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Nafion 117 stability under conditions of PEM water electrolysis at elevated temperature and pressure
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Nafion 117 stability under conditions of PEM water electrolysis at elevated temperature and pressure Jakub Malis, Petr Mazu´r, Martin Paidar, Tomas Bystron, Karel Bouzek* Department of Inorganic Technology, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic
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abstract
Article history:
In this work a systematic study of the behaviour of a Nafion 117 membrane under condi-
Received 26 August 2015
tions of elevated temperatures (up to 150 C) and pressure (up to 0.7 MPa) was carried out.
Received in revised form
Attention focused primarily on the ionic conductivity of the membrane in the proton form
11 November 2015
with exposure to the conditions under study for up to 800 h. The ion-exchange capacity,
Accepted 19 November 2015
morphology, FTIR and NMR spectra of the membrane were determined to explain the
Available online 13 January 2016
decline in conductivity observed over time. The techniques used did not reveal any chemical degradation of the membrane polymer. The morphological changes to the
Keywords:
membrane connected with excessive expansion of the internal structure of the polymer
Nafion 117
are assumed to be the reason for the phenomenon observed. Finally, to confirm the con-
Ionic conductivity
clusions derived, the membrane behaviour in a laboratory-scale water electrolysis cell was
Elevated temperature
studied under operating conditions corresponding to its prior characterization.
Elevated pressure
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Degradation
Introduction Within the last decade studies focusing on proton exchange membrane (PEM) water electrolysis have increased significantly. This research is motivated by the search for efficient, flexible and reliable high-capacity energy storage. This is connected with the need to develop a new, decentralized energy distribution grid capable of compensating for fluctuations caused by the increased capacity of unstable renewable energy sources [1]. According to the concept of the hydrogen economy, this problem could be solved by converting excessive electrical energy into the chemical energy of hydrogen by electrochemical decomposition of water. The hydrogen produced can be used either as a raw material in chemical industries, as a fuel for different applications, such as for car
propulsion, or it can be reconverted into water while recovering electrical energy in periods of excessive electricity demand [2]. Water electrolysis can be carried out under both alkaline and acidic conditions [1,3,4]. Alkaline water electrolysis has been a well-established industrial process for decades. For intermittent operation the acidic electrolysis, utilising a PEM membrane is, however, a more suitable option. This is mainly due to the high intensity and flexibility of this process, the compact, modular design of the cell and easy separation of the produced gases, and the high purity of the product. The main obstacle to the widespread implementation of this technology is the necessity to use an electrocatalyst based on platinum metals [3,5]. A promising method to reduce the electrocatalyst loading is to increase the operating temperature of the process, preferentially to above 100 C. The electrode reaction
* Corresponding author. E-mail address: [email protected] (K. Bouzek). http://dx.doi.org/10.1016/j.ijhydene.2015.11.102 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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kinetics, enhanced by the increased operating temperature, allows the catalyst loading to be reduced substantially, while at the same time increasing process efficiency and maintaining process intensity. The increased operating temperature can, however, cause significant complications, especially for the polymer electrolyte. Today, perfluorinated sulfonated membranes represent the sole option as a polymer electrolyte and, at the same time, as an electrode compartment separator in PEM water electrolysis [1e3]. This is due to their extraordinary properties, namely excellent chemical and mechanical stability in combination with high ionic conductivity. Nevertheless, this material is highly sensitive to its degree of swelling. Above 100 C under atmospheric pressure, i.e. above the normal boiling point of water, the relative humidity of its environment rapidly decreases with increasing temperature. It is connected with the drying out of the membrane, leading to a decline in its ionic conductivity and reduced lifetime [6]. This problem has been intensively studied, mainly in connection with the development of high-temperature PEM fuel cells. There, the problem of low membrane conductivity under conditions of low relative humidity was solved by the application of membranes based on phosphoric acid imbibed into a polymer matrix [7]. This type of membrane, however, has not proven to be sufficiently stable under water electrolysis conditions [8]. A promising alternative is to utilize elevated pressure, allowing water to be kept in the liquid state at the desired operating temperatures and thus avoiding the problem of insufficient membrane swelling. This approach is possible because, in contrast to fuel cell technology, the flooding of the electrodes by water represents a desirable effect in water electrolysis, as water represents the reactant at the anode and no gases have to be transported from the distribution channels to the catalytic layer. The long-term stability of a Nafion-type membrane under the conditions discussed here, i.e. at temperatures above 100 C and at elevated pressure, has, however, not yet been clarified. Hitherto, attention has mainly focused on the behaviour of the membrane in a PEM FC, i.e. on composite materials at least partly maintaining their conductivity even under low relative humidity conditions [9]. To a certain extent, the studies by Alberti et al. [10,11], dealing with the behaviour of the Nafion 117 membrane at temperatures up to 160 C and relative humidity up to 100%, represent an exception. At the same time, however, in these studies the membrane was sandwiched between the electrodes and exposed to different pressures. The authors reported a significant decline in membrane conductivity during its exposure. Since chemical degradation of the membrane materials was excluded, the behaviour was ascribed to the anisotropic swelling of the membrane along the plane of its surface. This phenomenon was not, however, discussed in greater detail. The absence of chemical degradation is in agreement with the results of spectroscopic experiments published by Morita and Kitagawa [12], reporting that chemical degradation commenced at 280 C. On the other hand, for example, there are some reports indicating presence of SO2 in the exhaust gases of low temperature PEM FC (utilising Nafion 112 membrane) at 70 C. As a main reason was mentioned action of H2O2 formed in the system [13].
Alberti et al. [14,15] continued to study the properties of a Nafion-type membrane exposed to elevated temperatures and various degrees of humidity. In the first step [14], mainly the influence of hydrothermal treatment of Nafion 117 on its water uptake and related mechanical properties of the membrane were studied. Later, he investigated the colligative properties of a Nafion 117 membrane in a broad range of temperatures in relation to its visco-elastic properties [15]. One of the main conclusions was that, above approx. 80 C, Nafion 117 must be considered as an ionomer exhibiting viscous behaviour. These conclusions are also supported by recent results of Matos et al. [16], who measured conductivities of Nafion 115 at 100% relative humidity in temperature range of up to 180 C. They found two distinct Arrhenius-like conductivity regions for temperatures below and above 90 C. Moreover, the structural changes in the membrane that occurred at elevated temperature led to changes in its conductivity when experiment was repeated. This suggests the thermal treatment history effects membrane conductivity. Weber et al. approached in their work the effect of membrane humidity and pressure in thickness direction at temperatures below 100 C [17]. Conductivity improved with increasing temperature and membrane humidity. Pressure of up to 8 MPa was found to have no clear effect on the membrane proton conductivity in membrane. All the above-mentioned data were determined primarily for a gaseous environment typical of fuel cell operation. The conclusions drawn are, therefore, only partly relevant to PEM water electrolysis technology. It is thus of interest to evaluate the stability of the Nafion 117 membrane as a typical representative of membranes based on perfluorinated sulfonated polymers under conditions relevant to PEM water electrolysis at elevated temperature and pressure in a liquid water environment. At the same time, the chemical stability of the polymer under the conditions under study needs to be evaluated. The first information on the behaviour of the Nafion 117 membrane under the conditions of interest was provided in our previous work [18]. It is also worth mentioning that there is still no total agreement reached in the community on the anisotropic behaviour of Nafion membranes. Comparison of in-plane and out-of-plane conductivities has been subject of several works. Differences can be attributed to measurement procedure, equilibration times, or membrane history such as large pressure applied in one direction (e.g. (hot)pressing). These aspects can change distances between ion clusters in the bulk of the membrane and therefore induce anisotropy of its conductivity. Several authors report that if Nafion membranes were not subjected to pressure load, identical values of inplane and out-of-plane conductivities were obtained [19,20]. Additionally not all authors considered cell and cell/membrane resistance in order to obtain true Nafion conductivity when performing out-of-plane conductivity measurement. Moreover all these measurements were performed in air atmosphere humidified into the different degree. The results are thus not directly applicable to membrane immersed in liquid water. The aim of this study is to extend this research and to provide more detailed information on the behaviour of Nafion 117. Particularly, we aimed to study Nafion behaviour in liquid
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water in temperature range of up to 160 C and pressures up to 0.7 MPa. These conditions should mimic PEM water electrolysis operating conditions, i.e. ensure exposure of the membrane to the liquid water at given temperature and pressure. Selection of these conditions was motivated by enhancement of the electrode reactions kinetics and potential membrane conductivity in order to intensify electrolysis process and/or possibly reduce catalyst loading.
Experimental Membrane in-plane conductivity determination at elevated temperature and pressure The in-plane ionic conductivity of the Nafion 117 membrane sample (9 52 mm2) was determined in a longitudinal direction by electrochemical impedance spectroscopy (EIS) in a four-electrode arrangement. A Solartron SI 1260 Frequency Response Analyzer connected to a Solartron SI 1287 Electrochemical Interface (Solartron Analytical, UK) was used for this purpose. Impedance spectra were recorded in a frequency range of 65 kHz to 0.1 Hz using a perturbing signal amplitude of 10 mV. A schematic sketch of the experimental set-up used during these experiments is shown in Fig. 1. Demineralized water was pumped from the reservoir (1) by an HPLC pump (2) to the tempered part of the set-up. The residence time of the water in the hydraulic circuit was long enough to ensure its equilibration with the temperature set in the tempered box. Teflon tubing was used to avoid any contamination of the water. Prior to entering the cell, water passed through the absorber containing 28 cm2 of the scarified membrane in the Hþ form (4). The demineralised water in the absorber was equilibrated with the membrane sample in order to remove any impurities potentially present before it entered the conductivity cell. The typical weight of the membrane sample tested was 0.1 g. During the experiment which lasted for
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several hundreds of hours about 45 dm3 of water flowed through the cell. Therefore, even traces of ions present in the demineralised water could substantially contaminate the measured sample should they be exchanged for the Hþ present in the membrane. Subsequently, water entered the conductivity cell (5), formed by a flow-through channel, in which the membrane sample was positioned. On each end of the sample two platinum wires were pressed against the membrane surface, one acting as a terminal and the second as a sensing electrode. At opposite ends of the samples these electrodes were positioned on opposite sides of the membrane (8). Demineralised water flowing continuously through the cell washed all sides of the membrane as well as the attached electrodes continuously and applied isotropic pressure to the sample. The conductivity cell was made of polyether-ether-ketone (PEEK). After leaving the conductivity cell, the water passed through the back-pressure valve (6) controlling the pressure in the conductivity cell (5) and flowed into the reservoir of spent water (7). The pressure in the pressurized conductivity part of the set-up was recorded by a manometer (3). After assembling the cell and connecting it to the experimental set-up, the system was pressurized by the water. Subsequently it was heated to the desired temperature. When determining the dependence of membrane in-plane conductivity on temperature, the measurement was initiated 5 min after the temperature in the tempering box had reached the desired value. The short delay served to minimise any possible changes to the membrane sample during conditioning. The measurement was repeated at 5-min intervals. The maximum conductivity value determined was considered as the membrane conductivity under the given conditions. The temperature range of 30e160 C was studied. The long-term in-plane conductivity measurements started 2 h after the system had reached operating temperature. This delay allowed the membrane to equilibrate with the experimental conditions. The evolution of membrane conductivity in time was determined at temperatures of 110, 130 and 150 C and at pressures of 0.5 and 0.7 MPa. Typically the conductivity experiments lasted for 800 h. The measurements were taken every 2 h.
