Study on electrochemical impedance spectroscopy and cell voltage

Prévia do material em texto

Electrochimica Acta 426 (2022) 140837
Available online 10 July 2022
0013-4686/© 2022 Elsevier Ltd. All rights reserved.
Study on electrochemical impedance spectroscopy and cell voltage 
composition of a PEM SO2-depolarized electrolyzer using graphite felt as 
diffusion layer 
Xifeng Ding a, Songzhe Chen a,b,*, Laijun Wang a,b, Ping Zhang a,b,* 
a Institute of Nuclear and New Energy Technology, Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education, Tsinghua University, Beijing 
100084, China 
b Tsinghua University-Zhang Jiagang Joint Institute for Hydrogen Energy and Lithium-Ion Battery Technology, Tsinghua University, Beijing 100084, China 
A R T I C L E I N F O 
Keywords: 
SO2-depolarized electrolysis 
Electrochemical impedance spectroscopy 
Cell voltage 
Overpotential 
Polarization impedance 
A B S T R A C T 
SO2-depolarized electrolysis (SDE) is the hydrogen-producing step of the hybrid sulfur (HyS) process. In this 
study, electrochemical impedance spectroscopy (EIS) is used to study the composition of cell voltage of SDE 
electrolyzers, which apply porous graphite felts as diffusing layers. By measuring the open circuit potential, the 
possible side reactions and the methods to avoid the side reactions are discussed from the perspectives of 
thermodynamics and kinetics. According to EIS results obtained at different voltages, the impedance and its 
components are determined by equivalent circuit fitting calculation. The obtained impedance values of different 
cells agree very well with their SDE performance results. The voltage loss in the SDE process is split into ohmic 
impedance loss, kinetics impedance loss and concentration polarization impedance loss. The influences of the 
graphite felt compression ratio, operating temperature and anolyte flow rate on the impedance and performance 
of SDE cell are investigated. 
1. Introduction 
Hydrogen is a widely used industrial raw material as well as a 
promising clean energy carrier with great application potential [1–3]. In 
recent years, the global demand for hydrogen has increased sharply, 
consequently the development of reliable large-scale hydrogen pro-
duction technology became a very important topic. Because hydrogen 
production basing on fossil energy is accompanied by a large amount of 
CO2 emissions, renewable hydrogen production basing on water 
decomposition has attracted great attention. 
Since water is a very stable compound, the decomposition of water is 
usually accompanied by harsh reaction conditions, such as high tem-
perature or high voltage. Thermochemical water splitting cycles are 
proposed to produce hydrogen at relatively mild conditions [4,5], by 
combining several chemical reactions to form a close cycle, while the net 
reaction is the decomposition of water. 
Among the numerous proposed thermochemical cycles, the hybrid 
sulfur (HyS) process is a relatively simple thermochemical cycle process 
[6–8], containing only two independent chemical reactions, namely, 
sulfuric acid decomposition and SO2 depolarization electrolysis (SDE) as 
follows: 
H2SO4→SO2 + H2O+ 1 / 2O2 (1) 
2H2O+ SO2→H2SO4 + H2 (2) 
The net reaction of the whole process is: 
H2O→H2 + 1 / 2O2 (3) 
Compared with other thermochemical cycles, all the materials 
involved in the HyS process are fluid, and the engineering application of 
this cycle is also relatively easier. Thus, HyS process is considered one of 
the most promising thermochemical cycles. 
In this process, the hydrogen production step is the SDE, whose half- 
cell reactions are described by Eqs. (4) and (5). 
Anode: 
SO2 + 2H2O→SO2−
4 + 4H+ + 2e− (4) 
Cathode: 
2H+ + 2e− →H2 (5) 
* Corresponding authors. 
E-mail addresses: chenszh@mail.tsinghua.edu.cn (S. Chen), zhangping77@mail.tsinghua.edu.cn (P. Zhang). 
Contents lists available at ScienceDirect 
Electrochimica Acta 
journal homepage: www.journals.elsevier.com/electrochimica-acta 
https://doi.org/10.1016/j.electacta.2022.140837 
Received 24 May 2022; Received in revised form 3 July 2022; Accepted 10 July 2022 
mailto:chenszh@mail.tsinghua.edu.cn
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Electrochimica Acta 426 (2022) 140837
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The cathode reaction of the SDE process is the same as that of pure 
water electrolysis. However, for the anodic reaction, i.e. the oxidation of 
SO2 to H2SO4, the reversible potential ΦθH2SO4/SO2 = 0.158 V [9], 
significantly lower than the reversible potential ΦθO2/H2O = 1.23 V 
[10] for the anodic reaction of pure water electrolysis, i.e. the oxidation 
of water to oxygen. 
