Role of Surface Chemistry on Nanoparticle Dispersion and Vanadium

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Applications of Polymer, Composite, and Coating Materials

Role of Surface Chemistry on Nanoparticle Dispersion and Vanadium Ion Crossover in Nafion Nanocomposite Membranes Allison Jansto, and Eric M Davis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11297 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 30, 2018

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ACS Applied Materials & Interfaces

Role of Surface Chemistry on Nanoparticle Dispersion and Vanadium Ion Crossover in Nafion Nanocomposite Membranes Allison Jansto and Eric M. Davis* Department of Chemical and Biological Engineering, Clemson University, Clemson, South Carolina 29634 * To whom correspondence should be addressed; E-mail: [email protected]

KEYWORDS: Nafion; nanocomposite membranes; ion permeability; nanoparticles; dispersion state ABSTRACT: While the introduction of nanoparticles into Nafion membranes has proven to be a viable method to tune the ion selectivity in energy storage technologies such as the vanadium redox flow battery (VRFB), there still remains a limited understanding of the fundamental mechanism by which the nanoparticles selectively restrict ion crossover. Herein, the surface chemistry and loading of SiO2 nanoparticles (SiNPs) were systematically varied to elucidate the relationship between nanoparticle dispersion (or dispersion state) and vanadium ion permeability in Nafion nanocomposite membranes. Specifically, nanoparticle surface functionalization was altered to achieve both attractive (amine-functionalized) and repulsive (unfunctionalized and sulfonic acid-functionalized) electrostatic interactions between the SiNPs and the ionic groups of Nafion. At a nanoparticle loading of 5 wt%, membranes containing unfunctionalized and aminefunctionalized SiNPs demonstrated ~25% reduction in vanadium ion permeability as compared to unmodified Nafion. Drastically different dispersion states were observed in the electron microscopy images of each nanocomposite membrane, where most notably, aggregates on the order of 500 nm were observed for membranes containing amine-functionalized SiNPs (at all nanoparticle loadings). Results from this work indicate that both the dispersion state and surface

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chemistry of the SiNPs play a critical role in governing the vanadium ion transport in these ionomer nanocomposite membranes.

Introduction Polymer electrolyte membranes (PEMs) are ubiquitous in energy storage devices such as the vanadium redox flow battery (VRFB),1 a promising grid-scale storage technology for alternative energy sources such as wind and solar.2,3 The current state-of-the-art PEM utilized in VRFBs is Nafion, a perfluorosulfonic acid ionomer with a hydrophobic polytetrafluoroethylene (i.e., Teflon) backbone and pendant perfluoroalkyl ether side chains terminated with hydrophilic sulfonic acid (SO3H) groups.3–5 The Teflon backbone provides chemical and mechanical stability under the highly acidic and oxidative conditions of VRFBs,6 while the SO3H groups coalesce in the presence of water to form ionic channels that facilitate proton conduction. That is, Nafion nanophase segregates into a hydrophobic Teflon region and a hydrophilic ionic region.7,8 One standing issue with Nafion is high vanadium ion permeability (i.e., crossover of vanadium ions between electrolyte solutions), which decreases the lifetime and efficiency of VRFBs.4,9 The electrolyte solutions on either side of the Nafion membrane contain only vanadium ions of distinct valence states, where the concentration of the different valence states in the electrolyte solutions governs the electric potential across the membrane. Crossover of vanadium ions from the anolyte to the catholyte (and vice versa) reduces the overall potential of the flow battery. Therefore, minimizing crossover of these ions is paramount to long-term battery operation (i.e., 10,000+ cycles).10 Various modifications to Nafion have been explored to address vanadium ion crossover, including blending Nafion with other polymers,11 layer-by-layer composites of other polymers with Nafion,12,13 and incorporating inorganic fillers to form Nafion nanocomposites.9,14,15 Nafion

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nanocomposites are of particular interest as these materials have been shown effective at reducing methanol crossover in direct methanol fuel cells (DMFCs).16–19 Analogous to the issues involving vanadium ion crossover, methanol crossover results in degraded fuel cell performance.20 The incorporation of inorganic nano-fillers into Nafion such as titania (TiO2), silica (SiO2), and other metal oxides have been studied extensively for DMFC applications,18 from which two common synthesis routes have emerged: sol-gel and solution casting. In the former, a condensation process utilizes nanoparticle precursors that react in situ to create nanoparticles inside an already-formed (extruded) Nafion membrane.21,22 In solution casting, a dense nanocomposite membrane is formed by casting of a solution/dispersion of polymer (in this case Nafion) and nanoparticles and allowing the solvent to fully evaporate.23–25 Relative to unmodified Nafion membranes, Nafion-SiO2 and Nafion-Si/TiO2 nanocomposite membranes have shown substantially reduced vanadium ion crossover.9,14 Using the sol-gel method, Teng et al.14 showed that the inclusion of Si/TiO2 resulted in a ten-fold reduction in vanadium ion crossover, from a value of 36.9 × 10-7 cm2/min in unmodified Nafion 117 (extruded Nafion membrane) to a value of 4.3 × 10-7 cm2/min in the Nafion-Si/TiO2 nanocomposite membranes. In work by Trogadas et al.4 on solution-cast Nafion membranes, it was shown that the inclusion of SiO2 nanoparticles (SiNPs) resulted in a five-fold reduction in vanadium ion crossover, from a value of 30 × 10-9 cm2/sec in unmodified Nafion to a value of 6.2 × 10-9 cm2/sec in Nafion membranes containing 20 wt% SiNPs. However, to date, it remains unclear how these nano-fillers act to alter the vanadium ion crossover in Nafion nanocomposite membranes. Similar to what has been speculated regarding the success of Nafion nanocomposites at reducing methanol crossover in DMFCs,25–27 it has been hypothesized that the nanoparticles reside in the ion-conducting domains of Nafion where they

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block the crossover of larger, hydrated vanadium ions via size exclusion.9,14,28,29 Recent findings from small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) call this hypothesis into question as they have shown that the NPs in sol-gel nanocomposite membranes are often significantly larger than the hydrophilic channels of Nafion.3,30 In addition, studies employing solution-cast nanocomposite membranes have utilized nanoparticles with diameters several times larger than the ionic domains of Nafion.3,31 Such findings underscore the need for more systematic investigations on the relationship between nanoparticle surface chemistry, dispersion, and vanadium ion crossover. The effect of nanoparticle dispersion (or dispersion state) within Nafion (‘host’ polymer) has yet to be fully interrogated in its relation to ion selectively for both solution-cast and sol-gel nanocomposite

membranes.

Poor

dispersion

states

(i.e.,

high

degree

of

agglomeration/aggregation of NPs in the membrane) can indicate unfavorable interactions between the nanoparticles and the host polymer, whereas a good dispersion state (i.e., low degree of agglomeration/aggregation of NPs in the membrane) is indicative of favorable interactions between the two immiscible phases. Surface functionalization of NPs has been explored as a means to tune the compatibility of the non-miscible NP phase within the host polymer and, in turn, alter the resultant dispersion state.32–35 Further, in the case of Nafion, NP surface functionalization may also influence the location of the nanoparticles within the hydrated nanophase-segregated membranes. That is, depending on the particular NP surface chemistry, it may be more energetically favorable for particles to reside in either the hydrophobic or hydrophilic domain.36,37 Composite membranes have also been created with an immiscible embedded phase by utilizing fillers that interact weakly with the polymer through van der Waals or hydrogen

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bonding forces.38–40 Of direct relevance to this study, acid-functionalized particles have been demonstrated as having good acid stability and hydrophilicity in Nafion, as well as improving proton conductivity of the Nafion composite membrane.26,41 For non-charged systems (i.e., systems involving neutral polymers, not ionomers like Nafion), numerous studies have shown success in improving the dispersibility of NPs in a polymer matrix by grafting brushes of the analogous polymer to the surface of the NPs.32–35 For example, Green et al.35 demonstrated the ability to directly control the wetting of poly(dimethysiloxane) (PDMS) on SiNPs by varying both the density of PDMS chains grafted onto the SiNPs, as well as the molecular weight of the PDMS matrix. This study seeks to elucidate the relationship between dispersion state and vanadium ion crossover of solution-cast Nafion nanocomposites by systematically varying the surface chemistry and NP loading of the SiNPs. Solution casting enables direct control over the size and surface chemistry of the NPs prior to incorporation into the Nafion matrix. To influence interactions between the SiNPs and Nafion, the NP surface chemistry was altered to direct electrostatic interactions (either repulsive or attractive) with the sulfonic acid end groups of the ion-conducting phase in Nafion. Surface functionalization was confirmed using Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy in combination with zeta potential. In addition, vanadium ion crossover through the various nanocomposite membranes was measured by ultraviolet-visible (UV-vis) spectroscopy. Finally, the dispersion state of nanoparticles in the final nanocomposite membranes was characterized with TEM.