Additional methods of membrane characterization
Fig. 1 e Schematic sketch of the experimental set-up for membrane conductivity determination at elevated temperature and pressure; 1 e reservoir of deionized water, 2 e HPLC pump, 3 e manometer, 4 e absorber, 5 e conductivity cell, 6 e back-pressure valve, 7 e reservoir of spent deionized water, 8 e electrodes.
The ion-exchange capacity (IEC) of the membrane prior to and after the degradation experiment was evaluated potentiometrically [21]. The activated membrane sample in the Hþ form was plunged into 125 cm3 of 0.1 mol dm3 NaCl solution. The potential response of a Ross combined glass electrode (Orion, USA) immersed in the solution was recorded by a Keithley 6514 electrometer (Keithley, UK) with high input impedance (200 TU). The recorded value of the glass electrode potential was converted into the amount of Hþ ions displaced from the membrane into the solution by means of a calibration curve. The experiments were performed under argon atmosphere. The IEC was evaluated on the basis of the equilibrium concentration of the Hþ ions in the solution and the weight of the dry membrane sample. The membrane morphology in the dry state was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A Hitachi S4700 microscope was used to obtain
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SEM pictures. After the experiments, the membrane samples were stored in a desiccator for a week. Prior to analysis, they were sputtered by a 20 nm thick layer of Au/Pd to avoid the surface charging. The AFM experiments were carried out using a Veeco CP-II instrument (Veeco, USA). Fourier transform infrared (FTIR) spectra of membrane samples were obtained using a NICOLET IS10 (Thermo Scientific, USA) spectrometer in reflection mode on the ATR crystal. The spectra were collected after 64 scans for both the background and the samples in the wave number range between 4000 and 650 cm1 with a wave number resolution of 4 cm1. Background subtraction was used. Measurements were carried out at laboratory temperature with samples in equilibrium with air humidity. Nuclear magnetic resonance (NMR) data were obtained using Oxford/Varian 300 MHz NMR (Varian, UK) with a 5 T superconducting solenoid unit. Samples were prepared by dissolving the membrane in isopropyl alcohol at 200 C using an autoclave with a Teflon lining. The dissolution took 12 h. 19 F NMR spectra of solutions containing 5 wt.% of Nafion 117 were measured at laboratory temperature. Each spectrum was collected from 128 scans with 10 s delay and 300 MHz pulse. A change of membrane volume during the long-term conductivity measurements was calculated from the membrane dimensions prior to and after experiments. Membrane thickness was determined by screw micrometer which applied pressure of around 165 kPa to the sample. Membrane length and width were measured by calliper.
Water electrolysis The membrane was tested in a laboratory-scale single cell water electrolysis set-up. Gold-plated titanium end-plates with parallel flow channels with an active area of 4 cm2 were used. As a cathode, a commercially available platinum-based gas diffusion electrode HT ELAT (BASF, USA) with a platinum catalyst loading of 0.5 mg cm2 was used. The anode, based on titanium felt activated by IrO2 (0.8 mg cm2), was made in-house. The Nafion 117 membrane was used as a polymer electrolyte. The cell was constricted to a momentum force of 5 Nm. Operating temperatures of 110 and 150 C were used. The electrolysis operating pressure was set by a back pressure regulator to 0.5 MPa in both the anode and cathode compartments. Demineralized water, preheated to its operational temperature prior to entering the cell, was circulated through the anode compartment. Long-term electrolysis at a cell voltage of 1.7 V was performed in order to determine the stability of the system. During the electrolysis load curves and EIS spectra were measured every 1 and 48 h, respectively. The load curve was measured between 1.3 and 1.75 V in 0.05 V steps (lasting for 60 s to allow the current to stabilise) by a Statron 3251.0 stabilized power source (Statron, Germany). The EIS spectra were measured at a cell voltage of 1.5 V with a perturbing signal frequency of 65 kHze100 Hz using 10 mV amplitude.
the membrane sample in deionized water at 80 C for 60 min and in 3 wt.% H2O2 at 60 C for 30 min. Subsequently, the membrane was immersed in 0.05 mol dm3 H2SO4 solution at 60 C for 30 min in order to obtain Nafion in the Hþ form. Finally, excess H2SO4 was removed from the sample by soaking it in deionized water at 60 C for 120 min. In this way activated membranes were stored in deionized water at room temperature. All chemicals used in this study were of analytical or higher grade. H2SO4, H2O2 and isopropyl alcohol were purchased from Lach-Ner (CZ). Deionized water was prepared in a demineralization station constructed in-house and it had a conductivity of 0.9 mS cm1 at 20 C.
Results and discussion Membrane conductivity at elevated temperature and pressure The values of the Nafion 117 membrane in-plane conductivities determined at different temperatures and pressures of 0.1, 0.5 and 0.7 MPa shortly after exposition to experimental conditions are summarized in Fig. 2. As expected, the conductivity increased with rising temperature. Whereas at 30 C it has a value of approximately 9.9 S m1 for all the pressures studied, at 160 C it reaches values above 37 S m1. It should be mentioned that the precision of the conductivity determination decreases with increasing temperature and pressure due to the reasons discussed later. At the highest temperatures the conductivity value can be underestimated by about 10%. Nevertheless, the data still show clear trends. The increase in membrane in-plane conductivity with temperature is caused by the improved mobility of the protons in the polymer phase. This is related to a decrease in pore fluid viscosity on the one hand and at the same time by a reduction of the activation barrier for transport of protons between the hydrophilic clusters forming the internal structure of the membrane [11].
Materials and chemicals used A Nafion 117 membrane (Ion Power, USA) was used as a polymer electrolyte in this study. Prior to application, the membrane was activated [22]. Activation began by immersing
Fig. 2 e Dependence of ionic conductivity of the fully hydrated membrane in the Hþ cycle on temperature at three pressures; the pressure values are indicated in the inset.
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The in-plane conductivity dependence on temperature, shown in Fig. 2, clearly deviates from the simple exponential shape expected according to the Arrhenius law. Additionally, the conductivity does not seem to be influenced by external pressure on the time scale of the measurement. A more detailed analysis of the conductivity data is presented in Fig. 3. At both elevated pressures two distinct domains can be recognised in the Arrhenius plot. The low temperature domain ranging from 30 to 70 C is characterised by activation energy of about 9 kJ mol1. Similar conductivities and activation energy were found by other authors for Nafion 11X membrane types [16,23,24]. The activation energy of about 10 kJ mol1 corresponds to proton Grotthus conduction mechanism [16]. For the high temperature domain localised between 110 and 160 C the value of approximately 16 kJ mol1 was found. The temperature range of 90e110 C represents the transition between these two domains and corresponds well with the glass transition temperature (Tg) reported in the literature for Nafion in the Hþ-form. At this point the mechanical properties of the Nafion 117 polymer need to be discussed in dependence on temperature. Generally the Tg of Nafion 117 in the Hþ-form is reported to be located between 110 and 120 C [25,26]. The situation is, however, more complex. According to Osborn et al. [27], the Tg of Nafion membrane of equivalent weight (EW) 1100 in the Hþ-form is located close to 20 C (b-relaxation). This corresponds to the polymer backbone motion in the framework of a physically cross-linked (hydrogen-bond)
Fig. 3 e Arrhenius plot for the conductivity of the membrane in a proton cycle under conditions shown in Fig. 2; lines indicate linear fit of the data, pressure values are indicated in the inset.
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network. The temperature above which the hydrogen-bond network becomes dynamic (a-relaxation) is located close to 100 C. Any morphological reorganizations are only possible above this temperature. It should be noted that the a-relaxation strongly predominates over b-relaxation. Due to these facts, in the present study a-relaxation is denoted as the Tg. The determined increase in activation energy of proton transport from 9 to 16 kJ mol1 suggest possible difference in proton transport mechanism below and above the Tg. Below the Tg and at reasonably high degree of membrane swelling the Grotthus proton conduction mechanism is most important [28]. Above the Tg the situation is not fully clear. We may expect more complex arrangement and involvement of all three possible transport mechanisms is possible. In the expanded hydrophilic domains migration in a solution and Grotthus mechanism may be considered to play an important role. Between the domains surface hopping through the hydrophobic channels may play a primary role. It strongly depends on the extent of the swelling and of the corresponding structural changes. Very little, if any, dependence of the membrane conductivity on the pressure was observed in the time-scale of these experiments, which suggests a slow rate of membrane swelling. Moreover, the relatively high value of Nafion 117 conductivity represents a promising aspect with respect to its intended utilization as a polymer electrolyte in mediumtemperature water electrolysis, i.e. in the range of 100e200 C. Independence of Nafion in-plane conductivity on pressure on short-time scale was observed also by Weber et al. [17], although their experiments covered temperature region of up to 100 C only. Besides this positive aspect, the danger of the conductivity of the Nafion 117 membrane decline at elevated temperature and pressure must be taken seriously [10,11]. This aspect may disqualify such a type of membrane for an envisaged application. Therefore, in the next step, long-term conductivity measurements of the Nafion 117 membrane were performed. The results are summarized in Fig. 4. Contrary to the results showed in Fig. 2, where conductivities measured shortly after condition equilibration are present, in Fig. 4 the effect of pressure is substantial. At a pressure of 0.5 MPa the highest initial conductivity was observed at a temperature of 110 C. The fact that the initial conductivities determined for temperatures of 130 and 150 C are lower may be explained by the rapid initial decrease in conductivity with time at these two high temperatures. The samples had been equilibrated for 2 h under the conditions of the experiment before the conductivity was recorded. During this period, which was necessary for the cell to reach the required temperature, the conductivity decline had already started, as can be deduced from the early stage of the obtained dependence, see Fig. 4. On the other hand, at a temperature of 110 C, the membrane conductivity remained approximately constant during the initial 350 h and thus it exceeded the values determined for the higher temperatures. A pressure of 0.7 MPa led to a similar conductivity degradation rate for all temperatures studied and, therefore, such a discrepancy did not arise. Two basic findings clearly follow from Fig. 4: (i) value of inplane Nafion membrane conductivity at elevated temperature and pressure declines in time and (ii) the rate of decline is
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Fig. 4 e Dependence of the in-plane conductivity of the membrane on time of exposure to elevated pressure at different temperatures; temperature and pressure values indicated in the inset.
