PBI-based polymer electrolyte membranes fuel cells

PBI-based polymer electrolyte membranes fuel cells

Electrochimica Acta 52 (2007) 3910–3920 PBI-based polymer electrolyte membranes fuel cells Temperature effects on cell performance and catalyst stabi...

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Electrochimica Acta 52 (2007) 3910–3920

PBI-based polymer electrolyte membranes fuel cells Temperature effects on cell performance and catalyst stability Justo Lobato ∗ , Pablo Ca˜nizares, Manuel A. Rodrigo, Jos´e J. Linares Chemical Engineering Department, University of Castilla-La Mancha, Campus Universitario s/n, 13004 Ciudad Real, Spain Received 14 September 2006; received in revised form 6 November 2006; accepted 8 November 2006 Available online 14 December 2006

Abstract In this work, it has been shown that the temperature (ranging from 100 to 175 ◦ C) greatly influences the performance of H3 PO4 -doped polybenzimidazole-based high-temperature polymer electrolyte membrane fuel cells by several and complex processes. The temperature, by itself, increases H3 PO4 -doped PBI conductivity and enhances the electrodic reactions as it rises. Nevertheless, high temperatures reduce the level of hydration of the membrane, above 130–140 ◦ C accelerate the self-dehydration of H3 PO4 , and they may boost the process of catalyst particle agglomeration that takes place in strongly acidic H3 PO4 medium (as checked by multi-cycling sweep voltammetry), reducing the overall electrochemical active surface. The first process seems to have a rapid response to changes in the temperature and controls the cell performance immediately after them. The second process seems to develop slower, and influences the cell performance in the “long-term”. The predominant processes, at each moment and temperature, determine the effect of the temperature on the cell performance, as potentiostatic curves display. “Long-term” polarization curves grow up to 150 ◦ C and decrease at 175 ◦ C. “Short-term” ones continuously increase as the temperature does after “conditioning” the cell at 125 ◦ C. On the contrary, when compared the polarization curves at 175 ◦ C a continuous decrease is observed with the “conditioning” temperature. A discussion of the observed trends is proposed in this work. © 2006 Elsevier Ltd. All rights reserved. Keywords: Polybenzimidazole; High temperature PEMFCs; Cell performance; Temperature effects; Impedance spectra; Catalyst stability

1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are receiving a growing interest for many potential power sources applications, both stationary and portable, because of their high power density, high-energy conversion efficiency and low emission level. Thus, over the last decade, there has been a very intense research activity focused on improving the quality of all the elements (polymer electrolyte membranes, electrocatalysts, electrode composition, and gas diffusion media) and overcoming some limitations that this technology has [1–6]. One of these limitations comes from the most traditional polymeric material used as membrane, Nafion® (DuPont Inc.). In a simplified way, this polymer consists of a hydrophobic Teflon backbone onto which hydrophilic sulphonic acid groups have



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0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.11.014

been attached, so that it can be divided into marked regions with hydrophobic and hydrophilic characteristics, respectively [7]. Proton conduction in perfluorinated membrane is directly linked to the presence of water, so that it is not advisable to work at temperatures above 90 ◦ C at atmospheric pressure when these types of materials are used. Unfortunately, working below this temperature makes necessary the use of pure feed streams, since the presence of poisons in traces causes an abrupt decay in the cell performance. In order to overcome this limitation, it has been proposed to raise the operational temperature [8–11]. This increase implies that all the materials used for this purpose must withstand those conditions (thermal stability), apart from having the adequate properties for their use in PEMFCs (e.g. proton conductivity for the polymeric membrane, catalytic activity for the electrocatalyst, chemical stability, mechanical stability, reliability, durability). Polybenzimidazole (PBI), the polymer used in this work, can be included within the group of polymeric electrolytes proposed for high temperature PEMFCs. PBI, when impregnated with

