Effect of flow regime of circulating water on a proton exchange membrane electrolyzer

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Effect of flow regime of circulating water on a proton exchange membrane electrolyzer H. Ito a,*, T. Maeda a, A. Nakano a, Y. Hasegawa a,b, N. Yokoi b, C.M. Hwang c, M. Ishida c, A. Kato d, T. Yoshida e a

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba 305-8564, Japan b Tokyo Institute of Technology, 4259 Nagatsuda, Midori-Ku, Yokohama 226-8503, Japan c University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8573, Japan d Takasago Thermal Engineering Co., Ltd., 3150 Iiyama, Atsugi 243-0213, Japan e Daiki Ataka Engineering Co., Ltd. 11 Shintoyofuta, Kashiwa 277-8515, Japan

article info

abstract

Article history:

The flow characteristics of circulating water in a proton exchange membrane (PEM) elec-

Received 24 May 2010

trolyzer were experimentally evaluated using a small cell and two-phase flow theory.

Received in revised form

Results revealed that when a two-phase flow of circulating water at the anode is either slug

25 June 2010

or annular, then mass transport of the water for the anode reaction is degraded, and that

Accepted 27 June 2010

the concentration overvoltage increases at higher current density compared to that when

Available online 31 July 2010

the flow is bubbly. In a serpentine-dual flow field, when both phases of the two-phase flow are assumed laminar, then the increase in pressure drop caused by the increase in gas

Keywords:

production can be explained relatively well using the LockharteMartinelli method with the

Proton exchange membrane

Chisholm parameter. The optimal flow rate of circulating water was also discussed based

Electrolysis

on mass balance analysis.

Flow field

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Two-phase flow

1.

Introduction

Hydrogen as a clean and environmentally acceptable fuel is expected be one of the most promising energy carriers in the near future. A proton exchange membrane (PEM) electrolyzer as a hydrogen production device has demonstrated higher efficiency and higher current density capability compared to an alkaline electrolyzer. PEM electrolyzers have been extensively researched and developed for more than 30 years [1,2]. Although certain PEM electrolyzers have been commercially marketed, the need for higher efficiency and flexible operation has prompted active study of electrocatalysts [3e7], high

pressure operation [8e11], modeling [12e14] and stack development [15e17]. Because higher current density capability is desirable for hydrogen production using unstable renewable energy sources, such as solar and wind active research has also focused on the system set-up and operation of an integrated hydrogen production system consisting of a PEM electrolyzer and a photovoltaic and/or wind power source [18e23]. In a PEM electrolyzer, the PEM serves as the electrolyte as well as the barrier between the hydrogen and oxygen. Because PEM is acidic, half-cell reactions during electrolysis can be expressed as follows:

* Corresponding author. Tel.: þ81 29 861 7262; fax: þ81 29 851 7523. E-mail address: [email protected] (H. Ito). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.06.103

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Nomenclature A a C D F G i L M nchan ndrag p Q Re T X x z

electrode surface area, m2 cross-sectional area of flow channel, m2 Chisholm’s parameter hydraulic diameter of channel, m Faraday constant, C mol1 mass flow flux in channel, kg m2 s1 current density, A m2 length of channel, m molar weight, kg mol1 the number of channels in parallel in flow field drag coefficient of water in membrane pressure, Pa flow rate of circulating water, m3 s1 (l min1) Reynolds number cell temperature, K LockharteMartinelli parameter quality location along the channel from the inlet, m

Greek symbols l coefficient of friction loss m viscosity, Pa s

H2 O/2Hþ þ 1=2O2 þ 2e 2Hþ þ 2e /H2

at anode

at cathode

(1) (2)

Liquid water is introduced at the anode and dissociated into molecular oxygen, protons, and electrons. Solvated protons formed at the anode migrate through the membrane to the cathode where they are reduced to molecular hydrogen. During migration of protons through the membrane, water molecules accompany the protons through the membrane from the anode to cathode due to an electric field. Thus, the PEM is kept wet without an outside water supply to the cathode, and water as a reactant is supplied only at the anode during typical operation of a PEM electrolyzer. De-ionized liquid water supplied to the anode flows through the channel in the anode separator (bipolar plate), and diffuses to the surface of the electrode via a porous layer. Oxygen gas generated at the electrode diffuses to the channel and is entrained by the flow of liquid water. After the flow exits the cell, the oxygen gas is separated from liquid water at an accumulator. The separated liquid water is then re-supplied to the cell, and thus liquid water is essentially re-circulated between the anode of the cell and the accumulator by a circulation pump. The capacity of this pump can be determined based on its pressure drop (head) and the flow rate of the circulating water. Although there is no general standard for this flow rate, a typical flow rate might be up to 10 times that of the water consumed at the anode. Several groups [17,24,25] have studied the pressure drop of circulating water in the channel of a PEM electrolysis cell, but have examined the pressure drop of only liquid water as a function of flow rate, but not that of gas production from the electrode. Most recently, Nie et al. [26,27] did numerical analysis of two-phase flow in the channel of a PEM electrolyzer and pointed out that

r FL z

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density, kg m3 two-phase flow multiplier the ratio of the flow rate of the circulating water to the amount of water needed for anode reaction (Eq. (21))

