Accepted Manuscript

Title: Mesostructured HSO3-functionalized TiO2-P2O5 sol-gelfilms prepared by evaporation induced self-assembly methodwith high proton conducting

Author: Jadra Mosa Mario Aparicio Alicia Duran YolandaCastro

PII: S0013-4686(15)01102-0DOI: http://dx.doi.org/doi:10.1016/j.electacta.2015.04.169Reference: EA 24923

To appear in: Electrochimica Acta

Received date: 17-3-2015Revised date: 17-4-2015Accepted date: 29-4-2015

Please cite this article as: Jadra Mosa, Mario Aparicio, Alicia Duran, Yolanda Castro,Mesostructured HSO3-functionalized TiO2-P2O5 sol-gel films prepared by evaporationinduced self-assembly method with high proton conducting, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2015.04.169

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Mesostructured HSO3-functionalized TiO2-P2O5 sol-gel films prepared by evaporation induced self-assembly method with high proton conducting

Jadra Mosa, Mario Aparicio, Alicia Durán, Yolanda Castro

1 Instituto de Cerámica y Vidrio (CSIC). C/ Kelsen 5 Campus de Cantoblanco 28049 Madrid, Spain

*Dr. Jadra Mosa Instituto de Cerámica y Vidrio (CSIC), C/ Kelsen 5. 28049 Madrid, Spain Tel.: +34 917355840; fax: +34 917355843 E-mail address: jmosa@icv.csic.es

Abstract

Ordered mesoporous coatings, especially those structured as channels in three

dimensions, could be applied as thin proton conducting membranes for application in

proton exchange membrane fuel cells (PEMFC). This kind of materials shows high

proton conductivity due to their favourable architecture with small and interconnected

porosity. Transparent and crack-free TiO2-P2O5 sol-gel coatings with thickness around

200 nm were obtained by Evaporation induced self-assembly (EISA) method using

Pluronic-127 as pore generating agent. X- Ray diffraction, SAXS and TEM analysis

confirm the presence of ordered porous mesostructured films with a cubic Fd3m

structures. Spectral ellipsometry and EEP measurements were used to determine the

film thickness, refractive index, pore size, pore volume and specific surface area. EIS

four-probe method was used to measure the proton conductivity. The textural properties

(pore size in the range of mesoporous, high specific surface area and high pore volume)

combined with inherent characteristics of sol-gel materials (high concentration of OH

groups and release of protons mainly from Ti-OH and P-OH bonds) lead to a high

proton conductivity, with maximum values around 0.11 and 0.20 S/cm depending on the

phosphorous precursors (PCl3 or H3PO4), measured at 140ºC and 80%RH. Acid

functionalisation of mesoporous TiO2-P2O5 thin-films, with HSO3 groups, increases the

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proton conductivity up to 0.79 S/cm at 140ºC and 80%RH presenting high long-term

stability. This work shows a processing guideline to design functionalized-inorganic

mesostructured thin-films with high proton conductivity at high temperature.

Keywords: mesostructured thin-films; EISA; inorganic proton conductors; PEMFC;

four probe EIS; sol-gel

1. Introduction

Proton exchange membrane fuel cells (PEMFC) can be used in many technological

applications because of their high efficiency and low level of contamination respect to

conventional systems [1-3]. Perfluorosulfonic ionomers such as Nafion® are considered

the most valuable electrolytes for operating at temperature below 80ºC. However, these

membranes present several limitations that include a high cost and low stability during

prolonged use at temperatures higher than 80ºC. On the other hand, the use of Nafion as

membranes requires the use of fuels to pure hydrogen avoiding the use of alternatives

combustibles such as ethanol or methanol, and the humidification has to be maintained

in values around 100% RH [4].

The development of alternative membranes able to work at higher temperatures would

increase the reaction kinetics and the carbon monoxide tolerance of platinum electro-

catalyser. One possible route is to incorporate an inorganic component to the polymeric

membrane arriving to a hybrid organic-inorganic nanocomposite with higher thermal

stability [5-9]. On the other side, the development of pure inorganic proton-conducting

materials could be also a process to reach higher chemical and thermal stability [10].

In recent years, great improvements in the preparation of novel meso-structured and

mesoporous materials [11] have attracted more attention, especially from the discovery

of the M41S family of materials [12,13]. The processing of inorganic mesoporous films

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on different substrates, with high thermal, chemical and mechanical stability, opens up

new possibilities and interesting applications in microelectronics [14], nano-photonics

[15] and, especially, in membrane fuel cells [16].

