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Review
An overview of cathode material and catalysts suitable forgenerating hydrogen in microbial electrolysis cell
Anirban Kundu a, Jaya Narayan Sahu b,*, Ghufran Redzwan a, M.A. Hashim b
a Institute of Biological Sciences, University of Malaya, 50603 Kuala Lumpur, MalaysiabDepartment of Chemical Engineering, University of Malaya, 50603 Kula Lumpur, Malaysia
a r t i c l e i n f o
Article history:
Received 12 September 2012
Received in revised form
3 November 2012
Accepted 5 November 2012
Available online xxx
Keywords:
MEC
Cathode
Cathode catalyst for hydrogen
evolution
* Corresponding author. Tel.: þ60 3 79675295E-mail addresses: [email protected], ja
Please cite this article in press as: Kundhydrogen in microbial electrolysis cej.ijhydene.2012.11.031
0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.11.0
a b s t r a c t
Bio-electrohydrogenesis through Microbial Electrolysis Cell (MEC) is one of the promising
technologies for generating hydrogen from wastewater through degradation of organic
waste by microbes. While microbial activity occurs at anode, hydrogen gas is evolved at the
cathode. Identifying a highly efficient and low cost cathode is very important for practical
implication of MEC. In this review, we have summarized the efforts of different research
groups to develop different types of efficient and low cost cathodes or cathode catalysts for
hydrogen generation. Among all the materials used, stainless steel, Ni alloys Pd nano-
particle decorated cathode are worth mentioning and have very good efficiency. Industrial
application of MEC should consider a balance of availability and efficiency of the cathode
material.
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction through the external circuit. There are two aspects of this BES.
Energy is considered as the lifeline of modern era. Non-
renewable energy sources like coal, oil and natural gas are
excessively used to meet the world’s energy requirements.
This has come out with an increased rate of depletion of the
natural resources. This is a global concern as the future
generations would definitely be at threat if the natural
resource is depleted at this alarming rate. There are of course
alternatives for non-renewable energies. Utilization of solar
energy, wind, tidal, geothermic and bio energy, which in
principle are renewable, are possible alternatives.
Bio-electrochemical system (BES) possesses a tremendous
potential for energy generation and simultaneous treatment
of wastewater containing organic substances. Microbes
present in the anode degrade the organic substance and
release electron to the solid anode surface which travel
; fax: þ60 3 [email protected] (J.N.
u A, et al., An overviewll, International Journ
2012, Hydrogen Energy P31
One in which electricity is produced known as MFC and
another where hydrogen is produced known as Microbial
Electrolysis Cell (MEC) [1]. Cathode is one of the most impor-
tant parts of the MEC where hydrogen as well as different
chemical compounds are produced. In recent time numbers of
researches are directed toward development and application
of a low cost cathode or cathode catalyst for practical utili-
zation in MECs for hydrogen generation. In this article the
material and catalyst used by different research groups for
hydrogen generation and result obtained are discussed.
2. Hydrogen as fuel
Recently interest on hydrogen as a fuel has increased a lot
mainly due to the reason that hydrogen has the highest
Sahu).
of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 2 ) 1e1 32
gravimetric energy density of any known fuel and is compat-
ible with electrochemical and combustion processes for
energy conversion without creating environmental pollution
and contributing to climate change by producing carbon-based
emissions. Hydrogen has a high energy density by weight yet
when it is burnt in engines produces almost no pollution. An
Otto cycle internal-combustion engine has a maximum effi-
ciency of about 38%, 8% higher than a gasoline internal-
combustion engine when the former is run on hydrogen [2].
Hydrogen is a colorless, odorless gas that accounts for almost
75 percent of the entire universes’ mass. However it is avail-
able on Earth only in combination with other elements like
oxygen, carbon and nitrogen and must be separated from
these other elements to obtain pure hydrogen to be used.
Although, hydrogen is not a primary source of energy like coal,
rather it is an energy carrier like electricity. Hydrogen fuel cells
and related hydrogen technologies provide the essential link
between renewable energy sources and sustainable energy
services. NASA has used liquid hydrogen as fuel for space
shuttle and rockets since the 1970s [3,4]. Hydrogen is used
there as fuel to power the electrical systems and as byproduct
pure water is produced, used by the crew as drinking water.
The market of hydrogen fuel is increasing and according to
a market research company the total world production of
hydrogen as a chemical constituent and as an energy source
was valued at $120 billion in 2010. An expected increase at
a 6.3% compound annual growth rate it may reach a value of
$163 billion in 2015 [5]. This seems to be very promising not
only for the business but also for the academicians because
a chunk of this money will be invested in the R&D section
where the researcher can research on novel technology to
increase the power output and machineries to use the
hydrogen as fuel at lesser cost. There are different types of
processes for hydrogen production like steam reforming,
partial oxidation, plasma reforming, water electrolysis, water
thermolysis, photocatalytic water splitting, sulfureiodine
Fig. 1 e Schematic d
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cycle. Biomass and waste streams can in principle be con-
verted into bio-hydrogen with biomass gasification, steam
reforming or biological conversion like biocatalysed electrol-
ysis or fermentative hydrogen production. Fermentative
hydrogen production is a well-known process. Besides dark
and photo fermentation, bio-electrohydrogenesis (electrolysis
using microbes) is another possibility. The biocatalysed elec-
trolysis process is based on a biological anode where electro-
chemically active microorganisms transfer electrons from
organic substrate directly to a solid surface and the get
combined with Hþ ions at cathode in absence of oxygen.
Microbial electrohydrogenesis provides a completely new
approach for hydrogen generation from biomass, such as bio-
waste andwastewater, and accomplishing waste treatment at
the same time.
3. MEC system
Microbial Electrolysis Cell (MEC) technology represents
a new form of clean and green energy by generating elec-
tricity and hydrogen from what would otherwise be
considered waste. In bio-electrochemical systems bacteria
oxidize organic matter and release carbon dioxide and
protons into solution and electrons to anode [6]. The elec-
trons released in anode flows through an external electrical
circuit to the cathode where they are used in reduction of
oxygen. When oxygen is supplied to the cathode, current
can be produced and the system is known as MFC. However,
in absence of oxygen, and electrochemically enhancing the
cathode potential in an MFC circuit it is possible to produce
hydrogen (Fig. 1) directly by reduction of protons and elec-
trons produced by the bacteria [3,7e9]. This approach greatly
reduces the energy needed to make hydrogen directly from
organic matter compared to that required for hydrogen
production from water via electrolysis. In a typical MFC, the
iagram of MEC.
w of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 2 ) 1e1 3 3
open circuit potential of the anode is 0.30 V [10,11]. When
hydrogen production occurred at the cathode, the half
reactions occurring at the anode and cathode, with acetate
oxidized at the anode, is as follows [7]:
Anode : C2H4O2 þ 2H2O/2CO2 þ 8e� þ 8Hþ (1)
Cathode : 8Hþ þ 8e�/4H2 (2)
Producing hydrogen at the cathode requires a potential of at
least E0 ¼ 0.41 V (NHE) at pH 7.0 [12], so hydrogen can theo-
retically be produced at the cathode by applying a circuit
voltage greater than 0.11 V (i.e., 0. 41e0.30 V). This voltage is
substantially lower than that needed for hydrogen derived
from the electrolysis of water, which is theoretically 1.21 V at
neutral pH. In practice, 1.8e2.0 V is needed for water hydro-
lysis under alkaline solution conditions due to overpotential
at the electrodes [7].
