α-NiO/Ni(OH)2/AgNP/F-Graphene Composite for Energy Storage Application

The α-NiO/Ni(OH)2/AgNP/F-graphene composite, which is silver nanoparticles preanchored on the surface of fluorinated graphene (AgNP/FG) and then added to α-NiO/Ni(OH)2, is investigated as a potential battery material. The addition of AgNP/FG endows the electrochemical redox reaction of α-NiO/Ni(OH)2 with a synergistic effect, resulting in enhanced Faradaic efficiency with the redox reactions of silver accompanied by the OER and the ORR. It resulted in enhanced specific capacitance (F g–1) and capacity (mA h g–1). The specific capacitance of α-NiO/Ni(OH)2 increased from 148 to 356 F g–1 with the addition of AgNP(20)/FG, while it increased to 226 F g–1 with the addition of AgNPs alone without F-graphene. The specific capacitance of α-NiO/Ni(OH)2/AgNP(20)/FG further increased up to 1153 F g–1 with a change in the voltage scan rate from 20 to 5 mV/s and the Nafion-free α-NiO/Ni(OH)2/AgNP(20)/FG composite. In a similar trend, the specific capacity of α-NiO/Ni(OH)2 increased from 266 to 545 mA h g–1 by the addition of AgNP(20)/FG. The performance of hybrid Zn–Ni/Ag/air electrochemical reactions by α-NiO/Ni(OH)2/AgNP(200)/FG and Zn-coupled electrodes indicates a potential for a secondary battery. It results in a specific capacity of 1200 mA h g–1 and a specific energy of 660 W h kg–1, which is divided into Zn–Ni reactions of ∼95 W h kg–1 and Zn–Ag/air reactions of ∼420 W h kg–1, while undergoing a Zn–air reaction of ∼145 W h kg–1.


INTRODUCTION
Energy storage devices have been developing rapidly due to a growing market for portable electronic devices including electric vehicles, mobile instruments, and large-scale energy storage applications. Typical energy storage devices depend on the conversion of chemical energy to electricity defined by electrochemical redox reactions in fuel cells, batteries, and electrochemical pseudocapacitors (i.e., supercapacitors). Supercapacitors (SCs) are useful components for portable electronic devices, providing high power densities, long life cycles, and short charging times. Despite relatively poor storage capacities, hybrid supercapacitors have attracted increasing attention over the last decade. 1−11 The differences between SCs and batteries are due to their distinctly different energy storage mechanisms, although they have similar components such as electrodes, separators, and electrolytes. For example, Li-related batteries like Li-ion and Li-metal have high energy densities but low power densities. In contrast, SCs have high power densities with long-term cycle stability but low energy densities. A lot of research has focused on enhancing the specific energy of battery−supercapacitor hybrid (BSH) devices by modification of the electrode potential with optimizing material properties in a complementary strategy. 1−4 The research field of supercapacitors has focused on replacing RuO 2 with NiO, Co 3 O 4 , MnO 2 , Co(OH) 2 , or Ni(OH) 2 as low-cost alternative materials for energy storage. Ni attracts our attention because it is abundant and environmentally friendly, and it has also proved its electrochemical properties through lots of research. For example, Ni(OH) 2 undergoes electron transfer reactions, behaving like a capacitor that stores energy via redox (i.e., Faradaic) reactions. Ni(OH) 2 has two polymorphs, αand β-phases. β-phase consists of well-ordered stacked layers of Ni(OH) 2 , while α-Ni(OH) 2 has randomly stacked and positively charged layers aligned along the c-axis. Exchangeable anions and water molecules are intercalated in the spaces between layers to provide overall electroneutrality in α-Ni(OH) 2 . Lee et al. reported the specific capacitance of α-Ni(OH) 2 , which is dependent on the size of the intercalated anions between layers (e.g., the largest anion of SO 4 2− yields a low specific capacitance while giving the highest specific capacitance with the intercalation of the smallest anion of Cl − ). 