under identical conditions is w3-fold slower than the same reaction with MV. This comparison is of value because it shows the effect that charge and viologen redox potential has on the rate of viologen-catalyzed carbohydrate oxidation. MV has a stronger oxidizing potential (0.46 V) compared
Fig. 3 shows the variation in the rate of glucose oxidation by a fixed concentration of MV as a function of increasing glucose
Fig. 2. The reaction of MMV with MHA at pH 12 and 25 C. MMV was held constant at 12.5 mM and the mM MHA concentration was varied from 1.3 (purple, ), 3.1 (red, -), 6.25 (green, :) and 12.5, A). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. The reaction of 6.25 mM MV as a function of glucose concentration at pH 12 and 40 C. The glucose mM concentration was 1.55 (orange, C), 3.13 (-, red), 6.25, (blue,A), 10.0 (purple, ), 12.5 (green, :), and 25 (blue, U). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
G.D. Watt / Renewable Energy 63 (2014) 370e375
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3.5. Relative carbohydrate reactivity
Fig. 4. The reaction of 6.25 mM MV and 2.5 mM glucose as a function of temperature. The reaction was conducted at pH 12 in 0.50 M potassium phosphate buffer at temperatures ( C): 40 (purple, ), 45 (green, :), 50 (red, -) and 55 (blue, A). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. The reaction of MV and glucose at 40 C as a function of pH. The concentrations of MV and glucose were held constant at 6.25 mM and the pH was varied from 10.2 (red, -), 10.5 (orange, C), 11 (blue, H), 11.5(blue, þ), 11.8 (pink, -), 12 (blue, A), 13 (green, :). The same reaction but at a MV and glucose concentration of 31.5 mM (purple, ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to MMV (0.81 V) [6] and when combined with the decrease of one unit of charge it is seen that MV oxidizes carbohydrates faster than MMV under the same conditions as verified by examining the rate constants in Table 1. This same result was observed previously by following these two reactions electrochemically [10], where MMV reacted w2-fold slower than MV Figs. 7 and 8.
Fig. 6. The reaction of MMV as a function of glucose concentration at pH 12 and 40 C. MMV was 6.25 mM and the mM glucose concentrations were: 6.25 (green, :), 12.5 (red, -) and 25.0 (blue, A). The lag phase was 200, 70 and 15 s, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Glucose is a six-carbon aldohexose with a six member cyclic ring structure but converts to a linear form with an aldehyde group on carbon number 1. Fructose is a six-carbon ketohexose that exists in a five-member cyclic ring structure with a keto group on carbon number 2 in its linear structure. Both form ene-diols at pH w10. Initially it was thought that ring opening of the six and five membered rings would determine the relative reactivity of glucose and fructose, respectively. The two reactions were compared at 40 C under identical conditions and as shown in Table 1 fructose reacts at a similar rate as that of glucose. However, ribose is an aldopentose that also forms a five member ring similar to that in fructose but reacts w13 times faster than glucose and fructose. Factors other than simple ring opening must be influencing the overall reactivity. At pH 12 and 25 C the reactions of both MMV and MV with glucose, fructose and ribose range from no reaction for glucose to very slow reaction for fructose and ribose. Under these same conditions, MV reacts with MHA 1.5 times faster than with MMV and both reactions are sufficiently rapid to sustain reactions for a viable fuel cell at this temperature. However, at 40 C and pH 12, MV reacts with glucose 3.3-times faster than MMV and under the same conditions both MMV and MV react with MHA at rates too rapid to measure. Based on the rate constants in Table 1, carbohydrate reactivity with MV at pH 12 and 40 C relative to glucose, follows the decreasing rate of reaction: DHA (w35) > GLY (w32) > MHA (30) > ribose (13) > fructose w glucose (1). In general, MMV follows a similar trend but the rates are 1.5e3.0 times slower. In the legend to Table 1 rate constants of 0.31 and 0.056 M1 s1 are given for the reaction of glucose with MV and MMV at pH 12 and 55 C, respectively. Both increase w7-fold from 40 to 55 C and maintain a difference of w3 in their reactivity toward glucose. Both oxidize glucose at rates compatible with viable fuel cell development (See below). The results suggest that the rates of both types of viologens with carbohydrates consist of contributions from the pKa for enediol formation, the relative reactivity of the resulting ene-diol, stability of the cyclic ring structure (where present), and perhaps other factors not yet identified. The results obtained with glycerol under identical conditions used In Table 1 show that the rate is much slower than with glucose and that while glycerol undergoes reaction with both MV and
Fig. 7. The reaction of 6.25 mM MMV with 2.5 mM glucose at 40 C in 0.50 M potassium phosphate buffer pH 12 (green, :), 0.10 M KOH (blue, A), and 1.0 M KOH (red, -). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. Temperature variation of the reaction of 6.25 mM MMV with 6.25 mM glucose in 0.50 M potassium phosphate buffer pH 12. The reaction temperatures were: 40 (green, :), 45 (red, -), 47 (blue, H), 50 (purple, ) and 55 (blue, A) C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
