nanomaterials Article Polyoxometalate-Stabilized Silver Nanoparticles and Hybrid Electrode Assembly Using Activated Carbon Sara Goberna-Ferrón 1, * , Laia Cots 2 , Marta Perxés Perich 3 , Jun-Jie Zhu 2 and Pedro Gómez-Romero 2 1 Instituto Universitario de Tecnología Química (CSIC-UPV), Universitat Politècnica de València, Avda. De los Naranjos s/n, 46022 Valencia, Spain 2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; pedro.gomez@icn2.cat (P.G.-R.) 3 Materials Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, 3584 CG Utrecht, The Netherlands * Correspondence: sgobfer@itq.upv.es Abstract: The intersection between the field of hybrid materials and that of electrochemistry is a quickly expanding area. Hybrid combinations usually consist of two constituents, but new routes toward more complex and versatile electroactive hybrid designs are quickly emerging. The objective of the present work is to explore novel triple hybrid material integrating polyoxometalates (POMs), silver nanoparticles (Ag0 NPs), and activated carbon (AC) and to demonstrate its use as a hybrid electrode in a symmetric supercapacitor. The tri-component nanohybrid (AC/POM-Ag0 NPs) was fabricated through the combination of AC with pre-synthesized ∼27 nm POM-protected Ag0 NPs (POM-Ag0 NPs). The POM-Ag0 NPs were prepared using a green electrochemical method and char- acterized via UV-vis and IR spectroscopy, electron microscopy, dynamic light scattering (DLS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). Afterward, the AC/POM-Ag0 NPs ternary nanocomposite material was constructed and characterized. The electrochemical behavior of AC/POM-Ag0 NPs’ modified electrodes reveal that the nanomaterial is electroactive and exhibits a moderately higher specific capacitance (81 F/g after 20 cycles) than bare AC electrodes (75 F/g) in a symmetrical supercapacitor configuration in the voltage range 0 to 0.75 V and 20 mV/s, demonstrating the potential use of this type of tri-component nanohybrid for Citation: Goberna-Ferrón, S.; Cots, L.; Perxés Perich, M.; Zhu, J.-J.; electrochemical applications. Gómez-Romero, P. Polyoxometalate-Stabilized Silver Keywords: hybrid nanomaterials; triple hybrid materials; polyoxometalate; Ag nanoparticles; Nanoparticles and Hybrid Electrode activated carbon; electroactive materials; supercapacitor Assembly Using Activated Carbon. Nanomaterials 2023, 13, 2241. https:// doi.org/10.3390/nano13152241 1. Introduction Academic Editor: Domenica Tonelli Multifunctional hybrid materials have consolidated as a very versatile and promising Received: 10 July 2023 class of materials. The hybrid approach allows for a reinforcing combination of properties Revised: 28 July 2023 of dissimilar components in synergic combinations. The compositions are ample and Accepted: 2 August 2023 flexible, including magnetic, semiconductor, organic, inorganic, or metallic materials [1–4]. Published: 3 August 2023 Their applications are equally varied, ranging from automotive and structural applications to electronics, biomedical, and energy applications. In this context, the intersection be- tween the area of hybrid materials and electrochemistry is a quickly expanding field with Copyright: © 2023 by the authors. key applications, from sensors to energy storage and conversion [5,6]. Regarding energy Licensee MDPI, Basel, Switzerland. storage, different devices based on hybrid materials have been recently developed includ- This article is an open access article ing aqueous Zn-ion batteries [7], lithium–selenium batteries [8], supercapacitors [9–11], or distributed under the terms and ammonium-ion batteries [12]. Various strategies have been made to enhance the electro- conditions of the Creative Commons chemical performance of energy storage systems, from hybrid materials to hybrid devices, Attribution (CC BY) license (https:// providing enhanced energy and power densities by combining battery and supercapac- creativecommons.org/licenses/by/ itor materials and storage mechanisms [13]. Hybrid combinations of electroactive and 4.0/). conductive materials for energy storage applications include a wide variety of inorganic Nanomaterials 2023, 13, 2241. https://doi.org/10.3390/nano13152241 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2023, 13, 2241 2 of 17 species, from extended oxides (or oxyanion phases) to polyoxometalate (POM) clusters, but also conducting organic polymers or carbon materials [14]. Frequently, these hybrid combinations consist of two constituents, but the path to further complexity is also begin- ning to be explored. For example, triple hybrid materials integrating conducting polymers, oxides, and inorganic molecules [15]; ternary hybrids of MXene, activated carbon (AC), and POMs [16]; ternary nanocomposites of single-walled carbon nanotubes, polyaniline, and silver nanoparticles (NPs) [17]; or electrocatalytic films of conducting polymer/POM/Pt NPs grown layer by layer [18]. Thus, multimaterial combinations with novel nanoarchitec- tures may provide a differential advantage when it comes to fully exploiting the properties of intrinsically but only partly excellent electrode materials. POMs are oxoanion clusters of early transition metals (especially V, Mo, and W) that can range from 1 to 5 nm in size and frequently present a rich and reversible redox chemistry. POMs display an extraordinary range of physicochemical properties and are some of the most promising building blocks for functional nanomaterials [19]. Although these clusters can be used for a wide range of applications, the most important functionality of POMs involves their unique electrochemical behavior [20]. Due to the high stability of their redox states, they can participate in fast reversible multielectron transfer reactions, making them potential candidates to achieve a high capacity for energy storage applications [21]. However, they usually exist as salts and solid acids with negligible electronic conductivity and high solubility. Thus, their use as electrode materials requires anchoring onto a variety of conducting substrates such as conducting polymers [22], or carbon materials including nanocarbons or activated carbons (AC) as matrices [23]. AC is a general term commonly applied to a wide family of amorphous carbon materials with excellent and unique textural properties (i.e., specific surface area, porosity, and pore size distribution) [24]. In recent decades, special attention has been paid to the use of ACs as low-cost electrode materials in electrochemical energy storage and conversion devices, mainly supercapacitors [25], lithium and sodium ion batteries [26], and fuel cells [27]. Amongst other aspects, electrical conductivity generally plays a crucial role in the suitability and performance of ACs as electrode materials. Noble metals are promising for (photo-)electrochemical applications, because of their good conductivity, plasmonic behavior, and chemical stability [28]. However, due to the scarcity and high cost of noble metals, their integration with other sustainable and available materials is considered one of the most attractive ways to optimize their properties and minimize their consumption. Additionally, using metal nanostructures (i.e., NPs) increases the overall surface-to-volume ratio and hence reduces the metal loading. In fact, noble metal NPs have been widely used as conductive dopants in electrode materials for energy storage [29]. Silver has attracted special attention because of its unique physical prop- erties, electrical and thermal conductivity, and low toxicity. Among noble metals, silver is very attractive for electrochemical applications because of its higher conductivity and relatively lower cost [30]. Interestingly, POMs have been exploited for directing the syn- thesis of metallic NPs (M0 NPs) [31–33]. POM-protected M0 NPs have received enormous attention because of their unique chemical and physical properties with applications in (electro)catalysis [34,35], electrochemical biosensors [36], and biomedicine [37]. However, their use in electrochemical energy storage remains largely unexplored. To the best of our knowledge, there is only one example of an electrode material based on POM-protected Ag0 NPs for Li-ion batteries [38]. Herein, POM clusters, Ag0 NPs, and AC materials are combined into a single entity through nanoengineering. The strategy to prepare the novel tri-component nanohybrid material is to combine AC and POM-protected Ag0 NPs (POM-Ag0 NPs), pre-synthesized using a green method in which the POM acts both as a reductant and as a stabilizing agent. First, the synthesis of POM-Ag0 NPs was optimized by studying the effects of the reactant’s concentration and the redox potential of the selected POM on the size of the formed NPs. Then, the AC/POM-Ag0 NPs tri-component nanohybrid was prepared and fully character- ized. Finally, the electrochemical performance of the nanohybrid material was tested, and Nanomaterials 2023, 13, 2241 3 of 17 our results revealed that the nanocomposite is electroactive and exhibits an appreciably im- proved performance compared with bare AC in a symmetrical supercapacitor configuration due to the synergistic effect of the three components in the final nanocomposite. 