Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes

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Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes Arjun Sunil Rao a,⇑, K.R. Rashmi b, D.V. Manjunatha a, A. Jayarama b, V. Veena Devi Shastrimath c, Richard Pinto a a Department of Electronics and Communication Engineering, Alva’s Institute of Engineering and Technology, Moodbidri, Karnataka (Affiliated to Visvesvaraya Technological University, Belagavi), India b Department of Physics, Alva’s Institute of Engineering and Technology, Moodbidri, Karnataka (Affiliated to Visvesvaraya Technological University, Belagavi), India c Department of Electronics and Communication Engineering, NMAM Institute of Technology, Nitte, Karkala Taluk, Karnataka (Affiliated to Visvesvaraya Technological University, Belagavi), India

a r t i c l e

i n f o

Article history: Received 5 January 2020 Received in revised form 3 February 2020 Accepted 6 February 2020 Available online xxxx Keywords: Polyvinyl alcohol Passive methanol fuel cells Nafion membrane Methanol permeability Proton conductivity

a b s t r a c t This paper presents the effect of polyvinyl alcohol (PVA) coated Nafion membranes on their water uptake, swelling and proton conductivity for various PVA coating thicknesses. These studies show that the optimum coating thickness of PVA on Nafion is 2 mm. Methanol permeation studies show that 2 mm thick PVA coating forms a barrier for methanol and significantly reduces methanol permeation through the membranes. Further, passive methanol fuel cells are tested with 2 mm thick PVA coat on Nafion as proton exchange membranes and their polarization plots show a significant enhancement in power as compared to the methanol fuel cells with pristine Nafion due to reduction in methanol crossover. Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Laser Deposition: Nanostructures, Hetero-structures and 2D layers.

1. Introduction Methanol fuel cells (MFCs) have received attention over the last decade because of their high efficiency and high power density generation capacity [1–3]. MFCs have provision for miniaturization/scaling laws [4], and MFC is therefore, an evolving technology [5,6]. They consume methanol as fuel together with oxygen to generate electrical power and hence, they are considered as electrochemical devices. MFCs are fuel cells that use Nafion proton exchange membrane (PEM). Pt-Ru nanoparticles aid the splitting of methanol at anode into carbon-di-oxide, electrons and protons [7]. The electrons travel through the outer electrical circuit while the protons diffuse through the PEM to the cathode and generate electrical power. At the cathode, e
, H+ and O2 combine to generate water with Pt acting as nano-catalyst [8]. The most important membrane presently used in FCs as PEMs are perfluorosulfonic acid (PFSA) membranes (such as Nafion);

⇑ Corresponding author. E-mail address: [email protected] (A.S. Rao).

Nafion has excellent mechanical strength in addition to excellent stability. They have backbone of hydrophobic fluorocarbon chains along with hydrophilic sulfonic acid groups [9]. These hydrophilic groups contribute to proton conductivity. The conductivity of the protons will be small with low water content and the membranes with hydrophilic groups having higher water content will have higher proton conductivity [10,11]. On the other hand, the membrane will be mechanically compromised by excess hydrophilic groups [12]. Hence, an optimal density of hydrophilic groups should be present in the PEM along with their optimum crosslinking density [13]. When the PEM absorbs water, hydrophilic domains swell facilitating the protons to conduct. There has been extensive study on how water sorption affects proton conductivity of nafion [14–23]. Hence, by studying the hydration of PEM, proton conductivity is evaluated followed by the power density of MFCs. Water soluble polymers (WSPs), like Poly (styrene sulfonic acid), chitosan (CS), Poly (vinylpyrrolidone) (PNVP), poly (ethylene glycol) (PEG), poly (2-acrylamido-2-1-propanesulfonic acid) (PAMPS), polyvinyl alcohol (PVA), etc, have lately become progressively interesting to both academia and industry, as it is possible to use them in soft material applications [24–27]. WSPs that are hydro-

https://doi.org/10.1016/j.matpr.2020.02.093 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Laser Deposition: Nanostructures, Hetero-structures and 2D layers.

