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Effect of asymmetric variation of operating parameters on EED
cell for HI concentration in IeS cycle for hydrogen production
Pradeep Kumar Sow, Anupam Shukla*
Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, India
a r t i c l e i n f o
Article history:Received 11 May 2012
Received in revised form
17 July 2012
Accepted 18 July 2012
Available online 11 August 2012
Keywords:
Electro-electrodialysis
IeS cycle
Open circuit voltage
a b s t r a c t
EED process for HI concentration was studied for the effect of individual operatingparameters such as I2/HI ratio, concentration of HIxHI=H2O, temperature and pressure.
Studies were conducted in an asymmetric system where the effects of operating param-
eters were varied for anolyte and the catholyte separately. Open circuit voltage (OCV) was
found to be a contributor toward the net potential drop across the EED cell. Ohmic resis-
tance was found to decrease with increase in I2/HI ratio in catholyte and was found to
increase with increase in I2/HI ratio in anolyte. Increase in xHI=H2 O decreased the resistance
for anolyte section whereas caused an increase in resistance for catholyte section. Increase
in temperature reduced the voltage drop and the resistance across the EED cell. A non-zero
differential pressure between the two compartments of the cell increased the resistance
across the cell without affecting the OCV value. Electrode potential studies at the graphite
electrodes showed an increase in the electro potential with increase in the iodine
concentration and decrease with the increase in the HI concentration. Energy required for
concentrating acid increased linearly with current density favoring operation at lowcurrent densities. Energy consumed in overcoming OCV contributed substantial fraction of
the total energy consumed in EED process at lower current densities.
Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction
At present worlds energy architecture is heavily dependent
on the nonrenewable sources and mostly on the fossil fuel
reserves. Fossil fuels suffer from few other problems mostnotably carbon dioxide emissions [1]. To address the growing
energy need, search for alternative energy sources as
well as energy carriers has accelerated in past few decades
[2,3]. Other energy sources being explored such as wind,
hydro etc. still constitute a very limited fraction in the total
energy generated, which does not lead us to a large scale
energy solution [4,5]. One of the most widely researched
energy carriers is hydrogen owing to the higher energy density
per unit mass and environmentally benign combustion
product [5e8].
Most of the hydrogen requirement of the present time is
met by the steam reforming process that suffers from a major
drawback of CO2 as a byproduct [5,9]. Proposed alternativesinvolve hydrogen production from water by using a variety of
processes like water splitting process, fermentation of
biomass, from bio-ethanol etc. [6e12]. Large scale hydrogen
production by closed loop thermo-chemical cycles has
attracted a lot of attention owing to their lower heat demand
compared to direct thermal decomposition of water [13e15].
Thermo-chemical cycles essentially constitute the application
of both heat and chemicals for breaking down water into
* Corresponding author. Tel.: 91 11 26596290; fax: 91 11 26581120.E-mail address: anupam@chemical.iitd.ac.in (A. Shukla).
Available online at www.sciencedirect.com
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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 e n e r g y 3 7 ( 2 0 1 2 ) 1 3 9 5 8 e1 3 9 7 0
0360-3199/$ e see front matter Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijhydene.2012.07.068
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hydrogen and oxygen at temperatures much lower than the
direct decomposition of water. The high temperature needed
for the process can be supplied by nuclear energy from high
temperature gas cooled reactor. More than 200 thermo-
chemical processes have been proposed till date. Iodi-
neeSulfur (IeS) cycle, proposed by General Atomics (GA) is
found to be one of the most energy efficient [16]. IeS process
essentially consists of three major reactions which are the
Bunsen reaction, Hydroiodic acid decomposition and sulfuric
acid decomposition [13].
Bunsen reaction : I2 SO2g 2H2O!2HIaq: H2SO4aq:
(1)
Sulphuric acid decomposition : H2SO4aq:! H2Ol
SO2g
12
O2 (2)
Hydroiodic acid decomposition : 2HIaq:!H2 I2 (3)
In the above reaction scheme, iodine and SO2 act as recy-
cling agents and are recycled back from decomposition
sections to Bunsen reactor in a continuous operation. In thetraditional Bunsen reaction step of the IeS cycle, excess
amount of iodine and water is used for facilitating the phase
separation between the two acid phases and making the
reaction spontaneous, respectively [13,16]. The traditional IeS
cycle has lower efficiency compared to the theoretically
calculated value primarily due to downstream problems
created by these excess of reactants [17,18]. Although highest
fraction of total heat required is consumed in the sulfuric acid
decomposition section, it is generally believed that theres
little room for further optimization of that section [19]. A
significant amount of energy is also consumed in the
concentration of HIx solution coming out of Bunsen reactor.
The HIx from the Bunsen reaction stage is a pseudo-azeotropicmixture thereby increasing the heat requirement substan-
tially to affect the concentration process [17,20]. Corrosive
nature of the HIx solution further increases technical diffi-
culties especially material selection. One of the major tech-
nical challenges for efficient IeS process is an energy efficient
process for separation and concentration of hydroiodic acid
coming out of the Bunsen reaction.
Following process developments have been suggested in
literature to concentrate the HIx solution from the Bunsen
reaction:
a) Extractive distillation using phosphoric acid as the
extracting agent [13].
b) RheinischeWestfalische Technische Hochschule (RWTH),
suggested the feasibility of concentrating HIx solution
using reactive distillation under pressurized condition [18].
c) Electro-electrodialysis (EED) using ion exchange
membranes [21].
The EED process for concentration of HIx suggested by
Onuki et al. [21] was found effective in concentrating the HIxfor the HI decomposition step. The energy consumption was
found to be lower than the other alternatives described above.
EED process consists of a two-compartment cell separated by
an ion exchange membrane which performs the function of
a selective barrier between the HI-concentrated and the HI-
depleted electrolytes allowing only the selective passage of
hydronium ions. Reversible redox reaction at the anode and
cathode [Eqn (4)] involves conversion of iodine to iodide ion
and vice versa. Cation exchange membrane was preferred
over anion exchange membrane owing to lower resistance to
ion transport and lower energy consumption [21].
