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Scanning Electrochemical Microscopy Study of H2O2 Byproduct during O2 Reduction at Pt/C-Nafion Composite Cathode Akira Kishi, Mitsuhiro Inoue, and Minoru Umeda* Department of Materials Science and Technology, Faculty of Engineering, Nagaoka UniVersity of Technology, Kamitomioka 1603-1, Nagaoka, Niigata 940-2188, Japan ReceiVed: September 17, 2009; ReVised Manuscript ReceiVed: NoVember 27, 2009
In this study, we investigated the effect of a Nafion ionomer on H2O2 byproduct generation during O2 reduction at a Pt/C-Nafion composite electrode, which is used as the cathode of practical polymer electrolyte fuel cells. The generated H2O2 was detected by scanning electrochemical microscopy (SECM). First, O2 reduction voltammetry was conducted using a rotating ring-disk electrode (RRDE) with the Nafion-coated Pt disk electrode. The results show that the diffusion-limited current density of O2 reduction and the amount of H2O2 generated decrease with increasing thickness of the Nafion coating layer. A decrease in diffusion-limited current density is also obtained using Nafion-coated Pt microdisk electrodes. However, SECM measurement using these Pt microdisk electrodes as a generator indicates that increasing the thickness of the layer suppresses the amount of H2O2 generated, despite the amount of O2 consumption slightly decreasing. Subsequently, another SECM measurement was carried out using a porous microelectrode packed with the Pt/C-Nafion composite having 0, 13.4, or 28.6 wt % Nafion ionomer. The obtained results demonstrate that an increase in the Nafion ionomer content leads to a drastic decrease in the amount of generated H2O2 without a decrease in O2 consumption. In a semiquantitative analysis of H2O2 generation during O2 reduction, the smallest percentage of H2O2 generated is obtained for the Pt/C-Nafion composite with 28.6 wt % Nafion ionomer (0.1%). According to this result, increasing the content of the Nafion ionomer in the Pt/C-Nafion composite is highly effective for suppressing the generation of H2O2. 1. Introduction For the cathode catalyst of polymer electrolyte fuel cells (PEFCs), Pt and Pt-based alloys are commonly used for O2 reduction.1,2 The reduction of O2 at the Pt electrode generates H2O; however, it is accompanied by the production of a small amount of H2O2 as a byproduct.3–6 The generated H2O2 has been reported to degrade not only the polymer electrolyte membrane7–10 but also the Pt used for the electrocatalyst.11 Therefore, it is worthwhile investigating in detail the H2O2 generation mechanism at the Pt electrocatalyst with the aim of preventing the degradation of PEFC cell performance. For this reason, we have focused our attention on a composite of Nafion ionomer and Pt-supported carbon (Pt/C), which is generally used at the PEFC cathode. In the present study, the amount of H2O2 generated during O2 reduction was evaluated by scanning electrochemical microscopy (SECM), while varying the composition ratio of Nafion ionomer and Pt/C. When O2 is reduced in an aqueous solution, it is well-known that O2 adsorbs on Pt then is reduced via a 4-electron reaction (“direct 4e- pathway”).4–6 A 2-electron reaction also simultaneously occurs (“series 2e- reduction”). The thus-generated H2O2 diffuses into the electrolyte or is reduced to H2O by a second 2-electron reaction (“series 4e- reduction”). The generation of H2O2 frequently occurs in a negative potential region.3–5,12,13 It has also been reported that H2O2 is generated in solidstate PEFCs,11 which leads to the degradation of the materials used in PEFCs.7–11 For example, the Nafion used as a polymer electrolyte membrane is degraded by the decomposition of its side chain and main chain, which is induced by the attack of * Corresponding author. E-mail:
[email protected]. Tel/Fax: +81-258-47-9323.