Experimental Materials. Ethanol (200 proof, anhydrous), sulfuric acid (H2SO4) (95–98% ACS Reagent), fuming sulfuric acid (reagent grade), Nafion stock solution (Nafion® DE 2021, 20 wt% in

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mixture of lower aliphatic alcohols and water), magnesium sulfate (MgSO4) (anhydrous, ReagentPlus, ≥99.5%), vanadium (IV) oxide sulfate hydrate (VOSO4) (97%), and tin (II) chloride (SnCl2) (reagent grade, 98%) were purchased from Sigma Aldrich. Methyl isobutyl ketone (MIBK) (certified ACS solvent), sodium hydroxide (NaOH, ACS certified), and phenolphthalein

were

purchased

from

Fisher

Scientific.

(3-

trimethoxysilylpropyl)diethylenetriamine (TMSDEA) was purchased from Gelest. Glycidyl phenyl ether (GPE) (99%)was purchased from Acros Organics, and hydrochloric acid (HCl) (3.0N) and sodium chloride (NaCl, ACS certified) were purchased from VWR Analytical. Unfunctionalized silica nanoparticles (SiNPs) (colloidal silica in methanol; MTST grade; ,avg = 11 nm) were obtained from Nissan Nanomaterials. Deionized water (DI, resistivity ≈18 MΩ⋅cm) was used for all synthesis procedures and vanadium crossover experiments. Phenyl-SO3 Nanoparticle Functionalization. Using a method previously developed,41 500 mg of SiNP was dried and re-dispersed as a 15 wt% solution in MIBK via sonication. GPE (1.25 g) was added to the NP suspension and the solution was briefly sonicated. Following this, 0.100 mol% (relative to GPE) SnCl2 catalyst was added and the solution was magnetically stirred for 3 hours at 140 °C. Subsequently, the nanoparticle solution was cooled, centrifuged for 20 minutes at 13,000 rpm, and then washed with DI water twice. The wet NPs were treated with fuming sulfuric acid (1.63 g) at room temperature, and the solution was stirred for 18 hours. The reaction was ceased by pouring the NP solution into ice water bath. The precipitate was collected by centrifugation (30 minutes at 14,500 rpm) and washed with water. After washing, the nanoparticles were dried under dynamic vacuum for 2–4 hours at 100 °C. After this, the oven was turned off, and the nanoparticles were left under static vacuum to cool to room temperature.

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Tri-Amine Nanoparticle Functionalization. 150 mL ethanol, 10 mL water, and 0.5 g silica nanoparticles (as 30.6 wt% solution) were added to a round bottom flask, which was capped with a rubber septum. The solution was then sonicated for 30 minutes to suspend the nanoparticles in solution. After sonication, 35 µL TMSDEA was added to the solution, which was then stirred for 24 hours at room temperature. After stirring, the solution was centrifuged for 20 minutes at 14,000 rpm and washed three times with DI water. The nanoparticles were then dried under dynamic vacuum for 2–4 hours at 110 °C. After this, the oven was turned off, and the nanoparticles were left under static vacuum to cool to room temperature. Film Preparation. The as-received Nafion stock solution was poured into a Teflon Petri dish and allowed to evaporate in the fume hood for 24 hours to remove the stock solvent mixture. The remaining Nafion solid was re-dispersed in pure ethanol to create a 10 wt% Nafion dispersion. All nanocomposite membranes were cast from this new stock solution. To create nanocomposite membranes, SiNPs were suspended in Nafion solution by sonication for at least 30 minutes prior to casting. The Nafion-nanoparticle suspensions were then cast onto quartz windows, covered by funnel with Kim-wipe flue, and allowed to evaporate overnight. The dried nanocomposite films were first annealed at 140 °C for 2 hours under dynamic vacuum. After 2 hours, the oven was turned off and the films were left under dynamic vacuum for an additional 30 minutes. The films were then left under static vacuum to cool to room temperature. The films were equilibrated in DI water overnight before use. Hydrated film thicknesses were on the order of 30–100 µm. Vanadium Ion Crossover Analysis. A custom-built diffusion cell (Permegear Franz cell, Bethlehem, PA) was used for the measurement of the permeability of vanadium ions. As depicted in Figure 1, the 15 mL receiving cell was filled with 1.5 M MgSO4 in 3 M H2SO4, and

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Figure 1. Schematic of Permegear Franz permeation cell used for vanadium ion crossover experiments.

the 1 mL donating cell was filled with 1.5 M VOSO4 in 3 M H2SO4 with membrane sandwiched between the cells.

Aliquots were sampled via the side arm of the receiving cell at regular time intervals. The concentration of vanadium ions (specifically V4+) in each aliquot was measured via UV-vis spectrometry (VWR, UV-3100PC), scanning from 1100 to 400 nm. A prominent peak at 760 nm is observed in the spectrum and can be attributed to V4+ ions.4,42 The absorbance at this wavelength is directly related to the concentration of V4+ ions. Following UV-vis characterization, the aliquots were placed back into the receiving cell. The concentration of V4+ ions (mol/L) in the receiving cell was measured over time using UV-vis spectroscopy (see Figure 8


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S1 in Supporting Information). From these data, the permeability of vanadium ions can be calculated from the following equation

() = ,

(1)

where and () are the vanadium ion concentration in the donating and receiving cells, respectively, and are the area and thickness of the membrane, respectively, is the permeability of vanadium ions (cm2/s), and is the volume of the receiving cell. This expression assumes that (1) the permeation in the membrane has reached pseudo-steady state, (2) vanadium ion permeability is independent of ion concentration, (3) CD ≫ CR(t), and (4) the reduction in CD over the length of the experiment is negligible.43 Nanoparticle Surface Functionalization Characterization. Nanoparticle functionalization was analyzed by FTIR spectroscopy using a Thermo Scientific Nicolet iS50R FT-IR equipped with Specac Golden Gate attenuated total reflectance (ATR) attachment. All spectra were collected using a liquid nitrogen-cooled mercury-cadmium-telluride detector with 64 scans per spectrum at a resolution of 4 cm-1. Nanoparticle surface chemistry was further analyzed by measuring the zeta potential under acidic pH (in this case, pH ≈ 2) using a Malvern Zetasizer Nanoseries (Nano-ZS). This pH was selected based on the apparent pH of Nafion stock solution (pH ≈ 2), which was measured by a Hach Sension 5048 pH probe. As the nanoparticle dispersions were not stable in water for long periods of time, zeta potential measurements were taken immediately following sonification of each solution. Nanocomposite Membrane Characterization. Ion exchange capacity (IEC) experiments were performed according to literature.44 Briefly, the membrane was dried under vacuum at 80

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°C for 24 hours, massed, and immersed in 1 M NaCl for 24 hours. Next, the membrane was removed from the NaCl solution, and the remaining solution was titrated with 0.01 M NaOH with phenolphthalein (1% (w/v) in a mixture of 1:1 water:ethanol). The IEC for each membrane was calculated as follows

IEC =

NaOH NaOH , dry

(2)

where NaOH is the volume of titrated NaOH solution in L, NaOH is the concentration of the NaOH solution in mol/L , and dry is the dry mass of the membrane in g. Similar to IEC experiments, equilibrium acid uptake experiments were carried out by first drying the films under vacuum at 80 °C for 24 hours, recording their dry mass, then immersing the films in 3 M H2SO4 for 24 hours. After 24 hours, the films were removed from the sulfuric acid, patted dry to remove any residual liquid on the surface, and their mass was recorded. The equilibrium acid uptake of each membrane was calculated using the following equation

% uptake=

(hydrated -dry ) × 100% , dry

(3)

where hydrated is the mass of the film in g after 24 hours in immersed in the sulfuric acid. Nanoparticle Dispersion (or Dispersion State) in Nanocomposite Membranes. TEM was used to directly image (i.e., in real space) the distribution of nanoparticles in each of the nanocomposite membranes. Samples were prepared by solution casting 0.05–0.1 wt% Nafionnanoparticle ethanol solutions onto lacey carbon grids (Electron Microscopy Services, Hatfield, PA), which were allowed to dry for at least 24 hours before following the annealing procedure

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described previously. Samples were analyzed using a Hitachi 9500 high-resolution TEM (HRTEM), which was operated at 300 kV, with an emission current of 8.0 µA, and the filament at 29.1 V. Exposure time for each image was 2.0 seconds. The images obtained from TEM were analyzed using ImageJ to determine particle size. In ImageJ, the images were modified in the following order: auto local threshold (Sauvola, radius = 50, white objects on black background), de-speckle noise, remove noise outliers (radius = 3), enhance contrast, auto-threshold, analyze particles, region of interest color coder (broadly applicable routines application, measuring Feret from 1–20 nm using “royal” look up table).

