High-Performance 3D Nanostructured Silver Electrode for Micro-Supercapacitor Application

High-Performance 3D Nanostructured Silver Electrode for Micro- Supercapacitor Application Ana Silvia González, Javier García, Victor Vega, Rafael Caballero Flores, and Victor M. Prida* Cite This: ACS Omega 2023, 8, 40087−40098 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: In the current energy crisis scenario, the development of renewable energy forms such as energy storage systems among the supercapacitors is an urgent need as a tool for environmental protection against increasing pollution. In this work, we have designed a novel 3D nanostructured silver electrode through an antireplica/replica template-assisted procedure. The chemical surface and electrochemical properties of this novel 3D electrode have been studied in a 5 M KOH electrolyte. Microstructural characterization and compositional analysis were studied by SEM, energy-dispersive X-ray spectroscopy, XRD technique, and Kripton adsorption at −198 °C, together with cyclic voltammetry and galvanostatic charge−discharge cycling measurements, Coulombic efficiency, cycle stability, and their leakage current drops, in addition to the self-discharge and electrochromoactive behavior, were performed to fully characterize the 3D nanostructured electrode. Large areal capacitance value of 0.5 F/cm2and Coulombic efficiency of 97.5% are obtained at a current density of 6.4 mA/cm2for a voltage window of 1.2 V (between −0.5 and 0.8 V). The 3D nanostructured silver electrode exhibits excellent capacitance retention (95%) during more than 2600 cycles, indicating a good cyclic stability. Additionally, the electrode delivers a high energy density of around 385.87 μWh/cm2and a power density value of 3.82 μW/cm2and also displays an electrochromoactive behavior. These experimental results strongly support that this versatile combined fabrication procedure is a suitable strategy for improving the electrochemical performances of 3D nanostructured silver electrodes for applications as micro-supercapacitors or in electrochemical devices. 1. INTRODUCTION One of the highest priorities of energetic plan action that affects worldwide concerns the transformation to renewable energies and optimized management of resources in order to reduce the carbon fingerprint and mitigate climate change. This transformation not only pretends to substitute all energy sources with renewables without endangering the natural development of the society. In fact, it pretends an electrification of as many economical areas such as automotive or heating and cooling industries among others. Considering electricity as the primary energy source, it is becoming increasingly more important to find new and more efficient ways of electrical energy storage. Many of the limits associated with batteries and their declining credibility as storage solutions have led to alternative energy markets predicting supercapacitors to replace batteries entirely in the middle term. 1,2 Supercapacitors, also known as ultracapacitors or electrochemical capacitors, exhibit outstanding properties for use as energy storage systems due to their high power density, fast charge−discharge rates, and excellent cycle stability, plus other qualities. They can be divided into two large groups: electric double-layer capacitors (EDLCs) and pseudocapacitors.· 3 EDLCs employed their very thin double-layer structure to store the electrical charges, while pseudocapacitors store the electrical charges through fast and reversible redox reactions. For this, the pseudocapacitors show much higher capacitance compared with the capacitive capability of double-layer capacitors where a much lower degree of electron transfer occurs, 4,5 showing that they are considered as an excellent source of potential candidates like energy storage devices in many research areas such as in energy sources for portable devices, memory power sources, energy power recovery and regeneration systems, or electrical energy distribution machines and storage devices. 6 Nowadays, investigations about pseudocapacitors are addressed to the discovery of new advanced nanostructured materials presenting high effective surface areas or different morphologies with enhanced energy densities or improving the capacitance of pseudocapacitors. In the context of the analysis about which materials could serve as optimal electrodes for pseudocapacitors, four key factors need to be taken into consideration: (1) high Received: April 3, 2023 Accepted: September 29, 2023 Published: October 17, 2023 Article http://pubs.acs.org/journal/acsodf © 2023 The Authors. Published by American Chemical Society 40087 https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 This article is licensed under CC-BY-NC-ND 4.0 Downloaded via UNIV DE SEVILLA on June 6, 2024 at 12:09:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. surface area for the redox reactions, (2) doping of the materials to increase the redox state and their conductivity, (3) a wide potential window, and (4) a high charge/discharge rate. 7−11 About the different materials, conducting polymers and transition-metal oxides can be found to meet these requirements; however, metal oxides are generally considered the main candidates that can be used as electrode materials in supercapacitors because of the wide variety of oxidation states that allows for redox charge transfer, together with other excellent properties to be used as electrodes in supercapacitor devices for energy storage applications (such as high specific surface area, high conductivity, abundant reserves, environmental geniality, etc.). Therefore, among the different metal oxides, the most widely used oxides as electrodes for pseudocapacitors include tin oxide (SnO2), ruthenium oxide (Ru2O), cobalt oxide (Co3O4), nickel oxide (NiO), iron oxide (Fe2O3), vanadium oxide (V2O5), copper oxide (CuO), tungsten oxide (WO3), iridium oxide (IrO2), molybdenum oxide (MoO3), manganese oxide (MnO2), and so forth. 