Design Strategy for Zinc Anodes with Enhanced Utilization and

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Design Strategy for Zinc Anodes with Enhanced Utilization and Retention: Electrodeposited Zinc Oxide on Carbon Mesh Protected by Ionomeric Layers Daniel Stock, Saustin Dongmo, Dominik Damtew, Martina Stumpp, Anastasiia Konovalova, Dirk Henkensmeier, Derck Schlettwein, and Daniel Schröder ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01117 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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ACS Applied Energy Materials

Design Strategy for Zinc Anodes with Enhanced Utilization and Retention: Electrodeposited Zinc Oxide on Carbon Mesh Protected by Ionomeric Layers Daniel Stocka,b, Saustin Dongmoa,b, Dominik Damtewc, Martina Stumppb,c, Anastasiia Konovalovad,e, Dirk Henkensmeierd,e, Derck Schlettweinb,c and Daniel Schröder*a,b a

Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich-BuffRing 17, D-35392 Giessen, Germany. b

Center for Materials Research (LaMa), Justus Liebig University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany.

c

Institute of Applied Physics, Justus Liebig University, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany.

d

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarangro 14gil5, Seongbukgu, Seoul 02792, Republic of Korea. e

Division of Energy & Environment Technology, KIST school, University of Science and Technology, Seoul 02792, Republic of Korea.

Abstract In order to establish secondary zinc-oxygen batteries as sustainable and cost-efficient future energy storage technology, the cycle life of zinc anodes must be further improved. We show that using a three-dimensional carbon mesh as a host structure for the active material zinc oxide and then coating it homogeneously with an ionomeric, hydroxide-conductive confinement layer yields unprecedented cycling stability. Long-term stable charge/discharge of the zinc anode can only be achieved by using this order of compounds: oxidized zincate species that would otherwise leech into the bulk electrolyte are directly confined at the electron conductive host structure by the applied ionomeric coating. We confirm with operando X-ray diffraction measurements that the defined layer of electrodeposited active material (zinc oxide) can be converted efficiently into zinc during charge – and reversed then back to zinc oxide during discharge – directly on the carbon host. We evidence high utilization of the active material (up to 93% based on initial amount of zinc oxide) and enhanced capacity retention (four times higher compared to uncoated anodes after 30 cycles), tested for coin-type cell batteries with optimal amount of ionomeric coating. Analyses by

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means of scanning electron microscopy and cyclic voltammetry are used to prove that the polymer applied is chemically and electrochemically stable. In addition, permeability measurements prove low permeation rates for zincate ions for the tested ionomeric membranes and zinc-oxygen cells without zincate species in the bulk electrolyte indicate the confinement of zincate ions during cycling – keeping them near the electrochemical active surface area where they are needed.

Keywords metal air battery; zinc air battery; zinc anode; carbon host; polymer; protected anode

1. Introduction Advantages of zinc-based secondary batteries include environmental friendliness, abundance of the constituent elements and safe operation.1,2 Over the last decades a multitude of primary and secondary batteries with zinc (Zn) anodes has been investigated (Zn/NiOOH, Zn/MnO2, Zn/Ag2O, Zn/O2).

3–8

Especially, secondary zinc-oxygen (Zn/O2) batteries become a

promising candidate for energy storage in electronic applications because of a large theoretical specific energy with cost-efficient and recyclable materials. However, secondary Zn/O2 batteries are not commercialized so far. Besides limited stability of the gas diffusion electrode (GDE) with bifunctional catalyst, the repeated charge and discharge of the Zn anode is mainly limited due to high solubility of oxidized Zn species in alkaline electrolytes. 8–16 Miyazaki et al. applied an anion-exchange ionomer (AEI), which is a ionomeric hydroxideconducting polymer (IHCP) with a fraction of cationic head groups covalently bonded to the carbon backbone, on the Zn anode and introduced the benefit of selective permeation for hydroxide ions (OH−) and zincate ions ([Zn(OH)4]2−) to suppress dendrite growth of Zn anodes.17 Moreover, the IHCP as OH−-conductive binder led to higher cycling stability for their slurry-based Zn anodes. We recently revealed – with model electrodes and X-ray photoelectron spectroscopy – that it is possible to confine [Zn(OH)4]2− during discharge as a uniform ZnO layer at the interface between IHPC and Zn, preventing the loss of [Zn(OH)4]2− into the bulk electrolyte and thus unwanted shape change of the entire electrode.18 However, the formed ZnO interlayer is electronically insulating and can – if formed homogeneously around the active material Zn during discharge – decrease the amount of utilizable active material. To ensure high utilization of the active material and thus high energy density of the Zn anode, we recently proposed various host structures – with a


2

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lightweight carbon mesh (C mesh) being the most promising one – as suitable for the electrodeposition of ZnO layers with defined thickness.19 ZnO as discharge product exhibits a higher molar volume than Zn metal, meaning that the volume of the active material expands around 1.58 times at discharge.20,21 Starting the cycling with ZnO instead of Zn implies starting with the maximum volume possible avoiding further expansion of the active material upon cycling and, therefore, provides best prospects to avoid the formation of cracks. In this work, we now transfer both aforementioned studies to the practically relevant electrode level: Using a C mesh as a host structure for the active material and then coating it with HPC leads to microstructured HPC-modified Zn anodes with high retention and utilization of the active material resulting in increased cycling stability.

2. Experimental Methods Preparation of IHCP-modified Zn anodes. The herein presented HPC-modified Zn anode consists of a C mesh host structure, on which we deposit a uniform layer of ZnO and subsequently cover it with a homogeneous layer of IHCP. The detailed preparation protocol is as follows: ZnO layers were prepared by electrodeposition from a nitrate-based electrolyte (compare Stumpp et al.