Results Nanoparticle Functionalization. The relationship between electrostatic interactions (both attractive and repulsive), the dispersion state of the final nanocomposite membranes, and vanadium ion transport was investigated for three distinct surface chemistries. The nomenclature for each of the SiNPs is summarized in Table 1.

Table 1. Nomenclature for the unfunctionalized and functionalized SiO2 nanoparticles utilized in solution-cast Nafion nanocomposite membranes. SiO2 Nanoparticle Surface Chemistry

Nomenclature

Unfunctionalized SiO2

UF-SiNP

Phenyl-SO3 Functionalization

PS-SiNP

Tri-Amine Functionalization

TA-SiNP

An illustration of the different surface-functionalized SiO2 nanoparticles (SiNPs), along with reaction schemes for the nanoparticle surface modification, is presented in Figure 2. The

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Figure 2. (a) Illustration of the surface chemistry for the unfunctionalized SiNPs (UF-SiNPs). Reaction scheme for the surface functionalization of SiNPs with (b) amine (triamino) groups (TA-SiNPs) and (c) with sulfonic acid groups (PS-SiNPs). illustration in Figure 2(a) depicts an unfunctionalized silica nanoparticle (UF-SiNPs). Figure 2(b and c) show the reaction scheme for the triamine-functionalized silica nanoparticles (TA-SiNPs) and the phenyl-sulfonic acid-functionalized silica nanoparticles (PS-SiNPs), respectively. Under acidic conditions, amine-functionalized NPs are expected to attractively interact with the negatively-charged sulfonic acid groups in Nafion, especially as the Teflon domain is apolar. This attractive interaction could aid in the segregation of the TA-SiNPs into the ionic domains, disrupting the transport of large, hydrated vanadium ions in the membrane. It is unclear at this point how the negatively-charged surface of the sulfonic acid-functionalized NPs will affect their dispersion state within the Nafion membrane. From a simple surface charge standpoint, the negative charges on the NP surface should yield repulsive (or weak) electrostatic interactions with the ionic groups in Nafion. However, the sulfonic acid groups on the surface of the SiNPs

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may result in more favorable interactions between the Nafion and the NPs, which could lead to improved dispersibility of the NPs in the polymer matrix. To confirm successful functionalization of the SiNP surface, IR spectra of the unfunctionalized and functionalized nanoparticles were collected using FTIR-ATR spectroscopy. The spectra obtained from this analysis are presented in Figure 3, where the spectra in Figure 3(a) have been offset for clarity. The spectrum of the UF-SiNPs (solid blue line in Figure 3(a)), i.e., the unmodified nanoparticles, shows three distinct IR bands in the fingerprint region of the spectrum (~1500–650 cm-1). These IR bands can be attributed to the stretching modes of the Si– O–Si (1200–1000 cm-1), Si–OH (964 cm-1), and Si–O (797 cm-1) groups on the surface of the unfunctionalized SiNPs.28,41 The IR spectrum of the PS-SiNPs (dashed green line in Figure 3(a)) indicates successful functionalization of the SiNP surface with the phenyl-SO3 chemistry.45–47

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Figure 3. FTIR spectra of (a) UF-SiNPs (solid blue line) and PS-SiNP (dashed green line), as well as (b) TA-SiNPs (solid red line). The insert shows IR bands that can be assigned to various stretching and bending modes of the phenyl ring.

Additional IR bands located at 1599 cm-1, 1496 cm-1, 753 cm-1, and 691 cm-1 are characteristic of the stretching and bending modes of the phenyl ring (the two lowest wavenumbers can be attributed to the 1, 2, 4-trisubstituted phenyl ring).41,48 The IR band at 1459 cm-1 can be attributed to the scissoring vibrational mode of the H–C–H alkyl group in the bridging chain of the PS-SiNPs. Additionally, the IR band centered at approximately 1080 cm-1, in comparison with the UF-SiNP, shows an increase in relative intensity and a broadening of the peak, which is indicative of the emergence of new IR band. Although it is largely obscured by the peak associated with the SiNPs, this new peak (or shoulder) can be attributed to the S=O stretching vibrational mode from the sulfonic acid groups attached to the phenyl rings.

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The IR spectrum of the TA-SiNPs (solid red line in Figure 3(b)) also indicates successful functionalization of the SiNP surface with the tri-amine chemistry. The spectral range in Figure 3(b) was expanded to highlight the presence of a broad IR band (centered around 3400 cm-1) associated with the N–H stretching mode of the amine groups, as well as infrared bands associated with the alkyl groups on the bridging chain (C–H and H–C–H stretching modes at 2967 cm-1 and 2930 cm-1, respectively). Additionally, the presence of three new IR bands in the fingerprint region further indicate successful functionalization of the surface with the tri-amine chemistry. As highlighted in the inset in Figure 3(b), the new IR band at ~1630 cm-1 can be attributed to the H–N–H bending vibrational mode of primary amines, while the new IR band at ~1510 cm-1 can be assigned to the N–H bending vibrational mode of secondary amines, both of which are present in the chain grafted to the surface of the SiNPs.28,49 Finally, the new IR band at ~1470 cm-1 can be attributed to the H–C–H scissoring vibrational mode of the alkyl groups in the bridging chain. SiNP surface functionalization was also confirmed by measuring the zeta potential (,) of the unfunctionalized and functionalized nanoparticles. As seen in Table 2, zeta potential values range from approximately −30 mV for the PS-SiNPs, to approximately +30 mV for the TASiNPs. Nanoparticles with zeta potentials of ≤ −30 mV are considered to be highly anionic (i.e., they have a negatively charged surface), while those with zeta potentials of ≥ +30 mV are considered to be highly cationic (i.e., they have a positively charged surface).50 Due to the acidity of the Nafion casting-solution, as well as the highly acidic nature of VRFBs, the nanoparticles were dispersed in a solution of acidified (with HCl) deionized water with a pH ≈ 2. While the apparent pH of the Nafion solution was approximately 2, the local pH within the ionomer membrane, once hydrated, may be significantly lower. Using the theoretical IEC of

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1100 equivalent weight (EW) Nafion (0.9 mmol/g Nafion) for a membrane with 20% water uptake (0.2 g water/g Nafion), a value of pH ≈ -0.65 is obtained for the local pH inside the membrane. The local pH within Nafion may be low enough to affect the protonation of the SiNPs (based on their specific surface chemistry). However, as it is impossible to determine the exact pH each NP inside the ionomer experiences, the discussion moving forward will assume an average pH ≈ 2.

Table 2. Zeta potential of unfunctionalized and functionalized silica nanoparticles dispersed in water under acidic conditions (pH ≈ 2). Nanoparticle

Zeta Potential (mV)

UF-SiNPs

−13.9

TA-SiNPs

+31.2

PS-SiNPs

−29.9

Under these conditions, both the PS-SiNPs should have a strong negative charge, the UF-SiNPs should have a moderately negative to neutral charge (pH ≈ 2 is close to the isoelectric point of silica NPs),51 while TA-SiNPs should have a strong positive charge. The zeta potential of the UF-SiNPs was determined to be −13.9 mV, which is consistent with the surface charge of unfunctionalized (i.e., untreated) silica at this pH.51,52 Compared to the value for the UF-SiNPs, the lower zeta potential (more negative) for the PS-SiNPs (, = −29.9 mV) indicates successful sulfonic-acid functionalization of the SiNPs, as this chemistry should create a highly anionic surface. In contrast, the zeta potential value (, = +31.2 mV) for the TASiNPs indicates these nanoparticles have highly cationic surfaces. Together with the

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aforementioned FTIR analysis, the zeta potentials obtained indicate successful functionalization of the SiNPs with the different chemistries.