Interconnected porous sol-gel materials prepared by a surfactant-assisted method are

expected to exhibit high proton conductivity. The pore, with a high concentration of

hydroxyl groups, should allow a way for the proton conduction through the membrane.

The high specific surface area and pore volume, together with the periodic order of the

pore structure, can increase the proton conductivity of coating such as silica films

[17,18]. Ordered films exhibit higher proton conductivity across the entire range of

temperatures respecting to disordered films likely due to the relatively higher surface

area and pore volume of the ordered thin-film [19]. Moreover, titania-phosphorus

structure appears as a good alternative to enhance the adsorption of water molecules and

to improve the proton conduction [20,21].

Our further interest in this field is the fabrication of fast-proton inorganic conducting

films on suitable substrates, which can perform a portable fuel cell system. Since the

high conductivities of the inorganic thin-films can be attributed to proton hopping

between the hydroxyls groups of the surface and water molecules absorbed in the pores,

the pore structure is essential for determining proton conductivities [22]. Therefore, the

development of proton exchange membranes with high conductivity, and performance

stability at high temperature is a very critical component in this area [23]. It has been

reported that the cell resistance decreases with reducing electrolyte membrane

thickness, this implicating that cell performance depends on the thickness of electrolyte

membrane, so the thinner coating is, the higher will be the cell performance [24]. Thus,

a key contribution to develop high performance HT-PEMFC lies in creating thin film

electrolyte membrane [23]. Several high-temperature PEMs have been developed

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through modification of inorganic oxides grafted by sulfonic groups as potential

electrolyte membranes for high temperature operation due to their high chemical and

structural stability. However, most of them could not work at high temperature

operation due to their low chemical and thermal stability [23].

In this work, we report the preparation and characterisation of mesostructured TiO2-

P2O5 thin-films via sol-gel using the evaporation-induced self-assembly (EISA) method

[25-27]. The different parameters implicated in the sol-gel synthesis and deposition

process have been evaluated. An additional step was performed for grafting mesoporous

thin-films with HSO3 groups. The electrochemical behaviour and long-term stability of

the thin-films have been studied by EIS four-probe method and interpreted taking into

account the chemical and porous structure of the films. The main aim of this study is to

evaluate if mesoporous inorganic TiO2-P2O5 thin-films are suitable for PEM

applications and that pore sizes and surface functionalisation with strong acidic groups

may be a good alternative avenue to enhance proton conduction.

2. Experimental

Synthesis: TiO2-P2O5 thin-films, containing 10 mol % of P2O5, were prepared by sol-gel

using the dipping process combined by evaporation induced self-assembly method

(EISA). Titanium tetrachloride (TiCl4, ABCR) was used as source of titanium and

phosphoric acid (85%) (H3PO4, Aldrich) or phosphorus trichloride (PCl3, Aldrich) as

sources of phosphorus. A tri-block copolymer (EO)20(PO)70(EO)20 (Pluronic127,

Aldrich ) was used as template and pore generating agent. First, TiCl4 was mixed with

ethanol in a molar ratio 1TiCl4: 5EtOH, to control the exothermic character of TiCl4.

Then, PCl3 or H3PO4 precursors were mixed with ethanol in a molar ratio of (PCl3 or

H3PO4)/EtOH 1:50, and then, mixed to TiCl4: EtOH solution. Finally, Pluronic-F127

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was added and dissolved, and distillated water added drop by drop. The final molar ratio

was fixed to 1TiCl4: 1(PCl3 or H3PO4): 60 EtOH: 0.004 F127: 10 H2O.

On the other hand, F127-free sols were obtained following a similar process but without

the addition of surfactant.

Analytical methods: The stability of the sols was studied through the evolution of

viscosity with time, using an A&D equipment (SV 1A model).

Films were prepared by dip-coating onto glass-slides (8cm2) and silicon wafer (111) (5

and 10 cm2) at a constant withdrawal rate of 16 cm/min. Relative humidity (RH) was

measured and controlled during the deposition process using a dip-coater chamber.

Coatings were deposited at 30 and 20-70 % RH in order to study the disorder-to-order

transition Coatings were deposited at 30 and 20-70 % RH. In the last case, initial RH

was maintained at 20-30 % up to finish the evaporation of solvent, and then RH was

raised up to 70% in order to produce a different nanosegregation process [25-27]. The

quantity of water in the atmosphere (the relative humidity) of deposition process

determines the mesoporosity structure. The films were further dried at 150ºC for 48 h

and 350ºC for 3h to remove the solvents and template, and promote the condensation.

