- Email: [email protected]
Porous electrode improving energy efficiency under electrode-normal magnetic field in water electrolysis
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 2 7 8 0 e2 2 7 8 6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Porous electrode improving energy efficiency under electrode-normal magnetic field in water electrolysis Hong-bo Liu a,*, Haotian Xu a, Liang-ming Pan b,**, Ding-han Zhong a, Yang Liu b a
School of Civil Engineering and Architecture, Southwest Petroleum University, Chengdu, China Key Laboratory of Ministry of Education of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing, China
b
highlights Porous electrode improving water electrolysis efficiency under magnetic field. Micro-MHD convection exists within porous electrode. Smaller current brings larger voltage reduction under normal-to-electrode magnetic field.
article info
abstract
Article history:
The porous electrodes (Ni, Cu) with 110 pores per inch (PPI) are adopted in water elec-
Received 5 April 2019
trolysis for hydrogen production under normal-to-electrode magnetic field. The result
Received in revised form
shows that the voltage drop between electrodes can be reduced up to 2.5% under 0.9 T field,
22 June 2019
and the electric energy efficiency is improved correspondingly. Based on the numerical
Accepted 3 July 2019
simulation method, the micro-magnetohydrodynamic (micro-MHD) convection induced by
Available online 26 July 2019
Lorentz force within the porous structure is found. The results showed that although the apparent current direction is parallel to magnetic field outside the porous electrode, the
Keywords:
electric field may be distorted within the porous structure, and the Lorentz force is involved
Porous electrode
near the rib of the micro structure where the current is not parallel to the magnetic field
Water electrolysis
any more. Micro-MHD plays the role of strengthening the mass transfer and facilitating
Micro-magnetohydrodynamic
bubble to eject from the porous structure, which results in the cell voltage decreasing. The
convection
combined application of porous electrode and magnetic field should be potential to further
Energy efficiency
improve energy efficiency of water electrolysis for hydrogen production. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the process of energy utilization, people's demand for clean energy is increasingly urgent with the aggravation of
man-made environmental destruction. Renewable energy based hydrogen energy is one of the most promising solutions to the environmental issues as a carbon free fuel. Only 4e5% of total hydrogen production is being done by water electrolysis [1], although the process of water electrolysis is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-b. Liu), [email protected] (L.-m. Pan). https://doi.org/10.1016/j.ijhydene.2019.07.024 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 2 7 8 0 e2 2 7 8 6
simple and reliable [2]. Two factors are the premise of the development of electrolysis water for hydrogen production in the future, the first one is the enough electricity supply which comes from primary clean energy such as wind energy and solar energy [3]; the second one is the large reduction of electrolytic energy consumption which may refer to the catalytic electrode, gas-liquid management and electrolytic cell design [4]. Recent years, wind power plant combined with electrolysis for hydrogen production has been developed in many countries [5,6]. If the energy consumption can be further reduced, water electrolysis for hydrogen production should play more important role of clean energy utilization in the future. Although gas product management is only one aspect of water electrolysis, it has a great effect on the reduction of energy consumption in electrolysis. The gas product can increase the ohmic voltage drop and the electrode overpotential at the same time due to the non-conducting characteristics [7]. Many new technologies have been used widely to enhance phase separation, such as ultrasonic field [8], super gravity field [9] and magnetic field [10]. The magnetohydrodynamic (MHD) convection induced by Lorenz force results in the enhancement of electrochemical reaction and gas separation [11e13]. Iida et al. [14] superimposed a high magnetic field of 5 T to water electrolysis. Large reduction of cell voltage was achieved, especially in alkaline solution and at high current density. Lin et al. [15] found that magnetic effect was more significant by shortening inter-electrode distance. Ferromagnetism such as nickel is better cathode material than paramagnetism (platinum) and diamagnetism (graphite) under magnetic field. Koza et al. [16,17] found the magnetic field had the effect of improving desorption of hydrogen bubbles from a transparent electrode surface and reducing the fraction of bubble coverage. Matsushima [18] found the gaslayer thickness and local gas fraction of layer were reduced due to MHD convection near the flat electrode surface, and