Simultaneously recover metal ions and harvest electricity via K

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Simultaneously recover metal ions and harvest electricity via K-Carrageenan@ZIF-8 membrane Zhuoyi Li, Yi Guo, Xiaobin Wang, Peipei Li, Wen Ying, Danke Chen, Xu Ma, Zheng Deng, and Xinsheng Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12501 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Simultaneously recover metal ions and harvest electricity via K-Carrageenan@ZIF8 membrane Zhuoyi Li, a† Yi Guo,a† Xiaobin Wang,a Peipei Li,a Wen Ying,a Danke Chen,a Xu Ma,a Zheng Deng,a Xinsheng Penga,* aState

Key Laboratory of Silicon Materials, School of Materials Science and

Engineering, Zhejiang University, Hangzhou 310027, China.

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ABSTRCT The spent LIBs contains a significant amount of valuable metals such as lithium and cobalt. However, how to effectively recover the valuable metals and minimize the environment pollution simultaneously is still a challenge. In this work, a natural biopolymer K-Carrageenan is introduced into a stable MOFs ZIF-8 to form a composite (KCZ) membrane for selectively separating Li+ from Co2+ and efficiently harvesting the concentration gradient energy simultaneously. The prepared KCZ membrane shows Li+ ionic conductivity up to 1.70×10-5 S/cm, five orders of magnitude higher than 1.1 × 10-10 S/cm of pristine ZIF-8, with Li+ flux of 0.342 mole m-2 h-1 and selectivity of about 8.29 for Li+ over Co2+. Moreover, this asymmetric KCZ/AAO (anodic alumina oxide) membrane exhibits good output power up to 3.54 μW when employed as a concentration-gradient-energy harvesting device during separation process. Hence, the KCZ membrane shows great potential in applying for advanced separation and concentration gradient energy harvesting simultaneously. KEY WORDS: spent LIBs, valuable metal recycle, K-Carrageenan@ZIF-8 membrane, cation selectively separation, electricity harvesting

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INTRODUCTION The extensive application of Lithium-ion batteries (LIBs)1-3 is inevitably led to treat the spent LIBs in time.4-5 Especially, the valuable metals exist in such waste, such as metal lithium and cobalt.6-9 Therefore, recovering these valuable metals has attracted worldwide

attention.

In

general,

the

three

technologies,

hydrometallurgy,

pyrometallurgy and biometallurgy processes have been developed for separating and recovering valuable metals from spent LIBs.10 Hydrometallurgy is more favorable than pyrometallurgy and biometallurgy processes since it can achieve lower poisonous gas emission and energy consumption. A typical hydrometallurgical process often contains the following major steps: (1) discharge/dismantle the spent LIBs. (2) Separate the electrode material with current collector. (3) Acid leaching of the pretreated cathode materials. (4) Recycle and purify metals from leachate. At present, the methods of recovery and purification of valuable metals mainly include flotation, precipitation, solvent extraction and electrodeposition.11-13 Although these methods can effectively recover valuable metals in the leachate, they will ineluctably cause the secondary pollution. Recently, we have introduced poly(sodium-p-styrene sulfonate) (PSS) into porous HKUST-1 metal-organic frameworks (MOFs), PSS@HKUST-1 membrane, and achieved a high Li+ flux with good selectivity from divalent cations.14 This may provide a new avenue for solving the above intractable problems, namely, selectively separate Li+ ions from other transition metal cations in the LIBs cathode material leachate. However, although the PSS@HKUST-1 membrane possesses a relatively high

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selectivity and Li+ ion flux, the chemical stability of HKUST-1 is relatively poor. By the way, the synthesis of PSS will cause certain environmental pollution in the process of sulfonation. Thus, a natural biopolymer with similar function should be more environmentally friendly. Therefore, preparing a membrane that has both good chemical stability and high ion selectivity by an environmentally friendly, simple and efficient preparation process are critical to be considered and developed. The fact is that utilizing this cation-selective membrane to separate Li+ ions from other transition metal cations in the LIBs cathode material leachate will induce concentration gradient and generate corresponding concentration gradient power. It is well known that the concentration gradient power is ubiquitous in nature and one of the most common place it generates is where the river meets the sea. Therefore, the effectively collection of this renewable and sustainable concentration-gradient energy has attracted great attentions,15-17 which inspired us to make good use of the generated concentration gradient that contains substantial power during the ionic separation process. Thus, separating Li+ ions from other metal cations in the LIBs cathode material leachate efficiently and collecting the generated concentration gradient power simultaneously would make the process more sustainable and attract great attention. In this work, a natural biopolymer K-Carrageenan18 was introduced into a well-known stable MOFs ZIF-8 to form composite MOFs membrane via a solid confinement process19 for selectively separating Li+ from Co2+ and efficiently harvesting the concentration gradient energy during the separation process. In K-Carrageenan@ZIF8 (KCZ) composite membrane, K-Carrageenan possesses excellent stability and

