A New Future for Carbohydrate Fuel Cells

Renewable Energy 72 (2014) 99e104

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Renewable Energy

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Review A new future for carbohydrate fuel cells

G.D. Watt

Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA article info abstract

Article history: The development of renewable energy sources to reduce our dependence on limiting fossil fuel reserves Received 23 August 2013 continues to be a critical research initiative. Utilizing the abundant high energy content of carbohydrates Accepted 16 June 2014 contained in biomass (cellulose and hemicellulose) must be considered to be an important contribution Available online 18 July 2014 to our overall energy budget. Carbohydrate-derived furan-based liquid fuels and especially ethanol are becoming important added components forming gasoline blends to lower overall fossil fuel use. Alter- Keywords: nate renewable energy processes that more efﬁciently use the carbohydrate energy content are desirable Renewable energy and would lower the overall carbohydrate input requirement for energy production. Recently, new Alkaline carbohydrate fuel cells ﬁ Electricity from biomass catalysts have shown the feasibility of ef ciently transporting the 24 electrons in glucose to fuel cell Viologen catalysis electrodes making possible the direct conversion of the stored energy in carbohydrates into electricity with the benign formation of carbonate and water as products. The conversion of glycerol, a byproduct of biodiesel production, into three-carbon carbohydrates provides another opportunity to produce electricity from an abundant carbohydrate source. New developments in catalyst systems promise to make carbohydrate fuel cells an important part of future energy strategies. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The emerging energy contributions from wind farms, photo- voltaic cells for direct electricity production, and other ongoing The United States in particular and the world in general, do not uses of solar energy are making important contribution to our suffer from a shortage of energy. The problem has been our narrow overall energy demands and will become more important as historical focus on using the convenient but finite reserves of fossil technical developments lower costs and improve efficiency. How- fuels on an ever-larger scale to meet our increasing industrial and ever, these processes currently suffer from limited scale and transportation energy needs. Energy shortages arose in the 1970s intermittency, which results when there is no energy production on when petroleum-based products became in short supply due to windless and overcast days and especially at nighttime, when solar market limitations, thereby interrupting and temporarily slowing energy is absent. These latter problems require the development of worldwide commerce. From these initial events, petroleum-supply large, convenient and low-cost methods to store excess electrical problems have continued on an almost decade-by-decade time energy when resources are available, and releasing it when they are frame and the urgency to develop the earth's other ample resources not [4]. is now fully recognized. The development of new energy sources requires that new The enormous scale of current fossil fuel use is now associated technologies be developed to exploit them. The important question with climate problems, environmental degradation, and geopolit- then becomes “What abundant, renewable and sustainable energy ical problems, which in turn create market instability and greater resource is available on the enormous scale needed to replace the competition for remaining fossil fuel reserves [1e3]. The recent vast and reliable, but dwindling, fossil fuel reserves”? catastrophic crisis in the nuclear power industry in Japan and its Biomass has the potential to become both a viable short- and worldwide repercussions threaten to decrease energy production long-term resource to replace fossil fuels during the transition from from this important, reliable and well-integrated energy source and fossil fuel use to alternative energy sources [5,6]. Biomass is an adds impetus to the need to develop alternative energy sources. extensive and almost endlessly renewable resource that is formed daily by converting CO2 and H2O into stable and harvestable, high- energy organic molecules through photosynthesis. This natural 0 0 Abbreviations: MV, oxidized methyl viologen (1, 1 -dimethyl-4,4 -bipyridine); process represents a large, stable “storable source” of solar energy. MVr, reduced methyl viologen; MMV, monomethyl viologen; SHE, standard Glucose and other carbohydrates, in the form of free sugars, cel- hydrogen electrode; Wh/kg, Watt-hours/kilogram. E-mail address: [email protected]. lulose and hemi cellulose, account for up to 70% of the organic http://dx.doi.org/10.1016/j.renene.2014.06.025 0960-1481/© 2014 Elsevier Ltd. All rights reserved. 