the rising detachment velocity reduces residential time of bubbles on electrode surface and diminishes bubble coverage [19]. Additionally, Bidin [20] found that Lorentz force from external magnetic field causes the water molecule to spin and generate eddy current which generates repulsion force on the hydrogen bonding and enhances the hydrogen production in water splitting. At the same time, the development of the researches of the structure and surface catalysis of electrode is also attractive and exciting in respect of the cell voltage reduction in water electrolysis. Alexander et al. [21] found Black Silicon (BS) has great potential to be used as water splitting materials due to its morphology consisting of needles and wells. Trompoukis et al. [22] found the porous materials has the advantage of lower ionic Ohmic losses compared to dense ones in solar water splitting, and analyzed how micrometer scale pore dimensions could greatly reduce Ohmic losses. Zhou et al. [23] gave the method of growing ternary molybdenum sulfoselenide particles on self-standing porous nickel diselenide foam and achieved a reduced electrode overpotential in water splitting. The efficiency of photoelectrochemical water splitting can be greatly enhanced with reduced graphene oxide (rGO) in BiVO4/rGO photoanode [24]. Porous nickel electrode and 3D carbon foam electrodes are adopted for alkaline water
22781
electrolysis [25,26], and the lower overpotential is obtained due to the efficient electrolyte transport into the whole electrode matrix concurrent with an ability to quickly dispose oxygen bubbles into the electrolyte. Saleh's simulation of oxygen evolution reaction at porous anode electrode found that the electrode potential is highly dependent on the degree of bubble formation within the bed matrix of porous structure [27], and Kadyk [28] further gave the analysis of how to enhance gas removal from porous electrode in electrochemical gas evolution system. If the porous electrode is used combined with the magnetic field in water electrolysis, it may facilitate the gas production elimination and further reduce the cell voltage. The authors [29] have given the experiment of water electrolysis with the porous copper electrode under the parallelto-electrode magnetic field, and the improved electrolysis efficiency up to 3.4% is achieved. In this work, the porous metal electrode combined with normal-to-electrode magnetic field is used for water electrolysis, the reduced cell voltage is achieved. The analysis may help us to form a more comprehensive understanding about the combined application of magnetic field and porous electrode, which is promising and meaningful for water electrolysis in the future.
Experimental setup Water electrolysis is carried out in the two-electrode cell (35 35 20 mm) with 2 mm and 2.5 mm intervals of electrodes if the nickel and copper electrodes are adopted as cathode respectively. As shown in Fig. 1(a), two electrodes are parallel to each other in the cell with the magnetic field perpendicular to the electrodes. The magnetic field is implemented by a set of two large permanent magnets (150 150 150 mm), and the homogeneous field is up to 0.9 T within the cell domain. The magnetic field is measured by a Gaussmeter (0e2000 mT, ±0.2%, SJ300, SJ Technology Co., Ltd.), to ensure that the intensity of the magnetic field is uniform in the electrolyzer. The platinum sheet (15 5 0.2 mm) is adopted at the anode side, and both of the porous nickel and copper with the size of 15 5 1 mm are used as cathode for hydrogen desorption, the electrodes are embedded into the surface of polylactic acid sheet through 3D printing technology. The experiment is performed in galvanostatic condition, and the voltage drop between two electrodes is monitored simultaneously as the frequency of 5 Hz. Water electrolysis is conducted in KOH aqueous solution of 0.5 mol/L and 1.0 mol/L. The apparent current density through working electrode is within the range of 130e670 A/m2. The porous electrode is with 110 pores per inch (PPI) as shown in Fig. 1 (b), and the porosity of electrode is about 96%. Before experiment, the porous electrode is immersed in acetone, and then it is cleaned with 0.5 mol/L H2SO4 aqueous solution in ultrasonic. As shown in Fig. 1(b), the microstructure of the porous electrode has many hollow cells, which consists of pore and ribs. In experiment, the liquid electrolyte is completely filled into the pore and the cell once the electrode is immersed into the aqueous solution.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 2 7 8 0 e2 2 7 8 6
shown in Fig. 2. The abstracted structure is with the same porosity of the electrode used in experiment, ε ¼ 96%(ε ¼ V0VV , 0 where V0 is the apparent volume the porous electrode occupied, and V is the absolute volume the ribs occupied).
Results and discussion
Fig. 1 e Schematic diagram of the electrolytic cell (a) and the microstructure of the porous nickel electrode captured by the scanning electron microscopy (SEM) (b).