Thin-films were sulfonated by immersion by films between 10 and 80 minutes in 0.3M

solution of trimethylsilylchlorosulfonate ((CH3)3SiSO3Cl) (from Aldrich) in 1,2-

dichloroethane, then washed several times with deionized water. Finally, thin-films

were dried in an oven at 80ºC overnight.

Low angle -XRD diffraction patterns of mesoporous films were recorded on a Bruker

D8 using Cu K radiation (= 1.514 Å) in θ-2θ scan mode, using a step size of

0.02º/min and a collection mode of 0.5 s/step in the 2θ range 0.5-5.5º.

Small-angle X-ray scattering was recorded with a S3-MICRO SWAXS camera system

(HECUS X-ray Systems, Graz, Austria). Cu KR radiation (1.542 Å) was generated with

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a GENIX fine microfocus (0.1542 mm) and detected by a punctual focalization FOX

2D operating at 50 kV and 1 mA.

Ellipsometric and Environmental Ellipsometric Porosimetry (EEP) measurements were

performed using a spectral ellipsometer (M-2000UTM, J.A. Co., Woollam) modified

with a system that allow controlling the relative humidity (Humidity Generator HG-1,

Michel Instruments) to characterise films deposited onto silicon substrates to study the

variation of refractive index (n) at = 700 nm and thickness (t) as a function of RH.

The spectra are measured in the visible region between 250 - 950 nm at a fixed incident

angle of 70°. The analysis of data was performed with the WVase32 software. The pore

volume and the adsorption-desorption isotherms were further obtained by considering

the Bruggeman Effective Medium Approximation model. The pore size distributions are

calculated utilising a modified Kelvin equation, taking into account an ellipsoidal pore

geometry. Finally, an estimation of the specific surface area was calculated [28].

Transmission electron microscope (TEM, H-7100 Hitachi, Japan) and Field-Emission

Scanning Electron Microscope (FE-SEM,) coupled to EDX analysis was used to

characterise the mesostructured films. Samples were obtained by scratching the films

and depositing the particles on carbon-coated copper grids. The Ionic Exchange

Capacity (IEC) of the hybrid membrane was determined by acid-base titrations. The

EIC was calculated according to IEC= nH+ / Wdry, where IEC (mmol∙g-1) is the number

of milimoles of H+ (nH+) per membrane weight. Thin-films were converted to H+ ionic

form by inmersion in HCl 1 N for 8 hours. Afterwards, they were soaked in 50 mL of

NaCl 1 M for 24 hours to produce the exchange of protons and sodium ions. Finally, 50

mL of the solution was titrated with NaOH 0.005 M until equivalent point. Thin-films

were regenerated with 0.1 N HCl, washed with water and dried up to a constant weight

using a vacuum oven at 80ºC.

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Proton conductivity was measured as a function of temperature at a constant relative

humidity of 80% by electrochemical impedance spectroscopy (EIS) with a Gamry

Reference 600 Potentiostat. The measurements were performed using a four-probe

device, with Pt wire electrodes (0.33 mm diameter). In this method, the two inner

probes served as voltage sensors, and the two outer Pt wires were used as current

injectors. The film was sandwiched between two Teflon blocks. The amplitude of the

AC signal was 50 mV over frequency range from 0.1Hz to 1 MHz (10point/decade).

The measurements were performed using the four-electrode method inside a humidity

control chamber keeping the humidity constant at 80% [29,30] in the temperature range

20-140ºC. Samples were introduced in measurement chamber without humidification

treatment. Temperature and RH have been measured using a Rotronic HYGROCLIP

HK 25. Measurement was performed three times to obtain an average. The resistance

(R) was obtained from the intersection of the semicircle with real impedance axis in the

Nyquist plots by fitting using Gamry Echem Analyst Software. Sulfonated thin-films

were measured up to 80 days as a function of temperature and keeping constant the RH

at 80% to investigate the possible swelling of electrolytes.

3. Results and discussion

3.1. Structural characterisation

Homogeneous and transparent sols were obtained for phosphorus precursors with and

without surfactants. The stability of the sols was evaluated through viscosity

measurements at 25°C as a function of aging time. Since all the sols are highly diluted,

they maintain a constant viscosity around 2 mPa.s during more than 1 month, revealing

an excellent stability and the possibility of use for long time.

