Article pubs.acs.org/JPCC
Proton-Selective Ion Transport in ZSM‑5 Zeolite Membrane Zhi Xu,†,∥,⊥ Ioannis Michos,∥,† Zishu Cao,† Wenheng Jing,‡ Xuehong Gu,‡ Kevin Hinkle,§ Sohail Murad,§ and Junhang Dong*,† †
Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221, United States ‡ State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China § Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, United States S Supporting Information *
ABSTRACT: The ionic ZSM-5 zeolite membranes were investigated for proton-selective ion separation in electrolyte solutions relevant to redox flow batteries. The zeolite membrane achieved exceptional selectivity for proton over V4+ (VO2+), Cr2+, and Fe2+ via size-exclusion at the zeolitic channel openings, and remarkably low area specific resistance resulted from its hydrophilic surface, copious extraframework protons, and micron-scale thickness. The ZSM-5 membrane, as a new type of ion separator, demonstrated substantially reduced self-discharge rates and enhanced efficiencies for the all-vanadium and iron−chromium flow batteries as compared to the benchmark Nafion membrane. Findings of this research show that ionic microporous zeolite membranes can potentially overcome the challenge of trade-off between ion selectivity and conductivity associated with conventional polymeric ion separators.
V5+(VO2+)/V4+(VO2+) in the all-vanadium RFB (VRFB), or different, e.g., Fe2+/Fe3+ and Cr4+/Cr3+ in the iron−chromium RFB (FCRFB).4−6 The ion separator allows the transfer of balancing ions, commonly protons, while preventing the reactive metal ions from crossing over between the two electrodes. Conventional RFB ion separators are thick membranes of ionic polymers, most famously Nafion, the sulfonated fluoropolymer−copolymer widely considered as a benchmark material possessing good stability and high ion conductivity.7−10 In the ionic polymer, such as Nafion, nanometer-sized water channels (>2 nm in width11) form by self-organization of the sulfonated side chain terminals under hydration. These water channels allow for fast proton transport but provide limited ion selectivity because of their relatively large width and structural flexibility that in turn cause metal ion crossover to increase self-discharge and decrease efficiency for the RFB.12 In recent years, extensive efforts have been made in searching for ion separators with perfect ion selectivity, low area specific resistance (ASR), and long-term stability to improve the energy efficiency, power density, lifetime, and ultimately cost-effectiveness of the RFBs.13,14 Here we show an aluminum-containing MFI-type zeolite membrane as a new type of ion separator that has the potential to simultaneously achieve near-perfect proton selectivity and low ASR with chemical and structural stabilities desirable for RFBs. Zeolites are aluminosilicate crystals containing uniform subnanometer-sized pore systems defined by specific frame-
1. INTRODUCTION Redox flow batteries (RFBs) are promising for large-scale electrical energy storage, which is necessary for broader penetration of renewable solar and wind powers into the commercial electric grids.1−3 In the RFB operation, electrochemical reactions of the red/ox ion couples, i.e., Mam/Mam+1 and Mcn/Mcn+1, occur at the negative and positive electrodes residing on the two sides of a membrane ion separator as schematically illustrated in Figure 1. The metal ions, Ma and M c, could be elementally identical, e.g., V2+/V 3+ and
Received: September 16, 2016 Revised: November 4, 2016 Published: November 7, 2016
Figure 1. Schematic showing the principle of RFB operation for electrical energy storage in renewable power systems. © XXXX American Chemical Society
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DOI: 10.1021/acs.jpcc.6b09383 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. (A) Cross-sectional SEM image of the ZSM-5 zeolite membrane on porous alumina; (B) cross-sectional SEM image of the ZSM-5 zeolite membrane on graphite-coated carbon cloth; (C) ZSM-5 membrane surface after being immersed in 2 M H2SO4 solution for different times: ((a) 0 days, (b) 2 days, (c) 20 days, and (d) 180 days); and (D) XRD patterns of the ZSM-5 zeolite particles after being immersed in 2 M H2SO4 solution for different times (0−180 days).
