Interplay between Composition, Structure, and Properties of New

Article pubs.acs.org/Macromolecules

Interplay between Composition, Structure, and Properties of New H3PO4‑Doped PBI4N−HfO2 Nanocomposite Membranes for HighTemperature Proton Exchange Membrane Fuel Cells Graeme Nawn,† Giuseppe Pace,†,‡ Sandra Lavina,†,§ Keti Vezzù,∥ Enrico Negro,†,§ Federico Bertasi,†,‡,§ Stefano Polizzi,§,⊥ and Vito Di Noto*,†,§ †

Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, I-35131 Padova (PD), Italy CNR-IENI, Via Marzolo 1, I-35131 Padova (PD), Italy § Consorzio Interuniversitario Nazionale per la Scienza e la Tecnolgia dei Materiali, INSTM, Florence, Italy ∥ Veneto Nanotech S.C.p.a., Via San Crispino, 106, I-35129, Padova (PD), Italy ⊥ Dipartimento di Scienze Molecolari e Nanosistemi, Università di Venezia, Calle Larga S. Marta, Dorsoduro 2137, I-30123 Venezia (VE), Italy ‡

S Supporting Information *

ABSTRACT: Polybenzimidazole (PBI) has become a popular polymer of choice for the preparation of membranes for potential use in high-temperature proton exchange membrane polymer fuel cells. Phosphoric acid-doped composite membranes of poly[2,2′(m-phenylene)-5,5′-bibenzimidazole] (PBI4N) impregnated with hafnium oxide nanofiller with varying content levels (0−18 wt %) have been prepared. The structure− property relationships of both the undoped and acid-doped composite membranes are studied using thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the presence of nanofiller improves the thermal and mechanical properties of the undoped membranes and facilitates a greater level of acid uptake. The degree of acid dissociation within the aciddoped membranes is found to increase with increasing nanofiller content. This results in a conductivity, at 215 °C and a nanofiller level x ≥ 0.04, of 9.0 × 10−2 S cm−1 for [PBI4N(HfO2)x](H3PO4)y. This renders nanocomposite membranes of this type as good candidates for use in high temperature proton exchange membrane fuel cells (HT-PEMFCs). particular the CO sensitivity of the anodic catalyst.1 These issues have created a strong driving force for the development of membranes that are capable of operating at elevated temperatures (>100 °C). At elevated temperatures not only have the sluggish reaction kinetics of the oxygen reduction reaction (ORR) been observed to improve,2 but an increased tolerance toward CO has also been observed.3 This could enable a switch to the use of fuels from reformed fuel feedstocks. Also, the preclusion of water omits the necessity for humidification and cooling units. This could lead to dramatically decreasing the fabrication and operating costs of mid−high temperature fuel cells. Some common strategies aimed at achieving higher operating temperatures for PEMFCs that focus on membrane manipulation include; modifying PFSA based membranes, using alternative sulfonated polymers, and using thermally stable acid−base polymer membranes. The former two strategies are

1. INTRODUCTION Proton conducting membranes (PEMs) are electrolytic media that, for three primary reasons, play an essential role in proton exchange membrane fuel cells (PEMFCs): (1) they guarantee the ionic transport between the anode and the cathode, (2) act as a barrier to keep the feedstock gases and products separate from each other, and (3) provide support for the electrode catalysts in the membrane electrode assembly (MEA) at the heart of PEMFCs. For these reasons it is imperative that PEMs possess high ionic conductivity, low gas permeability, and good mechanical properties if they are to be utilized for fuel cell applications. Current state-of-the-art commercial PEMs, e.g. Nafion, are based on perfluorosulfonic acid (PFSA). They require high levels of humidification in order to conduct protons and as a result limit fuel cell operating temperatures to below 100 °C. This thermal constraint gives rise to a number of challenges such as low cathode efficiency, high materials cost, challenging system design including the necessity of cooling and humidification units, and finally the need of additional fuel processing due to the low tolerance of fuel impurities, in © 2014 American Chemical Society

Received: September 12, 2014 Revised: December 10, 2014 Published: December 22, 2014 15

dx.doi.org/10.1021/ma5018956 | Macromolecules 2015, 48, 15−27

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benefits alone could significantly decrease the overall fabrication and running costs of PEMFCs. Although hafnia may be a more expensive filler in comparison to other fillers being studied for high-temperature PEMFC applications, the cost of hafnia would not preclude its use as inorganic filler in PEMFCs should it bring about more desirable attributes and improved performance than its more economical counterparts. HfO2 is basic in nature and stable under acid conditions; thus, it is a good nanofiller candidate to improve the thermal and mechanical properties of ion-exchange membranes. It can be coordinated to the nitrogen atoms of the PBI4N polymer chains in order to increase the basic sites for the proton exchange process and improve H3PO4 uptake in bulk membranes. The relationship between composition and structural, thermal, mechanical, and electrical properties or both the undoped and acid-doped nanocomposite membranes are studied.

