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Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of high-temperature polymer electrolyte membrane fuel cells
international journal of hydrogen energy xxx (xxxx) xxx
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Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of high-temperature polymer electrolyte membrane fuel cells Arvind Kannan, David Aili, Lars N. Cleemann, Qingfeng Li, Jens Oluf Jensen* Department of Energy Conversion and Storage, Technical University of Denmark, Fysikvej, Building 310, 2800 Kgs. Lyngby, Denmark
highlights
graphical abstract
Acid reservoir in acid doped HTPEM fuel cell. Significantly improved stability in the course of 10,000 h. Small degradation rates of 2.3 e4.1 mV h-1 at 180 C.
article info
abstract
Article history:
High-temperature polymer electrolyte membrane fuel cells with phosphoric acid doped
Received 21 May 2019
polybenzimidazole (PBI) are made with three-layered membranes. The central membrane
Received in revised form
layer is meant as an acid reservoir made from direct cast PBI with a higher acid content than
18 October 2019
the outermost layers, which are post doped membranes acting as barrier layers to limit the
Accepted 24 October 2019
acid transport out of the central layer. Cells with three-layered membranes and others with
Available online xxx
normal single layered membranes are tested at 170 and 180 C. At both temperatures, the cells with three-layered membranes show significantly lower voltage decay rates than the corre-
Keywords:
sponding cells with single-layered membranes. Post doped PBI membranes based on linear or
PEM fuel cell
thermally crosslinked PBI are used for the barrier layers of the three-layered membranes and
Polybenzimidazole
for the single-layered membranes in the test series at 180 C. The acid loss rates assessed by
Phosphoric acid reservoir
acid collection at the fuel cell exhaust, are rather comparable. At 180 C, the cells are tested for
Membranes
up to 10,000 h and voltage decay rates of 2.3 and 4.1 mVh-1 are measured for the cells with
Durability
three-layered membranes and 14 and 11 mVh-1 for cells with single-layered membranes. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J.O. Jensen). https://doi.org/10.1016/j.ijhydene.2019.10.186 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Introduction Polymer electrolyte membrane fuel cells (PEMFC) have attracted much attention due to their promise as energy conversion devices for both mobile and stationary applications [1,2]. High temperature PEMFC (HT-PEMFC), operating in the 120e200 C range, possess excellent tolerance to CO [3] from reformat hydrogen and allow for simplified water and thermal management [4]. They are typically constructed around a proton conducting membrane based on polybenzimidazole (PBI) imbibed with phosphoric acid. The proton transport between the electrodes is made possible by the phosphoric acid and the proton conductivity increases with increasing phosphoric acid content [5]. Phosphoric acid (PA) is much less volatile than water and that is the reason why the cells can be operated at higher temperature. However, PA is still mobile in the PBI membrane and the catalyst layers [6,7]. When the doped membrane is assembled with two gas diffusion electrodes forming a membrane-electrode-assembly (MEA), the doping acid redistributes within the MEA from the membrane to the electrodes [8]. In this way, proton conducting pathways in the catalyst layer and three-phase-boundary regions at the interface between the electrodes and the membrane are created. This acid redistribution phenomenon is often referred to as the break-in or activation process at start-up when the cell performance slowly improves to reach a steady state within up to a few hundred hours [9,10]. This is a highly desired process, but the wetting of the catalyst layer also lead to a large area exposure of the PA to evaporation and surface diffusion. Loss of doping acid by minute evaporation and diffusion processes are degradation mechanisms the HTPEMFC has in common with the phosphoric acid fuel cell. The transport pattern for PA in the cell is complex. Besides diffusion and evaporation, acid migration towards the anode is a phenomenon known from the phosphoric acid fuel cells. Part of the charge transport in the PA phase is assigned to negative hydrogen phosphate ions, which recombine with protons on the anode side resulting an accumulation of PA there. The effect has been verified in HT-PEMFC too [11,12]. The change of PA content on the anode side affects the performance due to changing degrees of wetting and flooding of the anode catalyst layer as reported by Halter et al. [13]. Thus, acid re-distribution or re-location takes place throughout the entire life of the cell. It initially secures the conditions for ionic conductivity in the catalyst layer during activation and later as a general feature playing a role in the acid loss mechanisms over long term [14,15]. Other mechanisms, like catalyst sintering [16,17] catalyst dissolution [18e20] and carbon support corrosion [21e23], the cell shares with the common low-temperature PEMFC. Acid loss is believed to be a dominating factor for performance loss during long term operation. In some studies, acid is collected from the exhaust gases of the fuel cell [24] and in that connection it should be stressed that only a fraction of the acid lost from the membrane and the catalyst layer is collected at the exit [15]. Fuel cell durability targets range from 10,000 h for automotive application to 80,000 h for stationary application [25].
For further development of HT-PEMFC, significant attention has been on understanding degradation and in development of strategies to improve durability [26]. Limiting the phosphoric acid loss from the polymer matrix is thus a key strategy for improving the endurance and thereby to obtain a reliable and cost efficient system [27]. Long-term durability of HT-PEMFCs has been demonstrated under constant current operation at temperatures of 150e160 C. Schmidt et al. [28] and Oono et al. [29] have reported tests over 17,000 h, with an average degradation rate of 3e6 mV h1. Yu et al. [24] reported a degradation rate of 4.9 mV h1 at 160 C. At further elevated operating temperatures, the degradation is significantly accelerated, but the literature is limited. At 170 C, a degradation rate of 10 mV h1 was reported [29]. At 170 C, Martin et al. reported 5 and 15 mV h1 over 2e3000 h for electro-sprayed cells [30] and more recently, 4.5 mV h1 over 2000 h [31]. It should be noted that the decay rates increased to 12 and 20 mV h1 during prolonged testing. Schmidt et al. reported 20 mV h1 at 180 C with cells influenced by H2S and CO [4]. At 190 C, 60 mV h1 has been reported [24,29]. A pronounced effect of current density as well as gas flow rates has been measured at 160 and 180 C, but quantified in terms of acid loss rates instead of voltage decay rates [32]. The acceleration of degradation during long-term operation has to a large extent been attributed to the cumulative acid loss and the resultant membrane thinning and catalyst malfunctioning due to a low acid content in the catalyst layers [14,26]. Oono et al. [14,27] investigated phosphoric acid redistribution and loss in MEAs operated at 150 C and 200 mA cm2 before and after the long term tests using electron probe micro-analysis and transmission electron microscopy. Their results showed a decline in the PA content in the membrane and the electrodes with time. They indicated that acid depletion from the membrane into the electrodes could be the reason for degradation of cells during long term tests. Thermal curing of the PBI membranes prior to acid doping impart features which are fundamental characteristics of a thermoset resin including complete insolubility, high resistance to swelling and improved mechanical toughness [33]. The improved physicochemical characteristics of the membranes after curing were further illustrated by a dramatically improved long term durability of the corresponding fuel cell MEAs [34,35]. Søndergaard et al. [34] recently reported a test with linear and thermally cured (crosslinked) membranes for over 13,000 h at 160 C with air and hydrogen stoichiometries of 4 and 2 respectively. A degradation rate of 2.6 mV h1 and 0.5 mV h1 for the first 9,200 h was observed for linear and crosslinked variants respectively. In this paper, a novel method for lowering the degradation due to acid loss is presented. The idea is to prepare a layered membrane with a core layer with a higher acid content, functioning as an acid reservoir, and with surface layers (linear or crosslinked) limiting the leaching of acid into the electrode structures. The core layer of the membrane is prepared by the direct casting from phosphoric acid. Direct casting from polyphosphoric acid was developed by Benicewicz et al. [36,37] and leads to extremely high doping levels that cannot be obtained by post doping, i.e. doping by imbibing after casting. Membranes prepared by direct casting
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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are morphologically different from the conventional film cast and acid imbibed membranes [37,38]. Direct cast membranes can be used as the entire membrane [24], but due to low mechanical strength, such membranes are normally thicker than post doped membranes. Therefore, the resulting area specific resistance of the fuel cell membranes becomes comparable despite the higher specific conductivity of the direct cast membrane with the highest acid content. The purpose of the outermost layers of the sandwich membrane is to provide mechanical strength and to slow down the acid transport out of the membrane system as compared to a one layer entirely direct cast membrane. It is expected that the electrodes will face a membrane interface similar to that with a usual post doped membrane and that wetting, activation and the acid loss rate therefore will be similar. The expected benefit is that the higher total acid content will allow for acid loss for a longer time before it becomes critical.