12 It has been attributed to charge repulsion between the intercalated anion and OH − in the alkaline electrolyte. 5 In comparison, the electrochemical performance of the primary catalyst embodied in the electrode (EC) is dependent on the size and shape of the electrode materials. For example, α-Ni(OH) 2 normally exhibits better electrochemical performance than β-Ni(OH) 2 . Batteries that are α-Ni(OH) 2 -based undergo a capacity decrease over charge−discharge cycles due to a phase transformation into the β-phase under alkaline conditions because of the greater thermodynamic stability of β-Ni(OH) 2 compared to the α-phase Ni(OH) 2 . 13−15 To retard the phase transformation from α to β phase, the metal ion dopants such as Co 3+ , Mn 3+ , Zn 2+ , Cd 2+ , and Al 3+ are often added to stabilize the crystal structure of α-Ni(OH) 2 . 16 −23 On the other hand, the Faradaic property of the metal hydroxide is relatively low compared to the metal oxide due to an intrinsic poor electronic conductivity. The surface-modified α-NiO/Ni(OH) 2 composite, which contains NiO partially, has been reported to have superior pseudocapacitive properties, indicating a relatively high Faradaic current with enhanced surface area compared to Ni(OH) 2 itself. 15 The fabrication of Ni(OH) 2 electrodes normally requires an adhesive binder such as a conventional conducting polymer. However, it still gives low conductivity, resulting in inefficient electron transfer between the electroactive material and the current collector. Carbonaceous additives have been used to compensate for the low conductivities. 24,25 Alternatively, inorganic colloidal nanocrystalline semiconductors have served as choice materials for electronic and optical devices due to their useful physical and electrochemical properties to improve conductivity. The nanocrystals could be readily mass-produced and used for device manufacturing, but still challenging to use for efficient electronic charge transfer from one to another in case the bulky organic surface ligand is capped on nanocrystals, which serves as an insulator. As reported, the electronic coupling between nanocrystals could be dramatically increased by the substitution of insulating organic molecules with novel inorganic molecules as reported previously. 26 Metallic silver, Ag, can be used as an efficient bifunctional catalyst material in metal−air batteries since it often improves the performance of the OER and the ORR in aqueous alkaline media. The ORR occurs via direct or indirect processes. The direct ORR is a near-four-electron transfer reduction of O 2 to OH − , while the indirect ORR is a two-electron transfer oxygen reduction, which is a lower efficiency pathway due to the disproportionation of HO 2 − . 27 AgNPs have proven to be excellent electrocatalysts in the ORR as they catalyze the fourelectron pathway, reducing oxygen to OH − . 28 In addition, the halogenated graphene nanoplate like F-graphene has been reported to exhibit remarkable electrocatalytic activities for the oxygen reduction reaction (ORR) in alkaline electrolytes with long-term cycle stability. In detail, the halogenation into the sp 2 -hybridized orbitals of carbon regulates the surface electronic structure that plays a key role in ORR activities. As evidence, fluorine-functionalized graphene (F-graphene) has proven to be an even better four-electron ORR catalyst than Pt/C, which is known as a promising ORR catalyst. 33−36 In this study, we developed the electrode α-NiO/Ni(OH) 2 / AgNP/FG, which is composed of α-NiO/Ni(OH) 2 as the primary component with the silver nanoparticles anchored to fluorinated graphene (AgNP/F-graphene). This is likely a polymer binder-free electrode as-prepared successfully using a TiO 2 nanogel instead of a conventional conductive polymer. The AgNPs anchored to F-graphene (AgNP/FG) promise an enhanced OER and ORR. This particular composite of α-NiO/Ni(OH) 2 /AgNPs/F-graphene results in an increased specific capacitance and specific capacity compared to the α-NiO/Ni(OH) 2 core electrode. 