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Effects of magnetic field on water electrolysis using foam electrodes
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
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Effects of magnetic field on water electrolysis using foam electrodes Yang Liu, Liang-ming Pan*, Hongbo Liu, Tianming Chen, Siyou Yin, Mengmeng Liu Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing, 400044, China
article info
abstract
Article history:
Alkaline water electrolysis using foam electrodes was performed under the influence of a
Received 6 October 2018
uniform magnetic field. The motivation of doing this work is to combine magnetic field
Received in revised form
with foam electrode to see if it can get unexpected higher water electrolysis efficiency. The
9 November 2018
result shows that the energy consumption was reduced by about 3.4% and some unique gas
Accepted 13 November 2018
producing properties of foam electrodes were exhibited under the parallel-to-electrode
Available online xxx
magnetic field. The magnetohydrodynamic (MHD) convection induced by the Lorentz force can accelerate the detachment of bubbles, which are capable to be generated both on
Keywords:
the surface and in the interior of foam electrodes simultaneously. Due to the uneven
Water electrolysis
distribution of Lorentz forces, a circulating flow was formed in the electrolyzer, which is
Foam electrode
special designed to make full use of the circulating flow. The flow scoured the inner space
Magnetic field
of the foam electrodes so that the void fraction was reduced and the reaction overpotential
Magnetohydrodynamic convection
was decreased. It should be potential to significantly improve the energy efficiency of the
Electrolysis efficiency
hydrogen manufacturing via water electrolysis and provide new ideas for the engineering design of electrolyzer. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen manufacturing process via water electrolysis is the simplest way to produce high-purity hydrogen up till the present moment. Water electrolysis is easy to realize in largescale applications and zero emissions can be achieved by this hydrogen producing process [1,2]. It is also a good energy storage method with the rapid development of smart grid at present [3e5]. The production of high purity hydrogen through water electrolysis plays an irreplaceable role in many aspects, such as aerospace [6,7], food industry, polypropylene production [8] etc. In addition, water electrolysis can be used as
an intermediary link of a sustainable distributed clean energy supply system. For example, it may play a great role in the development of new energy vehicles [9], internal combustion engine (ICE) vehicles and heat engines [10]. However, water electrolysis is not widely applied because of its relatively low efficiency compared with fossil fuels reforming [11]. The electrolysis energy consumption of 1 m3 (normal condition) hydrogen produced by industrial electrolyzer is 4.5e5.0 kWh and the corresponding efficiency is not higher than 61.5% by calculation [12]. There are many methods developed by many researchers to reduce the energy consumption, such as intensive electrode arrangement [1], porous foam electrode,
* Corresponding author. E-mail address: [email protected] (L.-m. Pan). https://doi.org/10.1016/j.ijhydene.2018.11.103 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
2
international journal of hydrogen energy xxx (xxxx) xxx
imposing supergravity field [13] or magnetic field, ultrasonic vibration [14] and many other ways. No matter what method is used to reduce energy consumption, bubble management is always a very important problem [15]. Especially when a higher current density is applied, a large part of the cell resistance is caused by electrolytically generated bubbles [16,17]. When the generated bubbles are attached to the electrode surface, the active area of electrodes will be reduced. In addition, the bubbles reduce the effective conductivity of the electrolyte on account of their electrical insulation. That is, with the increase of void fraction in the cell, the conductivity of electrolyte decreases. In comparison with the conventional plate electrode, the foam electrode has a better performance because of its increased electrode surface area [18,19]. Larger specific surface area means a higher hydrogen evolution activity. In addition, the electrolysis reaction can occur simultaneously on the surface and inside of the foam electrode because of its special structure. But in practice the inner space of foam electrode will be blocked by the bubbles generated inside, so the electrolyte can hardly reach the inner surface of the electrode to participate in the reaction. As a result the advantages of foam electrodes cannot be brought into full play [20,21]. Mahmoud M. Saleh [22] found that the gas bubbles, generated within the porous electrode, aggravate the nonuniform reaction distribution by increasing the effective resistance of electrolyte. Mass transfer resistance further restricts the electrolyzer efficiency since a non-uniformly distributed potential can result in mass transfer limited current locally within the pores. Therefore, the predicted limiting current values are lower and have a non-negligible gap with the theoretical limiting current of the electrode. Like other methods, bubble management is still a very important issue. Thus how to accelerate the detachment of bubbles inside the electrode has become the most worth studying problem. It has been shown by many researches that imposing external field can reduce the coverage of bubbles on the electrode surface and then reduce the electric energy consumed by water electrolysis significantly [23e27]. Under the influence of magnetic field, the mass transfer of H2(aq) on the electrode surface is enhanced, and the bubble coverage on the cathode surface decreases, resulting in the decrease of the electrode potential [24]. And the MHD convection, a forced convection induced by Lorentz force, is capable to reduce electrode bubble coverage, void fraction, and hence cell resistance [16,17,28e32]. A lot of mechanism studies have also been done to explain the effects induced by magnetic field [33e37]. Compared with many other methods, imposing external magnetic field is the most economical way because ordinary permanent magnet can meet the requirements of magnetic field strength. But few researchers combine magnetic field with other methods. Noriah Bidin et al. reported that the combination of magnetic field and optic field has exhibited superior results, and the output of hydrogen is nine times higher than that of conventional water electrolysis [38]. This promotion is very impressive but providing optic field is a huge energy consuming process itself, so it is not of great engineering value to combine magnetic field with optic field.
