Supercapacitors Based on Metal Electrodes Prepared from

Apr 16, 2010 - (5) The SSA of these electrodes can be very high(1) (up to 2000 m2/g), ... mm × 15 mm)(21) to form a film whose bluish-purple color re...
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Supercapacitors Based on Metal Electrodes Prepared from Nanoparticle Mixtures at Room Temperature Hideyuki Nakanishi and Bartosz A. Grzybowski* Department of Chemical and Biological Engineering and Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208

ABSTRACT Films comprising Au and Ag nanoparticles are transformed into porous metal electrodes by desorption of weak organic ligands followed by wet chemical etching of silver. Thus prepared electrodes provide the basis for supercapacitors whose specific capacitances approach 70 F/g. Cyclic voltammetry measurement yield “rectangular” I-V curves even at high scan rates, indicating that the supercapacitors have low internal resistance. Owing to this property, the supercapacitors have a high power density ∼12 kW/kg, comparable with that of the state-of-the-art carbon-based devices. The entire assembly protocol does not require high-temperature processing or the use of organic binders. SECTION Energy Conversion and Storage

ltracapacitors or supercapacitors (SCs) are electrochemical capacitors that store energy via the formation of electric double layers (EDLs) over electrodes presenting high specific surface area (SSA). SCs are characterized by a specific capacitance, Csp, and energy density, ε, several orders of magnitude higher than those of common dielectric capacitors, DCs (ε ≈ 10 Wh/kg for SCs versus ε ≈ 3  10-2 Wh/kg for DCs1). While energy density stored in SCs is lower than that of batteries,1 SCs can provide higher power densities (ω ≈ 10 kW/kg for SCs versus ω ≈ 1 kW/kg for batteries1). Furthermore, the fact that supercapacitors can be charged/discharged rapidly and have lifetimes measured in millions of charge/discharge cycles makes them attractive candidates for applications in portable electronics, hybrid vehicles, and industrial equipment where large powers are needed.1,2 Because the specific capacitance and energy density of SCs scale linearly with the electrode's electroactive area, much effort has been devoted to developing high SSA electrodes, typically made of carbon materials such as carbonblack particles,3 carbon nanotubes,4 or carbon aerogels.5 The SSA of these electrodes can be very high1 (up to 2000 m2/g), and the specific capacitances of the resulting SCs range from a few to ∼300 F/g. On the other hand, coalescence of the carbon precursors into continuous, porous networks requires high-temperature treatment (e.g., >2500 °C for graphitizing carbons6) and in some cases chemical surface modification of the carbon particles7 and of the metal current collector.8 Contact resistance across the carbon-binder and carbonmetal interfaces has also been an issue since it dissipates power and effectively decreases energy/power densities9 (the maximal power density is inversely proportional to the equivalent series resistance, ESR10-12). Here, we describe a method in which SC electrodes are prepared from metal nanoparticles (NPs) at room temperature. In our procedure, a binary mixture of Au and Ag NPs is


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first cast onto a desired conductive substrate, and the NPs are fused together by desorption of weak organic ligands from the NPs' cores. Subsequently, one of the metals is removed by wet chemical etching to leave behind a high SSA structure of the other metal. SCs based on thus prepared electrodes have specific capacitance approaching 70 F/g (ε ≈ 2.4 Wh/kg) and high power density (ω ≈ 12 kW/kg) suitable for surge-power delivery application.9,13 These parameters compare favorably with those reported for dealloyed AuAg “sponges”, for which coarsening during anodization results in relatively low Csp ≈ 3-10 F/g and SSA ≈ 10-15 m2/g.14-16 On the other hand, the specific capacitance of our NP-based devices is significantly lower than ∼200 F/g for SCs based on aerogels incorporating ruthenium oxides17 (though these ruthenium composites are extremely expensive and the cost of a vehicle-sized SC made of RuO2 is estimated at $1 million2). Our solution deposition method does not require the use of binders between the NPs, does not suffer from noticeable coarsening during wet chemical etching, and appears generic to metal NPs protected with weak organic ligands. To deposit SC electrodes, solutions of Au NPs (particle diameter 5.6 ( 0.8 nm; particle concentration 10.9 mM in terms of metal atoms) and of Ag NPs (5.6 ( 0.8 nm; 14.6 mM) were prepared as described previously18 and were stabilized in toluene by excess of dodecylamine (DDA) and decanoic acid (DA), respectively19,20 (Figure 1a). After washing the NPs to remove excess surfactants, the particles were redispersed in toluene, and their solutions were mixed at desired proportions (quantified by the molar content of silver, χAg). Subsequently, 100 μL of the binary AuDDA/AgDA NP mixture was Received Date: March 28, 2010 Accepted Date: April 14, 2010 Published on Web Date: April 16, 2010


