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  • 1. 19. Electrochemistry at Nanostructured Diamond Electrodes: Characterization and Applications Kensuke Honda and Akira Fujishima 19. 1. Introduction Nanomaterials have many possible applications for analytical chemistry [1], and for electronic, optical, and mechanical devices [2]. In particular, nanomaterials and electrochemistry have a long shared history (e.g., the use of finely dispersed Pt particles as catalysts in fuel cell electrodes). This cChapter deals specifically with electrochemical applications of the template-synthesized nanostructured diamond. We begin with the basic electrochemical properties of nanostructured diamond electrodes. Two possible electrochemical applications are discussed. 19. 2. Fabrication of Nanostructured Diamond 19. 2. 1. Template synthesis of nanostructured materials 1
  • 2. There are numerous chemical methods for preparing nanomaterials [2, 3]. A number of researchers have been studying a method termed “template synthesis” [3]. Traditionally, this method has entailed synthesizing the nano-ordered structure of a desired compounds or material by use of a nanoscale template. Recently, the template method has been used with the pores in a microporous solid as a nanoscopic mold [3]. Many materials are available for the template materials [3, 4]. Pore diameter sizes range from Å to micrometers. Out of the many available template materials, anodic alumina (Al2O3) has been commonly used used as a template [5, 6]. When grown on high-purity aluminum, anodic alumina has a hexagonal pattern of cylindrical pores. Pore diameters from ~10 to ~400 nm can be synthesized. Recent improvements in the degree of ordering obtainable for a hole array has increased the attractiveness of such materials for nanofabrication. 19.2. 2. Fabrication procedure of nanostructured diamond Figure 19.2.1. shows the procedure for the fabrication of nano- porous diamond films (diamond nanohoneycomb) by template synthesis using with porous alumina membranes. Ordered thorough-hole anodic porous alumina membranes were laid on the top of the synthetic diamond films, and then deep holes were etched into the film by useing of an oxygen plasma treatment. 2
  • 3. 19. 2. 3. Polishing of polycrystalline diamond films Nanohoneycomb structures were fabricated from polished polycrystalline films. The polishing of the as-deposited films was Thorough-hole porous anodic alumina mask Oxygen Plasma Polished boron-doped diamond thin film Nano-honeycomb diamond carried out by the Namiki Precision Jewel Co., Ltd., Tokyo, Japan, by use of a proprietary process. The films polished by this process are extremely smooth, with height variations on the order of ca. 1 nm. Fig. 19.2.1 Schematic diagrams of the fabrication procedure for the nano-honeycomb diamond electrode 19. 2. 4. Preparation of the anodic alumina mask Anodic porous alumina is formed via the anodization of Al in an appropriate solution. The preparation of the thorough-hole porous anodic alumina mask has been described [7]. The pore interval of porous alumina, in other words, the cell size, was determined by 3
  • 4. the applied voltage used for anodization [7]: the cell size has a good linear relationship with the applied voltage, where the proportionality constant of cell size per unit applied voltage is approximately 2.5 nm V-1. In a previous survey, self-ordering has been observed to occur under limited voltage conditions, which were specific to the solution used for anodization; self-ordering takes place at 25 V in sulfuric acid solution with a 65- nm cell size, at 40 V in oxalic acid solution with a 100- nm cell size, and at 195 V in phosphoric acid with 500- nm cell size [7]. An aluminum sheet (10 × 50 × 30 mm: 99.999%; Nilaco) was electropolished in a mixed solution of perchloric acid ([60%)] and ethanol (1:4 in volume) at constant current conditions of 100 mA cm-2 at a temperature below 10℃ for 4 min. Anodization was conducted under constant voltage conditions (40 V in a 0.3 M oxialic acid solution for 10 h) using a DC source (Metronix 410A-350). The temperature of the electrolyte was maintained at 0 ℃ during anodization using with a cooling system (EYELA CTP-20). After anodization the surface was protected against etching using a coating layer made of a mixture of nitrocellulose and polyester resin in ethyl acetate, butyl acetate and heptane. The Al layer was removed in a saturated HgCl 2 solution. Then, the bottom part of the anodic porous alumina membrane was removed in 5 wt% phosphoric acid at 30℃ for 60 min, after which the coating layer was dissolved in acetone, to form a thorough-hole membrane. 19. 2. 5. Oxygen plasma etching process 4
  • 5. The oxygen plasma etching of the diamond films was conducted with an RF−driven (13.56 GHz) plasma etching apparatus (Samco BP-1, Japan) [8]. The diamond specimen with mask was placed on one of the planar electrodes in the plasma chamber. Oxygen plasma etching was carried out for 15 min. The operating oxygen pressure was 20.0 Pa, and the plasma power was 150 W. 19. 3. Impedance Characteristics of the Nanoporous Honeycomb Diamond and Application as an Electrical Double−-Layer Capacitor Fabrication of nanostructured diamond 19. 3. 1. Fabrication of nanostructured diamond Nanoporous materials [8-10] have attracted much recent interest, including that stemming from possible electrochemical applications [11, 12]. The electrochemical capacitor [13, 14] is a natural application for nanoporous structures. Activated carbons have been the most extensively examined capacitor materials over the past decade [13, 15]. Another possible approach involves improving the performance of activated carbon-based capacitors through modification of the electrolyte. In order to increase the specific energy, organic electrolytes have been examined due to the larger available operating voltage range (ca. 2.5 V) [13], however, the discharge performance of such capacitors is much lower than those obtained with aqueous electrolytes, due to the high resistance of the electrolyte. The conductivity of aqueous electrolytes is at least one order of magnitude greater than those of organic electrolytes. Thus, it would be desirable to have an 5