I2 2e%
cathode
anode2I (4)
Studies on EED cell also suggested that the resistance of theEED cell increases with increase in the iodine content of the
HIx feed solution in a symmetric system (identical anode and
cathode compartments and identical initial concentration of
anolyte and catholyte) [22]. Increase in the resistance was
ascribed to thereducedmobility of the iodide ion which forms
poly-iodide complex with iodine. Studies on the effect of
temperature as an independent operating variable on the EED
process were also reported in literature [23,24]. Increase in
temperature decreased both the transport number of the
protons through the membrane and the ohmic resistance of
the cell. The decrease in the resistance was due to the
increased thermal energy and hence increased the mobility of
the ions in the solution. Studies also suggested that themembrane potential drop decreases (w80%) as the operating
temperature is increased from 293 to 373 K [24]. A study was
carried out to determine the effect of graphite electrodes of
different BET surface area (1400 and 700 m2/gm) and different
I2/HI ratio [25]. It was found that the electrode with higher BET
surface area was more energy efficient in concentrating HIxsolution. Tanaka et al. studied the use of radiation grafted
membrane for the EED process based concentration of HIxsolution [26]. It was shown in the above mentioned studies
that the radiation grafted membrane offered lower resistance
as compared to Nafion 117 membrane.
Following information about the EED system is not avail-
able in the literature:
Nomenclature
a, b Tafel constants
Eeq/a equilibrium potentials at anode
Eeq/c equilibrium potentials at cathode
Ej potential drop across membrane
I2/HI ratio ratio of molarities of I2 to HI
Rsm combined solution and membrane resistance
OCV open circuit voltage
Vcell cell voltage drop
xHI=H2O mole fraction of HI on iodine free basis
ha anode reaction overpotential
hc cathode reaction overpotential
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a) Studies were done in a system where the initial concen-
tration of anolyte and the catholyte sections was same. In
actual continuous process in conjugation with other IeS
cycle components, the steady state concentration of HI in
the catholyte section of EED would be higher
xHI=H2O > 0:157 than the anolyte section.
b) Studies on effect of I2/HI ratio on cell performance were
carried out by simultaneously varying the ratio in both theanolyte and catholyte sections. Thus, the effect of I2/HI
ratio of each of the anolyte and the catholyte section is not
known independently.
The presentwork focuses on the effects of the independent
variables on cell voltage where the physical parameters of
anolyte and catholyte were varied separately as shown in
Table 1. The concentration of HI in the catholyte chamber
xHI=H2 Ow0:185 was kept higher than the azeotropic concen-
tration xHI=H2Ow0:157 thus augmenting the information
available for the system in literature. In addition, effect of
pressure of the anolyte and the catholyte sections is also re-
ported. Effect of the electrolyte flow rate was studied sepa-rately for determining the mass transport resistance in terms
of cell voltage drop. Studies on the equilibrium potential at the
electrodes were done separately to determine the effect of
concentration of electroactive components on the OCV value
generated.
2. Theory
EED process for concentration of hydroiodic acid consumes
electrical energy. Optimizing electrical energy consumed in
EED is necessary to improve the overall performance of the
IeS process. Energy consumption depends upon the voltage
drop across the cell and the current efficiency. The cell voltage
drop (Vcell) is due to (a) the total ohmic resistance offered by
the solution and the membrane (Rohm), (b) overpotential at the
electrodes, and (c) the open circuit voltage (OCV) value and (d)
voltage drop required to overcome the mass transfer or the
diffusion resistance (Vmt) to the transport of the reactants
from the bulk to the electrode surface.
Vcell can be written as
Vcell ha hc Vohm Vmt OCV (5)
where ha and hc arethe overpotentialat the anode andcathode
respectively, Vohm is the ohmic potential drop, Vmt is the mass
transfer potential drop and OCV is the open circuit potential.
Vohm along with Vmt varies linearly with current [27].
Vohm Vmt iRohm Rmt iRsm (6)
The electrode overpotential at higher current density (such
that the reverse reaction at the electrode is negligible) can berepresented using Tafel equation [27]. Combined Tafel equa-
tion for both the electrodes can be expressed as
hnet hahc aaba lniacbc lniab lni (7)
where a is sum of ac and aa and b is sum of bc and ba. OCV
consists of three different potentials [27,28].
OCV Eeq=a Eeq=c Ej (8)
where Eeq/a and Eeq/c are the equilibrium potential at the
anode and cathode respectively and Ej represents potential
difference across the membrane. Studies conducted by
Tanaka and Onuki [28] on the equilibrium potential for the
EED cell suggested it to be independent of the current density.
Empirical relationship developed is given by
OCV 4:7 106Texp
1:6 103=T ln
0BBBBB@
xHI;AxHI;C
2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
xI2 ;AxI2;C
s1CCCCCA (9)
Using Eqns. (6) and (7), Eqn. (5) can be written as:
Vcell Da Dblni i Rsm OCV (10)
3. Experimental
Two different sets of experiments were done. The first set
involved experiments on an EED cell while the second set of
experiments were done to determine behavior of electrodes in
contact with their respective electrolytes.
3.1. Materials and instrumentation
Electrolyte solutions were prepared using 55 wt% hydroiodic
acid (AR grade) supplied by CDH Pvt. Ltd., and iodine
Table 1 e Different independent operating variables varied in the EED experiments. Table showing the values of theoperating variables under three headers of low mid and high. Values of other parameters were: I2/HI ratio anolyte: 0.73, I2/HI ratio catholyte: 0.43, xHI=H2O anolyte: 0.112, xHI=H2O, catholyte: 0.183, temperature:293 K, anolyte pressure:1 bar, catholytepressure: 1 bar, differential pressure: 0 bar.
