New LDPE based anion-exchange membranes for alkaline solid polymeric electrolyte water electrolysis

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 9 9 2 e1 5 0 0 2

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New LDPE based anion-exchange membranes for alkaline solid polymeric electrolyte water electrolysis Marco Faraj a, Massimiliano Boccia b, Hamish Miller b, Francesca Martini a,c, Silvia Borsacchi a,c, Marco Geppi a,c, Andrea Pucci a,c,d,* a

Dipartimento di Chimica e Chimica Industriale, Universita` di Pisa, Via Risorgimento 35, 56126 Pisa, Italy Acta S.p.A., Via di Lavoria 56/G, 56042 Crespina (PI), Italy c INSTM, Unita` di Ricerca di Pisa, Via Risorgimento 35, 56126 Pisa, Italy d CNR NANO, Istituto Nanoscienze-CNR, Piazza San Silvestro 12, 56127 Pisa, Italy b

article info

abstract

Article history:

In this work, a new LDPE based anion-exchange membrane was prepared by UV-induced

Received 26 May 2012

grafting of vinylbenzyl chloride (VBC) functional monomers and their successive conver-

Received in revised form

sion into quaternary ammonium sites with 1,4-diazabicyclo(2.2.2)octane (Dabco). After thin

20 July 2012

film formation this material was used to prepare a MEA (membrane electrode assembly) for

Accepted 5 August 2012

use in an alkaline membrane water electrolyzer. The membrane was primarily charac-

Available online 27 August 2012

terized by electron microscopy, FTIR and solid state NMR techniques in order to determine its structural and phase properties whereas electrochemical parameters were evaluated

Keywords:

and compared to a commercial benchmark membrane. Experimental data showed that the

Polyethylene based anion-exchange

electrochemical performance of the LDPE-VBC-Dabco membrane was comparable to that

membrane

measured for the best commercial material, and enabled its use in an electrolytic cell for

Vinylbenzyl chloride photografting

hydrogen production. The results obtained in the electrolytic cell showed a constant

1,4-Diazabicyclo(2.2.2)octane

hydrogen production rate of about 30 cc/min over more than 500 h. However, the long time

amination

stability of the LDPE-g-VBC-Dabco membrane needs still to be improved in the alkaline

Alkaline membrane water

environment of the working cell.

electrolyzer

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

There are three main types of water electrolysis technologies that can be distinguished by the actual electrolyte used in the electrolysis cell [1]: (a) alkaline electrolysis, with a liquid electrolyte (usually KOH 20e40 wt%) [2]; (b) polymer electrolyte membrane (PEM) electrolysis with an acidic ionomer electrolyte (e.g. NAFION) [3,4]; and (c) high-temperature (HT) steam electrolysis, with a solid oxide electrolyte. Today, large scale production of hydrogen by electrolysis is dominated by the alkaline electrolysis technology. Compared

with alkaline electrolysers, PEM electrolysers offer better performance for hydrogen generation in small and efficient units that have niche applications. Compared to the large scale alkaline electrolysers PEM electrolysers based on acid electrolyte membranes like NAFION do not use a liquid electrolyte and have the advantage of high current densities and high cell efficiencies [5,6]. The recent development of alkaline exchange membranes (AEMs) and their application in alkaline membrane fuel cells (AMFCs) have been driven by the need of lowering the cost of the materials in order to make fuel cells competitive with

* Corresponding author. Dipartimento di Chimica e Chimica Industriale, Universita` di Pisa, Via Risorgimento 35, 56126 Pisa, Italy. Tel.: þ39 0 502219270; fax: þ39 0 502219260. E-mail address: [email protected] (A. Pucci). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.012

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existing battery technology [7e10]. The strongly acidic nature of the NAFION ionomer and membrane needs the use of noble metal catalysts mainly platinum. The application of AEMs permits the use of cheap base metal catalysts and faster electrode kinetics. Interestingly, AEMs have not been widely applied in water electrolysis where similar gains should also be possible [11,12]. Recently, ACTA SpA has commercially launched an alkaline membrane water electrolyzer (AMWE, Figure S1) which operates with non precious metal catalysts at both the anode and cathode and utilizes an alkaline exchange membrane together with a dilute aqueous K2CO3 electrolyte which guarantees ionic conductivity along with pH buffering (around 10). The system exploits the advantages of both alkaline electrolysis (low cost materials) and PEM electrolysers (compact, higher current densities and differential pressurization of gases up to 30 bar). In order to match the excellent cell efficiencies and high current densities of the PEM electrolysers, a further development is essential especially in ionomer and membrane materials along with the anode and cathode catalyst materials. As part of continued efforts to develop improved membrane materials in this article we describe the preparation and characterization of an AEM based upon a low-density polyethylene (LDPE) backbone by using vinylbenzyl chloride (VBC) as functional groups incorporated by UV-induced radical grafting in the presence of benzophenone. Commodity low cost polymeric materials, such as polyethylene (PE) were selected because they are known to be very stable in severe electrochemical environments and exhibit an interesting combination of hydrophobicity, negligible swelling, good mechanical properties under demanding environments, and are easily functionalized by radical grafting processes [13]. The grafted chlorobenzylic moieties were then converted into the anion exchange sites by quaternization with aliphatic diamines such as the 1,4-diazabicyclo(2.2.2)octane (Dabco) thus providing an anion exchange thermoplastic membrane in the form of a thin sheet (50e100 microns), in which the Dabco groups can act as a cationic crosslinker as well [9]. The functionalized material was characterized by means of spectroscopic and microscopic investigations and the final properties of this new AEM were determined in terms of water uptake (WU), ion-exchange capacity (IEC), ionic conductivity (sIP and sTP), hydrogen permeability and electrolytic cell tests. All results were compared with those obtained for a standard commercial material used as benchmark.