Experimental Membranes The core layer of the layered membranes was prepared by direct casting from phosphoric acid. The membrane was tape cast from a hot solution of poly(2,20 -m-phenylene5,50 -bibenzimidazole), mPBI, (5e10 wt %) in phosphoric acid (150e200 C) on a glass substrate. After cooling to room temperature under ambient conditions, the obtained membrane was equilibrated in ortho-phosphoric acid (85 wt% H3PO4) at room temperature. This process resulted in a membrane with an acid content of about 30e37 PA molecules per polymer repeat unit. This is hereafter referred to as the acid doping level (ADL). The surface layers of the three-layered membranes were based on post doped PBI. In some cases the surface layers were thermally cured at about 350 C for 10 min to give a crosslinked structure with higher strength and a denser structure to limit acid diffusion. All membranes for the surface layers were post-doped in 85 wt% PA at room temperature for more than 500 h (until use). After equilibration, the acid content corresponded to 10 and 12 PA molecules per polymer repeat unit for the membranes based on crosslinked and linear (i.e. non-crosslinked) PBI, respectively. Reference membranes of linear PBI were obtained from Danish Power Systems. In the following, membranes are referred to at 1L, 3L, 1X and 3X where the digit refers to the number of layers and “L” and “X” to linear and crosslinked, respectively. The thicknesses of the membranes and membrane layers, and their ADL are listed in Table 1.
Conductivity of membranes In-plane conductivity measurements were carried out by a four-probe conductivity cell. A symmetric square wave current was applied through platinum foil contact points in a frequency range from 6 to 10 kHz. The in-plane voltage drop was measured via platinum probes in contact with the membrane sample 1 cm apart. The sample was fixed onto the conductivity cell in a closed glass tube having inlet and outlet
3
for humidified gas. The cell was placed in an oven at the desired temperature for 1 h prior to data acquisition to ensure thermal and humidity equilibrium. Conductivities were measured at 120 C, 140 C, 160 C and 180 C at a controlled water partial pressure of 0.3 bar absolute.
Preparation of gas diffusion electrodes For preparation of gas diffusion electrodes (GDE) catalytic inks were prepared by mixing the catalyst (Pt 60 wt% on carbon black, Johnson Matthey, HiSPEC 9100) and ethanol (96% v/v) as solvent in the ratio 1:50 (w/w). The ink was then subjected to ultra-sonication for 1 h for good dispersion. Electrodes were prepared by spraying the catalytic ink over the microporous layer of the gas diffusion layer (GDL) with an active surface of 25 cm2. An ultrasonic spraying robot (Sonotec Exactacoat) was used with an ink flow rate of 0.25 ml min1 and a needlesubstrate distance of 13 cm. This has previously demonstrated high reproducibility in producing thin film coatings [18]. The Pt loading was about 0.25 mg cm2 for the anode and about 0.78 mg cm2 for the cathode. Gas diffusion layers (carbon cloth) incorporated with a wet-proofed microporous layer (H23C2 or H24C3) obtained from Freudenberg, Germany, was used as electrode substrates for both anode and cathode. The GDL used for the different cells can be seen in Table 1.
MEA fabrication The MEA was assembled directly in the fuel cell hardware with 8 bolts of 8 mm and a torque of 2 Nm by sandwiching the reservoir membrane inserted between the two post doped surface layers and electrodes without prior hot-pressing. The bipolar plates consisted of machined graphite plates with 5 parallel channels in a serpentine flow pattern with incorporated gaskets made of fluorinated elastomer (Viton®). A nonreactive sub gasket frame made of polyimide was introduced in order to reinforce the fringe of the membrane along the periphery of the electrodes. The active area of the resulting MEAs was 23 cm2.
Fuel cell characterization The fuel cells were mounted onto an in-house build test rack. The set point temperature for each cell was maintained using CAL 3300 PID temperature controllers. Brooks GF 80 thermal mass flow controllers were used to control the feeding gases without prior heating, humidification or pressurization. The fuel cells were controlled and monitored using a LabVIEW interface during steady state operation and polarization. The cells were tested under steady state operation with a galvanostatic current load of 200 mA cm2 at 170 C or 180 C. These temperatures were chosen slightly above the usual 160 C to accelerate degradation. The polarization curves were taken with the electronic load operating in galvanostatic mode and the voltage was recorded after 90 s at each set current density. For current densities above 200 mA cm2, flow rate stoichiometries were maintained as 1.5 for hydrogen and 2 for air. For current densities of 200 mA cm2 and below, flow rates equivalent to 200 mA cm2 were used. Electrochemical impedance spectroscopy (EIS) was carried out with a frequency response analyzer coupled to a
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Table 1 e Overview of the tested cells. Gas diffusion layer materials used, membrane thickness and acid doping level of the layers. Cell (Tested at) 1L (170 C) 3L (170 C) 1L (180 C) 1X (180 C) 3L (180 C) 3X (180 C)
GDL
Doped membrane thickness
ADL
(The layers, mm)
(The layers)
Anode/cathode H23C2/H23C2 H23C2/H23C2 H23C2/H24C3 H23C2/H24C3 H23C2/H24C3 H23C2/H24C3
80 43 80 80 90 90
±7 ± 5 (16 ± 2 þ 11 ± 1 þ 16 ± 2) ±7 ±7 ± 9 (24 ± 3 þ 42 ± 3 þ 24 ± 3) ± 9 (24 ± 3 þ 42 ± 3 þ 24 ± 3)
10.4 10.4, 37, 10.4 11.5 10.2 11.3, 31, 11.9 10.2, 31, 10.4
potentiostat/galvanostat (Versastat 4) with an AC amplitude of 200 mA (8.7 mA cm2) in the frequency range of 100 kHz 10 mHz. In order to assess the phosphoric acid loss during fuel cell operation, the exhaust gas stream was bubbled through a water bath of 100 g at room temperature and the water with condensed acid was collected every month. The product water sample containing phosphoric acid was analyzed with respect to the phosphorus content by inductively coupled plasma optical emission spectroscopy (ICP-OES). Voltage decay rates and impedance change rates were determined from the slopes of manually fitted model lines, which can be seen in Supplementary Information.
Results Conductivity of the individual membrane layers The proton conductivity of each of the individual membrane layers of the sandwich at their respective acid doping levels are shown in Fig. 1. As one can expect, specific conductivity scales with acid doping level. The direct cast membranes meant for the core of the sandwich thus possess significantly higher conductivity than the post doped membranes due to the higher doping level. It is also noteworthy that the crosslinked post-doped membrane, having a lower doping level than the linear (non-crosslinked) membrane, has the lowest conductivity. This is expected too, because crosslinking reduces the swelling capability and consequently the acid uptake under fixed doping conditions.
Fuel cell durability tests at 170 C The first set of durability tests were performed at 170 C with cells made from a three-layered membrane (3L) and a singlelayered membrane (1L), respectively. The development of the cell voltages at constant current, 200 mA cm2, is plotted in Fig. 2. The 3L membrane was composed of an ~11 mm direct cast reservoir layer and two post doped linear PBI barrier layers of ~16 mm each, giving a total membrane thickness of ~43 mm. The single layered membrane was ~80 mm thick. All electrodes were made from H23C2 GDLs. The cumulatively collected acid form the exhausts is plotted on the secondary axis. For the 1L based cell, the cell voltage decay started around 250 h and lasted for rest of the test, i.e. to 2506 h, and the average cell voltage decay rate in that period was 30 mV h1. The decay trend lines used for assessing the decay rates are
Fig. 1 e In-plane proton conductivity of the individual membrane layers for use in the three-layered membrane. The water partial pressure was 0.3 bar.
shown in Supplementary Information. The activation period of the cell based on the novel 3L membrane lasted until around 700 h where an instability of the curve appears and was followed by a linear decay until 2865 h, with a voltage decay rate of 6.7 mV h1. Due to a test bench event at 2865 h it had to be stopped. The acid collection rates were very similar for the two cells and practically unchanged during the entire period with measurements. It was 63 ng cm2 h1 for 1L and 73 ng cm2 h1 for 3L. For 1L, the rate increased slightly in the last interval in accordance with a slightly faster voltage decay towards the end of that period. Extrapolation to zero time suggests initial losses resulting in additional acid collections of 180 mg cm2 for 1L and 60 mg cm2 for 3L. The most likely explanation to this is that acid is squeezed out during assembly, leading to higher acid emission rates during the initial few hundreds of hours. The faster activation and higher cell voltage of the 1L cell point in the same direction. The total acid collection rates and the minor specific contribution from the anode are listed in Table 2. Acid in the cathode exhaust was strongly dominating. The contribution from the anode
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 2 e Cell voltages over time for cells with a three-layered membrane (3L) and a single-layered membrane (1L). The cells were operated at a constant current load of 200 mA cm¡2 at 170 C with gas stoichiometries of 1.5 for hydrogen and 2 for air. The cumulative acid collection at the exhausts is plotted on the secondary axis.