Unfortunately, in the actual SDE operation, the cell voltage is 
significantly higher than the theoretical reversible potential. The voltage 
higher than the reversible potential, in other words, overpotential, is 
directly related to the energy efficiency of the SDE process. It is neces-
sary to study the composition of the overpotential and its influencing 
factors. For a SDE cell, the total cell voltage U could be split into four 
parts as expressed by Eq. (6) [11]: 
U=Ueq + i⋅RA + ηa + ηc (6) 
where Ueq is the reversible potential of SDE, RA is the ohmic impedance 
and i⋅RA is the ohmic impedance overpotential, while ηa and ηc represent 
the anode and cathode polarization overpotentials, respectively. i⋅RA, ηa 
and ηc form the overpotential consumed by the SDE cell. 
For the SDE process, i⋅RA includes the voltage loss caused by inherent 
impedance of each component of the cell, such as the electrodes, cata-
lytic layers, proton exchange membrane, electrolyte, etc. ηA and ηC can 
be divided into kinetics impedance voltage loss and concentration po-
larization impedance voltage loss. Kinetics impedance, also known as 
charge transfer impedance, refers to the charge transfer potential dif-
ference caused by the inability of electrochemical reaction to complete 
instantly. Concentration polarization impedance, also known as diffu-
sion impedance, is the resistance caused by the difference in material 
concentration between the liquid phase and the surface of membrane 
electrode. During SDE process, a small amount of SO2 in anolyte will 
cross the MEA into cathodic region and undergo reduction reactions, 
thus forming an internal galvanic cell. The galvanic potential offsets part 
of the working voltage in the SDE process, leading to an apparent rise of 
cell voltage. 
Electrochemical impedance spectroscopy (EIS) has been applied for 
the study on pure water electrolysis system extensively [12]. However, 
the impedance of the SDE cell and its influencing factors are rarely re-
ported. In our previous work [13,14], EIS were conducted on SDE cells 
with carbon papers as the diffusion layers. However, the performance of 
the cells was poor and the overall impedance is quite large. In addition, 
crossover of SO2 and the subsequent side reactions were not considered. 
In a recent cooperating research work [15] among the Savannah River 
National Laboratory (SRNL), Sandia National Laboratories and the 
University of South Carolina (USC), EIS technology was used to measure 
the overpotential of gas-phase feed SDE cells. Researchers adopted a 
simplified method to work out the composition of overpotential. With 
EIS, the ohmic impedance was measured at high frequency, then the 
potential loss on ohmic impedance was calculated. The reversible po-
tential and ohmic impedance potential loss were subtracted from the 
total cell voltage, obtaining the kinetics overpotential. However, over-
potential caused by side reactions, concentration polarization, etc. was 
not discussed in details. 
For the purpose of further understanding of SDE process and 
improvingthe performance, it is of great importance to carry out sys-
tematical impedance composition research and identify/quantify the 
kinetics overpotential, concentration polarization overpotential and 
side reaction overpotential of SDE process. In our previous work [16], 
porous graphite felts were used to replace carbon papers as the diffusion 
layers of the liquid-phase feed SDE cells, which were proved to possess 
quite high performance and the performance is dramatically influenced 
by graphite felt compression ratio, operating temperature, anodic fluid 
flow rate and so on. It is necessary and also interesting to study the 
impedance and overpotential of above SDE cells. In this study, in situ EIS 
tests are conducted on these cells. The composition of cell voltage and 
overpotential is discussed in depth, based on EIS results and the 
equivalent circuit fitting calculations. Meantime, the influence of cell 
structure and operating conditions on each component of the cell 
voltage is investigated. 
2. Experimental section 
2.1. Chemicals and test solution 
Analytically pure 98% sulfuric acid (Beijing Tongguang Fine 
Chemical Co., LTD.) is diluted to 30 wt.% with deionized water, and 
then saturated with SO2 (99.99%, liquefied air (Tianjin) Co., LTD.) to 
obtain anolyte for SDE cells. 
2.2. Electrolyzers and experiment apparatus 
The EIS test objects, i.e., the 4 SDE electrolyzer cells, are the same as 
those used in our previous work [8], which reveals the performance of 
these cells equipped with porous graphite felts as diffusion layers. The 
electrolyzers adopt symmetrical structure. Fig. 1(a) gives an exploded 
sectional view of the half part of an electrolyzer. 