MMV, the rate and the extent of reaction at 25e50 C are not sufficient to warrant its use in a viologen catalyzed fuel cell. 3.6. Stability and redox potential of MMV compared to MV Consulting the Pourbaix Diagram for glucose and other carbohydrates and MV and MMV in Ref. [13] establishes that the redox potential for glucose oxidation becomes more favorable with increasing pH, while that of MV and MMV remains invariant at 0.46 V and 0.81 V. Consistent with the diagram, the experimental results reported here, show that MV and MMV are effective oxidants of glucose and other carbohydrates only at pH > 10. This is supported by direct measurements using the stronger oxidant benzyl viologen (BV, 0.36 V), which slowly oxidizes glucose at pH 9.0, consistent with its stronger oxidizing ability. While both MV and MMV are theoretically viable for use as carbohydrate fuel cell catalysts, the instability of MV above pH 12 limits its use to the pH range 11e12, whereas MMV is stable and active from pH 11 to 1.0e 3.0 M KOH. 4. Discussion Previous results have demonstrated that viologens effectively catalyze carbohydrate oxidation by oxygen or fuel cell electrodes under alkaline conditions [8e11]. This result provided the impetus for developing a simple fuel cell operating at w40% conversion efficiency of DHA and glucose to carbonate and producing 20e 40 mamp/cm2 using homogeneous MV as catalyst [8]. We report here the rate law for the oxidation of a variety of carbohydrates in alkaline solution using two different viologen types: dialkyl viologens as exemplified by MV and monoalkyl viologens using MMV. Previous reports have discussed the redox potentials, the relative stabilities and the heterogeneous rate constants for these two types of viologens reacting with electrodes [9,10,13]. Table 1 summarizes kinetic results for the viologen-carbohydrate interactions and demonstrates that the rates of carbohydrate oxidation by both monoalkyl and dialkyl viologen catalysts are rapid, demonstrating the feasibility of using viologens as alkaline fuel cell catalysts. During this investigation it was found that the viologen type (dialkyl vs monoalkyl) has distinct advantages and disadvantages for alkaline fuel cell development. When reacted with the same carbohydrate at pH 12 and 40 C, MV reacts 1.5e3.0-fold faster than MMV. As the current flow in a fuel cell depends on the rate at which the carbohydrate is oxidized, a higher current flow would result using MV as a catalyst than with MMV. However, consulting a Pourbaix diagram in Ref. [13] raises another important consideration. Carbohydrate oxidation becomes more favorable at pH > 12, where MV is unstable with respect to conversion to MMV [10,15].
MV would produce a higher current flow but its use is limited to pH 12 or below. These are conditions where the voltage output from carbohydrate oxidation is not optimal. The disadvantage of slower carbohydrate oxidation and consequently lower current flow using MMV as catalyst is countered by increased stability and a more efficient use of the available chemical energy derived from carbohydrate oxidation at high pH. Another advantage of MMV compared to MV is the more favorable redox potential. MMV has a redox potential near 0.81 V, whereas that of MV is 0.46 V [13]. When compared to the redox potential of 1.2 V generated by a carbohydrate at pH 14, the potential of MMV more closely approaches that of the carbohydrate than does MV. The result is that a more efficient transfer of electrochemical energy from carbohydrate to fuel cell electrodes occurs with MMV (70%) than with MV (38%). For comparison, Table 5 in Ref. [13] shows that at pH 12 where the carbohydrate potential is 0.93 V, a voltage transfer from carbohydrate to MMV and MV is 92% and 66%, respectively. At pH 12, MMV is an excellent carbohydrate fuel cell catalyst but to cover a higher pH range with the opportunity to maximize power production from a carbohydrate oxidation, monoalkyl viologen catalysts with redox potentials approaching 1.2 V will be required. We are currently examining methods to move the potential of monoalkyl viologens to be more compatible with the potential generated by carbohydrates above pH 12 in order to more efficiently transfer carbohydrate energy to fuel cell electrodes. If successful and if the rates of carbohydrate oxidation by these enhanced monoalkyl viologens are rapid, they can be used to maximize carbohydrate fuel cell performance at high pH. Fig. 4 shows that the rate of glucose oxidation by MV and MMV increases 7-fold from 40 to 55 C and reaches a rate sufficient for fuel cell use. This can be shown by using rate law (1), with k ¼ 0.31 M1 s1 for MV reacting with glucose at 55 C from Table 1 and assuming MV and glucose concentrations of 1.0 M. Under these conditions, electrons are removed from glucose at a rate of 0.31 M/ s. If this reactivity occurs in 1.0 L of solution on a 1.0 cm2 electrode, with a solution column of 1.0 cm, a current density of 30 mamp/cm2 is calculated vs 29 mamp/cm2 previously measured in a simple fuel cell [1]. Under the same conditions, MMV would produce a current density w30% that of MV. Table 1 indicates that the homogeneous rate constants for carbohydrate oxidation by viologens span a range of values depending on the carbohydrate type and conditions but are similar to the heterogeneous electron-transfer rate constants (kw0.20 cm/s)for electron transfer from viologen to electrodes [13,15]. This useful approximation suggests that both the heterogeneous and homogeneous rates are favorable for utilizing carbohydrates as available and renewable fuels for electrical production via an alkaline carbohydrate fuel cell. It is important to emphasize that the above calculation only estimates the rate of electron production at the anode using homogeneous MV as catalyst and the least reactive carbohydrate glucose as a fuel source. In actual fuel cell operation, the reduction of oxygen at the cathode and ion transport through the separating membrane and other factors also contribute to fuel cell performance. However, a >35-fold increase in anodic fuel cell output compared to glucose could be realized by using DHA and GLY, both of which are derived from abundant glycerol produced as a byproduct from biodiesel production. The above considerations have focused on utilizing homogeneous viologens for carbohydrate oxidation with subsequent electron transfer to fuel cell electrodes. A significant improvement in fuel cell performance can be anticipated using immobilized viologens on electrodes. This process not only minimizes viologen diffusion processes but enhances viologen-carbohydrate interaction with an accompanying increase in carbohydrate oxidation efficiency and enhanced electron transfer to fuel cell electrodes. This