19,22

): first,

cylindrical C mesh samples with 14 mm diameter and 0.33 mm thickness (113 g/m², SCCG 5N, SAATI) were placed into an in-house manufactured sample holder to ensure uniform electrical contact.

19

By these means, a restricted sample area with a diameter of 12 mm was

in contact with the electrolyte solution onto which ZnO was deposited. Before electrodeposition, all C mesh samples were cleaned for 15 min in ethanol inside an ultrasonic bath and afterwards rinsed with distilled water. Deposition of ZnO was carried out from 0.1 mol dm−3 aqueous zinc nitrate solution (Zn(NO3)2 hexahydrate, ≥ 99%, Sigma Aldrich) at 70 °C in a temperature-controlled cell. The solution was deaerated by purging nitrogen gas through a glass frit before ZnO deposition. C mesh samples were placed as working electrode into the electrolyte solution together with a platinum counter electrode and a RedRod reference electrode (0 mV vs Ag/AgCl; saturated KCl; REF201, Radiometer Analytical). Galvanostatically controlled pulses were applied to reduce nitrate anions, produce hydroxide ions and thereby precipitate crystalline, homogeneous ZnO films around the carbon filaments of the C mesh using an IviumStat potentiostat/galvanostat with the following pulse parameters: current density of − 7.6 mA cm−2 (based on the calculated surface area of the woven C mesh, which was accessible for the electrolyte), on- and off-pulse time of 0.01 s and total deposition time of 600 s – resulting in a total capacity of 6.4 mAh that corresponds to


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9.8 mg active material ZnO (corresponding to 29 wt.% of the total anode mass including C mesh, ZnO and IHCP) on the C mesh (13.2 mg). After deposition, samples were rinsed with deionized water and dried at room temperature for 12 h. The IHCP was introduced into the samples by a drop coating technique in an in-house manufactured sample holder in such a way that IHCP was only coated on the sample area with deposited ZnO (12 mm diameter). Both sides of the sample were either coated with AS4 (5 wt.% in 1-propanol; Tokuyama), KOH doped meta-polybenzimidazole (mPBI; 2 wt.% with 2.5 wt.% KOH in ethanol; Dapozol,

Danish

Power

Systems),

KOH

doped

poly[(1-(4,4´-diphenylether)-5-

oxybenzimidazole)-benzimidazole] (PBI-OO; 4 wt.% in dimethylacetamide; Fumatech) or FAA3 (10 wt.% in N-methyl-2-pyrrolidone; uncrosslinked Fumion, Fumatech). After IHCP coating, samples were dried for 48 h. Typically, all samples were coated with the same mass of dry IHCP (11.4 ± 0.6 mg). To obtain the OH− form of the IHCPs, IHCP-modified ZnO/C samples were immersed in [Zn(OH)4 ]2−-saturated KOH solution (4 mol dm−3; Alfa Aesar) for 1.5 h and rinsed with [Zn(OH)4 ]2−-free aqueous KOH (4 mol dm−3) before cell assembly. We assume that all IHCPs were loaded fully with OH− within this time, which should yield sufficient OH−-conductivity.

Assembly of Zn/NiOOH and Zn/O2 cells. A modified two electrode test cell (Toyo System) with an inserted coin-type cell casing (in-house manufactured; see Figure S1) and the following cell assembly procedure was used for Zn/NiOOH and Zn/O2 cells: first, a polytetrafluoroethylene disk (PTFE, 13 mm diameter) was placed on the backside of the electrochemical active area of the Zn anode to avoid direct contact with a current collector made of tin (Sn). Afterwards, the sample was fixed between a Sn-ring (outer diameter 16 mm and inner diameter 12 mm, 125 µm thickness, 99.99+%, Chempur) and a Sn-disk (16 mm diameter) used as current collector. By this means, circular electrical connection between the Zn anode and the Sn current collector was guaranteed. Two laminated separators (14 mm diameter; Celgard® C5550) were soaked in aqueous [Zn(OH)4 ]2−-saturated KOH (4 mol dm−3) and placed in between the Zn anode and the respective cathode. Zn/O2 cells were exclusively prepared with aqueous KOH (4 mol dm−3) without ZnO additive to avoid pore blocking due to precipitation of ZnO at the GDE surface.20 For Zn/NiOOH cells a commercial nickel oxide hydroxide/nickel hydroxide electrode (NiOOH/Ni(OH)2; Panasonic) with a cup-shaped nickel foil (Ni; 25 µm thickness, 99%, Alfa Aeasar) as current collector were used.23,24 For Zn/O2 cells a GDE (10 mm diameter; similar to previously by us reported 25,26

) with the bifunctional Sr2CoO3Cl catalyst was used and placed on top of the separator. A


4

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Ti-mesh (10 mm diameter, 0.076 mm wire thickness, Alfa Aeasar) as current collector was used at the cathode side. The current collectors on the cathode side were electrically contacted with a stainless steel spring. Overall, approximately 0.40 mL of electrolyte was used for a Zn/NiOOH cell and 0.25 mL for a Zn/O2 cell. For operando X-ray diffraction (XRD) of Zn/NiOOH cells, an electrochemical operando XRD cell from a previous study with cupshaped Ni foil as cell casing was used (see Figure S2).27 After cycling all samples were carefully rinsed with ethanol and dried at room temperature before further ex situ analysis.