Briefly, graphite plates (Fig. 1(b), Shanghai Hongfeng Industrial Co., 
Ltd., China) with 5 cm × 5 cm square groove of different depths, 0.8 or 
1.0 mm, are used as polar plates of the electrolyzers. Viton gaskets have 
square holes of the same outline size as the grooves in graphite plates, 
namely, 5 cm × 5 cm. The Viton gaskets are 0.55 or 1.10 mm thick 
originally and 0.5 or 1.0 mm thick after compressed during cell as-
sembly. The MEA is of “sandwich” with three layers, i.e. a thin film of 
catalyst, a membrane (Nafion membrane N117CS (E. I. du Pont)), 
another film of catalyst. Catalyst amounts on both sides of MEA are the 
same, 0.42 mg Pt/cm2. The diffusion layer, namely graphite felt of 5 cm 
× 5 cm × 2.5 mm is placed in the interior square space enclosed by polar 
plate, gasket and MEA, as illustrated by Fig. 1(a). During assembly, the 
deformation of polar plate and MEA is very limited, therefore above 
space is compressed due to the reduction of the gasket’s thickness. Ac-
curate control the final thickness of the gaskets of a SDE cell is achieved 
with the help of a vernier caliper. After assembly processes, the gasket 
thickness of the 4 cells A, B, C and D are 1.0, 1.0, 0.5 and 0.5 mm, 
respectively. Since the compress of the space is well controlled, the 
graphite felt wrapped around the interior space will be compressed 
along with it, and the compression ratio of graphite felt could also be 
controlled as a specific value. 
The structural parameters of the 4 cells are listed in Table 1. In the 
table, compression ratio (rc) of graphite felt is calculated as the ratio of 
the reduced thickness of a compressed graphite felt to the original 
thickness. It should be noted that the key difference among the 4 SDE 
cells is their different rc of graphite felts, and the values of Cell A, B, C 
and D are 20%, 28%, 40% and 48%, respectively. 
The EIS test facility is illustrated by Fig. 2, with photograph and 
schematic diagram. Anolyte, SO2 saturated 30wt.% sulfuric acid solu-
tion is contained in a quartz reservoir with temperature controlled 
heating jacket. 
The mounted electrolyzer is also equipped with heater, thermo-
couple, and PID digital temperature controller (type SR1, Shimaden Co., 
Ltd., Japan). The tubes connecting reservoir and SDE cell are wrapped 
by foam pipe for heat preservation. A pump (WT600-2J, Baoding Longer 
Precision Pump Co., Ltd, China),) is equipped to circulate the anolyte 
between the tank and the anodic region of the electrolyzer cell. The EIS 
tests are driven by an electrochemical workstation (Zennium pro, with a 
4-quadrant power potentiostat PP241, Zahner Scientific Instruments, 
Germany). 
2.3. EIS test method 
In situ EIS method is used to measure the impedance composition of 
the SDE cell with the anode as the working electrode, and cathode as the 
reference electrode and counter electrode. A constant voltage (CV) mode 
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is used to provide periodic amplitude over a set frequency range around 
a set voltage. The frequency range is 100 kHz to 0.1 Hz, and the DC 
voltage amplitude is 10 mV. 
The SDE process is sensitive to electrolytic voltage. When no elec-
trolytic voltage is applied, the electrolysis process will not take place 
spontaneously, and the SDE cell is a galvanic cell. The open-circuit 
voltage of the electrolytic cell is first measured, as an important elec-
trochemical indicator for side reactions. At this time, the current is 
formed by the charge transfer driven by the redox reactions on the two 
sides of the cell. The expected SDE reaction will not occur until the 
applied cell voltage can overcome the potential of side reactions and the 
reversible potential of the expected reaction. 
Before EIS experiment, 30 wt.% sulfuric acid in the reservoir is 
preheated to the desired temperature. SO2 is then bubbled into the 
sulfuric acid for enough time ensuring the sulfuric acid is saturated. 
Anodic solution is pumped into anode side of the cell at flow rate of 
240mL/min. During the test, there is no input on the cathode side, where 
the fluid is hydrogen product and a small amount of water permeating 
through the MEA from anode side. After the measurement of OCP using 
the electrochemical workstation, the cell voltage of 1.00 V is applied to 
the electrolyzer in constant voltage (CV) mode. Keep the system running 
for more than 30 minutes to stabilize. Then, the parameters (anodic fluid 
flow rate, cell voltage, frequency range and the DC voltage amplitude) 
are set and the EIS test is triggered. In this work, EIS tests are conducted 
at different cell voltages (OCP, 0.40 V, 0.70 V, 1.00 V and 1.20 V), 
temperatures (25◦C – 80◦C) and anodic fluid flow rates (160 – 360 mL/ 
min). 
