Self-Generated Nanoporous Silver Framework for High-Performance

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Self-Generated Nanoporous Silver Framework for HighPerformance Iron Oxide Pseudocapacitor Anodes Jae Young Seok, Jaehak Lee, and Minyang Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03725 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

Self-Generated Nanoporous Silver Framework for HighPerformance Iron Oxide Pseudocapacitor Anodes Jae Young Seok, Jaehak Lee, and Minyang Yang*. Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

KEYWORDS: Nanoporous silver, Silver halide, Electroreduction, Pseudocapacitor, Iron oxide

*

Corresponding author, Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The rapid development of electric vehicles is increasing the demand for next-generation fast-charging energy storage devices with a high capacity and long-term stability. Metal oxide/hydroxide pseudocapacitors are the most promising technology because they show a theoretical capacitance that is 10-100 times higher than that of conventional supercapacitors, and rate capability and long-term stability that are much higher than those of Li-ion batteries. However, the poor electrical conductivity of metal oxides/hydroxides is a serious obstacle for achieving the theoretical pseudocapacitor performance. Here, a nanoporous silver (np-Ag) structure with a tunable pore size and ligament is developed using a new silver halide electroreduction process. The structural characteristics of np-Ag (e.g., high specific surface area, electric conductivity, and porosity), are desirable for metal-oxide-based pseudocapacitors. This work demonstrates an ultra-high capacity, fast-charging, and long-term cycling pseudocapacitor anode by developing an np-Ag framework and depositing a thin layer of Fe2O3 on its surface (npAg@Fe2O3). The np-Ag@Fe2O3 anode shows ~608 F g-1 at 10 A g-1, and ~84.9% of the capacitance is retained after 6000 charge-discharge cycles. This stable and high capacity anode, which can be charged within a few tens of seconds, is a promising candidate for next-generation energy storage devices.

INTRODUCTION In recent years, with the boom of electric vehicles, the demand for high-performance energy storage devices with ultrafast charging and long lifetimes has been explosively increasing.1-3 Unfortunately, commercialized lithium-ion batteries or other secondary batteries have a high


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capacity at a low charging rate, while their overall performance is significantly degraded and their lifetime is simultaneously shortened at a high charging rate. Supercapacitors are one of the most promising next-generation energy storage devices because of their high power density, extremely high cycle life, and safe operation.4-6 Supercapacitors can be classified into two main groups: (1) electrical double-layer capacitors (EDLCs) that use activated carbon, carbon nanotubes, graphene, and mesoporous carbon7,8 and (2) pseudocapacitors that use transition metal oxides/hydroxides and conducting polymers.9,10 Generally, EDLCs can offer an extremely high power density (fully charging within a few seconds) and excellent cyclic stability (maintaining over 90% of the initial capacity up to 10000 cycles). However, the energy density of EDLCs (typically 5-10 W h kg-1) is much lower than that of conventional Li-ion batteries (150-200 W h kg-1); this is a serious obstacle for the widespread use of supercapacitors.11-13 Pseudocapacitors that use transition metal oxides/hydroxides are superior alternatives because they can provide an energy density that is 10-100 times higher than that of EDLCs because of Faradaic redox reactions.14-16 However, these transition metal oxides have several drawbacks, such as a low electrical conductivity, poor rate capability, and inferior cyclic stability. Therefore, nano-sized materials (such as nanoparticles, wires, porous materials)17-19 or hybrid materials (such as core-shell, core-branch, and sandwiched structures) have been used to improve the properties of transition metal oxides.20-23 Among these strategies, using transition metal oxides decorated on a three-dimensional (3D) porous conductive structure is known that the most effective method for enhancing the performance. This rationally designed electrode structure improves the charge and ion transport because of its well-interconnected ligament and pore structures, and helps to release the stress generated during electrochemical cycling, thus providing exceptionally high-performance devices with long-term stabilities.24-27