Vanadium Ion Permeability in Nafion Nanocomposite Membranes. Vanadium ion permeation experiments were performed for unmodified Nafion (i.e., membranes without SiNPs) and Nafion nanocomposite membranes at 1 wt%, 3 wt%, and 5 wt% NP loading for each distinct surface chemistry. Permeation experiments were carried out by sandwiching a membrane between a vanadium ion donating cell (V4+ ion source) and a receiving cell (vanadium sink) (see Figure 1 in the Experimental Section for more details). Table 3. Vanadium ion permeability for unmodified Nafion and Nafion nanocomposite membranes at various NP loadings and surface chemistries. Standard deviations for the vanadium ion permeability are associated with repeat measurements on at least four separate films. -8

Membrane

2

Permeation (10 cm /s) 0 wt%

1 wt%

3 wt%

5 wt%

Unmodified Nafion

0.81 ± 0.05

–

–

–

UF-SiNP

–

1.0 ± 0.2

0.90 ± 0.04

0.60 ± 0.04

1.0 ± 0.01

0.98 ± 0.05

1.14 ± 0.08

1.1 ± 0.2

1.05 ± 0.06

0.62 ± 0.05

PS-SiNP TA-SiNP

– –

From Table 3, it can be seen that the permeability of vanadium ions through unmodified Nafion was determined to be = 0.81 × 10-8 cm2/s, which falls within the range reported in literature for solution-cast membranes (0.78–0.90 × 10-8 cm2/s).3 At the lowest SiNPs loading (1 wt%), the mean vanadium permeability was ~25% higher for all nanocomposite membranes as compared to unmodified Nafion. This result is counterintuitive from the perspective of the Maxwell model,53,54 which predicts a decrease in penetrant (in this

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case, vanadium ions) transport through the membrane with the introduction of an impermeable filler such as the SiNPs. The Maxwell model has historically been used as a benchmark for the theoretical reduction in diffusion/permeability in composite materials. However, the Maxwell model does not take into account chemical interactions between the diffusing species and the impermeable filler, nor the chemical functionality of the filler, both which may play a significant role for these systems.53,54 Note, unexpected increases in other membrane properties, such as proton conductivity and methanol permeability, at low NP loadings have been previously reported. It has been suggested that the nanocomposite permeability is governed by the balance between membrane swelling, which leads to an increase the hydrophilic channel size of Nafion, and membrane tortuosity, which arises from the introduction of the nanoparticles.55,56 Additionally, if the standard deviation of the calculated permeability is considered, one could argue that the vanadium ion permeability essentially remains unchanged at a NP loading of only 1 wt% (excluding Nafion membranes containing PS-SiNPs). The vanadium ion permeability for PS-SiNPs nanocomposite membranes remains relatively constant, even as the NP loading is increased from 1 wt% ( ≈ 1.0 × 10-8 cm2/s) to 5 wt% ( ≈ 1.14 × 10-8 cm2/s). In contrast, for the same NP loadings, the vanadium ion permeability for membranes containing UF-SiNPs and TA-SiNPs decreases by approximately 40% and 45%, respectively. Furthermore, the calculated permeabilities for films with 5 wt% UF-SiNP and TASiNP are approximately 25% lower that the vanadium ion permeability of unmodified Nafion membrane. The changes in vanadium ion permeability (as compared to unmodified Nafion) may be, in part, due to differences in the Donnan potential (more specifically, the membrane potential) for each membrane.57 The Donnan equilibrium refers to the concentration of ions (or charges) in the

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membrane relative to that in the adjacent ionic solution. Donnan equilibrium theory suggests that counter-ions (those ions with an opposite charge to the fixed charge in the membrane) experience an electrostatic potential that increases their flux through the membrane, while co-ions (those ions with the same charge as the fixed charge in the membrane) exhibit a reduced flux through the membrane (i.e., Donnan exclusion principle).57,58 To gain insight into how the concentration of ‘available’ charges varies among the different membranes, the IECs of the unmodified Nafion and Nafion nanocomposites (for 1 wt% and 5 wt% NP loading) were measured. In addition, the equilibrium acid uptake was measured for each membrane. The results of this analysis are presented in Table 4. Table 4. Ion exchange capacity (IEC) and equilibrium acid uptake of unmodified Nafion and Nafion nanocomposite membranes. Ion Exchange Capacity (meq/g)

Acid Uptake (%)

Unmodified Nafion 1 wt% UF-SiNP 5 wt% UF-SiNP

1.03 ± 0.03

6.5 ± 0.4

0.95 ± 0.02

7.0 ± 1.0

0.92 ± 0.02

9.0 ± 0.2

1 wt% PS-SiNP

1.10 ± 0.05

7.0 ± 0.4

5 wt% PS-SiNP

0.93 ± 0.04

7.4 ± 0.8

1 wt% TA-SiNP

1.02 ± 0.03

9.0 ± 0.6

5 wt% TA-SiNP

0.86 ± 0.01

9.0 ± 1.0

Sample

Note, while the EW of the Nafion (Nafion® DE 2021) used in this study was 1100, the IEC obtained experimentally (1.03 ± 0.03 meq/g), equates to an EW ≈ 970. However, this IEC value falls within the range of total acid capacity listed on the technical sheet for Nafion® DE 2021 (0.95–1.01 meq/g). Let us first compare the results for unmodified Nafion to those of membranes containing 1 wt% and 5 wt% PS-SiNPs. As previously shown from the zeta potential measurements, the PS-SiNPs were confirmed to have a highly anionic (i.e., highly negatively-

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charged) surface (, ≈ −30 mV). However, as seen from the IEC values in Table 4, there was no obvious correlation between NP loading and the concentration of available negative charges in the membrane. Specifically, the IEC of Nafion membranes containing 1 wt% PS-SiNPs was slightly higher (~5 %) than that of the unmodified Nafion membranes (1.10 ± 0.05 meq/g vs. 1.03 ± 0.03 meq/g), indicating that these membranes have a higher concentration of available sulfonic acid groups (or fixed negative charges). The higher IEC may explain, in part, the observed increase in vanadium ion permeability for the 1 wt% PS-SiNPs membranes. Contrary to what might be expected, the addition of more negatively-charged PS-SiNPs to the ionomer membrane did not result in an increase in the IEC for membranes containing 5 wt% PS-SiNPs. In fact, the addition of more PS-SiNPs resulted in a decrease in IEC to a value that was lower than that of unmodified Nafion (0.93 ± 0.04 meq/g vs. 1.03 ± 0.03 meq/g). This result is quite interesting as these membranes exhibited the highest vanadium ion permeability of all membranes investigated, suggesting the presence of an additional mechanism causing the observed increase in vanadium permeability at higher PS-SiNP loadings. One possibility is that above a ‘critical’ PS-SiNP loading, the presence of the NPs alters the hydrophilic (ionic) network which forms during the drying process (i.e., solvent evaporation). That is, for this particular surface chemistry, the presence of the PS-SiNPs may promote increased connectivity between ionic domains, thereby reducing the tortuosity of the nanocomposite membrane. In addition to differences in the IECs, differences in the equilibrium acid uptake among the different membranes may provide additional insight into the higher vanadium ion permeability observed for Nafion membranes containing PS-SiNPs. It has been suggested that increased acid uptake leads to increased swelling of the ionic domain, resulting in less selective ion transport through the membrane.59 As seen in Table 4, results from this analysis indicate that the

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equilibrium acid uptake does not vary significantly among the different membranes, ranging from 6.5 ± 0.4% (for unmodified Nafion) to 9.0 ± 0.6% (for 1 wt% TA-SiNPs). With such a small difference among all of the membranes, no discernable correlation between ion permeability and acid uptake can be established. Instead, we posit that the increased vanadium ion permeability of the PS-SiNPs membranes is a result of two phenomena: (1) at lower PSSiNPs loadings, the addition of negative charges into the membrane results in an increase in the IEC, and thus an increase in the vanadium ion permeability due to the Donnan potential and (2) at higher PS-SiNPs loadings (~5 wt%), the presence of the PS-SiNPs promotes increased connectivity of the ionic channels, which leads to a decrease in membrane tortuosity, and in turn, an increase in vanadium ion permeability. Next, let us compare the results for unmodified Nafion to those of membranes containing 1 wt% and 5 wt% TA-SiNPs. As previously shown from the zeta potential measurements, the TASiNPs were confirmed to have a highly cationic (i.e., positively-charged) surface (, ≈ +30 mV). As seen in Table 4, we do not observe a measurable change in the IEC of the membrane until the TA-SiNP loading is increased to 5 wt%, where the measured IEC was seen to be approximately 15% lower than that of the unmodified Nafion membrane (0.86 ± 0.01 meq/g vs. 1.03 ± 0.03 meq/g).