In our study we used both foam electrodes and external magnetic fields to conduct water electrolysis experiments. Neither of these two methods requires additional energy input. In addition, no expensive proton exchange membranes [39] and electrocatalysts [40] are used in our study, and the method we use to improve the efficiency is not limited by geography and time [41]. The electrolyte is driven by Lorenz force, which will be generated as long as there is an orthogonal component between the magnetic field and the electric field, to flow. In other words, Electrolyte selfactuation can be realized. Some unique phenomena have been exhibited under the magnetic field. And the bubble behavior especially the motion of large bubbles is discussed prominently.
Experimental setup The water electrolysis was carried out in a two-electrode cell (40 40 40 mm), which consists of a transparent plastic cell containing 70.0 mL of aqueous 4.24 M KOH solution and two foam electrodes (110 PPI, JSD Co., Ltd.). A piece of foam copper was cut as the cathode and a nickel one as the anode. The two uniform-sized (20 20 2 mm) electrodes were arranged inside the cell walls to face each other. As shown in Fig. 1 there is an interval between electrode bracket and the wall on each side so that the bubbles can release freely on both sides of the electrode. A series of through-holes were drilled on the two electrode brackets to guarantee that the momentum of the electrolyte can exchange across the brackets. A mirror, which was attached to an acrylic bar and settled in the middle of the cell, has a 45 angle with the horizontal plane, so the high
Fig. 1 e Schematic diagram of the electrochemical cell employing a mirror to reflect the images of the cathode to the high speed camera above.
Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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The electrolysis was performed galvanostatically. The constant current is supplied by an electrochemical workstation (PARSTAT 4000 potentiostat/galvanostat, Wuhan Corrtest Instruments Corp., Ltd). The images of bubble behaviors, reflected by the mirror, were recorded by a highspeed camera with speed of 500 fps. Electrode frame with known size is used to measure the size of bubbles generated on the cathode. The images were recorded after the water electrolysis start at least 15s to ensure the process is in a stable state.
Results and discussion Fig. 2 e Cell voltage vs current density curves for water electrolysis with and without a 0.9 T magnetic field in 0.2 M KOH solution.
speed camera can record the phenomenon on the cathode right above. A pair of two large neodymium-iron-boron (NdFeB) permanent magnets (80 106 124 mm each one) is placed on both sides of the electrolyzer to superimpose a homogenous magnetic field (0.9 T) and make the magnetic field parallel to the electric field. The magnetic field was measured by a Gaussmeter (0e2000 mT, ±0.2%, SJ300, SJ Technology Co., Ltd.), to ensure that the intensity of the magnetic field is equal everywhere in the electrolyzer.
