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Evaluation of hydrogen production via electrolysis with ion exchange membranes
Journal Pre-proof Evaluation of hydrogen production via electrolysis with ion exchange membranes B. Yuzer, H. Selcuk, G. Chehade, M.E. Demir, I. Dincer PII:
S0360-5442(19)32115-2
DOI:
https://doi.org/10.1016/j.energy.2019.116420
Reference:
EGY 116420
To appear in:
Energy
Received Date: 19 July 2019 Revised Date:
19 October 2019
Accepted Date: 22 October 2019
Please cite this article as: Yuzer B, Selcuk H, Chehade G, Demir ME, Dincer I, Evaluation of hydrogen production via electrolysis with ion exchange membranes, Energy (2019), doi: https://doi.org/10.1016/ j.energy.2019.116420. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Evaluation of Hydrogen Production via Electrolysis with Ion Exchange Membranes
B. Yuzera,b, H. Selcuka G. Chehadeb, M.E. Demirb, I. Dincerb,c, a
Department of Environmental Engineering, Engineering Faculty, Istanbul University-Cerrahpasa, Istanbul, Turkey
b
Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada c
Department of Mechanical Engineering, Yildiz Technical University, Istanbul, Turkey
Abstract In this study, the ion exchange membranes are proposed and tested in an electrolysis process for hydrogen production from acidic and alkaline solutions. The results of the experiments are compared to evaluate the effect of ion exchange membranes on the performance of the electrolysis process. This study shows that ion exchange membranes can increase the performance of the electrolysis reactor and supply high pH differences between compartments due to the membrane’s feature of low electrical resistance and high resistance to pH changes. Anion exchange membrane, cation exchange membrane, and bipolar membrane are used individually as a separator between anode and cathode chamber of electrolysis reactor to evaluate the effect of these ion exchange membranes on system efficiency. Also, the comparison of using ion-exchange membranes to generate hydrogen in the acidic-alkaline electrolysis reactor is studied for the first time in this study. The electrolysis reactor is tested using various electrochemical techniques and analyzed thermodynamically. The maximum hydrogen production rate is determined with the bipolar membrane as 11.4 mmol/h, while the highest energy and exergy efficiencies are found for the reactor configuration with anion exchange membrane as 82% and 68%, respectively.
Keywords: Electrolysis, Hydrogen Production, Ion Exchange Membranes, Efficiency
1
1. Introduction The combustion of carbon-based fuels such as coal and petroleum releases carbon dioxide (CO2) to the atmosphere causing extreme weather patterns while the waste products are severely polluting the environment. In order to move towards a sustainable and environmentally benign world, it is essential to replace carbon-based fuels with sustainable green energy. Hydrogen, the simplest element in the periodic table, consists of a large proton and a single electron charge on its valence shell, is considered an energy-efficient low polluting fuel [1]. However, hydrogen does not present inherently in nature. It is found in water and many fuels such as gasoline, natural gas, methanol, and propane. Thus, hydrogen requires some processes to be produced. Methods, such as renewable and nuclear energy, photo-assisted electrolysis, biochemical, reverse electro-electrodialysis, electro-thermal, photo-thermal, and thermochemical water splitting are primarily carbon-free, thus are considered promising candidates for a diversified energy supply [2–6]. The electrolysis contributes to 4% of the global hydrogen production, while it is an efficient way to store and transport energy. Researches continue to improve the overall system efficiency by using different electrode materials and membrane separators between anode and cathode compartments. Coupling alternatives, storage mediums and electrolyzer membrane assemblies are progressing to ensure better efficiencies. Kotowicz et al. [7] proposed a methodology to determine the efficiency of hydrogen production systems that takes into account the power needs of its subsidiary systems. They presented a set of laboratory test results on a PEM water electrolyzer-based hydrogen generator for a wide range of system loadings. The output characteristics of the electrolyzers, including that of the entire hydrogen generator, were calculated using the power supply monitored for its auxiliary devices. Based on the results of the experimental tests, the authors have proposed generalized characteristics of hydrogen generator efficiency.
Mohammadi and Mehrpooya [8] have reviewed different methods of coupling
2
electrolyzers with various renewable energy sources to determine the most efficient system and it is indicated that the most promising renewable energy source to produce hydrogen is hydroelectric.
In addition, types of membrane and electrolytes can affect the reactor
performance considerably. Ion-exchange membranes (IEMs) have gained much interest in electromembrane processes such as electrodialysis, diffusion dialysis, energy conversion, energy storage, and also in electrolysis. Ogungbemi et al. [9] have discussed the use of membranes in fuel cell applications to produce energy and concluded that composite membranes can increase the cell voltage and overall performance of the cell by up to 11% and 17%, respectively. Sakr et al.[10] have used acrylic separator and polymeric membrane to separate the anode and cathode compartments in an alkaline water electrolysis cell. They have reported that the hydrogen production efficiency was higher when using the membrane as a separator instead of the acrylic separator. IEMs are divided into 3 subsets of membranes: anion exchange membrane (AEM), cation exchange membrane (CEM), and bipolar membrane (BPM). Generally, AEMs and CEMs are used in the electrolysis process to produce chlorine and caustic soda or hydrogen and oxygen. Also, they are the key elements in energy storage processes like batteries. There are two types of batteries reported by Strathman [11], such as “concentration flow battery” and neutralization flow battery.” AEM and CEM are usually used in concentration flow batteries, and the open cell voltage depends on the ion concentration gradients of the two adjacent compartments composed by IEMs. Weng et al. [12] have used both AEM and CEM as separators in a three-electrolyte electrochemical energy storage system and indicated that the proposed method had reduced the internal resistance of the battery system, which resulted in higher Coulumbic and voltage efficiencies. On the other hand, BPMs are used in neutralization flow batteries [13]. The electrical potential is generated by the difference between H+ and OH- ion concentrations between the anion- and
3
cation-exchange layers of the BPM, called “open voltage” of the battery. The open cell voltage of this battery depends on the H+ and OH- ion concentrations, and open-cell voltage is the main difference between these two batteries. Li et al. [14] investigate the performance of a modified BPM as diaphragms for hydrogen production from water electrolysis. The related BPM is synthesized by plasma-induced polymerization, and the performance analysis is conducted by measuring the cell voltage and H2 production rate for cells operated with or without the presence of the BPMs, as a function of current densities. The results show that the hydrogen production efficiency of the cell is improved by about 10-20% by using the modified BPMs as diaphragms. AEMs can be used in electrochemical applications, mostly in fuel cells due to their ability to work in low-temperature mediums and non-precious metals as a catalyst [15,16]. The most dominant transport mechanisms in AEMs are diffusion, migration, convection, and Grotthuss mechanism, which transport OH- through the membrane surface. Recently, Hibbs et al. [17] developed AEMs by varying the amount of ammonium, resulting in high ion exchange capacity. The study was compared to that of proton exchange membrane (PEM). The results show that the self-diffusion coefficient was higher in AEM, but the binding-water in the membrane was lower when compared to PEM. Ito et al. [18] have increased the hydrogen pressure in an AEM electrolyzer in order to test electrolysis performance, and they indicate that the hydrogen barrier ability of the AEM is superior to a PEM. The lower mobility of the OH- is associated with slow water diffusion. Grew and Chiu, [19] conducted research to identify the factoring limit for OHtransport in the AEM. Their study concluded that the Grotthuss mechanism contributes to the proton transport in aqueous solution and the main mechanism for OH- transport. However, other researchers showed that the OH- transport is limited to the solvation shell and discompose the hydrogen bond network [20,21]. Thus, it can be said that AEMs contributes to the OH- transport and hydrogen network in an alkali solution. Taking the mechanism of an AEM in a fuel cell and