100 G.D. Watt / Renewable Energy 72 (2014) 99e104 content of biomass [7,8]. Glucose and other hexoses (six carbon Unfortunately, up to the present time, only very low electrical po- carbohydrates) have a high-energy content of 4420 Wh/kg and the wer production has been reported. The prospect of deriving elec- still plentiful pentoses, tetroses, and trioses have abundant but trical energy directly from carbohydrates is important and is the proportionately less energy. Biomass is therefore a resource that focus of the remaining discussion. rivals fossil fuels on the enormous scale available that is capable of providing the appropriate high-energy content that is needed to meet present and predicted energy needs [5e8]. Given this po- 3. Electrical power from carbohydrate fuel cells tential for providing large scale energy for societies needs, what process or processes are the most advantageous to exploit this How is electricity produced from carbohydrates? One approach fi resource in a sustainable manner? is to blend biomass at 15% or more with coal in coal- red power The possibility of using carbohydrates from biomass for trans- plants [15,16]. While this approach is feasible, decreases coal use portation energy and for electrical energy production has been and lowers overall greenhouse gas emissions, it is not the most fi considered for decades [5e8]. Although progress is being made in ef cient use of carbohydrate energy. A more promising approach several areas of renewable energy there remains a need for further was reported more than 60 years ago when electricity was pro- effective and low-cost catalyst systems to be developed to carry out duced from glucose using a simple Pt-based fuel cell [6]. the complex chemical and/or electrochemical conversions A fuel cell is an electrochemical device that catalytically removes required. However, this important objective continues to attract electrons from an energy-rich fuel source at one electrode interest and stimulate efforts toward its development. Two general compartment (the anode) and transfers them to a second electrode research efforts have been and are being actively explored for compartment (the cathode), where they combine with an oxidizer, converting the energy content of carbohydrates into convenient which is typically oxygen [17]. Fig. 1 shows this process in the form energy sources to meet society's needs. of a schematic diagram. The combination of two half-cell reactions produces a cell voltage equivalent to the sum of the two half-cell 2. Transportation fuels potentials and generates an electrical current in the external circuit. The two half-cell reactions for glucose and oxygen (Reactions e The direct formation of high-energy liquid fuels from carbohy- 1 2, pH 12) show that oxidation of the glucose fuel by oxygen drates with sufficient energy content, volatility, and stability for produces a cell voltage of 1.45 V. Catalysts for both half-cell re- fi safe use in internal combustion engines is a promising research actions are critical for smooth and ef cient fuel-cell operation objective to supplement or replace gasoline as a transportation fuel. [17,18]. Preliminary and important steps have been taken and processes þ ¼ 2 þ þ have been reported that produce high-energy, furan-based liquid Anodic Reaction C6H12O6 36 OH 6CO3 24 H2O 24 e fuels [9e11]. Conversion efficiencies of ~ 50e80% have been re- E ¼ 0:93 V ported, and overall useful fuels are emerging. While promising, (1) better catalysts and more efficient second-generation processes are needed to increase efficiency, produce greater volume, and lower Cathodic Reaction 6O2 þ 24 e þ 12 H2O ¼ 24 OH E ¼ 0:52 V costs to more competitive levels. (2) Ethanol production now accounts for 90% of carbohydrate- derived liquid fuels and is used to supplement or even replace þ þ ¼ 2 þ current petroleum-based fuels [12,13]. However, more than 50% of Overall Reaction C6H12O6 6O2 12 OH 6CO3 12 H2O the original energy content of glucose is lost during the conversion E ¼ 1:45 V to and separation of ethanol from the dilute fermentation mixture (3) [12,13]. Until glucose and other carbohydrates are easily separated from the cellulosic and hemi cellulosic content of biomass and efficiently utilized, the production of ethanol will likely continue to require subsidies and rely on agriculturally-derived sugars and starches that compete with food supplies. The previous discussion regarding liquid fuels has presented the possibility, desirability and promise of large-scale energy production of liquid transportation fuels from energy rich and renewable carbohydrates derived from biomass. However, even if this objective is successful, the inefficient utilization of the initial energy content of the parent carbohydrate remains an important problem. This has been documented for ethanol use, where only 10e15% of the original carbohydrate energy content is actually utilized in vehicle propulsion [14]. An important recent assessment of bio- energy use concludes [14e16] that using biomass to power electric automobiles is three times more efficient and more environmental compatible than using ethanol for this purpose. More efficient energy use of the initial carbohydrate energy content is clearly desirable and when achieved will actually lower the amount of carbohydrate resources needed. Fig. 1. Schematic representation of a carbohydrate fuel cell. The anode compartment Carbohydrates have not been used for electrical energy pro- on the left receives the carbohydrate fuel to be oxidized. This compartment contains duction because effective and low-cost catalyst systems have not the viologen catalyst in contact with the anodic current collecting electrode. The cathodic compartment on the right receives electrons removed from the fuel and been developed to carry out the required complex electrochemical catalytically transfers them to oxygen forming OH . The semi-permeable membrane conversions. However, this important objective continues to attract separating the electrochemical compartments allows for transfer of OH to facilitate interest and to stimulate efforts toward its development [17,18]. reaction and maintain ionic balance. G.D. Watt / Renewable Energy 72 (2014) 99e104 101