The water electrolysis using foam electrodes was conducted in 4.24 M KOH solution for 5 min galvanostatically and the electric potential difference between cathode and anode, also known as the cell voltage, was recorded by the electrochemical workstation. Fig. 2 shows the cell voltage at a number of current densities without (0 T) and with magnetic fields (0.9 T). As expected, when the magnetic field was imposed the cell voltage became lower than that without a magnetic field. The degree of cell voltage reduction become larger with the increasing current density in a magnetic field. When the current density is greater than 1250 A/m2, the cell voltage is reduced by about 3.4%. Under the condition of constant working current, reducing the voltage of electrolyzer is the fundamental way to reduce the energy consumption and improve the efficiency of
Fig. 3 e Set of optical images demonstrating a time sequence, which shows the process of a large H2 bubble dashing out of the cathode. The picture that a large bubble just appear was chosen as 0 s (1000 A/m2). Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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electrolysis. In alkaline electrolyzer, the cell voltage is composed of the following parts [42]: DU ¼ E0 þ jhc j þ jha j þ I$ 0
X
R
(1)
where E is the theoretical decomposition voltage; jhcj and jhaj are the polarization overpotential on the cathode and anode, P respectively; I· R is the ohmic resistance includes electrolyte resistance (Re), membrane resistance (Rm), gas phase resistance (Rg) and electric wire resistance (Rc). The difference between foam electrodes and common plane electrodes is that foam electrodes have a certain space in its interior because of its unique structure. So the gas product, hydrogen and oxygen, is able to generate on the surface and inside the electrode simultaneously. The bubbles generated on the surface of or inside the foam electrodes play a dual role. On the one hand, they are gas product what we are aim to obtain. On the other, because of the near-zero conductivity ratio of the gas phase and the liquid phase, the occurrence of gas phase will lead to the decrease of the comprehensive conductivity of electrolyte. And at the same
time, the adhesion of bubbles on the electrode surface changed the actual contact area between the electrode surface and the liquid phase [30]. In addition, bubbles can disturb current distribution and isolate active sites from reaction ion, so they induce a micro-convection to push away electrolyte in a radial direction [43]. The phenomenon leads to high reaction overpotential and large ohmic voltage drop. Aldas also confirmed by numerical investigation which shows that bubbles layer adsorbed decreases gas evolution rate seriously on electrode surface [27]. Thus this makes the abovementioned jhcj, jhaj and Rg relatively large. So the energy consumption of water electrolysis will be greatly reduced if bubbles can be released in time. In Fig. 1, the magnetic field and electric field were set to be an orthogonal position to give the Lorentz force an upward direction. Gas bubbles evolved on the surface of the foam electrodes will be flushed away by the magnetohydrodynamics (MHD) convection, which is the same as in the case of using plate electrodes. And it has been proved by many scholars that this effect makes a remarkable improvement of
Fig. 4 e The number and diameter of large H2 bubbles produced inside the foam cathode is different under the condition of magnetic field and no magnetic field. (a) B ¼ 0 T, j ¼ 1000 A/m2; (b) B ¼ 0.9 T, j ¼ 1000 A/m2; (c) The curve of the number of larger bubbles (N1, d > 0.9 mm) and smaller bubbles (N2, d > 0.9 mm) with the current density. Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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the surface coverage so as to improve the efficiency of water electrolysis [44] [25e27]. A big bubble will dash out from the pore when the volume of hydrogen bubble, generated inside the cathode, is beyond the internal space of the foam electrode (as shown in Figs. 3 and 4). The same phenomenon also happens on the anode predictably. As can be seen in Fig. 3, A large bubble in the lower right part of the cathode appeared completely in merely 4 frames (500 fps). Thus it can be known that the extruded bubbles had a quite large horizontal velocity (approximately 0.5 m/s according to the pictures taken) when they were squeezed out of the foam electrodes. It is expected that this kind of large bubbles can penetrate the concentration boundary layer and intensely agitate the electrolyte in the vicinity of the electrodes. This effect may not only drastically decrease the supersolubility of dissolved H2 gas but also provide fresh electrolytes to the surface of the foam electrodes. So it is obvious that this effect, which is unique to the foam electrodes, can evidently lower both the reaction overpotential and the concentration overpotential of dissolved hydrogen gas. And more than that, this effect may work in coordination with MHD convection to accelerate the detachment of bubbles on the surface. On the premise of the same porosity of the foam electrode and the same current density (1000 A/m2), there is a distinct difference between the cases with and without a magnetic field, see Fig. 4(a) and (b). The diameter of the large squeezed bubbles is smaller when there is a magnetic field compared with the case that no magnetic field imposed. The bubbles are divided into two categories here: d 0.9 mm and d < 0.9 mm. As can be seen in Fig. 4(c), the number of bubbles in two diameters increases with the increase of current density. Under 0.9 T magnetic field, the number of bubbles with larger diameter is smaller than that without magnetic field, but the bubbles with smaller diameters are more. The