Pseudo-capacitance of silver oxide thin film electrodes in ionic liquid for electrochemical energy applications

The energy storage potential of silver oxide (Ag 2O) thin film electrodes, deposited via radio frequency reactive magnetron sputtering, were inves tigated in an ionic electrolyte (1-Ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide for supercapacitor application. X-ray diffraction (XRD), Raman spectroscopy, X-ray photoe lectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR) tool s were used to evaluate the structural and oxide phases present in the sputtered silver ox ide thin film electrodes. The growth mode, morphology, surface area, wettability and surface e n rgy of the deposited nano-structure silver oxide thin films were confirmed with scannin g electron microscope (SEM), BrunauerEmmetter-Teller (BET), goniometer and tensiometer. Furthermore, ion diffusion, Faradaic redox reactions and capacitance of the sputtered th in films exposed to 1-Ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ionic electrolyte, were monitored with electrochemical impedance spectroscopy (EIS) and cy clic voltammetry (CV). SEM micrograph depicts that silver oxide thin films exh ibit columnar growth mode, with wettability analysis revealing that Ag 2O thin film is hydrophilic, an excellent electroche mical behaviour indicator. Cyclic voltammetry measurement s show that Ag 2O thin films exhibit a specific capacitance of 650 F/g at higher sputterin g power, demonstrating its promising potential as an active electrode for supercapacitor application.


Introduction
The population of the world is expected to be on the increase as the years go by, therefore the use of energy is expected to increase with population density [1]. The fuel cell and battery technologies have been excellent storage systems for decades, however, the primary concern with them is that they are not suitable for burst power applications, due to their low power density output [2][3][4]. Storage devices like supercapacitors with their enormous power density and excellent cycle life are designed and engineered to solve the present and future energy storage problem [3][4]. The energy storage mechanism in supercapacitor could either be via charge separation in the Helmholtz double layer or reverse faradaic redox (oxidationreduction) reaction on the electrode surface [5][6]. Supercapacitor performance depends on the kind of electroactive material used for the fabrication of the electrodes. This impacts the level of capacitive performance, energy density and power density of the supercapacitor [7].
The active electrode materials used for supercapacitor processing are grouped into carbon, conducting polymers and transition metal oxide-based materials, with carbon-based materials predominantly used for processing EDLC type of supercapacitors. The conducting polymers and transition metal oxide materials are mainly used for the pseudocapacitor kind of supercapacitor, while the combination of the three materials is for processing the composite electrodes for hybrid supercapacitors. Activated carbon is one the most investigated carbon materials for EDLC because of their low cost, high surface area and good electrical properties, but yields lower energy density and are unsuitable for high-temperature use [8].
Investigating alternative base material such as silver oxide that is cheap and has the potential to exhibits electrochemical behaviour close to that of ruthenium oxide is the aim of this with silver oxide (Ag 2 O) been the most stable amongst them [14]. The ability for silver oxide to change and adopt different oxidation states (+1, +2), facilitates the energy storage ability of Ag 2 O. Silver oxide has previously been studied by various researchers for different applications such as anti-microbial agent [15][16][17], due to the release of silver ion (Ag + ), reactive oxygen spices (ROS) and the hydroxyl group, which are produced from redox M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT reaction. Fuji et al. [18] and Kim et al. [19] reported silver oxide widely used in optical disk storage due to their photoactivation properties. Her et al. [20] integrated silver oxide thin films into super resolution near field structures, for optical memory applications. Silver oxides nanostructured particles have also served as a protective coating material, to stop the degradation of zinc oxide-based photodetectors [21]. Silver oxide thin film coating has been deployed successfully as a substrate for surface-enhanced Raman spectroscopy for molecular level detection [22]. Silver oxide nanomaterials have been studied and found to be highly conductive, making them suitable for battery cell applications [23][24][25]. Porous morphology, good conductivity, good thermal stability and reasonable wettability are some of the characteristics [23][24][25][26][27][28][29], attributed to silver oxide, making it a promising electroactive material for pseudocapacitor applications. In this work, the energy storage potential of Ag 2 O thin films produced by reactive magnetron sputtering was investigated in ionic electrolyte to determine its electrochemical performance and suitability for supercapacitor application.

Experimental Investigation
The thin film disposition parameters, structural characterization and electrochemical measurement details are described by Oje et al. [28]. The charge-discharge measurement was carried out for 10000 cycles for an applied current and voltage window of 10mA and 1V respectively. Furthermore, the following voltage range was used for the cyclic voltammetry analysis -1000mV to 1000mV, for scan rate of 2mV/s, 5mV/s and 10mV/s respectively.