Ongoing studies continue to describe fuel-cell systems of provides the basis for evaluating the advantages and disadvantages various types for carbohydrate oxidation using a wide variety of of these two processes. The Pourbauix diagram conveniently sum- noble metal catalysts and metal alloy composites [19e26]. Three marizes the variation in redox potentials as a function of pH for the major problems have impeded their development: 1) incomplete various half-cell reactions involved in a carbohydrate fuel cell. fuel oxidation limits overall power production. The reaction is typically glucose / gluconic acid, with only two of the twenty-four 5.1. Vanadium catalyzed carbohydrate oxidation at low pH electrons initially present being used for power generation; 2) poisoning of the metallic catalyst by-products of carbohydrate The first of these processes operates at low pH using a vana- oxidation (CO and other organic compounds), which terminates diumeplatinum catalytic system, which readily converts carbohy- catalyst function and 3) the high cost and low availability of the drates to CO2 and H2O with generation of 0.66 V open-circuit noble metals and metallic components used as catalysts, which potential and operating at 54% efficiency [34]. This fuel-cell system forms economic barriers to their use. has several important advantages. First, the vanadium catalyst is The first of these problems results in electrical energy production inexpensive and readily available. Second, complete oxidation of the of only ~ 8% (2 electrons of the 24 available) of the electrical energy carbohydrate to CO2 and water occurs. Third, the kinetics of the fuel- theoretically available from carbohydrate oxidation. Catalysts that cell reactions is rapid. However, a costly co-catalyst of platinum black more effectively catalyze complete electron removal (oxidation) is required and at low pH, CO2 is released; bubbles could potentially from the carbohydrate reservoir are needed, but development has interfere with the electrochemical reactions and disrupt smooth fuel- been very slow. The second problem is being addressed by forming cell operation. The energy efficiency of this fuel cell is higher (54%) metallic composites and formulations to prevent catalyst inhibition than using bio mass energy for electrical production via the thermal and corrosion by undesirable side reactions [24,26]. Nano- route (35e40%), because Carnot Cycle losses are avoided. structured materials are also being investigated [26], but catalyst poisoning and degradation continue to be serious challenges. Only 5.2. Viologen catalyzed carbohydrate oxidation at alkaline pH finding efficient, abundant and less expensive carbohydrate- oxidation catalysts can solve the third problem. While efforts to The viologen catalyst functions as a mediator that transfers further advance this traditional and valuable carbohydrate oxidation electrons from the carbohydrate to the fuel cell electrode according approach continues, two new paradigms in carbohydrate oxidation to Reaction (1). The simplest viologen is dimethyl viologen (1,10- catalysts and fuel cell development are showing promise. dimethyl-4, 40-dipyridyl, MV), shown in Fig. 3.MVfirst is reduced by carbohydrate to MVr by a one-electron energy transfer step. The 4. Biofuel cells MV redox potential for this process is 460 mV vs the standard hydrogen electrode (SHE) and is pH independent, as shown in Biofuel cells follow traditional fuel-cell design concepts of Fig. 2. MVr is an intensely blue (Lmax ¼ 603 nm), stable free combining both anodic and cathodic electrochemical half-cells [17]. radical, which is readily oxidized by O2 and requires anaerobic Noble metals with their present limitations are not necessary and condition for its formation. MVr also readily transfers electrons to are replaced by biological catalysts consisting of immobilized en- metallic electrodes to form MV, making it ideal as a mediator for zymes or whole microorganisms [27]. Electricity is generated by carbohydrate fuel cells [35,36]. Reaction (4) shows the complete oxidation of such biofuels as glucose, glycerol or ethanol, coupled to 2 oxidation of glucose by MV to form MVr, CO3 and H2O during fuel the reduction of oxygen to water. Biofuel cells overcome the three cell operation. MVr then transfers a single electron to the current- problems outlined above by completely oxidizing carbohydrates collecting electrode of the fuel cell, reforming MV by Reaction (5). and other organic molecules to CO2 and H2O, possessing high specificity for the organic fuel used and by using catalysts that are þ þ ¼ 2 þ þ C6H12O6 24MVo 36OH 6CO3 24H2O 24MVr (4) renewable at relatively low cost. While overall successful, biofuel cells suffer from two serious limitations. The catalysts used are derived from biological sources and are easily inactivated under ambient conditions and consequently have short catalyst lifetimes. The more serious problem is that they have limited reaction capacity yielding low power production [27e30]. While power production is low it is sufficient for low power applications, such as implanted medical devices and small electronic equipment [31], but it cannot compete in high- power applications.

