Utilization of solar energy for continuous bioethanol production for energy applications

Article abstract of DOI:10.1039/c6ra00389c

The focus of the present research is to develop energy-efficient, sustainable, and continuous-flow bioethanol production based on solar energy. Solid-state fermentation of glucose was performed in a specially designed solar-energy-driven continuous flow reactor. Aqueous glucose solutions of 10 and 20 wt% were fed into the reactor bed containing baker's yeast (Saccharomyces cerevisiae), resulting in 4.7 and 8.7 wt% ethanol yields, respectively. The bioethanol produced was separated from the yeast bed soon after its formation by an evaporation-condensation process. High ethanol yields (91.2 and 85.5% of the theoretical yield, respectively) indicate the atom-efficiency of the process. No loss in the activity of yeast was observed even after two months of continuous operation of the reactor. The current study demonstrates an energy-efficient methodology for bioethanol production utilizing solar energy. The bioethanol obtained (8.7 wt%, ca. 2 M) was further tested in alkaline-acid direct ethanol fuel cells operated at 303 K, resulting in a power density value of 330 mW cm^-2 at a modest open circuit voltage value of 1.65 V (65.5% voltage efficiency).

Full text of DOI:10.1039/c6ra00389c

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Utilization of solar energy for continuous bioethanol production forView Article Online
DOI: 10.1039/C6RA00389C
energy applications
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Betina Tabah[a], Indra Neel Pulidindi[a], Venkateswara Rao Chitturi[c], Leela Mohana
Reddy Arava[c], and Aharon Gedanken* [a, b]
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[a]Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
[b]Department of Materials Science and Engineering, National Cheng Kung
University, Tainan 70101, Taiwan
[c]Department of Mechanical Engineering, Wayne State University, Detroit, MI
48202, USA
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*Corresponding author:
Tel: +972-3-5318315; Fax: +972-3-7384053; e-mail: gedanken@mail.biu.ac.il
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Solar-energy driven solid-state fermentation was developed for continuous bioethanol
production from glucose. Bioethanol was tested in alkaline-acid direct ethanol fuel
cells for its potential as fuel.
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Abstract
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The focus of the present research is to develop energy efficient, sustainable, and
continuous flow bioethanol production based on solar energy. Solid-state fermentation
of glucose was performed in a specially designed solar-energy driven continuous flow
reactor. Aqueous glucose solutions of 10 and 20 wt.% were fed into the reactor bed
containing the baker's yeast (Saccharomyces cerevisiae), resulting in 4.7 and 8.7 wt.%
ethanol yields, respectively. The bioethanol produced was separated from the yeast
bed soon after its formation by an evaporation-condensation process. High ethanol
yields (91.2 and 85.5% of the theoretical yield, respectively) indicate the atom
efficiency of the process. No loss in the activity of yeast was observed even after two
months of continuous operation of the reactor. The current study demonstrates an
energy efficient methodology for bioethanol production utilizing the solar energy. The
bioethanol obtained (8.7 wt.%, ca. 2 M) was further tested in alkaline-acid direct
ethanol fuel cells operated at 303 K, resulting in a power density value of 330
mW/cm2 at a modest open circuit voltage value of 1.65 V (65.5% voltage efficiency).
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Keywords: Biofuel; solar reactor; solid-state fermentation; Saccharomyces
cerevisiae; direct alcohol fuel cells
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Introduction
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Bioethanol production is important not only because it could be blended with gasoline
but also it could be a feedstock for the production of C2 hydrocarbons. In near future,
biofuels, especially bioethanol and biobutanol, could form the vital feedstock for long
chain hydrocarbons and also for a variety of biochemicals that are currently solely
being produced from oil.1-2 The potential global production of bioethanol from crop
wastes and crop residues is estimated at 491 GL year-1 which could replace 32% of
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total gasoline consumption. Bioethanol, because of its high gravimetric energy
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density (30 MJ/kg), high octane value, and high combustion efficiency (the anti-knock
index for gasoline: 87, for ethanol: 99), is one of the most promising alternatives to
conventional transportation fuels.4-5 In addition, the use of ethanol produced from
biomass as a transport fuel can reduce CO2 buildup.6 Bioethanol can be blended with
petrol or used as neat alcohol in dedicated engines and it is excellent fuel for advanced
future flexible-fuel hybrid vehicles.7 In comparison to gasoline, ethanol contains only
a trace of sulfur; therefore, ethanol blended with gasoline helps to decrease the overall
emission of SOx.8 Global annual bioethanol production reached nearly 115 billion
liters in 2015.9 Hence, breakthroughs in bioethanol production technologies are
beneficial for the socio-economic well-being of humankind.