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phosphoric acid, presents some interesting properties such as acceptable conductivity up to 200 ◦ C, and thermal and chemical stability at temperatures above that temperature. Thus, acid-doped PBI can be used as polymeric electrolyte in High Temperature PEMFC [12–15]. One possible limitation of this system is the H3 PO4 stability within the PBI matrix, as phosphoric acid is known to start to self-dehydrate at 140 ◦ C [16], generating oligomers of the original acid (e.g. pyrophosphoric acid), which have got lower conductivity [13]. Consequently, this undesirable process would produce a gradual decay of the cell performance. Thus, this process may degrade the cell performance with time. Yet, there are other processes which also impair the performance stability of a fuel cell, such as the sintering of catalyst particles, loss of catalytic active components, poisoning of the electrocatalysts by impurities, variation of the hydrophobic/hydrophilic properties of catalyst and gas diffusion layers, etc. [17]. For example, in the case of phosphoric acid fuel cells (PAFC), loss of activity has been ascribed in part to particle agglomeration apart from phenomena of leaching out of the acid [18–21]. Although H3 PO4 -doped PBI fuel cell systems are included within PEMFCs, they are not distant from PAFC, since the membrane must be impregnated with a large amount of acid in order to be proton conductor [12,15,22,23]. In addition, electrodes are also impregnated with acid since they contain PBI to provide proton conduction to the catalytic layer. Thus, platinum electrocatalysts in H3 PO4 -doped PBI-based PEMFC systems are immersed in a severe acidic medium. In this work, the influence of the temperature (from 100 to 175 ◦ C) on the performance of a PBI-based PEMFC has been analysed. For that, before carrying out any electrochemical measurement, the system was left for 24 h in one of the four temperatures at a constant voltage of 0.5 V (“conditioning” procedure). Thus, it is intended to permit that all the elements of the membrane-electrode-assembly (MEA) (electrocatalyst and polymeric electrolyte) can achieve a stationary (stable) or, at least, nominal (pseudo-stationary) regime. Thereby, all the subsequently obtained results (polarization curves and impedance spectra recorded at each temperature of the four ones used in this work) will be related to the temperature in which the system was conditioned, independent of the momentary temperature that was used to perform those measurements. Besides, in order to determine the catalyst stability, a cyclic voltammetry (CV) study was carried out, consisting of performing hundreds of consecutive cycles, from which it was possible to observe the changes in the catalytic activity. Before and after the cycling CV tests, X-ray diffraction patterns of the electrodes were taken to determine any change in the structure of the catalyst during the CV measurements. 2. Experimental 2.1. Preparation of the membrane-electrode-assemblies In order to prepare the electrodes, the following procedure was followed. On top of a gas diffusion media (Toray Graphite

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Paper, TGPH-120, 350 ␮m thick, 20% wet-proofed, ETEK Inc.), the electrodes were deposited by N2 -spraying a microporous layer (MPL) consisting of 1 mg/cm2 Vulcan XC-72R Carbon Black (Cabot Corp.) and 40% PTFE (TeflonTM Emulsion Solution, Electrochem Inc.). Next, they were also deposited by N2 -spraying a catalyst layer, composed of Pt/C catalyst (20% Pt on carbon black, ETEK-Inc.), PBI ionomer (5 wt.% PBI in N,N -dimethylacetamide, DMAc) and DMAc as a dispersing solvent. After depositing the catalyst layer, the electrodes were dried at 190 ◦ C for 2 h. Afterwards, the electrodes were wetted with a solution of 10% H3 PO4 with a loading of 30 mg/cm2 . Electrodes were left to absorb the acid overnight. The electrodes prepared were divided into four pieces. Two of them were used to prepare the membrane-electrode-assembly (MEA) whereas the other two were destined for the CV and structural studies. For the preparation of the MEA, a PBI membrane was taken out from an 80 wt.% phosphoric acid bath. Doping level acquired by the membrane was 6.5 molecules of acid per polymer repeating unit. The superficial acid onto the membrane was thoroughly wiped off with filter paper, and subsequently, it was used to prepare the MEA. In order to fabricate it, the doped membrane was sandwiched between a couple of electrodes, hot-pressing the whole system at 150 ◦ C and 100 kg/cm2 for 7 min. Once the MEA was ready, it was inserted into the cell. The active area of the electrodes was 4.65 cm2 . 2.2. Fuel cell tests Cell hardware consisted of two bipolar plates made of graphite (Ralph Coidan, UK) into which it was machined channel with parallel geometry. Within the graphite plates, heating rods were fitted in order to heat the cell up. During the performed measurements, the cell was fed with pure hydrogen and oxygen at a flow rate of 0.2 l/min and atmospheric pressure. Temperature was controlled with the aid of a temperature controller (CAL 3300, Cal Controls Ltd., UK). The procedure to perform polarization curves and impedance spectra can be depicted as follows. Firstly, the cell was kept at one temperature for 24 h, monitoring the current at a constant potential of 0.5 V. Afterwards, polarization curves and impedance spectra were consecutively recorded at the four temperatures used in this study, starting from the temperature in which the cell was conditioned and continuing from the lowest temperature to the highest one. Polarization curves were measured with a potentiostat/galvanostat Autolab PGSTAT30 (Ecochemie, The Netherlands). Unfortunately, this limited the upper limit of the current to 1 A, so that curves had to be stopped at a current density of 0.215 A/cm2 . Impedance spectra were recorded by the Frequency Response Analyzer (FRA) Module of the potentiostat/galvanostat at a potential of 0.5 V. Frequency ranged from 10 kHz down to 10 mHz, with a potential wave of 0.05 V. Once the cell swept the four temperatures, it was again left for 24 h at the next temperature. The conditioning procedure was carried out from the lowest temperature (100 ◦ C) to the highest one (175 ◦ C). Experiments were labelled as collected in Table 1.

