Improvement of Yeast-Biofuel Cell Output by Electrode Modifications
Yolina V. Hubenova,*,†Rashko S. Rashkov,‡Vasil D. Buchvarov,‡Marina H. Arnaudova,‡
Sofia M. Babanova,§and Mario Y. Mitov§
Department of Biochemistry and Microbiology, PloVdiV UniVersity, 24 Tsar Asen str., 4000 PloVdiV, Bulgaria,
Institute of Physicochemistry, Bulgarian Academy of Sciences, 11 Acad.G.BoncheV str., 1113 Sofia, Bulgaria,
and Department of Chemistry, South-West UniVersity, 66 IVan MihajloV str., 2700 BlagoeVgrad, Bulgaria
In this study, a methodology for electrodeposition of nickel nanostructures on carbon felt was developed on
the base of pulse plating technique. Different in size, shape, and distribution, Ni-island nanostructures were
deposited varying the potential, current, pulse duration, and cycle reiteration. The biocompatibility and
nontoxicity of the newly created materials toward Candida melibiosica yeast cells was proven. The prepared
Ni-nanomodified carbon felts were investigated as anodes in a two-chamber mediatorless yeast-biofuel cell.
Maximum power density values of 720 and 390 mW/m2were achieved with the electrodes modified under
galvanostatic and potentiostatic conditions, respectively, against 36 mW/m2for the nonmodified ones. The
better biofuel cell performance obtained with the Ni-modified electrodes is assigned to an improved electron
Biofuel cells, more popular as microbial fuel cells (MFCs),
represent an innovative technology for simultaneous electricity
generation and organic waste purification.1-4The principle of
the MFC is based on the direct conversion of the biochemical
energy of living cells into electrical energy. The utilization of
entire microorganisms as natural biocatalysts, the operation at
ambient temperatures, and the use of neutral electrolytes and
inexpensive carbon-type electrodes are the biggest advantages
of the MFCs over chemical fuel cells. The low electrical output,
however, is the major drawback for their wide application. The
improvement of the electron transfer from the microorganisms,
while oxidizing the biodegradable organic matter, to the anode
is considered to be one of the most important factors for
increasing the MFC efficiency. Physically, the extracellular
transport of the electrons toward the anode can either occur
through the use of soluble electron shuttles or through membrane-
bound electron shuttling compounds.2An advance in the
understanding of the electron transfer mechanisms has been
recently achieved discovering that some metal reducing micro-
organisms like Fe(III) reducing bacteria can realize a direct
electron transfer using the anode as a final electron acceptor.5,6
In the natural environment, these microorganisms produce
energy for their growth and reproduction by coupling the
oxidation of organic compounds to the reduction of insoluble
metal and metalloid oxides.7Unlike natural external electron
acceptors such as Fe(III) or Mn(IV) oxides, the anodes in MFCs
do not participate in mineral dissolution reactions, and the
electron transfer rates can be estimated. The anodes also provide
a stable source of electron acceptor and do not generate reduced
products.8The microorganisms, which are able to transfer
electrons from reduced substrates to a solid electrode as part of
their energy-generating respiration, are known as anode-respiring
The achieved up to now MFC power outputs are ranging
within over 3 orders of magnitudesfrom milliwatts per square
meter to several watts per square meter.3The obtained opera-
tional characteristicsscell voltage, current, and power densitys
depend on many variables, which may be divided into two
groups: (i) MFC type (single or two-chamber), construction
(batch or flow mode), and used components (electrodes,
membranes) and (ii) used biocomponentsspure microbial
culture or microbial society, cultivation conditions including
medium and substrates, pH, temperature, use of artificial soluble
electron mediator, etc.
As a novel technology developed to decrease the energy cost
of the wastewater purification, the most of the reported results
are obtained with prokaryotic strains associated to water
reservoirs: from genera Geobacter (Geobacter sulfurreducens,
Geobacter metallireducens), Desulfuromonas (Desulfuromonas
acetoxidans), Shewanella (Shewanella putrefaciens), etc.2,3,6,10
Investigations with Thermincola strain JR and Geobacillus
strain S2E show that, members of Firmicutes produce current
comparable or even higher than that of either Geobacter or
Shewanella MFC communities.8New strains like Rhodoferax
ferrireducens,11capable of oxidizing various substrates while
at the same time efficiently producing a continuous, usable
electrical current, are on and on discovered.