Hence, thermodynamic analysis shows that, the addition
of greater than 0.11 V to the power that generated by bacteria
(0.3 V) will generate hydrogen gas at the cathode. However it
is observed that voltages of 0.2 V are needed because of
electrode over potentials. This hydrogen evolution process
provides a route for extending bio-hydrogen production past
the endothermic barrier imposed by the microbial formation
of fermentation dead-end products, such as acetic acid
and same exoelectrogenic bacteria can be used that was
used in MFCs and not exposing the cathode to oxygen
[7,10,13]. Although, some researchers [7e9,14] referred to the
MEC as a biocatalyzed electrolysis cell (BEC) or a bio-
electrochemically assisted microbial reactor (BEAMR) but
Logan et al. [1] defined the process as electrohydrogenesis or
microbial electrolysis to emphasize that in an MEC there is an
electrically driven hydrogen evolution process that is distinct
from fermentation.
4. Cathode material and catalyst used inMEC for hydrogen evolution
The hydrogen evaluation reaction (HER) reaction occurs at
the cathode. MEC construction includes one anodic chamber
with the anode; one cathodic chamber with cathode; or
a single chamber with both anode and cathode [15]; an
external electrical power source; and an electronic separator.
Cathode acts as the primary electron donor in an MEC system
[7,8,16].
A variety of parameters can limit current density in MECs,
for example, distance between the anode and cathode, elec-
trolyte resistivity (including the membrane), ARB (Anode-
respiring bacteria) density on the anode, and the specific
surface area of the electrodes, cathode material and catalyst
[17e20]. The base material of cathode and the catalyst
applied on it have significant effect on the performance of the
MECs. The base material of the cathode may be graphite,
titanium of other conductive materials. The hydrogen
evolution reaction (HER) on plain carbon electrodes is very
slow, requiring a high overpotential to drive hydrogen
production. To reduce the over potential a catalyst is used on
the cathodes [1].
Please cite this article in press as: Kundu A, et al., An overviewhydrogen in microbial electrolysis cell, International Journj.ijhydene.2012.11.031
4.1. Platinum as cathode catalyst
Platinum is commonly used as the catalyst due to its low
overpotential (�0.05 V at 15 Am�2) for the HER under opti-
mized conditions (for phosphate buffer at pH 6.2, for ammonia
it was at pH 9.0) of mass transport [21]. Platinum catalyzed
electrodes are commercially available e.g., E-TEK, USA;
Magneto Special Anodes, The Netherlands; Alfa Aesar,
Germany but can easily be prepared in the laboratory [22,23]
by mixing commercially available platinum (e.g., 10 wt% Pt/
C, E-TEK) with a chemical binder (5% Nafion solution or 2%
PTFE solution). Hydrogen production rates obtained
3.12 � 0.02 m3 m�3 reactor liquid day �1at Eap ¼ 0.8 V with
a cathodic hydrogen recovery rH2, cat ¼ 96% and an overall
energy efficiency of hEþS ¼ 75% by using Pt catalyst on carbon
cloth in a single chamber MEC of volume 28 ml [24]. Titanium
mesh having surface area 400 cm2, thickness 1 mm, specific
surface area 1.7 m2m�2 with platinum catalyst were used by
Rozendal et al. (2007) in two-chamber and one-chamber
configurations, with hydrogen production rates up to
0.3 m3 m�3 reactor liquid day�1at an applied voltage of 1.0 V
[8,15]. Selembo et al. (2010) applied platinum catalyst of
particle size 0.002 mm mixing with 400 mL and 50 mg carbon
black on carbon cloth using a brush to obtain hydrogen
production rate 1.6 � 0.0 m3 m�3 reactor liquid day�1,
Columbic efficiency 85.0 � 6.4% [25].
However, there aremany disadvantages to using platinum,
including the high cost and the negative environmental
impacts incurred during mining/extraction [26]. Platinum can
also be poisoned by chemicals such as sulfide, which is
a common constituent of wastewater [1]. Therefore, finding
out alternative of Pt catalyst for commercialization of the MEC
systemwas very important. Differentmaterials were tested as
cathode in MECs. In the following sections different alterna-
tives of platinum which has been used by different research
groups has been discussed in detail.
4.2. Microbial biocathode
Concepts of a cathode where microorganism will act as the
catalyst were first developed in the 1960s but no significant
progress was made [27]. In the recent years researchers have
studied and explored several metabolic processes present in
the cathode, stepping toward a possibility to develop
a cathode catalyzed by microorganisms or biocathode [28].
Microorganisms that can produce hydrogen are found in
a large variety of environments [29] and contain hydrogenases
that catalyze the reversible reaction given in Eq. (3).
2Hþ þ 2e�4H2 (3)
Purified hydrogenases have been successfully used on carbon
electrodes as a catalyst for hydrogen production [30e33].
Rozendal et al. [34] describes thedevelopmentof amicrobial
biocathode for hydrogen production from a naturally selected
mixed culture of electrochemically active microorganisms. An
MEC half cell with carbon felt electrodes was constructed with
a biological anode and fed with acetate and hydrogen at the
initial phase. The operation started in a batch mode and latter
shifted to continuousmode.Hexacyanoferrate(III)was reduced
of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 2 ) 1e1 34
at the cathode. Once the anodic current was stable, the acetate
and hydrogen supply was stopped, and the polarities of anode
and cathode were reversed, resulting in a biocathode and
chemical anode as schematically described in Fig. 2.
At a cathode potential of �0.7 V, the average current
generation of the biocathode was about �1.2 Am�2, average
volumetric hydrogen production rate during the hydrogen
yield tests was about 0.63 m3 H2/m3 cathode liquid volume/
day for the biocathode chamber and 0.08 m3 H2m�3 cathode
liquid volume/day for the control electrode chamber. A carbon
monoxide inhibition test was conducted to prove that the
biocathode is microbial in origin and expected microbial bio-
cathode behavior was observed.