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Table 1 Series of experiments performed in this work Conditioning temperature (◦ C)

Fuel cell measurement temperature (◦ C)

Label for the experiment

100 (C1)

100 125 150 175

P11 P12 P13 P14

125 (C2)

100 125 150 175

P21 P22 P23 P24

150 (C3)

100 125 150 175

P31 P32 P33 P34

175 (C4)

100 125 150 175

P41 P42 P43 P44

2.3. Thermogravimetric analysis (TGA) measurements TGA analyses were performed on a Perkin-Elmer Thermogravimetric Analyzer TGA7 equipped with a Gas Selector and a Thermal Analysis Controller TAC7/DX. A small piece of a PBI·6.5 H3 PO4 membrane was heated from 25 ◦ C up to 200 ◦ C

with dry air flowing, with a heating ramp of 0.5 ◦ C/min. Besides, another two samples of the same membrane were heated up to 150 and 175 ◦ C and kept at that temperature for 10 h in order to follow the evolution of the weight with time. 2.4. Catalyst stability 2.4.1. Cyclic voltammetric study CV measurements were performed as follows. First of all, the electrochemical active surface, EAS, of the electrodes was measured in 1 M H2 SO4 . Next, 500 cyclic voltammetries were carried out in 85 wt.% phosphoric acid (Panreac). At the end of the 500 cycles, EAS of the catalyst was again measured in 1 M H2 SO4 . All the measurements were performed on the glass-halfcell shown in Fig. 1 at room temperature. As it can be seen, it consists of a three-electrode system, with the working electrode located in a Teflon holder, whose disposition enable the circulation of N2 at the rear of the electrode. The counter electrode was a gold-foil and the reference electrode used in the work was a Standard Calomel Electrode, although reported potential values are referred to the Standard Hydrogen Electrode (SHE). Prior to any measurement, the system was thoroughly purged with nitrogen. Afterwards, cyclic voltammograms were all recorded between 0 and 1.2 V versus SHE at a scan rate of 0.05 V/s by using a potentiostat/galvanostat Autolab 30. Structural characterisation of the electrodes before and after the CV study were carried out on a rotating anode Philips PW-

Fig. 1. Schematic diagram of the half-cell used to perform the half-cell analyses.

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3. Results and discussion

seems to counteract any drying out phenomenon of the MEA, so that the membrane is under a stable hydration level. Both at 150 and 175 ◦ C, the cell behaviour at 0.5 V for 24 h is similar. Initially, as at 125 ◦ C, there is an increase of the current density. However, once the maximum is reached, the current density continuously drops at different rates depending on the temperature. The higher the temperature, the steeper the decay is. Furthermore, another interesting feature is the time necessary to attain the maximum, 180 min at 150 ◦ C and 15 min at 175 ◦ C. These two facts lead to think about a progressive electrolyte dehydration process taking place. The higher the temperature, the faster and more severe the process is expected to be. This explains why at 175 ◦ C a shorter period is necessary for this process to be noticeable, as well as the higher current density decay rate. Ma et al. [16] reported that working under anhydrous conditions, PBI proton conductivity decreases above 130–140 ◦ C due to the formation of pyrophosphoric acid, according to the following reaction:

3.1. Temperature effect on the “conditioning” curves

2H3 PO4 → H4 P2 O7 + H2 O

Fig. 2 shows the “conditioning” curves for a period of 24 h at a constant voltage of 0.5 V. According to the nomenclature used in Table 1, this corresponds to curves C1, C2, C3, and C4. From the figure, it can be seen that at 100 and 125 ◦ C the respective current densities reach stable values, whereas at 150 and 175 ◦ C, after an initial increase, current densities drop almost at constant rate (1 mA/(cm2 h) at 150 ◦ C and 2.8 mA/(cm2 h) at 175 ◦ C), indicating a degradation process of the MEA at those high temperatures. Focusing on the behaviour of the cell at each “conditioning” temperature, some interesting trends can be seen. Thus, at 100 ◦ C, there is an initial decay of the current density. This could be due to the handling procedure follows just before inserting the MEA in the cell. After pressing the MEA, this was mounted in the cell, in a process which requires certain time. Acid-doped PBI is known to be highly hygroscopic [12,24–26], so that it rapidly takes up water from the environment humidity. The loss of that absorbed water during the previous process to the start of the test could explain the initial losses of current density, as water is an active element in the proton conduction in PBI membranes [12,16,23,27]. Next, the current starts to go up slightly. This may be explained in terms of the water vapour generated in the cathode from the electrochemical reduction of oxygen. Water would partially rehydrate the membrane, increasing, accordingly, its conductivity and the overall cell performance. Contrary to 100 ◦ C, at 125 ◦ C, the current density increases until attaining an almost stable current density value. The initial increase could be due to initial non-equilibrium conditions when the test starts. Catalyst activation and the increase in the PBI conductivity (intrinsic effect of the temperature on this [12,16,22–24,27]) may explain the initial enhancement in the performance, until a hypothetic global equilibrium is reached by all the MEA elements. In this situation, assuming that the hydration level of the polymer electrolyte is the determinant of the cell performance, water vapour produced in the cathode

Thus, the origin of the degradation process at the highest temperatures (150 and 175 ◦ C) can be this undesirable process, which generates the less conductive pyrophosphoric acid species [16,28] The higher the temperature is, the faster the process becomes, as it will be shown later on. In summary, there are two opposite trends. On the one hand, higher temperatures improve the cell performance because of the intrinsically greater electrolyte conductivity and enhanced reaction kinetic. On the other hand, electrolyte dehydration deteriorates cell performance because of the decrease in the conductivity, becoming more predominant and faster as the temperature becomes higher. Moreover, as it will be displayed in the last section, redistribution in the catalyst dispersion may be another factor threatening the stable performance of the cell. The prevailing process will determine the “long-term” behaviour of the cell, as it can be seen in Fig. 2. The different behaviour at 100 ◦ C was earlier explained.

Fig. 2. Potentiostatic curves of the cell during the conditioning procedure at each temperature.

˚ Cu K␣) 2θ between 5◦ and 1700 diffractometer (λ = 1.5418 A, 75◦ .

(1)

3.2. Temperature effect on “Post-24 h” polarization curves Firstly, the term “Post-24 h” is used since, in this section, polarization curves at the same temperatures at which the cell was conditioned after 24 h are compared. Despite at 150 and 175 ◦ C real steady-state conditions have not been attained yet after 24 h, the trend towards which the system tends (nominal regime) can be observed. In consequence, results could be considered, at least, partially representative of the effect of the temperature on the cell performance. Hence, polarization curves could be also named as “pseudo-steady-state” or “long-term” ones. Fig. 3(a) displays the polarization curves corresponding to the experiments P11, P22, P33, and P44. As it can be seen, cell performances grow when temperatures increase from 100 to 150 ◦ C. Thus, at 0.6 V, current densities are 27.6 mA/cm2 at 100 ◦ C, 31.6 mA/cm2 at 125 ◦ C, and 32.2 mA/cm2 at 150 ◦ C.

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Fig. 3. (a) Polarization curves, (b) impedance spectra for: () P11; () P22; () P33; () P44; (c) equivalent circuit used to fit the experimental data shown in (b); (d) relative change of the ohmic resistance () and the cathodic polarization resistance (); Values of the corresponding resistances are displayed, with the polarization resistance in brackets.

However, at 175 ◦ C, current density is 28.3 mA/cm2 . In principle, it should be expected that an increase of the cell performance as temperature goes up as it was previously reported in literature [12–14,22,29–31], as consequence of higher proton conductivity through the PBI membrane and enhanced electrodes kinetic. Between 100 and 125 ◦ C, a notable increase of the cell performance can be seen, whereas between 125 and 150 ◦ C the increase is sensitively smaller and at 175 ◦ C performance decays compared to 150 ◦ C. In order to help to interpret the fuel cell results, Fig. 3(b) collects the Nyquist plots corresponding to P11, P22, P33, and P44 experiments. With it, it is possible to discern between the contribution of each one of the influential elements on the fuel cell performance (ohmic, kinetic and mass transport limitations) [32–34]. In order to fit the experimental data, Fig. 3(c) shows the proposed equivalent circuit. This consists of a combination of an uncompensated resistance, accounting for the ohmic resistance of the system, with a parallel circuit containing a charge transfer resistance and a constant phase element related to the hydrogen oxidation, added in order to explain the small loop present at high frequency as shown by Boillot et al. [35], and finally, another parallel circuit containing a new constant phase element and the polarization resistance for the oxygen reduction reaction (ORR). The presence of a unique loop for the ORR impedes the distinction between the relative contributions of the charge transfer and diffusional processes, becoming both together in the polarization resistance [32,33,36,37]. Fig. 3(d) shows the collected values of the ohmic and oxygen reduction polarization resistances after fitting the experimental values to the equivalent circuit. Analysing the trend and the values of the ohmic resistance in Fig. 3(d), it can be seen that it decreases between 100 and