The application of eukaryotic yeast cells in MFCs is still rare.
Investigations with the yeast species Saccharomyces cereVisae,
Hansenula anomala, and Arxula adeniniVorans have been
reported.4,12-15Recently, we have found that Candida meli-
biosica 2491, a special strain of Candida yeast, appropriate for
purification of hard plant waste rich of phytate, possesses
electrogenic properties and could be used as a biocatalyst in
MFC.16For improvement of the electron transfer in yeast MFCs,
a supplement of exogenous mediator is usually needed. How-
ever, the addition of artificial mediators is not preferable as they
are generally too expensive and they could be toxic at higher
concentrations or can be degraded over longer time periods.12,16-18
Of all components of MFC, the electrode materials play a
crucial role in the electricity generation.19Although much
research has focused on cathode modification and optimization
of bacteria inoculation,20according to Qiao et al. the anode
modification results in considerable contribution to the overall
MFC performance.21The anode material and its structure can
significantly affect bacteria attachment, electron transfer, and
* Towhomcorrespondenceshould be addressed.Phone:
+359898982716. E-mail: email@example.com.
‡Bulgarian Academy of Sciences.
Ind. Eng. Chem. Res. 2011, 50, 557–564
2011 American Chemical Society
Published on Web 06/15/2010
substrate oxidation.22Up to now, various carbon materials such
as cloth, felt, paper, and rods are most commonly applied as
MFC anodes due to their biocompatibility, chemical stability
in a microbial inoculum mixture, and good conductivity.
However, these materials possess small electrocatalytic activity
toward the anode microbial reactions. Thus, modification of the
carbon electrodes is the main approach to improve their
performance. It is worth noting that the electrocatalytic effects
of different metal modified carbon materials have not been
widely examined yet.
Nickel is one of the essential microelements participating in
several metabolic pathways as a cofactor for the enzymes
involved. Ni-containing enzymes catalyze five distinct biological
activities including reversible hydrogen oxidation, interconver-
sion of carbon monoxide and carbon dioxide, methane genera-
tion, urea hydrolysis and superoxide dismutation.23Like other
transition metals, however, excess Ni is toxic to cells; thus,
synthesis of these Ni-enzymes requires the presence of carefully
controlled Ni-processing mechanisms that range from selective
transport of Ni into the cells to productive insertion of Ni into
the apoproteins.24From another point of view, nickel-based
materials have found important industrial applications as elec-
trocatalysts in batteries, fuel cells, electrolyzers, and electrosyn-
thesis devices due to the well-established surface oxidation
properties of nickel.25-27Up to now, however, nickel-based or
modified electrodes have been rarely tested in MFCs.28
The aim of this study was to develop methods for elec-
trodeposition of nickel-island structures on carbon felt and to
examine the performance of the prepared nanomodified materials
as anodes in mediatorless yeast-biofuel cell. In order to evaluate
the electrocatalytic properties of modified materials, the recorded
MFC operational characteristicssopen cell voltage (OCV),
maximum power, and current densityswere compared with
those obtained with nonmodified carbon felt anodes.
2. Materials and Methods
2.1. Electrodeposition of Nickel Nanostructures on Carbon
Felt. Nickel was electrodeposited on carbon felt (SPC-7011, 30
g/m2, Wei?gerber GmbH & Co. KG) by a pulse plating
technique, varying the voltage, current, and pulse duration. A
nickel sulfamate electrolyte containing 80 g/L Ni2+, 35 g/L
H3BO3, and 10 g/L NiCl2(pH 4) was used. The electrolysis
was carried out in a conventional three-electrode cell with
platinum-titanium mesh counter electrode and saturated calomel
electrode (SCE) as a reference using a Gamry G750 poten-
tiostat-galvanostat (Gamry Instruments, US). The electroplating
bath temperature was kept constant at 50 °C by thermostat. The
following three regimes were determined as appropriate for
deposition of nickel island structures:
(i) double potentiostatic pulse (E1) -1.8 V vs SCE for 0.5 s
and E2) -0.8 V vs SCE for 10 s);
(ii) multiple potentiostatic pulse (E1) -1.8 V vs SCE for
0.25 s and E2) -0.2 V vs SCE for 0.25 s, 10 cycles);
(iii) multiple galvanostatic pulse (i1) -40 mA/cm2for 1s
followed by 1 s pause, 10 cycles).