The experimental result shows that the magnetic field has the effect of reducing the cell voltage within the current range of 130e670 A/m2. The voltage drop reduces up to 2.5% under current density of 130 A/m2 if the nickel electrode is adopted. As shown in Fig. 3, as the current increases, the magnetic field influence becomes weak, and the difference in potential tends to decrease with and without magnetic field influence. Additionally, the magnetic field has more obvious effect on the nickel electrode compared with the copper one although both of them are with the same porosity. Of course, the larger concentration of electrolyte can significantly reduce the cell voltage due to the increased conductivity of electrolyte which brings the smaller ohmic voltage drop between anode and cathode. The result seems to be interesting and amazing. As known before, if the magnetic field is perpendicular to the electrode, the Lorentz force can't be induced, if no other force is introduced in the solution, the convection is only driven by the concentration difference due to the gas dispersion in liquid phase and gas ascending motion due to buoyancy force. In this experiment, the seemingly nonexistent force makes difference and the cell voltage decreases to some extent. By means of numerical simulation, the electric field and the
Numerical simulation The numerical strategy used to analysis the magnetic field influence on bubble behavior has been used in previous works. The Lorentz-force-driven flow is governed by the incompressible NaviereStokes equation in the steady state, ! V $ ðr V Þ ¼ 0
(1)
!! ! !T 2 ! ! V $ ðr V V Þ ¼ Vp þ V$m V V þ V V V $ V I þ r! g þ FL 3 (2) ! ! ! where I is the unit tensor, and Lorentz force F L ¼ j B is introduced as a user-defined external body force. The electric potential (f) equation is governed by the standard scalar transport equation,V ,ðse VfÞ ¼ 0, wherese is electrical conductivity. The above NaviereStokes and electric potential equations should be discretized by a finite-volume method. The SIMPLE algorithm is used to calculate the pressure and the velocities. More details about the calculation strategy can be found in previous work [30]. The density and dynamic viscosity of 0.5 mol/L KOH aqueous solution are set to 1022 kg/m3 and 1.112 103 (Pa∙s), and the electrolyte conductivity is 11.46 S/ m [31]. According to the porous electrode used in experiment, the microstructure of electrode is abstracted as the model
Fig. 2 e Geometric model used in the simulation (D ¼ 1 mm, H ¼ 3 mm) and the simplified microstructure, ε ¼ 96%.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 2 7 8 0 e2 2 7 8 6
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Fig. 3 e Voltage drop between electrodes vs. current density.
induced Lorentz force is calculated, and the micro-MHD convection within the porous nickel electrode is analyzed. It can be found from Lin's analysis [15] that the actual magnetic field between the electrodes of ferromagnetism material is not the same with the field around the diamagnetism electrode if the same external magnetic field is adopted. The nickel electrode (ferromagnetism) has better preference under magnetic field compared with the diamagnetism and paramagnetism ones due to the unchanged magnetic field around the electrode surface which arouses larger Lorentz force. In this simulation, the difference of magnetic property of nickel (ferromagnetism) and copper (diamagnetism) is not involved, and the field direction and magnitude within the porous electrode is considered as the same with the external magnetic field everywhere. The apparent current density through the anode and cathode is set to 670 A/m2. According to the normalized geometric construction of the porous electrode shown in Fig. 2, the actual current density through the rib surface of the porous structure is about 289 A/m2, and it is assumed that the current is along the normal direction at the rib surfaces of the porous electrode. The current line distribution around part of the ribs is shown in Fig. 4. It is found that although the apparent current off the porous structure is parallel to the Z-axis direction, i.e. the magnetic field direction, the current line direction is distorted within the porous electrode and not parallel to the magnetic field any more due to the complex layout of the ribs within the porous electrode. The current flux is larger at the surface compared with the space away from the rib. It also has to be said that the actual structure within the porous electrode used in the experiment is more complex than the simplified model in the simulation, and the current distribution is more complex and irregular. The Lorentz force is involved where the current is not parallel to magnetic field. As shown in Fig. 5, the force around the rib surfaces of porous structure reaches to about 850 N/m3, the force intensity is weakened away from the rib surface due to the smaller current flux.
Fig. 4 e Current lines distribution around the ribs within the porous structure.
If no other force such as the bubble buoyancy and natural convection is referred in the simulation, the Lorentz force may induce a rotated flow within the structure, as shown in Fig. 6. The velocity is up to 0.34 mm/s within the region and it is about 0.15 mm/s at the center of pore. The flow is clockwise in the central of Plane1, and the electrolyte is rotated in the opposite direction outside the central region. At Plane2, the forced flow is all anticlockwise. The maximum of velocity is just occurred within each cell. Due to the flow boundary layer, the velocity is reduced close to the rib surface. The simulation result above shows that the micro convection involved by Lorentz force exists within each cell of the porous structure. Certainly, the actual porous structure is more complex with disordered arrangement. But what is certain is that the micro convection must have happened if only the cage-like structure exists within the porous metal material, and what's more, no matter what is the exact form of the micro structure. As what has been known [29,32], if the magnetic field orientation is parallel to the electrode surface, the micro-MHD convection may result in the reduction of cell voltage to large extent due to the gas bubble elimination between electrodes, and the reduction becomes larger as the current and field increase. In a word, larger force makes gaseous product elimination more efficient, which results in smaller cell potential drop. However, the result shows that the extent of cell voltage reduction is larger under lower current density condition in this experiment. As described in Fig. 7(a), the nucleation occurs at the rib surface simultaneously once the hydrogen concentration in the solution becomes supersaturated as the electrolysis going on. According to Vogt's analysis [33], the first nucleation is
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 2 7 8 0 e2 2 7 8 6
Fig. 5 e Lorentz force magnitude (contour) and orientation (arrow lines) around the ribs of the porous structure (N/m3).