4
reversing the reaction into an electrolysis cell, the transport phenomena of the OH- across the membrane from the anode and formation of H+ on the cathode can be attained. In an AEM electrolysis reactor, the cathode and anode form the external circuit, which provides DC power. The internal circuit is separated by the AEM in an alkaline electrolyte. Water at the anode side combines with the transported OH- across the membrane, and electron transfer occurs in the external circuit; thus water and oxygen are the products of the half-reaction. While at the cathode side, the hydrogen network bond breaks to form hydrogen ions with electron transfer hydrogen ions become hydrogen gas, which forms on the electrode surface [22]. The half-reactions take place when the theoretical, applied potential is greater than Gibbs free energy. However, in practice, the minimum applied potential is higher than the theoretical potential due to the ohmic resistance and to overcome the kinetics of the electrolyte, which is typically higher than 1.85 V at normal operating conditions [23]. CEMs are made mainly from cationic groups which are directly deposited on the polymer backbones, and they preclude the passage of anions and allows cations from passing through the membrane. CEMs are mostly functional in the diffusion dialysis for base recovery. Recent studies suggested that the sulfonic acid group is the best cation-exchange functional groups due to its strong acidity and high conductivity [24,25]. Although developed technologies on chaintype characterization for CEMs, low cation conductivity is apparent and restricting the potential practical applications in electrodialysis. In order to improve the functionality and conductivity of CEMs Bae et al. [26] proposed an interconnected cation-conducting channel on the backbone material using sulfonation co-polymers, the CEMs showed high proton conductivity due to the inter-connected ionic channels. However, BPM, which is composed of an anion layer, a cation layer, and an interfacial layer between the two layers, can generate hydroxide ions (OH-) and protons (H+) simultaneously making it the best candidate for producing acid and base. Water
5
dissociation occurs throughout the interface of the BPMs showing promising technology that is still under investigation. BPMs are similar to p-n type junction semiconductors, though the junction formed at the BPM interfacial layer (IL) creates a depletion region causing the bias over potential. BPMs are generally prepared by layer by layer casting of cation exchange layer and anion exchange layer, in the presence of heat, pressure and/or adhesive. This results in a thin layer commonly known as the interfacial layer between the anion and cation layer. When an electrical potential is applied to the membrane, ions are removed from the interfacial layer between the two membrane layers. When only water remains at the interfacial layer, the water dissociates to protons and hydroxide ions continuously due to the driving force of an electrical potential gradient in the BPM. The transportation of the electrical charges is driven by these protons and hydroxide ions generated in the BPM. The IL is developed according to chemical composition and structure based on the mechanism of proton transfer. Chabi et al. [27] have prepared BPMs and assessed their suitability for solar-driven water-splitting process. It is revealed that the interface between CEM and AEM produces a large electric field, which enables fast water dissociation.
In this study, the effects of ion-exchange membranes on the electrolysis
process are evaluated comparatively by using acid (H2SO4) and base (NaOH) solutions as electrolytes in the cathode and anode compartments, respectively. In the literature, studies generally focus on the use of one type of ion-exchange membrane to produce hydrogen in the electrolysis process [28–30]. However, in this particular research, three different types of ionexchange membranes (anion exchange membrane, cation exchange membrane, and bipolar membrane) are tested and compared with each other for the first time by using acidic and alkaline electrolytes at the cathode and anode compartments, respectively. Strong acidic, sulfonic acid based cation exchange membrane, and strong basic ammonium based anion exchange membrane are selected in this study. The bipolar membrane consists of the laminate of the cation and the anion exchange membrane layer. The performance evaluation of the membranes is 6
performed due to their impact on the hydrogen production process. The membranes are characterized by electrochemical methods. The pH gradient between anode and cathode compartments reduces the required energy and thermodynamic voltage for hydrogen production [31] because the system consumes the acid and base, which reduces the initial applied electrochemical potential difference. Ion exchange membranes provide pH gradients between anolyte and catholyte solutions. However, the consumed acid and base have to be replaced or regenerated. At this point, BPM can be used to increase the acidity and alkalinity of the electrolytes by producing OH- and H+ ions continuously. Another important parameter is the distance between the anode and cathode electrodes. Two different distances (0.8 cm and 2.2 cm) are tested to observe the effect on hydrogen production via electrolysis process. 2. Experimental apparatus and procedure In this section, the design and construction processes of the reactor, preparation processes of the electrolyte solutions, data collecting instruments, and performance assessment methods are described under sub-sections. 2.1.Electrolysis Reactor Electrodialysis cell (PCCell GmbH, Heusweiler, Germany) is modified by installing a graphite bipolar plate as the anode (64 cm2) and perforated 304 stainless steel (29 cm2) to produce hydrogen gas as in Figure 1. The electrolysis process performance is evaluated based on the conditions affecting hydrogen production, such as the following: •
Using monopolar and bipolar ion exchange membranes for the separation of anode and cathode chamber.
•
The distance between anode and cathode electrodes are adjusted to 2.2 cm and 0.8 cm
7
The adjustment of the distance between two electrodes is made by using thick and thin rubber frames. Also, the solution volume of the electrolysis reactor is increased from 50 mL to 140 mL by increasing the distance between the electrodes.
H2 Gas Flow Meter
H2 Collection Column
O2 Gas Electrolysis Reactor Ion exchange membrane Anode (+)
Cathode (-)
Potentiostat
Data Storage
Figure 1 Schematic presentation of the experimental setup and parts of the electrolysis reactor.
Ion exchange membranes are placed between anode and cathode sides and separated by rubber frames in order to adjust the distance between anode and cathode electrodes. The generated hydrogen gas is collected from the channels placed on the electrolysis reactor. The electric power supplied by a potentiostat to the electrolysis reactor and all the data obtained stored in a computer connected to the potentiostat. Also, the open circuit potential (OCP), cyclic voltammetry (CV) graphs, and current densities are monitored for all ion exchange membranes at two different electrode distances by using potentiostat. The hydrogen gas production is measured 8
by gas flow meter connected to the exit of the reactor and then collected in a column to measure the total collected gas volume by water displacement method (Figure 1). 2.2.Ion exchange membranes Commercial ion exchange membranes such as anion exchange membrane, cation exchange membrane, and bipolar membrane are purchased from PCCell GmbH, Germany. The ionexchange membranes are produced for standard desalination applications in the electrodialysis process; however, in this study, they are tested for hydrogen production in the electrolysis reactor. The AEM is highly alkaline and composed of ammonium group, while the CEM is highly acidic and composed of sulfonic acid. These membranes are the standard ion exchange membranes for the water desalination processes. Using these membranes in the electrolysis process will provide integration of different processes such as water treatment and hydrogen production at the same time. The properties of ion exchange membranes are given in Table 1 derived from the supplier. Table 1. Properties of ion-exchange membranes.