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Bioethanol production, using various substrates by free or immobilized cells of
bacteria (Clostridium sp.) or yeasts (Saccharomyces sp., Zymomonas sp.), has been
intensively studied over the past two decades. Fermentation using immobilized cells
prevents substrate inhibition. In immobilized systems, the use of higher
concentrations of carbohydrates is feasible and the recovery of the biocatalysts is
simple; therefore, the biocatalysts can be reused for many fermentation cycles.10
Saccharomyces cerevisiae is a facultative anaerobe able to live on various fermentable
and non-fermentable carbon sources. When it is grown on fermentable substrates,
such as glucose, the metabolic energy originates from glycolysis.11 Additionally, S.
cerevisiae is the most effective ethanol-producing microorganism for hexose sugars
including glucose, mannose, and galactose. S. cerevisiae is a yeast with high ethanol
productivity, high tolerance to ethanol and to the inhibitory compounds present in the
hydrolysate of lignocellulosic biomass.12
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A common problem in bioethanol production is the separation of
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microorganisms from the broth. Solid-state fermentation is the fermentation involving
solids in absence (or near absence) of free water. However, substrates must have
enough moisture to support growth and the metabolism of microorganisms. Solid-
state fermentation, with its low energy requirements, produces less wastewater and is
environment-friendly. In addition, in solid-state fermentation, microbial cultures are
closer to their natural habitats and it is easy to separate them from the broth.13
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Solar energy is a sustainable energy source and exploiting it for biofuels
production has economic and environmental advantages. Thus, the use of alternate
green, sustainable, and renewable resources such as biomass and solar energy could
be a solution to meet the growing energy demands. To the best of our knowledge, we
are the first to utilize solar energy for bioethanol production.14 The objective of the
present study is to develop a continuous flow solid-state glucose fermentation process
driven by solar energy for production of bioethanol in a specially-designed reactor.
The special design of the reactor facilitated in situ separation of ethanol from the yeast
bed by evaporation-condensation process. Moreover, the bioethanol produced was
demonstrated as a potential fuel for operating direct ethanol fuel cells.
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Experimental
The solar reactor
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The solar reactor was designed and fabricated to perform solid-state glucose
fermentation (flow system) and to continuously separate the aqueous ethanol solution
from the yeast bed by an in situ evaporation-condensation mechanism (Fig. 1). Two
aspects, namely, the geometry and the dimensions were crucial in the operation of the
reactor for effective production as well as separation of ethanol. The reactor was
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fabricated using aluminum blocks so that it was lightweight and non-corrosive. A
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nearly right-angle triangular geometry (with 275 mm base and 127 mm height) was
selected. The height of the reactor was kept much lower than the base to facilitate the
condensation of ethanol vapor (from the first chamber) onto the top glass surface.
Such geometry facilitated free flow of the condensate from the top surface of the
reactor to the second chamber, where ethanol was collected (See ESI, Fig. S1 for a
detailed design and depiction of the components of the solar reactor). Tabah et al.
previously reported the principle of operation of the reactor.14
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Solid-state glucose fermentation
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Glucose (D-Glucose, product no. G8270, Sigma-Aldrich, Israel) solution reservoir (2
L, 10 or 20 wt.%), was connected to the solar fermentation reactor (Fig. 1a). The first
chamber of the reactor had 75 g of Saccharomyces cerevisiae (instant baker's yeast
purchased from the local supermarket) covered with activated carbon cloth (Kynol®,
90 g/m2, 0.43 mm thickness, >1800 m2/g specific surface area) (Fig. 1b) into which
the glucose solution was continuously fed (2.8 mL/h flow rate). The yeast was not
supplemented with any other additional nutrients and the pH of the glucose solutions
was 7. Fermentation took place in this chamber and the ethanol produced evaporated
to the top flat glass surface of the reactor, which allowed the solar radiation into the
bed. The ethanol droplets that condensed on the glass plate were collected in the
second chamber of the reactor, which had an outlet for ethanol collection (see ESI,
Fig. S1 for the design of the solar reactor).