Please cite this article as: A. S. Rao, K. R. Rashmi, D. V. Manjunatha et al., Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.02.093

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philic in nature are used in membranes requiring high water retention capacity [28]. PVA is the world’s largest volume of synthetic WSP formed by polyhydroxy polymer. Outstanding adhesion of PVA to cellulose materials enables it to be highly resistant to grease, solvents and oil adhesive and coating material. Furthermore, owing to its high electrical resistance, varied range of crystallinity, better film forming efficiency and high crystal module, it is used in artificial fibers, paper, cloths, coatings and binders [29]. PVA is commonly chosen as a cross-linker due to its hydrophilic properties, capacity to form films, and availability of reactive functionalities for irradiation induced cross-linking, thermal or chemical treatment [30]. The monomer-containing hydroxyl groups of PVA and dialdehyde are crosslinkable under acidic conditions by acetal reactions among the groups of aldehyde and hydroxyl [31]. Furthermore, PVA hydroxyl groups and polymer or monomer carboxylic acid groups form a network structure through dehydration between hydroxyl and carboxylic acid groups. This contributes to robust attachment during thermally triggered reactions [30]. PVA has also been used as alcohol esterification agent to crack the azeotrope of alcohol and water due to the high affinity of water for alcohol [32]. Hence it is a very desirable source for making PEMs in MFCs. In proton conductivity and methanol permeability studies, Pivovar et al. [33] investigated the capacity of PVA as a PEM in MFCs. Their results showed that the dense structure of PVA membranes caused by hydrogen bonding in intra- and inter- molecular level showed much strong barriers to methanol than pristine Nafion. It has been reported that the PVA-based anion exchange membrane has strong hydrophilic properties and film forming ability due to its capacity to form cross-connections with accessible hydroxyl groups [34]. PVA is a low cost substance with excellent properties of mechanical stability and chemical resistance [35]. A downside of PVA is that it lacks negatively charged ions — in contrast with commonly used Nafion; it is therefore, a weak proton conductor. For this purpose, to boost both its proton conductivity and its hydrophilicity, it is important that proton sources are integrated into PVA. To this end, Li et al. [36] developed phosphotungstic acid (PWA) doped PVA membranes. Nevertheless, their mechanical strength is limited by the high swelling of the membrane. PVA is typically integrated into the PEM as a crosslinker to form a 3D dense structure by aldol condensation [37–47] or esterification [48–54]. In addition, it has been shown that the level of crosslinking in PVA-based membranes can be easily controlled by successive treatments (esterification). These membranes demonstrate that the existence of the dense network can effectively control swelling; this is particularly useful in terms of inhibiting methanol crossover. PEMs with optimum properties were therefore constructed by adding PVA as a cross-linking mechanism. The conductivity of these membranes was much lower relative to pristine Nafion 117. Nevertheless, the PVA-based membranes compensate for the poor conductivity of protons by showing stronger barrier to methanol compared to Nafion, thereby reducing fuel losses [55]. In addition, these membranes provide a substantial reduction in the cost of production. There is also the possibility of doping these membranes with acids to obtain conductivities similar to pristine Nafion [56]. This paper explores the effect of PVA coating on Nafion, its water uptake, swelling and methanol permeability of the composite PEMs. Based on these studies the optimum thickness of PVA on Nafion is determined and it is found to be 2 lm. Further, we explore the effect of optimal coating thickness of PVA on Nafion on power enhancement of MFCs due to reduced permeation of methanol. Our experiments show that PVA coated composite Nafion membranes with 2 lm thick PVA layer, result in substantial improvement of power density of MFCs operated in passive conditions.