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Thin films with high optical quality, transparency and without precipitates were

deposited by dip-coating combined with EISA method at different relative humidity

conditions and a withdrawal rate of 16 cm/min. The refractive index (n) and the

thickness (t) of the films were determined by ellipsometric measurements. Table 1

shows the ellipsometric parameters, porosity properties as well as contact angle as a

function of phosphorus precursor and deposition relative humidity, for reference and

porous coatings treated at 350ºC for 3 hours. Reference coatings are obtained to F127-

free sol and sintered following the same process.

In the case of reference coatings, it is observed that both, the thickness and refractive

index vary with the relative humidity conditions and P precursors (Table 1). The

thickness is lower for coatings deposited at 30% RH and for PCl3 precursor, 74 nm.

However, maximum thickness of 100 nm was obtained for H3PO4 films at 20-70 % RH.

On the other hand, refractive indexes of 1.99-1.91 are obtained very close to the index

of dense titania sol-gel coating (2.2) which is the major phase in the composition under

study. The results suggest that the coatings are non-porous and could be considered as

reference coating.

For coatings prepared with the sols incorporating F127 and deposited at 30 and 20-70%

RH, the refractive index varied between 1.70-1.80, lower than reference material (1.99)

indicating the presence of porosity. The thickness is higher than 100 nm (reference

coating). The addition of surfactants promotes an increase in the coating thickness and a

simultaneous decrease in its refractive index. The porosity allows relaxing the stresses

created during drying and heat treatment thus arriving to increase the thickness.

Concerning to contact angle, smaller values indicate higher hydrophilic character of the

coatings. In this case, all the coatings present contact angles below 20º, being highly

hydrophilic.

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The porosity properties of the films were determined by EEP measurements. The

adsorption-desorption isotherms were obtained from the variation of thickness and

refractive index, induced by the change of water partial pressure [28]. Figure 1 shows

the adsorption-desorption isotherms for TiO2-P2O5 films obtained for PCl3 and H3PO4,

deposited at 20-70%RH and sintered at 350ºC. Both films have a hysteresis loops

attributed to the presence of porous in the range of meso-porous materials and indicative

of reversible type IV-adsorption/desorption isotherm [31-33]. The relative pressure

(P/P0) rises slowly up to value of 0.3 for PCl3 films and a little faster up to 0.4 for

H3PO4 coatings, indicating the presence of microporosity. From these points a rapid

increase is detected, associated with the filling of the pores with water by capillary

condensation. From P/P0 0.65 the volume remains almost constant for both samples.

Desorption curves present a hysteresis loop, due to a small amount of moisture are

retained. The decrease of P/P0 is quite sharps according to loop type H1 considering the

classification of Everett [34].

Similar behavior was obtained for samples deposited at 30% RH (not shown) and for

both precursor of phosphorous.

The corresponding pore size distributions was determined from adsorption and

desorption branches and plotted in Figure 2 for TiO2-P2O5 thin films deposited at 20-

70%RH for both P precursors and sintered at 350ºC. They were determined using the

Kelvin equation modified to take into account the surface tension, water contact angle

and presence of absorbed water layer at the pore surface [28]. The geometric model

applied for such structure was ellipsoidal pore geometry. Bruggeman effective medium

approximation (BEMA) model was applied to ellipsometric data to determine the pore

volume. Total pore volumes between 20-23 vol. % and pore size in the range of 2.3-3.5

nm were obtained. Table 1 summarises the values of pore volume, pore size and specific

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surface area for all the thin-films. In general, the films obtained with PCl3 precursor

have pore size higher than H3PO4 precursor. Respect to specific surface area [m2/cm3]

calculated from the slopes (microporous and mesoporous) of the t-plot curve and

considering the pore size (), pore volume (Vporous) and an ellipsoidal pore geometry

were calculated and high Ss of 171 - 110 m2/cm3 was obtained, associated with the low

pore size. The t-plot was obtained taking each point on the isotherm and multiplying the

fraction of P/P0 at that point by the thickness of the monolayer (t)[28].

Fig. 3 shows the low-angle XRD patterns of films sintered at 350ºC for 3h. The

difractrograms of the films prepared with PCl3 precursor for both humidity conditions

show two reflection peaks at 2.58 and 2.86º associated to (111) and (200) reflections of

a cubic phase Fd3m and with a unit cell of around 8.6 nm calculated using the peak

(200). On the other hand, three broad peaks at 2θ= 2.86, 3.10 and 3.38 were observed

for the H3PO4 precursor at 20-70% RH deposition, indicating a low defined meso-

structure. However, these peaks can be identified and indexed to (200), (210) and (211)

reflections of an ordered three cubic structure, Pm3n [17]. The size of the unit cell is

estimated to be around 9.5 nm. No defined structure was observed for H3PO4 precursor

at 30% RH. These results indicate that different phosphorous precursors lead to

different mesostructure geometries, but in both cases, the interconnected porous

structure may allow a suitable way for fast proton conduction. For phosphoric acid

precursor, relative humidity determines the porosity order. The films prepared with

F127-free sols do not present diffraction peaks (not showed here), demonstrating that

these coatings do not have ordered structural since surfactants are not incorporated in

the synthesis of sol.