the subject of numerous reviews, and for more information readers are directed elsewhere.4−7 Improved materials for operating at elevated temperature (ca. 120 °C) were achieved by preparing a variety of composite membranes consisting of Nafion doped with heteropolyacids1−4 or oxides such as SiO2,5 TiO2,6 ZrO2,6 and HfO2.7 Among the simple metal oxide-doped membranes, [Nafion(HfO2)n]7 showed the best properties in terms of elastic modulus and conductivity. Recently, it was demonstrated that Nafion[(M1mOn)·(M2xOy)z] membranes based on [(M1mOn)· (M2xOy)z] “core−shell” nanoparticles with a “core” of a hard oxide (M1mOn) (M1 = Zr, Si) covered by a thin layer of a soft oxide (M2xOy) (M2 = Si, Hf, Ta) show reduced water uptake and improved mechanical properties and proton conductivity as compared to pristine Nafion and [Nafion(MxOy)n] membranes. One of the more successful acid−base polymers developed so far is phosphoric acid-doped poly[2,2′-(m-phenylene)-5,5′bibenzimidazole] (PBI4N). Owing to its impressive mechanical and thermal properties (Tg = 425−436 °C),20 PBI4N has found a variety of applications ranging from those in the textiles industry21 to blood dialysis, reverse osmosis,22 valve coating in the oil industry,23 and more recently as a component in supercapacitors and gas separators.24,25 Pure PBI4N is an electrical insulator that can be rendered a proton conductor upon doping with strong acids owing to its inherent basicity (pKa ≈ 5.5 of the conjugate acid).26,27 While a variety of acids have been investigated for the purposes of doping PBI4N, the most suitable for high temperature fuel cell applications is phosphoric acid. This is owing to both its increased thermal stability (up to 200 °C) and higher proton conductivity with respect to other acids.28 As an electrolyte for PEMFCs, PBI4N membranes were first demonstrated by Wainright et al. followed by Samms et al. to be able to operate at elevated temperatures (100−200 °C) and therefore under anhydrous conditions.29,30 What followed has been a boom in research covering areas such as functionalization, structural and mechanical properties, acid doping and water uptake, fuel crossover, thermal stability, and water drag.4,5,29,31−38 In general, a higher level of acid uptake by the membrane results in improved conductivity. A variety of methods aimed at improving the acid uptake of PBI4N membranes have been explored. Modifications to the monomer unit such as replacement of the phenol ring with pyridine (yielding PBI5N), grafting on sulfonate groups or introducing Lewis basic linkers have all been shown to improve the properties of resulting membrane.39,40 An alternative strategy involves the incorporation of inorganic nanofillers into the polymer matrix. The use of phosphotungstic acid, imidazole-functionalized silica, and zirconium phosphates have all resulted in composite PBI4N membranes with improved characteristics over pristine PBI4N membranes. Higher levels of acid and water retention at elevated temperatures, improved mechanical and conductive properties, and protection for the catalysts from CO poisoning have all been reported.26,33,41−43 This investigation incorporates hafnium oxide nanofiller into the PBI4N matrix at various weight percentages to create composite PBI4N membranes. The primary motivation behind this study is to investigate whether the presence of hafnia nanofiller could facilitate the use of composite PBI4N membranes in high temperature proton exchange membrane fuel cells, a byproduct of which could be a diminished (or zero) dependence on humidification units and cooling systems as well as the possibility to use reformed fuel feedstocks. These

2. EXPERIMENTAL SECTION 2.1. Materials. Poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole], hereafter called PBI4N, was purchased from Celazole as a concentrated gum in dimethylacetamide (DMAc) with a LiCl stabilizer (1.5 wt %). The polymer has a molecular weight in excess of 30 000 to give better mechanical properties.44 Hafnium oxide (>98%), ophosphoric acid (85 wt %), and all solvents were purchased from Sigma-Aldrich and used as received. 2.2. Preparation of the Pristine PBI4N Membrane. The stock gum was diluted in DMAc to give a solution of 1.0 g/mL. This solution was then cast using a doctor blade onto a stainless steel plate within an aluminum border of dimensions 10 cm × 10 cm and a thickness of ca. 330 μm. The plate was then put into an oven at 80 °C for 1 h before the temperature was raised to 110 °C, and the plate was left for an additional hour. The plate was then removed from the oven and, after allowing it to cool, was then submerged into a room temperature water bath until the membrane lifted free. The membrane was then immersed in an 80 °C water bath of doubly distilled water for 3 h, with the water being changed after every hour. The membrane was then blotted dry before being hot pressed at 120 °C and 1500 PSI for 10 min. Finally, the membrane was dried under vacuum at 120 °C for 12 h and stored under an argon atmosphere. It is important that the membranes are free of any residual DMAc as the solvent can coordinate to the metal sites on the surface of the HfO2 nanoparticles and subsequently affect the properties of the nanocomposite membranes.38,45 2.3. Preparation of the Hafnium Oxide Nanofiller. HfO2 powder (ca. 7.0 g) was dry milled in a tungsten carbide milling jar containing three tungsten carbide spheres. Four 5 min cycles with a 10 s break before a direction change were used. Approximately 6 mL of DMAc was then added directly into the milling jar to create a white paste. The paste was further milled for 1 h using the same 5 min cycles as the dry milling. The paste was then rinsed into a glass jar, and further DMAc was added to give a total volume of ca. 150 mL. The suspension was then shaken vigorously before being left to settle overnight. The supernatant was then decanted from the mixture leaving behind a white residue. The milky looking supernatant was concentrated under reduced pressure at 44 °C to yield approximately 10 mL of white HfO2 nanofiller suspension. The concentration of the suspension was found to be 0.16 g/mL by thermogravimetric analysis. 2.4. Preparation of PBI4N−HfO2 Composite Membranes, [PBI4N(HfO2)x]. The PBI4N gum and HfO2 nanofiller were combined in DMAc to yield a mother suspension consisting of 35 wt % HfO2 and a PBI4N concentration of 1 g/mL. Working suspensions of varying nanofiller composition (3, 5, 8, 11, and 18 wt %), all with a PBI4N concentration of 1 g/mL, were prepared by diluting aliquots of the mother suspension with a solution of PBI4N in DMAc. The working suspensions were then cast and treated following the same procedure as that for the pristine PBI4N membrane. The obtained membranes were in the thickness range of 65−85 μm and stored 16


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under an argon atmosphere. The nanofiller content (x) is defined as the number of moles of hafnium oxide per repeat unit of PBI4N. 2.5. Preparation of Acid-Doped Membranes, [PBI4N(HfO2)x](H3PO4)y. A circular portion of membrane (diameter = 6 cm) was placed between two glass plates and immersed in 85 wt % ophosphoric acid for 72 h at room temperature. Following the acid imbibing, the membrane was blotted dry and placed under vacuum for 12 h at 120 °C. The acid-doped membranes were in the thickness range 80−100 μm and stored under an argon atmosphere. The acid doping level (y), defined as the number of moles of H3PO4 per repeat unit of PBI4N, was calculated by comparing the weights of the undoped membranes to that after acid imbibing and drying. The weight increase is therefore attributed to phosphoric acid alone as all traces of water have been eliminated during the drying process (see section 5.7).

Scanning electron microscopy (SEM) images were collected with backscattered electrons and recorded using a Cambridge Stereoscan 250 Mark 1 electron microscope at an acceleration voltage of 20 kV. High-resolution transmission electron microscopy (HR-TEM) analysis was executed at 300 kW with a Jeol 3010 apparatus mounting a Gatan slowscan 794 CCD camera. Samples were suspended in isopropyl alcohol, and a 5 μL drop of the suspension was deposited onto a holey carbon film supported on 3 mm copper grid. Elemental analyses (C, H, and N) were conducted using a FISONS EA-1108 CHNS-O instrument. The hafnium content of the final membranes was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using an ICP SPECTRO Arcos with EndOnPlasma torch. Sample mineralization was carried out by dissolving an aliquot of each material in hot aqua regia. The analyses were performed using the method of standard additions. The emission lines were λ(Li) = 670.78 nm and λ(Hf) = 264.141 nm.