2. Experimental 2.1. Fabrication of passive MFCs We designed passive MFCs to test the performance improvement of MFCs having PVA-coated Nafion PEM. Fig. 1(a) shows a schematic of passive MFC used by us. Fig. 1(b) shows the exploded view. We employed trapezoidal shaped channels etched in Si wafers (h100i orientation) with methanol having 7 M concentration as fuel at anode as described by our previous work [4]. Micro-electromechanical systems (MEMS) technology as defined in ref. [11] was used to fabricate channels. A significant MFC subsystem, the membrane electrode assembly (MEA), consists of a PEM inserted between two gas diffusion layers (GDLs) had an area of active region 2.89 cm2. The MFC has a porous GDL that controls flow of methanol, an effective catalyst consisting of layer of Pt-Ru nanoparticles for splitting methanol into H+ and e-, and a PEM for proton transmission as illustrated in Fig. 1. The PEMs used are Nafion 117, Nafion 1035 and Nafion 212 with thicknesses of 183 mm, 90 mm and 50 mm, respectively with and without PVA coating. The schematic reveals a PVA film coating on both sides of Nafion as well. For the reaction of O2, H+ and e- into water, an efficient catalyst (Pt) was loaded on cathode GDL. A layer of Chrome (10 nm) followed by a layer of Gold (150 nm) was sputtered on the Si channels to take power output from the cell. Aluminum reservoirs were used for storage and continuous supply of fuel for MFC operation. We used K-type thermocouples as illustrated in Fig. 1 to measure the cell temperature.

2.1.1. Preparation of GDLs loaded with catalyst GDLs used were commercially obtained porous carbon paper (Toray). Commercially purchased black metal Pt and black Ru nano catalysts were bought from the Fuel Cell Store. At anode, nano catalyst mixture of Pt-Ru with 1:1 ratio were loaded to GDL as catalyst and at cathode Pt nanoparticle was loaded to GDL. The catalyst nanoparticle solutions were prepared by suspending the nanoparticles in isopropyl alcohol (IPA) in discrete round bottom glass flasks. The mixtures were sonicated at 25 °C for 1 h for uniform suspension of the nanoparticles in IPA. The suspensions were coated on GDLs with catalyst densities of 3.5 mg cm
2 and evaporation of solvent (IPA) was achieved by post baking process.

2.1.1.1. Composite PEM preparation. Nafion membranes were cut to a size of 1.7 cm  1.7 cm and washed with acetone, IPA, followed by a baking process at 100 °C. PVA solution was prepared as follows: 1 g of PVA powder (molecular weight 85,000–124,000, Sigma) and 5 mL of de-ionized (DI) water were taken in a glass bottle. The glass bottle was placed on a hot plate with continuously stirring at 80 °C until PVA powder dissolves completely in DI water. This solution was coated on both sides of Nafion by dip-coating method followed by drying at 100 °C. The single dip-coat introduces a coating thickness of 2 mm each on both sides of Nafion membranes. Multiple dip-coating of Nafion membranes was done to achieve PVA coating of 4 mm and 6 mm. Cross-sectional SEM representation of the PVA-coated Nafion membrane is illustrated in Fig. 2. MFCs were assembled as follows (refer Fig. 1(b)): anode-side aluminum reservoir, Cr-Au deposited Si channel, GDL (GDL surface coated with catalyst facing PEM), PEM, GDL (GDL surface coated with catalyst facing PEM), Cr-Au deposited Si channels and cathode aluminum reservoir. The assembly was carried out in a clean room (class 1000-ISO6) maintained with 50 % RH and 25 °C. The assembled MFCs have been tested to study their performance.

Please cite this article as: A. S. Rao, K. R. Rashmi, D. V. Manjunatha et al., Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.02.093

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(a)

(b) Fig 1. (a) Schematic, and (b) exploded view of MFC showing PVA coated Nafion PEM.

2.2. Uptake of water and swelling The procedure for determining the water-uptake of the composite PEMs is as follows: initial pre-baking of samples at 100 °C for 1 h on a hot plate and followed by its dry weight measurement (Wdry); soaking of the samples for 24 h in DI water so that they reach equilibrium, followed by its wet weight measurement (Wwet). By determining the difference in their weight between the wet and dry PEMs using Equation (1), the water-uptake was determined [56].

Water
uptakeð%Þ ¼

W wet
W dry  100 W dry

ð1Þ

Likewise, Equation (2) was used to calculate the swelling ratio by determining the differences in the thickness of wet and dry composite PEMs [57]. The thickness of the PEMs was measured using a micrometer.

Swelling ratioð%Þ ¼

t wet
t dry  100 tdry

ð2Þ

where t wet is the membrane sample thickness after 24 h soak in DI water and t dry is the dry membrane sample thickness. For PVA coatings of thicknesses 0 lm, 2 lm, 4 lm and 6 lm on Nafion 117 membrane, water absorption and swelling ratios were determined as shown in Table 1.