Levels Independent operating variable varied
I2/HI ratioanolyte
I2/HI ratiocatholyte
xHI=H2Oanolyte
xHI=H2Ocatholyte
Temperature(K)
Anolyte pressure(bar)/Differential
pressure (Pano Pcatho) (bar)
Catholyte pressure(bar)/Differential pressure
(Pano Pcatho) (bar)
Low 0.56 0.26 0.08 0.145 293 1 (0) 1 (0)
Mid 0.73 0.33 0.093 0.16 308 1.5 (0.5) 1.5 (0.5)
323
High 0.92 0.43 0.112 0.183 341 2 (1) 2 (1)
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(AR grade) supplied by Fischer Chemicals Ltd. without any
further purification. Nafion 117 (supplied by Electrochem Inc.,
USA) was used in the EED cell as a separator between the
compartments. Flexible graphite sheets supplied by ONS
Engineers & Consultants India Ltd., Mumbai were used as the
electrodes. Sodium thiosulphate and sodium hydroxide
(AR grade) procured from Fischer Chemicals Ltd. were used
without further purification for determination of hydroiodicacid and iodine concentration. An autotitrator (make: Mettler
Toledo, model: DL-15) was used for the estimation of hydro-
iodic acid concentration and iodometric titration was used for
the determination of the iodine content in the sample. A
potentiostat (make: Gamry model: reference 600) was used for
the measurement of electrode potentials. A d.c. power supply
(Instek GPS-2303) was used for the EED experiments.
3.2. Electro-electrodialysis experiment
Experimental setup shown in Fig. 1 can be divided into three
major sections, namely the anolyte loop, the catholyte loop
and the two-compartment EED cell. Two leak-free (Durion
pressure plus) bottles were used as reservoirs for the anolyte
and thecatholyte. Thereservoirs were mounted on a hot plate
magnetic stirrer for providing necessary heat to increase the
reservoir temperature and mixing. Electrolyte solutions from
the reservoir were pumped to the EED cell through silicon
tubing using variable flow peristaltic pumps (Miclins PP 30 EX).
Nitrogen was used for pressurizing the anolyte and the cath-
olyte sections by monitoring pressure with the help of pres-
sure gauges connected to each reservoir. Digital temperature
sensors were used to continuously monitor the electrolytes
temperature.
The two-compartment EEDcell wasfabricated using Teflon
and the compartments were separated by Nafion 117
membrane. The membrane was supported with two neoprene
rubber gaskets to prevent the electrolyte leakage from the EED
cell under pressure. Each of the Teflon compartment housed
a graphite electrode. The graphite electrodes were ultra-
sonicated and dried in air oven for 10 h before use. Baffles
made of Teflon were placed inside both compartments of thecell to improve the flow of the electrolyte through the cell and
avoid channeling or dead zones. Active area of the cell was
10 cm2.
Before the start of the experiment, the anolyte and the
catholyte reservoirs were filled with aqueous HIx solution and
all the tubings connecting reservoir to the cell were purged
with nitrogen for 10 min. After that the purge lines were
closed and the pressures of the reservoirs were adjusted to the
required values. The heaters and stirrers were switched for
raising the electrolyte solution to desired temperature. Both
the electrolytes were circulated continuously in their respec-
tive loops (reservoir to their compartment of the cell and back
to reservoir)using peristaltic pumps. After the system reacheda steady state (constant temperature), the voltage drop across
the cell was noted as the open circuit voltage (OCV). There-
after, electro-electrodialysis was started by applying
a constant current to the EED cell. Current density was varied
from 0.002 to 0.275 A/cm2 and the corresponding steady state
cell voltage was recorded to obtain the currentevoltage (IeV)
curve. Each experiment was repeated thrice and the average
values of the resulting data have been reported.
Table 1 shows the values of the various parameters that
were used for the EED experiments. xHI=H2O in catholyte was
kept higher at 0.183 while for anolyte it was 0.112. I2/HI ratio
was kept 0.43 for the catholyte whereas it was kept higher for
the anolyte at 0.73. The operating temperature was kept at293 K with both the anolyte and catholyte under atmospheric
pressure (differential pressure 0 bar).
First set of experimentswere done at different flow ratesof
anolyte and catholyte. Two types of flow rate studies on the
cell voltage drop were conducted. In the first type, experi-
ments were done with identical flow rates for both anolyte
and catholyte. The linear flow velocity was varied from 0.95 to
3.53 cm/s. In the second type, flow rate was varied in one of
the sections (in the linear velocity range of 0.95e3.53 cm/s)
keeping the flow rate in the other section constant (linear
velocity 3.53 cm/s).
Further experiments were done with anolyte and catholyte
linear velocity of 3.53 cm/s. Experiments were done by varyingthe xHI=H2O of anolyte and catholyte separately keeping the I2/
HI ratio constant at 0.73 for the anolyte and 0.43 for the
catholyte section. The anolyte xHI=H2 O was varied in the range
of 0.08e0.113 whereas the catholyte xHI=H2 O was varied
between 0.145 and 0.183.
For experiments to determine the effect of I2/HI ratio, the
xHI=H2O values for both the anolyte and the catholyte were held
constant at 0.112 and 0.183 respectively. The I2/HI in the
anolyte was varied in the range of 0.56e0.92 whereas its value
for the catholyte section was varied from 0.26 to 0.43.
EED experiments were also done by varying pressures of
anolyte and catholyte separately using nitrogen gas. In these
experiments, the xHI=H2 O values for anolyte and catholyte were
1: Anolyte reservoir
2: Catholyte reservoir
3: EED cell
4: Heating plate and stirrer
5: Nitrogen cylinder
6: Pressure Gauge
7: Digital Temperature Indicator8: Peristaltic Pump
9: Valve
13
4 4
5
5
6 6
77
8 8
99
2
Fig. 1 e A schematic diagram of the EED experimental
setup used for concentration of hydroiodic acid.