XRD, Raman, FTIR and XPS results
The diffraction peaks for the crystal structure of Ag 2 O thin films sputtered on microscope glass slides, were established using the standard international centre for diffraction data card number (ICCD CARD number: 041-1104). The Braggs peaks on the Ag 2 O thin films deposited at 250W, 300W, 350W and at oxygen flow rates of 10sccm reveal that the deposited silver oxide thin films are crystalline as shown in Figure 1a, with peaks at (111),  [31], attribute to increase in crystal grain size as deposition power increases. The crystal grain orientation of the silver oxide changes from (111) to (200) crystal plane, an indication that the sputtered silver oxide thin film is nonhomogeneous [29,[31][32][33]. This also suggests that the non-uniformity of the sputtered Ag 2 O thin films increase with deposition power.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
Furthermore, Ingham et al. [34], reported that the shift in the 2θ angles of the crystal planes of silver oxide in Figure 1a is due to the sputtered Ag 2 O thin films nonhomogeneous and columnar growth structure. The packing arrangement in the cubic structure of silver oxide deposited at all the conditions are in cubic shape at the (111), (002) and (200) crystal planes [29,32,35,[36][37][38]. This gives rise to the cubic interstitial site, where electrolyte diffusion can take place, thereby encouraging redox reaction process.
The Raman spectroscopy of Ag 2 O thin films shown in Figure 1b  occupy the cubic interstitial holes [34,[39][40]. There is a shift in the Raman peaks as the deposition power increases for silver oxide thin films produced at radio frequency power of 250W, 300W and 350W at 10sccm oxygen flow rate. This is due to the ability of the grain boundaries trapping more oxygen molecules at higher deposition power for the silver oxide thin film deposited at 350W 10sccm. Martina et al. [41], attribute the 915 cm -1 , 954 cm -1 and 987 cm -1 Raman spectra peaks to chemosorbed atomic/molecular oxygen species end bond on deposited Ag 2 O thin films with (002) and (200)

SEM Results and BET Surface Area.
A scanning electron microscope (SEM) was deployed to reveal the microstructural  Figure 3. The columnar growth structure, island formation and segregation of silver and oxygen atoms lead to void creations. This island growth exhibited by silver oxide thin films, allow the molecules to be bonded strongly to each other, enabling the clustering of the atoms. As the deposition power increases, the pores on the silver oxide thin films become more pronounced, and atoms agglomerate forming a larger island. Rebelo et al. [36], believes this is due to the growth of stable nuclei to the maximum and the ability of more oxygen molecules to diffuse at higher deposition power. The improved roughness and mesopores as deposition power increases, enhance conductivity, improves ion diffusion and facilitates the redox reaction [31,[53][54][55][56][57][58][59]. This is an indication that Ag 2 O 350W 10sccm will offer more surface area for electrode/electrolyte interaction, resulting in higher specific capacitance. BET analysis was used to evaluate the surface area and the average pore size of the deposited silver oxide thin films and the results presented in Table 1. The BET experimental analysis reveals that the surface area and average pore size of the sputtered Ag 2 O thin films, increases as deposition power increases as depicted in Table 1. Dimitrijević et al. [60] and Agasti et al. [61], attributes this to ions been more energetic as sputtering power increases from 250W to 350W, resulting in bigger surface area and more probability of nanoclusters. Arjomandi et al. [62] suggest that higher surface area allows faster transport through the electrolyte ions and this increases the supercapacitor performance [63], supporting SEM results. Furthermore, Zhao et al. [64], reported that electrode surface area and pore size are directly connected to specific capacitance, with more surface area presenting more active sites for interfacial faradaic reaction and charge storage. An indication that energy density can be improved by increasing specific capacitance, a direct impact of surface area and pore size improvement.  Surface energy analysis was performed using three approaches namely: Fowkes, Wu and acid-base approach. Figure 5 and Table 2 reveal that, the total surface energy ) of the Ag 2 O thin films increased as deposition power increases [29]. The wettability of the deposited Ag 2 O thin films depends on the overall contribution of the polar component to the entire surface energy. The higher the polar component contribution to the overall surface energy, the better the wettability of the films [69] as shown in Figure 5a, 5b and Table 2.

Wettability and Surface Energy Analysis
Silver oxide thin film sputtered at 350W, show higher polar component contribution to the  Table 2.  Pawar et al. [75], reported that the lower impedance of silver oxide thin film sputtered at 350W 10sccm (22.14Ω) is an indication of its high conductivity and the ability to offer more surface area for ion diffusion for better electrochemical supercapacitor performance [76]. The This leads to lower contact angle and higher surface energy values as depicted in Figure 4 and Figure 5 respectively.