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hydrophilicity to generate efficient ionic pathways through the porous ZIF-8 membrane. The prepared KCZ membrane shows good Li+ ion conductivity and proper selectivity of Li+ over Co2+ (8.29). When assembling this cation-selective membrane with anodic aluminum oxide (AAO) as a concentration gradient-driven energy device, this asymmetric KCZ/AAO membrane exhibits good output power density. Hence, the successful synthesis of such a KCZ membrane can inspire more efforts to prepare MOFs membrane systems for the advanced separation and concentration gradient energy harvesting simultaneously. RESULTS AND DISCUSSION ZIF-8 has a high channel density while low ionic conductivity due to the hydrophobic channels. K-Carrageenan,18 a natural biomolecule with abundant hydrophilic sulfonated groups is a promising material for modifying the hydrophobic channels of ZIF-8. Nevertheless, the intractable challenge is how to incorporating the KCarrageenan with long chain and large conformation size into ZIF-8 crystals without destroy their crystal structure. Herein, we use a chemical vapor deposition (CVD) method20-21 to grow KCZ membrane in-situ. Negatively charged K-Carrageenan was tightly attached to positively charged zinc hydroxide nanostrands (ZHNs), which is the precursor of ZIF-8, through electrostatic interaction in the mixed solution. Then the hybrid K-Carrageenan/ZHNs membrane (Figure 1A) was formed by filtering the mixture solution on a AAO substrate. Finally, after hanging the hybrid on KCarrageenan/ZHNs membrane/AAO in 2-methylimidazole (2-MI) vapor at 120 oC for 24 h, a KCZ membrane was successfully prepared. The K-Carrageenan is stable without

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decomposition which was proved by its thermogravimetric analysis (Figure S1). Here, the target KCZ membranes were named as KCZ-5%, KCZ-10% and KCZ-15%, where 5, 10 and 15, respectively, mean the weight percent of the K-Carrageenan in ZIF-8 membranes. The XRD patterns and SEM images of KCZ membranes confirmed the structure of ZIF-8 was preserved after incorporation of K-Carrageenan. As shown in Figure 2A, the XRD patterns of KCZ membranes is consistent with that of ZIF-8. This means that the inclusion of amorphous K-Carrageenan do not change the crystalline phase of ZIF-8. The crystals shape and size of ZIF-8, KCZ-5% and KCZ-10% membranes are similar as shown in the SEM images (Figure S2A, Figure S2B, Figure 1B, Figure 1C, Figure S2C, Figure S2D, respectively). The thicknesses of them are approximately 1 μm. Consequently, the inclusion of K-Carrageenan makes no detrimental effect on the structure of ZIF-8 membrane. The confinement of K-Carrageenan in ZIF-8 membrane is confirmed by the Energydispersive X-ray spectroscopy (EDXS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Brunauer–Emmett–Teller (BET) surface areas. EDXS mapping analysis of the cross-section of KCZ-10% membrane (Figure 1D) shows that element S originated from K-Carrageenan is distributed evenly in the KCZ membrane. In the XPS spectra of KCZ membrane (Figure 2B and Figure S3A), the peaks at 166 eV are ascribed to S originated from K-Carrageenan. Moreover, the XPS spectra of KCZ membrane obtained by depth dissection analysis display apparent peaks of sulfur, indicating the K-Carrageenan is uniformly distributed in the

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Figure 1. SEM images of (A) K-Carreageenan/ZHNs, (B) the surface and (C) the cross-section of KCZ-10% membrane. (D) The EDS mappings of cross-section of KCZ-10% membrane.