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Carbonaceous materials (e.g., graphene, carbon nanotubes, and mesoporous carbon)28-31 have been widely investigated as conductive frameworks for pseudocapacitive materials, but several disadvantages, such as a large contact resistance between interfaces, hydrophobic nature, and complex structure, lead to inefficient charge and mass transfer. Alternatively, 3D porous metallic frameworks show potential because of their excellent ohmic conductance, hydrophilic nature, and superior mechanical stability. As evidence, previous work showed outstanding capacitive properties by coupling the 3D porous metallic scaffolds with electrochemically active materials.32-33 In this work, we developed a new fabrication concept of nanoporous silver (np-Ag) with controllable ligament and pore sizes ranging from 50 to 300 nm via one-step electroreduction of silver halide (AgX). The phenomenon of np-Ag structure formation through electroreduction of AgX is described and analyzed. The formation mechanism was analyzed using various measurements, and a parametric study was conducted to optimize the nanoporous structure. Fe2O3, a low-cost and high-capacitance material,34-35 was then uniformly decorated on the np-Ag surface by electrodeposition. The deposited quantity of Fe2O3 was well controlled to optimize the electrochemical performance. Taking the unique structural advantages of np-Ag, the npAg@Fe2O3 anode demonstrates high specific capacitance, rate capability, and cyclic stability, compared with previously reported pseudocapacitor anodes using Fe2O3 materials and 3D conductive scaffold.36-41

RESULTS AND DISCUSSION Fabrication and Characterization of Nanoporous Silver Framework. Figure 1a schematically illustrates the fabrication procedure of the np-Ag structure. AgCl and AgBr were


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dissolved in an aqueous solution of ammonium hydroxide (~14 vol%) and were then used as silver halide precursor solutions (AgX ionic precursors). AgI is minimally soluble in ammonium hydroxide; therefore, AgI powder was ball-milled to 1-2-µm-sized particles and was then dispersed in an N-methyl-2-pyrrolidone solution. The AgX precursor solution was then dropcasted onto a thin silver film and annealed at 90 °C to evaporate the solvents and re-precipitate the AgX materials. Densely deposited AgX particles were electro-reduced in a 1

M

Na2SO4

aqueous electrolyte with a three-electrode electrochemical system composed of a saturated calomel electrode (SCE) as a reference and platinum mesh (Pt) as a counter electrode. Surprisingly, the nanoporous-shaped silver structure was self-generated by a simple process of electrochemical reduction of bulk AgX particles. The detailed experimental procedure and images of the experimental samples are shown in Figure S1. Additionally, digital camera images of samples of each AgX electroreduction experiment are shown in Figure S2. The structural formation mechanism has not been elucidated; therefore, we assumed a plausible mechanism and conducted various experiments to identify the mechanism. A schematic diagram of the proposed formation mechanism is shown in Figure 1b. As a representative schematic of the partly reduced state (left image in Figure 1b), the sphere is composed of np-Ag (reducedAgX), which is the net-shaped gray part, and bulk AgX, which is the orange part. As shown in the left scheme of Figure 1b, electrons were injected into AgX using a three-electrode electrochemical system, and the AgX material was reduced to pure silver according to Equation (1). AgX + e− → Ag(s) + X− (X = Cl, Br, I)

(1)

As electroreduction proceeded, many halide ions at the reaction interface immediately dissolved into the aqueous electrolyte solution through diffusion, and the reduced silver atoms