2. Materials and Methods 2.1. Synthesis of POM-Ag0 NPs 2.1.1. Preparation of Reduced POM Solutions The POMs selected for this work are Phosphotungstic acid H3 [PW12 O40 ] (abbreviated PW12 3− ) and silicotungstic acid H4 [SiW12 O40 ] (abbreviated SiW12 4− ), both from Sigma- Aldrich (St. Louis, MO, USA) (≥99.9% trace metals basis). First, POM solutions were elec- trochemically reduced. In a typical experiment, an aqueous solution of POM (50 mL, 1 mM) without any supporting electrolyte was placed in a bulk electrolysis cell (from BASi). A coiled platinum wire (within a fritted glass isolation chamber) and Ag/AgCl electrode were used as counter and reference electrodes. Bulk electrolysis was performed with a BioLogic potentiostat applying the necessary potential to reduce the POM by 1 or 2 e− until intensity values reached a plateau. Potential O2 interference was kept to a minimum with vigorous Ar blanketing over the entire process. Upon electrolysis, the solution turned blue due to the formation of reduced POM (red POM). The applied potential was selected at the end of the corresponding 1e− or 2e− cathodic peak and before the following reaction’s half-wave potential; that way, the kinetics of the desired reaction are favored while preventing the initiation of the next reduction (Figure S1). The concentration of red POM was measured pre- cisely in each case using UV-vis absorption spectrometry (Figure S2) by measuring the char- acteristic absorbance of the blue red POM species (PW12 O40 4− ε752nm = 2000 M−1 cm−1 ; PW12 O40 5− ε653.6nm = 4400 M−1 cm−1 ; and SiW12 O40 6− ε625 nm = 3800 M−1 cm−1 ) [39]. 2.1.2. Synthesis of NPs Silver nanoparticles were obtained by injecting 1mL of a de-aerated aqueous stock solution of AgNO3 (Sigma-Aldrich, 99.9999% trace metals basis) into the freshly prepared red POM solution (45 mL) that served as both the reductant and protecting ligand. The AgNO3 stock solution concentration was varied from 4.05 to 405 mM to obtain final Ag+ /red POM ratios from 0.1 to 10. The initially blue solutions turned green and finally yellow, orange, or brown depending on the Ag+ /red POM ratio utilized (Figure S3), which is indicative of NP formation, within a period between seconds and minutes. The solutions were stirred under Ar atmosphere for 1 h and allowed to stand for 4 days in closed 50 mL centrifuge tubes and then filtered using a 0.45 µm syringe filter before UV-vis, microscopy, and DLS characterization. The residual free POM and unreacted Ag+ in solution were removed using centrifugation (at 10,500 rpm, 30 min). The supernatant was removed via pipette and an equal volume of pure water was added in its place. The process was repeated 3 times and the final collected POM–Ag0 NPs were re-dispersed into water for characterization (XRD, XPS, IR, and CV) and for the preparation of POM–Ag0 NPs/AC hybrids. 2.2. Preparation of Hybrid Material POM-Ag0 NPs synthesized using a ratio of Ag+ /red POM = 1 (red POM = PW12 4− ) were used to prepare POM–Ag0 NPs/AC hybrids. The NPs were re-dispersed in water (half of the initial reaction volume) and 60 mg of activated carbon (Cabot Corporation, Alpharetta, GA, USA) was added to yield a concentration of AC of 3mg/mL. The mixture was left under vigorous stirring in a closed flask overnight. Then, the solution was vacuum filtered using a 45 µm filter and washed thoroughly with water. The solid was dried under vacuum at 80 ◦ C. 2.3. Preparation of Hybrid Electrodes and Assembly of Coin Cells The conventional electrodes were fabricated by mixing the active material (AC or AC/POM–Ag0 NPs), carbon black, and PVDF (poly(vinylidene fluoride), average molecule Nanomaterials 2023, 13, 2241 4 of 17 weight ≈ 534,000 from Sigma-Aldrich) at a weight ratio of 85:5:10. The mixture was first formed as a slurry by adding a few drops of N-methyl-2-pyrrolidone (anhydrous, ≥99.5%, Sigma-Aldrich), and then coated onto aluminum foil (>99%, 18 µm, Goodfellow, Hamburg, Germany) and dried under vacuum at 120 ◦ C for 12 h. The thickness of the coated POM– Ag0 NPs/AC layer on Al foil was 53.25 µm and the loading masses of both electrodes were 4.5 mg. The thickness of the coated AC layer was 20 µm with loading masses of around 3 mg. CR2032 coin cells were used to fabricate symmetric supercapacitors. All the electrodes were pressed at 3MPa before assembling. Additionally, 1 M H2 SO4 served as the electrolyte. Cyclic polarization (CP) was performed in a two-electrode cell to evaluate the capacitive performance in devices. The calculation of capacitance is presented in Supplementary Materials Equation (S1). 2.4. Instrumentation All the electrochemical tests were conducted on Biologic VMP3 multichannel potentio- stat. Conventional cyclic voltammetry (CV) was performed in a three-electrode configuration. UV-vis absorption spectrum was recorded in the range of 350–900 nm using a Cary 4000 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) and disposable PMMA cuvettes (transmittance range between 300 and 900 nm). Attenuated total re- flectance Fourier transform infrared spectroscopy (ATR-FTIR) in the range between 450 and 4000 cm−1 using a Bruker Tensor 27 spectrometer (Bruker, Billerica, MA, USA), 32 running scans were collected at a resolution of 4 cm−1 . The NP dispersion sample was measured by drop-depositing ethanol-suspended NPs on the diamond prism and drying in air before the measurement. UV-vis and ATR-FTIR data were analyzed using Spectragryph optical spectroscopy v1.2.15 software [40]. Dynamic light scattering (DLS) measurements were carried out using Zetasizer Nano ZS and disposable PMMA cuvettes. Electron microscopy was performed with an FEI Magellan 400L XHR SEM (FEI Com- pany, Hillsboro, OR, USA), operated in bright field STEM mode, with a voltage of 20 kV; water dispersions of NPs were drop-deposited on a 300-mesh copper substrate with a car- bon membrane (Lacey Carbon) and evaporated in air for 24 h. Scanning electron microscopy (SEM) images of hybrid materials were obtained using an SEM Quanta 650 FEG (FEI Com- pany, Hillsboro, OR, USA); solid samples were dispersed in ethanol, drop-deposited on a silicon chip, and evaporated in air for 24 h. The silicon chips were previously washed with ethanol, sonicated for a few minutes, and heated at 80 ◦ C for ~1 h. All microscopy images were analyzed using ImageJ v1.53t software [41]. Powder X-ray diffraction (XRD) analyses were performed using an X’Pert PRO PANa- lytical (Malvern Panalytical, Malvern, UK) with Cu Kα (45 kV and 40 mA) radiation. The samples in solution were prepared with the drop deposition (20 µL) of a filtered ethanol- resuspended solution of nanoparticles on a silicon substrate and air-evaporated for at least 24 h. For hybrid analyses, the solid was mortared for 5 min and directly deposed on the silicon substrate. X-ray photoelectron spectroscopy (XPS) measurements were performed using a SPECS PHOIBOS 150 hemispherical analyzer (SPECS Surface Nano Analysis GmbH, Berlin, Ger- many) with Al Kα radiation (1486.74 eV). Solid samples were prepared by placing the powders on a Kapton tape. The NP dispersion sample was drop-deposited on a silicon chip and evaporated in air for 24 h before measurement. Data were analyzed using CasaXPS v2.3.16Dev52 software [42]. 