Electrochemical characterization. Electrochemical characterization was performed using a SP-300 potentiostat/galvanostat (Biologic) at room temperature. Zn/NiOOH cells with an initial OCV of 1.10 V (discharged state) were galvanostatically charged to 1.98 V and discharged to 1.40 V at different current densities. All cells were cycled inside the laboratory where temperature control was set to 25°C. For operando X-ray diffraction measurements, Zn/NiOOH cells were cycled within the potential window of 1.40 to 1.98 V at ± 2.7 mA cm−2. Zn/O2 cells with an initial OCV of 1.00 V (discharged state) were galvanostatically cycled under active supply of humidified O2 (150 sccm; purity 5.0, Praxair) within the potential window of 0.90 V – 2.10 V at ± 2.2 mA cm−2. All current values were divided by the geometrical area of the smaller electrode (12 mm diameter for Zn anode for Zn/NiOOH cells and 10 mm diameter for GDE for Zn/O2 cells) to obtain the herein reported current densities. Cyclic voltammetry experiments were recorded in a three-electrode measurement setup (SVC-3 Voltammetry cell with PTFE cap, ALS Co., Ltd) containing a volume of 10 mL aqueous KOH solution (4 mol dm−3) with a Hg/HgO reference electrode (4 mol dm−3 KOH; RE-61AP, ALS Co., Ltd) and a Pt counter electrode (23 cm wire; ALS Co., Ltd).

Characterization of the hydroxide-conductive polymer membranes. All IHCP membranes were pretreated in 4 mol dm−3 KOH solution for 48 h. Hydroxide conductivity was estimated by using impedance spectroscopy (see Supporting Information). Stress-strain curves were obtained by a Comtech model QC-508E.28 To prevent drying of the membranes, they were constantly wetted with electrolyte during the measurement. [Zn(OH)4 ]2− permeability measurements for different IHCP membranes were carried out in a permeation measurement cell consisting of two 100 mL compartments, which were separated by a membrane with an active area of 2.99 cm² (cell 1) or 4.91 cm2 (cell 2). Compartment A was filled with 100 mL of 0.1 mol dm−3 NaCl in 4 mol dm−3 KOH solution and compartment B with 100 mL of 0.1 mol dm−3 ZnCl2 in 4 mol dm−3 KOH solution. The solutions in both


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compartments were stirred. The concentration of [Zn(OH)4 ]2− in compartment A was monitored by colorimetry:

29

3 mL samples of compartment A were taken repeatedly after

different periods of time and the chelating agent dithizone (Sigma Aldrich), dissolved in chloroform, was added. In a next step, the colored complex was extracted with chloroform and analyzed by UV/Vis spectroscopy (530 nm) to estimate the concentration cA of zinc ions (Zn2+) in compartment A (additional information can be found in the Supporting Information).

X-ray diffraction. As-prepared and cycled anodes as well as operando Zn/NiOOH cells were analyzed using an Empyrean X-ray powder diffractometer (Cu-Kα, 40 kV, 40 mA; PANalytical) in Bragg-Brentano geometry. Operando XRD patterns of Zn and ZnO reflections were continuously recorded in intervals of 1 mAh between 2θ angle range from 31° to 44°. The combination of battery cycling and XRD allows real time determination of the chemical nature of formed crystalline charge/discharge products.

Scanning electron microscopy. Microstructure images of as-prepared anodes and of cycled anodes were obtained with a Merlin high-resolution scanning electron microscope (SEM; Carl Zeiss AG, Germany). Moreover, energy-dispersive X-ray spectroscopy (EDX; 50 mm² Silicon Drift Detector X-Max, Oxford Instruments) was used for ex situ elemental analysis of the samples. Microstructure images of uncoated ZnO/C samples were obtained on a JEOL JSM-7500 F scanning electron microscope with focused ion beam (FIB) system (JIB 4601 F, JEOL) and the cross section image of the FAA3-modified ZnO/C sample was obtained on a TESCAN XEIA3 Triglav with plasma FIB. All other cross section images were prepared with a surgical knife.


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3. Results and Discussion Characterization of as-prepared Zn anodes. To ensure high utilization of the active material, the use of an electronic conductive host structure as a matrix with high surface area can be beneficial.19,30–33 In the present study, we used C mesh substrates with defined geometry as host structure for controlled and uniform electrodeposition of the active material ZnO. The benefit of starting with ZnO instead of Zn is to start with the active material at its maximum volume possible to avoid further expansion upon cycling. To characterize the morphology and distribution of electrodeposited ZnO on the C mesh host structure, as-prepared ZnO/C samples were investigated using SEM in combination with FIB etching. Figure 1a shows that ZnO is deposited throughout the entire C mesh. In addition, each single C filament of the woven C fiber (7 ±1 µm diameter) is covered with a polycrystalline and homogeneous layer of ZnO without cracks. From Figure 1a we estimate an average thickness of the ZnO layer of 1.5 ± 0.5 µm. Further, we observe a homogeneous and smooth interface between ZnO and C without cracks. Hence, the electrical contact needed for cycling of the entire active material ZnO is ensured. In order to investigate the electrochemical behavior of the as-prepared ZnO/C samples, we performed operando XRD analysis during battery operation of a Zn/NiOOH cell (see Figure S2 for setup used).

27

Figure 1b shows the first charge and discharge of the Zn/NiOOH

operando XRD cell with as-prepared ZnO/C sample as anode. Significant overcharging of the cell likely originates from the known formation process of the NiOOH/Ni(OH)2-electrode as well as from chemical side reactions due to the nature of the half-open cell setup (not observed in closed Zn/NiOOH cells; compare Figure S6).23,34 Figure 1c exhibits the associated XRD patterns and gray dots indicate the voltage and capacity of the operando cell at the time the respective XRD was measured in Figure 1b. Before cycling, only ZnO reflections (blue) are observed, proving the crystalline nature of the electrodeposited ZnO. During charge, the intensity of ZnO reflections decreases while that of all Zn reflections (red) increases continuously. After approximately 12 mAh, solely Zn reflections appear and no ZnO reflections can be observed anymore in the XRD pattern. At this capacity we presume complete conversion of ZnO to Zn, which implies that a full charge of the ZnO/C sample is achieved. Conversely during discharge, the intensity of Zn reflections decreases while the intensity of all ZnO reflections increases again. After approximately 5 mAh of discharge (corresponding to 514 mAh g−1 based on the deposited mass of ZnO and 78.5% depth of discharge [DoD]), the XRD pattern possesses solely ZnO reflections proving the complete discharge of the anode.