publications.19,20 The precursor solution was obtained by mixing 11.3 mL of tetrapropylammonium hydroxide (TPAOH, 1 M, Aldrich), 19.2 mL of tetraethyl orthosilicate (TEOS, Acros), 60 mL of DI water, and 0.458 g of NaAlO2 (Aldrich), where TPAOH is the structure directing agent (SDA).19 The αalumina and carbon cloth substrates were immersed in the precursor solution contained in a Teflon-lined autoclave, and the hydrothermal synthesis was conducted at 180 °C under autogenous pressure for 6 h in both in situ and secondary growth syntheses. The resultant zeolite membranes were washed, dried, and then calcined at 500 °C for 6 h to remove the SDA from the zeolitic channels. Detailed procedures for the membrane fabrication are provided in the Supporting Information. Membrane Characterizations. The zeolite membranes were examined for pinholes and cracks by helium permeation before SDA removal and room temperature H2/CO2 separation after calcination, respectively.21,22 The scanning electron microscopy (SEM, S4800, Hitachi), energy dispersive X-ray spectroscopy (EDS, Noran NSS 2.2, Thermo Scientific), and Xray diffraction (XRD, Miniflex 600, Rigaku) were used to examine the membrane thickness, crystal morphology, elemental composition, and crystal phase of the zeolites, respectively. The membranes were then tested by the conventional ion diffusion and electrochemical impedance spectroscopy (EIS; Gamry Reference-600) methods in the RFB cell to determine their ion transport selectivity and ion conductivity.18,23 The ion diffusion permeation measurements used a feed solution of 4/7 M VOSO4 + 4/7 M H2SO4 and a permeate side solution of 1 M MgSO4 for determining proton and vanadium ion fluxes, and a feed solution of 1 M FeCl2 and 1 M CrCl2 in 2 M of HCl and a permeate side solution of 1.67 M MgCl2 for measuring the fluxes of proton and iron and chromium ions. The MgSO4 and
work structures. The MFI-type zeolites have an interconnected three-dimensional pore system where nearly cylindrical channels of ∼0.56 nm diameter run straight the in b-direction.15 This pore dimension is close to the largest size, which is expected to realize perfect size exclusion for the hydrated multivalent metal ions ([Mn+·xH2O]; kinetic diameters dk > 0.6 nm) and lower transport resistance for hydrated protons (mainly in the form of hydronium, H3O+, dk ∼ 0.27 nm).16,17 In our recent study, the concept of size-exclusion-enabled protonselective ion transport in the VRFB electrolyte solutions was verified by the pure-silica MFI zeolite (i.e., silicalite) membrane because its nonionic nature avoids electrostatic complications.18 However, the silicalite membrane exhibited large ASR (e.g., ∼4.59 Ω·cm2 for a membrane thickness of ∼6.5 μm) because of the lack of exchangeable protons in its structure and the hydrophobic surface that severely impedes water and proton (H3O+) from entering the zeolitic channels. The ZSM-5 zeolites are structural analogues of silicalite obtained by Al3+ substitution for Si4+ in the framework at various Si/Al atomic ratios (>1.0). The resultant framework [AlO2]− tetrahedrons are electrically balanced by exchangeable extraframework cations X+ ([AlO2]−·X+), which also makes the surface highly hydrophilic.
2. METHODS Membrane Synthesis. The ZSM-5 membranes were synthesized on homemade porous α-alumina disc (1.2 mm thick, 2.5 cm in diameter, average pore diameter of ∼0.1 μm, and porosity of 30−33%) by the in situ crystallization method and on graphite nanoparticle-coated carbon cloth (AvCarb©MGL370, Fuel Cell Earth) by the seeded secondary growth method. The methods for zeolite synthesis precursor preparation and hydrothermal treatment for membrane formation are similar to those reported in our previous B
DOI: 10.1021/acs.jpcc.6b09383 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 3. (A) Si/Al and Na/Al ratios of the ZSM-5 crystals as a function of time in 2 M H2SO4 solution; (B) results of ion diffusion for aluminasupported ZSM-5 membranes using feed solutions of (VO)SO4 (2 M) in 2 M H2SO4 (2 M) and FeCl2 (1 M) and CrCl2 (1 M) in 2 M HCl; (C) OCV decay curves for the VRFB and FCRFB equipped with ZSM-5 zeolite and Nafion-117 ion separators; and (D) EIS measurements for the VRFB and FCRFB cells equipped with various ion separators.
and FCRFB equipped with the zeolite and Nafion-117 ion separators were examined for Coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) via charge− discharge measurements at a current density of 30 mA/cm2. The zeolite membrane equipped RFBs were also tested for cyclic operations at a currently density of 120 mA/cm2. The open-circuit voltage (OCV) decay (i.e., self-discharging) rates and the polarization curves of the fully charged RFBs were measured following the procedures established in the literature.23
MgCl2 solutions were used in the permeate side to balance the ionic strength and minimize osmotic pressure differences between the two sides of the membrane. The ion flux is given by Ji = (ΔCi/Δt)·VMgSO4·(1/Amemε), where ε is 1 for unsupported Nafion film and the zeolite membrane supported on carbon cloth and is the substrate porosity (0.30−0.33) for membranes supported on alumina disc. The EIS measurements were conducted in the RFB electrolyte solutions, which were the same as those used for battery performance tests. The isolation of the membrane resistance from the whole cell resistance obtained by the EIS analysis was achieved by the common methodology using the model of resistors in series.24 RFB Performance Tests. The cell structure used for RFB operation tests was the same as that described in our previous publication.23 The circular ion separator was sandwiched between two carbon electrodes with an active area of 2.54 cm2. The electrodes were made of carbon felt (No. 42107, Alfa Aesar), which were pretreated by the literature methods.25,26 The cutoff voltages were 1.7 and 0.8 V for charging and discharging, respectively, for the VRFB, and were 1.25 and 0.6 V for charge and discharge, respectively, for the FCRFB. The VRFB used a solution of 2 M V2+/V3+ sulfates in 2 M H2SO4 as anolyte and a solution of 2 M V5+/V4+ (VO2+/VO2+) sulfates in 2 M H2SO4 as catholyte. The FCRFB used a solution of 1 M Cr2+/Cr3+ chlorides in 2 M HCl for anolyte and a solution of 1 M Fe2+/Fe3+ chlorides in 2 M HCl for catholyte. The VRFB
3. RESULTS AND DISCUSSION According to the SEM observations in Figure 2A and B, the average thicknesses (δ) of the apparently intergrown zeolite layers were ∼6.5 and ∼4.0 μm on the porous alumina and carbon substrates, respectively. All membranes were confirmed to be free of pinholes and cracks by the observations of extremely low helium permeance (
