Uncovering the Structure of Nafion–SiO₂ Hybrid Ionomer Membranes for Prospective Large‐Scale Energy Storage Devices

Abstract

4064 wileyonlinelibrary.com thermal stability. [ 5 ] However, Nafi on suffers from low ion selectivity which leads to vanadium crossover in the VRFB and decreases energy effi ciency of the battery over time. [ 3 ] There have been several efforts to combat the low ion selectivity of Nafi on for use as the IEMs in VRFBs. These include creating Nafi on blends with other polymers, [ 6 ] incorporating inorganic fi llers, [ 3,7,8 ] layered composites at interfaces, [9][10][11] as well as synthesizing entirely new classes of Nafi on-inspired membranes. [ 2 ] More specifi cally, sol-gel derived Nafi on-SiO 2 and Nafi on-Si/TiO 2 hybrid membranes were observed to signifi cantly reduce the permeability of vanadium ions and water, while minimally impacting ion exchange capacity (IEC) and proton conductivity, thus improving battery performance. [ 3,7 ] It has been suggested that these fi llers selectively incorporate into the ion-conducting domains of Nafi on, effectively altering (or disrupting) the ion/water transport domains. It is thought that creation of SiO 2 in the ion transport channels acts to reduce vanadium crossover by a "size exclusion" mechanism, impeding the transport of vanadium ions and creating a more ion-selective medium with better crossover resistance. There have been many studies of this type of Nafi on-SiO 2 nanocomposite using infrared [12][13][14][15][16][17] and/or nuclear magnetic resonance (NMR) [ 14,15,17 ] spectroscopy to identify the appropriate mechanism and gain a deeper understanding of the local environment of the SiO 2 and Nafi on phases. These spectroscopic investigations have provided great insight into the local interactions of the inorganic nanoparticles and the Nafi on; however, there is limited information on the nanostructure of these Nafi on-SiO 2 composite membranes.Recently, Ladewig et al. [ 18 ] published a structural characterization study of Nafi on-SiO 2 nanocomposite membranes created using this sol-gel chemistry approach. They employed both small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) to resolve the structure of the ionic domains in the Nafi on composite membranes. While little difference was observed between the X-ray scattering curves of the neat and modifi ed Nafi on, SANS showed a broadening and reduction in intensity of the ionomer peak, as well as a slight shift in the ionomer peak position to higher q (i.e., reduction in the d-spacing of this correlation peak, which can be indicative of Nafi on nanocomposite membranes are attractive candidates for the ion conducting phase in energy storage devices such as vanadium redox fl ow batteries. Herein, vanadium crossover and Nafi on-SiO 2 nanostructure are quantifi ed in a series of hybrid membranes created via solution-casting Nafi on fi lms with discrete SiO 2 nanoparticles, as well as membranes created using an in situ silica sol-gel condensation process. The crossover of vanadium ions is suppressed in all Nafi on membranes with the SiO 2 inorganic phase when compared to unannealed, neat N...

The Structure–Activity Relationship in Membranes for Vanadium Redox Flow Batteries

“…Introducing inorganic dopants into Nafion membrane will decrease the size of ionic regions or ion transfer channels and thus effectively reducing the transference of vanadium ions via size exclusion mechanism. However, a different result was reported by Davis' group . They compared the performance of various Nafion/SiO 2 hybrid membranes prepared using sol–gel method and solution‐casting method with dispersing discrete SiO 2 nanoparticles.…”

“…However, the proton transport is also blocked as the fillers act as physical barriers for the proton diffusion. [25] They compared the performance of various Nafion/SiO 2 hybrid membranes prepared using sol-gel method and solution-casting method with dispersing discrete SiO 2 nanoparticles. [11] In order to improve the performance of membrane without losing the proton conductivity, ultrathin polytetrafluoroethylene (PTFE)/Nafion/silica composite membranes (≈25 µm) were prepared via an in situ sol-gel method.…”

“…However, a different result was reported by Davis' group. [25] They compared the performance of various Nafion/SiO 2 hybrid membranes prepared using sol-gel method and solution-casting method with dispersing discrete SiO 2 nanoparticles. The results showed that the discrete SiO 2 nanoparticles in solution-casting membranes were too large to fill into the ionic domains of Nafion.…”

Unveiling the multiscale morphology of chemically stabilized proton exchange membranes for fuel cells by means of Fourier and real space studies

“…The hybridization strategy, however, can provide better control over the nanostructure and functional properties. 12 To name a few, one can cite: (i) an improvement of water retention with hygroscopic particles as ZrP, SiO 2 , TiO 2 , ZrO 2 (precipitated within pre-formed PFSA and sPEEK membranes or in the polymer dispersion 13,14 ), (ii) an enhancement of the mechanical properties with SiO 2 llers [15][16][17][18] for PEMPC, (iii) a reduction of vanadium crossover for vanadium redox ow batteries (again SiO 2 llers), 19 (iv) or a mitigation of the chemical degradation (cerium-NPs). 20 In addition, the aforementioned issues like the dispersion or elution of llers/nanocharges can be overcome if a SG network is grown in situ in a host ionomer membrane instead of dispersing inorganic particles into an ionomer solution previous to membrane casting.…”

Development of perfluorosulfonic acid polymer‐based hybrid composite membrane with alkoxysilane functionalized polymer for vanadium redox flow battery

“…On the other hand, further developments on fluoro‐based composite membranes are still underway for achieving high performance due to good proton transportation and high chemical stability. To overcome the vanadium permeation–related issues in Nafion membranes, inorganic nano materials such as TiO 2 , SiO 2 , mesoporous silica, silica PWA, silica‐ZrO 2 , ZrO 2 nanotube, and graphene oxide have been introduced into the Nafion polymer matrix as additives. However, the performance and efficiency of these systems are inadequate because of dispersion of inorganic additives in the Nafion polymer matrix .…”