2.4. Equivalent circuit fitting 
Zahner Analysis software is used to fit the Nyquist curves resulted 
from EIS tests, assisting to build the equivalent circuits. The ohm 
impedance, anode/cathode kinetics impedance and concentration po-
larization impedance are determined by equivalent circuit fitting 
calculations. 
3. Results and discussion 
3.1. Establishment of equivalent circuit model 
According to Eq. (6), impedance is divided into ohm impedance, 
anode impedance and cathode impedance. Both anode and cathode 
impedance could be further divided into anode kinetics impedance, 
cathode kinetics impedance and concentration polarization impedance. 
With different cell voltage applied, the SDE process is in different 
states. When applying relatively low cell voltage, the SDE process is 
controlled by kinetics and the kinetics impedance accounts for most of 
the total impedance [17]. Under high applied voltage, the electro-
chemical reaction rate and SO2 consumption rate accelerate signifi-
cantly, resulting in concentration polarization impedance between the 
volume phase of electrolyte and catalyst layer of MEA. At this time, SDE 
process is in combined control stage of kinetics and mass transfer, 
concentration polarization impedance contributes a lot to the total 
impedance [18]. Obviously,cell voltage is an important variable to SDE. 
Taking the EIS results of cell C at 60 ◦C and 360 ml/min as examples, 
the Nyquist curves obtained can be divided into three types when 
different cell voltages are applied, as shown in Fig. 3. The tested voltages 
ranges from open circuit voltage to 1.20 V. The highest voltage 1.20 V is 
set to ensure the stable and secure application of graphite materials in 
the SDE cells. 
When the cell works under open circuit voltage, the Nyquist curve 
consists of a short straight line in the high frequency region and an 
incomplete arc in the middle and low frequency regions, as shown in 
Fig. 3(a). At this time, the electrolytic process has not occurred, the open 
circuit voltage direction is opposite to the applied voltage of the elec-
trolytic cell, and the measured internal resistance of a galvanic cell, so it 
Fig. 1. Structure of SDE electrolyzer 
(a) assembling of graphite plate, gasket, graphite felt, and MEA; (b) graphite plate with square groove. 
Table 1 
Structural parameters of the SDE cells. 
Cell number A B C D 
Origin graphite felt thickness (mm) 2.5 2.5 2.5 2.5 
Depth of groove in carbon plates (mm) 1.0 0.8 1.0 0.8 
Gasket thickness in assembled cells (mm) 1.0 1.0 0.5 0.5 
Graphite felt thickness in assembled cells (mm) 2.0 1.8 1.5 1.3 
Compression ratio of graphite felt rc (%) 20 28 40 48 
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Fig. 2. EIS test facility (a) photograph; (b) schematic diagram. 
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Fig. 3. Nyquist diagram at different cell voltages on Cell C (rc = 40%) at 60◦C and anolyte flow rate of 360 ml/min 
(a) OCP; (b) 0.40 V; (c) 0.70 V; (d) 1.00 V; (e) 1.20 V. 
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is necessary to judge the possible reaction of the galvanic cell according 
to the measured value of the open circuit voltage. 
Once the voltage applied to the cell can overcome the sum of the 
open circuit voltage and reversible electromotive force, the electrolytic 
reaction is triggered. When the voltage is at a lower level, namely 0.40 
or 0.70 V, as shown in Fig. 3(b) and (c), the curve consists of a short 
straight line in the high-frequency region and a complete arc in the 
middle and low frequency regions, representing ohmic impedance and 
electrochemical polarization impedance (i.e., dynamic impedance), 
respectively. 
When the voltage is set at a higher level, 1.00 or 1.20 V, the obtained 
Nyquist curves are of different shape, as shown in Fig. 3(d) and (e). The 
curve consists of four parts, i.e., the straight line of the high frequency 
area, two incomplete arcs appear in the middle and low frequency re-
gions, and a straight line appears in the low-frequency region. 
At low electrolytic voltages such as 0.40 V and 0.70 V, the anodic 
electrochemical reaction is slow, and the electrolytic process is in the 
dynamic control stage. The equivalent circuit is shown in Fig. 4(a). 