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All in all, we demonstrate that the used C mesh is suitable as host structure for alkaline Znbased anodes: electrical contact is ensured due to homogeneous and reproducible ZnO deposition and thus reversible conversion of ZnO to Zn can be achieved during charge/discharge.

Figure 1. Morphological, electrochemical and structural characterization of ZnO/C samples: (a) scanning electron micrographs combined with FIB etching of as-prepared ZnO/C sample. The sample possesses a homogeneous, polycrystalline ZnO layer around every C filament. (b) First cycle of an operando XRD Zn/NiOOH cell with C/ZnO sample as anode within the potential window of 1.40 – 1.98 V at ± 2.7 mA cm−2. (c) Operando XRD patterns show increasing and decreasing intensity of ZnO and Zn reflections, proving the electrochemical conversion of all ZnO to Zn during charge and ACS Paragon Plus Environment

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conversely during discharge. The discharge capacity (based on the electrodeposited mass of ZnO) corresponds to 514 mAh g−1.

Physical and electrochemical characterization of hydroxide-conductive polymers. To minimize [Zn(OH)4]2− movement into the bulk electrolyte, the added coating on the electrode surface must possess certain properties.17,18 The coating must be chemically stable in alkaline electrolytes and should exclusively transport OH⁻ and H2O to maintain the working principle of the alkaline Zn anode. In addition, the IHCP must be mechanically stable to accommodate the expected volume change during repeated transformation from ZnO to Zn and back. One class of materials that fulfills the aforementioned properties are ionomeric hydroxideconducting polymers (IHCPs). However, research on advanced IHCPs for application in Zn anodes is relatively scarce. In this work we focus on two different kinds of commercially available IHCPs and their potential ability to minimize [Zn(OH)4]2− movement into the bulk electrolyte: two AEIs (FAA3 and AS4), which are single ion conductors, showing a high selectivity for anions, and two ion-solvating polymers (mPBI and PBI-OO), which absorb significant amounts of KOH from alkaline solutions and conduct both cations and anions. To analyze the physical and chemical properties of the four different IHCPs (see Table 1) as well as the unwanted permeation of [Zn(OH)4]2−, the IHCPs were used in membrane form. In the case of AS4, the correlated commercially available membrane A201 is additionally crosslinked and has a higher ion exchange capacity (1.6 meq g−1 compared to 0.8 meq g−1). The Young’s modulus (elastic modulus) E and the tensile strength σts of the IHCP membranes (see Figure S3), which are measures for the stiffness of the material and the stress they can withstand before breaking, were estimated from stress-strain curves and are given in Table 1. mPBI is highly plasticized due to the high uptake of KOH, which implies high elasticity as well as low E and σts. FAA3 and PBI-OO absorb less KOH, and therefore both are much stiffer and tougher, which makes them suitable to successfully accommodate the volume change of the active material during cycling. The crosslinking of the A201 membrane results in the highest σts. Table 1. Chemical and physical properties of the hydroxide-conductive polymers (IHCP) applied in this work: composition of the IHCP membranes after pretreatment in 4 mol dm−3 KOH solution for 48 h, resulting KOH concentration in the IHCP membrane as well as the measured hydroxide conductivity σmem (compare 4 mol dm−3 KOH solution: 570 mS cm−1); respective calculations shown in the Supporting Information.35 In addition, Young’s modulus E and the tensile strength σts of the respective membranes are given. Both mechanical parameters were calculated from stress-strain ACS Paragon Plus Environment

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curves (Figure S3) and are a measure for how the IHCP will accommodate the volume change of the active material during cycling.

IHCP

Composition of the soaked KOH conc. in Hydroxide

Tensile

Young's

membranes / wt%

the membrane

conductivity

strength σts

modulus

/ wt%

σmem /

/ MPa

E

/

KOH

Water

Polymer

3.55

29.34

67.11

10.8

21.4

73.65

123.13

FAA-3

4.47

17.17

78.36

20.7

8.6

25.77

546.57

PBI-OO

7.61

41.11

51.29

15.6

10.9

22.74

485.97

mPBI

14.11

43.72

42.17

24.4

43.2

12.43

4.51

A201 (AS4)

*

−1

MPa

mS cm

*for a membrane made directly from uncrosslinked AS4, a higher water uptake and therefore higher conductivity but reduced tensile strength and Young's modulus than for A201 are expected.