reason for these differences is that almost all the ions move between two electrodes, and based on Gauss's theorem the electric field is centrally distributed between the anode and the cathode so that the Lorentz force, FL¼B j, generated here is much greater than the other parts of the electrolyzer. Then a Lorentz-force-driven convection was induced similar to the pumping effect [45,46]. As a result, the electrolyte was driven to form a circulatory flow. It flows through the through-holes, which were drilled on the electrode brackets, and then the interval between the brackets and the wall of the electrolytic cell. After that, the electrolyte passes through the pores of the foam electrodes, and this phenomenon shows that the electrolyte driven by MHD convection has the effect of scouring the inner space of the foam electrodes. Because it is very difficult to shoot the whole process of the circulating flow, a diagrammatic sketch is used as shown in Fig. 5. This effect helps to carry the bubbles which generated inside the foam electrodes away before they reach the maximum volume. So when the magnetic field imposed, less bubbles with larger diameter and more bubbles with smaller diameter are generated compared with the case with no magnetic field. In addition, this effect can bring fresh electrolyte [25] from the upper part of the electrolytic cell to the
5
electrodes. Apparently by increasing the active surface area inside the foam electrodes, decreasing the supersolubility of dissolved gas and reducing the reaction overpotential and then the energy consumption can be reduced. According to Saleh [22], the two electrodes in the electrolyzer are a kind of gas evolving flow-through porous electrodes, and it is worth noting that no external driving force (e.g. pressure head) is needed in our case. The bubbles formed inside the foam electrodes have a considerable influence on the electrolysis process. A key parameter k, the conductivity of internal electrolyte of foam electrode, can be expressed as[22]. k ¼ ðq εÞ3=2
(2)
where q is the porosity of foam electrode and ε is the gas void fraction within the foam electrode (dimensionless). Due to the thickness of the foam electrode used for electrolysis is not as large as packed bed electrode, the gas void fraction is assumed to be independent of the position within the foam electrode. It is obvious that the bubbles formed inside the foam electrodes have a direct impact on the conductivity of internal electrolyte of foam electrode. The conductivity decreases with the increase of void fraction. The void fraction ε represents the amount of gas production, which is in proportion to the current density i. The
Fig. 5 e Diagrammatic sketch of the circulating flow (blue arrows) on one side of the electrolyzer, which is caused by the uneven distributed Lorentz force (FL). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article).
Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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current density of each position inside the foam electrode is equal to that of the surface because of its small thickness. In this case ε is also impacted by the electrolyte flow rate as Equations (3) and (4) [22]. ε¼
q 1þF
F ¼ Qg=Io
(3) (4)
In the above equations F is the dimensionless bubble group, and in its expression Q is the electrolyte flow rate and g ¼ 7.87C cm3 at standard pressure and temperature [47]. Small values of F suggest significant bubble formation. It is indicated by Eqs. (2)e(4) that the void fraction and porosity are equal when there is no electrolyte passes through the foam electrode. It means that the inner space of the foam electrode is occupied by bubbles generated by electrolysis although it has a large specific surface area. So it is almost impossible for the electrolyte to reach the inner surface to participate in the electrolysis. As a result, the advantages of foam electrodes are greatly weakened. And at a certain current density F is proportional to Q, so at high flow rates the bubble group F is large. The bubbles are carried away by the electrolyte which means void fraction ε has a small value. Thus the pore electrolyte conductivity k is increased i.e. the ohmic potential drop is reduced [48]. So Saleh [22] recommended it is effective to operate the cell with a high flow rate electrolyte scouring the foam electrodes. The intention is to decrease the adverse effects of bubbles and increase the limiting current. As elaborated in this paper, this goal can be achieved by taking advantage of magnetohydrodynamic (MHD) convection and foam electrodes without any external driving force. Moreover, the application of this technology in industrial electrolyzer is not difficult and has a great potential to be widely applied.
Conclusions Alkaline water electrolysis using porous foam metals as electrodes was performed when a magnetic field (B ¼ 0.9 T) was applied. Some unique gas producing properties of foam electrodes in a magnetic field were investigated. It is found that the energy consumption of water electrolysis was considerably reduced at high current density by MHD convection. There is space for water electrolysis inside the foam electrode in addition to its surface. When the internal bubbles are large enough, they will dash out at a quite high speed and agitate the electrolyte intensely, and the magnetic field may strengthen this effect. Moreover, a circulating flow was induced in the electrolyzer because of the uneven distributed Lorentz force, and the flow was able to scour the inner space of the foam electrodes. The effects mentioned above, unique while coupling foam electrodes and magnetic field, can accelerate the detachment of bubbles and decrease the reaction overpotential. The results are advantageous for the development of new type highefficiency electrolyzer.