M A N U S C R I P T
A C C E P T E D ACCEPTED MANUSCRIPT Furthermore, the stability of the prepared silver oxide thin films was tested for 10000 cycles, at 10mA applied current, for voltage window of 1V. Figure 7 (a, b and c) show the initial and last ten cycles charge-discharge measurements for the three-silver oxide thin film electrodes, with a 2% drop between the initial and the last 10 cycles potential for the entire 10000 cycles.  with redox peak voltages at -0.6 to -0.8V and at +0.6 to 0.8V, an indication of charge transfer process taking place. Abedin et al. [79], attributes this increase in peak current to the oxidation of the [EMIM] + cation, a reduction reaction product. These redox peak voltages are within the standard oxidation-reduction potential reported by Amor et al. [80] on silver oxide redox peak. Nanostructure silver oxide film prepared at 350W 10sccm has higher peak current density compared to silver oxide thin films sputtered at 250W 10sccm and 300W

Cyclic Voltammetry
10sccm as shown in Figure 8. This is because at 350W 10sccm, the deposited silver oxide electrode offers more surface area for electrolyte/electrode interaction, resulting in better capacitance performance. Using equation 1, the specific capacitance of the various prepared Where is the potential scan rate (mV/s), is the potential range, stands for the current response and m the weight of the electrode. Using equation 1, specific capacitance of Ag 2 O at 250W 10sccm at 2mV/s, 5mV/s and 10mV/s are 617 F/g, 519 F/g and 429 F/g respectively. At a scan rate of 2mV/s, 5mV/s and 10mV/s, silver oxide thin films sputtered at 300W 10sccm gave specific capacitance of 623 F/g, 540 F/g and 489 F/g respectively.
Furthermore, cyclic voltammetry analysis of silver oxide thin films prepared at 350W 10sccm, resulted in specific capacitance of 650 F/g, 591 F/g and 531 F/g, at a scan rate of 2mV/s, 5mV/s and 10mV/s respectively. It is obvious from the specific capacitance calculation that the scan rate affects the oxidation/ reduction peak current height and the shape of the curve, with a scan rate of 10mV/s offering more peak current but less capacitance. This is because a limited number of ions are allowed to diffuse into the microstructure of the silver oxide thin films at a higher scan rate (10mV/s), reducing the redox reaction process [80]. The pseudocapacitance behaviour is revealed on the cathodic and anodic sides of the CV curves, by the peak current at each of voltage level.  [28,82], on silver oxide-based supercapacitors. The measured specific capacitance from cyclic voltammetry analysis for silver oxide thin films still depends on the electrode processing method, surface area, pore size and ion size [28,[82][83][84].

Conclusions
In this research, radio frequency magnetron sputtering was successfully used to fabricate thin film electrodes based on nanostructure silver oxide materials, for energy storage application, supercapacitor to be precise. XRD, Raman spectroscopy, XPS and FTIR reveals that the oxide phases belong to silver oxide. The scanning electron micrograph indicates that the deposited silver oxide thin films exhibited Volmer-Weber mechanism growth mode, with film roughness and pores increasing with deposition power. The BET surface area measurement shows that as the deposition power increases the surface area and average pore size, with Ag 2 O thin films prepared at 350W exhibiting better wettability, an indication of strong interaction between the electrode/electrolyte. Furthermore, electrochemical impedance spectroscopy, charge-discharge and cyclic voltammetry in (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide reveal that Ag 2 O thin films produced at 350W, possesses lower charge transfer resistance, good cycle life and a specific capacitance of 650 F/g at 2mV/s scan rate. The enhance specific capacitance at higher deposition power can be linked to silver oxide thin films sputtered at 350W 10sccm offering more surface area, more pore size and reduced charge transfer resistance for oxidation-reduction reaction.
Supercapacitor plays important role in energy storage technology because of the high-power boost it offers as a stand-alone or complementary energy storage device in hybrid cars, trains, space tools, airplanes, windmills, cranes and consumer electronic gadgets. The specific capacitance of 650 F/g offered by silver oxide thin film, demonstrates its electrochemical potential to be used as an active electrode for supercapacitor processing. This is extremely important for energy recovery systems such as car dynamic braking systems, where supercapacitors excellent life cycle, paves the way for it to be used to extend the lifespan of battery storage technology.