[Zn(OH)4]2− permeation was monitored in a permeation measurement cell with two compartments (see Figure 2a) by colorimetric analysis (additional information in Figure S4). Figure 2a shows the estimated [Zn(OH)4]2− permeability P for all tested IHCP membranes. All of the here tested commercially available IHCP membranes allow a certain permeation of [Zn(OH)4]2−. The lowest P value, and hence a very low permeation rate for [Zn(OH)4]2−, is observed for FAA3. This low P may originate from the low water uptake in this IHCP membrane (see Table 1). Because high swelling of the polymer increases the pore size and the free volume in the hydrated polymer and decreases the strongly correlated selective permeation of ions through the polymer with higher water uptake.36 Higher P values for the other tested IHCP membranes also likely result from the formation of small cracks and partial decomposition of the IHCP membranes. Partial decomposition of the IHCP layer leads to the formation of cracks, and enables direct flux of [Zn(OH)4]2− into compartment A. In general, it is known that IHCPs are to some extent prone to degradation in alkaline solution (nucleophilic reaction with OH−, e.g. Hofmann elimination mechanism) during long term operation.37–41 Figure 2b shows cyclic voltammetry (CV) experiments on single ZnO/C yarns (compare Figure 1a for nomenclature) as a model electrode. The C yarns were loaded with ZnO according to the same procedure as the C meshes and subsequently coated with one of the respective IHCPs. All CVs were performed in an electrochemical cell with excess aqueous KOH (4 mol dm−3) electrolyte. The shown CVs prove that all used IHCPs and the C mesh


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host structure are electrochemically stable in the potential range between − 1.60 and − 0.80 V versus Hg/HgO reference electrode. Moreover, higher current peaks in the 5th and 20th CV cycle for both Zn deposition and dissolution in comparison to a ZnO/C yarn sample without IHCP as a reference (gray curve) imply that IHCP-modified samples show enhanced cycling stability – as the used electrolyte contained no additional [Zn(OH)4]2−. Especially, FAA3 and PBI-OO show a current spike in the cathodic sweep region between −1.06 to −1.10 V, which can be attributed to a closed ZnO layer (see Figure S5 for 1st CV cycle). 16,18 This implies that those two IHCPs successfully confine the active material at the interface between IHCP and C. Summing up, particularly FAA3 combines high chemical and mechanical stability with high KOH concentration in the polymer as well as low [Zn(OH)4]2− permeation through the polymer.

Figure 2. Chemical and electrochemical characterization of the used IHCPs: (a) permeability P of [Zn(OH)4]2− estimated from the monitored progression of the [Zn(OH)4]2− concentration in compartment A of the permeation cell for all tested IHCPs with schematic depiction of the used ACS Paragon Plus Environment

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permeation cell. Especially, FAA3 shows very low values of P for the permeation of [Zn(OH)4] 2−. (b) Cyclic voltammetry experiments for uncoated and different IHCP-modified ZnO/C yarn as model electrodes in aqueous KOH (4 mol dm−3) electrolyte (see schematic depiction on the left-hand side; based on single C yarn with 0.35 mg ZnO, which corresponds to a theoretical capacity of 0.23 mAh). CVs experiments start with cathodic sweep from OCP at 4 mV s−1 scan rate. CV cycle number five shows both a Zn deposition peak between − 1.40 and − 1.50 V and a Zn dissolution peak between – 1.35 and – 1.05 V for all samples. This implies that all IHCPs are electrochemically stable in the potential range between − 1.60 and − 0.80 V vs Hg/HgO reference. In addition, CV cycle number 20 shows for all IHCP-modified samples higher current peaks for both Zn deposition and dissolution than the uncoated sample (gray curve) indicating higher retention of the active material for IHCP-modified samples.

Electrochemical characterization of IHCP-modified Zn anodes. In a next step, we introduced a homogeneous coating with one out of the four IHCPs, but with the same mass of the respective IHCP (11.4 ± 0.6 mg), on the ZnO/C samples to investigate their impact on cycling stability. Thereby, the IHCP-modified Zn anodes yield a maximum possible capacity of 189.7 mAh ganode−1 based on the total weight of the anode including C mesh, ZnO and IHCP. In the case of a Zn/NiOOH cell this corresponds to an area-specific energy of 9.34 mWh cm−2, an energy density of 313.5 mWh ganode−1 and a power density of 243 mW ganode−1. The morphology of as-prepared anodes with IHCP was investigated using SEM combined with FIB etching (Figure 3a) to demonstrate that the surface is covered homogeneously with IHCP. Single ZnO/C filaments are covered with a layer of IHCP indicating sufficient adhesion to the ZnO layer. To investigate the cycling stability of the uncoated ZnO/C sample as Zn anode and the impact of the different IHCPs, Zn/NiOOH cells (see Figure S1 for cell casing) have been galvanostatically cycled. Cycling was performed without capacity limitation (Figure 3b,c and detailed charge/discharge profiles in Figure S6). After 30 charge/discharge cycles, the Zn/NiOOH cells were disassembled and the cycled anodes were analyzed ex situ by SEM (Figure 3d). Figures 3b, c show the discharge capacity Qdis (based on the electrodeposited mass of ZnO) and the respective Coulombic efficiency η for all anodes within 30 cycles. The uncoated sample yields an initial Qdis value of 525 mAh g−1, before continuously decreasing to 63 mAh g−1 after 15 cycles with an averaged Coulombic efficiency ηav of 77.2% within 30 cycles. Afterwards, shallow cycling with Qdis values less than 58 mAh g−1 is possible; however, almost all active material ZnO is dissolved into the electrolyte and cannot be ACS Paragon Plus Environment

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restored during charge.9,10 This observation is in line with the SEM image of the uncoated sample in Figure 3d. There, only bare C filaments without residual active material ZnO/Zn are attained after cycling. To ensure that the dissolved ZnO in the aqueous KOH (4 mol dm−3) electrolyte does not significantly affect Qdis of the Zn anodes as it can also be converted to Zn on the host structure in theory, a cell with an uncoated C mesh as anode was cycled (compare Figure S7; discharge not possible). To provide evidence that ZnO was converted completely to Zn and reversed during cycling in the case of IHCP-modified Zn anodes, we also performed operando XRD analysis and DoD-dependent electrochemical impedance spectroscopy measurements for a Zn/NiOOH operando cell with an IHCP-modified sample (see Figure S8 and S9b). In addition, we disassembled a Zn/NiOOH cell with IHCP-modified Zn anode after initial charge and proved the presence of Zn and the absence of ZnO by means of XRD, SEM and EDX (see Figure S10). IHCP-modified samples coated with various types of IHCP yield initial Qdis values between 457 mAh g−1 and 611 mAh g−1 (Figure 3b) and possess higher cycling stability with