GISAXS measurement was used to characterize the morphology and ordering of porous

films (Fig. 4). Fig. 4 a, shows GISAXS patterns of thin film obtained at different

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deposition RH with both precursors of P (PCl3 and H3PO4). Only one peak is observed

for TiO2-P2O5 films (PCl3 precursor) at 30 % RH making difficult the identification of

order or not-order mesoporous coatings. In the case of TiO2-P2O5 films obtained with

H3PO4 precursor at 20-70 % RH and 30%RH, GISAXS patters show a small and not-

well defined broad peak. However, TiO2-P2O5 film obtained with PCl3 precursor at 20-

70 %, exhibits two diffraction peaks with relative positions at q, √3/√4 and √3/√8,

probably associated with a diamond cubic structure type (Fd3m). This structure is

agreed with the XRD results. The distance parameter of cubic cell, was calculated as the

inverse of the slope when plotting 1/d as a function of (h2+k2+l2)0.5, where d is the d-

spacing and h, k, l are the Miller indices (Fig. 4b). Parameter dimension of 14.4 nm was

determined for PCl3 deposited at 30% RH.

This periodicity reduction between type of precursors and deposition conditions may be

due to distortion of the ordered mesoporous structure [35] associated with the

contraction of the structure porous during the sintering conditions. This indicates that

sintering condition has a substantial impact on the geometry of the porous.

In order to clarify the SAXS and XRD characterisations, TEM was performed. Fig. 5

shows the TEM images of TiO2-P2O5 films prepared with chloride and phosphoric acid

precursors at different relative’s humidity. All the microphotographs show light and

dark sections corresponding to porous and matrix, respectively. A periodic meso-

structure of the film was observed for all the coating except to for H3PO4 at 30%RH,

confirming the results obtained by XRD and clarifies the GISAXS measurements. The

films have uniform and homogeneous pore size with a diameter lower than 5 nm

separated by 6 nm sol-gel matrix strips. EDX analysis was performed to confirm the

chemical composition of the films. Several authors report big loses of P precursors in

inorganic compositions, mainly due to fast evaporation of P alkoxides. Table 2 shows

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the ratio TiO2/P2O5 for thin-films prepared with sols from both P precursors and at

different RH. P-content decreases for all the coatings, although a maximum P-content

was obtained for films prepared with PCl3 at 20-70% of relative humidity. In general,

phosphorus loses are higher for thin films deposited at low relative humidity (30%).

This decrease in P-content can be affecting the mesoporosity structure.

3.2. Electrochemical characterisation

Thin films prepared from phosphorous trichloride and phosphoric acid precursors as P

precursors and deposited at 20-70%RH were selected for the electrochemical

characterisation because of their porosity properties: presence of mesostructuring, pore

size, and high surface area.

Fig. 6 shows the Nyquist impedance plot for PCl3 precursor thin-film deposited at 20-

70%RH treated at 350ºC. Measurements were performed using a four-probe test cell at

different temperatures (40-140ºC) and keeping the relative humidity constant at 80%

[36]. No humidification process was performed before measurements; each one was

repeated three times and the average value is used as resistance (R). The protonic

conductivity is calculated taking into account the distance between Pt wire electrodes

(0.92 cm) (L), the thickness of the coating (t) and the wide of the substrate (W), using

the following equation: σ = L/RWt. Four-probe EIS offers an advantage over two-probe

EIS because it is believed to eliminate contact impedances. In the two-probe cell

configuration, the current-generating electrodes also serve as the voltage-measuring

probes, and thus interfacial impedance is expected to dominate in the lower frequency

range. In contrast, with a four-probe cell configuration, the voltage measuring probes

are connected through a high impedance device so that negligible current flows across

these interfaces [36]. Only one well-defined time constant at high frequency

corresponding to electrical response of materials can be observed in all the temperature

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range. Typical diffusion behaviour at low-frequency range does not appear in the four

probes method, so protonic resistance can be simply calculated, interpolating the high-

frequency semicircle without considering the influence of low-frequency impedances.