about 4 mm which is much smaller than the rib diameter of porous structure used in the experiment. The greater current results in more nucleation at the time time around the rib. The cell of the porous structure is larger than the newborn bubble, and it smaller than the mature bubble before release. The micro nucleation of bubbles will fight for space during growing up and the coalescence is inevitable especially under the condition of larger current density. Once the coalescence
happens, the cell is easy to be completely occupied by the single immature bubble as shown in Fig. 7(b). The big one is hard to be expel promptly until it is big enough to overcome the increased surface tension force through bubble buoyancy as what has been observed in another work [29], In other words, greater current makes more cells easy to be blocked by single big bubble and more bubbles release from the electrode with larger diameters. The smaller the current, the smaller the
Fig. 6 e Velocity field distribution (contour) and streamlines (arrow lines) within the microstructure (V, 10¡5m/s).
Fig. 7 e Schematic diagram of simultaneous nucleation of bubbles (a) and bubbles coalescence into a big one within the porous unit (b), and the expelled big bubble preparing to release from the outer surface of porous electrode (c).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 2 7 8 0 e2 2 7 8 6
number of larger bubbles (r > 0.45 mm) at the outer surface of porous electrode [29]. Of course, if the current is smaller and the external forced convection is present, a large number of minute bubbles are expelled before merging to a big one, i.e. almost no visible big bubbles is found at the outer surface of the porous electrode. It is obvious that the internal micro convection almost has no effect on the big bubble adhering at the outer surface of the porous electrode as shown in Fig. 7(c). The big bubble can only be removed with the bubble collision and the gas-liquid flow through the electrode outer surface. As what has been observed in experiment [29], if the current becomes larger, more large bubble exists at the outer surface of porous electrode which make more contribution for the increase of the ohmic voltage drop and electrode over-potential [10]. In conclusion, Although the driving effect of the Lorentz force becomes smaller as the current decreases, less simultaneous nucleation of micro bubbles occur within the cell of porous structure, and less coalesced big bubbles with the same size of the cell are present at the interior and outer space of the porous electrode for which the Lorentz force is powerless. So the perpendicular-to-electrode magnetic field shows more obvious effect on the cell voltage decrease within the experiment current range.
Conclusions In conclusion, if the porous electrode is used in water electrolysis for hydrogen, the cell voltage can be reduced up to 2.5% under the current density of 130 A/m2 even when the magnetic field is perpendicular to electrode surface. The smaller current density makes larger voltage reduction within the experiment current range of 130e670 A/m2. The simulation result shows that although the apparent current direction is parallel to the magnetic field, the Lorentz force is still aroused within the porous structure due to the electric field distortion around the rib of the porous structure, and the Lorentz-force-induced flow within micro structure plays the role of expelling micro bubbles off the pores of porous structure. It is foreseeable that if the porous structure electrodes are adopted both at the anode and cathode side, the cell voltage can be further reduced, and the electric energy efficiency can be more improved in water electrolysis.
Acknowledgement
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The authors are grateful for the support of the Natural Science Foundation of China (Grant No: 51706190, 51676020), and the support of Young Scholars Development Fund of SWPU (Grant No: 201699010120).
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[1] Souza DE, Roberto F, Padilha, Janine C, Goncalves, Reinaldo S, et al. Electrochemical hydrogen production from water
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cells for scalable solar water splitting. Sol Energy Mater Sol Cells 2018;182:196e203. Zhou H, Yu F, Huang Y, Sun J, Zhu Z, Nielsen RJ, et al. Efficient hydrogen evolution by ternary molybdenum sulfoselenide particles on self-standing porous nickel diselenide foam. Nat Commun 2016;7:12765. Soltani T, Tayyebi A, Lee B-K. Efficient promotion of charge separation with reduced graphene oxide (rGO) in BiVO4/rGO photoanode for greatly enhanced photoelectrochemical water splitting. Sol Energy Mater Sol Cells 2018;185:325e32. € m R, Siljebo W, Shchukarev A, Pham TN, Sharifi T, Sandstro Kordas K, et al. Robust hierarchical 3D carbon foam electrode for efficient water electrolysis. Sci Rep 2017;7:6112. Kim JH, Lee JN, Yoo CY, Lee KB, Lee WM. Low-cost and energy-efficient asymmetric nickel electrode for alkaline water electrolysis. Int J Hydrogen Energy 2015;40:10720e5. Saleh MM. Simulation of oxygen evolution reaction at porous anode from flowing electrolytes. J Solid State Electrochem 2007;11:811e20.