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fixed at 0.112 and 0.183 respectively and the I2/HI ratio in the
anolyte and catholyte were kept at 0.73 and 0.43 respectively.
Experiments were also done at four different temperatures
between 293 and 341 K. Magnetic stirrer was used to keep the
temperature as well as the concentration uniform inside the
reservoirs.
The values of the different operating variables in the
different EED experiments are summarized in Table 1.
3.3. Electrode potential studies
Equilibrium potential of graphite electrode in different
concentration HIx solutions was measured in a three electrode
cell using a potentiostat/galvanostat (make: Gamry, model:
Reference 600) Ag/AgCl/KCl (3 M) electrode was used as the
reference electrode and Pt wire as counter electrode. Graphite
strip of 2 cm2 area was used as the working electrode. The
electrodes were first washed in distilled water then ultra-
sonicated for 10 min to remove all the adsorbed impurities.
The cleaned electrode was then dried in an air oven for 10 h
before use in experiments.In this case, two different sets of experiments were done.
First set consisted of measurement of electrode potential on
HIx solutions with constant iodine concentration of 0.63M and
varying HI concentration between 2.2 and 5.5 M. The second
set usedsolutions with constant HI concentration (2.75 M) and
varying iodine concentration between 0.32 and 1.9 M.
4. Results and discussion
In a continuous IeS cycle, EED operates along with the other
units of the IeS process and the concentration of a stream at
any point of flowsheet remains constant at the steady state.Thus, in the EED operation, various points (along length of the
stack) will have different concentrations of anolyte and
catholyte. The outlet concentration of HIx solution from
Bunsen reaction that goes to EED unit is nearly azeotropic
xHI=H2 Ow0:155. Different flowsheets have been proposed in
literature where the anolyte stream of EED is obtained by
mixing a part of Bunsen outlet HIx stream with liquid stream
coming out of flash/distillation or from HI decomposer (after
H2 removal). These streams are rich in iodine and also contain
water. Thus the EED anolyte inlet xHI=H2O is expected to be
lower than 0.15. Also, the anolyte xHI=H2O decreases along the
length of EED unit. In this work therefore xHI=H2O in the anolyte
section was taken as 0.08, 0.093 and 0.112. xHI=H2 O for catholytewas taken as 0.145, 0.160 and 0.183 since the HI concentration
of the catholyte increases along the length of EED unit. Also,
optimal value for EED exit concentration was reported to be
between 0.175 and 0.197 [29]. xHI=H2O ratio of 0.183 represents
above-azeotropic concentration and the vapor in equilibrium
with this solution has high HI mole fraction (>60 mol%) [30].
Therefore, this solution can be further concentrated using
distillation.
A lower value of I2/HI ratio was used as compared to the
value in the outflow stream of traditional Bunsen reaction
(w4). This was done because newer schemes of carrying out
Bunsen reaction like membrane electrolysis are being
proposed in literature and in these schemes a lower value of
I2/HI molar ratio (w0.5) is used [31]. Also even for traditional
scheme of Bunsen reaction (where I2/HI ratio is more), sepa-
ration schemes are reported where HI rich phase is subjected
to flash separation first. During flashing a large amount of
iodine is separated from the HIx solution. The HIx solution
from the flash column is further concentrated before being
sent for decomposition. Thus even with traditional scheme of
Bunsen reaction, the EED operation can be used for concen-trating HIx solution after flash operation [14,32]. Concentra-
tion of anolyte iodine increases along the length of EED stack
and hence I2/HI molar ratios of 0.56, 0.73 and 0.92 were used
in this work. On the other hand, concentration of catholyte
iodine decreases along the length of EED stack. Therefore
0.26, 0.33 and 0.43 were selected as catholyte I2/HI molar
ratios.
The effect of different concentrations of both HI and iodine
on Vcell is required to calculate energy required for EED oper-
ation. Therefore in this work, experiments were performed
with higher concentration of HI in catholyte and lower
concentration in anolyte. Effect of different operating
parameters was obtained using anolyte and catholyte flowrates at which mass transfer effects were minimal. One of the
desired targets is to operate EED at conditions where the mass
transfer resistance is reduced to a minimum. Mass transfer
resistance causes a non-linear increase in the cell voltage
(with current density). Mass transfer effects can be reduced by
changing the flow rates of anolyte and catholyte.
4.1. Effect of catholyte and anolyte flow rate
Fig. 2 shows variation of Vcell with (identical) flow rates of
electrolytes in the anolyte as well as the catholyte at two
different current densities (0.01 A/cm2 and 0.175 A/cm2). Vcell
decreased as the linear velocity of electrolytes was increasedfrom 0.95 to 3.53 cm/s. This was due to the decrease in mass
transfer resistance with increased convection. In another set
of experiments, flow rates of anolyte and catholyte were
varied separately. The studies were done at four different
linear velocities (0.95, 1.89, 2.78 and 3.53 cm/s). While the flow
rate ofone side of the cell was varied theflow rate of other side
was kept constant at the highest flow rate (3.53 cm/s). Fig. 3
shows the Vcell variation with anolyte and catholyte flow
rates respectively. Vcell decreased as the linear velocity of the
catholyte increased from 0.95 to 3.53 cm/s. On the other hand,
change in Vcell with increase in anolyte flow rate was insig-
nificant. This suggested that the mass transfer resistance of
the cell was dominated by the cathode side. Cathode sidemass transfer resistance was dominant because of the
concentration and nature of the reacting species. Therefore,
drop in Vcell with increase in anolyte flow rate (reduced
thickness of diffusion boundary layer) was insignificant.
Decrease in Vcell was however small (w30 mV) for 0.175 A/cm2
and insignificant (w8 mV)for low current density (0.01 A/cm2).