accelerated by increasing temperature and pressure. At temperatures of 130 and 150 C the rapid exponential decline immediately after the conductivity reading begins is followed by a slow linear decline in membrane conductivity. On the other hand, a stable conductivity value for a relatively long period of time, followed by a sudden accelerated decrease, can be observed at 110 C, especially at a pressure of 0.5 MPa. There are different possible explanations for the observed phenomena. The first possibility considered is thermal degradation of the polymer, namely its desulfonation [12]. If this was the case, the effect of the pressure would be significantly less pronounced and the predominant influence on the observed conductivity decrease would have to be that of temperature. However, this explanation does not correspond with the long-term conductivity evolution observed at temperatures of 110 and 130 C, where the conductivity decline strongly depends on the applied pressure, see Fig. 5. A second and more likely explanation is connected with the change in the mechanical properties of the membrane at or near the Tg, i.e. the temperature above which the polymer loses its mechanical stability. The reported glass Tg region of 110e120 C [25,26] corresponds to the lower limit of the temperature range covered within the long-term degradation experiments. This indicates that the mechanical properties of the polymer material may represent a critical issue, especially at elevated operational pressure, where the membrane
Fig. 5 e Comparison of dependence of the in-plane conductivity s of the membrane on time; temperature and pressure are indicated in the inset.
polymer tends to swell excessively. At a temperature well below the Tg the internal forces in the polymer do not allow an excessive increase in the internal void volume and the polymer is mechanically stable. If, however, the Tg is exceeded, the situation changes dramatically. Under such conditions the internal forces may be surpassed by the solvation forces of the functional sulfo-groups, resulting in an expansion of the hydrophilic domains of the polymer. Therefore, at 150 C the higher pressure accelerates excessive swelling more progressively than at 130 C. Such an explanation is in agreement with the conclusions of Alberti et al. [14]. The proposed hypothesis of excessive membrane swelling is also supported by the observed increase in the dimensions of the membrane sample during the long-term
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conductivity measurements. Information on the relative increase in the dimensions and volume of the membrane observed during the long-term conductivity measurements is summarized in Table 1. The values in the table clearly document that the membrane dimensions increase in all directions. The most extensive swelling can be observed in the membrane thickness, where an increase of up to 24% was determined. Swelling in other directions, except for the membrane length at temperature and pressure of 150 C and 0.7 MPa, respectively, did not exceed 10%. The membrane dimensions determined after the end of the degradation measurement, and consequently the calculated volume, will most likely differ from the corresponding values in the conductivity cell. This also leads to uncertainty regarding the membrane conductivity determined, as mentioned at the beginning of this section. On the other hand, membrane sample dimension changes qualitatively show the abovedescribed dependence of the irreversible swelling extent on the temperature and pressure, i.e. the higher temperature and pressure in the system during measurement, the higher membrane swelling. The volume changes presented in Table 1 may also explain the conductivity differences visible in Fig. 5 for pressures of 0.5 and 0.7 MPa at 110 C. At a pressure of 0.5 MPa a volume increase of only 20e40% was observed, while at 0.7 MPa it was already about 35e50%. It was shown earlier that water acts as a plasticiser [26,29]. Therefore, its increasing content in the membrane structure leads to a reduction of the Tg value. Thus the reason for behaviour observed here is that the amount of water present in the membrane and influencing its properties is dependent on the external pressure. In other words, a pressure of 0.5 MPa at a temperature of 110 C is not sufficient to swell the membrane to such a degree as to bring about a rapid decrease in the Tg to, or below, 110 C. For this reason, almost 400 h of exposure are needed at 0.5 MPa and 110 C to achieve such a situation. From this point onwards excessive swelling connected with a conductivity decline is initiated. As follows from the data presented so far, whereas an initial increase in membrane swelling has a positive effect on its conductivity, longer exposure results in a rapid decline of this characteristic. These two phases of membrane swelling have to be clearly distinguished. The first swelling phase leads to a restructuring of the hydrophilic domains (change in the orientation of the hydrophilic domains [25,30]) in the membrane structure and causes increased ionic mobility in its interior. The second phase includes irreversible changes in the internal structure of the polymer material and the overall
membrane performance. This is connected with a loss of interaction between the polymer backbone chains, resulting in a downgrade of its mechanical properties [31]. It is not yet fully clear whether the difference in the membrane behaviour at the two higher pressures during the long-term conductivity measurement is caused exclusively by the substantial changes in the polymer material internal structure, or whether the change in the mechanical properties of the polymer reduced the accuracy of the conductivity measurement due to the irregular contact between the polymer electrolyte membrane and the electrodes. This fact also renders an evaluation of the conductivity data significantly more complex because the sample dimensions were measured at atmospheric pressure prior to and after the experiment. Moreover, the sample dimensions required to recalculate the measured ohmic resistance of the membrane to its conductivity were assumed to change linearly in time during the experiment. This assumption introduces another error into the conductivity data presented. Nevertheless, it is an acceptable compromise, allowing the dimensional changes to be included in the evaluation of the experiment. A close relationship between the membrane conductivity and its swelling is documented by the data summarized in Fig. 6. The data on membrane conductivity and sample dimensions obtained at the end of the long-term conductivity experiment, and thus free from errors caused by volume interpolation, were used to construct this diagram. It is evident that the ionic conductivity of Nafion 117 decreases linearly with increasing final to initial volume ratio of the sample at pressure of 0.5 MPa. This is in agreement with the postulated explanation of the decline in membrane conductivity with time of exposure. At pressure of 0.7 MPa, however, the conductivity does not show any clear dependence on the membrane volume changes determined after the experiment. It is very likely that the membrane swelling is partially reversible so that, during cooling and depressurising of the system, the membrane partially shrinks. This will be truth especially at pressure 0.7 MPa and temperatures 130 and 150 C. As a result, the actual membrane volume at high temperature and pressure can differ greatly from that measured at ambient room conditions.
Membrane morphology In the first instance, an attempt was made to identify any possible changes in membrane morphology using SEM.
Table 1 e Changes in Nafion membrane dimensions during long-term conductivity measurements at elevated pressure and temperature. Membrane: a e length, b e width, c e thickness. Pressure
Temperature
Dimensions [mm] Before
Volume changes After
[MPa]
[ C]
a
b
c
a
b
c
[%]
0.5
110 130 150 110 130 150
50.3 50.1 49.2 49.7 50.2 48.3
9.9 9.8 9.7 10.2 9.5 9.6
0.203 0.211 0.206 0.205 0.209 0.207
51.3 52.1 52.9 52.7 54.3 55.1
10.2 10.4 10.4 10.9 9.9 10.1
0.231 0.246 0.249 0.250 0.253 0.256
119.6 128.7 139.3 138.2 136.5 148.4
0.7
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Fig. 6 e Dependence of Nafion 117 conductivity at the end of the long-term conductivity experiment as a function of the sample volume; V stands for the sample volume at the end and V0 at the beginning of the experiment.
Images of both the membrane cross-section and its surface were obtained. In neither case did a SEM photograph of a sample exposed to elevated temperature and pressure reveal any changes in membrane morphology. It can be concluded that either they are not visible ex-situ on the dry membrane or they are present on a significantly smaller scale than that provided by the SEM image. Therefore, the SEM images are not shown here. More detailed information was obtained by AFM, a significantly more sensitive technique. The surface morphology of the membrane prior to and after exposure to a temperature of 150 C and pressure of 0.7 MPa is shown in Fig. 7. When comparing individual surface scans, a basic trend is clearly visible. Exposure to the experimental conditions leads to an approximately three-fold increase in the roughness of the membrane surface. Instead of relatively minor stochastic irregularities originating from the membrane production by thermal extrusion visible on the freshly activated membrane (Fig. 7A, B), numerous protrusions appear on the surface of the exposed membrane (Fig. 7C, D). Their origin may well be explained by excessive swelling of the polymer, leading to local expansion of less mechanically stable internal hydrophilic micelles. Even during subsequent drying of the sample for one week over silica preceding the AFM experiment, the surface changes originating from the excessive swelling have not completely recovered.
Membrane ion-exchange capacity The above explanation of the performance of the Nafion 117 membrane under elevated temperature and pressure has so far been supported indirectly by the morphological changes in the membrane sample. The development of the IEC of the membrane after conductivity measurements, i.e. after exposure to elevated temperature and pressure, may provide additional information. The IEC values obtained for membranes before and after long-term degradation are summarized in Fig. 8. Two main observations can be made from this
figure: (i) the IEC value reaches a maximum at a temperature of 110 C; (ii) membranes exposed to a pressure of 0.5 MPa have a higher IEC value compared to those exposed to 0.7 MPa. These results are consistent with data published by Kwon et al. [25]. As already discussed in chapter 3.1, with the initial swelling the orientation of the proton transport channels changes. It opens the polymer structure, enabling it to access hitherto closed proton-conductive domains. This explains why the IEC increases after exposure to the experimental conditions at temperatures below the Tg. At higher temperatures and pressures the IEC values decline to below the level obtained for the freshly activated membrane. This is explained by isolation of part of the ion-exchange groups from the surrounding solution, thus incurring the loss of these ion-exchange sites. This is caused by the collapse of the structure above the Tg, resulting from excessive swelling of part of its structure characterised by lower mechanical strength. Chemical degradation can be excluded at temperatures of 110 and 130 C because, at a pressure of 0.5 MPa, the IEC exceeds that of the freshly activated membrane. After exposure to 150 C the IEC is lower than that of the freshly activated membrane for both pressures under study. Nevertheless, since the IEC values obtained for these pressures are different, the reason may be considered to be different from thermally induced chemical degradation of the polymer.
Membrane polymer characterization by FTIR Direct information on possible changes in the polymer structure and thus its chemical degradation can be provided by FTIR spectroscopy. The spectra obtained for the freshly activated Nafion 117 membrane and for the membrane exposed to elevated temperature and pressure are summarized in Fig. 9. Individual vibration lines were assigned, as indicated in Fig. 9, with the help of [32]. As can be seen from the spectra, there are no significant deviations between the activated sample and samples exposed to the conditions studied. Potential chemical degradation of the membrane polymer is most likely to take place on the side-chains carrying functional groups. This is due to the extreme chemical stability of the poly(tetrafluoroethylene) backbone. Hence, the spectra changes related to polymer degradation must be anticipated mainly in lines 900e1140 cm1 sym. val. and 1270e1060 cm1 asym. val. corresponding to the eCeOeCe bond. In the case of degradation, it would most probably be replaced by eCOOH groups (visible as carboxylic eOH groups due to lines at 3550e3500 cm1 and 3300e3130 cm1 or as carboxylic eC]O groups due to lines at 1800e1750 cm1 [12,33]. Alternatively a cleavage of the eCeSeOe bond (lines at 1350e1550 cm1) would have the same result, i.e. carboxylation of the side chain [32]. The slight variation of the line intensities at 1620 and 3400 cm1 corresponds to the different hydration degree of the membrane samples. In summary, no significant degradation-related changes, such as the appearance of new lines corresponding to polymer degradation products, can be identified in the spectra obtained. This finding supports the earlier assumption that the deterioration of membrane conductivity is not related to chemical degradation of the polymer, but more likely to its structural changes.
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Fig. 7 e Morphology of the Nafion 117 membrane surface (sputtered by a 20 nm thick layer of Au/Pd) obtained using AFM (A,B) prior to and (C,D) after exposure to a temperature 150 C and pressure 0.7 MPa for 850 h. (A,C) picture area of 10 £ 10 mm2, (B,D) picture area of 2 £ 2 mm2.
