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Development and application of flexible integrated microsensor as real-time monitoring tool in proton exchange membrane water electrolyzer
Renewable Energy 143 (2019) 906e914
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Development and application of flexible integrated microsensor as real-time monitoring tool in proton exchange membrane water electrolyzer Chi-Yuan Lee a, *, Chia-Hung Chen b, Shih-Chun Li a, Yu-Syuan Wang a a b
Department of Mechanical Engineering, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan, Taiwan, ROC HOMYTECH Global CO., LTD, Taoyuan, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2019 Received in revised form 16 April 2019 Accepted 16 May 2019 Available online 18 May 2019
The proton exchange membrane (PEM) water electrolyzer has such advantages as simple system, low operating temperature and small-scale hydrogen production according to real time requirement, and the hydrogen production process is clean, meeting the environmental requirements. The PEM water electrolysis hydrogen production is the reverse reaction of fuel cell, but the water electrolysis requires high operating voltage, the resistance is likely to generate a lot of waste heat, and the nonuniform current density results in hot spots, the internal temperature rises, accelerating the decomposition of hydrogen molecules, the water electrolyzer is likely to age and fail. In addition, four important physical parameters (temperature, flow, voltage and current) in the running water electrolyzer can influence its performance and life, but the present bottleneck is external, theoretical, simulated or single measurement, the authentic information in the water electrolyzer cannot be obtained accurately and instantly. This study uses micro-electro-mechanical systems (MEMS) technology to develop a flexible integrated (temperature, flow, voltage and current) microsensor applicable to the high voltage and electrochemical environment in water electrolyzer, which is integrated with a 20 mm thick polyimide (PI) film material. The real-time microscopic diagnosis and measurement in the PEM water electrolyzer can measure the internal local temperature, voltage, current and flow distribution uniformity instantly and accurately, so as to optimize the operating conditions and analysis. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Flexible integrated microsensor Real-time microscopic diagnosis PEM water electrolyzer
1. Introduction In recent years, renewable energy has evolved as a trend of new energy use, including solar energy, geothermal, wind power, hydraulic power, tide, biomass energy and hydrogen energy. The hydrogen is a colorless, tasteless, odorless and nontoxic flammable gas energy, it is a secondary energy, there is plenty of it in natural world, mostly forming compounds with other elements, such as oxygen and carbon, the end product of hydrogen energy use is water. In recent years, it is universally recognized that the hydrogen energy is a very promising alternative energy among clean energy sources to reduce the dependence on petroleum, air pollution and greenhouse gas emission [1]. At present, the hydrogen is derived
* Corresponding author. Department of Mechanical Engineering, Yuan Ze University, 135 Yuan-Tung Road, Chung-Li, 32003 Taoyuan, Taiwan, ROC. E-mail address: [email protected] (C.-Y. Lee). https://doi.org/10.1016/j.renene.2019.05.071 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
mainly from fossil material recombination, coal gasification, industrial byproducts and water electrolysis. The new hydrogen production methods include photocatalytic water decomposition, biomass pyrolysis, microbial fermentation and high temperature water splitting. However, the advanced hydrogen production method is still in the development phase, there is still a long distance to commercialization. Therefore, in the next 20e30 years, the hydrogen will be produced still by conventional methods [2]. In comparison to conventional water electrolysis, the PEM water electrolyzer has such advantages as high energy efficiency, high production, simple and safe system and low operating temperature [3]. However, the physical quantities in the running PEM water electrolyzer have a key influence on its performance and life. For example, the hydrogen production efficiency of water electrolyzer is reduced if the flow is too low, and the waterproof quality of anticorrosion microporous layer fails if the flow distribution is nonuniform, the anticorrosion microporous layer is damaged, so that the overall impedance increases and the performance
C.-Y. Lee et al. / Renewable Energy 143 (2019) 906e914
degrades. According to references, the operating temperature is a key factor in the performance of water electrolyzer. If the temperature is too lower than the working temperature of membrane electrode assembly (MEA), the performance of PEM water electrolyzer cannot be enhanced. The MEA may be damaged if the temperature is too high, and the material of bipolar plate can be corroded and dissolved in the harsh high voltage chemical environment [4e8]. Therefore, this study plans to develop a flexible integrated microsensor which can measure the local temperature, flow, voltage and current operating conditions in the water electrolyzer instantly and accurately, and the internal information can be fed back instantly, so that the water electrolyzer control system can be adjusted to optimum operating parameters immediately.
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temperature can be expressed as Eq. (3).