  • 6. electrode material with high capacitance and a wide working potential range in highly conductive aqueous electrolytes. The most promising material thus far considered appears to be diamond. Diamond possesses a wide potential window in aqueous [16, 17] and nonaqueous [18] media and extreme electrochemical stability [19]. Although as-deposited polycrystalline diamond exhibits very low capacitance [17], here we have demonstrated that the capacitance can be increased drastically by producing high-aspect-ratio cylindrical pores in the electrode through oxidative etching. In the present work, we have carried out the electrochemical characterization of the diamond honeycomb electrodes using cyclic voltammetry and impedance measurements. 19.3.2. Film characterization Scanning electron microscopy−Figure 19.3.1 shows SEM images of the three types of diamond nanohoneycomb films. Highly uniform, well-ordered arrangements of holes, with a hexagonal close-packed pattern, are clearly seen in these figures. Nanoporous boron-doped diamond films with various pore diameters (30 nm to 400 nm) and pore depths (50 nm to 3 µm) were fabricated by etching polished polycrystalline diamond films through porous alumina masks with an oxygen plasma. Among the three honeycomb films that we have fabricated, the film with a pore diameter of 60 nm and depth of 500 nm has the most highly ordered structure, in terms of both the shapes of the individual pores as well as the overall arrangement (Fig. 6
  • 7. 19.3.1B, honeycomb pore dimension type 60 × 500 nm). The average pore density was 1 × 10 10 -2 cm . Based on the pore dimensions and pore density, the surface area was estimated to be a factor of 10.5 times larger for the honeycomb film compared to a flat, polished surface. The film with 30-nm pores has a lower porosity (i. e., roughness factor), due to the small diameter of the pores, the larger intervals and the shallower pore depth (Fig. 7
  • 8. pores, the larger intervals and the shallower pore depth (Fig. Α−α A-a Α−β A-b 300 νµ 300 nm 237 νµ 237 nm Β−α B-a Β−β B-b 300 νµ 300 νµ Χ−α Χ−β 1.5 µµ 1.5 1.5 µµ Fig. 19.3.1. SEM images of a highly boron-doped nanohoneycomb diamond electrode.(a) top view, (b) oblique view at a 45° tilt angle for pore types (A) 30 × 50 nm, and (B) 60 nm × 500 nm and (C) 400 nm × 3 µm. Nanohoneycomb films observed by SEM were fabricated from free-standing polished diamond 8
  • 9. 19.3.1A, pore type 30 × 50 nm). The average pore density of this film is 2.78 × 1010 cm-2. Based on the pore dimensions and pore density, the surface area for this film was estimated to be only a factor of 2.11 times larger compared to a flat, polished surface. However, in the case of the honeycomb with 400-nm diameter pores, the latter are very closely spaced, and some pores have merged to form larger ones (Fig. 19.3.1C, pore type 400 nm × 3 µm). In this case, due to the larger pore depth, the porosity is much greater than that of the other two films. Although the pore density is only 4 × 108 cm-2, this film has a high roughness factor, (15.6). µ −2 4 mA cm-2 4 Α χ µ (a) As-deposited diamond ( α ) Α σ − δ ε π ο σ ι τ ε δ δ ι α µ ο ν δ ) −2 -2 (β) Πορε τψπε 30∼ 50 νµ (b) Pore type 30 ~50 nm (χ) Πορε τψπε 60∼ 500 νµ (δ) Πορε τψπε 400νµ ∼~3 (d) Pore type 400nmm 3 µµ Χυρρεντ δενσιτψ 0.4 ς −0.5 0.4 - 0.5 ς(mA cm Current density V (µΑVχµ −2.5 −1.5 − 0.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 Ποτεντιαλ(ς ϖ . Αγ/ΑγΧλ σ ) Potential (V vs. Ag/AgCl) Fig. 19.3.2. Cyclic voltammograms for (a) as-deposited diamond and pore types (b) 30 × 50 nm, (c) 60 × 500 nm and (d) 400 nm × 3 µm;. eElectrolyte ,: 1 M H 2 SO 4 ; sweep rate,: 100 mV s -1 . Arrows indicate the potentials at which the impedance measurements were carried out. 9
  • 10. Cyclic voltammetry−Because the advantage of diamond in the double-layer capacitor application is its wide working potential window, we have examined the current-potential behavior for the honeycomb films (Figure 19.3.2A). Interestingly, the working potential window for the honeycomb films remained essentially the same as that for the as-deposited film, even after extended oxygen plasma treatment. Table 19.3.1. Comparison of double-layer capacitance and specific energy for various types of carbon-based electrodes. Χδλ, µΦ χµ −2 Potential window −1 ∆ς , ς φ οµ χψ ιχ ρ χλ (γεοµ ετριχ) Χδλ, Φ γ −2 Roughness ϖ τ µ ετρ ολαµ ψ φ οµ ρ φ οµ ρ Εδλ, µ ϑχµ α β χ δ − δ 1 −1 ε factor (Ερεδ, Εοξ) ιµ πεδανχε ιµ πεδανχε (γεοµ ετριχ) Εδλ, ϑγ Εδλ, ϑγ -2 Ασ−δεποσ εδ δια ονδ φ µ ιτ µ ιλ 4.0 3.04 (−1.24, 1.80) 12.9 5.94 ~ 10 -1 G s y c rb nG 0 la s a o C-2 2 7 (-1.0 , 1.4 ) .4 3 4 5 .1 5 1.6 ~ 10 8 -2 H GZYA OP 1.9 (-0 4 1.2 ) 3 .6 , 9 7.02 1.2 ~ 10 6 Ac a dc rb n tiv te a o 1.0(-0 0 ) .7, .3 10 -4 0 0 0 5 -2 0 0 0 P retyp 3 ? 5 n o e 0 0 m 2.11 2 (-1.12 1.5 ) .70 , 8 129 9.12 0 6 .4 9 3 .3 3 15 .9 0 3 P retyp 6 ? 5 0n o e 0 0 m 10.9 2 2(-1.0 , 1.5 .6 5 7) 1.8 ~ 10 3 14.5 6 9 .2 4 .9 9 6 .0 3 3 P retyp 70? 75 n o e 0 m 16.7 2 1 (-1.0 , 1.5 ) 2 0 ~ 10 .6 5 6 .9 17.9 9.12 61.1 72.8 P retyp 4 0n ? 3 m o e 0 m m 15.6 2 6(-0 5 1.6 ) 3 1 ~ 10 .4 .8 , 0 .9 3 74.6 11.8 24 2 .8 18 .1 5 Dire t e h dd m n c tc e ia o d f 4.0 3 (-1.3 , 1.8 ) .17 4 3 28 3 1.20 (n m s o a k) a Values obtained from cyclic voltammograms measured at 100 mV s-1 . The definition of potential window is ∆V < 2 mA V-1 cm-2 (data from Fig. 19.3.2). b Values obtained by AC impedance analysis at 0.4 V vs. Ag/AgCl (data from Fig. 19.3.2). c The specific capacitance for a hypothetical through-hole diamond membrane. d The specific energy was estimated from the equation, Edl = 1/2×Cdl×(∆V)2. e The specific energy for a thorough-hole membrane estimated from pore parameters and the differential capacitance of 200 µF cm-2. f Etched for 1 min. SEM showed no significant roughening of the surface. 10