This result is quite intriguing, as these membranes exhibited one of the lowest

vanadium ion permeabilities (25% reduction in vanadium ion permeability relative to unmodified Nafion membranes). At higher TA-SiNP loadings (~5 wt%), it appears that the electrostatic interactions between the negatively-charged sulfonic groups and the positivelycharged TA-SiNPs lead to some fraction of the available ionic groups in Nafion being sequestered, thereby hindering their ability to participate in (i.e., facilitate) ion transport through the membrane. Previous research has supported this notion, where it was postulated that the

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sulfonic acid groups in Nafion may become inaccessible for vanadium ion transport due to their interactions with nanoparticles.60 The IEC of membranes containing 1 wt% TA-SiNPs was equivalent to that of the unmodified Nafion membranes (1.02 ± 0.03 meq/g vs. 1.03 ± 0.03 meq/g), which may explain why these membranes have relatively similar vanadium ion permeabilities (when the standard deviation of each calculated permeability is considered). Finally, let us compare the results for unmodified Nafion to those of membranes containing 1 wt% and 5 wt% UF-SiNPs. As previously shown from the zeta potential measurements, the UFSiNPs surface has a sparse anionic charge distribution (i.e., hydroxyl groups are partially protonated at a pH near the isoelectric point of silica; , ≈ −14 mV). As seen in Table 4, the IECs of Nafion membranes containing 1 wt% and 5 wt% UF-SiNPs were lower than that of unmodified Nafion. For example, the IEC of membranes containing 5 wt% UF-SiNPs was determined to be approximately 10% lower than that of the unmodified Nafion membranes (0.92 ± 0.02 meq/g vs. 1.03 ± 0.03 meq/g). The lower IEC for these membranes may explain, in part, the reduced vanadium ion permeability as compared to unmodified Nafion. However, the decrease in vanadium ion permeability for this particular membrane cannot entirely be explained by the lower concentration of fixed negative charges. This is underscored by the fact that the IECs for Nafion membranes containing 1 wt% and 5 wt% UF-SiNPs were similar (0.95 ± 0.02 meq/g vs. 0.92 ± 0.02 meq/g), even though the vanadium ion permeability values for these two membranes differed by approximately 40%. For these nanocomposite membranes, we posit that: (1) at lower UF-SiNPs loadings, the presence of the UF-SiNPs promotes increased connectivity of the ionic channels, which leads to a decrease in membrane tortuosity, and in turn, an increase in vanadium ion permeability and (2) at higher UF-SiNPs loadings (~5 wt%), interactions between the negatively-charged sulfonic groups and the partially protonated UF-SiNPs lead to

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some fraction of the available ionic groups in Nafion being sequestered, thereby hindering their ability to participate in (i.e., facilitate) vanadium ion transport through the membrane.

Nanoparticle Dispersion in Nafion Membrane (Dispersion State). To gain additional insight into the permeability trends observed for the nanocomposite membranes, TEM was used to investigate the effect of the NP surface chemistry on the dispersion state of the Nafion nanocomposite membranes at NP loadings of 1 wt%, 3 wt%, and 5 wt%. TEM results are shown in Figures 4–6 for Nafion nanocomposite membranes containing UF-SiNPs, PS-SiNPs, and TASiNPs, respectively. The scale bar in each TEM image is 100 nm. TEM images of unmodified Nafion (Nafion containing no SiNPs) were also collected and can be found in the Supporting Information (see Figure S2).

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As seen in the TEM images in Figure 4(a–c), the dispersion state of the UF-SiNPs changes significantly as the NP loading is increased from 1 wt% to 5 wt%. The TEM images of the 1 wt% and 3 wt% membranes (Figure 4(a and b), respectively) show that, for the most part, the NPs are well-dispersed in the Nafion membrane. However, as seen most clearly in Figure 4(c), there are a number of small, 2–3 NP aggregates, which are ~20–30 nm in total size; however, no 24


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significant aggregation is observed. Other than the limited number of small aggregates observed in Figure 4(c), the UF-SiNPs are predominantly gathered into loosely-grouped (or diffuse) clusters containing ~10–20 NPs. This can be most clearly seen by the diffuse grouping of NPs in the top left of Figure 4(c), which stretches approximately 300 nm from end to end of the grouping. From the TEM image in Figure 4(c), it appears the diffuse clusters are primarily comprised of larger UF-SiNPs (> 10 nm in diameter), while the smallest sized NPs (< 5 nm in diameter) show good dispersion in areas of the membrane where there are no diffuse clusters or small aggregates. The diffuse NP clusters observed in the 5 wt% UF-SiNPs nanocomposite membrane may also help explain the measured reduction in permeability (~25% reduction compared to unmodified Nafion). The combination of the fixed charges in the membrane (Donnan effect) and NP aggregation will be addressed more thoroughly later on in this section. Analysis of the TEM images shown in Figure 4(a–c) was carried out using ImageJ software to obtain a more quantitative characterization the NP size and dispersion state for each NP loading. As shown in Figure 4(d –f), colors were assigned to the NP phase according to NP size using a gradient from dark blue (for NPs < 7 nm in diameter), to dark red (for NPs > 12 nm in diameter), to grey (for NPs/aggregates ≥ 20 nm in diameter). Color-coding of particles (and aggregates) in the modified images is used to more clearly highlight the dispersion state of the UF-SiNPs shown in the original TEM images. The distribution of the frequency of each NP size obtained from this analysis is given as histograms in Figure 4(g–i) for NP loadings of 1 wt%, 3 wt%, and 5 wt%, respectively. The histograms show that for the 1 wt% and 3 wt% membranes, the average diameter of the UFSiNPs particles is ~3–5 nm. For the membrane containing 5 wt% UF-SiNPs, the presence of larger NPs and 2–3 NP aggregates is more obvious in the modified image (Figure 4(f)), as well

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as in the histogram (Figure 4(i)), where relative to the 1 wt% and 3 wt% nanocomposite membranes, the occurrence of NPs and NP aggregates ≥ 20 nm in size increases significantly. Figure 5(a–c) shows the TEM images for membranes containing PS-SiNPs at NP loadings of 1 wt%, 3 wt%, and 5 wt%, respectively. From these images, relatively little aggregation is observed, especially for membranes containing 1 wt% and 3 wt% PS-SiNPs. At the highest NP loading (5 wt%), we see an increase in the presence of small, 3–4 NP aggregates, though we do not observe the global (diffuse) grouping of NPs seen for membranes containing 5 wt% UFSiNPs (Figure 4(c)). This is further highlighted by the color-coded TEM images (Figure 5(d–f)) and the accompanying histograms (Figure 5(g–i)). Relative to nanocomposite membranes containing UF-SiNPs, the observed dispersion state of the PS-SiNPs in the Nafion membrane is improved at all NP loadings. The frequency of larger NPs is relatively low, where the average diameter of the NPs was ~5–7 nm for each of the PSSiNP loadings. Similar to the UF-SiNP nanocomposite membranes, the smallest NPs (< 5 nm) exhibited better dispersibility than the larger NPs (> 10 nm in diameter). That is, the smallest NPs are more evenly dispersed throughout the TEM image and do not tend to aggregate or cluster with other NPs. Larger NPs and small, 3–4 NP aggregates ~20–30 nm in diameter are observed at NP loadings of 3 wt% and 5 wt%. Furthermore, the frequency of larger PS-SiNPs with diameters ≥ 20 nm increases significantly for membranes containing PS-SiNPs loading of 5 wt%.

In

contrast

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to

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the diffuse clustering observed in nanocomposite membranes containing UF-SiNP at 5 wt% loading, the PS-SiNPs appear to be well-dispersed throughout the TEM image. This result suggests that energetically favorable interactions must exist between the PS-SiNPs and Nafion. Here, favorable interactions between the sulfonic acid groups in Nafion and the sulfonic acid groups on the surface of PS-SiNPs result in a better dispersion of these NPs within the

27



nanocomposite membrane.34,61,62 This results is not surprising as favorable interactions between nanofillers and host polymers have been exploited to reduce NP aggregation and improve dispersibility in a wide variety of polymer-nanofiller systems.32,33,63,64

TEM images of Nafion membranes containing TA-SiNPs at NP loadings of 1 wt%, 3 wt%, and 5 wt% are shown in Figure 6(a–c).

