Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis

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Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis Munazza Mohsin a,d,**, Rizwan Raza c,*, M. Mohsin-ul-Mulk b, Abida Yousaf a, Viktor Hacker d a

Department of Physics, Lahore College for Women University, Lahore, 54000, Pakistan Department of Management Sciences, COMSATS University Islamabad, Lahore Campus, Lahore, 54000, Pakistan c Clean Energy Research Lab (CERL), Department of Physics, COMSATS University Islamabad, Lahore Campus, Lahore, 54000, Pakistan d Fuel Cell Laboratory, Institute of Chemical Engineering and Environmental Technology, Graz University of Technology, 8010, Graz, Austria b

highlights  PEM assemblies were investigated through accelerated voltage cycling test.  Best results at different relative humidity at 70  C fuel cell temperature.  Limiting current density value through polarization curves.  Fuel cell performance is depressed by decreasing RH from 100 to 33%.

article info

abstract

Article history:

This paper presents the diagnostic results of single polymer electrolyte membrane fuel cell

Received 29 September 2018

assemblies characterized by polarization curves. Single PEM fuel cell assemblies were

Received in revised form

investigated through accelerated voltage cycling test at different values of relative hu-

3 August 2019

midity. The fuel cells are tested at different humidity level. The cells are discussed in this

Accepted 29 August 2019

paper with analysis results at different relative humidity at atmospheric pressure. This

Available online xxx

represents a nearly fully humidified, a moderately humidified, and a low humidified condition, respectively. This technique is useful for diagnosing the main sources of loss in MEA

Keywords:

development work, especially for high temperature/low relative humidity operation where

Fuel cell

several sources of loss are present simultaneously. All the fuel cells showed better per-

Polarization

formance in terms of limiting current density value through polarization curves when

Humidity

oxygen was fed to the cathode side of each cell instead of air. The results indicate that the

Electrode kinetics

performance of the fuel cell could be depressed significantly by decreasing RH from 100 to 33%. Decrease in RH can result in slower electrode kinetics, including electrode reaction and mass diffusion rates, and higher membrane resistance. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponsing author. ** Corresponsing author. Lahore College for Women University, Lahore, 54000, Pakistan. E-mail addresses: [email protected] (M. Mohsin), [email protected] (R. Raza). https://doi.org/10.1016/j.ijhydene.2019.08.246 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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Introduction Ever increasing population of the world, diminishing size of fossil fuel resources and rapid change in global climate due to the utilization effects of the fossil fuels have moved the world in burgeoning the demand for clean and sustainable energy [1,2] (see Table 1). Renewable Energy is the energy that will never run out. The main kinds of renewable energy sources are Wind, Solar, Geothermal, and Biomass. Hydrogen is a clean fuel and efficient energy storage medium for a wide variety of applications. Hydrogen can be produced from the water and other renewable sources. This is available in abundance in the universe [1]. Fuel Cell is the electrochemical device that converts chemical energy of the fuel (Hydrogen) into electricity at high efficiency without combustion. Fuel cells are viewed as viable power sources for many applications including transport, distributed power generations and portable electronics [3,4]. There are various types of fuel cells including Phosphoric Acid (PAFC), Molten Carbonate (MOFC), Alkaline (AFC), Solid Oxide (SOFC) and Proton Exchange Membrane fuel cell (PEMFC). These types are generally characterized by electrolyte (substance between cathode and anode) material, which serves as a bridge of the ion exchange that generates electric current. Electrolytes are very important material for any electrochemical device including PEM fuel cell [5,6]. PEM fuel cell is a low temperature (60  Ce80  C) fuel cell in which a proton conducting membrane is used as the electrolyte [7]. PEM is also known as the solid polymer or polymer electrolyte membrane fuel cell. A PEM fuel cell contains an electrolyte that is a layer of solid polymer (usually sulfonic acid, commercial name is Nafion TM) that allows proton to be transmitted from one face to the other. Among several types of fuel cells, the PEM fuel cell has been recommended as an alternative power source for electric vehicles and mobile phones due to its high efficiency [8,9]. The commercial breakthrough of fuel cell is hindered by the high price of fuel cell components. After basic study on PEM fuel cell the focus of research would be to lower price by developing new materials for electrode and membrane which ultimately would improve the performance of the fuel cell. Research and development on low cost electrode is very important for reducing the ultimate cost of PEM fuel cell [10,11]. Among different types of fuel cells, the Polymer Electrolyte Membrane (PEM) fuel cells have the capability to convert the chemical energy of the fuel (hydrogen), effectively into electrical energy with the formation of pure water as the only byproduct. Their significant features of high-power density, low operating temperature, easy and fast start-up/shut-down processes