Rt ¼ R0 ð1 þ a1 DT þ a2 DT þ a3 DT þ /Þ
(3)
DT ¼ t t0
(4)
2. Sensing principle of flexible integrated microsensor
wherein Rt is the resistance value (U) at t C; R0 is the resistance value (U) at 0 C; a1, a2 and a3 are the temperature coefficient of resistance (%/ C); DT is the temperature difference to the reference temperature 0 C; t is the temperature at t C; t0 is the temperature at 0 C. Eq. (3) shows the relationship between the temperature and resistance value of conductor is nonlinear, but if the conductor resistance value of RTD is in the linear range, Eq. (3) can be reduced to Eq. (5).
2.1. Micro temperature sensor
Rt ¼ R0 ð1 þ a1 DTÞ
The micro temperature sensor used in this study is a resistance temperature detector (RTD), the electrode is snake structure, as shown in Fig. 1. The RTD material is Au for its stable chemical properties, simple process, high linearity and low cost. The resistance value of general metallic conductors can be expressed as Eq. (1).
R¼ r
L A
wherein the physical significance of temperature coefficient a1 is the sensitivity of sensor, so Eq. (5) can be changed to Eq. (6).
a1 ¼ ðRt R0 Þ=R DT 0
(6)
wherein a1 is the sensitivity of sensor (1/ C) [9].
(1)
wherein r is the resistivity; L is the conductor length (m); A is the conductor cross-section area (m2). The resistance value of most metallic conductors increases with ambient temperature, this characteristic is the “temperature coefficient of resistance” of conductor, defined as Eq. (2).
a ¼
(5)
1 dr r0 dT
(2)
wherein a is the temperature coefficient of resistance; r0 is the resistivity at 0 C. Therefore, the relation between the resistivity of conductor and
2.2. Micro flow sensor This study uses hot-wire micro flow sensor, for simple structure, and the process, drive and signal output circuit design is easy, it has become the mainstream of research on micro flow sensor. The main measurement structure is resistance heater, the heat source is generated by constant voltage input, a stable temperature field is formed. In the flow field, the temperature field generated by the heater varies with the forced heat convection of fluid. If the external heat for heater is constant, the resistance value of heater decreases as the fluid flow increases. The heat supplied to the hot wire is controlled, the temperature difference between hot wire and flow remains fixed, the heating power increases with fluid flow, and the flow is converted into electric signal output by constant temperature circuit design. In short, the hot-wire flow sensor is designed based on the positive correlation between the thermal energy dissipation rate of hot wire and the fluid flow. The principle of hotwire micro flow sensor is shown in Fig. 2. According to King’s law, the relationship between thermal energy dissipation rate and fluid flow rate is expressed as Eq. (7).
Q ¼ Iw Rw ½a þ bðvÞ Tw Tg
Fig. 1. Schematic diagram of RTD structure.
(7)
wherein Q is the electric power from the external power supply; Iw is the electric current through hot wire; Rw is the resistance of hot wire; a is the heat transfer coefficient of fluid; b(v) is the convection coefficient of fluid; v is the fluid flow rate; Tw is the temperature of hot wire; Tg is the fluid temperature. However, the heat transfer reaction on the hot wire surface in the heat transfer process is very complex, so there is not yet a specific computing equation for deducing the relationship between average flow velocity and the complex reaction on hot wire surface. Therefore, the hot wire current is extracted directly in this study, the relationship between current and liquid flow is established, and the flow is not calculated from the power and temperature by Eq. (7), so that the error values resulted from equipment accuracy and calculation process can be reduced effectively [10].
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Fig. 4. Schematic diagram of sensing principle of micro current sensor.
Fig. 2. Sensing principle of hot-wire micro flow sensor.
2.3. Micro voltage sensor The micro voltage sensor used in this study is a miniaturized voltmeter probe, which is an extension conductor. A 600 mm 600 mm sensing area is exposed at the foremost end of micro voltage sensor, the rest of conductor is insulated by insulating layer, so as to make sure the detected voltage is from local location of water electrolyzer, the voltage extracted by the foil probe is from the foremost end of conductor. Fig. 3 is the schematic diagram of micro voltage sensor. 2.4. Micro current sensor The micro current sensor used in this study is a miniaturized galvanometer probe, which is an extension conductor, and a 600 mm 600 mm sensing area is exposed at the foremost end, the rest of conductor is insulated by insulating layer. The micro current sensor penetrates into the water electrolyzer, series connected to the external instrument to form a circuit, the internal current can be measured, as shown in Fig. 4. 3. Fabrication process of flexible integrated microsensor
Fig. 5. Optical microscope image of flexible integrated micro sensor.