Qdis values higher than 165 mAh g−1 (corresponds to 25% of the initial amount of ZnO) after 25 cycles compared to the uncoated sample (yellow region in Figure 3b). AS4- and FAA3modified samples provide values higher Qdis than 177 mAh g−1 with ηav of nearly 90% even after 30 cycles (see Table 2). This ηav value is 13% higher compared to the uncoated sample and close to reported values for commercial Zn/NiOOH cells. 42


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Figure 3. IHCP-modified ZnO/C samples and cycling performance of Zn/NiOOH cells with uncoated and IHCP-modified ZnO/C samples within 30 cycles together with ex situ analyzed scanning electron micrographs (11.4 ± 0.6 mg of respective IHCP, ± 4.4 mA cm−2, voltage window of 1.40 – 1.98 V): (a) exemplary IHCP-modified ZnO/C sample (here FAA3). The IHCP is homogeneously distributed and covers every single C filament. (b) Discharge capacity Qdis related to the mass of ZnO and to the entire anode mass (C mesh, ZnO, IHCP). (c) Respective Coulombic efficiency. (d) Scanning electron micrographs of uncoated and IHCP-modified Zn anodes after 30 cycles. Overall, Zn/NiOOH cells with IHCP-modified Zn anode yield higher Qdis values and improved cycling stability. Especially, FAA3-modified samples show significantly higher Qdis values after 30 cycles compared to the uncoated sample, which evidences that [Zn(OH)4]2− can be kept near the electrode surface with IHCP. On the contrary, uncoated samples do possess no more active material after cycling.


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The initial decrease of the Qdis value for all IHCP-modified samples to 354 ± 8 mAh g−1 (blue region in Figure 3b) within the first five cycles likely originates from the dissolution of active material ZnO from spots without IHCP coating (not observed for higher amount of IHCP; compare Figure 4). To assess the efficiency of the anode exclusively without influence of the counter electrode, we calculated the average utilization of the anode active material, herein referred to as χ, for all samples. χ is the averaged discharge capacity within 30 cycles (based on the electrodeposited mass of ZnO) divided by the theoretical capacity of ZnO (658 mAh g−1). All IHCP-modified samples possess approximately twice as high values of χ compared to the uncoated sample (23.3%); AS4 and FAA3 show the highest χ values of 50.6 and 50.0%, demonstrating that the active material can be accessed and electrochemically converted very well. To further emphasize that all IHCPs increase the retention of active material ZnO and, therefore, the cycling stability of our novel IHCP-modified Zn anode, we introduce and evaluate the value β. β is the ratio of the discharge capacity after 30 cycles (based on the electrodeposited mass of ZnO) and the theoretical capacity of ZnO for all samples. It is given in Table 2. Particularly noteworthy is the value for β of 38.5% for the FAA3-modified sample. It reflects a very high achievable retention of the active material ZnO: 3.9 times more active material is left on the anode after 30 cycles compared to the uncoated sample. This high retention is in line with the permeation measurement in Figure 2a – as FAA3 possesses the lowest P value. Figure 3d depicts the morphology of cycled IHCP-modified samples (see Figure S11 for additional EDX analysis): for all samples the IHCPs are still present on the surface. Especially for FAA3 it appears that the IHCP is still homogeneously distributed around the entire surface area of the C mesh indicating that the active material ZnO is present underneath the IHCP layer. This observation implies that FAA3 is mechanically stable to withstand the expected mechanical stress resulting from the volume change due to ZnO/Zn conversion during cycling. As a result, it yields the highest retention of active material ZnO (β > 38%) after 30 cycles. In contrast, spots without IHCP coating and with agglomerated ZnO/Zn crystals are observed for AS4, PBI-OO and mPBI indicating lower mechanical and chemical stability.

Table 2. Estimated utilization and retention of active material ZnO for uncoated and IHCP-modified ZnO/C samples as anode with averaged Coulombic efficiency ηav within 30 cycles. The parameter χ is


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defined as ratio of the averaged Qdis within 30 cycles (related to nominal mass of ZnO) and the theoretical initial discharge capacity and is thus a measure for the utilization of the active material ZnO. The parameter βi is given as ratio of Qdis after i cycles (related to nominal mass of ZnO) and the theoretical initial discharge capacity (658 mAh g−1) and is thus a measure for the retention of the active material ZnO.

sample

AS4

FAA3

PBI-OO

mPBI

uncoated

ηav / %

89.0

94.0

88.5

77.2

77.2

χ/%

50.6

50.0

40.4

36.1

23.3

β/%

27.9

38.5

24.2

15.1

9.8

All in all, our novel IHCP-modified Zn anodes with incorporated IHCP show improved cycling performance: Especially, FAA3, which combines chemical and mechanical stability with a low permeation rate of [Zn(OH)4]2− through the IHCP layer, shows high utilization and retention of the active material by up to 4 and 2 times, respectively, in comparison with an uncoated Zn anode. Based on the total weight of anode (including C mesh, ZnO and ionomeric layer) the anode with FAA3 yields an achieved specific capacity of 92.1 mAh ganode−1 averaged over 30 cycles. In the case of a Zn/NiOOH cell this corresponds to an averaged area-specific energy of 5.17 mWh cm−2, an energy density of 152 mWh g−1 and a power density of 206 mW ganode−1. Overall, the presented data implies that the benefit of using an IHCP on a Zn anode can be successfully transferred to large area secondary Zn anodes.