Subscripts circ circulation cons consumed drag drag exit exit of channel G, g gas phase in two-phase flow water H2O i component of flow, gas or liquid L, l liquid phase in two-phase flow L0 liquid water oxygen O2 TP two phase Abbreviations GDL gas diffusion layer MEA membrane electrode assembly PEM proton exchange membrane PEMFC proton exchange membrane fuel cell

increasing the mass flow rate for oxygen production causes an increase in the pressure drop over the flow field. However, there is no detailed description concerning experimental conditions such as electrode area, temperature, and current density. In contrast, gas production by electrolysis using flowing electrolyte solution has been comprehensively investigated both analytically and experimentally, and the effect of gas production on the flow characteristics is known at the micro-scale [28e31]. According to those works, gas bubbles form and grow at the electrode surface as a result of electrochemical reaction and leave the surface when buoyancy and shear forces acting on a bubble exceed the interfacial tension force. In addition, the flow of electrolyte solution with bubble formation can change from laminar to turbulent throughout the length of the cell. Nagai et al. [32] has developed a correlation between the void fraction and current density in an alkaline electrolysis cell, whose electrolyte is a solution of KOH. Roy et al. [33] evaluated the voltage loss caused by gas evolution as the ohmic overpotential at an alkaline electrolyzer and presented an empirical equation of bubble voltage loss and current density by referring to the correlation by Nagai et al. [32]. Although these works are instructive for our present study, there is a distinct difference between the alkaline and PEM electrolyzer, because in a PEM electrolyzer, the electrolyte is not a solution but a membrane. Thus, results obtained for cells with electrolytic solution, such as alkaline electrolyzers, cannot be directly applied to PEM electrolyzers. In our present study, to optimize the design and operation of a PEM electrolyzer, we evaluated the flow characteristics of circulating water in the channel during gas production at the macro-scale. First, the effect of the flow characteristics on the cell performance and pressure drop was investigated experimentally, and then the optimal flow rate of the circulating water was evaluated based on mass balance analysis.

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flow field, and serial repetition of vertical upward and downward in the serpentine flow fields. By using these different flow fields, the flow velocity in the channel could be changed drastically even when the flow rate was kept constant. The cell temperature was controlled by a thermocouple inserted in the body of the cell (item 1 in Fig. 1) and by electric heaters on the cover plates of the cell (item 2). De-ionized water was supplied for electrolysis. Liquid water was circulated only at the oxygen electrode (anode) by an electromagnetic pump (item 9). Water flow rate was controlled by a control valve (item 8) and a flow meter (item 7). Water supplied to the cell was heated to the same temperature as the cell by a pre-heating tank (item 6), and the pipe in the upper region of the inlet was wrapped with an electric heater (item 5) to keep the water temperature the same as the cell temperature. Two-phase flow of gas and liquid was released from the exit of the cell at both electrodes. Hydrogen and oxygen gases were separated by respective accumulators (item 10) and vented to the atmosphere. The stop valve of the hydrogen side (item 4) was closed except during the experiment to study the effect of water circulation at both the hydrogen and oxygen sides, when the stop valve was opened, and the flow rate and temperature of liquid water were controlled the same way as that at the oxygen electrode side. DC power for electrolysis of water was supplied from a power source to the cell. Cell voltage was measured at each current density and the flow rate of the generated hydrogen was measured using a soap-film flow meter (SF-1U, Horiba) before the hydrogen was vented. Pressure drop of the circulated water between the inlet and exit was measured using a differential pressure transducer (DP-15, Validyne). Signals from the transducer were amplified and acquired every 0.5 s for 2 min and the average value was used for the analysis of pressure drop.