study uses the PI film with small expansion coefficient as the flexible substrate of microsensor. 3.1. PI film cleaning and Cr/Au film evaporating First, the PI film substrate is cleaned in organic solvent ethanol, and then put in the preheated organic solvent acetone, the highly volatile organic solvent acetone is blown off by nitrogen gun. The metal is evaporated by EBS-500 electron-beam evaporator of JST company, and the metal evaporation source is heated. When the metal evaporation source temperature approaches the melting point, the evaporative capacity is very high, these evaporated metal particles are used to deposit metal film on the sample. 3.2. Photolithography The positive photoresist is coated on the sample uniformly by spin coater. The photolithography comprises soft bake, exposure and development steps, so as to define the pattern of the integrated
This study uses MEMS technology to integrate temperature, flow, voltage and current sensing structures. As the flexible integrated microsensor shall be soaked in DI water for a long time, this
Fig. 3. Schematic diagram of micro voltage sensor.
Fig. 6. Correction curves of micro temperature sensor.
C.-Y. Lee et al. / Renewable Energy 143 (2019) 906e914
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microsensor. 3.3. Au/Cr etching The photoresist film (AZP 4620) can form a wet etching mask after photolithography process, the commercial Type-TFA Au etching solution and Cr-7T Cr etching solution are used for etching Au/Cr to remove unwanted metal film, and then the photoresist film is removed by acetone. 3.4. Protection layer definition In order to avoid the flexible integrated microsensor being destroyed by the closing pressure of end plate when it is embedded in the PEM water electrolyzer, this study uses PI (PI 7920) with high mechanical strength which is applicable to high chemical environment as the flexible integrated microsensor protection layer, and uses photolithography process to define the sensing area of micro voltage and current sensors. The integrated flexible microsensor is shown in Fig. 5.
Fig. 7. Correction curves of micro flow sensor.
4. Reliability test for flexible integrated microsensor 4.1. Temperature correction In order to make the correction environment closer to the practical situation, this study uses program controlled constant temperature and humidity testing machine as the basis of correction environment. In the operation of water electrolyzer, the channel is full of DI water, so the humidity is fixed at 100% in the course of temperature correction. The temperature correction range of micro temperature sensor is 75 Ce120 C, the interval is 5 C. The NI PXI 2575 data capture equipment is used to extract 10 signals. The correction curves of micro temperature sensor are shown in Fig. 6, the correction curves are highly linear. 4.2. Flow calibration
Fig. 8. Flexible integrated microsensor embedding positions.
The flow correction range is 0e30 ml/min, measured once at intervals of 5 ml/min, the obtained correction curves of micro flow sensor are shown in Fig. 7, where It is the current measured when
Fig. 9. Upstream outlet temperature curve.
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Fig. 10. Midstream temperature curve.
Fig. 11. Downstream inlet temperature curve.
Fig. 12. Upstream outlet flow curve.
C.-Y. Lee et al. / Renewable Energy 143 (2019) 906e914
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Fig. 13. Midstream flow curve.
the flow is f, Ir is the current at the flow reference point. 4.3. Flexible integrated microsensor embedded in PEM water electrolyzer Because the cathode is a hydrogen generating reaction site and there is not much research on the cathodic reaction in the literature. The flexible integrated microsensor is embedded in the cathode flow channel, and the sensor is fixed to the runner plate. Finally, the PEM water electrolyzer is closed uniformly by the closing pressure of end plate, so that the signals of flexible integrated microsensor can be extracted stably. Fig. 8 shows the flexible integrated microsensor embedding positions.
tested for about one hour, the signal is captured every 5 s. The results are shown in Fig. 9, Fig. 10 and Fig. 11. It is observed that the temperature is lower than 80 C at first, because in order to make sure the water is free of impurities, the experimental equipment uses water excluded heating, insulated by a glass tube, due to the thermal conductivity of glass and the effect of constant temperature water tank, the temperature reaches 80 C in about 5 min. The downstream inlet temperature is high in the reaction process, because the water is injected in the downstream of PEM water electrolyzer, the water reacts a lot in downstream, the heat released is high, the upstream temperature drops gradually as the heat is transferred in water.
5.2. Local flow distribution of PEM water electrolyzer 5. Real-time microscopic diagnosis of PEM water electrolyzer 5.1. Local temperature distribution of PEM water electrolyzer The DI water at 80 C is admitted into the water electrolyzer,
Fig. 12, Fig. 13 and Fig. 14 show the local flow distribution of PEM water electrolyzer. It is observed that the flow rate at downstream inlet is higher at first, that at midstream and upstream outlet is lower. It may because of the design of channel, the fluid is
Fig. 14. Downstream inlet flow rate curve.
