Materials. Poly(ether ether ketone) (PEEK) and poly(ether sulfone) (PES) were provided by Changchun Jilin University Special Plastic Engineering Research. Sulfonated poly(ether ether ketone) (SPEEK) was prepared by direct sulfonation of PEEK with sulfuric acid (98%) at 40 oC for 3 hours. Potassium hydroxide, N, N-dimethylacetamide (DMAc) were purchased from Tianjin Damao Chemical Reagent Factory. Magnesium chloride, aluminum chloride, sodium hydroxide, sodium ferrocyanide, potassium ferricyanide and zinc oxide were bought from Kermel Chemical Reagent Factory. Potassium chloride (99.5%), sodium chloride (99.5%), calcium chloride dihydrate, was purchased from Aladdin, Shanghai. These reagents were supplied with analytical grade (AR).
Synthesis of LDHs. MgAl-Cl-LDH nanoparticles were prepared using the co-precipitation and hydrothermal treatment method38. To prepare MgAl-Cl-LDH nanoparticles, a mixture of 0.3 mol L−1 MgCl2 and 0.1 mol L−1 AlCl3 solution (10 mL) was quickly added to 40 mL of 0.15 mol L−1 NaOH solution with 10 minutes stirring, then the LDH slurry was separated by centrifugation and washed with deionized water. The dispersed solution was transferred into a stainless Teflon-lined stainless steel autoclave for hydrothermal reaction at 100 oC for 16 h. Then a transparent, homogenous suspension containing MgAl-Cl-LDH nanoparticles was obtained, followed by filtration separation, deionized water washing and freeze drying for use. MgAl-OH-LDH was prepared by soaking MgAl-Cl-LDH in 3 mol L−1 NaOH solution at 40 oC for 72 h, then freeze drying for use.
Preparation of porous support (Base). The poly(ether sulfone) (PES) porous membrane was prepared by the phase inversion method. The polymers (10 wt.% SPEEK in the polymer and 35 wt.% for polymer concentration) were first dissolved in DMAc, then casted the above solution onto a clean glass plate at room temperature with humidity less than 30%. Afterwards, the plate was immersed into water to form the PES membrane.
Preparation of LDHs-coated composite membrane. The as-prepared LDHs nanoparticles were first dispersed in ethanol and sonicated for 4 h using a sonic bath (KQ5200) to form LDH nanosheets. Then a certain amount of 1.0 wt.% Nafion solution served as binder was added into the above suspension (the mass ratio of LDHs/Nafion is 8/2). The 1.0 wt.% Nafion solution was prepared by diluting 5.0 wt.% Nafion dispersion (Dupont, D-520) with isopropanol (IPA). The resulted suspension was sonicated for another 4 h and formed the LDHs dispersion (The dispersion concentration is 40 mg mL−1). Then 1 mL of above LDHs dispersion was slowly and evenly sprayed onto the prepared porous support, forming the LDH-M.
LDHs characterization. The high resolution diffraction of LDHs were recorded on the Titan Themis G3 ETEM (Thermo Scientific Company) at 80 kV with a Cs corrector for parallel imaging (CEOS GmbH). High-resolution transmission electron microscopy (HRTEM, JEM-2100) was conducted to characterize the morphology of the LDHs. X-ray diffractometer (D8 ADVANCE ECO; RIGAKU, Japan) was used to detect the powder XRD patterns of MgAl-Cl-LDH and MgAl-OH-LDH nanoparticles, which has a monochromatic Cu-Kα radiation source at 40 kV and 40 mA and scan rate of 10° min−1. The disappearance of chlorine element was detected using energy-dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) were conducted on a Thermo ESCALAB 250XI at 150 W (Al Kαradiation, 1486.6 eV).
Membrane Characterization. Field-emission scanning electron microscopy (FE-SEM, JEOL 6360LV, Japan) and energy-dispersive X-ray spectroscopy (EDS) were used to detect the morphologies of prepared membranes. The membranes were treated by breaking them in liquid nitrogen and sprayed with gold to obtain the cross-sections before imaging. The thickness of the LDHs flaker layer on the substrate after cycling (400 cycles, 200 mA cm−2) was measured by FE-SEM, the membrane was collected by disassembling the battery and washed with ultra-pure water. The contact angle meter (POWEREACH, China) was used to clarify the wettability between membranes and the electrolyte (3 mol L−1 NaOH).
Membrane Conductivity. Electrochemical impedance spectroscopy (EIS) testing station (Solartron SI 1260 and SI 1287) was used to test the membrane conductivity as reported44. The range of frequency was set to be 1 ~ 100 KHz. Pieces of membrane were sandwiched between two round titanium plates with 1.5 cm in diameter. The membrane conductivity was calculated as the following equation.
σ (S cm−1 ), L (cm), R (Ω) and A (cm2) are the conductivity, the thickness, the resistance and the effective area of the membrane, respectively.