make PEM fuel cells a promising candidate to the next generation power source for transportation, portable and stationary applications [12,13]. In order to decrease the price of a cell the metal content of lower platinum in the catalyst layers and inexpensive bipolar plates are essential. The main factor limiting the lifetime of PEMFCs in terms of performance is the loss of active catalyst. But, acute letdown of the cell is usually caused by the failure of polymer electrolyte membrane (PEM) [14]. It is essential for the membrane to give low gas porousness, high proton conductivity and electrical protection. Further, it needs to oppose mechanical stress because of swelling and contracting during humidity cycling furthermore, breaking down due radical assault. Mechanical degradation can be smothered through joining of an e-PTFE support layer into the membrane [15,16]. One of the most essential issues which restrains the commercialization of PEMFC-based power frameworks is the water management in fuel cell. Numerous logical works are committed to examination of the impact of air relative humidity and the operational temperature on cell performance [23]. Among the studies in the literature, Xu H et al. investigated the 5 cm2 active area proton exchange membrane fuel cell and proved that by increasing the RH from 20% to 100% the cell performance was improved in terms of decrease in membrane resistance (from 0.407 U cm2 to 0.092 Ucm2) and electrode resistance (from 0.203 U cm2 to 0.041 U cm2) [2]. Zhang et al., characterized the cell by subjecting it under different symmetric RH conditions and concluded that by decreasing RH there was great decline in performance of fuel cell [17]. Yan et al., elaborated that the cathode RH has great impact on the performance of fuel cell as compare to the anode relative humidity [18]. Amirinejad M et al., reported the other way around that the anode RH has considerable effect on the performance of the fuel cell than the cathode RH by analyzing a 5 cm2 active area cell, subjected under different operating conditions [19]. Some more researcher has reported for high temperature-based PEM work tested a 5 cm2 active area PEM fuel cell at temperature from 75 to 120  C with less than 50% RH by using composite membrane and found better performance of Nafion/Zeolite composite membrane than pure Nafion Membrane [20]. It was observed that humidity changes at anode side has great impact on the performance of the fuel cell than that of cathode side [21]. It was analysed that the cell performance can be improved by increasing the anode RH [22]. In this paper the detail analysis of Polarization curves of PEM fuel cell and different losses has been addressed. Also, the comparison of different MEA at different values of relative humidity has been discussed.

Experimental Table 1 e Voltage Cycles Completed by F type Fuel Cells at different values of RH. Fuel Cell F F1 F2

% RH

Voltage Cycles Completed

92% 80% 90%

10,000 12,570 17,536

In this work, ten fuel cells, each of area 5 cm2 were assembled. In seven fuel cells (G, G1, G2, G3, G5, G6, & G7) MEA (X)-micro reinforced composite expanded polytetrafluoroethylene polymer electrolyte membrane was used while in other three cells F, F1 and F2 MEA (Y) a perfluorinated sulfonic acid membrane was used. MEA (X) has catalyst loading of 0.4 mg Pt/cm2 on both anode and cathode side and MEA (Y)

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has catalyst loading, 0.6 mg Pt/cm2 on cathode and 0.3 mg Pt/ cm2 on anode. MEAs were activated at 0.6 V (15min) and 0.4 V (30min) and OCV (1min) for 6e8 cycles. The cells were examined through the Accelerated Voltage Cycling Test. Before starting the potential cycling experiment each cell was characterized by recording the reference characterization curves. The activation of the MEA and the characterisation curves (Polarization Curve, EIS, Cyclic Voltammogram and Hydrogen Diffusion Measurement) of the cell are recorded by connecting the fuel cell to the characterisation test stand. All the characterisation curves were recorded by using cathode as working electrode and anode as counter electrode. The pressure of the gases was maintained between 1 and 2 bar, almost (1.5 bar) with stoichiometric coefficients of 1.5 for H2, 2 for O2 and 2.2 for air respectively. The fuel cell was operated at 70  C. The temperature of the humidifier was set at 68  C. Thus, the value of relative humidity (RH) applied for all characterization curves was 90%. The fuel cell was purged both on anode and cathode sides with dry and wet nitrogen before starting the experiment and after finishing the experiment. The experimental fuel cell testing rig is shown in Fig. 1. Then the fuel cell was connected to the other test stand for potential cycling tests. At this test stand a parallel connection of transistors was used with each fuel cell in order to change its voltage from 0.7 V to 0.9 V and vice versa after every 30 s. Therefore, one voltage cycle was set to complete in 1 min (See Fig. 2). The cells were tested at 70  C under H2/air (H2 at 50 ml/min and air at 100 ml/min for each fuel cell), and with different values of relative humidity. The pressure of the gases (H2, N2 and Air) was maintained between 1 and 2 bar (almost 1.5 bar). By setting the temperature of the humidifier at different values the potential cycling test series was conducted at different values of relative humidity.