  • 11. We have chosen the criterion for the definition of potential window to be that the slope of the CV at 100 mV s -1 is < 2 mA V-1 cm-2. The potential windows for various electrodes, estimated in this manner, are summarized in Table 19.3.1. The potential windows for as-deposited diamond (3.04 V) and the 30 × 50-nm pore honeycomb (2.70 V) are appreciably larger than those for either GC (2.47 V) or HOPG (1.93 V) [17, 20]. The values for the honeycomb diamond electrodes were somewhat smaller (340 to 580 mV) than that for as-deposited diamond due in part to the less negative potential limits (Table 19.3.1). As a result, these porous structures exhibited wide electrochemical potential windows (ca. 3.0 V) in aqueous electrolytes, being somewhat smaller than unetched, as-deposited diamond electrodes, independent of pore structure. The double layer capacitive current for the diamond honeycomb was a factor of 18 to 20 larger than that for the as- deposited diamond electrode due to the surface roughness of the nanohoneycomb structure. We shall next explore this difference in greater detail using impedance measurements. 19.3.3. Impedance measurements Impedance plots−Figure 19.3.3 shows experimental impedance plots (complex plane representation) obtained for both the as- deposited and the honeycomb diamond electrodes at 0.4 V. The plots for the pore types, 60 × 500 nm (Fig. 19.3.3-c), 70 × 750 nm (not shown), and 400 nm × 3 mm (Fig. 19.3.3-d), exhibit two distinct domains: a high frequency domain, where the impedance behavior is that expected for a cylindrical pore electrode, with a 11
  • 12. characteristic linear portion at a 45° angle, and a low frequency domain, where the behavior is that expected for a flat electrode ) 12.0 ) 2 0.010 α 12.0 2 χµ χµ χ 0.013 Ω Ω 5 8.0 3 8.0 Ζ ( 10 0.025 Ζ ( 10 0.025 4.0 Ιµ 4.0 Ιµ − 0.050 − 0.050 0.10 0.10 5 10 Ηζ 105Ηζ 0.0 0.0 0.0 4.0 8.0 0.0 4.0 8.0 Re Z (105 Ω χµ 2) Ρ ε Ζ ( 1 0 Ρε Ζ Ρ ε Ζ (103 ( 1 0 Ω χµ 2) ) 22.5 9.0 ) 2 χµ β 2 δ χµ Ω Ω 15.0 4 6.0 3 0.010 0.010 Ζ ( 10 7.5 3.0 10 Ζ( Ιµ − 0.025 Ιµ 0.050 − 0.025 0.050 0.10 0.10 105Ηζ 105Ηζ 0.0 0.0 0.0 7.5 15.0 0.0 3.0 6.0 Ρ ε Ζ (104 Ω χµ 2) Ρ ε Ζ (103 Ω χµ 2) [21]. Fig. 19.3.3. Complex-plane plots of the impedance for electrodes of (a) as-deposited diamond and pore types (b) 30 × 50 nm, (c) 60 × 500 nm, and (d) 400 nm × 3 µm, at +0.4 V vs. Ag/AgCl. Experimental data points (○) and simulated curves (solid lines), were calculated on the basis of equivalent circuits involving modified transmission line models (see text), are shown. The parameters used in the calculated curves are given in Table 19.3.2. 12
  • 13. The impedance plots for the pore type 30 × 50 nm electrode, however, exhibit only a high frequency domain, with a characteristic linear portion at a 45° angle (Fig. 19. 3.3-b). In this case, even at low frequencies, the potential oscillations have negligible influence beyond a certain depth (penetration depth). At cylindrical-pore electrodes, the capacitance tends to reach an intrinsic limiting value at very low frequencies. The values were calculated in the low frequency limit (0.01 Hz) from the imaginary component of the impedance with the relation Z = --i/(ωC). The results are summarized in Table 19. 3. 1. The double layer capacitance values per unit area discussed in this paper are based on the geometric area, except where explicitly stated otherwise. The capacitance values were found to increase with increasing roughness factor, based on the pore dimensions. Among the electrodes examined, the honeycomb with 400 nm × 3 µm pores yielded a maximum capacitance value of 3.91 × 103 mF cm-2, which is a factor of ca. 400 larger than that for the as- deposited surface. For the porous film with 30-nm diameter pores, there was only a very small effect of the pore structure on the capacitance due to the high pore impedance. Table 19. 3. 1 shows that the specific capacitance value (74.6 F g-1) estimated for the 400 nm × 3 µm pore type honeycomb is comparable to those typical for activated carbon electrodes, which range from 100 to 400 F g-1 [22]. In terms of device applications, the ability to store energy is important, and the larger available potential range for diamond (> 3.0 V) compared to those for other forms of carbon (ca. 1.0 V for activated carbon [37]) becomes an advantage. Energy densities 13
