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Impact of key deposition parameters on the morphology of silver foams prepared by dynamic hydrogen template deposition
Electrochimica Acta 55 (2010) 6383–6390
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Impact of key deposition parameters on the morphology of silver foams prepared by dynamic hydrogen template deposition Serhiy Cherevko, Chan-Hwa Chung ∗ Advanced Materials and Process Research Center for IT, School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 14 June 2010 Accepted 19 June 2010 Available online 1 July 2010 Keywords: Silver foam Supercapacitor Electrodeposition Hydrogen
a b s t r a c t The potentiostatic deposition of porous Ag foams from a new stable thiocyanate based bath using hydrogen bubbles as a dynamic template during deposition was investigated. The influence of the electrolyte content, deposition potential and deposition time on the micro- and nanoscale morphology of the Ag form was examined. The formation of three main morphological forms on the nanoscale level: dendrites, a framework of identical particles, and agglomerates of inhomogeneous particles with big Ag granules distributed on the foam surface, was demonstrated by the analysis of the scanning electron microscopy (SEM) data. The quality of the structures obtained was examined by energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). It was found that the experimental parameters had a huge effect on the morphology of Ag on both the micro- and nanoscale. The structures with the most efficient geometry from the technological point of view were obtained with the correct combination of parameters, viz. high concentrations of NH4 + and Ag+ in conjunction with a sufficient deposition potential and time. Foams with roughness factors as high as 1100 were obtained, showing a high geometrical area normalized double-layer capacitance and relatively high gravimetric capacitance of 22 mF cm−2 and 2.4 F g−1 , respectively. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Due to their high surface area, porous and especially nanoporous materials have attracted a great deal of attention over the last decade. This interest comes from the great impact of such materials in electrocatalysis. Besides the increase in the reaction rate brought about by their huge surface area, nanostructured porous electrodes can also have a significant effect on the kinetics of a reaction, which is usually related to the high activity of the nanostructures and large amount of active sites [1,2]. However, the rate of an electrochemical reaction is not always proportional to the surface area of the electrode. Indeed, as shown by numerous researchers [3–6], the efficient utilization of highly porous electrodes can only be expected when the reaction is sluggish. For fast, diffusion controlled reactions, usually only a small increase in the reaction rate is observed. Even for kinetically controlled reactions, an inefficient electrode geometry leads to additional ohmic and bubble overvoltages, mass transfer limitations, and, due to the possible hydrophobicity, a decrease of the electrolyte accessible surface area. All of these effects decrease the efficiency when utilizing porous electrodes. As an example, it was found that for the Raney–Nickel electrode used for the hydrogen evolution reaction,
∗ Corresponding author. Tel.: +82 31 290 7260; fax: +82 31 290 7272. E-mail address: [email protected] (C.-H. Chung). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.054
only 1.5% of the available surface area was utilized under certain conditions [7]. It is clear that improving the electrode geometry is a critical step in the development of porous materials for use as electrodes in electrocatalysis. It has been suggested that an ideal porous electrode should have micro- and nanoscale-features. A non-uniform microscopic porosity in the micrometer range, where the pore sizes of the electrode decrease with increasing distance from the front surface to the inner space, as well as porosity on the nanoscale which brings about an increase in the surface area and electrode activity is needed. Recently, such electrodes were produced by applying simple electrochemical techniques. It was found that electrodeposition at very high overpotentials in aqueous solutions results in the formation of porous foams with walls composed of a huge concentration of dendrites. With this method, highly porous Ni electrodes with microscopic morphologies appropriate for cathodes in water electrolysis were produced [8]. More recently, Shin et al. proposed the electrodeposition of self-supported 3D foams of copper, tin and copper-tin alloy by using the so-called hydrogen template [9,10]. It was shown while the hydrogen bubbles, generated at the electrode, are attached to the electrode, deposition can occur within the interstitial spaces between the bubbles, thus replicating them. Due to the constant evolution of hydrogen, the bubbles continue to grow and, at some point, detach from the electrode. The size of such bubbles increases with time, leading to the formation of non-uniform porous structures. This technique was also applied to the deposition of porous
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silica [11,12], but the main research was focused on the deposition of porous copper. The mechanism of deposition and the influence of the experimental parameters on the copper morphology were investigated. Nikolic et al. [13–17] studied the effect of such parameters on the formation of copper dendrites, foams, and honeycomb and dish-like structures. Also, it was shown that the mechanical strength and branch sizes of the dendrites of the copper foam can be improved by using additives such as Cl− , NH4 + , acetic acid, polyethylene glycol, 3-mercapto-1-propane sulfonic acid, and cetyltrimethylammonium bromide [18–20]. More recently, the influence of the deposition regime (constant or pulsating overpotential) on the morphology of copper honeycomb-like structures was presented [21,22]. In our recent communication, we applied the concept of the hydrogen template for porous silver deposition [23]. It was shown that the morphology of the silver electrodes can vary with the amount of NH4 + , which was used to increase the hydrogen evolution rate. The aim of the current work is to study the effect of the deposition parameters (deposition potential and deposition time) and electrolyte content on the morphology of the porous silver. The final goal is to find a trend in the optimization of conditions for the production of silver electrodes most suitable for electrochemical applications, in areas where Ag electrodes were already employed, such as alkaline and direct borohydride fuel cells [24–26], double layer supercapacitors [27,28], batteries [29], sensors [30] or electrodes for electrocatalysis [31–33]. This is accomplished by analysis of the electrode morphology and by measuring the electrolyte accessible surface area. 2. Experimental 2.1. Reagents Potassium thiocyanate, ammonium chloride, sodium citrate, potassium fluoride, sodium hydroxide were obtained from Sigma–Aldrich. Silver sulfate was purchased from Samchun Pure Chemicals (Korea). All of the solutions were prepared in deionized (DI) water. All chemicals were used as received.
Table 1 Composition of electrolytic solutions for deposition of porous Ag. Reagents
Concentration, M
Ag2 SO4 KSCN NH4 Cl C6 H5 Na3 O7 ·2H2 O
0.01–0.06 1.5 0–2.5 0–0.03
Ag2 SO4 , 1.5 M KSCN, 0.5 M NH4 Cl, and 0.01 M C6 H5 Na3 O7 ·2H2 O, which we call the “standard bath” and any modifications of this “standard bath” used in this work will be highlighted in the text. The deposition was performed at room temperature from an unstirred solution. The deposition potential was varied in the range E = −1 to −4 V vs. Ag/AgCl. The deposition time was in the range of t = 15–120 s. After the deposition, the electrodes with deposited Ag foams were washed with DI water and dried under nitrogen gas flow. 2.4. Roughness factor evaluation All of the electrochemical experiments were carried out at room temperature in a standard three-electrode system. For the measurement of the surface area, two solutions were used: 1 M sodium hydroxide and 0.1 M potassium fluoride aqueous electrolytes. However, it was found that due to the presence of a large amount of highly active sites, the typical voltammetric profile of the silver foam in sodium hydroxide solution contains numerous Faradaic features which make it difficult to estimate the surface area. Thus, for the estimation of the surface area, only the data obtained from the CV taken in the potassium fluoride electrolyte was analyzed. Prior to the voltammetric experiments, the electrolytes were deaerated by purging with pure nitrogen gas for 20 min and, during the measurements, a stream of nitrogen gas was passed over the solution. 3. Results and discussion 3.1. Structure characterization
2.2. Instruments The electrochemical deposition and electrochemical measurements were performed using an electrochemical workstation (Zahner® Elektrik IM6ex, Germany). Ag/AgCl and Hg/HgO reference electrodes were utilized for the electrodeposition and surface area measurements, respectively, and a Pt plate was used as the counter electrode. The chemical composition was measured by energy dispersive X-ray analysis (EDS) performed in a field emission scanning electron microscope (FESEM) (JEOL JSM-7000F) with which the morphology of the deposited films was also examined. The structural properties were examined by X-ray diffractometry (XRD) (Bruker AXS with Cu K␣ radiation at 40 kV and 40 mA). 2.3. Electrode preparation The method of synthesizing the porous Ag foam presented herein is similar to the one reported in our previous works [23,34]. Briefly, Ag foams were deposited on a Pt/Ti/Si 5 mm × 25 mm electrode. A 10 nm thick Ti layer served as an adhesion layer and a 200 nm thick Pt thin film was deposited onto a Si(1 0 0) oriented wafer using electron beam evaporation. To prepare the working electrode, a 2 mm radius circle on the Pt surface was isolated and the edges and back side of the electrode were covered by insulating varnish. The deposition was carried out in constant potential mode. The electrolytes used for the deposition of the Ag foams are summarized in Table 1. Hereafter, the electrolyte containing 0.01 M
Due to the high rate of hydrogen evolution on a Pt substrate in the presence of a large amount of NH4 + ions in a thiocyanate based Ag bath, porous silver foams can be deposited galvanostatically, as described in our previous report [23]. However, as was pointed out, the high cathodic currents needed to obtain a sufficiently high hydrogen evolution rate result in the precipitation of sulfur or sulfur complexes. To eliminate such undesired effects, a small amount of sodium citrate was added to the solution in the present study. We found that by adding sodium citrate in the concentration range from 0.01 M to 0.03 M, the formation of precipitates could be avoided without any detrimental effect on the foam morphology. Thus, the electrolyte used in our previous work was modified by adding sodium citrate at a concentration of 0.01 M. The mechanism of bath stabilization is not clear. We found that the addition of sodium citrate does not change the CV profile of Ag deposition/stripping on the Pt electrode. We believe, that the sodium citrate stabilizes the byproducts of Ag deposition, which would otherwise precipitate. Before moving to the analysis of the influence of the experimental conditions on the microporous structure of the deposits, we would like to focus in detail on the evaluation of the nanoscopic features. During the electron microscopy investigation, we found that the nanoscale morphologies of all of the electrodeposited Ag foams can be categorized into three basic groups, namely dendrites, a framework of identical particles, and agglomerates of inhomogeneous particles with big Ag granules distributed on the foam
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Fig. 1. SEM images of three typical morphologies on microlevel: (a) dendrites, (b) small powder, (c) mixed powder obtained by deposition: from “standard bath” over (a) 15 s and (c) 90 s, and (b) from bath containing 0.75 M of NH4 Cl. (d) Typical EDS spectra and (e(C)) XRD pattern taken from electrodeposited Ag foam. For comparison, the XRD pattern taken from the substrate (e(A)) and standard XRD data for poly crystalline Ag JCPDS-004-0783 (e(B)) are also shown.