0.9V IR-Free V

30s

0.7V

30s

Fig. 2 e Change in potential between 0.7 V and 0.9 V (1 voltage cycle ¼ 1 min).

During the test series each fuel cell was disconnected from the test stand and its characterisation curves were recorded after specific number of voltage cycles like 1000, 3000, 5000, 7500, 10000 and 15000 voltage cycles etc. After recording the characterisation curves, the cell was connected again to the other test stand for potential cycling test and this process was stopped when the cell potential became less than 0.9 V.

Results and discussion Electrochemical analysis Three fuel cells F, F1 and F2 were assembled by using MEA of type H25-F. The fuel cell F was tested at 92% RH, F1was operated at 80% RH and F2 was investigated at 90% RH. During the test series, after the completion of 1000, 3000, 5000, 7500, 10000 and 15000 voltage cycles the accelerated voltage cycling test was stopped and the fuel cells were characterised by polarisation curves, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The fuel cell F worked till 10,000 voltage cycles, F1 and F2 worked longer and stopped working after 12,570 voltage cycles and 17,536 voltage cycles respectively.

Polarization curves for F-Type fuel cell Polarization curves were recorded by feeding the cathode of the fuel cell with oxygen and then with air.

Polarization curve with oxygen supply at cathode Fuel cell F-92% RH. Fig. 3 illustrates the comparison of all

Fig. 1 e Single PEM fuel cell test stand.

polarization curves with oxygen at cathode for the fuel cell F. The graph indicates that the limiting current density with the fresh MEA is less than its value after 1000 voltage cycles but afterward the maximum current density decreases with the increase in the number of voltage cycles. The current density versus voltage curve at 10,000 voltage cycles shows the effect of mass transport limitation due to poor water balance. It is also visible from this figure that the current density versus potential curves, till 3,000 voltage cycles show large linear region of ohmic drop but the curves after 5000, 7500 and 10,000 voltage cycles exhibit a short region of ohmic losses with less values of limiting current densities. Fig. 3 also shows the power density curves for the fuel cell F with oxygen supply at its cathode. It is visible from the figure that the power density increased in the beginning from zero cycles to 1,000 voltage cycles. It might be due to proper humidification of the fuel cell at 1,000 VC. After that, the power density decreased until 7,500 VC.

Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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Current Density(A/cm2) PC-F- O2 before cycling

PC-F-O2- after 1000 cycles

PC-F-O2 after 3000 cycles

PC-F-O2 after 5000 cycles

PC-F-O2 after 7500 cycles

PC-F-O2 after 10000 cycles

PD-F-O2 before cycling

PD-F-O2 after 1000 cycles

PD-F-O2 after 3000 cycles

PD-F-O2 after 5000 cycles

PD-F-O2 after 7500 cycles

PD-F-O2 after 10000 cycles

Fig. 3 e Polarization Curves and power density curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Oxygen supply at cathode of the fuel cell F (92% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5cm2, Relative Humidity ¼ 92%, Pressure ¼ Ambient (1.013 bar). Since polarization curve and power density curve are two different expressive ways for showing the same behaviour of the fuel cell, therefore, for all other cells investigated by accelerated voltage cycling tests, only polarization curves were plotted and discussed.

Fuel cell F1-80% RH. Fig. 4 shows the comparison of all the Polarization Curves with oxygen at cathode for the fuel cell F1. The graph indicates that the current density increased until

3,000 voltage cycles. It means that at this stage the cell became completely activated. Then the value of the current density decreased (2.39 A/cm2 to 2.35 A/cm2 at 10,000 voltage cycles) but afterward there is a sudden and prominent decrease. Its value fell to 1.75 A/cm2 at 12,570 voltage cycles. The reason of this decrease was sudden accidently shut downed the humidifier heater and the heater for the cell. The fuel cell was operated the whole night with 25  C cell temperature and 22  C cathode humidifier temperature.