Membrane Type Functional group
Anion exchange Cation exchange Strong basic Strong acidic Ammonium Sulfonic acid
Perm selectivity KCl (0.1/0.5 N) Acid (0.7/3 N) Resistivity / W cm2 Active membrane area, cm2 Water content (wt %) Max operational temperature, ℃ Thickness, µm Membrane size, mm Ionic form
2.3.Electrolyte Solutions
9
>0.95
>0.95
≈1.8 64 ≈14 60 180-220 110 x 110 Cl-
≈2.5 64 ≈9 50 160-200 110 x 110 Na+
Since the resistivity of electrolyte solutions affect the energy losses in the electrolysis process, the overall energy losses in an electrolysis reactor can be minimized when acidic and basic electrolytes are used on the cathode and anode sides of the reactor respectively [32]. Anode compartment is filled with 1 M NaOH, and the cathode compartment is filled with 1 M H2SO4 as electrolyte solutions. 2.4.Experimental Procedure One ion exchange membrane is installed to the electrolysis reactor between the anode and cathode compartments and replaced with another after one set of the experiment ended. After the assembling of the reactor, cathode and anode compartments are filled with H2SO4 (1M) and NaOH (1M) electrolytes, respectively. First, the OCP and CV graphs obtained for the electrolysis reactor with each membrane by using the potentiostat. Then, electrolyte solutions are replaced with the new ones, and the hydrogen production capability of the electrolysis reactor with each membrane is monitored by applying the constant voltage to the reactor for 30 minutes under the same conditions. The applied potentials are decided according to the CV graphs that give the required current density to initiate the hydrogen production, and the obtained hydrogen production rates are extrapolated to one hour. All the experiments are conducted at the standard ambient temperature (25 oC) and pressure (1 atm). At these conditions, Faraday, energy, and exergy efficiencies are calculated. 2.5.Efficiency and performance assessment The energy, exergy, and Faraday efficiencies of the electrolysis system are calculated in order to evaluate the system performance from various perspectives. The Faraday efficiency is an important parameter to compare system efficiencies in the electrolysis process. It refers to the value of the transformed current in hydrogen evaluation reaction, and it is the ratio of hydrogen
10
volume obtained during the experiment to the theoretical hydrogen volume that is produced at the applied current. The Faraday efficiency is calculated by:
=
× × ×
(1)
Here; VH2 is the hydrogen volume produced during the experiment (L), I is the electric current (A), t is the experiment time (sec), Vm is the molar volume of the hydrogen (L), a is the number of electrons transformed to produce hydrogen gas at the cathode, and F is the Faraday constant (96485 A s/mol). The energy efficiency is the ratio of the energy values of useful products obtained in an electrolysis process to the total amount of consumed energy in the electrolysis process. The following formula is used to calculate the energy efficiency of the system: =
.
(2) ! Here is the energy efficiency is the hydrogen mol flow (mol/sec) is the higher heating value of the hydrogen (kJ/mol) and ! is the amount of energy that is given to the system, work rate (kW). Similarly, the exergy efficiency is the ratio of the exergy values of useful products obtained in an electrolysis process to the total amount of exergy inlet to the electrolysis process. The following formula is used to calculate the exergy efficiency of the system: . #$ (3) ! Here is the exergy efficiency is the hydrogen mol flow (mol/sec) #$ is the exergy value of the hydrogen (kJ/mol) and ! is the amount of energy that is given to the system (kW). =
3. Results and Discussion The testing of OCP is commonly performed before any electrochemical characterization to obtain information about the nature of the reactor. Since the OCP is dependent on the dissolved
11
oxidant and metal passivity, it is used as an indicator of such conditions. Strathmann [11] reported that the open cell voltage of the concentration flow battery is proportional to the concentration difference of the two adjacent ionic solutions separated by the AEM or CEM. Also, the OCP is calculated as 0.23 V assuming monovalent salt concentration is 1 mol in brine solution at the concentrate compartment and 0.01 mol at the dilute concentration. Similar OCP results are obtained for the configurations with IEM and 2.2 cm of electrode distance, as shown
800
900 800 700 600 500 400 300 200 100 0
600
CEM Potential, mV
Potential, mV
in Figure 2. The lowest OCP is observed as average 150 mV for the AEM membrane.
AEM BPM
CEM
400
AEM 200
BPM
0 -200 0
1
2
3
0.0
4
0.5
Time, min (a)
1.0
1.5 2.0 Time, min
2.5
3.0
3.5
(b)
Figure 2. Open circuit potential graph for ion exchange membranes in electrolysis process (a) distance between anode and cathode electrodes is 2.2 cm, (b) distance between anode and cathode electrodes is 0.8 cm (anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
Also, it can be said that BPM exhibits the p-n type junction semiconductors’ [33] or neutralization flow battery properties [11]; thus, the highest open-circuit voltage is observed around 850 mV for BPM which is the similar value reported in the literature [33]. This means the higher electrical potential is generated by BPM, and electrolysis system is charged by BPM due to the acid and base concentration gradient at the BPM surfaces. When the distance between anode and cathode electrodes decreases to 0.8 cm, the OCP potentials of the monopolar membranes decrease to a negative value, and OCP of the BPM is reduced to around 700 mV (Figure 2 (b)). This indicates that the lower distance between 12
electrodes results in the discharge of the electrical potential in the electrolysis cell, and the required overpotential decreased by this way. Also, when the distance between electrodes decreases to 0.8 cm, almost the same results of the OCP are obtained for the AEM and CEM. The experimental protocol to conduct CV experiment is performed by sweeping the potential of the stainless steel electrode/working electrode back and forth within a pre-set regime, and recording the current flow. The recorded peaks present in both the forward and reverse scan refer to the corresponding oxidation-reduction reactions. The cyclic voltammetry graph obtained at the 2.2 cm electrode distance, given in Figure 3. The required voltage in order to get the same current density values for the ion exchange membranes is the highest for BPM and lowest for the AEM. 35 BPM
Current Density, mA/cm2
30
AEM
25
CEM 20 15 10 5 0 500
1000
1500
2000
2500
3000
3500
Potential, mV
Figure 3 Cyclic voltammetry graph for ion exchange membranes in the electrolysis process (distance between anode and cathode electrodes is 2.2 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
The increase in the system performance is observed from the CV graphs given in Figure 4 when the distance between electrodes decreased to 0.8 cm. The higher current densities obtained with lower voltages when the electrode distance is reduced. This improvement is caused by the decrease of electrolyte resistivity by decreasing the distance.