[28] Kadyk T, Bruce D, Eikerling M. How to enhance gas removal from porous electrodes? Sci Rep 2016;6:38780. [29] Liu Y, Pan L-m, Liu H, Chen T, Yin S, Liu M. Effects of magnetic field on water electrolysis using foam electrodes. Int J Hydrogen Energy 2019;44:1352e8. [30] Liu H-b, Hu Q, Pan L-m, Wu R, Liu Y, Zhong D. Electrodenormal magnetic field facilitating neighbouring electrochemical bubble release from hydrophobic islets. Electrochim Acta 2019;306:350e9. [31] Gilliam RJ, Graydon JW, Kirk DW, Thorpe SJ. A review of specific conductivities of potassium hydroxide solutions for various concentrations and temperatures. Int J Hydrogen Energy 2007;32:359e64. [32] Iida T, Matsushima H, Fukunaka Y. Water electrolysis under a magnetic field. ChemInform 2010;38:112e5. € Balzer RJ. The limits of the analogy between [33] Vogt H, Aras O, boiling and gas evolution at electrodes. Int J Heat Mass Transf 2004;47:787e95.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Porous electrode improving energy efficiency under electrode-normal magnetic field in water electrolysis Hong-bo Liu a,*, Haotian Xu a, Liang-ming Pan b,**, Ding-han Zhong a, Yang Liu b a
School of Civil Engineering and Architecture, Southwest Petroleum University, Chengdu, China Key Laboratory of Ministry of Education of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Chongqing, China
b
highlights Porous electrode improving water electrolysis efficiency under magnetic field. Micro-MHD convection exists within porous electrode. Smaller current brings larger voltage reduction under normal-to-electrode magnetic field.
article info
abstract
Article history:
The porous electrodes (Ni, Cu) with 110 pores per inch (PPI) are adopted in water elec-
Received 5 April 2019
trolysis for hydrogen production under normal-to-electrode magnetic field. The result
Received in revised form
shows that the voltage drop between electrodes can be reduced up to 2.5% under 0.9 T field,
22 June 2019
and the electric energy efficiency is improved correspondingly. Based on the numerical
Accepted 3 July 2019
simulation method, the micro-magnetohydrodynamic (micro-MHD) convection induced by
Available online 26 July 2019
Lorentz force within the porous structure is found. The results showed that although the apparent current direction is parallel to magnetic field outside the porous electrode, the
Keywords:
electric field may be distorted within the porous structure, and the Lorentz force is involved
Porous electrode
near the rib of the micro structure where the current is not parallel to the magnetic field
Water electrolysis
any more. Micro-MHD plays the role of strengthening the mass transfer and facilitating
Micro-magnetohydrodynamic
bubble to eject from the porous structure, which results in the cell voltage decreasing. The
convection
combined application of porous electrode and magnetic field should be potential to further
Energy efficiency
improve energy efficiency of water electrolysis for hydrogen production. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the process of energy utilization, people's demand for clean energy is increasingly urgent with the aggravation of
man-made environmental destruction. Renewable energy based hydrogen energy is one of the most promising solutions to the environmental issues as a carbon free fuel. Only 4e5% of total hydrogen production is being done by water electrolysis [1], although the process of water electrolysis is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-b. Liu), [email protected] (L.-m. Pan). https://doi.org/10.1016/j.ijhydene.2019.07.024 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 2 7 8 0 e2 2 7 8 6
simple and reliable [2]. Two factors are the premise of the development of electrolysis water for hydrogen production in the future, the first one is the enough electricity supply which comes from primary clean energy such as wind energy and solar energy [3]; the second one is the large reduction of electrolytic energy consumption which may refer to the catalytic electrode, gas-liquid management and electrolytic cell design [4]. Recent years, wind power plant combined with electrolysis for hydrogen production has been developed in many countries [5,6]. If the energy consumption can be further reduced, water electrolysis for hydrogen production should play more important role of clean energy utilization in the future. Although gas product management is only one aspect of water electrolysis, it has a great effect on the reduction of energy consumption in electrolysis. The gas product can increase the ohmic voltage drop and the electrode overpotential at the same time due to the non-conducting characteristics [7]. Many new technologies have been used widely to enhance phase separation, such as ultrasonic field [8], super gravity field [9] and magnetic field [10]. The magnetohydrodynamic (MHD) convection induced by Lorenz force results in the enhancement of electrochemical reaction and gas separation [11e13]. Iida et al. [14] superimposed a high magnetic field of 5 T to water electrolysis. Large reduction of cell voltage was achieved, especially in alkaline solution and at high current