DOI: 10.1021/jz1004076 |J. Phys. Chem. Lett. 2010, 1, 1428–1431

Figure 2. (a) A scheme and dimensions of a SC assembled using porous metal electrodes. (b) Typical time dependencies of the voltage during discharge of SCs. Different curves correspond to electrodes made of NP films having different contents of Ag, χAg. The linearity of these semilogarithmic plots indicates exponential decay of the voltage with time. (c) Specific capacitances, Csp, calculated from the data shown in (b). The masses of the used electrodes are 455, 380, 105, 120, 50, 20, and 20 μg for χAg = 0, 0.40, 0.63, 0.72, 0.79, 0.87, and 0.93, respectively. (d) Cyclic voltammograms of a χAg = 0.87 supercapacitor recorded at different scan rates (270 mV/s, dashed line; 550 mV/s, dotted line; 1900 mV/s, dark-violet solid line).

Figure 1. (a) Schemes of the NPs used in this study and their coalescence upon desorption of the stabilizing surfactants. (b,c) (left) Optical images of NP electrode precursor films (15 mm  15 mm), (middle) their representative SEM images, and (right) the corresponding UV-vis spectra. The data in (b) are for the asprepared films, and (c) is for films after methanol immersion and NP coalescence. (d) Representative SEM images of films after Ag etching, (left) low SSA for χAg = 0.1 and (right) high SSA for χAg = 0.87. SEM scale bars are 50 nm in (b) and (c), 200 nm in (d), and 50 nm in the insets of (d).

two identical electrodes (symmetric system), the specific capacitance, Csp, in Farads per gram was calculated as Csp = (2/m)  C,30 where m is the mass (g) of one metal electrode. Figure 2c illustrates that the values of Csp depended on the content of silver, χAg, in the electrodes and that the maximal Csp was ∼70 F/g when χAg=0.87. The corresponding maximal energy density was ε=2.4 Wh/kg at the operating voltage of Vop=1.0 V. Power densities that can be achieved by NP-based suparcapacitors were estimated by means of cyclic voltammetry. Figure 2d shows typical cyclic voltammograms (CVs) of a χAg= 0.87 SC recorded at different scan rates. The fact that the CV curves are approximately symmetric and rectangular, even at scan rates as high as 1900 mV/s, indicates that the chargedischarge process is governed by a non-Faradic reaction (electric double layer formation) and that internal resistance is small. The power density estimated from the inner area of the CV loop12 is ∼12 kW/kg (for scan rate 1900 mV/s). To understand the experimental trends, we first explain the transformation of NP films into mesoporous metal electrodes. Before immersion in methanol, the bluish-purple color of the film reflects the surface plasmon resonance (SPR) of the NPs (cf. Figure 1b); after immersion, however, the color changes to gold-yellow, and the characteristic SPR peak disappears from the film's UV-vis spectrum, indicating the coalescence of proximal NPs (Figure 1c). As we have shown before,31,32 NP coalescence is due to the desorption of loosely bound DDA and DA surfactants from the metal NP cores; when devoid of the stabilizing organics, these cores “melt” in order to minimize the total surface area and the unfavorable gold-solvent surface energy.33 The desorption process can be related to the

drop-cast onto a nickel foil (15 mm  15 mm)21 to form a film whose bluish-purple color reflected the plasmon properties of the constituent, aggregated NPs20,22 (Figure 1b). After toluene evaporated, the NP film was placed in 20 mL of methanol for ∼1 h at room temperature. This procedure changed the film's color to gold (Figure 1c) and also rendered it stable to both toluene and methanol (as well as other common solvents). After washing with toluene and methanol several times, the NP films were immersed in a Ag etchant (1:1:3 v/v/v ammonium hydroxide/hydrogen peroxide/ethanol) to selectively remove silver. This procedure gave mesoporous23,24 metal electrodes (pore size ≈ 10 nm;10,25-28 Figure 1d, right) of thickness ≈ 200 nm (measured by profilometry) and the electroactive SSA as high as 350 m2/g (estimated by dividing Csp ≈ 70 F/g by the typical value of EDL capacitance, ∼0.2 F/m2 10,29). The SCs were assembled by placing an ion-permeable separator (∼100 μm cellulose film, Kimberly-Clark) between two metal electrodes and immersing this system in 6 M KOH electrolyte solution (Figure 2a). After fully charging at a voltage below 1.2 V (field strength E ≈ 12 kV/m), the SC was discharged through a resistor (typically a resistance of R ≈ 820 Ω) connected in parallel with the SC. During selfdischarging,10 the voltage V decayed exponentially with time t, V(t)/V(t=0) = exp(-t/RC). The capacitance C was then determined from the slope of ln(V(t)/V(t=0)) versus t plots (Figure 2b). Since the SCs assembled in this work consisted of