Table 2 e Summary of test results of all cells. Cell (Test at) 1L (170 C) 3L (170 C) 1L (180 C) 1X (180 C) 3L (180 C) 3X (180 C) a b
Resistance increase rate (mU cm2 h1)
Acid collection rate (ng cm2 h1)
Test time
Voltage decay rate
(hours)
(mV h-1)
Series
Polarization
Cathode
Anode
2,506 2,864 5,512 10,440 10,462 10,462
30 6.7 14 11 2.3 4.1
27 13 23 7e9 7e9 7e9
(25) e (þ70) (100) e (þ27) 11e56 22e56 5e27 ~ 7b
63 73 105 93a 87a 77a
1.4 9.4 0.64 0.32 0.81 0.54
Acid collection rates until 6500 h after which the curves become less linear. With an initial increase rate of ca. 45 mU cm2h1 the first 1500 h only.
exhaust was below one percent of the total collection in most cases. The cells were subjected to periodical EIS measurements and the high frequency intercept with the real axis of the Nyquist plot (zero phase shift) was taken as the series resistance (ohmic resistance) of the cell during the test. Although it comprises contributions from contact and electronic resistances in the electrodes, it is assumed that the major part of the series resistance can be attributed to the electrolyte and changes are thus assumed to mostly reflect changes in electrolyte resistance. The interpretation of the low frequency intercept is less simple since it might contain contributions from both charge transfer and mass transport [2,39]. The total polarisation resistance was calculated by subtraction of the high frequency intercept resistance from that of the low frequency intercept. In all cases, the high frequency intercepts were clear, in contrast to the low frequency intercepts in some cases. Thus, the assessment of polarization resistance was more uncertain than of the series resistance. Polarization resistance is affected by the acid content and reactant diffusion, since a good acid distribution and sufficient reactant concentrations in the catalyst layer is important for the catalyst utilization [40,41]. The Nyquist plots used for the
assessment of resistances can be found in Supplementary Information. Fig. 3 shows the development of the area specific series and polarization resistances obtained from the Nyquist plots for cells with the 1L and 3L membranes. The initial ohmic resistance of the cell with the 3L membrane was 88 mU cm2 and the resistance of the cell with 1L membrane was 156 mU cm2. An area specific resistance of 88 mU cm2 was lower than what has previously been reported in the literature for HT-PEMFC, but it was only about 43 mm thick. Values reported typically range from 120 to 300 mU cm2 [2]. The series resistance of the cells increased at a rate of 27 mU cm2 h1 and 13 mU cm2 h1 for 1L and 3L membranes respectively (see Supplementary Information for trend lines). The change of the polarization resistances followed a less regular non-linear pattern, increasing for 1L and decreasing for 3L. This behavior can only partly be ascribed to uncertainty of the assessment, and must also be an effect of acid redistribution in the catalyst layer (local flooding or depletion) and the usual PEMFC degradation mechanisms like corrosion and particle growth. Fig. 4 shows polarization curves of the two cells at 669 and 693 h. The total resistances from EIS in the two cases at this time correspond reasonably to the slope of the polarization curves at
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 3 e Evolution of area specific resistances of cells with the single-layered membrane and the three-layered membrane operated at 200 mA cm¡2 and 170 C. The resistances were determined by EIS (see Supplementary Information).
Fig. 4 e Polarization curves of cells with singeelayered and three-layered membranes operated close to 700 h at 200 mA cm¡2 and 170 C.
200 mA cm2 (600e700 mU cm2). It is striking that the two polarization curves are so similar.
Fuel cell tests at 180 C The first test series at 170 C was made with a three-layered membrane that was much thinner than the single-layered membrane. This complicated comparison. For the second test series the thickness of the three-layered membrane was increased to match that of the single-layered membrane. The ex-situ thickness of the core layer was about 42 mm and the thickness of the surface layers (linear and crosslinked) were about 24 mm. The strength of the reservoir layer was improved by increasing the polymer content from 5 wt % to 8 wt % resulting in an ADL of 31. Moreover, some new materials were introduced; the GDL material on the cathode
side was changed from H23C2 to H24C3 in all cells, because a higher acid retention was previously shown for this material [15] and thermally cured PBI, i.e. crosslinked [33,35], was used for barrier layers (3X) as well as for single layer (1X) beside the linear PBI (1L and 3L). Finally, the durability testing was carried out at 180 C to accelerate the degradation processes further. In Fig. 5, there are four curves representing three cells tested for more than 10,000 h, and the 1L cell which was stopped after 5,512 h. All four cells had in common that after an activation period of some hundred hours followed by some initial degradation, the degradation rate became quite constant. From around 2,500 h the rates were roughly constant up to around 9,200 h, or shortly after, where they tended to increase somewhat. Such a behavior after ca. 9,000 h at same current density, was recently reported by Søndergaard et al. [32], but at 160 C and gas stoichiometries of 2 and 4. It was suggested then that the time for this change of rate is determined by the amount of gas passing through the cell. These steady state voltage decay rates below 9,000 h were 14 mV h1 for 1L and 11 mV h1 for 1X. These rates are reasonably well in line with the few reports at comparable temperatures cited above in the introduction. The cells with layered membranes showed much lover voltage decay rates, namely 2.3 mV h1 for 3L and 4.1 mVh1for 3X. The acid collection rates were in all cases comparable with a tendency to lower collection rates for the three-layered cells (77 and 87 ng cm2 h1) as compared to the single layered cells (93 and 105 ng cm2 h1). No significant indication of an initial acid squeeze-out, like in the previous series, was seen. Acid collection at the anode side was in all cases less than one percent of that at the cathode side. See Table 2. After around 2,900 h, there was a power failure for the test bench computer. The data acquisition was resumed after 20 h and the tests were continued. The cells 1X, 3L and 3X were tested in parallel, whereas the 1L cell was tested earlier. There was a power cut for few minutes around 7,390 h and 7,413 h possibly affecting the three cells the same way.
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 5 e Cell voltages and acid collection at constant current density of 200 mA cm¡2 at 180 C with flow stoichiometries of 1.5 for hydrogen and 2 for air. The membrane combinations are indicated on the figure as 1 and 3 for number of layers and L and X for linear and crosslinked membrane or barrier layers.