density. Lin et al. [15] found that magnetic effect was more significant by shortening inter-electrode distance. Ferromagnetism such as nickel is better cathode material than paramagnetism (platinum) and diamagnetism (graphite) under magnetic field. Koza et al. [16,17] found the magnetic field had the effect of improving desorption of hydrogen bubbles from a transparent electrode surface and reducing the fraction of bubble coverage. Matsushima [18] found the gaslayer thickness and local gas fraction of layer were reduced due to MHD convection near the flat electrode surface, and the rising detachment velocity reduces residential time of bubbles on electrode surface and diminishes bubble coverage [19]. Additionally, Bidin [20] found that Lorentz force from external magnetic field causes the water molecule to spin and generate eddy current which generates repulsion force on the hydrogen bonding and enhances the hydrogen production in water splitting. At the same time, the development of the researches of the structure and surface catalysis of electrode is also attractive and exciting in respect of the cell voltage reduction in water electrolysis. Alexander et al. [21] found Black Silicon (BS) has great potential to be used as water splitting materials due to its morphology consisting of needles and wells. Trompoukis et al. [22] found the porous materials has the advantage of lower ionic Ohmic losses compared to dense ones in solar water splitting, and analyzed how micrometer scale pore dimensions could greatly reduce Ohmic losses. Zhou et al. [23] gave the method of growing ternary molybdenum sulfoselenide particles on self-standing porous nickel diselenide foam and achieved a reduced electrode overpotential in water splitting. The efficiency of photoelectrochemical water splitting can be greatly enhanced with reduced graphene oxide (rGO) in BiVO4/rGO photoanode [24]. Porous nickel electrode and 3D carbon foam electrodes are adopted for alkaline water
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electrolysis [25,26], and the lower overpotential is obtained due to the efficient electrolyte transport into the whole electrode matrix concurrent with an ability to quickly dispose oxygen bubbles into the electrolyte. Saleh's simulation of oxygen evolution reaction at porous anode electrode found that the electrode potential is highly dependent on the degree of bubble formation within the bed matrix of porous structure [27], and Kadyk [28] further gave the analysis of how to enhance gas removal from porous electrode in electrochemical gas evolution system. If the porous electrode is used combined with the magnetic field in water electrolysis, it may facilitate the gas production elimination and further reduce the cell voltage. The authors [29] have given the experiment of water electrolysis with the porous copper electrode under the parallelto-electrode magnetic field, and the improved electrolysis efficiency up to 3.4% is achieved. In this work, the porous metal electrode combined with normal-to-electrode magnetic field is used for water electrolysis, the reduced cell voltage is achieved. The analysis may help us to form a more comprehensive understanding about the combined application of magnetic field and porous electrode, which is promising and meaningful for water electrolysis in the future.
Experimental setup Water electrolysis is carried out in the two-electrode cell (35 35 20 mm) with 2 mm and 2.5 mm intervals of electrodes if the nickel and copper electrodes are adopted as cathode respectively. As shown in Fig. 1(a), two electrodes are parallel to each other in the cell with the magnetic field perpendicular to the electrodes. The magnetic field is implemented by a set of two large permanent magnets (150 150 150 mm), and the homogeneous field is up to 0.9 T within the cell domain. The magnetic field is measured by a Gaussmeter (0e2000 mT, ±0.2%, SJ300, SJ Technology Co., Ltd.), to ensure that the intensity of the magnetic field is uniform in the electrolyzer. The platinum sheet (15 5 0.2 mm) is adopted at the anode side, and both of the porous nickel and copper with the size of 15 5 1 mm are used as cathode for hydrogen desorption, the electrodes are embedded into the surface of polylactic acid sheet through 3D printing technology. The experiment is performed in galvanostatic condition, and the voltage drop between two electrodes is monitored simultaneously as the frequency of 5 Hz. Water electrolysis is conducted in KOH aqueous solution of 0.5 mol/L and 1.0 mol/L. The apparent current density through working electrode is within the range of 130e670 A/m2. The porous electrode is with 110 pores per inch (PPI) as shown in Fig. 1 (b), and the porosity of electrode is about 96%. Before experiment, the porous electrode is immersed in acetone, and then it is cleaned with 0.5 mol/L H2SO4 aqueous solution in ultrasonic. As shown in Fig. 1(b), the microstructure of the porous electrode has many hollow cells, which consists of pore and ribs. In experiment, the liquid electrolyte is completely filled into the pore and the cell once the electrode is immersed into the aqueous solution.