Jeremiasse et al. [35] studied a full biological MEC where
both the anodic as well as cathodic reactions were catalyzed
by microorganisms to understand the difference of perfor-
mance predicted from the electrochemical half cell by
Rozendal et al. [34] using the same experimental set up. Two
set ups (MEC1 and MEC2) were used in this 1600 h study. The
cathode chamberwas inoculated 600 h after the inoculation of
anode chamber. During the 52-h yield test, MEC 1 had an
average current density of 0.79 Am�2 and produced 0.08 L of
hydrogen; and MEC 2 had an average current density of
0.84 Am�2 and produced 0.11 L of hydrogen. Hydrogen
production rate was 0.03 Nm3/m3 reactor liquid/day to
0.04 Nm3m�3 reactor liquidday�1. At an applied cell voltage of
0.8 V, both MECs had a current density of 3.3 Am�2. According
to the authors this result is compatible with the result of
Tartakovsky et al. (3.2 A m�2 at 0.85 V) [36].
Jeremiasse et al. [37] further investigated for the faster
start-up of microbial biocathode. Two attributes were studied,
the effects of cathode potential and carbon source. The former
attribute had no effect on the start-up time but the latter
reduced the start-up time almost 2 folds. According to the
researchers of this study higher biomass yield from acetate
may have helped the faster start-up of the microbial bio-
cathode which was supported by thermodynamic calcula-
tions. To increase the H2 production rate, a flow through
biocathode fed with acetate was investigated. This biocathode
produced 2.2m3 H2 m�3 reactor day�1 at a cathode potential of
�0.7 V versus NHE. In Table 1 the experimental conditions
used for biocathodes are tabulated.
Fig. 2 e Three phase biocathode start-up procedure. A) Start-up
inoculation with a mixed culture of electrochemically active mi
polarity reversal to a hydrogen-producing biocathode and adap
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4.3. Use of stainless steel as cathode
In the research to find out alternative for expensive Platinum
catalyst it was found that first row transition metals are very
useful due to their stability, abundance in nature, low cost,
and low toxicity to living organisms. The most promising
materials identified so far are nickel and stainless steel alloys
for their low cost, easy availability, low overpotentials, and
stability in highly alkaline solutions [39].
Olivares-Ramırez et al. [40] worked on three different type
of stainless steel each with different metal composition. SS
304, SS 316 and SS 430 containing 9.25%, 12%, and 0.75% of
nickel respectively were used for hydrogen evaluation reac-
tion (HER) in the alkaline electrohydrolyzer. From the cyclic
voltametry experiments modification of active surface of the
stainless steel and cyclic voltammograms well defined
cathode and anode peaks corresponding to hydrogen and
oxygen evolution zones in the cathodic (�1500 mV to
�1350 mV/NHE) and anodic (400e500 mV/NHE) regions
respectively were reported. The hydrogen evolution rate
which was analyzed by means of the graphical mineral oil
volume displacement as a function of time reveals that SS 316
is the best cathode electrode at various applied voltage among
SS 304, SS 316 and SS 430 and it was attributed to its higher
nickel content than the others used in the experiment.
Selembo et al. [39] studied different stainless steel alloys
304, 316, 420 and A286 and nickel alloys 201, 400, 625 and HX at
neutral pHcondition. Thecathodesweremadebycutting sheet
metal in to 3.8 cm diameter disks. Sheet metal of flat surface
was used to eliminate the influence of factors like catalyst
particle size, binding efficiency etc. The experiments were
conductedat either 0.6Vor 0.9 V at 30 �Cconstant temperature.
According to the results obtained SS A286 is the best
alloy in terms of overall hydrogen recovery (rH2,
COD ¼ 62 � 6%), cathodic hydrogen recovery (rH2,
cat ¼ 61 � 3%), energy efficiency relative to electrical input
(hE¼ 107� 5%), overall energy recovery based both electric and
substrate inputs (hEþS ¼ 46 � 3%), Current density
(IV ¼ 222 � 4 A m�3), hydrogen production rate
(Q¼ 1.50� 0.04m3m�3 day�1), andH2 content (80� 2%) at 0.9 V.
SS 304 also showed better result in comparison to platinum
sheet metal. SS alloys A286 (21.2 � 2.2 ml) and 304 produced
of an acetate and hydrogen-oxidizing bioanode after
croorganisms, B) adaptation to hydrogen oxidation only, C)
tation. Adopted from Ref. [34].
w of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
Table
1e
Experim
entalco
nditionsforth
etestin
gofbioca
thode.
MECsy
stem
Typeofelectro
deuse
dTypeofelectro
lyte
use
dTem
peratu
re(�C)
pH
Modeofoperation
Reference
Anode
Cath
odeandca
thode
catalyst
Anodech
am
ber
Cath
odech
amber
Electro
chemical
halfce
ll
Graphitefelt
Electro
chemically
activatedmicro
bes
ongraphitefelt
100mM
potassium
ferricyanide/
ferrocy
anide
solution
Standard
micro
bial
nutrientso
lutiona
e7
Initiallyfed-batch
modelater
continuousmode
[34]
Twoch
ambered
Electro
chemically
micro
beson
graphitefelt
Electro
chemically
activatedmicro
bes
ongraphitefelt
Standard
micro
bial
nutrientso
lutionaþ
1.36gL�1
NaCH
3COO$3
H2O
Standard
micro
bial
nutrientso
lutionaþ
0.84gL�1
NaHCO
3
30
7Continuous
mode
[35]
a0.74gL�1KCl,0.58gL�1NaCl,0.68gL�1KH
2PO
4,0.87gL�1K2HPO
4,0.28gL�1NH
4Cl,0.1
gL�1MgSO
4$7H
2O,0.1
gL�1CaCl 2$2H
2O,and1mLL�1ofatrace
elementmixtu
re[38].
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 2 ) 1e1 3 5
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, An overviewtional Journ
19.1 � 1.1ml hydrogen at an applied voltage of 0.9 V compared
to Platinum sheet metal producing18.9 � 5.4 ml. Ni 625 per-
formed better than the othermetals in terms of total hydrogen
gas production at 0.6 V applied voltage by producing 6.61mlH2.
Performance of SSA268 andNi 625were enhancedwhennickel
oxide was electrodeposited on the surface of the sheet metal.
Gas production increased from 9.4 to 25 ml for SS A286 and
from 16.2 to 25 ml for Ni 625 at an applied voltage of 0.6 V.