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simultaneously attached to adjacent silver structures. This process was schematically shown in the right image of Figure 1b. Because the ion diffusion rate in solution is much slower than the silver growth rate, the silver structure is formed before halide ions are removed by diffusion. At this moment, halide ions occupy some volumetric space and interrupt the isotropic growth of the silver structure at the reaction interface. As a result, the volume of halide ions remains as voids after the electroreduction process, and thus induces a porous structure rather than a bulk form. Additionally, the surface energy of silver is very low; therefore, the np-Ag structure, which has a high surface area, could be in a stable state. Figure 1c shows a scanning electron microscopy (SEM) image of a partly reduced AgBr sample (corresponding to that shown in Figure 1b), with the reduced (porous) region; the unreduced (bulk) region is clearly separated by a red dashed line, which represents the reaction interface. To determine whether the suggested mechanism is correct, we investigated the following issues: i) Identifying whether the chemical reaction is a pure single reduction reaction of AgX. ii) Verifying that the chemical composition of the bulk region is AgX and that the porous region is pure Ag. iii) Confirming whether the halide ion size affects the resultant nanoporous structure. First, we conducted X-ray diffraction (XRD), linear sweep voltammetry, and ultra-precision mass measurements to determine the chemical reaction of the process. Figure 1d shows the XRD patterns of the as-deposited (black line), partly reduced (blue line), and completely reduced (red line) AgBr samples. As the electroreduction proceeded, the AgBr peaks (blue circle) gradually decreased while the Ag peaks (red triangle) increased; no unknown peaks were observed. For fully reduced AgBr (r-AgBr), no AgBr peaks were visible and only Ag peaks were present. In addition, the XRD analysis for AgCl and AgI with the same graphical notation are shown in Figure S3a and S3b, respectively. AgCl and AgI were directly converted to Ag without any


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intermediate product through an electroreduction process (this is consistent with the XRD results for AgBr). Ultra-precision mass measurements of each experimental sample, as shown in Figure S4, support the above XRD results. Figure 1e shows the linear sweep voltammetry results for each AgX material. Linear sweep voltammetry was performed from 0.2 to -0.6 V in a 1

M

Na2SO4 electrolyte with an SCE as a reference and Pt mesh as a counter electrode. The red curves, green curves, and blue curves correspond to the AgCl, AgBr, and AgI samples, respectively. The tangent of the curve and its intersection with the x-axis represent the reaction potential of the electrochemical process. The tangent lines are indicated by arrows for each curve; they intersect with the x-axis at 0, -0.16, and -0.39 V (vs. SCE), respectively. The standard reduction potentials of the silver halides are -0.0157 V for AgCl, -0.1667 V for AgBr, and 0.3902 V for AgI (vs. SCE), which correspond with the experimental results. Additionally, the slopes of the curves after the reaction potentials are a single slope, indicating that the electrochemical reactions are single reactions without any side reactions for all silver halides. Finally, the XRD, mass, and linear sweep voltammetry measurements confirmed that the reaction process was a simple electrochemical reduction reaction of AgX without any side reactions or intermediates. To verify the chemical composition of the bulk and porous regions, we conducted SEM and energy dispersive X-ray spectroscopy (EDS) mapping measurements, and the results are presented in Figure 2. An SEM image of partly reduced AgCl and its corresponding EDS mapping image are presented in Figure 2a and b, respectively. The bulk and porous regions are distinguished by a yellow dashed line (Figure 2a); the Ag component is represented by green dots and the Cl component is represented by red dots (Figure 2b). This result intuitively shows that the unreduced region was composed of a nearly equal amount of silver and chlorine, and the


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reduced area was composed of pure silver. For the partly reduced AgBr and AgI samples, SEMEDS measurements using the same graphical notation are presented in Figure 2d and e, and Figure 2g and h, respectively. The unreduced region in the bulk form is composed of both the Ag and Br (or I) components, whereas the reduced region in the porous form consists of only the Ag component; this is similar to the results for the AgCl sample. A high-magnification SEM image of the completely reduced AgCl sample is shown in Figure 2c. It shows a nanoporous structure with a ligament sizes of 150-350 nm, pore sizes of 100-200 nm, and porosity of about 35%. On the other hand, AgBr and AgI samples were prepared under the same experimental conditions (electrolyte and reduction current) as the AgCl samples, and their SEM images are shown in Figure 2f and 2i, respectively. The nanoporous structures of the AgBr and AgI samples are quite different from that of the AgCl sample. Figure 2f (AgBr sample) shows a nanoporous structure with a ligament size of 75-120 nm, pore size of 75-120 nm, and porosity of about 50%. Figure 2i (AgI sample) shows a nanoporous structure with a ligament size of 50-100 nm, pore size of 70-200 nm, and porosity of about 65%. These structural characteristics of each reduced-AgX were summarized with respect to ligament diameter, pore size, and porosity in Table 1. Notably, the obvious structural tendency was found by comparing the SEM images of r-AgCl, r-AgBr, and r-AgI. As the size of the halide ions (Cl < Br < I) increases, the ligaments become smaller, the pore size increases with respect to the ligament size, and the porosity increases accordingly. To clarify the fundamental reason for these structural differences, we conducted further experiments by changing the process conditions, such as the reduction current or electrolyte, for each AgX material. SEM images for each reduced AgX sample at different reduction currents are shown in Figure S5. As the process conditions change, the basic structural characteristics of each reduced AgX are maintained, with minor structural