Scanning electron microscopy (SEM) using Leo 1455VP and
Leo Supra 55VP microscopes with energy dispersion X-ray
(EDX, Oxford Inca 200 instrument, Software INCA-Vers.4) was
applied for characterization of the surface morphology of
modified carbon felt materials. The samples were examined
before and after being used as electrodes in MFC. In the latter
case, 1 nm-thick Pt/Pd layer was sputter-coated onto the
specimens using a Cressington Ressington Sputter Coater 208
2.2. Cell Cultivation and Cytotoxicity Test. Candida meli-
biosica 2491 was cultivated in YPfrumedium in an incubator at
28 °C using an orbital shaker, 120 rpm, for receiving of cell
biomass quantity. The YPfru medium consisted of 0.5% yeast
extract, 0.8% peptone, 167 mM fructose, pH 7. The cells were
collected by centrifugation at 5000g for 10 min and washed twice,
and the concentrate was kept under distilled water at low temper-
ature (4 °C) for culture synchronization under starving conditions.
The cell vitality was examined microscopically using 0.4% trypane
blue dye solution and a Trinocular Microscope Magnum-T
equipped with an Si-3000 camera. The yeast biomass quantity was
determined spectrophotometrically by measurement of optical
density at λ ) 600 nm. A unified quantity of yeast concentrate,
The newly synthesized nickel-carbon felt materials were
analyzed for cytotoxicity toward yeast cells by applying the agar
diffusion assay,29,30but with the following modification: instead
paper discs, specimens electrodeposited under both galvanostatic
and potentiostatic conditions of modified Ni-carbon felt as well
as Ni-wire and nonmodified carbon felt were used. The samples
were put in a contact with the yeast monolayer on agar medium
and then incubated at 28 °C for at least 48 h. The cells’ growth
was comparatively characterized by morphology, viability, and
proliferation. At the same time, the potential cytotoxicity of the
Ni2+ions to the yeast cells was also tested. The yeast cultivation
was carried out as upper described with addition of Ni2+
(NiSO4) in concentrations up to 100 nmol/L, corresponding to
the quantity of the electrodeposited nickel on carbon felt. The
cell development in the presence and absence of Ni2+ions was
followed up and compared by measuring the yeast culture optical
density OD600with time.
2.3. Yeast-Biofuel Cell Studies. The electrocatalytic effects
of the island type nickel modifications were studied by using
the nanomodified carbon felt materials as anodes in a recently
designed two-chamber MFC (Figure 1).
The tested carbon felt sample was fixed in a plastic holder
and assembled in the anodic compartment of the fuel cell. The
one-side round-shaped hole of the holder assured a projected
surface area of 4.5 cm2, exposed to the anolyte. A sheet of
nonmodified carbon felt with the same dimensions as the anode
was assembled in the cathodic compartment. For simplification
of the next expose, the electrodes modified by galvanostatic
pulse deposition, potentiostatic pulse technique, and nonmodified
ones will be denoted as GME, PME, and NME, respectively.
A buffered suspension of Candida melibiosica 2491 yeast cells
in YPfru medium (pH 7) was used as an anolyte in a batch
operation mode and 0.1 M potassium ferricyanide served as a
terminal electron acceptor in the cathode compartment. The
anodic and cathodic chambers, each with a volume 13 cm3, were
separated by a proton-exchange membrane (Nafion 117, Du
Figure 1. Two-chamber MFC used in the bioelectrochemical experiments.
Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011
In the progress of the yeast culture development, polarization
measurements under different loads ranging from 100 kΩ to
10 Ω were carried out at each 6 h by using a decade resistor
box. The external resistances were changed through 2 units at
each decade. The cell voltage was recorded 5 min after switching
a given resistance by using digital multimeter DMM2700
(Keithley Instruments Inc., US). Current density, i (A/m2), was
calculated as i ) V/RA, where V (V) is the cell voltage recorded,
R (Ω) is the external resistance applied, and A (m2) is the
projected electrode surface area. For each pair Vsi, the power
density, P (W/m2), was estimated according to P ) iV. The
obtained experimental data were plotted as polarization curves
V vs i and power curves P vs i.