Fig. 15. Upstream outlet voltage curve.
Fig. 16. Midstream voltage curve.
Fig. 17. Downstream inlet voltage curve.
C.-Y. Lee et al. / Renewable Energy 143 (2019) 906e914
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electrolyzer. The operating voltage is set as 2.5 V in this study, the results are shown in Fig. 15, Fig. 16 and Fig. 17. It is observed that the voltage fluctuates around 2.5 V, the relative temperature is relatively stable, stable electrochemical reaction in the battery can be deduced.
5.4. Local current density distribution of PEM water electrolyzer
Fig. 18. Upstream outlet current density curve.
When rated voltage is given, the current density in PEM water electrolyzer displays local reaction. Fig. 18, Fig. 19 and Fig. 20 show the local current density distribution of PEM water electrolyzer. It is observed that the downstream inlet current density fluctuates largely, it may because the water flows fast at the inlet, the reaction is unstable. The current density at the upstream outlet is a little higher than that in midstream and downstream by about 0.178 A/ cm2, meaning the fluid is fully reacted in midstream and upstream.
6. Conclusion
Fig. 19. Midstream current density curve.
transferred smoothly and fast at the inlet of snakelike runner [11], it is stabilized at 8e10 with time. 5.3. Local voltage distribution of PEM water electrolyzer An external constant voltage is required for operating the water
This study uses MEMS technology to develop a high temperature electrochemical environment resistant flexible integrated microsensor applicable to the inside of PEM water electrolyzer, the micro temperature, flow, voltage and current sensors are integrated into a 20 mm thick PI film substrate successfully, and the PI (Fujifilm Durimide® PI 7920) resistant to the corrosion of electrochemical environment is used as protection layer. This flexible integrated microsensor has four functions, and such advantages as corrosion resistance, small size, high sensitivity, good temperature tolerance, real-time measurement and arbitrary placement. Three flexible integrated microsensors are embedded in the cathode channel plate of water electrolyzer successfully without influencing the operation of water electrolyzer, and the temperature and flow are corrected, and the correction curves are highly linear. The local temperature, flow, voltage and current information in the water electrolyzer is extracted successfully by NI PXI 2575 data acquisition unit in the operation process of water electrolyzer. The measured signal is exported from the data acquisition system to computer for analysis. The measured signal is expected to be fed back and controlled instantly by chip control system in the future, so as to prevent failure mode and to optimize performance.
Fig. 20. Downstream inlet current density curve.
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Acknowledgements This work was accomplished with much needed support and the authors would like to thank for the financial support by Ministry of Science and Technology of R.O.C. through the grant MOST 1072221-E-155-056, 106-2221-E-155-019, 104-2623-E-155-004-ET, 102-2221-E-155-033-MY3, 103-2622-E-155-006-CC2, 103-2622-E155-018-CC2, 104-2622-E-155-004, 104-2622-E-155-007-CC2, 105ET-E-155-002-ET, 105-2221-E-155-005-8, 105-2622-8-155-003TE3 and 105-2218-E-155-012. In addition, we would like to thank the YZU Fuel Cell Center and NENS Common Lab, for providing access to their research facilities. References [1] Key World Energy Statistics, 2017. [2] C. Hua, Y.H. Wu, Outline of Renewable Energy, Wu Nan Books, 2013. [3] C.Y. Lai, Social Technology Development Trend Analysis of Hydrogen, Science & Technology Policy Research and Information Center, 2016. [4] C. Rozain, P. Millet, Electrochemical characterization of polymer electrolyte
membrane water electrolysis cells, Electrochim. Acta 131 (2014) 160e167. [5] S. Siracusano, V. Baglio, N. Briguglio, G. Brunaccini, A.D. Blasi, A. Stassi, R. Ornelas, E. Trifoni, V. Antonucci, A.S. Arico, An electrochemical study of a PEM stack for water electrolysis, Int. J. Hydrogen Energy 37 (2012) 1939e1946. deau, N. Guillet, S. Chelghoum, D. Thoby, F. Fouda-Onana, [6] M. Chandesris, V. Me Membrane degradation in PEM water electrolyzer: numerical modeling and experimental evidence of the influence of temperature and current density, Int. J. Hydrogen Energy 40 (2014) 1353e1366. [7] F.E. Chakik, M. Kaddami, M. Mikou, Effect of operating parameters on hydrogen production by electrolysis of water, Int. J. Hydrogen Energy 42 (2017) 25550e25557. [8] J.S. Wilson, Sensor Technology Handbook, Butterworth-Heinemaann, 2005. [9] Q. Feng, X.Z. Yuan, G. Liu, B. Wei, Z. Zhang, H. Li, H. Wang, A review of proton exchange membrane water electrolysis on degradation mechanisms and mitigation strategies, J. Power Sources 366 (2017) 33e55. [10] F.H. Liu, Flexible Microsensor for Real-Time Microscopic Diagnosis in High Temperature Proton Exchange Membrane Fuel Cell Stack and Remote System Verification, Master’s Thesis, Mechanical Engineering Institute. Yuan Ze University, 2014. [11] H. Ito, T. Maeda, A. Nakano, Y. Hasegawa, N. Yokoi, C.M. Hwang, M. Ishida, A. Kato, T. Yoshida, Effect of flow regime of circulating water on a proton exchange membrane electrolyzer, Int. J. Hydrogen Energy 35 (2010) 9550e9560.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Development and application of flexible integrated microsensor as real-time monitoring tool in proton exchange membrane water electrolyzer Chi-Yuan Lee a, *, Chia-Hung Chen b, Shih-Chun Li a, Yu-Syuan Wang a a b