12−14 Among these metal oxides, ruthenium oxide has attracted a lot of attention due to its high conductivity, good thermal stability, and excellent electrochemical performance; however, its high environmental toxicity and high cost hinder its commercialization as an electroactive material for supercapacitor applications. Consequently, current novel investigations are being done in search of other alternatives for electrode materials in pseudocapacitors. Among these alternatives, silver generates high interest as it exhibits an electrochemical behavior close to that of ruthenium oxide, is cheaper than ruthenium oxide, and is more environmentally sustainable. 15,16 Also, silver in its different oxidation states (AgO, Ag2O, and Ag2O2) is more conductive than other transition-metal oxides investigated for pseudocapacitor applications and has more ability to change and adopt different oxidation states (+1, +2), providing the energy storage ability. Furthermore, from the electrochemical perspective, silver is very reactive and forms different oxide phases such as AgO, Ag3O4, Ag4O3, Ag2O3, and Ag2O, where Ag2O is the most stable among them. 17−22 In addition, silver has been used in previous studies as a component of electrodes; also, silver nanowires are commonly used in flexible and wearable electronic devices. Nanocomposites of silver nanoparticles with graphene, carbon nanotubes, metal oxides, or conducting polymers have been tested for their use in several energy storage applications. Also, silver has been reported to be used with metal oxides including MnO, NiO, CuO, and Co3O4, where the presence of silver alleviates problems associated with the low electrical conductivity of metal oxides. 23−31 Therefore, the main objective of this work deals with investigating the excellent qualities displayed by silver for its use like an electrode for pseudocapacitors because this novel research line possesses currently a real interest from a scientific point of view. Several fabrication methods can be employed to synthesize metal oxides for supercapacitor electrodes with different morphologies. Some of these methods include screen printing, hydrothermal, electrophoretic deposition, chemical precipitation, solvothermal, chemical bath deposition, microwaveassisted deposition, and sonochemical methods. 32−36 In addition, the use of nanostructured templates in combination with the previous methods offers the possibility of designing new nanostructured electrodes with high surface areas and optimized morphologies, where the interface contact between the electrode and electrolyte can be promoted. Furthermore, the development of a nanoporous structure of the electrode allows the liquid electrolyte to easily diffuse into the electrode material, thus increasing its specific surface area, which results in huge interest for applications in supercapacitors, sensors, catalysis, lithium-ion batteries, and adsorption. 37,38 In a Figure 1. Schematic picture of the subsequent processes for the fabrication of a 3D nanostructured silver electrode: nanoporous alumina template grown during the anodization procedure (a); vacuum thermal evaporation of the Ag seed layer on the top surface of the AAO template forming the nanostructured Ag antidot film platform (b); infiltration of poly(methyl methacrylate) (PMMA) and chloroform (3 g of PMMA in 50 mL of chloroform) solution inside the nanoholes of Ag antidot film platform (c); Al- and Al2O3-selective chemical etchings (d); silver electrodeposition for the replicated template (e); and final replica of the 3D nanostructured silver electrode (f). ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40088 previous work by Zhao et al., a self-supported metallic MnO2@ Ni-nanostructured electrode was developed by following a replica/antireplica synthesis technique based on nanoporous anodic aluminum oxide (AAO) templates, where the highly oriented nanoporous structure of the former AAO templates was demonstrated to be highly desirable for the metallic electrodes in supercapacitor applications. 39 Also, the recent work made by Brzozka et al. compares between the electrocatalytic properties of nanostructured metallic electrodes made by highly ordered arrays of Ag nanowires and hemispherical Ag nanocups, which were fabricated through the AAO template-assisted electrochemical deposition method in the case of Ag nanowire arrays, or the metallic sputtering deposition technique for the hexagonal ordered arrangement of Ag nanohemispheres. 40 Taking into account that surface area and morphology have a major influence on the capacity of the electrode material, it becomes important to implement a fabrication process able to provide nanostructured materials with high surface area, uniform pore size, and highly ordered parallel alignment, together with well-controlled porosity, in order to achieve excellent performance as electrochemical capacitor systems. 