Membrane polymer characterization by
19
F NMR
NMR spectroscopy represents an alternative, highly sensitive tool for determining the structure of chemical molecules. Therefore, it was selected as a complement to FTIR spectroscopy. NMR spectra were taken of the dissolved Nafion 117 membrane prior to and after exposure. Two approaches to spectra analysis were used to identify possible changes in the polymer structure. First, the obtained spectra of freshly
Fig. 8 e Ion-exchange capacity of the Nafion 117 membrane prior to and after exposure to elevated temperature and pressure; individual conditions of exposure are indicated at the x-axis description, exposure duration can be derived from Fig. 4.
activated and exposed membranes were compared [34]. Degradation of the polymer would result in the appearance of a new signal in the spectra. Spectra of the Nafion 117 solutions
Fig. 9 e IR spectra of the fresh Nafion 117 membrane and samples after exposure to elevated temperature and pressure; the experimental conditions the sample was exposed to are indicated in the graph, selected absorption bands are indicated in the spectrum obtained for the sample exposed to 150 C and 0.7 MPa. Indexes a and s stand for asymmetric and symmetric vibrations, respectively.
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before and after exposure are shown in Fig. 10. Close examination of these spectra revealed no differences that would indicate polymer degradation. The second approach involved a comparison of the intensity of two characteristic signals. Any change in their ratio would indicate a change in the content of the corresponding part of the molecule contained in the polymer. For this analysis the integral values of signals corresponding to the part of the main backbone where the side chain is connected, i.e. OCF2CF2 (83 ppm), and side chains where functional groups are connected, i.e. CF2CF2SO3H (121 ppm), were selected. The ratios of the integral values of these signals had a value of 1.454917 and 1.454926 for Nafion 117 prior to and after exposure to 150 C and 0.7 MPa for 850 h. For all the other conditions studied, the results obtained by the two approaches were identical and it can thus be concluded that no chemical degradation of the ionomer occurs or, if so, its extent is lower than 5%.
Water electrolysis at elevated temperature and pressure The above results indicate that chemical degradation of the polymer does not present a problem in the application of a Nafion 117 membrane in a water electrolysis cell working at temperatures above 100 C (the upper temperature limit studied here is 150 C). To keep this material sufficiently conductive it is necessary to work under high enough pressure so that the water inside the cell remains in a liquid state. However, under such conditions the mechanical stability of the membrane and its ability to separate gases produced at the anode and the cathode represent a crucial problem.
Fig. 10 e 19F NMR spectrum of Nafion 117 membrane prior to and after exposure to 150 C and 0.7 MPa.
Fig. 11 summarises the results of testing the stability over time of Nafion 117 under conditions of laboratory electrolysis. It shows the development of the current density in the electrolyser in time. The initial current density observed at 150 C and at a cell voltage of 1.7 V represents 190% of the value determined at 110 C. This can be explained by the more sluggish reaction kinetics at a lower temperature. It was not possible to extract the desired kinetic information from the EIS spectra precisely. However, it can be said that activation resistance at 110 C and 150 C was about 0.2 U and 0.05 U (at 1.5 V), respectively. Importantly, these values did not change much on the timescale of the electrolyses. The EIS spectra were recorded at 1.5 V because the currents at 1.7 V exceeded the current range of the impedance frequency analyser used. Therefore, the values obtained are only qualitatively relevant for electrolysis operated at 1.7 V. Particularly, the activation resistance values at 1.7 V will be several times lower than those determined at 1.5 V. A rapid decline in current density occurred immediately at the start of electrolysis at 150 C. With time the current density decrease slowed down. After 162 h of the experiment the cell operating at 150 C was short-circuited. Ohmic resistance of the cell, represented mainly by the membrane resistance, increased steadily from 0.04 U at the beginning of the experiment to about 0.07 U at the moment just before the short circuit. A likely explanation for the observed phenomena is that the decreased mechanical stability of the Nafion membrane at this temperature in combination with the pressure of the electrodes (applied during cell assembly) led to thinning of the Nafion membrane. This eventually resulted in the cell short-circuiting. Simultaneously with the membrane thinning the conductivity of the Nafion material was decreasing due to disruption of its inner structure, as discussed in the previous sections. These two effects had a contrary consequence for the measured cell resistance. Another potential contribution to the electrolysis cell performance decline represents an
Fig. 11 e Time dependence of the performance of the laboratory PEM water electrolyser with a Nafion 117 membrane at a cell voltage of 1.7 V and pressure 0.5 MPa in both compartments; titanium felt anode with 0.8 mg cm¡2 of IrO2, HT ELAT gas diffusion electrode with Pt loading of 0.5 mg cm¡2, temperature is stated in the inset.
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increase in the contact resistance between the IrO2 catalyst and titanium felt support. The performance of the electrolysis cell operated at a temperature of 110 C remained relatively stable for the first 160 h. This agrees very well with the range of the almost constant value of the ohmic cell resistance which remained between 0.05 and 0.06 U. At the same time this is in accordance with the Nafion conductivity data discussed in previous sections. After 160 h the current density decline became accelerated and it eventually dropped from about 500 mA cm2 to 110 mA cm2. Despite the low current density the electrolysis was running even after 380 h of operation. This performance decay was accompanied by a faster increase in the cell ohmic resistance to more than threefold the initial value, see Fig. 12. This is in agreement with the results of Stassi [35] who, in the context of fuel cell tests, reported that the decrease in the conductivity of the Nafion membrane was accelerated by the decrease in the crystallinity of its material. In the present study, such a decrease is accelerated by the increasing amount of water being absorbed by the membrane, i.e. by the increasing operational pressure. The overall electric resistance in the electrolyser, calculated from the cell voltage and the current, is illustrated in Fig. 12. In the first 160 h the values of the overall resistance increased from 0.12 to 0.21 U at 150 C and from 0.21 to 0.32 U at 110 C. Then the cell operated at 150 C was shortcircuited and a sharp increase in the overall cell resistance from about 0.32 up to 1.16 U was observed at 110 C. The overall cell resistance at the beginning of the experiment corresponds approximately to the sum of the determined ohmic and polarisation resistance. At the end of the experiment the sum represents only about 35% of the overall cell resistance. However, it has to be kept in mind that the ohmic resistance was determined at a cell voltage of 1.5 V, while electrolysis was operated at 1.7 V, where a higher current flows through the electrolyser. Thus, the remaining part of the overall cell resistance is likely due to the blockage or
Fig. 12 e Evolution of the overall and ohmic resistance of the laboratory PEM water electrolysis cell with a Nafion 117 membrane over time; operating pressure 5 bar; EIS experiments performed at a cell voltage of 1.5 V using perturbation signal amplitude of 10 mV; titanium felt anode with 0.8 mg cm¡2 of IrO2, HT ELAT gas diffusion electrode with Pt loading of 0.5 mg cm¡2.
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inactivation of part of the anode at higher voltage and throughout the electrolysis. This results in locally significantly enhanced current densities in the parts of the electrode remaining active. From a global (i.e. overall cell) point of view locally enhanced current density appears as an increase in the apparent overall cell resistance. The electrode blockage mentioned may be a consequence of the accumulation of produced gas in the electrode structure. A similar effect can also occur by a partial blockage of the structure of a porous anode catalyst layer due to excessive swelling/degradation of the recast Nafion used as a binder of the catalyst under conditions of electrolysis. Recast Nafion is known to have lower stability than a Nafion membrane [36,37]. In summary, although elevated pressure allows the use of the membrane at a temperature above 100 C and thus a short-term improvement of its conductivity, on the other hand it is responsible for its rapid deterioration. Shortcircuiting of the cell is, therefore, most probably caused by gradual thinning of the membrane as a result of the polymer becoming plastic at a temperature higher than its Tg and being pressed out of the space between the electrodes by the pressure applied on the system during its assembly. Parallel to the stability of the Nafion membrane, the stability of the recast Nafion in the catalyst layer has to be taken into account.
Conclusions The results of the experiments conducted show that, in agreement with Alberti et al. [10,11], a decrease in conductivity of the Nafion 117 membrane with increasing time of its exposure to an elevated temperature (110e150 C) and pressure (0.5e0.7 MPa) in a liquid water environment is caused by changes in the internal structure of the polymer and not by its chemical degradation. Structural changes are related mainly to excessive swelling of the polymer occurring at temperatures exceeding its Tg. Additional study is needed to identify the exact nature of the structural changes occurring in the polymer during the excessive swelling. Nevertheless, the results obtained within the framework of this study show that the application of polymer electrolyte materials based on perfluorinated sulfonated acids in the process of water electrolysis at a temperature above 100 C is possible at elevated pressure ensuring sufficient membrane hydration. Significant attention has, however, to be paid to the stability of the mechanical properties of the polymer to prolong cell life time. An interesting option offers also utilisation of the short side chain perfluorinated sulfonic acids showing better viscoelastic properties at elevated temperature. This will be the subject of further study.
Acknowledgement Financial support of this work by the Grant Agency of the Czech Republic within the framework of the Project No. 1502407J and by the Specific University Research MSMT No. 20/ 2015 is gratefully acknowledged.