29 3.0 g of urea was added and mixed via magnetic stirring until a homogeneous green transparent solution was obtained. The green transparent solution was transferred to a Teflon-lined autoclave and then heated at 110°C for 15 h. The solid product was washed several times with each Milli-Q water and acetone and then dried in the oven at 80−100°C overnight to obtain α-Ni(OH) 2 . The calcination of α-Ni(OH) 2 was carried out at 260, 280, and 300°C for 2 h. 15 The mixed phase of α-NiO/Ni(OH) 2 calcined at 280°C was used as a major material for the current study of energy storage applications. 2.4. Synthesis of the TiO 2 Nanogel. TiO 2 was synthesized according to the following procedure. Titanium tetraisopropoxide, Ti(OCH(CH 3 ) 2 ) 4 , was diluted with isopropyl alcohol (IPA), and then it was added drop by drop into Milli-Q water in an M H2O /M TTIP mass ratio of 110 and acidified with acetic acid to pH = 2. After complete hydrolysis, the suspension was heated at 90°C for 4 h with vigorous stirring. TiO 2 particles were collected by centrifugation that yields 5 wt % sol−gel conc., which is defined as a TiO 2 nanoglue. 76,77 2.5. Electrode Fabrication. The α-NiO/Ni(OH) 2 electrode was prepared with the slurry of α-NiO/Ni(OH) 2 and TiO 2 nanogel mixture with an estimated mass ratio of ∼2.5:1 in ethanol and it was coated onto the carbon paper and then heat-treated at 250°C for 1 h. The α-NiO/Ni(OH) 2 /AgNP electrode was prepared in a similar way as mentioned above. A α-NiO/Ni(OH) 2 /AgNP/F-graphene composite electrode was prepared using the same technique with a slurry mixture in an estimated mass ratio of 1:2.5:1−1.2 for AgNP/FG, α-NiO/ Ni(OH) 2 , and TiO 2 nanogel. Annealing was carried out at 250°C for 1 hr. The proportion of AgNPs to the total mass of active material including α-NiO/Ni(OH) 2 , TiO 2 , and Fgraphene is about 1 % for AgNP (20), ∼1.5% for AgNP (30), and ∼10 % for AgNP(200). The Zn/C electrode was prepared by applying a slurry coating on the carbon paper with a mixture of Zn powder, TiO 2 nanogel, and F-graphene in a mass ratio of 3:5:0.15. After fabrication of α-NiO/Ni(OH) 2 /AgNP/FG electrodes, the surface obtained was a thin layer coated with as little Nafion as possible, followed by heat treatment at 130°C

Synthesis of AgNP/F-Graphene and α-NiO/Ni-
for an hour. The schematic illustration of the fabrication and synthesis process of α-NiO/Ni(OH) 2 /AgNP/FG composite electrode is provided in Figure S1.
2.6. Analytical Methods. The crystal structure of α-NiO/ Ni(OH) 2 was confirmed with an X-ray diffractometer (XRD, PANalytical X-ray diffractometer, X'Pert Pro) equipped with Cu Kα radiation (λ = 1.50405 Å). The mesoporosity of α-NiO/Ni(OH) 2 and α-Ni(OH) 2 was confirmed by Brunauer− Emmett−Teller (BET) analysis, where the isotherms of nitrogen adsorption−desorption profile are observed with a distinct hysteresis loop in the high range of relative pressure between 0.5 and 1.0. The BET was carried out using a Micromeritics TriStar II surface area and porosity analyzer. The electrode surface morphology was investigated via SEM using a ZEISS 1550VP Field Emission Scanning Electron Microscope, with energy-dispersive X-ray spectroscopy (EDS, INGA Oxford) to identify the materials. Fourier transform infrared spectroscopy (FTIR) was carried out on a Nicolet iS50 spectrometer (Thermo Scientific, USA) with an integrated diamond crystal as an accessory. UV−vis diffuse reflectance spectra were determined using a Shimadzu UV-2101PC (dual-beam) equipped with an integrating sphere attachment (Shimadzu ISR-260), which is used to obtain reflection and transmittance measurements of liquid and solid samples. BioLogic Science potentiostats (VSP and SP 50) were used to acquire cyclic voltammograms and charge−discharge profiles.