41−45 In this work, we report on a novel strategy to prepare 3D nanostructured silver electrodes (3D-NSEs) for supercapacitors using a fast, facile, and cost-effective antireplica/replica process where we have used AAO as precursor templates for the fabrication of electrodes. The electrochemical and surface chemical properties of the 3D nanostructured silver electrode were systematically studied in a 5 M KOH electrolyte. The surface morphologies, capacitive properties, and redox couples of the 3D-NSEs were characterized by SEM, XRD, Kripton adsorption at −198 °C, cyclic voltammetry (CV), galvanostatic charge−discharge cycling measurements, Coulombic efficiency, cycle stability, working voltage, self-discharge, and electrochromoactive behavior. The fabricated 3D-NSE exhibited a high areal capability of 0.5 F/cm2and 97.5% Coulombic efficiency at a current density of 6.4 mA/cm2and excellent charge−discharge cycling stability with 95% capacitance retention after 2600 cycles. Also, the electrode delivers a high energy density of around 385.87 μWh/cm2and a power density value of 3.82 μW/cm2and also displays an electrochromoactive behavior. These results suggest that this 3D-NSE presents excellent qualities for its use in energy storage devices such as supercapacitors and secondary batteries. 2. EXPERIMENTAL PROCEDURE 2.1. Synthesis of 3D Nanostructured Silver Electrode. The 3D-NSE has been fabricated by following an antireplica/ replica procedure employing nanoporous AAO as precursor templates. 39,40,44 This fabrication method consists of a series of processes, where the first one corresponds to the preparation of porous alumina membrane by a two-step electrochemical anodization process. 46 The additive fabrication process flow is schematized in Figure 1a−f. The nanoporous AAO templates have been fabricated by employing the well-known two-step anodization (mild anodization) process using 0.3 M oxalic acid as the electrolyte of the anodization bath with a pH of 0.5. 39,40,44 High-purity aluminum discs (Al 99.999%) were used as source substrates for the fabrication of the nanoporous AAO membrane (0.5 mm in thickness and 25 mm in diameter). Before the anodization process, the aluminum substrates were first cleaned by sonication in isopropanol and ethanol and after this electropolished in a mixture of HClO4 and ethanol (25:75) to smoothen the surface for 300−500 s at 5°C under a voltage of 20 V applied between the foil and a platinum counter-electrode. After that, the Al substrates were cleaned with distilled water (resistivity value: 18.2 MΩ/cm). Subsequently, the foils were first anodized in 0.3 M oxalic aqueous solution under a voltage of 40 V at a temperature of 1−3°C for 24 h. Then, the alumina grown during the first anodization process was chemically etched by a mixed aqueous solution of 0.2 M CrO3and 0.6 M H3PO4at 35 °C for 24 h. The second anodization step was conducted under similar circumstances as the first one but lasting for 1 h, leading to a highly hexagonally ordered nanopore arrangement in the AAO template. Afterward, the diameter of the former nanopores was enlarged through a pore-widening process in phosphoric acid (5% in volume) at 30 °C for 35 min. After this chemical treatment, the final pore diameter of nanoporous AAO samples is estimated to be around 70 nm (Figure 1a). In order to create a working electrode for Ag electrodeposition, a 50 nm thick Ag seed layer was deposited by vacuum thermal evaporation process, which involves the evaporation of the metal on the top surface of the nanoporous anodic alumina membranes used as patterned templates, 47 thus forming a nanostructured silver antidot film platform (Figure 1b). Subsequently, the nanoholes of the Ag antidot film layer were then filled at room temperature by a solution of poly(methyl methacrylate) (PMMA) and chloroform (3 g of PMMA in 50 mL of chloroform) (Figure 1c). Following this, two selective chemical etchings were carried out to remove the Al substrate and the alumina layer. An aqueous mixture of 50 mL of HCl (37%) and 0.3 M CuCl2was employed to perform chemical etching and partially remove the Al substrate, by exposing an area of around 1.8 cm2of the AAO backside. The alumina layer was further removed by immersing the samples in an acidic solution of 0.2 M CrO3and 0.6 M H3PO4(Figure 1d). Then, the metallic silver antidot film platform was electroplated using the silver nanolayer as an electrode in a commercial silver electrolyte (silver volume, 28.7 g/L) bath under a voltage of −1.7 V for 6 h (Figure 1e), employing a two-electrode electrochemical cell, performed at room temperature. After the subsequent removal of the polymer by immersion in chloroform, a 3D nanostructured silver electrode was finally obtained (Figure 1f). Thus, it can be seen from Figure 1f that the 3D nanostructured silver electrode shows an irregular columnar nanostructure grown with roughness along the sides of the columns. It is because of the way in which silver is electroplated into the nanopatterned polymer template, where the irregular and rough columnar shape of Ag oxide nanostructure tries to replicate the nanopatterning of the former alumina template. 2.2. Morphological and Microstructural Characterization and Chemical Analysis of 3D-NSEs. The structure and morphology of the 3D nanostructured silver electrode have been first investigated by SEM in a JEOL 6610LV microscope, equipped with an Oxford INCA EDX microanalysis system. The SEM system is fitted with a tungsten filament electron gun, which operates at an acceleration voltage of 20 kV. Top and bottom surface views and cross-sectional images of the samples were obtained. The chemical characterization of the 3D-NSE was performed by local EDX analysis, which allows us to determine the elemental chemical distribution on the sample. ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40089 XRD were also performed as a complementary technique to analyze the crystallographic microstructure of the sample. The XRD patterns of the 3D-NSE were obtained by an X-ray powder diffractometer with an XPERT-PRO diffractometer system within the angle range of 20°< 2θ< 90°. The effective surface area of the electrode sample was estimated accordingly by krypton (Kr) gas adsorption measurements carried out at 75 K (−198 °C) in an ASAP2420 volumetric apparatus. Previous to this analysis, the sample was degassed at 150 °C for 15 h under vacuum. The motivation for using this characterization method is that the krypton adsorption method of measurement is much more sensitive at these temperatures and can be applied to assess surface areas down to at least 0.05 m2, so it is considered a standard method for performing low surface area analysis. 48 2.3. Electrical Properties of Nanostructured Ag Electrode. 2.3.1. Cyclic Voltammetry. The cyclic voltammetry (CV) measurements performed in the 3D-NSE have been carried out with a Potenciostat AMEL instrument (Model 7050). Ten sequential cycles were programmed at the different scan rates of 10, 20, 30, 40, 50, and 70 mV/s in a potential window ranging from −0.5 V up to 0.8 V versus the Ag/AgCl reference electrode. The areal capacitance is proportional to the integral of the CV curve and can be determined from the charge value Q. Thus, Qis identified as the difference between the area under the charge curve and the area under the discharge curve data ranging between the upper and lower limits of the potential window. Thus, the areal capacitance can be obtained from eqs 1−3: 49−56 i k j j jy { z z z = · =Q I t t V d ; v (1) = ·Q I V 1 v d (2) i k j j j j j j j y { z z z z z z z = · = · · · CQ a V C I V a V ; d v (3) where Iis the current (A), tis the time (s), Vis the voltage (V), v is the scan rate parameter (V/s), Cis the areal specific capacitance (F/cm2), ais the surface area of the device in the single electrode in cm2, and it is fixed at 1 cm2for this study, and Qis the charge/discharge value (C). 2.3.2. Charging/Discharging Measurements. The charging/discharging measurements of the nanostructured electrode were carried out by means of a source measure unit (SMU) KEITLEY (Model 2010) with a current density of 6.4 mA cm−2and a voltage window of 1.2 V. From the charging/ discharging measurements, the specific capacitance value was calculated according to eq 4: 49−56 = · ·C I t a V( )/( ) d (4) where C(F/cm2) is the areal specific capacitance, I(A) is the discharge current, td(s) is the discharging time, a(cm2) is the surface area of the device in the single electrode given in cm2 which is fixed at 1 cm2in this study, and ΔV(V) is the voltage window for discharge. 2.3.3. Coulombic Efficiency and Cycling Stability. The Coulombic efficiency of the 3D-NSE was also measured in an SMU KEITLEY system (Model 2410). The Coulombic efficiency can be calculated through the following equation: 49−56 = t t d c (5) where tcand tdrepresent the charge and discharge times, respectively. The leakage current (LC) of supercapacitors corresponds to the tiny current that flows when the rated voltage continues to be applied to the capacitor. The leakage current is equivalent to the charging current required to maintain the supercapacitor at the specified voltage value. It is measured in amperes (A). The leakage current was measured in a Potenciostat AMEL instrument (Model 7050) by keeping the supercapacitor at a constant voltage value of 0.7 V and the measured time interval for 2 h. When the device has a high internal resistance and the charging source is disconnected from the supercapacitor, the device can start losing its charge. This is called the selfdischarge characteristic that consists of a voltage drop of the charged capacitor after a time period with no load condition. The self-discharge of the electrode was measured in a SMU KEITLEY instrument (Model 2400). The electrode was charged at 0.7 V, and the measured time was 5 h. The motivation for measuring the leakage current and selfdischarge of the supercapacitor is due to the importance for practical applications of the device. To calculate the energy density (Ed) and power density (Pd) of the samples, eqs 6 and 7were used: 49−56 = ·E C V 1 2 d 2 (6) =P E t d d d (7) where C(0.5 F/cm2) is the areal specific capacitance from charging/discharging measurements, Edis the energy density, Pdis the power density, tdis the discharge time, and Vis the potential window of the three-electrode measurement system (1.2 V). 2.4. Optical Characterization of Nanostructured Silver Electrodes. Finally, the electrochromoactive behavior of 3D-NSEs was analyzed. For this study, we apply a staircase voltage ramp across the electrode to induce redox reactions, therefore modifying the energy bands that interact with the visible light and thus changing the surface color of the electrode. In this particular case, the samples were analyzed by measuring the reflected light on the sample as a function of the applied potential. As a photometric measurement of luminous intensity, the luminance of the electrodes has been calculated using an RGB (red, green, and blue) code 8: 57 = × · + · + · Luminance(%) 100 (0.2126 R 0.7152 G 0.0722 B) (8) Such a color change experienced by the 3D-NSE during the charging/discharging measurements can be an excellent indicator of proper functioning of the nanostructured Ag electrode. 