3. INSTRUMENTS AND METHODS Thermogravimetric (TG) analyses were carried out using a highresolution (HR) thermobalance TGA 2950 (TA Instruments). Analyses were conducted under either a nitrogen or dry air flow at a rate of 100 cm3 min−1. TG profiles were collected over a temperature range of 25−950 °C with the heating rate varied from 40 to 0.001 °C min−1, depending on the first derivative of the weight loss. Approximately 3 mg of undoped or 6 mg of acid-doped membrane was loaded into an alumina bucket on an open platinum pan. The samples were prepared inside a glovebox under an argon atmosphere before being transferred quickly to the TG analyzer. This minimized exposure to the outside atmosphere as PBI4N membranes have previously been observed to be hygroscopic in nature.24,46 Modulated differential scanning calorimetry (MDSC) analyses were performed using a DSC Q 20 (TA Instruments) equipped with a liquid nitrogen cooling system. Measurements were carried out using a modulated mode over a temperature range of −150−200 °C with a ± 1.000 °C modulation every 60 s. The measurements of the undoped samples were made by loading approximately 8 mg of sample inside a hermetically sealed aluminum pan. For the phosphoric acid-doped membranes, approximately 15 mg of sample was loaded into a chromium(VI)-coated hermetically sealed aluminum pan. All samples were prepared inside a glovebox under an argon atmosphere. Fourier transform−infrared attenuated total reflectance (FT-IRATR) spectra were collected using a Nicolet FT-IR Nexus spectrometer operating at a resolution of 2 cm−1. The spectra were obtained in Single Bounce ATR mode with an inert atmosphere Golden Gate accessory (Specac). All samples were prepared in a glovebox under an argon atmosphere. The spectra were normalized with respect to the band at 1528 cm−1, which is shown to be largely unaffected by the nanofiller content. Dynamic mechanical analyses (DMA) was conducted using a DMA Q 800 (TA Instruments) equipped with a clamp specifically designed for testing films in the tension mode. Spectra were collected by applying a sinusoidal deformation of amplitude 4 μm and 1 Hz in 5 °C intervals over a thermal range of −150 to 350 °C. A rectangular sample (width = 7 mm, length = 30 mm) was subjected to a preloading forces of 0.05 N. The viscoelastic behavior of the samples was quantified in terms of elastic modulus (E′), loss modulus (E″), and loss factor (tan δ, defined as E″/E′). X-ray patterns of membrane films and powder nujol mulls were collected via wide-angle X-ray scattering (WAXS) within the 2θ range of 5°−70°. A GNR analytical instrument (mod. eXplorer) using a Cu Kα1 source of λ = 1.5406 Å with a current of 30 mA and potential of 40 kV was used. All samples were prepared under an argon atmosphere. Broadband electrical spectroscopy (BES) data were collected over the frequency range of 0.1 Hz−1 MHz using a Novocontrol Alpha A Analyzer. The electrical spectra were measured in the thermal range between −105 and 225 °C (for the acid-doped membranes) and −105 and 195 °C (for the undoped membranes) using 10 °C intervals with accuracy greater than ±0.1 °C. Circular samples of diameter 13 mm were compressed between two platinum cylindrical electrodes. All samples were prepared under an argon atmosphere.

4. QUANTUM MECHANICAL CALCULATIONS Optimized geometries and infrared spectra of molecular models were evaluated using density functional theory (DFT) methods on the basis of an all-electron code using the DMol3 program.47The internal modes are identified by animating the atomic motion of each calculated mode using the features available in the Materials Studio package. Double numerical plus polarization basis set, gradient-correlated (GCA) BLYP functional was used. 5. RESULTS AND DISCUSSION 5.1. HR-TEM and SEM Analyses. The method employed in this investigation to prepare the HfO2 nanofiller results in particles or particle aggregates of an average size distribution between ca. 50 and 150 nm, whose morphology is shown in Figure 1.

Figure 1. TEM images of HfO2 nanofiller. Magnification: (a) 12000×; (b) 25000×.

The incorporation of HfO2 into the PBI4N matrix results in composite membranes with a homogeneous nanofiller distribution, with nanofiller particles or particle aggregates remaining mostly in the 50−150 nm range. This suggests a good compatibility between the two phases at this nanofiller level (Figure 2).

Figure 2. SEM images of [PBI4N(HfO2)0.127]. Magnification: (a) 5000×; (b) 20000×; (c) 250000×. 17


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Following treatment with phosphoric acid, the nanofiller remains homogeneously distributed throughout the membrane. Particle size is retained with no evidence of further aggregation observed within the scanning region shown (Figure 3).

Figure 3. SEM images of [PBI4N(HfO2)0.127](H3PO4)13. Magnification: (a) 5000×; (b) 20000×; (c) 250000×.

Whether it be undoped or doped with phosphoric acid, the polymer matrix is able to embed effectively the filler nanoparticles and nanoparticle aggregates. Thus, relatively strong chemical interactions are expected to occur between the various components of the hybrid membranes, as also witnessed by the swelling around the nanoparticles and nanoparticle aggregates upon doping with phosphoric acid (Figure 3c). 5.2. Acid Doping. In one repeat unit of PBI4N there are two Lewis basic nitrogen atoms that can be protonated resulting in bonded acid. Any additional acid uptake is thought to remain as free acid, in equilibrium with the ionic species (Figure 4).47,48 It has been shown that the presence of free acid increases the conductivity of the acid-doped membranes over those possessing only bound acid, i.e., a doping level of y = 2.37

Figure 5. Phosphoric acid doping level of [PBI4N(HfO2)x](H3PO4)y with different amounts of HfO2 nanofiller, x.

polymer matrix up to a certain loading level (x ≈ 0.172). Above this level, nanofiller-rich domains may form which increases the free volume within the membrane. This in turn may result in the increased acid uptake as the phosphoric acid fills these voids. The increase in acid uptake by the composite membranes, with respect to pristine PBI4N, and therefore increase in the amount of free acid is shown to manifest itself in the conductivity measurements (see section 5.8). The ability of membranes to retain acid was revealed by immersing the membranes in doubly distilled water for 60 min at room temperature. Results show (Figure SI-1) that after this procedure49,50 a reasonable amount of acid is still present in bulk material that can guarantee an acceptable value of membrane conductivity (y ≈ 4). 5.3. Thermogravimetric Analysis. TG profiles of pristine PBI4N and [PBI4N(HfO2)x], conducted under a nitrogen atmosphere, all exhibit three mass losses (Figure 6). The first,

Figure 4. Repeat unit of PBI4N showing Lewis basic nitrogen atoms (green), “bonded acid” (blue), and “free acid” (red) at a doping level of 4.