Please cite this article as: A. S. Rao, K. R. Rashmi, D. V. Manjunatha et al., Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.02.093

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3. Results and discussions 3.1. Properties of water uptake and swelling

PVA coat Nafion membrane

Fig 2. SEM image of PVA coated Nafion 117 membrane showing the cross section.

2.3. Proton conductivity Following procedure was followed to investigate how water uptake affects proton conductivity. The procedure is as follows: initial soaking of composite PEMs for 24 h in DI water at 25 °C so that they became wet followed by their resistance (R) measurement. The resistance was measured by four-probe system. Using Equation (3) the proton conductivity (r) has been calculated [9]:

r¼

t RA

ð3Þ

where t is the composite PEM thickness and A is the electrode surface area in contact with the membrane samples. 2.4. Permeation of methanol Methanol permeability of composite PEMs was calculated by standard procedure with the use of two-compartment diffusion cells [13]. The composite PEMs were stabilized by saturating them in DI water for 24 h prior to the measurements. Cell A of diffusion cell was filled with 7 M concentration of methanol solution, while cell B was filled with DI water. While experiment, both the compartments were continuously stirred. A density meter determined the rise in the concentration of methanol in DI water in cell B. The permeability of methanol (P) was determined using Equation (4) [58].

P¼

Uptake of water is the total amount of water molecules bound to the surface of membrane and interstitial sites [59]. It is mostly associated with the integral hydrophilic or hydrophobic functional chains that associates with a membrane and increases with the formation of water molecules attaching with hydrogen [60]. The absorption of water helps to transfer H+ through the membrane. However, excessive water will unpleasantly affect the mechanical stability of the membrane. Nafion 117 reported 16 ± 1.2% of the least uptake of water. In the case of Nafion 117, there exists a relationship between the built-in -SO3H groups and the –OH water groups that gives rise to uptake water [61]. Water sensitivity rises with the comparative increased abundance of hydroxide groups on the PVA-coated membrane surface resulting in its highest capacity for water absorption [59,34]. This result would suggest that the strong hydrophilic nature of PVA molecule, the PVA nanofibers within the composite membranes often swell in some degree. Similarly, the swelling reflects the same tendency as the water uptake. Swelling ratios for Nafion 117 membrane with PVA coating thicknesses 0 mm, 2 mm, 4 mm and 6 mm are found to 13 ± 0.7%, 14 ± 0.9%, 19 ± 0.4% and 23 ± 0.7% respectively, as shown in Fig. 3 (a) and are summarized in Table 1. The increase in the swelling ratio of the Nafion membrane with an increase in the thickness of PVA coating, as seen in the figure, is in good agreement with the results of water uptake.

1 DC b LV b   Ca Dt A

ð4Þ

Where methanol concentration in first cell (cell A) is C a ,DDCtb is the change in concentration of methanol in cell B (as a function of time) in the lapse of time, the volume of second cell is Vb, the surface area of composite PEMs is A and L is its thickness.

3.2. Proton conductivity Proton conductivity is an important property for PEMs of fuel cells. As mentioned earlier, proton conductivities were determined at 25 °C for Nafion 117 membrane with PVA coating thicknesses 0 lm (pristine nafion), 2 lm, 4 lm and 6 lm and were found to be 18 ± 0.7  10
2 S cm
1, 14 ± 0.9  10
2 S cm
1, 10 ± 1.1  10
2 S cm
1 and 7 ± 0.7  10
2 S cm
1, respectively, which are illustrated in Fig. 3(b). The proton conductivity of the PVA-coated Nafion composite membranes is less than that of the usual values found for pristine Nafion membranes owing to the unsulfonated PVA coating on the Nafion membrane. The PVA layer proton conductivity is about 10
10 S cm
1, which compares to those reported in the literature [61]. Proton conductivity of PVA is very weak because the sulfonic groups are absent. As a result, the measurements of conductivity have shown lower values compared to pristine Nafion. The interpretation of this action is due to the mere functionalization of PVA molecules that hardly contribute to proton conduction. However, there is some conductivity in the layer due to the presence of PVA. Had the PVA been sulfonated, the diffusion of methanol across its thickness would have risen sharply [62] and therefore the PVA coating would not have shown much resistance to methanol permeation.