3. RESULTS AND DISCUSSION 3.1. Microstructure and Chemical Analysis of 3D- NSEs. Figure 2a,b shows the SEM images corresponding to the ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40090 3D-NSE with nanohole structure by replicating the former nanoporous AAO template, before (Figure 2a) and after (Figure 2b) the stability tests. From these SEM images it is possible to see that the 3D-NSE structure hardly changed after its performance. In order to determine the chemical composition of the 3D-NSE, EDX microanalysis was performed on the top surface of the sample before and after the stability tests. Figure 2c,d shows the EDX profile where only the presence of Ag has been detected, which confirms the successful fabrication approach for the nanostructured Ag electrodes and that the chemical composition does not change after the stability tests. The XRD patterns provide information about the crystallographic microstructure of the 3D-NSE before and after the stability tests (Figure 3). According to the literature consulted, the XRD peaks appeared at 2θof 37.99°, 44.2°, 64.4°, 77.4°, and 81.7°, which can be attributed to (111), (200), (220), (311), and (331) crystallographic planes of face-centered cubic (fcc) silver crystals, respectively, and XRD peaks at 2θof 39.57°and 46.38°can be attributed to a small portion of cubic crystallographic phase corresponding to Ag2O, indicating that after the stability tests there may be some remnants of silver oxide. 58−60 From the XRD results, it was found that the 3D- NSE possesses a crystallographic plane of face-centered cubic (fcc) structure and that the preferred growth orientation of the 3D-NSE is fixed at the (111) direction. The effective surface area of the 3D-NSE sample was estimated by a Krypton gas adsorption isotherm measured at 75 K (−198 °C), which is shown in Figure 4. The specific surface area was calculated by applying the Brunauer− Emmet−Teller equation in the relative pressure range between 0.1 and 0.3. 48 The obtained value for the specific surface area of the 3D-NSE is 0.0344 ±0.0014 m2/g. Based on the results obtained through the krypton gas adsorption isotherms measured at 75 K, it is clear that the entire surface area of the 3D-NSE device in the single electrode in cm2is potentially reactive to the reversible redox reactions, which are the typical reactions that take place in these kinds of supercapacitor electrodes, in contrast to the double-layer electrodes where the double-layer charging is related to the capacitive-like nature of the electrode/electrolyte interface. 3.2. Electrochemical Properties of 3D-NSEs. To investigate the electrochemical properties of the nanostructured electrode, CV experiments were performed. The CV Figure 2. SEM top-view surface images corresponding to the 3D-NSE before (a) and after (b) the stability tests. EDX microanalysis was performed on the top surface of the 3D-NSE before (c) and after (d) the stability tests. Figure 3. XRD patterns of the 3D-NSE before (red) and after (blue) the stability tests. Figure 4. Krypton gas adsorption isotherm measured at 75 K for the 3D-NSE sample. ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40091 curves were measured in 5 M KOH electrolyte at different scan rates (10, 20, 30, 40, 50, and 70 mV/s), with the potential window ranging from −0.5 V up to 0.8 V. The electrochemical measurements were performed in a three-electrode electrochemical cell, with the 3D nanostructured silver electrode as the working electrode, a Pt foil as the counter electrode, and Ag/AgCl as the reference electrode. The motivation for employing the Pt foil as a counter electrode is due to the platinum being commonly employed as a counter-electrode material. The use of platinum as an electrode is due to its excellent properties, such as inertness and nonreactiveness, resistance to acid attack and nonirritants, together with a long-lasting electrode, and so forth. 61 With respect to the reference electrode, Ag/AgCl is a widely used reference electrode because it is simple, inexpensive, very stable, and nontoxic; also, it works in a wide temperature range (other reference systems like the Hg/Hg2Cl2electrode can be used only at temperatures no higher than 60−70 °C). In addition, it is a nonpolarizable DC reference electrode because it has a well-defined DC potential not very dependent on the DC current flow. 62 Figure 5a shows the CV curves of the 3D-NSE. From the CV analysis, it is possible to see how all curves exhibit different redox peaks, which are responsible for the pseudocapacitance behavior exhibited by the 3D-NSE, because the areal capacitance is proportional to CV areas (eqs 1−3). However, it can be seen from the CV curves how the characteristic capacitance of the 3D-NSE is very different from the typical ideal rectangular shape shown by the electric double-layer capacitors. 37 If we analyze Figure 5a, it is possible to see four anodic current peaks (two minor ones (A1and A2) and two major ones (A3and A4)), in the anodic section of the CV curve, prior to the oxygen evolution potential. However, in the reverse potential scan, we can see an activated anodic current peak (A5) and two cathodic contributions (C1and C2), prior to the hydrogen evolution potential. 63,64 The A1anodic peak may be associated with the electrodissolution of Ag into [Ag(OH)2]on the basis of diffusion of soluble [Ag(OH)2] and of adsorption and desorption of OH 63,64 by following the respective reactions 9and 10: + [ ] +Ag 2OH Ag(OH) e ads 2 ads (9) [ ] [ ]Ag(OH) Ag(OH) 2 ads 2 aq (10) The A2anodic peak may be associated with the electroformation of a monolayer of Ag2O coming from the precipitation of [Ag(OH)2] from its supersaturated solution at the electrode surface. 