Energy recovery based on electrical input (hE) increased from
3.1% (SS A286) and 31% (Ni 625) to 137% for SS A286 and Ni 625
both with nickel oxide coating. Volumetric hydrogen produc-
tion rates (Q) also improved from 0.01 (SS A286) and 0.1 (Ni 625)
to 0.76m3H2m�3 day�1 for bothnickel oxidemodifiedmetals at
0.6 V. Methane gas production was reduced from 6.8 to 4.1 ml
for SS A286 and from 7.7 to 4.2 ml for Ni 625. These results are
very promising in terms of good performance and at low cost
which is an absolute requirement for large scale application.
Call et al. [41] selected high nickel containing SS 304 to use
in an MEC. They compared the performance of high surface
area SS brush with Pt-catalyzed carbon cloth (Pt/C) cathodes
and also examined the effect of material composition on
current productionwith SS brush and graphite brush cathode.
Hydrogen production rates and overall energy recoveries ob-
tained here for a reactor with 100% loaded SS brush oriented
vertically above and parallel to the core of the anode was
Q ¼ 1.7 � 0.1 m3H2m�3d�1 and overall energy recovery was
hEþS ¼ 78 � 5%.
Stainless steel mesh SS 304 woven and expanded mesh
having composition 0.08%C, 2%Mn, 1%Si, 18e20%Cr, and
8e11%Ni, was tested for their suitability as cathodes in MECs
as cathode [42]. SSmesh has higher surface area as well as flat
nature like flat sheet therefore closer space between anode
and cathode can be arranged. Linear sweep voltammetry (LSV)
result shows that �0.67 � 0.01 V of voltage is required to
generate substantial current and cyclic voltammetry (CV)
tests results measured that electrochemically active areas for
all mesh ranged from 12.23 to 23.26 cm2. Matsuda’s equation
(Eq. (4)) was applied to obtain the peak current, ip (A) and
effective surface area of the working electrodes:
ip ¼ 0:4464� 10�3n3=2F3=2AðRTÞ�1=2DR1=2C�Rv
1=2 (4)
where n ¼ 1 is the number of electrons transferred,
F ¼ 96,487 C/mol e� Faraday’s constant, R ¼ 8.314 J mol�1 K�1
the gas constant, T ¼ 303 K the temperature, CR* ¼ the initial
ferrocyanide concentration, and v ¼ the scan rate.
SS mesh no 60 with a wire diameter of 0.019 cm and a pore
size of 0.023 cm produced the highest current densities and
hydrogen production rate (3.3 � 0.4 m3H2/m3d, 2.1 � 0.3 m3H2/
m3d, and 0.8 � 0.1 m3H2m�3d�1) at the three different applied
voltages of 1.2 V, 0.9 V, 0.6 V respectively with respect to other
different meshes. When tested in MEC SS mesh no 60 was
used the Coulombic efficiency obtained was 101 � 1%. At
applied voltage 0.9 Ve1.2 V the cathodic hydrogen recoveries
(rcat) was almost 100%. The energy recovery relative to elec-
tricity input (hw%) reached the highest value of 232 � 9% at
0.6 V and the highest overall energy efficiency (hwþs%) of
74 � 4% at 0.9 V. A similar overall efficiency of 72 � 11% was
reached at 0.6 V. The SS mesh showed good stability as the
SEM-EDSmeasurement shows little variation the composition
of the SS after 15 cycles at applied voltage of 0.9 V.
of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 2 ) 1e1 36
Study on microbial corrosion showed that hydrogen
evolution reaction enhanced due to deprotonation of phos-
phate species on stainless steel [43,44]. When stainless steel
was combined with phosphate species it was found that the
cathodic deprotonation of phosphate identified in corrosion
could be exploited to enhance H2 production in neutral pH
electrolysis. The high concentration of phosphate species
used in combination with a stainless steel cathode allowed
high current density for hydrogen evolution and hydrogen
evolution rates of up to 4.9 Lh�1m�2 in saline solutions at pH
8.0, a relevant pH value for MEC operation [45]. The presence
of phosphate species and other weak acids has recently been
shown to have a beneficial effect in MEC, because the charged
species increase the electrolyte conductivity and also reduce
the overpotential on Pt-graphite cathodes [46]. The cathode
was tested in an MEC that used a microbial anode formed
from marine sediment and operated at high salinity.
Combination of bicarbonate buffer and SS 304 cathodes
with mesh no 60 provided good performance compared to
MECs containing Pt catalysts or phosphate buffers [47]. The
maximum volumetric hydrogen production rate of
Q ¼ 1.40 � 0.13 m3 H2-m�3 d�1, electrical energy efficiency of
hE ¼ 155 � 8%, overall energy efficiency of hEþS ¼ 68 � 3%, and
a Coulombic Efficiency (CE) ¼ 87 � 5% with MECs using SS
cathodes and BBS were similar to values produced with either
PBS or a Pt catalyst at Eap ¼ 0.9 V. Hence inexpensive SS
catalysts and carbonate buffers can achieve performance
similar to those achievedwith Pt and phosphate bufferswhich
are expensive as well as there is discharge limit of phosphate
in the environment which make phosphate buffer unsuitable
to be used for wastewater treatment. A comparison of the
testing conditions of stainless steel as cathode is tabulated in
Table 2.
4.4. Use of other metals and materials as cathode andcathode catalyst
Tungsten carbide (WC), inspite of performing less than plat-
inum as catalyst, is considered as catalyst material since
1960s because of its low price and insensitivity toward catalyst
poisons like H2S and CO [48]. In an half cell experiment with
conventional three electrode system maximum current
density of �118 mA cm�2 at overpotential 760 mV and
26 mA cm�2 at overpotential 300 mV the electrocatalytic
activity for tungsten carbidewas observed. Interestingly when
different composition of the tungsten carbide was tested
a significant relation between WC content and HER current
densities obtained [49].
Nickel and Ni-alloy for hydrogen production in MECs was
tested for the development of efficient, stable and cost-
effective cathodes by different research groups. Hu et al. [50]
developed cathodes by electrodepositing NiMo and NiW onto
a three-dimensional carbon-fiber-weaved cloth material and
were evaluated at neutral pH using electrochemical cells.
These electrodeswere also examined for hydrogen production
in single chamber tubular MECs with cloth electrode assem-
blies (CEA). While similar performances were observed in
electrochemical cells, NiMo cathode exhibited better perfor-
mances than NiW cathode in MECs. At an applied voltage of
0.6 V, the MECs with NiMo cathode accomplished a hydrogen
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production rate of 2.0 m3 m�3 reactor liquid day�1at current
density of 270 Am�3 (12 A m�2), which was 33% higher than
that of the NiW MECs producing 1.5 m3 m�3 reactor liquid
day �1and slightly lower than that of the MECswith Pt catalyst
producing 2.3 m3 m�3 reactor liquid day�1. The overall
hydrogen recoveries were 65%, 55% and 48% for theMECswith
NiMo, NiW, and Pt catalysts, respectively at the applied
voltage of 0.6 V. NiMo and NiW MECs ran for over 30 h and no
methane was detected by the end of the batches.