Impact of used amount of IHCP and applied current density on the practical cycling performance. From a practical point of view, the coating thickness of a closed IHCP layer should be as low as possible to minimize the penalty in energy density due to additional weight and increased resistance for ion transport. On the other hand, the coating thickness must be as thick as necessary to confine [Zn(OH)4]2− and thus to achieve high retention of the active material during cycling. To investigate the influence of the applied amount of IHCP on the cycling performance of IHCP-modified Zn anodes, additional Zn/NiOOH cells with AS4 and FAA3 samples have been cycled. Figure 4a shows the specific discharge capacity, Qdis, for cells with different amounts of IHCP. The applied amount of IHCP is normalized to m0, which is the arbitrarily chosen mass of 11.4 mg. The uncoated sample can be cycled 10 times until less than 165 mAh g−1 can be achieved, which corresponds to a loss of 75% of the initial amount of ZnO. By contrast, as the ACS Paragon Plus Environment

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proportion of IHCP increases, the achievable cycle number until Qdis < 165 mAh g−1 can be significantly increased up to 90 cycles for 2.9 · m0 FAA3. This implies, that by increasing the amount of IHCP – and thus the coating thickness – the barrier for selective ion permeation of OH− and [Zn(OH)4]2− can be increased (as reported by Miyazaki et al.17) and consequently the flux of [Zn(OH)4]2− into the bulk electrolyte can be reduced. This observation underlines the essential requirements for an IHCP layer in Zn anodes18: a closed, homogeneous and chemically stable IHCP layer for the confinement of [Zn(OH)4]2− is essential to increase cycling stability and retention of the active material. If the IHCP does not fully cover the entire active material, [Zn(OH)4]2− will migrate into the bulk electrolyte and thus Qdis decreases rapidly within the first five cycles (see Figure 4a). Only a high amount of IHCP, such as 2.9 · m0 FAA3, can ensure that the entire amount of ZnO can be covered. Hence, Qdis does not decay within the few first cycles. On the downside, this increased amount of FAA3 yields lower values of Qdis within the first five cycles and indicates that probably not all ZnO is electrochemically converted to Zn. This we attribute to a limited OH− supply for FAA3. To analyze the impact of electrode kinetics and OH− supply on the performance of IHCPmodified ZnO/C samples, Zn/NiOOH cells with FAA3- and AS4-modified anodes (1.0 · m0) at different current densities j from 2.2 to 15.9 mA cm−2 were cycled. Figure 4b shows the specific discharge capacity Qdis,1 of the first cycle at different j, which is a measure of the initial utilization of the active material. Both IHCPs show the same trend: increasing the current density from 2.2 up to 25.0 mA cm−2 results in lower Qdis,1 indicating lower utilization of the active material, which we attribute to limited mass transport of OH− through the IHCP. Comparing both IHCPs (see Table 1 for respective OH− conductivity), higher Qdis,1 are observed for AS4 than for FAA3 at the same current density. In line with the trend for Qdis,1, electrochemical impedance spectroscopy performed on Zn/NiOOH cells before galvanostatic cycling prove a higher ionic resistance for IHCP-modified samples compared to uncoated samples (see Figure S9a). In addition, Figure 4c depicts Qdis,30 after 30 cycles, which is a measure of the retention of active material. For both IHCPs Qdis,30 decreases for higher j values, whereas anodes with FAA3 possess higher values than anodes with AS4 until j < 13 mA cm−2 (blue line). This behavior might be attributed to higher chemical and mechanical stability as well as to the lower permeation rate of [Zn(OH)4]2− (see Figure 2). For j > 13 mA cm−2 the mass transport of OH− through the IHCP likely limits the utilization of the active material.


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Summing up, our analysis hints that the achievable cycling stability for operation of IHCPmodified Zn anodes in Zn/NiOOH cells depends on the nature of the IHCP and can be limited by the mass transport of OH− through the IHCP layer. Hence, balancing between IHCP coating thickness and applied current density is crucial for optimizing the practical cycling performance of IHCP-modified Zn anodes.

Figure 4. Cycling performance of Zn/NiOOH cells with IHCP-modified Zn anodes (AS4, FAA3) for varied amount of the IHCP (normalized to m0 = 11.4 mg) and current density j (voltage window of 1.40 – 1.98 V): (a) specific discharge capacity Qdis (based on the electrodeposited mass of ZnO) for AS4- and FAA3-modified Zn anodes with different amount of IHCP at 4.4 mA cm−2. Higher IHCP amount leads to higher retention of the active material, but can also result in lower utilization of the active material due to limited accessibility for OH−. Qdis for cells with FAA3- and AS4-modified Zn anode (1.0 · m0) at different j for (b) the first cycle (Qdis,1) and (c) after 30 cycles (Qdis,30). Higher j values lead to limited mass transport of OH− through the IHCP layer and with that to decreased utilization of the active material. Thus, both considered parameters have to be balanced to enhance the cycling performance and stability of IHCP-modified Zn anodes.