surface, as shown in Fig. 1(a–c), respectively. The growth of dendrites was observed only in the initial stages of foam formation, that is at a small deposition time, at a low concentration of NH4 + , and at a low overpotential. Under such deposition conditions, the amount of hydrogen gas was not enough to produce significant stirring of the solution layer close to the electrode. Thus, the deposition of Ag was primarily controlled by diffusion rather than kinetics. This result is in good agreement with the well known fact that the growth of Ag dendrites usually happens as a result of diffusion controlled deposition. By increasing the H2 evolution rate, the decrease in the thickness of the diffusion layer makes the mass-transfer of Ag+ ions faster, leading to the formation of compact particles. This likely compresses the dendrites and results in the formation of denser structures. Finally, in the kinetics controlled reaction regime (mainly at protrusive structures of deposited foams) the deposition of large agglomeration becomes possible. To ensure that the electrodeposited foams did not contain undesirable contaminants, EDS analysis was performed. The typical EDS spectra clearly showing the absence of impurities are shown in Fig. 1(d). Further confirmation of the silver quality was obtained by X-ray diffractometry (Fig. 1(e(C))). Except for the XRD peaks from the Pt/Ti/Si substrate (Fig. 1(e(A))), the XRD pattern obtained from the Ag foam contains only the peaks from pure Ag (fcc) phase. The position of the peaks matches well with the standard XRD pattern taken from polycrystalline Ag (JCPDS 004-0783) (Fig. 1(e(B))). As was found from SEM analysis, the size of the dendrite branches and the particle dimensions vary significantly with the deposition time or deposition potential. Surprisingly, it was found that the mean crystallite size for all of the electrodeposited foams does not vary greatly and was equal to 22.8 ± 1.7 nm. This result is based on the investigation of twelve samples with different microscopic morphologies deposited under different plating conditions. The mean crystallite size was estimated from the full width at half maximum height (FWHM) of the (1 1 1), (2 0 0), and (2 2 0) peaks according to the Debye–Scherrer equation [35]. Such a result is to be expected when taking into account the fact that both the big particles and dendrite branches are composed of smaller grains.
3.2. Influence of deposition parameters on the microstructure of Ag foams As shown in a previous study [23], varying the amount of NH4 + in the Ag electrolyte plays a significant role in the formation of the porous Ag foams. However, the foams formed were relatively thin, which made it difficult to obtain high surface area electrodes. Adjusting the other experimental parameters is, therefore, necessary to produce highly efficient electrodes for electrocatalysis. In the following sections, the influence of the NH4 + concentration, deposition potential, deposition time, and the amount Ag+ ions on the microstructure of the film are described. 3.2.1. Influence of NH4 + concentration As in the case of the galvanostatic deposition [23], in the potentiostatic deposition at E = −3 V and t = 30 s from the “standard bath” where NH4 + concentration was varied from 0 to 2.5 M, the morphology of the Ag foam varied drastically depending on the amount of NH4 + in the electrolyte, as shown in Fig. 2. Under such deposition conditions, foams with ordered pores were only obtained at ammonium chloride concentrations of 0.5 M and higher. For the chosen deposition time, the formation of porous films by galvanostatic deposition also starts at this concentration. The tendency of the pore size to decrease with increasing NH4 + concentration can be clearly seen, as presented by the solid line in Fig. 3. Due to the decrease of the pore size, the number of pores over a defined area increases, as shown by the dotted line in Fig. 3. The increase in the number of surface pores, however, eventually reaches saturation as a result of the simultaneous decrease in the pore wall thickness (or interpore distance) and hole size. The average pore size is in the range of 10 m to 20 m which is two times smaller than that found for Ag foams grown by galvanostatic deposition at j = 1 A cm−2 [23]. The current during potentiostatic deposition increased with increasing NH4 + concentration, as shown in Fig. S1(a) in the Supporting Materials. The current–time transients are very noisy because of the disturbances brought about by the change in the surface area and the change in hydrodynamic con-
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Fig. 2. SEM images of Ag structures deposited from electrolytes containing (a) 0.1 M, (b) 0.25 M, (c) 0.5 M, (d) 0.75 M, (e) 1 M, (f) 1.5 M, (g) 2 M, and (h) 2 M NH4 Cl.
ditions due to the evolution of hydrogen. The effect of the increase of the deposition current on the concentration of ammonium chloride is complex and consists of a few regions with different slopes, as shown in Fig. S1(b) in the Supporting Materials. Here, the average current density jav calculated as jav =
1 t
t
j(t)dt
(1)
0
was used [15]. A significant increase in the current was observed for NH4 + concentrations of 0.5 M and higher which correlates with the porous structures formation. As we showed in a previous study [23], the Ag deposition efficiency at very high overpotentials is very low, which means that the increase in current is almost entirely due to the rise in the hydrogen evolution rate. As was extensively described in previous works [15,17,23], a high rate of hydrogen evolution results in the change of the hydrodynamic conditions in the solution layers near the electrode, thus decreasing the diffusion layer thickness and increasing the rate of transfer of metal ions to the electrode.
Fig. 3. The dependences of the surface pore diameter (solid line) and number of surface pores over an area of 0.09 mm2 (dotted line) on the concentration of NH4 Cl.
It was shown that the increase of the density and the decrease of the surface tension of the solution leads to a decrease in the hydrogen bubble break-off diameter, which results in the formation of smaller holes [15]. However, we believe that the observed change in pore size is controlled by the current variation (from 0.7 A cm−2 to 1.2 A cm−2 for 0 and 2.5 M of NH4 + , respectively), as the contribution of the current density to the variation of the hole size is much larger than that of the solution density and surface tension. On the nanoscale level, the morphology changes from dendrites to small particles with simultaneous increase in the foam wall compactness, as NH4 + concentration is increased. 3.2.2. Influence of deposition potential Fig. 4 shows the evolution of Ag foam morphology obtained from the “standard bath” at various deposition potentials for t = 60 s. Pores are obtained for the potential is E ≤ −2 V. With increasing deposition potential, the morphology of the foams, in the terms proposed by Nikolic et al. [15,17], changes from honeycomb-like to a combined honeycomb- and dish-like structures. The population of the holes for the dish-like foams is lower than that for the honeycomb-like structures. Also, it can be seen that the evolution of the pore multilayers slows down for E < −3 V. The foam deposited at higher voltages consists of very compact deposits and has a relatively small amount of holes. It is clear that increasing the overpotential results in an increase of the hydrogen evolution, as the current density was observed to constantly increase (Fig. S2(a and b) in the Supporting Materials). Due to the complicated nature of the electrodeposition (competitive Ag+ reduction and H2 evolution reactions), it is difficult to explain the observed result. However, taking into account the high efficiency of H2 generation, we can say that the current–potential profile is governed mainly by the hydrogen evolution rate. Under such experimental conditions, the formation of dish-like structures was unexpected. Increasing the current density should result in the decrease of the break-off diameter of the hydrogen bubbles and the formation of smaller holes [15]. The opposite effect was observed as it is evident that the formation of dish-like holes was caused by the formation of H2 with a larger break-off diameter at the prolonged times needed for their detachment from the electrode surface.