5. New generation of carbohydrate fuel-cell cells and catalysts

The desired catalyst for high power fuel cell application is one that efficiently catalyzes Reaction 1 and related reactions, is not readily inhibited or degraded, is abundant and inexpensive, is easily handled, is non-toxic and is environmentally compatible. These are demanding constraints, but if fulfilled they could lead to a new era in carbohydrate fuel cell development and provide processes to efficiently utilize the vast stored energy of carbohydrates for both small- and large-scale electricity generations. Fig. 2. The Pourbauix Diagram describes the variation of half-cell potential with increasing pH for various half-cell reactions involved in carbohydrate fuel cell oxida- Recently, two carbohydrate fuel-cell systems operating at tion and oxygen reduction. MV is the dialkyl viologen and MMV is the mono alkyl different pH conditions and using different catalytic approaches have substituted viologen. The diagram shows that the respective half-cell potentials of both been reported that meet many of the above criteria [32e34]. Fig. 2 viologens are pH independent. 102 G.D. Watt / Renewable Energy 72 (2014) 99e104

Fig. 3. A schematic representation of the structure of oxidized methyl viologen (MVo) on the left and a one-electron reduction to form the stable, free radical reduced form (MVr) on the right. MVr then transfers the electron and the accompanying electrochemical energy to the anodic current collecting electrode.