  • 14. have been calculated for all of the various types of electrodes examined in the present work in terms of the geometric areas (Table 19. 3. 1). Taking the capacitance values (Cdl) from the impedance measurements and the potential window values (∆V) from the CV measurements, the energy densities (per unit geometric area) for the actual diamond honeycomb double-layer capacitors for a full cell were calculated by use of the formula Edl = 0.5 × Cdl × (∆V)2. Assuming that the free-standing diamond honeycomb films with though-holes were available for the pore geometries examined here, we have estimated hypothetical values for the specific capacitance for the various honeycomb samples (i. e., per unit mass) (Table 19.3.1). These range from 33.3 to 224.8 J g -1. Due to the large working potential range, the specific energies for the honeycomb diamond electrodes fall nearly in the same range as that for typical activated carbon-based capacitors (50 - 200 F g--1). Because of the wide electrochemical potential window in aqueous electrolytes and the high capacitance, honeycomb diamond electrodes are promising candidates for electrochemical capacitor applications. Numerical simulations−The double-layer charging process for a porous electrode consisting of cylindrical pores can be simulated with the use of the transmission line model [24-26]. If the cylindrical pores are characterized by radius r, length l and number of pores n, the mathematical form for the transmission line model is Z = W coth(γl) (19.3.1) 14
  • 15. where W and γ are defined as (RZ)1/2 and (R/Z)1/2, respectively. Here, 1/Z is jωC, and R and C are the resistance and capacitance per unit pore depth and are expressed by 1/(nπr2κ) and 2πrnCdpore, respectively. κ is the electrolyte conductivity and Cdpore is the differential double-layer capacitance in the pores. The impedance can be simulated by use of the geometric parameters of the cylindrical pores observed by SEM. Ρ ρεξτ Ρ εαχτιον ρεσ τανχε ισ Ρ σεξτ Χδ εξτ Πορε δεπτη λ Σεριεσ ρεσ τανχε ισ Ελεχτρολψτε Πορε διαµ ετερ χονδυχτιϖ ιτψ Ρ σπορε Rspore κ δ Χδ πορε Cdpore Ρ ρπορε ∆ιφφερεντιαλ Ρ εαχτιον χαπαχιτανχε ρεσ τανχε ισ Τρανσ ισ ιον λ µ οδελ µ σ ινε Transmission line model Fig. 19.3.4. Equivalent circuit based on the transmission line model, including both a Faradaic charge-transfer reaction and double-layer charging in the honeycomb diamond electrode The calculated impedance curves for the various honeycomb electrodes are shown in Fig. 19. 3. 3, together with the experimental curves. Figure 19. 3. 4 shows an equivalent circuit employed to reproduce the impedance plots for honeycomb diamond electrodes. Table 19. 3. 2 summarizes the values of the fitting parameters and the average relative errors for the 15
  • 16. calculated curves. The calculated curves are in good agreement with the experimental curves. The areal capacitances of the pore walls (Cdpore), falling in the range 120 to 230 mF cm-2, were on the same order as that of the 1-min direct-etched diamond surface (see Table 19. 3. 1). This capacitance enhancement for the plasma-etched surfaces is due to contributions from oxygen-containing functional groups and various types of defects generated on the surface during the plasma treatment. Usually, the electrolyte conductivities inside the honeycomb pores, as determined by impedance, range from 15 to 180 mS cm-1, which are of the same order of magnitude as the bulk sulfuric acid conductivity. However, in the case of the pore type 30 × 50 nm film, the electrolyte conductivity was estimated to be only 70 mS cm-1, based on the fitting (Table 19. 3. 2). For the equivalent circuit used for the porous electrodes, the pore impedance is usually determined only by the value of the electrolyte conductivity. In the case of the 30-nm pore diameter nano−honeycomb, the pore impedance has drastically increased. Using a transmission-line model for double-layer charging within the pores, we were able to simulate the experimental impedance curves. The diamond honeycomb structures appear to be good approximations to an ideal cylindrical pore-type electrode. Table 19.3.2. Parameters used for fitting the impedance results in the complex plane (Fig. 19.3.3),   based on the modified transmission line model (Fig. 19.3.4). Series ∆ιφερεντιαλ φ Ρεαχτιον Σερ ιεσ ∆ιφερ φ εντιαλ Ρεαχτιον Τιµε χαπαχιτανχε φορ ρεσιστανχε ρ εσιστανχε φ χαπαχιτανχε ορ ρεσιστανχε resistance for χονσταντφορ Τιµε Αϖ αγε ερ εξτερναλ φ εξτερναλ πορεσ, ορ φ πορεσ, ορ φ πορε, ορ external εξτ εξτερναλ πορε πορε χονσταντ πορε Πορε Πορ Πορε ε Ελ ψ ρελτιϖ εχτρολτε αε surface, συρφ Χδ , συρφ αχε, αχε, συρα φ χε, Ρσ , Χδ , φ πορε, Ρρ , ορ διαµετερ, δεπτη, δενσιτψ, χονδυχτιϖ , ερ ιτψ ρορ, 2 µΦχµ −2 εξτ 2 2 µΦχµ −2 τπορε, σ Ω χµ 2 −2 κ µΣ χµ −1 (%) Type of equivalent circuit Rs , Ω χµ τεξτ, µσ Ρρ , Ω χµ Ω χµ ext δ, νµ λ νµ ν, χµ , Ασ−δεποσ διαµονδ ιτεδ 85.2 12.9 1.10 Πυρε τρ ανσµισσιον λ ινε 140 − 60 500 1.0 ~ 107 15 µοδελ 4 Poretyp 30 0n e ?5 m 3 .5 5 29 29 .6 - 1.42 ~ 10 120 5 .5 38 - 30 50 2.8 ~ 1010 0.07 13.1 P type60 0 n ore ?5 0 m 213 60 9.16 - 71.0 140 5.15 - 60 50 1.0 ~ 107 0 15 9.75 3 Poretype4 n ?3mm 00 m 69 3 160 50.8 - 3 ~ 10 .20 20 3 4.75 - 40 0 3 0 4.8 ~ 108 00 180 8.94 16
  • 17. 19. 4. Electrochemical Properties of Pt−Modified Nanohoneycomb Diamond and Applications as a Size- Selective Sensor Materials Diamond possesses morphological stability at extreme anodic and cathodic potentials and corrosion resistance in both acidic and alkaline conditions, without any evidence of structural degradation [27]. Polycrystalline diamond is ideally suited as a current collector for batteries [28] or as an electrocatalyst support for fuel cells [29] and for electrosynthesis. Diamond, because of its extremely high packing density, is almost completely impervious to insertion of ions. In order to achieve high catalyst loadings and large surface areas, use of porous diamond supports is advantageous for applications in electrocatalysis. In this section, we report the use of conductive nanoporous honeycomb diamond as a support for Pt nanoparticles for electrocatalytic applications. In the present work, nanohoneycomb diamond electrodes with various pore diameters were modified with Pt nanoparticles and their size-selective electrocatalytic properties were studied. The catalytic activity and reaction kinetics for oxygen reduction and alcohol oxidation were found to be dependent on the pore dimensions. 19.4.1. Film characterization Scanning electron microscopy−Platinum nanoparticles were deposited in the pores of the diamond nano-honeycomb film using the following method. The nanohoneycomb films were immersed 17