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enhanced reactivity was previously demonstrated by using viologen polymers, which increased carbohydrate oxidation efficiency by w25% [9]. Electrodes containing immobilized viologens are currently being investigated for enhanced fuel cell operation and their performance will be reported later. Acknowledgments The Department of Chemistry and Biochemistry and the Office of Creative Research at Brigham Young University supported this research. References [1] Kamat PV. Meeting the clean energy demand: nanostructure architectures for solar energy conversion. J Phys C 2007;111:2834e60. [2] Campbell CJ. Petroleum and people. Popul Environ 2002;24:193e207. [3] Dresselhaus MS, Thomas IL. Alternative energy technologies. Nature 2001;414:332e7. [4] Lichtenthaler FW, Peters S. Carbohydrates as green raw materials for the chemical industry. CR Chim 2004;7:65e90. [5] Field CB, Campbell JE, Lobell DB. Biomass energy: the scale of the potential resource. Trends Ecol Evol 2007;23:65e72.
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[6] Xuan J, Leung MKH, Leung DYC, Ni M. A review of biomass-derived fuel processors for fuel cell systems. Renew Sustain Energ Rev 2009;13:1301e13. [7] Larsson R, Folkesson B. A catalytic oxidation of sugar by vanadium (IV). J Mol Catal 2005;229:183e90. [8] Wheeler DR, Nichols J, Hansen D, Andrus M, Choi S, Watt GD. Viologen catalysts for a direct carbohydrate fuel cell. J Electrochem Soc 2009;10:B1201e7 carbohydrate oxidation during viologen-catalyzed O2-oxidation, 2010 carbohydrate oxidation during viologen-catalyzed O2-oxidation: mechanistic insights. Renew Energy 2010;156:B1201eB1207. [9] Watt GD, Hansen D, Dodson D, Andrus M, Wheeler D. Electrical energy from carbohydrate oxidation during viologen-catalyzed O2-oxidation: mechanistic insights. Renew Energ 2010;362:1523e8. [10] Read A, Hansen D, Aloi S, Pitt WG, Wheeler DR, Watt GD. Monoalkyl viologens are effective carbohydrate O2-oxidation catalysts for electrical energy generation by fuel cells. Renew Energ 2012;46:218e23. [11] Scott D, Liaw BY. Harnessing electric power from monosaccharides-a carbohydrate-air alkaline fuel cell mediated by redox dyes. Energ Environ Sci 2009: 10e5. [12] Michaelis L, Hill ES. The viologen indicators. J Gen Physiol 1932;16:859. [13] Hansen DC, Pan Y, Stockton J, Pitt WG, Wheeler DR. Cyclic voltammetry investigation of organic species considered for use as catalysts in directcarbohydrate fuel cells. Renew Energ 2012;159:H834e41. [14] Hunig S, Gross J, Schenk W. Diquartzrsalze der Bipyridyle and Dipyridylathylene. Synthese und Polarographie. Liebigs Ann Chem 1973:324e8. [15] Monk PMS. The viologens, physiochemical properties, synthesis and applications of the salts of 4, 40 -bipyridine. Chichester, England: John Wiley & Sons; 1998.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Kinetic evaluation of the viologen-catalyzed carbohydrate oxidation reaction for fuel cell application Gerald D. Watt Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2013 Accepted 20 September 2013 Available online 17 October 2013
The use of abundant carbohydrates as resources for the production of electrical energy is an important area of research and development. Until recently only limited success has been reported in developing efficient catalysts for use in carbohydrate fuel cells. Viologens are active catalysts in transferring the abundant, low-potential (w1.0 V) electrons stored in carbohydrates (24 electrons/glucose) to O2 or fuel cell electrodes in alkaline solution. To maximize electrical production from an alkaline carbohydrate fuel cell, it is essential to understand the variables determining the rate of electron transfer from the carbohydrate fuel to the viologen catalysts and then to the current collecting electrodes. Electron transfer from viologens to electrodes is a rapid process, so here we report a kinetic investigation evaluating the kinetics of oxidation of various carbohydrates with viologens under a variety of conditions, including viologen type. At a fixed temperature and pH, a first order reaction in both viologen and carbohydrate was observed. In general, carbohydrates with fewer than 5 carbon atoms react rapidly at room temperature and below but those with 5 carbons or more react more slowly and require temperatures of 40 e55 C. The results demonstrate that viologen oxidation of carbohydrates is sufficiently rapid that viable electrical power can be derived from alkaline carbohydrate fuel cells. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Carbohydrate fuel cell Carbohydrate oxidation Kinetics Fuel cell catalyst
1. Introduction Climate change issues and their projected negative effects on the economic and social development of world populations and also the need for developing national energy independence are creating the impetus to search for abundant, alternative energy sources. Solar, nuclear and wind energy sources are being actively utilized and will likely continue making important contributions to the world’s energy needs [1e3]. However, one viable energy resource that has yet to be efficiently developed is production of electricity from the energy present in abundant and bio renewable carbohydrates [4e6]. Glucose has 24 low-potential (w1.0 V) electrons per molecule that could be utilized for the efficient production of electricity via fuel cell technology but until recently inexpensive, functional and readily available catalysts have not been forthcoming to exploit this renewable resource [7e11]. When fully maximized, this approach would bypass the relatively low conversion efficiency of glucose energy into ethanol and directly access the abundant chemical energy of glucose as electricity.