2.

Experimental part

2.1.

Instrumentations

2.1.1.

UV equipment

UV irradiation was carried out employing a 400 W high pressure mercury lamp (Polymer 400 Helios Italquarz) with an energy irradiated output at a distance of 15 cm of 11,000 and 12,400 mW cm2 at 254 and 365 nm, respectively.

2.1.2.

FT-infrared (FT-IR) spectroscopy analysis

Infrared spectra were performed with a Fourier transform spectrometer PerkineElmer Spectrum One on neat and

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functionalized LDPE films. Attenuated Total Reflectance Fourier Transform Infrared (ATR/FTIR) spectra were recorded on polymer films with the help of a PerkineElmer Spectrum One spectrometer fitted with Universal ATR (UATR, DiComp crystal) accessory.

2.1.3.

Scanning electron microscopy (SEM)

The scanning electron microscopy (SEM) analysis was performed with a Jeol 5600-LV microscope, equipped with Oxford energy dispersive X-ray spectroscopy (EDS) microprobe, at the Chemical Engineering Department of Pisa University.

2.1.4.

Differential scanning calorimetry (DSC)

The DSC analyses were performed under nitrogen flux (80 mL/ min) with a Mettler-Toledo/DSC 822e equipped with a cooling system. The calibration was performed with Zinc and Indium. Heating and cooling thermograms were carried out at a standard rate of 10  C/min.

2.1.5.

Solid state NMR (SSNMR)

SSNMR experiments were carried out on a dual-channel Varian InfinityPlus 400 spectrometer, equipped with a 7.5 mm Cross Polarization/Magic Angle Spinning (CP/MAS) probehead, working at 400.03 MHz for proton, at 79.47 MHz for silicon-29 and at 100.61 MHz for carbon-13. All the experiments were performed under MAS and high-power decoupling conditions, with a MAS frequency of 5 kHz, a decoupling field of about 30 kHz and a 1H 90 pulse duration of 5.1 ms. The 13C CP/MAS spectra were obtained acquiring 20,000 transients with a recycle delay of 5 s, using contact times (ct) of both 1 ms and 50 ms. The 13C-Delayed CP/MAS spectra were recorded inserting a 100 ms delay between the 1H 90 pulse and the contact time, thus allowing the magnetization of proton nuclei having very short spinespin relaxation times to completely dephase [14]. Proton spin-lattice in the laboratory (T1) and in the rotating (T1r) frame relaxation times were measured using the inversion recovery-CP/MAS [15] and the variable spin-lock time-CP/MAS [16] sequences, respectively. For these measurements each spectrum was recorded using a ct of 1 ms, and acquiring 100 and 400 transients for the inversion recovery-CP/MAS and the variable spin-lock time-CP/MAS experiments, respectively. For these measurements each spectrum was recorded using a ct of 1 ms, and acquiring 100 and 400 transients for the inversion recovery-CP/MAS and the variable spin-lock time-CP/MAS experiments, respectively. The 2D 13Ce1H FSLG-HETCOR [17] spectrum was recorded acquiring 80 rows and 400 transients. Where not specified, the experiments were performed at room temperature and using air as spinning gas. TMS was used as a primary chemical shift reference for all nuclei, while hexamethylbenzene and adamantane as secondary references for 13C, 29Si and 1H, respectively.

2.2.

Materials

Low density polyethylene (LDPE) films were provided by Exxon Chemical (LD 158 JD) and are characterized by a thickness of 60  5 mm, a density of 0.925 g/cm3, a Melt Index (190  C/ 2.16 kg) of 2.0 g/10 min, a peak melting temperature of 111  C and number and weight average molecular weight of 19,000

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and 70,600, respectively. LDPE films were purified from surface impurities by washing with acetone for 1 h, dried under reduced pressure, and then cut into circular samples of about 7 cm of diameter. 4-Vinylbenzyl chloride (VBC) (4-(chloromethyl)styrene, Aldrich, 90%) and divinylbenzene (Merck) were purified by washing twice with 25% aqueous sodium hydroxide, and then thoroughly washed with distilled water until neutral. Finally the monomer was distilled at reduced pressure and stored under nitrogen over molecular sieves at 20  C. Benzophenone (99%), diphenyl ether (99%), 1,4diazabicyclo(2.2.2)octane (Dabco, 98%), were obtained from Aldrich and used without purification.

2.3.

Preparation of the anion exchange membranes

2.3.1.