Resistor R1 represents the ohmic impedance of the circuit, including the 
inherent resistance of the electrolytic cell element and the impedance 
caused by the solution and cathode bubbles. Resistance R2 represents 
the total kinetics impedance of the circuit, i.e. the dynamic impedance. 
This model does not involve diffusion impedance and does not need to 
consider concentration polarization. Therefore, it is suitable for low cell 
voltage and low current density, as well as operation at low tempera-
tures such as 25◦C, at which the electrochemical reaction rate is slow. 
However, when the electrolytic voltage is high (1.00 V and 1.20 V), 
the reaction rate increases, and the reactant consumption rate increases, 
making the mass transfer rate gradually unable to meet the needs of 
maintaining the reactant concentration on the catalyst surface, and the 
SDE process transfers to the diffusion control stage. The equivalent 
circuit model at this point is shown in Fig. 4(b). Where resistance R1 still 
represents the ohmic impedance of the circuit, and resistors R2 and R3 
represent the kinetics impedance of the anode and cathode respectively 
[19]. 
R2 corresponds to the anodic kinetics impedance, which appears in 
the high-frequency region. The reason is that the oxidation of SO2 in 
acidic aqueous solution is more difficult than the reduction of H+. 
Therefore, a relatively short period of low voltage process will also 
significantly affect the rate of SO2 oxidation reaction. Accordingly, R3 
represents the dynamic impedance of the cathode and appears in the 
following relatively lower frequency region. 
R4 represents the concentration polarization impedance, and the 
capacitor in series with resistor R4 represents the volumetric reactance 
of the concentration gradient region formed on the surface of the MEA 
due to concentration polarization. SDE reaction only occurs on the 
surface of MEA. With increasing electrolytic voltage, the chemical re-
action rate on the surface of the MEA accelerates so that the gap between 
the SO2 concentration on the surface of the MEA and the solution body 
expands rapidly, forming a concentration gradient of SO2 supply 
shortage, thus forming a capacitor that can block direct current in the 
local area. 
3.2. Open-circuit voltage measurement 
The open-circuit voltages of each cell are measured at different 
anolyte flow rates and operating temperatures. At open-circuit voltage, 
the electrolytic cells are of equilibrium state, and no polarization occurs 
on the electrode. In the absence of an applied voltage, the measured 
voltage of the electrolytic cell is considered to have two sources: 1) 
hydrogen ions or other charged substances on anode side cross the MEA 
into the cathode side, forming a concentration cell; 2) the occurrence of 
the following side reactions at cathode side [20,21]: 
SO2 + 4H+ + 4e− →S+ 2H2O E0 = 0.45V (7) 
SO2 + 6H+ + 6e− →H2S+ 2H2O E0 = 0.35V (8) 
Thus, the following two groups of REDOX pairs could form galvanic 
cells. 
Pt
⃒
⃒SO2(aq)
⃒
⃒SO2−
4 (aq) ‖ SO2(aq)
⃒
⃒S(s)
⃒
⃒Pt 
Pt
⃒
⃒SO2(aq)
⃒
⃒SO2−
4 (aq) ‖ SO2(aq)
⃒
⃒H2S(aq)
⃒
⃒Pt 
As the reversible potential of SO2 oxidation to SO4
2− in the standard 
state is 0.158 V, according to electrochemical theory, the reversible 
potentials of the above two galvano cells are approximately 0.292 V (i.e. 
0.45 V - 0.158 V = 0.292 V). When the open circuit voltage is signifi-
cantly lower than the above value, the side effects can be considered to 
be negligible. 
Fig. 5 shows the open-circuit voltage of each electrolytic cell at 
different anodic flow rates when the electrolytic temperature is 60◦C. 
Under the same conditions, the open-circuit voltage increases with 
increasing compression ratio of graphite felt in the electrolytic cell. 
Whether charged particles such as H+ diffuse to the cathode or the 
reduction side reaction of SO2 occurs at the cathode, trans-membrane 
diffusion of materials is required. Therefore, the lower pole spacing is 
beneficial to shorten the trans-membrane distance and strengthen the 
trans-membrane diffusion effect. However, when the pole spacing is 
compressed to a certain extent, the contribution of the shortening of pole 
spacing to the enhancement of membrane crossing begins to signifi-
cantly decrease, which is reflectedin the same open-circuit voltage of 
electrolytic cell C and electrolytic cell D under the same conditions. 
Fig. 4. Equivalent circuit models 
(a) model 1: low electrolytic voltage; (b) model 2: high electrolytic voltage. 
Fig. 5. Open circuit voltages at 60 ◦C and different anolyte flow rates 
(Cell A, rc = 20%; Cell B, rc = 28%; Cell C, rc = 40%; Cell D, rc = 48%). 