Department of Mechanical Engineering, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan, Taiwan, ROC HOMYTECH Global CO., LTD, Taoyuan, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2019 Received in revised form 16 April 2019 Accepted 16 May 2019 Available online 18 May 2019
The proton exchange membrane (PEM) water electrolyzer has such advantages as simple system, low operating temperature and small-scale hydrogen production according to real time requirement, and the hydrogen production process is clean, meeting the environmental requirements. The PEM water electrolysis hydrogen production is the reverse reaction of fuel cell, but the water electrolysis requires high operating voltage, the resistance is likely to generate a lot of waste heat, and the nonuniform current density results in hot spots, the internal temperature rises, accelerating the decomposition of hydrogen molecules, the water electrolyzer is likely to age and fail. In addition, four important physical parameters (temperature, flow, voltage and current) in the running water electrolyzer can influence its performance and life, but the present bottleneck is external, theoretical, simulated or single measurement, the authentic information in the water electrolyzer cannot be obtained accurately and instantly. This study uses micro-electro-mechanical systems (MEMS) technology to develop a flexible integrated (temperature, flow, voltage and current) microsensor applicable to the high voltage and electrochemical environment in water electrolyzer, which is integrated with a 20 mm thick polyimide (PI) film material. The real-time microscopic diagnosis and measurement in the PEM water electrolyzer can measure the internal local temperature, voltage, current and flow distribution uniformity instantly and accurately, so as to optimize the operating conditions and analysis. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Flexible integrated microsensor Real-time microscopic diagnosis PEM water electrolyzer
1. Introduction In recent years, renewable energy has evolved as a trend of new energy use, including solar energy, geothermal, wind power, hydraulic power, tide, biomass energy and hydrogen energy. The hydrogen is a colorless, tasteless, odorless and nontoxic flammable gas energy, it is a secondary energy, there is plenty of it in natural world, mostly forming compounds with other elements, such as oxygen and carbon, the end product of hydrogen energy use is water. In recent years, it is universally recognized that the hydrogen energy is a very promising alternative energy among clean energy sources to reduce the dependence on petroleum, air pollution and greenhouse gas emission [1]. At present, the hydrogen is derived
* Corresponding author. Department of Mechanical Engineering, Yuan Ze University, 135 Yuan-Tung Road, Chung-Li, 32003 Taoyuan, Taiwan, ROC. E-mail address: [email protected] (C.-Y. Lee). https://doi.org/10.1016/j.renene.2019.05.071 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
mainly from fossil material recombination, coal gasification, industrial byproducts and water electrolysis. The new hydrogen production methods include photocatalytic water decomposition, biomass pyrolysis, microbial fermentation and high temperature water splitting. However, the advanced hydrogen production method is still in the development phase, there is still a long distance to commercialization. Therefore, in the next 20e30 years, the hydrogen will be produced still by conventional methods [2]. In comparison to conventional water electrolysis, the PEM water electrolyzer has such advantages as high energy efficiency, high production, simple and safe system and low operating temperature [3]. However, the physical quantities in the running PEM water electrolyzer have a key influence on its performance and life. For example, the hydrogen production efficiency of water electrolyzer is reduced if the flow is too low, and the waterproof quality of anticorrosion microporous layer fails if the flow distribution is nonuniform, the anticorrosion microporous layer is damaged, so that the overall impedance increases and the performance
C.-Y. Lee et al. / Renewable Energy 143 (2019) 906e914
degrades. According to references, the operating temperature is a key factor in the performance of water electrolyzer. If the temperature is too lower than the working temperature of membrane electrode assembly (MEA), the performance of PEM water electrolyzer cannot be enhanced. The MEA may be damaged if the temperature is too high, and the material of bipolar plate can be corroded and dissolved in the harsh high voltage chemical environment [4e8]. Therefore, this study plans to develop a flexible integrated microsensor which can measure the local temperature, flow, voltage and current operating conditions in the water electrolyzer instantly and accurately, and the internal information can be fed back instantly, so that the water electrolyzer control system can be adjusted to optimum operating parameters immediately.