13
100
Current Density, mA/cm2
Current Density, mA/cm2
35 30 25
AEM
20
CEM
15 10 5
80 BPM 60 40 20 0 -20
0 0
500
1000 Potential, mV
1500
0
2000
1000
2000
3000
4000
Potential, mV (b)
(a)
Figure 4. Cyclic voltammetry graph for (a) monopolar membranes and (b) bipolar membrane in electrolysis process (distance between the anode and cathode electrodes is 0.8 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
After electrochemical characterization, the reactor is subjected to an overpotential, and the stainless steel electrode in polarized cathodically. Under such conditions, electrons flow through the outer circuit. The magnitude of the current passing through the working electrode and the counter electrode is recorded. The constant voltage values applied to the electrolysis reactor to produce hydrogen, and the change in current densities are monitored continuously. Based on the OCP and CV results where the distance of electrodes is 0.8 cm, the applied voltages are set as 2.7V and 1.9V for BPM and monopolar membranes, which correspond to the same level of current densities. The current density values for ion exchange membranes while the distance between electrodes is 2.2 cm are given in Figure 5 (a). In the literature, the applied current density values for alkaline and polymer membrane electrolyzers vary between 200-400 mA/cm2 and 600-2000 mA/cm2 [23]. The obtained current densities are low with respect to the results given in literature; however, the hydrogen production is still high with ion-exchange membranes. When the distance between the electrodes is decreased to 0.8 cm, higher current density values are obtained with ion-exchange membranes, as shown in Figure 5 (b). The applied potential for the BPM is 2.7 V and 1.9 V for the monopolar membranes. It is interesting that similar to the 14
OCP measurements, the same trend is seen with the AEM and the CEM for current density values. Almost the same current density values obtained with monopolar membranes. Since the current density values decrease by the time with monopolar membranes, the stable current density values obtained with the BPM. 30
14
Current Density, mA/cm2
Current Density, mA/cm2
16 AEM CEM BPM
12 10 8 6 4 2 0 0
10 Time, min 20
30
(a)
25 20 15 10
BPM AEM CEM
5 0 0
10
Time, min (b)
20
30
Figure 5. Graph of current densities for ion exchange membranes in electrolysis process during hydrogen production (a) the distance between anode and cathode electrodes is 2.2 cm, applied potential for BPM: 3.7 V, monopolar membranes 1.9 V and (b) the distance between anode and cathode electrodes is 0.8 cm, applied potential for BPM: 2.7 V, monopolar membranes 1.9 V (anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
Hydrogen production rates of ion exchange membranes are given in Figure 6 (a) for the distance between electrodes is 2.2 cm. The highest production rate is obtained with the BPM; however, the applied potential is twice of the monopolar membranes. The Faraday efficiencies of the BPM and the AEM are calculated as 100% for both while it is calculated as 98% for the CEM, as shown in Figure 6 (b). The energy efficiencies of the electrolysis process with ion-exchange membranes are given in Figure 6 (c), and the same values are obtained (77%) for the AEM and the CEM. On the other hand, 41% of energy efficiency is achieved with the BPM. The same trend is seen for the exergy efficiency values since 63% of exergy efficiencies obtained with the AEM and the CEM. 34% of exergy efficiency is calculated for the BPM (Figure 6 (d)).
15
100 Faraday Efficiency, %
H2 Production Rate, mmole/h
10
5
0
98
96 AEM
CEM
BPM
BPM
90 80 70 60 50 40 30 20 10 0 BPM
AEM
AEM
CEM
(b)
Exergy Efficiency, %
Energy Efficiency, %
(a)
CEM
90 80 70 60 50 40 30 20 10 0 BPM
(c)
AEM
CEM
(d)
Figure 6. Graphs of (a) hydrogen production rates, (b) Faraday efficiencies, (c) energy efficiencies, (d) exergy efficiencies for ion exchange membranes in electrolysis process during hydrogen production (applied potential for BPM: 3.7 V, monopolar membranes 1.9 V distance between anode and cathode electrodes is 2.2 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
As it is expected, the hydrogen production rates are increased with the decreasing electrode distance. The hydrogen production rate is given in Figure 7 (a) and calculated as 11.4 mmole/h with the BPM and 8.2 mmole/h for the AEM. The Faraday efficiencies of the BPM and the AEM are calculated as 100% for both, as shown in Figure 7 (b). Energy efficiency for the BPM is increased by the decreasing electrode distance; however, it is still low with respect to monopolar membranes (Figure 7 (c)). The exergy efficiency values are given in Figure 7 (d). The highest energy and exergy efficiencies are found 81% and 67%, respectively. It is evident that BPM has the lowest exergy efficiency value while it has the highest hydrogen production rate.
16
100 Faraday Efficiency, %
H2 Production Rate, mmole/h
15
10
5
0
98
96 BPM
CEM
AEM
BPM
90 80 70 60 50 40 30 20 10 0 BPM
CEM
AEM
(b)
Exergy Efficiency, %
Energy Efficiency, %
(a)
CEM
AEM
90 80 70 60 50 40 30 20 10 0 BPM
(c)
CEM
AEM
(d)
Figure 7. Graphs of (a) hydrogen production rates, (b) Faraday efficiencies, (c) energy efficiencies, (d) exergy efficiencies for ion exchange membranes in electrolysis process during hydrogen production (applied potential for BPM: 2.7 V, monopolar membranes 1.9 V, distance between anode and cathode electrodes is 0.8 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
The results of this study are compared with literature. Nazemi et al. [31] investigate the monopolar alkaline electrolyser operates with acidic and basic electrolyte types at room temperature where the electric current density is 20 mA/cm2. The electric potential of the electrolyser is reported as between 1.2V-2.2V and the hydrogen conversion rate of 98.8%. In this study, current density is obtained in the range of 0.10- 0.20 A/cm-2 at the cell potential of 1.9V for monopolar membranes. Similarly the conversion rate is obtained in the same range as presented in Figure 7 (b). Li et al.[29] investigated an electrochemically neutralized energyassisted alkaline electrolyze with bipolar membrane assembly. The electrolyser used acidic and basic electrolytes similar to this study and operated under 3.0V to obtain 10 mA/cm2 of current 17
density with a Faraday efficiency of 100%. In this study, 15 mA/cm2 of current density is obtained under 3.7V of cell potential and a Faraday efficiency of 100% which show similar trend with data published in the literature. 4. Conclusions In this study, the performances of the AEM, CEM, and BPM in the electrolysis process for hydrogen production are evaluated comparatively. Also, the effect of distance between anode and cathode electrodes on the electrolysis process is examined. The open-circuit potential is measured as 850 mV, and this is resulted in a higher overpotential for the BPM due to the water dissociation at the junction of the ion exchange membranes. The BPM membrane shows a stable current density value as 20 mA/cm2 during hydrogen production. On the other hand, the current density value decreases from 20 mA/cm2 to 7 mA/cm2 as a result of the replacement of the membrane to monopolar ion-exchange membranes. The higher hydrogen production obtained as 11.4 mmol/h with the BPM; however, the lowest energy and exergy efficiencies were calculated as 55% and 45% for the BPM, respectively. The distance between anode and cathode electrodes affects system performance. When the distance decreased from 2.2 cm to 0.8 cm, the energy and exergy efficiencies, the hydrogen production rates, and the current density values increased around 34%, 32%, 47%, and 42% in the electrolysis process, respectively. Using ion-exchange membranes in the acid/base electrolysis process for hydrogen production gives promising results. For further studies, using spent acid and alkali liquors from the metal industry in this process will decrease the environmental impact of those waste solutions while producing hydrogen energy.
18
Acknowledgment The authors would like to thank the EU, and the Natural Sciences and Engineering Research Council of Canada for funding, in the frame of the collaborative international Consortium ECOSAFEFARMING financed under the ERA-NET WaterWorks2015 Cofunded Call. This ERA-NET is an integral part of the 2016 Joint Activities developed by the Water Challenges for a Changing World Joint Programme Initiative (Water JPI). Nomenclature F
Faraday constant (96485 A s/mol)
I HHV t a V !
Electric current (A) Higher heating value (kJ/mol) Time (sec) Number of electrons transformed to produce hydrogen Mol flow rate (mol/sec) Volume (L)
IEMs AEM CEM BPM PEM IL PEG PVA OCP CV
Ion-exchange membranes Anion exchange membrane Cation exchange membrane Bipolar membrane Proton exchange membrane Interfacial layer Polyethylene glycol Polyvinyl alcohol Open circuit potential Cyclic voltammetry
Work rate (kW) Greek Letters η Efficiency Acronyms
Subscripts and Superscripts En Energy Ex Exergy m H2
Molar Hydrogen
19
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22
Research Highlights •
A novel system for hydrogen production with ion exchange membranes is designed.
•
Comparative assessment of various ion exchange membranes on the system performance.
•
Electrochemical analysis of the electrolysis reactor with ion exchange membranes.