Acknowledgments The authors are grateful for the support of the Natural Science Foundation of China (Grant No: 51676020, 51706190, 51706025) and the Chongqing Natural Science Foundation (Grant No: cstc2015jcyjBX0130).
references
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Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Effects of magnetic field on water electrolysis using foam electrodes Yang Liu, Liang-ming Pan*, Hongbo Liu, Tianming Chen, Siyou Yin, Mengmeng Liu Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing, 400044, China
article info
abstract
Article history:
Alkaline water electrolysis using foam electrodes was performed under the influence of a
Received 6 October 2018
uniform magnetic field. The motivation of doing this work is to combine magnetic field
Received in revised form
with foam electrode to see if it can get unexpected higher water electrolysis efficiency. The
9 November 2018
result shows that the energy consumption was reduced by about 3.4% and some unique gas
Accepted 13 November 2018
producing properties of foam electrodes were exhibited under the parallel-to-electrode
Available online xxx
magnetic field. The magnetohydrodynamic (MHD) convection induced by the Lorentz force can accelerate the detachment of bubbles, which are capable to be generated both on
Keywords:
the surface and in the interior of foam electrodes simultaneously. Due to the uneven
Water electrolysis
distribution of Lorentz forces, a circulating flow was formed in the electrolyzer, which is
Foam electrode
special designed to make full use of the circulating flow. The flow scoured the inner space
Magnetic field
of the foam electrodes so that the void fraction was reduced and the reaction overpotential
Magnetohydrodynamic convection
was decreased. It should be potential to significantly improve the energy efficiency of the
Electrolysis efficiency
hydrogen manufacturing via water electrolysis and provide new ideas for the engineering design of electrolyzer. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen manufacturing process via water electrolysis is the simplest way to produce high-purity hydrogen up till the present moment. Water electrolysis is easy to realize in largescale applications and zero emissions can be achieved by this hydrogen producing process [1,2]. It is also a good energy storage method with the rapid development of smart grid at present [3e5]. The production of high purity hydrogen through water electrolysis plays an irreplaceable role in many aspects, such as aerospace [6,7], food industry, polypropylene production [8] etc. In addition, water electrolysis can be used as
an intermediary link of a sustainable distributed clean energy supply system. For example, it may play a great role in the development of new energy vehicles [9], internal combustion engine (ICE) vehicles and heat engines [10]. However, water electrolysis is not widely applied because of its relatively low efficiency compared with fossil fuels reforming [11]. The electrolysis energy consumption of 1 m3 (normal condition) hydrogen produced by industrial electrolyzer is 4.5e5.0 kWh and the corresponding efficiency is not higher than 61.5% by calculation [12]. There are many methods developed by many researchers to reduce the energy consumption, such as intensive electrode arrangement [1], porous foam electrode,
* Corresponding author. E-mail address: [email protected] (L.-m. Pan). https://doi.org/10.1016/j.ijhydene.2018.11.103 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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imposing supergravity field [13] or magnetic field, ultrasonic vibration [14] and many other ways. No matter what method is used to reduce energy consumption, bubble management is always a very important problem [15]. Especially when a higher current density is applied, a large part of the cell resistance is caused by electrolytically generated bubbles [16,17]. When the generated bubbles are attached to the electrode surface, the active area of electrodes will be reduced. In addition, the bubbles reduce the effective conductivity of the electrolyte on account of their electrical insulation. That is, with the increase of void fraction in the cell, the conductivity of electrolyte decreases. In comparison with the conventional plate electrode, the foam electrode has a better performance because of its increased electrode surface area [18,19]. Larger specific surface area means a higher hydrogen evolution activity. In addition, the electrolysis reaction can occur simultaneously on the surface and inside of the foam electrode because of its special structure. But in practice the inner space of foam electrode will be blocked by the bubbles generated inside, so the electrolyte can hardly reach the inner surface of the electrode to participate in the reaction. As a result the advantages of foam electrodes cannot be brought into full play [20,21]. Mahmoud M. Saleh [22] found that the gas bubbles, generated within the porous electrode, aggravate the nonuniform reaction distribution by increasing the effective resistance of electrolyte. Mass transfer resistance further restricts the electrolyzer efficiency since a non-uniformly distributed potential can result in mass transfer limited current locally within the pores. Therefore, the predicted limiting current values are lower and have a non-negligible gap with the theoretical limiting current of the electrode. Like other methods, bubble management is still a very important issue. Thus how to accelerate the detachment of bubbles inside the electrode has become the most worth studying problem. It has been shown by many researches that imposing external field can reduce the coverage of bubbles on the electrode surface and then reduce the electric energy consumed by water electrolysis significantly [23e27]. Under the influence of magnetic field, the mass transfer of H2(aq) on the electrode surface is enhanced, and the bubble coverage on the cathode surface decreases, resulting in the decrease of the electrode potential [24]. And the MHD convection, a forced convection induced by Lorentz force, is capable to reduce electrode bubble coverage, void fraction, and hence cell resistance [16,17,28e32]. A lot of mechanism studies have also been done to explain the effects induced by magnetic field [33e37]. Compared with many other methods, imposing external magnetic field is the most economical way because ordinary permanent magnet can meet the requirements of magnetic field strength. But few researchers combine magnetic field with other methods. Noriah Bidin et al. reported that the combination of magnetic field and optic field has exhibited superior results, and the output of hydrogen is nine times higher than that of conventional water electrolysis [38]. This promotion is very impressive but providing optic field is a huge energy consuming process itself, so it is not of great engineering value to combine magnetic field with optic field.