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Fig. 4. SEM images of Ag foams deposited from the “standard bath” at applied potentials E of (a) −1.5 V, (b) −2 V, (c) −2.5 V, (d) −3 V, (e) −3.5 V, and (f) −4 V vs. Ag/AgCl. The deposition time was t = 60 s.
On the nanoscale level small overpotentials result in the growth of dendrites because of diffusion controlled growth (low solution mixing rate). Due to the increase in the hydrogen evolution rate and, thus, the increase in the mass transfer rate of Ag to the electrode at higher potentials, the nanoscale structure changes to a compact particle-like one with uniform particles at moderate overpotentials and mixed particles at high overpotentials. Considering their possible electrochemical applications, we believe that the foams deposited at E = −3 V should have the most efficient reagent accessible geometry. 3.2.3. Influence of deposition time Shown in Fig. 5 are the Ag foams electrodeposited from the “standard bath” at an applied potential of E = −3 V and different deposition times in the range of 15–120 s. As seen in Fig. 5(a), even for deposition times over 15 s, a thin porous foam of Ag is formed on the electrode surface. The foam walls consist of relatively sparse structures. The tendency in the morphology change on the nanoscale, and the difference in the density of the Ag structure in the wall region, as
shown by two typical examples for films deposited for 30 s and 90 s, are shown in the inset pictures of the corresponding images, respectively. As shown in Fig. 1(a–c), the former one consists of dendrites and the latter one of non-uniform particles. The difference in their density is also evident. With increasing deposition time, there is also a significant change in the microstructure. For deposition times shorter than 60 s, pore multilayer structures are formed, whose surface pore size increases with increasing deposition time. When the deposition time is longer than 60 s, there is also a change in the wall thickness. At this point the wall thickness of a pore is comparable to its diameter. The honeycomb-like structures formed within a short time are modified to a structure in which the holes are covered on the edges. It should be noted that without taking the first few seconds of deposition into account the current profile does not change significantly over time, as shown in Fig. S3 in the Supporting Materials. However, it is obvious that with increasing deposition time, the amount of available Pt sites drastically diminishes. This can result in the decrease of the H2 evolution efficiency and the hydrogen bubbles can be enclosed by Ag.
Fig. 5. SEM images of Ag foams deposited from the “standard bath” at an applied potential E = −3 V with deposition times of (a) 15 s, (b) 30 s, (c) 45 s, (d) 60 s, (e) 90 s, and (f) 120 s.
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Fig. 6. SEM images of Ag foams deposited from the electrolytes containing NH4 + (a–c) 0.5 M and (d–f) 2.5 M, and Ag+ (a and d) 0.01 M, (b and e) 0.03 M, and (c and f) 0.06 M. The deposition potential and time were −3 V and 30 s, respectively.
It should be noted that such a change in morphology is undesirable, as it can inhibit the transfer of the electroactive species to the inner layers of the electrode. Thus, improvement of the electrode morphology (increase of the active area) cannot be obtained by simply prolonging the deposition time.
3.2.4. Influence of Ag+ amount Lastly, the effect of the amount of silver salt was investigated. It is believed that increasing the concentration of Ag+ can decrease the efficiency of hydrogen evolution [16] and, thus, increase the rate of silver deposition. It should be noted that the solubility of Ag2 SO4 is very poor and strongly depends on the amount of KSCN. At a KSCN concentration of 1.5 M, the highest concentration of soluble Ag was less than 0.1 M. The dependence of the foam morphology on the amount of Ag+ was studied in the electrolytes with concentrations of NH4 + 0.5 M and 2.5 M. The corresponding SEM images are presented in Fig. 6(a–c) and (d–f), respectively. At a low rate of hydrogen evolution (low concentration of NH4 + ), the rise in the deposition rate with increasing amount of silver ions is evident. Due to the coalescence of the hydrogen bubbles during deposition, each new layer in the foam structure consists of large pores. However, at a concentration of 0.06 M, the hydrogen evolution rate is insufficient, resulting in sparse structures. It is likely that the deposition rate is limited by the rate of transfer of Ag ions, which is lower at a low hydrogen evolution rate. The metal in this case tends to grow on the pore walls, making them thicker, instead of building them up to form new layers. Foams with completely different morphologies were formed when the amount of ammonium chloride was sufficiently high, as can be seen in Fig. 6(d and e). Even though the efficiency of hydrogen evolution probably decreases at higher concentrations of silver, it is still very high (the current in this case is significantly higher, as shown in Fig. S4(a and b) in the Supporting Materials for the deposition from the electrolyte with NH4 + concentrations of 0.5 M and 2.5 M, respectively), which allows for high rates of Ag transfer and deposition. Films grown under such conditions have a pronounced 3D structure with increasing pore size diameter toward the outer surface. Thus, the formation of a layered structure with a highly efficient layered morphology can be achieved.
3.3. Roughness factor evaluation It is obvious that because of their microporous and nanoscopic structure, the surface area of metal foams prepared by dynamic hydrogen template deposition is significantly higher than the corresponding geometrical area of the electrode. So far, we concentrated mostly on the investigation of the microstructure by electron microscopy analysis. Three different morphologies on the nanoscale and various morphological forms on the micro-scale were found. Nonetheless, the electrochemical analysis of the real surface area or electrochemically accessible area is no less important. The electrochemically accessible area is significant in that this is the area that will likely be utilized in most electrochemical applications of such metal foams. Moreover, for some applications, such as double layer capacitors or electrocatalysis with sluggish kinetics, the improvement of device performance is directly proportional to the area of the electrode. It is convenient to use the roughness factor, Rf , the ratio of the real active surface area to the geometrical area of the electrode, as a quantitative parameter of the electrode surface area. In this study, we used a method of evaluating the roughness factor based on the measurement of the double layer capacitance of the Ag foam in KF solution. Even though there are other methods [36] and the current method has some limitations related to the oxide electrodes [37], the relatively broad so-called double-layer region on Ag electrodes makes it possible to measure the surface roughness of the Ag foams. Another advantage of this method is that it is a non-destructive process. We want to emphasize that, due to the presence of a large amount of active sites, some Faradaic features in the double-layer region were observed when the NaOH electrolyte was utilized. On the other hand, it is likely that the specific adsorption of F− ions decreases the amount of such Faradaic processes. The voltammetric profile in 0.1 M KF solution was undoubtedly capacitive. Fig. 7(a) shows the CVs taken from the Ag foam electrode deposited from the “standard bath” with 0.03 M Ag+ and 2.5 M of NH4 + at different scan rates, as indicated in the figure. Evidence of the capacitive nature of the current is provided by the linear proportionality between the scan rate and current measured at the center of the potential window, i.e. −0.25 V vs. Ag/AgCl, as shown in Fig. 7(b). The variation of the roughness factor of the Ag foams with the experimental parameters is presented in Fig. 8. The corre-
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Fig. 7. (a) Typical cyclic voltammetry data for the Ag foam in 0.1 M KF solution at scan rates of 10–160 mV s−1 and (b) corresponding dependence of current j0.25 on the scan rate.
sponding CVs are presented in Fig. S5 in the Supporting Materials. The Rf was calculated from the CV data using the formula: Rf =
Cexp Cspec
Cexp =
j0.25 dE/dt
(2)
(3)
where Cexp is the experimentally obtained capacitance normalized to the geometrical surface area, j0.25 = (janodic + jcathodic )/2 is the current density at E = 0.25 V vs. Ag/AgCl, and Cspec is the specific capacitance of the smooth silver/electrolyte interface (Cspec = 20 × 10−6 F cm−2 ) [38]. It can be seen that the increase in the Ag deposition rate in all cases results in the deposition of films with higher surface areas. Nonetheless, the effect of the deposition parameters is different in each case. As could be predicted from the SEM data, the Ag foams electrodeposited from the electrolyte in the presence of a high concentration of NH4 + have a higher surface area than the ones obtained at a low concentration of ammonia chloride, as shown in Fig. 8(a). The highest obtained Rf was about 180 at an NH4 + concentration of 2.0 M where saturation takes place. Fig. 8(b) shows the dependence of Rf on the deposition potential. It can be seen that the Rf increases almost linearly and reaches values of around 300 at −4 V. The similar linear proportionality of Rf with the deposition time is shown in Fig. 8(c) with a maximum at 120 s of ∼370.