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1H and 13C NMR analyses
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Aliquots were collected from the reactor (ethanol outlet) at regular time intervals and
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analyzed by H and C NMR spectroscopy. D2O was used as a solvent and spectra
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were recorded on Bruker Avance DPX 300 at room temperature. HCOONa (97%,
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Product No. 107603, Sigma Aldrich, Israel) was used as an internal standard in H
NMR spectroscopy for quantification of ethanol.14,15
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Gas chromatography (GC) analysis
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The quantification of ethanol present in the product was also performed by gas
chromatography (Gas chromatograph Varian 3900). The GC was equipped with a
Varian Chrompack capillary column (25 m x 0.63 mm x 10 m) and a flame
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ionization detector (FID). The detector temperature was fixed at 200 C and helium
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was used as a carrier gas. The initial oven temperature was 80 C for 0.5 min,
reaching 160 oC with a heating rate of 20 oC/min and remaining at this temperature for
10 min. The column oven end time was 14.5 min. The injection temperature was 140
oC (with a split ratio of 50 and column flow 2.0 mL/min). 10 L of sample was
injected into the chromatograph during each analysis. The chromatograms were
recorded and the peak responses were measured. Product identification was done by
comparing the retention time of the analyte with that of the authentic sample. The
ethanol yield was calculated from the calibration plot deduced from standard ethanol
(AR, Cat. No. 05250502, BioLab, Israel).
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Preparation of 30 wt.% Pd1Ni1/C and 50 wt.% Pd1Au1/C electrocatalysts
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Bimetallic palladium electrocatalysts were prepared by NaBH4 reduction method.
Metal salts, specifically K2PdCl4, HAuCl4.3H2O, and anhydrous NiCl2 were used as
precursors. In a typical synthesis, desired amounts of metal salts were dissolved in
ethylene glycol and added to the vulcan XC-72 carbon dispersed in ethylene glycol.
The resulting suspension was then stirred for 1 h. Thereafter, NaBH4 dissolved in
ethylene glycol was added slowly under continuous stirring at room temperature.
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After 3 h, the suspension was filtered, washed thoroughly with ethanol and dried
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under vacuum. Finally, the materials were heat-treated at 773 K under H2/N2 (9:1) for
1 h. The prepared materials were characterized using X-ray diffraction (XRD-Rigaku
Miniflex II), scanning electron microscopy (SEM-JEOL JSM-7500F), and
transmission electron microscopy (TEM-JEOl JEM 2010) techniques.
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Membrane electrode assembly (MEA) fabrication
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The as-synthesized 30 wt.% Pd1Ni1/C and 50 wt.% Pd1Au1/C catalysts were used to
fabricate three-dimensional (3D) structured anode and cathode, respectively. Briefly,
a homogeneous mixture consisted of Pd1M1/C (M = Ni and Au), isopropanol, and 5
wt.% Nafion was spray-coated on the 3D macroporous Ni foam layer-wise and dried.
The active material loading on the anode and cathode was maintained to be 1.0 and
3.5 mg/cm2, respectively. MEA was fabricated by sandwiching cation exchange
membrane (pre-treated Nafion211, 25 μm thickness) between anode and cathode. Pre-
treatment of Nafion211 was conducted by immersing it in 10 wt.% NaOH solution
and heating at 353 K for 1 h followed by washing with de-ionized water several times.
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Alkaline-acid direct ethanol fuel cell (AA-DEFC) measurements
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During the operation of fuel cell, alkalized bioethanol (2 M bioethanol + 5 M NaOH)
and acidified hydrogen peroxide (4 M H2O2 + 1 M H2SO4) were fed at anode and
cathode, respectively. Flow rate of both bioethanol and hydrogen peroxide was
maintained to be 2 mL/min using peristaltic pump and performance of the fuel cell
was measured at two different temperatures, 303 and 333 K. The MEA was
conditioned at a constant current density until the open circuit voltage (OCV) became
steady and then current-voltage (I-V) polarization curves were recorded by applying
potential from OCV to 0.2 V.