3. Results and Discussion 3.1. Synthesis and Morphological Characterization of POM-Ag0 NPs Reductions of metal salts can be carried out with previously reduced POMs (red POM), in which the POM acts as both a reductant and a stabilizing agent [43]. Reduction of POMs can be accomplished photochemically [44], chemically [45], or electrochemically [46]. Among them, the electrochemical method represents a green, facile, and scalable approach. in which the POM acts as both a reductant and a stabilizing agent [43]. Reduction of POMs can be accomplished photochemically [44], chemically [45], or electrochemically [46]. Among them, the electrochemical method represents a green, facile, and scalable ap- proach. In a simplified view, the silver NP synthesis can be sketched using the following equation (and it is represented in Scheme 1a): Nanomaterials 2023, 13, 2241 5 of 17 POM + Ag+  oxPOM + Ag0 red (1) Then, in the stabilization process, the adsorption of a layer of POM anions on the In a simplified view, the silver NP synthesis can be sketched using the following equation nanoparticle surface provides both charge stabilization and steric stabilization. Of course, (and it is represented in Scheme 1a): this reaction is only feasible for metal ions with a higher reduction potential than the re- duction potential of the POM used,POM which+ is Agindeed + → oxthe POM case for0the pair Ag/Ag+ (E0 = 0.557 (1) + Ag red V vs. SCE). Scheme 1. Schematic representation of (a) the synthesis of POM-Ag0 NPs in which POM serves as both Scheme a reducingrepresentation 1. Schematic and stabilizingofagent (a) theand (b) theof synthesis preparation POM-Ag0 NPsprocedure of the in which POM tri-component serves as AC/POM-Ag 0 NPsstabilizing hybrid material. Not(b)in scale; POM is 1 nm in size and 0 Ag tri-component NPs are around both a reducing and agent and the preparation procedure of the AC/POM-Ag NPs hybrid 27–43 nm 0depending onmaterial. Not the initial in scale; reagent POM is 1 nm in size and Ag0 NPs are around 27– concentration. 43 nm depending on the initial reagent concentration. Then, in the stabilization process, the adsorption of a layer of POM anions on the nanoparticle 3.1.1. surface Influence of provides bothofcharge the Concentration Ag+ stabilization and steric stabilization. Of course, this In reaction the firstisset only of feasible for metal experiments, ions with [redPOM] was akept higher reduction constant at 1potential mM, and than thethe+ reduction Ag red +/ potential of the POM used, which is indeed the case for the pair POM molar ratio values were varied from 0.1 to 10. The POM selected for this workAg/Ag 0 = 0.557 V vs. SCE). is a(Eheteropoly acid with the chemical formula H3[PW12O40] (with its phosphotungstate anion abbreviated 3.1.1. Influence of as the PWConcentration 123−); the one-electron-reduced of Ag+ anion PW124− was generated elec- trochemically and used directly as the reducing agent (redPOM). Figure 1a shows+/three In the first set of experiments, [red POM] was kept constant at 1 mM, and the Ag +red POM UV-vis spectra of Ag NPs synthesized by changing the initial concentration of Ag . For molar ratio values were varied from 0.1 to 10. The POM selected for this work is a heteropoly Ag+/redPOM = 1, an absorption peak was observed centered at 395.1 nm, featuring a typical acid with the chemical formula H3 [PW12 O40 ] (with its phosphotungstate anion abbreviated surface plasmon resonance associated with spherical or quasi-spherical Ag0 NPs [47]. as PW12 3− ); the one-electron-reduced anion PW12 4− was generated electrochemically and When the ratio is decreased to 0.1, there is a shift to longer wavelengths (427.5 nm) that used directly as the reducing agent ( POM). Figure 1a shows three UV-vis spectra of Ag indicates an increase in the size of the red NPs and a decrease in the intensity, indicating a NPs synthesized by changing the initial concentration of Ag+ . For Ag+/ red POM = 1, an smaller concentration of NPs. For Ag+/redPOM = 10, a broad absorption peak appears at absorption peak was observed centered at 395.1 nm, featuring a typical surface plasmon 400.5 nm with an intermediate intensity, which suggests that the NPs have an intermedi- resonance associated with spherical or quasi-spherical Ag0 NPs [47]. When the ratio is ate size and concentration between ratios 0.1 and 1. Furthermore, a second weak and very decreased to 0.1, there is a shift to longer wavelengths (427.5 nm) that indicates an increase in broad band indicated via a shoulder at ca. 550 nm emerges, which is characteristic of the the size of the NPs and a decrease in the intensity, indicating a smaller concentration of NPs. coupling For Agof+/thePOMsurface plasmons = 10, a broad of aggregated absorption NPs peak [48]. at 400.5 nm with an intermediate appears red intensity, which suggests that the NPs have an intermediate size and concentration between ratios 0.1 and 1. Furthermore, a second weak and very broad band indicated via a shoulder at ca. 550 nm emerges, which is characteristic of the coupling of the surface plasmons of aggregated NPs [48]. Nanomaterials 2023, 13, x FOR PEER REVIEW 6 of 17 Nanomaterials 2023, 13, 2241 6 of 17 Figure 1. UV-vis spectra of POM-Ag0 NPs synthesized by (a) changing the initial concentration of Ag+ (Different Ag+/redPOM ratios) using PW124− as redPOM; and (b) by changing the reducing power of the used POM while keeping the Ag+/redPOM ratios = 1. These results agree with STEM data (Figure 2 and Table 1) where the size distribution obtained from the Gaussian fit to the STEM histograms shows that the size of the NPs follows1.the Figure trend UV-vis 0.1 > of spectra 10POM-Ag > 1 of Ag0 +/ NPsredPOM ratios. Aggregation/polydispersity synthesized can also be by (a) changing the initial concentration of Figure 1. UV-vis spectra of POM-Ag0 NPs synthesized by (a) changing the initial concentration of appreciated Ag + (Different inAgthe+/ images POM and reflected ratios) using PWin the 12 4 red ratios) using PW124− as redPOM;red − standard as POM; deviation, and (b) which by is changing high the for ratios reducing Ag+ (Different Ag+/redPOM and (b) by changing the reducing power 0.1 and 10 used compared withkeeping ratio the1. Surprisingly, Ag+/ the UV-vis spectra (Figure 1a) for ofpower the used of the POM POM while while keeping the Ag+/red POM red POM ratios = 1. ratios = 1. Ag+/redPOM = 0.1 did not show the characteristic band (at 550 nm) of aggregated NPs, probablyThesedue results to the agree small with STEM dataindicated concentration (Figure 2 by andtheTable low 1) where the size distribution These results agree with STEM data (Figure 2 and Table 1) intensity where theofsize light absorption. distribution obtainedDLS from data the Gaussian (Table 1) were fit tocollected also the STEM for histograms all the NPs. shows The that the mean size size for of +/thePOM Ag NPs obtained from the Gaussian fit to the +/ STEM histograms shows that the size of theredNPs follows ratios the the 1 and trend 0.1 0.10.1 agrees > 10 > 1 of Ag red STEM data. However, the DLS for ratio = 10also POM ratios. Aggregation/polydispersity can follows trend > 10 with > 1 ofUV-vis Ag+/redPOM and ratios. Aggregation/polydispersity can also bere- be appreciated in the images and reflected in the thestandard deviation, which is high for appreciated in the images and reflected in themeet sulted in a size quality report that did not standard quality criteria deviation, dueis which tohigh a too-polydis- for ratios ratios perse 0.1 sampleand 10 compared for cumulant with ratio 1. Surprisingly, the UV-vis spectra (Figure 1a)UV- for 0.1 and +/ 10 compared with analysis, ratio 1. in agreement with Surprisingly, the