Table 1 Uptake of water, swelling and conductivity of proton of PVA coated Nafion 117. Summarized is also the permeation of methanol in all the three Nafion membranes with various PVA coating thicknesses. Thickness of PVA (mm)

0 2 4 6

Water uptake (%)

16.44 ± 1.2 21.21 ± 0.9 25.74 ± 1.6 31.6 ± 1.2

Swelling ratio (%)

13.19 ± 0.7 14.2 ± 0.9 19.7 ± 0.4 23.3 ± 0.7

Methanol permeability (cm2 s
1)

Proton conductivity (S cm
1)

Nafion 212

Nafion 1035

Nafion 117

6.6 ± 0.3 3 ± 0.2 1.9 ± 0.2 1 ± 0.4

5.1 ± 0.15 2.1 ± 0.2 1.3 ± 0.4 0.7 ± 0.2

3.4 ± 0.2 1.4 ± 0.4 0.8 ± 0.3 0.4 ± 0.3

18.39  10-2 14.63  10-2 10.4  10-2 7.63  10-2

Please cite this article as: A. S. Rao, K. R. Rashmi, D. V. Manjunatha et al., Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.02.093

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Fig 3. Nafion 117 membrane (a) water uptake and swelling ratio, and (b) proton conductivity, for various PVA coating thicknesses.

3.3. Permeation of methanol Permeation of methanol that results in methanol crossover by PEMs, is a serious setback that concerns MFC efficiency [63,64]. In general, membranes with excellent proton conductivity have ionic cluster containing H+, –OH and group of sulfonic acid that favours crossing over of methanol from anode to cathode and get oxidized resulting in minimizing MFC efficiency [65]. The measurement of permeation of methanol in Nafion membrane for all the three thicknesses with numerous PVA coatings are illustrated in Fig. 4 and summarized in Table 1. The methanol permeability of pristine Nafion 212, Nafion 1035 and Nafion 117 membranes was observed to be 6.6 ± 0.3  10
6 cm2 s
1, 5.1 ± 0.1  10
6 cm2 s
1 and 3.4 ± 0.2  10
6 cm2 s
1, respectively, with the last value commonly appropriate for MFC operation. Nafion membrane coating with a PVA layer will further reduce the permeability of methanol. As shown in Fig. 4, methanol permeation in 2 lm PVA-coated membranes is decreased by about 50% relative to that of pristine Nafion membranes. 3.4. Passive MFC performance The efficiency of MFC depends heavily on how efficiently the membrane conducts protons. It also depends on how resistive it

Fig 4. Permeability of methanol in all the three Nafion membranes with various thicknesses of PVA coating.

is to methanol permeation. The conductance of proton depends on the membrane’s conductivity and thickness. The emphasis here is to reduce permeation of methanol by adding a coat of PVA on Nafion with no much loss in its proton conductivity. Our proton conductivity tests shown above demonstrate that 2 lm thick PVA coating on the Nafion membrane has a small reduction in the composite membrane’s proton conductivity (Fig. 3(b)). In fact, our study on permeation of methanol has shown that pristine Nafion with 2 lm thick PVA coating significantly reduces the composite membranes’ methanol permeability relative to pristine Nafion membranes. This means that it is possible to introduce PVA coating of thickness 2 lm on Nafion and these composite membranes can be used as PEMs. There has been extensive study of the efficiency of passive MFCs having pristine Nafion with the above said thicknesses. The polarization plots are illustrated in Fig. 5(a) and (b). Passive MFC output with three separate Nafion membranes with a thickness of 50, 90 and 183 lm with and without optimum coating (2 lm thickness) of PVA was analysed by obtaining the polarization plots shown in Fig. 5(a) and Fig. 5(b) and also outlined in Table 2. From the polarization plots in Fig. 5, it is evident that the voltage and power density rise with the Nafion thickness decrease. It is logical since the power density is highly dependent on the membrane’s proton conductance, which means greater the conductance, higher the power density. This is reflected distinctly in Fig. 5(b), where pristine Nafion 212 is 50 lm thick and displays higher power densities than pristine Nafion with 90 lm and 183 lm thicknesses. Furthermore, with 2 lm thick PVA coating of all three thicknesses on Nafion membranes, the voltages as well as the power density are very significantly improved as seen in the polarization plots. Better PVA-coated sample results were obtained for composite membranes of 50 lm and 90 lm. Improvement in the efficiency of passive MFC with PVA coating on Nafion 117 is very minimal since it has low proton conductivity (revealed in the high internal membrane resistance for proton conductivity as observed in the ohmic region of polarization plots) and low crossover of methanol without the coat. Therefore, methanol crossover through Nafion membrane with PVA coating is limited insignificantly. On the other hand, it is interesting to note that even with a small decrease in proton conductivity of 2 lm thick PVA coated 50 lm and 90 lm thick Nafion composite membranes, the voltage and power densities of passive MFCs are significantly increased. This is because of the high proton conductance of lower-thickness Nafion membranes. While 90 lm thick Nafion has high proton conductivity (ion exchange capacity, 1.01 meq g
1) relative to 50 lm thick