The series and polarization resistances over time determined by EIS for cells are plotted in Fig. 6. The initial series resistance of the cells 1L, 3L and 3X are at similar levels around 110 mU cm2. The 1X cell shows higher series resistance (170 mU cm2) as expected due to lower acid doping level. For the three-layered membranes however, crosslinking of the barrier layers does not seem to affect the series resistance at all. The series resistance of the three cells develop linearly by 7e9 mU cm2 h1 over 10,000 h. The 1L cell had the same initial series resistance as the cells with layered membranes, but the value increases much faster than the other three. Looking at the polarization resistance, it appears that this is where the main differentiation between the cells takes places. The three-layered cells degrade only slowly while the single layered cells rise in resistance at a higher and ever increasing rate. Polarization curves were measured after an activation period of 500 h for all the cells except 1L for which is was performed after 800 h as shown in Fig. 7. Nevertheless, there is no distinct performance variation between 500 h and 800 h for this cell as we can see from the durability curves. Two trends are clear from the polarization curves. The 1L cell with the crosslinked membrane has the highest slope of the linear part of the curves, which corresponds well with the lower conductivity of the crosslinked material (Figs. 1 and 6). The other trend is that the cells with three-layered membranes show earlier mass transport limitations (curve bending at high current density) than the cells with single-layered membranes. The reason for this is less obvious, but should likely be sought in the acid distribution in the catalyst layer. It is also striking that the open circuit voltages (OCV) vary a lot. The mixed potential resulting from gas crossover has a strong effect on OCV because of the steep slope of the polarization curve at small current densities. The single-layer based cells appear to reflect that with the crosslinked denser membrane having the highest OCV, but for the three-layer based cell, the order is reversed. Anyway, the OCV is often one of the least
reproducible properties of HT-PEMFC and it is better to refrain from drawing strong conclusion. The acid collection rates shown in Fig. 5 were slightly lower for 3L and 3X than for 1L and 1X. If we assume that this is a significant effect, then we can additionally assume that losses of acid in the form of evaporation and surface diffusion are also lower for 3L and 3X as a consequence of a lower acid content in the catalyst layer. The acid content in the 3L and 3X may thus be less than ideal and result in earlier mass transport effects due to a somewhat poorer utilization of the catalyst material. Had the problem been flooding, then a higher acid content would be expected to lead to a higher acid loss rate indicated by a higher acid collection rate.
Discussion The work was initiated on the idea that the direct cast reservoir layer allows for the containment of more acid (more acid to lose) and the post doped barrier layers secure that the loss rate is similar to that in cells with single post doped membranes. The latter part of the hypothesis was based on the assumption that the interface between the membrane and the catalyst layer remains unchanged. The study did show significantly lower voltage decay rates over up to more than 10,000 h, but the simple assumption that the increased durability would be realized because it takes longer to deplete the membrane with a reservoir, was not verified during the 10,000 h. In the experimental series at 180 C, the acid collection rates were in the same order for the four cells and this supports only one part of the hypothesis. Had the cells been operated much longer with no other degradation effect, it can be expected that eventually the cells with a reservoir would last longer due to postponed lethal depletion. However, the faster degradation seen for the single layered cells was clearly not an effect of acid depletion inside the membrane, but instead an effect on the polarization.
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 6 e Evolution of area specific resistances of cells with single-layered membrane and three-layered membrane operated at 200 mA cm¡2 and 180 C. The resistances were determined by EIS. The membrane combinations are indicated on the figure as 1 and 3 for number of layers and L and X for linear and crosslinked barrier layers.
Fig. 7 e Polarization curves of cells measured at 180 C. For 1X, 3L and 3X after 500 h and for 1 L after 800 h.
Whether the faster increase of polarization of the singlelayered cells was a result of catalyst degradation or increased mass transport (or both) cannot be determined from Fig. 7, but regardless, it is likely an effect of acid redistribution in the catalyst layer. Over long term operation, acid is transported from the catalyst layer by evaporation and surface diffusion is balanced by simultaneous replenishing from the membrane. This balance is only strictly maintained if the properties of the acid containing membrane are unaltered. If the replenishment is slightly diminished (or increased) a different acid content in the catalyst layer will be the result and consequently lead to different polarization behavior. With a reservoir with high phosphoric acid activity right behind, the doping level of the membrane layer that faces the catalyst layer is maintained closer to constant than
in a layer without a backing reservoir. With a reservoir the catalyst layer is kept optimal for a longer period. Crosslinked PBI was chosen for alternative barrier layers to limit the acid loss further as compared to linear PBI due to slower diffusion through the dense texture [34]. Surprisingly, this did not lead to a significant further increase of durability. However, after realizing that the main cause for increased overvoltage was in the catalyst layer, it makes sense, as the crosslinked barrier layer may be slightly less efficient in acid replenishment. Thus, acid transport out of the membrane should not be limited too much, only optimized for best catalyst layer performance. The single-layered cells degraded faster, mainly via a polarization effect. In terms of series resistance, the 1L cell started at a lower value (coincidently coinciding with that of the three-layered cells), but increased much faster than the 1X cell. This is in accordance with the lower acid mobility in the crosslinked membrane [34]. If we take a closer look at the four acid collection rates at 180 C, it is striking that both threelayered cells had 17% lower acid collection rates than their corresponding single-layered cells, linear and crosslinked respectively. Likewise both cells with crosslinked membrane components had acid collection rates 11% lower than the corresponding cells with linear membrane components. This trend is not fully reflected in the voltage decay rates. The three-layered cells did decay much slower than the singlelayered cells, but the 3X cell, despite a lower collection rate, decayed faster that the 3L cell. Coming back to the first test series at 170 C, it can be concluded that the higher stability of the 3L cells was also verified here. Likewise, the hypothesis that the acid loss rate (assumed indicated by the acid collection rate) is supported, but apart from that, the results give a less clear picture than the more systematic series at 180 C, especially the EIS results. However, the series resistances, which are the most well
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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defined and most trustworthy indicator, resembled those in the 180 C test series. The series resistance of the 1L cell was much higher, but again, the membrane was also much thicker than the 3L membrane. Despite the difference in thickness, the 1L cell increased its series resistance by twice the rate of the 3L cell. The story told by the development of the polarization is more complex, but the polarization of the 1L increased sharply while that of the 3L was not only maintained like in the 180 C test series, it was actually decreasing. Apparently, the acid redistribution improved in this case. Finally, we make a few notes on the comparison of the test series at 170 C with that at 180 C. Higher rates of voltage decay and acid collection must be expected the higher the temperature, other things being equal. In this study, the 1L and 3L cells in the 170 C series both had more than twice the voltage decay rates than the 1L and 3L cells in the 180 C series. The reason the two series are not fully comparable is that (1) the membrane of the 3L (170 C) was thinner than the other membranes used and (2) the different choice of GDL, where the GDLs used at 180 C were chosen for improved acid retention properties. This may at a first glance appear in poor accordance with the acid collection rates for these cells, which were the lowest of the entire study (63 and 73 ng cm2 h1). However, the initial portion of acid presumably resulting from a pre-test squeeze-out was omitted, as the collection rates were naturally based on linear fits of the rest of the curves. If we compare the total collected acid at time 2,500 h, where the cells at 170 C were stopped, the amount from the 1L and 3L cells at 170 C were ca. 350 and 250 mg cm2, respectively, while only 200e270 mg cm2 was collected from the four cells tested at 180 C after the same period of time.
Conclusions High-temperature PEM fuel cells with three-layered PBI membranes, with the middle layer acting as an acid reservoir is a way to improve durability as compared to conventional cells with single layered post doped PBI membranes. This was verified at constant current operation at 200 mA cm2 at 180 C over testing periods of up to 10,000 h with voltage decay rates of 2.3 and 4.1 mV h1 for the cells with three-layered membranes and 14 and 11 mV h1 for cells with single-layered membranes. The rate of acid loss from the cells, as assessed by in-line acid collection at the exhausts, is largely independent on the presence of an internal reservoir layer. Both linear and thermally crosslinked PBI were used for single-layered membranes as well as for the outer layers (the barrier layers) of the three-layered membranes. The acid collection rate was in both cases 11% lower with crosslinked PBI than with linear PBI, but a lower voltage decay rate was not seen when using crosslinked barrier layers in the threelayered cell. The dominating effect leading to the lower voltage decay rates of the cells with three-layered membranes was on the electrode polarization resistance which grew at a lower rate within the 10,000 h of testing. The initial hypothesis that the acid reservoir of the center membrane layer will extend the lifetime due to a postponed acid depletion, could not be verified in the course of the time available for the test (10,000 h).
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The authors believe that this effect will ultimately play a role if the cells are operated for even longer periods of time.