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changes. As the reduction current (silver growth rate) increased, partially aggregated ligaments were more frequently observed. Meanwhile, Figure S6 shows SEM images of each reduced AgX sample with different electrolytes (1

M

Li2SO4, Na2SO4, and NiSO4) under the same reduction

current. However, these electrolytes do not directly participate in the reaction at the potential of the AgX reduction process (they merely assist the charge transfer); therefore, they do not affect structural formation, and no significant differences were observed in the resultant nanoporous structure. By considering these various observations, we concluded that this single electrochemical reduction of AgX leads to the np-Ag structure, which is dominantly determined by the size of the halide ion. These experimental results agree with the hypothesis for nanoporous formation that was suggested above. In addition, we have investigated thermal and electrochemical stability of the np-Ag framework, and the results are presented and discussed in Figure S7 and Figure S8, respectively. Nanoporous Metal/Oxide Hybrid Anodes. Fe2O3 (np-Ag@Fe2O3), a promising low-cost anode material with a high theoretic capacitance, was coated on np-Ag to demonstrate a highperformance supercapacitor anode using np-Ag. Here, the np-Ag structure prepared from AgBr reduction was used to fabricate the anode because it can provide a high capacity, conductivity, and mechanical stability because of its high specific area and interconnected structure. The structure of r-AgCl has a relatively low specific area because of its structural characteristics (e.g., large ligaments and small pores), and r-AgI exhibits an unreliable electrochemical performance during device operation because of the mechanically unstable and weak structural interconnection. Meanwhile, Fe2O3 was electrodeposited because it provides a low-cost, highly conformal coating on a textured surface, and the deposition amount can be easily controlled. SEM images of np-Ag@Fe2O3 by varying amount of deposited Fe2O3 are shown in Figure 3a-c;


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the np-Ag structure was uniformly covered with the active materials while maintaining its unique porous structure, and slightly aggregated Fe2O3 morphology could be observed (Figure 3c) when excessive amounts of Fe2O3 was deposited on the np-Ag framework. Here, 0.45-1.8 mg/cm2 signified the loading mass of Fe2O3, and the weight of np-Ag for a typical fabrication process was 0.92 mg/cm2, thus the weight ratio between np-Ag and Fe2O3 was calculated and presented for each case in figure 3a-c. The loading mass of Fe2O3 will increase, as larger amount of np-Ag framework are deposited. In addition, to clearly confirm the conformal deposition of Fe2O3 on the surface of the nanoporous structure, high-angle annular dark-field imaging-scanning transmission electron microscopy (HAADF-STEM) was performed; the results and their corresponding EDS maps for a single ligament of np-Ag@Fe2O3 are shown in Figure 3d-g. From the mapping result, the Fe and O elements were uniformly distributed on the np-Ag ligament surface, which verified the conformal deposition of Fe2O3 on the structure. X-ray diffraction (XRD) measurements and X-ray photoelectron spectroscopy (XPS) were conducted to study the crystallinity and the electronic state of np-Ag@Fe2O3, and the results are shown in Figure 3h-k. From the XRD patterns of np-Ag@Fe2O3 (Figure 3h), besides the diffraction peaks from silver, all other diffraction peaks in the XRD patterns can be indexed to γFe2O3 (JCPDS card No. 39-1346). The wide-range scan of np-Ag@Fe2O3 (Figure 3i) shows the electronic states of three elements (Ag, Fe, and O), and no other impurities were detected. The Fe 2p spectrum (Figure 3j) contains two main peaks (Fe 2p 3/2 and 2p 1/2) at 710.8 and 724.4 eV, respectively, accompanied by satellite peaks. This result corresponds to previous analysis of Fe3+ without the Fe2+ state.42-43 In Figure 3k, the principle peak of O 1s was located at 530.3 eV and originated from the metal oxide (O2-), and the small shoulder peak located at 531.3 eV correlated with the surface hydroxyl group (-OH). Furthermore, the Ag 3d spectrum was measured to