Abbreviations: MV, dimethyl viologen; MMV, monomethyl viologen; BV, benzyl viologen; MHA, monohydroxy acetone; DHA, dihydroxy acetone; GLY, glyceraldehyde. E-mail address: [email protected]. 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.09.025
Recently a class of organic compounds known as viologens (1, 10 dialkyl-4, 40 -bipyridilium salts) were shown to function as catalysts in alkaline solution (pH > 10) and to rapidly remove electrons from carbohydrates and transfer them to metallic electrodes [8e11]. A simple un-optimized fuel cell was constructed which attained greater than 40% conversion of the chemical energy of carbohydrates into electrical energy [8]. Subsequent studies have shown that the viologens initially oxidize the terminal end of the ene-diol form of the carbohydrate and then continue a one-carbon, step-wise oxidation until the carbohydrate was fully oxidized to carbonate and formate [9]. Higher electrical production was observed at pH values > 12, but at these higher pH values the dialkyl viologens that were initially used as fuel cell catalysts were unstable to the slow loss of one of the alkyl substituents, resulting in formation of monoalkyl substituted viologens [10]. In contrast to the dialkyl derivatives, the resulting monoalkyl viologens are stable in strong base and, importantly, were found to also catalyze carbohydrate oxidation [10]. This important discovery demonstrated that the monoalkyl viologens could effectively catalyze carbohydrate oxidation at high pH, where greater electrical energy is produced. However, the rate of carbohydrate oxidation was slower than that of the dialkyl viologens [10]. The rate of electron transfer from the carbohydrate to form the reduced viologen catalyst and the rate from the reduced viologen to the electrode are important reactions in defining power production
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from an alkaline carbohydrate fuel cell. To create an efficient alkaline carbohydrate fuel cell, the kinetics of these two component reactions must be rapid and their reaction rates maximized. The electrochemical reaction between viologens and electrodes has been extensively studied [12e15] and is rapid so electron transfer from carbohydrate to viologen will likely be a factor in controlling current flow in an alkaline carbohydrate fuel cell. Kinetic information to maximize the fuel cell design and to project power production from such a fuel cell is essential. Here, we report the rate of carbohydrate oxidation by various viologen catalysts as a function of a number of variables, including temperature, variation of reactant concentrations, pH and viologen type. The results establish that both monoalkyl and dialkyl viologens react rapidly with carbohydrates near room temperature and demonstrate that an efficient alkaline carbohydrate fuel cell (pH 12e14) is feasible to utilize the abundant and bio renewable carbohydrate resources obtainable from biomass. 2. Materials and methods The rate of reaction between dimethyl viologen (methyl viologen, MV, Sigma), and glucose (6), fructose (6), ribose (5), xylose (4), dihydroxy acetone (DHA, 3), monohydroxy acetone (MHA, 3) and glyceraldehyde (GLY, 3) was determined by measuring the increase in absorbance of reduced MV at 730 nm (ε ¼ 2.17 103 cm1 M1) using an HP 5483 spectrophotometer to collect the absorbance-time data sets. The number of carbon atoms in each carbohydrate type is given in parentheses. Reaction with glycerol was also examined. Initial rates were measured in triplicate during the first 200e500 s of the reaction and used to determine the reaction order and evaluate rate constants. The uncertainty of the triplicate measurements was 5e9%. Reactions were run from 25 to 55 C and at pH values from 9 to 12 (0.50 M phosphate buffer) and at >pH 12 (0.10e1.0 M KOH). Pseudo first order conditions in both viologen and carbohydrate were used at viologen/carbohydrate and carbohydrate/viologen ratios of 20/1 to 5/1. Variations of initial rates with variation of carbohydrate (viologen) concentration while holding the viologen (carbohydrate) concentration constant provided the information necessary to determine the reaction order in both reagents. Some