Electrochemical study of the PdO nanoflake thin film

Figure 5-1(a) shows the SEM image of as-deposited PdO nanoflakes on a carbon fiber selected from the carbon cloth shown in the inset. The uniform PdO thin film sputter-deposited on the carbon cloth is composed of continuously interconnected flake-like nanostructures as shown by the magnified SEM image in Fig. 5-1(b). The PdO nanoflakes are very thin with a thickness of ~15-20 nm according to the SEM image, and XRD analysis (not shown) indicates that the room-temperature deposited PdO thin film is amorphous. The nanoflake-like morphology of the as-deposited PdO thin film results in a large surface area, providing enormous electrochemical active sites. Since PdO is a p -type semiconductor and has a low electric resistivity, the PdO nanoflake thin film imposes little

Figure 5-1 SEM images of (a) the as-deposited PdO nanoflakes on a carbon fiber selected from the carbon cloth shown in the inset; (b) a magnified SEM image of PdO nanoflakes deposited on the carbon fiber.

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difficulty in the electrochemical measurement and the electrodeposition of Pt nanoparticles.

To study the electrocatalytic activity of the PdO nanoflake thin film toward MOR in acidic media, we first performed CV measurements for the thin film in an aqueous solution of 0.5 M H2SO4, and the CV result is presented in Fig. 5-2. The CV measurement has a scan potential range between -0.2 − 0.9 V vs. SCE with the scan rate at 20 mV s^-1. The cyclic voltammogram shows that, after the first cycle scan, the PdO thin film exhibits a CV behavior characteristic for metallic Pd. To examine if metallic Pd was electrochemically formed on the PdO nanoflake thin film in the acidic solution, we studied chemical states present on the PdO thin film by XPS. Figure 5-3 shows Pd 3d XPS spectra of the PdO thin film before and after the CV measurement. The dashed lines in the figure mark the binding energies of the Pd(0), Pd(II) and Pd(IV) states, which are referred to the literature [96-98]. The as-deposited nanoflake thin film has the Pd 3d5/2 peak maximum at 336.5 eV, suggesting that the PdO and/or Pd(OH)₂ is the primary chemical states on the film surface.

Because the Pd 3d doublet has a very broad peak width, PdO2 is likely also present in the as-deposited film. When the PdO thin film is polarized at 0.1 V in the 0.5 M H₂SO₄

Figure 5-2 The Cyclic voltammogram of the PdO nanoflake thin film in the aqueous solution of 0.5 M H₂SO₄. The scan rate was 20 mV s^-1.

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solution for 30 min, the film surface is reduced to the metallic Pd state as shown by the dramatic change in the shape of the Pd 3d peak profile, which has a clear signal component situated at 335.2 eV. After the first cycle of the CV scan, the Pd 3d3/2 peak is narrowed with the peak maximum at 335.2 eV, indicating that the PdO thin film is extensively reduced. The reduced thin film has the Pd 3d doublet peak intensity slightly higher on the high binding energy side than a metallic Pd thin film, which was sputter-deposited on the carbon cloth for comparison; this suggests that the PdO and Pd(OH)2 states still contribute Pd 3d signal to the XPS spectrum of the reduced PdO electrode. Because of the detection of the Pd(II) state, the metallic Pd surface layer electrochemically formed on the PdO thin film has a thickness less than the probe depth of the Pd 3d photoelectron, which is about 5 nm when the Mg Kα line is used as the x-ray excitation source. As a consequence, the metallic Pd surface layer must decisively govern the CV behavior of the PdO nanoflake thin film after the first CV cycle.

Palladium has CV responses in acidic media similar to Pt [43, 80, 99]. In a cyclic voltammogram of metallic Pd in the H2SO4 solution, three pairs of CV peaks at potentials below 0.9 V are observed; the three peak pairs are associated with three redox reactions:

hydrogen adsorption/desorption, hydrogen absorption/evolution and Pd oxidation/ PdO reduction. The anodic and cathodic peaks below 0 V vs. SCE are due to hydrogen desorption and adsorption reactions, respectively, on metallic Pd. Metallic Pd has a great hydrogen absorption capability; the cathodic and anodic peaks due to the hydrogen absorption/evolution reactions may span a large potential range (-0.1− 0.3 V vs. SCE) depending on the thickness of the metallic Pd layer, and overlap the peaks due to hydrogen adsorption/desorption reactions [100-102]. However, because the metallic Pd surface layer formed on the PdO nanoflake electrode is very thin as discussed above, the hydrogen absorption/evolution reactions must have a very small contribution to the CV peaks at potentials below 0.02 V. The anodic wave beginning around 0.45 V results from the

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oxidation of metallic Pd during the positive sweep. Since the CV scan limit (0.9 V vs.