Thus no significant advantage could be gained by operating at
higher flow rates at low to moderate current densities. Further
experiments were therefore performed at anolyte and cath-
olyte flow rates of 3.53 cm/s. Also, flow rates of both anolyte
and catholyte were kept same, to ensure that no local pressure
gradient across the membrane are generated at any point
inside the cell.
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4.2. Effect of variation ofxHI=H2O
The effect of HI concentration was considered in form of
xHI=H2O because the azeotropic concentration of HIx solution is
conveniently expressed using this ratio. Vcell variation withchange in HI concentration was measured by independently
changing the xHI=H2O of the anolyte and the catholyte. Even
though the anode and cathode reaction were reverse of each
other, OCV value was non-zero due to unequal concentrations
of anolyte and catholyte. OCV value reduced by about 19%
(Fig. 4(b)) as the xHI=H2O of catholyte was reduced from 0.183 to
0.145. Fig. 4(a) shows the measured Vcell at current densities
between 0.001 A/cm2 and 0.275 A/cm2 for xHI=H2O values of
0.183, 0.16 and 0.145 (anolyte xHI=H2O was kept constant at
0.112). Measured Vcell values were fitted using Eqn. (9) and the
values of parameters of the equation are listed in Table 2. The
value of the Tafel parameters Db (of order 104) and Da
(of order 103) was negligible. Using these parameter values
the calculated electrode overpotential values were very small
fraction of the Vcell (e.g., 0.2% ofVcell at 0.275 A/cm2). Ie
Vdataof the cell, therefore, can be approximated with a linear
response as given below:
Vcell i Rnet OCV (11)
Total resistance equivalent of the cell (Rnet) and the
regressed value of open circuit voltage (V0) are also given in
Table 2. Rnet increased (Fig. 4(b)) from 0.145 to 0.183 U/cm2 as
the catholyte xHI=H2O increased from 0.145 to 0.183. OCV was
significant proportion of the Vcell ranging from 12 to 14% at
higher current densities to w75% at lower current density of
(0.02 A/cm2).
Variation in Vcell with change was measured at three
different anolyte xHI=H2O (0.112, 0.093 and 0.08). IeV data
0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.176
0.178
0.180
0.182
0.184
0.186
Voltage(V
)
Linear flow velocity (cm/sec)
Catholyte
Anolyte
0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.720
0.725
0.730
Voltage(V)
Linear flow velocity (cm/sec)
Catholyte
Anolyte
a
b
Fig. 3 e Effect of independent variation in flow rate of
anolyte and catholyte on cell potential drop at current
density of (a) 0.01 A/cm2, and (b) 0.175 A/cm2. Linear
velocity of one of the compartments was fixed at 3.53 cm/s
when that of the other was varied.
0.5 1.0 1.5 2.0 2.5 3.0 3.50.178
0.180
0.182
0.184
0.186
0.188
0.190
Voltage(V)
Linear flow velocity (cm/sec)
0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.720
0.725
0.730
0.735
0.740
0.745
0.750
0.755
Voltage(V)
Linear flow velocity (cm/sec)
a
b
Fig. 2 e Variation of cell potential drop with simultaneous
change in linear flow velocity of anolyte and catholyte at
current density of (a) 0.01 A/cm2, and (b) 0.175 A/cm2.
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showed linear trend indicating the combined electrodes
overpotential were insignificant fraction of total Vcell(Fig. 5(a)). In contrast to the trend observed in the catholyte
section, Vcell was higher at lower xHI=H2 O. OCV value increased
(Fig. 5(b)) considerably (w55.5%) on reducing xHI=H2 O from
0.112 to 0.08. This can be attributed to a sharp change in thechemical potential of the anolyte. This is important in light of
the fact that OCV value constituted a major fraction ofVcell at
lower current densities (w70% at 0.02 A/cm2) and remained
non-negligible even at higher current densities (w15% at
0.275 A/cm2). Vcell data at different current density was fitted
using Eqn. (11) to obtain the Rnet value. Fig. 5(b) shows the Rnetvalues for different values of anolyte xHI=H2O. Rnet decreased
by w10% as the xHI=H2 O was reduced from 0.112 to 0.08.
Increase in the cell voltage with time of operation has been
reported in literature [21e25]. The above reported results
suggested that increase in cell potential was due to devel-
opment of differential concentration across the membrane of
the EED.
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0.0
0.2
0.4
0.6
0.8
1.0
Voltage(V
)
Current density (A/cm2)
xHI/H O
: 0.183
xHI/H O
: 0.160
xHI/H O
: 0.145
0.14 0.16 0.18
0.10
0.12
0.14
0.14 0.16 0.18
OCV
Rnet
xHI/HO
(catholyte)
OpenCircuitVoltage(OCV)
(V)
3.00
3.05
3.10
3.15
3.20
3.25
ResistanceR
net(-cm2)
a
b
Fig. 4 e Effect ofxHI=H2O in catholyte on (a) cell voltage drop,
and (b) OCV and calculated value of cells ohmic resistance
(Rnet).
Table 2 e Table shows the simulated values of thederived parameters using the Tafel equation and thelinear fitting for varying xHI=H2O in the catholyte section.
xHI=H2O(catholyte)
Parameters Tafel equation Linear fit
0.183 OCV (V) 0.137
Rnet (U
/cm
2
) 3.103 3.09962Da (V) 0.0019 0.0008
Db 0.0002263 e
0.16 OCV 0.117
Rnet 3.045 3.05279
Da 0.0012 0.00064
Db 0.0002208 e
0.145 OCV 0.111
Rnet 3.001 3.008
Da 0.0013 0.0002
Db 0.0002194 e
0.00 0.05 0.10 0.15 0.20 0.25 0.300.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage(V)
Current density (A/cm2)
xHI/H O
: 0.112
xHI/H O
: 0.093
xHI/H O : 0.080
0.08 0.10 0.12
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.240.08 0.10 0.12
OCV
R
xHI/HO
(anolyte)
OpenCircuitVoltage(O
CV)
(V)
3.05
3.10
3.15
3.20
3.25
3.30
3.35
3.40
3.45
ResistanceR
net(-cm2)
a
b
Fig. 5 e Effect ofxHI=H2O in anolyte on (a) cell voltage drop,
and (b) OCV and calculated value of cells ohmic resistance
(Rnet).