•
A comprehensive energy and exergy efficiencies assessment.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0360-5442(19)32115-2
DOI:
https://doi.org/10.1016/j.energy.2019.116420
Reference:
EGY 116420
To appear in:
Energy
Received Date: 19 July 2019 Revised Date:
19 October 2019
Accepted Date: 22 October 2019
Please cite this article as: Yuzer B, Selcuk H, Chehade G, Demir ME, Dincer I, Evaluation of hydrogen production via electrolysis with ion exchange membranes, Energy (2019), doi: https://doi.org/10.1016/ j.energy.2019.116420. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Evaluation of Hydrogen Production via Electrolysis with Ion Exchange Membranes
B. Yuzera,b, H. Selcuka G. Chehadeb, M.E. Demirb, I. Dincerb,c, a
Department of Environmental Engineering, Engineering Faculty, Istanbul University-Cerrahpasa, Istanbul, Turkey
b
Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada c
Department of Mechanical Engineering, Yildiz Technical University, Istanbul, Turkey
Abstract In this study, the ion exchange membranes are proposed and tested in an electrolysis process for hydrogen production from acidic and alkaline solutions. The results of the experiments are compared to evaluate the effect of ion exchange membranes on the performance of the electrolysis process. This study shows that ion exchange membranes can increase the performance of the electrolysis reactor and supply high pH differences between compartments due to the membrane’s feature of low electrical resistance and high resistance to pH changes. Anion exchange membrane, cation exchange membrane, and bipolar membrane are used individually as a separator between anode and cathode chamber of electrolysis reactor to evaluate the effect of these ion exchange membranes on system efficiency. Also, the comparison of using ion-exchange membranes to generate hydrogen in the acidic-alkaline electrolysis reactor is studied for the first time in this study. The electrolysis reactor is tested using various electrochemical techniques and analyzed thermodynamically. The maximum hydrogen production rate is determined with the bipolar membrane as 11.4 mmol/h, while the highest energy and exergy efficiencies are found for the reactor configuration with anion exchange membrane as 82% and 68%, respectively.
Keywords: Electrolysis, Hydrogen Production, Ion Exchange Membranes, Efficiency
1
1. Introduction The combustion of carbon-based fuels such as coal and petroleum releases carbon dioxide (CO2) to the atmosphere causing extreme weather patterns while the waste products are severely polluting the environment. In order to move towards a sustainable and environmentally benign world, it is essential to replace carbon-based fuels with sustainable green energy. Hydrogen, the simplest element in the periodic table, consists of a large proton and a single electron charge on its valence shell, is considered an energy-efficient low polluting fuel [1]. However, hydrogen does not present inherently in nature. It is found in water and many fuels such as gasoline, natural gas, methanol, and propane. Thus, hydrogen requires some processes to be produced. Methods, such as renewable and nuclear energy, photo-assisted electrolysis, biochemical, reverse electro-electrodialysis, electro-thermal, photo-thermal, and thermochemical water splitting are primarily carbon-free, thus are considered promising candidates for a diversified energy supply [2–6]. The electrolysis contributes to 4% of the global hydrogen production, while it is an efficient way to store and transport energy. Researches continue to improve the overall system efficiency by using different electrode materials and membrane separators between anode and cathode compartments. Coupling alternatives, storage mediums and electrolyzer membrane assemblies are progressing to ensure better efficiencies. Kotowicz et al. [7] proposed a methodology to determine the efficiency of hydrogen production systems that takes into account the power needs of its subsidiary systems. They presented a set of laboratory test results on a PEM water electrolyzer-based hydrogen generator for a wide range of system loadings. The output characteristics of the electrolyzers, including that of the entire hydrogen generator, were calculated using the power supply monitored for its auxiliary devices. Based on the results of the experimental tests, the authors have proposed generalized characteristics of hydrogen generator efficiency.
Mohammadi and Mehrpooya [8] have reviewed different methods of coupling
2
electrolyzers with various renewable energy sources to determine the most efficient system and it is indicated that the most promising renewable energy source to produce hydrogen is hydroelectric.
In addition, types of membrane and electrolytes can affect the reactor
performance considerably. Ion-exchange membranes (IEMs) have gained much interest in electromembrane processes such as electrodialysis, diffusion dialysis, energy conversion, energy storage, and also in electrolysis. Ogungbemi et al. [9] have discussed the use of membranes in fuel cell applications to produce energy and concluded that composite membranes can increase the cell voltage and overall performance of the cell by up to 11% and 17%, respectively. Sakr et al.[10] have used acrylic separator and polymeric membrane to separate the anode and cathode compartments in an alkaline water electrolysis cell. They have reported that the hydrogen production efficiency was higher when using the membrane as a separator instead of the acrylic separator. IEMs are divided into 3 subsets of membranes: anion exchange membrane (AEM), cation exchange membrane (CEM), and bipolar membrane (BPM). Generally, AEMs and CEMs are used in the electrolysis process to produce chlorine and caustic soda or hydrogen and oxygen. Also, they are the key elements in energy storage processes like batteries. There are two types of batteries reported by Strathman [11], such as “concentration flow battery” and neutralization flow battery.” AEM and CEM are usually used in concentration flow batteries, and the open cell voltage depends on the ion concentration gradients of the two adjacent compartments composed by IEMs. Weng et al. [12] have used both AEM and CEM as separators in a three-electrolyte electrochemical energy storage system and indicated that the proposed method had reduced the internal resistance of the battery system, which resulted in higher Coulumbic and voltage efficiencies. On the other hand, BPMs are used in neutralization flow batteries [13]. The electrical potential is generated by the difference between H+ and OH- ion concentrations between the anion- and
3
cation-exchange layers of the BPM, called “open voltage” of the battery. The open cell voltage of this battery depends on the H+ and OH- ion concentrations, and open-cell voltage is the main difference between these two batteries. Li et al. [14] investigate the performance of a modified BPM as diaphragms for hydrogen production from water electrolysis. The related BPM is synthesized by plasma-induced polymerization, and the performance analysis is conducted by measuring the cell voltage and H2 production rate for cells operated with or without the presence of the BPMs, as a function of current densities. The results show that the hydrogen production efficiency of the cell is improved by about 10-20% by using the modified BPMs as diaphragms. AEMs can be used in electrochemical applications, mostly in fuel cells due to their ability to work in low-temperature mediums and non-precious metals as a catalyst [15,16]. The most dominant transport mechanisms in AEMs are diffusion, migration, convection, and Grotthuss mechanism, which transport OH- through the membrane surface. Recently, Hibbs et al. [17] developed AEMs by varying the amount of ammonium, resulting in high ion exchange capacity. The study was compared to that of proton exchange membrane (PEM). The results show that the self-diffusion coefficient was higher in AEM, but the binding-water in the membrane was lower when compared to PEM. Ito et al. [18] have increased the hydrogen pressure in an AEM electrolyzer in order to test electrolysis performance, and they indicate that the hydrogen barrier ability of the AEM is superior to a PEM. The lower mobility of the OH- is associated with slow water diffusion. Grew and Chiu, [19] conducted research to identify the factoring limit for OHtransport in the AEM. Their study concluded that the Grotthuss mechanism contributes to the proton transport in aqueous solution and the main mechanism for OH- transport. However, other researchers showed that the OH- transport is limited to the solvation shell and discompose the hydrogen bond network [20,21]. Thus, it can be said that AEMs contributes to the OH- transport and hydrogen network in an alkali solution. Taking the mechanism of an AEM in a fuel cell and