The evolution of the surface area with the amount of Ag+ in the electrolyte was found to be different for the two electrolytes with different concentrations of NH4 + . It can be seen in Fig. 8(d) that, at a low concentration of Ag+ , the deposited foams have similar Rf values. However, the situation changes drastically when the Ag+ concentration is 0.045 M and 0.06 M. In the latter case, the difference in the roughness factor is approximately a factor of two (∼600 and ∼1100). It was shown by Brevnov [27,28] that electrodeposited high surface area silver films can be used as double-layer capacitors with a high-frequency response. Therefore, besides the capacitance normalized to the geometric surface area, it would be worthwhile to estimate the gravimetric capacitance. To do so, the Ag foam electrodeposited from an electrolyte containing 0.045 M Ag and 2.5 M NH4 + was weighed. It was found that the weight of the foam was around 5.6 mg cm−2 . Thus, the gravimetric surface area is around 11.8 m2 g−1 and the gravimetric capacitance is 2.4 F g−1 . The geometric surface normalized capacitance was approximately 13.2 mF cm−2 . The Ag foam deposited from the electrolyte with an [Ag+ ] of 0.06 M had the highest value of the geometric surface normalized capacitance of around 22 mF cm−2 . The electrode composed of interconnected silver particles with a size of around 200 nm described by Brevnov and Olson [28] had corresponding capacitances of 1.7 ± 0.2 mF cm−2 and 0.7 F g−1 , respectively. On the other hand, using porous silver films with particles 30 ± 7 nm resulted in the improvement of the capacitances
Fig. 8. Evolution of roughness factor Rf of Ag foam on the (a) NH4 + concentration, (b) time, (c) potential, and (d) Ag+ ions amount in solution with NH4 + of 0.5 M (solid line) and 2.5 M (dotted line).
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to 2.9 ± 0.1 mF cm−2 and 3.9 ± 0.1 F g−1 , respectively [27]. It is clear that because of the multilayer structure and very high surface area the capacitance normalized by the geometric area obtained in our work is significantly higher than the previously reported values. Similarly, the value of the gravimetric capacitance obtained was higher than that of the film of 200 nm Ag particles and lower than that of the film of 30 ± 7 nm Ag particles obtained by Brevnov et al. Our result is in good agreement with the works of Brevnov, as the Ag foams consist of particles with sizes smaller than 200 nm, but larger than 30 nm. Also, it is worth noting that the voltammetric profile of a typical Ag foam is capacitive in a potential window of at least 0.9 V, as shown in Fig. S6 in the Supporting Materials. It is evident that to improve the gravimetric capacitance, the feature sizes of the particles and the sizes of the micropores should be decreased, which could probably be achieved by using additives and surfactants [18–20]. 4. Conclusions The morphology of the Ag foam showed strong dependence on the electrolyte composition and deposition parameters. The NH4 + concentration was crucial for the formation of porous structures. Without NH4 + and at NH4 + concentrations lower than 0.25 M, the deposits consisted of relatively uniformly distributed sparse dendrites. At [NH4 + ] of 0.25 M and 0.5 M, the structure was no longer uniform and changed to honeycomb-like at higher concentrations. Further increasing the amount of NH4 + resulted in the formation of compact pores with pore sizes varying from 20 m to 10 m. The nanoscopic features changed from dendrites to relatively small uniform particles. All of these modifications resulted in the evolution of the roughness factor from around 25 to 180. As the deposition potential was increased, similar changes in the nanoscale morphology were found. However, at relatively high potentials, the appearance of large agglomerates on the surface of the Ag foams was observed. The purely honeycomb-like microstructure changed to a mixed one with some fraction of dish-like holes. The roughness factor increased with increasing deposition potential to around 300 at −4 V. In the case of the variation of the deposition time, after relatively long deposition times, the appearance of large agglomerates was observed on the surface of the Ag foams, as in the case of high potentials. However, the change in the microstructure was completely different from that observed before. A high deposition time resulted in the trapping of the hydrogen bubbles within the Ag matrix, which led to the overgrowth of Ag within the edges of the hydrogen bubbles. Thus, the pore wall thickness increased and became comparable to the sizes of the holes. The roughness factor increased gradually to about 370 at deposition time over 120 s. The most drastic change was found when the amount of Ag was increased from 0.01 M to 0.06 M, especially with the simultaneous increase in the concentration of NH4 + from 0.5 M to 2.5 M. Even for high Ag concentration, when the NH4 + concentration is low, the film is sparse. Completely different morphological forms were observed at high concentration Ag+ and NH4 + .
Acknowledgements This work was supported by New & Renewable Energy R&D program (2009-T100100282) under the Ministry of Knowledge Economy, Republic of Korea Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2010.06.054. References [1] A. Ahern, L. Nagle, D. Burke, J. Solid State Electrochem. 6 (2002) 451. [2] L.D. Burke, J.A. Collins, M.A. Horgan, L.M. Hurley, A.P. O’Mullane, Electrochim. Acta 45 (2000) 4127. [3] Y. Bai, Y. Sun, C. Sun, Biosens. Bioelectron. 24 (2008) 579. [4] S. Cherevko, C.-H. Chung, Sens. Actuators B: Chem. 142 (2009) 216. [5] M. DeLeo, A. Kuhn, P. Ugo, Electroanalysis 19 (2007) 227. [6] S. Park, T.D. Chung, H.C. Kim, Anal. Chem. 75 (2003) 3046. [7] K. Lohrberg, P. Kohl, Electrochim. Acta 29 (1984) 1557. [8] C.A. Marozzi, A.C. Chialvo, Electrochim. Acta 45 (2000) 2111. [9] H.-C. Shin, J. Dong, M. Liu, Adv. Mater. 15 (2003) 1610. [10] H.C. Shin, M. Liu, Adv. Funct. Mater. 15 (2005) 582. [11] W.-Z. Jia, K. Wang, Z.-J. Zhu, H.-T. Song, X.-H. Xia, Langmuir 23 (2007) 11896. [12] S. Yang, W.-Z. Jia, Q.-Y. Qian, Y.-G. Zhou, X.-H. Xia, Anal. Chem. 81 (2009) 3478. [13] N. Nikolic, K. Popov, L. Pavlovic, M. Pavlovic, J. Solid State Electrochem. 11 (2007) 667. [14] N.D. Nikolic, G. Brankovic, M.G. Pavlovic, K.I. Popov, Electrochem. Commun. 11 (2009) 421. [15] N.D. Nikolic, G. Brankovic, M.G. Pavlovic, K.I. Popov, J. Electroanal. Chem. 621 (2008) 13. [16] N.D. Nikolic, L.J. Pavlovic, M.G. Pavlovic, K.I. Popov, Electrochim. Acta 52 (2007) 8096. [17] N.D. Nikolic, K.I. Popov, L.J. Pavlovic, M.G. Pavlovic, J. Electroanal. Chem. 588 (2006) 88. [18] J.-H. Kim, R.-H. Kim, H.-S. Kwon, Electrochem. Commun. 10 (2008) 1148. [19] Y. Li, W.-Z. Jia, Y.-Y. Song, X.-H. Xia, Chem. Mater. 19 (2007) 5758. [20] H.-C. Shin, M. Liu, Chem. Mater. 16 (2004) 546. [21] N. Nikolic, G. Brankovic, V. Maksimovic, M. Pavlovic, K. Popov, J. Solid State Electrochem. 14 (2009) 331. [22] N.D. Nikolic, G. Brankovic, V.M. Maksimovic, M.G. Pavlovic, K.I. Popov, J. Electroanal. Chem. 635 (2009) 111. [23] S. Cherevko, X. Xing, C.-H. Chung, Electrochem. Commun. 12 (2010) 467. [24] F. Bidault, A. Kucernak, J. Power Sources 195 (2010) 2549. [25] E. SanlI, H. C¸elikkan, B. Zühtü Uysal, M.L. Aksu, Int. J. Hydrogen Energy 31 (2006) 1920. [26] N. Wagner, M. Schulze, E. Gülzow, J. Power Sources 127 (2004) 264. [27] D.A. Brevnov, J. Electrochem. Soc. 153 (2006) C249. [28] D.A. Brevnov, T.S. Olson, Electrochim. Acta 51 (2006) 1172. [29] X. Jin, J. Lu, Y. Xia, P. Liu, H. Tong, J. Power Sources 102 (2001) 124. [30] W. Lian, L. Wang, Y. Song, H. Yuan, S. Zhao, P. Li, L. Chen, Electrochim. Acta 54 (2009) 4334. [31] J. Geng, Y. Bi, G. Lu, Electrochem. Commun. 11 (2009) 1255. [32] M.-C. Tsai, D.-X. Zhuang, P.-Y. Chen, Electrochim. Acta 55 (2010) 1019. [33] F.-H. Yeh, C.-C. Tai, J.-F. Huang, I.W. Sun, J. Phys. Chem. B 110 (2006) 5215. [34] S. Cherevko, C.-H. Chung, Talanta 80 (2010) 1371. [35] A.L. Patterson, Phys. Rev. 56 (1939) 978. [36] A.J. Motheo, S.A.S. Machado, M.H.V. Kampen, J.R. Santos, J. Braz. Chem. Soc. 4 (1993) 122. [37] S. Trasatti, O.A. Petrii, Pure Appl. Chem. 63 (1991) 711. [38] A. Bard, L. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Willey, New York, 2001, p. 233.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Impact of key deposition parameters on the morphology of silver foams prepared by dynamic hydrogen template deposition Serhiy Cherevko, Chan-Hwa Chung ∗ Advanced Materials and Process Research Center for IT, School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 March 2010 Received in revised form 14 June 2010 Accepted 19 June 2010 Available online 1 July 2010 Keywords: Silver foam Supercapacitor Electrodeposition Hydrogen
a b s t r a c t The potentiostatic deposition of porous Ag foams from a new stable thiocyanate based bath using hydrogen bubbles as a dynamic template during deposition was investigated. The influence of the electrolyte content, deposition potential and deposition time on the micro- and nanoscale morphology of the Ag form was examined. The formation of three main morphological forms on the nanoscale level: dendrites, a framework of identical particles, and agglomerates of inhomogeneous particles with big Ag granules distributed on the foam surface, was demonstrated by the analysis of the scanning electron microscopy (SEM) data. The quality of the structures obtained was examined by energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). It was found that the experimental parameters had a huge effect on the morphology of Ag on both the micro- and nanoscale. The structures with the most efficient geometry from the technological point of view were obtained with the correct combination of parameters, viz. high concentrations of NH4 + and Ag+ in conjunction with a sufficient deposition potential and time. Foams with roughness factors as high as 1100 were obtained, showing a high geometrical area normalized double-layer capacitance and relatively high gravimetric capacitance of 22 mF cm−2 and 2.4 F g−1 , respectively. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Due to their high surface area, porous and especially nanoporous materials have attracted a great deal of attention over the last decade. This interest comes from the great impact of such materials in electrocatalysis. Besides the increase in the reaction rate brought about by their huge surface area, nanostructured porous electrodes can also have a significant effect on the kinetics of a reaction, which is usually related to the high activity of the nanostructures and large amount of active sites [1,2]. However, the rate of an electrochemical reaction is not always proportional to the surface area of the electrode. Indeed, as shown by numerous researchers [3–6], the efficient utilization of highly porous electrodes can only be expected when the reaction is sluggish. For fast, diffusion controlled reactions, usually only a small increase in the reaction rate is observed. Even for kinetically controlled reactions, an inefficient electrode geometry leads to additional ohmic and bubble overvoltages, mass transfer limitations, and, due to the possible hydrophobicity, a decrease of the electrolyte accessible surface area. All of these effects decrease the efficiency when utilizing porous electrodes. As an example, it was found that for the Raney–Nickel electrode used for the hydrogen evolution reaction,
