Electrochemical Performance of ZIF-8 Coated Zn Anode in a Solid-State Zn Air Battery

The growing research on the application of flexible electronics, namely, flexible metal-air batteries has been evident throughout the recent decade. In this study, Zeolitic imidazolate framework-8 (ZIF-8) as a metal-organic framework was used as a masking agent to inhibit the zincate ions at the negative electrode’s vicinity. Polyacrylamide/polyacrylic acid gel copolymer was used as the solid-state electrolyte and a commercial Pt-coated gas diffusion layer (GDL) as the cathode compartment with ZIF-8 coated zinc nanoparticles being the negative electrode. Scanning electron microscopy was conducted to study the shape deformation of the anode. The electrochemical performance of the battery was evaluated using a galvanostatic charge/discharge test and electrochemical impedance spectroscopy. This research resulted in a capacity of 117.71 mAh at discharge rate of 5 mA cm−2 and 13 successful charge/discharge cycles at the rate of 50 mA cm−2. In addition, the Na-PAA/PAM copolymer solid-state electrolyte was reported to have ionic conductivity of 300 mS cm−1.

A shift from fossil fuels to renewable energy has been proven to be crucial due to the increasing awareness of fossil fuel's environmental impacts. 1 Batteries, with a considerable share in electrochemical energy storage systems, have been studied to a large extent on a material level aiming to bring fossil fuels to an end. In recent years, metal-air batteries received attention due to their low cost ($100 kW −1 h −1 with a potential cost as low as $10 kW −1 h −1 in the future), safety (aqueous electrolyte), and superior energy density. [2][3][4][5] Rechargeability is the feature of interest for the metal-air batteries that is directly connected to the negative electrode and electrolyte's composition. [5][6][7] Zn foil is the dominant form of the negative electrode being used in this field of research as well as in grid-scale applications. 8 However, Zn nanoparticles elicit better electrochemical performance, meaning higher energy density and higher negative electrode utilization. For instance, 86% anode utilization was reported for a fibrous Zn electrode compared to regular Zn powders with 60% utilization. 9 As a result of the zincate ion's high solubility and their non-uniform distribution in the electrolyte, the anode's surface tends to deform resulting in sharp dendrites, which can cause a short circuit in the system. 10,11 This study focuses on the deforming prevention of shape changes resulting in the battery's low energy efficiency. Thus, necessary anode and electrolyte modifications have been reported to be beneficial in terms of rechargeability or cycle life. Regarding the former, employing masking agents, using alloy metals as additives, and different anode structures are the most widely utilized alterations. 5,12 New types of materials called metal-organic frameworks with a wide variety of properties have been reported to be effective masking agents and active materials in case of electrochemical performance. [13][14][15][16][17] Yuksel et al. presented a new anode structure using pyrolyzed ZIF-8 as a masking layer in a study in order to increase the negative electrode's cycle life in an aqueous electrolyte. Namely, highly reversible anode structures with uniform charge distribution were reported in his research. 18 Masking agents were only studied in aqueous metal air batteries and their performance in solid state electronics is still unclear.
With a semi-open system such as metal-air batteries, water evaporation, and electrolyte loss are of great concern while using aqueous electrolytes. Solid-state metal air batteries have been used in flexible electronics extensively and they prevent electrolyte leakage by trapping the solution inside their network. [19][20][21][22] Several polymers have been identified as good candidates for solid-state electrolytes such as polyvinyl alcohol (PVA), 23 polyacrylic acid (PAA), 24 and polyacrylamide (PAM). 25 Multiple factors should be considered when deciding on a flexible solid-state electrolyte. Specifically, mechanical strength, ionic conductivity, and water retention ability determine the overall performance of a solid-state electrolyte.
Herein, we report a new structural design for Zn anode using the same ZIF-8 masking agent with a Na-PAA/PAM solid-state electrolyte. A ZIF-8 layer, which is a type of metal-organic framework (MOF), was used in this paper as a masking agent on the negative electrode. The electrochemical performance of this negative electrode was evaluated via galvanostatic charge/discharge test (GCD) and discharge curves in comparison to pure Zn nanoparticles and Zn foil anodes. It is worth noting that a great portion of the Zn air batteries' research has been dedicated to the positive electrode and the membrane compartment of Zn air batteries. Thus, a shift of focus to the negative electrode is recommended by several scholars. 4,26 Overall, the main goal of this paper is to enhance the cycle life and capacity of a solid-state zinc-air battery using a MOF layer operating as a mask for zincate ions, in the presence of a solid-state polymer electrolyte. A thumbnail sketch of the battery's structure is delineated in Fig. 1. In addition, material characterization tests were reported, such as X-ray diffraction spectroscopy (XRD) and scanning electron microscopy (SEM), and electrochemical characterizations were reported that entail discharge test, galvanostatic charge/ discharge test (GCD), electrochemical impedance spectroscopy (EIS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Experimental
A negative electrode was prepared using Zn nanoparticles as active materials and carboxymethylcellulose (CMC) as the binder. Pt coated gas diffusion layer was the primary positive electrode in this system with the gel polymer electrode playing the role of the separator as well as the electrolyte. A metal organic framework as the masking agent that was spray coated on top of the zinc nanoparticles. Detailed electrochemical and material methodologies were discussed in supporting information.