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DOI: 10.1021/jz1004076 |J. Phys. Chem. Lett. 2010, 1, 1428–1431

desorption equilibrium constant KD=[NP][S]/[NPS-S], where [NP] stands for the concentration of the “free” binding sites on NP surface, [S] is the concentration of DDAor DA surfactants in solution, and [NPS-S] denotes the surface sites occupied by S. For 5.6 nm NPs, there are ∼460 binding sites per particle, and KD in alcoholic solution is ∼10-2 M for DDA and ∼8  10-2 for DA. On the basis of this data and using the experimental concentrations of surfactants, it can be calculated that upon soaking in methanol, over 99% of the surfactants are desorbed, enabling the coalescence of the proximal “naked” NPs. When the Au/Ag composite is transformed into a SC electrode by chemical etching of silver, the SSA of the metal electrode is determined by the silver content, χAg. The SEM image in the left portion of Figure 1d illustrates the situation when χAg is low; in this case, the SSA after etching is also low, and so is the specific capacitance of the SC. When, however, χAg increases (Figure 1d, right), the SSA and Csp also increase. The specific capacitance reaches its maximal value of ∼70 F/g when χAg=0.87 (Figure 2c). Further increase in χAg causes a slight decrease in Csp, likely because the mesoporous Au network is not mechanically sturdy and collapses. This mechanical fragility imposes some limitations on our method. For example, when electrodes made of smaller, 2 nm Au/Ag NPs were prepared, we expected that their SSA and the specific capacitance would be higher than those for electrodes made of 5 nm NPs. In reality, for χAg=0.9, the specific capacitance was only ∼20 F/g. We attribute this decrease to the collapse of the very fine porous mesh during etching.29 Our SCs made of binder-free, all-metal electrodes are characterized by a power density (ω ≈ 12 kW/kg) comparable to or higher than that of state-of-the-art carbon-based SCs (e.g., ∼7 kW/kg for a MWNT-based supercapcitor34 and ∼10 kW/ kg for a graphene-based supercapcitor35). Because the CV curves in Figure 2d show no noticeable Ir drop upon reversing scan direction, this relatively large value of ω can be attributed12 to small internal resistance r. This resistance usually has three contributions, (i) ionic resistance of the electrolyte in a porous material, (ii) contact resistance between the current collector, electrode, and binders, and (iii) resistance of the electrode itself. For carbon-based SCs, the contact resistance of the electrodes is often dominant (Fcontact ≈ 1-100 Ω cm and Fcontact/Fbulk ≈ 104-106 for carbon nanotubes dropcast on metal electrodes36-38) and limits the power density (although it can be lowered to some extent by high-temperature treatment, ∼1000 °C).12 In our system, NPs devoid of the surfactants and binders form metal-metal, low-resistance contact with the deposition Ni surface (the resistivity of binary Au/Ni alloys is < 5.0  10-5 Ω cm39). Finally, the relatively large pore size (much larger than the size of hydrated ions, 6-7.6 Å; see ref 12), high ionic conductivity of aqueous KOH electrolyte (∼1 S/cm; see ref 6), and small internal resistance of the porous metal also contribute to the low value of r. In summary, we fabricated SCs based on mesoporous, allmetal electrodes. While the specific capacitances approaching 70 F/g are slightly lower than those using state-of-the-art carbon-based electrodes, the power densities are at least comparable and, in principle, suitable for surge-power delivery applications. One of the most appealing features of our

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method is that it does not require high-temperature processing and/or the use of binding resins. The method appears generic to other types of metal NPs, and in the future, it would certainly be desirable to extend it to particles less expensive than gold (e.g., Cu/Ag systems).

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the Nonequilibrium Energy Research Center, which is an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0000989.

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