Fed-batch operation MFC experiments with continuously
switched constant load resistance (1 kΩ) were also carried out.
Every 24 h, half of the anolyte suspension was replaced with a
fresh yeast-free YPfrumedium. During these experiments, the
anode potential was monitored with time.
Cyclic voltammetry (CV) experiments were performed by
switching the MFC anode as a working and the cathode as a
counter electrode. The working electrode potentials were
measured against Ag/AgCl reference. CV measurements of Ni-
modified carbon felt as well as massive nickel in a phosphate
buffer (Phi, pH 7) and YPfrumedium without yeast cells were
also carried out. At the end of the MFC experiments, the yeast
suspension (anolyte) was centrifuged at 5000g for 10 min, and
the obtained fractionssthe supernatant, containing exhausted
medium and excreted metabolites, and the cell pellet, washed
twice and resuspended in the same volume fresh YPfrumedium,
were analyzed by means of CV. Usually, a scan rate of 2 mV/s
was applied. CV experiments were carried out using a PJT 35-2
potentiostat-galvanostat (Radiometer-Tacussel) with an IMT
101 electrochemical interface and VoltaMaster 2 data acquisition
3. Results and Discussion
Nickel island nanostructures different in size and distribution
were produced by using the above described three regimes of
electrodeposition. According to the double pulse method,31the
first pulse E1) -1.8 V vs SCE was sufficient to create nuclei
and the subsequent second one was essential for the growth of
the already existing nuclei. Under such conditions, the size of
deposited nickel crystallites varied between 20 and 70 nm
(Figure 2a). Decreasing the amplitude and the duration of the
second pulse and increasing the number of reiterations, bigger
size crystallites (about 100 nm) disposed on rare occasions have
been obtained (Figure 2b). A formation of dense deposit from
separate nanocrystallites was achieved under galvanostatic
conditions by alternation of cathodic current pulses (-40 mA/
cm2) and power breakdowns (Figure 2c).
The specific application of new materials in MFCs requires
an implementation of preliminary cytotoxicity analysis. The
applied agar diffusion assay showed that the yeast cells, which
were in a contact with the Ni-modified carbon felt materials on
agar medium, grew unchanged. No zones of inhibition of cells’
proliferation were observed, which proved the yeast viability
in the presence of the examined materials. In addition, the
development of Candida melibiosica 2491 yeast strain in YPfru
medium without and with the addition of Ni2+ions was followed
up by means of spectrophotometric analyses. The presence of
Ni2+in the yeast suspension does not disturb significantly the
cell development, and it is commensurable to the growth of the
yeast/YPfrucontrol (Figure 3). The observed subtle distinction
is in the limits of the normal diversion of culture’s development.
The resistance of Candida yeast to nickel ions could be explained
with the known ability of fungi and microorganisms to prevent
heavy metals’ toxicity by production of metal-binding proteins,
organic and inorganic precipitation, active transport, and intra-
cellular reorganization.32At high nickel concentrations, Candida
species are able to switch on mechanisms for restriction of metal
entry into the cells by (i) reducing metal uptake or increasing
metal efflux; (ii) metal immobilization, e.g., cell wall adsorption,
extracellular precipitation of secondary minerals; (iii) extracel-
lular metal sequestration by exopolysaccharides and other
extracellular metabolites.33On the basis of the obtained results
from the conducted cytotoxicity tests, it has been concluded
that the prepared Ni-modified carbon felt materials are nontoxic
and biocompatible toward Candida melibiosica 2491 yeast cells
and may be applied as anodes in a yeast-biofuel cell.
