Poly(AAc-co-DMAPMA) membrane (PADMA) is synthesized by free radical aqueous copolymerization of acrylic acid (AAc) and N-3-[dimethylamino)propyl]-methacrylamide (DMAPMA) to check its stability and conductivity. The hydrogel membrane characterized physically to study morphology by SEM, thermal stability by TGA and mechanical stability by measuring compressive strength and ionic conductivity by electrochemical impedance spectroscopy in alkaline as well as in acidic environment at different temperatures. The compression modulus of the hydrogel membrane is 24 kPa at pH = 1.0 and 16 kPa at pH = 7.0, and stable (no fracture) till 72% deformation. The PADMA hydrogel membrane ionic conductivity increased with the increase in temperature and structurally stable up to 190°C. Improvement in ionic conductivity is observed after the heat treatment of the membrane. Compared with ionic conductivity of Nafion® (SE512), the PADMA membrane found to be inferior. However, the PADMA hydrogel membrane conductivity was greater (~1 × 10 <sup>-4</sup>S/cm) at low and high pH compared with neutral pH (~1 × 10 -5S/cm) indicating the possibility of using the membrane as either a proton and hydroxyl ion conductor.
Fuel cell technology has been considered as a clean source of energy compared to conventional energy converting device. Out of many types of fuel cells which are developed, proton exchange membrane fuel cell (PEMFC) is most promising in terms of efficiency in energy conversion [
The development of high temperature polymer electrolyte membrane fuel cells is a recent research area. The interest in the development is due to the numerous advantages of PEMFC technology operating above 100˚C [
The present study is focused on the development of a novel and low cost polymer electrolyte membrane. These are hydrogel types of membranes, and to date the hydrogel membrane is rarely applied as PEM for fuel cell application. Nikolic et al. [
The present study focuses on the synthesis of the poly(AAc-co-DMAPMA) membrane (PADMA), which is cheap and easy to prepare from acrylic acid and N-3-[dimethylamino)propyl]-methacrylamide. The PADMA hydrogel membrane is preliminary characterized by SEM, TGA, mechanical strength, and ionic conductivity at different temperature and pH for the possible use as electrolyte in fuel cells.
The membrane was synthesized using acrylic acid (AAc) (G.S. Chemicals, India), N-3-[dimethylamino) propyl]- methacrylamide (DMAPMA) (Aldrich, USA), ammonium persulphate (APS) (Qualigens Fine Chemicals, India), N,N,N′,N′-tetramethyl ethylene diamine (TEMED) (SRL Pvt. Ltd, India ). AAc and DMAPMA were purified before use by vacuum distillation. Other chemicals were used as received from the source.
The membrane was prepared by free radical aqueous copolymerization of AAc and DMAPMA; using APS and TEMED as the initiator and accelerator, respectively. The reaction, as shown in the
three edges of a pair of glass plates. The open side along the remaining edge was partially closed by a smaller spacer, leaving two orifices at the corners. Nitrogen gas was purged through one of the orifices, while the other was used to inject the reaction mixture under gentle stream of nitrogen. The mold was then placed vertically in a thermostated water bath at 41˚C ± 1˚C and dipped up to the height of reaction mixtures. Nitrogen purging was stopped after 15 min and the orifices were closed using paraffin grease. After 24 h, the PADMA membrane was removed from the mold, cut into pieces, washed in regularly changed distilled water for 3 days to remove the unreacted monomers, and dried in vacuum. The thickness of the membrane was 830 μm.
ZEISS EVO Series scanning electron microscope (Model EVO 50) was used to investigate the morphology of the membrane. To evaluate the morphology, the membrane was swollen in an appropriate solution. The swollen sample was lyophilized to remove any solvent or water from the sample and kept in vacuum till silver sputtering treatment.
The thermal stability of the membrane was investigated using a PerkinElmer Pyris 6 TGA with a heating rate of 20˚C/min from 25˚C to 800˚C.