Experimental

Fig. 1 shows a schematic diagram of the PEM electrolyzer system used here. This electrolyzer is a small single cell originally designed as a proton exchange membrane fuel cell (PEMFC). The configuration of the cell is a commonly used PEMFC configuration, consisting of a membrane electrode assembly (MEA), gas diffusion layers (GDLs), and separators (bipolar plates) with flow channels. Preparation of the MEA was the same as that of a PEMFC, that is, catalytic electrodes were hot pressed to both surfaces of the membrane. Nafion 115 was used as the PEM. Iridium oxide (IrO2) electrocatalyst was used for the oxygen electrode, and platinum catalyst was used for the hydrogen electrode. The surface area of the electrode was 27 cm2. To prevent possible corrosion, titanium-felt coated with Pt was used as the gas diffusion layer of the anode, whereas carbon paper (Toray TGP-H-090) was used as the GDL of the cathode. Porosity of each GDL was approximately 0.75. In the PEM electrolyzer, the GDLs were actually fully saturated by liquid water during electrolyzer operation, and their main roles were electric conduction between the electrode and the separator and efficient gas transport from the electrode to the flow channels of the separator. The effect of the velocity of circulating liquid water on the electrolyzer performance and pressure drop was evaluated for three kinds of separators, each with a different flow field (Fig. 2); (a) serpentine-single flow field, (b) serpentine-dual flow field, and (c) parallel flow field, when the flow filed of the both electrode sides was the same. The number of flow channels in parallel was 1, 2, and 26, respectively, and for all three flow fields, the cross-section of the channel was square and 0.01 cm2 in area (Fig. 1d). In the present cell set-up, the flow in the separators was vertical upward in the parallel

O2 Vent

H2 Vent

11 1

3 DP

Anode

O2 gas

10

Cathode

2

Water + H2 gas

2 10

Water

4

8

9

6 DC power supply

6

Water

5

5 7

H2 gas

Water + O2 gas

7 8

9

Fig. 1 e Schematic diagram of experimental setup to evaluate circulating water characteristics in a PEM electrolyzer system: (1) electrolytic cell (PEM electrolyzer), (2) electric heaters for the cell, (3) differential pressure transducer, (4) stop valve, (5) electric heaters of the pipes, (6) preheating tanks, (7) flow meters, (8) control valves, (9) pumps, (10) accumulators, (11) H2 gas flow meter.

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1.7

52 mm

A

T = 70C

1.6

V [V]

52 mm

a

1.5

Serpentine-single Serpentine-dual Parallel

1.4

52 mm

0

200

400

600

800

1000

-2

i [mA cm ] 1.7 52 mm

B

T = 80C

V [V]

1.6

b

1.5

52 mm

Serpentine-single Serpentine-dual Parallel

1.4

52 mm

0

200

400

600

800

1000

-2

i [mA cm ] Fig. 3 e Characteristics of current density (i) and voltage (V) of a PEM electrolyzer with different flow fields at a cell temperature (T ) of 70 (A) and 80  C (B), a flow rate of circulating water (Q) at the anode of 25 ml/min.

c

1mm

MEA GDL (Current collector) Flow channel 1mm 1mm

d Fig. 2 e Flow field of the separator (bipolar plate); (a) serpentine-single, (b) serpentine-dual, and (c) parallel. (d) Cross-sectional dimensions for all three flow fields.

3.

Results and discussion

3.1.

Current-voltage characteristics

Fig. 3 shows the measured current-voltage (i-V) characteristics of the PEM electrolyzer measured at a cell temperature of 70 and 80  C for the three different flow fields at a circulating water

flow rate (Q) of 25 ml/min. Difference in the i-V curves is clearly evident at the higher current density region. When the cell temperature is 70  C (Fig. 3 (A)), the difference in cell voltage at 1.0 A cm2 between the serpentine-single and -dual flow fields and between the serpentine-dual and parallel flow fields was approximately the same, about 20 mV. These two voltage differences at 80  C of the cell temperature (Fig. 3 (B)) are 35 mV and 20 mV respectively. At the both cell temperatures of 70 and 80  C, the measured cell voltage at 1.0 A cm2 has the reproducibility within 3 mV. An overvoltage in the high current density region was larger when the flow velocity of the circulating water in the channel was higher, because the flow rate per channel, and thus the flow velocity, is significantly different for different types of flow field and depends on the number of channels in parallel even when the flow rate is the same. This suggests that an increase in overvoltage in the high current density region is caused by an increase in concentration overvoltage and suggests a relation between the flow velocity and the concentration overvoltage. Fig. 4 shows the i-V characteristics of the PEM electrolyzer at different Q and cell

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temperature T for the serpentine-single flow field. The flow rate Q had no apparent effect on voltage difference at 80  C cell temperature, though had a slight effect (<10 mV) at 70  C. This suggests that the voltage difference in different flow fields in Fig. 3 cannot be explained only by the effect of flow velocity. In these experiments, hydrogen production rate was measured using a soap-film meter. Although the hydrogen production rate was too small to be measured precisely when i < 200 mA cm2, the Faraday efficiency was higher than 99.5% regardless of the type of flow field when i > 200 mA cm2.

3.2.