24 MVr ¼ 24 MVo þ 24 e ðelectrodeÞ (5) Fig. 4. The proposed mechanism for the stepwise, four-electron oxidation of carbon number 1 of each successive carbohydrate as catalyzed by the viologen catalyst [39]. Carbonate is released and the four electrons released from each step form four MVr, 6O2 þ 24 e þ 12 H2O ¼ 24 OH (6) which then transfers their electrons to the anodic electrode. In the last step 8e are produced and produce 8 MVr. þ þ ¼ 2 þ C6H12O6 6O2 12 OH 6CO3 12H2O (7) The resulting electrons flow through the external circuit to the catalytically oxidized [32]. Alcohols, aldehydes and carboxylic acids cathode and reduce O2 to OH according to Reaction (6) to com- were not oxidized, but significantly, none of these latter molecules plete the overall reaction sequence of glucose oxidation. Reaction inhibited the catalytic reaction. These initial studies also estab- (4) shows that 36 OH are required in the anodic half-cell reaction, lished that at MV/carbohydrate ratio of 1.0 carbohydrate oxidation but 24 are then produced by Reaction (6) at the cathode. This gives efficiency was 30e50% but at a ratio of 10, the efficiency increased a net requirement of 12 OH per glucose, which is required for the to >80%. Oxidation efficiency is defined as the number of electrons overall Reaction (7). Fig. 2 shows that as the pH increases, the actually transferred to O2 or fuel cell electrodes divided by the anodic carbohydrate half-cell potential increases, as does the number initially available in the carbohydrate fuel. This unusual cathodic reduction of O2 to OH . The combined effect of increasing behavior was explained considering Reaction (4), which shows that pH increases the energy production from an alkaline carbohydrate 24 MV are required to oxidize a single glucose molecule. At a ratio fuel cell. Open circuit potentials of 1.45 and 1.75 V are predicted at of 1.0, MV must cycle 24 times to completely oxidize each glucose pH 12 and 14, respectively. molecule. If the rate of MVr oxidation to MV becomes limiting, glucose is incompletely oxidized and a lower overall oxidation ef- 6. Mechanism of carbohydrate oxidation ficiency results. More MV present at higher ratios facilitates efficient carbohydrate oxidation and minimizes formation of The mechanism of glucose oxidation by viologen catalysis was unreactive intermediates. This conclusion was verified using viol- reported [37] to consist first of oxidation of the C1 carbon atom of ogen polymers, which effectively produce a higher localized con- the linear form of the carbohydrate to carbonate with subsequent centration of viologen monomer units and resulted in a >25% formation of the next carbohydrate one-carbon atom shorter. This enhancement of carbohydrate oxidation relative to an equivalent cascading process, summarized in Fig. 4, then continues through homogeneous monomer concentration [32]. These results suggest the successive carbohydrate chain until carbonate is formed in the that electrodes densely covered with immobilized viologen cata- last oxidation step. Shorter chain length carbohydrates can enter lysts would not only facilitate more complete glucose oxidation and this reaction cascade at any point and be fully oxidized. During minimize byproduct formation, but the viologen layer would actual viologen-catalyzed carbohydrate oxidation, the last step was transfer electrons directly to the fuel cell electrode, thereby mini- found to produce both carbonate and formate in about equal mizing diffusion processes. amounts, instead of the idealized reaction shown in Fig. 4.In Dialkyl viologens are valuable catalysts for carbohydrate fuel- e contrast to Reaction sequence 4 7, the released CO2 in alkaline cell operation but two critical shortcomings were recognized that solution is converted to soluble carbonate and remains in solution, limit their use in further alkaline fuel-cell development at higher thereby preventing bubble formation. An important observation is pH values. The midpoint potentials of dialkyl viologens are in the that reaction sequence 4e7 continues without interruption even in range of 460 to 550 mV [36] and the potential generated by 1 M carbonate, demonstrating that viologen catalysis is not reaction (4) is 0.93 V at pH 12 (Fig. 2). With dialkyl viologens as inhibited by product formation [32]. It is important to note that this catalysts, the electrode experiences the potential of the mediating mechanism applies to all carbohydrates examined so far and es- viologen (~0.50 V) and not that produced by the carbohydrate tablishes that electrical energy can be obtained from a wide di- (0.93 V, pH 12). This results in a potential drop of ~0.43 V, with a versity of carbohydrates present in both cellulose and corresponding decrease in the energy obtainable from carbohy- hemicellulose derived from biomass. Initial reports [32] demon- drate oxidation. A closer alignment of viologen and carbohydrate strated the utility of dialkyl viologens as carbohydrate catalysts at potentials would enhance energy recovery. Synthetic methods for pH 10e12 but subsequent studies have established two limitations altering dialkyl viologen structures that move their potentials associated with their use [38]. closer to that of the carbohydrate are known [35,36] and theoretical methods for modifying the viologen potentials are currently being 7. Versatility and limitations of dialkyl viologen catalysts investigated. These efforts should provide viologen catalysts with potentials that fit closer to that of the carbohydrate and provide Initial studies using dialkyl viologens established that carbohy- more efficient energy transfer for future alkaline fuel cell drates ranging from three to six carbon atoms were easily improvement. G.D. Watt / Renewable Energy 72 (2014) 99e104 103

The second problem is more serious and requires a fundamental production from carbohydrate fuel cells is clearly a more efﬁcient change to the catalyst. Dialkyl viologens function well at pH 10e12, and less competitive use of carbohydrate energy than is ethanol but at pH > 12, they become unstable and slowly release one alkyl production. substituent to form monoalkyl viologens. While this chemical An important ongoing research objective [2] is to obtain glucose behavior limits the viability of dialkyl viologens to the pH range and other carbohydrates from cellulose and hemicellulose hydro- 10e12 and prevents the more efﬁcient use of carbohydrate energy lysis, thereby minimizing the problem of competition between obtainable at higher pH values (Fig. 2), this problem presents other glucose use for energy production and its use in food products. opportunities. Until this objective is reached, continued focus on maximizing electrical energy production from abundant and diverse carbohy- 8. Mono alkyl viologens as carbohydrate fuel-cell catalysts drates is an important research effort.