Please cite this article as: A. S. Rao, K. R. Rashmi, D. V. Manjunatha et al., Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.02.093

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(a)

(b)

Fig 5. Polarization plots of passive MFC showing: (a) voltage vs. current density, and (b) power density vs. current density for pristine Nafion and composite membranes with 2 lm thick PVA coated Nafion membranes as PEM.

Table 2 Description of the power densities at room temperature of passive MFCs. MEA area (cm2)

Fuel rate (mL min
1)

Methanol concentration (M)

Nafion thickness (mm)

Highest power density (mW cm
2)

References

1 0.18 4 0.39 0.39 9 0.39 4 2.89 1 2.89 0.39 2.89 2.89 2.89 5 2.89

NS 1.38 NS 1.38 2.76 NS 5.52 NS 25 1 25 8.28 25 25 25 80 25

4 3 3 3 3 2 3 3 7 2 7 3 7 7 7 2 7

183 183 183 183 183 183 183 183 183 183 183 183 90 90 50 50 50

1.7 2 2.2 2.45 2.8 3.3 3.7 3.8 4.28 4.9 5.19 5.8 11.67 19.35 23.26 26.3 30.25

[68] [69] [70] [69] [69] [70] [69] [70] Our worka [71] Our workb [69] Our workc Our workd Our worke [72] Our workf

NS- Not Specified. a PEM: pristine Nafion 117. b PEM: Nafion 117 coated with 2 lm PVA. c PEM: pristine Nafion 1035. d PEM: Nafion 1035 coated with 2 lm PVA. e PEM: pristine Nafion 212. f PEM: Nafion 212 coated with 2 lm PVA.

Nafion (ion exchange capacity, 0.95 meq g
1), the latter has higher proton conductance due to its lower thickness expressed in Fig. 5 [66,67]. The power densities achieved here are higher than those stated in the literature and summarized in Table 2 as well. The MFC power enhancement with PVA coating was demonstrated in the composite membranes by a significant reduction in the permeation of methanol. This drop of methanol permeation with PVA coating is maximal in thinner Nafion (50 lm and 90 lm thick) relative to thicker Nafion (183 lm thick) as shown in Fig. 4 and the polarization plots in Fig. 5. It is therefore of great interest to improve the power output of passive MFCs by pristine Nafion membranes with optimal PVA coating as PEMs. 4. Conclusion In the present work, a new PEM is prepared by coating Nafion membranes with optimal PVA coating thickness. For Nafion

membranes with various PVA coating thickness, the uptake of water, swelling and proton conductivity were determined. Studies on water uptake have showed that the composite membranes tend to absorb more water as the PVA coating thickness on Nafion increases. Further, the studies on swelling ratio have also shown that the composite membranes swell in water as the PVA coating thickness in Nafion membrane increases. The trend of swelling ratio matches that of water uptake. These results conclude that the hydrophilicity of composite membrane increases as the PVA coating thickness on Nafion membrane. Proton conductivity of PVA composite membranes is less than that of pristine Nafion membrane which is due to the absence of sulfonic groups in PVA layer. In addition to this, the composite membranes were tested for methanol permeability for various PVA coating thicknesses. It was seen that even with 2 mm thick coat of PVA on Nafion membrane, the methanol permeability reduced significantly by 50%. Hence, these studies show that the optimum coating thickness of