reactions displayed lag phases (time periods during which no reaction was observed) of 20e500 s. After this lag phase, the reactions proceeded normally and the straight-line portions of the progress curves were used to evaluate the rate of the reaction. Because viologens are reduced by one electron to form stable free radicals and the carbohydrates studied yield 12 (three-carbon carbohydrates) or 24 (six-carbon carbohydrates) electrons/carbohydrate upon oxidation, the electron/viologen ratios are larger by factors of 12 or 24 than the molar ratios used. The reaction between monomethyl viologen (MMV) and carbohydrates was conducted as described above for MV at the same pH and temperatures values. For both reactions types, anaerobic solutions were prepared by several evacuationeflush cycles of argon of the carbohydrate solution in the desired buffer and then equilibrated at the selected temperature. When thermal conditions were established, viologen was added to initiate the reaction. The results presented next first consider reactions of MV and MMV with three-carbon carbohydrates followed by reactions of MV and MMV with six-carbon carbohydrates. 3. Results 3.1. MV reaction with three-carbon carbohydrates
Fig. 1. The reaction of 12.5 mM MV with MHA at pH 12 and 25 C. MV was held constant at 0.0125 M and at DHA concentration of: 2.5 mM (purple, ), 5.0 (red, -), 6.25 (blue, A), 10.0 (green, :) and 12.5 (blue, H). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
were too rapid to conveniently measure so the reaction of MHA was used to represent the oxidation reaction of all three-carbon carbohydrates. Fig. 1 shows kinetic traces for MV oxidation of MHA at a fixed MV concentration but at various MV/carbohydrate molar ratios at 25 C. The reactions show several important features. At low concentrations, the rate for some reactions does not begin immediately upon addition of MV but instead a lag period of up to 40 s is required before the reaction begins. This lag phase varies inversely with the MHA concentration, so at concentrations of MHA at > 6.25 mM, it is eliminated. Once the lag phase is complete the reaction accelerates and the kinetic traces form straight-line sections before the reaction slows as MV is completely reduced. Rate constants were evaluated from the straight line sections of the progress curves, giving an overall rate law that was first order in both MHA and MV. The curving portion of the progress curve as the reaction goes to completion was also analyzed and confirmed the first order behavior in each reactant. Equation (1) summarizes the rate equation at a fixed pH observed for the general reaction of carbohydrates with viologens that were studied and Table 1 summarizes the measured rate constants for 3 as well as 5- and 6-carbon carbohydrates discussed below.
Rate ¼ ½carbohydrate1 ½viologen1
(1)
The second feature of interest is that after the lag phase, the final absorbance change generally increases slightly with increasing MHA concentration at a fixed MV concentration. This effect results from the redox equilibrium between the MV oxidant and the MHA reductant, which shifts the equilibrium to more reduced MV at higher MHA concentrations. Conducting identical reactions with DHA or GLY in place of MHA produced rapid kinetic behavior similar to that for MHA but the rates of reaction were too rapid to analyze. Table 1 Second order rate constants for the reaction of MV and MMV with the carbohydrates listed in the first column. All reactions were run in 0.50 M potassium phosphate buffer pH 12. The rate constants for the reaction of glucose with MV and MMV at 55 C were 0.31 and 0.085 M1 s1, respectively.
Carbohydrate MHA (3)b Ribose (5) Glucose (6) Fructose (6) a
Three carbon carbohydrates react rapidly with MV at 25 C and pH 10e12 in the order DHA > GLY > MHA. The first two reactions
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b c
MV
MV
MMV
MMV
ka (25 C) 0.056 0.031 NDc 0.041
ka (40 C) 1.31 0.57 0.043 0.044
ka (25 C) 0.034 0.0012 NDc NDc
ka (40 C) NDc NDc 0.013 0.18
The units of the second order rate constant k are M1 s1. ( ), indicates the number of carbon atoms in the carbohydrate. The reactions are either too slow or to rapid to measure.