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The TEM images indicate a good dispersion state for these nanocomposite membranes, as little NP aggregation is observed. Additionally, the TA-SiNPs appear to be evenly dispersed within the Nafion membrane (i.e., approximately equal distances between individual NPs), even at the highest NP loading of 5 wt%. In contrast to the images shown in Figure 4 and Figure 5, the TASiNPs appear to be relatively monodisperse in size (~5 nm in diameter), and the occurrence of larger NPs (> 10 nm in diameter) is low. This is most easily seen in the color-coded TEM images and histograms shown in Figure 6(d–f) and Figure 6(g–i), respectively. Furthermore, the presence of large agglomerations of NPs is not found in the TEM images shown in Figure 6. As seen in Figure 6, the TA-SiNPs are, on average, similar in size to the ionic channels (domains) in Nafion (ionic channels ~3–5 nm in size). Similar to what was observed for membranes containing UF-SiNPs and PS-SiNPs, the smallest NPs (< 5 nm in diameter) are more uniformly dispersed over the entire TEM image. In the case of the TA-SiNPs, the smaller NPs can more easily segregate into the ionic domains during the evaporation process (i.e., membrane formation), where they interact strongly with the sulfonic acid groups of Nafion, essentially fixing them in more dispersed positions throughout the membrane and rendering those ionic groups unavailable to assist in ion transport. Note, the TEM images shown in Figure 4, Figure 5, and Figure 6 do not fully capture the degree to which the NP surface chemistry governs the aggregation of particles throughout the entire membrane, as these images only capture a small fraction of the entire nanocomposite. For example, while the TEM images in Figure 6(a–c) show well-dispersed TA-SiNPs of relatively uniform size, there still exists the possibility that these nanocomposite membranes contain larger nanoparticle aggregates in portions of the membrane not pictured. To more thoroughly address

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this issue for all nanocomposite membranes, an abundance of additional TEM images were collected at the highest NP loading of 5 wt% for all surface chemistries.

Figure 7. TEM images of Nafion membranes containing (a)–(c) UF-SiNPs, (d)–(f) PSSiNPs, and (g)–(i) TA-SiNPs at NP loadings of 1 wt%, 3 wt%, and 5 wt%. All scale bars for TEM images are 100 nm.

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Figure 7 shows the TEM images of Nafion membranes containing UF-SiNPs (Figure 7(a–c)), PS-SiNPs (Figure 7(d–f)), and TA-SiNPs (Figure 7(g–i)) at a NP loading of 5 wt%. Aggregation of SiNPs is observed for all nanocomposite membranes in Figure 7, though the degree to which the NPs aggregate, as well as the overall size of the NP aggregates, varies based on surface chemistry. As seen in Figure 7(a–c), the UF-SiNP dispersion state in the Nafion membrane is analogous to what was observed in the TEM image in Figure 4(c) (membrane with 5 wt% UFSiNPs). That is, the TEM images for membranes with UF-SiNPs show both close aggregation of NPs, as well as more diffuse clusters of NPs. The formation of large aggregates of the UF-SiNPs at higher loadings seems to be inhibited by the favorable interactions between the partially protonated (i.e., moderately negative) surfaces of the UF-SiNPs and the negatively-charged sulfonic acid groups in Nafion. The TEM images for membranes with 5 wt% PS-SiNPs (presented in Figure 7(d–f)) show that NPs are consistently well-dispersed throughout several areas of the membrane. This is attributed to the energetics of interaction between the NPs and the Nafion chains, where the sulfonic acid groups on the surface of the NPs lead to energetically favorable interactions with Nafion, thereby improving their dispersion within the membrane. In contrast, the TEM images of Nafion membranes containing 5 wt% TA-SiNPs (Figure 7(g–i)) show a drastically different picture of the NP dispersion state than what was presented in Figure 6(c). Please note, the TEM image in Figure 7(i) shows part of the lacey carbon support, which should not be mistaken for imperfections in the nanocomposite film itself. The TEM images in Figure 7(g–i) show large regions of NP aggregation, some as large as 400–500 nm in size. It is interesting to note that while large aggregates (> 400 nm) are frequently observed in the TEM images of nanocomposite membranes containing TA-SiNPs, the

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TA-SiNPs that are not part of these large aggregates appear to be relatively well-dispersed. In regards to changes in the transport properties of membranes containing these large TA-SiNP aggregates, previous studies in the field of membrane-based separations have shown that aggregation of NPs at higher loadings oftentimes leads to significant decreases in membrane performance (e.g., water permeability (or flux)).30,65,66 Therefore, the reduction in vanadium ion permeability at 5 wt% loading (~25% reduction compared to unmodified Nafion) may be due to a combination of a lower IEC, as well as the presence of large aggregates which could act to disrupt the formation of the ionic channels. However, at this point, it is unclear if these two mechanisms are mutually exclusive. The disruption of the ionic channels is likely caused by a decrease in conformational entropy of the Nafion chains, as they are required to wrap around the large NP clusters/aggregates as the dense membrane forms (i.e., during solvent evaporation). The reduction in conformational freedom of the Nafion chains likely hinders ionic network formation, resulting in a more tortuous path for ion transport.7,67 Additionally, the reduced IEC of Nafion membranes containing 5 wt% TASiNPs suggests the presence of strong interactions between the ionic domains of Nafion and the TA-SiNPs, resulting in a reduction of the sulfonic acid groups available for ion transport. These findings indicate that a balance between the enthalpic forces (interactions between the NP and Nafion) and the entropic forces (conformational ‘freedom’ of the Nafion chains) must be achieved for these nanoparticles to have the greatest impact on restricting vanadium ion crossover. As such, further investigations are necessary to accurately elucidate the ‘ideal’ balance of thermodynamic forces in these ionomer nanocomposite membranes.

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Conclusions In this study, the surface chemistry and loading of SiO2 nanoparticles (SiNPs) were systematically varied to elucidate the relationship between nanoparticle dispersion (or dispersion state) and vanadium ion permeability in solution-cast Nafion nanocomposite membranes. Specifically, the surface functionalization of the SiNPs was altered to achieve both attractive (amine-functionalized) and repulsive (unfunctionalized and sulfonic acid-functionalized) electrostatic interactions between the SiNPs and the ionic groups of Nafion. Successful functionalization of the NP surface with the selected chemistries was confirmed with both FTIRATR spectroscopy and zeta potential measurements. Values obtained from zeta potential measurements confirmed a highly anionic surface (, ≈ −30 mV) for the PS-SiNPs, a partially protonated surface (i.e., weakly anionic; , ≈ −14 mV) for the UF-SiNPs, and a highly cationic surface (, ≈ +30 mV) for the TA-SiNPs. Reduced vanadium ion crossover was measured for Nafion membranes containing 5 wt% unmodified silica nanoparticles (UF-SiNPs) and 5 wt% amine-functionalized silica nanoparticles (TA-SiNPs). In contrast, accelerated vanadium ion crossover was measured for Nafion membranes containing sulfonic acid-functionalized silica nanoparticles (PS-SiNPs) and was not a function of the PS-SiNP loading. At low PS-SiNP loadings, the higher vanadium ion permeability was attributed to an increase in the IEC of the nanocomposite membrane (Donnan effect), while at higher PS-SiNP loadings, the presence of the NPs may act to promote the formation of the ionic network, leading to the observed increase in vanadium ion permeability. Furthermore, the lower vanadium ion permeabilities for membranes containing 5 wt% UF-SiNPs and 5 wt% TA-SiNPs was attributed, in part, to a decrease in the IECs of these nanocomposite membranes (Donnan effect).

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As imaged by TEM, each SiNP surface chemistry resulted in a distinct (and different) dispersion state and was shown to be a weak function of NP loading. TEM images for membranes containing UF-SiNPs showed both close (tightly-packed) aggregation of NPs, as well as diffuse clusters of NPs, where some these diffuse clusters were as large as 300 nm from end to end. PS-SiNPs exhibited good dispersion within the Nafion membrane for all NP loadings (i.e., relatively uniform dispersion with minimal aggregation), which was attributed to thermodynamically favorable interactions between sulfonic acid groups on the NPs and in Nafion. While areas of uniform dispersion were observed in Nafion membranes containing aminemodified nanoparticles, on average, the positively-charged TA-SiNPs tended to coalesce into large, tightly-packed aggregates, as compared to the partially protonated UF-SiNPs or negatively-charged PS-SiNPs. In regions of severe NP aggregation, these tightly-packed TASiNPs aggregates were observed to be > 400–500 nm in diameter. The dispersion states for membranes containing UF-SiNPs (diffuse clusters on the order of 100s of nm) and TA-SiNPs (tightly-packed aggregates on the order of 100s of nm) appear to directly influence vanadium ion permeability. We believe the higher concentration of these clusters/aggregates at 5 wt% results in a decrease in conformational entropy of the Nafion chains, as the Nafion chains are required to wrap around the large NP clusters/aggregates as the dense membrane forms (i.e., during solvent evaporation). The reduction in conformational freedom of the Nafion chains likely results in a more tortuous path for ion transport. Results from this study underscore the need for further investigations on the interplay between the NP surface chemistry, the dispersion state, and the ion selectivity of ionomer nanocomposite membranes.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Raw data for vanadium ion permeation experiments, additional TEM images.

Corresponding Author *E-mail: [email protected]

ORCID Allison Jansto: 0000-0002-0264-0410 Eric M. Davis: 0000-0002-5633-5489

Notes The authors declare no competing financial interest.

