Currently, direct methanol fuel cells (DMFC) represent an inventive alternative for current power sources among the other fuel cells (FCs) types. DMFC has significant importance in recent few years because of its low operating temperature, high-power density, high energy conversion efficiency and become less expensive fuel [1]. Rather, DMFC is considered one of the important tools that take a part in turning to carbon-neutral operations [2].
The electrolytic membrane is the operating heart of the cell, it is responsible for conducting protons (H+) from the anode part to the cathode. Some of the important characteristics of the polyelectrolytic membrane are good conductors for protons (H+), electrically insulators, very low methanol cross-over chemically stable, and withstand during cell operating conditions [3].
However, a major restriction of DMFC is the dropout of methanol through the electrolytic membrane and the production of highly reactive polymeric electrolytes with optimum water management. Nowadays Nafion® is widely used as an electrolyte in polymer electrolyte membrane fuel cell (PEMFC) but is limited by methanol crossover and water management problems [4, 5].
The polyelectrolytic membrane can be produced using different types of polymers after the modification process to enhance the desired character and improves their ionic conductivity. Modification can be done through blending and/or chemical treatment [6]. Introducing inorganic materials such as HAP [7], CNT [8] or GO [9] with polyelectrolytic membrane leads to improving protonic conductivity and mechanical stability in addition to enhancing barrier effect which is the important property of preventing fuel cross-over [10] .
For more effectiveness, functionalization of the inorganic blend is the key factor to producing a filler with high surface active (interfacial area), good compatibility and prevented sedimentation or agglomeration during the polyelectrolytic membrane production process [11]. CNTs are one of the most attractive materials as inorganic membrane fillers owing to their high aspect ratio and surface area with low density. It has a limited dispersion in the polymeric membrane owing to its low functional groups. The polymeric blend can be improved by the addition of CNTs or modified CNTs through the functionalization of CNTs with -COOH, -OH, or -SO3H groups [12, 13]. Not only mechanical or thermal stability improved but also ionic conductivity growing as a result of micro-channels provided by polymer blend around CNTs. Rambabu et al. [14] discussed modifying CNTs by introducing -SO3H groups using polystyrene sulfuric acid to prepare the SPEEK nanocomposite membrane. The protonic conductivity and mechanical stability were improved particularly with increasing of sulfonation process rather than overall performance [10].
Another attractive filler is the activated carbon (AC) molecular sheet. The advantage of AC is attributed to its excellent thermal [9], mechanical [15] and transport properties. However, AC is difficult to exfoliate in a polymer matrix due to its high cohesive force between them. Thus, AC is functionalized with other groups such as (-SO3H) to impart specific features and interactions [16].
Rahul et al. [17] stated that, the salt incorporation in polymeric membrane produces relatively higher proton conductivity (~ 10− 5 to ~ 10− 3 S/cm), flexible geometry and electrochemical stability. The benefit of protonic conducting components is their advanced dynamics for the proton association. The protonic conducting blends as (NH4+) are good proton conducting sources in the development of some types of polymeric membranes [18]. Leena et al. [19] provided that NH4SCN can rising the (H+) transferred across cell parts through the polymeric membrane. several studies explained that [20] the transportation based on polymeric ammonium complexes system is mainly due to NH3-H+.
PVA-ZnO based polyelectrolyte DMFC membrane was prepared by Diyan Ul Imaan [21] through impregnating zinc oxide nanoparticles in a cross-linked PVA matrix. The obtained membrane possesses high water uptake (105%), IEC (0.78 meq/g) and low ionic conductivity (3.9 mS/cm). In the same way, Bijay P. Tripathi and Vinod K. Shahi [13] prepared N-p-carboxy benzyl chitosan- silica- PVA proton exchange membrane for DMFC by sol-gel method. The obtained membrane exhibits low ionic conductivity as a result of weak proton conducting carboxylic groups. The conductivity and stability improved through the introduction of high charge density (-SO3H) groups. Asnag et al. [22] successfully prepared PVA-CMC based nanocomposite film via a solution casting technique for electrical and optical properties. Polyacrylamide/Nafion electrolyte membrane [4] shows an increase in water and methanol uptake with raising the amount of Polyacrylamide in the membrane. In contrast, the IEC shows decreasing by inserting Polyacrylamide into Nafion®.
Therefore, in this work, the research was conducted to find an alternative membrane for DMFC based on sulfonated CMC and PVA. Due to the non-toxicity, cheapness, biodegradability, and good matrix forming of CMC and PVA, they are selected for this study. Acrylamide (AA) blend is added to improve the ionic conductivity and facilitate proton transportation across the membrane through the formation of positively charged ammonium (NH4+) ions [4, 23]. The blending of CMC with PVA and AA followed by sulfonation using chlorosulfonic acid (CSA) with the addition of carboxylated carbon nanotube (CCNT) or sulfonated activated carbon (SAC) as fillers to improve the blocking methanol crossover and enhance the IEC and proton transportation property of the membrane with enhanced electrochemical performance.
Modification of CMC/PVA/AA based membranes through introducing of CCNT and SAC are providing reinforcement and will give a winding path for methanol cross-over through the membrane in addition maintain water swell-ability which may be overcome the methanol-crossover and water management fuel cell problems [24]. Furthermore, the introducing of CCNT and SAC can improve the mechanical and thermal stability in addition to its involvement to enhance the ionic conductivity by providing a large interfacial area and ionic channels.