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The variation in rate of MV with MHA was also conducted at pH 9e12. The reaction at pH 9 occurs at a rate that is just measurable but the rate increases rapidly with pH and attains maximum velocity at pH 12. This pH dependent behavior was previously explained as due to the carbohydrate undergoing proton ionization with a pKa of w10 to form the reactive ene-diol form of the carbohydrate [8]. For MV, the reaction conditions were limited to pH < 12 because dimethyl-substituted viologens begin to slowly hydrolyze and lose an alkyl group above this pH to form monoalkyl viologens [10]. The reaction of MHA with MV as a function of temperature was not studied above 25 C because the reactions were essentially complete upon mixing. However, the reaction proceeds easily at 5 C indicating small activation energy for MHA oxidation by MV. 3.2. MMV reaction with three-carbon carbohydrates Fig. 2 shows the reaction of MHA with MMV at pH 12 and 25 C as a function of MHA concentration. The results show that a significant lag phase occurs at low MHA concentrations but it is shortened by an increase in MHA concentration. At MHA concentrations >50 mM, the lag phase is eliminated and the reactions begin immediately. The lag phase can also be eliminated by increasing the temperature. Once the reaction starts, a linear section of the curve results from which first order behavior in both MHA and MMV was determined. The rate constant for this reaction is given in Table 1 and shows that the MMV reacts w3-fold slower than MV, which is consistent with previous electrochemical oxidation measurements [10]. The slower rate probably arises because MMV is a mono cation whereas MV is a dication and interaction with the negatively charged ene-diol of MHA with MMV is decreased. Another factor is that the reduction potential of MV (0.46 V) is more positive than that of MMV (0.81 V) so that the carbohydrate oxidation driving force is less for MMV, resulting in a slower rate. However, during fuel cell operation, electrons are rapidly transferred from reduced MMV to the anodic electrode and the equilibrium rapidly shifts in favor of product formation, so that complete oxidation of the carbohydrate occurs. In addition, fuel cell operation would be conducted at reagent concentrations 0.1 M or greater, so that the lag phase behavior would be completely eliminated and the reactions would proceed rapidly in accordance with the rate constants in Table 1. 3.3. MV reaction with six-carbon carbohydrates
concentration at 40 C and pH 12. Several important features are observed. First, at the concentrations of MV and glucose examined, some reactions only begin after 100e500 s, even though all reagents are present. The reaction begins after a time delay in an inverse glucose concentration dependent manner. At higher glucose concentrations, the lag phase is eliminated and the reaction begins immediately. For those reactions with a lag phase, raising the concentrations of both MV and glucose while maintaining the same ratio eliminates the lag phase. Temperature increase also eliminates the lag phase. This unusual rate behavior is likely a consequence of the low concentrations required by the optical method used to follow the rate because of the high extinction coefficients of reduced MV. Under the much higher concentrations (0.10 M or more) of fuel cell operation, the lag phase is completely eliminated and reactions proceed immediately. From the straight-line sections of the uninhibited reactions in Fig. 3 and the linear dependence after the lag phase in the other reactions, a first order dependence on glucose and MV was determined as outlined above with rate constants shown in Table 1. The rate of oxidation of glucose by MV was also examined as a function of pH between 10 and 12. The reaction does not precede below pH 10 but increases rapidly at pH 11 and reaches a maximum rate at pH 12. The reaction was run under identical conditions at 40 and 45 C at pH 11 and 12 using phosphate and carbonate buffers. The reaction was 1.3 times faster in carbonate than in phosphate buffer. This is an important result because carbonate would be a product during fuel cell operation but the results show that there is no inhibition relative to phosphate even in 0.50 M carbonate buffer. Fig. 4 shows the variation in rate for MV oxidation of glucose at pH 12 as a function of temperature. At temperatures below 40 C the rate is very slow. However, the rate increases rapidly between 40 and 50 C. Surprisingly, there is only slight enhancement in rate at 55 C. The results suggest that at pH 12, 50e55 C is the optimum temperature for the MV-glucose reaction for fuel cell use Fig. 5. 3.4. MMV reaction with six-carbon carbohydrates The oxidation of six-carbon glucose using MMV (þ1) is shown in Fig. 6 and was shown also to be first order in glucose and MMV. This reaction can be compared to the same reaction with MV (2þ) in Fig. 3. The reaction of MMV under identical conditions is w3-fold slower than the same reaction with MV. This comparison is of value because it shows the effect that charge and viologen redox potential has on the rate of viologen-catalyzed carbohydrate oxidation. MV has a stronger oxidizing potential (0.46 V) compared
Fig. 3 shows the variation in the rate of glucose oxidation by a fixed concentration of MV as a function of increasing glucose
Fig. 2. The reaction of MMV with MHA at pH 12 and 25 C. MMV was held constant at 12.5 mM and the mM MHA concentration was varied from 1.3 (purple, ), 3.1 (red, -), 6.25 (green, :) and 12.5, A). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. The reaction of 6.25 mM MV as a function of glucose concentration at pH 12 and 40 C. The glucose mM concentration was 1.55 (orange, C), 3.13 (-, red), 6.25, (blue,A), 10.0 (purple, ), 12.5 (green, :), and 25 (blue, U). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3.5. Relative carbohydrate reactivity
Fig. 4. The reaction of 6.25 mM MV and 2.5 mM glucose as a function of temperature. The reaction was conducted at pH 12 in 0.50 M potassium phosphate buffer at temperatures ( C): 40 (purple, ), 45 (green, :), 50 (red, -) and 55 (blue, A). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. The reaction of MV and glucose at 40 C as a function of pH. The concentrations of MV and glucose were held constant at 6.25 mM and the pH was varied from 10.2 (red, -), 10.5 (orange, C), 11 (blue, H), 11.5(blue, þ), 11.8 (pink, -), 12 (blue, A), 13 (green, :). The same reaction but at a MV and glucose concentration of 31.5 mM (purple, ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
to MMV (0.81 V) [6] and when combined with the decrease of one unit of charge it is seen that MV oxidizes carbohydrates faster than MMV under the same conditions as verified by examining the rate constants in Table 1. This same result was observed previously by following these two reactions electrochemically [10], where MMV reacted w2-fold slower than MV Figs. 7 and 8.