  • 18. Α−α Α−β 3 3 µµ 600 νµ 600 nm Β−α B-a Β−β B-b 300 νµ 300 nm 300 νµ 300 nm Χ−α C-a Χ− Cβ -b 600 νµ 600 nm 600 νµ 600 nm Fig. 19.4.1. SEM images of Pt-modified highly boron-doped diamond electrodes: (A) top view for Pt-modified as- deposited diamond electrode at (a) low and (b) high magnification;. (a) top view; (b) oblique view at a 45° tilt angle for pore types (B) 60 × 500 nm, and (C) 400 nm × 3 µm. 18
  • 19. in a 73-mM H2PtCl6 aqueous solution for 8 hours. After immersion, the film was dried in air, and the Pt ions were reduced to the metal by a 3-h exposure to flowing H2 gas at 580 °°C. This process results in the incorporation of platinum nanoparticles on the external surface and on the pore walls. Figure 19.4.1 shows SEM images of three types of Pt- modified diamond films that were fabricated from as-deposited diamond and nanoporous diamond films. Figure 19.4.1A (a and b) shows images of the as-deposited diamond surface with dispersed Pt nanoparticles (as-deposited diamond / Pt ). The Pt nanoparticles, located mainly at the grain boundaries, have diameters from 10 to 150 nm. There are also very small Pt deposits (10-50 nm) on the grain surface. The two nanohoneycomb films (Fig. 19.4.1, B and C) are shown both as top views (left) and oblique views (right). The top views show highly uniform, well-ordered arrangements of holes, with a hexagonal close-packed pattern [26]. The Pt deposits are predominantly present in the pores rather than on the external surface, as seen by comparing the top and oblique views. The oblique views of the edges of the honeycomb films clearly show the well-defined cylindrical pores, with relatively large numbers of Pt deposits on the pore walls. In the SEM images of both nanohoneycomb / Pt films (honeycomb pore dimension type 60 × 500 nm / Pt and 400 nm × 3 µm / Pt), the homogeneous distribution of Pt nanoparticles on the inner walls of the honeycomb pores is clearly evident. For pore type 60 × 500 nm, due to the small pores, Pt deposits as small as 10 to 40 nm were 19
  • 20. obtained. In contrast, on the as-deposited diamond surface, the Pt deposits ranged up to 150 nm. Hence, honeycomb films provide better dispersion of Pt deposits. Table 19.4.1. Comparison of the number of exposed surface Pt atoms for Pt-modified as-deposited diamond and Pt- modified nano-honeycomb diamond electrodes Desorption of Number of Roughness hydrogen surface Pt atom Pt surface area -2 15 -2 2 factor / mC cm (geo.) / 10 cm (geo.) / cm (real) As-deposited diamond / Pt 3 0.61 3.77 2.88 Pore type 600~500 nm / Pt 10.9 1.89 11.8 9.02 Pore type 400 nmm µµ / Πτ ~3 15.9 2.84 17.8 13.6 Πολ χρψ ταλινε Π ψ σ λ τ 0.68 4.21 3.21 Background cyclic voltammetry−Background cyclic voltammograms were obtained in 1 M H2SO4 solution at a sweep rate of 50 mV s-1. The voltammetric features of Pt-modified diamond are characteristic of Pt metal, with Pt oxide formation in the +0.7 to +1.2 V region, the reduction of Pt oxide at ca. +0.5 V, and the adsorption and desorption of hydrogen between 0 and --0.18 V (not shown). Integration of the oxidation charge associated with the desorption of hydrogen between 0 and -0.18 V yielded a value of 1.89 mC cm-2 for pore type 60 × 500 nm / Pt. This charge can be used to calculate the number of exposed surface Pt atoms, which was estimated to be 1.18 × 1016 cm-2 (geometric area) using a standard value of 210 mC cm-2, which corresponds to a calculated value of 1.30 × 1015 atoms cm-2 for polycrystalline Pt. The values determined for the diamond / Pt 20
  • 21. and polycrystalline Pt from the cyclic voltammograms are summarized in Table 19.4.1. Interestingly, the number of exposed surface Pt atoms per unit geometric area observed on the as- deposited diamond / Pt was close (ca. 90%) to that for polycrystalline Pt. Thus, this as-deposited / Pt film was expected to exhibit similar electrocatalytic activity compared to Pt metal. However, this was not the case, as discussed later. 19. 4. 2. Electrocatalysis with Pt-modified diamond: cyclic voltammetry 2 - cm 0.4 mA (A) 0.0 -0.4 -0.8 Current density / -1.2 -0.5 0.0 0.5 1.0 1.5 Potential / V vs. Ag/AgCl 2 - cm 1.0 mA 0.5 (B) 0.0 -0.5 -1.0 -1.5 Current density / -2.0 -0.5 0.0 0.5 1.0 1.5 Potential / V vs. Ag/AgCl Fig. 19.4.2. Cyclic voltammograms for (A) as-deposited diamond before Pt deposition (dotted line), after Pt deposition (solid line) and (B) Pt-nanoparticle-filled nano- honeycomb 60 × 500 nm (dot-dashed line), 400 nm × 3 µm 21
  • 22. (solid line); e. Electrolyte,: oxygen-saturated 1 M H 2 SO 4 ; sweep rate,: 50 mV s -1 ; geometric surface area,: 0.071 cm 2 . 2 - cm 1.2 mA (A) 0.8 0.4 0.0 Current density / -0.4 -0.5 0.0 0.5 1.0 1.5 Potential / V vs. Ag/AgCl 2 - 10 cm 8 (B) mA 6 4 2 0 Current density / -2 -0.5 0.0 0.5 1.0 1.5 Potential / V vs. Ag/AgCl Fig. 19.4.3. Cyclic voltammograms for (A) as-deposited diamond before Pt deposition (dotted line), after Pt deposition (solid line) and (B) Pt-nanoparticle-filled nano- honeycomb 60 × 500 nm (dot-dashed line), 400 nm × 3 µm (solid line); e. Electrolyte,: 2 M methanol + 1 M H 2 SO 4 ; sweep rate,: 50 mV s -1 ; geometric surface area,: 0.071 cm 2 . 22