LDPE photografting procedure

In a typical experiment, about 0.300 g of LDPE film were soaked in a 8 cm diameter Petri dish containing 6 mL of a solution (amount required to effectively dip all the surface and thickness of the film) composed by 3 mL of distilled VBC (0.021 mol), 3 mL of diphenyl ether (0.019 mol) and the 0.7 mol% of benzophenone at the temperature of 75e80  C for 30 min. Then, the temperature was lowered to 70  C and the Petri dish was put at a fixed position 15 cm below the focal point of the UV lamp and irradiated for 30 min. After cooling down to room temperature, the functionalized films (LDPE-g-VBC) were washed with large amounts of chloroform and acetone to effectively remove unreacted VBC and the poly(vinylbenzyl chloride) oligomer obtained during the reaction. The film was finally dried in a desiccator for 24 h until constant weight.

2.3.2.

Amination of the functionalized LDPE films

The LDPE-g-VBC film was immersed into a 1 M Dabco methanol solution at 60  C for 72 h, operating with amine/eCH2Cl mol. ratio of about 100. The unreacted amine present on the membrane surface was removed by washing with excess of water and methanol and then dried in a desiccator for 48 h until constant weight.

2.4.

Characterization of the anion exchange membranes

Deionized (DI) water (r  10 M U cm) was used in all experiments. A multi-channel potentiostat/galvanostat, having an impedance channel and a 4 A current booster (VMP3 from BioLogic SA) was used to perform Electrochemical Impedance Spectroscopy (EIS) measurements and the determination of hydrogen permeability. For both through plane conductivity and permeability measurements, different kinds of MEAs (Membrane Electrode Assemblies) were prepared with the anion exchange membranes as described in the respective sections. The results of characterization data obtained for the new LDPE-g-VBC-Dabco membrane are compared with data obtained for a commercially available benchmark alkaline membrane supplied by Tokuyama Corporation.

50  C for 30 min in order to convert the membrane to the OH form. The membrane was then washed using N2-degassed water (the nitrogen atmosphere was required for anionic membrane to prevent CO2 poisoning) and soaked for 30 min in a measured volume of a standardized aqueous HCl solution, whereby the OH in the membrane is neutralized by a part of the acid present. The excess of HCl was then titrated using a standardized aqueous KOH solution. End-point was detected by visual or potentiometric methods. The IEC was then calculated as follows: IECðmeq=gÞ ¼

where meqHCl and meqKOH represent respectively the milliequivalent of HCl solution used for soaking the membrane during neutralization of its OH groups and the milliequivalent of KOH solution used for the titration of the excess of acid; gpolymer is the dry weight of the membrane determined after 12 h drying at 50  C using a four-digit analytical balance.

2.4.2.

Determination of the water uptake (WU)

The dry weight (Wdry) of the polymer film was measured after 12 h drying at 50  C using a four-digit analytical balance. The film was then activated in 1 M K2CO3 aqueous solution at 50  C for 30 min in order to exchange its counter-anions to the carbonate form. Temperature control was ensured using a thermostatic oil bath. Wet weight (Wwet) was measured after conditioning in DI water (r  10 M U cm) at the required temperature for 30 min. Before weighing, the excess of water was removed from the membrane surface using humidified filter paper. Water uptake, as mass percentage, was calculated using the following equation:

WUð%Þ ¼

Wwet  Wdry ,100 Wdry

where wet film weight was averaged over 3 measurements.

2.4.3.

Determination of the in-plane (sIP) conductivity

In plane conductivity was measured using an EIS technique. A piece of membrane was soaked in 1 M K2CO3 aqueous solution for 24 h at 50  C and after washing abundantly with DI water, was inserted in a Bekktech PTFE 4-probe flow-cell, assembled with a 5 cm2 cell hardware supplied by Fuel Cell Technologies. Impedance spectra were performed until a constant conductivity value was obtained. The membrane was washed with DI water between each measurement in order to remove any excess electrolyte that remained absorbed in the membrane which is slowly released by the effect of electrical drag during the measurement. Figure S2(a) shows a typical Nyquist plot obtained for such measurements: the high-frequency arc represents a parallel RC circuit arising from membrane bulk resistance and capacitance, while the low-frequency arc is an artifact coming from the Pt-wire/AM interfacial impedance [18]. The membrane resistance was calculated by fitting the impedance data to an RC circuit between 100 kHz and 10 kHz (i.e. before the low-frequency arc) to prevent artifacts and calculated as follows: sIP ðS=cmÞ ¼

2.4.1.

Determination of the ion exchange capacity (IEC)

The IEC was determined by acidebase back-titration. A piece of the polymer film was activated in 1 M KOH aqueous solution at

meqHCl  meqKOH gpolymer

0:425ðcmÞ HFRðUÞ,wðcmÞ,tðcmÞ

where HFR represents the measured resistance (from the impedance spectra; Figure S2(a), t the thickness of the

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membrane, w the width of the piece of membrane used for measurements. A multi-channel potentiostat/galvanostat, having an impedance channel and a 4 A current booster, (VMP3 from Bio-Logic SA) was used to record electrochemical impedance spectra.

2.4.4.