Flow regime of two-phase flow

The flow in an anode channel becomes a gaseliquid twophase flow consisting of produced oxygen and circulating water. The flow regime of two-phase flow changes depending on the quality, mass flux, or superficial velocity of each phase and is classified by the form of interface between the two phases as follows: bubbly flow, slug flow, churn flow, annular flow, and mist flow [34]. The flow regime affects the i-V characteristics of a PEM electrolyzer. Here, we discuss flow in the channel as a two-phase flow and evaluate its characteristics. When water electrolysis occurs in a PEM electrolyzer, liquid water without gas flows into the inlet of the channel, and then entrains oxygen gas from the electrode facing the channel. The quality of the two-phase flow (x) at an arbitrary location along the channel is defined as: x¼

Gg Gl þ Gg

(3)

where Gg and Gl denote the mass flux of the gas and liquid, respectively. Assuming that Q is constant throughout the length of the channel and that the current (i) distribution in the electrode is uniform and thereby gas production is also uniform, x increases linearly from zero at the inlet to xexit at the exit. This assumption is the same as that used to analyze the two-phase flow in an evaporating channel under uniform

Serpentine-single

heating [35]. Therefore, if xexit can be determined, then x at any location along the channel can be easily predicted. To determine xexit, we calculated the amount of gas production, water consumption, and water transport from the anode to cathode based on the entire area of the electrode. Note that the following analysis considers only a single cell. According to Faraday’s law, the production rate of oxygen in an electrolyzer cell is directly proportional to the transfer rate of electrons at the anode, which in turn is equivalent to the electrical current in an external circuit. When the Faraday efficiency is assumed to be unity, all oxygen gas generated from the anode is entrained by the flow in the channel. Hence, the mass flux of oxygen gas at the exit can be calculated as: GO2 ¼

iAMO2 4Fnchan a

where i is the current density, A is the entire area of the electrode, MO2 is the molar weight of oxygen, F is Faraday constant, nchan is the number of channels in parallel, and a is the cross-sectional area of the channel. However, part of the circulating water is consumed by the reaction expressed by Eq. (1). The consumption rate of water is also proportional to i. Thus, the total mass flux of consumed water per channel can be described as Gcons ¼

iAMH2 O 2Fnchan a

ndrag ¼ 0:0134  T þ 0:03

V [V]

T = 70 C Q = 25 ml/min Q = 50 ml/min T = 80 C Q = 25 ml/min Q = 50 ml/min

1.4

0

200

400

600

800

1000

-2

i [mA cm ] Fig. 4 e Characteristics of current density (i) and voltage (V) of a PEM electrolyzer with serpentine-single flow field at a cell temperature (T ) of 70 and 80  C and a flow rate of circulating water (Q) at the anode of 25 and 50 ml/min.

(6)

Part of the circulating water in an anode channel eventually moves to the cathode by electrolysis. The total mass flux of the water dragged by electro-osmosis can be calculated as: Gdrag ¼ ndrag

1.5

(5)

where MH2 O is the molar weight of water. When the proton moves through the PEM, water molecules also accompany the proton through the membrane by the electric field. This phenomenon is well known as electro-osmosis [36]. The electro-osmotic drag coefficient ndrag is defined as the number of water molecules “dragged” per Hþ. Onda et al. [12] found that ndrag depends only on the membrane temperature under PEM electrolyzer conditions, and experimentally obtained the following correlation [12]:

1.7

1.6

(4)

iAMH2 O Fnchan a

(7)

Because ndrag is approximately between 4.5 and 5.0 at a temperature range from 333 to 353 K, Gdrag is about 10 times that of Gcons. The xexit can then be predicted by the total mass balance of the above-mentioned terms of GO2 , Gcons, and Gdrag and the mass flux of circulating water through the channel (Gcirc), which is regulated by the flow meter in our experiments. In addition, when the flow field is the serpentine-dual or parallel, the circulating water introduced into the cell is assumed to be allotted to the each parallel channels evenly. Finally, substituting these mass fluxes into Eq. (3) yields xexit of the two-phase flow in the channel: xexit ¼

GO2 Gcirc  Gcons  Gdrag þ GO2

(8)

where Gcirc is considered constant even when oxygen gas is entrained. In our analysis, we assumed that x changes linearly

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along the channel from zero to xexit. Superficial velocities of the gas and liquid ( jg and jl, respectively) strongly depend on the type of flow field and can be calculated along the channel as follows: jg ¼ Gx=rO2 jl ¼ Gð1  xÞ=rH2 O