1 0.9

F1-O2 before cycling F1-O2 after 1000 VC F1-O2 after 3000 VC F1-O2 after 5000 VC F1-O2 after 7500 VC F1-O2 after 10000 VC F1-O2 after 12570 VC

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Current Density (A/cm2)

Fig. 4 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Oxygen supply at cathode of the fuel cell F1 (80% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 80%, Pressure ¼ Ambient (1.013 bar). Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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Fuel cell F2-90% RH. Fig. 5 shows all current densities/voltage curves for the fuel cell F2 when oxygen was supplied to the cathode of the fuel cell. Here again the current density of the fresh MEA was less than its value after 1,000 voltage cycles. After 1,000 VC the current density reached the maximum but decreased until 5,000 voltage cycles. The current density/ voltage curve after 7,500 voltage cycles operation showed zig zag changes in the ohmic loss portion due to membrane inefficiency. After that, the current density decreased until 17,536 voltage cycles. The current density versus voltage curve at 17,536 voltage cycles indicates a lot of activation losses as the curve intersects the voltage axis at 0.815 V instead of at 0.943 V (value for all other curves). This shows a prominent effect of catalyst loss after 17,536 voltage cycles. The reasons of the catalyst loss, nonlinear ohmic losses and low current densities were that the cell was operated between 15,000 and 17,536 voltage cycles at 25  C cell temperature instead of 70  C and that the cathode humidifier temperature was 22  C instead of 68  C. This happened accidently. Polarization curve with air supply at cathode Polarisation curves were also recorded when air was fed to the cathode side of the fuel cells F, F1 and F2.

Fuel cell F-92% RH. In the above Fig. 6 the Shift of the maximum current density and the maximum power density can be seen. The current density and power density of the fuel cell with the fresh MEA is less than after 1000 and 3000 voltage cycles. These maximum values are the signs of proper and complete humidification and activation of the MEA. Afterward the current density and power density decreased and at 10,000 voltage cycles the minimum values were obtained. The current density decreased from 2 A/cm2 to 1.47 A/cm2 and the power density from 0.6 W/cm2 to 0.3 W/cm2. It can also be observed from the current versus voltage curve after 10,000 voltage cycles that the open circuit voltage (OCV) decreased from its maximum value of 0.98 V to 0.91 V. This means that catalyst deteriorated.

Fuel cell F1-80% RH. Fig. 7 shows all current densities/voltage curves for the fuel cell F1 when air was supplied to the cathode of the fuel cell. The current density of the fresh MEA increased until 1,000 voltage cycles (1.25 A/cm2). Afterward, till 7,500 voltage cycles it stayed constant and it decreased slowly. A gross decrease was observed after 12,570 voltage cycles. The reason of the sharp reduction after 12,570 VC was that the fuel cell F1 was operated at cell temperature 25  C and cathode humidifier temperature 22  C due to the accidently shut down of the heaters. The decline of the OCV from 0.957 V to 0.863 V shows the catalyst degradation of the fuel cell F1 operated at 80% relative humidity. Fuel cell F2-90% RH. Fig. 8 illustrates all current density/ voltage curves for the fuel cell F2 when air was supplied to the cathode of the fuel cell. The value of the current density for the fresh MEA is large. Then it decreased with the increase of number of voltage cycles till 7500 voltage cycles. After 10,000 voltage cycles there is a sudden increase in the value of current density. This current density/voltage curve exhibits a large linear ohmic region. The curves after 15,000 and 17,536 voltage cycles also show the same characteristic with less value of maximum current density. The reduction in the value of maximum current density throughout the experiment is due to membrane deterioration. The decrease in the OCV from the start to the end (0.952 Ve0.857 V) of the experiments is a measure of catalyst degradation. Polarization curves for G-Type fuel cells Polarization curve with oxygen supply at cathode Fuel cell G-81% RH. The fuel cell G was operated at 81% relative humidity with 70  C cell temperature. Fig. 9 illustrates the comparison of all Polarization Curves with oxygen supply at cathode of the fuel cell G. The figure shows the decrease in the current density with the increase in the number of the voltage