∗ Corresponding author. Tel.: +82 31 290 7260; fax: +82 31 290 7272. E-mail address: [email protected] (C.-H. Chung). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.054
only 1.5% of the available surface area was utilized under certain conditions [7]. It is clear that improving the electrode geometry is a critical step in the development of porous materials for use as electrodes in electrocatalysis. It has been suggested that an ideal porous electrode should have micro- and nanoscale-features. A non-uniform microscopic porosity in the micrometer range, where the pore sizes of the electrode decrease with increasing distance from the front surface to the inner space, as well as porosity on the nanoscale which brings about an increase in the surface area and electrode activity is needed. Recently, such electrodes were produced by applying simple electrochemical techniques. It was found that electrodeposition at very high overpotentials in aqueous solutions results in the formation of porous foams with walls composed of a huge concentration of dendrites. With this method, highly porous Ni electrodes with microscopic morphologies appropriate for cathodes in water electrolysis were produced [8]. More recently, Shin et al. proposed the electrodeposition of self-supported 3D foams of copper, tin and copper-tin alloy by using the so-called hydrogen template [9,10]. It was shown while the hydrogen bubbles, generated at the electrode, are attached to the electrode, deposition can occur within the interstitial spaces between the bubbles, thus replicating them. Due to the constant evolution of hydrogen, the bubbles continue to grow and, at some point, detach from the electrode. The size of such bubbles increases with time, leading to the formation of non-uniform porous structures. This technique was also applied to the deposition of porous
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silica [11,12], but the main research was focused on the deposition of porous copper. The mechanism of deposition and the influence of the experimental parameters on the copper morphology were investigated. Nikolic et al. [13–17] studied the effect of such parameters on the formation of copper dendrites, foams, and honeycomb and dish-like structures. Also, it was shown that the mechanical strength and branch sizes of the dendrites of the copper foam can be improved by using additives such as Cl− , NH4 + , acetic acid, polyethylene glycol, 3-mercapto-1-propane sulfonic acid, and cetyltrimethylammonium bromide [18–20]. More recently, the influence of the deposition regime (constant or pulsating overpotential) on the morphology of copper honeycomb-like structures was presented [21,22]. In our recent communication, we applied the concept of the hydrogen template for porous silver deposition [23]. It was shown that the morphology of the silver electrodes can vary with the amount of NH4 + , which was used to increase the hydrogen evolution rate. The aim of the current work is to study the effect of the deposition parameters (deposition potential and deposition time) and electrolyte content on the morphology of the porous silver. The final goal is to find a trend in the optimization of conditions for the production of silver electrodes most suitable for electrochemical applications, in areas where Ag electrodes were already employed, such as alkaline and direct borohydride fuel cells [24–26], double layer supercapacitors [27,28], batteries [29], sensors [30] or electrodes for electrocatalysis [31–33]. This is accomplished by analysis of the electrode morphology and by measuring the electrolyte accessible surface area. 2. Experimental 2.1. Reagents Potassium thiocyanate, ammonium chloride, sodium citrate, potassium fluoride, sodium hydroxide were obtained from Sigma–Aldrich. Silver sulfate was purchased from Samchun Pure Chemicals (Korea). All of the solutions were prepared in deionized (DI) water. All chemicals were used as received.
Table 1 Composition of electrolytic solutions for deposition of porous Ag. Reagents
Concentration, M
Ag2 SO4 KSCN NH4 Cl C6 H5 Na3 O7 ·2H2 O
0.01–0.06 1.5 0–2.5 0–0.03
Ag2 SO4 , 1.5 M KSCN, 0.5 M NH4 Cl, and 0.01 M C6 H5 Na3 O7 ·2H2 O, which we call the “standard bath” and any modifications of this “standard bath” used in this work will be highlighted in the text. The deposition was performed at room temperature from an unstirred solution. The deposition potential was varied in the range E = −1 to −4 V vs. Ag/AgCl. The deposition time was in the range of t = 15–120 s. After the deposition, the electrodes with deposited Ag foams were washed with DI water and dried under nitrogen gas flow. 2.4. Roughness factor evaluation All of the electrochemical experiments were carried out at room temperature in a standard three-electrode system. For the measurement of the surface area, two solutions were used: 1 M sodium hydroxide and 0.1 M potassium fluoride aqueous electrolytes. However, it was found that due to the presence of a large amount of highly active sites, the typical voltammetric profile of the silver foam in sodium hydroxide solution contains numerous Faradaic features which make it difficult to estimate the surface area. Thus, for the estimation of the surface area, only the data obtained from the CV taken in the potassium fluoride electrolyte was analyzed. Prior to the voltammetric experiments, the electrolytes were deaerated by purging with pure nitrogen gas for 20 min and, during the measurements, a stream of nitrogen gas was passed over the solution. 3. Results and discussion 3.1. Structure characterization
2.2. Instruments The electrochemical deposition and electrochemical measurements were performed using an electrochemical workstation (Zahner® Elektrik IM6ex, Germany). Ag/AgCl and Hg/HgO reference electrodes were utilized for the electrodeposition and surface area measurements, respectively, and a Pt plate was used as the counter electrode. The chemical composition was measured by energy dispersive X-ray analysis (EDS) performed in a field emission scanning electron microscope (FESEM) (JEOL JSM-7000F) with which the morphology of the deposited films was also examined. The structural properties were examined by X-ray diffractometry (XRD) (Bruker AXS with Cu K␣ radiation at 40 kV and 40 mA). 2.3. Electrode preparation The method of synthesizing the porous Ag foam presented herein is similar to the one reported in our previous works [23,34]. Briefly, Ag foams were deposited on a Pt/Ti/Si 5 mm × 25 mm electrode. A 10 nm thick Ti layer served as an adhesion layer and a 200 nm thick Pt thin film was deposited onto a Si(1 0 0) oriented wafer using electron beam evaporation. To prepare the working electrode, a 2 mm radius circle on the Pt surface was isolated and the edges and back side of the electrode were covered by insulating varnish. The deposition was carried out in constant potential mode. The electrolytes used for the deposition of the Ag foams are summarized in Table 1. Hereafter, the electrolyte containing 0.01 M
Due to the high rate of hydrogen evolution on a Pt substrate in the presence of a large amount of NH4 + ions in a thiocyanate based Ag bath, porous silver foams can be deposited galvanostatically, as described in our previous report [23]. However, as was pointed out, the high cathodic currents needed to obtain a sufficiently high hydrogen evolution rate result in the precipitation of sulfur or sulfur complexes. To eliminate such undesired effects, a small amount of sodium citrate was added to the solution in the present study. We found that by adding sodium citrate in the concentration range from 0.01 M to 0.03 M, the formation of precipitates could be avoided without any detrimental effect on the foam morphology. Thus, the electrolyte used in our previous work was modified by adding sodium citrate at a concentration of 0.01 M. The mechanism of bath stabilization is not clear. We found that the addition of sodium citrate does not change the CV profile of Ag deposition/stripping on the Pt electrode. We believe, that the sodium citrate stabilizes the byproducts of Ag deposition, which would otherwise precipitate. Before moving to the analysis of the influence of the experimental conditions on the microporous structure of the deposits, we would like to focus in detail on the evaluation of the nanoscopic features. During the electron microscopy investigation, we found that the nanoscale morphologies of all of the electrodeposited Ag foams can be categorized into three basic groups, namely dendrites, a framework of identical particles, and agglomerates of inhomogeneous particles with big Ag granules distributed on the foam
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Fig. 1. SEM images of three typical morphologies on microlevel: (a) dendrites, (b) small powder, (c) mixed powder obtained by deposition: from “standard bath” over (a) 15 s and (c) 90 s, and (b) from bath containing 0.75 M of NH4 Cl. (d) Typical EDS spectra and (e(C)) XRD pattern taken from electrodeposited Ag foam. For comparison, the XRD pattern taken from the substrate (e(A)) and standard XRD data for poly crystalline Ag JCPDS-004-0783 (e(B)) are also shown.