Hydrolysis of dialkyl viologens in 1.0 M KOH quantitatively 10. Direct utilization of carbohydrate energy produces monoalkyl viologens [37,38]. Addition of carbohydrate to the hydrolysis mixture resulted in catalytic oxidation of carbohy- Assuming 70% fuel-cell conversion efficiency, a kg of food drate by the newly formed monoalkyl viologens [40]. Monoalkyl quality glucose costing $610/metric ton can produce 3200 kWh of viologens present an advantage and a disadvantage compared to electricity. This translates to $0.20/kwh, which is comparable to their dialkyl analogs. The disadvantage is that the rate of carbo- typical electricity costs. The cost of charging the batteries of an hydrate oxidation by monoalkyl viologens is only 50% that of the electrical vehicle using glucose fuel cell energy would be only dialkyl viologens [37,38]. This in part is due to the advantage that slightly higher than using conventional electrical sources and accrues from the properties of the monoalkyl viologens. The po- would decrease with the cost of glucose. Glucose and other car- tential of monoalkyl viologens at pH 12 is ~0.81 V [41], much bohydrate fuel sources derived from the cellulosic and hemi- closer to that generated by the carbohydrate. This more favorable cellulosic content of biomass could significantly reduce the costs, potential consequently presents an opportunity for more efficient making carbohydrate-derived electricity very competitive. transfer of energy from the carbohydrate to the fuel-cell electrode. The use of glycerol in electricity production could lower elec- At pH 12, the energy transfer efficiency from glucose to mono- trical costs even further because of its low commodity price. methyl viologen is 0.81/0.93 ¼ 0.87, compared to 0.50/0.93 ¼ 0.54 Glycerol prices are volatile but are now near $0.11/kg and are for dialkyl viologens. The slower rate is probably a consequence of edging lower with increasing biodiesel production. Glyceraldehyde the smaller driving force that results from the two potentials being and dihydroxy acetone are three-carbon carbohydrates readily closer together. Another factor possibly slowing the rate of reaction derived from glycerol, but they contain only half of the energy of is that the dialkyl viologens have two positively charged quaternary glucose at 2210 kwh/kg. However, both are more efficiently used nitrogen reaction centers (Fig. 2), but the monoalkyl viologens have with the viologen catalyst because they readily undergo reaction at only one. Overall, the more favorable redox potential, combined room temperature and even at 10 C [37]. Using glyceraldehyde and with the greater stability in strong base, makes the monoalkyl dihydroxy acetone derived from glycerol, assuming electricity viologens more efficient catalysts for carbohydrate fuel cell oper- production from a fuel cell at 70% efficiency, yields electricity at ation, in spite of their slower kinetics. $0.071/kwh, well below current electricity prices. The continual decrease in the price of glycerol, coupled to mandated biodiesel 9. Carbohydrate fuels production, should lower the price of electricity and make energy from this source very competitive and sustainable. Significant Glucose currently is the most viable carbohydrate for fuel cell technological advances must be made to arrive at this more effi- use because of its high energy content and abundance but all car- cient use of glucose energy (and that of carbohydrates in general) bohydrates that have been examined undergo catalytic oxidation by alkaline fuel-cell technology but the promise is there and the by dialkyl or monoalkyl viologens under basic conditions [32,40]. promising fuel-cell technology to attain this goal has been Biodiesel manufacturing is mandated both in Europe and the demonstrated [32,37,40]. United States to increase significantly in the next decade. Glycerol is produced as an abundant byproduct (~ 0.11 kg/L of biodiesel) that 11. Summary currently creates glycerol overproduction, which in turn leads to a fall in glycerol prices and subsequent expensive disposal issues Carbohydrates derived from biomass are abundant, renewable [42e44]. Glycerol is easily converted to glyceraldehyde and di- and and have high energy content. Direct utilization of the energy monohydroxy acetone [45,46], and these three carbohydrates content of carbohydrates for electrical energy production via fuel readily produce electricity using a viologen-catalyzed fuel cell cell technology is a more efficient process than conversion to liquid [32,40]. Energy production from diverse carbohydrate derived from organic fuels with subsequent use in low efficiency internal com- cellulose and himicellulose has the distinct advantage of utilizing a bustion engines. Combustion of liquid fuels derived from carbo- wide range of carbohydrates with very favorable overall energy hydrates in internal combustion engines only utilizes 10e15% of the production efficiency. initial carbohydrate energy, whereas conversion into electricity via The conversion of glucose to ethanol is a successful industrial- fuel cells and use with electric engines could increase the carbo- ized process that produces ~50 billion liters of ethanol annually. hydrate energy use efficiency 3e5-fold. This would significantly However, the organisms used in the fermentation process are very reduce the use of agriculturally derived carbohydrates. However, specific for ethanol production from glucose, and they are not utilization of carbohydrates derived from cellulose and hemicel- currently viable for use with the wide variety of other abundant lulose for electrical energy production provides a greater oppor- carbohydrates derived from hemicellulose. 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