In our study we used both foam electrodes and external magnetic fields to conduct water electrolysis experiments. Neither of these two methods requires additional energy input. In addition, no expensive proton exchange membranes [39] and electrocatalysts [40] are used in our study, and the method we use to improve the efficiency is not limited by geography and time [41]. The electrolyte is driven by Lorenz force, which will be generated as long as there is an orthogonal component between the magnetic field and the electric field, to flow. In other words, Electrolyte selfactuation can be realized. Some unique phenomena have been exhibited under the magnetic field. And the bubble behavior especially the motion of large bubbles is discussed prominently.
Experimental setup The water electrolysis was carried out in a two-electrode cell (40 40 40 mm), which consists of a transparent plastic cell containing 70.0 mL of aqueous 4.24 M KOH solution and two foam electrodes (110 PPI, JSD Co., Ltd.). A piece of foam copper was cut as the cathode and a nickel one as the anode. The two uniform-sized (20 20 2 mm) electrodes were arranged inside the cell walls to face each other. As shown in Fig. 1 there is an interval between electrode bracket and the wall on each side so that the bubbles can release freely on both sides of the electrode. A series of through-holes were drilled on the two electrode brackets to guarantee that the momentum of the electrolyte can exchange across the brackets. A mirror, which was attached to an acrylic bar and settled in the middle of the cell, has a 45 angle with the horizontal plane, so the high
Fig. 1 e Schematic diagram of the electrochemical cell employing a mirror to reflect the images of the cathode to the high speed camera above.
Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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The electrolysis was performed galvanostatically. The constant current is supplied by an electrochemical workstation (PARSTAT 4000 potentiostat/galvanostat, Wuhan Corrtest Instruments Corp., Ltd). The images of bubble behaviors, reflected by the mirror, were recorded by a highspeed camera with speed of 500 fps. Electrode frame with known size is used to measure the size of bubbles generated on the cathode. The images were recorded after the water electrolysis start at least 15s to ensure the process is in a stable state.
Results and discussion Fig. 2 e Cell voltage vs current density curves for water electrolysis with and without a 0.9 T magnetic field in 0.2 M KOH solution.
speed camera can record the phenomenon on the cathode right above. A pair of two large neodymium-iron-boron (NdFeB) permanent magnets (80 106 124 mm each one) is placed on both sides of the electrolyzer to superimpose a homogenous magnetic field (0.9 T) and make the magnetic field parallel to the electric field. The magnetic field was measured by a Gaussmeter (0e2000 mT, ±0.2%, SJ300, SJ Technology Co., Ltd.), to ensure that the intensity of the magnetic field is equal everywhere in the electrolyzer.