Fig. 6. The reaction of MMV as a function of glucose concentration at pH 12 and 40 C. MMV was 6.25 mM and the mM glucose concentrations were: 6.25 (green, :), 12.5 (red, -) and 25.0 (blue, A). The lag phase was 200, 70 and 15 s, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Glucose is a six-carbon aldohexose with a six member cyclic ring structure but converts to a linear form with an aldehyde group on carbon number 1. Fructose is a six-carbon ketohexose that exists in a five-member cyclic ring structure with a keto group on carbon number 2 in its linear structure. Both form ene-diols at pH w10. Initially it was thought that ring opening of the six and five membered rings would determine the relative reactivity of glucose and fructose, respectively. The two reactions were compared at 40 C under identical conditions and as shown in Table 1 fructose reacts at a similar rate as that of glucose. However, ribose is an aldopentose that also forms a five member ring similar to that in fructose but reacts w13 times faster than glucose and fructose. Factors other than simple ring opening must be influencing the overall reactivity. At pH 12 and 25 C the reactions of both MMV and MV with glucose, fructose and ribose range from no reaction for glucose to very slow reaction for fructose and ribose. Under these same conditions, MV reacts with MHA 1.5 times faster than with MMV and both reactions are sufficiently rapid to sustain reactions for a viable fuel cell at this temperature. However, at 40 C and pH 12, MV reacts with glucose 3.3-times faster than MMV and under the same conditions both MMV and MV react with MHA at rates too rapid to measure. Based on the rate constants in Table 1, carbohydrate reactivity with MV at pH 12 and 40 C relative to glucose, follows the decreasing rate of reaction: DHA (w35) > GLY (w32) > MHA (30) > ribose (13) > fructose w glucose (1). In general, MMV follows a similar trend but the rates are 1.5e3.0 times slower. In the legend to Table 1 rate constants of 0.31 and 0.056 M1 s1 are given for the reaction of glucose with MV and MMV at pH 12 and 55 C, respectively. Both increase w7-fold from 40 to 55 C and maintain a difference of w3 in their reactivity toward glucose. Both oxidize glucose at rates compatible with viable fuel cell development (See below). The results suggest that the rates of both types of viologens with carbohydrates consist of contributions from the pKa for enediol formation, the relative reactivity of the resulting ene-diol, stability of the cyclic ring structure (where present), and perhaps other factors not yet identified. The results obtained with glycerol under identical conditions used In Table 1 show that the rate is much slower than with glucose and that while glycerol undergoes reaction with both MV and
Fig. 7. The reaction of 6.25 mM MMV with 2.5 mM glucose at 40 C in 0.50 M potassium phosphate buffer pH 12 (green, :), 0.10 M KOH (blue, A), and 1.0 M KOH (red, -). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. Temperature variation of the reaction of 6.25 mM MMV with 6.25 mM glucose in 0.50 M potassium phosphate buffer pH 12. The reaction temperatures were: 40 (green, :), 45 (red, -), 47 (blue, H), 50 (purple, ) and 55 (blue, A) C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
MMV, the rate and the extent of reaction at 25e50 C are not sufficient to warrant its use in a viologen catalyzed fuel cell. 3.6. Stability and redox potential of MMV compared to MV Consulting the Pourbaix Diagram for glucose and other carbohydrates and MV and MMV in Ref. [13] establishes that the redox potential for glucose oxidation becomes more favorable with increasing pH, while that of MV and MMV remains invariant at 0.46 V and 0.81 V. Consistent with the diagram, the experimental results reported here, show that MV and MMV are effective oxidants of glucose and other carbohydrates only at pH > 10. This is supported by direct measurements using the stronger oxidant benzyl viologen (BV, 0.36 V), which slowly oxidizes glucose at pH 9.0, consistent with its stronger oxidizing ability. While both MV and MMV are theoretically viable for use as carbohydrate fuel cell catalysts, the instability of MV above pH 12 limits its use to the pH range 11e12, whereas MMV is stable and active from pH 11 to 1.0e 3.0 M KOH. 4. Discussion Previous results have demonstrated that viologens effectively catalyze carbohydrate oxidation by oxygen or fuel cell electrodes under alkaline conditions [8e11]. This result provided the impetus for developing a simple fuel cell operating at w40% conversion efficiency of DHA and glucose to carbonate and producing 20e 40 mamp/cm2 using homogeneous MV as catalyst [8]. We report here the rate law for the oxidation of a variety of carbohydrates in alkaline solution using two different viologen types: dialkyl viologens as exemplified by MV and monoalkyl viologens using MMV. Previous reports have discussed the redox potentials, the relative stabilities and the heterogeneous rate constants for these two types of viologens reacting with electrodes [9,10,13]. Table 1 summarizes kinetic results for the viologen-carbohydrate interactions and demonstrates that the rates of carbohydrate oxidation by both monoalkyl and dialkyl viologen catalysts are rapid, demonstrating the feasibility of using viologens as alkaline fuel cell catalysts. During this investigation it was found that the viologen type (dialkyl vs monoalkyl) has distinct advantages and disadvantages for alkaline fuel cell development. When reacted with the same carbohydrate at pH 12 and 40 C, MV reacts 1.5e3.0-fold faster than MMV. As the current flow in a fuel cell depends on the rate at which the carbohydrate is oxidized, a higher current flow would result using MV as a catalyst than with MMV. However, consulting a Pourbaix diagram in Ref. [13] raises another important consideration. Carbohydrate oxidation becomes more favorable at pH > 12, where MV is unstable with respect to conversion to MMV [10,15].