KCZ membrane. As shown in the FTIR spectroscopy of KCZ (Figure 2C and Figure S3B), the peak at 3443cm−1 is belonged to O-H stretching.22 The 2978 and 2919 cm−1 peaks are assigned to the C–H stretching vibration of the alkane group in the Carrageenan. The amide I peak at 1645 cm−1 appears in the spectra of Carrageenan. The 1254 cm−1, 923 cm−1 and 846cm−1 peaks correspond to the sulfate ester groups,23 3,6-anhydro-D-galactose and galactose-4-sulfate,24 respectively. The N2 sorption isotherms of ZIF-8, KCZ-5%, KCZ-10% and KCZ-15% at 77 K shown in Figure S3C display approximately type I isotherms. The corresponding pore sizes distributions are shown in Figure S3D. The pore sizes of KCZ membranes are similar to that of ZIF-8. The BET surface area of ZIF-8, KCZ-5%, KCZ-10% and KCZ-15% are 1091.19,

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1082.61, 1004.76 and 900.61 m2/g (Figure 2D), respectively. The BET surface area and micropore volume of KCZ membranes decrease with the increasing content of KCarrageenan indicating the K-Carrageenan was successfully incorporated into ZIF-8 membranes.

Figure 2. (A) The XRD patterns of ZIF-8, KCZ-5%, KCZ-10% and KCZ-15 membranes. (B) The XPS spectra of ZIF-8 and KCZ-10%. (C) The FTIR spectra of K-Carrageenan, ZIF-8 and KCZ10%. (D) The BET surface area and micropore volume of ZIF-8, KCZ-5%, KCZ-10% and KCZ15%, respectively.

The cationic conductivities of pristine ZIF-8 and KCZ membranes were measured for a membrane based device25 in aqueous solution of metal chloride at 25 oC. The ionic conductivities through these KCZ membranes calculated from I–V curves (Figure S5) are in accordance with the values measured by alternating current impedance (Figure S6 and Figure S8). All the KCZ membranes exhibit higher Li+ ionic conductivity than

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Figure 3. (A) Li+ ionic conductivity of the ZIF-8, KCZ-5%, KCZ-10% and KCZ-20% in 0.5M LiCl solution. (B) I–V curves of KCZ-10% membrane in 0.5 M LiCl and 0.5 M CoCl2 solution respectively. (C) I-V curves of the KCZ-10% membrane under a concentration gradient (1M/1μM). (D) The shortcircuit current (ISC) and open-circuit voltage (VOC) of the cell. The high concentration (CoCl2 and LiCl ) mixed solution fixed at 1 M is placed in the KCZ-10% side. The low-concentration (LiCl) solution fixed at 1 μM is placed in the AAO side.

that of pristine ZIF-8. With the increment of K-Carrageenan, the Li+ ionic conductivity increases first and then decreases. The highest Li+ ionic conductivity of KCZ membrane with K-Carrageenan content of 10 wt% is 1.70 × 10-5 S/cm at room temperature, five orders of magnitude higher than 1.1 × 10-10 S/cm of pristine ZIF-8 (Figure 3A). This attributes to the fact that the more the content of Carrageenan, the more negatively charged channel surface of the KCZ membrane, which is more negative than that of pristine ZIF-8 (Figure S4). The highly negatively charged channel surface of the KCZ

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membrane definitely facilitates capturing the cations from solutions and transporting them through ZIF-8 crystals. On the contrary, the transportation of negatively charged anions into the ZIF-8 was hindered, making KCZ membrane a cation-selective membrane. However, further increase K-Carrageenan content will lead to the formation of some voids and cracks (Figure S2E and Figure S2F), which is not good for ionic separation. To explore the transportation mechanism of Li+ through the KCZ membranes, the Li+ ionic conductivity of KCZ-10% membrane was measured at different temperatures, which shows an Arrhenius-like behavior with an activation energy of 0.37 eV (Figure S7). Therefore, the transportation of Li+ is via Grotthuss mechanism26 that hopping of Li+ is between sulfonate groups of K-Carrageenan threaded in ZIF-8 membrane. Typically, the ideal separation factors of two cations is considered as their ionic conductivity ratios. Figure 3B and Figure S8 show that the KCZ-10% membrane exhibits much higher ionic conductivity of Li+ than Co2+ with separation factors of 26.39 for Li+/Co2+. Possessing such high separation factors indicates that the KCZ-10% membrane has great potential for separating Li+ from Li+/Co2+ mixtures. The separation selectivity for the binary ions of Li+/ Co2+ was 8.23, measured by inductively coupled plasma mass spectroscopy (ICPMS) (Table S1) with a Li+ flux of 0.34 mol m-2 h-1. This performance is comparable or even higher than those reported in previous works27-29 (Table S2), indicating good Li+ transportation property of KCZ membrane. In this separation process, the Li+ and Co2+ are in hydrated form and their hydrated diameters are 0.764 and 0.846 nm, respectively.30 However, the pore entrance of ZIF-8 is only