The water electrolysis using foam electrodes was conducted in 4.24 M KOH solution for 5 min galvanostatically and the electric potential difference between cathode and anode, also known as the cell voltage, was recorded by the electrochemical workstation. Fig. 2 shows the cell voltage at a number of current densities without (0 T) and with magnetic fields (0.9 T). As expected, when the magnetic field was imposed the cell voltage became lower than that without a magnetic field. The degree of cell voltage reduction become larger with the increasing current density in a magnetic field. When the current density is greater than 1250 A/m2, the cell voltage is reduced by about 3.4%. Under the condition of constant working current, reducing the voltage of electrolyzer is the fundamental way to reduce the energy consumption and improve the efficiency of
Fig. 3 e Set of optical images demonstrating a time sequence, which shows the process of a large H2 bubble dashing out of the cathode. The picture that a large bubble just appear was chosen as 0 s (1000 A/m2). Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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electrolysis. In alkaline electrolyzer, the cell voltage is composed of the following parts [42]: DU ¼ E0 þ jhc j þ jha j þ I$ 0
X
R
(1)
where E is the theoretical decomposition voltage; jhcj and jhaj are the polarization overpotential on the cathode and anode, P respectively; I· R is the ohmic resistance includes electrolyte resistance (Re), membrane resistance (Rm), gas phase resistance (Rg) and electric wire resistance (Rc). The difference between foam electrodes and common plane electrodes is that foam electrodes have a certain space in its interior because of its unique structure. So the gas product, hydrogen and oxygen, is able to generate on the surface and inside the electrode simultaneously. The bubbles generated on the surface of or inside the foam electrodes play a dual role. On the one hand, they are gas product what we are aim to obtain. On the other, because of the near-zero conductivity ratio of the gas phase and the liquid phase, the occurrence of gas phase will lead to the decrease of the comprehensive conductivity of electrolyte. And at the same
time, the adhesion of bubbles on the electrode surface changed the actual contact area between the electrode surface and the liquid phase [30]. In addition, bubbles can disturb current distribution and isolate active sites from reaction ion, so they induce a micro-convection to push away electrolyte in a radial direction [43]. The phenomenon leads to high reaction overpotential and large ohmic voltage drop. Aldas also confirmed by numerical investigation which shows that bubbles layer adsorbed decreases gas evolution rate seriously on electrode surface [27]. Thus this makes the abovementioned jhcj, jhaj and Rg relatively large. So the energy consumption of water electrolysis will be greatly reduced if bubbles can be released in time. In Fig. 1, the magnetic field and electric field were set to be an orthogonal position to give the Lorentz force an upward direction. Gas bubbles evolved on the surface of the foam electrodes will be flushed away by the magnetohydrodynamics (MHD) convection, which is the same as in the case of using plate electrodes. And it has been proved by many scholars that this effect makes a remarkable improvement of
Fig. 4 e The number and diameter of large H2 bubbles produced inside the foam cathode is different under the condition of magnetic field and no magnetic field. (a) B ¼ 0 T, j ¼ 1000 A/m2; (b) B ¼ 0.9 T, j ¼ 1000 A/m2; (c) The curve of the number of larger bubbles (N1, d > 0.9 mm) and smaller bubbles (N2, d > 0.9 mm) with the current density. Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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the surface coverage so as to improve the efficiency of water electrolysis [44] [25e27]. A big bubble will dash out from the pore when the volume of hydrogen bubble, generated inside the cathode, is beyond the internal space of the foam electrode (as shown in Figs. 3 and 4). The same phenomenon also happens on the anode predictably. As can be seen in Fig. 3, A large bubble in the lower right part of the cathode appeared completely in merely 4 frames (500 fps). Thus it can be known that the extruded bubbles had a quite large horizontal velocity (approximately 0.5 m/s according to the pictures taken) when they were squeezed out of the foam electrodes. It is expected that this kind of large bubbles can penetrate the concentration boundary layer and intensely agitate the electrolyte in the vicinity of the electrodes. This effect may not only drastically decrease the supersolubility of dissolved H2 gas but also provide fresh electrolytes to the surface of the foam electrodes. So it is obvious that this effect, which is unique to the foam electrodes, can evidently lower both the reaction overpotential and the concentration overpotential of dissolved hydrogen gas. And more than that, this effect may work in coordination with MHD convection to accelerate the detachment of bubbles on the surface. On the premise of the same porosity of the foam electrode and the same current density (1000 A/m2), there is a distinct difference between the cases with and without a magnetic field, see Fig. 4(a) and (b). The diameter of the large squeezed bubbles is smaller when there is a magnetic field compared with the case that no magnetic field imposed. The bubbles are divided into two categories here: d 0.9 mm and d < 0.9 mm. As can be seen in Fig. 4(c), the number of bubbles in two diameters increases with the increase of current density. Under 0.9 T magnetic field, the number of bubbles with larger diameter is smaller than that without magnetic field, but the bubbles with smaller diameters are more. The reason for these differences is that almost all the ions move between two electrodes, and based on Gauss's theorem the electric field is centrally distributed between the anode and the cathode so that the Lorentz force, FL¼B j, generated here is much