1 F2-O2 before cycling F2-O2 after 1000 VC F2-O2 after 3000 VC F2-O2 after 5000 VC F2-O2 after 7500 VC F2-O2 after 10000 VC F2-O2 after 15000 VC F2-O2 after 17536 VC

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Fig. 5 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Oxygen supply at cathode of the fuel cell F2 (90% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ Ambient (1.013 bar). Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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Current Density(A/cm2) PC-F-Air before cycling

PC-F-Air after 1000 cycles

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PC-F-Air after 10000 cycles

PD-F-Air before cycling

PD-F-Air after 1000 cycles

PD-F-Air after 3000 cycles

PD-F-Air after 5000 cycles

PD-F-Air after 7500 cycles

PD-F-Air after 10000 cycles

Fig. 6 e Polarization Curves and power density curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Air supply at cathode side of the fuel cell F (92% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 92%, Pressure ¼ Ambient (1.013 bar).

1

F1-air before cycling F1-air after 1000 VC F1-air after 3000 VC F1-air after 5000 VC F1-air after 7500 VC F1-air after 10000 VC F1-air after 12570 VC

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Fig. 7 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Air supply at cathode side of the fuel cell F1 (80% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 80%, Pressure ¼ Ambient (1.013 bar).

cycles till 3000 voltage cycles. The current density versus voltage curve after 5000 voltage cycles indicates an activation loss as the curve intersects the voltage axis at 0.821 V instead of at 0.971 V (OCV value before cycling). This shows a prominent effect of catalyst loss after 5000 voltage cycles.

Fuel cell G1-90% RH. The fuel cell G1 was operated at 90% relative humidity. Fig. 10 shows all polarization curves with

oxygen feed at the cathode side of the fuel cell. The current density is maximum with the fresh MEA but a continuous decrease in the maximum current density can be observed with the increase of number of voltage cycles. A prominent decrease in the open circuit voltage (OCV) after 7500 voltage cycles is obvious from the Fig. 10. This shows the effect of catalyst loss during potential cycling test. The current density versus voltage curves after 5,000 and 7,500 voltage cycles also

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Fig. 8 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Air supply at cathode of the fuel cell F2 (90% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ Ambient (1.013 bar).

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Fig. 9 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Oxygen supply at cathode of the fuel cell G (81% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ Ambient (1.013 bar).

present the effect of mass transport limitation due to poor water balance.

Fuel cell G2-70% RH. Fig. 11 presents all the polarization curves when oxygen was supplied at the cathode of the fuel cell G2, tested at 70% RH. This cell worked till 3,000 VC. The current density is maximum as 2.96 A/cm2 before voltage cycling test but later it reduces with the enhancement in the number of voltage cycles. The current density versus voltage curve after 3,000 VC intersects the potential axis at lesser value (0.861 V) showing catalytic loss at that time. All three curves have maximum current densities at 0.15 V.

Fuel cell G3-90% RH. Details of all polarization curves with oxygen feed at the cathode of the fuel cell G3 operated at 90% RH, are shown in Fig. 12. The current density with the fresh MEA is maximum but there is gradual decrease in it with the increase in number of voltage cycles. The current density/ voltage characteristics of MEA till 1500 VC show almost the same behaviour, large linear ohmic region without diffusion losses, only with small decrease in the value of maximum current density at 0.15 V but afterward the drastic decrease in the current density after 2000, 3000, 5000 and 7350 voltage cycles can be observed as well as these curves show more diffusion losses due to mass transport limitations.

Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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Potential (V)

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Fig. 10 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Oxygen supply at cathode of the fuel cell G1 (90% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ Ambient (1.013 bar).

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Current Density (A/cm2) Fig. 11 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Oxygen supply at cathode of the fuel cell G2 (70% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 70%, Pressure ¼ Ambient (1.013 bar).