The exanimation of the prepared nanomodified Ni-carbon felt
materials as anodes in a mediatorless yeast-biofuel cell has
shown that in the progress of MFC tests the operational
characteristics (OCV, maximum power, and current density)
increase, reaching maxima at the late log-phase and the
beginning of stationary phase (24th to 30th hour) of the yeast
culture development. Despite the observed variations in nickel
islands’ size and distribution, the results obtained with the anode
materials modified under the both potentiostatic regimes show
unsubstantial differences (less than 10%), which determines the
later on presentation and discussion of only one type of them
Figure 2. SEM images of nickel-modified carbon felt obtained under: (a)
double potentiostatic pulse; (b) multiple potentiostatic pulse; (c) multiple
galvanostatic pulse conditions.
Figure 3. Development of Candida melibiosica in YPfru medium in the
presence and absence of Ni2+ions.
Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011
(related to Figure 2a). Comparing the MFC performance using
GME and PME, higher operational characteristics were achieved
with the anodes modified under galvanostatic conditions.
Typically, after inoculation of the MFC the OCV values
stabilized at about 400 mV for the both types of modified
anodes. In the progress of the experiment, the OCV grew up to
660 and 540 mV when using GME and PME, respectively. The
recorded polarization curves, presented in Figure 4a, reveal that
the use of former electrodes results in lower internal cell
resistance, which may be assigned to the higher nickel loading
and better conductivity of the modified carbon felt in this case.
The better performance of GME also finds expression in almost
twice higher short circuit current density of 5.5 A/m2in
comparison with 3 A/m2for PME and fifteen times higher than
0.38 A/m2for NME (Figure 4b). The highest maximum power
density values of 720 and 390 mW/m2were achieved with GME
and PME, respectively, against 36 mW/m2for the NME. These
values exceed even those obtained when the artificial mediator
MB was added to the anolyte suspension and nonmodified
carbon felt anode was used (Figure 5b).
The MFC outputs grew up to the 30th hour of inoculation,
then the power and current densities began to diminish. Because
the experiments were carried out in a batch mode, the possible
explanation of the performance deterioration may be assigned
to an exhaustion of nutritious ingredients in the medium as well
as to a cell density inhibition. In order to eliminate the effect
of these factors, we performed experiments in a fed-batch
operation mode, in which the MFC was continuously loaded
with 1 kΩ external resistance and half of the yeast suspension
was replaced with a fresh medium every 24 h. When working
with a fixed external resistance, the evolution of the electrode’s
potential as a function of microorganisms’ kinetics can be
monitored.34Usually, at the beginning of such an experiment,
a steep drop of the anode potential in the negative direction
was observed (Figure 5). Similar decreases of up to 500 mV of
the anode potential in an MFC have been observed within just
a few hours,35,36which indicates that communities adapt quickly
to large fluctuations in the potential of the electron acceptor.
The subsequent opposite shift and stabilization of the potential
may be connected with the ability of the microorganisms to
drive the system toward an attainable activity that allows energy
capture for growth and maintenance.37The repetitive half-
replacement of the anolyte with fresh yeast-free medium results
in a similar variation of the anode potential with a tendency
toward decrease of the steady-state values, which is a precondi-
tion for long-term MFC operation.
The anode biofilm formation is considered to contribute to
more effective electron transfer mechanism.1-3,6,10The SEM
observations of modified and nonmodified carbon felt specimens
that have been applied as anodes in MFC confirmed the
formation of yeast biofilm on the electrode surface. The SEM
images, shown in Figure 6, illustrate the aptitude of Candida
melibiosica rapidly to create biofilm on a carbon surface. At
the 30th hour of cultivation, the biofilm is not in a matured
phase yet and the yeast cells are keeping on geminating and
spreading on the electrode surface. An oval shape and a size of
Candida melibiosica cells up to 7 µm have been determined. It
should be pointed out that the yeast cells grow well and gemmate
both on nonmodified and Ni-modified carbon felt anode. The
observed budding cells on the GME surface (Figure 6b)
demonstrate once again the biocompatibility of this strain to
the electrodeposited nickel. Harrison et al. supposed that the
biofilm formation may be one of the strategies for metal
Figure 4. Polarization and power curves of a Candida melibiosica yeast-biofuel cell by using (a) GME, PME and (b) NME, NME + 0.9 mM Methylene
Blue, as anodes.