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At the same time, when the anodic fluid flow rate is low (< 360 mL/ 
min), the open circuit voltage is significantly less than 0.192 V. When 
the anodic fluid flow rate is relatively low, the side reaction can be 
almost ignored. This is because when the anodic fluid flow rate is low, 
the trans-membrane diffusion is weak, and the SO2 concentration of the 
cathode is too low to form the side reaction galvanic cell. 
Fig. 6 shows the open-circuit voltage of cell C at different tempera-
tures and anodic flow rates. At the same temperature, the larger the 
anodic flow rate, the more significant the trans-membrane diffusion, the 
higher the chemical potential of side reactions, and the higher the open- 
circuit voltage. The effect of temperature on the open-circuit voltage is 
twofold. On the one hand, an increase in temperature decreases the 
solubility of SO2, the concentration of SO2 in the anodic solution and the 
concentration gradient between the anode and cathode, which inhibits 
diffusion across the membrane. On the other hand, the increase of 
temperature will improve the mass transfer rate, so that SO2 cross the 
membrane intensified, at the same time, the activity of MEA also in-
creases with the increase of temperature, so that the electrochemical 
reaction rate is accelerated, improving the galvanic potential. Therefore, 
under the same conditions, from 25 to 80◦C, the open-circuit voltage 
shows a trend that it decreases at first and then increases. The highest 
value of open-circuit voltage obtained on Cell C in this work is 0.287 V, 
at 25◦C, and anolyte flow rate of 360 mL/min. This measured value is 
very close to above mentioned theoretical value 0.292 V. 
Therefore, for the SDE process, the total potential composition 
relation described in Eq. (6) should be decomposed into: 
U = Ueq + iRA + ηa + ηc + ηs (9) 
where ηS represents the galvanic potential of side reactions in the SDE 
process. 
Because side reactions are detrimental to the SDE process, they need 
to be avoided. To suppress the spontaneous reaction in the galvanic cell, 
an initial voltage should be applied to the electrolytic cell before feeding 
anolyte, and the initial voltage should be higher than the reversible 
potential of the side reaction. This operation has two effects. First, from 
a thermodynamic point of view, the initial voltage inhibits the side re-
actions. Second, in terms of kinetics, a higher voltage in advance can 
accelerate the consumption rate of SO2 at the beginning of electrolysis, 
reduce the concentration gradient of SO2 between the anode and cath-
ode, slowing down the diffusion rate across the membrane. 
According to the highest value of open-circuit voltage obtained in 
this work (0.287V) and the reversible potential of the SDE process, the 
lowest working voltage of the SDE cells involved in this work should not 
be less than 0.400 V. 
3.3. EIS research under working voltage 
3.3.1. Influence of electrolytic voltage 
According to the selected working voltage after the open-circuit 
voltage experiment, the impedance composition of each electrolytic 
cell under different working voltages was measured. Fig. 7 shows the 
impedance composition of electrolytic cells A – D at different electrolytic 
voltages, 60◦C and 360 mL/min anolyte flow rate. 
Ohm impedance is unchangeable at the same temperature and flow 
rate, for a specific electrolytic cell. When the electrolytic voltage is low 
(0.400 V), due to the extremely low rate of the SDE reaction, the SDE 
process is applicable to equivalent circuit model 1, and the kinetics 
impedance is dominant. When the electrolytic voltage rises, equivalent 
circuit model 2 is applicable. In addition, due to the rapid SO2 con-
sumption rate in the initial stage of 1.20 V, the concentration of re-
actants on the surface of the catalytic layer drops sharply, increasing the 
SO2 concentration difference between the catalytic layer and the solu-
tion body. At the same time, under the condition of a constant anodic 
flow rate, the concentration of SO2 in the catalytic layer could not be 
supplemented in a timely manner, resulting in a decrease in the elec-
trochemical reaction rate and an increase in the anodic kinetics 
impedance. 
3.3.2. Influence of compression ratio of graphite felt 
Fig. 8 shows the impedance composition of electrolytic cells A – D at 
1.00 V and anolyte flow rate of 360 mL/min. The total impedance order 
of the 4 electrolytic cells is C < D < B < A, which is in good agreement 
with the performance order of the 4 cells as illustrated by Fig. 9. For cell 
A which has the lowest compression ratio of graphite felt, the anode 
electrochemical impedance accounts for the highest proportion in the 
total impedance. While in other 3 cells, the concentration polarization 
impedance accounts for the highest proportion. 