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determine whether the np-Ag structure was oxidized during the Fe2O3 electrodeposition process (Figure S9). Deconvolution of the Ag 3d spectrum shows that the two peaks for Ag 3d 5/2 and 3d 3/2 were centered at 368.2 and 374.2 eV, respectively; this corresponds to Ag without any Ag2O or AgO states.44-45 Performance Evaluation of the Anode. The electrochemical behavior of np-Ag@Fe2O3 was examined using a three-electrode system with 2.5

M

Li2SO4 as the electrolyte. The potential

window was set to 1 V to avoid irreversible reactions (e.g., H2 generation) that would deteriorated the electrode when the potential window exceeded 1 V (Figure S10). Figure 4a shows the cyclic voltammetry (CV) curves of bare np-Ag and np-Ag@Fe2O3 electrodes at a scan rate of 50 mV s-1, and the inset shows the magnified CV curve for bare np-Ag. The CV curve of bare np-Ag contains a rectangular curve for the electric double-layer capacitance, while the CV curve of np-Ag@Fe2O3 shows the apparent pseudocapacitive behavior of Fe2O3. The shape of CV curves were highly consistent with previous studies of pseudocapacitors based on Fe2O3 electrode with a potential range of 1 V.34,36,47 Here, the enclosed area of the CV curve represents the capacitance, and the capacitance increases considerably after Fe2O3 deposition. The pseudocapacitive mechanism of the Fe2O3 based on Li+ as working ions, has been studied in previous works, and the reaction can be expressed as following equation (2).41,48 Fe2O3 + xLi+ + xe- ↔ Lix+Fe2-x3+Fex2+O32-

(2)

The effect of the scan rate on the electrochemical performance of np-Ag@Fe2O3 was investigated in Figure 4b, presenting liver-shaped curves, and the shapes of these CV curves did not change as the scan rate increased from 10 to 200 mV s-1. It indicates that the fast charge and ion diffusion response are due to the unique interconnected porous structure and exceptionally high electronic conductivity of silver.


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The fast reversible pseudocapacitive reaction mainly occurs on the surface and near-surface region of the metal oxide, and Fe2O3 is insulating; therefore, controlling the deposition amount of Fe2O3 is important for optimizing the supercapacitor performance. The galvanostatic chargedischarge curves of np-Ag@Fe2O3 with various mass loadings of Fe2O3 were measured at the same current density (8 mA cm-2) (Figure 4c), and the curves of varying charge-discharge current densities are shown in Figure S11. All the charge-discharge curves are symmetric with a high coulombic efficiency, and the gravimetric capacitances calculated from the CV and chargedischarge curves agree at each charging rate. The capacitive features of np-Ag@Fe2O3 were examined by varying the mass loading of Fe2O3; the rate capability of the three samples was calculated, and the results are shown in Figure 4d. Each sample show high capacitance with 701.4, 615.7, and 551 F g-1 at the scan rate of 10 mV s-1, respectively. When the scan rate changes from 10 to 200 mV s-1, the np-Ag@Fe2O3 sample (mass loading of 0.45 mg cm-2) exhibits an exceptionally high capacitance retention of 86.1%. By depositing Fe2O3 at a higher mass loadings (0.9 mg cm-2 and 1.8 mg cm-2), the capacitance retention was greatly reduced by 72.9% and 41.1%, respectively. Electrochemical impedance spectroscopy (EIS) shows a similar trend. In the high frequency region of the Nyquist plots (Figure 4e), the semicircle diameter in the real part gradually increases with increasing mass loading of Fe2O3, indicating that the charge transfer resistance increases because of the thicker Fe2O3 layer. In the low frequency region, the slope of the curve indicates the electrolyte and proton diffusion resistances. As the amount of Fe2O3 increases, the slope of the curve decreases, indicating a higher resistance to ion/proton diffusion. From the EIS results, the thick Fe2O3 deposition greatly increases the resistance to both charge transfer and ion diffusion, thus the Faradaic reaction is disturbed at high charging rate conditions, which is consistent with the result of the rate capability.