125 ◦ C as expected, whereas it increases when temperature goes from 125 to 175 ◦ C, more notably between 150 and 175 ◦ C. The increase of the ohmic resistance could be attributed to the gradual formation of the pyrophosphoric acid during the “conditioning” procedures at 150 and 175 ◦ C, as explained earlier. Having a look at the oxygen reduction polarization resistance, another important parameter which also influences cell performance, it can be seen that this resistance decreases as temperature goes up. Thus, at 100 ◦ C this resistance is 1.77 , whereas at 175 ◦ C this is 0.49 . However, it can be seen that the higher the temperature, the less prominent the decrease is. Higher temperatures imply faster electrode kinetics, which must entail a reduction in Rp,c , as it can be mainly seen when temperature increased from 100 to 150 ◦ C. However, at 175 ◦ C the decrease is almost negligible, which can be explained in terms of the notable increase of the electrolyte resistance. It has been classically proposed in literature [33,38] that the catalytic layer is a “3-phases” system in which in order to have the maximum active area, it is necessary to have the catalyst particle in contact with an electron conductor (carbon support), ionic conductor (electrolyte) and a good access for the reactant gases (overall porous structure). If any of these elements fails (in this case, the electrolyte conductivity) the catalyst active area drops with the consequent reduction in the catalytic activity and increases in Rp,c . Furthermore, it has been reported in literature that a reduction in the water content in H3 PO4 -doped PBI diminishes the diffusion coefficient of oxygen through it [39]. Thereby, it is expected that at higher temperatures mass transfer processes become more difficult, which would also account for the augment in Rp,c . Finally, with the aid of the trends shown in Fig. 3(d), the behaviour of the polarization curves can be interpreted. Between

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100 and 125 ◦ C, the increase in the cell performance is due to the simultaneous reduction of both the ohmic resistance (R ) and the polarization resistance (Rp,c ). Between 125 and 150 ◦ C, the increase is appreciably less remarkable, since R slightly increases and the decrease in the Rp,c is not as notorious as before. Finally, between 150 and 175 ◦ C, there is a notable increase in R and the reduction of Rp,c is minimal, which could explain the decrease in the cell performance between those temperatures. 3.3. “Short-term” cell performances Herein, the term “short-term” cell performance is used to refer to the subsequent experiments performed after “conditioning” the cell at any temperature. In the case where polarization curves are carried out at different temperatures than the “conditioning” curves, cell conditions are momentarily changed, and measurements are carried out immediately. Hence, the response of the system (“short-term” one) will be purely dynamic. The system does not have enough time to reach equilibrium, so that any parameter obtained from these analyses will be associated to the “conditioning” temperature. Two kinds of studies have been performed. On one hand, after conditioning the cell at one temperature, the immediate response of the system at the four temperatures used in this study will be analysed. On the other hand, the cell performance at a unique temperature after conditioning the cell at the four temperatures will be studied.

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3.3.1. Temperature effect on the “short-term” response of the system after the “conditioning” process Fig. 4(a) displays the polarization curves after conditioning the cell at 125 ◦ C (experiments P21, P22, P23, and P24). As it can be seen, there is a continuous increase in the performances of the cell. Thus, at 0.6 V, the current density is 11.7 mA/cm2 at 100 ◦ C, whereas at 175 ◦ C and the same potential the current density is 76.3 mA/cm2 . Besides, the increase in the performance is constant as temperature increases in contrast to the “pseudosteady-state” polarization curves. Fig. 4(b) shows the Nyquist plots for the considered experiments. In Fig. 4(c), the relative variation of R and Rp,c and their absolute values in brackets have been displayed. As it can be seen, both resistances decrease as temperature goes up, indicating in the first case an increase in the electrolyte conductivity. This seems to confirm that the electrolyte degradation process is a progressive one and requires certain time in order to fully develop, and therefore, values of R are making the classical effect of the temperature on the PBI conductivity [12,14,16,22,27,30,31] clear. In the case of Rp,c , the increase in the temperature comes accompanied by an enhancement in the electrode performance, reducing that resistance consequently. Besides, in this case, as commented, electrolyte dehydration does not occur, and the mass transfer processes should improve with the increase in the temperature, accounting as well for the decrease in Rp,c . Thus, “short-term” response of the system is dominated by the expected intrinsic effects of the temperature (faster electrodic kinetic and higher electrolyte conductivity).