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Results and discussion
Continuous flow bioethanol production in a solar energy-driven reactor
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The maximum theoretical yield of ethanol expected from the fermentation of 10 wt.%
glucose solution is 5.1 wt.% and of 20 wt.% glucose solution is 10.2 wt.% (See ESI,
Eq. S1). However, due to different metabolic pathways operative in the yeast, in
addition to ethanol, secondary metabolites such as glycerol and acetic acid are also
formed reducing the process efficiency. The concentration of aqueous ethanol
collected at regular time intervals using 10 and 20 wt.% glucose feed are plotted in
Fig. 2. The average ethanol yield from 10 wt.% glucose feed was 4.7±0.2 wt.% (1 M,
91.2% of the theoretical yield) and from 20 wt.% glucose feed was 8.7±0.9 wt.% (1.9
M, 85.5% of the theoretical yield). The high yields of ethanol could be due to the
provision in the reactor for the in situ separation of ethanol from the fermentation
broth which facilitates the forward reaction of ethanol formation in accordance with
Le Chatelier’s principle. Since all the experiments were performed during winter (mid
December to mid February), the average temperature was 20 °C during the day and 13
°C during the night. Although the fermentation was continuous, majority of the
evaporation process occurred during the day and was negligible during the night.
After reaching maximum ethanol yield possible with 10 wt.% glucose feed,
the reactor was then fed with 20 wt.% aqueous glucose solution with the same yeast
bed for another month. No loss in the activity of yeast was observed even after two
months of continuous operation of the solar-energy driven fermentation reactor with
different glucose feed solutions. Unlike the cultured yeasts grown in the laboratory,
the yeast strain used in the current experiments is the commercial instant baker’s
yeast. Moreover, the yeast was taken in large excess which could be the reason for the
sustained activity for two months utilizing a sole carbon source, glucose. It should be
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noted that the yeast bed was always in solid state, thus there was no efflueDnOtI:a1n0.d103i9t/C6RA00389C
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was very convenient to change the feed solutions between the experiments.
Increasing the concentration of glucose feed also increased the ethanol yield
(from 4.7 wt.% to 8.7 wt.%) indicating the efficiency of the fermentation process,
resulting in bioethanol yields close to the theoretical values (91.2 and 85.5% of the
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theoretical yield, respectively). Representative H NMR spectra of the ethanol
produced from 10 and 20 wt.% aqueous glucose solutions (collected on 25th day) are
shown in Fig. 3. A peak centered at 1.19 ppm (3H, t) and a peak centered at 3.66 ppm
(2H, q) are typical of the presence of ethanol. The singlet peak at 8.40 ppm is
attributed to HCOONa, an internal standard used to quantify ethanol amount, and the
peak at 4.80 ppm corresponds to the solvent.
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In addition to H NMR, the product was also analyzed by 13C NMR
spectroscopy. Typical 13C NMR spectra of the product obtained from 10 and 20 wt.%
aqueous glucose feed solutions (collected on 10th day) are depicted in Fig. 4 (a and b)
with comparison to authentic ethanol (Fig. 4c). The characteristic peaks of –CH3
group at 17 ppm and –OCH2 group at 58 ppm indicate that ethanol is the only
fermentation product. No trace of glucose (61 (C6), 70 (C4), 72, 73 (C2), 75 (C3), 76
(C5), 92 (C1α), and 96 (C1β) ppm) was observed in the product (see ESI, Fig. S2 for
13C NMR spectrum of authentic glucose). The absence of glucose or yeast in the
product signifies the role of solar radiation in separating the aqueous ethanol formed
in the fermentation chamber by means of evaporation and condensation processes.
Absence of typical peaks of secondary metabolites glycerol and acetic acid in both 1H
NMR (Fig. 3a and 3b) and 13C NMR (Fig. 4a and 4b) spectra of the fermentation
product again indicates the product purity and its possible direct use for energy related
applications such as fuel cells operation. (See ESI, Fig. S3 and Fig. S4 for 1H and 13C
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NMR spectra of authentic glycerol and acetic acid).