(9)

where G is the total mass flux of two-phase flow and defined as Gg þ Gl, and rO2 and rH2 O are the densities of oxygen and water, respectively. Fig. 5 shows the flow regime map for a small diameter channel (diam. ¼ 2.05 mm) reported by Mishima and Hibiki [37], and shows our calculated data for an anode channel for the three different flow fields. In the case of the parallel flow field, because both jg and jl per channel were relatively small compared to those in either of the serpentine flow fields (single or dual), the flow regime was apparently bubbly flow from the inlet to the exit of the channel when i < 1.0 A cm2. The flow became slug near the exit when i > 2.0 A cm2, although data was not measured at this location in our experiments. In the case of the serpentine flow field (either single or dual), the flow regime of two-phase flow in the channel changed from bubbly to slug flow near the exit at i z 0.2 A cm2, and 80% of the flow in the entire area changed to slug flow at i z 1.0 A cm2. On the other hand, when the flow rate (Q) is increased at the serpentine-single as shown in Fig. 4, the flow regime is not changed drastically as shown in c and d in Fig. 5. These results indicate that there is the initiate relationship between the flow regime and the electrolysis performance. In the case of bubbly flow, since gas bubbles is dispersed in a continuum of liquid, there is no remarkable degradation in liquid water distribution on the electrode surface. On the other hand, at slug flow, the bubbles grow by coalescence and, ultimately, they become of the same order of

1

10

d

Inlet

0

10

Mist

Annular

Slug

Bubbly

-1

jl [m s ]

current density of

-2 0.2 A cm -2 0.5 A cm -2 1.0 A cm -2 2.0 A cm -2 3.0 A cm

-1

10

a

Churn

-2 -3

-2

10

jg D ng

!  0:301 jl D ¼ 1:199  106 nl

(10)

where vg and vl are kinetic viscosities of gas and liquid, respectively, and D is the hydraulic diameter of the channel. Although this criterion was obtained for vertical flow in a relatively large diameter pipe (50.8 mm in diameter), it can be used as a reference criterion for our case and thus is also plotted in Fig. 5. This figure shows that the flow regime of two-phase flow does not change to mist flow regardless of the type of flow field even when i is considerably high (w3.0 A cm2).

3.3.

Exit value at

b

10

diameter as the channel and must hinder the supply of reactant water to the electrode surface at a certain degree. And when the flow becomes annular, the liquid flows only on the walls of the channel as a film, the water supply must be restricted rather severely. Note that the effect of bends in the serpentine flow field on the flow velocity is not considered in this analysis. Trabold [38] pointed out that slug flow is essentially stationary at a 180 bend corner in a serpentine flow field during certain operating conditions of PEMFC, and thus completely blocks the channels over a significant fraction of the entire active area of the electrode. In our experiments, the number of 180 bends per channel was 26 in the serpentine-single flow field, but only 6 in the serpentine-dual flow field. The difference in i-V characteristics between the serpentine-single and -dual flow fields in Fig. 3 might be explained by this difference in the number of bends. The walls of the channel must be kept wet not only for slug flow but also for annular flow so that the electrolyzer cell can operate without damage to the membrane even under such flow conditions, although the concentration overvoltage might become larger compared to the case of bubbly or slug flow. However, the wall might become dry when the flow is annular-mist or mist. Chien and Ibele [39] proposed the following criterion for the transition of flow regime from annular to annular-mist flow [39]:

Pressure drop of circulating water

Exit

c

10

9555

-1

10

0

10

1

10

2

10

3

10

4

10

-1

jg [m s ] Fig. 5 e Flow regime map expressed by superficial velocity of gas ( jg) and liquid ( jl) presented by Mishima and Hibiki [37], the correlation by Chien and Ibele (dashed line) [39], and jg and jl in the channel at 70  C from inlet to exit at each current density (symbols) calculated in the present study for parallel (a), serpentine-dual (b), serpentine-single (c) flow fields at a flow rate of circulating water (Q) of 25 ml/ min, and serpentine-single flow field at Q of 50 ml/min (d ).

Fig. 6 shows the pressure drop versus i for the three different flow fields for Q ¼ 25 ml/min and cell temperature (T ) of 60  C. The pressure drop significantly depended on the type of flow field, when the exit back pressure was approximately atmospheric. In the case of serpentine-single flow field, the pressure drop was significantly high, because the flow velocity of flowing water was high and the length of channel was long compared to that of parallel flow field, because the number of channels in parallel was as few as 1 as shown in Fig. 2. The serpentine-single flow field is typical for PEMFC, when the fluid is humidified gas (H2 and O2/Air). The pressure drop of gas is much smaller than that of liquid water and the high velocity of a gas becomes an advantage for drawing off the liquid water produced at cathode of PEMFC. In our experiments, the pressure drop was significantly small with the parallel flow field, because the flow rate per channel was small and the length was relatively short, because the number of channels in the parallel flow field is as many as 26. In the case of serpentine dual-line flow field, because both

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3.4.