The method of acid doping employed in this investigation results in a doping level of 11 for [PBI4N](H3PO4)11. This is consistent with the doping levels described in the literature for the same doping time and method.5,28,29,37,38 For the composite membranes the doping levels achieved ranges from y = 13 to y = 21 and appears dependent on the amount of nanofiller within the membrane (Figure 5). In the nanofiller range 0 < x ≤ 0.08 the doping level is observed to increase, reaching a maximum of 13 at x = 0.08. This suggests that the basic nature of the hafnium oxide increases the phospholicity of the composite membrane with respect to that of pristine PBI4N. As the hafnia content continues to increase, 0.08 < x < 0.17, there is little change observed in the acid uptake level. As the amount of nanofiller increases further, the doping level rises sharply, achieving a doping level of 21 mol of phosphoric acid per repeat of PBI4N at a nanofiller loading level of x = 0.322. This sharp increase in acid uptake may suggest that the hafnia nanofiller can only been homogeneously distributed within the

Figure 6. TG profiles for [PBI4N(HfO2)x] measured under N2 from 30 to 950 °C with the derivative dwt %/dT (inset).

in the temperature range 20−200 °C, is likely due to desorption of H2O both on the surface and within the membrane matrix. It has also been reported that in this thermal range loss of CO2 occurs, possibly as a result of thermal decomposition of the carboxyl containing polymer chain end groups.30 This first mass loss is on the order of 1 wt % for all membranes and highlights 18


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losses. The first mass loss between 20 and 120 °C is attributed to the elimination of water. Similar to the undoped membranes, this first mass loss corresponds to only 1% for all membranes indicative of an efficient drying process. The second mass loss within a temperature range of 180−240 °C is attributed to the dehydration of phosphoric acid as it converts to pyrophosphoric acid.5,30

the efficiency of the drying process used in this investigation. The second mass loss occurs between 550 and 650 °C and is indicative of benzene elimination during the start of thermal decomposition of the PBI4N matrix. Further thermal decomposition occurs between 650 and 800 °C where mass losses have been attributed to elimination of HCN, NH3, CH4, CH3CN, and possibly CH3NC.30,46 The onset of the thermal decomposition of the polymer is observed to increase with increasing nanofiller content (Figure 7). This suggests that the hafnium oxide interacts with the PBI4N chains in such a way that increases the overall thermal stability of the polymer matrix.

2H3PO4 ⇌ H 2O + H4P2O7

Not only is this mass loss not observed in the TG profiles of the undoped samples, but the TG profile of neat phosphoric acid also exhibits a mass loss in this temperature range. The third mass loss between 300 and 580 °C is attributed to the onset of thermal decomposition of the PBI4N matrix as observed in the TG profiles of the undoped membranes. The final mass loss occurring in the thermal range 580−690 °C is attributed to a combination of continued PBI4N matrix decomposition along with dehydration of the pyrophosphoric acid during the formation of poly(phosphoric acid).5,30 Again, the TG profile of neat phosphoric acid exhibits a mass loss in this range: H4P2O7 ⇌ H 2O + 2HPO3

The loss in weight attributed to the dehydration of phosphoric acid (180−240 °C) gives an indication to the degree of acid doping within the membrane. By plotting the weight loss in this temperature range as a function of hafnium oxide nanofiller content, a trend reminiscent to that of acid uptake is observed (Figure SI-3). The acid-doped membranes are observed to exhibit a higher degree of thermal stability than their undoped analogues. The average decomposition temperature, taken as the most intense first derivative, is 609 °C for the undoped membranes but 640 °C for those that have been acid-doped. This is in good agreement with the literature and attributed to the stronger hydrogen bonds formed between the imidazole rings and the acid molecules than those formed between the polymer chains themselves.24,42,48 The TG analysis profiles of the acid-doped membranes, conducted under an oxidizing atmosphere, also exhibit four transitions and can be found in the Supporting Information (Figure SI-4). 5.4. Modulated Differential Scanning Calorimetry. MDSC analysis of all membranes was conducted, and that of pristine PBI4N and [PBI4N(HfO2)0.077], as well as their doped analogues, is discussed below as representative examples. Reference to other samples is made as required. All other profiles can be found in the Supporting Information (Figures SI-5 and SI-6). The total heat flow and the reversible component are shown to aid distinguishing between glass transition temperatures (Tg) and other thermal events. A Tg for undoped PBI4N occurs at 177 °C. This is slightly higher in temperature than the corresponding Tg observed for [PBI4N(HfO2)0.077], which is observed at 172 °C (Figure 9). As reported in the literature, the main Tg of PBI4N is expected at 420−430 °C20 and corresponds to the order−disorder transition of the bulk crystallized domains of the polymer. This event, which is expected in our membranes, is not mentioned or discussed in this report. Our attention is focused on the thermal transitions detected at T < 210 °C, which is significantly diagnostic of the polymer−hafnia interactions. The Tg at ca. 177 °C is a very interesting phenomenon which witnesses that the PBI4N molecules here proposed exhibit bulk

Figure 7. A comparison of the undoped polymer matrix decomposition temperature with increasing nanofiller content (temperature values taken as the peak of the first derivative occurring at ca. 600 °C).

