Novel proton exchange membranes based on PVC for microbial fuel cells (MFCs)

2.1 Experimental material

Silica, citric acid and DMF were purchased from Merck, Mumbai, India and were of analytical grade and used without further purifications. PWA (H₃PW₁₂O₄₀·nH₂O) and PVC (Mol Wt ~48,000) were purchased from Sigma, USA. De-ionized water was used for all the experiments performed.

2.2 Membrane preparation

The polymeric films of pure PVC and different compositions of PVC complexed with silica, citric acid and PWA were prepared in various weight percent ratios by solution casting method using dimethylformamide (DMF) as a solvent. The preparation was preceded by dissolving PVC resin into DMF and stirring until a homogenous solution is obtained. This was followed by dispersing a predetermined amount of silica (1, 2, 5 and 10%, w/w), citric acid (10 and 20%, v/w) and PWA acid (1, 3 and 5%, w/w) (Table 1) to the PVC solution with constant stirring at room temperature (40°C) which continued for 1 h. The solution was then allowed to cool below room temperature (20°C) for 6 h, so as to form the gel, and then cast onto polypropylene dishes. The solvent was evaporated by keeping the dishes in an oven at a temperature of 50°C for 1 day followed by 4 days at room temperature.

2.2.1 Leaching test of PWA from polymeric membranes for samples J to M

A sample of the synthesized membrane containing PWA (2×2 cm) was immersed in 100 ml of water, and the concentration of PWA in water was determined at a regular interval using a UV-visible spectrophotometer (Agilent Technologies, Double Beam, Cary 300 UV-Vis), taking absorbance measurements at 292 nm. A calibration curve relating the PWA concentration to absorbance at 292 nm was used to calculate the PWA concentration in the aqueous phase (Figure 1).

Figure 1:

Amount of PWA leached out.

2.3 Membrane characterization

2.3.1 Morphological studies

Surface morphologies of the prepared membranes (or polymeric films) were investigated by scanning electron microscopy (SEM) (Figure 2).

Figure 2:

SEM of synthesized membranes. (A, B) Pure PVC, (C, D) PVC +10% silica, (E, F) PVC +5% silica +5% PWA.

2.3.2 Tensile strength

Mechanical stability of membrane M and Nafion was determined by using an Instron Universal Materials Testing System (model 3369) (Figure 3).

Figure 3:

Tensile test for Nafion 117 and membrane M. Tensile test for (A) Nafion-117 and (B) membrane M.

2.3.3 Water uptake analysis

The water uptake analysis was carried out by taking the dry and wet weight ratio of the prepared membranes. First, the weights of the dry membranes were taken. Then, the membranes were immersed for 24 h in a glass beaker (100 ml) with distilled water (50 ml) at room temperature. The samples were taken out and surface water was removed with absorbent paper to obtain the wet weight. The percentage of water uptake was calculated by the following equation [32]:

Water uptake (%)=(Ww−Wd/Wd)×100

where W_w is the weight after immersing the samples in water and W_d is the dry weight, respectively. The results are reported in Figure 4.

Figure 4:

Water uptake of prepared membranes.

2.3.4 Ion exchange capacity (IEC)

The IEC of the prepared membranes was evaluated using titration. The membranes were converted to their protonic form by immersing in a 1 m HCl solution for 24 h, and then the membranes were washed with distilled water to remove excess acid. Subsequently, the membranes were placed in a 0.1 m NaCl solution for 24 h, and then the solution was titrated with 0.01 m NaOH. The IEC was calculated using the following equation [33]:

IEC (mequiv/g dry membrane)=VNaOH×Conc. of NaOH/m

where m is the mass (g) of dry membrane. The results are given in Figure 5.

Figure 5:

IEC of prepared membranes.

2.4 Application in MFC

2.4.2 MFC construction

2.4.2.1 Two chambers with membrane

The setup consisted of two ARISTO airtight plastic containers (1500 ml) with holes of 1-inch diameter cut in the center of their broader face. The chambers were connected by an arrangement of PVC pipes (9-inch length). Synthesised membranes were inserted into the union by easy dissembling of union. Anode (graphite) and cathode (copper) were inserted into the chamber by making the required holes in the cover of the airtight containers. The electrodes were connected with an aluminum wire (16.54×10⁻⁴ Ω). The wires were than attached to the two electrodes of the multimeter (UNI-T) with an electrical tape (Steel Grip). All the vacant surfaces were sealed using M-Seal (PIDLITE) to make the anode chamber completely anaerobic.

2.4.2.2 Anolyte composition

The anode compartment was filled with medium containing NaCl (10 g/l), tryptone (10 g/l), yeast extract (5 g/l), glucose (100 g/l) and cysteine (0.75 g/l).