Fig. 4. (a) Polarization curves, (b) impedance spectra for: () P21; () P22; () P23; () P24; (c) relative change of the ohmic resistance () and the cathodic polarization resistance (); values of the corresponding resistances are displayed, with the polarization resistance in brackets.

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This could be explained taking into account that curves were continuously recorded without any “break” once the “conditioning” process finished. In consequence, as shown by Fig. 4(c), the simultaneous reduction in the ohmic and cathodic polarization resistance accounts for the increase in the cell performance that takes place when the temperature is immediately and consecutively increased after the “conditioning” process at one fixed temperature (125 ◦ C). 3.3.2. “Short-term” response of the system after conditioning at each temperature In order to carry out these measurements, the polarization curves and impedance spectra of the cell at 175 ◦ C (the most severe temperature) will be compared after the corresponding C1, C2, C3, and C4 processes (curves P14, P24, P34, and P44). Thus, Fig. 5 shows the polarization curves (Fig. 5(a)) and Nyquist plots (Fig. 5(b)) already fitted to the equivalent circuit (Fig. 3(c)). Fig. 5(c) shows the relative changes in the ohmic resistance and the polarization resistance for the cathodic reaction as in Figs. 3(d) and 4(c). As the temperature of the polarization curves and impedance spectra is always the same, it is expected that results mainly reflect the effects of the “conditioning” process (“long-term” ones). As it can be seen in Fig. 5(a), there is a decrease in the cell performance at 175 ◦ C as the temperature of the “conditioning” increases. At 0.6 V, the current density is 69.6 mA/cm2 after conditioning the cell at 100 ◦ C, whereas after conditioning at 175 ◦ C the corresponding current density is 28.3 mA/ cm2 .

In order to interpret the fuel cell results, results of the impedance spectra are analysed (Fig. 5(b and c)). In them, it can be seen that the ohmic resistance continuously increases with the “conditioning” temperature. This leads to think that H3 PO4 doped PBI is undergoing a progressive and constant dehydration process during the “conditioning” process. When temperature increases from 100 to 125 ◦ C, it is possible that a reduction in the hydration level of the electrolyte takes place, as a result of a larger evaporation rate that is not compensated by the water produced in the cathode in comparison to that at 100 ◦ C. At 150 and 175 ◦ C, the increase in R can be explained in terms of the self-dehydration of phosphoric acid to lead to the formation of pyrophosphoric acid. In the case of Rp,c , the increase could be explained in terms of a reduction in the catalytic activity. As commented, the electrolyte is fundamental in order to have a good electrode performance, so that its degradation is expected to impair the charge transfer process, maybe, through a reduction in the electrochemical active surface of the electrode. Larger mass transfer limitations also account for the increase in Rp,c as the diffusion coefficient of oxygen is expected to decrease. In addition, as it will be pointed out later on, catalyst agglomeration may be another factor responsible for this decrease. Finally, as earlier, relative changes in R and Rp,c help to understand more easily the polarization curves. As it can be seen in Fig. 5(c), as “conditioning” temperature increases, ohmic and cathodic polarization resistances also increase. Furthermore, the higher the temperature, the more notable the increases are. This lead to the conclusion that the progressive electrolyte dehydration happening in the MEA is mainly responsible for the decay in the performance in this set of experiments.

Fig. 5. (a) Polarization curves, and (b) impedance spectra for: () P14; () P24; () P34; () P44; (c) relative change of the ohmic resistance () and the cathodic polarization resistance (); values of the corresponding resistances are displayed, with the polarization resistance in brackets.

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Fig. 6. Thermogravimetric analysis performed on a PBI·6.2 H3 PO4 membrane.