907
temperature can be expressed as Eq. (3).
Rt ¼ R0 ð1 þ a1 DT þ a2 DT þ a3 DT þ /Þ
(3)
DT ¼ t t0
(4)
2. Sensing principle of flexible integrated microsensor
wherein Rt is the resistance value (U) at t C; R0 is the resistance value (U) at 0 C; a1, a2 and a3 are the temperature coefficient of resistance (%/ C); DT is the temperature difference to the reference temperature 0 C; t is the temperature at t C; t0 is the temperature at 0 C. Eq. (3) shows the relationship between the temperature and resistance value of conductor is nonlinear, but if the conductor resistance value of RTD is in the linear range, Eq. (3) can be reduced to Eq. (5).
2.1. Micro temperature sensor
Rt ¼ R0 ð1 þ a1 DTÞ
The micro temperature sensor used in this study is a resistance temperature detector (RTD), the electrode is snake structure, as shown in Fig. 1. The RTD material is Au for its stable chemical properties, simple process, high linearity and low cost. The resistance value of general metallic conductors can be expressed as Eq. (1).
R¼ r
L A
wherein the physical significance of temperature coefficient a1 is the sensitivity of sensor, so Eq. (5) can be changed to Eq. (6).
a1 ¼ ðRt R0 Þ=R DT 0
(6)
wherein a1 is the sensitivity of sensor (1/ C) [9].
(1)
wherein r is the resistivity; L is the conductor length (m); A is the conductor cross-section area (m2). The resistance value of most metallic conductors increases with ambient temperature, this characteristic is the “temperature coefficient of resistance” of conductor, defined as Eq. (2).
a ¼
(5)
1 dr r0 dT
(2)
wherein a is the temperature coefficient of resistance; r0 is the resistivity at 0 C. Therefore, the relation between the resistivity of conductor and
2.2. Micro flow sensor This study uses hot-wire micro flow sensor, for simple structure, and the process, drive and signal output circuit design is easy, it has become the mainstream of research on micro flow sensor. The main measurement structure is resistance heater, the heat source is generated by constant voltage input, a stable temperature field is formed. In the flow field, the temperature field generated by the heater varies with the forced heat convection of fluid. If the external heat for heater is constant, the resistance value of heater decreases as the fluid flow increases. The heat supplied to the hot wire is controlled, the temperature difference between hot wire and flow remains fixed, the heating power increases with fluid flow, and the flow is converted into electric signal output by constant temperature circuit design. In short, the hot-wire flow sensor is designed based on the positive correlation between the thermal energy dissipation rate of hot wire and the fluid flow. The principle of hotwire micro flow sensor is shown in Fig. 2. According to King’s law, the relationship between thermal energy dissipation rate and fluid flow rate is expressed as Eq. (7).
Q ¼ Iw Rw ½a þ bðvÞ Tw Tg
Fig. 1. Schematic diagram of RTD structure.
(7)
wherein Q is the electric power from the external power supply; Iw is the electric current through hot wire; Rw is the resistance of hot wire; a is the heat transfer coefficient of fluid; b(v) is the convection coefficient of fluid; v is the fluid flow rate; Tw is the temperature of hot wire; Tg is the fluid temperature. However, the heat transfer reaction on the hot wire surface in the heat transfer process is very complex, so there is not yet a specific computing equation for deducing the relationship between average flow velocity and the complex reaction on hot wire surface. Therefore, the hot wire current is extracted directly in this study, the relationship between current and liquid flow is established, and the flow is not calculated from the power and temperature by Eq. (7), so that the error values resulted from equipment accuracy and calculation process can be reduced effectively [10].
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Fig. 4. Schematic diagram of sensing principle of micro current sensor.