Figure 5. Variation of potential of modified MFC anode (GME) with time
in a fed-batch operation mode. The arrows indicate the refreshment of the
Figure 6. SEM images of Candida melibiosica biofilm on the surface of
(a) NME and (b) GME.
Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011
resistance and/or tolerance of yeasts.38Metal-chelator precipi-
tates could be formed in biofilms following exposure to the
heavy metals like Ni2+and Cu2+.38This suggests that Candida
cells may adsorb metal cations from their surroundings and that
sequestration in the extracellular matrix may contribute to
resistance. Moreover, this natural ability of Candida melibiosica
probably assists the achieved improvement of MFC outputs.
Recently, cyclic voltammetry is more frequently used to study
the complex electron transfer phenomena between living
microorganisms and electrodes in MFCs.39,40In this study, CV
measurements were performed to elucidate the differences in
the performance of Ni-modified and nonmodified carbon felt
materials as MFC anodes. Cyclic voltammograms recorded at
times corresponding to the optimum MFC characteristics
achieved are presented in Figure 7. The CV patterns obtained
with Candida melibiosica yeast suspension/nonmodified carbon
felt electrode are characterized by appearance of anodic and
cathodic peak at +225 and -45 mV vs Ag/AgCl, respectively
(Figure 7a). Similar quasi-reversible behavior has been also
observed on graphite electrodes.16In a contrast, when Ni-
nanomodified carbon felt electrodes were applied, only an anodic
peak with a long shoulder after the maximum appeared in the
voltammograms (Figure 7b). The position of this anodic peak
is at more negative potentials than that obtained with NME,
which indicates a participation of different electroactive species
in the electrochemical reaction in the case of modified and
nonmodified electrodes. At the same time, the anodic peak
heights achieved with the different electrodes decrease in the
order GME-PME-NME corresponding to their performance
as MFC anodes.
To get more information concerning the observed electro-
chemical behavior, CV measurements of the fractions, obtained
by centrifugation of the anolyte suspension after the end of MFC
experiments, were also carried out. CV plots recorded with the
collected supernatants and resuspended cell pellets are presented
in Figure 8. An anodic and a cathodic peak at the same potentials
as those associated with the quasi-reversible behavior of the
yeast suspension/NME system (Figure 7a) appeared only in the
voltammograms obtained with the correspondent supernatant
(Figure 8a), but not with the resuspended yeast pellets (Figure
8b). This suggests that the electrochemical activity of the NME
bioanode is due to oxidation/reduction of excreted metabolite
(endogenously generated mediator) in the medium. The cyclic
voltammograms obtained with the fractions from anolytes
contacted with Ni-modified electrodes, however, have not given
clear evidence for the electron transfer mechanism. A not well-
shaped anodic peak at more positive potentials than that of yeast
suspension/Ni-modified carbon felt appeared in the voltammo-
grams recorded with supernatant (Figure 8a). Surprisingly, a
reversible electrochemical behavior, characterized by the ap-
pearance of oxidation and reduction peaks at +85 and +25 mV
(vs Ag/AgCl), respectively, was observed for the yeast pellets
(Figure 8b). A similar CV performance of the yeast suspension/
Ni-modified anode has been noticed at earlier periods (between
Figure 7. Cyclic voltammograms of Candida melibiosica yeast/YPfrususpension after 24 h yeast-MFC operation obtained with (a) NME; (b) GME and
PME. The scan rate was 2 mV/s.
Figure 8. Cyclic voltammograms of (a) the supernatant obtained by centrifugation of the anolyte (Candida melibiosica yeast/YPfrususpension); (b) resuspended
yeast cells after 30 h MFC operation with NME and GME anodes. The CV of yeast suspension with GME recorded at the 18th hour of MFC operation is
shown for comparison. A scan rate of 2 mV/s.
Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011
the12th and 18th hour) of yeast-MFC operation, but at the 24th
hour and later, the system is characterized by the irreversible
behavior, shown in Figure 7b.
To examine the hypothesis for participation of the electrode-
posited nickel in the anodic reaction, the CV behavior of Ni-
modified carbon felt as well as massive nickel electrodes in
neutral phosphate buffer solution and YPfru medium without
yeast cells was studied. The obtained voltammograms are shown
in Figure 9.