The corresponding TG experiments of the undoped membranes conducted under an oxidizing atmosphere allows for the nanofiller content to be quantified (Figure SI-2). After heating in excess of 600 °C, the only remaining material is that of the white hafnium oxide residue. The results are consistent with those obtained from the combustion and ICP-AES analyses. The TG analysis curves of PBI4N(H3PO4)y and [PBI4N(HfO2)x](H3PO4)y as well as that of H3PO4 (85 wt %) are shown in Figure 8. All membranes exhibit four observable mass

Figure 8. TG profiles for PBI4N(H3PO4)y and [PBI4N(HfO2)x](H3PO4)y measured under N2 from 30 to 950 °C with the derivative d wt %/dT (inset). 19


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Figure 9. MDSC profiles for pristine PBI4N (left) and [PBI4N(HfO2)0.077] (right) showing heat flow (red) and reversing heat flow (blue). Tg marked by an asterisk.

Figure 11. Effect of nanofiller content (x) on the glass transition temperature of both undoped (circles) and acid-doped (squares) PBI4N−HfO2 composite membranes.

different polymer domains that undergo order−disorder transitions at different temperatures. Indeed, this phenomenon suggests that the hafnia has a slight plasticizing effect, likely as a result of disrupting the intermolecular cross-links between PBI4N chains. It has been observed that polymer blends that restrict the PBI4N interchain interactions also result in lowering glass transition temperatures.51 After doping, both PBI4N(H3PO4)11 and [PBI(HfO2)0.077](H3PO4)13 exhibit the expected drop in Tg to −39 and −46 °C, respectively (Figure 10).52 This decrease in Tg is attributed to

increase with increasing amounts of nanofiller. As previously discussed, in the nanofiller range 0 ≤ x ≤ 0.172 the phosphoric acid uptake increases. This is mirrored by a decrease in the Tg. However, while the acid doping level was then observed to increase sharply as the nanofiller content exceeded 0.172, the anticipated decrease in Tg is not observed. Instead, the Tg actually increases, reaching a value of −35.8 °C. This may suggest that as the nanofiller content exceeds 0.172 the postulated phase separation resulting in nanofiller rich domains that limit the freedom of the polymer chains to move. A similar trend is also observed for the undoped composite membranes. Up to a nanofiller content of x = 0.172 the Tg decreases, and then at x > 0.172 the Tg rises lightly. 5.5. X-ray Analysis. For all the undoped membranes two peaks are observed, centered at ca. 17.6° and ca. 24.9°. The broad nature of these signals indicates the presence of some ordered nanodomains. However, similar to other membranes based on PBI4N, the membranes in this study are largely amorphous in nature. The two peaks are attributed to planes (200) and (110) of parallel stacked benzimidazole units that are slightly staggered with respect to each other.39 Both signals are found to be sensitive to the amount of nanofiller embedded within the polymer matrix. As the nanofiller level increases, the 2θ value for (200) increases from 16.8° for pristine PBI4N to 18.4° for [PBI4N(HfO2)0.322] and that of (110) increases from 21.1° to 23.7°. This corresponds to a decrease in the interplanar distances from 5.3 to 4.8 Å for (220) and 4.2 to 3.7 Å for (110) (Figure SI-7). Indeed, a similar trend has been reported in the literature for composite membranes of silica.25,41 This further supports the hypothesis that the hafnia acts as a mild plasticizing agent. By disrupting the side-to-side chain interactions, the bibenzimidazole units are allowed reorientate themselves in a more planar geometry (see section 5.7). The combination of hydrogen bond disruption and planar orientation of the aromatic groups facilitates a greater freedom and the formation of ordered domains of stacked polymer chain. For the undoped composite membranes, sharp, intense peaks corresponding to that of hafnium oxide are observed with intensities increase with increasing filler content (Figure 12). These signals align well with those observed for the neat hafnium oxide nanofiller as well as that of monoclinic HfO2.54 This indicates that the hafnia has not undergone any structural

Figure 10. MDSC profiles for PBI4N(H3PO4)11 (left) and [PBI4N(HfO2)0.077](H3PO4)13 (right) showing heat flow (red) and reversing heat flow (blue). Tg marked by an asterisk.

the phosphoric acid disrupting the interactions between the polymer chains, forcing chain separation resulting in an increased polymer mobility. The plasticizing effect of the phosphoric acid also manifests itself in the increased elastic nature of the acid-doped membranes with respect to the undoped membranes. As the temperature approaches 200 °C, there is a strong transition that likely corresponds to the dehydration of phosphoric acid. This transition is not observed in the MDSC profiles of the undoped membranes, and in addition, phosphoric acid is observed to dehydrate about this temperature from the TG analyses (Figure 8). There is a more significant drop in the Tg for the composite membranes of higher nanofiller content (Figure 11). This is perhaps expected given that the acid doping level is observed to 20


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The acid groups (H3PO4 and H2PO4−) are thought to be evenly distributed along the polymer chains (see section 5.7). This arrangement may promote aggregation of the protonated polymer chains (PBI4NH+) along the direction perpendicular to bibenzimidazolium plane. This results in small lamellar domains which gives rise to the broad signals observed in the WAXS spectra (Figure SI-8). The shifting and broadening of these signals upon acid doping are also observed in the literature for both pristine and other composite PBI4N membranes.25,55,56 5.6. Dynamic Mechanical Analysis. The mechanical properties of the undoped membranes were studied by dynamic mechanical analysis (DMA). A storage modulus (E′), at −150 °C, of greater than 4500 MPa is observed for all samples. Mechanical stability is maintained up to approximately 200 °C (Figure SI-10). Four mechanical relaxations (α1, β1, α2, and β2) are observed in both the storage modulus (E′) and loss modulus (E″) as well as in tan δ. Given the temperature that these transitions occur and in conjunction with BES data (see forthcoming paper), they are assigned as the following: (i) α1 (T ≈ 300 °C), long-range segmental motions of PBI4N chains; (ii) β1 (T ≈ 100 °C), relaxation attributed to interchain interations involving the dipole moment of phenyl benzimidazole repeat units of PBI4N; (iii) α2 (T ≈ 20 °C), short-range segmental motion; and (iv) β2 (T ≈ −70 °C), local fluctuations of dipole moments associated with bibenzimidazole unit. The storage modulus is observed to be affected by both the temperature and the level of nanofiller (x) (Figure 14). For all

Figure 12. WAXS spectra of pristine PBI4N, [PBI4N(HfO2)x] and HfO2.

change upon embedding within the PBI4N matrix. Using the Rietveld analysis, the sizes of the hafnium oxide particles are determined to be in the order of 70 nm in diameter. This is in good agreement with that observed by high-resolution microscopy. After doping the membranes with phosphoric acid, the signals attributed to HfO2 remain at the same 2θ values as those observed for the undoped composite membranes, albeit with significantly reduced intensity (Figure 13). This suggests that

Figure 14. Effect of nanofiller content and temperature on the storage modulus of [PBI4N(HfO2)x].