Theory of two-phase pressure drop

There are numerous bends in the serpentine flow fields as shown in Fig. 2, and an acceleration pressure drop might be caused by a bend. However, the measured pressure drop of liquid flow (at i ¼ 0) even in a serpentine-single flow field is represented well by the theoretical value in which only frictional pressure drop is considered. Thus, we discuss only the frictional pressure drop in the flow channel. The frictional pressure gradient of single-phase flow of gas ( g) or liquid (l ) in a channel can be expressed as 

Fig. 6 e Pressure drop (Dp) versus current density (i) with different flow fields at a cell temperature (T ) of 60  C, and circulating flow rate (Q) at the anode of 25 ml/min.

the flow rate and the length of each channel are in the region of serpentine-single flow field and parallel flow field, the pressure drop was between that of these other two types of flow field. Because the high hydraulic pressure would act on the membrane and thus a high power capacity would be needed by the circulation pump, the parallel flow field is much more suitable for a PEM electrolyzer than is the serpentine flow field from the viewpoint of pressure drop, as well as from i-V characteristics. In addition, the pressure drop gradually increased as i increased (Fig. 6). In the case of serpentine dual-line flow field, the pressure drop at i ¼ 1 A cm2 was over 3 times larger than that at i ¼ 0. Although the pressure drop with the parallel flow field also tended to increase with increasing i, the drop was too small for quantitative evaluation in the small cell used in our study. However, such small pressure drop with a parallel flow field indicates that pressure drop caused by a nozzle and diffuser respectively at the inlet and exit of the cell could be negligible for the other two flow fields and that the change in pressure drop with a serpentine-single or -dual flow field is caused only by the channel portion facing the electrode. Based on previous studies on gaseliquid two-phase flow [34], the increase in pressure drop observed in our study can be attributed to the increase in voids in the fluid. Hence, analysis of the increase in pressure drop based on the theory of two-phase flow can be used in our study. Pressure drop is significantly large in a large-scale cell even when the flow field is parallel [24]. In that case, the increase in pressure drop caused by an increase in i must also be relatively large, and quantitative evaluation of this increase in pressure drop is important for the design and operation of PEM electrolyzer systems. As mentioned in Sections 3.1 and 3.2, although serpentine flow fields are an unrealistic option for PEM electrolyzer cells, the pressure drop increase with such flow fields is relatively high and thus suitable for comprehensive analysis of the increase in pressure drop. Therefore, in our present study, analysis of the increase in pressure drop involved data for the serpentine-single and -dual flow fields.

 li G2i dp ¼ dz i 2Dri

i ¼ g or l

(11)

where G and r respectively denote mass flux and density of gas or liquid phase, and D is the hydraulic diameter of the channel. The friction factor li strongly depends on whether the flow is laminar or turbulent, and can be obtained for each flow regime as follows: li ¼ 56:9=Rei for laminar flow li ¼ 0:3164Re0:25 for turbulent flow i


(12)

where Rei is the Reynolds number of the i phase. In the correlation for laminar flow, the numerator for a cylindrical channel is typically 64; however, for a rectangular channel, a value of 56.9 is chosen in this case. Correlation for turbulent flow is known as the Blasius equation. The Reynolds number for each phase is defined as Rei ¼

Gi D mi

(13)

The most widely used correlation for two-phase pressure drop is the correlation by LockharteMartinelli [40], who defined a two-phase multiplier (FL) as FL ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðdp=dzÞTP ðdp=dzÞL

(14)

where ðdp=dzÞTP is the frictional pressure gradient due to the two-phase flow, and ðdp=dzÞL is the pressure gradient if the liquid is flowing alone in the channel. FL is correlated in terms of a parameter X defined as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðdp=dzÞL X¼ ðdp=dzÞG

(15)

where ðdp=dzÞG is the pressure gradient if the gas is flowing alone in the channel. Different correlation equations are used for X depending on whether the phase flowing alone in the channel is turbulent or laminar, and can be obtained by substituting Eqs. (11 and 12) into Eq. (15) as listed in Table 1. To correlate FL with X, the equation suggested by Chisholm and Laird [41] is F2L ¼ 1 þ

C 1 þ X X2

(16)

where C is the Chisholm parameter whose constant values have been proposed for each flow regime as listed in Table 1. The increase in pressure drop due to the gas production as shown in Fig. 6 can be explained based on pressure drop analysis for an evaporating channel [34]. In this case, it is

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 5 5 0 e9 5 6 0

convenient to evaluate the pressure gradient ðdp=dzÞL0 for the liquid phase flowing at the total mass velocity G (¼ Gg þ Gl) as a standard. Then a parameter FL0 defined as FL0 ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðdp=dzÞTP ðdp=dzÞL0

(17)

where ðdp=dzÞL0 is expressed as   dp ll G2 ¼ dz L0 2Drl

(18)

The relationship between FL0 and FL can be obtained from Eqs. (3), (11), (12), (14) and (17) for each flow regime as follows: F2L0 ¼ ð1  xÞF2L for case of LL or LT F2L0 ¼ ð1  xÞ1:75 F2L for case of TL or TT


(19)

where the abbreviations LL, LT, TL, and TT are as defined in Table 1. Consequently, the ratio of pressure drop over a channel length L is obtained as Z DpTP 1 L 2 ¼ F dz DpL0 L 0 L0

(20)

The theoretical value of DpTP =DpL0 is calculated as follows. First, xexit is calculated using Eq. (8) at an arbitrary i and a linear x profile from 0 to xexit over a full length L. Then, the X profile is obtained by applying the appropriate x at each location to the corresponding equation in Table 1. Then, F2L can be determined by the corresponding C in Table 1 and Eq. (16), and F2L0 by Eq. (19). Finally, DpTP =DpL0 can be calculated by integrating F2L0 over L as expressed in Eq. (20).