greater than the other parts of the electrolyzer. Then a Lorentz-force-driven convection was induced similar to the pumping effect [45,46]. As a result, the electrolyte was driven to form a circulatory flow. It flows through the through-holes, which were drilled on the electrode brackets, and then the interval between the brackets and the wall of the electrolytic cell. After that, the electrolyte passes through the pores of the foam electrodes, and this phenomenon shows that the electrolyte driven by MHD convection has the effect of scouring the inner space of the foam electrodes. Because it is very difficult to shoot the whole process of the circulating flow, a diagrammatic sketch is used as shown in Fig. 5. This effect helps to carry the bubbles which generated inside the foam electrodes away before they reach the maximum volume. So when the magnetic field imposed, less bubbles with larger diameter and more bubbles with smaller diameter are generated compared with the case with no magnetic field. In addition, this effect can bring fresh electrolyte [25] from the upper part of the electrolytic cell to the
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electrodes. Apparently by increasing the active surface area inside the foam electrodes, decreasing the supersolubility of dissolved gas and reducing the reaction overpotential and then the energy consumption can be reduced. According to Saleh [22], the two electrodes in the electrolyzer are a kind of gas evolving flow-through porous electrodes, and it is worth noting that no external driving force (e.g. pressure head) is needed in our case. The bubbles formed inside the foam electrodes have a considerable influence on the electrolysis process. A key parameter k, the conductivity of internal electrolyte of foam electrode, can be expressed as[22]. k ¼ ðq εÞ3=2
(2)
where q is the porosity of foam electrode and ε is the gas void fraction within the foam electrode (dimensionless). Due to the thickness of the foam electrode used for electrolysis is not as large as packed bed electrode, the gas void fraction is assumed to be independent of the position within the foam electrode. It is obvious that the bubbles formed inside the foam electrodes have a direct impact on the conductivity of internal electrolyte of foam electrode. The conductivity decreases with the increase of void fraction. The void fraction ε represents the amount of gas production, which is in proportion to the current density i. The
Fig. 5 e Diagrammatic sketch of the circulating flow (blue arrows) on one side of the electrolyzer, which is caused by the uneven distributed Lorentz force (FL). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article).
Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103
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current density of each position inside the foam electrode is equal to that of the surface because of its small thickness. In this case ε is also impacted by the electrolyte flow rate as Equations (3) and (4) [22]. ε¼
q 1þF
F ¼ Qg=Io
(3) (4)
In the above equations F is the dimensionless bubble group, and in its expression Q is the electrolyte flow rate and g ¼ 7.87C cm3 at standard pressure and temperature [47]. Small values of F suggest significant bubble formation. It is indicated by Eqs. (2)e(4) that the void fraction and porosity are equal when there is no electrolyte passes through the foam electrode. It means that the inner space of the foam electrode is occupied by bubbles generated by electrolysis although it has a large specific surface area. So it is almost impossible for the electrolyte to reach the inner surface to participate in the electrolysis. As a result, the advantages of foam electrodes are greatly weakened. And at a certain current density F is proportional to Q, so at high flow rates the bubble group F is large. The bubbles are carried away by the electrolyte which means void fraction ε has a small value. Thus the pore electrolyte conductivity k is increased i.e. the ohmic potential drop is reduced [48]. So Saleh [22] recommended it is effective to operate the cell with a high flow rate electrolyte scouring the foam electrodes. The intention is to decrease the adverse effects of bubbles and increase the limiting current. As elaborated in this paper, this goal can be achieved by taking advantage of magnetohydrodynamic (MHD) convection and foam electrodes without any external driving force. Moreover, the application of this technology in industrial electrolyzer is not difficult and has a great potential to be widely applied.
Conclusions Alkaline water electrolysis using porous foam metals as electrodes was performed when a magnetic field (B ¼ 0.9 T) was applied. Some unique gas producing properties of foam electrodes in a magnetic field were investigated. It is found that the energy consumption of water electrolysis was considerably reduced at high current density by MHD convection. There is space for water electrolysis inside the foam electrode in addition to its surface. When the internal bubbles are large enough, they will dash out at a quite high speed and agitate the electrolyte intensely, and the magnetic field may strengthen this effect. Moreover, a circulating flow was induced in the electrolyzer because of the uneven distributed Lorentz force, and the flow was able to scour the inner space of the foam electrodes. The effects mentioned above, unique while coupling foam electrodes and magnetic field, can accelerate the detachment of bubbles and decrease the reaction overpotential. The results are advantageous for the development of new type highefficiency electrolyzer.
Acknowledgments The authors are grateful for the support of the Natural Science Foundation of China (Grant No: 51676020, 51706190, 51706025) and the Chongqing Natural Science Foundation (Grant No: cstc2015jcyjBX0130).
references
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Please cite this article as: Liu Y et al., Effects of magnetic field on water electrolysis using foam electrodes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.103