Figure 13. WAXS spectra of [PBI4N(HfO2)0.172] (black) and [PBI4N(HfO2)0.172](H3PO4)13 (red) with the signals corresponding to the stacked planes of PBI4N and PBI4NH+ (220, 110) shown in blue and green, respectively. Spectra for the other samples can be found in the Supporting Information (Figure SI-8).

values of x the storage modulus decreases with increasing temperature. This is likely as a result of the interchain interactions being overwhelmed by thermal motions. At a constant temperature, a drop in E′ is observed upon increasing x from 0 to 0.09. This suggests that as the nanofiller content increases, the strengthening intermolecular interactions between PBI4N chains are decreased. This is consistent with the MDSC and WAXS data. However, as x increases from 0.13 to 0.30, E′ also increases. This may be a consequence of the phase separation where the formation of inorganic rich domains may influence the elastic modulus of the membranes. This is owing to the concurrent increase of both the density of polymer−

there is no change to the hafnia morphology upon PBI4N complexation with phosphoric acid. The 2θ values for planes (200) and (110) are observed to increase to 19.2° (200) and 23.7° (110). This corresponds to decreased interplanar distances of 4.6 and 3.9 Å, respectively. However, because the acid doping level used in this study is so high and phosphoric acid is known to have a strong plasticizing effect, there is no observable change in 2θ with increasing x (Figure SI-7). 21


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(HfO2)0.127](H3PO4)13 will be discussed as representative examples with reference to other samples where necessary. In general, the bands in the absorption spectra of the composite membranes appear at the same frequency as those of pristine PBI4N (Figure 16). In addition, for all membranes no

nanofiller cross-links and the effect of interdomain steric hindrance. It is observed that α1, α2, and β2 are largely unaffected by varying nanofiller content; consequently, it can be assumed that they are unaffected by the disruption of intermolecular crosslinking. β1 is affected by nanofiller content and exhibits a trend reminiscent of those observed for the storage modulus. That is, in the range 0 < x < 0.13 there is a decrease followed by an increase as x > 0.13 This suggests that as the disruption of interchain interactions increases, and the mobility of the chains increases, relaxations involving local fluctuations of repeat units are also increased (Figure SI-11). Despite numerous attempts to measure their storage modulus, the mechanical properties of the acid-doped membranes were not obtained. Unfortunately, no reproducible or accurate results could be obtained owing to the slippery surface of the samples when mounted in the DMA clamp. The reduction in mechanical properties of PBI4N membranes with doping levels greater than two, especially at elevated temperatures, has previously been observed. It is thought that the presence of the acid results in increased separation of PBI4N backbones, thus reducing the formation of strengthening intermolecular interactions.5,27,44,58 5.7. Infrared Spectroscopy Analysis. FT-ATR-IR spectra experiments were conducted on both sides of all undoped and acid-doped membranes to probe whether there was any significant difference in nanofiller dispersion at the surface of each face. The spectra are found to be super imposable, suggesting that the nanofiller is homogeneously distributed throughout the membrane. This is in good agreement with the HR-SEM analysis (Figure 2). Complete spectral assignment of the undoped PBI4N and that of the protonated species (PBI4NH+) has been achieved using correlative analysis27,28,48,58−61 supported by DFT calculations. The different conformation of PBI4N and PBI4NH+ chains were obtained by modulating the dihedral angles (ψ and φ) between molecular planes of a model consisting of two repeat units (Figure 15). Good agreement is

Figure 16. Normalized FT-IR-ATR spectra for pristine PBI4N (blue) and [PBI(HfO2)0.127] (red). A comparison in the range 3600−2300 cm−1 is also shown (inset).

signals are observed at ca. 3616 or 2940 cm−1, indicating that the membranes are free of both water and DMAc. This is in agreement with the TG analysis. The broad feature roughly occurring between 3500 and 2000 cm−1 contains observable peaks at 3385, 3036, 2960, 2920, and 2850 cm−1 (Figure 16). The first band is assigned as the stretching frequency of the non-hydrogen-bonded N−H group, with the second band assigned to that of the N−H group involved in hydrogen bonding. The band at 2920 cm−1 is attributed to the stretching of C−H groups associated with the aromatic rings, with the remaining two bands tentatively assigned as vibrational overtones involving the conjugation between benzene and imidazole rings.27,28,53,58,59,62 Because of the broadness of this area, care has to be taken when drawing conclusions; however, an increase in the intensity of the peak at 3385 cm−1 is observed upon increasing nanofiller content. This suggests a diminished amount of hydrogen bonding involving the N−H group, in favor of non-hydrogen-bonded N−H. This supports the notion that the HfO2 nanofiller acts as a plasticizer by disrupting the interchain hydrogen bonds. A variety of in-plane (ip) and out-of-plane (oop) bands have been identified between 1600 and 400 cm−1 (Figure SI-11). Two in-plane bands occurring at 1421 and 1439 cm−1 that are attributed to δXHip(II) + δCH ip(BII) and [δXH + νCC]IP(II) + δCHIP(BII), on the basis of DFT calculations, are shown in Figure 17. The intensity of these bands is observed to be dependent on the level of nanofiller content (x). As x increases from 0 to 0.12, the intensity of the two bands also increases (Figure SI-13). This suggests that as the hafnia interrupts the side-to-side chain interactions, there are fewer restrictions for the in-plane vibrational motions. Figure 18 shows two out-of-plane (oop) modes identified at 792 and 684 cm−1 that are attributed to the CHoop stretch of the BII mode and the XHoop stretch of the II mode, respectively. Bands associated with the Bu modes of monoclinic HfO2 can also be found in this region (Figure SI-12)63, one of which, occurring at 760 cm−1, is also shown below. All three

Figure 15. Identification of normal modes in PBI4N (ϕ indicates the dihedral angle between adjoining benzimidazole units within a repeat unit).