Moreover, the fermentation products were further analyzed using GC. The
chromatograms of the aliquots of products from 10 and 20 wt.% glucose feeds
collected on 25th day of the fermentation are shown in Fig. 5 (a and b). For
comparison, the chromatogram of authentic ethanol is also shown in Fig. 5c. The
appearance of the peak at 6.9 min retention time in both analytes confirms the
presence of ethanol. The ethanol yields from 10 and 20 wt.% glucose feed on 25th day
were calculated (from the calibration plot) as 4.8 wt.% (1 M) and 8.5 wt.% (1.9 M),
respectively. The concentration of the analytes determined from GC analyses agreed
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well with H NMR analyses confirming the authenticity of the methodology used for
ethanol estimation (see ESI, Table S1 and Table S2 for comparison).
Novel methods have been explored for bioethanol production by various
research groups for making the processes commercially feasible.16 Pulidindi et al.
reported 2.3 times increase in the fermentation rate of glucose (20 wt.%) using
sonication (compared to a stirred reaction).17 Tabah et al. studied the effect of stirring
speed on the kinetics of glucose (20 wt.%) fermentation and reported two times
enhancement of fermentation rate using an ultraturrax compared to the incubation at
30 °C. The acceleration of fermentation was also observed using other carbohydrates
such as sucrose and molasses.15 Choi et al. reported a maximum of 31.6% ethanol
yield from starch using fermentative bacteria in their study about fermentative
bioenergy production using ultrasonication on microalgae biomass Scenedesmus
obliquus.18 Nakashima et al. developed a direct bioethanol production from cellulose
by arming yeast using ionic liquid pretreatment, producing approximately 90%
ethanol yield.19 Saifuddin and Hussain studied microwave assisted bioethanol
production from starch and improved the yield of ethanol by 45.5% when compared
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to the non-microwave process.20 Despite being novel, all abovementioned DmOeI:t1h0.o10d3Vs9ie/Cw6ARrtAic0le0O38nl9inCe
are based on highly-controlled reactions and state-of-the-art instruments requiring
electrical energy input.
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In our previous study, we reported 84% of theoretical yield for conversion of
starch (5 wt.%) to ethanol in a single step batch fermentation process by using the
solar reactor at 30-35 °C.14 In the current study, not only solar energy was used to
obtain close to theoretical ethanol yields (91.2 and 85.5%) from the glucose feeds (10
and 20 wt.%) but also the bioethanol production was a continuous flow solid-state
process. Even though solar energy is unstable in terms of limited day hours and
seasons, the present study demonstrates the operability and the sustainability of the
process even in winter season with lower temperatures (20 °C). Moreover, no
electricity was consumed for the operation of the fermentation reactor making the
whole process green, sustainable, and most importantly cost effective. Thus, this
process is economically feasible, having no energy requirement other than solar
energy -which is free- for bioethanol production.
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One of the main issues associated with bioethanol production is its purification
from fermentation broths.21 Various approaches for extraction and distillation of
ethanol are being studied.21-22 It is important to note that, although further distillation
is needed to attain anhydrous ethanol, the unique advantage of the use of solar energy
in the current methodology is that no additional extraction process is required to
separate the aqueous ethanol from the fermentation broth. The formed ethanol in the
yeast bed is simultaneously evaporated and condensed to the glass panel of the reactor
from which it flows down into a separate chamber with an outlet for ethanol
collection. The lower pressure in the reactor, relative to the atmospheric pressure,
facilitates the evaporation of ethanol at a temperature much lower than the boiling
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point of ethanol (78.5 °C). Lowering of boiling point of ethanol as a functiDoOnI: o10f.10i3tVs9ie/Cw6ARrtAic0le0O38nl9inCe
vapor pressure and the principle of operation of the reactor were described by Tabah
et. al.14 A maximum of 8.72 wt.% (~2 M) ethanol was produced in the current process
through fermentation of 20 wt.% glucose solution and the potential of the bioethanol
generated was tested for fuel cell applications.
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Alkaline-acid direct ethanol fuel cell (AA-DEFC) performance at 303 and 333 K
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AA-DEFCs have drawn much attention due to the high power density, faster kinetics
of ethanol oxidation in alkaline media, low activation loss and faster kinetics of
hydrogen peroxide reduction in acidic media due to two-electron transfer, use of non-
Pt electrocatalysts, low fuel cross-over and high theoretical cell voltage (2.52 V) than
other types of fuel cells.23-25 These fuel cells are characterized by oxidation of ethanol
at anode (CH3CH2OH + 12OH− → 2CO2 + 9H2O + 12e−; Eo = −0.74 V) and reduction
of hydrogen peroxide at cathode (6H2O2 + 12H+ + 12e− → 12H2O; Eo = +1.78 V).