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references
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[19] Ma S, Siroma Z, Tanaka H. Anisotropic conductivity over inplane and thickness directions in Nafion-117. J Electrochem Soc 2006;153:A2274e81. [20] Cooper KR. Progress toward accurate through-plane membrane resistance and conductivity measurement. ECS Trans 2009;25:995e1007. [21] Karas F, Hnat J, Paidar M, Schauer J, Bouzek K. Determination of the ion-exchange capacity of anion-selective membranes. Int J Hydrogen Energy 2014;39:5054e62. S, Cı´lova Z, Bouzek K. Preparation of a novel [22] Moravcova composite material based on a Nafion® membrane and polypyrrole for potential application. J Appl Electrochem 2005;35:991e7. [23] Halim J, Bu¨chi FN, Haas O, Stamm M, Scherer GG. Characterization of perfluorosulfonic acid membranes by conductivity measurements and small-angle X-ray scattering. Electrochim Acta 1994;39:1303e7. [24] Zawodzinski Jr TA, Derouin C, Radzinski S, Sherman RJ, Smith VT, Springer TE, et al. Water uptake by and transport through Nafion® 117 membranes. J Electrochem Soc 1993;140:1041e7. [25] Kwon O, Wu S, Zhu D. Configuration changes of conducting channel network in Nafion membranes due to thermal annealing. J Phys Chem B 2010;114:14989e94. [26] Yang C, Srinivasan S, Bocarsly AB, Tulyani S, Benziger JB. A comparison of physical properties and fuel cell performance of Nafion and zirconium phosphate/Nafion composite membranes. J Membr Sci 2004;237:145e61. [27] Osborn SJ, Hassan MK, Divoux GM, Rhoades DW, Mauritz KA, Moore RB. Glass transition temperature of perfluorosulfonic acid ionomers. Macromolecules 2007;40:3886e90. [28] Choi P, Jalani NH, Datta R. Thermodynamics and proton transport in nafion, II. Proton diffusion mechanisms and conductivity. J Electrochem Soc 2005;152:E123e30. [29] Jalani NH, Dunn K, Datta R. Synthesis and characterization of Nafion®-MO2 (M ¼ Zr, Si, Ti) nanocomposite membranes for higher temperature PEM fuel cells. Electrochim Acta 2005;51:553e60. [30] Bass M, Berman A, Singh A, Konovalov O, Freger V. Surface structure of Nafion in vapor and liquid. J Phys Chem B 2010;114:3784e90. € nsel H, [31] Schmidt K, Schoberth HG, Ruppel M, Zettl H, Ha Weiss TM, et al. Reversible tuning of a block-copolymer nanostructure via electric fields. Nat Mater 2008;7:142e5. [32] Socrates G. Infrared characteristics group frequencies. John Wiley & Sons; 1980, ISBN 0-471-27592-1. [33] Li J, Yang X, Tang H, Pan M. Durable and high performance Nafion membrane prepared through hightemperature annealing methodology. J Membr Sci 2010;361:38e42. [34] Chen Q, Schmidt-Rohr K. 19F and 13C NMR signal assignment and analysis in a perfluorinated ionomer (Nafion) by Twodimensional solid-state NMR. Macromolecules 2004;37:5995e6003. [35] Stassi A, Gatto I, Passalacqua E, Antonucci V, Arico AS, Merlo L, et al. Performance comparison of long and shortside chain perfluorosulfonic membranes for high temperature polymer electrolyte membrane fuel cell operation. J Power Sour 2011;196:8925e30. [36] Siroma Z, Fujiwara N, Ioroi T, Yamazaki S, Yasuda K, Miyazaki Y. Dissolution of Nafion membrane and recast Nafion film in mixtures of methanol and water. J Power Sour 2004;126:41e5. [37] Kannan R, Parthasarathy M, Maraveedu SU, Kurungot S, Pillai VK. Domain size manipulation of perflouorinated polymer electrolytes by sulfonic acid-functionalized MWCNTs to enhance fuel cell performance. Langmuir 2009;25:8299e305.
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Nafion 117 stability under conditions of PEM water electrolysis at elevated temperature and pressure Jakub Malis, Petr Mazu´r, Martin Paidar, Tomas Bystron, Karel Bouzek* Department of Inorganic Technology, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic
article info
abstract
Article history:
In this work a systematic study of the behaviour of a Nafion 117 membrane under condi-
Received 26 August 2015
tions of elevated temperatures (up to 150 C) and pressure (up to 0.7 MPa) was carried out.
Received in revised form
Attention focused primarily on the ionic conductivity of the membrane in the proton form
11 November 2015
with exposure to the conditions under study for up to 800 h. The ion-exchange capacity,
Accepted 19 November 2015
morphology, FTIR and NMR spectra of the membrane were determined to explain the
Available online 13 January 2016
decline in conductivity observed over time. The techniques used did not reveal any chemical degradation of the membrane polymer. The morphological changes to the
Keywords:
membrane connected with excessive expansion of the internal structure of the polymer
Nafion 117
are assumed to be the reason for the phenomenon observed. Finally, to confirm the con-
Ionic conductivity
clusions derived, the membrane behaviour in a laboratory-scale water electrolysis cell was
Elevated temperature
studied under operating conditions corresponding to its prior characterization.
Elevated pressure
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Degradation
Introduction Within the last decade studies focusing on proton exchange membrane (PEM) water electrolysis have increased significantly. This research is motivated by the search for efficient, flexible and reliable high-capacity energy storage. This is connected with the need to develop a new, decentralized energy distribution grid capable of compensating for fluctuations caused by the increased capacity of unstable renewable energy sources [1]. According to the concept of the hydrogen economy, this problem could be solved by converting excessive electrical energy into the chemical energy of hydrogen by electrochemical decomposition of water. The hydrogen produced can be used either as a raw material in chemical industries, as a fuel for different applications, such as for car
propulsion, or it can be reconverted into water while recovering electrical energy in periods of excessive electricity demand [2]. Water electrolysis can be carried out under both alkaline and acidic conditions [1,3,4]. Alkaline water electrolysis has been a well-established industrial process for decades. For intermittent operation the acidic electrolysis, utilising a PEM membrane is, however, a more suitable option. This is mainly due to the high intensity and flexibility of this process, the compact, modular design of the cell and easy separation of the produced gases, and the high purity of the product. The main obstacle to the widespread implementation of this technology is the necessity to use an electrocatalyst based on platinum metals [3,5]. A promising method to reduce the electrocatalyst loading is to increase the operating temperature of the process, preferentially to above 100 C. The electrode reaction
* Corresponding author. E-mail address: [email protected] (K. Bouzek). http://dx.doi.org/10.1016/j.ijhydene.2015.11.102 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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kinetics, enhanced by the increased operating temperature, allows the catalyst loading to be reduced substantially, while at the same time increasing process efficiency and maintaining process intensity. The increased operating temperature can, however, cause significant complications, especially for the polymer electrolyte. Today, perfluorinated sulfonated membranes represent the sole option as a polymer electrolyte and, at the same time, as an electrode compartment separator in PEM water electrolysis [1e3]. This is due to their extraordinary properties, namely excellent chemical and mechanical stability in combination with high ionic conductivity. Nevertheless, this material is highly sensitive to its degree of swelling. Above 100 C under atmospheric pressure, i.e. above the normal boiling point of water, the relative humidity of its environment rapidly decreases with increasing temperature. It is connected with the drying out of the membrane, leading to a decline in its ionic conductivity and reduced lifetime [6]. This problem has been intensively studied, mainly in connection with the development of high-temperature PEM fuel cells. There, the problem of low membrane conductivity under conditions of low relative humidity was solved by the application of membranes based on phosphoric acid imbibed into a polymer matrix [7]. This type of membrane, however, has not proven to be sufficiently stable under water electrolysis conditions [8]. A promising alternative is to utilize elevated pressure, allowing water to be kept in the liquid state at the desired operating temperatures and thus avoiding the problem of insufficient membrane swelling. This approach is possible because, in contrast to fuel cell technology, the flooding of the electrodes by water represents a desirable effect in water electrolysis, as water represents the reactant at the anode and no gases have to be transported from the distribution channels to the catalytic layer. The long-term stability of a Nafion-type membrane under the conditions discussed here, i.e. at temperatures above 100 C and at elevated pressure, has, however, not yet been clarified. Hitherto, attention has mainly focused on the behaviour of the membrane in a PEM FC, i.e. on composite materials at least partly maintaining their conductivity even under low relative humidity conditions [9]. To a certain extent, the studies by Alberti et al. [10,11], dealing with the behaviour of the Nafion 117 membrane at temperatures up to 160 C and relative humidity up to 100%, represent an exception. At the same time, however, in these studies the membrane was sandwiched between the electrodes and exposed to different pressures. The authors reported a significant decline in membrane conductivity during its exposure. Since chemical degradation of the membrane materials was excluded, the behaviour was ascribed to the anisotropic swelling of the membrane along the plane of its surface. This phenomenon was not, however, discussed in greater detail. The absence of chemical degradation is in agreement with the results of spectroscopic experiments published by Morita and Kitagawa [12], reporting that chemical degradation commenced at 280 C. On the other hand, for example, there are some reports indicating presence of SO2 in the exhaust gases of low temperature PEM FC (utilising Nafion 112 membrane) at 70 C. As a main reason was mentioned action of H2O2 formed in the system [13].
Alberti et al. [14,15] continued to study the properties of a Nafion-type membrane exposed to elevated temperatures and various degrees of humidity. In the first step [14], mainly the influence of hydrothermal treatment of Nafion 117 on its water uptake and related mechanical properties of the membrane were studied. Later, he investigated the colligative properties of a Nafion 117 membrane in a broad range of temperatures in relation to its visco-elastic properties [15]. One of the main conclusions was that, above approx. 80 C, Nafion 117 must be considered as an ionomer exhibiting viscous behaviour. These conclusions are also supported by recent results of Matos et al. [16], who measured conductivities of Nafion 115 at 100% relative humidity in temperature range of up to 180 C. They found two distinct Arrhenius-like conductivity regions for temperatures below and above 90 C. Moreover, the structural changes in the membrane that occurred at elevated temperature led to changes in its conductivity when experiment was repeated. This suggests the thermal treatment history effects membrane conductivity. Weber et al. approached in their work the effect of membrane humidity and pressure in thickness direction at temperatures below 100 C [17]. Conductivity improved with increasing temperature and membrane humidity. Pressure of up to 8 MPa was found to have no clear effect on the membrane proton conductivity in membrane. All the above-mentioned data were determined primarily for a gaseous environment typical of fuel cell operation. The conclusions drawn are, therefore, only partly relevant to PEM water electrolysis technology. It is thus of interest to evaluate the stability of the Nafion 117 membrane as a typical representative of membranes based on perfluorinated sulfonated polymers under conditions relevant to PEM water electrolysis at elevated temperature and pressure in a liquid water environment. At the same time, the chemical stability of the polymer under the conditions under study needs to be evaluated. The first information on the behaviour of the Nafion 117 membrane under the conditions of interest was provided in our previous work [18]. It is also worth mentioning that there is still no total agreement reached in the community on the anisotropic behaviour of Nafion membranes. Comparison of in-plane and out-of-plane conductivities has been subject of several works. Differences can be attributed to measurement procedure, equilibration times, or membrane history such as large pressure applied in one direction (e.g. (hot)pressing). These aspects can change distances between ion clusters in the bulk of the membrane and therefore induce anisotropy of its conductivity. Several authors report that if Nafion membranes were not subjected to pressure load, identical values of inplane and out-of-plane conductivities were obtained [19,20]. Additionally not all authors considered cell and cell/membrane resistance in order to obtain true Nafion conductivity when performing out-of-plane conductivity measurement. Moreover all these measurements were performed in air atmosphere humidified into the different degree. The results are thus not directly applicable to membrane immersed in liquid water. The aim of this study is to extend this research and to provide more detailed information on the behaviour of Nafion 117. Particularly, we aimed to study Nafion behaviour in liquid
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water in temperature range of up to 160 C and pressures up to 0.7 MPa. These conditions should mimic PEM water electrolysis operating conditions, i.e. ensure exposure of the membrane to the liquid water at given temperature and pressure. Selection of these conditions was motivated by enhancement of the electrode reactions kinetics and potential membrane conductivity in order to intensify electrolysis process and/or possibly reduce catalyst loading.