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shown in Fig. 2. The abstracted structure is with the same porosity of the electrode used in experiment, ε ¼ 96%(ε ¼ V0VV , 0 where V0 is the apparent volume the porous electrode occupied, and V is the absolute volume the ribs occupied).
Results and discussion
Fig. 1 e Schematic diagram of the electrolytic cell (a) and the microstructure of the porous nickel electrode captured by the scanning electron microscopy (SEM) (b).
The experimental result shows that the magnetic field has the effect of reducing the cell voltage within the current range of 130e670 A/m2. The voltage drop reduces up to 2.5% under current density of 130 A/m2 if the nickel electrode is adopted. As shown in Fig. 3, as the current increases, the magnetic field influence becomes weak, and the difference in potential tends to decrease with and without magnetic field influence. Additionally, the magnetic field has more obvious effect on the nickel electrode compared with the copper one although both of them are with the same porosity. Of course, the larger concentration of electrolyte can significantly reduce the cell voltage due to the increased conductivity of electrolyte which brings the smaller ohmic voltage drop between anode and cathode. The result seems to be interesting and amazing. As known before, if the magnetic field is perpendicular to the electrode, the Lorentz force can't be induced, if no other force is introduced in the solution, the convection is only driven by the concentration difference due to the gas dispersion in liquid phase and gas ascending motion due to buoyancy force. In this experiment, the seemingly nonexistent force makes difference and the cell voltage decreases to some extent. By means of numerical simulation, the electric field and the
Numerical simulation The numerical strategy used to analysis the magnetic field influence on bubble behavior has been used in previous works. The Lorentz-force-driven flow is governed by the incompressible NaviereStokes equation in the steady state, ! V $ ðr V Þ ¼ 0
(1)
!! ! !T 2 ! ! V $ ðr V V Þ ¼ Vp þ V$m V V þ V V V $ V I þ r! g þ FL 3 (2) ! ! ! where I is the unit tensor, and Lorentz force F L ¼ j B is introduced as a user-defined external body force. The electric potential (f) equation is governed by the standard scalar transport equation,V ,ðse VfÞ ¼ 0, wherese is electrical conductivity. The above NaviereStokes and electric potential equations should be discretized by a finite-volume method. The SIMPLE algorithm is used to calculate the pressure and the velocities. More details about the calculation strategy can be found in previous work [30]. The density and dynamic viscosity of 0.5 mol/L KOH aqueous solution are set to 1022 kg/m3 and 1.112 103 (Pa∙s), and the electrolyte conductivity is 11.46 S/ m [31]. According to the porous electrode used in experiment, the microstructure of electrode is abstracted as the model
Fig. 2 e Geometric model used in the simulation (D ¼ 1 mm, H ¼ 3 mm) and the simplified microstructure, ε ¼ 96%.
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Fig. 3 e Voltage drop between electrodes vs. current density.
induced Lorentz force is calculated, and the micro-MHD convection within the porous nickel electrode is analyzed. It can be found from Lin's analysis [15] that the actual magnetic field between the electrodes of ferromagnetism material is not the same with the field around the diamagnetism electrode if the same external magnetic field is adopted. The nickel electrode (ferromagnetism) has better preference under magnetic field compared with the diamagnetism and paramagnetism ones due to the unchanged magnetic field around the electrode surface which arouses larger Lorentz force. In this simulation, the difference of magnetic property of nickel (ferromagnetism) and copper (diamagnetism) is not involved, and the field direction and magnitude within the porous electrode is considered as the same with the external magnetic field everywhere. The apparent current density through the anode and cathode is set to 670 A/m2. According to the normalized geometric construction of the porous electrode shown in Fig. 2, the actual current density through the rib surface of the porous structure is about 289 A/m2, and it is assumed that the current is along the normal direction at the rib surfaces of the porous electrode. The current line distribution around part of the ribs is shown in Fig. 4. It is found that although the apparent current off the porous structure is parallel to the Z-axis direction, i.e. the magnetic field direction, the current line direction is distorted within the porous electrode and not parallel to the magnetic field any more due to the complex layout of the ribs within the porous electrode. The current flux is larger at the surface compared with the space away from the rib. It also has to be said that the actual structure within the porous electrode used in the experiment is more complex than the simplified model in the simulation, and the current distribution is more complex and irregular. The Lorentz force is involved where the current is not parallel to magnetic field. As shown in Fig. 5, the force around the rib surfaces of porous structure reaches to about 850 N/m3, the force intensity is weakened away from the rib surface due to the smaller current flux.
Fig. 4 e Current lines distribution around the ribs within the porous structure.