2.4.2.3 Catholyte composition

The cathode compartments were filled with an equivalent amount of distilled water with electrodes dipped in them.

2.4.2.4 Recording of data

The multimeter (UNI-T) was used to get the readings at the 1-h interval, and the voltage and current were recorded in units of mV and mA, respectively. The power density was calculated by using the equation

P=(V×I)/A

where V is the observed voltage, I is the observed current and A is the area of electrode dipped in the medium (5.91 cm²). The results are given in Figures 6–8.

Figure 6:

Combined OCV curve of selected prepared membranes applied in MFCs.

Figure 7:

Polarization curve of selected prepared membranes applied in MFC. Polarization curves of (A) membrane F, (B) membrane K, (C) membrane L and (D) membrane M.

Figure 8:

Power density curve of selected prepared membranes applied in MFC. Power density curves of (A) membrane F, (B) membrane K, (C) membrane L and (D) membrane M.

3.1 Leaching of PWA from polymeric membrane

Three membranes (K, L and M) containing different amounts of PWA (1, 3 and 5%, w/w, respectively) were subjected to the leaching test to determine the amount of PWA coming out of the membrane. The amounts of PWA leached out at different time periods are shown in Figure 1. The graph shows that the amount of PWA leached out of the membrane increased linearly with time until 16 h, and thereafter no significant leaching was observed. The results show that the amount of PWA left in the membranes was sufficient to provide the enhanced conductivity of membranes.

3.2 Membrane morphology

The membranes are characterized by SEM. Figure 2A–F presents the morphologies of the representative membranes at 2500 magnitudes. SEM micrographs of the three samples – PVC, PVC and silica and PVC, silica and PWA – are shown in Figure 2A–F. In Figure 2A and B, a layered structural characteristic of PVC can be identified, and a smooth and crack-free surface is seen clearly. The images in Figure 2C and D show the morphology of the PVC containing 10% silica where the individual particles of the silica can be seen, resulting in a granular morphology, which is homogenously distributed all over the surface. The average size of the granules is 6–8 μm. In Figure 2E and F, the morphology of the PVC membrane along with silica and PWA is shown, which has a homogenously distributed surface.

3.3 Tensile test

All the prepared membranes contain PVC (2 g), and the mechanical strength of the membrane is due to the PVC matrix. So, the tensile test of membrane M (membrane showing better IEC) and Nafion was carried out. Figure 3 shows the stress – strain curves of the synthesized membrane M and Nafion-117 in the dry state. The synthesized membrane M showed high tensile strength (62 MPa) as compared to Nafion-117 (29 MPa). The higher tensile strength is attributed to the presence of the PVC matrix.

3.4 Water uptake

Water uptake capacity of the prepared membranes is shown in Figure 4. The water uptake of pure PVC is low (4.02%) as compared to those of all the prepared membranes. This is due to the strong hydrophobic character of the groups present there. Meanwhile, the water uptake is observed to increase steadily with the increase in concentration of SiO₂, due to the formation of Si-OH bonds, which promotes water retention. Out of the four membranes (membranes B–E) where varying % of SiO₂ was introduced, 10% showed a maximum water uptake of 55.8% (membrane E), with comparable water uptake of 46.6% (membrane D) at 5%. Therefore, these two concentrations were further used for preparing other membranes. This is also supported by the literature where water uptake increases with an increase in the % of SiO₂ [39].

Further, when citric acid was used (membranes F–I), water uptake reduces as compared to that of SiO₂. It shows that the citric acid molecules are chemically bonded to the host polymer, leading to increased cross-linking and reduction in water uptake [40]. Upon the addition of PWA, water uptake increases. This was an expected result since PWA is hydrophilic; therefore, water uptake should increase with an increase in PWA wt% [41], [42].

3.5 Ion exchange capacity

IEC is an important parameter for PEMs. A high IEC is required to ensure high proton conductivity. The results of the IEC are shown in Figure 5. The IEC of the membranes with varying percentage of silica (membranes B to E) are somewhat better than that of virgin membrane A. This is also supported by the literature where the introduction of SiO₂ improved the ionic conductivity of membranes at room temperature [43]. The IEC values of the PWA-doped-PVC system vary from 0.63 to 0.875 meq/g. It can be seen that IEC increases almost linearly with PWA doping. Increase in IEC with PWA content is an expected trend since PWA acts as the proton source here. The maximum IEC value of 0.875 meq/g is obtained with 5% PWA (w/w) but is lower than that of Nafion-117 (0.98 meq/g) [44].

3.6 Fuel cell analysis