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0.34 nm,31 far smaller than the sizes of hydrated Li+ and Co2+. Under this circumstance, the hydrated Li+ and Co2+ are supposed to dehydrate so as to successfully enter the pores of ZIF-8 since the size of Li+ and Co2+ are 0.12 and 1.44 nm after dehydration.31 Moreover, compared with Li+ whose binding affinity to sulfonate groups is only 1, the binding affinity of sulfonate groups to Co2+ ions is 3.74.32 In fact, cationic binding affinity to sulfonate groups is decisive for the permselectivities of Li+ and Co2+ , since the cation which has higher binding affinity is easier to form the cation–sulfonate pair and is harder to dissociate and transport. As a result, the good selectivity of Li+ over Co2+ is contributed by synergistic effort of the size sieving effect and the binding affinity, even though the size-sieving effect make less contribution. Given the cation-selective KCZ membrane with good selectivity of Li+ over Co2+ and asymmetric surface charge that the KCZ side was negatively charged while the AAO side was positively charged.33 In order to harvest the energy generated by the concentration gradient, we further develop the asymmetric KCZ/AAO membranes into an energy harvesting device (Scheme 1). The asymmetric KCZ/AAO membrane was installed between two compartment cell filled with concentrated and diluted solutions, respectively (Figure S9). Besides, a pair of Ag/AgCl electrodes with saturated KCl bridges were inserted in two compartments. The I−V scans under a series of concentration gradients across the membrane were investigated. The corresponding short-circuit current (ISC, the intercept on the current axe) which represents the net current flow when no external voltage is applied and open-circuit voltage (VOC, the intercept on the voltage axe) can be directly obtained from the I-V plots (Figure 3C).15

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Scheme 1. The illustration of KCZ membrane used for separating Li+ from Co2+ ions and collecting the concentration gradient power simultaneously.

The VOC include two parts:34 the redox potential (Eredox) and the diffusion potential (Ediff) (Figure S10). Eredox is arisen from the different potential drop at the interface between electrode and solution while Ediff is generated by the different diffusion rates of the anions and cations. The solution in the low-concentration side will be negatively charged while positively charged in the high-concentration side. This will lead to a potential which generated between the inlet and the outlet of the nanochannels35 since the Li+ and Co2+ mainly diffuse from the high-concentration side to the lowconcentration side. As a result, the occurrence of redox reactions on the surfaces of Ag/AgCl electrode is inevitable and the generated electrons will transfer to an external circuit so as to keep the solutions in electroneutral.36 The employment of Ag/AgCl electrodes with saturated KCl bridges offset the electrode potential, so Eredox can be ignored, Ediff=Voc. The concentration (LiCl) on the AAO side was fixed at 1 μM while

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the concentration (LiCl and CoCl2) on the KCZ side ranges from 10 μM to 1 M. When the concentration gradient was adjusted from 10 to 106 (Figure 3D), the corresponding VOC and ISC gradually increase from 15 to 78 mV and 0.045 to 37.79 µA, respectively. The harvested power can be used to supply an external load resistor as well. The current density, membrane resistance and output power of energy harvesting device based on the KCZ membranes were investigated in more detail. Figure 4A indicates that the current density is decreased when the load resistance increases. The output power density is calculated as PL =I2RL. When the external load resistance is equal to the resistance of the KCZ membranes (Figure 4B), the obtained output power is maximum. Apparently, the resistance of the KCZ membrane (Figure S12) is far lower than reported 30 kΩ15 due to the use of homemade porous Ag/AgCl membrane electrodes (Figure S11) to minimize the inner resistance with the resistance of KCZ-10% is only 50 Ω. The corresponding maximum output power density the KCZ-10% membrane based generator (Figure 4C) reached up to 45.16 mW/m2, which is in consistence with the result of Li+ ionic conductivity of KCZ membranes. Moreover, we further explored the stability of KCZ-10% membrane in the separation and energy harvesting process. As shown in Figure S13, the maximum output power density of the generator decreased slightly (about 20%) after 4 days, indicating that the structure of KCZ-10% membrane was not damaged during the process.37 The good stability of KCZ-10% membrane was also proved by the SEM images (Figure S14) and XRD patterns (Figure S15) of KCZ10% membrane after the long-term test since the crystals of ZIF-8 are intact and the XRD pattern matches well with that of ZIF-8.