Uniaxial compression is a useful technique to evaluate the mechanical strength of the polymer electrolyte membranes, especially hydrogels. TA-XT2i Texture Analyzer (Stable Micro Systems, UK ), with a 5 kg load cell was used to analyze the compressive strength of the hydrogel. The force measurements accuracy and the distance resolution of the instruments were 0.0025% and 0.0025 mm, respectively. The membrane was separately equilibrated in buffered solution of pH 1.0 and 7.0. Circular discs of 15 mm diameter were punched off with the help of a sharp edged puncher. Uniaxial compression experiment was performed on the cut sample of the swollen membrane at 25˚C. Cylindrical aluminium probe with a diameter of 35 mm was used for compression. The pre- test speed, test speed and post-test speed were set up at 2.00 mm/s, 1.00 mm/s and 1.00 mm/s respectively, with an acquisition rate of 200 points/s. The force necessary for compressing the discs at 0.8 mm was recorded. The stress values (σ) were determined using Equation (1) [
where, F is the force and A is the cross-sectional area of the strained specimen. The parameters generated by the instrument were force and time/displacement. Those information were then converted to elastic modulus, E, by using following Equation (2).
E was determined from the slope of the stress-strain relationship. The macroscopic deformation ratio (λ) was calculated as λ=Lt/L0. Here, Lt and L0 are the length of the deformed and undeformed specimen respectively.
Ionic conductivity of the membrane was measured in a conductivity cell in which a strip of the membrane was placed in between two pair of platinum strips separated by 0.5 cm. The bulk conductivity of the hydrogel and Nafion® 512 membrane was measured using frequency response analyser (FRA). AC impedance measurements were carried out between 100 Hz and 30 kHz. The thickness of the PADMA membrane was 625 μm after pressing and that of Nafion® 512 was 133 μm. The platinum strips acted as connector for resistance measurement. The top and bottom cover of the conductivity cell is attached with plate heater to measure conductivity with the variation of temperature. The hydrogel membrane was held in the conductivity cell at the desired temperature to reach the steady state before every measurement. The details of the conductivity measurement cell and technique of measurement is given in [
SEM micrographs of the membrane are shown in
Buffer A | Buffer B | Buffer C | |||
---|---|---|---|---|---|
pH | 0.2 M HCl (mL) | pH | 0.1 M NaOH (mL) | pH | 0.1 M NaOH (mL) |
2.00 | 13.0 | 6.00 | 11.2 | 10.00 | 36.6 |
8.00 | 93.4 |
380˚C. This is most probably due to the degradative reactions involving the pendant groups in the polymer chains, such as decarboxylation. The TGA thermogram indicates that the membrane loses ~50% of its weight at this stage. The second step of weight loss, most probably due to back bone degradation, starts at ~400˚C. There- fore, PADMA membrane can be safely used up to 190˚C.
Compression experiment was done to determine the system hardness (Fmax) and compressive elastic modulus (E). Profiles obtained from a typical experiment with the swollen PADMA membrane at pH 1.0 and 7.0 are shown in
respectively at 0.8 mm compression.
Linear stress-strain relationship was observed at low strain, and is shown as inset. The elastic modulus (E) of the membrane was found to be around 24 kPa and 16 kPa at pH 1.0 and pH 7.0, respectively. During the operation of a fuel cell, electrodes are pressed against the polymer electrolyte membrane, and this put the polymer under compressive stress. The elastic modulus of the well known nafion membrane ranges from 0.5 MPa to 1.28 MPa for different testing conditions [
has been shown that the hydrogel comprising DMAPMA units are temperature sensitive [
The PADMA hydrogel membrane is synthesized for the possible use in fuel cells as electrolyte. The morphological analysis shows that the membrane surface is composed of fused nanogels and the inner part of the membrane has porous and spongy structures. The PAMDA membrane is thermally stable up to 190˚C and possesses excellent compressive strength. The ionic conductivity of PADMA membrane is measured as 9.76 × 10−5 S/cm at 80˚C and pH 10.4. After heat treatment, the membrane conductivity increases to 2.17 × 10−4 S/cm at the same temperature (80˚C) and pH (10.4). The ionic conductivity of the PADMA membrane increases with the increase in temperature. The higher ionic conductivity at low (=2.2) and high pH (=10.6) indicates that the membrane may be used both as proton and hydroxyl ion conductor.
Authors would like to acknowledge financial support of UK India Education Research Initiative, Department of Science and Technology, Govt. of India.