• OH and •OOH radicals generated from H2O2.7–10 The presence of H2O2 causes Pt dissolution from the cathode catalyst,11 resulting in a decrease in the Pt electrochemical surface area and the deposition of Pt particles in the polymer electrolyte membrane.11,14–16 To minimize the above types of degradation, various attempts to suppress the generation of H2O2 have been made, for instance, the reduction of the amount of impurities included in the electrocatalyst and in gases supplied to the PEFCs17 and the development of a novel electrocatalyst that does not cause H2O2 generation.18–20 As mentioned above, the electrode layers in the PEFCs are composed of a Pt/C-Nafion composite, which is prepared by mixing the Pt/C electrocatalyst and Nafion ionomer. On the basis of the electrode structure, the effects of the Nafion ionomer on H2O2 generation during O2 reduction at a Pt plate21–23 or a Pt/C electrocatalyst13,24,25 have been investigated. Particulate catalysts within polymers have also been investigated by SECM.26 However, the Pt electrodes used in these studies were prepared by covering the Pt plate or Pt/C electrocatalyst loaded on a carbon disk electrode with a Nafion ionomer layer, which has a different structure from that of the above-mentioned Pt/CNafion composite. Therefore, to clarify the effect of the Nafion ionomer on H2O2 generation in practical PEFCs, the amount of generated H2O2 byproduct should be measured using the Pt/CNafion composite. To accomplish this objective, we investigated the effect of the Nafion ionomer used in the Pt/C-Nafion composite on H2O2 generation during the reduction of O2 by SECM. First, O2 reduction voltammetry was performed using a rotating ringdisk electrode (RRDE) for reference. Subsequently, the amount of H2O2 generated during O2 reduction was evaluated by SECM
10.1021/jp909010q 2010 American Chemical Society Published on Web 12/22/2009
H2O2 Byproduct during O2 Reduction
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using Pt microdisk electrodes as a generator. Another SECM measurement was conducted using porous microelectrodes (PMEs) packed with Pt/C-Nafion composites.27 Finally, a semiquantitative analysis of the H2O2 generation was performed using the obtained results, from which the generation of H2O2 is found to be dramatically suppressed when the Nafion to Pt/C weight ratio increases. 2. Experimental Section 2.1. Preparation of Electrodes. For the RRDE measurement, a Pt ring-disk electrode (HR-200, Hokuto Denko) was used. The geometric surface areas of the ring and disk electrodes were 0.265 and 0.196 cm2, respectively. A Nafion coating layer was prepared only on the disk electrode by casting 5 wt % Nafion solution (ion exchange capacity: 0.476 mmol g-1, Wako) using a microsyringe, which was then dried for 12 h at room temperature in air. The thickness of the obtained coating layer was estimated using a laser scanning microscope (OLS1200, Olympus).28 The refractive index of the Nafion coating layer used for estimation was 1.34, which was obtained from the published thicknesses of the Nafion 112, 115, and 117 membranes divided by their thicknesses measured by using the laser scanning microscope.29,30 The Pt microdisk electrode used as a generator in SECM was prepared as follows.31 A Pt wire of 50 µm diameter was inserted into the tip of a glass capillary (φ1.6 mm). Subsequently, the tip of the capillary was heat-sealed. After the tip of the microelectrode was polished using lapping films, a Nafion coating layer was prepared on the electrode using a dip coater (VLA-ST-45-06-0100, Aiden) in the same manner as that used to coat the RRDE. The other type of generator used in SECM, i.e., the PME packed with a Pt/C-Nafion composite, was prepared as follows.27 A gold wire of 50 µm diameter was inserted into a glass capillary and heat-sealed by decompressing the air inside the glass. By using lapping films, the tip of the capillary was then polished. Subsequently, the tip of the gold electrode was etched in 1 mol dm-3 HCl aqueous solution at a current density of 0.1 A cm-2 for 300 s, resulting in a cavity with a depth of 20 µm. To prepare the Pt/C-Nafion composite including Nafion ionomer with weight contents of 0, 14.3, and 28.6 wt %, the Pt/C catalyst (Pt: 45.9 wt %, Tanaka Kikinzoku Kogyo) was mixed with the Nafion ionomer diluted by a mixed solution of methanol, 2-propanol, and Milli-Q water (weight ratio of 1:1:1). After the mixture was dried for 1 h at 140 °C under atmospheric pressure, the obtained powder was completely filled in the microcavity of the PME so that the top of the cavity could be smooth, as shown in Figure 1. 2.2. Electrochemical Measurements. In electrochemical measurements, a Pt wire and Ag/Ag2SO432 were applied as counter and reference electrodes, respectively. All electrode potentials in this report are with reference to the reversible hydrogen electrode potential (RHE) at the same temperature. Moreover, the obtained currents are normalized by the geometric surface area of the working electrode unless otherwise stated. Prior to the measurements, the working electrode was electrochemically cleaned in 0.5 mol dm-3 H2SO4 by successive potential cycles for 1 h between 0.04 and 1.4 V vs RHE at a sweep rate of 50 mV s-1. O2 reduction at the RRDE33–35 was conducted in O2-saturated 0.5 mol dm-3 H2SO4. The O2 reduction voltammogram was obtained by performing a negative-direction sweep of the Pt disk electrode potential from 1.2 V vs RHE at a rate of 30 mV s-1. During the measurement, the Pt ring electrode potential
Figure 1. Schematic illustration of porous microelectrode and Pt microdisk electrode used as generator and detector, respectively, in SECM measurement.