surface, as shown in Fig. 1(a–c), respectively. The growth of dendrites was observed only in the initial stages of foam formation, that is at a small deposition time, at a low concentration of NH4 + , and at a low overpotential. Under such deposition conditions, the amount of hydrogen gas was not enough to produce significant stirring of the solution layer close to the electrode. Thus, the deposition of Ag was primarily controlled by diffusion rather than kinetics. This result is in good agreement with the well known fact that the growth of Ag dendrites usually happens as a result of diffusion controlled deposition. By increasing the H2 evolution rate, the decrease in the thickness of the diffusion layer makes the mass-transfer of Ag+ ions faster, leading to the formation of compact particles. This likely compresses the dendrites and results in the formation of denser structures. Finally, in the kinetics controlled reaction regime (mainly at protrusive structures of deposited foams) the deposition of large agglomeration becomes possible. To ensure that the electrodeposited foams did not contain undesirable contaminants, EDS analysis was performed. The typical EDS spectra clearly showing the absence of impurities are shown in Fig. 1(d). Further confirmation of the silver quality was obtained by X-ray diffractometry (Fig. 1(e(C))). Except for the XRD peaks from the Pt/Ti/Si substrate (Fig. 1(e(A))), the XRD pattern obtained from the Ag foam contains only the peaks from pure Ag (fcc) phase. The position of the peaks matches well with the standard XRD pattern taken from polycrystalline Ag (JCPDS 004-0783) (Fig. 1(e(B))). As was found from SEM analysis, the size of the dendrite branches and the particle dimensions vary significantly with the deposition time or deposition potential. Surprisingly, it was found that the mean crystallite size for all of the electrodeposited foams does not vary greatly and was equal to 22.8 ± 1.7 nm. This result is based on the investigation of twelve samples with different microscopic morphologies deposited under different plating conditions. The mean crystallite size was estimated from the full width at half maximum height (FWHM) of the (1 1 1), (2 0 0), and (2 2 0) peaks according to the Debye–Scherrer equation [35]. Such a result is to be expected when taking into account the fact that both the big particles and dendrite branches are composed of smaller grains.
3.2. Influence of deposition parameters on the microstructure of Ag foams As shown in a previous study [23], varying the amount of NH4 + in the Ag electrolyte plays a significant role in the formation of the porous Ag foams. However, the foams formed were relatively thin, which made it difficult to obtain high surface area electrodes. Adjusting the other experimental parameters is, therefore, necessary to produce highly efficient electrodes for electrocatalysis. In the following sections, the influence of the NH4 + concentration, deposition potential, deposition time, and the amount Ag+ ions on the microstructure of the film are described. 3.2.1. Influence of NH4 + concentration As in the case of the galvanostatic deposition [23], in the potentiostatic deposition at E = −3 V and t = 30 s from the “standard bath” where NH4 + concentration was varied from 0 to 2.5 M, the morphology of the Ag foam varied drastically depending on the amount of NH4 + in the electrolyte, as shown in Fig. 2. Under such deposition conditions, foams with ordered pores were only obtained at ammonium chloride concentrations of 0.5 M and higher. For the chosen deposition time, the formation of porous films by galvanostatic deposition also starts at this concentration. The tendency of the pore size to decrease with increasing NH4 + concentration can be clearly seen, as presented by the solid line in Fig. 3. Due to the decrease of the pore size, the number of pores over a defined area increases, as shown by the dotted line in Fig. 3. The increase in the number of surface pores, however, eventually reaches saturation as a result of the simultaneous decrease in the pore wall thickness (or interpore distance) and hole size. The average pore size is in the range of 10 m to 20 m which is two times smaller than that found for Ag foams grown by galvanostatic deposition at j = 1 A cm−2 [23]. The current during potentiostatic deposition increased with increasing NH4 + concentration, as shown in Fig. S1(a) in the Supporting Materials. The current–time transients are very noisy because of the disturbances brought about by the change in the surface area and the change in hydrodynamic con-
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Fig. 2. SEM images of Ag structures deposited from electrolytes containing (a) 0.1 M, (b) 0.25 M, (c) 0.5 M, (d) 0.75 M, (e) 1 M, (f) 1.5 M, (g) 2 M, and (h) 2 M NH4 Cl.
ditions due to the evolution of hydrogen. The effect of the increase of the deposition current on the concentration of ammonium chloride is complex and consists of a few regions with different slopes, as shown in Fig. S1(b) in the Supporting Materials. Here, the average current density jav calculated as jav =
1 t
t
j(t)dt
(1)
0
was used [15]. A significant increase in the current was observed for NH4 + concentrations of 0.5 M and higher which correlates with the porous structures formation. As we showed in a previous study [23], the Ag deposition efficiency at very high overpotentials is very low, which means that the increase in current is almost entirely due to the rise in the hydrogen evolution rate. As was extensively described in previous works [15,17,23], a high rate of hydrogen evolution results in the change of the hydrodynamic conditions in the solution layers near the electrode, thus decreasing the diffusion layer thickness and increasing the rate of transfer of metal ions to the electrode.
Fig. 3. The dependences of the surface pore diameter (solid line) and number of surface pores over an area of 0.09 mm2 (dotted line) on the concentration of NH4 Cl.
It was shown that the increase of the density and the decrease of the surface tension of the solution leads to a decrease in the hydrogen bubble break-off diameter, which results in the formation of smaller holes [15]. However, we believe that the observed change in pore size is controlled by the current variation (from 0.7 A cm−2 to 1.2 A cm−2 for 0 and 2.5 M of NH4 + , respectively), as the contribution of the current density to the variation of the hole size is much larger than that of the solution density and surface tension. On the nanoscale level, the morphology changes from dendrites to small particles with simultaneous increase in the foam wall compactness, as NH4 + concentration is increased. 3.2.2. Influence of deposition potential Fig. 4 shows the evolution of Ag foam morphology obtained from the “standard bath” at various deposition potentials for t = 60 s. Pores are obtained for the potential is E ≤ −2 V. With increasing deposition potential, the morphology of the foams, in the terms proposed by Nikolic et al. [15,17], changes from honeycomb-like to a combined honeycomb- and dish-like structures. The population of the holes for the dish-like foams is lower than that for the honeycomb-like structures. Also, it can be seen that the evolution of the pore multilayers slows down for E < −3 V. The foam deposited at higher voltages consists of very compact deposits and has a relatively small amount of holes. It is clear that increasing the overpotential results in an increase of the hydrogen evolution, as the current density was observed to constantly increase (Fig. S2(a and b) in the Supporting Materials). Due to the complicated nature of the electrodeposition (competitive Ag+ reduction and H2 evolution reactions), it is difficult to explain the observed result. However, taking into account the high efficiency of H2 generation, we can say that the current–potential profile is governed mainly by the hydrogen evolution rate. Under such experimental conditions, the formation of dish-like structures was unexpected. Increasing the current density should result in the decrease of the break-off diameter of the hydrogen bubbles and the formation of smaller holes [15]. The opposite effect was observed as it is evident that the formation of dish-like holes was caused by the formation of H2 with a larger break-off diameter at the prolonged times needed for their detachment from the electrode surface.