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4.3. Effect of the variation of I2/HI ratio
Fig. 6 shows OCV and currentevoltage data respectively, at
three different I2/HI ratios (0.26, 0.33 and 0.43) in the catholyte.
OCV values were significant part of the Vcell and contributed
up to 60% to Vcell at lower current densities (up to 0.02 A/cm2)
andw15% at higher current densities. However, OCV did not
change appreciably (decreased by w2%) with increase in I2/HIratio (Fig. 6(b)). Vcell varied linearly with current density and
the data was regressed linearly. Rnet for different I2/HI ratio
(Fig. 6(b)) decreased with increase in iodine concentration.
Fig. 7 shows OCV and currentevoltage data respectively, at
three different I2/HI ratios (0.56, 0.73 and 0.92) in the anolyte.
OCV increased by 40% as the I2/HI ratio was increased from
0.56 to 0.92. Difference in iodine concentration of the two
sections became large as the anolyte iodine concentrationwas
increased. This may be the reason for significant increase in
OCV on changing anolyte iodine concentration while an
insignificant increase with change of catholyte iodine
concentration. Vcell varied linearly with current density and
the Rnet values obtained at different I2/HI ratio are shown in
Fig. 7(b). In contrast to trend for iodine increase in catholyte,
Rnet value increased with increase in iodine concentration of
the anolyte.
Increase in the iodine concentration affected the ohmicresistance of the cell as well as the OCV. Anolyte iodine
concentration affected Vcell more strongly as compared to the
catholyte iodine concentration. EED cell resistance was re-
ported to increase with increase in iodine concentration [22].
Separate increase in iodine concentration of anolyte and
cathode done in this work, suggested that cell resistance
increased more by increase in iodine concentration of anolyte
than the catholyte.
4.4. Effect of pressure on the cell voltage
Effect of pressure on cell voltage has been analyzed by
defining differential pressure as:
0.00 0.05 0.10 0.15 0.20 0.25 0.300.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage(V)
Current density (A/cm2)
I2/HI : 0.26
I2/HI : 0.33I2/HI : 0.43
0.25 0.30 0.35 0.40 0.450.142
0.143
0.144
0.145
0.146
0.147
0.148
0.149
0.150
0.151
0.1520.25 0.30 0.35 0.40 0.45
OCV
R
I2/HI ratio (Catholyte)
OpenCircuitVoltage(OCV)
(V)
3.10
3.15
3.20
3.25
3.30
R
esistanceR
net(-cm2)
a
b
Fig. 6 e Effect of I2/HI ratio in catholyte on (a) cell voltage
drop, and (b) OCV and calculated value of cells ohmic
resistance (Rnet).
0.00 0.05 0.10 0.15 0.20 0.25 0.300.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage(V
)
Current density (A/cm2)
I2/HI : 0.56
I2/HI : 0.73
I2/HI : 0.92
0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95
0.09
0.10
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95
OCV
R
I2/HI ratio (Anolyte)
OpenCircuitVoltage(OCV)
(V)
3.0
3.1
3.2
3.3
3.4
3.5
ResistanceR
net(-cm2)
a
b
Fig. 7 e Effect of I2/HI ratio in anolyte on (a) cell voltage
drop, and (b) OCV and calculated value of cells ohmic
resistance (Rnet).
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PDiff Panolyte Pcatholyte (12)
Differential pressure was created by pressurizing either the
anolyte or the catholyte and keeping the other section at
atmospheric pressure (1 bar). Experiments were done at five
differential pressures between 1 bar and 1 bar. In addition
the currentevoltage data was also measured at zero differ-
ential pressure but with both the anolyte and catholyte at2 bar pressure and theresponse was found to be similar to that
with that of the base case. OCV was unaffected by differential
pressure changes. Fig. 8(a) shows the currentevoltage data at
different differential-pressures. Vcell varied linearly with
current density for all the differential pressures. Rnet value
obtained from linear regression of current voltage data was
least for zero differential pressure and increased for both
positive and negative values of differential pressure (Fig. 8(b)).
Increase in Rnet was more for positive differential pressure
(6.2% for 1 bar compared to 4% for 1 bar at 0.275 A/cm2). Rnetvalue remained unchanged at zero differential pressure for
both 1 bar and 2 bar pressure of the system. A possible
explanation for increase in Rnet at non-zero differential pres-
sures can be sticking of iodine to the membrane. A differential
pressure caused iodine to deposit on membrane at the higher
pressure side and the amount deposited increased with
increase in differential pressure. Presence of iodine on
membrane surface increased the resistance of the membrane.
No specific experiments were conducted to determine the
effect of iodine sticking. However iodine was found on themembrane as indicated the change in color of the membrane
to light brown after the experiment.
4.5. Effect of temperature
Fig. 9(a) shows the currentevoltage data at four different
temperatures (293 K, 308 K, 323 K and 341 K). OCV value of the
EED cell did not change with increase in the temperature
(Fig. 9(b)). This suggested that the change in chemical poten-
tial of both the anolyte and catholyte with temperature was
nearly same. Vcell decreased with increase in temperature and
the difference was higher at higher current densities (w11% at
0.275 A/cm2). Insignificant change of OCV suggested that drop
0.00 0.05 0.10 0.15 0.20 0.25 0.300.0
0.2
0.4
0.6
0.8
1.0
1.2
V
oltage(V)
Current density (A/cm2)
PDIff
: 0 Bar
PDIff
: + 0.5 Bar
PDIff : + 1 BarP
DIff: - 0.5 Bar
PDIff
: - 1 Bar
-2 -1 0 1 2
0.14
0.16
-2 -1 0 1 2
OCV
R
Differential Pressure (bar)
OpenCircuitVoltage(OCV)
(V)
3.15
3.30
3.45
R
esistanceR
net(-cm2)
a
b
Fig. 8 e Effect of differential pressure on (a) cell voltage
drop, and (b) OCV and calculated value of cells ohmic
resistance (Rnet).