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In the viewpoint of areal capacitance related to the total capacity (Figure S12), the npAg@Fe2O3-containing low quantity Fe2O3 (0.45 mg cm-2) showed a low capacitance, which is expected because a low Fe2O3 mass provides less Faradaic reaction. On the other hand, npAg@Fe2O3 with a high mass loading (1.8 mg cm-2) exhibited a high capacitance under slow charge rate conditions, but the capacitance decreased sharply with increasing charge rate. The excessive amount of Fe2O3 provided an adverse effect on the ionic and electronic conductivities. Additionally, this side effect of excessive amount of Fe2O3 can be observed by the shape change of CV curve according to the scan rate (Figure S13). In the case of high mass loading (1.8 mg cm-2), the redox peak of CV curves was shifted significantly by increasing of the scan rate, that implies the slow charge transport rate and ion diffusion response during the electrochemical reaction. To achieve an optimized electrochemical performance, the amount of Fe2O3 should be determined to minimize excessive loading; this delays the electrochemical reaction and reduces the power density. The best electrochemical performance is observed for 0.9 mg cm-2 npAg@Fe2O3. Furthermore, the important characteristic of supercapacitor performance—the cyclic stability of np-Ag@Fe2O3 with a mass loading of 0.9 mg cm-2—was also investigated, and the results are shown in Figure 4f. An excellent cyclic stability was observed for np-Ag@Fe2O3, which can maintain 84.9% of its initial capacitance after 6000 cycles of charge-discharge. During the first 100 cycles, the capacitance rapidly decreased to approximately 86.5% of the initial capacitance and then stabilized; the capacitance was maintained for 6000 cycles without degradation. This was confirmed by comparing the SEM images of the samples before and after the cyclic measurements. Roughened surface and surface debris of electrodeposited Fe2O3 rapidly diminished the capacitance during the initial cycle. The morphology of as-deposited Fe2O3 was rough and loose, while that of Fe2O3 after 100 cycles was smooth and tightly coupled


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to the np-Ag structure (Figure 5a-b). The charging and discharging curves at the early and latter stages during the cyclic stability measurement are shown in the inset of Figure 4f. The performance of np-Ag@Fe2O3 was higher than that of most previously reported Fe2O3/conductive support-based supercapacitors in terms of the capacitance, rate capability, and cycle stability. A summary of the supercapacitor performances with respect to different iron oxide-based pseudocapacitors is presented in Table S1 to facilitate comparisons with previous work.

CONCLUSIONS We presented a novel route to fabricate np-Ag structures with controllable ligaments and pore sizes from 50 to 300 nm. To understand the mechanism of pore generation during the electrochemical reduction, np-Ag structures were fabricated under various conditions, and we analyzed that the structure formation was mainly determined by the size of halide ions and the growth rate of the silver. Parametric studies were simultaneously conducted to optimize the npAg structures as effective conductive supports for supercapacitor electrodes. Conformal deposition of Fe2O3 onto np-Ag using a facile electrodeposition method developed high-quality np-Ag@Fe2O3 supercapacitor electrodes. The exceptionally thin ligaments and interconnected pore structures of highly conductive silver enhanced the electrochemical kinetics, thus producing a high-performance supercapacitor. The np-Ag@Fe2O3 anode revealed an ultra-high specific capacitance (616 F g-1) without sacrificing its high rate capability and excellent cyclic stability (84.9% retention for 6000 cycles). These results imply that np-Ag@Fe2O3 has a great potential for a high-performance supercapacitor electrode, which has long-term stable operation, especially at extremely high charge and discharge rates. Furthermore, we are under developing a


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nanoporous structure based on the low-cost metallic materials (e.g., copper, nickel) in freestanding film configuration with highly improved mechanical stability fabricated by this facile electroreduction method. This unique and controllable nanoporous metallic structures would arouse large interests in various energy and environmental applications, such as supercapacitors, Li-ion batteries, fuel cells and electrocatalysts.