Two anodic peaks, corresponding to electrooxidation of
metallic nickel most probably to Ni(II) and Ni(III), appear in
voltammograms obtained with massive nickel during the first
scan in the potential range between -400 and +600 mV vs
Ag/AgCl (Figure 9a). In the subsequent cycles, these peaks
disappear, which indicates a formation of an insoluble protective
film on the nickel surface. At the same potential range, no peaks
are present in the voltammograms obtained with the electrode-
posited Ni-carbon felt electrodes (Figure 9b, solid line). The
possible explanation of this behavior is that the dispersed nickel
has already been oxidized, while exposed to the air. When the
potential sweep starts from more negative values, however, two
anodic and one cathodic peak appear in the voltammograms
(Figure 7b, dashed line). We consider that while passing through
the potential region where hydrogen evolution reaction takes
place, the evolved hydrogen reduces the oxidized nickel on the
surface, and then, when the potential is swept in the anodic
direction the reduced nickel undergoes subsequent electrooxi-
dation to different oxidation states. The cathodic peak in the
next reverse scan corresponds to reduction of oxidized Ni from
higher to lower oxidation state. Similar differences in the CV
behavior of massive and deposited nickel (peak potentials, peak
separations, and peak current ratios) have been often observed
and contributed to the thickness and dispersion of the deposition,
depending on the method of preparation as well as the nature
and surface area of the support.41It is worthwhile to note,
however, that none of the observed peaks in the voltammograms
recorded with Ni-modified electrodes in the presence of yeast
cells (Figures 7b and 9b) coincide with those obtained in buffer
solution (Figure 9b). Therefore, the electrochemical behavior
of the yeast suspension/Ni-modified electrode system could not
be assigned to a direct electrooxidation of the electrodeposited
nickel. Furthermore, the absence of peaks in the CVs obtained
with modified carbon felt electrodes in the YPfru medium or
buffered solution of alcohol without yeast cells (Figure 9c)
indicates that neither the medium ingredients nor the product
of fructose fermentation undergo electrochemical oxidation on
this type of anode.
The observed differences in the electrochemical activity of
modified and nonmodified electrodes more probably suggest a
change of the yeast metabolism in the presence of nickel
resulting in the expression of specific substance(s) and formation
of outer-membrane associated metalloprotein(s).42We suppose
that nickel in different oxidation states improves the electron
transportation via two possible mechanisms: acting as an
electron acceptor, similar to the interaction between metal
reducing bacteria and insoluble metal oxides,7and/or due to
adaptive mechanisms as a response to Ni2+, resulting in
facilitated electron transfer across the cell membrane. Other
studies also indicate the capability of anode-respiring cultures
to adapt their metabolism and the mechanisms of electron
Figure 9. Cyclic voltammograms of (a) massive nickel in phosphate buffer (pH 7); (b) GME in phosphate buffer (pH 7) with different cathodic switch
potentials; (c) GME in YPfruand 5%EtOH/Phi, pH 7. The scan rate is 2 mV/s.
Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011
transfer to changes in the anode potential in order to maximize
the biological energy gain.43
On the basis of the obtained results in the present study, it
may be concluded that the application of a pulse plating
technique provides the opportunity for preparation of new
biocompatible nickel-nanomodified carbon felt materials, pos-
sessing high electrocatalytic activity as anodes in a yeast-biofuel
cell. The generated power densities using Ni-modified carbon
felt anodes exceed over an order of magnitude those obtained
with nonmodified ones and are comparable or even higher than
values achieved by other yeast-biofuel cells reported until now.
The improved performance of the modified electrodes is
associated with the presence of nickel on the anode surface,
switching over specific cell metabolic processes, facilitating the
electron transfer mechanism.
Further investigations, aiming at optimization of electrode-
posits’ composition and structure, electrode conductivity and
stability, better understanding of the possible mechanisms of
interactions between the microorganisms and the modified
materials, and identification of secreted metabolites as well as
long-term MFC-experiments, are in progress.
This study was funded by the program “New power sources
and energy-saving technologies” of the National Science Fund
of Bulgaria through contract D002-163/2008 and partially
supported by DFG contract BUL-436 112/38/04.
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ReVised manuscript receiVed May 22, 2010
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Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011