Determination of the through-plane (sTP) conductivity

MEAs for testing were prepared using a piece of anion exchange membrane and a commercially available anion exchange ionomer (AS4 from Tokuyama corporation) solubilized in a solvent mixture. MEAs were prepared using a commercial 40 wt% Pt on carbon catalyst (BASF) as follows: the catalyst was mixed with a certain amount of water, isopropanol, and AS4 ionomer solution, and then sonicated to obtain a homogeneous ink. The obtained ink was coated onto a PTFE sheet and then transferred using a decal method to both sides of the membrane (hot pressing conditions: 105  C, 250 kg/cm2, for 10 min) to obtain the Catalyst Coated Membrane (CCM) (the final Pt loading was 0.5 mg/cm2 on both sides). SGL carbon paper (25BC) with a microporous layer was used as diffusion medium during the cell measurements. A standard MEA, prepared as described above, was soaked in 1 M K2CO3 aqueous solution for 24 h at 50  C and after washing abundantly with DI water, was inserted in a 5 cm2 cell hardware supplied by Fuel Cell Technologies. Figure S2(b) shows a typical Nyquist Plot obtained for such measurements: at high frequencies, a transmission line is observed (i.e. a 45 line), coming from the ionic resistance of the electrode, while at low frequencies the graph approaches a vertical line, due to the electrode capacitance [19]. The contact resistance was measured using a cell assembled without the membrane (Rcontact z 5 mU). The membrane resistance was calculated by extrapolating the 45 line to the real axis and subtracting the measured contact resistance with the following equation:

sTP ðS=cmÞ ¼

tðcmÞ ½HFRðUÞ  Rcontact ðUÞ,Aðcm2 Þ

where HFR represents the measured resistance (from the impedance spectra; Figure S2(b)), t the thickness of the membrane, A the area of the electrodes within the MEA [20]. A multi-channel potentiostat/galvanostat, having an impedance channel and a 4 A current booster, (VMP3 from Bio-Logic SA) was used to record electrochemical impedance.

2.4.5.

Determination of the hydrogen permeability

MEAs for permeability testing were prepared using a piece of anion exchange membrane and a commercially available anion exchange ionomer (AS4 from Tokuyama Corp.) solubilized in a solvent mixture. MEAs were prepared using a commercial 40 wt% Pt on carbon catalyst (BASF) as follows: The catalyst was mixed with a certain amount of water, isopropanol, and AS4 ionomer solution, and then sonicated to obtain a homogeneous ink. The obtained ink was coated directly onto a sheet of SGL carbon paper (25BC) with a microporous layer (the final Pt loading was 0.5 mg/cm2). The MEA was formed by sandwiching a piece of membrane between two 5 cm2 pieces of coated SGL carbon paper (25BC) with a microporous layer in the fuel cell hardware. A standard MEA prepared as described above, was soaked in 1 M K2CO3 aqueous solution for 24 h at 50  C and after washing

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abundantly with DI water, was inserted in a 5 cm2 cell hardware supplied by Fuel Cell Technologies. The cell hardware was designed to allow the flow of humidified hydrogen to the cathode and humidified nitrogen to the anode. The permeability measurement was performed through a limiting current value determination [21]. A voltage scan was performed at the anode side in order to measure the current derived from the crossover of hydrogen from the cathode to the anode, which was determined by measuring the plateau current obtained. The limiting current (IL), expressed in Ampere is related to the permeability coefficient of the membrane to hydrogen ðkH2 Þ according to the following equation: kH2 ¼

IL ,l A,n,F,pH2

where l represents the thickness of the membrane expressed in cm, A is the electrode area in cm2, n is the number of electrons involved in the hydrogen oxidation reaction, F represents the Faraday constant (96,485 A s) and pH2 the partial pressure of the hydrogen (all measurements were performed at atmospheric pressure (100 kPa)). Consequently, the permeability coefficient of a membrane can be expressed as mol cm cm2 s1 kPa1. The voltammetry spectra were recorded with a scan voltage from 0 to 0.8 V at a scan rate of 20 mV/s.

2.4.6.

Electrolytic cell tests

An appropriately sized sample of the LDPE-g-VBC-Dabco polymer film was used to prepare a MEA and tested in a home-made alkaline membrane water electrolysis test cell. The membrane was mounted in the test cell with cathode and anode electrodes prepared with proprietary non PGM catalysts produced by ACTA SpA (Figure S1). A picture of the cell setup is shown in Figure S1. The MEA was formed by pressing the two electrodes onto the membrane in the test cell and closing under controlled force. Tests were run at a cell temperature of 45  C with a flowing aqueous solution of 1 wt% K2CO3 at the anode side with no liquid electrolyte at the cathode side. The water required for hydrogen production at the cathode was provided by osmosis of the solution from the anode through the membrane. The exit hydrogen gas was pressurized to 20 bar during the running of the test. The anode or oxygen evolution side was kept at atmospheric pressure. The production of hydrogen was monitored by measuring the flow of hydrogen produced at the cathode side. The durability of each MEA was examined by running the electrolysis cells under the standard conditions described above at a constant current density of 460 mA/cm2. The cell potential was monitored along with hydrogen production rate. All electrical parameters were controlled and recorded using the Multi-Channel Battery Testing System Arbin BT2000 by Arbin Instruments.

3.

Results and discussion

3.1.