The ionic transport properties. The ionic transport properties of LDH-M was investigated using Gamry Interface 3000. The current - voltage (I-V) profile was recorded when the membrane was sandwiched between two cells soaking with a gradient of 1|3 mol L−1 NaOH solution. Two Ag/AgCl reference electrodes filled with saturated KCl solution and two salt bridges filled with saturated KCl solution were employed to eliminate the potential drop. Thus the open-cell voltage of the device (V0) is equal to the value of diffusion potential (Vd) resulted from the NaOH concentration gradient, which can be calculated as the following equation.
R, T, F, 𝑡Na+, 𝑡𝑂𝐻− 𝑎𝑛𝑑 𝛥 are the gas constant, temperature, faraday constant, Na+ transference number, OH− transference number and activity gradient (the mean ion activity coefficient was considered since the concentration of NaOH solution is high), respectively.
The permeability of different ions and hydroxide ions. The feed solution: 1 mol L−1 salt solution in ultra-pure water, including KCl, NaCl, CaCl2, MgCl2 and K3Fe(CN)6 with a varied hydrated diameter of cations or anions, respectively. The permeate side: ultra-pure water. The ion concentration of the diffusion side was then calculated as following equation46.
κ is the conductivity of solution in the diffusion side, and c is the ion concentration. The conductivity of the diffusion side with a certain concentration was first measured. Then the slope of the plot of conductivity and concentration was the molar conductivity Λm of metal chloride in the diffusion side.
The permeability of hydroxyl ions across the membranes was also tested using an osmosis cell, and the hydroxyl ions concentration was measured by Mettler Toledo pH meter. The feeding solution and the diffusion side were using 3 mol L−1 NaOH and ulta-pure water, respectively.
Electrochemical performance of the alkaline zinc-iron flow battery. The AZIFB was assembled by sandwiching the prepared membrane between two carbon felt electrodes (3×3 cm2), clamped by two graphite plates. The composite membrane with LDHs was facing the positive side of the battery. The negative and positive electrolytes were 40 mL 0.4 mol L−1 Zn(OH)42− + 3 mol L−1 OH− and 40 mL 0.8 mol L-1 Fe(CN)64− + 3 mol L−1 OH−, respectively. Charge-discharge tests were carried out on ArbinBT 2000 at different current densities (80 ~ 200 mA cm−2). Constant charge capacity was controlled through time cutoff (50 minutes at 80 mA cm−2 and 20 minutes at 200 mA cm−2) during charge process, while the discharge process was ended with a cut-off voltage of 0.1 V. The polarization curves were conducted by charging to 90%, 60%, 30% SOC at 40 mA cm−2 and discharging at different current densities, respectively.
Models for LDHs. Mg2Al(OH)6Cl·2H2O (model-0) is a commonly layered double hydroxide (MgAl-Cl-LDH). The MgAl-Cl-LDH was constructed from the 3 × 3 × 1 supercells of brucite (Mg(OH)2) but substituting 1/3 Mg2+ with Al3+, the induced positive charge after substitution was balanced by interlayer Cl− with the accompany with water molecules, and fully relaxed subsequently. The unit cell parameters for MgAl-Cl-LDH model were referred to Wang’s work47. After the optimization, the Cl− was all replaced by the OH− (MgAl-OH-LDH, model-1, Fig. 5a) to investigate the conductivity of OH− in LDHs layer. It should be noted that the theoretical simulations were explored under hydrated environment, and hence each layer of MgAl-OH-LDH structure (model-1) was composed of 3 hydroxide anions and 8 water molecules. Additionally, to further study the selectivity of LDHs layer, the Zn(OH)42− and Fe(CN)64− replaced respective 2 and 4 Cl− on the basis of model-0 to obtain model-2 and model-3, respectively. The three models were accurately optimized using advanced periodic density functional theory (DFT). The optimized models were all shown in Supplementary Fig. 11.
Ab initio molecular dynamics and optimization. The optimization as well as ab initio molecular dynamics (AIMD) simulation were carried out with the mixed Gaussian plane wave scheme using the CP2K package (version 4.1)48-50. Then, the Perdew, Burke, Ernzrhof (PBE) exchange-correlation functional51, and the DZVP-MOLOPT-SR basis set with Goedecker-Teter-Hutter (GTH) pseudo potentials52 were used. During the calculation, the Grimme D3 correction53 with zero damping was applied to account for the dispersion interactions, as well as the plane wave cutoff energy and relative cutoff were 650 Ry and 60 Ry, respectively. The three structures (model-1, model-2, and model-3) were relaxed before performing AIMD simulation. During the AIMD process, a 20 ps simulation with a time step of 0.5 fs and a coupling time constant of 100 fs was performed in the NVT ensemble at 298 K, and controlled by the Nosé-Hoover thermostat54. The trajectories were recorded every step to analyze the mean square displacement.