Results and Discussion
XRD analysis.-Three different samples were tested with XRD analysis denoted as S1, S2, and S3 which are before (S1), after two hours of (S2) soaking, and after one day (S3) of soaking in the KOH z E-mail: staser@ohio.edu * Electrochemical Society Member. solution for ICP analysis. As shown in Fig. 2A, sample S1 contains zinc peaks at 2θ of 31, 43, 45 and 54 degrees, ZIF-8 peaks at 2θ of 8, 16, and 25 degrees, and zinc oxide peaks at 32, 34, 36, 47, 63, and 68 degrees. Pure zinc peaks' intensities were reduced after the experiments due to the reaction of all the zinc nanoparticles with KOH in the solution. The oxygen content was drastically increased due to the oxidation of the electrode. Zinc oxide peaks were also identified in the second sample as the discharge products. Moreover, ZIF-8 peaks were still consistent in S2 (after two hours of analysis) which demonstrates the chemical stability of the masking agent (ZIF-8), although, the peaks finally disappeared after a long time of soaking in KOH solution (two days). Graphite peaks were detected at 2θ of 22, 24, 46, and 42 degrees after two days which shows that a great portion of the materials was detached from the carbon paper and transferred into the KOH solution. This issue can be alleviated in future studies via increasing the binder concentration and the conductivity of the anode materials should be considered.
SEM.-Morphology and shape deformation were analyzed by the SEM at 50 μm and 100 μm scales. Figure 2B illustrates the pure zinc electrode with no ZIF-8 masking on it. A complete deformation and dendrite formation was obviously seen in Fig. 2C after the complete discharge of the system. On the other hand, Fig. 2D shows the zinc anode coated with ZIF-8 before the ICP analysis, and Fig. 2E was used to demonstrate the relatively stable structure after the ICP analysis (Soaking in the 5 M KOH). Better shape stability and dendrite depression were the corresponding results of the ZIF-8 masking agent in terms of chemical and structural characterization. In addition, EDS spectra of each image were provided, respectively showing the elements existing in each SEM image. The zinc utilization was also studied with EDS mapping as can be seen in Figs. 2F-2G.
Discharge test.-Three 100% depths of discharge (DOD) at the rate of 5mA cm −2 were taken at three different anode and electrolyte structures (Zn/ZIF-8 with SSE, pure Zn with SSE, and Zn/ZIF-8 with separator) assembled in a solid-state zinc-air battery as shown in Fig. 3A. With a similar discharge current, the anode with ZIF-8 coating and SSE was reported to have a longer lifetime before full discharge; Thus, a higher capacity resulted (117.71 mAh) compared to the pure zinc electrode with SSE (88.97 mAh) and Zn/ZIF-8 with a separator (82 mAh). The discharge test clearly illustrates the higher capacity of the ZIF-8 coated anode with SSE in a single 100% DOD discharge. A longer lifetime and better capacity demonstrate improved electrochemical performance of the SSE and the negative electrode structure in the battery. The latter was examined with the porous shape of ZIF-8 nanoparticles that hinder the zincate ions' passage throughout the electrolyte. ZIF-8 nanoparticles were synthesized in the way that zincate ions were blocked while hydroxide ions were allowed to pass the structure. This masking layer prevents anode deformation or dendrite formation which results in a longer lifetime of the battery under certain discharging conditions. Higher capacities in the battery with SSE compared to a separator bodes well for the promising performance of SSEs in terms of lifetime enhancements. Water retention capacity and ionic conductivity are two pivotal factors affecting the capacity of a battery. In this study, the water retention capability was ensured via carboxylic acid groups existing in polyacrylamide (PAM) and the ionic conductivity of the SSE was guaranteed using polyacrylic acid (PAA).