The amplitude of the semicircle decreases when temperature increases, meaning proton

mobility is improved.

Fig. 7 shows the conductivity of PCl3 and H3PO4 thin-films deposited at 20-70%RH and

heat-treated at 350ºC. Measurements were performed in the range 25-140ºC at 80%RH.

Conductivity increases with temperature up to 140ºC (full symbols). When temperature

is gradually decreased back to room temperature (RT), higher conductivity values are

obtained (empty symbols) for both compositions. Films synthesized using H3PO4

precursor present a higher conductivity in all the temperature range compared with

chloride precursor. The mesostructured thin-films present similar pore structures with a

cubic ordered mesoporous, but H3PO4 shows lower pore size and higher surface area,

this affecting in the electrochemical behaviour. Proton conductivity increases with the

amount of water molecules and hydroxyl groups (Ti-OH or P-OH) due to the release of

protons [37-38]. Protons, dissociated from hydroxyl bonds on the pores surface, are

transferred by hopping between hydroxyl groups and water molecules, resulting in a

high conductivity [39]. The water vapour condenses inside the pores through capillary

condensation, and this occurs at lower vapour pressures for smaller pore size. Proton

conductivity usually increases with increasing amounts of adsorbed water that promotes

proton transport through the membranes.

The conductivity of the studied films suggests the convenient of creating proton-

conducting paths, with trapped water within porosity on low pore size structures. The

molecular water retained likely provides high proton mobility, contributing to an

increase and stabilization of proton conductivity in the whole range of temperatures.

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Both compositions present high values of proton conductivity at high temperature

(140ºC) 0.11 and 0.20 S/cm for PCl3 and H3PO4, respectively. The electronegativity of

the elements involved (P > Ti) is a key factor to explain their influence in the proton

mobility, that increases when protons are strongly hydrogen bonded [40,41]. It should

be also considered that hydroxyl groups attached to elements of group V have a higher

acidity compared to Ti-OH. Thus, the associated proton in the P-OH groups is more

strongly bonded with neighbouring water molecules, which should increase its mobility.

Conductivity increases with increasing temperature in both compositions indicating that

there is a thermal activated process (Arrhenius behaviour), but the dependence is not

linear. Indeed, the conductivity slope changes between 65ºC and 95ºC more accentuated

for PCl3 that present bigger pore size, likely delay the filling of mesopores. Above 90ºC,

the mesoporosity is filled with water, and the conductivity reaches a maximum, that

maintains up to 140ºC. In both cases, the increase of the water partial pressure inside the

pores could generate a water confinement [42].

The proton conduction mechanism is water assisted, and the hydration state and water

adsorption of the materials are crucial factors for a suitable electrochemical

performance. At high temperature, the interconnected mesoporosity network is

especially relevant because the presence of water inside the pores allows increasing the

proton conductivity [41]. In the cooling branch, both compositions present a hysteresis

cycle, conductivity values remain higher than those obtained when heating. In the whole

temperature range proton conductivity decreases gradually when temperature is lowered

[43-45]. For PCl3 thin film a bigger slope change is observed between 90ºC and 60ºC,

related to desorption of water from the pore walls [45]. H3PO4 films present higher

conductivity at low temperature respecting to PCl3 ones, almost two orders of

magnitude, mainly due to the smaller mesopore size and higher specific surface area,

15

which represents a bigger physical barrier to desorption phenomena [45]. The effect of

mesostructuring of pore is not evident, although ordered structures shows smaller pores

size and higher specific surface areas.

Fig. 8 shows the proton conductivity of PCl3 and H3PO4 thin-films before and after

functionalization by immersion in sulfonated solutions for 30 min. Measurements were

performed in the temperature range of 20-140ºC keeping constant RH at 80%, also

using four-probe technique. The optimized sulfonation conditions were previously

determined tuning the immersion in the sulfonated solutions between 10 and 80

minutes, and then measuring the proton conductivity at 100°C and 80% RH (not

shown). Conductivity presents a Gaussian behaviour reaching a maximum at 30

minutes, after which it decreases slowly. IEC increases with the sulfonation time,

suggesting a higher sulfonation degree. For H3PO4 precursor, it becomes as high as 2.21

mmol/ g when the membrane is sulfonated for 30 min. However, PCl3 precursor

produces a maximum value of 0.87 mmol/ g for a sulfonation time of 30 min. After 30

min, conductivity and IEC decrease probably due to an excessive water uptake that

leads to a dilution effect lowering the concentration of sulfonic acid groups. In the Fig.