METHODS Materials. AgCl, AgBr, and AgI powders were purchased from Sigma-Aldrich and used without further purification. AgCl or AgBr powder was dissolved in an ammonium hydroxide solution (14%, Sigma-Aldrich) at a concentration of 20 mg mL-1. AgCl or AgBr was dissolved in an ionic form in the ammonium hydroxide solution according to Equation (3) or (4) (X = Cl, Br). AgX + 2NH3 → {Ag(NH3)2}1+ + X1AgX + 2NH3 → {Ag(NH3)2}1+ + {AgX2(NH3)2}1-

(3) (4)

The solubility of AgI was significantly lower than that of AgCl and AgBr in the ammonium hydroxide solution, and this material was prepared differently. AgI powder was ball-milled to small particle sizes (1 to 2 µm) and dispersed in N-methyl-2-pyrrolidone (>99.0%, SigmaAldrich). Li2SO4·H2O (>99.0%), Na2SO4 (>99.0%), and NiSO4·6H2O (>99.0%) were purchased from Sigma-Aldrich and used as received. Li2SO4, Na2SO4, and NiSO4 were each dissolved in deionized water at a concentration of 1 M, and used as a reduction electrolyte. Fe2(SO4)3·xH2O (~97%), NaOH (>97%), and triethanolamine (>99%) were purchased from Sigma-Aldrich. These were then dissolved in DI water to prepare the following solutions: 0.09 M Fe2(SO4)3, 2 M NaOH, and 0.1

M

triethanolamine, and used as an electrolyte for electrodeposition of active

material (Fe2O3) on the np-Ag surface.


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Fabrication. The nanoporous silver (np-Ag) structure was fabricated by electroreduction of AgX particles, which were deposited on conductive substrates (e.g., silver, stainless steel, and carbon). In a typical synthesis procedure, the AgX precursor solution (0.1 mL) was drop-casted on the thin silver film. It was then dried on a hotplate at 90 °C for 20 min. The AgX ionic solution was transformed into AgX particles by evaporating the ammonia components from the precursor solution. Precipitated AgX particles were electrochemically reduced using a three-electrode electrochemical system. A saturated calomel electrode (SCE) was used as a reference, and Pt mesh was used as the counter electrode. AgX particles were reduced in an aqueous electrolyte containing 1

M

Na2SO4 (or 1

M

Li2SO4, 1

M

NiSO4) at a constant reduction current of 2.5 mA

cm-2. A three-electrode system, which contained Hg/HgO as a reference and Pt mesh as a counter electrode, was used for the electrodeposition of Fe2O3. Step voltages (0.15 V for 50 s, 0.175 V for 50 s, 0.2 V for 50 s, and 0.23 V for 500-2000 s) were subsequently applied to np-Ag for conformal deposition of Fe2O3. Characterization. Samples were characterized using scanning electron microscopy (SEM, FEI Nova 230, 5-15 kV), transmission electron microscopy (TEM, FEI Tecnai F30 ST) with an energy dispersive X-ray spectroscopy (EDS) detector, X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific K-alpha), and X-ray diffraction (XRD, RIGAKU D/MAX-2500). Characteristics related to the electrochemical properties were evaluated using an electrochemical workstation (Ivium-n-Stat) with a three-electrode configuration. The mass of the electrode materials was measured using an ultra-precision microbalance (Radwag XA 4Y, d = 0.01 mg). Evaluation of the electrochemical performance. Electrochemical performance measurements were performed using an electrochemical workstation (Ivium-n-Stat) at room temperature in a 2.5 M Li2SO4 aqueous electrolyte. The np-Ag@Fe2O3 anode was directly attached to a metal clip


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and used as the working electrode; a Pt mesh and SCE were used as the counter electrode and reference electrode, respectively. The specific capacitance in the three-electrode test was calculated according to the following equations.

