Reorientation of Magnetic Graphene Oxide Nanosheets in Crosslinked Quaternized Polyvinyl Alcohol as Effective Solid Electrolyte

Jia-Shuin Lin, Wei-Ting Ma, Chao-Ming Shih, Bor-Chern Yu, Li-Wei Teng, Yi-Chun Wang, Kong-Wei Cheng, Fang-Chyou Chiu, Shingjiang Lue
2016 Energies  
This work aims to clarify the effect of magnetic graphene oxide (GO) reorientation in a polymer matrix on the ionic conduction and methanol barrier properties of nanocomposite membrane electrolytes. Magnetic iron oxide (Fe 3 O 4 ) nanoparticles were prepared and dispersed on GO nanosheets (GO-Fe 3 O 4 ). The magnetic GO-Fe 3 O 4 was imbedded into a quaternized polyvinyl alcohol (QPVA) matrix and crosslinked (CL-) with glutaraldehyde (GA) to obtain a polymeric nanocomposite. A magnetic field was
more » ... magnetic field was applied in the through-plane direction during the drying and film formation steps. The CL-QPVA/GO-Fe 3 O 4 nanocomposite membranes were doped with an alkali to obtain hydroxide-conducting electrolytes for direct methanol alkaline fuel cell (DMAFC) applications. The magnetic field-reoriented CL-QPVA/GO-Fe 3 O 4 electrolyte demonstrated higher conductivity and lower methanol permeability than the unoriented CL-QPVA/GO-Fe 3 O 4 membrane or the CL-QPVA film. The reoriented CL-QPVA/GO-Fe 3 O 4 nanocomposite was used as the electrolyte in a DMAFC and resulted in a maximum power density of 55.4 mW·cm −2 at 60 • C, which is 73.7% higher than that of the composite without the magnetic field treatment (31.9 mW·cm −2 ). In contrast, the DMAFC using the CL-QPVA electrolyte generated only 22.4 mW·cm −2 . This research proved the surprising benefits of magnetic-field-assisted orientation of GO-Fe 3 O 4 in facilitating the ion conduction of a polymeric electrolyte. (such as Tokuyama A210) [4] [5] [6] or hydroxide-conducting membranes [7-9] provide advantages over proton-exchange membranes (PEMs), such as lower-cost membranes, reduced methanol cross-over, easy water management, and non-platinum catalyst [10] [11] [12] . Several nanofillers have been blended into membrane electrolytes to enhance the ionic conductivity. Huang et al. fabricated a polyvinyl alcohol (PVA) composite containing modified carbon nanotubes (m-CNTs) and showed that PVA/m-CNTs doped with 6 M potassium hydroxide (KOH) had a conductivity value 51.9% higher than that of PVA [13] . Lue et al. [14] incorporated fumed silica (FS) into a PVA membrane and formed a PVA/20% FS composite. The membrane had a higher conductivity than PVA (0.058 S·cm −1 vs. 0.018 S·cm −1 , respectively). Yang et al. [15] prepared a quaternized polyvinyl alcohol/alumina (QPVA/Al 2 O 3 ) nanocomposite polymer membrane and the membrane had an excellent electrochemical performance compared with pristine QPVA. In our previous publications, we decorated graphene oxide-iron oxide (GO-Fe 3 O 4 ) on QPVA polymer to form an electrolyte membrane. The QPVA/GO-Fe 3 O 4 membrane had better cell performance (172 mA·cm −2 vs. 51 mA·cm −2 ) and conductivity (0.0305 S·cm −1 vs. 0.0159 S·cm −1 ) over pristine QPVA [16] . Membranes incorporating nanofiller can increase the polymer free volume, and this might increase the ionic diffusivity within the membranes, which further enhances conductivity [17] . Many nanofillers in polymeric membranes can suppress methanol crossover in DMFCs. For example, iron oxide (Fe 3 O 4 ) decorated CNTs in PVA membrane showed decreased methanol permeability [10]. Yuan et al. [18] layered poly(diallyldimethylammonium chloride) (PDDA) and GO nanosheets onto the surface of Nafion membrane. The methanol permeation across the composite membrane decreased in comparison to the pristine Nafion. Yang et al. [19] fabricated PVA/TiO 2 membranes with a permeability values of the order of 10 −8 cm 2 ·s −1 , which is one order of magnitude smaller than that of a PVA membrane. Recently, GO has gained substantial attention for its noticeable thermal [20], mechanical and barrier properties [21]. GO is exfoliated from graphite [22-25] and it has been used in many applications, including supercapacitors [26], water treatment [27], Li ion batteries [28], and fuel cells [29-35]. Nair et al. [21] and Paneri et al. [36] proved that GO membranes can successfully block methanol, ethanol and hexane permeation while allowing water diffusion. Lin and Lu [37] reported that the methanol permeability of hot-pressed Nafion-GO decreased 41% relative to the pristine Nafion. Yuan et al. [18] reported that a GO-coating on a Nafion membrane lowered its methanol permeability. This indicates that GO can suppress alcohol crossover in membranes. However, some researchers indicate that the addition of GO filler decreased the composite's conductivity [22, 37, 38] , whereas some determined that GO could increase the ion conductivity [39] . In our previous work, QPVA membranes were fabricated and applied in a direct methanol alkaline fuel cell (DMAFC) as membrane electrolytes [5, 40] . With the introduction of quaternary ammonium groups (-N + (CH 3 ) 3 ) onto the PVA matrix, QPVA exhibits anion-exchange functional groups [40] and better conductivity than pristine PVA [15] . These functional groups facilitate hydroxide ion (OH − ) transport through the membrane via the Grotthuss mechanism [41] . We also demonstrated that KOH-doped QPVA possessed excellent chemical stability in the Fenton test and maintained a high open circuit voltage (OCV) in a long-term DMAFC test (~230 h) [40] . Controlling the GO orientation in the polymer matrices is critical for better methanol suppression through increase of the apparent aspect ratio of the GO [22]. Reducing the methanol permeability directly lowers the cell over-potential and improves the cell voltage and power density [42] . With the objective of decreasing methanol crossover and improving conductivity, we prepared a well-aligned, CL-QPVA/GO-Fe 3 O 4 membrane using an external magnetic field. Magnetic Fe 3 O 4 nanoparticles were decorated onto GO nanosheets (GO-Fe 3 O 4 ). Guo et al. [43] reported that ultrafiltration (UF) membranes containing magnetic particles were prepared with a magnetic field and demonstrated that this magnetic-field-assisted orientation can be achieved at an industrial scale. The resulting GO-Fe 3 O 4 nanofillers were blended with QPVA polymer solution and cast into films. A magnetic field was applied to re-orient the nanofillers before the membranes solidified. To improve the stability of the
doi:10.3390/en9121003 fatcat:guqjlps2afcxdixh72tbonnxnm