Catal Lett (2014) 144:242–247 DOI 10.1007/s10562-013-1150-z

A Facile Route to Fabricate Effective Pt/IrO2 Bifunctional Catalyst for Unitized Regenerative Fuel Cell Fan-Dong Kong • Sheng Zhang • Ge-Ping Yin Jing Liu • Ai-Xia Ling

•

Received: 4 September 2013 / Accepted: 17 October 2013 / Published online: 23 November 2013 Ó Springer Science+Business Media New York 2013

Abstract In order to fabricate effective bifunctional oxygen catalyst, IrO2 nanoparticles have been synthesized by hydrothermal method, and Pt/IrO2 bifunctional catalyst is then prepared by a microwave-assisted polyol process. X-ray diffraction and transmission electron microscopy are employed to characterize the catalysts, which reveal that Pt with a particle size of 2–3 nm is deposited on IrO2 surface. Electrochemical tests indicate that Pt/IrO2 bifunctional catalyst possesses much higher catalytic activity and durability towards both oxygen reduction reaction and oxygen evolution reaction than pure Pt or pure IrO2. Kinetic analysis shows that the oxygen reduction reaction on Pt/IrO2 catalyst mainly follows four-electron pathway. Keywords Iridium dioxide  Iridium dioxide supported platinum  Bifunctional oxygen catalyst  Unitized regenerative fuel cell

1 Introduction Unitized regenerative fuel cell (URFC) is an energy conversion and storage system that serves both as a fuel cell (H2 ? O2 ? H2O ? E) and an electrolyzer (H2O ? E ? H2 ? O2) [1, 2]. In comparison to secondary batteries [3], URFC system possesses many advantages such as higher F.-D. Kong  J. Liu  A.-X. Ling School of Fundamental Sciences, Jining Medical University, No. 16, He-Hua Street, Jining 272000, Shandong, China e-mail: [email protected] F.-D. Kong  S. Zhang  G.-P. Yin (&) School of Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China e-mail: [email protected]

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specific energy density (3,600 Wh kg-1 in theory), larger capacity, non self-discharge, long-term storage property [4– 6]. For space use, it can provide power supports for devices operation and orbital adjustment. It can also provide the living necessities (O2, H2O) for astronauts at the space station [7, 8]. For terrestrial use, URFC stores energy intermittently from solar energy, wind energy as well as tidal energy [9]. It creates a green, low-carbon model for power generation. This system can be used in special fields, for example, power supply for off-grid remote regions, communication stations, etc. [10–12]. In URFC system, the bifunctional oxygen catalyst has proved to be the key technology because the electrochemical processes on oxygen electrode are extremely complex. During these processes, two different reaction mechanisms for oxygen evolution (OER: H2O ? O2) and oxygen reduction (ORR: O2 ? H2O) have been observed, and both are involved a number of oxygen species (O2-, O22-, O, O-, O2-), which lead to the difficulties of the mechanisms research [13, 14]. Practically, a bifunctional oxygen catalyst usually contains two different components which catalyze the reactions of ORR and OER respectively. At present, Pt/IrO2 is commonly used as a bifunctional oxygen catalyst, in which Pt mainly catalyzes the oxygen reduction reaction and IrO2 catalyzes the oxygen evolution reaction [15–17]. For Pt preparation, there have been numerous reports appeared and the desired results have substantially been obtained [18–20]. For IrO2 preparation, however, there has not been a particularly effective approach reported. Conventional techniques include precipitation method and calcination method, etc. Nevertheless, the final IrO2 powder from any of them displays an agglomeration state with large range of particle size distribution. In addition, the active sites on the catalyst surface are lowered, or even