0.00 0.05 0.10 0.15 0.20 0.25 0.300.0
0.2
0.4
0.6
0.8
1.0
T : 293 K
T : 308 KT : 323 K
T : 341 K
Current density (A/cm2)
Voltage(V)
290 300 310 320 330 3400.10
0.11
0.12
0.13
0.14
0.15290 300 310 320 330 340
OCV
R
Temperature (K)
OpenCircuitVoltage(O
CV)
(V)
2.7
2.8
2.9
3.0
3.1
3.2
ResistanceR
net(-cm2)
a
b
Fig. 9 e Effect of cell operating temperature on (a) cell
voltage drop, and (b) OCV and calculated value of cells
ohmic resistance (Rnet).
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in Vcell with temperature was entirely due to the decrease in
the ohmic resistance of the cell. Fig. 9(b) shows the Rnet value
of the cell at different temperatures obtained by linear
regression of the currentevoltage data. Rnet decreased by 14%
for a temperature difference of 48 K. Similar trends have been
reported in literature [23,24].
4.6. Equilibrium potential studies on graphite electrode
OCV contributed a substantial fraction to the Vcell, thus it is
important to understand how OCV is affected by different
operating variables. As discussed above, OCV was not affected
by pressure and temperature was changed significantly with
the I2/HI ratio as well as the xHI=H2O. As discussed earlier [Eqn.
(8)] the net OCV generated is function of the electrode equi-
librium potential along with the membrane potential. Exper-
iments were done in a three electrode setup to determine how
electrode potential (against Ag/AgCl reference electrode)
varied with concentration of HI and iodine. The experimen-
tation was done in two sets, one with constant HI concen-
tration and the other with constant iodine concentration.
Fig. 10 shows the electrode potential at iodine concentrations
between 0.32 M and 1.9 M (HI concentration was 2.75 M).
Maximum increase in the electrode potential (Eeq) in the
experimental concentration range was found to bew13%. The
rate of increase however reduced at higher iodine concen-
tration. The average rate of increase per unit change in iodine
molarity was found to be around 19:6 mV=MI2 . Fig. 11 shows
electrode potentials at HI concentrations between 2.2 and
5.5 M (at constant iodine concentration of 0.63 M). The elec-
trode potential showed a progressive increase (by about 53%)
with the increase in HI concentration in the experimental
range along with average rate of decrease was about 42 mV/
MHI. Thus, electrode potential increased with increase in
iodine concentration and decreased with increase in the HI
concentration. Also, the electrode potential was affected more
strongly by the HI concentration. This explains smaller
change in OCVwith change I2/HI ratio compared to the xHI=H2O.
4.7. Optimal operating parameters
EED consumes electric energy which itself is obtained at 40%
efficiency from heat source. Hence energy consumption in
EED will significantly affect the overall efficiency of the IeS
process. Recent simulation studies reported in literature also
suggested that the reduction in potential drop required in EED
cell results in major increase in theoverall IeS cycle efficiency
[33]. For comparison of the EED process, energy required is
expressed in terms of the heat equivalent the total electricenergy required to concentrate enough acid that on decom-
position gives one mole of H2. Since one mole of HI on
decomposition gives half mole of H2, heat equivalent of energy
required can be written as:
UH2 2FVcellhIhH
(13)
where hI is the current efficiency and hH is the efficiency of
conversion of heat to electricity (taken as 0.45). Reported
current efficiency values varies in the range of 0.8e0.95
[21e24]. Presentstudies showed that OCV and ohmic potential
drop constitutedmajor fraction of the total potential drop. Cell
potential drop reduced on loweringxHI=H2O of the catholyte, sothe exit catholyte xHI=H2 O of EED should be kept at the
minimum value after which simple distillation can be used
effectively for further concentration of HI. Optimal catholyte
exit xHI=H2 O value can only be found by simultaneous energy
calculation over EED and distillation of HIx solution. Optimal
value for EED exit concentration was reported to be between
0.175 and 0.197 [29]. The catholyte exit xHI=H2 O value of around
0.18 is in the range of optimal values. EED cell potential drop
increased with decrease in anolyte xHI=H2 O. So the inlet anolyte
xHI=H2 O must be kept at the highest possible value which is the
exit concentration of Bunsen reaction i:e:; xHI=H2Ow0:15. The
change in HI concentration of catholyte and anolyte of EED is
related more or less in stoichiometric way. Thus, an exit
0.0 0.5 1.0 1.5 2.0220
230
240
250
260
HI - 2.75 MEeq
(mVvsAg/
AgClref.)
Iodine Concentration (M)
Fig. 10 e Equilibrium potential of the graphite electrode in
HIx solutions of different iodine concentration and a fixed
HI concentration of 2.75 M. The potential reported is wrt
Ag/AgCl reference electrode.
2 3 4 5 6
120
160
200
240
280
I2
- 0.63 M
Eeq
(mVvsAg/AgClref.)
HI Concentration (M)
Fig. 11 e Equilibrium potential of the graphite electrode in
HIx
solutions of different HI concentration and a fixed
iodine concentration of 0.63 M. The potential reported is
wrt Ag/AgCl reference electrode.