SCE) is far below the potential required for the formation of PdO2 (1.4 V) [43, 93, 103], the Pd(II) state must be the primary oxidation state on the PdO thin film in the potential range between 0.45 and 0.9 V. Electro-oxidation of metal Pd in acidic media is proposed to proceed via the following reactions [43, 93, 98]:

Pd + H2O → PdO + 2H⁺ + 2e⁻ ( 5 - 1 ) Pd + 2H₂O → Pd(OH)2 + 2H⁺ + 2e⁻ ( 5 - 2 ) The cathodic peak between 0.15−0.45 V in the reverse sweep is due to the reduction of the surface oxide and hydroxide, which are electrochemically formed during the preceding positive sweep. As to be discussed later, the presence of the hydroxide adspecies is important to the enhancement in the electrocatalytic activity of Pt nanoparticles toward MOR, which were electrodeposited on the PdO thin film.

Figure 5-3 Pd (3d) XPS spectra of the PdO nanoflake thin film: (a) the as-deposited PdO thin film; (b) the PdO thin film polarized at 0.1 V for 30 min in the 0.5 M H₂SO₄ solution; (c) the PdO thin film after one CV scan cycle; and (d) a metallic Pd thin film deposited on the carbon cloth and cleaned with Ar sputtering for 400 sec. The dashed lines mark the binding energies of the Pd(0), Pd(II) and Pd(IV) states, which are referred to the literature.

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Previous study shows that Pd has an electrocatalytic behavior similar to Pt for electro-oxidation of methanol, ethanol and formic acid in acidic or alkaline media [104-106]. Figure 5-4 shows a cyclic voltammogram of the PdO thin film in an aqueous solution of 1 M CH3OH + 0.5 M H2SO4. After the first scan cycle, the CV profile of the PdO thin film is similar to that of a Pt electrode. We have performed a CV measurement in the potential range between 0.4 – 0.9 V, where PdO reduction is unimportant, and the result shown in the inset of Fig. 5-4 indicates that PdO has little electrocatalytic activity toward MOR. Therefore the anodic peak situated at 0.58 V is due to MOR on the metallic Pd electrode only. Metal Pd sites are produced on the nanoflake thin film as a result of the PdO reduction during the potential sweep from -0.2 to 0.45 V. Methanol chemisorption may then take place on these metal Pd sites, and MOR starts around 0.33 V. The MOR current density increases with the CV cycle number, indicating that the repetitive PdO reduction and Pd oxidation during the CV scan increase Pd electrocatalytic sites on the PdO

Figure 5-4 The cyclic voltammogram of the PdO thin film in the aqueous solution of 1 M CH3OH + 0.5 M H2SO4. The inset shows a CV curve with a scan range of 0.4 – 0.9 V at the tenth scan cycle. The scan rate was 20 mV s^-1.

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thin film. The MOR current density increases by a factor of 1.54 after 10 cycles of the CV scan compared with the first CV cycle.

Figure 5-5 Schematic illustration of the electrochemical reaction steps for the PdO thin film in the aqueous solution of 1 M CH₃OH + 0.5 M H₂SO₄; (1) formation of a metallic Pd surface layer on the PdO thin film at potentials below 0.33 V; (2) electro-oxidation of chemisorbed methanol starts around 0.33V accompanied with carbonaceous residue formation; (3) free Pd sites are oxidized at potentials above 0.45 V; (4) the MOR on the PdO thin film is completed at 0.8 V; (5) the carbonaceous adspecies are oxidized at potentials below 0.6 V in the reverse CV sweep; (6) the PdO reduction begins at ~0.45 V; (7) more methanol molecules are chemisorbed on the electrode; (8) hydrogen adsorption and desorption take place at potentials below 0 V.

The dimension of the block representing the carbon residue denotes the amount of the poisoned sites.

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In the reverse sweep, an anodic peak starts at 0.65 V with the peak maximum at 0.46 V.