Facile Route to Fabricate Effective Pt/IrO2 Bifunctional Catalyst

destroyed due to the high-temperature treatment. So these kinds of IrO2 catalysts usually exhibit lower catalytic activities towards oxygen evolution reaction. As for Pt/IrO2 preparation, physical mixture of Pt and IrO2 has usually been used but, as demonstrated, agglomerates and poor dispersions for each component are observed. Therefore, exploring a facile efficient strategy for preparation of IrO2 as well as Pt/IrO2 catalyst with a quantity of active sites and high catalytic activity is a great challenge. In the present work, IrO2 nanocatalyst has been synthesized through hydrothermal method under a mild condition, and Pt/IrO2 bifunctional catalyst has subsequently been prepared by reduction of Pt precursor and supporting Pt particles onto IrO2 surface through microwave-assisted polyol process. We focus on the understanding of the influence of single Pt or IrO2 on Pt/IrO2 bifunctional catalyst performance. The unique advantages of the as-prepared Pt/ IrO2 catalyst include: (1) IrO2 is synthesized under a mild condition (at lower temperature), which allows obtaining high active surface area and higher OER activity, and avoid destroying active sites in higher-temperature calcination in other methods. Meanwhile, such an IrO2 material facilitates Pt nanoparticles to be supported on it. (2) The reaction associated with H2PtCl6 reduction to form Pt/IrO2 catalyst is performed in organic medium, which allows Pt uniformly supporting on IrO2 surface with ultrafine nanoparticles (2–3 nm), and favoring ORR process. This method provides a new insight into the fabrication of a high-activity catalyst.

2 Experimental Section 2.1 Preparation of Catalyst 2.1.1 Preparation of Nanosized IrO2 Catalyst Hydrothermal technique was employed to synthesize nanosized IrO2 particles. In a typical process, the desired amount of H2IrCl66H2O was added into the mixed solution containing water and ethylenediamine (5 wt%), and the mixture was consecutively stirred for 1 h. Then, the pH of the mixture was adjusted to 7.5 with 0.5 M NaOH solution. After stirring for 0.5 h, the mixture was transferred to an autoclave for hydrothermal reaction at 180 °C for 5 h. When it was cooled to room temperature, the resulting product was centrifuged and washed with alcohol/water solution until no Cl- ions were detected. Subsequently, it was subjected to heat-treatment at 350 °C for 30 min. 2.1.2 Preparation of Pt/IrO2 Bifunctional Catalyst Pt/IrO2 (1:1 by weight) bifunctional catalyst was prepared by a microwave-assisted polyol process (MAPP) [21] using

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IrO2 (from Sect. 2.1.1) as a support and H2PtCl66H2O as a Pt precursor. Briefly, 50 mg of IrO2 was dispersed in 60 mL of glycol/isopropyl alcohol (4:1 by volume) solution under ultrasonic treatment for 1 h to form uniform ink. Then, 6.75 mL of 0.038 M H2PtCl6/EG solution was added with subsequently stirring for 3 h. The pH of the ink was adjusted to 12.00 by adding 1 M NaOH/EG solution dropwise. After being purged with argon gas for 15 min to remove the dissolved oxygen, the ink was heated in a microwave oven for 50 s. The solution was cooled down to room temperature by continuously stirring, followed by adding 0.1 M HNO3 solution to adjust the pH to 3–4. It is further stirred for 12 h. Finally, the solution was centrifuged and washed repeatedly with ultrapure water (Millipore, 18.2 MX cm) until no Cl- ions were detected. The obtained Pt/IrO2 catalyst was dried in a vacuum oven for 5 h at 80 °C. As comparison, the pure Pt catalyst was also prepared by the same method as Pt/IrO2 catalyst, i.e., prepared by a MAPP, which has been proved to be a reliable method for ultrafine Pt nanoparticles preparation. 2.2 Material Characterization The synthesized IrO2 and Pt/IrO2 catalysts were characterized using the X-ray diffraction (XRD) technique (D/ max-rB, Japan) with a Cu Ka radiation source operated at 45 kV and 100 mA. The tests were carried out in the angle (2h) range from 10° to 90° at a scan rate of 4° min-1 with an angular resolution of 0.05° of the 2h scan. Transmission electron microscopy (TEM) images of the catalysts were taken on a JEOL TEM-1200EX (Netherlands) system with a spatial resolution of 1 nm. The operating voltage was 120 keV for low-resolution tests and 300 keV for high-resolution tests. Before testing, the samples were finely ground and ultrasonically dispersed in ethanol to form a uniform ink. Then, a drop of the ink was deposited and dried on the standard carbon membrane substrate. 2.3 Electrochemical Measurements The electrochemical measurements were performed at 25 °C in a three-electrode electrochemical cell controlled by CHI-650C (China) system. A glassy carbon rotating disk electrode (GRDE) (3 mm diameter) loaded with catalyst was employed as the working electrode. The catalyst ink was prepared by blending 6.0 mg of the catalyst with 3.0 mL of the mixed alcohol/water (1:1 by volume) solvent in an ultrasonic bath. Then, 12 lL of the catalyst ink was deposited onto the glassy carbon disk by three times. After the ink had dried in argon atmosphere, 5 lL of a Nafion solution (0.5 wt%, Alfa Aesar) was coated onto the catalyst

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Pt (311)

Pt (220)

IrO2 (211)

IrO2 (101) IrO2 (200) Pt (111) Pt (200)

IrO2 (110)

Intensity / a.u.

layer to ensure better adhesion of the catalyst on the glassycarbon substrate. A platinum foil and an Hg/Hg2SO4 electrode were used as the counter and reference electrodes, respectively. All potentials in this work were reported as referenced to the normal hydrogen electrode.