aggregation the UV-vis spectra observed (Figure 1a) via for Ag vis red POM and STEM. = 0.1 did not show the characteristic band (at 550 nm) of aggregated NPs, Ag +/redPOM = 0.1 did not show the characteristic band (at 550 nm) of aggregated NPs, probably due to the small concentration indicated by the low intensity of light absorption. probably due to the small concentration indicated by the low intensity of light absorption. DLS data (Table 1) were also collected for all the NPs. The mean size for Ag+/redPOM ratios 1 and 0.1 agrees with UV-vis and STEM data. However, the DLS for ratio = 10 re- sulted in a size quality report that did not meet the quality criteria due to a too-polydis- perse sample for cumulant analysis, in agreement with the aggregation observed via UV- vis and STEM. Figure 2. Bright-field STEM images (top) and size distribution (bottom) obtained from the Gaus- sian fit to the STEM histograms of POM-Ag0 NPs synthesized by using Ag+/ red POM ratios of 0.1 (a), 1 (b), and 10 (c). The red POM used in the synthesis was PW12 4− . Nanomaterials 2023, 13, 2241 7 of 17 Table 1. Summary of the main parameters obtained from UV-vis, STEM, and DLS data. UV-vis STEM Gaussian Fit DLS Ratio Plasmon Mean Particle Mean Size red POM Std. Dev. PDI Mean PDI Ag+/ red POM Band (nm) Size (nm) Count Number (nm) PW12 4− 0.1 427.50 43.07 32.81 140 0.58 39.17 0.21 PW12 4− 1 395.10 27.41 5.90 221 0.05 20.40 0.29 PW12 4− 10 400.50 32.51 22.59 143 0.48 PW12 5− 1 401.10 27.62 11.23 104 0.17 28.62 0.22 SiW12 6− 1 408.00 12.12 0.29 DLS data (Table 1) were also collected for all the NPs. The mean size for Ag+/ red POM ratios 1 and 0.1 agrees with UV-vis and STEM data. However, the DLS for ratio = 10 resulted in a size quality report that did not meet the quality criteria due to a too-polydisperse sample for cumulant analysis, in agreement with the aggregation observed via UV-vis and STEM. It has been reported that the rate of Ag+ reduction strongly affects the initial nucleation of silver particles [49]. A low rate of reduction is achieved for high Ag+/ red POM ratios (low concentration of red POM), where crystal growth is favored and leads to larger and more polydisperse or aggregated particles. This is in agreement with our observations for Ag+/ red POM molar ratio = 10.0; where aggregated/polydispersed NPs are observed that are larger than for a ratio = 1. On the other hand, a faster reduction of Ag+ (low Ag+/ red POM ratio and high [red POM]) should result in smaller and more uniform NPs since the nucleation process is enhanced more than the growth of silver NPs, in agreement with our observations for Ag+/ red POM molar ratio = 1. Following this trend, we would assume that an even faster rate of reaction, as expected for Ag+/ red POM molar ratio = 0.1, would result in even smaller NPs since the nucleation process is faster. However, the low concentration of large polydisperse NPs observed for ratio = 0.1 may be a consequence of two factors that can increase the instability of NPs: (i) Excess equivalents of red POM anions can react when exposed to air to give peroxide and eventually hydroxide. These can oxidatively degrade M0 NPs [32]. Furthermore, (ii) smaller M0 NPs are generally more amenable to oxidations and degradation [49]. Previous studies using other red POM and metal salts have shown that varying the ini- tial reagent concentrations may generate different nanostructures (sphere NPs, nanowires, nanoplates, and polygon-shaped nanostructures) [50,51]. However, under the experimen- tal conditions used here, only nanospheres were generated, although with various final concentrations, sizes, and degrees of polydispersity depending on the initial concentration of reactants. Considering these results, a ratio of Ag+/ red POM = 1 produces spherical NPs with high concentration and low polydispersity. With such a ratio, the influence of the different degree of reduction of the same POM and the influence of the kind of POM from the same series was studied. 3.1.2. Influence of the Reducing Power of the POM The effect of the degree of reduction of the same POM was studied using the two- electron-reduced PW12 5− , generated electrochemically from PW12 3− , and compared with the one-electron-reduced PW12 4− . The effect of the use of a different POM was performed using another heteropoly acid with the chemical formula H4 [SiW12 O40 ] (abbreviated SiW12 4− ); the two-electron-reduced SiW12 6− was generated electrochemically and used directly as red POM. In this second set of experiments, [red POM] was kept constant at 1 mM, with Ag+ /red POM = 1. The redox potential (E0 ) values for PW12 4− , PW12 5− , and SiW12 6− are +0.005, −0.25, and −0.39 V (vs. SCE), respectively [43]. It has been reported that more negative redox potentials lead to faster electron transfer to Ag+ and smaller particles [49]. Even though the use of PW12 5− generated very similar NPs (size, polydispersity, and concentration, see Nanomaterials 2023, 13, x FOR PEER REVIEW 8 of 17 Nanomaterials 2023, 13, 2241 8 of 17 1b and S4 and Table 1) than the NPs obtained using PW124- as redPOM agent, the higher negative redox potential of SiW126- generated smaller NPs (DLS size ~12 nm, Table 1) with lowFigure 1b and Figure concentration and S4 and aggregation some Table 1) than indicated the NPs obtained by the using PW12 4− as low-intensity red POM broad agent, UV-vis 6 − plasmon peak (Figure 1b); these smaller NPs, however, had a strong tendency to aggre-nm, the higher negative redox potential of SiW 12 generated smaller NPs (DLS size ~12 Table gate when1) with theylow concentration were isolated viaand some aggregation centrifugation indicated by the and re-dispersed low-intensity in water and STEM broad UV-vis plasmon peak (Figure 1b); these smaller NPs, however, had a measurements could not be obtained. Therefore, the best conditions chosen for the syn- strong tendency to aggregate thesis of Ag NPs when they using were as a POM isolated via centrifugation a reducing andagent and stabilizing re-dispersed were: (i)ina water and ratio of Ag+/redPOM = 1 and (ii) the use of PW124- as redPOM, since a further reduction of the POMthe STEM measurements could not be obtained. Therefore, the best conditions chosen for to synthesis PW+/ of Ag NPs using a POM as a reducing and stabilizing agent were: (i) a ratio of 125- yielded similar results. POM-Ag0 NPs synthesized under these conditions were Ag red POM chosen for further = 1physicochemical of PW12 4− as red POM, since a further reduction of the and (ii) the use characterization. − POM to PW12 yielded similar results. POM-Ag0 NPs synthesized under these conditions 5 3.2.were chosen for further Physicochemical physicochemical Characterization of POM-Ag characterization. 0 NPs 3.2.1. 3.2.Identification Physicochemicalof Characterization Metallic Silver on POM-Ag00 NPs of POM-Ag NPs XRD 3.2.1. analysis wasofperformed Identification to identify Metallic Silver the crystallinity on POM-Ag 0 NPs of the nanostructures. The XRD pattern recorded from a drop-coated film of the POM-Ag0 NPs sample on a silicon XRD analysis was performed to identify the crystallinity of the nanostructures. The substrate is shown in Figure 3. The (111), (200), (220), and (311) Bragg reflections of face- XRD pattern recorded from a drop-coated film of the POM-Ag0 NPs sample on a silicon centered cubic (fcc) metallic silver are clearly observed (JCPDS file No. 00-004-0783). The substrate is shown in Figure 3. The (111), (200), (220), and (311) Bragg reflections of face- silver peaks are indexed and indicate that the sample studied was highly pure Ag0 NPs centered cubic (fcc) metallic silver are clearly observed (JCPDS file No. 00-004-0783). The without any impurities. In addition, an average crystallite size distribution of 24.7 nm was silver peaks are indexed and indicate that the sample studied was highly pure Ag0 NPs obtained from the XRD data using the Scherrer equation [52], in perfect agreement with without any impurities. In addition, an average crystallite size distribution of 24.7 nm the