Experimental Membrane in-plane conductivity determination at elevated temperature and pressure The in-plane ionic conductivity of the Nafion 117 membrane sample (9 52 mm2) was determined in a longitudinal direction by electrochemical impedance spectroscopy (EIS) in a four-electrode arrangement. A Solartron SI 1260 Frequency Response Analyzer connected to a Solartron SI 1287 Electrochemical Interface (Solartron Analytical, UK) was used for this purpose. Impedance spectra were recorded in a frequency range of 65 kHz to 0.1 Hz using a perturbing signal amplitude of 10 mV. A schematic sketch of the experimental set-up used during these experiments is shown in Fig. 1. Demineralized water was pumped from the reservoir (1) by an HPLC pump (2) to the tempered part of the set-up. The residence time of the water in the hydraulic circuit was long enough to ensure its equilibration with the temperature set in the tempered box. Teflon tubing was used to avoid any contamination of the water. Prior to entering the cell, water passed through the absorber containing 28 cm2 of the scarified membrane in the Hþ form (4). The demineralised water in the absorber was equilibrated with the membrane sample in order to remove any impurities potentially present before it entered the conductivity cell. The typical weight of the membrane sample tested was 0.1 g. During the experiment which lasted for
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several hundreds of hours about 45 dm3 of water flowed through the cell. Therefore, even traces of ions present in the demineralised water could substantially contaminate the measured sample should they be exchanged for the Hþ present in the membrane. Subsequently, water entered the conductivity cell (5), formed by a flow-through channel, in which the membrane sample was positioned. On each end of the sample two platinum wires were pressed against the membrane surface, one acting as a terminal and the second as a sensing electrode. At opposite ends of the samples these electrodes were positioned on opposite sides of the membrane (8). Demineralised water flowing continuously through the cell washed all sides of the membrane as well as the attached electrodes continuously and applied isotropic pressure to the sample. The conductivity cell was made of polyether-ether-ketone (PEEK). After leaving the conductivity cell, the water passed through the back-pressure valve (6) controlling the pressure in the conductivity cell (5) and flowed into the reservoir of spent water (7). The pressure in the pressurized conductivity part of the set-up was recorded by a manometer (3). After assembling the cell and connecting it to the experimental set-up, the system was pressurized by the water. Subsequently it was heated to the desired temperature. When determining the dependence of membrane in-plane conductivity on temperature, the measurement was initiated 5 min after the temperature in the tempering box had reached the desired value. The short delay served to minimise any possible changes to the membrane sample during conditioning. The measurement was repeated at 5-min intervals. The maximum conductivity value determined was considered as the membrane conductivity under the given conditions. The temperature range of 30e160 C was studied. The long-term in-plane conductivity measurements started 2 h after the system had reached operating temperature. This delay allowed the membrane to equilibrate with the experimental conditions. The evolution of membrane conductivity in time was determined at temperatures of 110, 130 and 150 C and at pressures of 0.5 and 0.7 MPa. Typically the conductivity experiments lasted for 800 h. The measurements were taken every 2 h.
Additional methods of membrane characterization
Fig. 1 e Schematic sketch of the experimental set-up for membrane conductivity determination at elevated temperature and pressure; 1 e reservoir of deionized water, 2 e HPLC pump, 3 e manometer, 4 e absorber, 5 e conductivity cell, 6 e back-pressure valve, 7 e reservoir of spent deionized water, 8 e electrodes.
The ion-exchange capacity (IEC) of the membrane prior to and after the degradation experiment was evaluated potentiometrically [21]. The activated membrane sample in the Hþ form was plunged into 125 cm3 of 0.1 mol dm3 NaCl solution. The potential response of a Ross combined glass electrode (Orion, USA) immersed in the solution was recorded by a Keithley 6514 electrometer (Keithley, UK) with high input impedance (200 TU). The recorded value of the glass electrode potential was converted into the amount of Hþ ions displaced from the membrane into the solution by means of a calibration curve. The experiments were performed under argon atmosphere. The IEC was evaluated on the basis of the equilibrium concentration of the Hþ ions in the solution and the weight of the dry membrane sample. The membrane morphology in the dry state was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A Hitachi S4700 microscope was used to obtain
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SEM pictures. After the experiments, the membrane samples were stored in a desiccator for a week. Prior to analysis, they were sputtered by a 20 nm thick layer of Au/Pd to avoid the surface charging. The AFM experiments were carried out using a Veeco CP-II instrument (Veeco, USA). Fourier transform infrared (FTIR) spectra of membrane samples were obtained using a NICOLET IS10 (Thermo Scientific, USA) spectrometer in reflection mode on the ATR crystal. The spectra were collected after 64 scans for both the background and the samples in the wave number range between 4000 and 650 cm1 with a wave number resolution of 4 cm1. Background subtraction was used. Measurements were carried out at laboratory temperature with samples in equilibrium with air humidity. Nuclear magnetic resonance (NMR) data were obtained using Oxford/Varian 300 MHz NMR (Varian, UK) with a 5 T superconducting solenoid unit. Samples were prepared by dissolving the membrane in isopropyl alcohol at 200 C using an autoclave with a Teflon lining. The dissolution took 12 h. 19 F NMR spectra of solutions containing 5 wt.% of Nafion 117 were measured at laboratory temperature. Each spectrum was collected from 128 scans with 10 s delay and 300 MHz pulse. A change of membrane volume during the long-term conductivity measurements was calculated from the membrane dimensions prior to and after experiments. Membrane thickness was determined by screw micrometer which applied pressure of around 165 kPa to the sample. Membrane length and width were measured by calliper.
Water electrolysis The membrane was tested in a laboratory-scale single cell water electrolysis set-up. Gold-plated titanium end-plates with parallel flow channels with an active area of 4 cm2 were used. As a cathode, a commercially available platinum-based gas diffusion electrode HT ELAT (BASF, USA) with a platinum catalyst loading of 0.5 mg cm2 was used. The anode, based on titanium felt activated by IrO2 (0.8 mg cm2), was made in-house. The Nafion 117 membrane was used as a polymer electrolyte. The cell was constricted to a momentum force of 5 Nm. Operating temperatures of 110 and 150 C were used. The electrolysis operating pressure was set by a back pressure regulator to 0.5 MPa in both the anode and cathode compartments. Demineralized water, preheated to its operational temperature prior to entering the cell, was circulated through the anode compartment. Long-term electrolysis at a cell voltage of 1.7 V was performed in order to determine the stability of the system. During the electrolysis load curves and EIS spectra were measured every 1 and 48 h, respectively. The load curve was measured between 1.3 and 1.75 V in 0.05 V steps (lasting for 60 s to allow the current to stabilise) by a Statron 3251.0 stabilized power source (Statron, Germany). The EIS spectra were measured at a cell voltage of 1.5 V with a perturbing signal frequency of 65 kHze100 Hz using 10 mV amplitude.
the membrane sample in deionized water at 80 C for 60 min and in 3 wt.% H2O2 at 60 C for 30 min. Subsequently, the membrane was immersed in 0.05 mol dm3 H2SO4 solution at 60 C for 30 min in order to obtain Nafion in the Hþ form. Finally, excess H2SO4 was removed from the sample by soaking it in deionized water at 60 C for 120 min. In this way activated membranes were stored in deionized water at room temperature. All chemicals used in this study were of analytical or higher grade. H2SO4, H2O2 and isopropyl alcohol were purchased from Lach-Ner (CZ). Deionized water was prepared in a demineralization station constructed in-house and it had a conductivity of 0.9 mS cm1 at 20 C.
Results and discussion Membrane conductivity at elevated temperature and pressure The values of the Nafion 117 membrane in-plane conductivities determined at different temperatures and pressures of 0.1, 0.5 and 0.7 MPa shortly after exposition to experimental conditions are summarized in Fig. 2. As expected, the conductivity increased with rising temperature. Whereas at 30 C it has a value of approximately 9.9 S m1 for all the pressures studied, at 160 C it reaches values above 37 S m1. It should be mentioned that the precision of the conductivity determination decreases with increasing temperature and pressure due to the reasons discussed later. At the highest temperatures the conductivity value can be underestimated by about 10%. Nevertheless, the data still show clear trends. The increase in membrane in-plane conductivity with temperature is caused by the improved mobility of the protons in the polymer phase. This is related to a decrease in pore fluid viscosity on the one hand and at the same time by a reduction of the activation barrier for transport of protons between the hydrophilic clusters forming the internal structure of the membrane [11].
Materials and chemicals used A Nafion 117 membrane (Ion Power, USA) was used as a polymer electrolyte in this study. Prior to application, the membrane was activated [22]. Activation began by immersing
Fig. 2 e Dependence of ionic conductivity of the fully hydrated membrane in the Hþ cycle on temperature at three pressures; the pressure values are indicated in the inset.
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The in-plane conductivity dependence on temperature, shown in Fig. 2, clearly deviates from the simple exponential shape expected according to the Arrhenius law. Additionally, the conductivity does not seem to be influenced by external pressure on the time scale of the measurement. A more detailed analysis of the conductivity data is presented in Fig. 3. At both elevated pressures two distinct domains can be recognised in the Arrhenius plot. The low temperature domain ranging from 30 to 70 C is characterised by activation energy of about 9 kJ mol1. Similar conductivities and activation energy were found by other authors for Nafion 11X membrane types [16,23,24]. The activation energy of about 10 kJ mol1 corresponds to proton Grotthus conduction mechanism [16]. For the high temperature domain localised between 110 and 160 C the value of approximately 16 kJ mol1 was found. The temperature range of 90e110 C represents the transition between these two domains and corresponds well with the glass transition temperature (Tg) reported in the literature for Nafion in the Hþ-form. At this point the mechanical properties of the Nafion 117 polymer need to be discussed in dependence on temperature. Generally the Tg of Nafion 117 in the Hþ-form is reported to be located between 110 and 120 C [25,26]. The situation is, however, more complex. According to Osborn et al. [27], the Tg of Nafion membrane of equivalent weight (EW) 1100 in the Hþ-form is located close to 20 C (b-relaxation). This corresponds to the polymer backbone motion in the framework of a physically cross-linked (hydrogen-bond)
Fig. 3 e Arrhenius plot for the conductivity of the membrane in a proton cycle under conditions shown in Fig. 2; lines indicate linear fit of the data, pressure values are indicated in the inset.