found between the computations and experimentally determined band. Despite the extensive and complex coupling of internal coordinates constituting the normal modes of PBI4N, it is possible to identify three types of “pure” normal modes. These are assigned as BI (modes associated with phenyl rings of benzimidazole groups), BII (vibrational modes of metasubstituted phenyl rings), and II (vibrations attributed to bibenzimidazole groups) (Figure 15). A limited number of spectral features will be discussed in this section with the full IR assignment being found in the Supporting Information (Figures SI-11 and SI-12). The spectra of pristine PBI4N, [PBI4N(HfO 2 ) 0.127 ], and [PBI4N22


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However, a significant change in the calculated intensity of the band is predicted. The highest intensity calculated to be at a higher dihedral angle with a decrease in intensity as the dihedral angle approaches 0° (Figure 19). Experimentally, the band is

Figure 17. Normalized FT-IR-ATR spectra of pristine PBI4N (red) and [PBI4N(HfO2)0.127] (blue) and that of neat HfO2 (black) showing the in-plane bands at 1439 and 1421 cm−1. Figure 19. Effect on intensity and band position of the out-of-plane stanching of XHoop with increasing dihedral angle: intensity (squares) and calculated wavenumber (circles).

observed at 458 cm−1, which agrees well with the calculations. In addition, with increasing nanofiller content the intensity of the band decreases as the nanofiller content reaches a maximum at x = 0.322 (Figure SI-15). This suggests that as the hafnium oxide disrupts the interchain hydrogen bonding, the aromatic groups do indeed adopt a more coplanar conformation facilitating stacking in a less staggered fashion. The doping levels employed in this investigation (y ≥ 11) result in IR spectra for the acid-doped membranes being dominated by bands associated with phosphoric acid (Figure 20). The spectrum of [PBI4N(HfO2)0.127](H3PO4)13 is shown as a representative example for composite membranes with the others found in the Supporting Information (Figures SI-16 and SI-18). In agreement with other studies, bands above 2000

Figure 18. FT-IR-ATR spectra of pristine PBI4N (red) and [PBI4N(HfO2)0.127] (blue) and that of neat HfO2 (black) showing the out-of-plane bands at 792, 684, and 458 cm−1.

out-of-plane bands are observed to decrease in intensity as the level of hafnia content increases (Figure SI-14). As the hafnia disrupts the hydrogen bonding between chains, the ring stacking becomes less staggered and the interplanar distances decreases (see section 5.5). This results in a greater restriction for the out-of-plane modes, resulting in diminished band intensity. As expected, the band attributed to hafnia is observed to increase in intensity with increasing hafnia content. As a consequence of the interrupted side-to-side interchain hydrogen bonding and in order to allow the less staggered ring stacking observed by WAXS to be adopted, it would be expected that the aromatic rings within the repeat units become more coplanar. This would correspond to decreasing dihedral angles between the aromatic units. DFT calculations were conducted on a bibenzimidazole model with varying dihedral angles ranging from coplanar to orthogonal, i.e., 0°−90°, in order to probe how the chain conformation may affect the band δX−Hoop stretch of mode II. Computations suggest that the frequency of the vibration remains at approximately 458 cm−1 regardless of the dihedral angle between aromatic groups.

Figure 20. Normalized FT-IR-ATR spectra for [PBI4N(HfO2)0.127](H3PO4)13 (red) and 85% H3PO4(aq) (black). A comparison in the range 1700−1400 cm−1 for [PBI4N(HfO2)0.127] (blue) and [PBI4N(HfO2)0.127](H3PO4)13 (red) (inset). 23


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cm−1 are attributed to O−H stretches as well as the rather complex N−H modes of the benzimidazolium units.28,58,59,64 It should be noted that similar to the undoped membranes, the acid-doped derivatives do not exhibit any signs of water presence, in agreement with the HR-TG analyses. This is of upmost importance because having the acid-doped membranes in this dry condition best recreates the operating conditions in high-temperature PEMFCs. In the region 1650−1450 cm−1 differences can be observed for the undoped and acid-doped membranes. Bands attributed to the −CC− and −CN− modes as well as bands associated with the conjugation between phenyl and imidazole units undergo a shift in stretching frequency upon going from imidazole to imidazolium units as the electronic density of the aromatic groups changes (Figure 20 (inset)). With such high doping levels (n = 11−21), both bonded (H2PO4−) and free (H3PO4) acid are expected to be present within the membrane (Figure 5). Using the tetramers [H3PO4]4 and [H3PO4]3[H2PO4]− as model compounds, good agreement is found between the phosphate bands obtained computationally and those observed experimentally.64 This allows for complete decomposition of the phosphate region (1320−520 cm−1) and assignment of bands associated with both dissociated and undissociated acid (Figure SI-17). The stretching frequency of νPO has been shown to be sensitive to the dielectric constant (ε) of the surrounding environment.64,65 DFT calculations were conducted on the band assigned as νPO + νP(OH)2 using the COSMO (conduction like screening model)66,67 approach (Figure 21).

there is a significant amount of undissociated acid within the membrane, the acid is not present in an environment of high dielectric constant such as that of bulk phosphoric acid domains but rather dispersed evenly along the interfaces between the inorganic nanoparticles and polymer domains and along the polymer chains. In particular, with an increase in x comes an increase in the density of inorganic-rich domains. It is these domains that result in the observed dramatic increase in acid uptake. Initially the acid groups interact aligning along the surface of the nanofiller. However, once all the basic sites become occupied, the excess acid groups then form clusters like those that are postulated to be evenly distributed along the polymer chains. Having identified the bands of both dissociated and undissociated acid, it is possible to gauge the degree of acid dissociation (f1) using the equation A1007 f1 = A1007 + A1090 where A1007 is the area under the band of the νPO + νP(OH)2 mode of [(H3PO4)3(H2PO3)−] observed at 1007 cm−1 and A1090 is the area under the band of the νPO + νP(OH) mode of [H3PO4]4 observed at 1090 cm−1. As the filler level increases, the basic nature of the membrane also increases. The result of this is that more acid is drawn into the dissociated state (Figure 22). The increase in dissociated acid renders the composite

Figure 22. Dependence on the fraction ( f1) of dissociated phosphoric acid with varying hafnia content (x). Figure 21. Change in calculated (squares) and experimental (circles) stretching frequency of ν(PO) in environments of differing dielectric constant. Frequency and dielectric constant of observed band indicated by the dashed line. The experimental values of dielectric constant and ν(PO) are taken from refs 64 and 65 and were measured using H3PO4 solutions with different solvents.