Fig. 2. Sensing principle of hot-wire micro flow sensor.
2.3. Micro voltage sensor The micro voltage sensor used in this study is a miniaturized voltmeter probe, which is an extension conductor. A 600 mm 600 mm sensing area is exposed at the foremost end of micro voltage sensor, the rest of conductor is insulated by insulating layer, so as to make sure the detected voltage is from local location of water electrolyzer, the voltage extracted by the foil probe is from the foremost end of conductor. Fig. 3 is the schematic diagram of micro voltage sensor. 2.4. Micro current sensor The micro current sensor used in this study is a miniaturized galvanometer probe, which is an extension conductor, and a 600 mm 600 mm sensing area is exposed at the foremost end, the rest of conductor is insulated by insulating layer. The micro current sensor penetrates into the water electrolyzer, series connected to the external instrument to form a circuit, the internal current can be measured, as shown in Fig. 4. 3. Fabrication process of flexible integrated microsensor
Fig. 5. Optical microscope image of flexible integrated micro sensor.
study uses the PI film with small expansion coefficient as the flexible substrate of microsensor. 3.1. PI film cleaning and Cr/Au film evaporating First, the PI film substrate is cleaned in organic solvent ethanol, and then put in the preheated organic solvent acetone, the highly volatile organic solvent acetone is blown off by nitrogen gun. The metal is evaporated by EBS-500 electron-beam evaporator of JST company, and the metal evaporation source is heated. When the metal evaporation source temperature approaches the melting point, the evaporative capacity is very high, these evaporated metal particles are used to deposit metal film on the sample. 3.2. Photolithography The positive photoresist is coated on the sample uniformly by spin coater. The photolithography comprises soft bake, exposure and development steps, so as to define the pattern of the integrated
This study uses MEMS technology to integrate temperature, flow, voltage and current sensing structures. As the flexible integrated microsensor shall be soaked in DI water for a long time, this
Fig. 3. Schematic diagram of micro voltage sensor.
Fig. 6. Correction curves of micro temperature sensor.
C.-Y. Lee et al. / Renewable Energy 143 (2019) 906e914
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microsensor. 3.3. Au/Cr etching The photoresist film (AZP 4620) can form a wet etching mask after photolithography process, the commercial Type-TFA Au etching solution and Cr-7T Cr etching solution are used for etching Au/Cr to remove unwanted metal film, and then the photoresist film is removed by acetone. 3.4. Protection layer definition In order to avoid the flexible integrated microsensor being destroyed by the closing pressure of end plate when it is embedded in the PEM water electrolyzer, this study uses PI (PI 7920) with high mechanical strength which is applicable to high chemical environment as the flexible integrated microsensor protection layer, and uses photolithography process to define the sensing area of micro voltage and current sensors. The integrated flexible microsensor is shown in Fig. 5.
Fig. 7. Correction curves of micro flow sensor.
4. Reliability test for flexible integrated microsensor 4.1. Temperature correction In order to make the correction environment closer to the practical situation, this study uses program controlled constant temperature and humidity testing machine as the basis of correction environment. In the operation of water electrolyzer, the channel is full of DI water, so the humidity is fixed at 100% in the course of temperature correction. The temperature correction range of micro temperature sensor is 75 Ce120 C, the interval is 5 C. The NI PXI 2575 data capture equipment is used to extract 10 signals. The correction curves of micro temperature sensor are shown in Fig. 6, the correction curves are highly linear. 4.2. Flow calibration
Fig. 8. Flexible integrated microsensor embedding positions.
The flow correction range is 0e30 ml/min, measured once at intervals of 5 ml/min, the obtained correction curves of micro flow sensor are shown in Fig. 7, where It is the current measured when
Fig. 9. Upstream outlet temperature curve.
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Fig. 10. Midstream temperature curve.
Fig. 11. Downstream inlet temperature curve.
Fig. 12. Upstream outlet flow curve.
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Fig. 13. Midstream flow curve.
the flow is f, Ir is the current at the flow reference point. 4.3. Flexible integrated microsensor embedded in PEM water electrolyzer Because the cathode is a hydrogen generating reaction site and there is not much research on the cathodic reaction in the literature. The flexible integrated microsensor is embedded in the cathode flow channel, and the sensor is fixed to the runner plate. Finally, the PEM water electrolyzer is closed uniformly by the closing pressure of end plate, so that the signals of flexible integrated microsensor can be extracted stably. Fig. 8 shows the flexible integrated microsensor embedding positions.
tested for about one hour, the signal is captured every 5 s. The results are shown in Fig. 9, Fig. 10 and Fig. 11. It is observed that the temperature is lower than 80 C at first, because in order to make sure the water is free of impurities, the experimental equipment uses water excluded heating, insulated by a glass tube, due to the thermal conductivity of glass and the effect of constant temperature water tank, the temperature reaches 80 C in about 5 min. The downstream inlet temperature is high in the reaction process, because the water is injected in the downstream of PEM water electrolyzer, the water reacts a lot in downstream, the heat released is high, the upstream temperature drops gradually as the heat is transferred in water.