8, the conductivity increases with increasing temperatures for all studied films. Proton

conductivity is enhanced one order of magnitude for functionalised TiO2-P2O5-SO3- thin

films reaching about one order of magnitude higher than these mesoporous TiO2-P2O5

films. Proton conduction data of sulfonated films can be filled to Arrhenius plots and

similar activation energy values are obtained 0.99 eV and 0.91 eV for H3PO4 and PCl3,

respectively. Pore size keeps its influence, H3PO4 presenting higher conductivity values

in the whole temperature range, 0.79 S/cm at 140ºC respecting 0.29 S/cm for PCl3 films.

The increase of proton conductivity in functionalised films is attributed to the presence

of hydrophilic domains in the three-dimensional mesoporous TiO2-P2O5-SO3H

16

framework that may form dynamic cross-links in hydrophilic polar cages and accelerate

the proton migration along the interconnecting channels [21]. Proton transport is

supported mainly via hopping of protons from one water molecule to the next following

the Grotthuss mechanism [43]. The diffusion of H3O+ ions may also facilitate the proton

transport. Temperature increases strongly affecting both processes, the diffusion is

faster, and SO3H groups can easier contact to the next neighboring groups, enhancing

the proton hopping. The channel geometry of the pores in which the SO3H groups are

anchored helps to attach and keep adsorbed water and, thus, facilitates the proton

transport. Mesoporous functionalised thin-film presents an inorganic network that

supports the presence of sulfonic acid groups within the pores. These functionalised-

nanoarchitectures allow proton channel continuity in the pores and facilitate the proton

transfer with lower activation energy respecting to mesoporous TiO2-P2O5 thin-film.

Furthermore, the high proton conductivity measured in both samples, seems to be

related to not only to a high density of SO3H groups and also to the presence of with the

physical adsorbed water and Ti-OH and P-OH surfaces [46 ]

3.3 Long-term stability of functionalized films A main concern about the incorporation of sulfonic acid groups into electrolytes is

related to the possibility of degradation of the electrolyte with the water generated in the

cathode of the fuel cell, thus likely producing a rapid drop of proton conductivity and

then a shortcut.

TiO2-P2O5-SO3H thin films synthesised using H3PO4 precursors were tested after

immersion in water at room temperature up to 80 days. The proton conductivity was

measured at different temperatures (90, 104, 120 and 140ºC) and 80% RH as a function

of immersion time (Fig. 9). As observed, the conductivity of the functionalised film

remains near constant during the long storage in water. Proton conductivity after 80

17

days maintained its high values, between 0.26 and 0.57 S/cm at 90ºC and 140ºC,

respectively. This confirms that mesoporous TiO2-P2O5- SO3H thin-films present high

stability in full hydrated conditions during long periods. This behavior reveals that

sulfonic groups remain anchored in the mesoporous TiO2-P2O5 network for a long term

likely due to the condensation of protons and water inside the porosity, and the

homogeneous distribution of SO3- groups in the porous structure. Thus, mesoporous

TiO2-P2O5 membranes thin-films functionalised with SO3H groups present high proton

conductivity, suggesting that these materials can be proposed as good candidates to be

used in PEMFC up to 140ºC, thus improving the operational conditions and allowing

the use of reformed H2 or even ethanol and methanol as fuels. Furthermore, presence of

high amount of water and SO3- into the mesopores could fill up the mesopores of the

thin-films and suppress the crossover of the gas.

4. Conclusions

Mesostructured TiO2-P2O5 proton-conducting thin-films were successfully obtained

using Pluronic-127 as surfactant and EISA method and tuning relative humidity

deposition have successfully prepared. Cubic order symmetry was identified by XRD,

SAXS and TEM. EDX analysis reveals up to 90% of initial phosphorus is incorporated

in the final films by using both P precursors (PCl3 and H3PO4).

Thin-films with pore sizes of 2.2-3.5 nm and higher specific surface area (170 m2/cm3)

were obtained.

Proton conductivity increases with temperature (at 80%RH) but does not follow a pure

Arrhenius behaviour. A hysteresis cycle appears being conductivity values of cooling

branch higher than heating branch. This is attributed to restrictions in the diffusion of

proton species by the small pores and water confinement inside porosity. PCl3 and

18

H3PO4 thin-films deposited at 20-70%RH present the highest proton conductivities at

140ºC, 0.11 and 0.20 S/cm. Proton conductivity was increases by and additional order

of magnitude through functionallisation of the films with HSO3 groups. In this case, the

high proton conductivity, 0.29 and 0.79 S/cm for PCl3 and H3PO4, respectively

measured at 140ºC is related not only to the high density of SO3H groups but also to the

adsorbed water and high concentration of hydroxyl bonds (Ti-OH and P-OH).