direct measurements of particle size from the STEM images. Finally, particle size was obtained from the XRD data using the Scherrer equation [52], in perfect agreement measurements using three different techniques (STEM, DLS, and XRD) led to very close with the direct measurements of particle size from the STEM images. Finally, particle size values. On the other hand, diffraction peaks corresponding to the PW12 structure were not measurements using three different techniques (STEM, DLS, and XRD) led to very close observed; this is common for other POM-M0 NPs [50,53,54] and it is most likely related to values. On the other hand, diffraction peaks corresponding to the PW12 structure were not theobserved; lack of anthis extended is commonlong-range for otherorder POM-Mcrystal structure 0 NPs of POMs, [50,53,54] and it istogether withrelated most likely the strong diffraction of the metallic bulk NP compared with the weak dispersion of to the lack of an extended long-range order crystal structure of POMs, together with the X-rays with the thin strong POM layer diffraction onmetallic of the the surface. bulk NP compared with the weak dispersion of X-rays with the thin POM layer on the surface. Figure 3. XRD pattern of the synthesized POM-Ag0 NPs. Indexed peaks correspond to metallic silver. Figure 3. XRD pattern of the synthesized POM-Ag0 NPs. Indexed peaks correspond to metallic sil- ver. The electrochemical behavior of POM-Ag0 NPs was studied using cyclic voltammo- grams (CVs) from −0.6 to 1.0 V in 0.5 mM sulfuric acid at 20 mV/s on GC electrode The electrochemical drop-casted with the NPsbehavior of POM-Ag and coated 0 NPs was studied using cyclic voltammo- with Nafion. Figure 4a shows CVs corresponding to grams (CVs) and free POM fromPOM-Ag −0.6 to 01.0 V inThe NPs. 0.5CVmMofsulfuric the freeacid POMatshows 20 mV/s theon GCcharacteristic three electrode drop- redox casted waves [55]. The CV of POM-Ag NPs shows an anodic peak at positive potentialstothat with the NPs and coated with 0 Nafion. Figure 4a shows CVs corresponding freecan POM and POM-Ag be attributed 0 NPs. The CV of the free POM shows the three characteristic redox to the reversible formation of silver oxides on silver nanoparticles [56,57]. waves [55]. The CV of POM-Ag 0 NPs shows an anodic peak at positive potentials that can Successive cycling leads to a decrease in the peak currents, (Figure 4b); a possible interpre- tation could be that the formation of silver oxides on the Ag surface gradually becomes Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 17 be attributed to the reversible formation of silver oxides on silver nanoparticles [56,57]. Nanomaterials 2023, 13, 2241 Successive cycling leads to a decrease in the peak currents, (Figure 4b); a possible inter- 9 of 17 pretation could be that the formation of silver oxides on the Ag surface gradually becomes irreversible due to the complete passivation of the metallic surface by a thin layer of insu- lating oxide. Moreover, irreversible due to thethecomplete CVs show two weakofshoulders, passivation the metallica cathodic surface by one at ca.layer a thin −0.25ofVinsu- andlating an anodic one oxide. (even less Moreover, thedefined) CVs show at ca. two0 weak V. This may involve shoulders, a larger a cathodic one at ca. −0.25 V polarization thanandin the free POM, an anodic oneassociated (even less with its adsorption defined) at ca. 0 V. in themay This Ag surface. involve Furthermore, the a larger polarization peakthan in theVfree at −0.25 POM, associated vs. Ag/AgCl withthe agrees with its peak adsorption observedin the by Ag surface. Kulesza Furthermore, et al. at 0.0 V vs. the RHEpeak −0.25 [57].atNote V after that vs. Ag/AgCl successiveagrees cycling with the peak (Figure observed 4b), the by Kulesza weak shoulder et al.Vatbe- at −0.25 0.0 V comes even more defined, suggesting that POMs remain on the surface of Ag NPs even V vs. RHE [57]. Note that after successive cycling (Figure 4b), the weak shoulder at − 0.25 afterbecomes complete even more defined, passivation of the suggesting Ag surface.that POMs remain Nevertheless, on the surface the weakly definedoffeatures Ag NPsofeven after complete the POM could be apassivation consequence of the Ag of (i) surface. the diminishedNevertheless, potentialthe weakly window duedefined features of to hydrogen the POM evolution could be catalyzed a consequence with of (i) silver [58], and (ii)the thediminished thin layer ofpotential window due POMs protecting thetosurface hydrogen of Agevolution 0 NPs thatcatalyzed may notwith sufficesilver for[58], and (ii)with detection the CV. thin Therefore, layer of POMsotherprotecting the surface characterization of Ag0 NPs techniques that to are used may not the prove suffice for detection presence of POMswith CV. Therefore, stabilizing the Ag0 other NPs. characterization techniques are used to prove the presence of POMs stabilizing the Ag0 NPs. Figure 4. (a) Comparative cyclic voltammogram (CV) with glassy carbon electrode modified with Figure free4.POM (a) Comparative (PW12 3− ) orcyclic POM-Agvoltammogram (CV) with glassy 0 NPs. (b) Consecutive CVs of carbon POM-Ag electrode modified 0 NPs, the with arrow indicates free the POM (PW 123−) or POM-Ag0 NPs. (b) Consecutive CVs of POM-Ag0 NPs, the arrow indicates the increase in the CV cycle number. A scan rate of 20 mV/s and 0.5 M H2 SO4 medium were used. Pt increase in the CV cycle number. A scan rate of 20 mV/s and 0.5 M H2SO4 medium were used. Pt wire and Ag/AgCl electrodes were employed as counter and reference electrodes, respectively. wire and Ag/AgCl electrodes were employed as counter and reference electrodes, respectively. 3.2.2. Presence of POM on POM-Ag0 NPs 3.2.2. Presence Figureof POM on 5 shows thePOM-Ag XPS W 4fNPs 0 and Ag 3d core-level spectra recorded from drop-coated Figure 5 shows the 0 XPS films of the POM-Ag NPs solutionW 4f and Ag 3doncore-level a silicon spectra chip. The recorded Ag 3dfrom drop-coated core-level spectrum films of the POM-Ag 0 NPs solution on a silicon chip. The Ag 3d core-level spectrum (Fig- (Figure 5a) resolves into two spin-orbit components. The Ag 3d5/2 and 3d3/2 peaks occur ure at 5a)a resolves into and BE of 368.4 two 374.4 spin-orbit components.These eV, respectively. The Ag 3d5/2 indicate values and 3d3/2thatpeaks occur silver at a is present BE of only368.4 andmetallic in the 374.4 eV, respectively. form, indicating These values indicate the formation 0 NPs of Agthat silver [59].isFigure present 5bonly showsin the the metallic W4f spectrumform, indicating recorded fromthe formation the POM-Ag 0 film. of Ag 0 NPs [59]. Figure 5b shows the W4f spec- This spectrum can be resolved with two trum recordedcomponents spin-orbit from the POM-Ag with a0 film. 4f7/2 This spectrum binding energycan be of (BE) resolved 35.8 eVwithwhichtwocorresponds spin-orbit to components W(VI) specieswith a[60], 4f7/2pointing binding energy (BE) of 35.8 to the presence eV fully of the which corresponds oxidized PW12 to 3− W(VI) spe- the stabilizing 0 ciesAg [60],NPs. The relative pointing atomic composition to the presence of the analyzed of the fully oxidized PW12 deposit 3- stabilizingis 78.2 the at Ag% NPs. 0 and 21.8 Theat % of silver relative atomicandcomposition tungsten, respectively of the analyzed(notedeposit that XPS is a surface is 78.2 at % and analysis 21.8 at %technique of silverand and this composition does not correspond toathe whole POM-covered 0 NP but to just a certain Agand tungsten, respectively (note that XPS is surface analysis technique this composition doesdepth from the surface). not correspond These observations to the whole POM-covered support Ag0 NP our proposal but to just athat PW12depth certain served as both from a reducing the surface). and observations These an encapsulatingsupportmolecule. our proposal that PW12 served as both a reducing and an encapsulating molecule. Nanomaterials 2023, 13, x FOR PEER REVIEW 10 of 17 Nanomaterials 2023, 13, 2241 10 of 17 XPS 0 NPs. Experimental data (black Figure Figure 5. 