40,41 Furthermore, the A3anodic current peak can be ascribed to the nucleation and growth of a Ag2O multilayer following the chemical reaction 11: 63,64 + + +2Ag 2OH Ag O H O 2e 22 (11) Finally, the A4anodic peak may be associated with the electrooxidation of Ag2O and the formation of AgO in accordance with the following chemical reaction 12: 63,64 + + +Ag O 2OH 2AgO H O 2e 22 (12) On the other hand, the A5anodic peak appeared at the reverse scan that has been reported in the same potential range of the A3peak, which can be associated with the continuous nucleation and growth of an Ag2O film as a result of electrooxidation of the basal Ag metal. 63,64 Paying attention to the cathodic section of the CV studied, it is possible to observe two reversible reactions. First, the C1cathodic peak could be ascribed to the electroreduction of AgO into Ag2O, indicating that this peak is conjugated to the A4anodic peak, according to the chemical reaction 13: 63,64 + + +2AgO H O 2e Ag O 2OH 22 (13) Finally, the more negative C2cathodic peak can be associated with the processes involved in the electroreduction of Ag(I) oxygen species into metallic Ag. 64 In Figure 5a, it becomes noticeable that with the increasing sweeping rates, the anodic and cathodic potential peaks shift in the more anodic and cathodic directions, respectively, thus decreasing the capacitance. The lower capacitance for high scan rates (Figure 5b) is a consequence of the migration and diffusion of electrolyte ions restricted at the surface of the electrode, whereas bulk diffusion occurs at lower scan rates, yielding larger capacitance values. 65,66 This electrochemical behavior indicates the presence of pseudocapacitance, where the electron transfer process of the redox reactions is very dependent on the voltage scan rate, caused by the polarization in the electrode material. The shapes of the CV curves reveal that the characteristic capacitance is well distinguished from that of the electric double-layer capacitance, where the CV curve is normally close to an ideal rectangular shape. 67−69 Galvanostatic charge−discharge (GCD) curves of the 3D- NSE are shown in Figure 6a. The cycling performances of the samples were estimated at a current density of 6.4 mA cm−2. The GCD curve exhibited a symmetric shape, which is indicative of an excellent reversibility behavior and an optimal capacitive behavior of the material. 70 Figure 5. CV curves of the 3D-NSE measured in 5 M KOH aqueous solution at different scan rates within a potential window ranging from −0.5 V up to 0.8 V with respect to the Ag/AgCl reference electrode (a); variation of the area-specific capacitance with the scan rate (b). ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40092 Also, the nearly symmetric potential−time relation of the galvanostatic charge/discharge curves implies low polarization value of the nanostructured Ag electrode and high charge− discharge Coulombic efficiency from it, 71,72 and the charge− discharge platforms are also consistent with the redox peaks that can be observed in the CV curves, indicating that redox reactions occur during the energy storage and release processes. 73−75 Base on the obtained GCD curves, it is possible to calculate the Coulombic efficiency, which was approximately 97.5%, demonstrating the high efficiency of the fabricated 3D-NSE. Additionally, it is possible to show from Figure 6b that the 3D- NSE also delivers good cycling stability with a capacitance retention of 95% and that no significant areal capacitance is lost after 2600 cycles. Finally, the areal capacitance calculated from the charge/discharge curves (eq 4) reaches a high value of 0.5 F/cm2, confirming that the 3D-NSE exhibits a high areal capacitance performance. We employed the areal capacitance value expressed in (F/cm2) because it is a very important indicator of the performance of a supercapacitor and for its potential use as an electrode device. 49−56 From Table 1, it can be seen that the 3D-NSE shows a highly competitive energy storage areal capacity, compared with other silver-based electrodes reported in previous publications. 51−55 Additionally, the 3D-NSE shows stable long-term electrochemical performance and cycle stability. Also, a bending cycle test was performed to demonstrate the mechanical stability of the fabricated electrode. Therefore, all of the above allows us to state that the 3D-NSE exhibits an ideal nanostructure with a high electrochemical performance and long-term stability, without the degradation of its mechanical structure. Table 2 lists the specific capacitance values versus the voltage window at the current density of 6.4 mA/cm2, appreciating the existing linear relationship between the areal capacitance and cell voltage window for the 3D-NSE, where the areal capacitance values increase by increasing the voltage window due to the pseudocapacitance effect contribution to the entire capacitance, which is a typical phenomenon in pseudocapacitors. 76 Considering the potential applications of this novel 3D nanostructured silver electrode, its leakage current and selfdischarge values were both measured in a three-electrode electrochemical cell (Figure 7). The leakage current was measured at a constant voltage of 0.7 V for 2 h through an electrochemical cell. The obtained data shown in Figure 7a indicate that first, the leakage current drops rapidly to gradually stabilize and to reach a value close to 0.07 mA. Such a low leakage current value indicates an optimal capacitor performance, 77 which could be attributed to few shuttle reactions caused by the impurities in electrode materials. 