Pt/IrO2

Prior to recording, the catalyst was activated through cyclic voltammetry (CV) scanning at 50 mV s-1 between 0.05 and 1.2 V until steady CV curves were obtained. The cyclic voltammograms were recorded in an argon-saturated 0.5 M H2SO4 solution at 20 mV s-1. The linear sweep voltammograms (LSVs) for ORR were recorded in an oxygen-saturated 0.5 M H2SO4 solution with a series of rotating speeds at 5 mV s-1 between 1.1 and 0.4 V. The LSVs for OER were recorded in a 0.5 M H2SO4 solution at 5 mV s-1 between 1.2 and 1.65 V. The APCT for ESA degradation was recorded before and after 5,000 cycles at 50 mV s-1 between 0.05 and 1.2 V, and that for OER degradation was recorded before and after 5,000 cycles at 50 mV s-1 between 1.2 and 1.6 V. [22, 23].

Pt

3 Results and Discussion IrO2 20

40 60 2 theta / degree

3.1 Physical Characterization

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Fig. 1 X-ray diffraction patterns of Pt/IrO2, Pt, and IrO2 catalysts

The XRD pattern of the Pt/IrO2 catalyst is presented in Fig. 1. As comparison, the XRD patterns of pure IrO2 and

Fig. 2 TEM (a) and HRTEM (b) images of Pt/IrO2 catalyst 5 10

(a)

(b) 0 -1

J / mA mg

-1

J / mA mg Pt

5 0 -5 Pt/IrO2 Pure Pt Pure IrO2

-10 -15

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-5 -10 -15

-25

E / V vs. SHE

Fig. 3 The CV and the LSV curves of Pt/IrO2, pure Pt, and pure IrO2 catalysts

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Pt/IrO2 Pure Pt Pure IrO2

-20

0.4

0.5

0.6

0.7

0.8

E / V vs. SHE

0.9

1.0

1.1

Facile Route to Fabricate Effective Pt/IrO2 Bifunctional Catalyst

Pt are also provided in the same figure. For Pt/IrO2 catalyst, the diffraction peaks of Pt and IrO2 coexist in the Pt/IrO2 curve, which indicates that Pt metal has been successfully supported on IrO2 surface. Four major peaks at 39.6°, 46.3°, 67.4°, and 81.2° are assigned to the diffractions from the (111), (200), (220), and (311) planes of the face-centered cubic Pt metal (JCPDS No. 65-2868). The particle size of Pt metal is estimated to be 2.4 nm according to the Scherer equation. On the other hand, the four major broadened peaks at 28.0°, 34.8°, 40.0°, and 54.1° are assigned to the diffractions from the (110), (101), (200), and (211) planes of the amorphous IrO2 (JCPDS No. 65-2822). Figure 2 shows the TEM image and HR-TEM image of Pt/IrO2 catalyst. From Fig. 2a, we can observe that Pt and IrO2 particles are combined with each other and both are uniformly dispersed. From Fig. 2b, the Pt particles with a distinct crystal lattice (d (111) = 0.22 nm) and the IrO2 particles with amorphous state (judged by short-distance or disordered array) can be easily recognized. The particle size of Pt ranges from 2 to 3 nm, which is consistent with the XRD data. In addition, the crystal structure of Pt/IrO2 catalyst can be confirmed by the electron diffraction pattern (the inset) in which the diffused circles are produced by amorphous IrO2 and the bright diffraction dots are produced by Pt crystallite.