5. XPS WW4f4f (a)(a) and and AgAg 3d3d (b)(b) core-level core-level spectra spectra ofof POM-Ag POM-Ag 0 NPs. Experimental data (black dots) dots) are are shown shown with with envelopes envelopes (black (black lines), lines), component component fitting fitting (red (red lines), lines), andand background background (grey (grey line). line). Theinteraction interactionofof POM species with the 0 NP was studied with FTIR spectroscopy. The POM species with the AgAg 0 NP was studied with FTIR spectroscopy. Figure Figure 6 allows 6 allows us us to compare to compare the spectra the spectra of theofneat the POM neat POM (PW12)(PW 12 ) species species and their and their stabi- stabilized POM-Ag 0 NPs. Thepositions band positions and assignments lized POM-Ag 0 NPs. The band and assignments basedbased on theon the literature literature [61] [61] are are indicated indicated in Figurein Figure 6. The 6. PWThe PW12 structure 12 structure (Keggin(Kegginstructure)structure) contains contains four types four types of of oxygen oxygen atoms: theatoms: central the centralatoms oxygen oxygen (Oatoms (Oa ), corner-sharing a), corner-sharing bridgingbridging oxygen(O oxygen atoms atoms (Ob ), b), edge- edge-sharing bridging oxygen atoms (O ), and terminal sharing bridging oxygen atoms (Oc), and terminal oxygen atoms (Od). The central c oxygen atoms (O d ). The central oxygen oxygen the connects connects P to a the P to aThe W atom. W atom. next twoThetypesnext two types atoms of oxygen of oxygen atoms bridge twobridge W atoms twoinW atoms in adjoining octahedra. Finally, the terminal oxygen atom adjoining octahedra. Finally, the terminal oxygen atom is bonded to only one W atom [62]. is bonded to only one W atom In the spectrum[62]. In the spectrum of PW of PW12, four bands are observed12 , four bands are observed at 1078, 979, at 1078, 979, 902, and 804 cm−1. The band 902, and 804 cm −1 . The band positions are well correlated with the data in the literature [61]. The positions are well correlated with the data in the literature [61]. The spectrum of POM- 0 NPs shows the presence of the main IR peaks for the fingerprint Agspectrum 0 NPs shows of POM-Ag the presence of the main IR peaks for the fingerprint region of PW12, region of PW12 demonstrating the, demonstrating presence of thethe POM presence on the of the POM silver on the silversome NP. Furthermore, NP. Furthermore, differences some differences in the spectra appear due to the presence of the NPs, which suggests that in the spectra appear due to the presence of the NPs, which suggests that the detected the detected POM was most likely interacting with silver. The band at 804 cm−1 splits POM was most likely interacting with silver. − 1 The band at 804 cm−1 splits into two compo- into two components at−1 785 and 817 cm in the spectrum of POM-Ag0 NPs. This band nents at 785 and 817 cm in the spectrum of POM-Ag NPs. This band corresponds to a 0 corresponds to a triply degenerated mode involving the W-Oc -W motion [63]. This band triply degenerated mode involving the W-Oc-W motion [63]. This band splitting can be splitting can be rationalized by a decrease in the molecular symmetry of PW12 due to rationalized by a decrease in the molecular symmetry of PW12 due to the interaction of Oc the interaction of O atoms with the silver surface [64]. The band at 902 cm−1 involving atoms with the silver csurface [64]. The band at 902 cm−1 involving motions of O b atoms is motions of Ob atoms is slightly red-shifted to 895 cm−1 in the spectrum of POM-Ag0 NPs, slightly red-shifted to 895 cm−1 in the spectrum of POM-Ag0 NPs, which corroborates the which corroborates the lowering of the symmetry due to the interaction of Ob atoms with lowering of the symmetry due to the interaction of Ob atoms with silver. The bands at 1078 silver. The bands at 1078 and 979 cm−1 due to the central Oa and terminal Od oxygen atoms, and 979 cm−1 due to the central Oa and terminal Od oxygen atoms, respectively, show neg- respectively, show negligible shifts, suggesting that these oxygen atoms are not involved in ligible shifts, suggesting that these oxygen atoms are not involved in the interaction with the interaction with silver. Thus, the FTIR spectra indicate that the bridge atoms Ob and Oc silver. take aThus, part inthetheFTIR spectraofindicate interaction PW12 with thatthe theAgbridge 0 NPs,atoms while O b and Oc take a part in the the distal Od atoms are farther interaction away fromofthe PW 12 with the Ag NPs, while the distal Od atoms are farther away from the surface. 0 The participation of the bridge oxygen atoms in the interaction with surface. the silverThesurface participation was alsoofobserved the bridge oxygen for Ag 0 NPsatoms in the stabilized viainteraction Keggin-type with the silver silicotungstate surface was also observed for Ag 0 NPs stabilized via Keggin-type silicotungstate anions anions H4 [SiW12 O40 ] (SiW12 ) [64]. Therefore, by using complementary characterization Htechniques, 4[SiW12O40] (SiW12) [64]. Therefore, by using complementary characterization we have demonstrated the synthesis of pure POM-Ag0 NPs where thetech- POM niques, we have acts both as a reducingdemonstrated and anthe synthesis ofagent. encapsulating pure POM-Ag0 NPs where the POM acts both as a reducing and an encapsulating agent. Nanomaterials 2023, 13, x FOR PEER REVIEW 11 of 17 Nanomaterials 2023, 13, x FOR PEER REVIEW 11 of 17 Nanomaterials 2023, 13, 2241 11 of 17 Figure 6. Comparative FTIR spectra of free POM (PW123−) and POM-Ag0 NPs. FigureFigure 6. Comparative FTIR spectra of freeofPOM 3−) and3− (PW12(PW POM-Ag 0 NPs. 0 6. Comparative FTIR spectra free POM 0 12 ) and POM-Ag NPs. 3.3. Preparation and Characterization of AC/POM-Ag NPs Hybrid Material 3.3.Physisorption Preparation and 0 NPs Hybrid Material 3.3.3.3.1. Preparation ofCharacterization POM-Ag0 of and Characterization NPs of onAC/POM-Ag AC 0 NPs Hybrid AC/POM-Ag Material 0 NPs on AC 3.3.1. 3.3.1. Physisorption POM-Ag Physisorption 0 NPsof wereof incorporated POM-Ag POM-Ag 0 NPs on into AC to create a tri-component nanohybrid ma- AC terial. The POM-Ag 0 incorporation NPs wereinto AC was incorporated performed intotoAC tousing create a aphysisorption tri-component method nanohybrid(repre- POM-Ag NPs were incorporated into 0 AC create a tri-component nanohybrid ma-mate- sented rial. in TheScheme 1b), incorporation depositing into AC pre-made was NPs performed on AC using through a the physisorption terial. The incorporation into AC was performed using a physisorption method (repre- vigorous method stirring of a (represented suspension. sented ininScheme Scheme The 1b), samples 1b),depositing were depositing centrifuged pre-made pre-made NPsand NPs on washed on AC AC through throughwiththe pure the water stirring vigorous vigorous before subse- stirring ofaa sus- of pension. quent The samples characterization. were Figure centrifuged 7 shows a and washed representative with SEM suspension. The samples were centrifuged and washed with pure water before subse- pure image, water where before the subsequent POM-Ag 0 characterization. Figure 0 NPs quent are evenly distributed characterization. Figure7on shows the AC 7 shows a representative a substrate. SEM However, representative SEMimage, SEMwhere image, size thethe POM-Ag distribution where POM-Ag NPs data 0in-are evenly distributed that they tend onto the AC aggregate substrate. to an However, average SEM size of size 45.83 distribution NPs are evenly distributed on the AC substrate. However, SEM size distribution data in-the dicates nm in data comparison indicates with that they tend to 0 aggregate to an average size of 45.83 nm in comparison with the free POM-Ag 0 free POM-Ag NPs (average size of 27.41 mm, Figure 2 and dicates that they tend to aggregate to an average size of 45.83 nm in comparison with the Table 1). This aggregating free NPs (average phenomenon POM-Ag 0 NPs size on AC of 27.41 may (average besizemm, Figure attributed of 27.41 2 and tomm, the Table lack 21). of strong Figure andThis aggregating interaction Table phenomenon the support. on withaggregating 1). This AC may be attributed to the lack of strong interaction with phenomenon on AC may be attributed to the lack of strong interaction with the support. the support. Figure 7. SEM image with a one-micron scale bar showing an even distribution of nanoparticles on Figure 7. SEM image with a one-micron scale bar showing an even distribution of nanoparticles on AC AC (a) (a) andand size size distribution distribution (b)(b) obtained obtained from from thethe Gaussian Gaussian fit the fit to to the SEMSEM histograms histograms of AC/POM- of AC/POM- FigureAg7.0SEM image with NPs nanohybrid a one-micron material. scale bar showing an even distribution of nanoparticles on ACAg(a) NPs and nanohybrid material. 