RESULTS AND DISCUSSION
urea. SDS was used as a structure-directing reagent, while urea was added to increase the pH, thus forcing precipitation of Ni(OH) 2 in a water−ethanol mixture. In the presence of excess ammonia, a tetrahedral ammonia complex, Ni(NH 3 ) 4 2+ , was formed. The NH 3 ligands of Ni(NH 3 ) 4 2+ undergo hydrogen bonding with dodecyl sulfate, which eventually leads to a complete ligand exchange of ammonia by dodecyl sulfate anions. Under these conditions, Ni(OH) 2 nanosheets were formed. With aging, the nanosheets incorporate dodecyl sulfate between the nanosheet layers, leading to the formation of microspheres due to Ostwald ripening. 15,37 α-Ni(OH) 2 was characterized by XRD, FTIR, and BET analysis. Dehydration of Ni(OH) 2 was carried out with annealing at ≥260°C, and it formed a mixed phase of NiO and α-Ni(OH) 2 with a color change from green to dark gray (or dark green) depending on the temperature and time. The annealing at 300°C for 2 h resulted in mostly NiO as identified with the XRD peaks at 38, 41, and 61, corresponding to the (111), (200), and (220) crystalline phases of NiO (Figure 1a−d). This is in accordance with the FTIR spectrum as no characteristic peak of O−H stretching was detected from >NiO−H at 3600 cm −1 (Figure 1b). 12 The residual products were still detected at 1600, 1490, 1310, and 2170 cm −1 , which are the bending or stretching modes of N−H, C−H, S�O, and SCN, respectively, and those are presumably the residue of decomposed dodecyl sulfate and urea. 38 The TiO 2 nanogel might catalyze the decomposition of SCN − at the surface of α-NiO/Ni(OH) 2 during the electrode fabrication process.
The surface area of α-Ni(OH) 2 and α-NiO/Ni(OH) 2 was confirmed by Brunauer−Emmett−Teller (BET) analysis. The isotherm of nitrogen adsorption−desorption has a hysteresis loop over the relative pressure range of 0.5 to 1.0 and resulted in a surface area of 225 m 2 g −1 for NiO and 149 m 2 g −1 for the mixed phase of α-NiO/Ni(OH) 2 . The surface area of α-Ni(OH) 2 dried at <100°C without a sintering process is obtained as 88 m 2 g −1 (Figure 1c).

Evaluation of Electrochemical
Cyclic voltammetry (CV) curves for the electrodes were obtained under identical experimental conditions to determine the effect of AgNPs and F-graphene on the redox reactions of α-NiO/Ni(OH) 2 ( Figure 3). The electrodes were coated with a 5% Nafion solution to avoid a mass loss or migration of α-NiO/Ni(OH) 2 . The CV curve of α-NiO/Ni(OH) 2 alone ( Figure 3a) has a pair of redox peaks that is consistent with the oxidation of Ni 2+ to Ni 3+ at 0.41 V and the reduction of Ni 3+ to Ni 2+ at 0.13 V in a reversible process, which is reproducible while repeating CV cycles ( Figure 3b). The redox potential of α-NiO/Ni(OH) 2 is shifted to a higher potential region with increased current density upon the addition of AgNPs and AgNP/FG (Figure 3c). This is attributed to the electrochemical and physical properties of AgNPs and F-graphene. As a consequence, the specific capacitance increases in the following order: . The silver nanoparticles (AgNPs) in AgNP(20)/Fgraphene are distributed quite evenly over the surface of fluorinated graphene with their particle size, d < 20 nm, as shown in SEM images. AgNPs anchored to F-graphene seem to maintain a nanosize characteristic owing to the twodimensional hexagonal lattice structure of F-graphene having sp 2 -hybridized orbitals. In detail, the electrons located in a p z orbital in graphene most likely form a π bond that hybridizes together to form a πand π*bands, allowing its exceptional electronic properties. In the case of fluorine-substituted graphene, some free-moving electrons in a p z orbital interact with fluorine, leading to the formation of σ-bonds directed toward the z-axis, which is perpendicular to the graphene plane. Thus, the electronic and structural properties of the Fgraphene may allow the silver nanoparticles to preserve quantized electrophysical properties. 