  • 23. The effectiveness of the Pt-modified diamond electrodes for the electrocatalysis of fuel cell reactions was examined. We have tested their electrocatalytic activities for O2 reduction and alcohol oxidation. Figure 19.4.2 compares O2 reduction currents for the as-deposited diamond, the as-deposited diamond / Pt, honeycomb 60 × 500 nm / Pt and the 400 nm × 3 µm / Pt electrodes in 1 M H2SO4 saturated with oxygen at a sweep rate 50 mV s-1. In the absence of the Pt nanoparticles, essentially no O2 reduction is observed over this potential range, as diamond is known to have low catalytic activity for O2 reduction. In contrast, at diamond / Pt composite electrodes, large O2 reduction current is observed at potentials characteristic for Pt electrocatalysis in this solution. The cathodic current density for the 400 nm × 3 µm / Pt electrode (ca. -1.8 mA cm-2, geometric) was nearly twice as large as that for the as-deposited diamond / Pt electrode (ca. -1.0 mA cm-2, geometric), and this is due to the high surface area. Based on the number of surface Pt atoms per unit geometric area, which was ca. five times greater for pore type 400 nm × 3 µm film than for the as-deposited diamond, a similar factor could be possible for the peak current, but this is clearly not expected, due to mass transport limitations. For methanol oxidation (2 M in 1 M H2SO4), cyclic voltammograms were obtained at the as-deposited diamond, the as-deposited diamond / Pt, 60 × 500 nm / Pt and the 400 nm × 3 µm / Pt electrodes (Fig. 19.4.3). At an as-deposited diamond film, no methanol oxidation was observed; diamond is known to have 23
  • 24. low activity for methanol oxidation. In the case of the nonporous diamond / Pt electrode, a large anodic peak was observed at ca. 0.9 V, attributable to methanol oxidation. The Pt-containing film is known to be electroactive for methanol electrooxidation [30, 31]. The Pt nanoparticles supported on the diamond electrode provide the catalytic activity for methanol oxidation in acid solution. The oxidation current for the 400 nm × 3 µm / Pt electrode (ca. 7.0 mA cm-2, geometric) was greatly enhanced compared to the as- deposited diamond / Pt (ca. 1.1 mA cm-2, geometric) and was found 2 - cm 20 (A) mA 15 10 5 0 Current density / -0.5 0.0 0.5 1.0 1.5 Potential / V vs. Ag/AgCl 2 - cm 0.8 mA (B) 0.4 0.0 -0.4 Current density / -0.8 -0.5 0.0 0.5 1.0 Potential / V vs. Ag/AgCl to be ca. 16 times higher than that for the Pt polycrystalline electrode (ca. 0.44 mA cm-2, geometric) (Fig. 19. 4. 3). 24
  • 25. Fig. 19.4.4. Cyclic voltammograms for (A) ethanol oxidation and (B) 2-propanol oxidation for as-deposited diamond / Pt (dotted line), Pt-nanoparticle-filled nano- honeycomb 60 × 500 nm (dot-dashed line), and 400 nm × 3µm (solid line); e. Electrolyte,: 2 M ethanol or 2 M 2- propanol + 1 M H 2 SO 4 ; sweep rate,: 50 mV s -1 ; geometric surface area,: 0.071 cm 2 . At the as-deposited diamond / Pt electrode, the peak current is proportional to the square root of the scan rate, indicating that the oxidation of methanol at this electrode is controlled by diffusion. In contrast, at both the 60 × 500 nm /Pt and the 400 nm × 3 µm / Pt electrodes, the current densities deviate from the linear curve at higher sweep rates. This behavior is thought to be caused by the nanoporous structure effect for methanol mass transport inside the pores. This effect is expected to be dependent on the size of the reacting molecules. Therefore, the oxidation reactions of larger size alcohols were also investigated. For example, ethanol oxidation was examined. Cyclic voltammograms were obtained for the as-deposited diamond / Pt, 60 × 500 nm /Pt and 400 nm × 3 µm / Pt electrodes in 2 M ethanol in 1 M H2SO4 (Fig. 19. 4. 4A). The Pt-modified diamond electrodes show elecrocatalysis for ethanol oxidation [32]. It can be seen that the oxidation current for pore type 400 nm × 3 µm / Pt was ca. 4 times higher than that for as-deposited diamond / Pt, but the 25
  • 26. oxidation current for pore type 60 × 500 nm / Pt was suppressed, being only ca. 0.6 times of that for as-deposited diamond / Pt. The expected current enhancement due to the nanohoneycomb roughness was not observed for this pore type. In addition, 2-propanol oxidation was examined. Figure 19.4.4B shows cyclic voltammograms obtained for as-deposited diamond / Pt, 60 × 500 nm /Pt and 400 nm × 3 µm / Pt electrodes in 2 M 2-propanol in 1 M H2SO4. In this case [33], it can be seen that the oxidation currents for as-deposited diamond / Pt, 60 × 500 nm / Pt and 400 nm × 3 mm / Pt electrodes are all similar, and therefore, there was no enhancement due to the honeycomb roughness for either nanohoneycomb / Pt electrode. In order to better illustrate the nanostructure effect for the electrocatalytic reactions examined here, peak current ratios were used. (Figure 19.4.5) These values (Rp) are the ratios of the peak current densities for the honeycomb diamond / Pt electrodes (Iph) to that for the as-deposited diamond / Pt (Ipa), normalized by the ratio of number of surface Pt atoms exposed (Nh and Na, respectively) using the formula Rp = (Iph / Ipa) × (Na / Nh). This could be considered to be an indicator of the fraction of surface Pt atoms that are actually actively involved in the electrocatalytic reaction. In the case of methanol oxidation, at both honeycomb diamond / Pt electrodes, approximately all of the surface Pt atoms appear to be available for the catalytic reaction. 1.4 1.2 1 0.8 0.6 0.4 Peak Current Ratio 0.2 0 Methanol Ethanol 2-Propanol Oxygen M e t h a n o l E t h a n o l 2 - P r o p a n o l O x y g e n Oxidation Oxidation Oxidation Reduction O x i d a t i o n O x i d a t i o n O x i d a t i o n R e d u c t i o n 26