Acknowledgement The authors gratefully acknowledge the Danish ForskEl program for funding via the project SmartMEA (no. 2014-1-12218). The authors moreover thank Sinh Hy Nguyen at DTU Environment for analyzing samples with ICP-OES and Danish Power Systems for providing PBI membranes for reference.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.186.
references
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Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of high-temperature polymer electrolyte membrane fuel cells Arvind Kannan, David Aili, Lars N. Cleemann, Qingfeng Li, Jens Oluf Jensen* Department of Energy Conversion and Storage, Technical University of Denmark, Fysikvej, Building 310, 2800 Kgs. Lyngby, Denmark
highlights
graphical abstract
Acid reservoir in acid doped HTPEM fuel cell. Significantly improved stability in the course of 10,000 h. Small degradation rates of 2.3 e4.1 mV h-1 at 180 C.
article info
abstract
Article history:
High-temperature polymer electrolyte membrane fuel cells with phosphoric acid doped
Received 21 May 2019
polybenzimidazole (PBI) are made with three-layered membranes. The central membrane
Received in revised form
layer is meant as an acid reservoir made from direct cast PBI with a higher acid content than
18 October 2019
the outermost layers, which are post doped membranes acting as barrier layers to limit the
Accepted 24 October 2019
acid transport out of the central layer. Cells with three-layered membranes and others with
Available online xxx
normal single layered membranes are tested at 170 and 180 C. At both temperatures, the cells with three-layered membranes show significantly lower voltage decay rates than the corre-
Keywords:
sponding cells with single-layered membranes. Post doped PBI membranes based on linear or
PEM fuel cell
thermally crosslinked PBI are used for the barrier layers of the three-layered membranes and
Polybenzimidazole
for the single-layered membranes in the test series at 180 C. The acid loss rates assessed by
Phosphoric acid reservoir
acid collection at the fuel cell exhaust, are rather comparable. At 180 C, the cells are tested for
Membranes
up to 10,000 h and voltage decay rates of 2.3 and 4.1 mVh-1 are measured for the cells with
Durability
three-layered membranes and 14 and 11 mVh-1 for cells with single-layered membranes. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (J.O. Jensen). https://doi.org/10.1016/j.ijhydene.2019.10.186 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Introduction Polymer electrolyte membrane fuel cells (PEMFC) have attracted much attention due to their promise as energy conversion devices for both mobile and stationary applications [1,2]. High temperature PEMFC (HT-PEMFC), operating in the 120e200 C range, possess excellent tolerance to CO [3] from reformat hydrogen and allow for simplified water and thermal management [4]. They are typically constructed around a proton conducting membrane based on polybenzimidazole (PBI) imbibed with phosphoric acid. The proton transport between the electrodes is made possible by the phosphoric acid and the proton conductivity increases with increasing phosphoric acid content [5]. Phosphoric acid (PA) is much less volatile than water and that is the reason why the cells can be operated at higher temperature. However, PA is still mobile in the PBI membrane and the catalyst layers [6,7]. When the doped membrane is assembled with two gas diffusion electrodes forming a membrane-electrode-assembly (MEA), the doping acid redistributes within the MEA from the membrane to the electrodes [8]. In this way, proton conducting pathways in the catalyst layer and three-phase-boundary regions at the interface between the electrodes and the membrane are created. This acid redistribution phenomenon is often referred to as the break-in or activation process at start-up when the cell performance slowly improves to reach a steady state within up to a few hundred hours [9,10]. This is a highly desired process, but the wetting of the catalyst layer also lead to a large area exposure of the PA to evaporation and surface diffusion. Loss of doping acid by minute evaporation and diffusion processes are degradation mechanisms the HTPEMFC has in common with the phosphoric acid fuel cell. The transport pattern for PA in the cell is complex. Besides diffusion and evaporation, acid migration towards the anode is a phenomenon known from the phosphoric acid fuel cells. Part of the charge transport in the PA phase is assigned to negative hydrogen phosphate ions, which recombine with protons on the anode side resulting an accumulation of PA there. The effect has been verified in HT-PEMFC too [11,12]. The change of PA content on the anode side affects the performance due to changing degrees of wetting and flooding of the anode catalyst layer as reported by Halter et al. [13]. Thus, acid re-distribution or re-location takes place throughout the entire life of the cell. It initially secures the conditions for ionic conductivity in the catalyst layer during activation and later as a general feature playing a role in the acid loss mechanisms over long term [14,15]. Other mechanisms, like catalyst sintering [16,17] catalyst dissolution [18e20] and carbon support corrosion [21e23], the cell shares with the common low-temperature PEMFC. Acid loss is believed to be a dominating factor for performance loss during long term operation. In some studies, acid is collected from the exhaust gases of the fuel cell [24] and in that connection it should be stressed that only a fraction of the acid lost from the membrane and the catalyst layer is collected at the exit [15]. Fuel cell durability targets range from 10,000 h for automotive application to 80,000 h for stationary application [25].
For further development of HT-PEMFC, significant attention has been on understanding degradation and in development of strategies to improve durability [26]. Limiting the phosphoric acid loss from the polymer matrix is thus a key strategy for improving the endurance and thereby to obtain a reliable and cost efficient system [27]. Long-term durability of HT-PEMFCs has been demonstrated under constant current operation at temperatures of 150e160 C. Schmidt et al. [28] and Oono et al. [29] have reported tests over 17,000 h, with an average degradation rate of 3e6 mV h1. Yu et al. [24] reported a degradation rate of 4.9 mV h1 at 160 C. At further elevated operating temperatures, the degradation is significantly accelerated, but the literature is limited. At 170 C, a degradation rate of 10 mV h1 was reported [29]. At 170 C, Martin et al. reported 5 and 15 mV h1 over 2e3000 h for electro-sprayed cells [30] and more recently, 4.5 mV h1 over 2000 h [31]. It should be noted that the decay rates increased to 12 and 20 mV h1 during prolonged testing. Schmidt et al. reported 20 mV h1 at 180 C with cells influenced by H2S and CO [4]. At 190 C, 60 mV h1 has been reported [24,29]. A pronounced effect of current density as well as gas flow rates has been measured at 160 and 180 C, but quantified in terms of acid loss rates instead of voltage decay rates [32]. The acceleration of degradation during long-term operation has to a large extent been attributed to the cumulative acid loss and the resultant membrane thinning and catalyst malfunctioning due to a low acid content in the catalyst layers [14,26]. Oono et al. [14,27] investigated phosphoric acid redistribution and loss in MEAs operated at 150 C and 200 mA cm2 before and after the long term tests using electron probe micro-analysis and transmission electron microscopy. Their results showed a decline in the PA content in the membrane and the electrodes with time. They indicated that acid depletion from the membrane into the electrodes could be the reason for degradation of cells during long term tests. Thermal curing of the PBI membranes prior to acid doping impart features which are fundamental characteristics of a thermoset resin including complete insolubility, high resistance to swelling and improved mechanical toughness [33]. The improved physicochemical characteristics of the membranes after curing were further illustrated by a dramatically improved long term durability of the corresponding fuel cell MEAs [34,35]. Søndergaard et al. [34] recently reported a test with linear and thermally cured (crosslinked) membranes for over 13,000 h at 160 C with air and hydrogen stoichiometries of 4 and 2 respectively. A degradation rate of 2.6 mV h1 and 0.5 mV h1 for the first 9,200 h was observed for linear and crosslinked variants respectively. In this paper, a novel method for lowering the degradation due to acid loss is presented. The idea is to prepare a layered membrane with a core layer with a higher acid content, functioning as an acid reservoir, and with surface layers (linear or crosslinked) limiting the leaching of acid into the electrode structures. The core layer of the membrane is prepared by the direct casting from phosphoric acid. Direct casting from polyphosphoric acid was developed by Benicewicz et al. [36,37] and leads to extremely high doping levels that cannot be obtained by post doping, i.e. doping by imbibing after casting. Membranes prepared by direct casting
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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are morphologically different from the conventional film cast and acid imbibed membranes [37,38]. Direct cast membranes can be used as the entire membrane [24], but due to low mechanical strength, such membranes are normally thicker than post doped membranes. Therefore, the resulting area specific resistance of the fuel cell membranes becomes comparable despite the higher specific conductivity of the direct cast membrane with the highest acid content. The purpose of the outermost layers of the sandwich membrane is to provide mechanical strength and to slow down the acid transport out of the membrane system as compared to a one layer entirely direct cast membrane. It is expected that the electrodes will face a membrane interface similar to that with a usual post doped membrane and that wetting, activation and the acid loss rate therefore will be similar. The expected benefit is that the higher total acid content will allow for acid loss for a longer time before it becomes critical.
