Internal Microscopic Diagnosis of Accelerated Aging of Proton Exchange Membrane Water Electrolysis Cell Stack

The hydrogen production reaction of the proton exchange membrane (PEM) water electrolysis cell stack is the reverse reaction of the fuel cell, but the water electrolysis operation requires high pressure, and the high pressure decomposes hydrogen molecules, thus aging or causing failure in the water electrolysis cell stack. In addition, there are five important physical parameters (current, voltage, flow, pressure and temperature) inside the water electrolysis cell stack, which can change the performance and shorten the life of the cell stack. However, the present techniques obtain data only by external simulation or single measurement; they cannot collect the internal real data in operation instantly and accurately. This study discusses the causes for aging or failure, and develops an internal real-time microscopic diagnosis tool for accelerated aging of the PEM water electrolysis cell stack. A flexible integrated (current, voltage, flow, pressure and temperature) microsensor applicable to the inside (high voltage and electrochemical environment) of the PEM water electrolysis cell stack is developed by using micro-electro-mechanical systems (MEMS) technology; it is embedded in the PEM water electrolysis cell stack for microscopic diagnosis of accelerated aging, and 100-h durability and reliability tests are performed. The distribution of important physical parameters inside the PEM water electrolysis cell stack can be measured instantly and accurately, so as to adjust it to the optimal operating conditions, and the local aging and failure problems are discussed.


Introduction
With the progress of human civilization, as energy becomes more and more important, the demand increases greatly, and people use fossil fuels, such as petroleum, natural gas and coal, to meet the huge demand, so that a lot of CO 2 is emitted, which induces global warming. Therefore, renewable energy has become the key point of current research, including hydrogen energy, solar energy, hydraulic power, wind power, geothermal, tide and biomass energy. Hydrogen is one of the optimal sustainable energy carriers, and hydrogen is a zero carbon energy carrier; it is applicable to low-carbon transportation and industrial decarbonization. Generally, burning fossil fuels to produce hydrogen will generate a lot of CO 2 , so carbon-free hydrogen production is the present research objective. Electrolytic technology can transform low-carbon electricity, which is unlikely to be stored and transmitted into hydrogen. It is friendlier to the environment than traditional hydrogen production methods (e.g., coal gasification and natural gas reforming). In addition, the intermittent power problems in wind energy and solar energy can be solved, and hydrogen is generated continuously, so as to relieve the outage problem. electrolysis cell will influence its performance [19]. However, the present techniques obtain data only by external simulation or single measurement; they cannot collect the internal real data in operation instantly and accurately. This study uses MEMS technology to further develop a flexible integrated (current, voltage, flow, pressure and temperature) microsensor applicable to the inside of the PEM water electrolysis cell stack, which is embedded in the PEM water electrolysis cell stack for microscopic diagnosis of accelerated aging.

Research Methods
The micro current, voltage, flow, pressure and temperature sensors are integrated into a 5-in-1 microsensor, as shown in Figure 1, by using MEMS technology, which is embedded in the PEM water electrolysis cell stack; the local distributions of internal current, voltage, flow, pressure and temperature are measured simultaneously, and a 100-h accelerated aging test is performed. Temperature sensing area is 750 µm × 600 µm; voltage sensing area is 600 µm × 600 µm; current sensing area is 600 µm × 600 µm; flow sensing area is 750 µm × 600 µm and pressure sensing area is 850 µm × 850 µm.
Micromachines 2020, 11, x 3 of 13 material optimization, especially film thickness, have now reached a critical point, requiring a lot of effort to make small improvements [14][15][16][17][18]. The current, voltage, temperature and flow inside the PEM water electrolysis cell will influence its performance [19]. However, the present techniques obtain data only by external simulation or single measurement; they cannot collect the internal real data in operation instantly and accurately. This study uses MEMS technology to further develop a flexible integrated (current, voltage, flow, pressure and temperature) microsensor applicable to the inside of the PEM water electrolysis cell stack, which is embedded in the PEM water electrolysis cell stack for microscopic diagnosis of accelerated aging.