39 In contrast, the silver particles that formed on the surface of α-NiO/Ni(OH) 2 are about 10 times larger than those on F-graphene, and they are oxidized and formed Ag 2 O clusters in strong alkali electrolytes. As a reason, the surface of α-NiO/Ni(OH) 2 is partially   Figure 3g). The specific capacitance that is inversely proportional to the voltage scan rate is due to a Faradaic redox reaction limited by the diffusion rate of OH − . A Nafion-free electrode was prepared using more TiO 2 nanoglue to avoid any mass loss of the active material during electrolysis on the Nafion-free α-NiO/Ni(OH) 2 /AgNP(20)/ FG electrode. Repeated CV cycles over 2 h confirm that the α-NiO/Ni(OH) 2 /AgNP(20)/FG electrode is stable without Nafion coating. The Nafion-free composite electrode exhibited improved current efficiency compared to the Nafion-coated electrode and resulted in an increased specific capacitance of up to 979 F g −1 . In addition, a reduced voltage scan rate from 20 to 5 mV/s resulted in a further increased specific capacitance of up to 1153 F g −1 . Figure 4 summarizes the specific capacitances of α-NiO/Ni(OH) 2 family electrodes, which are dependent on the voltage scan rates with the composition of active materials, and the presence and absence of coated Nafion. Since Nafion affects the current efficiency, we minimized Nafion usage. (20)/FG was explored based on the voltage profiles of charge/discharge cycles carried out using a standard three-electrode setup in 1.0 M KOH electrolyte. The discharge of α-NiO/Ni(OH) 2 electrodes takes place initially at the potential of 0.29 V and −0.45 V vs Ag/AgCl, which is consistent with a sequential reduction of Ni 3+ to Ni 2+ and Ni 2+ to Ni 0 . The electrode with added silver nanoparticles, AgNP (10), to the core α-NiO/Ni(OH) 2 resulted in an extended plateau at −0.43 V (vs Ag/AgCl) (Figure 5a). In fact, it is possible for some of the nanosilver particles, Ag 0 , to be oxidized to Ag + as the species, AgOH, at pH 14 (i.e., formation constants for AgOH, β 1 = 10 2 M −1 and Ag(OH) 2 − , β 2 = 10 4 M −2 ) in 1.0 M KOH. AgOH forms a dark brown silver(I) oxide (Ag 2 O, log K sp = −7.7) via (2 AgOH ⇌ Ag 2 O + H 2 O, K = 10 5.75 ). Thus, the extended plateau at −0.43 V (vs Ag/AgCl) is attributed to the reduction of Ag 2 O to Ag occurring at a potential close to the reduction of Ni 2+ to Ni 0 . On the other hand, the α-NiO/Ni(OH) 2 /AgNP(20)/FG electrode exhibits extended plateaus at the potential of 0.29 and −0.45 V vs Ag/AgCl (Figure 5b). The AgNP(20)/FG, which has preanchored silver nanoparticles on the surface of Fgraphene, seems to cause a synergistic effect on the overall electrochemical reactions due to the interaction of their electrophysical properties, not only between F-graphene and AgNP but also between AgNP/F-graphene and α-NiO/ Ni(OH) 2 . The highest specific capacity with the α-NiO/ Ni(OH) 2 electrode is obtained at the current density of 1.1 A g −1 , while the α-NiO/Ni(OH) 2 /AgNP(20)/FG reaches its highest specific capacity at the current density of 2.1 A g −1 . It results in a 1.4-fold enhanced specific capacity compared to α-NiO/Ni(OH) 2 itself (Figure 5c,d).