membranes better proton conductors with respect to aciddoped pristine PBI4N (see section 5.8). To these authors knowledge this is the first time that complete assignment as well as quantification of the degree of acid delocalization within acid-doped PBI4N membranes has been achieved. 5.8. Conductivity Measurements. A complete study of the electrical relaxation phenomena including modes of conductivity studied by broadband electrical spectroscopy (BES) will be the focus of a forthcoming paper. As a brief highlight, it was found that at elevated temperatures the conductivity of the all the composite membranes is higher than that of the nanofiller free PBI4N membrane. Conductivity is observed to increase in the region 0 < x < 0.1 and then decreases slightly (Figure 23). This is reminiscent of the MDSC data where the degree of plasticity increases in the same region before decreasing at higher filler levels. This may further

There is good agreement between the computations and the experimental data in the low dielectric constant region (ε < 10) with less agreement observed as the dielectric constant of the surrounding medium increases. Nevertheless, the observed stretching frequency of 1223 cm−1 suggests an environment within the acid-doped membranes of a dielectric constant that is approximately in the range of 3−7. This is in good agreement with the reported dielectric constant for PBI4N of ca. 3.2.68 This suggests that even at high doping levels (y ≥ 11), where 24


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temperature of the Tg was significantly decreased due to the strong plasticizing effect of the phosphoric acid. DMA analyses of the undoped membranes shows that the storage modulus decreases in the filler range 0 < x ≤ 0.9 as the strengthening interchain hydrogen bonds are interrupted. At higher filler levels the formation of inorganic rich domains results in an increase in the storage modulus. Four mechanical relaxations were observed, one of which (β1) was found to be sensitive to nanofiller content. IR measurements assisted by DFT calculations allows for complete spectral assignment of the undoped and acid-doped membranes. For the undoped membranes a variety of out-ofplane and in-plane stretching bands were shown to be affected by hafnia content. The out-of-plane bands are observed to decrease in intensity with increasing nanofiller levels with the in-plane bands increasing in intensity as x increases from 0 to 0.1 before the intensity remains largely constant. The dihedral angle between benzimidazole units is also observed to be affected by nanofiller content. It is suggested that as the filler content increases, the aromatic units adopt a more coplanar conformation. For the acid-doped membranes the region corresponding to the phosphate bands was fully decomposed, and this enables the identification of bands corresponding to both dissociated and undissociated acid. In addition, it was found that the level of undissociated acid increases with increasing nanofiller content. This increased acid uptake of the composite membranes with respect to PBI4N is reflected in higher conductivity values. The combination of all the analyses leads us to draw the conclusion that the hafnia nanofiller acts as a plasticizer in PBI4N composite membranes. The nanofiller disrupts the sideto-side interchain hydrogen bonding interactions allowing the aromatic units to adopt a more coplanar structure that facilitates a lower degree of staggering of the aromatic planes. The effect seems to be maximized at a nanofiller level of x = 0.08. As the nanofiller content exceeds this value a phase separation occurs, resulting in domains of PBI4N−nanofiller and domains of pure nanofiller. These areas of differing composition are observed to alter the properties of the membrane. This work extends the approach of incorporating inorganic nanofiller into PBI-based membranes in order to improve the properties of PEMs for potential PEMFCs. The ability to provide and keep a high phosphoric acid level even after removal of all traces of water renders composite membranes of PBI4N as good candidates for high temperature fuel cells in particular.

Figure 23. Dependence on the hafnia content (x) of the conductivity of [PBI4N(HfO2)x](H3PO4)y (σDC(T)) at 215 °C. The dotted line corresponds to the conductivity of the pristine PBI4N membrane (x = 0).

suggest that the incorporation of nanofiller at lower filler levels, x < 0.15, introduces defects into the membrane morphology that facilitate proton transfer and chain relaxations. As the level of nanofiller increases, regions of nanofiller aggregates are introduced that no longer improve the conductivity pathway for protons. The highest conductivity is observed at a nanofiller content of x = 0.04. The conductivity at 215 °C of [PBI/ (HfO2)0.04]/(H3PO4)12 is found to exceed 9.0 × 10−2 S cm−1, which is higher than that observed for PBI4N under the same conditions, 4.8 × 10−2 S cm−1.

6. CONCLUSIONS Composite PBI4N membranes with well-dispersed hafnium oxide nanofiller have been prepared and subsequently doped with phosphoric acid, [PBI(HfO2)x](H3PO4)y. The presence of the basic nanofiller in the composite membranes enables them to achieve higher doping levels than those achieved by pristine PBI4N. A steady increase in acid uptake is observed in the nanofiller range 0 < x ≤ 0.77. The acid uptake level then reaches a plateau before rising sharply at nanofiller levels in excess of x = 0.13. The reason for the sharp increase in acid uptake is thought to be as a result of the formation of basic inorganic rich domains that increase the free volume within the membrane. The thermal stability of the membranes was probed by HRTG analysis, and the results indicate a stability of up to 400 °C for the undoped and 180 °C for the acid-doped membranes, respectively. WAXS analysis found that the morphology of the hybrid membranes is largely amorphous but with some regions of ordered nanodomains. The interplanar distances (200 and 110) are found to decrease with increasing hafnia content for the undoped membranes, and the interplanar distances were decreased significantly following acid doping with no dependence on hafnia content. MDSC measurements for the undoped membranes revealed one Tg that was found to decrease with increasing nanofiller content up to x = 0.13. At higher nanofiller levels the Tg was observed to increase. For the acid-doped membranes the

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ASSOCIATED CONTENT

S Supporting Information *

HR-TG, MDSC, WAXS, and DMA analyses not shown in the main article; the full FT-IR-ATR assignment of all undoped and acid-doped membranes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (V.D.N.). Notes

The authors declare no competing financial interest. 25


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ACKNOWLEDGMENTS This research was funded by the Strategic Project of the University of Padova “Materials for Membrane-Electrode Assemblies to Electric Energy Conversion and Storage Devices (MAESTRA)” and by Veneto Nanotech SCpA (Venice). The authors extend their most sincere thanks to the staff of the electrical and mechanical workshops of the Department of Chemical Sciences of the University of Padova for their skillful technical assistance, with a particular reference to Eng. A. Doimo, Mr. C. Comaron, Mr. L. Dainese, and Mr. P. Roverato.

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