Research Methods
The micro current, voltage, flow, pressure and temperature sensors are integrated into a 5-in-1 microsensor, as shown in Figure 1, by using MEMS technology, which is embedded in the PEM water electrolysis cell stack; the local distributions of internal current, voltage, flow, pressure and temperature are measured simultaneously, and a 100-h accelerated aging test is performed. Temperature sensing area is 750 μm × 600 μm; voltage sensing area is 600 μm × 600 μm; current sensing area is 600 μm × 600 μm; flow sensing area is 750 μm × 600 μm and pressure sensing area is 850 μm × 850 μm.

Sensing Principle of Micro Temperature Sensor
The sensing principle of micro temperature sensor is that when the ambient temperature rises, as Au has a Positive Temperature Coefficient (PTC), the resistivity increases with temperature; this characteristic results from the "temperature coefficient of resistance" (TCR) of the conductor.

Sensing Principle of Micro Humidity Sensor
The electrode form of the capacitive micro humidity sensor used in this study is the interdigitated electrode structure. There is a humidity sensitive thin film above the electrode. When the water vapor absorbed by the humidity sensitive thin film increases, the dielectric constant increases with ambient humidity; the humidity can be obtained by calculating the changed capacitance value.

Process Development of Flexible Integrated Microsensor
The current, voltage, flow, pressure and temperature sensing structures are integrated by using MEMS technology in this study. The process of the flexible integrated microsensor equipment is shown in Figure 2.
(a) PI thin film cleaning

Sensing Principle of Micro Temperature Sensor
The sensing principle of micro temperature sensor is that when the ambient temperature rises, as Au has a Positive Temperature Coefficient (PTC), the resistivity increases with temperature; this characteristic results from the "temperature coefficient of resistance" (TCR) of the conductor.

Sensing Principle of Micro Humidity Sensor
The electrode form of the capacitive micro humidity sensor used in this study is the interdigitated electrode structure. There is a humidity sensitive thin film above the electrode. When the water vapor absorbed by the humidity sensitive thin film increases, the dielectric constant increases with ambient humidity; the humidity can be obtained by calculating the changed capacitance value.

Process Development of Flexible Integrated Microsensor
The current, voltage, flow, pressure and temperature sensing structures are integrated by using MEMS technology in this study. The process of the flexible integrated microsensor equipment is shown in Figure 2. The PI (Polyimide) thin film substrate is cleaned with organic solvent ethanol, and then it is put in the hot organic solvent acetone, and the acetone is volatilized.
The metal is evaporated by electronic beam evaporator as shown in Figure 3; the 100 Å thick Cr and 1000 Å Au are deposited at deposition rate of 0.1 Å/s.

(c) Photolithography
The positive photoresist is coated on the sample uniformly by spin coater, as shown in Figure 4, and the pattern of integrated microsensor is defined by using photolithography, as shown in Figure  5.  The PI (Polyimide) thin film substrate is cleaned with organic solvent ethanol, and then it is put in the hot organic solvent acetone, and the acetone is volatilized.
The metal is evaporated by electronic beam evaporator as shown in Figure 3; the 100 Å thick Cr and 1000 Å Au are deposited at deposition rate of 0.1 Å/s. The PI (Polyimide) thin film substrate is cleaned with organic solvent ethanol, and then it is put in the hot organic solvent acetone, and the acetone is volatilized.
The metal is evaporated by electronic beam evaporator as shown in Figure 3; the 100 Å thick Cr and 1000 Å Au are deposited at deposition rate of 0.1 Å/s.

(c) Photolithography
The positive photoresist is coated on the sample uniformly by spin coater, as shown in Figure 4, and the pattern of integrated microsensor is defined by using photolithography, as shown in Figure  5.