0 size distribution (b) obtained from the Gaussian fit to the SEM histograms of AC/POM- Ag0 NPs nanohybrid material.was further characterized by XRD and XPS. The XRD patterns of pure The nanohybrid The nanohybrid was further characterized by XRD and XPS. The XRD patterns of AC and AC/POM-Ag0 NPs are displayed in Figure 8. The AC pattern shows broad peaks pure TheAC and AC/POM-Ag nanohybrid 0 NPs are displayed in Figure 8. The AC pattern shows broad was further of amorphous carbon (2θ = ∼characterized 24◦ and 44◦ ) by XRD [65]. and After XPS. the The XRD patterns incorporation of the NPs,of the peaks of amorphous pure characteristic AC and AC/POM-Ag carbon (2θ NPs are = ∼24° and 44°) [65]. After the incorporation of the NPs, todisplayed the Bragg in Figure 8.ofThe AC pattern Ag fccshows broad 0 peaks assigned reflections metallic structure emerge, the of peaks characteristic amorphous peaks carbon assigned (2θ 0 = ∼24°to and the 44°) Bragg reflections [65]. After the of metallic Ag of incorporation fccthe structure NPs, suggesting that POM-Ag NPs have been incorporated into the AC support. As in the case theemerge, suggesting that POM-Ag 0 NPs have been incorporated into the AC support. As characteristic of free POM-Agpeaks 0 NPsassigned (Figure 3), todiffraction the Braggpeaks reflections of metallic corresponding AgPW to the fcc12 structure structure were emerge, not suggesting that POM-Ag observed. Therefore, 0 NPs have been incorporated into the AC support. As the presence of the POM-protecting ligands in the AC/POM-Ag0 NPs hybrid material was confirmed using XPS. in the case of free POM-Ag0 NPs (Figure 3), diffraction peaks corresponding to the PW12 in the case ofwere structure free POM-Ag 0 NPs (Figure 3), diffraction peaks corresponding to the PW12 not observed. Therefore, the presence of the POM-protecting ligands in the structure were not AC/POM-Ag observed. 0 NPs hybridTherefore, the presence material was confirmedofusing the POM-protecting XPS. ligands in the Nanomaterials 2023, 13, 2241 AC/POM-Ag0 NPs hybrid material was confirmed using XPS. 12 of 17 Figure 8. XRD patterns of bare AC (bottom) and AC/POM-Ag0 NPs. Small peaks marked with filled XRD patterns 0 NPs. Small peaks marked with filled Figure Figure circles and8. 8. XRD patterns asterisks bare of ACbare ofcorrespond toAC (bottom)(bottom) minorand and andAC/POM-Ag AC/POM-Ag AgCl 0 NPs. Small SiO2 impurities, peaks marked with filled respectively. circles and asterisks circles correspond and asterisks to minor correspond to AgCl minorand SiOand AgCl 2 impurities, respectively. SiO2 impurities, respectively. The Ag 3d3/2 and 3d5/2 doublet can be easily observed in the XPS spectrum of the as- The preparedAgThe3d Ag 3/2 and AC/POM-Ag 3d3d3/25/2and 3dhybrids 0doublet NPs doublet 5/2can can observed be(Figure easily be The 9). easily 3dobserved in the XPSinspectrum the XPS spectrum of the as- of the 5/2 level is located at 368.25 eV and 0 thatas-prepared prepared ofAC/POM-Ag AC/POM-Ag NPs eV, hybrids NPs hybrids (Figure that 9). (Figure 9). Theis3dpresent The 3d 5/2 level levelatis368.25 is located locatedeVatand 368.25 eV 0 5/2in 3d3/2 at 374.25 confirming silver only the metallic form. The ofand 3d3/2that thatpresence of 3d3/2 atof374.25 eV, atconfirming 374.25 eV, confirming that silver that is silveronly present is present in tungsten was also detected, and the W4f7/2 and W4f5/2 doublet with binding the only in the metallic metallic form. The form. presence Theofpresence tungsten ofwas tungsten also was alsoand detected, detected, the W4f and 7/2 andtheW4f W4f5/27/2 and W4f doublet with 5/2 doublet with binding energies of 35.69 and 37.63 eV, respectively. These values indicate that the tungsten is in energies binding of 35.69energies andform of 35.69 37.63 eV,VI)and 37.63 eV, respectively. respectively. valuesThese values thatindicate that the tungsten its fully oxidized (W POM,These in the VI corroborating indicate the formation theoftungsten ternary is in hybrids its fullyis in its oxidized fullyformoxidized (W VI ) form in the (W POM, ) in the POM, corroborating corroborating the formation the of formation ternary of ternary hybrids of AC/POM-Ag NPs. Therefore, 0 0 NPs. the above characterization techniques confirm that POM- hybrids of of AC/POM-Ag 0 AC/POM-Ag Therefore, the above characterization techniques confirm Ag0 NPs wereNPs. Therefore, successfully 0 the above incorporated characterization homogeneouslytechniques onto AC toconfirm that POM- form a tri-component that Ag0nanohybrid POM-Ag NPs were successfully NPs were successfully incorporated homogeneously onto AC to form a material. incorporated homogeneously onto AC to form a tri-component tri-component nanohybrid material. nanohybrid material. Figure 9. XPS W 4f (a) and Ag 3d (b) core-level spectra of AC/POM-Ag0 NPs. Experimental data Figure 9. XPS W 4f (a) and Ag 3d (b) core-level spectra of AC/POM-Ag0 NPs. Experimental data Figure (black 9. XPS dots)4fare (a)shownAgwith envelopes (black lines), ofcomponent fitting (red lines), and background (black dots)Ware shownandwith 3d (b) core-level envelopes (black spectra AC/POM-Ag lines), component fitting0 NPs. Experimental (red lines), data and background (black (grey dots) (grey line). are shown with envelopes (black lines), component fitting (red lines), and background line). (grey line). 3.3.2. Electrochemical Properties of Hybrid Electrodes The promising application of the AC/POM-Ag0 NPs composite as a hybrid electrode was preliminarily tested by evaluating the electrochemical performance. To this aim, sym- metric cells were assembled by sandwiching a separator soaked in 1M H2 SO4 electrolyte between two identical electrodes (AC or AC/POM-Ag0 NPs). Figure 10a shows cyclic polarization (CP) curves for AC and AC/POM-Ag0 NPs symmetric cells at a constant scan rate of 20 mV/s. The shape of the CP curves for the AC symmetric cell is rectangular due to the contribution of the electric double-layer capacitance (EDLC). On the other hand, quasi-rectangular CP curves are observed for the AC/POM-Ag0 NPs symmetric cell, with various redox waves originating from Ag0 NPs that overlap with a rectangular (capacitive) envelope. This CP structure indicates the coexistence of both charge-storing mechanisms, faradaic and EDLC. It is worth noting that the observed waves from silver become smaller with increasing cycle numbers and become imperceptible, indicating some degree of in- Nanomaterials 2023, 13, 2241 stability, as observed for free POM-Ag0 NPs (see Figure 4b). As discussed above (see Sec- 13 of 17 tion 3.2.1 and Figure 4b), this may be a consequence of silver oxide formation on the Ag surface with the POMs remaining on the surface of the NPs. In fact, there is a correlation between various theredox decreasewaves in originating waves fromfrom silverAg (Figure 0 NPs that10a) overlap during thewithfirst few cycles (capacitive) a rectangular with an increase in the specific capacitance calculated from the area in envelope. This CP structure indicates the coexistence of both charge-storing mechanisms, the CP (Figure 10b). This mayfaradaic be explained by the difference in specific capacitance of the thin and EDLC. It is worth noting that the observed waves from silver become smaller layer of silver oxide compared to silver.