Different electrocatalysts were explored to improve the kinetics of redox reactions and
thereby the power density.24,26,27 Recently, Zhao’s research group demonstrated a high
power density of 360 mW/cm2 at 333 K using PdNi/C at anode with a loading of 1.0
mg/cm2 and Pt/C at cathode with a loading of 3.9 mg/cm2.24 In the present work,
PdNi/C was used for the oxidation of bioethanol at anode and PdAu/C for the
reduction of hydrogen peroxide at cathode. The reason for using PdAu/C rather than
Pt/C is to avoid the decomposition of hydrogen peroxide into oxygen and water.28,29
Moreover, it is less expensive than Pt.30
Schematic representation of AA-DEFCs is depicted in Fig. 6a. The system is
composed of alkalized bioethanol and acidified hydrogen peroxide compartments
separated by a cation conducting membrane. In both anode and cathode
compartments, 3D-structured electrodes were used to facilitate the diffusion of
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reactant species and also improve the electrochemical active surface area for redox
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reactions. XRD analysis confirmed phase purity of the materials (see ESI, Fig. S5).
Particle size was observed to be ca. 5-6 nm for 30 wt.% Pd1Ni1/C and 50 wt.%
Pd1Au1/C catalysts based on TEM images (Fig. 6b and 6c) and the 3D-structured
configuration of the electrodes was seen from SEM images (Fig. 6d and 6e).
I-V polarization and power density curves of AA-DEFCs operated at 303 and
333 K are shown in Fig. 6f. The open circuit voltage (OCV) was observed to be
around 1.65 V (i.e. 65.5% voltage efficiency) at both operating temperatures. A
maximum power density of 330 mW/cm2 was observed at 303 K. The OCV and
power density observed in AA-DEFCs configuration is higher than the values
reported in the literature for acid DEFCs and alkaline DEFCs.14,30-32 The improved
performance can be attributed to the avoidance of the mixed-potential phenomena.
When the operating temperature increased from 303 to 333 K, the power density
increased from 330 to 410 mW/cm2. The enhanced performance is attributed to the
faster electrochemical kinetics of the redox (ethanol oxidation and H2O2 reduction)
reactions at the electrode compartments, improved membrane conductivity, increased
reactant delivery and product removal rates. The performance values observed in the
present study are higher than the values reported in literature (see ESI, Table S3 for
comparison). This is possibly due to the high catalytic activity of bimetallic Pd
electrocatalysts and 3D-structured electrode configuration.
The superior features of the current approach for bioethanol production and its
application are (i) the bioethanol production process being a continuous flow which
could be easily adopted for industrial applications for large scale production, (ii)
obtaining high concentrations of bioethanol (~2 M) by feeding 20 wt.% glucose
solution into the reactor, (iii) using solar energy for fermentation and for separation of
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the formed ethanol, (iv) achieving high ethanol yields without electricity
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consumption, (v) using the same microorganism (without any additional nutrients) for
a long time (two months) without loss in the activity, (vi) no production of any
polluting effluent due to the solid-state fermentation, (vii) demonstrating the potential
of the produced bioethanol as fuel for operating AA-DEFCs, and (viii) achieving
current and power density values as high as 700 mA/cm2 and 330 mW/cm2 at a
modest OCV of 1.65 V.
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Conclusions
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Biofuels may replace current transportation fuels and the major commercial biofuel is
bioethanol. A solar-energy driven solid-state continuous flow bioethanol production
process was described in the current study. Solar energy was not only used to drive
the glucose fermentation reaction but also to separate the bioethanol in situ from the
broth. Separation of ethanol from the yeast bed was possible soon after its formation
by an evaporation-condensation process. No traces of the reactant (glucose) or
secondary metabolites (glycerol or acetic acid) were observed in the product. The
average ethanol yield was 91.2 and 85.5% of the theoretical yield for 10 and 20 wt.%
of glucose solutions, respectively. Thus, even at low operation temperatures (~20 °C),
solar-energy based solid-state fermentation is a promising method for bioethanol
production. In addition to the production of 2 M bioethanol from 20 wt.% glucose
feed, the application of this bioethanol was demonstrated for the operation of the
DEFCs. The OCV was observed to be 1.65 V which corresponds to 65.5% voltage
efficiency. The current and power density values derived from the fuel cell operated at
room temperature were 700 mA/cm2 and 330 mW/cm2, respectively. Thus, a new
avenue was explored for decentralized power supply based on solar energy.