Preparation

A 60 mm thick LDPE film was firstly soaked at 75e80  C with the functionalizing monomer, i.e. 4-vinylbenzyl chloride (VBC),

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containing the 0.7 mol% of benzophenone photoinitiator and diphenyl ether, this last used to favor polymer swelling and VBC/initiator uptake. Moreover, owing to its chemical structure, i.e. absence of radically sensitive aliphatic or benzylic groups, diphenyl ether resulted inert under the reaction conditions. The temperature in this first step was strictly maintained below 80  C to avoid early melting of the LDPE film. The film was then irradiated at 70  C by using a high pressure mercury UV lamp (400 W) for 30 min. Since Petri dish is mostly UV transparent and thanks to the metal reflective surface of the heating plate, all the portion of LDPE film was effectively affected by irradiation. The mechanism of the grafting reaction is well reported in the literature [22e24]; it initiates with the UV light absorption by the embedded benzophenone followed by its excitation to a singlet state and successive transformation to a triplet (biradical) state, which is able to extract hydrogen from the PE matrix. Then, the macroradical adds a molecule of VBC thus initiating the growth of the grafted chain. The ungrafted poly(vinylbenzyl chloride) oligomers generated during the functionalization reaction were separated from the LDPE-g-VBC samples by repeated washing with chloroform and acetone [9].

3.2.

FT-IR, SEM and DSC characterization

The functionalized film was analyzed by FT-IR spectroscopy which showed the typical signals attributed to the presence of grafted chlorobenzylic moieties; for example, the stretching of the C]C aromatic double bonds at 1610 and 1510 cm1, the bending of the eCH2Cl at 1265 cm1, the bending of the C] CeH units at 840 cm1 and the stretching CeCl at 675 cm1 can be clearly detected along with the strong absorption at 1465 cm1 attributed to the bending of the LDPE methylene units (Fig. 1). The morphology of the LDPE-g-VBC film and the amount of grafted VBC units expressed as % by weight of chlorine atoms were evaluated by SEM coupled EDS. The analysis showed the presence of a rough surface attributed to the grafting of the VBC units on the LDPE matrix (Fig. 2a), and its magnification (Fig. 2b) evidenced the intimate contact between the two layers. For example, the EDS analysis showed a chlorine content at the surface of about 16 wt% which decreased to 6.9 wt.% in the bulk of the material, as expected. However, even if the energy irradiated output of the UV-lamp tends to gradually decrease within the LDPE layer, the measured chlorine content in the bulk of the functionalized films resulted approximately constant for all the samples (Fig. 2c). The analysis of a pristine LDPE film was also reported for comparison (Figure S3). DSC (Fig. 3) performed on pristine and functionalized LDPEg-VBC films showed only the endothermic peak of the LDPE matrix pointed at 110  C in both cases. However, the lower melting endotherm calculated for the functionalized film (DH ¼ 104.2 J/g for LDPE and 87.1 J/g for LDPE-g-VBC) suggests that polymer crystallinity could be partially affected by the functionalization process. Benzyl chloride groups grafted onto the LDPE backbone were converted into quaternary ammonium groups by using

Fig. 1 e FT-IR spectra and attribution of the main signals of an LDPE film before and after functionalization with VBC.

a large excess of a tertiary amine (amine/eCH2Cl mol. ratio w 100) such as the 1,4-diazabicyclo(2.2.2)octane (Dabco) as analogously reported in literature [9,25,26]. Comparing FTIR spectra before and after quaternization reaction with Dabco, the peak at 1265 cm1, associated to CH2eCl wagging, disappears whereas new peaks at 1313 and 1058 cm1, assignable to amine absorptions, show up (Figure S4).

3.3.

SSNMR characterization

In order to get more insights into the structural, phase and dynamic properties of the LDPE-g-VBC-Dabco membrane, several high-resolution SSNMR experiments were carried out on the LDPE-g-VBC-Dabco system as well as on the LDPE-gVBC intermediate and the pristine LDPE film. In Fig. 4a the 13 C CP/MAS at ct ¼ 1 ms spectra of LDPE, LDPE-g-VBC and LDPE-g-VBC-Dabco are reported. All the spectra show an intense signal at about 31e33 ppm ascribable to PE carbons. Furthermore in the spectrum of LDPE-g-VBC a signal at about 46 ppm and a group of signals at about 120e150 ppm ascribable to VBC CH2Cl and aromatic carbons, respectively, can be observed [27]. The latter signals are also present in the spectrum of LDPE-g-VBC-Dabco together with two signals at 45.8 and 52.9 ppm ascribable to Dabco carbons, one and two bonds far from the ammonium moiety, respectively [28]. Moreover, the disappearance of the CH2Cl in passing from the spectrum of LDPE-g-VBC to that of LDPE-g-VBC-Dabco, and the corresponding appearance of those arising from Dabco carbons, indicate that the amination reaction was complete, in agreement with FT-IR results. On the other hand, the similar intensity shown by the two Dabco signals at 45.8 and 52.9 ppm suggests that most of Dabco does not cross-link, likely due to the relevant molar excess of amino groups with respect to the grafted chlorobenzylic moieties (that is, amine/eCH2Cl mol. ratio w 100) used during the amination process. In order to obtain information on the phase and dynamic behavior of the different sample components, 13C selective experiments were performed. In particular 13C CP/MAS at ct ¼ 1 ms and 50 ms and 13C Delayed-CP/MAS spectra were acquired. While in the 13C CP/MAS spectrum at ct ¼ 1 ms

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Fig. 3 e DSC curves of pristine LDPE and functionalized LDPE-g-VBC films.