3.4. Study of the thermal stability of H3 PO4 -doped-PBI in the range of the fuel cell operating conditions So far, drops in the performance with the temperature have been mainly explained in terms of losses of water in the electrolyte due to self-dehydration of orthophosphoric acid to produce the less conductive pyrophosphoric acid [16,28], according to the reaction shown in Eq. (1). In order to evaluate the stability of the acid-doped-membrane (PBI·6.5 H3 PO4 ) and set the temperature in which the dehydration process begins, a TGA analysis at a slow heating ramp was performed (0.5 ◦ C/min) from 25 ◦ C up to 200 ◦ C under a dry air atmosphere. Results are shown in Fig. 6. As it can be seen, there are two well-defined weight decays in the curve. The first one starts from the initial temperature of the TGA (25 ◦ C) and goes up to 55 ◦ C. This loss of weight is related to the free water contained within the polymer because of both the hydrophilic nature of the polymer and the hygroscopic characteristics of the phosphoric acid [12,16,28,40]. Then, approximately, at 130 ◦ C, a second loss of weight appears. This one is ascribed to the self-dehydration reaction, forming the above-mentioned pyrophosphoric acid [12,16,28,40]. In consequence, the initial decrease observed at 100 ◦ C in Fig. 2 can be fully attributed to losses of free water as it was previously explained. Then, at 150 and 175 ◦ C, as displayed by TGA analysis, the membrane is already undergoing self-dehydration of the acid. This will notably reduce the electrolyte conductivity, and therefore, the current density is drawn from the cell, as it can be seen in Figs. 2, 3(a) and 5(a). Fig. 7 shows the evolution of the weight losses with time at 150 and 175 ◦ C. As it can be seen, the higher the temperature, the more rapid the dehydration process is. This is in agreement with the results shown in Fig. 2, where the rate of performance decay is greater at 175 ◦ C than at 150 ◦ C. However, there could be a remaining question: why is stability in weight reached so fast in Fig. 7 and it is not attained in Fig. 2? In theory, current density should decrease until reaching a stable value in which the membrane does not self-dehydrate any longer. It may be thought that this value should be similar to the time in which a stable weight is reached in Fig. 7. However, it has to be taken into account that conditions in a TGA analysis and in a fuel cell are very different. In the TGA, pieces of membranes are directly

Fig. 7. Evolution of the weight of the PBI·6.2 H3 PO4 membrane with time at 150 ◦ C (solid line) and 175 ◦ C (dashed line).

in contact with a flow of dry air. This implies that the dehydration process is accelerated compared to the fuel cell, where the electrolyte in the membrane is protected by the electrode. Similarly, the dehydration of the electrolyte in the catalytic layer is slowed down by the gas diffusion and the micro-porous layer. Water produced in the cathode also slows the electrolyte dehydration process in a high-scale fuel cell. This may explain why there is still a continuous decrease in the current density at 150 and 175 ◦ C even after 24 h of test, despite stability in the weights shown in Fig. 7 is attained relatively rapid. 3.5. Catalyst stability. Cyclic voltammetric study Catalytic stability is another issue to be considered in phosphoric acid-doped PBI-based PEMFC system as it is in PAFC [18–21]. The catalyst in this kind of cells is impregnated with H3 PO4 in order to make the PBI contained in the catalytic layer proton conductor. Also, this layer is in contact with a membrane whose phosphoric acid concentration is 15 M [12,24]. The extremely acid environment may accelerate the degradation of the catalyst. A multi-cycling sweep voltammetry test has been carried out in order to determine the Pt catalyst stability. Fig. 8(a) shows the cyclic voltammograms as the test in concentrated phosphoric acid proceeded. Fig. 8(b) displays the relative decrease of the area under the hydrogen desorption peak, considering as reference the area of the first cycle (A1 ). The area of these peaks is related to the electrochemical active surface of the electrode. It can be seen that there is a progressive decrease of the active area of the electrode as the CV study goes by. Dehydration cannot be the origin of losses of catalytic activity, since PBI is continuously in contact with a concentrated phosphoric acid solution. In consequence, losses of activity have to be explained in terms of Pt dissolution–redeposition and Pt migration through the surface [41]. The extremely corrosive medium in which measurements were performed will cause losses of active area due to particle sintering when the electrode is polarised [42]. In addition, a better distinction between different hydrogen desorption peaks (corresponding to different crystalline orientations [43]) can be appreciated. Kinoshita [44] reported that the increase in

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Fig. 9. Cyclic voltammetry performed on electrodes in 1 M sulphuric acid. Table 2 Electrochemical active surface of the electrodes before and after the 500 cycles performed on the electrodes Measurement

Electrochemical active surface (m2 /g Pt)

Before the 500 cycles After the 500 cycles

18.67 8.25

Fig. 8. (a) Cyclic voltammetry performed on electrodes in 85% phosphoric acid medium; (b) relative evolution of the electrode active area vs. the number of cycles.