[1] with increasing cycleNevertheless, numbers andthe specific become capacitance indicating imperceptible, is moderately somehigher degreefor of in- AC/POM-Ag 0 NPs hybrid electrodes than for 0 bare AC electrodes; stability, as observed for free POM-Ag NPs (see Figure 4b). As discussed above while the specific capac-(see itance of AC Section is 75 3.2.1 andF/g, AC/POM-Ag Figure 0 NPs reach 81 F/g after 20 cycles. This result indicates 4b), this may be a consequence of silver oxide formation on the Ag thatsurface the hybridwith the POMs remaining on the exceeds material is electroactive and surface of to the some degree NPs. AC there In fact, in a symmetrical is a correlation supercapacitor between the decrease in waves from silver (Figure 10a) during the firstvariations configuration. The change in the specific capacitance with few cyclesinwiththe an scanincrease rate is shown in Figure S5; the specific capacitances of the AC and in the specific capacitance calculated from the area in the CP (Figure 10b). This AC/POM-Ag 0 NPs cellsmay decrease with increasing be explained current density. by the difference in specific For AC/POM-Ag capacitance of the0 NPs, the highest value thin layer of silver oxide wascompared 83.5 F/g attoa silver scan rate [1].ofNevertheless, 2 mV/s, whichthe further decreased specific to 77.5 capacitance is F/g at a scanhigher moderately rate of for 50 mV/s. AC/POM-Ag 0 NPs hybrid This reduction of theelectrodes specific capacitance than for bare at AC highelectrodes; scan rateswhile has been attributed the specific capaci- to the tancelow ofdiffusion AC is 75 F/g, of theAC/POM-Ag electrolyte ion.0 NPs Due to the reach timeafter 81 F/g constraint, 20 cycles. theThis ionicresult motion in indicates the that electrolyte during the high-rate charge–discharge process is the hybrid material is electroactive and exceeds to some degree AC in a symmetrical always limited by diffu- sion, and only theconfiguration. supercapacitor outer active surface is utilized The change in thefor charge specific storage. Slowing capacitance down the with variations in the scanscan raterate canisallow shown thein electrolyte Figure S5; theto penetrate into pores more specific capacitances of thethoroughly AC and AC/POM-Ag and to make 0 NPs greater cellscontact decrease withwith theincreasing internal surface current ofdensity. the electrode For AC/POM-Ag 0 NPs,in material, resulting more the charge highest value stored wason theF/g 83.5 surface of the at a scan electrode rate of 2 mV/s, (larger whichcapacitance) [66]. further decreased to 77.5 F/g at a scan rate of 50Conclusively, mV/s. This reduction this studyofprovides the specific capacitance promising at high scan preliminary datarates has beenthe supporting attributed use of to thisthe typelowofdiffusion ternary of the electrolyte hybrid materials ion. Due to theand as electrodes timedemonstrating constraint, the the ionic motion in potential of the electrolyte during the high-rate charge–discharge process other POM/M NPs/nanocarbon composites for electrochemical applications. Further- 0 is always limited by diffusion, andthe more, only the outerofactive possibility tunningsurface is utilized the degree for charge of silver storage. oxidation (i.e., Slowing down the electrochemical pre-scan rate canwhile treatment) allowkeeping the electrolyte to penetrate intact the POM-protectinginto pores more opens ligands, thoroughly and to make new interesting greater routes for contact with the internal other applications such as surface of the electrode material, resulting in more charge stored electrocatalysis. on the surface of the electrode (larger capacitance) [66]. Figure 10. (a) CV curves of electrodes in symmetric coin cells using 1M H2 SO4 electrolyte and collected at 20 mV/s, the arrows indicate the increase in the CV cycle number. (b) Variation in the specific capacitance as a function of CV cycle number. Conclusively, this study provides promising preliminary data supporting the use of this type of ternary hybrid materials as electrodes and demonstrating the potential of other POM/M0 NPs/nanocarbon composites for electrochemical applications. Furthermore, the possibility of tunning the degree of silver oxidation (i.e., electrochemical pre-treatment) while keeping intact the POM-protecting ligands, opens new interesting routes for other applications such as electrocatalysis. 4. Conclusions In summary, uniform and sized-controlled silver nanoparticles were prepared using a green method in which the POM acts both as a reductant and as a stabilizing agent. Control of the size and homogeneity of the particles was achieved by varying the silver concentration and the redox potential of the POM. Novel ternary AC/POM-Ag0 NPs nanohybrids were successfully fabricated using a facile method. Extensive characterization of the new nanomaterial was performed using a range of spectroscopic and imaging Nanomaterials 2023, 13, 2241 14 of 17 techniques, which demonstrated unambiguously that POMs are attached to the Ag0 NP surface. The use of this nanocomposite as a hybrid electrode has been explored, and its electrochemical performance reveals that the material is electroactive and exhibits moderately higher performance than bare AC electrodes in a symmetrical supercapacitor configuration. Our results are proof of concept that the electrochemical properties of the tri-component nanohybrid material hold great prospects in electrochemical applications. Future studies will explore new approaches to improve the stability and cyclability of the hybrid electrodes by developing new covalent and non-covalent linkage modes between POMs and metallic surfaces/nanocarbon. Furthermore, nanoparticle size, shape, and composition can be used to both tailor and improve the properties of new composite materials. In this context, a more systematic study of their synergistic properties and structure–activity relationships will significantly help develop the full potential of hybrid materials, electrodes, and devices for electrochemical applications. Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano13152241/s1, Figure S1: Cyclic Voltammetry results of PW12 in water; Figure S2: UV-vis spectroscopy signal of reduced and oxidized states of the POM; Fig- ure S3: Photography of the reaction mixtures resultant from the synthesis of POM-Ag0 NPs; Figure S4: STEM image and size distribution of POM-Ag0 NPs synthesized using PW12 5− ; Figure S5: Variation of specific capacitance of bare AC and AC/POM-Ag0 NPs symmetric cells with scan rate; Equation (S1): Calculation of the capacitance. References [55,67] are cited in the Supplementary Materials. Author Contributions: Conceptualization, S.G.-F. and P.G.-R.; methodology, S.G.-F.; formal analy- sis, S.G.-F., L.C., M.P.P. and J.-J.Z.; investigation, S.G.-F., L.C., M.P.P. and J.-J.Z.; resources, P.G.-R.; writing—original draft preparation, S.G.-F.; writing—review and editing, S.G.-F. and P.G.-R.; visual- ization, S.G.-F.; supervision, S.G.-F. and P.G.-R.; funding acquisition, S.G.-F. All authors have read and agreed to the published version of the manuscript. Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 894270. This research was funded by the postdoctoral fellowship program Beatriu de Pinós, funded by the Commission for Universities and Research of the Department of Innovation, Universities, and Enterprise of the Generalitat de Catalunya and by the Horizon 2020 program of research and innovation of the European Union under the Marie Skłodowska-Curie grant agreement No 801370. Data Availability Statement: Data available on request. The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. References 1. Abdullah, N.H.; Shameli, K.; Abdullah, E.C.; Abdullah, L.C. Solid Matrices for Fabrication of Magnetic Iron Oxide Nanocom- posites: Synthesis, Properties, and Application for the Adsorption of Heavy Metal Ions and Dyes. Compos. Part B Eng. 2018, 162, 538–568. [CrossRef] 2. Liu, W.; Cheng, L.; Li, S. Review of Electrical Properties for Polypropylene Based Nanocomposite. Compos. 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