78,79 On the other hand, self-discharge of a supercapacitor refers to the gradual decrease in the voltage across the supercapacitor that occurs when the supercapacitor device is left in an open circuit. Therefore, self-discharge may reflect the efficiency that can be present in a supercapacitor as an energy storage device. 78 The self-discharge profile of the 3D-NSE obtained under an open-circuit voltage condition is shown in Figure 7b. As can be observed in the inset of Figure 7b, the 3D-NSE maintains approximately 50% of the initial charge potential Figure 6. GCD curves of the 3D-NSE at a current density of 6.4 mA cm−2(a) and cycle performance of the 3D-NSE during 2600 GCD cycles (b). Table 1. Comparison of the 3D-NSE Characteristic Values with Other Silver-Based Electrodes Reported in Previous Publications electrodes electrolyte current density specific capacitance cycle stability reference 3D-NSE 5 M KOH 6.4 mA/cm2500 mF/cm295% (2600 a ) this work Ag/CuxO/CF-40 3 M KOH 2 mA/cm21192 mF/cm293.3% (10,000) 51 Ag/CNFs 6 M KOH 100 mA/cm230.6 mF/cm2104% (10,000) 52 PANI/Ag/CNF aerogel 1 M H2SO4176 mF/cm2-53 Ag NWs/WO31 M H2SO413.6 mF/cm272.6% (5000) 54 Ag/Au/polypyrrole - 5.8 mA/cm20.58 mF/cm293% (800) 55 a Bending cycles. Table 2. Specific Capacitance Values for 3D-NSE versus the Voltage Window (ΔV) at a Current Density of 6.4 mA/cm2 ΔV(V) C(F/cm2) 1.2 0.5 1 0.3 0.8 0.2 ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40093 after 60 min of the self-discharge process, suggesting that charge redistribution plays an important role, since most of the voltage reduction can be due to the redistribution of charge carriers deep inside the holes of pores during longtime discharging, which is very typical in pseudocapacitors. All these effects are reflected in the lack of the self-discharge rates diminishing with longer hold times. 80−82 Other important parameters for the performance characterization of a capacitor device are related to its energy density (Ed) and power density (Pd) values. The energy-storage performance for our 3D-NSE was evaluated from the Ragone plot shown in Figure 8. Noticeably, we obtained for the 3D- NSE an energy density value of 385.87 μWh/cm2(eq 6) at the power density of 3.82 μW/cm2(eq 7). The obtained values of areal energy and power densities for our 3D-NSE are very competitive, if we compare them with the previous ones reported for other types of electrodes used for supercapacitor applications. 83−89 Considering all these results, it is observed that the capacitance behavior of our 3D-NSE provides superior energy density performance as compared to EDLC capacitors because the Faradaic process occurs not only on the surface but also inside of these electrodes. Thus, on the pseudocapacitive materials, it is very important to design and synthesize a nanostructure that provides improved access to these reaction centers and shortens the ion diffusion lengths. All of these justify that the peculiar nanostructured design featured by our 3D-NSE would be especially appropriate for improving the redox charge-transfer process that occurs inside of the electrode, decreasing the electronic diffusion distances, which causes the 3D-NSE to have relatively large energy densities as compared to the previously reported silver electrodes. At the same time, this behavior provides a lower power density because these faradaic processes are normally slower than nonfaradaic processes, which are associated with the intrinsic character of the electrochemical reactions that take place in these types of electrodes. 90,91 In addition, hybrid supercapacitors with an asymmetrical electrode configuration (where one electrode can be an electrostatic carbon material and the other one can be a faradaic capacitance material) begin to wake up a huge interest in numerous research investigations to capitalize on both electrode materials’ advantages for improving the overall cell voltage, energy, and power densities. 92 3.3. Electrochromoactive Behavior of 3D-NSEs. Finally, we also analyze the electrochromoactive behavior of the fabricated 3D-NSE. Figure 9 shows the corresponding color change experienced by the 3D-NSE during a cyclic voltammetry measurement taken at a scan rate of 70 mV/s. The obtained data indicate that the luminance of the device decreases (the electrode gets darker) as the 3D-NSE is charged (CL). On the other hand, when the 3D-NSE is discharged, the luminance increases (DL), which implies that the electrode is getting brighter, acquiring a grayish color. Therefore, the color changes of the electrode reflect the charging/discharging state of the 3D-NSE. It is because the color change, which takes place during the cycles of the charging/discharging process, is associated with the different silver oxides formed during the redox process exhibited by the 3D-NSE. 93 Figure 7. Leakage current curve for the 3D-NSE at a voltage value of 0.7 V and kept constant for 2 h (a). Self-discharge curve of the 3D- NSE under open-circuit conditions for 5 h after being charged at a voltage value of 0.7 V and the variation in the charge during the selfdischarge process (inset) (b). Figure 8. Ragone plot showing the classification of the 3D-NSE in comparison to previously reported electrodes used for supercapacitor applications. Figure 9. Electrochromoactive behavior exhibited by the 3D-NSE during the measurement taken at cyclic voltammetry for a scan rate of 70 mV/s. ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40094 Additionally, the video supported in the Supporting Information displays the electrochromoactive behavior exhibited by the 3D-NSE during one cycle of the charge/ discharge operation mode. For all of these reasons, this nanostructured Ag electrode could be considered like a smart material because it reacts to external stimuli, such as the electric field, changing its optical properties, given that its color changed during the capacitor performance (see video in Supporting Information). Furthermore, this fabulous quality can also be used to test the correct electrode operation because the visual effect of color change is related with the electrical charge storage through the fast and reversible redox reactions and, therefore, is a direct indicator that the electrode is properly operating. 94,95 Overall, this work is technologically very inspiring and represents the significant progress in achieving a high areal specific capacitance in an electrode film with several microns of thickness, which provides a nice manifestation on how to optimize the fundamental electrochemistry process (redox reaction, charge diffusion, etc.) by properly tailoring the electrode architecture. 96−98 4. CONCLUSIONS These experimental results reveal the feasibility to fabricate a metallic 3D-NSE with improved electrochemical properties for outstanding supercapacitor devices through facile and costeffective template-assisted antireplica/replica processing. Its electrochemical performance as a supercapacitor electrode was systematically investigated, exhibiting a high areal capacitance value of 0.5 F cm−2and a good Coulombic efficiency (97.5%). In addition, the 3D-NSE exhibits excellent capacitance retention (95%) after 2600 cycles at a current density of 6.4 mA cm−2, indicating a good cyclic stability due to the excellent reversibility exhibited by this 3D nanostructured Ag electrode. Along with this, the 3D-NSE delivers a high energy density of around 385.87 μWh/cm2at a power density of 3.82 μW/cm2 and also displays an electrochromoactive behavior. Furthermore, we have demonstrated that this nanostructured Ag electrode exhibits excellent electrochemical performance, such as high areal capacitance, increased energy density, and power density, as well as long and stable cycle life, together with added electrochromoactive behavior, which proves its suitability as an advanced electrode material for pseudosupercapacitor applications. Considering the results reported in this work, the fabricated nanopatterned metallic 3D-NSE can be considered as a very promising material for its implementation in novel energy storage systems such as micro-supercapacitors or in electrochemical devices. ■ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c02235. Electrochromoactive behavior exhibited by the 3D-NSE during one cycle of charge/discharge operational mode. It also appreciates the visual effect of color change related with the electrical charge storage through fast and reversible redox reactions, which indicates that the 3D-NSE is properly operating (MP4) ■AUTHOR INFORMATION Corresponding Author Victor M. Prida −Depto. de Física, Facultad de Ciencias, Universidad de Oviedo, 33007 Oviedo, Spain; orcid.org/ 0000-0001-5541-8816; Email: [email protected] Authors Ana Silvia González −Depto. de Física, Facultad de Ciencias, Universidad de Oviedo, 33007 Oviedo, Spain Javier García −Depto. de Física, Facultad de Ciencias, Universidad de Oviedo, 33007 Oviedo, Spain Victor Vega −Laboratorio de Membranas Nanoporosas, Servicios Científico-Técnicos, Universidad de Oviedo, 33006 Oviedo, Spain Rafael Caballero Flores −Depto. Física de la Materia Condensada, Facultad de Física, Universidad de Sevilla, 41080 Sevilla, Spain Complete contact information is available at: https://pubs.acs.org/10.1021/acsomega.3c02235 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Spanish MCINN research grant no. PID2019−108075RB- C32/AEI/10.13039/501100011033. Regional government of Principado de Asturias under FICYT project ref.: SV-PA-21- AYUD/2021/51399. Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENTS This work was supported by Spanish MCINN under research grant no. PID2019-108075RB-C32/AEI/10.13039/ 501100011033. The authors thank FICyT from Principado de Asturias for the financial support under project SV-PA-21- AYUD/2021/51399 and particularly A.S.G. for her research contract. The authors also wish to thank Dr. A. Castro-Muniz (INCAR-CSIC) for his scientific and technical support on the 3D-NSE characterization. ■REFERENCES (1) Zhong, Y.; Xia, X.; Mai, W.; Tu, J.; Fan, H. J. Integration of Energy Harvesting and Electrochemical Storage Devices. Adv. Mater. Technol. 2017,2(12), No. 1700182. (2) Karthikeyan, S.; Narenthiran, B.; Sivanantham, A.; Bhatlu, L. D.; Maridurai, T. Supercapacitor: Evolution and Review. Mater. Today: Proc. 2021,46 (9), 3984−3988. (3) Forouzandeh, P.; Kumaravel, V.; Pillai, S. C. Electrode Materials for Supercapacitors: A Review of Recent Advances. Catalysts 2020,10 (9), 969. (4) Kumar, Y.; Rawal, S.; Joshi, B.; Hashmi, S. A. Background, Fundamental Understanding and Progress in Electrochemical Capacitors. J. Solid. State. Electrochem. 2019,23, 667−692. (5) Sung, J.; Shin, C. Recent Studies on Supercapacitors with Next- Generation Structures. Micromachines 2020,11 (12), 1125. (6) Banerjee, S.; De, B.; Sinha, P.; Cherusseri, J.; Kar, K. K. Applications of Supercapacitors in Handbook of Nanocomposite Supercapacitor Materials I; Kar, K. K., Ed.; Springer Cham: Germany, 2020; pp 341−350. (7) Wu, C.; Cai, J.; Zhang, Q.; Zhou, X.; Zhu, Y.; Shen, P. K.; Zhang, K. Hierarchical Mesoporous Zinc−Nickel−Cobalt Ternary Oxide Nanowire Arrays on Nickel Foam as High-Performance Electrodes for ACS Omega http://pubs.acs.org/journal/acsodf Article https://doi.org/10.1021/acsomega.3c02235 ACS Omega 2023, 8, 40087−40098 40095