As shown in Fig. 8(a) The ohmic impedance measured at 60◦C is of 
the order of cell A > cell B > cell D > cell C. The relationship between 
the ohmic impedance and the compression ratio of graphite felt is as 
follows: the ohmic impedance decreases first and then increases with the 
increase in the compression ratio of graphite felt. At 80◦C, the ohmic 
impedance of the electrolytic cell and the compression ratio of graphite 
decrease and fluctuated slightly. 
Ohm impedance can be considered to be composed of two parts, one 
is the ohm impedance of the cell components, including the inherent 
impedance of the components such as the graphite collector plate, 
diffusion layer, catalytic layer and proton exchange membrane, and the 
contact resistance caused by the contact between components. The other 
is the ohm impedance of anolyte. As the graphite felt gradually com-
pressed the contact resistance decreased. At the stage when the contact 
resistance was dominant, the compressed graphite felt could effectively 
reduce the contact resistance, thus reducing the overall ohmic imped-
ance. When the compression ratio of graphite felt is greater than 40%, 
the void ratio of graphite felt is significantly reduced, the flow space of 
solution in the diffusion layer is significantly compressed, and even local 
solution discontinuity occurs, which makes the solution impedance 
significantly increased; thus, the overall ohmic impedance increases. 
The kinetics impedance of the cathode decreases with increasing 
compression ratio of the graphite felt. With the continuous compression 
of graphite felt, the pole spacing shrinks, and the voltage gradient in-
creases at the same voltage. A higher potential on the catalytic layer 
speeds up the reaction rate and reduces the electrochemical polarization 
impedance. 
Anodic kinetics impedance and concentration polarization imped-
ance need to be discussed as a whole because the two impedance 
changes have a strong correlation. When the consumption rate of SO2 
increases, the anode kinetics impedance decreases. However, the 
Fig. 6. Open circuit voltages of cell C at different operating temperatures and 
anolyte flow rates. 
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Fig. 7. Impedance compositioncomparison of different electrolytic cells at different voltages, at 60◦C and anolyte flow rate 360 ml/min 
(a) Cell A; (b) Cell B; (c) Cell C; (d) Cell D. 
Fig. 8. Impedance composition comparison among different electrolytic cells 
(a) 1.00 V, 60◦C, 360 ml/min; (b) 1.00 V, 80◦C, 360 ml/min. 
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Electrochimica Acta 426 (2022) 140837
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difference in SO2 concentration between the catalytic layer surface and 
the solution body increases at the same time. When the concentration 
difference increases to a certain extent, the concentration polarization 
impedance will increase significantly. 
Fig. 10 shows the change in the sum of the anode kinetics impedance 
and concentration polarization impedance with the compression ratio of 
graphite felt. The sum of impedance decreases first and then increases 
with the increase of the compression ratio of graphite felt. This is mainly 
caused by the change in the sufficient degree of anodic fluid distribution 
caused by the increase in the compression ratio of graphite felt. 
3.3.3. Influence of anodic fluid flow rate 
Fig. 11 shows the impedance composition of cell C at different anodic 
flow rates at 60◦C and 1.00 V voltage. Ohmic impedance is unchanged at 
the same voltage and anolyte flow rate for a certain SDE cell. The ki-
netics impedance at the cathode is also constant because the concen-
tration of hydrogen ions in the cathode is considered to be sufficient and 
much larger than that of other species. The kinetics impedance and 
concentration polarization impedance at the anodes decrease with 
increasing anodic flow, which is due to the increase in SO2 supply, which 
not only accelerates the electrochemical reaction rate but also improves 
the mass transfer. 
3.3.4. Influence of electrolysis temperature 
Fig. 12 shows the impedance composition of electrolytic cell C at 
1.00 V, 360 mL/min anodic fluid flow rate, and different operating 
temperatures. Since the resistance of membrane and electrolyte solution 
increase with decreasing temperature, so the ohmic impedance is rela-
tively large at 25◦C. On the other hand, the catalyst and proton exchange 
membrane are not fully activated at 25◦C, so the SDE is in the kinetic 
control stage. 
Model 2 applies to the SDE process at 60◦C and 80◦C. The cathodic 
kinetics impedances at above two temperature points are approximately 
the same. At 80◦C, the chemical reaction rate constant is relatively high 
[22], thus consumption rate of the SO2 on catalytic layer is very fast. 