Acknowledgements. This research was funded by Clemson University Department of Chemical and Biomolecular Engineering start-up funds. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1246875.

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References (1)

(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)

Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.; Iwashita, N. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107, 3904–3951. Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577–3613. Davis, E. M.; Kim, J.; Oleshko, V. P.; Page, K. A.; Soles, C. L. Uncovering the Structure of Nafion-SiO 2 Hybrid Ionomer Membranes for Prospective Large-Scale Energy Storage Devices. Adv. Funct. Mater. 2015, 25, 4064–4075. Trogadas, P.; Pinot, E.; Fuller, T. F. Composite, Solvent-Casted Nafion Membranes for Vanadium Redox Flow Batteries. Electrochem. Solid-State Lett. 2011, 15, A5–A8. Skyllas-Kazacos, M.; Kasherman, D.; Hong, D. R.; Kazacos, M. Characteristics and Performance of 1 KW UNSW Vanadium Redox Battery. J. Power Sources 1991, 35, 399–404. Tang, Z.; Keith, R.; Aaron, D. S.; Lawton, J. S.; Papandrew, A. P.; Zawodzinski Jr., T. A. Proton Exchange Membrane Performance Characterization in VRFB; 23; 2012; Vol. 41, pp 25–34. Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. Rev. 2004, 104, 4535– 4586. Allen, F. I.; Comolli, L. R.; Kusoglu, A.; Modestino, M. A.; Minor, A. M.; Weber, A. Z. Morphology of Hydrated As-Cast Nafion Revealed through Cryo Electron Tomography. ACS Macro Lett. 2015, 4, 1–5. Xi, J.; Wu, Z.; Qiu, X.; Chen, L. Nafion/SiO2 Hybrid Membrane for Vanadium Redox Flow Battery. J. Power Sources 2007, 166, 531–536. Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928–935. Mai, Z.; Zhang, H.; Li, X.; Xiao, S.; Zhang, H. Nafion/Polyvinylidene Fluoride Blend Membranes with Improved Ion Selectivity for Vanadium Redox Flow Battery Application. J. Power Sources 2011, 196, 5737–5741. Luo, Q.; Zhang, H.; Chen, J.; Qian, P.; Zhai, Y. Modification of Nafion Membrane Using Interfacial Polymerization for Vanadium Redox Flow Battery Applications. J. Membr. Sci. 2008, 311, 98–103. Lu, S.; Wu, C.; Liang, D.; Tan, Q.; Xiang, Y. Layer-by-Layer Self-Assembly of Nafion–[CS– PWA] Composite Membranes with Suppressed Vanadium Ion Crossover for Vanadium Redox Flow Battery Applications. RSC Adv 2014, 4, 24831–24837. Teng, X.; Zhao, Y.; Xi, J.; Wu, Z.; Qiu, X.; Chen, L. Nafion/Organic Silica Modified TiO2 Composite Membrane for Vanadium Redox Flow Battery via in Situ Sol–gel Reactions. J. Membr. Sci. 2009, 341, 149–154. Sang, S.; Wu, Q.; Huang, K. Preparation of Zirconium Phosphate (ZrP)/Nafion1135 Composite Membrane and H+/VO2+ Transfer Property Investigation. J. Membr. Sci. 2007, 305, 118–124. Chen, D.; Hickner, M. A.; Agar, E.; Kumbur, E. C. Optimizing Membrane Thickness for Vanadium Redox Flow Batteries. J. Membr. Sci. 2013, 437, 108–113.

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Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Mai, Z.; Zhang, H.; Li, X.; Bi, C.; Dai, H. Sulfonated Poly(Tetramethydiphenyl Ether Ether Ketone) Membranes for Vanadium Redox Flow Battery Application. J. Power Sources 2011, 196, 482–487. (18) Tripathi, B. P.; Shahi, V. K. Organic–inorganic Nanocomposite Polymer Electrolyte Membranes for Fuel Cell Applications. Prog. Polym. Sci. 2011, 36, 945–979. (19) Thiam, H. S.; Daud, W. R. W.; Kamarudin, S. K.; Mohamad, A. B.; Kadhum, A. A. H.; Loh, K. S.; Majlan, E. H. Nafion/Pd–SiO2 Nanofiber Composite Membranes for Direct Methanol Fuel Cell Applications. Int. J. Hydrog. Energy 2013, 38, 9474–9483. (20) Heinzel, A.; Barragán, V. M. A Review of the State-of-the-Art of the Methanol Crossover in Direct Methanol Fuel Cells. J. Power Sources 1999, 84, 70–74. (21) Mauritz, K. A.; Stefanithis, I. D. Microstructural Evolution of a Silicon Oxide Phase in a Perfluorosulfonic Acid Ionomer by an in Situ Sol-Gel Reaction. 2. Dielectric Relaxation Studies. Macromolecules 1990, 23, 1380–1388. (22) Mauritz, K. A.; Warren, R. M. Microstructural Evolution of a Silicon Oxide Phase in a Perfluorosulfonic Acid Ionomer by an in Situ Sol-Gel Reaction. 1. Infrared Spectroscopic Studies. Macromolecules 1989, 22, 1730–1734. (23) Antonucci, P. L.; Aricò, A. S.; Cretı̀, P.; Ramunni, E.; Antonucci, V. Investigation of a Direct Methanol Fuel Cell Based on a Composite Nafion®-Silica Electrolyte for High Temperature Operation. Solid State Ion. 1999, 125, 431–437. (24) Stonehart, P.; Watanabe, M. Polymer Solid-Electrolyte Composition and Electrochemical Cell Using the Composition. 5523181, June 1996. (25) Dimitrova, P.; Friedrich, K. A.; Stimming, U.; Vogt, B. Modified Nafion®-Based Membranes for Use in Direct Methanol Fuel Cells. Solid State Ion. 2002, 150, 115–122. (26) Chen, Z.; Holmberg, B.; Li, W.; Wang, X.; Deng, W.; Munoz, R.; Yan, Y. Nafion/Zeolite Nanocomposite Membrane by in Situ Crystallization for a Direct Methanol Fuel Cell. Chem. Mater. 2006, 18, 5669–5675. (27) Rhee, C. H.; Kim, H. K.; Chang, H.; Lee, J. S. Nafion/Sulfonated Montmorillonite Composite: A New Concept Electrolyte Membrane for Direct Methanol Fuel Cells. Chem. Mater. 2005, 17, 1691–1697. (28) Lin, C.-H.; Yang, M.-C.; Wei, H.-J. Amino-Silica Modified Nafion Membrane for Vanadium Redox Flow Battery. J. Power Sources 2015, 282, 562–571. (29) Teng, X.; Zhao, Y.; Xi, J.; Wu, Z.; Qiu, X.; Chen, L. Nafion/Organically Modified Silicate Hybrids Membrane for Vanadium Redox Flow Battery. J. Power Sources 2009, 189, 1240–1246. (30) Ladewig, B. P.; Knott, R. B.; Hill, A. J.; Riches, J. D.; White, J. W.; Martin, D. J.; Diniz da Costa, J. C.; Lu, G. Q. Physical and Electrochemical Characterization of Nanocomposite Membranes of Nafion and Functionalized Silicon Oxide. Chem. Mater. 2007, 19, 2372–2381. (31) Muriithi, B.; Loy, D. Proton Conductivity of Nafion/Ex-Situ Sulfonic Acid-Modified Stöber Silica Nanocomposite Membranes As a Function of Temperature, Silica Particles Size and Surface Modification. Membranes 2016, 6, 12. (32) Frischknecht, A. L.; Hore, M. J. A.; Ford, J.; Composto, R. J. Dispersion of Polymer-Grafted Nanorods in Homopolymer Films: Theory and Experiment. Macromolecules 2013, 46, 2856– 2869. (33) Hore, M. J. A.; Ford, J.; Ohno, K.; Composto, R. J.; Hammouda, B. Direct Measurements of Polymer Brush Conformation Using Small-Angle Neutron Scattering (SANS) from Highly Grafted Iron Oxide Nanoparticles in Homopolymer Melts. Macromolecules 2013, 46, 9341– 9348.