Fuel cell G5-90% RH. Fuel Cell G5 was tested at 90% RH. This cell worked till 5,960 voltage cycles. Fig. 13 represents all the current density versus potential curves for the fuel cell G5. The value of maximum current density is highest before potential cycling test and then a gradual decrease in it can be observed from the Figure. The current density/voltage curves for the fresh MEA and after 1,000 voltage cycles show long linear ohmic region without diffusion losses but afterward the curves show more diffusion losses than the ohmic losses due to poorer water balance and limited gas supply. Fuel cell G6-60% RH. The fuel cell G6 was operated at 60% relative humidity with 70  C cell temperature. This fuel cell worked till 5,500 voltage cycles. Fig. 14 illustrates the detail of all polarization curves for the fuel cell G6 with oxygen feed at the

cathode side of the cell. The current density is maximum for the fresh MEA but the current density/voltage curve before potential cycling test shows more diffusion losses due to temporarily limited gas supply. This effect of mass transport limitation is also prominent for the current density/voltage curve after 400 voltage cycles. Later on a gradual reduction in the value of maximum current density with the increase in the number of voltage cycles is elaborated from the graphs. The curves after 5000 and 5500 voltage cycles present more diffusion losses due to mass transport limitations than the ohmic losses. The fuel cell G7 was tested at 33% relative humidity with 70  C cell temperature through potential cycling test. This fuel cell worked till 1500 voltage cycles with the set parameters. All the polarization curves obtained with oxygen supply at the cathode side of the cell are presented in the Fig. 15. The graphs

Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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4.50

5.00

Current Density (A/cm2)

Fig. 12 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with Oxygen supply at cathode of the fuel cell G3 (90% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature ¼ 70  C, Area of fuel cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ Ambient (1.013 bar).

1.1

PC-O2-G5-before cycling

1

PC-O2-G5-after 1000 VC

Potential (V)

0.9

PC-O2-G5-after 1500 VC

0.8

PC-O2-G5-after 2000 VC

0.7

PC-O2-G5 after 3000 VC

0.6

PC-O2-G5-after 5000 VC

0.5

PC-O2-G5-after 5960 VC

0.4 0.3 0.2 0.1 0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

2

Current Density (A/cm ) Fig. 13 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with oxygen supply at the cathode of the fuel cell G5 (90%RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 68  C, Cell Temperature <80  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ ambient (1.013 bar).

indicate the maximum current density with the fresh MEA. Afterward a regular decrease in the current density value can be observed. The current density verses voltage curve after 1500 voltage cycles shows the mass transport limitation behaviour with less ohmic drop.

Polarization curve with air supply at cathode Polarization curves were also recorded for the fuel cells G, G1, G2, G3, G5, G6 and G7 when air was supplied to the cathode of these cells whereas anode of these cells were fed with the hydrogen.

Fuel cell G-81% RH. The fuel cell G was investigated at 81% relative humidity and worked till 5000 voltage cycles. Fig. 16 shows all polarization curves with air supply at the cathode of the fuel cell G. The value of maximum current density is highest (2.25 A/cm2) for fresh MEA but with the increase in number of potential cycles the decrease in maximum current density value can be observed from Fig. 17. The current density versus potential curve after 5,000 voltage cycles touches the potential axis at lower point showing less value of OCV (0.798 V). It means, total catalyst loss has been occurred.

Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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1.1 PC-O2-G6-before cycling

Potential (V)

1

PC-O2-G6 after 400 VC

0.9

PC-O2-G6 after 1500 VC

0.8

PC-O2-G6 after 2000 VC PC-O2-G6 after 3000 VC

0.7

PC-O2-G6 after 5000 VC PC-O2-G6 after 5500 VC

0.6 0.5 0.4 0.3 0.2 0.1 0 0.00

0.50

1.00

1.50

2.00

2.50

Current Density (A/cm²)

Fig. 14 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with oxygen supply at the cathode of the fuel cell G6 (60%RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 59  C, Cell Temperature ¼ 70  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 60%, Pressure ¼ ambient (1.013 bar).

Potential (V)

1 0.9 9

PC-O2-G7-before cycling

08 0.8

PC-O2-G7-after 400 VC

07 0.7

PC-O2-G7-after 1000 VC

06 0.6

PC-O2-G7-after 1500 VC

05 0.5 04 0.4 03 0.3 02 0.2 01 0.1 0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

Current Density (A/cm²) Fig. 15 e Comparison of all polarization curves with oxygen supply at the cathode of the fuel cell G7 (33%RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 47  C, Cell Temperature ¼ 70  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 33%, Pressure ¼ ambient (1.013 bar).

Fuel cell G1-90% RH. The fuel cell G1 operated at 90% RH. Fig. 17 illustrates the graphical representation of all the polarization curves. For this cell, the value of maximum current density is greatest before potential cycling test. A decline in the value of

maximum current density can be seen with the proceeding of potential cycles. The current density/voltage curve after 5000 voltage cycles shows the mass transport limitation effect. This curve exhibits more diffusion losses than the ohmic drop. The

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international journal of hydrogen energy xxx (xxxx) xxx

Potential (V)

Fig. 16 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with air supply at the cathode of the fuel cell G (81%RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 81%, Pressure ¼ ambient (1.013 bar).