Please cite this article as: A. S. Rao, K. R. Rashmi, D. V. Manjunatha et al., Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.02.093

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PVA is 2 mm since the proton conductivity drop and swelling of composite membrane is least. The effect on passive MFC output of 2 lm thick PVA layer on Nafion membranes was tested by obtaining 25 °C polarization plots with 25 lL min
1 methanol flow at anode and air-breathing at cathode. Regarding passive MFCs with all three composite PEMs with 2 lm PVA layer, a very substantial improvement in both voltage and power density was observed. This is the first time that such an improvement in the power density of passive MFCs with unsulfonated PVA layer on Nafion membrane is accomplished. This was demonstrated by a significant drop in the permeability of methanol and thus crossover of methanol in composite PEMs contributing to a very substantial power density improvement. This finding of increasing power density on Nafion membranes by reducing permeation of methanol using optimal 2 lm PVA layer thickness is very favorable. The process of coating Nafion (i.e. dip-coating process) with PVA is very straightforward as this process requires no specialized tool. CRediT authorship contribution statement Arjun Sunil Rao: Conceptualization, Methodology, Validation, Investigation, Data curation, Writing - original draft. K.R. Rashmi: Methodology, Investigation, Data curation. D.V. Manjunatha: Investigation, Validation. A. Jayarama: Resources, Formal analysis. V. Veena Devi Shastrimath: Formal analysis. Richard Pinto: Project administration, Supervision, Visualization, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We thank Indian Nano-electronic Users’ Program of IIT Bombay, Tata Institute of Fundamental Research, Mumbai and MEMS lab of Alva’s Institute of Engineering and Technology, Moodbidri (affiliated to Visvesvaraya Technological University, Belagavi) for their generous support during the progress of this research. References [1] F. Bresciani, A. Casalegno, J.L. Bonde, M. Odgaard, R. Marchesi, A comparison of operating strategies to reduce DMFC degradation, Int. J. Energy Res. 38 (1) (2014) 117–124. [2] D’Urso C, Baglio V, Antonucci V, Arico‘ AS, Specchia S, Icardi UA, Saracco G, Spinella C, D’Arrigo G. Development of a planar lDMFC operating at room temperature. Int J hydrogen energy 2011;3(6):8088-8093. [3] D.S. Falcao, V.B. Oliveira, C.M. Rangel, A.M.F.R. Pinto, Experimental and modeling studies of a micro direct methanol fuel cell, Renewable Energy 74 (2015) 464–470. [4] Arjun Sunil Rao, K.R. Rashmi, D.V. Manjunatha, A. Jayarama, R. Pinto, Enhancement of power output in passive micro direct methanol fuel cells with optimized methanol concentration and trapezoidal flow channels, J. Micromech. Microeng. 29 (2019) 075006. [5] D.S. Falcao, J.P. Pereira, C.M. Rangel, A.M.F.R. Pinto, Development and performance analysis of a metallic passive micro-direct methanol fuel cell for portable applications, Int. J. Hydrogen Energy 40 (15) (2015) 5408–5415. [6] J.P. Esquivel, T. Senn, P. Hernández-Fernández, J. Santander, M. Lörgen, S. Rojas, B. Löchel, C. Cané, N. Sabaté, Towards a compact SU-8 micro-direct methanol fuel cell, J. Power Sources 195 (24) (2010) 8110–8115. [7] S.-S. Hsieh, I.-C. Chen, C.-H. Hwong, Development of a four-cell DMFC stack with airbreathing cathode and dendrite flow field, Int. J. Energy Res. 38 (13) (2014) 1693–1711. [8] M.A. Rafe Biswas, Robinson D. Melvin, Prediction of direct methanol fuel cell stack performance using artificial neural network, J Electrochem. Energy Conv. Storage 14 (3) (2017) 031008.

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Please cite this article as: A. S. Rao, K. R. Rashmi, D. V. Manjunatha et al., Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.02.093