Fig. 2 e (a) Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) of a LDPEg-VBC film and (b) its magnification; (c) scanning line for EDS chlorine analysis across the film fracture.

signals arising from both rigid and mobile domains should be present, in the 13C CP/MAS at ct ¼ 50 ms and in the 13C DelayedCP/MAS spectra signals arising from rigid (characterized by short cross-polarization times) and mobile domains (characterized by long proton T2) can be selectively observed, respectively. The 13C selective spectra of all the samples are

reported in Fig. 4a. As far as LDPE-g-VBC-Dabco is concerned, the quite high intensity of all the signals in the 13C CP/MAS at ct ¼ 50 ms spectrum and their substantial suppression in the 13 C Delayed-CP/MAS one indicate that PE, VBC and Dabco are all quite rigid. This is also confirmed by 1H MAS spectra (Figure S5), where the signals belonging to LDPE, VBC and Dabco start to be partially resolved only at a MAS frequency of 20 kHz. Almost the same behavior can be observed for LDPE and VBC signals in the 13C selective spectra of LDPE and LDPEg-VBC, suggesting that the average mobility of these components does not change significantly during the preparation process. As far as LDPE is concerned, it is known that in 13C high resolution SSNMR spectra its amorphous and crystalline fractions, characterized by LDPE chains experiencing fast trans-gauche interconformational jumps and in all-trans conformation, respectively, give rise to two easily distinguishable signals, the former at about 31 and the latter at about 33 ppm. As it can be observed in Fig. 4a, two signals ascribable to amorphous and crystalline LDPE fractions are well resolved in the 13C CP/MAS at ct ¼ 1 ms spectrum of LDPE, but they are no longer distinguishable in those of LDPE-g-VBC and LDPE-g-VBC-Dabco. In order to get a better understanding of the phase behavior of LDPE, spectral deconvolution of the 13 C CP/MAS at ct ¼ 1 ms spectra were performed, and the results are shown in Fig. 4b and Table 1. In particular, even if the 13C CP/MAS at ct ¼ 1 ms spectra are not quantitative, we found that, in agreement with the DCS results, both crystalline and amorphous PE fractions are present not only in LDPE, but also in LDPE-g-VBC and LDPE-g-VBC-Dabco, and that the crystalline fraction seems to slightly decrease in passing from LDPE and LDPE-g-VBC to LDPE-g-VBC-Dabco. On the other hand, we observed a 3e4 fold increase of the linewidth of crystalline PE signal in passing from LDPE to LDPE-g-VBC and LDPE-g-VBC-Dabco, suggesting that, similarly to what previously reported by other authors for neat LDPE at temperature higher than 25  C [29], in the functionalized systems the crystalline LDPE chains in the crystalline phase experience a motion with a characteristic frequency in the range of kHz, which interferes with the proton decoupling field. This

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Fig. 4 e (a) From bottom to top: 13C selective spectra of pristine LDPE, LDPE-g-VBC, and LDPE-g-VBC-Dabco films. For each sample, from bottom to top, 13C CP/MAS at ct of 1 ms and 50 ms, and 13C-Delayed CP/MAS spectra are reported. (b) From bottom to top: spectral deconvolution of the 13C CP/MAS at ct of 1 ms relative to LDPE, LDPE-g-VBC, and LDPE-g-VBC-Dabco.

hypothesis is further supported by 13C spectra acquired at increasing temperature (from 25  C to 70  C, here not reported), which show that the linewidth of crystalline PE signal is strongly dependent on the temperature, i.e. on the frequency of the motion (Figure S6). The behavior of VBC aromatic carbon signals in the spectra of LDPE-g-VBC and LDPE-g-VBC-Dabco (Fig. 5) is also worthy to be discussed. An evident change in the distribution of these signals in passing from LDPE-g-VBC to LDPE-g-VBC-Dabco can be observed. In particular, beside the expected shifts of a and c carbons to lower (from 135.1 ppm to 132.9 ppm) and higher

(from 145.4 ppm to 147.5 ppm) chemical shift, respectively, we have to hypothesize a splitting of the signal arising from b and b0 carbons (from one signals at about 128.4 ppm to two signals at about 125.4 and 132.9 ppm). This assignment was performed on the basis of the relative areas of the signals and of the 1He13C HETCOR spectrum recorded for LDPE-g-VBC-Dabco (Figure S7), where, for instance, a strong correlation peak between the 13C peak at 125.4 ppm and the 1H peak at about 6.8 ppm is present, suggesting that the 13C peak at 125.4 ppm arises from tertiary aromatic carbons. The presence of two very well distinct signals for b, b0 carbons in LDPE-g-VBCDabco seems to be ascribable to conformational rather than

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Table 1 e Chemical shift (d), linewidth (D), and integral (I ) determined for the signals of crystalline (c) and amorphous (a) LDPE through deconvolution of the 13C CP/MAS at ct of 1 ms spectra. I is calculated as the percentage of the integral relative to a (c) with respect to the sum of the integrals of a and c. a

LDPE LDPE-g-VBC LDPE-g-VBC-Dabco

c

d (ppm)

D (Hz)

I (%)

d (ppm)

D (Hz)

I (%)