If no other force such as the bubble buoyancy and natural convection is referred in the simulation, the Lorentz force may induce a rotated flow within the structure, as shown in Fig. 6. The velocity is up to 0.34 mm/s within the region and it is about 0.15 mm/s at the center of pore. The flow is clockwise in the central of Plane1, and the electrolyte is rotated in the opposite direction outside the central region. At Plane2, the forced flow is all anticlockwise. The maximum of velocity is just occurred within each cell. Due to the flow boundary layer, the velocity is reduced close to the rib surface. The simulation result above shows that the micro convection involved by Lorentz force exists within each cell of the porous structure. Certainly, the actual porous structure is more complex with disordered arrangement. But what is certain is that the micro convection must have happened if only the cage-like structure exists within the porous metal material, and what's more, no matter what is the exact form of the micro structure. As what has been known [29,32], if the magnetic field orientation is parallel to the electrode surface, the micro-MHD convection may result in the reduction of cell voltage to large extent due to the gas bubble elimination between electrodes, and the reduction becomes larger as the current and field increase. In a word, larger force makes gaseous product elimination more efficient, which results in smaller cell potential drop. However, the result shows that the extent of cell voltage reduction is larger under lower current density condition in this experiment. As described in Fig. 7(a), the nucleation occurs at the rib surface simultaneously once the hydrogen concentration in the solution becomes supersaturated as the electrolysis going on. According to Vogt's analysis [33], the first nucleation is
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Fig. 5 e Lorentz force magnitude (contour) and orientation (arrow lines) around the ribs of the porous structure (N/m3).
about 4 mm which is much smaller than the rib diameter of porous structure used in the experiment. The greater current results in more nucleation at the time time around the rib. The cell of the porous structure is larger than the newborn bubble, and it smaller than the mature bubble before release. The micro nucleation of bubbles will fight for space during growing up and the coalescence is inevitable especially under the condition of larger current density. Once the coalescence
happens, the cell is easy to be completely occupied by the single immature bubble as shown in Fig. 7(b). The big one is hard to be expel promptly until it is big enough to overcome the increased surface tension force through bubble buoyancy as what has been observed in another work [29], In other words, greater current makes more cells easy to be blocked by single big bubble and more bubbles release from the electrode with larger diameters. The smaller the current, the smaller the
Fig. 6 e Velocity field distribution (contour) and streamlines (arrow lines) within the microstructure (V, 10¡5m/s).
Fig. 7 e Schematic diagram of simultaneous nucleation of bubbles (a) and bubbles coalescence into a big one within the porous unit (b), and the expelled big bubble preparing to release from the outer surface of porous electrode (c).
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number of larger bubbles (r > 0.45 mm) at the outer surface of porous electrode [29]. Of course, if the current is smaller and the external forced convection is present, a large number of minute bubbles are expelled before merging to a big one, i.e. almost no visible big bubbles is found at the outer surface of the porous electrode. It is obvious that the internal micro convection almost has no effect on the big bubble adhering at the outer surface of the porous electrode as shown in Fig. 7(c). The big bubble can only be removed with the bubble collision and the gas-liquid flow through the electrode outer surface. As what has been observed in experiment [29], if the current becomes larger, more large bubble exists at the outer surface of porous electrode which make more contribution for the increase of the ohmic voltage drop and electrode over-potential [10]. In conclusion, Although the driving effect of the Lorentz force becomes smaller as the current decreases, less simultaneous nucleation of micro bubbles occur within the cell of porous structure, and less coalesced big bubbles with the same size of the cell are present at the interior and outer space of the porous electrode for which the Lorentz force is powerless. So the perpendicular-to-electrode magnetic field shows more obvious effect on the cell voltage decrease within the experiment current range.
Conclusions In conclusion, if the porous electrode is used in water electrolysis for hydrogen, the cell voltage can be reduced up to 2.5% under the current density of 130 A/m2 even when the magnetic field is perpendicular to electrode surface. The smaller current density makes larger voltage reduction within the experiment current range of 130e670 A/m2. The simulation result shows that although the apparent current direction is parallel to the magnetic field, the Lorentz force is still aroused within the porous structure due to the electric field distortion around the rib of the porous structure, and the Lorentz-force-induced flow within micro structure plays the role of expelling micro bubbles off the pores of porous structure. It is foreseeable that if the porous structure electrodes are adopted both at the anode and cathode side, the cell voltage can be further reduced, and the electric energy efficiency can be more improved in water electrolysis.
Acknowledgement
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The authors are grateful for the support of the Natural Science Foundation of China (Grant No: 51706190, 51676020), and the support of Young Scholars Development Fund of SWPU (Grant No: 201699010120).
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