(c) Photolithography
The positive photoresist is coated on the sample uniformly by spin coater, as shown in Figure 4, and the pattern of integrated microsensor is defined by using photolithography, as shown in Figure 5.
Cr-7T Cr etching solutions, manufactured by OM Group Incorporated (Cleveland, OH, USA), and then the photoresist is removed by acetone. The pattern of the dielectric layer is defined through the aforesaid photolithography process, the Cr/Au metal is evaporated again, and the unwanted metal is removed by wet etching process. Finally, the LTC9320 (FUJIFILM Electronic Materials Co, Hsin-Chu, Taiwan) with high mechanical strength and fit for high chemical environment is used as a protection layer, so as to avoid the flexible integrated microsensor being destroyed by the closing pressure of end plate inside the PEM water electrolysis cell stack. Afterwards, the sensing area of micro voltage and current sensors is defined by using the photolithography process. The stereogram of the flexible integrated microsensor is shown in Figure 6.   Cr-7T Cr etching solutions, manufactured by OM Group Incorporated (Cleveland, OH, USA), and then the photoresist is removed by acetone. The pattern of the dielectric layer is defined through the aforesaid photolithography process, the Cr/Au metal is evaporated again, and the unwanted metal is removed by wet etching process. Finally, the LTC9320 (FUJIFILM Electronic Materials Co, Hsin-Chu, Taiwan) with high mechanical strength and fit for high chemical environment is used as a protection layer, so as to avoid the flexible integrated microsensor being destroyed by the closing pressure of end plate inside the PEM water electrolysis cell stack. Afterwards, the sensing area of micro voltage and current sensors is defined by using the photolithography process. The stereogram of the flexible integrated microsensor is shown in Figure 6.   The pattern of the dielectric layer is defined through the aforesaid photolithography process, the Cr/Au metal is evaporated again, and the unwanted metal is removed by wet etching process. Finally, the LTC9320 (FUJIFILM Electronic Materials Co, Hsin-Chu, Taiwan) with high mechanical strength and fit for high chemical environment is used as a protection layer, so as to avoid the flexible integrated microsensor being destroyed by the closing pressure of end plate inside the PEM water electrolysis cell stack. Afterwards, the sensing area of micro voltage and current sensors is defined by using the photolithography process. The stereogram of the flexible integrated microsensor is shown in Figure 6.

Correction of Flexible Integrated Microsensor
After the sensing structure is successfully integrated into the PI film, the temperature and flow of flexible integrated microsensor are corrected, and the reliability is validated. The flexible integrated microsensor is connected to NI PXI 2575 data capture equipment of National Instruments (NI, Austin, TX, USA) to capture the microsensor data instantly, as well as the resistance and current of micro temperature and flow sensors. The system is measured and controlled by LabVIEW system design software, and the signals are processed and analyzed and exported to the computer. The correction curves of temperature and flow are drawn, respectively, as shown in

Correction of Flexible Integrated Microsensor
After the sensing structure is successfully integrated into the PI film, the temperature and flow of flexible integrated microsensor are corrected, and the reliability is validated. The flexible integrated microsensor is connected to NI PXI 2575 data capture equipment of National Instruments (NI, Austin, TX, USA) to capture the microsensor data instantly, as well as the resistance and current of micro temperature and flow sensors. The system is measured and controlled by LabVIEW system design software, and the signals are processed and analyzed and exported to the computer. The correction curves of temperature and flow are drawn, respectively, as shown in Figures 7 and 8.

Correction of Flexible Integrated Microsensor
After the sensing structure is successfully integrated into the PI film, the temperature and flow of flexible integrated microsensor are corrected, and the reliability is validated. The flexible integrated microsensor is connected to NI PXI 2575 data capture equipment of National Instruments (NI, Austin, TX, USA) to capture the microsensor data instantly, as well as the resistance and current of micro temperature and flow sensors. The system is measured and controlled by LabVIEW system design software, and the signals are processed and analyzed and exported to the computer. The correction curves of temperature and flow are drawn, respectively, as shown in Figures 7 and 8.