Preliminary studies also revealed successful results with higher amounts of glucose
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feed (14 wt.% ethanol from 30 wt.% glucose solution and 18 wt.% ethanolDfOroI: m10.104309/C6RA00389C
wt.% glucose solution) using current methodology. Future efforts will be devoted on
direct conversion of biomass (marine and terrestrial) to ethanol using the same
strategy. Utilization of solar energy -which is renewable, clean, and sustainable- for
bioethanol production is surely a significant leap towards the realization of an
industrially-adoptable process.
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Acknowledgements
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The authors acknowledge the financial support from the Israel Science Foundation
(ISF, Grant No. 598/12) and the Israel Ministry of Science, Technology and Space
(Grant Nos. 3-9802 and 3-99763). The authors thank Mr. Menachem Schneeberg,
Head of Mechanical Workshop, Bar-Ilan University, for fabricating the solar reactor.
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Associated Content
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Electronic Supplementary Information (ESI). Design of the solar reactor (Fig. S1),
theoretical yield of ethanol from glucose fermentation (Eq. S1), 13C NMR spectrum of
1
authentic glucose (Fig. S2), H NMR spectra of authentic glycerol and acetic acid
(Fig. S3), 13C NMR spectra of authentic glycerol and acetic acid (Fig. S4),
1
comparison of ethanol concentrations deduced from GC and H NMR analyses from
10 and 20 wt.% glucose fermentation (Table S1 and S2), powder XRD patterns
recorded for bimetallic Pd catalysts (Fig. S5), and AA-DEFC performances reported
in the literature (Table S3).
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1 D. J. Hayes, S. Fitzpatrick, M. H. Hayes and J. R. Ross, Biorefineries-Industrial
Processes and Products, 2006, 1, 139-164.
361
362
2 H. Su, G. Xu, H. Chen and Y. Xu, ACS Sustainable Chem. Eng., 2015, 3, 2002-
2011.
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363
364
365
366
367
368
369
370
3 A. Avami, Renew. Sust. Energ. Rev., 2013, 26, 761-768.
4 Y. H. P. Zhang, Process Biochemistry, 2011, 46, 2091-2110.
5 J. E. Anderson, D. M. DiCicco, J. M. Ginder, U. Kramer, T. G. Leone, H. E. Raney-
Pablo and T. J. Wallington, Fuel, 2012, 97, 585-594.
6 T. Hasunuma and A. Kondo, Biotechnol. Adv., 2012, 30, 1207-1218.
371
372
7 S. Nikolić, L. Mojović, M. Rakin, D. Pejin and J. Pejin, Food Chem., 2010, 122,
216-222.
373
374
8 P. Subhedar and P. R. Gogate, Ind. Eng. Chem. Res., 2013, 52, 11816-11828.
9 F. Talebnia, D. Karakashev and I. Angelidaki, Bioresour. Technol., 2010, 101,
375
376
4744-4753.
377
378
10 A. I. Galaction, A. M. Lupasteanu and D. Cascaval, Open Sys. Biol. J., 2010, 3, 9-
20.
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
11 G. Francesca, F. Magherini, T. Gamberi, M. Borro, M. Simmaco and A. Modesti,
Biochim. Biophys. Acta, Proteins Proteomics, 2010, 1804, 1516-1525.
12 M. Akinori, I. Hiroyuki, K. Tsutomu and S. Shigeki, Appl. Microbiol. Biotechnol.,
2009, 84, 37-53.
13 A. Pandey, Biochem. Eng. J., 2003, 13, 81-84.
14 B, Tabah, I. N. Pulidindi, V. R. Chitturi, L. M. R. Arava and A. Gedanken,
ChemSusChem, 2015, 8, 3497-3503.