Losses of cell performance coming from variation of EAS can be estimated by the following equation [46]:   EAS1 (4) V = b log EAS2

the separation of the peaks can be attributed to a growth in the number of atoms in the (1 1 1) and (1 0 0) crystal orientations. This reorientation may be occurring during the movement of the Pt atoms in the dissolution-redeposition/migration. Fig. 9 shows the cyclic voltammograms before and after the 500 cycles performed in H3 PO4 , in order to measure the electrochemical active surface as described elsewhere [45]. Table 2 collects the corresponding values of the electrochemical active surface. As it can be seen, after carrying out the 500 cycles, the catalyst has suffered degradation, with the consequent decrease of the EAS, because of the processes previously commented.

where V is the voltage drop, b the Tafel slope, EAS1 and EAS2 are the electrochemical active surfaces before and after the CV measurements, respectively. According to the value reported by Liu et al. [39] for b (100 mV dec−1 ), the voltage loss coming from losses of catalytic active surface is approximately 35 mV. Fig. 10 collects the XRD patterns in order to confirm whether particle sintering after the cycling process has taken place. This phenomenon causes an increase in the particle size, which reflects in a diffractogram by the growth of the peaks associated to the different crystallographic planes. Thus, XRD analyses of the samples before and after the CV measurements were carried out.

Fig. 10. X-ray diffraction patterns of the catalyst: (a) before cycling process; (b) after cycling process.

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Fig. 10 shows the diffraction patterns between 30◦ and 70◦ , where the peaks associated to (1 1 1), (2 0 0) and (2 2 0) crystalline orientations, at 40◦ , 46◦ and 67◦ , respectively, can be seen. As it can be seen, after the CV test, all the peaks become narrower and more distinguishable. By means of the Scherrer equation [47], the mean particle sizes can be easily calculated by: D=

0.9λ β cosθ

(5)

˚ θ the angle at where λ is the wavelength of X-ray (1.5406 A), the peak maximum and β is the width in radians of the peak at half height. However, uncertainties in the determination of the width of the peaks at half heights preclude the estimation of the particle size. Then, results are only qualitatively discussed. The narrower peaks after the CV study is indicative that particles redistribution (migration) has taken place in the catalytic layer during the measurement, with an agglomerating (sintering) result, and with a consequent decrease of the catalytic activity owing to the increase in the particle size. These studies, although performed on a strong acidic medium trying to mimic the conditions in which the catalytic layer is imbibed (electrodes in contact with a PBI membrane highly impregnated in H3 PO4 ), were carried out at room temperature. It is expected that the higher operating temperature in the cell will accelerate the particle sintering process already observed at room temperature. In consequence, this fact cannot be rejected at all in order to explain performance losses in high temperature H3 PO4 -doped PBI based systems. Work is under progress in order to establish what elements are responsible for the degradation process, so that this undesirable process can be minimised or eliminated. Stable long-term performance is the final goal of this ongoing research. 4. Conclusions In this work, it has been seen that the temperatures greatly influence the cell performances by (i) the electrolyte conductivity and electrodic kinetic are intrinsically improved by increasing the temperature, which enhances the overall cell performance, and, on the other hand, (ii) high temperatures favour the electrolyte dehydration (by reducing the hydration level of the membrane or by the self-dehydration of the phosphoric acid) and may accelerate the catalyst particle agglomeration, which impairs the cell performance. These processes present different responses with changes in the temperature. The first processes rapidly respond and determine the “short-term” behaviour of the cell, whereas the respond of the second processes is more progressive and influences the cell performance in the “long-term”. In consequence, in order to obtain, at least, partially meaningful results of the behaviour of the system with the temperature, it is necessary to leave it for “certain” time at the considered

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temperature. This allows the different processes to mature, as demonstrated by the “conditioning” curves, so that the predominant ones at each temperature control the cell performance, albeit the others cannot be neglected. Thus, in the “long-term” polarization curves, temperature increases the cell performance up to 150 ◦ C, whereas at 175 ◦ C, it drops. The combination of electrolyte dehydration and a possible catalyst agglomeration (“second group” processes) spoils the cell performance at the latter temperature, whereas at the others the intrinsic effects of the temperature control the cell performance. “Short-term” polarization curves after conditioning the cell at one temperature are controlled by the “first group” processes as their performances appreciably increase as temperature increases. On the contrary, “short-term” polarization curves performed at 175 ◦ C after conditioning the cell at each temperature also reflect the effects of the “second group” processes. Acknowledgement This work was funded by the Ministry of Education and Science of the Spanish Government through a project (CTM2004-03817) which includes a pre-doctoral grant awarded to J.J. Linares. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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