Accelerated Aging Test for PEM Water Electrolysis Cell Stack
In order to observe the local physical change inside the PEM water electrolysis cell stack, the flexible integrated microsensor is embedded above the runner plate of PEM water electrolysis cell stack without influencing the performance of the PEM water electrolysis cell stack (Figures 9 and  10). Finally, the PEM water electrolysis cell stack is closed uniformly by the closing pressure of the end plate, so that the signals of the flexible integrated microsensor can be captured stably. The flexible integrated microsensor is embedded in the PEM water electrolysis cell stack for a 100-h accelerated aging test, so as to understand the real-time microscopic distribution of important physical quantities from activation to aging of the PEM water electrolysis cell stack.

Accelerated Aging Test for PEM Water Electrolysis Cell Stack
In order to observe the local physical change inside the PEM water electrolysis cell stack, the flexible integrated microsensor is embedded above the runner plate of PEM water electrolysis cell stack without influencing the performance of the PEM water electrolysis cell stack (Figures 9 and 10). Finally, the PEM water electrolysis cell stack is closed uniformly by the closing pressure of the end plate, so that the signals of the flexible integrated microsensor can be captured stably. The flexible integrated microsensor is embedded in the PEM water electrolysis cell stack for a 100-h accelerated aging test, so as to understand the real-time microscopic distribution of important physical quantities from activation to aging of the PEM water electrolysis cell stack.

Accelerated Aging Test for PEM Water Electrolysis Cell Stack
In order to observe the local physical change inside the PEM water electrolysis cell stack, the flexible integrated microsensor is embedded above the runner plate of PEM water electrolysis cell stack without influencing the performance of the PEM water electrolysis cell stack (Figures 9 and  10). Finally, the PEM water electrolysis cell stack is closed uniformly by the closing pressure of the end plate, so that the signals of the flexible integrated microsensor can be captured stably. The flexible integrated microsensor is embedded in the PEM water electrolysis cell stack for a 100-h accelerated aging test, so as to understand the real-time microscopic distribution of important physical quantities from activation to aging of the PEM water electrolysis cell stack.

Accelerated Aging Test for PEM Water Electrolysis Cell Stack
In order to observe the local physical change inside the PEM water electrolysis cell stack, the flexible integrated microsensor is embedded above the runner plate of PEM water electrolysis cell stack without influencing the performance of the PEM water electrolysis cell stack (Figures 9 and  10). Finally, the PEM water electrolysis cell stack is closed uniformly by the closing pressure of the end plate, so that the signals of the flexible integrated microsensor can be captured stably. The flexible integrated microsensor is embedded in the PEM water electrolysis cell stack for a 100-h accelerated aging test, so as to understand the real-time microscopic distribution of important physical quantities from activation to aging of the PEM water electrolysis cell stack.

100-h Accelerated Aging Test Conditions for PEM Water Electrolysis Cell Stack
The performance of the PEM water electrolysis cell stack is tested at different temperatures (85 • C, 90 • C and 95 • C), as shown in Figure 11. It is found that the performance is enhanced as the temperature rises, but the performance declines at the back end after 95 • C, so 95 • C and a high voltage of 3 V are selected as 100-h accelerated aging test conditions, because a voltage of more than 3V will damage the catalyst.

100-h Accelerated Aging Test Conditions for PEM Water Electrolysis Cell Stack
The performance of the PEM water electrolysis cell stack is tested at different temperatures (85 °C, 90 °C and 95 °C), as shown in Figure 11. It is found that the performance is enhanced as the temperature rises, but the performance declines at the back end after 95 °C, so 95 °C and a high voltage of 3 V are selected as 100-h accelerated aging test conditions, because a voltage of more than 3V will damage the catalyst.  Figure 12 shows the local temperature and voltage test for 100-h accelerated aging of the PEM water electrolysis cell stack. It is found that the temperature rises gradually with the operating time. The temperature at the downstream inlet is apparently higher than other regions. The temperature begins to fall at about 3 h; this may because mass inflow of fluid at the beginning of the reaction results in intense reaction, and a lot of gas is generated, inducing hot stack, and the fluid reduces the heat over time, so the temperature drops. The midstream temperature fluctuation is slighter than the other regions, meaning that the runner design is relatively free of the hot stack problem. The upstream is nearer to the outlet and close to air which dissipates heat, so the temperature is, relatively, the lowest. This result shows that the local heat distribution inside the PEM water electrolysis cell stack is more severe and fluctuates largely at the downstream inlet. The voltage distribution is steadier than the temperature distribution, and the temperature is higher only at the downstream inlet; this may be because the fluid is sufficient, inducing higher voltage. The midstream and upstream have lower voltage, which may be the result of the ohmic resistance and charge transfer resistance of the anode [8].