Specific Capacity. The half-cell redox reaction of the α-NiO/Ni(OH) 2 /AgNP
The current density in the charge/discharge process is a crucial factor to achieve durable charge/discharge cycles, giving a reliable specific capacity. To explore how it affects electrochemical reactions, the charge/discharge voltage profiles of α-NiO/Ni(OH) 2 /AgNP(20)/FG are determined as a function of current density (Figure 6a). At the current density  (Figure 6c). On the other hand, a repeating charge/discharge cycle at the relatively high current density of 8.3 A g −1 leads to a higher oxidation status of silver like Ag 2 O 3 caused by the OER, which results in an additional reduction reaction of Ag 2 O 3 to AgO along with AgO to Ag 2 O, and Ag 2 O to Ag at the subsequent discharging process (Figure 6d). 40−42 A summary of the likely redox reactions involving α-NiO/ Ni(OH) 2 and AgNPs carried out at a low current density (e.g., 2.1 A g −1 ) in 1.0 M KOH is given below. The key AgNP reduction reactions include Ag 2+ to Ag + and Ag + to Ag 0 (eqs 6 and 7), while the nickel undergoes the redox reactions of eqs 9 and 10.  As mentioned above, the ORR in aqueous solutions proceeds by two alternative pathways: (i) a direct fourelectron reduction, which occurs on a metal catalyst in general, or (ii) a two-electron reduction to peroxide, followed by the 2e − reduction of H 2 O 2 /HO 2 − to OH − or alternative via a selfdisproportionation reaction of 2HO 2 − to yield 2 OH − and O 2 , which is more common in alkali solution. 43 AgNP is known to be an excellent electrocatalyst for the ORR that facilitates a direct four-electron reduction. 27 The addition of AgNP/F-graphene to α-NiO/Ni(OH) 2 produced an increase in the specific capacity by a factor of 1.7. Moreover, the TiO 2 nanoglue that is used to fabricate the composite electrode may have an impact on the specific capacity due to its dual functionality as a binder and high capacitive material. 47

Application to Zn-Based Rechargeable Batteries.
We investigated the charge/discharge voltage profiles of a hybrid Zn−Ni/Ag/air that is governed by two distinct pairs of Zn and α-NiO/Ni(OH) 2 /AgNP/FG. 48 cathode: overall: In Zn−Ag/air batteries, the cathodic OER and ORR play pivotal roles, and the ORR is known to be a major limitation to performance efficiency due to its kinetics limitations. 53−57 However, in this hybrid electrode system, it seems that the ORR overlapped with multiple silver reduction processes on AgNP/F-graphene. 43,45,58−61 In addition, the localized electromagnetic field around AgNPs can induce a plasmonic resonance that accelerates the kinetics of the ORR. 62 The ORR in the current study is clarified by the disappearance of O 2 bubbles on the electrode surface during the discharge process. TiO 2 nanoparticles and the carbon paper used as a current collector of the cathode may also contribute to the ORR, as extensively reported. 33,45,53−55,62−65 On the other hand, the Zn-ion battery exhibits electrochemical reactions, which involve the reversible insertion/ extraction of Zn 2+ into/from the cathode as stripped out from the Zn anode in the discharging process, and the high overpotential for the hydrogen evolution reaction (HER) renders highly reversible zinc stripping and deposition in aqueous media. 66−72 According to the principle of Zn-ion battery, the morphology of Zn could be considered as a major factor, determining Zn dissolution efficiency. Figure 7 shows the charge/discharge voltage profiles dependent on silver amounts and Zn materials. The change in voltage profile appears over the potential range of 0.4−1.6 V, where the Zn−air and silver redox reactions take place. In the case of a Zn−air battery, the oxidation of Zn 0 to Zn(OH) 4 2− takes place close to 1.2 V, leading to the formation of ZnO. The reductions of Ag 2 O 3 , AgO, and Ag 2 O take place at E < 1.5 V (eqs 19−21). The discharge voltage is often inversely related to the current density. 73 Discharge over the voltage range between 1.5 and 2 V involves primarily the Zn−Ni redox reactions including the reduction of α-NiOOH to α-Ni(OH) 2 at E o = 1.85 V and to α-NiO at E o = 1.6 V. The voltage profile with Zn(foil)−α-NiO/Ni(OH) 2 /AgNP(20)/FG-coupled electrode exhibits two distinct voltage plateaus. The first one beginning at 1.8 V is attributed to the transformation of α-NiOOH to α-Ni(OH) 2 and α-NiO, while the second plateau near 1.2 V is attributed to a reduction of Ag 2 O to Ag 0 (AgNP) (Figure 7a). On the other hand, Figure 7b shows the charge/ discharge voltage profiles with 1.5 times more Ag applied electrode (i.e., α-NiO/Ni(OH) 2 /AgNP(30)/FG) that is coupled with a Zn/C instead of a Zn(foil). Multivoltage plateaus indicate a charging/discharging time dependence. Similar changes are obtained with Zn(foam)−α-NiO/Ni-(OH) 2 /AgNP(200)/FG-coupled electrodes (Figure 7c). In this case, AgNP(200) has approximately 10 times more silver than AgNP (20), and Zn(foam) has a larger surface area than Zn(foil). Since relatively low current density is applied to the discharging process, the discharge proceeds longer than the charging process in the manner of matching the charging/ discharging specific capacity. Much expanded voltage plateaus are observed at the potentials of ∼1.5, 0.9, and ∼0.6 V, which are attributed to the reduction of Ag 2 O 3 to AgO, AgO to Ag 2 O, and Ag 2 O to Ag 0 , respectively, accompanying the ORR. At the potential range where a Zn-ion battery reaction concomitantly initiates with Zn dissolution (stripping out) to Zn 2+ , the voltage plateaus must be influenced by a dissolution ability of Zn(foil), Zn/C, and Zn(foam). 66−72 The CVs after charge/ discharge cycles with Zn(foam)−α-NiO/Ni(OH) 2 /AgNP-(200)/FG-coupled electrodes are shown in Figure S2.