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Zn/O2 cells with novel IHCP-modified Zn anode. To emphasize the beneficial impact of the IHCP-modified secondary Zn anodes presented here, we cycled Zn/O2 coin-type cells in a special configuration with an FAA3-modified Zn anode and aqueous KOH (4 mol dm−3) electrolyte without [Zn(OH)4]2− in the bulk. In this cell configuration with electrolyte that contains no [Zn(OH)4]2−, the active material ZnO can easily vanish into the alkaline electrolyte if not confined by an IHCP because of the high driving force for Zn species to dissolve into the electrolyte (compare also Figure S12 for Zn/NiOOH cells with [Zn(OH)4]2−free electrolyte).9,10,42 Figures 5a, b show representative charge/discharge profiles of a Zn/O2 cell with FAA3modified Zn anode as well as the observed specific capacity within 13 cycles. Cycling was performed using active supply of oxygen and constant charge capacities of 658 mAh g−1 (based on the electrodeposited mass of ZnO), which shall result in the complete conversion of ZnO to Zn for an ideal working Zn/O2 cell without any side reactions. Averaged specific discharge capacity Qdis of 351 mAh g−1 (corresponding to a DoD of 53.6%) and averaged Coulombic efficiency of 71.1% within 13 cycles are observed. The lower charge-discharge efficiency compared to the Zn/NiOOH cells (compare Table 2) we attribute to a higher Zn dissolution and corrosion rate – as there is no additional Zn species dissolved in the electrolyte.43 The rapid decrease in Qdis after 10 cycles likely originates from drying-out of the coin-type cell due to the limited amount of electrolyte used (in this case only 250 µL for Zn/O2 cells and 400 µL for Zn/NiOOH cells). However, the use of cells with a limited amount of electrolyte solutions is a practically relevant cell test compared to the commonly used flooded cells with an excess of electrolyte volume (see Table S1).23

Figure 5. Cycling performance of a Zn/O2 cell (FAA3-modified Zn anode, GDE with Sr2CoO3Cl 2−

catalyst 26) with aqueous KOH (4 mol dm−3) electrolyte without [Zn(OH)4] : (a) Charge/discharge ACS Paragon Plus Environment

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profiles (± 2.2 mA cm−2, voltage window of 0.90 – 2.10 V, capacity limit during charge of 658 mAh g−1 based on the electrodeposited mass of ZnO). (b) Specific discharge capacity Qdis (based on the electrodeposited mass of ZnO) corresponds to an averaged DoD of 53.6% within 13 cycles.

4. Conclusions We present a reversibly cycled zinc anode that benefits from the combination of two lightweight materials: a carbon mesh as host structure and the active material zinc oxide that is confined by a coating with ionomeric hydroxide-conductive polymer. The proposed design for the anode enables cycling with high stability and retention of the active material. Operando X-ray diffraction measurements on zinc-nickel oxide hydroxide cells with the presented anode prove that the uniformly distributed zinc oxide can be electrochemically converted to zinc during charge and conversely during discharge. Electrochemical cycling data of cells with applied ionomeric layer demonstrate improved cycling performance with high utilization and retention of the active material by up to 4 and 2 times, respectively, in comparison with an uncoated anode. Particularly, FAA3 with low permeation for zincate ions as well as high electrochemical and mechanical stability performed best during long-term cycling. In addition, our data indicate that the nature of the ionomeric layer and the mass transport of hydroxide through it can have a crucial impact on the cycling performance. All in all, our study identifies FAA3 to be the most suitable – commercially available – ionomeric hydroxide-conductive polymer for application in the reversible zinc anode to enhance cycling stability. Admittedly, intensive research on polymers can further propel the search for even better suited materials in this context: to enhance the cycling stability of secondary zinc-based batteries, shaping the mechanical and chemical stability, as well as the exclusive transport of hydroxide ions might be subject of future studies on existing and/or novel ionomeric materials.

ASSOCIATED CONTENT Supporting Information Experimental cell setup for Zn/NiOOH cells; experimental cell setup for operando XRD Zn/NiOOH cells; stress-strain curves of IHCP membranes; zincate ion permeation in IHCP membranes; cyclic voltammetry on IHCP-modified ZnO/C yarn model electrodes; charge/discharge profiles of Zn/NiOOH cells using different IHCP-modified Zn anodes; charge/discharge profiles of Zn/NiOOH cells using C mesh without electrodeposited ZnO; operando XRD on Zn/NiOOH cell with IHCP-modified ZnO/C sample; electrochemical ACS Paragon Plus Environment

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impedance spectroscopy on IHCP-modified Zn anodes; ex situ analysis of an IHCP-modified Zn anode after first charge step; elemental distribution on the surface of cycled IHCPmodified Zn anodes; Zn/NiOOH cells with IHCP-modified Zn anode with zincate-free electrolyte; survey of reported secondary Zn anode concepts comparing used amount of electrolyte and cycling performance

AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare the following competing financial interest(s): D. St., S. D. and D. S. have filed a provisional patent on the anode composition presented in this work. Acknowledgments The authors gratefully acknowledge financial support by the BMBF (Federal Ministry of Education and Research) within the project ‘Zisabi’ (03XP0086) and by the DFG via the GRK (Research Training Group) 2204 "Substitute Materials for sustainable Energy Technologies". Moreover, we acknowledge Kohei Miyazaki and Takeshi Abe (Kyoto University) for providing GDEs with Sr2CoO3Cl catalyst, and most importantly for fruitful discussions regarding the use of hydroxide conductive polymers in zinc electrodes. We thank Jürgen Janek and Bjoern Luerßen for fruitful discussions. We also acknowledge Rainer Straubinger and Kerstin Volz (University of Marburg), as well as Boris Mogwitz (University Giessen) for FIB-SEM measurements. References (1)

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TOC


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Figure 1 / Characterization of ZnO/C anode 105x135mm (300 x 300 DPI)


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Figure 2: Characterization of ionomeric polymers 78x77mm (300 x 300 DPI)



Figure 3: Characterization of electrochemical performance and anode morphology 149x186mm (300 x 300 DPI)


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Figure 4 / Impact of current density and coating thickness 107x98mm (300 x 300 DPI)



Figure 5 / Performance of Zn/air battery 72x47mm (300 x 300 DPI)


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Graphical Abstract (TOC) 35x15mm (300 x 300 DPI)


Design Strategy for Zinc Anodes with Enhanced Utilization and

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