Figure 11.
Performance curves of the PEM water electrolysis cell stack at different operating temperatures. Figure 12 shows the local temperature and voltage test for 100-h accelerated aging of the PEM water electrolysis cell stack. It is found that the temperature rises gradually with the operating time. The temperature at the downstream inlet is apparently higher than other regions. The temperature begins to fall at about 3 h; this may because mass inflow of fluid at the beginning of the reaction results in intense reaction, and a lot of gas is generated, inducing hot stack, and the fluid reduces the heat over time, so the temperature drops. The midstream temperature fluctuation is slighter than the other regions, meaning that the runner design is relatively free of the hot stack problem. The upstream is nearer to the outlet and close to air which dissipates heat, so the temperature is, relatively, the lowest. This result shows that the local heat distribution inside the PEM water electrolysis cell stack is more severe and fluctuates largely at the downstream inlet. The voltage distribution is steadier than the temperature distribution, and the temperature is higher only at the downstream inlet; this may be because the fluid is sufficient, inducing higher voltage. The midstream and upstream have lower voltage, which may be the result of the ohmic resistance and charge transfer resistance of the anode [8].  Figure 13 shows the local current density test for 100-h accelerated aging of the PEM water electrolysis cell stack. It is observed that the downstream outlet has a slightly higher current density than the midstream and upstream outlets. The performance of the PEM water electrolysis cell stack reaches its maximum limit about 0.408184 A/cm 2 after about 40 min, and then the reaction is stabilized. The performance begins to decline at about 55 h, meaning the MEA (Membrane  Figure 13 shows the local current density test for 100-h accelerated aging of the PEM water electrolysis cell stack. It is observed that the downstream outlet has a slightly higher current density than the midstream and upstream outlets. The performance of the PEM water electrolysis cell stack reaches its maximum limit about 0.408184 A/cm 2 after about 40 min, and then the reaction is stabilized. The performance begins to decline at about 55 h, meaning the MEA (Membrane Electrode Assembly) and runner plate of PEM water electrolysis cell stack have aged gradually, the external glass tube with Deionized water (DI) water has been blackened, and the dissolved graphite has been discharged from the outlet, as shown in Figure 14.  Figure 13 shows the local current density test for 100-h accelerated aging of the PEM water electrolysis cell stack. It is observed that the downstream outlet has a slightly higher current density than the midstream and upstream outlets. The performance of the PEM water electrolysis cell stack reaches its maximum limit about 0.408184 A/cm 2 after about 40 min, and then the reaction is stabilized. The performance begins to decline at about 55 h, meaning the MEA (Membrane Electrode Assembly) and runner plate of PEM water electrolysis cell stack have aged gradually, the external glass tube with Deionized water (DI) water has been blackened, and the dissolved graphite has been discharged from the outlet, as shown in Figure 14.   Figure 15 shows the local flow test for 100-h accelerated aging of the PEM water electrolysis cell stack. It is observed that the downstream outlet has the maximum flow, the runners maintain stable flow before 40 h, and the midstream runner flow begins to decrease rapidly after 40 h. Thus, it can be seen that the blocking of the reaction between the downstream and midstream may be a symptom of aging, and the aging becomes increasingly severe with time. The deposition of graphite plate and carbon paper results in deceleration of the runner blocking flow. The PEM water electrolysis cell stack anode outlet diagram in Figure 16 shows that the black graphite blocks the runner.  Figure 15 shows the local flow test for 100-h accelerated aging of the PEM water electrolysis cell stack. It is observed that the downstream outlet has the maximum flow, the runners maintain stable flow before 40 h, and the midstream runner flow begins to decrease rapidly after 40 h. Thus, it can be seen that the blocking of the reaction between the downstream and midstream may be a symptom of aging, and the aging becomes increasingly severe with time. The deposition of graphite plate and carbon paper results in deceleration of the runner blocking flow. The PEM water electrolysis cell stack anode outlet diagram in Figure 16 shows that the black graphite blocks the runner. cell stack. It is observed that the downstream outlet has the maximum flow, the runners maintain stable flow before 40 h, and the midstream runner flow begins to decrease rapidly after 40 h. Thus, it can be seen that the blocking of the reaction between the downstream and midstream may be a symptom of aging, and the aging becomes increasingly severe with time. The deposition of graphite plate and carbon paper results in deceleration of the runner blocking flow. The PEM water electrolysis cell stack anode outlet diagram in Figure 16 shows that the black graphite blocks the runner.   cell stack. It is observed that the downstream outlet has the maximum flow, the runners maintain stable flow before 40 h, and the midstream runner flow begins to decrease rapidly after 40 h. Thus, it can be seen that the blocking of the reaction between the downstream and midstream may be a symptom of aging, and the aging becomes increasingly severe with time. The deposition of graphite plate and carbon paper results in deceleration of the runner blocking flow. The PEM water electrolysis cell stack anode outlet diagram in Figure 16 shows that the black graphite blocks the runner.