It is concluded that functionalised mesostructured TiO2-P2O5 thin-films have a great

potential as electrolytes for fuel cells operating at high temperatures with a long-term

stability. The conductivity appears closely related to the specific surface area and pore

size than to the ordered mesostructure of the films. The dependence between pore size

and proton conductivity might open up a new route in the design of highly proton

conducting mesoporous inorganic thin-films.

Acknowledgements

The work was supported by PIFC00-08-00022 project, financed by the Agencia Estatal

CSIC, as well as European Social Found (ESF) contracts JAE-Tec of Aritz Iglesias and

JAE-Doc of Dr. Jadra Mosa. The authors also acknowledge Laura Peláez and Aritz

Iglesias for the experimental work and J. Esquena´s group for the experimental SAXS

measurements.

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Figure captions Figure 1 Water adsorption/desorption isotherms of TiO2-P2O5 thin-films treated at 350ºC a) using PCl3 and b) H3PO4 precursors and deposited at 20-70% RH. Figure 2 Pore size distribution deduced from EEP isotherms for TiO2-P2O5 coating heat-treated at 350ºC, a) PCl3 and b) H3PO4 precursors and deposited at 20-70 %RH. Figure 3 XRD patterns of TiO2-P2O5 mesoporous thin-films heat-treated at 350ºC for 3 hours as a function of P-precursors and RH of deposition. Figure 4 Small-angle X-ray scattering (SAXS) of TiO2-P2O5 thin films heat-treated at 350ºC (a) SAXS spectra and (b) crystallographic assignation of the peaks.

Figure 5 TEM micrographs of TiO2-P2O5 mesoporous coatings a) PCl3 as precursor at 30%RH, b) PCl3 as precursor at 20-70%RH and c) H3PO4 at 20-70%HR. Figure 6 Nyquist plots (four-probe method) for TiO2-P2O5 thin-films using PCl3 as precursor, deposited at 20-70%RH and heat-treated at 350ºC, for temperatures in the range 40-140ºC and keeping constant relative humidity (80%RH). Figure 7. Conductivity measurements of TiO2-P2O5 films using PCl3 and H3PO4 as precursors deposited at 20-70%RH and heat-treated at 350ºC. Temperature cycles were applied, heating from room temperature to 140ºC (up curves, filled symbols), and cooling back to room temperature (down curves, empty symbols). RH was keeping constant at 80%. Figure 8 Conductivity measurements of TiO2-P2O5 thin-films using PCl3 and H3PO4 as precursors, deposited at 20-70%RH and heat-treated at 350ºC compared to sulfonated thin films (30 minutes) in the heating branch. Figure 9 Proton conductivity as a function of immersion time measured at 80%RH for TiO2-P2O5 films using H3PO4 as precursors, deposited at 20-70%RH and sulfonated for 30 minutes. Table 1. Ellipsometric and porosity characterisation of TiO2-P2O5 films as a function of phosphorus precursor and deposition conditions, together with the contact angle.

Composition RH (%)

Thickness (nm)

Refractive index

Pore volume

(%)

Pore size (nm)

Ss (m2/cm3) Contact

angle

TiO2-P2O5 (PCl3) 30 131 1.72 22 3.4 110 9.9 TiO2-P2O5 (PCl3) 20-70 120 1.76 22 3.5 120 14.1 TiO2-P2O5 (PCl3) 30 74 1.94 -- -- -- 17.8 TiO2-P2O5 (PCl3) 20-70 90 1.91 -- -- -- 18.3 TiO2-P2O5 (H3PO4) 30 146 1.81 20 2.3 152 19.3 TiO2-P2O5 (H3PO4) 20-70 172 1.74 23 2.5 171 9.2 TiO2-P2O5 H3PO4 30 88 1.99 -- -- -- 11.7 TiO2-P2O5 H3PO4 20-70 101 1.95 -- -- -- 10.7

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Table 2. Chemical composition of mesoporous thin-films heat-treated at 350ºC for 3

hours.

Composition RH (%) Mass P2O5 (%) Mass TiO2 (%) TiO2-P2O5 (PCl3) 30 8 92 TiO2-P2O5 (PCl3) 20-70 9 91 TiO2-P2O5 (H3PO4) 30 6 94 TiO2-P2O5 (H3PO4) 20-70 8 92