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Figure 4. The collected power is transferred to supply an external load resistor. Current density (A) and output power (B) of energy harvesting device based on KCZ-5%, KCZ-10% and KCZ-20% membranes. The KCZ side was placed to the solution with high concentration (0.25 M LiCl and 0.25 CoCl2), while the AAO side was placed to the solution with low concentration (1 mM LiCl). (C) The maximum output power density of KCZ-5%, KCZ-10% and KCZ-20% membranes. Current density (D) and output power (E) of energy harvesting device based on KCZ-10% membrane under three different concentration gradients. The high-concentration solution fixed at 0.5 M was placed in the KCZ-10% side. Low-concentration solution is adjusted from 0.001M to 0.1 M LiCl. (F) The maximum output power density of KCZ-10% membrane at 5-fold, 50-fold, and 500-fold

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concentration gradients.

We then used the KCZ-10% membrane to proceed to test the influence of concentration gradient on energy conversion (Figure 4D, 4E and 4F). The high-concentration solution (0.25 M LiCl and 0.25 M CoCl2) was placed in the KCZ side. The low concentration solution (varied from 0.001 to 0.1 M LiCl) was placed on the AAO side. The output power densities were 45.16 mW/m2, 26.25 mW/m2, and 6.97 mW/m2 with efficiencies approximately 18.87%, 13.42% and 11.39% for the 5-fold, 50-fold, and 500-fold concentration gradients, respectively (Table S3). As we mentioned above, the occurrence of redox reactions on the surfaces of Ag/AgCl electrode is inevitable and the generated electrons will transfer to an external circuit to keep the solutions in electroneutral. Theoretically, the quantity of electric charge change generated by the Li+ and Co2+ transport through the KCZ membrane equals to that of electrons transport through the external circuit, namely the efficiency of ion transportation converted to electricity is 100% since Li+ and Co2+ transported through KCZ membranes from the high concentration side to low concentration side, while Cl- being blocked by the membrane. However, the efficiency of ion transportation converted to electricity is often less than 100% because Cl- will enter the membrane due to the inevitable existence of defects of KCZ membrane. Here, the efficiency of ion transportation converted to electricity is 78.9% as shown in Table S4 and Figure S16. As result, the low efficiency of ion transportation converted to electricity cause the low energy conversion efficiency. Anyway, as a proof-concept, it is really possible to utilize ionic KCZ membranes to selectively separate Li+ in from Co2+ from the LIBs cathode

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material leachate and harvest the concentration gradient energy simultaneously. CONCLUSION In summary, we successfully fabricated a K-Carrageenan@ZIF-8 membrane on AAO substrate by introducing K-Carrageenan into ZIF-8 membrane, which not only endow hydrophobic channels of ZIF-8 with good hydrophilic and highly negative charged surface but also makes no detrimental effect on the membrane structure and crystallinity. Besides, KCZ membrane maintained large surface area, high channel density and short channel length. All these excellent characteristics make KCZ membranes exhibit good performance when employed as separation membrane for selectively separates the Li+ and Co2+ in mixed solution and as functions of concentration gradients for energy harvest. KCZ membranes show: 1) Li+ ionic conductivity is five orders of magnitude higher than that of ZIF-8. 2) The low activation energy (0.37 eV) of Li+ ionic conductivity means Li+ transport through the KCZ-10% membranes is via a Grotthuss mechanism 3) KCZ-10% membranes display good selectivity of Li+/ Co2+ with a separation factor of 8.29 and a good flux of 0.34 mol m-2 h-1 for Li+. 4) The KCZ membrane-based system possesses extremely low membrane resistance around 50 Ω and shows good performance of concentration gradient energy conversion with output power of 3.54 μW when the utilization area of 0.785 cm2. This work delicately prepared a K-Carrageenan@ZIF-8 membrane, thus opening up applications in separation of leachate of spent LIBs and sustainable power generation. ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website. Materials, measurements and calculation details, the synthetic procedure of the K-Carrageenan@ZIF-8 membrane, the TG curves for the KCarrageenan, SEM images, XPS spectra, FT-IR spectra, N2 sorption isotherms, Zeta potential and Pore size distribution of KCZ membranes with different KCarrageenan content, I−V curves and A.C. impedance spectra for the samples, scheme of the electrochemical testing setup, the table of separation performance, lithium ionic flux and energy conversion efficiency for the KCZ membranes. AUTHOR INFORMATION †This

authors contributed equally to this work.

Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundations of China (NSFC 21875212, 21671171), Key program of National Natural Science and Foundation (NSFC 51632008), Major R & D plan of Zhejiang Natural Science Foundation (LD18E020001) and National Key Research and Development Program (2016YFA0200204),

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TABLE OF CONTENTS. One membrane two functions: K-Carrageenan@ZIF-8 membrane simultaneously selectively separates Li+ from Co2+ and efficiently harvests the concentration gradient energy from spent LIB cathode leachates.

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