Table 1 ESAs and Iks of Pt/IrO2 and pure Pt catalysts Catalysts

ESA/m2 g-1

Ik/mA mg-1 (0.85 V)

Ik/mA mg-1 (0.90 V)

Pt/IrO2

24.2

32.3

7.1

Pure Pt

10.7

19.1

5.9

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3.2 Electrochemical Analyses 3.2.1 Cyclic Voltammetry and the ORR Activity of Pt/IrO2 Catalyst The cyclic voltammograms (CVs) of Pt/IrO2, pure Pt, and pure IrO2 catalysts recorded in Ar-purged 0.5 M H2SO4 solution are shown in Fig. 3a. The typical hydrogen and oxygen adsorption/desorption behaviors of platinum can be clearly observed on the Pt/IrO2 and Pure Pt catalysts, but no such behaviors can be observed on pure IrO2 catalyst. The electrochemical surface area (ESA) of platinum can be calculated with coulombic charges accumulated during hydrogen adsorption and desorption after correcting for the double-layer charging current from the CVs according to the following equation [18]: ESA ¼

QH ; 0:21  WPt

ð1Þ

where QH (mC) is the charge produced by the hydrogen adsorption/desorption in the hydrogen region (0.05–0.4 V) of the CVs, 0.21 mC cm-2 is the electrical charge associated with monolayer adsorption of hydrogen on Pt, and WPt is the load of Pt on the working electrode. The calculated ESA data are shown in Table 1. It is obvious that the Pt/ IrO2 catalyst possesses much higher ESA than pure Pt catalyst (2.3 times). The reasons for higher ESA of Pt/IrO2 may be associated with Pt dispersion, Pt particle size, and the interaction between Pt and IrO2. The LSVs of Pt/IrO2, Pure Pt, and Pure IrO2 samples recorded in an O2-saturated 0.5 M H2SO4 solution are presented in Fig. 3b. It is clearly found that pure IrO2 has nearly no ORR activity, which is in accord with the data from CV (Fig. 3a). The LSV curves of Pt/IrO2 and pure Pt catalysts both display two polarization regions: one region with mixed kinetic/diffusion control (0.7–1.0 V), and the other region with mass transport control (\0.7 V). The 0.55

(a)

-1

0.45

-2

2

0.40

-1

0.50

0.35

500 rpm 1000 rpm 1500 rpm 2000 rpm 2500 rpm

-4 -5 -6

(b)

0.30

0.86 V 0.84 V 0.82 V 0.80 V 0.78 V

-1

-3

J / mA cm

J / mA cm

-2

0

0.25 0.20 0.15

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

E / V vs. SHE

0.06

0.08 -1/2

0.10

0.12

0.14

-1 -1/2

/( rad s )

Fig. 4 The LSV polarization curves (a), and Kouteckey–Levich curves (b) of Pt/IrO2 catalyst recorded in an O2-saturated 0.5 M H2SO4 solution at 5 mV s-1

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oxygen-reduction kinetic currents (Ik) can be calculated according to Koutecky-Levich equation (Sect. 3.2.3) with the mass-transport correction for rotating disk electrodes. The calculated kinetic currents are shown in Table 1. It can be seen that the kinetic currents obtained on Pt/IrO2, both at 0.85 and 0.90 V, are higher than those obtained on pure Pt, indicating that Pt/IrO2 has higher catalytic activity towards ORR. The enhanced ORR activity may also be

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200

Pt/IrO2 Pure Pt Pure IrO2

3.2.2 Kinetic Analysis of Oxygen Reduction Reaction on Pt/IrO2 Catalyst As is early reported, for a thin layer of the catalyst on GRDE in an O2-saturated solution, the relationship between the oxygen reduction current (I) and the rotating speed (x) of the GRDE follows Koutecky–Levich equation [24]: 1 1 1 1 1 ¼ þ ¼ þ I IK ID IK Bx1=2

ð2Þ

B ¼ 0:62nFAD2=3 m1=6 c

ð3Þ

J / mA mg

-1

150

associated with Pt dispersion, Pt particle size, and the interaction between Pt and IrO2.