4
reversing the reaction into an electrolysis cell, the transport phenomena of the OH- across the membrane from the anode and formation of H+ on the cathode can be attained. In an AEM electrolysis reactor, the cathode and anode form the external circuit, which provides DC power. The internal circuit is separated by the AEM in an alkaline electrolyte. Water at the anode side combines with the transported OH- across the membrane, and electron transfer occurs in the external circuit; thus water and oxygen are the products of the half-reaction. While at the cathode side, the hydrogen network bond breaks to form hydrogen ions with electron transfer hydrogen ions become hydrogen gas, which forms on the electrode surface [22]. The half-reactions take place when the theoretical, applied potential is greater than Gibbs free energy. However, in practice, the minimum applied potential is higher than the theoretical potential due to the ohmic resistance and to overcome the kinetics of the electrolyte, which is typically higher than 1.85 V at normal operating conditions [23]. CEMs are made mainly from cationic groups which are directly deposited on the polymer backbones, and they preclude the passage of anions and allows cations from passing through the membrane. CEMs are mostly functional in the diffusion dialysis for base recovery. Recent studies suggested that the sulfonic acid group is the best cation-exchange functional groups due to its strong acidity and high conductivity [24,25]. Although developed technologies on chaintype characterization for CEMs, low cation conductivity is apparent and restricting the potential practical applications in electrodialysis. In order to improve the functionality and conductivity of CEMs Bae et al. [26] proposed an interconnected cation-conducting channel on the backbone material using sulfonation co-polymers, the CEMs showed high proton conductivity due to the inter-connected ionic channels. However, BPM, which is composed of an anion layer, a cation layer, and an interfacial layer between the two layers, can generate hydroxide ions (OH-) and protons (H+) simultaneously making it the best candidate for producing acid and base. Water
5
dissociation occurs throughout the interface of the BPMs showing promising technology that is still under investigation. BPMs are similar to p-n type junction semiconductors, though the junction formed at the BPM interfacial layer (IL) creates a depletion region causing the bias over potential. BPMs are generally prepared by layer by layer casting of cation exchange layer and anion exchange layer, in the presence of heat, pressure and/or adhesive. This results in a thin layer commonly known as the interfacial layer between the anion and cation layer. When an electrical potential is applied to the membrane, ions are removed from the interfacial layer between the two membrane layers. When only water remains at the interfacial layer, the water dissociates to protons and hydroxide ions continuously due to the driving force of an electrical potential gradient in the BPM. The transportation of the electrical charges is driven by these protons and hydroxide ions generated in the BPM. The IL is developed according to chemical composition and structure based on the mechanism of proton transfer. Chabi et al. [27] have prepared BPMs and assessed their suitability for solar-driven water-splitting process. It is revealed that the interface between CEM and AEM produces a large electric field, which enables fast water dissociation.
In this study, the effects of ion-exchange membranes on the electrolysis
process are evaluated comparatively by using acid (H2SO4) and base (NaOH) solutions as electrolytes in the cathode and anode compartments, respectively. In the literature, studies generally focus on the use of one type of ion-exchange membrane to produce hydrogen in the electrolysis process [28–30]. However, in this particular research, three different types of ionexchange membranes (anion exchange membrane, cation exchange membrane, and bipolar membrane) are tested and compared with each other for the first time by using acidic and alkaline electrolytes at the cathode and anode compartments, respectively. Strong acidic, sulfonic acid based cation exchange membrane, and strong basic ammonium based anion exchange membrane are selected in this study. The bipolar membrane consists of the laminate of the cation and the anion exchange membrane layer. The performance evaluation of the membranes is 6
performed due to their impact on the hydrogen production process. The membranes are characterized by electrochemical methods. The pH gradient between anode and cathode compartments reduces the required energy and thermodynamic voltage for hydrogen production [31] because the system consumes the acid and base, which reduces the initial applied electrochemical potential difference. Ion exchange membranes provide pH gradients between anolyte and catholyte solutions. However, the consumed acid and base have to be replaced or regenerated. At this point, BPM can be used to increase the acidity and alkalinity of the electrolytes by producing OH- and H+ ions continuously. Another important parameter is the distance between the anode and cathode electrodes. Two different distances (0.8 cm and 2.2 cm) are tested to observe the effect on hydrogen production via electrolysis process. 2. Experimental apparatus and procedure In this section, the design and construction processes of the reactor, preparation processes of the electrolyte solutions, data collecting instruments, and performance assessment methods are described under sub-sections. 2.1.Electrolysis Reactor Electrodialysis cell (PCCell GmbH, Heusweiler, Germany) is modified by installing a graphite bipolar plate as the anode (64 cm2) and perforated 304 stainless steel (29 cm2) to produce hydrogen gas as in Figure 1. The electrolysis process performance is evaluated based on the conditions affecting hydrogen production, such as the following: •
Using monopolar and bipolar ion exchange membranes for the separation of anode and cathode chamber.
•
The distance between anode and cathode electrodes are adjusted to 2.2 cm and 0.8 cm
7
The adjustment of the distance between two electrodes is made by using thick and thin rubber frames. Also, the solution volume of the electrolysis reactor is increased from 50 mL to 140 mL by increasing the distance between the electrodes.
H2 Gas Flow Meter
H2 Collection Column
O2 Gas Electrolysis Reactor Ion exchange membrane Anode (+)
Cathode (-)
Potentiostat
Data Storage
Figure 1 Schematic presentation of the experimental setup and parts of the electrolysis reactor.
Ion exchange membranes are placed between anode and cathode sides and separated by rubber frames in order to adjust the distance between anode and cathode electrodes. The generated hydrogen gas is collected from the channels placed on the electrolysis reactor. The electric power supplied by a potentiostat to the electrolysis reactor and all the data obtained stored in a computer connected to the potentiostat. Also, the open circuit potential (OCP), cyclic voltammetry (CV) graphs, and current densities are monitored for all ion exchange membranes at two different electrode distances by using potentiostat. The hydrogen gas production is measured 8
by gas flow meter connected to the exit of the reactor and then collected in a column to measure the total collected gas volume by water displacement method (Figure 1). 2.2.Ion exchange membranes Commercial ion exchange membranes such as anion exchange membrane, cation exchange membrane, and bipolar membrane are purchased from PCCell GmbH, Germany. The ionexchange membranes are produced for standard desalination applications in the electrodialysis process; however, in this study, they are tested for hydrogen production in the electrolysis reactor. The AEM is highly alkaline and composed of ammonium group, while the CEM is highly acidic and composed of sulfonic acid. These membranes are the standard ion exchange membranes for the water desalination processes. Using these membranes in the electrolysis process will provide integration of different processes such as water treatment and hydrogen production at the same time. The properties of ion exchange membranes are given in Table 1 derived from the supplier. Table 1. Properties of ion-exchange membranes.