The possible reason for higher HER activity may be attrib-
uted to the higher loading of the catalysts than the Pt catalyst
[50]. According to the authors the uniform deposition of NiW
or NiMo increase the surface area of catalyst coating and
decrease cathodic over potential [51,52]. In a further study the
research group has shown that the optimal condition for
electrodeposition of NiMo on carbon cloth is Mo/Ni mass ratio
of 0.65 in electrolyte bath, an applied current density of
50 mA cm�2and electrodeposition duration of 10 min and
under this condition, the NiMo catalyst has a formula of
Ni6MoO3 with a nodular morphology [53].
Commercially available Nickel and stainless steel powder
with different sizes at different metal loadings were used on
carbon cloth to construct cathodes by Selembo et al. [25]. Two
cathode namely Ni 210 with 60 mg Ni in 267 mLN afion and Ni
210 þ CB with 60 mg Ni in 400 mL Nafion had the low over-
potential of �0.500 V and �0.683 V at this current density
range (�0.63 mAcm�2 ¼ �3.2 log Acm�2). Very similar current
densities produced with these two materials over the
complete range of applied voltages. These two metal powder
and binder combinations were used as cathodes in MECs.
Single chamber MECs made of Lexan were 4 cm long con-
taining 3 cmdiameter cylindrical-shaped chamberswere used
and anodes were ammonia treated graphite fibre brushes
(25 mm diameter � 25 mm length, 0.22 m2 surface area) made
with a titanium wire twisted core. The maximum current
increased for the two Ni cathodes over the first 6 cycles by as
much as 4.7 mA for Ni 210 and 4.1 mA for Ni 210 þ CB. The
surfaces of the Ni 210-based cathodesmaintained very similar
structures before and after 12 MEC cycles [25]. Nickel powder
was relatively stable with respect to corrosion when MECs
were continuously maintained under anaerobic conditions
but if exposed to air a nickel oxide layer is formed and can
become hydroxylated, forming Ni(OH)2 in water, and may get
dissolved. Different types of Ni alloy with different composi-
tion, chemically deposited on a GDL 25BC carbon paper was
evaluated by Manuel et al. [54]. A stable H2 production at rates
of 2.8e3.7 L LR�1 D�1 was obtained at a total catalyst load of
1 mg cm�2 containing Ni, Mo, Cr, and Fe on cathode. Only 5%
methane was present in the gas stream. When Ni loading was
of 0.6 mg cm�2 hydrogen production reached volumetric rate
of 4.1 L LR�1 D�1.
An optimized composition of 150 mM NiSO4$7H2O, 25 mM
NiCl2$6H2O, 500 mM H3BO3, and 1 mM CTAB at pH 4.5 was
used for Ni electrodeposition onto a porous carbon paper to
develop a low cost cathode capable of prolonged hydrogen
generation [55]. Hydrogen production as high as 5.4 L LR�1 d�1
with a corresponding current density of 5.7 A m�2 was ach-
ieved at a Ni load of 0.2e0.4 mg cm�2 under acetate non-
limiting conditions. The optimized Ni load and high porosity
of gas diffusion cathode providing higher surface area and
w of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
Table 2 e Experimental conditions for the testing of stainless steel cathode.
MEC system Type of electrode used Type of electrolyte used Temperature(�C)
pH Mode ofoperation
Reference
Anode Cathode andcathode catalyst
Anode chamber Cathode chamber
Electrochemical
half cell
Graphite felt Electrochemically
activated microbes
on graphite felt
100 mM potassium
ferricyanide/
ferrocyanide solution
Standard microbial
nutrient solution*
e 7 Initially fed-batch
mode later
continuous mode
[34]
Two chambered Electrochemically
microbes on
graphite felt
Electrochemically
activated microbes
on graphite felt
Standard microbial
nutrient solution*
þ 1.36 gL�1
NaCH3COO$3H2O
Standard microbial
nutrient solution*
þ 0.84 gL�1 NaHCO3
30 7 Continuous mode [35]
Single chamber Ammonia treated
graphite brushes
(25 mm diameter
� 25 mm
length, 0.22 m2
surface area)
Stainless steel alloys
and nickel alloys
(sheet metal cut
into 3.8 cm diameter
disks).
50-mM phosphate buffer solution (4.58 gL�1
Na2HPO4 and 2.45 gL�1 NaH2PO4$H2O),
0.31 gL�1 NH4Cl, 0.13 gL�1 KCl, and
trace vitamins and minerals.
30 7 Fed-batch [39]
Single chamber Ammonia treated
graphite brushes
The SS brush
cathodes
(grade 304 SS)
1 gL�1 Sodium acetate in a 50 mM
phosphate buffer medium (PBS; Na2HPO4,
4.58 gL�1; and NaH2PO4$H2O, 2.45 gL�1)
and nutrient solution (NH4Cl, 0.31 gL�1; KCl,
0.13 gL�1; trace vitamins and minerals.
30 7 Fed-batch [41]
Single chamber Ammonia treated
graphite brushes
(25 mm diameter
� 25 mm
length, 0.22 m2
surface area)
SS 304 woven and
expanded mesh
1 gL�1 of sodium acetate; 50 mM PBS
phosphate buffer (4.58 gL�1 Na2HPO4, and
2.45 gL�1 NaH2PO4$H2O), 0.31 gL�1 NH4Cl,
0.13 gL�1 KCl; and trace vitamins and
minerals
30 7 Fed-batch [42]
Double chamber Cylindrical felt
carbon anode
(0.25 m2
projected
surface area).
254SMO stainless
steel
Seawater at pH 7.5
added with marine
sediments from the
Atlantic Ocea
Water
containing
0.5 M sodium
phosphate
Room
temperature
8 Fed-batch [45]
Single chamber Ammonia treated
graphite brushes
(25 mm diameter
� 25 mm
length, 0.22 m2
surface area)
The SS brush
cathodes
(grade 304 SS)
1 gL�1 sodium acetate and Bicarbonate
buffer (BBS, 80 mM; 6.71 gL�1 NaHCO3,
0.31 gL�1 NH4Cl, 0.05 gL�1 Na2HPO4,
0.03 gL�1 NaH2PO4$H2O, pKa ¼ 8.34,
pH ¼ 8.9 as prepared)
30 8.9 Fed-batch [47]
* 0.74 gL�1 KCl, 0.58 gL�1 NaCl, 0.68 gL�1 KH2PO4, 0.87 gL�1 K2HPO4, 0.28 gL�1 NH4Cl, 0.1 gL�1 MgSO4$7H2O, 0.1 gL�1 CaCl2$2H2O, and 1 mLL�1 of a trace element mixture [38].
internatio
naljo
urnalofhydrogen
energy
xxx
(2012)1e13
7
Please
citeth
isarticle
inpress
as:
Kundu
A,etal.,
An
overview
ofca
thodem
ateria
land
catalysts
suita
ble
forgeneratin
ghydro
gen
inm
icrobial
electro
lysis
cell,
Intern
atio
nal
Journ
al
of
Hydro
gen
Energy
(2012),
http
://dx.doi.o
rg/10.10
16/
j.ijhydene.201
2.11.031
Table 3 e Experimental conditions for the testing of other metals and materials as cathode and cathode catalyst.