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.00

G1-air before cycling G1-air after 3000 VC G1-air after 5000 VC G1-air after 7500 VC

0.50

1.00

1.50

Current Density (A/cm2)

2.00

2.50

Fig. 17 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with air supply at cathode of the fuel cell G1 (90% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 68  C, Cell Temperature <80  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ ambient (1.013 bar).

1

G2-air-before cycling

0.9

G2-air after 1000 Vc

Potential (V)

0.8

G2-air after 3000 VC

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.00

0.50

1.00

1.50

2.00

2.50

Current Density (A/cm2 ) Fig. 18 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with air supply at cathode of the fuel cell G2 (70% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Cell Temperature <80  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 70%, Pressure ¼ ambient (1.013 bar). Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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international journal of hydrogen energy xxx (xxxx) xxx

current density/voltage curve after 7500 voltage cycles also presents the same behaviour with least value of OCV (0.865 V) and current density (0.49 A).

Fuel cell G2-70% RH. Fuel cell G2 was tested at 70% relative humidity. Through potential cycling test this cell gave performance till 3,000 voltage cycles having decreasing trend in maximum current density value with the progress in number of potential cycles as mentioned in Fig. 18. Fuel cell G3-90% RH. The fuel cell G3 operated at 90% relative humidity, gave performance till 7,350 voltage cycles. Throughout the potential cycling experiment the OCV value remained ever 0.9 V. The maximum current density value is

greatest for the fresh MEA (2.52 A/cm2) but reduces with the course of the experiment. The current density versus potential curve after 7,350 voltage cycles exhibits more concentration losses due to catalytic loss and poor water balance (see Fig. 19).

Fuel cell G5-90% RH. Fuel Cell G5 was tested at 90% relative humidity. This cell worked till 5,960 voltage cycles. The value of maximum current density with the fresh MEA is less than its value after 1,000 voltage cycles. This show the complete and proper humidification and activation of the MEA at this stage. Then a continuous decrease in the maximum value of current density can be seen from Fig. 20.

1 PC-Air-G3-before cycling

0.9

PC-Air-G3-after 1000 VC

Potential (V)

0.8 0.7

PC-Air-G3-after 1500 VC PC-Air-G3-after 2000 VC

0.6

PC-Air-G3-after 3000 VC PC-Air-G3-after 5000 VC

0.5

PC-Air-G3-after 7350 VC

0.4 0.3 0.2 0.1 0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

Current Density (A/cm2)

Fig. 19 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with air supply at cathode of the fuel cell G3 (90% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 68  C Cell Temperature ¼ 70  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ ambient (1.013 bar).

1 PC-Air-G5-before cycling

0.9

PC-Air-G5-after 1000 VC

Potential (V)

0.8

PC-Air-G5-after 2000 VC

0.7

PC-Air-G5-after 3000 VC

0.6

PC-Air-G5-after 5000 VC

0.5

Pc-Air-G5 after 5960 VC

0.4 0.3 0.2 0.1 0 0.00

0.50

1.00

1.50

2.00

Current Density (A/cm2)

2.50

3.00

Fig. 20 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with air supply at cathode of the fuel cell G5 (90% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 68  C Cell Temperature ¼ 70  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 90%, Pressure ¼ ambient (1.013 bar). Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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international journal of hydrogen energy xxx (xxxx) xxx

1 PC-Air-G6-before cycling

0.9

PC-Air-G6-after 400 VC

0.8

PC-Air-G6-after 1000 VC

Potential (V)

0.7

PC-Air-G6-after 1500 VC

0.6

PC-Air-G6-after 3000 VC

0.5

PC-Air-G6-after 5000 VC

0.4

PC-Air-G6-after 5500 VC

0.3 0.2 0.1 0 0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

Current Density (A/cm2)

Fig. 21 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with air supply at cathode of the fuel cell G6 (60% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 59  C Cell Temperature ¼ 70  C, Area of Fuel Cell ¼ 5 cm2, Relative Humidity ¼ 60%, Pressure ¼ ambient (1.013 bar).