Experimental Membranes The core layer of the layered membranes was prepared by direct casting from phosphoric acid. The membrane was tape cast from a hot solution of poly(2,20 -m-phenylene5,50 -bibenzimidazole), mPBI, (5e10 wt %) in phosphoric acid (150e200 C) on a glass substrate. After cooling to room temperature under ambient conditions, the obtained membrane was equilibrated in ortho-phosphoric acid (85 wt% H3PO4) at room temperature. This process resulted in a membrane with an acid content of about 30e37 PA molecules per polymer repeat unit. This is hereafter referred to as the acid doping level (ADL). The surface layers of the three-layered membranes were based on post doped PBI. In some cases the surface layers were thermally cured at about 350 C for 10 min to give a crosslinked structure with higher strength and a denser structure to limit acid diffusion. All membranes for the surface layers were post-doped in 85 wt% PA at room temperature for more than 500 h (until use). After equilibration, the acid content corresponded to 10 and 12 PA molecules per polymer repeat unit for the membranes based on crosslinked and linear (i.e. non-crosslinked) PBI, respectively. Reference membranes of linear PBI were obtained from Danish Power Systems. In the following, membranes are referred to at 1L, 3L, 1X and 3X where the digit refers to the number of layers and “L” and “X” to linear and crosslinked, respectively. The thicknesses of the membranes and membrane layers, and their ADL are listed in Table 1.
Conductivity of membranes In-plane conductivity measurements were carried out by a four-probe conductivity cell. A symmetric square wave current was applied through platinum foil contact points in a frequency range from 6 to 10 kHz. The in-plane voltage drop was measured via platinum probes in contact with the membrane sample 1 cm apart. The sample was fixed onto the conductivity cell in a closed glass tube having inlet and outlet
3
for humidified gas. The cell was placed in an oven at the desired temperature for 1 h prior to data acquisition to ensure thermal and humidity equilibrium. Conductivities were measured at 120 C, 140 C, 160 C and 180 C at a controlled water partial pressure of 0.3 bar absolute.
Preparation of gas diffusion electrodes For preparation of gas diffusion electrodes (GDE) catalytic inks were prepared by mixing the catalyst (Pt 60 wt% on carbon black, Johnson Matthey, HiSPEC 9100) and ethanol (96% v/v) as solvent in the ratio 1:50 (w/w). The ink was then subjected to ultra-sonication for 1 h for good dispersion. Electrodes were prepared by spraying the catalytic ink over the microporous layer of the gas diffusion layer (GDL) with an active surface of 25 cm2. An ultrasonic spraying robot (Sonotec Exactacoat) was used with an ink flow rate of 0.25 ml min1 and a needlesubstrate distance of 13 cm. This has previously demonstrated high reproducibility in producing thin film coatings [18]. The Pt loading was about 0.25 mg cm2 for the anode and about 0.78 mg cm2 for the cathode. Gas diffusion layers (carbon cloth) incorporated with a wet-proofed microporous layer (H23C2 or H24C3) obtained from Freudenberg, Germany, was used as electrode substrates for both anode and cathode. The GDL used for the different cells can be seen in Table 1.
MEA fabrication The MEA was assembled directly in the fuel cell hardware with 8 bolts of 8 mm and a torque of 2 Nm by sandwiching the reservoir membrane inserted between the two post doped surface layers and electrodes without prior hot-pressing. The bipolar plates consisted of machined graphite plates with 5 parallel channels in a serpentine flow pattern with incorporated gaskets made of fluorinated elastomer (Viton®). A nonreactive sub gasket frame made of polyimide was introduced in order to reinforce the fringe of the membrane along the periphery of the electrodes. The active area of the resulting MEAs was 23 cm2.
Fuel cell characterization The fuel cells were mounted onto an in-house build test rack. The set point temperature for each cell was maintained using CAL 3300 PID temperature controllers. Brooks GF 80 thermal mass flow controllers were used to control the feeding gases without prior heating, humidification or pressurization. The fuel cells were controlled and monitored using a LabVIEW interface during steady state operation and polarization. The cells were tested under steady state operation with a galvanostatic current load of 200 mA cm2 at 170 C or 180 C. These temperatures were chosen slightly above the usual 160 C to accelerate degradation. The polarization curves were taken with the electronic load operating in galvanostatic mode and the voltage was recorded after 90 s at each set current density. For current densities above 200 mA cm2, flow rate stoichiometries were maintained as 1.5 for hydrogen and 2 for air. For current densities of 200 mA cm2 and below, flow rates equivalent to 200 mA cm2 were used. Electrochemical impedance spectroscopy (EIS) was carried out with a frequency response analyzer coupled to a
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Table 1 e Overview of the tested cells. Gas diffusion layer materials used, membrane thickness and acid doping level of the layers. Cell (Tested at) 1L (170 C) 3L (170 C) 1L (180 C) 1X (180 C) 3L (180 C) 3X (180 C)
GDL
Doped membrane thickness
ADL
(The layers, mm)
(The layers)
Anode/cathode H23C2/H23C2 H23C2/H23C2 H23C2/H24C3 H23C2/H24C3 H23C2/H24C3 H23C2/H24C3
80 43 80 80 90 90
±7 ± 5 (16 ± 2 þ 11 ± 1 þ 16 ± 2) ±7 ±7 ± 9 (24 ± 3 þ 42 ± 3 þ 24 ± 3) ± 9 (24 ± 3 þ 42 ± 3 þ 24 ± 3)
10.4 10.4, 37, 10.4 11.5 10.2 11.3, 31, 11.9 10.2, 31, 10.4
potentiostat/galvanostat (Versastat 4) with an AC amplitude of 200 mA (8.7 mA cm2) in the frequency range of 100 kHz 10 mHz. In order to assess the phosphoric acid loss during fuel cell operation, the exhaust gas stream was bubbled through a water bath of 100 g at room temperature and the water with condensed acid was collected every month. The product water sample containing phosphoric acid was analyzed with respect to the phosphorus content by inductively coupled plasma optical emission spectroscopy (ICP-OES). Voltage decay rates and impedance change rates were determined from the slopes of manually fitted model lines, which can be seen in Supplementary Information.
Results Conductivity of the individual membrane layers The proton conductivity of each of the individual membrane layers of the sandwich at their respective acid doping levels are shown in Fig. 1. As one can expect, specific conductivity scales with acid doping level. The direct cast membranes meant for the core of the sandwich thus possess significantly higher conductivity than the post doped membranes due to the higher doping level. It is also noteworthy that the crosslinked post-doped membrane, having a lower doping level than the linear (non-crosslinked) membrane, has the lowest conductivity. This is expected too, because crosslinking reduces the swelling capability and consequently the acid uptake under fixed doping conditions.
Fuel cell durability tests at 170 C The first set of durability tests were performed at 170 C with cells made from a three-layered membrane (3L) and a singlelayered membrane (1L), respectively. The development of the cell voltages at constant current, 200 mA cm2, is plotted in Fig. 2. The 3L membrane was composed of an ~11 mm direct cast reservoir layer and two post doped linear PBI barrier layers of ~16 mm each, giving a total membrane thickness of ~43 mm. The single layered membrane was ~80 mm thick. All electrodes were made from H23C2 GDLs. The cumulatively collected acid form the exhausts is plotted on the secondary axis. For the 1L based cell, the cell voltage decay started around 250 h and lasted for rest of the test, i.e. to 2506 h, and the average cell voltage decay rate in that period was 30 mV h1. The decay trend lines used for assessing the decay rates are
Fig. 1 e In-plane proton conductivity of the individual membrane layers for use in the three-layered membrane. The water partial pressure was 0.3 bar.
shown in Supplementary Information. The activation period of the cell based on the novel 3L membrane lasted until around 700 h where an instability of the curve appears and was followed by a linear decay until 2865 h, with a voltage decay rate of 6.7 mV h1. Due to a test bench event at 2865 h it had to be stopped. The acid collection rates were very similar for the two cells and practically unchanged during the entire period with measurements. It was 63 ng cm2 h1 for 1L and 73 ng cm2 h1 for 3L. For 1L, the rate increased slightly in the last interval in accordance with a slightly faster voltage decay towards the end of that period. Extrapolation to zero time suggests initial losses resulting in additional acid collections of 180 mg cm2 for 1L and 60 mg cm2 for 3L. The most likely explanation to this is that acid is squeezed out during assembly, leading to higher acid emission rates during the initial few hundreds of hours. The faster activation and higher cell voltage of the 1L cell point in the same direction. The total acid collection rates and the minor specific contribution from the anode are listed in Table 2. Acid in the cathode exhaust was strongly dominating. The contribution from the anode
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 2 e Cell voltages over time for cells with a three-layered membrane (3L) and a single-layered membrane (1L). The cells were operated at a constant current load of 200 mA cm¡2 at 170 C with gas stoichiometries of 1.5 for hydrogen and 2 for air. The cumulative acid collection at the exhausts is plotted on the secondary axis.