MV would produce a higher current flow but its use is limited to pH 12 or below. These are conditions where the voltage output from carbohydrate oxidation is not optimal. The disadvantage of slower carbohydrate oxidation and consequently lower current flow using MMV as catalyst is countered by increased stability and a more efficient use of the available chemical energy derived from carbohydrate oxidation at high pH. Another advantage of MMV compared to MV is the more favorable redox potential. MMV has a redox potential near 0.81 V, whereas that of MV is 0.46 V [13]. When compared to the redox potential of 1.2 V generated by a carbohydrate at pH 14, the potential of MMV more closely approaches that of the carbohydrate than does MV. The result is that a more efficient transfer of electrochemical energy from carbohydrate to fuel cell electrodes occurs with MMV (70%) than with MV (38%). For comparison, Table 5 in Ref. [13] shows that at pH 12 where the carbohydrate potential is 0.93 V, a voltage transfer from carbohydrate to MMV and MV is 92% and 66%, respectively. At pH 12, MMV is an excellent carbohydrate fuel cell catalyst but to cover a higher pH range with the opportunity to maximize power production from a carbohydrate oxidation, monoalkyl viologen catalysts with redox potentials approaching 1.2 V will be required. We are currently examining methods to move the potential of monoalkyl viologens to be more compatible with the potential generated by carbohydrates above pH 12 in order to more efficiently transfer carbohydrate energy to fuel cell electrodes. If successful and if the rates of carbohydrate oxidation by these enhanced monoalkyl viologens are rapid, they can be used to maximize carbohydrate fuel cell performance at high pH. Fig. 4 shows that the rate of glucose oxidation by MV and MMV increases 7-fold from 40 to 55 C and reaches a rate sufficient for fuel cell use. This can be shown by using rate law (1), with k ¼ 0.31 M1 s1 for MV reacting with glucose at 55 C from Table 1 and assuming MV and glucose concentrations of 1.0 M. Under these conditions, electrons are removed from glucose at a rate of 0.31 M/ s. If this reactivity occurs in 1.0 L of solution on a 1.0 cm2 electrode, with a solution column of 1.0 cm, a current density of 30 mamp/cm2 is calculated vs 29 mamp/cm2 previously measured in a simple fuel cell [1]. Under the same conditions, MMV would produce a current density w30% that of MV. Table 1 indicates that the homogeneous rate constants for carbohydrate oxidation by viologens span a range of values depending on the carbohydrate type and conditions but are similar to the heterogeneous electron-transfer rate constants (kw0.20 cm/s)for electron transfer from viologen to electrodes [13,15]. This useful approximation suggests that both the heterogeneous and homogeneous rates are favorable for utilizing carbohydrates as available and renewable fuels for electrical production via an alkaline carbohydrate fuel cell. It is important to emphasize that the above calculation only estimates the rate of electron production at the anode using homogeneous MV as catalyst and the least reactive carbohydrate glucose as a fuel source. In actual fuel cell operation, the reduction of oxygen at the cathode and ion transport through the separating membrane and other factors also contribute to fuel cell performance. However, a >35-fold increase in anodic fuel cell output compared to glucose could be realized by using DHA and GLY, both of which are derived from abundant glycerol produced as a byproduct from biodiesel production. The above considerations have focused on utilizing homogeneous viologens for carbohydrate oxidation with subsequent electron transfer to fuel cell electrodes. A significant improvement in fuel cell performance can be anticipated using immobilized viologens on electrodes. This process not only minimizes viologen diffusion processes but enhances viologen-carbohydrate interaction with an accompanying increase in carbohydrate oxidation efficiency and enhanced electron transfer to fuel cell electrodes. This
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enhanced reactivity was previously demonstrated by using viologen polymers, which increased carbohydrate oxidation efficiency by w25% [9]. Electrodes containing immobilized viologens are currently being investigated for enhanced fuel cell operation and their performance will be reported later. Acknowledgments The Department of Chemistry and Biochemistry and the Office of Creative Research at Brigham Young University supported this research. References [1] Kamat PV. Meeting the clean energy demand: nanostructure architectures for solar energy conversion. J Phys C 2007;111:2834e60. [2] Campbell CJ. Petroleum and people. Popul Environ 2002;24:193e207. [3] Dresselhaus MS, Thomas IL. Alternative energy technologies. Nature 2001;414:332e7. [4] Lichtenthaler FW, Peters S. Carbohydrates as green raw materials for the chemical industry. CR Chim 2004;7:65e90. [5] Field CB, Campbell JE, Lobell DB. Biomass energy: the scale of the potential resource. Trends Ecol Evol 2007;23:65e72.
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