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catholyte xHI=H2 O of about 0.185 would correspond to an exit
anolyte xHI=H2 O of about 0.11e0.12. EED potential drop
increased with increase in anolyte I2/HI ratio and therefore it
must be kept at the minimum possible value. Mixing of recycle
streams from flash or HI decomposer with inlet stream of EED
will increase the I2/HI ratio of the anolyte and should be
avoided. Mass balance calculation reveals that for inlet xHI=H2 O
and I2/HI ratio of 0.155 and 0.5 respectively and outlet xHI=H2O of0.183, the outlet I2/HI molar ratio will be 0.26. Similarly, with
identical inlet for anolyte, the outlet I2/HI molar ratio will be
0.78. Thus for a Bunsen outlet stream (after partial iodine
removal in case of direct contact mode of Bunsen reaction) of
composition xHI=H2 Ow0:155 and I2/HI molar ratio of 0.5, the
minimum energy demand is expected for identical inlet ano-
lyte and catholyte streams and exit concentration of catholyte
ofxHI=H2 O of 0.185, I2/HI molar ratio of 0.26. The corresponding
anolyte exit concentration would be xHI=H2O of 0.115 and I2/HI
molar ratio of 0.78.
Cellpotentialdecreased with increase in the catholyte I2/HI
ratio from 0.26 to 0.43. The iodine concentration in the inlet
catholyte stream can be increased by mixing of recycle streamfrom the HI decomposer which is rich in iodine and has
comparatively lower water content. However, energy required
in flash operation (immediately after EED unit) increases with
increase in iodine content of the EED product stream. Thus
inlet iodine of catholyte stream cannot be increased indefi-
nitely to lower energy consumption of HI decomposition
section. Data from the I2/HI ratio patterns suggest that for
a value of 0.56 the EED potential drop shows the minimum for
all the variation. Energy calculations were therefore per-
formed for two different inlet catholyte stream concentra-
tions (as shown in Table 3). First concentration corresponds to
out stream from Bunsen reactor and the second inlet cath-
olyte stream composition is based on the mixing of recyclestream from HI decomposer. Transport number and electro-
osmotic flow are important parameters which determine
performance and energy consumption of EED cell. Their
measurements require long duration EED operations where
inlet and outlet EED streams concentration are sufficiently
different. For this reason, average values of these parameters
are reported in literature. Since the focus of this work is on
determination of variation of EED cell performance with
asymmetric variation of independent variables, point deter-
mination of transport number and electro-osmotic coefficient
could not be determined. Energy calculation was therefore
done using average value of transport number and current
efficiency value reported in the literature.
Potential drop varied almost linearly with current density;
hence the energy consumption increased linearly with
current density. The energy consumption based on the initial
voltage value can be further subdivided into the energy
consumed in overcoming the OCV, energy consumed in
concentration process (electrode reactions) and the energy
consumed in overcoming the ohmic resistance of the cell.
Fig. 12 shows bar chart of the energy consumed at threedifferent current densities of 0.01, 0.05 and 0.275 A/cm2 for
two different inlet catholyte stream compositions. The
UeOCV corresponds to the energy consumed in overcoming
the OCV. U-Act represents the cumulative sum of energy
consumed in concentration process and overcoming the
ohmic drop. Energy consumption with addition of recycle
stream (109 kJ/mol-H2 at a current density of 0.01 A/cm2) was
higher than the case where the recycle stream (91 kJ/mol-H2 at
a current density of 0.01 A/cm2) was added. The energy
consumed in overcoming the OCV is independent of the
applied current density. UeOCV was significant contributor to
the total energy consumption at low current density while the
U-Act dominated the energy consumption at higher currentdensity.
The above stated energy consumption values exclude the
pumping costs. Typical heat demand for the IeS cycle is re-
ported in the range of 600e650 kJ/mol-H2 [29]. Thus EED
operation would be feasible only at lower current densities.
OCV was a major component of EED potential drop at low
current densities and was affected strongly by the electrolyte
concentrations viz. I2/HI ratio and xHI=H2O while other oper-
ating parameters such as temperature and pressure had little
effect on OCV. It was found that the increase in the difference
in concentrations of the electroactive components on two
compartments of the cell resulted in higher OCV. This sug-
gested network streams and their relative mass flow in HIdecomposition of the cycle should be designed such that
difference in concentration of electrolytes in two compart-
ments of the EED cell be minimized. Increasing the tempera-
ture however reduced the ohmic resistance of the cell.
Table 3 e I2/HI ratio of the inlet catholyte and anolytestreams of EED with and without addition of recyclestream from HI decomposer. The other variablesremained arexHI=H2O (catholyte)e 0.183, xHI=H2O (anolyte)e0.112, DP e 0 bar, Te 293 K.
I2/HI
Anolyte Catholyte
Solution 1 0.73 0.26
Solution 2 0.73 0.43
0
100
200
300
400
500
600
700
S-
2
S-
2
S-
1
S-
1
S-
2
S-
1
EnergyRequirement
(kJ/mole-H2
)
0.01 A/cm2
0.05 A/cm2
0.275 A/cm2
U-OCV
U-Act
Fig. 12 e Bar chart representation of the energy consumed
in EED cell for solution 1 (S-1), and solution 2 (S-2) at three
different current densities 0.01, 0.05 and 0.275 A/cm2.
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5. Conclusion
Effects of different operating parameters on the EED cell
potential drop were investigated. Studies on the EED cell
with asymmetric variation of electrolyte concentration
added to the information available in literature on effect of
operating variables on performance of EED cell. OCV wasidentified as a major potential drop component in the EED
cell operation. It was also found that the equilibrium elec-
trode potential changes rapidly with concentration of redox
species and thus difference in concentration of these species
in the two compartments of the cell rapidly increased the
OCV of the cell. Increase in I2/HI ratio in catholyte from 0.25
to 0.45 resulted in slight lowering of the energy consump-
tion. In contrast, increase in I2/HI ratio in anolyte resulted in
increase in the energy consumption. The energy calculation
revealed that the operation at lower current density is
preferred and at lower current densities OCV form a signifi-
cant part of the EED cell potential and hence energy
consumption.
Acknowledgment
Authors acknowledge the financial support from ONGC
Energy Centre for carrying out this work.
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