Analysis of Internal Parts of PEM Water Electrolysis Cell Stack after Accelerated Aging Test
After the 100-h accelerated aging test for the PEM water electrolysis cell stack, the PEM water electrolysis cell stack is disassembled to ascertain the internal condition, as shown in Figures 17 and 18. It is obvious that the regions of the runner plate close to the midstream and downstream are corroded, meaning the reaction is intense.

Analysis of Internal Parts of PEM Water Electrolysis Cell Stack after Accelerated Aging Test
After the 100-h accelerated aging test for the PEM water electrolysis cell stack, the PEM water electrolysis cell stack is disassembled to ascertain the internal condition, as shown in Figures 17 and  18. It is obvious that the regions of the runner plate close to the midstream and downstream are corroded, meaning the reaction is intense.

Anode bipolar plate
Cathode bipolar plate

Conclusions
This study has successfully developed a flexible integrated microsensor by using MEMS technology. This flexible integrated microsensor has five sensing functions, it is resistant to high-temperature electrochemical environment, it can perform real-time measurement, and it can be placed in any position inside the PEM water electrolysis cell stack.
The flexible integrated microsensor was successfully embedded in the PEM water electrolysis cell stack, and the local important information inside the PEM water electrolysis cell stack was extracted successfully without influencing the operation. A 100-h accelerated aging test was performed. The test result showed that the key factors influencing the PEM water electrolysis cell stack were temperature and voltage. As the PEM water electrolysis cell stack aged internally, the

Conclusions
This study has successfully developed a flexible integrated microsensor by using MEMS technology. This flexible integrated microsensor has five sensing functions, it is resistant to high-temperature electrochemical environment, it can perform real-time measurement, and it can be placed in any position inside the PEM water electrolysis cell stack.
The flexible integrated microsensor was successfully embedded in the PEM water electrolysis cell stack, and the local important information inside the PEM water electrolysis cell stack was extracted successfully without influencing the operation. A 100-h accelerated aging test was performed. The test result showed that the key factors influencing the PEM water electrolysis cell stack were temperature and voltage. As the PEM water electrolysis cell stack aged internally, the nonuniform distribution of temperature and flow resulted in large temperature difference, lower current density, graphitization of the internal runner plate and carbon paper pyrolysis. The most severe condition occurred between the downstream inlet and the midstream.