MEC system Type of electrode used Type of electrolyte used Temperature(�C)
pH Mode ofoperation
Reference
Anode Cathode and cathodecatalyst
Anode chamber Cathode chamber
Electrochemical
half cell
Platinum electrodes
served as the
counter electrodes
Tungsten carbide
powder mixed
with 5% Nafion�
solution and hand-
pressed onto the
graphite disc.
100 mM H2SO4, and 100 mM sodiume
phosphate-buffer,
22e23 �C 7 e [49]
Single chamber Carbon cloth type A NiMo and NiW
on carbon cloth
5 gL�1 of sodium acetate trihydrate, 100 mM
phosphate buffer and other nutrients.
30 � 2 7 Fed-batch [50]
Single chamber Ammonia treated
graphite brushes
(25 mm diameter
� 25 mm length,
0.22 m2 surface area)
Nickel (2e10 mm),
nickel oxide NiOx
87302 (�74 mm),
and stainless steel
powder on carbon
cloth (Nafion used
as binder)
1 gL�1 acetate and a 50 mM phosphate buffer
nutrient medium
30 7 Fed-batch [25]
Double chamber
(membrane less)
Carbon felt measuring
(10 cm � 5 cm and
5-mm thick)
Ni-alloy catalyst ink
with 30% Nafion on
GDL 25BC
carbon paper
Sodium acetate (90.7 gL�1), yeast extract
(6.7 gL�1), NH4Cl (18.7 gL�1), KCl (148.1 gL�1),
K2HPO4 (64.0 gL�1), and KH2PO4 (40.7 gL�1).
30 e Continuous
flow
[54]
Double chamber
(membrane less)
Carbon felt measuring
(10 cm � 5 cm and
5-mm thick)
Ni-salts
electrodeposited
on GDL 25BC
carbon paper
Sodium acetate (90.7 gL�1), yeast extract
(6.7 gL�1), NH4Cl (18.7 gL�1), KCl (148.1 gL�1),
K2HPO4 (64.0 gL�1), and KH2PO4 (40.7 gL�1).
30 e Continuous
flow
[55]
Double Chamber
with anion
exchange
membrane
Graphite felt
(10 � 10 � 0.25 cm)
Ni foam
(10 � 10 � 0.2 cm,
1360 kg m�3)
2.72 gL�1
NaCH3COO$3H2O,
0.68 gL�1 KH2PO4,
0.87 gL�1 K2HPO4,
0.74 gL�1 KCl,
0.58 gL�1 NaCl,
0.28 gL�1 NH4Cl,
0.1 gL�1 CaCl2$2H2O,
0.01 gL�1 MgSO4$7H2O
and 0.1 mLL�1 of
a trace element mixture
0.1 M KCl 30 � 1 �C Not
controlled
Continuous
flow
[56]
Double chamber
with heterogeneous
anion exchange
membrane
Graphite felt of
(10 � 10 � 0.25 cm)
Alloys with
nickeleirone
molybdenum
(NiFeMo) and
cobaltemolybdenum
(CoMo) electrodeposited
on Cu sheet
2.72 gL�1 NaCH3COO$3H2O,
0.68 gL�1 KH2PO4, 0.87 gL�1
K2HPO4, 0.74 gL�1 KCl,
0.58 gL�1 NaCl, 0.28 gL�1
NH4Cl, 0.1 gL�1 CaCl2$2H2O,
0.01 gL�1 MgSO4$7H2O
and 0.1 mLL�1 of a trace
element mixture
0.1 M KCl 30 � 1 �C 6 Continuous
flow
[60]
internatio
naljo
urnalofhydrogen
energy
xxx
(2012)1e13
8Please
citeth
isarticle
inpress
as:
Kundu
A,etal.,
An
overview
ofca
thodem
ateria
land
catalysts
suita
ble
forgeneratin
ghydro
gen
inm
icrobial
electro
lysis
cell,
Intern
atio
nal
Journ
al
of
Hydro
gen
Energy
(2012),
http
://dx.doi.o
rg/10.101
6/
j.ijhydene.2
012.11.031
Double
chamber
withpro
ton
exch
ange
membrane
Activatedca
rbon
fibers
with
enrich
edbacteria
Pdnanoparticle
depositedonplain
carb
onpaper
NaAc,
0.1
gL�1;NH
4Cl,
0.310gL�1;KCl,0.130gL�1;
CaCl 2,0.010gL�1;MgCl 2$6H
2O,
0.020gL�1;NaCl,0.002gL�1;
FeCl 2,0.005gL�1;CoCl 2$2
H2O,
0.001gL�1;MnCl 2$4H
2O,
0.001gL�1;AlCl 3,0.0005gL�1;
(NH
4) 6Mo7O,0.003gL�1;H
3BO
3,
0.0001gL�1;NiCl 2$6H
2O,
0.0001gL�1;CuSO
4$5H
2O,
0.001gL�1;ZnCl 2,0.001gL�1.
50mM
phosp
hate
buffer
e7
Fed-batch
[66]
Single
chamber
Ammonia
treated
graphitebru
shes
(25mm
diameter
�25mm
length
,
0.22m
2su
rface
area)
MoS2ca
talyst
powderon
stainless
steel
andca
rboncloth
1gL�1so
dium
ace
tate,
50mM
phosp
hate
buffer
aswellastrace
vitamins
andminerals
e7
Fed-batch
[72]
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 2 ) 1e1 3 9
Please cite this article in press as: Kundu A, et al., An ovehydrogen in microbial electrolysis cell, Internationalj.ijhydene.2012.11.031
rviewJourn
better transport of hydrogen improved the hydrogen produc-
tion rate not allowing themicrobial conversion of hydrogen to
methane. The highly reproducible procedure of electrodepo-
sition could be effectively used for manufacturing large scale
cathodes for pilot scale and industrial scale MECs. The size
and density of Ni particles could be easily controlled by elec-
trodeposition time and/or the concentration of Ni salt. The
authorsmade the observation that Ni nuclei of 200 nm formed
nodular Ni structure of about 10 mm and after 45 days the
characteristics were unchanged [55]. They attribute the
stability of the electrode to the nodular structure of Ni
deposition.