Galvanostatic charge/discharge analysis (GCD).-The zinc-air battery's cycle life and round-trip efficiency were evaluated at a certain current density of 50 mA cm −2 for different anode structures with SEE as delineated in Fig. 3B. The charge/discharge cycle time was set to 30 min per cycle. As shown in Fig. 3B, the Zn foil barely tolerates the first cycle. This result was aligned with our expectation since active material utilization in the Zn foil is way lower than in the nanoparticle size. 15 Moreover, zinc oxide deposition onto the Zn foil also increases the resistance of the cell as the battery discharges. Namely, the charge transfer resistance was evaluated in this study to find out the effect of non-conductive ZnO on the surface of the Zn foil. The results were also in line with the anticipations such that 0.65 ohm was reported for the charge transfer resistance before a complete discharge experiment (100% DOD) and 4.3 ohm was reported to be the charge transfer resistance after the first complete discharge. In this regard, the battery with Zn foil anode lost its capacity and ended up reaching its cut-off voltage after the very first cycle as shown in Fig. 3B. On the other end, the battery with Zn nanoparticles coated onto a carbon cloth lasts four more cycles with similar specifications, which shows good cycling performance of nanoparticles. The theoretical voltage for a zinc-air battery is reported to be 1.65 V ( + Air electrode O ) and the higher charging voltage can be attributed to other parasitic reactions happening in the system as well as cathode overpotential. The stability of the charge/discharge process, although delineated in Fig. 3B. since the figure plateaus, and no difference was seen in the voltage gap. Nonetheless, using nanoparticles in a system begets self-discharge which is detrimental to the battery by consuming active materials in the way that it increases the dissolution rate of zinc particles into the electrolyte in the form of zincate ions. Coating ZIF-8 onto the pure zinc electrode was reported to be beneficial in terms of cycle life as illustrated in Fig. 3B with 13 consecutive cycles at the same condition. This masking agent allows the hydroxide ions to move through but blocks the zincate ions to travel within the gel polymer electrolyte. This behavior demonstrates the paramount importance of the ZIF-8 masking . X-ray diffraction (XRD) patterns for ICP analysis samples before soaking into 5 M KOH (S1) and after soaking into the 5 M KOH for four hours (S2). SEM images of (B) pure zinc anode with no masking agent before discharge analysis, (C) pure zinc anode at fully discharged state, (D) ZIF-8 coated anode before ICP analysis (Soaking in 5 M KOH), (E) ZIF-8 coated zinc anode after the ICP analysis which shows the stable structure of the anode after soaking in the 5 M KOH solution, (F) EDS map of pure Zn nanoparticles coated into the carbon cloth support with CMC binder before any experiment, (G) EDS map of Zn-ZIF anode after the battery's full discharge. agent in the charge/discharge cycling of the zinc-air battery and provides a new field of interest in the selection of anode materials and their electrochemical performance optimization.
Electrochemical impedance spectroscopy (EIS).-The ionic conductivity of the SSE is a determining factor in the electrochemical performance of a zinc-air battery. Different gel polymer electrolytes (GPEs) elicit certain ionic conductivities and this number relates to different factors such as polymer network, KOH concentration, and GPE/electrode contact. Increasing the KOH concentration will promote ionic conductivity since a higher number of charge transfer carriers will exist but it has a range of 3-6 M. Higher KOH concentration results in shape deformation of polymer network. 27 A PAA SSE delivered 38 mS cm −1 ionic conductivity at its best performance but higher water uptake was assured in the system. On the other hand, adding acrylic acid monomers to the polymer synthesis process provided higher ionic conductivity. Using the Eq. 1, the ionic conductivity of the Na-PAA/PAM SSE was calculated. EIS analysis was performed using the Solartron impedance analyzer with a frequency range of 10Hz-100MHz and an amplitude of 10 mV. All these experiments were operated using Z plot/Z view software. Membrane samples were cut in 2 cm*1 cm dimensions with 100 μ thickness, and they were sandwiched in between a Teflon cell with Pt electrodes. The whole cell was tightened with screws and the temperature was fixed at 25°C. 28 Where δ is the ionic conductivity of the membrane, A= surface area of the membrane d the membrane's thickness, R = High-frequency intercept in the Nyquist plot (Bulk resistance). The bulk resistance was calculated using the EIS test as shown in Figs. 3C-3E for different negative electrode configurations, and the ionic conductivity was reported to be 300 mS cm −1 . Bulk resistance was increased in all cases due to the passivation of the negative electrode after the first charge and discharge.
Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis.-The amount of zincate ions was measured using the ICP-AES test. This analysis shows the amount of zincate ions dissolved in 50 ml of 5 M KOH solution. Two different anode structures with and without ZIF-8 coating were prepared and soaked in 5 M KOH for four hours for the ICP-AES test. Two samples of two different batches were selected for the zincate ions' quantification at 25°C. Overall, eight measurements were prepared after the analysis with each sample tested twice. The average amount of zincate ions in the pure zinc batches was reported to be 2180 ppm while in the ZIF-8 coated batches this number was stated to be 1058 ppm. The results demonstrate the considerable difference in the number of zincate ions remaining in KOH solutions. Roughly, the concentration of zincate is twice in the KOH solution in which the pure zinc anode was soaked in. This decrease in the amount of zincate ions was attributed to the existence of ZIF-8 masking agents. This method was also studied by Wu et al. with a different carbon nanoshell material. They reported a new anodic structure for Zn-air batteries so that they could improve the charge/discharge stability of the battery. 29

Conclusions
In this study, we proposed a solid-state zinc-air battery with enhanced electrochemical performance in terms of capacity and cycle life. Using Na-PAA/PAM solid-state electrolyte was reported to be beneficial for the capacity of the zinc-air battery (82 mAh to 117.71 mAh) in this paper. The anode shape deformation was shown to be alleviated using ZIF-8 metal organic framework as a masking agent with an increase in a successful number of cycles that the battery can tolerate under 50 mA cm −2 galvanostatic charge/discharge test. These results show the feasibility of metal-organic frameworks as new types of masking agents for solid-state metal-air batteries. Utilizing conductive metal organic frameworks can be of interest in future studies since the metal organic frameworks suffer from low electronic conductivity. In addition, mixing solid-state electrolytes with room temperature ionic liquids can also be a good prospective study that can decrease the self-discharge and parasitic reaction in the system.