3.5.

Two-phase multiplier DpTP =DpL0

Fig. 7 shows the experimental values of the two-phase multiplier DpTP =DpL0 (symbols) with the serpentine-dual flow field compared with the values predicted using the LockharteMartinelli method (lines). The experimental values of DpTP =DpL0 were calculated using the ratio of actual pressure drop at each iðDpTP Þ and that at i ¼ 0ðDpL0 Þ as shown in Fig. 6. The predicted lines were calculated using the Chisholm parameter C ¼ 5, which is the value when both phases are laminar as shown in Table 1. As shown in Fig. 7, the experimental values at cell temperatures (T ) of 60 and 70  C are represented relatively well by the corresponding lines predicted by the LockharteMartinelli method. In our

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experiments, Q was relatively small (25 ml/min), and thus the Reynolds number of liquid phase (Rel) was between 400 and 500 and that of gas phase (Reg) at the exit was less than 50. Thus, both phases of the two-phase flow were considered laminar. Fig. 8 compares the experimental and predicted DpTP =DpL0 for the serpentine-dual flow field, when Q was twice that shown in Fig. 7 (i.e., Q ¼ 50 ml/min). For these experimental conditions, (Rel) z 1000, and (Reg) z 100 at the exit. This value of (Rel) is a critical value determining either a laminar or turbulent flow regime. According to previous studies on the gas production by electrolysis in aqueous solution [28e31], fluid flow with gas production can change from laminar to turbulent, and both turbulent and laminar flows can coexist across the same cell channel even when Rel is in the laminar region. Therefore, the difference between experimental and predicted values might be due to the partial change in flow regime from laminar to turbulent. To further study this result, we performed another series of experiments where water was circulated not only in the anode channel but also in the cathode channel. Fig. 8 shows the measured pressure drop at both circulation sides, showing that this pressure drop is similar to that with only anode circulation. Furthermore, there was no significant difference in the i-V characteristics as well, though the data is not shown here. These results indicate that the water circulation at the cathode side does not significantly affect the cell operation. Fig. 9 compares the experimental and predicted DpTP =DpL0 for the serpentine-single flow field at Q ¼ 25 ml/min and at Rel the same as that for the serpentine-dual and Q ¼ 50 ml/min shown in Fig. 8. Although Reg was twice that in the case of serpentine-dual and Q ¼ 50 ml/min (Fig. 8), the gas phase was considered laminar throughout the length of the channel. The predicted values, which assumed that both phases were laminar, differed from the experimental values at i > 300 mA cm2. This difference is unexpected because the turbulence under these conditions would be more developed than that shown in Fig. 8. The reason for this difference is not yet clear, though a possible explanation is that the different characteristics of the pressure drop are caused by the difference in the number and configuration of bends [42e44]. Because the effect of bends is expected to be remarkable with a two-phase flow rather than a single-phase liquid flow. The number of bends per channel in the serpentine-dual flow field was comparatively few compared with the serpentine-single flow field, and thus the flow characteristics in the

Table 1 e LockharteMartinelli parameter (X ) and Chisholm parameter (C ) at each flow regime of a two-phase flow.a Flow regime symbol

LL

Flow regime

X

C

Liquid phase

Gas phase

Laminar

Laminar

1=2 g 1=2 ml 1=2 ð1x ð rl Þ ðmg Þ x Þ

10

r

5

TL

Turbulent

Laminar

1=2 rg 1=2 ml 1=2 ð r Þ ðm Þ 0:075ðGml DÞ0:375 ð1x x Þ l l g

LT

Laminar

Turbulent

m 1=2 rg 1=2 ml 1=2 ð rl Þ ðmg Þ 13:4ðGggDÞ0:375 ð1x x Þ

12

Turbulent

0:875 rg 1=2 ml 0:125 ð rl Þ ðmg Þ ð1x x Þ

21

TT

Turbulent

a Laminar: Rel, Reg < 1000, turbulent: Rel, Reg > 2000.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 5 5 0 e9 5 6 0

Fig. 7 e Measured (symbols) and predicted (lines) twophase multiplier (DpTP/DpL0) versus current density (i) for the serpentine-dual flow field at different cell temperature (T ) at a flow rate of circulated water (Q) of 25 ml/min and a Chisholm parameter (C ) of 5 for the predicted values.