Table 2 e Summary of test results of all cells. Cell (Test at) 1L (170 C) 3L (170 C) 1L (180 C) 1X (180 C) 3L (180 C) 3X (180 C) a b
Resistance increase rate (mU cm2 h1)
Acid collection rate (ng cm2 h1)
Test time
Voltage decay rate
(hours)
(mV h-1)
Series
Polarization
Cathode
Anode
2,506 2,864 5,512 10,440 10,462 10,462
30 6.7 14 11 2.3 4.1
27 13 23 7e9 7e9 7e9
(25) e (þ70) (100) e (þ27) 11e56 22e56 5e27 ~ 7b
63 73 105 93a 87a 77a
1.4 9.4 0.64 0.32 0.81 0.54
Acid collection rates until 6500 h after which the curves become less linear. With an initial increase rate of ca. 45 mU cm2h1 the first 1500 h only.
exhaust was below one percent of the total collection in most cases. The cells were subjected to periodical EIS measurements and the high frequency intercept with the real axis of the Nyquist plot (zero phase shift) was taken as the series resistance (ohmic resistance) of the cell during the test. Although it comprises contributions from contact and electronic resistances in the electrodes, it is assumed that the major part of the series resistance can be attributed to the electrolyte and changes are thus assumed to mostly reflect changes in electrolyte resistance. The interpretation of the low frequency intercept is less simple since it might contain contributions from both charge transfer and mass transport [2,39]. The total polarisation resistance was calculated by subtraction of the high frequency intercept resistance from that of the low frequency intercept. In all cases, the high frequency intercepts were clear, in contrast to the low frequency intercepts in some cases. Thus, the assessment of polarization resistance was more uncertain than of the series resistance. Polarization resistance is affected by the acid content and reactant diffusion, since a good acid distribution and sufficient reactant concentrations in the catalyst layer is important for the catalyst utilization [40,41]. The Nyquist plots used for the
assessment of resistances can be found in Supplementary Information. Fig. 3 shows the development of the area specific series and polarization resistances obtained from the Nyquist plots for cells with the 1L and 3L membranes. The initial ohmic resistance of the cell with the 3L membrane was 88 mU cm2 and the resistance of the cell with 1L membrane was 156 mU cm2. An area specific resistance of 88 mU cm2 was lower than what has previously been reported in the literature for HT-PEMFC, but it was only about 43 mm thick. Values reported typically range from 120 to 300 mU cm2 [2]. The series resistance of the cells increased at a rate of 27 mU cm2 h1 and 13 mU cm2 h1 for 1L and 3L membranes respectively (see Supplementary Information for trend lines). The change of the polarization resistances followed a less regular non-linear pattern, increasing for 1L and decreasing for 3L. This behavior can only partly be ascribed to uncertainty of the assessment, and must also be an effect of acid redistribution in the catalyst layer (local flooding or depletion) and the usual PEMFC degradation mechanisms like corrosion and particle growth. Fig. 4 shows polarization curves of the two cells at 669 and 693 h. The total resistances from EIS in the two cases at this time correspond reasonably to the slope of the polarization curves at
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 3 e Evolution of area specific resistances of cells with the single-layered membrane and the three-layered membrane operated at 200 mA cm¡2 and 170 C. The resistances were determined by EIS (see Supplementary Information).
Fig. 4 e Polarization curves of cells with singeelayered and three-layered membranes operated close to 700 h at 200 mA cm¡2 and 170 C.
200 mA cm2 (600e700 mU cm2). It is striking that the two polarization curves are so similar.
Fuel cell tests at 180 C The first test series at 170 C was made with a three-layered membrane that was much thinner than the single-layered membrane. This complicated comparison. For the second test series the thickness of the three-layered membrane was increased to match that of the single-layered membrane. The ex-situ thickness of the core layer was about 42 mm and the thickness of the surface layers (linear and crosslinked) were about 24 mm. The strength of the reservoir layer was improved by increasing the polymer content from 5 wt % to 8 wt % resulting in an ADL of 31. Moreover, some new materials were introduced; the GDL material on the cathode
side was changed from H23C2 to H24C3 in all cells, because a higher acid retention was previously shown for this material [15] and thermally cured PBI, i.e. crosslinked [33,35], was used for barrier layers (3X) as well as for single layer (1X) beside the linear PBI (1L and 3L). Finally, the durability testing was carried out at 180 C to accelerate the degradation processes further. In Fig. 5, there are four curves representing three cells tested for more than 10,000 h, and the 1L cell which was stopped after 5,512 h. All four cells had in common that after an activation period of some hundred hours followed by some initial degradation, the degradation rate became quite constant. From around 2,500 h the rates were roughly constant up to around 9,200 h, or shortly after, where they tended to increase somewhat. Such a behavior after ca. 9,000 h at same current density, was recently reported by Søndergaard et al. [32], but at 160 C and gas stoichiometries of 2 and 4. It was suggested then that the time for this change of rate is determined by the amount of gas passing through the cell. These steady state voltage decay rates below 9,000 h were 14 mV h1 for 1L and 11 mV h1 for 1X. These rates are reasonably well in line with the few reports at comparable temperatures cited above in the introduction. The cells with layered membranes showed much lover voltage decay rates, namely 2.3 mV h1 for 3L and 4.1 mVh1for 3X. The acid collection rates were in all cases comparable with a tendency to lower collection rates for the three-layered cells (77 and 87 ng cm2 h1) as compared to the single layered cells (93 and 105 ng cm2 h1). No significant indication of an initial acid squeeze-out, like in the previous series, was seen. Acid collection at the anode side was in all cases less than one percent of that at the cathode side. See Table 2. After around 2,900 h, there was a power failure for the test bench computer. The data acquisition was resumed after 20 h and the tests were continued. The cells 1X, 3L and 3X were tested in parallel, whereas the 1L cell was tested earlier. There was a power cut for few minutes around 7,390 h and 7,413 h possibly affecting the three cells the same way.
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 5 e Cell voltages and acid collection at constant current density of 200 mA cm¡2 at 180 C with flow stoichiometries of 1.5 for hydrogen and 2 for air. The membrane combinations are indicated on the figure as 1 and 3 for number of layers and L and X for linear and crosslinked membrane or barrier layers.