Jeremiasse et al. [56] used Ni foam as cathode for gener-
ating high purity hydrogen at high volumetric production rate.
Ni foam was found to have high HER catalytic activity under
alkaline condition [57,58] and low electrical resistivity than
graphite or titanium [59]. It is also cheap and easily available.
The pH was not controlled to observe the change in over-
potential due to pH change if any. The performance of the
MEC was found to be 50 m3 m�3 MEC d�1 of hydrogen
production at current density 22.8 � 0.1 A m�2 and electrical
energy input of 2.6 kWhm�3 H2 at applied cell voltage 1 V. The
cathodic overpotential was found to be low and the authors
suggested that this may be due to a large specific surface area
of Ni foam (128 m2 m�2 projected area). The measured over-
potential for Ni foam was �0.14 V at 10.5 A m�2. Due to large
surface area the true surface current density decreased and
consequently the activation overpotential was lowered. It was
found that the cathode overpotential increased during long
term operation but the reason for that is unclear to the
authors. Thismay be due to poisoning of active surface area of
cathode, formation of Nickel hydride or due to hydrogen gas
bubbles, clogging the pores but it needs further investigation.
Jeremiasse et al. [60] investigated with nickel-
eironemolybdenum (NiFeMo) and cobaltemolybdenum
(CoMo) range of alloys as possible HER catalysts in an MEC
around neutral andmild alkaline pH although these alloys are
known to have a high catalytic activity under strong alkaline
conditions in water electrolysers [60e62]. The catalytic
activity of these alloys was compared in a 0.1 M phosphate
buffered electrolyte of pH 6. Near neutral pH, Cu sheet cath-
odes coated with NiMo, NiFeMo or CoMo alloy showed a high
catalytic activity for the HER compared to cathodes that
consist of only Ni. Mass transport limitation at near neutral pH
becomes a hindrance for catalytic activity. Their catalytic
activity is best exploited at alkaline pH where mass transport
is not limiting. This was demonstrated in an MEC with CoMo
coated Cu cathode and anion exchange membrane, which
reached a production rate of 50 m3 H2 m�3 MEC d�1 (at STP) at
an electricity input of 2.5 kWh m�3 H2. A Ni-based gas diffu-
sion cathode having Ni loading of 0.4 mg cm�2was used to
treat domestic wastewater in a continuous flow microbial
electrolysis cell. Wastewater treatment efficiency observed
was maximum of 76% COD reduction at organic load 441 mg
La�1 d�1 and applied voltage Vapp ¼ 0.75 V. The energy
consumption at high organic loading of 3120 mg La�1 d�1 was
0.77 Wh g COD�1 and at lowest organic load 243 mg La�1 d�1 it
became 2.20 Wh g COD�1 [63] which is comparable with the
work of Cusick et al. which was done with Pt-based Microbial
Electrolysis Cell [64].
of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
Table 4 e Performance of different cathode materials and cathode catalysts used in MEC.
Cathode materialsand catalyst
Cathodepotential (V)
H2 yield (%) Currentgenerated (I )
A m�3
(unless otherwisestated)
Q ¼ hydrogenproductionrate m3 m�3
reactorliquid day�1
(unless otherwisestated)
Columbicefficiency (%)
Cathodichydrogen
recovery rH2,cat (%)
Overallhydrogen
recovery rH2,COD (%)
Energyefficiencyrelative toelectrical
input hE (%)
Overallenergy recovery
based bothelectric andsubstrate
inputs hEþS (%)
Reference
Biocathode �0.7 e �1.2 Am�2 0.63 e 49 e e e [34]
Biocathode 0.8 e 3.3 Am�2 0.03e0.04 e 17e21 e e e [35]
Biocathode �0.7 e e 2.2 e [37]
Stainless steel A286 0.9 80 � 2 222 � 4 1.50 � 0.04 e 61 � 3 62 � 6 107 � 5 46 � 3 [39]
Stainless steel 304 0.9 77 � 1 100 � 4 0.59 � 0.01 e 53 � 1 49 � 0 90 � 2 38 � 1 [39]
Ni 625 0.9 67 � 9 160 � 22 0.79 � 0.27 e 43 � 9 41 � 13 75 � 16 31 � 8 [39]
SS A286 þ NiOx 0.6 76 � 2 130 � 21 0.76 � 0.16 e 52 � 4 56 � 2 137 � 12 48 � 3 [39]
Ni 625 þ NiOx 0.6 76 � 5 131 � 7 0.76 � 0.15 e 52 � 9 56 � 10 137 � 24 48 � 9 [39]
Stainless steel-brush 0.6 e 188 1.7 e 84 e 221 78 [41]
Stainless Steel
304 mesh þbicarbonate
buffer
0.9 e e 1.40 � 0.13 87 � 5 e e 155 � 8 68 � 3 [47]
NiMo on carbon-
fiber-weaved cloth
0.6 2.6 mol mol�1 270 2.0 75 86 e 182 65 [50]
NiW on carbon-fiber-
weaved cloth
0.6 2.2 mol mol�1 200 1.5 73 e 114 55 [50]
Ni 210 (60 mg Ni in
267 mL Nafion on
carbon cloth)
0.6 92 � 0 160 � 31 1.3 � 0.3 92.7 � 15.8 79 � 10 73 � 3 210 � 40 65 � 2 [25]
Ni 210 þ CB (60 mg
Ni in 400 mL Nafion
on carbon cloth)
0.6 92 � 1 139 � 2 1.2 � 0.1 83.8 � 1.2 94 � 5 79 � 5 252 � 12 73 � 4 [25]
Ni (0.6 gm chemically
deposited on a GDL
25BC carbon paper)
1.0 2.8 mol mol�1 3.60 Am�2 4.14 68.0 103.3 e e e [54]
Pd nanoparticle on
carbon cloth
0.6 e e 2.6 � 0.5 L m�2 d�1 56.0 � 10.2 46.4 � 8.5 26.0 � 4.8 e e [66]
MoS2 on carbon cloth e 92 � 3 10.7 � 1.2 Am�2 e e e 90 � 2 123 � 7 57 � 3 [72]
internatio
naljo
urnalofhydrogen
energy
xxx
(2012)1e13
10Please
citeth
isarticle
inpress
as:
Kundu
A,etal.,
An
overview
ofca
thodem
ateria
land
catalysts
suita
ble
forgeneratin
ghydro
gen
inm
icrobial
electro
lysis
cell,
Intern
atio
nal
Journ
al
of
Hydro
gen
Energy
(2012),
http
://dx.doi.o
rg/10.101
6/
j.ijhydene.2
012.11.031
i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 2 ) 1e1 3 11
Recent extensive works in the field of nanofabrication
provide a unique opportunity to develop efficient electrode
materials due to the fact that nanomaterial’s are very stable
not only structurally but also have stable electrical and
chemical properties [65].