Fig. 9 e Measured (symbols) and predicted (lines) twophase multiplier (DpTP/DpL0) versus current density (i) for the serpentine-single flow field at a cell temperature (T ) of 70  C and flow rate of circulated water (Q) of 25 ml/min. The Chisholm parameter (C ) was 5 for the predicted values.

serpentine-dual were closer to those in the parallel flow field. As described above (Section 3.4), the Lockhart-Martinelli method is simple and easy to apply. Good agreement between experimental and predicted results in the case of the serpentine-dual flow field indicates that the analysis method presented here can be applied to general large-scale cells with a parallel flow field. In typical cell operation with a parallel flow field, because the flow must be laminar throughout the entire length of the channel regardless of the scale, the Chisholm parameter (C ) should be set to 5.

3.6.

Fig. 8 e Measured (symbols) and predicted (lines) twophase multiplier (DpTP/DpL0) versus current density (i) for the serpentine-dual flow field at a cell temperature (T ) of 70  C and flow rate of circulated water (Q) of 50 ml/min. Measured values were obtained for water circulation only at oxygen electrode side (,) and at both the oxygen and hydrogen electrode sides (C). The Chisholm parameter (C ) was 5 for the two-phase flow regime of LL (liquid is laminar, gas is laminar) for the predicted values.

Flow rate of circulating water Q

The ratio (z) of the flow rate of the circulating water Q to the amount of water needed for anode reaction per single cell is an important index of PEM electrolyzer operation and can be defined as: z¼

Gcirc Gcons þ Gdrag

(21)

The relation between z and i can thus be calculated independent of the type of flow field, and is shown in Fig. 10. Under the present experimental conditions (Q ¼ 25 and 50 ml/min, i ¼ 0 to 1 A cm2), z is over 10. By substituting Eqs. (5) and (7) into Eq. (21), a general relationship between Q per unit area of the electrode (Q/A) and i at various values of z for a single cell can be obtained when the parameter nchan is eliminated from the relation. Fig. 11 shows this calculated relation. Theoretically,

Fig. 10 e Ratio of circulating water flow rate (z) defined by Eq. (21) as a function of current density (i) at various Q.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 9 5 5 0 e9 5 6 0

Fig. 11 e Flow rate of circulating water per unit area of the electrode (Q/A) and current density (i) at various values of ratio of circulating water flow rate (z) at a cell temperature T of 70  C.

a value of z ¼ 1 is sufficient for the water electrolysis reaction. However, during operation of an actual large-scale cell/stack, z is set at about 5 to ensure safety by preventing the membrane from drying out [45]. Assuming z ¼ 5, Q for the small cell used in our present study must be larger than 25 ml/ min during operation at i ¼ 3.0 A cm2. However, when Q is too high, the pressure drop increases and thus damages the electrode by high hydraulic pressure. Consequently, Q should be determined based on the maximum i for a given electrolyzer operation and on a proper value of z (i.e., 5) or it must be controlled by adjusting i so that z is kept constant. In addition, the flow velocity should be small enough to keep the flow bubbly. To keep the flow bubbly, the channel should be as deep as possible so that the crosssectional area of the channel is large, because a large number of channels in parallel are better for small xexit per channel and low pressure drop. In some cases of a large cell/stack, the circulating water also acts as cooling water [45], and therefore a heat balance between cooling demand and ability of the heat exchanger must be considered in determining the flow rate in those cases.

4.

Conclusions

An experimental study focused on the characteristics of circulating water in the channel of a proton exchange membrane (PEM) electrolyzer. The current-voltage (i-V) characteristics and pressure drop were measured using a small single cell with three different types of flow field (serpentine-single, serpentine-dual, and parallel). The following results for flow regime, frictional pressure drop, and flow rate were obtained: 1) There is the initiate relationship between the flow regime of circulating water in the channel and the electrolysis performance. When the two-phase flow of the circulating water at the anode is either slug or annular, mass transport of water for the anode reaction is degraded and the

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concentration overvoltage increases at higher current density. 2) Water circulation at the anode is sufficient for cell operation, and that at the cathode does not affect either the flow characteristics in the anode channel or the cell performance. 3) In the case of serpentine-dual flow field, when both phases of the two-phase flow are assumed laminar, the pressure drop increase due to the increase in gas production can be explained relatively well by the LockharteMartinelli method with the Chisholm parameter. This method can thus be applied to a general large-scale cell, even when the cell has a parallel flow field. 4) Ratio of the flow rate of the circulating water to the amount of water needed for anode reaction can be used as criterion to provide the appropriate flow rate for the circulating water. The flow rate of circulating water should be determined based on a given operating current density and a proper value of z (i.e., 5).

Acknowledgements The authors gratefully acknowledge the financial support from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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