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Fig. 4. SEM images of Ag foams deposited from the “standard bath” at applied potentials E of (a) −1.5 V, (b) −2 V, (c) −2.5 V, (d) −3 V, (e) −3.5 V, and (f) −4 V vs. Ag/AgCl. The deposition time was t = 60 s.
On the nanoscale level small overpotentials result in the growth of dendrites because of diffusion controlled growth (low solution mixing rate). Due to the increase in the hydrogen evolution rate and, thus, the increase in the mass transfer rate of Ag to the electrode at higher potentials, the nanoscale structure changes to a compact particle-like one with uniform particles at moderate overpotentials and mixed particles at high overpotentials. Considering their possible electrochemical applications, we believe that the foams deposited at E = −3 V should have the most efficient reagent accessible geometry. 3.2.3. Influence of deposition time Shown in Fig. 5 are the Ag foams electrodeposited from the “standard bath” at an applied potential of E = −3 V and different deposition times in the range of 15–120 s. As seen in Fig. 5(a), even for deposition times over 15 s, a thin porous foam of Ag is formed on the electrode surface. The foam walls consist of relatively sparse structures. The tendency in the morphology change on the nanoscale, and the difference in the density of the Ag structure in the wall region, as
shown by two typical examples for films deposited for 30 s and 90 s, are shown in the inset pictures of the corresponding images, respectively. As shown in Fig. 1(a–c), the former one consists of dendrites and the latter one of non-uniform particles. The difference in their density is also evident. With increasing deposition time, there is also a significant change in the microstructure. For deposition times shorter than 60 s, pore multilayer structures are formed, whose surface pore size increases with increasing deposition time. When the deposition time is longer than 60 s, there is also a change in the wall thickness. At this point the wall thickness of a pore is comparable to its diameter. The honeycomb-like structures formed within a short time are modified to a structure in which the holes are covered on the edges. It should be noted that without taking the first few seconds of deposition into account the current profile does not change significantly over time, as shown in Fig. S3 in the Supporting Materials. However, it is obvious that with increasing deposition time, the amount of available Pt sites drastically diminishes. This can result in the decrease of the H2 evolution efficiency and the hydrogen bubbles can be enclosed by Ag.
Fig. 5. SEM images of Ag foams deposited from the “standard bath” at an applied potential E = −3 V with deposition times of (a) 15 s, (b) 30 s, (c) 45 s, (d) 60 s, (e) 90 s, and (f) 120 s.
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Fig. 6. SEM images of Ag foams deposited from the electrolytes containing NH4 + (a–c) 0.5 M and (d–f) 2.5 M, and Ag+ (a and d) 0.01 M, (b and e) 0.03 M, and (c and f) 0.06 M. The deposition potential and time were −3 V and 30 s, respectively.
It should be noted that such a change in morphology is undesirable, as it can inhibit the transfer of the electroactive species to the inner layers of the electrode. Thus, improvement of the electrode morphology (increase of the active area) cannot be obtained by simply prolonging the deposition time.
3.2.4. Influence of Ag+ amount Lastly, the effect of the amount of silver salt was investigated. It is believed that increasing the concentration of Ag+ can decrease the efficiency of hydrogen evolution [16] and, thus, increase the rate of silver deposition. It should be noted that the solubility of Ag2 SO4 is very poor and strongly depends on the amount of KSCN. At a KSCN concentration of 1.5 M, the highest concentration of soluble Ag was less than 0.1 M. The dependence of the foam morphology on the amount of Ag+ was studied in the electrolytes with concentrations of NH4 + 0.5 M and 2.5 M. The corresponding SEM images are presented in Fig. 6(a–c) and (d–f), respectively. At a low rate of hydrogen evolution (low concentration of NH4 + ), the rise in the deposition rate with increasing amount of silver ions is evident. Due to the coalescence of the hydrogen bubbles during deposition, each new layer in the foam structure consists of large pores. However, at a concentration of 0.06 M, the hydrogen evolution rate is insufficient, resulting in sparse structures. It is likely that the deposition rate is limited by the rate of transfer of Ag ions, which is lower at a low hydrogen evolution rate. The metal in this case tends to grow on the pore walls, making them thicker, instead of building them up to form new layers. Foams with completely different morphologies were formed when the amount of ammonium chloride was sufficiently high, as can be seen in Fig. 6(d and e). Even though the efficiency of hydrogen evolution probably decreases at higher concentrations of silver, it is still very high (the current in this case is significantly higher, as shown in Fig. S4(a and b) in the Supporting Materials for the deposition from the electrolyte with NH4 + concentrations of 0.5 M and 2.5 M, respectively), which allows for high rates of Ag transfer and deposition. Films grown under such conditions have a pronounced 3D structure with increasing pore size diameter toward the outer surface. Thus, the formation of a layered structure with a highly efficient layered morphology can be achieved.
3.3. Roughness factor evaluation It is obvious that because of their microporous and nanoscopic structure, the surface area of metal foams prepared by dynamic hydrogen template deposition is significantly higher than the corresponding geometrical area of the electrode. So far, we concentrated mostly on the investigation of the microstructure by electron microscopy analysis. Three different morphologies on the nanoscale and various morphological forms on the micro-scale were found. Nonetheless, the electrochemical analysis of the real surface area or electrochemically accessible area is no less important. The electrochemically accessible area is significant in that this is the area that will likely be utilized in most electrochemical applications of such metal foams. Moreover, for some applications, such as double layer capacitors or electrocatalysis with sluggish kinetics, the improvement of device performance is directly proportional to the area of the electrode. It is convenient to use the roughness factor, Rf , the ratio of the real active surface area to the geometrical area of the electrode, as a quantitative parameter of the electrode surface area. In this study, we used a method of evaluating the roughness factor based on the measurement of the double layer capacitance of the Ag foam in KF solution. Even though there are other methods [36] and the current method has some limitations related to the oxide electrodes [37], the relatively broad so-called double-layer region on Ag electrodes makes it possible to measure the surface roughness of the Ag foams. Another advantage of this method is that it is a non-destructive process. We want to emphasize that, due to the presence of a large amount of active sites, some Faradaic features in the double-layer region were observed when the NaOH electrolyte was utilized. On the other hand, it is likely that the specific adsorption of F− ions decreases the amount of such Faradaic processes. The voltammetric profile in 0.1 M KF solution was undoubtedly capacitive. Fig. 7(a) shows the CVs taken from the Ag foam electrode deposited from the “standard bath” with 0.03 M Ag+ and 2.5 M of NH4 + at different scan rates, as indicated in the figure. Evidence of the capacitive nature of the current is provided by the linear proportionality between the scan rate and current measured at the center of the potential window, i.e. −0.25 V vs. Ag/AgCl, as shown in Fig. 7(b). The variation of the roughness factor of the Ag foams with the experimental parameters is presented in Fig. 8. The corre-
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Fig. 7. (a) Typical cyclic voltammetry data for the Ag foam in 0.1 M KF solution at scan rates of 10–160 mV s−1 and (b) corresponding dependence of current j0.25 on the scan rate.
sponding CVs are presented in Fig. S5 in the Supporting Materials. The Rf was calculated from the CV data using the formula: Rf =
Cexp Cspec
Cexp =
j0.25 dE/dt
(2)
(3)
where Cexp is the experimentally obtained capacitance normalized to the geometrical surface area, j0.25 = (janodic + jcathodic )/2 is the current density at E = 0.25 V vs. Ag/AgCl, and Cspec is the specific capacitance of the smooth silver/electrolyte interface (Cspec = 20 × 10−6 F cm−2 ) [38]. It can be seen that the increase in the Ag deposition rate in all cases results in the deposition of films with higher surface areas. Nonetheless, the effect of the deposition parameters is different in each case. As could be predicted from the SEM data, the Ag foams electrodeposited from the electrolyte in the presence of a high concentration of NH4 + have a higher surface area than the ones obtained at a low concentration of ammonia chloride, as shown in Fig. 8(a). The highest obtained Rf was about 180 at an NH4 + concentration of 2.0 M where saturation takes place. Fig. 8(b) shows the dependence of Rf on the deposition potential. It can be seen that the Rf increases almost linearly and reaches values of around 300 at −4 V. The similar linear proportionality of Rf with the deposition time is shown in Fig. 8(c) with a maximum at 120 s of ∼370.