5.2. Local flow distribution of PEM water electrolyzer 5. Real-time microscopic diagnosis of PEM water electrolyzer 5.1. Local temperature distribution of PEM water electrolyzer The DI water at 80 C is admitted into the water electrolyzer,
Fig. 12, Fig. 13 and Fig. 14 show the local flow distribution of PEM water electrolyzer. It is observed that the flow rate at downstream inlet is higher at first, that at midstream and upstream outlet is lower. It may because of the design of channel, the fluid is
Fig. 14. Downstream inlet flow rate curve.
Fig. 15. Upstream outlet voltage curve.
Fig. 16. Midstream voltage curve.
Fig. 17. Downstream inlet voltage curve.
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electrolyzer. The operating voltage is set as 2.5 V in this study, the results are shown in Fig. 15, Fig. 16 and Fig. 17. It is observed that the voltage fluctuates around 2.5 V, the relative temperature is relatively stable, stable electrochemical reaction in the battery can be deduced.
5.4. Local current density distribution of PEM water electrolyzer
Fig. 18. Upstream outlet current density curve.
When rated voltage is given, the current density in PEM water electrolyzer displays local reaction. Fig. 18, Fig. 19 and Fig. 20 show the local current density distribution of PEM water electrolyzer. It is observed that the downstream inlet current density fluctuates largely, it may because the water flows fast at the inlet, the reaction is unstable. The current density at the upstream outlet is a little higher than that in midstream and downstream by about 0.178 A/ cm2, meaning the fluid is fully reacted in midstream and upstream.
6. Conclusion
Fig. 19. Midstream current density curve.
transferred smoothly and fast at the inlet of snakelike runner [11], it is stabilized at 8e10 with time. 5.3. Local voltage distribution of PEM water electrolyzer An external constant voltage is required for operating the water
This study uses MEMS technology to develop a high temperature electrochemical environment resistant flexible integrated microsensor applicable to the inside of PEM water electrolyzer, the micro temperature, flow, voltage and current sensors are integrated into a 20 mm thick PI film substrate successfully, and the PI (Fujifilm Durimide® PI 7920) resistant to the corrosion of electrochemical environment is used as protection layer. This flexible integrated microsensor has four functions, and such advantages as corrosion resistance, small size, high sensitivity, good temperature tolerance, real-time measurement and arbitrary placement. Three flexible integrated microsensors are embedded in the cathode channel plate of water electrolyzer successfully without influencing the operation of water electrolyzer, and the temperature and flow are corrected, and the correction curves are highly linear. The local temperature, flow, voltage and current information in the water electrolyzer is extracted successfully by NI PXI 2575 data acquisition unit in the operation process of water electrolyzer. The measured signal is exported from the data acquisition system to computer for analysis. The measured signal is expected to be fed back and controlled instantly by chip control system in the future, so as to prevent failure mode and to optimize performance.
Fig. 20. Downstream inlet current density curve.
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Acknowledgements This work was accomplished with much needed support and the authors would like to thank for the financial support by Ministry of Science and Technology of R.O.C. through the grant MOST 1072221-E-155-056, 106-2221-E-155-019, 104-2623-E-155-004-ET, 102-2221-E-155-033-MY3, 103-2622-E-155-006-CC2, 103-2622-E155-018-CC2, 104-2622-E-155-004, 104-2622-E-155-007-CC2, 105ET-E-155-002-ET, 105-2221-E-155-005-8, 105-2622-8-155-003TE3 and 105-2218-E-155-012. In addition, we would like to thank the YZU Fuel Cell Center and NENS Common Lab, for providing access to their research facilities. References [1] Key World Energy Statistics, 2017. [2] C. Hua, Y.H. Wu, Outline of Renewable Energy, Wu Nan Books, 2013. [3] C.Y. Lai, Social Technology Development Trend Analysis of Hydrogen, Science & Technology Policy Research and Information Center, 2016. [4] C. Rozain, P. Millet, Electrochemical characterization of polymer electrolyte
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