100

50

0 1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

E / V vs. SHE

Fig. 5 LSV curves of Pt/IrO2, pure Pt, and pure IrO2 catalysts recorded in 0.5 mol L-1 H2SO4 at 5 mV s-1

10

(a)

Where I is the experimentally obtained current, Ik is the kinetic current, B is the levich slope, and x is the rotating speed of the GRDE, n is the number of electrons transferred per oxygen molecule. Figure 4a shows the ORR polarization curves of Pt/IrO2 catalyst measured at different rotating speeds. The polarization curves display the well-defined limiting currents as a function of rotating speeds. Figure 4b exhibits I-1–x-1/2 plots derived from Fig. 4a at different voltages. A linear relationship between I-1 and x-1/2 is observed, and the

15

Pure Pt 35.2% ESA degradation

Pt/IrO2 28.8% ESA degradation

10

5 -1

5

J / mA mg

-1

J / mA.mg

(b)

0 -5

-5 -10 Before life test After life test

-15

Before life test After life test

-10

0

-20 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

0.2

0.4

E / V vs. SHE

0.6

0.8

1.0

1.2

E / V vs. SHE

200

Pure IrO 2

100

200

20.4% Jp degradation

24.5% Jp degradation

150

Before life test After life test

50 0 -50 1.2

(d) Pt/IrO2 Before life test After life test

-1

J / mA mg

-1

150

J / mA mg

(c)

100 50 0

1.3

1.4

1.5

E / V vs. SHE

1.6

-50 1.2

1.3

1.4

1.5

1.6

E / V vs. SHE

Fig. 6 The ESA (a, b) and peak current density (c, d) degradation percentages of the catalysts before and after life tests

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Facile Route to Fabricate Effective Pt/IrO2 Bifunctional Catalyst

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approximate identical slopes indicate that oxygen reduction reaction on Pt/IrO2 catalyst follows the first order kinetics [25, 26]. In addition, the slope of the straight line, so-called ‘B-factor’, allows the number of electrons involved in the ORR to be estimated in terms of Eqs. (2), (3) and the literature data [25, 26]. The resulting n value is *3.8, suggesting a mainly four-electron reaction mechanism of ORR on Pt/ IrO2 catalyst and a low occurrence of H2O2 intermediate.

hydrothermal method have a uniform distribution with an amorphous state. (1) Ultrafine Pt particles (2–3 nm) are evenly supported on IrO2 surface with a face-centered cubic. (3) Pt/IrO2 bifunctional catalyst possesses much higher ORR and OER catalytic activities and durability than pure Pt or pure IrO2. (4) Kinetic analysis shows that the oxygen reduction reaction on Pt/IrO2 catalyst mainly follows the four-electron reaction mechanism.

3.2.3 The OER Activity of Pt/IrO2 Catalyst

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 21276058, 21106024, and 21173062).

Figure 5 shows the OER polarization curves of Pt/IrO2, pure Pt, and pure IrO2 catalysts. It can be seen that pure Pt has much lower catalytic activity towards OER but significantly high overpotential due to the formation of highresistance layer of platinum oxide (PtO2) [27]. The current densities of Pt/IrO2 and pure IrO2- measured at 1.55 V are 34.8 and 16.6 mA mg-1 respectively, i.e., the former is 2.1 times as high as the latter. The enhanced OER activity of Pt/IrO2 catalyst can be interpreted in terms of the synergic effect of Pt and IrO2. On one hand, IrO2 has large surface area but poor conductivity, which leads to limited active sites to be activated, therefore relatively lower OER activity. On the other hand, pure Pt metal has good conductivity but poor OER activity. So when Pt supported on IrO2 surface, Pt particles provide electron paths between IrO2 particles, which decreases the ohmic polarization and substantially enhances the OER activity. As previously proved [23], the APCT is an effective method to assess durability of catalysts. Figure 6a and b show ESA degradation percentages of pure Pt and Pt/IrO2 catalysts which are recorded before and after 5,000 cycles. The pure Pt has a degradation percentage of 35.2 %, whereas Pt/IrO2 has a degradation percentage of 28.8 %, indicating a higher ESA retention for Pt/IrO2 catalyst. Figure 6c and d present peak current density degradation percentages which are recorded before and after 5,000 cycles. The pure IrO2 has a degradation percentage of 24.5 % while Pt/IrO2 has a degradation percentage of 20.4 %, indicating that Pt/IrO2 possesses much higher OER durability than pure IrO2 catalyst. The higher durability of the as-prepared Pt/IrO2 may be interpreted in terms of the strong interaction between IrO2 and Pt nanoparticles.

4 Conclusions Iridium dioxide nanocatalyst has been synthesized by hydrothermal method, and Pt/IrO2 bifunctional catalyst has then been prepared by reduction of Pt precursor and supporting Pt nanoparticles onto IrO2 surface. Based on above discussions the conclusions can be drawn as follows: (1) IrO2 nanoparticles obtained under mild condition of

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