The supplemental test upon a surplus charging effect is performed with a Zn(foil)−α-NiO/Ni(OH) 2 /AgNP(20)/FGcoupled electrodes. Two conditions are applied: one, charging/ discharging at the identical condition for each cycle with increasing time in the sequence of 5, 10, 15, and 20 min (as shown in Figure S3a,b) and the other, unbalanced charging/ discharging time process, which discharges for about 5 min in each cycle while increasing the charging time in the sequence of 5, 10, 15, and 20 min, as shown in Figure S3c,d, respectively. The unbalanced charging/discharging time process indicates a surplus charge that affects the next charge/discharge cycles, resulting in a more expanded voltage plateau compared to that at the identical condition.
The charge/discharge voltage profiles for the half-cell electrochemical reactions of α-NiO/Ni(OH) 2 /AgNP(200)/ FG, which are applied to the lower current density for the discharge process, are shown in Figure S4. The voltage profile for the discharge process is defined as a portion of each reduction of Ag 2 O 3 to AgO, AgO to Ag 2 O, and Ag 2 O to Ag, respectively, and each plateau expands corresponding to increased charge current density (see Figure S4a,c). Figure  S4b is the half-cell electrochemical reaction of Zn(foam) as a reference. The CV (a) in Figure S4d is obtained after charge/ discharge cycles for 123 h. It indicates a peak broadening in the voltage range between −1.2 and − 0.4 V Ag/AgCl, which is comparable to the CV (b) obtained prior to the charge/ discharge cycles.  Figure 8b shows the voltage profiles obtained in a fresh 1 M KOH electrolyte with the rinsed electrode after the electrochemical reactions in Figure 8a for comparison. The representative specific capacities are presented in Figure 8c,d. As a result, the specific capacity based on the Zn−Ni electrochemical reactions is dependent on the charging current density, and the estimated specific capacity contribution of Zn−Ni is ∼ 53 mA h g −1 . The specific capacity exhibited by the Zn−Ag/air battery reaction is distributed proportionally to the reduction of Ag 2 O 3 to AgO, AgO to Ag 2 O, and Ag 2 O to Ag, which increases corresponding to the increased charging current density. The highest specific capacity is obtained as 1200 mA h g −1 at the charging current density of 1.75 Ag −1 (14 mA). Since Zn is not counted as active material due to the difficulty of measuring Zn mass involved in the battery reaction, it would result in a number of changes in the specific capacity and the specific energy presented herein. 62 In this study, Zn−Ni/Ag/air hybrid battery reactions with the α-NiO/Ni(OH) 2 /AgNP/FG electrode are investigated as a potential secondary battery in which the role of AgNPs is to transform a primary Zn−air battery performance into a rechargeable battery with the aid of F-graphene and TiO 2 nanoparticles. 50 The specific energies for Zn-based batteries are reported in the following order: Zn−air (theoretical 1370 W h kg −1 ; operational 470 W h kg −1 ) > Zn−Ag (150−250 W h kg −1 ) > Zn−Ni (100 W h kg −1 ). However, the addition of silver nanoparticles in Zn−Ni and Zn−air batteries would be a trade-off between power density and energy density. A hybrid Zn−Ni/Ag/air battery reaction with the coupled electrodes of Zn (foam) and α-NiO/Ni(OH) 2 28 This process is valuable to stabilize the electrodes with the completed redox reaction needed for the next charge/ discharge cycle. It is worth noting that the interaction between AgNPs and fluorinated graphene seems to cause a selfdischarge after Zn−Ag/air electrochemical reactions, and it is glaringly obvious corresponding to the increased current density, resulting in a decreased specific energy of up to 82 W h kg −1 as estimated. 33 This is indicated in Figure S5A with a red arrow. As a result, the specific energy of 740 W h kg −1 contributed by each battery reaction with the Zn(foam)−α-NiO/Ni(OH) 2 /AgNP(200)/FG-coupled electrodes eventually results in ∼ 660 W h kg −1 that is applicable. The representative specific energy is presented in Figure S6 (refer to Table S1).