The series and polarization resistances over time determined by EIS for cells are plotted in Fig. 6. The initial series resistance of the cells 1L, 3L and 3X are at similar levels around 110 mU cm2. The 1X cell shows higher series resistance (170 mU cm2) as expected due to lower acid doping level. For the three-layered membranes however, crosslinking of the barrier layers does not seem to affect the series resistance at all. The series resistance of the three cells develop linearly by 7e9 mU cm2 h1 over 10,000 h. The 1L cell had the same initial series resistance as the cells with layered membranes, but the value increases much faster than the other three. Looking at the polarization resistance, it appears that this is where the main differentiation between the cells takes places. The three-layered cells degrade only slowly while the single layered cells rise in resistance at a higher and ever increasing rate. Polarization curves were measured after an activation period of 500 h for all the cells except 1L for which is was performed after 800 h as shown in Fig. 7. Nevertheless, there is no distinct performance variation between 500 h and 800 h for this cell as we can see from the durability curves. Two trends are clear from the polarization curves. The 1L cell with the crosslinked membrane has the highest slope of the linear part of the curves, which corresponds well with the lower conductivity of the crosslinked material (Figs. 1 and 6). The other trend is that the cells with three-layered membranes show earlier mass transport limitations (curve bending at high current density) than the cells with single-layered membranes. The reason for this is less obvious, but should likely be sought in the acid distribution in the catalyst layer. It is also striking that the open circuit voltages (OCV) vary a lot. The mixed potential resulting from gas crossover has a strong effect on OCV because of the steep slope of the polarization curve at small current densities. The single-layer based cells appear to reflect that with the crosslinked denser membrane having the highest OCV, but for the three-layer based cell, the order is reversed. Anyway, the OCV is often one of the least
reproducible properties of HT-PEMFC and it is better to refrain from drawing strong conclusion. The acid collection rates shown in Fig. 5 were slightly lower for 3L and 3X than for 1L and 1X. If we assume that this is a significant effect, then we can additionally assume that losses of acid in the form of evaporation and surface diffusion are also lower for 3L and 3X as a consequence of a lower acid content in the catalyst layer. The acid content in the 3L and 3X may thus be less than ideal and result in earlier mass transport effects due to a somewhat poorer utilization of the catalyst material. Had the problem been flooding, then a higher acid content would be expected to lead to a higher acid loss rate indicated by a higher acid collection rate.
Discussion The work was initiated on the idea that the direct cast reservoir layer allows for the containment of more acid (more acid to lose) and the post doped barrier layers secure that the loss rate is similar to that in cells with single post doped membranes. The latter part of the hypothesis was based on the assumption that the interface between the membrane and the catalyst layer remains unchanged. The study did show significantly lower voltage decay rates over up to more than 10,000 h, but the simple assumption that the increased durability would be realized because it takes longer to deplete the membrane with a reservoir, was not verified during the 10,000 h. In the experimental series at 180 C, the acid collection rates were in the same order for the four cells and this supports only one part of the hypothesis. Had the cells been operated much longer with no other degradation effect, it can be expected that eventually the cells with a reservoir would last longer due to postponed lethal depletion. However, the faster degradation seen for the single layered cells was clearly not an effect of acid depletion inside the membrane, but instead an effect on the polarization.
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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Fig. 6 e Evolution of area specific resistances of cells with single-layered membrane and three-layered membrane operated at 200 mA cm¡2 and 180 C. The resistances were determined by EIS. The membrane combinations are indicated on the figure as 1 and 3 for number of layers and L and X for linear and crosslinked barrier layers.
Fig. 7 e Polarization curves of cells measured at 180 C. For 1X, 3L and 3X after 500 h and for 1 L after 800 h.
Whether the faster increase of polarization of the singlelayered cells was a result of catalyst degradation or increased mass transport (or both) cannot be determined from Fig. 7, but regardless, it is likely an effect of acid redistribution in the catalyst layer. Over long term operation, acid is transported from the catalyst layer by evaporation and surface diffusion is balanced by simultaneous replenishing from the membrane. This balance is only strictly maintained if the properties of the acid containing membrane are unaltered. If the replenishment is slightly diminished (or increased) a different acid content in the catalyst layer will be the result and consequently lead to different polarization behavior. With a reservoir with high phosphoric acid activity right behind, the doping level of the membrane layer that faces the catalyst layer is maintained closer to constant than
in a layer without a backing reservoir. With a reservoir the catalyst layer is kept optimal for a longer period. Crosslinked PBI was chosen for alternative barrier layers to limit the acid loss further as compared to linear PBI due to slower diffusion through the dense texture [34]. Surprisingly, this did not lead to a significant further increase of durability. However, after realizing that the main cause for increased overvoltage was in the catalyst layer, it makes sense, as the crosslinked barrier layer may be slightly less efficient in acid replenishment. Thus, acid transport out of the membrane should not be limited too much, only optimized for best catalyst layer performance. The single-layered cells degraded faster, mainly via a polarization effect. In terms of series resistance, the 1L cell started at a lower value (coincidently coinciding with that of the three-layered cells), but increased much faster than the 1X cell. This is in accordance with the lower acid mobility in the crosslinked membrane [34]. If we take a closer look at the four acid collection rates at 180 C, it is striking that both threelayered cells had 17% lower acid collection rates than their corresponding single-layered cells, linear and crosslinked respectively. Likewise both cells with crosslinked membrane components had acid collection rates 11% lower than the corresponding cells with linear membrane components. This trend is not fully reflected in the voltage decay rates. The three-layered cells did decay much slower than the singlelayered cells, but the 3X cell, despite a lower collection rate, decayed faster that the 3L cell. Coming back to the first test series at 170 C, it can be concluded that the higher stability of the 3L cells was also verified here. Likewise, the hypothesis that the acid loss rate (assumed indicated by the acid collection rate) is supported, but apart from that, the results give a less clear picture than the more systematic series at 180 C, especially the EIS results. However, the series resistances, which are the most well
Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186
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defined and most trustworthy indicator, resembled those in the 180 C test series. The series resistance of the 1L cell was much higher, but again, the membrane was also much thicker than the 3L membrane. Despite the difference in thickness, the 1L cell increased its series resistance by twice the rate of the 3L cell. The story told by the development of the polarization is more complex, but the polarization of the 1L increased sharply while that of the 3L was not only maintained like in the 180 C test series, it was actually decreasing. Apparently, the acid redistribution improved in this case. Finally, we make a few notes on the comparison of the test series at 170 C with that at 180 C. Higher rates of voltage decay and acid collection must be expected the higher the temperature, other things being equal. In this study, the 1L and 3L cells in the 170 C series both had more than twice the voltage decay rates than the 1L and 3L cells in the 180 C series. The reason the two series are not fully comparable is that (1) the membrane of the 3L (170 C) was thinner than the other membranes used and (2) the different choice of GDL, where the GDLs used at 180 C were chosen for improved acid retention properties. This may at a first glance appear in poor accordance with the acid collection rates for these cells, which were the lowest of the entire study (63 and 73 ng cm2 h1). However, the initial portion of acid presumably resulting from a pre-test squeeze-out was omitted, as the collection rates were naturally based on linear fits of the rest of the curves. If we compare the total collected acid at time 2,500 h, where the cells at 170 C were stopped, the amount from the 1L and 3L cells at 170 C were ca. 350 and 250 mg cm2, respectively, while only 200e270 mg cm2 was collected from the four cells tested at 180 C after the same period of time.
Conclusions High-temperature PEM fuel cells with three-layered PBI membranes, with the middle layer acting as an acid reservoir is a way to improve durability as compared to conventional cells with single layered post doped PBI membranes. This was verified at constant current operation at 200 mA cm2 at 180 C over testing periods of up to 10,000 h with voltage decay rates of 2.3 and 4.1 mV h1 for the cells with three-layered membranes and 14 and 11 mV h1 for cells with single-layered membranes. The rate of acid loss from the cells, as assessed by in-line acid collection at the exhausts, is largely independent on the presence of an internal reservoir layer. Both linear and thermally crosslinked PBI were used for single-layered membranes as well as for the outer layers (the barrier layers) of the three-layered membranes. The acid collection rate was in both cases 11% lower with crosslinked PBI than with linear PBI, but a lower voltage decay rate was not seen when using crosslinked barrier layers in the threelayered cell. The dominating effect leading to the lower voltage decay rates of the cells with three-layered membranes was on the electrode polarization resistance which grew at a lower rate within the 10,000 h of testing. The initial hypothesis that the acid reservoir of the center membrane layer will extend the lifetime due to a postponed acid depletion, could not be verified in the course of the time available for the test (10,000 h).
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The authors believe that this effect will ultimately play a role if the cells are operated for even longer periods of time.
Acknowledgement The authors gratefully acknowledge the Danish ForskEl program for funding via the project SmartMEA (no. 2014-1-12218). The authors moreover thank Sinh Hy Nguyen at DTU Environment for analyzing samples with ICP-OES and Danish Power Systems for providing PBI membranes for reference.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.186.
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Please cite this article as: Kannan A et al., Three-layered electrolyte membranes with acid reservoir for prolonged lifetime of hightemperature polymer electrolyte membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2019.10.186