30.8  0.1 31.0  0.1 31.1  0.1

280  30 340  10 380  30

26  4 26  1 36  4

32.6  0.1 32.8  0.1 32.8  0.1

440  10 1600  100 1600  100

74  4 74  1 64  4

system spin diffusion tends to average proton T1 and T1r of the various components to a different extent, depending on the dimensions of the corresponding domains. In particular single T1 and T1r values are observed when the system is homoge˚ and 10e20 A ˚ spatial ranges, respectively. neous on 100e200 A The results obtained for the LDPE-g-VBC-Dabco system are summarized in Table 2. Two significantly different T1r values have been determined for PE and Dabco protons, as expected, since the distance in the molecular structure between these compo˚ . On the contrary very similar T1 nents is of the order of 10 A values were measured for all protons in the sample, indicating ˚ a substantial homogeneity of the whole system on a 200 A spatial scale. By looking more closely at the data, however, it is possible to see that Dabco and amorphous LDPE protons have the same T1 value within the experimental error, while the value for crystalline LDPE is slightly higher. This suggests that the VBC-Dabco grafting preferentially occurred on the amorphous LDPE regions, rather than in the crystalline ones. Fig. 5 e Assignment of the VBC aromatic carbon signals in the 13C CP/MAS at ct of 1 ms spectra of LDPE-g-VBC (bottom) and LDPE-g-VBC-Dabco (top).

chemical effects. Indeed, it seems more likely the freezing of the inter-conformational motions of the VBC aromatic moiety following the functionalization with Dabco: under this hypothesis, the two peaks at 125.4 and 132.9 ppm would be therefore ascribable to b and b0 carbons on the two sides of the aromatic ring, experiencing quite different chemical environments due to the frozen conformation of the ring itself. Finally, in order to obtain information on the degree of homogeneity of the LDPE-g-VBC-Dabco membrane, proton T1 and T1r relaxation times have been measured by means of high resolution CP/MAS experiments. Indeed in a composite

3.3.1.

Electrochemical characterization

In order to determine the suitability of the new LDPE-g-VBCDabco membrane for use in alkaline membrane water electrolysis cells, firstly the material was fully characterized for the following parameters: IEC, WU, sIP, sTP and hydrogen permeability (Table 3). The values obtained for each parameter are compared with those obtained for a commercial available alkaline exchange membrane from Tokuyama Corporation. A reoccurring problem associated with the characterization of alkaline membranes is the ease in which the OH counter ions react with atmospheric CO2 to form bicarbonate 2 HCO 3 and carbonate CO3 anions according to the equation: 2 þ OH þ CO2 /HCO 3 4CO3 þ H

In order to avoid problems associated with this so-called carbonatation, degassed water was used for IEC determination. For the determination of WU, ionic conductivity (sIP and

Table 2 e Proton T1 and T1r relaxation times measured from 13C CP/MAS spectra for the LDPE-g-VBC-Dabco sample. The values obtained for the carbon signals arising from crystalline (c) and amorphous (a) LDPE and from Dabco (peaks at 45.8 and 52.9 ppm) are reported. T1 (s)

T1r (ms)

LDPE a 0.87  0.03

Dabco c 0.99  0.04

45.8 ppm 0.81  0.04

LDPE 52.9 ppm 0.82  0.05

a 1.54  0.03

Dabco c 1.97  0.05

45.8 ppm 3.25  0.08

52.9 ppm 3.08  0.06

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Table 3 e Comparison between the electrochemical characterization of a LDPE-g-VBC-Dabco membrane and a benchmark membrane as a function of temperature. T ( C) IEC (meq/g) WU (wt%)

 sIP (mS/cm) HCO 3 =CO3 form

 sTP (mS/cm) HCO 3 =CO3 form

Hydrogen permeability (mol cm cm2 s1 kPa1)  HCO 3 =CO3 form

30 30 45 60 30 45 60 30 45 60 30 45 60

LDPE-g- Benchmark VBC-Dabco 1.5 81 91 94 14 19 25 10 14 17 3.4  1013 5.8  1013 7.5  1013

1.8 61 70 75 15 19 25 6 11 19 2.1  1013 2.7  1013 3.3  1013

sTP) and hydrogen permeability, the membranes were converted to the carbonate ðCO2 3 Þ form in equilibrium with bicarbonate one ðHCO 3 Þ and a novel procedure based on an EIS technique was developed for these measurements. The value of IEC obtained for the LDPE-g-VBC-Dabco membrane is slightly lower than that measured for the benchmark membrane; the level of ion conducting functional groups incorporated into the polymer structure (IEC) has a direct impact on the other two important physical properties of the membrane, i.e. the WU and the conductivity. A membrane with a low IEC tends to suffer from low ionic conductivity while at the same time a high IEC can lead to the membrane to absorb too much water resulting in strong mechanical stresses (swelling and shrinking) which may lead to structural damage. A trade-off between these two properties is usually desired. The WU was measured at 3 different temperatures; (a) 30  C (the reference temperature for comparison with values from literature); (b) 45  C the standard working temperature of the electrolytic cell used for tests; and (c) 60  C as maximum temperature allowed. The values of WU obtained for the LDPE-g-VBC-Dabco membrane are slightly higher with respect to that of the benchmark membrane but still within a range in which the mechanical stability is not compromised by wetting and drying cycles. Normally an acceptable value of WU is assumed to be lower than 100% at temperatures below 50  C. Having a lower IEC with respect to the commercial material, the LDPE-g-VBC-Dabco membrane could be expected also to have a lower ionic conductivity; unexpectedly comparable values were obtained for both in-plane and through plane conductivity. For both materials the through plane conductivities were generally lower than the in plane values at the respective temperature due to the higher functionalization at the LDPE film surface as revealed from the SEM analysis. An important parameter in a working electrolytic cell is represented by hydrogen permeability. Its importance is related both to the efficiency (higher permeability, higher loss

in hydrogen production), and to safety aspects (0.84% of hydrogen in oxygen is the lower explosiveness limit). The values obtained for the LDPE-g-VBC-Dabco membrane are higher than the benchmark material and may be directly related to the higher WU or swelling of this material. This difference is also observed in a smaller hydrogen production for the LDPE-g-VBC-Dabco in the electrolysis cell test.