For Pt electrodes, the anodic peak in the reverse sweep is generally ascribed to oxidation of carbonaceous adspecies, which are incompletely oxidized intermediates formed in the forward sweep [20, 34, 39, 107]. However, some previous studies proposed that the anodic peak in the reverse scan resulted from methanol oxidation on metal Pt sites, which were reactivated during the reverse sweep from the upper limit potential of the CV scan [88, 108-109]. Because the PdO electrode has little electrocatalytic activity toward MOR between 0.4 – 0.9 V as discussed above, the large anodic current density measured in the potential range of 0.4 – 0.65 V (in the reverse sweep) is unlikely due to methanol oxidation on reactivated metal Pd sites, which should be absent on the PdO electrode at potentials above 0.45 V in the acidic solution. Therefore, we believe that the anodic peak in the reverse CV sweep is a result of oxidation of carbonaceous adspecies on the PdO electrode.

Accompanied with the anodic peak is a shoulder peak with an obvious peak maximum around 0.33 V, which is close to the peak potential of the PdO reduction (Fig. 5-2). Note that the MOR anodic peak has a shoulder feature as well; the shoulder peak is especially obvious for the tenth CV cycle. The MOR shoulder peak is situated around 0.45 V, which is around the onset potential of the Pd oxidation on the nanoflake thin film. Therefore the observation of the shoulder features for both the forward and the reverse anodic peaks is likely correlated with the Pd/ PdO redox reaction on the PdO electrode in the acidic solution.

For clarity, we schematically summarize in Fig.5-5 the electrochemical reaction steps for the PdO thin film in the aqueous solution of 1 M CH₃OH and 0.5 M H₂SO₄. First, a metallic Pd surface layer is formed on the nanoflake thin film after repetitive CV scans as a result of the PdO reduction at potentials below 0.45 V vs. SCE (step 1). The Pd surface layer has a thickness smaller than 5 nm. Like on a Pt catalyst, dissociative chemisorption of methanol may occur on the metallic Pd surface layer before the onset potential of MOR.

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When the electro-oxidation of chemisorbed methanol starts around 0.33V, some metal Pd sites become free from methanol adspecies and some sites may be blocked by carbonaceous residues due to incompletely oxidized MOR intermediates (step 2). The carbonaceous residues are likely CO adspecies with the linear (i.e., atop) and bridge adsorption structures [110-112]. Free Pd sites are oxidized at potentials above 0.45 V (step 3), and the thus formed PdO and/or Pd(OH)₂ may modify the electronic and chemical structures of neighboring metal Pd sites that are still occupied by methanol adspecies. The modifications may improve the electrocatalytic activity of Pd toward MOR via the electronic effect and the bi-functional mechanism [20, 23]. The change in the MOR activity is likely the cause resulting in the shoulder on the low potential side of the anodic MOR peak. The MOR on the nanoflake thin film is completed at 0.8 V, and metal Pd sites on the electrode surface are either oxidized or covered by carbonaceous residues (step 4), which can be oxidized at potentials below 0.6 V in the reverse CV sweep (step 5).

Because the PdO reduction is suppressed on the nanoflake thin film at potentials above 0.45 V, free Pd sites created as a result of the oxidation of carbonaceous residues will be immediately oxidized in the potential range of 0.45 - 0.6 V. When PdO reduction begins at ~0.45 V, free metal Pd sites are produced (step 6). The chemical phase transition from PdO to metal Pd on the electrode surface may modify the chemical and electronic structure of the area surrounding carbonaceous adspecies, and thus alter the electro-oxidation rate of the carbonaceous adspecies. Moreover, free metal Pd sites can facilitate the transformation of a neighboring atop-CO adspecies to a bridge-CO adspecies, which has been proposed to be an important reaction step for the CO electro-oxidation on Pt electrodes [113], resulting in an improved CO removal rate on the PdO electrode.

Therefore, the formation of free Pd sites due to the PdO reduction is a probable cause of the development of the shoulder near the reduction peak potential (0.33 V in Fig. 5-2). The free Pd sites also allow the dissociative chemisorption of methanol on the PdO electrode in

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the potential range of step 6. More methanol molecules can be chemisorbed on the electrode in the potential range between 0 V and 0.2 V, in which surface oxides are completely removed (step 7). Hydrogen adsorption is activated on Pd sites that are unoccupied by methanol adspecies at potentials below 0 V (step 8), and hydrogen adatoms are desorbed when the CV scan is reversed to the forward direction.