While the SO2 solubility at this temperature is very low, leading to a 
poor reactants supply to the catalytic layer. Thus, anode kinetics 
impedance and concentration polarization impedance at 80◦C are both 
greater than those at 60◦C. Above is consistent with the SDE perfor-
mance test results at different temperatures, as shown in Fig. 12. 
Fig. 9. SDE performance of different electrolytic cells [16] 
1.00 V, 60◦C, 360 ml/min; (b) 1.00 V, 80◦C, 360 ml/min. 
Fig. 10. Comparison of R2+R4 in different cells 
(a) 1.00 V, 60◦C, 360 ml/min; (b) 1.00 V, 80◦C, 360 mL/min. 
X. Ding et al. 
Electrochimica Acta 426 (2022) 140837
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4. Conclusions 
In this work, EIS technology is used to study the composition of cell 
voltage of SDE electrolyzers with porous graphite felt as diffusing layers. 
A close look is taken at the composition of overpotential. The influence 
of graphite felt compression ratio and operating conditions on the SDE 
cell voltage, together with the mechanism and control steps of SDE are 
discussed. The internal reasons for the performance differences of the 4 
electrolytic cells at different operating conditions are explained from the 
perspectives of electrochemistry, in other words, EIS or impedance. The 
following conclusions are obtained: 
1) The transmembrane diffusion of SO2 leads to the formation of side 
reaction galvanic cells possessing open circuit voltage up to 0.287 V. 
To avoid these side reactions, it is necessary to apply a sufficiently 
high cell voltage before feeding the cell with anolyte, such as 1.00 
and 1.2 V, at which the side reactions (galvanic cells) are inhibited 
thermodynamically, while the consumption rate of SO2 by the 
expected reaction (oxidation of SO2 to H2SO4) is accelerated 
dynamically, and SO2 crossover could be controlled effectively. 
2) The SDE process is sensitive to cell voltage. At a lower operating 
voltage such as 0.40 V and 0.70 V, the SDE process is in the kinetics 
control state, and the impedance mainly consists of ohm impedance 
and anode electrochemical polarization impedance. At higher oper-
ating voltage, the SDE process is transferred to the combined control 
stage of kinetics and mass transfer, and the impedance includes ohm 
impedance, anode and cathode kinetics impedance, and concentra-
tion polarization impedance. 
3) Graphite felt compression ratio, anolyte flow rate, and operating 
temperature have great effects on the different component of the 
impedance, consequently show a complex influence on the SDE 
performance. Increasing graphite felt compression ratio will 
decrease the ohm impedance of the felt, while increase that of 
electrolyte. Anolyte flow rate has little effect on ohm impedance and 
cathodic kinetics impedance, while the kinetics impedance and 
concentration polarization impedance at the anodes decrease with 
increasing anolyte flow ratio. Increasing temperature can decrease 
the ohm impedance effectively, while at 80◦C, poor SO2 solubility 
causes the obvious rise of anode kinetics impedance and concentra-
tion polarization impedance, bring out the degradation of SDE 
performance. 
CRediT authorship contribution statement 
Xifeng Ding: Methodology, Conceptualization, Data curation, 
Writing – original draft. Songzhe Chen: Methodology, Data curation, 
Writing – review & editing. Laijun Wang: Investigation, Validation, 
Supervision. Ping Zhang: Conceptualization, Methodology, Writing – 
review & editing. 
Declaration of Competing Interest 
The authors declare that they have no known competing financial 
interests or personal relationships that could have appeared to influence 
the work reported in this paper. 
Data availability 
The data is available upon request. 
Acknowledgment 
This work was financially supported by the National Key R & D 
Program of China (Grant No. 2018YFE0202001), the Chinese National 
Science & Technology Major Project (Grant No. ZX06901), and the 
Tsinghua University Initiative Scientific Research Program. 
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	Study on electrochemical impedance spectroscopy and cell voltage composition of a PEM SO2-depolarized electrolyzer using gr ...
	1 Introduction
	2 Experimental section
	2.1 Chemicals and test solution
	2.2 Electrolyzers and experiment apparatus
	2.3 EIS test method
	2.4 Equivalent circuit fitting
	3 Results and discussion
	3.1 Establishment of equivalent circuit model
	3.2 Open-circuit voltage measurement
	3.3 EIS research under working voltage
	3.3.1 Influence of electrolytic voltage
	3.3.2 Influence of compression ratio of graphite felt
	3.3.3 Influence of anodic fluid flow rate
	3.3.4 Influence of electrolysis temperature
	4 Conclusions
	CRediT authorship contribution statement
	Declaration of Competing Interest
	Data availability
	Acknowledgment
	References