3.3.2.

Electrolysis tests

The performance of the LDPE-g-VBC-Dabco membrane electrode assembly (MEA) was compared with a benchmark MEA prepared using the standard commercial alkaline exchange membrane (from Tokuyama Corporation). In Fig. 6 the voltage vs current density curves obtained at a 1 mV/s scan rate are compared for the two membranes. The performance is similar in both cases. Over the range of current densities applied the benchmark MEA performs better with a difference of 80 mV at a current density of 600 mA/cm2. The stability of each MEA was examined by running the electrolysis cells under the standard conditions described above at a constant current density of 460 mA/cm2. The cell potential was monitored along with hydrogen production rate. The performance over more than 500 h is shown in Fig. 7. In both cases an increase in cell potential is observed over time. The average increase in cell potential over the period of the test was 2 mV/day (benchmark) and 6 mV/day (LDPE-g-VBC-Dabco). The AC cell resistance monitored at 1 kHz for each cell is also reported. While the AC resistance of the benchmark cell remains relatively unchanged during the experiment (0.22e0.23 U cm2) that recorded for the LDPE-g-VBC-Dabco MEA shows a gradual increase from 0.30 to 0.43 U cm2. The resistance at this frequency generally represents the resistance of the membrane or membrane electrode interface rather than within the electrode layer itself. So we can tentatively ascribe the deterioration in performance of the LDPE-g-VBC-Dabco MEA to changes in

Fig. 6 e Current-voltage curves measured during water electrolysis using the ACTA electrolytic test cell; LDPE-gVBC-Dabco MEA (A) and the benchmark MEA (B) Tcell 45  C, scan rate 1 mV/s.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 4 9 9 2 e1 5 0 0 2

4.

Fig. 7 e Time stability test of electrolyzer at constant current density 460 mA/cm2: LDPE-g-VBC-Dabco MEA (upper continuous line) and the benchmark MEA (lower continuous line). The AC cell resistance at 1 kHz is reported on the right hand Y axis for the LDPE-g-VBC-Dabco cell (dark circles) and the benchmark cell (light circles). Tcell 45  C, H2 outlet pressure 20 bar.

the membrane, likely due to the strong alkaline environment responsible for quaternary ammonium sites degradation [10,30]. Monitoring of the hydrogen production was undertaken by measuring the flow of hydrogen at the cathode side with a set pressure of 20 bar. Fig. 8 compares the hydrogen production rate with the change in cell potential over the course of the experiment. Hydrogen production remains stable at around 30 cc/min. The hydrogen production rate for the benchmark MEA was higher and the difference can be ascribed to the increased hydrogen permeability of the LDPE-g-VBC-Dabco membrane as determined here where the hydrogen is not lost through leakage but through crossover to the anode side, a process accentuated by the differential pressure.

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Conclusions

A new LDPE based anion-exchange membrane was prepared by UV-induced grafting of vinylbenzyl chloride (VBC) functional monomers and their successive conversion into quaternary ammonium sites with 1,4-diazabicyclo(2.2.2)octane (Dabco). FT-IR, SEM, DSC and 13C CP/MAS characterization clearly shown that the functionalization of pristine LDPE with VBC and the amination of LDPE-g-VBC with Dabco effectively occurred at the first and second step of the preparation process, respectively. 13C selective experiments revealed that all the components are quite rigid and that both crystalline and amorphous LDPE domains are present. In particular, in agreement with the DSC results, the crystalline LDPE fraction seems to slightly decrease in passing from LDPE-g-VBC to LDPE-g-VBC-Dabco. Finally, from T1 and T1r measurements the LDPE-g-VBC-Dabco membrane is found to be substantially ˚ spatial range, with Dabco moieties homogeneous on a 200 A preferentially grafted to the amorphous LDPE regions. Electrochemical performance of the LDPE-VBC-Dabco membrane was comparable to that measured for the commercial material, and enabled its use in an electrolytic cell for hydrogen production showing a constant hydrogen production rate of about 30 cc/min over more than 500 h. The deterioration in performance of the LDPE-g-VBC-Dabco membrane was larger with respect to the commercial material likely due to a minor resistance under the alkaline environment of the working cell. Future development will move in this direction, by properly selecting quaternary ammonium sites more stable under working cell conditions.

Acknowledgments FM thanks GIDRM/Borse “Annalaura Segre” for partial financial support. MB thanks Tokuyama Corporation for the supply of alkaline membrane

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2012.08.012.

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Fig. 8 e Time stability test of the LDPE-g-VBC-Dabco electrolyzer at constant current density 460 mA/cm2 (solid line): The hydrogen production (cc/min) is reported on the right hand Y axis (circles). Tcell 45  C, H2 outlet pressure 20 bar.

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