15 B. Tabah, I. N. Pulidindi and A. Gedanken, J. Bioprocess. Biotech., 2015, 5, 232.
16 I. N. Pulidindi and A. Gedanken, Springer Book Series-Production of Biofuels and
Chemicals: Ultrasound, 2015, Eds: Z. Fang, L.-S. Fan, J. R. Grace, Y. Ni, N. R.
Scott, R. L. Smith, Jr., Ch. 6, 159-188.
17 I. N. Pulidindi, A. Gedanken, R. Schwarz and E. Sendersky, Energy Fuels, 2012,
26, 2352-2356.
18 J. A. Choi, J. H. Hwang, B. A. Dempsey, R. A. Abou-Shanab, B. Min, H. Song
and B. H. Jeon, Energ. Environ. Sci., 2011, 4, 3513-3520.
19 K. Nakashima, K. Yamaguchi, N. Taniguchi, S. Arai, R. Yamada, S. Katahira and
A. Kondo, Green Chem., 2011, 13, 2948-2953.
20 N. Saifuddin and R. Hussain, J. Mathematics & Statistics, 2011, 7, 198-206.
406
407
408
21 C. M. Neves, J. F. Granjo, M. G. Freire, A. Robertson, N. M. Oliveira, J. A.
Coutinho, Green Chem., 2011, 13, 1517-1526.
16

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DOI: 10.1039/C6RA00389C
409
410
411
412
413
414
415
416
417
418
419
22 J. Baeyens, Q. Kang, L. Appels, R. Dewil, Y. Lv and T. Tan, Prog. Energy
Combust. Sci., 2015, 47, 60-88.
23 L. An and T. S. Zhao, Energy Environ. Sci., 2011, 4, 2213-2217.
24 L. An, T.S. Zhao and Y.S. Li, Renew. Sustainable Energy Rev., 2015, 50, 1462-
1468.
25 L. An and T.S. Zhao, Int. J. Hydrogen Energy, 2011, 36, 9994-9999.
26 T. Maiyalagan and K. Scott, J. Power Sources, 2010, 195, 5246-5251.
27 E. Lee, I. S. Park, A. Manthiram, J. Phys. Chem., 2010, 114, 10634-10640.
28 K. Ye, D. Zhang, X. Wang, K. Cheng and D. Cao, RSC Adv., 2015, 5, 3239-3247.
29 L. Gu, N. Luo and G. H. Miley, J. Power Sources, 2007, 173, 77-85.
420
421
422
423
424
425
426
427
428
429
430
30 A. Brouzgou, A. Podias and P. Tsiakaras, J. Appl. Electrochem., 2013, 43, 119-
136.
31 C. V. Rao and B. Viswanathan, J. Phys. Chem. C, 2009, 113, 18907-18913.
32 C. V. Rao and B. Viswanathan, Electrochim. Acta, 2010, 55, 3002-3007.
431
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LIST OF FIGURES:
a)
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Fig. 1 Solar reactor (whole system) (a), open reactor with instant baker's yeast on
activated carbon cloth (b)
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Fig. 2 Time on stream studies of ethanol yield (wt.%) with 10 and 20 wt.% glucose
feed solutions (Tave: 20 °C/13 °C)
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Fig. 3 H NMR spectra of ethanol produced from (a) 10 wt.% glucose (collected on
25th day, 1.1 M) and (b) 20 wt.% glucose (collected on 25th day, 2.1 M) feed solutions
(Inset shows the ethanol peaks, a 3H (t) centered at 1.19 ppm and a 2H (q) centered at
3.66 ppm.)
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Fig. 4 C NMR spectra of (a) ethanol from 10 wt.% glucose solution (collected on
10th day, 1.1 M), (b) ethanol from 20 wt.% glucose solution (collected on 10th day, 1.8
M), and (c) authentic ethanol
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Fig. 5 Gas chromatograms of (a) ethanol from 10 wt.% glucose solution (collected on
25th day, 1 M), (b) ethanol from 20 wt.% glucose solution (collected on 25th day, 1.9
M), and (c) authentic ethanol
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Fig. 6 Schematic representation of AA-DEFCs operated with as-produced bioethanol
as fuel (a), TEM images of bimetallic Pd electrocatalysts (b and c), SEM images of
3D-structured anode and cathode (d and e), and current-voltage (I-V) polarization and
power density curves recorded at 303 and 333 K (f)
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