The overall discharge process can be broken down into a contribution from the Zn−Ni redox reactions that occur at the rates of 0.130 and 0.1025 (W h kg −1 s −1 ) for each redox reaction of Zn−Ni(OH) 2 and Zn−NiO and a contribution from the Zn−Ag/air reactions at the distinguished rates of 0.085, 0.072, and 0.035 (W h kg −1 s −1 ) for Zn-Ag 2 O 3 , Zn-AgO, and Zn-Ag 2 O redox reactions, respectively Lastly, a contribution from Zn−air at a discharge rate of 0.04 W h kg −1 s −1 including a much slower discharge rate of <0.02 W h kg −1 s −1 from either of Ag-F redox or/and the reduction of F-graphene as estimated. The representative data are presented in Figure  S5B.

CONCLUSIONS
In this study, we developed the electrode of α-NiO/Ni(OH) 2 / AgNP/FG that is composed of α-NiO/Ni(OH) 2 as a primary component with the silver nanoparticles preanchored to fluorinated graphene (AgNP/FG). This is a polymer binderfree electrode as-prepared using a TiO 2 nanogel instead of a conventional conductive polymer. The AgNPs anchored to Fgraphene (AgNP/FG) promise an enhanced OER and ORR since the electrophysical properties of F-graphene play a pivotal role, allowing AgNPs to participate in multiredox reactions. The TiO 2 nanogel used as glue improves the Coulombic efficiencies for the OER and the ORR as intrinsic electrocatalytic properties. This particular composite of α-NiO/Ni(OH) 2 /AgNP(20)/F-graphene results in enhanced specific capacitance (F g −1 ) and specific capacity (mA h g −1 ) compared to the α-NiO/Ni(OH) 2 core electrode, resulting in increased specific capacitance from 148 to 356 F g −1 and the specific capacity from 266 to 545 mA h g −1 . As for the specific capacitance, it is further increased up to 1153 F g −1 with a change of voltage scan rate from 20 to 5 mV/s and the Nafionfree α-NiO/Ni(OH) 2 /AgNP(20)/FG composite.
The performance of hybrid Zn−Ni/Ag/air electrochemical reactions by α-NiO/Ni(OH) 2 /AgNP(200)/FG and Zncoupled electrode indicates a potential for a secondary battery. It results in a specific capacity of 1200 mA h g −1 and a specific energy of 660 W h kg −1 , which is divided into Zn−Ni reactions of ∼95 W h kg −1 and Zn−Ag/air of ∼420 W h kg −1 , while undergoing a Zn−air reaction of ∼145 W h kg −1 .
cell and the coupled electrodes of Zn(foam) and α-NiO/Ni(OH) 2 /AgNP(200)/FG with a specific capacity that is dependent on the current density ( Figures S2−  S4); and results of the specific energy (W h Kg −1 ), which are the plot of specific energy and the energy draining rates for the active species of Ni and Ag, and the portion of specific energy contributed by the electrochemical reaction of Zn−Ni, Zn−Ag/O 2 , Zn− air, as listed on Table S1 (Figures S5−S6) (PDF)