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network. The temperature above which the hydrogen-bond network becomes dynamic (a-relaxation) is located close to 100 C. Any morphological reorganizations are only possible above this temperature. It should be noted that the a-relaxation strongly predominates over b-relaxation. Due to these facts, in the present study a-relaxation is denoted as the Tg. The determined increase in activation energy of proton transport from 9 to 16 kJ mol1 suggest possible difference in proton transport mechanism below and above the Tg. Below the Tg and at reasonably high degree of membrane swelling the Grotthus proton conduction mechanism is most important [28]. Above the Tg the situation is not fully clear. We may expect more complex arrangement and involvement of all three possible transport mechanisms is possible. In the expanded hydrophilic domains migration in a solution and Grotthus mechanism may be considered to play an important role. Between the domains surface hopping through the hydrophobic channels may play a primary role. It strongly depends on the extent of the swelling and of the corresponding structural changes. Very little, if any, dependence of the membrane conductivity on the pressure was observed in the time-scale of these experiments, which suggests a slow rate of membrane swelling. Moreover, the relatively high value of Nafion 117 conductivity represents a promising aspect with respect to its intended utilization as a polymer electrolyte in mediumtemperature water electrolysis, i.e. in the range of 100e200 C. Independence of Nafion in-plane conductivity on pressure on short-time scale was observed also by Weber et al. [17], although their experiments covered temperature region of up to 100 C only. Besides this positive aspect, the danger of the conductivity of the Nafion 117 membrane decline at elevated temperature and pressure must be taken seriously [10,11]. This aspect may disqualify such a type of membrane for an envisaged application. Therefore, in the next step, long-term conductivity measurements of the Nafion 117 membrane were performed. The results are summarized in Fig. 4. Contrary to the results showed in Fig. 2, where conductivities measured shortly after condition equilibration are present, in Fig. 4 the effect of pressure is substantial. At a pressure of 0.5 MPa the highest initial conductivity was observed at a temperature of 110 C. The fact that the initial conductivities determined for temperatures of 130 and 150 C are lower may be explained by the rapid initial decrease in conductivity with time at these two high temperatures. The samples had been equilibrated for 2 h under the conditions of the experiment before the conductivity was recorded. During this period, which was necessary for the cell to reach the required temperature, the conductivity decline had already started, as can be deduced from the early stage of the obtained dependence, see Fig. 4. On the other hand, at a temperature of 110 C, the membrane conductivity remained approximately constant during the initial 350 h and thus it exceeded the values determined for the higher temperatures. A pressure of 0.7 MPa led to a similar conductivity degradation rate for all temperatures studied and, therefore, such a discrepancy did not arise. Two basic findings clearly follow from Fig. 4: (i) value of inplane Nafion membrane conductivity at elevated temperature and pressure declines in time and (ii) the rate of decline is
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Fig. 4 e Dependence of the in-plane conductivity of the membrane on time of exposure to elevated pressure at different temperatures; temperature and pressure values indicated in the inset.
accelerated by increasing temperature and pressure. At temperatures of 130 and 150 C the rapid exponential decline immediately after the conductivity reading begins is followed by a slow linear decline in membrane conductivity. On the other hand, a stable conductivity value for a relatively long period of time, followed by a sudden accelerated decrease, can be observed at 110 C, especially at a pressure of 0.5 MPa. There are different possible explanations for the observed phenomena. The first possibility considered is thermal degradation of the polymer, namely its desulfonation [12]. If this was the case, the effect of the pressure would be significantly less pronounced and the predominant influence on the observed conductivity decrease would have to be that of temperature. However, this explanation does not correspond with the long-term conductivity evolution observed at temperatures of 110 and 130 C, where the conductivity decline strongly depends on the applied pressure, see Fig. 5. A second and more likely explanation is connected with the change in the mechanical properties of the membrane at or near the Tg, i.e. the temperature above which the polymer loses its mechanical stability. The reported glass Tg region of 110e120 C [25,26] corresponds to the lower limit of the temperature range covered within the long-term degradation experiments. This indicates that the mechanical properties of the polymer material may represent a critical issue, especially at elevated operational pressure, where the membrane
Fig. 5 e Comparison of dependence of the in-plane conductivity s of the membrane on time; temperature and pressure are indicated in the inset.
polymer tends to swell excessively. At a temperature well below the Tg the internal forces in the polymer do not allow an excessive increase in the internal void volume and the polymer is mechanically stable. If, however, the Tg is exceeded, the situation changes dramatically. Under such conditions the internal forces may be surpassed by the solvation forces of the functional sulfo-groups, resulting in an expansion of the hydrophilic domains of the polymer. Therefore, at 150 C the higher pressure accelerates excessive swelling more progressively than at 130 C. Such an explanation is in agreement with the conclusions of Alberti et al. [14]. The proposed hypothesis of excessive membrane swelling is also supported by the observed increase in the dimensions of the membrane sample during the long-term
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conductivity measurements. Information on the relative increase in the dimensions and volume of the membrane observed during the long-term conductivity measurements is summarized in Table 1. The values in the table clearly document that the membrane dimensions increase in all directions. The most extensive swelling can be observed in the membrane thickness, where an increase of up to 24% was determined. Swelling in other directions, except for the membrane length at temperature and pressure of 150 C and 0.7 MPa, respectively, did not exceed 10%. The membrane dimensions determined after the end of the degradation measurement, and consequently the calculated volume, will most likely differ from the corresponding values in the conductivity cell. This also leads to uncertainty regarding the membrane conductivity determined, as mentioned at the beginning of this section. On the other hand, membrane sample dimension changes qualitatively show the abovedescribed dependence of the irreversible swelling extent on the temperature and pressure, i.e. the higher temperature and pressure in the system during measurement, the higher membrane swelling. The volume changes presented in Table 1 may also explain the conductivity differences visible in Fig. 5 for pressures of 0.5 and 0.7 MPa at 110 C. At a pressure of 0.5 MPa a volume increase of only 20e40% was observed, while at 0.7 MPa it was already about 35e50%. It was shown earlier that water acts as a plasticiser [26,29]. Therefore, its increasing content in the membrane structure leads to a reduction of the Tg value. Thus the reason for behaviour observed here is that the amount of water present in the membrane and influencing its properties is dependent on the external pressure. In other words, a pressure of 0.5 MPa at a temperature of 110 C is not sufficient to swell the membrane to such a degree as to bring about a rapid decrease in the Tg to, or below, 110 C. For this reason, almost 400 h of exposure are needed at 0.5 MPa and 110 C to achieve such a situation. From this point onwards excessive swelling connected with a conductivity decline is initiated. As follows from the data presented so far, whereas an initial increase in membrane swelling has a positive effect on its conductivity, longer exposure results in a rapid decline of this characteristic. These two phases of membrane swelling have to be clearly distinguished. The first swelling phase leads to a restructuring of the hydrophilic domains (change in the orientation of the hydrophilic domains [25,30]) in the membrane structure and causes increased ionic mobility in its interior. The second phase includes irreversible changes in the internal structure of the polymer material and the overall
membrane performance. This is connected with a loss of interaction between the polymer backbone chains, resulting in a downgrade of its mechanical properties [31]. It is not yet fully clear whether the difference in the membrane behaviour at the two higher pressures during the long-term conductivity measurement is caused exclusively by the substantial changes in the polymer material internal structure, or whether the change in the mechanical properties of the polymer reduced the accuracy of the conductivity measurement due to the irregular contact between the polymer electrolyte membrane and the electrodes. This fact also renders an evaluation of the conductivity data significantly more complex because the sample dimensions were measured at atmospheric pressure prior to and after the experiment. Moreover, the sample dimensions required to recalculate the measured ohmic resistance of the membrane to its conductivity were assumed to change linearly in time during the experiment. This assumption introduces another error into the conductivity data presented. Nevertheless, it is an acceptable compromise, allowing the dimensional changes to be included in the evaluation of the experiment. A close relationship between the membrane conductivity and its swelling is documented by the data summarized in Fig. 6. The data on membrane conductivity and sample dimensions obtained at the end of the long-term conductivity experiment, and thus free from errors caused by volume interpolation, were used to construct this diagram. It is evident that the ionic conductivity of Nafion 117 decreases linearly with increasing final to initial volume ratio of the sample at pressure of 0.5 MPa. This is in agreement with the postulated explanation of the decline in membrane conductivity with time of exposure. At pressure of 0.7 MPa, however, the conductivity does not show any clear dependence on the membrane volume changes determined after the experiment. It is very likely that the membrane swelling is partially reversible so that, during cooling and depressurising of the system, the membrane partially shrinks. This will be truth especially at pressure 0.7 MPa and temperatures 130 and 150 C. As a result, the actual membrane volume at high temperature and pressure can differ greatly from that measured at ambient room conditions.
Membrane morphology In the first instance, an attempt was made to identify any possible changes in membrane morphology using SEM.
Table 1 e Changes in Nafion membrane dimensions during long-term conductivity measurements at elevated pressure and temperature. Membrane: a e length, b e width, c e thickness. Pressure
Temperature
Dimensions [mm] Before
Volume changes After
[MPa]
[ C]
a
b
c
a
b
c
[%]
0.5
110 130 150 110 130 150
50.3 50.1 49.2 49.7 50.2 48.3
9.9 9.8 9.7 10.2 9.5 9.6
0.203 0.211 0.206 0.205 0.209 0.207
51.3 52.1 52.9 52.7 54.3 55.1
10.2 10.4 10.4 10.9 9.9 10.1
0.231 0.246 0.249 0.250 0.253 0.256
119.6 128.7 139.3 138.2 136.5 148.4
0.7
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Fig. 6 e Dependence of Nafion 117 conductivity at the end of the long-term conductivity experiment as a function of the sample volume; V stands for the sample volume at the end and V0 at the beginning of the experiment.
Images of both the membrane cross-section and its surface were obtained. In neither case did a SEM photograph of a sample exposed to elevated temperature and pressure reveal any changes in membrane morphology. It can be concluded that either they are not visible ex-situ on the dry membrane or they are present on a significantly smaller scale than that provided by the SEM image. Therefore, the SEM images are not shown here. More detailed information was obtained by AFM, a significantly more sensitive technique. The surface morphology of the membrane prior to and after exposure to a temperature of 150 C and pressure of 0.7 MPa is shown in Fig. 7. When comparing individual surface scans, a basic trend is clearly visible. Exposure to the experimental conditions leads to an approximately three-fold increase in the roughness of the membrane surface. Instead of relatively minor stochastic irregularities originating from the membrane production by thermal extrusion visible on the freshly activated membrane (Fig. 7A, B), numerous protrusions appear on the surface of the exposed membrane (Fig. 7C, D). Their origin may well be explained by excessive swelling of the polymer, leading to local expansion of less mechanically stable internal hydrophilic micelles. Even during subsequent drying of the sample for one week over silica preceding the AFM experiment, the surface changes originating from the excessive swelling have not completely recovered.
Membrane ion-exchange capacity The above explanation of the performance of the Nafion 117 membrane under elevated temperature and pressure has so far been supported indirectly by the morphological changes in the membrane sample. The development of the IEC of the membrane after conductivity measurements, i.e. after exposure to elevated temperature and pressure, may provide additional information. The IEC values obtained for membranes before and after long-term degradation are summarized in Fig. 8. Two main observations can be made from this
figure: (i) the IEC value reaches a maximum at a temperature of 110 C; (ii) membranes exposed to a pressure of 0.5 MPa have a higher IEC value compared to those exposed to 0.7 MPa. These results are consistent with data published by Kwon et al. [25]. As already discussed in chapter 3.1, with the initial swelling the orientation of the proton transport channels changes. It opens the polymer structure, enabling it to access hitherto closed proton-conductive domains. This explains why the IEC increases after exposure to the experimental conditions at temperatures below the Tg. At higher temperatures and pressures the IEC values decline to below the level obtained for the freshly activated membrane. This is explained by isolation of part of the ion-exchange groups from the surrounding solution, thus incurring the loss of these ion-exchange sites. This is caused by the collapse of the structure above the Tg, resulting from excessive swelling of part of its structure characterised by lower mechanical strength. Chemical degradation can be excluded at temperatures of 110 and 130 C because, at a pressure of 0.5 MPa, the IEC exceeds that of the freshly activated membrane. After exposure to 150 C the IEC is lower than that of the freshly activated membrane for both pressures under study. Nevertheless, since the IEC values obtained for these pressures are different, the reason may be considered to be different from thermally induced chemical degradation of the polymer.