The evolution of the surface area with the amount of Ag+ in the electrolyte was found to be different for the two electrolytes with different concentrations of NH4 + . It can be seen in Fig. 8(d) that, at a low concentration of Ag+ , the deposited foams have similar Rf values. However, the situation changes drastically when the Ag+ concentration is 0.045 M and 0.06 M. In the latter case, the difference in the roughness factor is approximately a factor of two (∼600 and ∼1100). It was shown by Brevnov [27,28] that electrodeposited high surface area silver films can be used as double-layer capacitors with a high-frequency response. Therefore, besides the capacitance normalized to the geometric surface area, it would be worthwhile to estimate the gravimetric capacitance. To do so, the Ag foam electrodeposited from an electrolyte containing 0.045 M Ag and 2.5 M NH4 + was weighed. It was found that the weight of the foam was around 5.6 mg cm−2 . Thus, the gravimetric surface area is around 11.8 m2 g−1 and the gravimetric capacitance is 2.4 F g−1 . The geometric surface normalized capacitance was approximately 13.2 mF cm−2 . The Ag foam deposited from the electrolyte with an [Ag+ ] of 0.06 M had the highest value of the geometric surface normalized capacitance of around 22 mF cm−2 . The electrode composed of interconnected silver particles with a size of around 200 nm described by Brevnov and Olson [28] had corresponding capacitances of 1.7 ± 0.2 mF cm−2 and 0.7 F g−1 , respectively. On the other hand, using porous silver films with particles 30 ± 7 nm resulted in the improvement of the capacitances
Fig. 8. Evolution of roughness factor Rf of Ag foam on the (a) NH4 + concentration, (b) time, (c) potential, and (d) Ag+ ions amount in solution with NH4 + of 0.5 M (solid line) and 2.5 M (dotted line).
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to 2.9 ± 0.1 mF cm−2 and 3.9 ± 0.1 F g−1 , respectively [27]. It is clear that because of the multilayer structure and very high surface area the capacitance normalized by the geometric area obtained in our work is significantly higher than the previously reported values. Similarly, the value of the gravimetric capacitance obtained was higher than that of the film of 200 nm Ag particles and lower than that of the film of 30 ± 7 nm Ag particles obtained by Brevnov et al. Our result is in good agreement with the works of Brevnov, as the Ag foams consist of particles with sizes smaller than 200 nm, but larger than 30 nm. Also, it is worth noting that the voltammetric profile of a typical Ag foam is capacitive in a potential window of at least 0.9 V, as shown in Fig. S6 in the Supporting Materials. It is evident that to improve the gravimetric capacitance, the feature sizes of the particles and the sizes of the micropores should be decreased, which could probably be achieved by using additives and surfactants [18–20]. 4. Conclusions The morphology of the Ag foam showed strong dependence on the electrolyte composition and deposition parameters. The NH4 + concentration was crucial for the formation of porous structures. Without NH4 + and at NH4 + concentrations lower than 0.25 M, the deposits consisted of relatively uniformly distributed sparse dendrites. At [NH4 + ] of 0.25 M and 0.5 M, the structure was no longer uniform and changed to honeycomb-like at higher concentrations. Further increasing the amount of NH4 + resulted in the formation of compact pores with pore sizes varying from 20 m to 10 m. The nanoscopic features changed from dendrites to relatively small uniform particles. All of these modifications resulted in the evolution of the roughness factor from around 25 to 180. As the deposition potential was increased, similar changes in the nanoscale morphology were found. However, at relatively high potentials, the appearance of large agglomerates on the surface of the Ag foams was observed. The purely honeycomb-like microstructure changed to a mixed one with some fraction of dish-like holes. The roughness factor increased with increasing deposition potential to around 300 at −4 V. In the case of the variation of the deposition time, after relatively long deposition times, the appearance of large agglomerates was observed on the surface of the Ag foams, as in the case of high potentials. However, the change in the microstructure was completely different from that observed before. A high deposition time resulted in the trapping of the hydrogen bubbles within the Ag matrix, which led to the overgrowth of Ag within the edges of the hydrogen bubbles. Thus, the pore wall thickness increased and became comparable to the sizes of the holes. The roughness factor increased gradually to about 370 at deposition time over 120 s. The most drastic change was found when the amount of Ag was increased from 0.01 M to 0.06 M, especially with the simultaneous increase in the concentration of NH4 + from 0.5 M to 2.5 M. Even for high Ag concentration, when the NH4 + concentration is low, the film is sparse. Completely different morphological forms were observed at high concentration Ag+ and NH4 + .
Acknowledgements This work was supported by New & Renewable Energy R&D program (2009-T100100282) under the Ministry of Knowledge Economy, Republic of Korea Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2010.06.054. References [1] A. Ahern, L. Nagle, D. Burke, J. Solid State Electrochem. 6 (2002) 451. [2] L.D. Burke, J.A. Collins, M.A. Horgan, L.M. Hurley, A.P. O’Mullane, Electrochim. Acta 45 (2000) 4127. [3] Y. Bai, Y. Sun, C. Sun, Biosens. Bioelectron. 24 (2008) 579. [4] S. Cherevko, C.-H. Chung, Sens. Actuators B: Chem. 142 (2009) 216. [5] M. DeLeo, A. Kuhn, P. Ugo, Electroanalysis 19 (2007) 227. [6] S. Park, T.D. Chung, H.C. Kim, Anal. Chem. 75 (2003) 3046. [7] K. Lohrberg, P. Kohl, Electrochim. Acta 29 (1984) 1557. [8] C.A. Marozzi, A.C. Chialvo, Electrochim. Acta 45 (2000) 2111. [9] H.-C. Shin, J. Dong, M. Liu, Adv. Mater. 15 (2003) 1610. [10] H.C. Shin, M. Liu, Adv. Funct. Mater. 15 (2005) 582. [11] W.-Z. Jia, K. Wang, Z.-J. Zhu, H.-T. Song, X.-H. Xia, Langmuir 23 (2007) 11896. [12] S. Yang, W.-Z. Jia, Q.-Y. Qian, Y.-G. Zhou, X.-H. Xia, Anal. Chem. 81 (2009) 3478. [13] N. Nikolic, K. Popov, L. Pavlovic, M. Pavlovic, J. Solid State Electrochem. 11 (2007) 667. [14] N.D. Nikolic, G. Brankovic, M.G. Pavlovic, K.I. Popov, Electrochem. Commun. 11 (2009) 421. [15] N.D. Nikolic, G. Brankovic, M.G. Pavlovic, K.I. Popov, J. Electroanal. Chem. 621 (2008) 13. [16] N.D. Nikolic, L.J. Pavlovic, M.G. Pavlovic, K.I. Popov, Electrochim. Acta 52 (2007) 8096. [17] N.D. Nikolic, K.I. Popov, L.J. Pavlovic, M.G. Pavlovic, J. Electroanal. Chem. 588 (2006) 88. [18] J.-H. Kim, R.-H. Kim, H.-S. Kwon, Electrochem. Commun. 10 (2008) 1148. [19] Y. Li, W.-Z. Jia, Y.-Y. Song, X.-H. Xia, Chem. Mater. 19 (2007) 5758. [20] H.-C. Shin, M. Liu, Chem. Mater. 16 (2004) 546. [21] N. Nikolic, G. Brankovic, V. Maksimovic, M. Pavlovic, K. Popov, J. Solid State Electrochem. 14 (2009) 331. [22] N.D. Nikolic, G. Brankovic, V.M. Maksimovic, M.G. Pavlovic, K.I. Popov, J. Electroanal. Chem. 635 (2009) 111. [23] S. Cherevko, X. Xing, C.-H. Chung, Electrochem. Commun. 12 (2010) 467. [24] F. Bidault, A. Kucernak, J. Power Sources 195 (2010) 2549. [25] E. SanlI, H. C¸elikkan, B. Zühtü Uysal, M.L. Aksu, Int. J. Hydrogen Energy 31 (2006) 1920. [26] N. Wagner, M. Schulze, E. Gülzow, J. Power Sources 127 (2004) 264. [27] D.A. Brevnov, J. Electrochem. Soc. 153 (2006) C249. [28] D.A. Brevnov, T.S. Olson, Electrochim. Acta 51 (2006) 1172. [29] X. Jin, J. Lu, Y. Xia, P. Liu, H. Tong, J. Power Sources 102 (2001) 124. [30] W. Lian, L. Wang, Y. Song, H. Yuan, S. Zhao, P. Li, L. Chen, Electrochim. Acta 54 (2009) 4334. [31] J. Geng, Y. Bi, G. Lu, Electrochem. Commun. 11 (2009) 1255. [32] M.-C. Tsai, D.-X. Zhuang, P.-Y. Chen, Electrochim. Acta 55 (2010) 1019. [33] F.-H. Yeh, C.-C. Tai, J.-F. Huang, I.W. Sun, J. Phys. Chem. B 110 (2006) 5215. [34] S. Cherevko, C.-H. Chung, Talanta 80 (2010) 1371. [35] A.L. Patterson, Phys. Rev. 56 (1939) 978. [36] A.J. Motheo, S.A.S. Machado, M.H.V. Kampen, J.R. Santos, J. Braz. Chem. Soc. 4 (1993) 122. [37] S. Trasatti, O.A. Petrii, Pure Appl. Chem. 63 (1991) 711. [38] A. Bard, L. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Willey, New York, 2001, p. 233.