Development of Sulfonated Poly(vinyl alcohol)/MoS₂-Based Robust Composite Proton Exchange Membranes with Higher Selectivity

Abstract

Selectivity is an essential property of proton exchange membranes. It is the ratio of proton conductivity to methanol permeability. In this study, we prepared mechanically robust composite proton exchange membranes (CPEMs) based on sulfonated poly(vinyl alcohol) (SPVA) and exfoliated molybdenum disulfide (MoS 2 ) via solution casting. MoS 2 nanoplatelets act as reinforcing fillers and enhance the barrier effect toward methanol permeation. The MoS 2 loadings (wt %) were varied in the CPEMs to achieve higher selectivity by either enhancing the proton conductivity or reducing the methanol permeability of the membranes. To assess their physicochemical properties, the synthesized CPEMs were characterized for their water uptake, ion exchange capacity, proton conductivity, methanol permeability, and thermomechanical stabilities. The SPVA/MoS 2 -0.3% membranes with 0.3 wt % of MoS 2 loading showed a proton conductivity of 48 ± 7 mS/cm, which is the highest among all of the CPEMs and 1.8 times higher than that of SPVA tested at 25 °C and 98% RH. Due to the presence of MoS 2 nanoplatelets, the CPEMs offered better resistance to methanol cross-over compared to pristine SPVA membranes. The SPVA/MoS 2 -0.3% membranes exhibit a methanol permeability of 1.72 × 10 −6 cm 2 /s at 25 °C, resulting in 5 and 37% reductions in methanol cross-over and 68 and 80% increase in selectivity than pristine SPVA and Nafion 117 membranes, respectively. Moreover, the SPVA/MoS 2 -0.3% membranes provided good mechanical stability with an enhanced proton conductivity of 54 ± 5 mS/cm at 60 °C and 98% RH. Post-cross-linking of the SPVA/MoS 2 -0.3% membranes with glutaraldehyde was done to improve further their chemical, thermal, and mechanical stabilities for higher-temperature applications. The cross-linked CPEMs of 24X3.0SPVA/MoS 2 -0.3% exhibit considerably higher selectivity at elevated temperatures (ca. 80 °C) and 37 times higher selectivity than Nafion 117 at 60 °C. The above results indicate that the as-prepared SPVA/MoS 2 -0.3% and 24X3.0SPVA/MoS 2 -0.3% CPEMs have great potential for direct methanol fuel cell (DMFC) applications at moderately high temperatures.

Construction of Successive Proton Conduction Channels to Accelerate the Proton Conduction Process in Flexible Proton Exchange Membranes

“…It was speculated that the redistribution of PA molecules and the rearrangement of Kevlar nanofibers with SEBS polymer molecular chains resulted in reinforcement on the resistance to methanol molecule permeation. From Figure , the p values were comparable with those of the modified Nafion membranes, such as 1.54 × 10 –6 cm 2 /s for Nafion 212, 1.76 × 10 –6 cm 2 /s for SPVA/MoS 2 -0.3%, 2.0 × 10 –6 cm 2 /s for Nafion/CAW, and 2.75 × 10 –6 cm 2 /s for N-PVA/HWP20, even lower than 4.5 × 10 –6 cm 2 /s for commercial Nafion-117, but higher than 4.29 × 10 –7 cm 2 /s for (PDDA/PM)­SF 2 , 6.23 × 10 –7 cm 2 /s for SPEEK/PVA, 6.28 × 10 –7 cm 2 /s for PEMG-0.3, and 5.12 × 10 –7 cm 2 /s for SPP-4 …”

“…From Figure , the β values were 6.38 × 10 4 S s/cm 3 for (K/PA/S) 3 , 1.91 × 10 4 S s/cm 3 for (K/PA/S) 3 -f, and 3.48 × 10 4 S s/cm 3 for (K/PA/S) 3 -50. Additionally, the reported β values were 3.1 × 10 4 S s/cm 3 for SPVA/MoS 2 -0.3%, 6.9 × 10 4 S s/cm 3 for Nafion/CAW, 2.25 × 10 4 S s/cm 3 for N-PVA/HWP20, 0.95 × 10 4 S s/cm 3 for commercial Nafion-117, 0.87 × 10 4 S s/cm 3 for (PDDA/PM)­SF 2 , 4.17 × 10 4 S s/cm 3 for SPEEK/PVA, 5.06 × 10 4 S s/cm 3 for PEMG-0.3, and 9.84 × 10 4 S s/cm 3 for SPP-4 . Although the prepared composite membranes illustrated a higher methanol permeability, these membranes possessed higher β values, resulting from higher proton conductivity.…”