The current density versus potential curves till 2,000 voltage cycles show more linear ohmic losses but afterward the curves exhibit more concentration losses. The current density/voltage curve after 5,960 voltage cycles does not show any ohmic drop, only mass transport losses exist with poor water management. It means that total catalyst has been lost at this stage as the curve ends with the least value of maximum current density (0.21 A/ cm2) at 0.15 V.

before potential cycling test. It means the MEA became fully activated at that time, but the maximum current density value is not high (1.66 A/cm2) in Fig. 21 due to low value of relative humidity (60% RH). Later, there is a gradual reduction in the maximum current density value with the course of the experiment. Also, with the increase in the number of potential cycles, the current density verses potential curves show less ohmic drop but more mass transport limitation effect (Fig. 2).

Fuel cell G6-60% RH. Fuel cell G6, operated at 60% relative humidity, worked till 5,500 voltage cycles. This cell gave maximum performance after 400 voltage cycles instead of

Fuel cell G7-33% RH. The fuel cell G7 was operated at 33% relative humidity and with all other similar parameters this cell worked for just 1,500 voltage cycles. The current density

Potential (V)

1 0.9 9

PC_Air-G7-before cycling

08 0.8

PC-Air-G7-after 400 VC

07 0.7

PC-Air-G7-after 1000 VC

0.6 0 6

PC-Air-G7-after 1500 VC

0.5 0 5 0.4 0 4 03 0.3 02 0.2 01 0.1 0 0.00

0.50

1.00

1.50

2.00

2.50

Current Density (A/cm²) Fig. 22 e Polarization Curves after completion of specific number of voltage cycles (as mentioned inside the figure) with air supply at the cathode of the fuel cell G7 (33% RH), Stoichiometry (H2 ¼ 1.5, O2 ¼ 2.0, Air ¼ 2.2), Humidifier Temperature ¼ 47  C, Cell Temperature ¼ 70  C, Area of Fuel cell ¼ 5 cm2, Relative Humidity ¼ 33%, Pressure ¼ ambient (1.013 bar). Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246

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international journal of hydrogen energy xxx (xxxx) xxx

versus potential curve after 1,500 voltage cycles shows the least value of current density (see Fig. 22). [3]

Conclusion The Accelerated Voltage Cycling Test proved that lifespan of the fuel cells analyzed at 90% RH was proved to be maximum. Three fuel cells were assembled with F type MEA. Fuel cells F (92% RH), F1 (80% RH) and F2 (90% RH) worked for 10,000, 12570 and 17536 voltage cycles respectively. Among these three fuel cells F, F1 and F2, the lifetime of the fuel cell F2 operated at 90% relative humidity is proved as longest. Seven fuel cells assembled with same G type of MEA were investigated at different values of relative humidity. The fuel cells G, G2, G6 and G7 were operated at 81%, 70%, 60% and 33% relative humidity respectively whereas the fuel cells G1, G3, and G5 were tested at 90% RH. Among them G1 and G3 has maximum lifetime, more than seven thousand voltage cycles i.e., 7500 and 7350 VC respectively. On the whole the fuel cells tested at 90% RH show better performance. Fuel cell G7 was operated at 33% RH exhibited minimum lifespan just 1500 VC. All the fuel cells showed better performance in terms of limiting current density value through polarization curves when oxygen was fed to the cathode side of each cell instead of air. The limiting current density values did not reach 2 A/cm2 when the F type Fuel cell was examined with air at cathode whereas with oxygen at cathode these values exceeded 2 A/ cm2. For G type Fuel Cell the limiting current density value with O2 feed at cathode side of the cell is 4.46 A/cm2 whereas with air feed it is less than 3 A/cm2. Life time of thicker MEA (50 mm membrane thickness) and with higher Pt loading (0.6 mg/cm2) on cathode side was proved to be considerably higher than the life time of the thinner MEA (18 mm membrane thickness) with less amount of Pt loading (0.4 mg/cm2) on the cathode side. The maximum value of limiting current density was obtained for thinner MEA with less Pt loading. Fuel starvation was proved to be a significant reason of severe and permanent damage to the electrocatalyst of the fuel cell and ultimate performance degradation of the fuel cell.

[4]

[5]

[6]

[7] [8]

[9]

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[14]

[15]

[16]

Acknowledgement Author acknowledged the Higher Education Commission of Pakistan (HEC) for PhD scholarship.

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Please cite this article as: Mohsin M et al., Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.08.246