= × × × ∆ =

× ∆ × ∆

(5) (6)

Equation (5) was used for the cyclic voltammetry tests and Equation (6) was used for the galvanostatic charge-discharge test.49 In Equation (5) and (6), m is the mass of the active material, v is the scan rate (V/s), ∆V is the potential window, i is the current, and ∆t is the discharging time. These performances were calculated based on the mass and deposition area of the active material (Fe2O3). Electrochemical impedance spectroscopy (EIS) measurements were performed from 0.01 Hz to 1000 Hz with a 0.01-V perturbation. Charge-discharge measurements were conducted at a current density of 30 A g-1 to evaluate the cycling stability of the device.

Acknowledgements This work was supported by Basic Science Research Program (2015R1A2A1A05001840) funded by the National Research Foundation (NRF) under the Ministry of Science.

Supporting Information Available: Additional scheme and digital images of fabrication procedure, and SEM images, XRD, XPS, mass measurement results of np-Ag, and SEM-EDS mapping results, detailed CV and charge-discharge curves of np-Ag@Fe2O3 (Figure S1-S13).


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Summary of iron oxide (FeOx) based supercapacitor anode performance (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Schematic presentation to show (a) the fabrication procedure of nanoporous silver (npAg) frameworks and (b) the porous structure formation mechanism of np-Ag via AgX electroreduction. (c) SEM image of np-Ag in a partly reduced state composed of AgBr and npAg. (d) XRD results of the as-deposited state (black line), partly reduced state (blue line), and fully reduced state (red line) of AgBr. (e) Linear sweep voltammetry results from 0.2 to -0.6 V for the AgCl, AgBr, and AgI samples.


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Figure 2. SEM images and corresponding element mapping in the partly reduced state for (a, b) AgCl, (d, e) AgBr, and (g, h) AgI. High magnification SEM images of the fully reduced state of (c) AgCl, (f) AgBr, and (i) AgI.


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Figure 3. Characterization of the conformal deposition of Fe2O3 on the np-Ag structure. (a-c) SEM images of np-Ag@Fe2O3 with different deposition amount of Fe2O3. (d) HAADF-STEM image of a single ligament of np-Ag@Fe2O3 and (e-g) the corresponding STEM element maps. (h) XRD pattern of the np-Ag@Fe2O3. XPS results: (i) survey spectrum, (j) Fe 2p spectrum, and (k) O 1s spectrum for np-Ag@Fe2O3.


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Figure 4. Electrochemical characterizations of the np-Ag@Fe2O3 anode. (a) CV curves for bare np-Ag and np-Ag@Fe2O3 (with an active material mass of 0.9 mg cm-2) at a scan rate of 50 mV s-1; the inset shows an enlarged CV curve of the bare np-Ag. (b) CV curves of np-Ag@Fe2O3 (0.9 mg cm-2) at different scan rates from 10 to 200 mV s-1. (c) Galvanostatic charge-discharge curves of np-Ag@Fe2O3 with different mass loadings of Fe2O3: 0.45, 0.9, and 1.8 mg cm-2. (d) Rate properties and (e) EIS of the three samples. (f) Cycle stability of np-Ag@Fe2O3 during 6000 cycles; the insets show charge-discharge curves from an early cycle and latter cycle, respectively.


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Figure 5. SEM Images of the np-Ag@Fe2O3. The surface morphology of (a) the as-deposited sample, and (b) the sample after the electrochemical cycle test.


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Table 1. Summary of the structural properties of np-Ag prepared from each silver halides. Sample r-AgCl r-AgBr r-AgI

Ligament Pore size (nm) diameter (nm) 250 (± 100) 150 (± 50) 95 (± 25) 95 (± 25) 75 (± 25) 135 (± 60)

Porosity (%) ~ 35% ~ 50% ~ 65%


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Table of contents graphic


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Self-Generated Nanoporous Silver Framework for High-Performance

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