37



(34) Kumar, S. K.; Jouault, N.; Benicewicz, B.; Neely, T. Nanocomposites with Polymer Grafted Nanoparticles. Macromolecules 2013, 46, 3199–3214. (35) Green, D. L.; Mewis, J. Connecting the Wetting and Rheological Behaviors of Poly(Dimethylsiloxane)-Grafted Silica Spheres in Poly(Dimethylsiloxane) Melts. Langmuir 2006, 22, 9546–9553. (36) Balazs, A. C.; Emrick, T.; Russell, T. P. Nanoparticle Polymer Composites: Where Two Small Worlds Meet. Science 2006, 314, 1107–1110. (37) Kim, B. J.; Chiu, J. J.; Yi, G.-R.; Pine, D. J.; Kramer, E. J. Nanoparticle-Induced Phase Transitions in Diblock-Copolymer Films. Adv. Mater. 2005, 17, 2618–2622. (38) Kim, D. J.; Jo, M. J.; Nam, S. Y. A Review of Polymer–nanocomposite Electrolyte Membranes for Fuel Cell Application. J. Ind. Eng. Chem. 2015, 21, 36–52. (39) Di Vona, M. L.; Marani, D.; D’Ottavi, C.; Trombetta, M.; Traversa, E.; Beurroies, I.; Knauth, P.; Licoccia, S. A Simple New Route to Covalent Organic/Inorganic Hybrid Proton Exchange Polymeric Membranes. Chem. Mater. 2006, 18, 69–75. (40) Wen, J.; Wilkes, G. L. Organic/Inorganic Hybrid Network Materials by the Sol−Gel Approach. Chem. Mater. 1996, 8, 1667–1681. (41) Liu, Y.-L.; Hsu, C.-Y.; Su, Y.-H.; Lai, J.-Y. Chitosan−Silica Complex Membranes from Sulfonic Acid Functionalized Silica Nanoparticles for Pervaporation Dehydration of Ethanol−Water Solutions. Biomacromolecules 2005, 6, 368–373. (42) Furman, S. C.; Garner, C. S. Absorption Spectra of Vanadium(III) and Vanadium(IV) Ions in Complexing and Non-Complexing Media. J. Am. Chem. Soc. 1950, 72, 1785–1789. (43) Tricoli, V. Proton and Methanol Transport in Poly (Perfluorosulfonate) Membranes Containing Cs+ and H+ Cations. J. Electrochem. Soc. 1998, 145, 3798–3801. (44) Liu, S.; Wang, L.; Ding, Y.; Liu, B.; Han, X.; Song, Y. Novel Sulfonated Poly (Ether Ether Keton)/Polyetherimide Acid-Base Blend Membranes for Vanadium Redox Flow Battery Applications. Electrochimica Acta 2014, 90–96. (45) Peña, L.; Xu, F.; Hohn, K. L.; Li, J.; Wang, D. Propyl-Sulfonic Acid Functionalized Nanoparticles as Catalyst for Pretreatment of Corn Stover. J. Biomater. Nanobiotechnology 2014, 05, 8–16. (46) Gill, C.; Price, B.; Jones, C. Sulfonic Acid-Functionalized Silica-Coated Magnetic Nanoparticle Catalysts. J. Catal. 2007, 251, 145–152. (47) Safari, J.; Zarnegar, Z. An Environmentally Friendly Approach to the Green Synthesis of Azo Dyes in the Presence of Magnetic Solid Acid Catalysts. RSC Adv. 2015, 5, 17738–17745. (48) Larkin, P. Infrared and Raman Spectroscopy: Principles and Spectral Interpretation, Second Edition.; Elsevier: Stanford, CT, 2018. (49) Kang, T.; Lee, J. H.; Oh, S.-G. Dispersion of Surface-Modified Silica Nanoparticles in Polyamide-Imide (PAI) Films for Enhanced Mechanical and Thermal Properties. J. Ind. Eng. Chem. 2017, 46, 289–297. (50) Clogston, J. D.; Patri, A. K. Zeta Potential Measurement. In Characterization of Nanoparticles Intended for Drug Delivery; Methods in Molecular Biology; Humana Press, 2011; pp 63–70. (51) Pham, K. N.; Fullston, D.; Sagoe-Crentsil, K. Surface Charge Modification of Nano-Sized Silica Colloid. Aust. J. Chem. 2007, 60, 662. (52) Kim, K.-M.; Kim, H. M.; Lee, W.-J.; Lee, C.-W.; Kim, T.; Lee, J.-K.; Jeong, J.; Paek, S.-M.; Oh, J.-M. Surface Treatment of Silica Nanoparticles for Stable and Charge-Controlled Colloidal Silica. Int. J. Nanomedicine 2014, 9, 29–40.

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Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(53) Gonzo, E. E.; Parentis, M. L.; Gottifredi, J. C. Estimating Models for Predicting Effective Permeability of Mixed Matrix Membranes. J. Membr. Sci. 2006, 277, 46–54. (54) Chung, T.-S.; Jiang, L. Y.; Li, Y.; Kulprathipanja, S. Mixed Matrix Membranes (MMMs) Comprising Organic Polymers with Dispersed Inorganic Fillers for Gas Separation. Prog. Polym. Sci. 2007, 32, 483–507. (55) Krishnan, N. N.; Henkensmeier, D.; Jang, J. H.; Kim, H.-J. Nanocomposite Membranes for Polymer Electrolyte Fuel Cells. Macromol. Mater. Eng. 2014, 299, 1031–1041. (56) Casciola, M.; Bagnasco, G.; Donnadio, A.; Micoli, L.; Pica, M.; Sganappa, M.; Turco, M. Conductivity and Methanol Permeability of Nafion–Zirconium Phosphate Composite Membranes Containing High Aspect Ratio Filler Particles. Fuel Cells 2009, 9, 394–400. (57) Mafe, S.; Ramirez, P.; Tanioka, A.; Pellicer, J. Model for Counterion-Membrane-Fixed Ion Pairing and Donnan Equilibrium in Charged Membranes. J. Phys. Chem. B 1997, 101, 1851– 1856. (58) Donnan, F. G. Theory of Membrane Equilibria and Membrane Potentials in the Presence of NonDialysing Electrolytes. A Contribution to Physical-Chemical Physiology. J. Membr. Sci. 1995, 100, 45–55. (59) Vijayakumar, M.; Luo, Q.; Lloyd, R.; Nie, Z.; Wei, X.; Li, B.; Sprenkle, V.; Londono, J.-D.; Unlu, M.; Wang, W. Tuning the Perfluorosulfonic Acid Membrane Morphology for Vanadium Redox-Flow Batteries. ACS Appl. Mater. Interfaces 2016, 8, 34327–34334. (60) Schwenzer, B.; Zhang, J.; Kim, S.; Li, L.; Liu, J.; Yang, Z. Membrane Development for Vanadium Redox Flow Batteries. ChemSusChem 2011, 4, 1388–1406. (61) Ganesan, V.; Jayaraman, A. Theory and Simulation Studies of Effective Interactions, Phase Behavior and Morphology in Polymer Nanocomposites. Soft Matter 2013, 10, 13–38. (62) Martin, T. B.; Mongcopa, K. I. S.; Ashkar, R.; Butler, P.; Krishnamoorti, R.; Jayaraman, A. Wetting–Dewetting and Dispersion–Aggregation Transitions Are Distinct for Polymer Grafted Nanoparticles in Chemically Dissimilar Polymer Matrix. J. Am. Chem. Soc. 2015, 137, 10624– 10631. (63) Chung, H.-J.; Kim, J.; Ohno, K.; Composto, R. J. Controlling the Location of Nanoparticles in Polymer Blends by Tuning the Length and End Group of Polymer Brushes. ACS Macro Lett. 2012, 1, 252–256. (64) Akcora, P.; Liu, H.; Kumar, S. K.; Moll, J.; Li, Y.; Benicewicz, B. C.; Schadler, L. S.; Acehan, D.; Panagiotopoulos, A. Z.; Pryamitsyn, V.; Ganesan, V.; Ilavsky, J.; Thiyagarajan, P.; Colby, R. H.; Douglas, J. F. Anisotropic Self-Assembly of Spherical Polymer-Grafted Nanoparticles. Nat. Mater. 2009, 8, 354–359. (65) Cao, K.; Jiang, Z.; Zhao, J.; Zhao, C.; Gao, C.; Pan, F.; Wang, B.; Cao, X.; Yang, J. Enhanced Water Permeation through Sodium Alginate Membranes by Incorporating Graphene Oxides. J. Membr. Sci. 2014, 469, 272–283. (66) Yang, Y.; Zhang, H.; Wang, P.; Zheng, Q.; Li, J. The Influence of Nano-Sized TiO2 Fillers on the Morphologies and Properties of PSF UF Membrane. J. Membr. Sci. 2007, 288, 231–238. (67) F. Green, P. The Structure of Chain End-Grafted Nanoparticle / Homopolymer Nanocomposites. Soft Matter 2011, 7, 7914–7926.

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Table of Contents (TOC) Figure

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Role of Surface Chemistry on Nanoparticle Dispersion and Vanadium

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