was held at 1.3 V vs RHE to detect the H2O2 generated. The Pt disk and ring electrode potentials were controlled by a dual potentiostat (model 700B, ALS). The rotating speed of the RRDE was varied between 100 and 3600 rpm by a speed controller (HR-202, Hokuto Denko). For the SECM measurements, a φ50 µm Pt microelectrode and PME were used as a generator electrode. A φ3 µm Pt microelectrode used as a detector was installed on an arm of the SECM instrument (HV-404, Hokuto Denko). The electrode potentials of the generator and detector were controlled by a dual potentiostat (HA1010 mM2B, Hokuto Denko). Prior to the SECM measurements, a background cyclic voltammogram, used to estimate the Pt electrochemical surface area (ESA), was measured in N2-saturated 0.5 mol dm-3 H2SO4 using the electrochemically cleaned generator. From the obtained voltammogram, the ESA was calculated from the following equation:36
ESA ) Q/210
(1)
where Q is the coulomb charge associated with H adsorption/ desorption (in µC) and 210 is the coulomb charge of H adsorption/desorption per unit area of Pt (in µC cm-2). Subsequently, the electrolytic solution was saturated by O2 gas and the O2 reduction voltammograms were measured by performing a negative-direction sweep of the generator potential from the rest potential. The sweep rate was set at 1 mV s-1. The SECM measurement was carried out in O2-saturated 0.5 mol dm-3 H2SO4. O2 starvation was measured by the redox competition mode of SECM (RC-SECM) to evaluate the amount of O2 consumed at the generator electrode as follows.37,38 The electrode potentials of the generator and detector were set at 0.3 V vs RHE, at which the O2 reduction occurs. The detector was set at 10 µm above the top of the generator by detaching the detector once it attached to the generator and was moved in the X and Y directions at a rate of 10 µm s-1, as shown in Figure 1. The central position above the generator was determined as where the minimum O2 reduction current was observed at the detector. Subsequently, the detector was moved in the Z direction by 50 µm (see Figure 1), then the O2 reduction current was observed at the detector by scanning it into the X and Y directions. The amount of H2O2 byproduct generated during O2 reduction was measured in the sample generation-tip collection mode (SGTC) by the following procedure.26,38,39 The electrode potential of the detector was set at 1.3 V vs RHE to detect the H2O2
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Kishi et al. diffusion-limited current density, if, and the diffusion-limited current density, id, as follows:24,42
1/i ) 1/ik + 1/if + 1/id ) (1/ik + 1/if) + 1/(0.62nFD2/3Cυ-1/6ω1/2) ) (1/ik + 1/if) + 1/Bω1/2
Figure 2. RRDE measurement in O2-saturated H2SO4 solution (rotating speed: 1600 rpm). Upper: voltammograms of Pt ring electrode (electrode potential: 1.3 V vs RHE). Lower: O2 reduction voltammogram of Pt disk electrode with Nafion coating layer. Scan rate of Pt disk electrode potential: 30 mV s-1. The inset shows the Koutecky-Levich plots obtained at 0.3 V vs RHE.
generated during O2 reduction at the generator, which was held at 0.3 V vs RHE. The current generated due to the detection of H2O2 was measured by moving the detector in the X, Y, and Z directions under the same conditions as those in O2 starvation measurement. From the obtained results, the thickness of the H2O2 diffusion layer was estimated. 3. Results and Discussion 3.1. H2O2 Genration at Nafion-Coated RDE. First, the effects of the Nafion coating layer on the O2 reduction performance of the Pt electrode were investigated using the RRDE. The result of this investigation was used as a reference when investigating the effect of using microelectrodes. The lower graph in Figure 2 shows the O2 reduction voltammograms at the rotating Pt disk electrode with Nafion coating layers of 0, 0.7, and 1.3 µm thickness in O2-saturated 0.5 mol dm-3 H2SO4 solution. The rotating speed of the electrode and potential sweep rate were 1600 rpm and 30 mV s-1, respectively. The figure shows that the onset potential for O2 reduction is changed by coating Pt disk electrode surface with a Nafion layer.21,40,41 However, the reason for the change in onset potential has not yet been clarified. On the other hand, the diffusion-limited current of O2 reduction is observed at