Palladium, being the most platinum like metal and with
excellent catalytic property and high abundance, is of prime
importance. Huang et al. [66] investigated the feasibility of
using Pd nanoparticles for hydrogen evolution in MEC. Elec-
trochemical deposition of Pd was conducted in 0.1 M NaCl
solution containing 1.26 mM K2PdCl6 as the precursor on
a carbon clothwith amaximum loading calculated as 0.17mg,
or 0.0106 mg cm�2 assuming 100% electrodeposition. A low
surface density of Pd nanoparticles with uniform shape was
formed under the tested conditions. The Pd nanoparticles
coated cathode performed better with coulombic efficiency of
56.0 � 10.2% and hydrogen generation of 2.6 � 0.5 L m�2 d�1
than the Pt catalyst used in this study with coulombic effi-
ciency of 52.0 � 7.4% and hydrogen production of
2.1� 0.3 Lm�2 d�1. The catalytic efficiency of Pdwas almost 50
timesmore than Pt. Thismuch higher catalytic activity inMEC
for hydrogen evolution may be due to the surface nano-sized
morphology of Pd deposition and deposition condition [67].
The Ni-based nanomodified materials are promising elec-
trocatalysts for HER in near neutral electrolytes and could be
applied as cathodes in MECs [68]. The electrocatalytic prop-
erties of electrodeposited NiFe-, NiFeP- and NiFeCoP-
nanostructures onto carbon felt toward HER in neutral and
weak acidic solutions show higher catalytic activity than bare
carbon felt. The voltage needed to initiate hydrogen produc-
tion and the current production rates were in the range of
�0.60 and �0.75 V vs. Ag/AgCl as estimated from obtained
linear voltammograms. The highest current production rate
corresponding to 1.7 � 0.1 m3 m�3 reactor liquid day�1was
achieved with NiFeCoP/carbon felt electrodes. These nano-
structure modification results in improvement of the corro-
sion resistance in neutral phosphate buffer and weak acidic
(pH 5.5) acetate buffer solutions.
Research work on enzymes and quantum chemical simu-
lations suggested that molybdenum disulfide (MoS2) would
have a small free energy difference for HER [69e71]. Use of
MoS2 as a photocatalyst for hydrogen evolution is well-known
but it was used in electrochemical studies for HER by Tokash
et al. [72]. Based on LSV study in sodium perchlorate and
a phosphate buffer solution optimumMoS2 loading was 54mg
MoS2 and 60 mg carbon black, or 47 wt.% MoS2, with a surface
density of 45 gm�2 but considering the cost with the perfor-
mance of LSV and galvanostatic polarization tests the 33 wt.%
MoS2 composite cathode was chosen for tests in MECs. The
performance was compared with a typical Pt catalyst on
carbon cloth which maintained the same amount of carbon
black and Nafion binder as used in the MoS2 composite cath-
odes. At average current density of 10.0 � 1.6 Am�2 for Pt and
10.7� 1.2 Am�2 for MoS2, and the average fed-batch cycle time
14.7 � 0.4 h the gas compositions were H2 ¼ 90 � 6%,
CH4 ¼ 1 � 0.6%, and CO2 ¼ 8.7 � 5.4% for the Pt cathodes, and
H2¼ 92� 3%, CH4¼ 1� 0.8%, and CO2¼ 6.7� 2.3% for theMoS2cathodes. COD removals were 88 � 2% for MECs with Pt-based
cathodes and 90 � 2% for the MECs with MoS2 based cathodes.
Electrical energy efficiency for cathodeswas 123� 7%which is
Please cite this article in press as: Kundu A, et al., An overviewhydrogen in microbial electrolysis cell, International Journj.ijhydene.2012.11.031
less than platinum-based cathodes (158 � 14%). However,
overall efficiency of MoS2 cathodes was similar to platinum-
based cathodes with 61 � 5%. Considering the cost MoS2composite cathodes costs one fifth of the typical platinum
cathode. Table 3 describes the experimental conditions for
testing different material as cathode and cathode catalyst
other than stainless steel or bio-cathode.
5. Summary
Cathode is one of themost important parts of theMEC system.
Toward the practical implication ofMEC system the cost of the
cathode along with its higher efficiency are two very impor-
tant parameters which are of immense importance. Cathode
along with its catalyst may contribute almost 47%, which is
a substantial amount toward the total cost of the MEC system
[73]. Although it was well-known that platinum is a good
catalyst for hydrogen generation its cost prevented it to be
used in practical situation. In search of material having same
or more efficiency at much lower cost it was found out that
stainless steel or nickel alloys are good alternatives. Gas
diffusion cathode prepared by chemical deposition of Ni alone
can successfully be used for hydrogen production. Molyb-
denum disulfide, tungsten carbide, Ni alloys like NiMo, NiW,
NiFeMo are successfully used by different research groups for
hydrogen generation. Application of nanotechnology has
shown a new area of research. Electrodeposited palladium
nanoparticles as cathode catalyst for hydrogen generation
was successfully operated. Ni-based nanomodified materials
are also very promising electrlocatalyst for hydrogen
production. The comparison done in Table 4 disclose that
among all the cathode materials stainless steel, MoS2 and
nickel foam are worth mentioning owing to their easy avail-
ability, low cost and comparative efficiency with platinum
cathode catalyst. Stainless steel is probably the bettermaterial
in terms of its durability, structural strength and easy adapt-
ability to mass-manufacturing approaches. Although, pilot
scale application of these cathodes and cathode catalysts are
yet to be tested for their performance in real wastewater
treatment, this development of different low cost material to
be used as cathode or cathode catalyst has taken the possi-
bility of MECs to be used more and more in practical purposes
a step closer to be reality.
Acknowledgments
The authors are grateful to University of Malaya (Project no:
UM.C/HIR/MOHE/ENG/20) for providing the fund for the
research work.
r e f e r e n c e s
[1] Logan BE, Call D, Cheng S, Hamelers HVM, Sleutels THJA,Jeremiasse AW, et al. Microbial electrolysis cells for highyield hydrogen gas production from organic matter. EnvironSci Technol 2008;42:8630e40.
of cathode material and catalysts suitable for generatingal of Hydrogen Energy (2012), http://dx.doi.org/10.1016/
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 2 ) 1e1 312
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