  • 27. Fig. 19.4.5. Relationships of peak current ratios for oxygen reduction and alcohol oxidation for Pt- nanoparticles-filled nano-honeycomb (□) 60 ⋅ 500 nm and (○) 400 nm × 3 µm electrodes. The peak current ratio is defined as the ratio of the peak current density for the honeycomb / Pt to that for the as-deposited diamond /Pt and normalized by the ratio of the number of surface Pt atoms exposed. In contrast, for ethanol oxidation, the apparent fraction of active Pt atoms for 60 × 500 nm / Pt was only 0.2, which is three times lower than that for pore type 400 nm × 3 µm / Pt. This result indicates that there is a limitation on the ability of the ethanol molecules to access the Pt atoms located within the 60-nm pores. This effect is even more evident for the larger molecule, 2- propanol, which yielded an active Pt atom ratio of 0.1, even for 400 nm × 3 µm / Pt. These results clearly indicate an effect of molecular size for the honeycomb / Pt electrodes for the catalytic oxidation of alcohols. The electrocatalytic activities of the Pt- modified nanohoneycomb films were found to be dependent on the structural parameters of the honeycomb pores and the molecular sizes of the alcohols, indicating that the selectivity of the electrodes can be controlled by variation of the pore dimensions. Both nanohoneycomb / Pt electrodes showed high electrocatalytic activity for oxygen reduction and methanol oxidation. Hence, these electrodes have potential application in fuel cell development. 27
  • 28. 19.4.3. Electrocatalysis with Pt-modified diamond: impedance measurements In order to understand the characteristics of the electrocatalysis reaction inside the nanoporous electrodes, additional analysis of the ac impedance behaviour was carried out, and the penetration depths of reactant molecules in the nanohoneycomb pores for catalytic reactions and the reaction parameters for different pore structures were estimated. The ac impedance measurements for Pt-modified diamond electrodes 28
  • 29. 2 χµ 2.0 Ω (Α) 0.3 1 3 1 0.1 100 κΗζ 0.5 0.25 0.1 0.020 0.020 1 100 κΗζ Ζ100/κΗζ 10 0.1 0.0 0.3 0.6 0.9 0.05 Ιµ − 0.01 0.0 0.0 2.0 4.0 Ρ ε Ζ / 103 Ω χµ 2 2 χµ 0.6 Ω (Β) 0.5 3 1 0.1 0.05 100 κΗζ 1 0.5 Ζ / 10 0.1 0.02 1 0.1 0.02 Ιµ − 100 κΗζ 0.0 0.0 0.6 1.2 1.8 Ρ ε Ζ / 103 Ω χµ 2 2 χµ 1.3 Ω (Χ) 0.1 0.05 0.01 3 100 κΗζ 0.5 Ζ / 10 1 0.5 0.1 Ιµ − 0.05 0.01 0.0 100 κΗζ 0.0 1.3 2.6 3.9 Ρε Ζ Re Z / 103 Ω χµ 2 Fig. 19.4.6. Impedance plots for electrocatalytic reaction s: (A) the oxidation of methanol at 0.9 V vs. Ag/AgCl; (B) the oxidation of ethanol at 0.9 V vs. Ag/AgCl; and (C) the reduction of oxygen at 0.4 V vs. Ag/AgCl. Experimental data points are shown as open symbols for ( △ ) as-deposited diamond / Pt, (□) 60 × 500 nm / Pt, and (○) 400 nm × 3 µm/ Pt. The simulated curves, calculated on the basis of the equivalent circuit in Fig. 19.3.4, are shown as solid lines. The parameters are summarized in Tables 19.4.2 and 3. 29
  • 30. during catalytic reactions were carried out at the peak potentials obtained in the CV measurements (Figures 19.4.6A-C). The impedance plots for methanol oxidation (Figures 19.4.6A) consist mainly of parallel RC-type semicircles whose diameters (and thus the corresponding resistances) decrease with increasing roughness factor (ca. 3 for as-deposited diamond, 10.9 for the 60-nm pores and 15.9 for the 400-nm pores). The diameters of the semicircles (Ω cm2, based on the geometric area) decreased in order for the as- deposited diamond / Pt, pore type 60 × 500 nm / Pt, and pore type 400 nm × 3 µm / Pt, roughly estimated to be 2.9 × 103 Ω cm2, 6.5 × 102 Ω cm2, and 5.0 × 102 Ω cm2, respectively. These can be related to the charge transfer resistances (discussed later in detail), which decrease with increasing effective surface area for the charge transfer reaction. In contrast, for ethanol oxidation and oxygen reduction (Figures 19.4.6B and C) the diameters no longer follow the same order as the roughness. The impedance plots for the pore type 60 × 500 nm / Pt electrode trace the largest semicircles for both ethanol oxidation and oxygen reduction. The charge- transfer resistance per unit area for pore type 60 × 500 nm / Pt is now larger than that for pore type 400 nm × 3 µm / Pt due to a mass transfer effect, as discussed later. The impedance of a porous electrode can be simulated with the transmission line model, and the penetration depth can be evaluated [24]. For the non-porous Pt-modified as-deposited surface, the methanol oxidation reaction can be simulated as a 30
  • 31. simple Randles equivalent circuit comprising a parallel combination of a double layer capacitance and a semi-infinite Warburg impedance in series with a charge transfer resistance. For oxygen reduction, a simple Randles equivalent circuit was also used, because the reaction mechanism for oxygen reduction for the Pt electrode can be described by mass transport-controlled kinetics. The simulated curves are shown in Figs. 19.4.6(A-C). The fits are reasonably good, with the charge-transfer resistances (based on geometric area) Rr values shown in Table 19.4.2. Table 19.4.2. Parameters used for fitting the impedance results for an as-deposited diamond / Pt electrode in the impedance plots (Fig. 19.4.6), based on the Randles circuit. Series ∆ ιφ ερεντιαλ φ Reaction Diffusion resistance χαπαχιτανχε resistance resistance Electroatalytic reaction Rs / Ω χµ 2 Χδ / µ Φχµ −2 2 R r / W cm d / W cm2 3 Methanol Oxidation 38.2 70 1.90 ? 10 4.60? 10 2 Ethanol Oxidation 50.2 90 3.60 ? 10 2 1.45 ? 10 2 Oxygen Reduction 28.2 80 9.00 ? 10 2 3.55 ? 10 2 The impedance of a charge-transfer reaction at a porous electrode consisting of cylindrical pores is given in the previous section by Eq. (19.3.1) [23-26]. To simplify the calculations, the Faradaic impedance per unit real surface area was assumed to be potential-independent over a range of values that would exist along the entire pore, e. g., < 0.25 V, and thus consists only of a parallel combination of the charge transfer resistance and a double-layer capacitance, without a Warburg impedance. 31