Membrane Type Functional group
Anion exchange Cation exchange Strong basic Strong acidic Ammonium Sulfonic acid
Perm selectivity KCl (0.1/0.5 N) Acid (0.7/3 N) Resistivity / W cm2 Active membrane area, cm2 Water content (wt %) Max operational temperature, ℃ Thickness, µm Membrane size, mm Ionic form
2.3.Electrolyte Solutions
9
>0.95
>0.95
≈1.8 64 ≈14 60 180-220 110 x 110 Cl-
≈2.5 64 ≈9 50 160-200 110 x 110 Na+
Since the resistivity of electrolyte solutions affect the energy losses in the electrolysis process, the overall energy losses in an electrolysis reactor can be minimized when acidic and basic electrolytes are used on the cathode and anode sides of the reactor respectively [32]. Anode compartment is filled with 1 M NaOH, and the cathode compartment is filled with 1 M H2SO4 as electrolyte solutions. 2.4.Experimental Procedure One ion exchange membrane is installed to the electrolysis reactor between the anode and cathode compartments and replaced with another after one set of the experiment ended. After the assembling of the reactor, cathode and anode compartments are filled with H2SO4 (1M) and NaOH (1M) electrolytes, respectively. First, the OCP and CV graphs obtained for the electrolysis reactor with each membrane by using the potentiostat. Then, electrolyte solutions are replaced with the new ones, and the hydrogen production capability of the electrolysis reactor with each membrane is monitored by applying the constant voltage to the reactor for 30 minutes under the same conditions. The applied potentials are decided according to the CV graphs that give the required current density to initiate the hydrogen production, and the obtained hydrogen production rates are extrapolated to one hour. All the experiments are conducted at the standard ambient temperature (25 oC) and pressure (1 atm). At these conditions, Faraday, energy, and exergy efficiencies are calculated. 2.5.Efficiency and performance assessment The energy, exergy, and Faraday efficiencies of the electrolysis system are calculated in order to evaluate the system performance from various perspectives. The Faraday efficiency is an important parameter to compare system efficiencies in the electrolysis process. It refers to the value of the transformed current in hydrogen evaluation reaction, and it is the ratio of hydrogen
10
volume obtained during the experiment to the theoretical hydrogen volume that is produced at the applied current. The Faraday efficiency is calculated by:
=
× × ×
(1)
Here; VH2 is the hydrogen volume produced during the experiment (L), I is the electric current (A), t is the experiment time (sec), Vm is the molar volume of the hydrogen (L), a is the number of electrons transformed to produce hydrogen gas at the cathode, and F is the Faraday constant (96485 A s/mol). The energy efficiency is the ratio of the energy values of useful products obtained in an electrolysis process to the total amount of consumed energy in the electrolysis process. The following formula is used to calculate the energy efficiency of the system: =
.
(2) ! Here is the energy efficiency is the hydrogen mol flow (mol/sec) is the higher heating value of the hydrogen (kJ/mol) and ! is the amount of energy that is given to the system, work rate (kW). Similarly, the exergy efficiency is the ratio of the exergy values of useful products obtained in an electrolysis process to the total amount of exergy inlet to the electrolysis process. The following formula is used to calculate the exergy efficiency of the system: . #$ (3) ! Here is the exergy efficiency is the hydrogen mol flow (mol/sec) #$ is the exergy value of the hydrogen (kJ/mol) and ! is the amount of energy that is given to the system (kW). =
3. Results and Discussion The testing of OCP is commonly performed before any electrochemical characterization to obtain information about the nature of the reactor. Since the OCP is dependent on the dissolved
11
oxidant and metal passivity, it is used as an indicator of such conditions. Strathmann [11] reported that the open cell voltage of the concentration flow battery is proportional to the concentration difference of the two adjacent ionic solutions separated by the AEM or CEM. Also, the OCP is calculated as 0.23 V assuming monovalent salt concentration is 1 mol in brine solution at the concentrate compartment and 0.01 mol at the dilute concentration. Similar OCP results are obtained for the configurations with IEM and 2.2 cm of electrode distance, as shown
800
900 800 700 600 500 400 300 200 100 0
600
CEM Potential, mV
Potential, mV
in Figure 2. The lowest OCP is observed as average 150 mV for the AEM membrane.
AEM BPM
CEM
400
AEM 200
BPM
0 -200 0
1
2
3
0.0
4
0.5
Time, min (a)
1.0
1.5 2.0 Time, min
2.5
3.0
3.5
(b)
Figure 2. Open circuit potential graph for ion exchange membranes in electrolysis process (a) distance between anode and cathode electrodes is 2.2 cm, (b) distance between anode and cathode electrodes is 0.8 cm (anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
Also, it can be said that BPM exhibits the p-n type junction semiconductors’ [33] or neutralization flow battery properties [11]; thus, the highest open-circuit voltage is observed around 850 mV for BPM which is the similar value reported in the literature [33]. This means the higher electrical potential is generated by BPM, and electrolysis system is charged by BPM due to the acid and base concentration gradient at the BPM surfaces. When the distance between anode and cathode electrodes decreases to 0.8 cm, the OCP potentials of the monopolar membranes decrease to a negative value, and OCP of the BPM is reduced to around 700 mV (Figure 2 (b)). This indicates that the lower distance between 12
electrodes results in the discharge of the electrical potential in the electrolysis cell, and the required overpotential decreased by this way. Also, when the distance between electrodes decreases to 0.8 cm, almost the same results of the OCP are obtained for the AEM and CEM. The experimental protocol to conduct CV experiment is performed by sweeping the potential of the stainless steel electrode/working electrode back and forth within a pre-set regime, and recording the current flow. The recorded peaks present in both the forward and reverse scan refer to the corresponding oxidation-reduction reactions. The cyclic voltammetry graph obtained at the 2.2 cm electrode distance, given in Figure 3. The required voltage in order to get the same current density values for the ion exchange membranes is the highest for BPM and lowest for the AEM. 35 BPM
Current Density, mA/cm2
30
AEM
25
CEM 20 15 10 5 0 500
1000
1500
2000
2500
3000
3500
Potential, mV
Figure 3 Cyclic voltammetry graph for ion exchange membranes in the electrolysis process (distance between anode and cathode electrodes is 2.2 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
The increase in the system performance is observed from the CV graphs given in Figure 4 when the distance between electrodes decreased to 0.8 cm. The higher current densities obtained with lower voltages when the electrode distance is reduced. This improvement is caused by the decrease of electrolyte resistivity by decreasing the distance.