Membrane polymer characterization by FTIR Direct information on possible changes in the polymer structure and thus its chemical degradation can be provided by FTIR spectroscopy. The spectra obtained for the freshly activated Nafion 117 membrane and for the membrane exposed to elevated temperature and pressure are summarized in Fig. 9. Individual vibration lines were assigned, as indicated in Fig. 9, with the help of [32]. As can be seen from the spectra, there are no significant deviations between the activated sample and samples exposed to the conditions studied. Potential chemical degradation of the membrane polymer is most likely to take place on the side-chains carrying functional groups. This is due to the extreme chemical stability of the poly(tetrafluoroethylene) backbone. Hence, the spectra changes related to polymer degradation must be anticipated mainly in lines 900e1140 cm1 sym. val. and 1270e1060 cm1 asym. val. corresponding to the eCeOeCe bond. In the case of degradation, it would most probably be replaced by eCOOH groups (visible as carboxylic eOH groups due to lines at 3550e3500 cm1 and 3300e3130 cm1 or as carboxylic eC]O groups due to lines at 1800e1750 cm1 [12,33]. Alternatively a cleavage of the eCeSeOe bond (lines at 1350e1550 cm1) would have the same result, i.e. carboxylation of the side chain [32]. The slight variation of the line intensities at 1620 and 3400 cm1 corresponds to the different hydration degree of the membrane samples. In summary, no significant degradation-related changes, such as the appearance of new lines corresponding to polymer degradation products, can be identified in the spectra obtained. This finding supports the earlier assumption that the deterioration of membrane conductivity is not related to chemical degradation of the polymer, but more likely to its structural changes.
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Fig. 7 e Morphology of the Nafion 117 membrane surface (sputtered by a 20 nm thick layer of Au/Pd) obtained using AFM (A,B) prior to and (C,D) after exposure to a temperature 150 C and pressure 0.7 MPa for 850 h. (A,C) picture area of 10 £ 10 mm2, (B,D) picture area of 2 £ 2 mm2.
Membrane polymer characterization by
19
F NMR
NMR spectroscopy represents an alternative, highly sensitive tool for determining the structure of chemical molecules. Therefore, it was selected as a complement to FTIR spectroscopy. NMR spectra were taken of the dissolved Nafion 117 membrane prior to and after exposure. Two approaches to spectra analysis were used to identify possible changes in the polymer structure. First, the obtained spectra of freshly
Fig. 8 e Ion-exchange capacity of the Nafion 117 membrane prior to and after exposure to elevated temperature and pressure; individual conditions of exposure are indicated at the x-axis description, exposure duration can be derived from Fig. 4.
activated and exposed membranes were compared [34]. Degradation of the polymer would result in the appearance of a new signal in the spectra. Spectra of the Nafion 117 solutions
Fig. 9 e IR spectra of the fresh Nafion 117 membrane and samples after exposure to elevated temperature and pressure; the experimental conditions the sample was exposed to are indicated in the graph, selected absorption bands are indicated in the spectrum obtained for the sample exposed to 150 C and 0.7 MPa. Indexes a and s stand for asymmetric and symmetric vibrations, respectively.
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before and after exposure are shown in Fig. 10. Close examination of these spectra revealed no differences that would indicate polymer degradation. The second approach involved a comparison of the intensity of two characteristic signals. Any change in their ratio would indicate a change in the content of the corresponding part of the molecule contained in the polymer. For this analysis the integral values of signals corresponding to the part of the main backbone where the side chain is connected, i.e. OCF2CF2 (83 ppm), and side chains where functional groups are connected, i.e. CF2CF2SO3H (121 ppm), were selected. The ratios of the integral values of these signals had a value of 1.454917 and 1.454926 for Nafion 117 prior to and after exposure to 150 C and 0.7 MPa for 850 h. For all the other conditions studied, the results obtained by the two approaches were identical and it can thus be concluded that no chemical degradation of the ionomer occurs or, if so, its extent is lower than 5%.
Water electrolysis at elevated temperature and pressure The above results indicate that chemical degradation of the polymer does not present a problem in the application of a Nafion 117 membrane in a water electrolysis cell working at temperatures above 100 C (the upper temperature limit studied here is 150 C). To keep this material sufficiently conductive it is necessary to work under high enough pressure so that the water inside the cell remains in a liquid state. However, under such conditions the mechanical stability of the membrane and its ability to separate gases produced at the anode and the cathode represent a crucial problem.
Fig. 10 e 19F NMR spectrum of Nafion 117 membrane prior to and after exposure to 150 C and 0.7 MPa.
Fig. 11 summarises the results of testing the stability over time of Nafion 117 under conditions of laboratory electrolysis. It shows the development of the current density in the electrolyser in time. The initial current density observed at 150 C and at a cell voltage of 1.7 V represents 190% of the value determined at 110 C. This can be explained by the more sluggish reaction kinetics at a lower temperature. It was not possible to extract the desired kinetic information from the EIS spectra precisely. However, it can be said that activation resistance at 110 C and 150 C was about 0.2 U and 0.05 U (at 1.5 V), respectively. Importantly, these values did not change much on the timescale of the electrolyses. The EIS spectra were recorded at 1.5 V because the currents at 1.7 V exceeded the current range of the impedance frequency analyser used. Therefore, the values obtained are only qualitatively relevant for electrolysis operated at 1.7 V. Particularly, the activation resistance values at 1.7 V will be several times lower than those determined at 1.5 V. A rapid decline in current density occurred immediately at the start of electrolysis at 150 C. With time the current density decrease slowed down. After 162 h of the experiment the cell operating at 150 C was short-circuited. Ohmic resistance of the cell, represented mainly by the membrane resistance, increased steadily from 0.04 U at the beginning of the experiment to about 0.07 U at the moment just before the short circuit. A likely explanation for the observed phenomena is that the decreased mechanical stability of the Nafion membrane at this temperature in combination with the pressure of the electrodes (applied during cell assembly) led to thinning of the Nafion membrane. This eventually resulted in the cell short-circuiting. Simultaneously with the membrane thinning the conductivity of the Nafion material was decreasing due to disruption of its inner structure, as discussed in the previous sections. These two effects had a contrary consequence for the measured cell resistance. Another potential contribution to the electrolysis cell performance decline represents an
Fig. 11 e Time dependence of the performance of the laboratory PEM water electrolyser with a Nafion 117 membrane at a cell voltage of 1.7 V and pressure 0.5 MPa in both compartments; titanium felt anode with 0.8 mg cm¡2 of IrO2, HT ELAT gas diffusion electrode with Pt loading of 0.5 mg cm¡2, temperature is stated in the inset.
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increase in the contact resistance between the IrO2 catalyst and titanium felt support. The performance of the electrolysis cell operated at a temperature of 110 C remained relatively stable for the first 160 h. This agrees very well with the range of the almost constant value of the ohmic cell resistance which remained between 0.05 and 0.06 U. At the same time this is in accordance with the Nafion conductivity data discussed in previous sections. After 160 h the current density decline became accelerated and it eventually dropped from about 500 mA cm2 to 110 mA cm2. Despite the low current density the electrolysis was running even after 380 h of operation. This performance decay was accompanied by a faster increase in the cell ohmic resistance to more than threefold the initial value, see Fig. 12. This is in agreement with the results of Stassi [35] who, in the context of fuel cell tests, reported that the decrease in the conductivity of the Nafion membrane was accelerated by the decrease in the crystallinity of its material. In the present study, such a decrease is accelerated by the increasing amount of water being absorbed by the membrane, i.e. by the increasing operational pressure. The overall electric resistance in the electrolyser, calculated from the cell voltage and the current, is illustrated in Fig. 12. In the first 160 h the values of the overall resistance increased from 0.12 to 0.21 U at 150 C and from 0.21 to 0.32 U at 110 C. Then the cell operated at 150 C was shortcircuited and a sharp increase in the overall cell resistance from about 0.32 up to 1.16 U was observed at 110 C. The overall cell resistance at the beginning of the experiment corresponds approximately to the sum of the determined ohmic and polarisation resistance. At the end of the experiment the sum represents only about 35% of the overall cell resistance. However, it has to be kept in mind that the ohmic resistance was determined at a cell voltage of 1.5 V, while electrolysis was operated at 1.7 V, where a higher current flows through the electrolyser. Thus, the remaining part of the overall cell resistance is likely due to the blockage or
Fig. 12 e Evolution of the overall and ohmic resistance of the laboratory PEM water electrolysis cell with a Nafion 117 membrane over time; operating pressure 5 bar; EIS experiments performed at a cell voltage of 1.5 V using perturbation signal amplitude of 10 mV; titanium felt anode with 0.8 mg cm¡2 of IrO2, HT ELAT gas diffusion electrode with Pt loading of 0.5 mg cm¡2.
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inactivation of part of the anode at higher voltage and throughout the electrolysis. This results in locally significantly enhanced current densities in the parts of the electrode remaining active. From a global (i.e. overall cell) point of view locally enhanced current density appears as an increase in the apparent overall cell resistance. The electrode blockage mentioned may be a consequence of the accumulation of produced gas in the electrode structure. A similar effect can also occur by a partial blockage of the structure of a porous anode catalyst layer due to excessive swelling/degradation of the recast Nafion used as a binder of the catalyst under conditions of electrolysis. Recast Nafion is known to have lower stability than a Nafion membrane [36,37]. In summary, although elevated pressure allows the use of the membrane at a temperature above 100 C and thus a short-term improvement of its conductivity, on the other hand it is responsible for its rapid deterioration. Shortcircuiting of the cell is, therefore, most probably caused by gradual thinning of the membrane as a result of the polymer becoming plastic at a temperature higher than its Tg and being pressed out of the space between the electrodes by the pressure applied on the system during its assembly. Parallel to the stability of the Nafion membrane, the stability of the recast Nafion in the catalyst layer has to be taken into account.
Conclusions The results of the experiments conducted show that, in agreement with Alberti et al. [10,11], a decrease in conductivity of the Nafion 117 membrane with increasing time of its exposure to an elevated temperature (110e150 C) and pressure (0.5e0.7 MPa) in a liquid water environment is caused by changes in the internal structure of the polymer and not by its chemical degradation. Structural changes are related mainly to excessive swelling of the polymer occurring at temperatures exceeding its Tg. Additional study is needed to identify the exact nature of the structural changes occurring in the polymer during the excessive swelling. Nevertheless, the results obtained within the framework of this study show that the application of polymer electrolyte materials based on perfluorinated sulfonated acids in the process of water electrolysis at a temperature above 100 C is possible at elevated pressure ensuring sufficient membrane hydration. Significant attention has, however, to be paid to the stability of the mechanical properties of the polymer to prolong cell life time. An interesting option offers also utilisation of the short side chain perfluorinated sulfonic acids showing better viscoelastic properties at elevated temperature. This will be the subject of further study.
Acknowledgement Financial support of this work by the Grant Agency of the Czech Republic within the framework of the Project No. 1502407J and by the Specific University Research MSMT No. 20/ 2015 is gratefully acknowledged.
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