  • 32. For a charge transfer-controlled process, 1/Z = 1/R ct + jωC, and the reaction resistance Rct and capacitance C per unit pore depth are expressed by Rrpore/(2πr) and jω2πrnCdpore, respectively. Here, Rrpore is the charge transfer resistance with respect to the real surface area on the pore walls. A distinction between R r and Rrpore has been made, because we wish to apply the latter specifically to the pores only, where most of the Pt particles are located. From the geometric parameters of the cylindrical pores (i.e., diameter, depth and number density), which are obtainable from SEM observation, the impedance can be evaluated. Table 19.4.3. Parameters used for fitting the impedance results in the impedance plots (Fig. 19.4.6), based on the equivalent circuit in Fig. 19.3.4. Series resistance ∆ιφ ερεντιαλ φ for χαπαχιτανχε Σεριεσ ∆ιφ ερεντια Reaction φ λ external φ εξτερναλ ορ ρεσιστανχε χαπαχιτανχε resistance Electrolyte Penetration Average surface συρφαχε φ πορε ορ φ πορε ορ for pore conductivity πορε πορε depth relative ext ε ξτ Ρσ Χδ pore Rs Χδ Rr k l error / Ω χµ / µ Φ χµ / Ω χµ / µ Φ χµ 2 −2 2 −2 2 -1 Type of Electrodes / W cm / mS cm / mm /% Pure transmission line 166 1.80 ? 10 3 80 3 Methanol 60 ? 500 nm / Pt 400 120 400 166 1.80 ? 10 80 0.46 3.02 Oxidation 400 nm ? 3 mm / Pt 130 600 200 900 4.50 ? 10 3 162 2.69 6.75 3 Pure transmission line 200 3.50 ? 10 0.7 Ethanol 60 ? 500 nm / Pt 350 100 350 200 3.50 ? 10 3 0.7 0.19 22.48 Oxidation 400 nm ? 3 mm / Pt 120 480 180 820 1.40 ? 10 3 830 2.87 12.09 4 Pure transmission line 400 1.80 ? 10 0.5 4 Oxygen 60 ? 500 nm / Pt 400 240 400 400 1.80 ? 10 0.5 0.36 8.76 Reduction 400 nm ? 3 mm / Pt 140 650 100 260 6.20 ? 10 3 100 2.49 8.01 The calculated impedance curves for the honeycomb electrodes are shown in Fig. 19.4.6(A-C) together with the experimental curves. Figure 19.3.4 shows the equivalent circuit 32
  • 33. employed to simulate the impedance plots for the honeycomb / Pt electrodes. Table 19.4.3 summarizes the values of the fitting parameters and the average relative errors for the calculated curves. For 60 × 500 nm / Pt, the electrolyte conductivity in the pores of 80 mS cm-1, measured for methanol oxidation, decreased to that for ethanol oxidation (0.7 mS cm-1, Table 19.4.3). This result suggests that the conductivity ofassociated with the alcohol molecule in the 60-nm nanohoneycomb pores decreases with increasing molecular size. By use of the transmission line model, the penetration depth for the reaction can be calculated. The penetration depth is defined in previous section by Eq. (19.4.1) [24]. λ = |Zt|1/2 R-1/2 sec 1/2ϕ (19.4.1) where |Zt| and ϕ are the amplitude and the phase angle for the impedance of the transmission part, respectively. Table 19.4.3 summarizes the penetration depths for the various catalytic reactions. For pore type 400 nm × 3 µm / Pt, the penetration depths λ for all of the catalytic reactions were close to the actual pore depth of 3 µm, with the lowest value being 2.49 µm for O2 reduction (ca. 80 % of the pore depth). For pore type 60 × 500 nm / Pt, with 460 nm for methanol oxidation, λ was also close to the actual depth, indicating that almost all of the pore surface is available. In contrast, the λ value for ethanol oxidation was 190 nm, which is only 40 % of the total pore depth. This result suggests that pore type 60 × 500 nm is sensitive to the size of the alcohol molecule, so that λ decreases with increasing reactant size. 33
  • 34. For O2 reduction, λ decreases due to its low concentration. However, even so, half of the pore depth for type 60 × 500 nm was still available for ethanol oxidation and oxygen reduction. It is interesting to note that the charge transfer resistances Rr calculated for methanol oxidation for the Pt- modified diamond electrodes (ca. 1.8 - 4.5 kΩ cm2) are of the same order. In contrast, the Rr values calculated for ethanol oxidation and oxygen reduction for the honeycomb / Pt electrodes are significantly larger than that for as-deposited diamond / Pt. Also, an increase in Rr is observed with decreasing pore size. A possible explanation for the increase of the reaction resistance could be the relatively low concentration of the reactant near the active catalytic sites because of the limitation of mass transport by the nanoporous structure. In order to clarify the contribution of the ethanol concentration to the Rr values, we have examined the concentration dependence of the impedance behavior. A series of impedance plots for the Pt-modified as- deposited diamond electrode in C2H5OH + 1 M H2SO4 solution were obtained (not shown). The ethanol was varied in concentration from 0.02 to 2 M. The fact that the R r value (1.0 kΩ cm2) obtained for 0.2 M ethanol for the as-deposited diamond / Pt electrode is close to the value (1.4 kΩ cm2) for 400 nm × 3 µm / Pt in 2 M ethanol (Table 19.4.3) indicates that the ethanol concentration in the pores for the latter is one order of magnitude less than that in the bulk. Similarly, for pore type 60 × 500 nm / Pt, the Rr value (3.5 kΩ cm2) was of the same order as that for as- deposited diamond / Pt (2.8 kΩ cm2) in 0.02 M ethanol. This result 34
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