13
100
Current Density, mA/cm2
Current Density, mA/cm2
35 30 25
AEM
20
CEM
15 10 5
80 BPM 60 40 20 0 -20
0 0
500
1000 Potential, mV
1500
0
2000
1000
2000
3000
4000
Potential, mV (b)
(a)
Figure 4. Cyclic voltammetry graph for (a) monopolar membranes and (b) bipolar membrane in electrolysis process (distance between the anode and cathode electrodes is 0.8 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
After electrochemical characterization, the reactor is subjected to an overpotential, and the stainless steel electrode in polarized cathodically. Under such conditions, electrons flow through the outer circuit. The magnitude of the current passing through the working electrode and the counter electrode is recorded. The constant voltage values applied to the electrolysis reactor to produce hydrogen, and the change in current densities are monitored continuously. Based on the OCP and CV results where the distance of electrodes is 0.8 cm, the applied voltages are set as 2.7V and 1.9V for BPM and monopolar membranes, which correspond to the same level of current densities. The current density values for ion exchange membranes while the distance between electrodes is 2.2 cm are given in Figure 5 (a). In the literature, the applied current density values for alkaline and polymer membrane electrolyzers vary between 200-400 mA/cm2 and 600-2000 mA/cm2 [23]. The obtained current densities are low with respect to the results given in literature; however, the hydrogen production is still high with ion-exchange membranes. When the distance between the electrodes is decreased to 0.8 cm, higher current density values are obtained with ion-exchange membranes, as shown in Figure 5 (b). The applied potential for the BPM is 2.7 V and 1.9 V for the monopolar membranes. It is interesting that similar to the 14
OCP measurements, the same trend is seen with the AEM and the CEM for current density values. Almost the same current density values obtained with monopolar membranes. Since the current density values decrease by the time with monopolar membranes, the stable current density values obtained with the BPM. 30
14
Current Density, mA/cm2
Current Density, mA/cm2
16 AEM CEM BPM
12 10 8 6 4 2 0 0
10 Time, min 20
30
(a)
25 20 15 10
BPM AEM CEM
5 0 0
10
Time, min (b)
20
30
Figure 5. Graph of current densities for ion exchange membranes in electrolysis process during hydrogen production (a) the distance between anode and cathode electrodes is 2.2 cm, applied potential for BPM: 3.7 V, monopolar membranes 1.9 V and (b) the distance between anode and cathode electrodes is 0.8 cm, applied potential for BPM: 2.7 V, monopolar membranes 1.9 V (anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
Hydrogen production rates of ion exchange membranes are given in Figure 6 (a) for the distance between electrodes is 2.2 cm. The highest production rate is obtained with the BPM; however, the applied potential is twice of the monopolar membranes. The Faraday efficiencies of the BPM and the AEM are calculated as 100% for both while it is calculated as 98% for the CEM, as shown in Figure 6 (b). The energy efficiencies of the electrolysis process with ion-exchange membranes are given in Figure 6 (c), and the same values are obtained (77%) for the AEM and the CEM. On the other hand, 41% of energy efficiency is achieved with the BPM. The same trend is seen for the exergy efficiency values since 63% of exergy efficiencies obtained with the AEM and the CEM. 34% of exergy efficiency is calculated for the BPM (Figure 6 (d)).
15
100 Faraday Efficiency, %
H2 Production Rate, mmole/h
10
5
0
98
96 AEM
CEM
BPM
BPM
90 80 70 60 50 40 30 20 10 0 BPM
AEM
AEM
CEM
(b)
Exergy Efficiency, %
Energy Efficiency, %
(a)
CEM
90 80 70 60 50 40 30 20 10 0 BPM
(c)
AEM
CEM
(d)
Figure 6. Graphs of (a) hydrogen production rates, (b) Faraday efficiencies, (c) energy efficiencies, (d) exergy efficiencies for ion exchange membranes in electrolysis process during hydrogen production (applied potential for BPM: 3.7 V, monopolar membranes 1.9 V distance between anode and cathode electrodes is 2.2 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
As it is expected, the hydrogen production rates are increased with the decreasing electrode distance. The hydrogen production rate is given in Figure 7 (a) and calculated as 11.4 mmole/h with the BPM and 8.2 mmole/h for the AEM. The Faraday efficiencies of the BPM and the AEM are calculated as 100% for both, as shown in Figure 7 (b). Energy efficiency for the BPM is increased by the decreasing electrode distance; however, it is still low with respect to monopolar membranes (Figure 7 (c)). The exergy efficiency values are given in Figure 7 (d). The highest energy and exergy efficiencies are found 81% and 67%, respectively. It is evident that BPM has the lowest exergy efficiency value while it has the highest hydrogen production rate.
16
100 Faraday Efficiency, %
H2 Production Rate, mmole/h
15
10
5
0
98
96 BPM
CEM
AEM
BPM
90 80 70 60 50 40 30 20 10 0 BPM
CEM
AEM
(b)
Exergy Efficiency, %
Energy Efficiency, %
(a)
CEM
AEM
90 80 70 60 50 40 30 20 10 0 BPM
(c)
CEM
AEM
(d)
Figure 7. Graphs of (a) hydrogen production rates, (b) Faraday efficiencies, (c) energy efficiencies, (d) exergy efficiencies for ion exchange membranes in electrolysis process during hydrogen production (applied potential for BPM: 2.7 V, monopolar membranes 1.9 V, distance between anode and cathode electrodes is 0.8 cm, anolyte: 1 M NaOH, catholyte: 1 M H2SO4).
The results of this study are compared with literature. Nazemi et al. [31] investigate the monopolar alkaline electrolyser operates with acidic and basic electrolyte types at room temperature where the electric current density is 20 mA/cm2. The electric potential of the electrolyser is reported as between 1.2V-2.2V and the hydrogen conversion rate of 98.8%. In this study, current density is obtained in the range of 0.10- 0.20 A/cm-2 at the cell potential of 1.9V for monopolar membranes. Similarly the conversion rate is obtained in the same range as presented in Figure 7 (b). Li et al.[29] investigated an electrochemically neutralized energyassisted alkaline electrolyze with bipolar membrane assembly. The electrolyser used acidic and basic electrolytes similar to this study and operated under 3.0V to obtain 10 mA/cm2 of current 17
density with a Faraday efficiency of 100%. In this study, 15 mA/cm2 of current density is obtained under 3.7V of cell potential and a Faraday efficiency of 100% which show similar trend with data published in the literature. 4. Conclusions In this study, the performances of the AEM, CEM, and BPM in the electrolysis process for hydrogen production are evaluated comparatively. Also, the effect of distance between anode and cathode electrodes on the electrolysis process is examined. The open-circuit potential is measured as 850 mV, and this is resulted in a higher overpotential for the BPM due to the water dissociation at the junction of the ion exchange membranes. The BPM membrane shows a stable current density value as 20 mA/cm2 during hydrogen production. On the other hand, the current density value decreases from 20 mA/cm2 to 7 mA/cm2 as a result of the replacement of the membrane to monopolar ion-exchange membranes. The higher hydrogen production obtained as 11.4 mmol/h with the BPM; however, the lowest energy and exergy efficiencies were calculated as 55% and 45% for the BPM, respectively. The distance between anode and cathode electrodes affects system performance. When the distance decreased from 2.2 cm to 0.8 cm, the energy and exergy efficiencies, the hydrogen production rates, and the current density values increased around 34%, 32%, 47%, and 42% in the electrolysis process, respectively. Using ion-exchange membranes in the acid/base electrolysis process for hydrogen production gives promising results. For further studies, using spent acid and alkali liquors from the metal industry in this process will decrease the environmental impact of those waste solutions while producing hydrogen energy.
18
Acknowledgment The authors would like to thank the EU, and the Natural Sciences and Engineering Research Council of Canada for funding, in the frame of the collaborative international Consortium ECOSAFEFARMING financed under the ERA-NET WaterWorks2015 Cofunded Call. This ERA-NET is an integral part of the 2016 Joint Activities developed by the Water Challenges for a Changing World Joint Programme Initiative (Water JPI). Nomenclature F
Faraday constant (96485 A s/mol)
I HHV t a V !
Electric current (A) Higher heating value (kJ/mol) Time (sec) Number of electrons transformed to produce hydrogen Mol flow rate (mol/sec) Volume (L)
IEMs AEM CEM BPM PEM IL PEG PVA OCP CV
Ion-exchange membranes Anion exchange membrane Cation exchange membrane Bipolar membrane Proton exchange membrane Interfacial layer Polyethylene glycol Polyvinyl alcohol Open circuit potential Cyclic voltammetry
Work rate (kW) Greek Letters η Efficiency Acronyms
Subscripts and Superscripts En Energy Ex Exergy m H2
Molar Hydrogen
19
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Research Highlights •
A novel system for hydrogen production with ion exchange membranes is designed.
•
Comparative assessment of various ion exchange membranes on the system performance.
•
Electrochemical analysis of the electrolysis reactor with ion exchange membranes.
•
A comprehensive energy and exergy efficiencies assessment.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: