Conclusions

Ti1-xTaxO2–δ (x=0.05, 0.1 0.2, 0.3) nanoparticles with a fused aggregate structure which has lower NI values than 0.5 were synthesized by a flame oxide-forming method.

The maximum electrical conductivity of Pt/Ti1-xTaxO2–δ was obtained at x = 0.1, and the

- 56 - conductivity was as high as 10^–1 S cm^–1.

I investigated the heat treatment temperature dependency of the ORR activities for Pt/Ti1-xTaxO2–δ (x=0.1, 0.2) catalysts with RDE technique. Pt/Ti0.8Ta0.2O2–δ heat-treated at 900 °C showed the highest MA_{@0.9 V} (93 A g_Pt⁻¹), which exceeded the value for the Pt/Ti0.7Ta0.3O2–δ reported in a previous literature (62 A gPt−1

) [14]. The SA@0.9 V roughly doubled by increasing heat treatment temperature from 600 to 800 °C due to the formation of a Pt-Ti alloy by the diffusion of Ti from Ti_1-xTa_xO_2–δ supports to Pt particles. Pt/Ti0.9Ta0.1O2–δ heat-treated at 800 °C showed the highest SA@0.9 V (531 μA cm_Pt⁻²), which was 1.8 times higher value than that of Pt/CB (292 A g_Pt⁻¹) [19].

However, the MA@0.9 V for Pt/Ti0.9Ta0.1O2–δ heat-treated at 800 °C was not as high as that of Pt/CB due to the low ECSA value.

I fabricated MEAs using Pt/Ti1-xTaxO2–δ (x = 0.1, 0.2) heat-treated at 900 °C as the cathode catalysts to investigate the performance and durability of under a practical fuel cell conditions. The resistance of the cells with the MEAs drastically increased by changing the supply gases from H2 (anode)/N2 (cathode) to H2 (anode)/O2 (cathode).

This phenomenon can be explained that chemisorbed molecules, such as charged oxygen species (O²⁻, O⁻, O²⁻) are generated by the reduction of oxygen molecules to introduce a depletion layer with band bending on the Ti_1-xTa_xO_2–δ surface and the band bending prevents electrons from moving across the grain boundaries and the particles.

The high electrical resistance of the cathode catalyst layers brought about non-uniform ORR current distributions in the catalyst layers, resulting in the low MA@0.9 V and SA@0.9 V values of the MEAs. The durability was evaluated with a potential step cycling between 0.9 and 1.3 V vs. H2 anode. The ECSA degradation rates of Pt/Ti1-xTaxO2–δ (x = 0.1, 0.2) cathodes were significantly lower than that of Pt/GCB, which indicates Pt particles on the Ti1-xTaxO2–δ support are more stable at high potentials than on the GCB

- 57 -

support. However, the degradations in the cell resistance and the ORR activity were greater than those of Pt/GCB cathode. The atomic ratio of Ti³⁺ in Pt/Ti_1-xTa_xO_2–δ (x=0.1, 0.2) cathodes decreased after the potential cycling with XPS analyses. Therefore, I conclude that the performance degradation during the potential cycling is due to the decreasing of the electrical conductivity of Pt/Ti1-xTaxO2–δ (x = 0.1, 0.2) cathodes by the oxidation of Ti³⁺.

- 58 -

10 20 30 40 50 60 70 80 90

Intensity / arb. unit

2θ (Cu Kα) / degree Ti_0.9Ta_0.1O_2–δ

Ti_0.7Ta_0.3O_2–δ

Ti_0.8Ta_0.2O_2–δ^◆ ◆ Anatase (110) (101) (211)

Ti_0.95Ta_0.05O_2–δ^◆ _◆

d_XRD / nm S_XRD / m² g^-1 S_BET / m² g^-1 NI

Ti0.95Ta0.05O2–δ 37.9 37.2 14.8 0.40

Ti_0.9Ta_0.1O_2–δ 36.3 38.8 14.7 0.38

Ti0.8Ta0.2O2–δ 28.7 49.1 25.7 0.52

Ti_0.7Ta_0.3O_2–δ 26.3 53.5 28.2 0.53

Figure 3-1 X-ray diffraction patterns for Ti_1-xTa_xO_2–δ. The inset shows the relationship between the amount of Ta dopant and the lattice constant of rutile-phase TiO2

Table 3-1 Crystallite diameter (dXRD) and specific surface area measured by XRD (SXRD), specific surface area (S_BET) measured by BET and necking index (NI) for Ti_1-xTa_xO_2–δ.

- 59 -

10 20 30 40 50 60 70 80 90

Intensity / arb. unit

2θ (Cu Kα) / degree 400℃

600℃

900℃

800℃

(a) Pt/Ti_0.95Ta_0.05O_{2–δ ▼：Pt}

3Ti

▼ ▼

▼ ▼

▼

▼

38 39 40 41 42

400℃

600℃

900℃

800℃

Pt(111)

10 20 30 40 50 60 70 80 90

Intensity / arb. unit

2θ (Cu Kα) / degree 400℃

600℃

900℃

800℃

(b) Pt/Ti_0.9Ta_0.1O_{2–δ ▼：Pt}

3Ti

▼ ▼

▼ ▼

▼

▼

38 39 40 41 42

400℃

600℃

900℃

800℃

Pt(111)

- 60 -

10 20 30 40 50 60 70 80 90

Intensity / arb. unit

2θ (Cu Kα) / degree 400℃

600℃

900℃

800℃

(c) Pt/Ti_0.8Ta_0.2O_2–δ

38 39 40 41 42

400℃

600℃ 900℃

800℃

Pt(111)

10 20 30 40 50 60 70 80 90

Intensity / arb. unit

2θ (Cu Kα) / degree 400℃

600℃

900℃

800℃

(d) Pt/Ti_0.7Ta_0.3O_2–δ

38 39 40 41 42

400℃

600℃

900℃

800℃

Pt(111)

Figure 3-2 X-ray diffraction patterns for Pt/Ti1-xTaxO2–δ ((a) x=0.05, (b) x=0.1, (c) x=0.2, (d) x=0.3) heat-treated at various temperatures.

- 61 - (a) Pt/Ti0.95Ta0.05O2–δ

(b) Pt/Ti_0.9Ta_0.1O_2–δ

(c) Pt/Ti_0.8Ta_0.2O_2–δ

(d) Pt/Ti0.7Ta0.3O2–δ

20 nm

20 nm 20 nm 20 nm

(a)- 400℃ (a)- 600℃ (a)- 800℃ (a)- 900℃

20 nm

20 nm 20 nm 20 nm

(b)- 400℃ (b)- 600℃ (b)- 800℃ (b)- 900℃

20 nm

20 nm 20 nm 20 nm

(c)- 400℃ (c)- 600℃ (c)- 800℃ (c)- 900℃

20 nm

20 nm 20 nm 20 nm

(d)- 400℃ (d)- 600℃ (d)- 800℃ (d)- 900℃

Figure 3-3 TEM images for Pt/Ti1-xTaxO2–δ ((a) x=0.05, (b) x=0.1, (c) x=0.2, (d) x=0.3) heat-treated at various temperatures.

- 62 - 0

1 2 3 4 5 6 7 8 9

200 400 600 800 1000

Average diameter of Pt/ nm

Heat treatment temperature (℃)

0 0.1 0.2 0.3 0.4

El ec tric al c onduc ti v ity / S c m

-1

Ta dopant, x 10

^-3

10

^-2

10

^-1

10

⁰

10

^-4

Pt/Ti_0.95Ta_0.05O_2-δ Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/Ti_0.7Ta_0.3O_2-δ

Figure 3-4 Relationship between the average diameter of Pt particles and the heat treatment temperature for Pt/Ti1-xTaxO2–δ.

Figure 3-5 Relationship between the apparent electrical conductivity and the amount of Ta dopant for Pt/Ti1-xTaxO2–δ heat-treated at 900 °C.

- 63 -

0.0 0.2 0.4 0.6 0.8 1.0 1.2

E/ V vs. RHE Pt/Ti_0.8Ta_0.2O_2-δ

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2

i/ mA cm-2

E/ V vs. RHE Pt/Ti_0.9Ta_0.1O_2-δ (a)

400℃

600℃ 800℃

900℃

400℃

600℃ 800℃

900℃

0 10 20 30 40 50 60

200 400 600 800 1000

ECSA/ m2 gPt-1

Heat treatment temperature (℃) (b)

0 0.2 0.4 0.6 0.8

200 400 600 800 1000

ECSA/STEMratio

Heat treatment temperature (℃) (c)

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ

Figure 3-6 (a) Cyclic voltammograms and relationship between the (b) ECSA, (c) ECSA/STEM

ratio and the heat treatment temperature for Pt/Ti1-xTaxO2–δ.

- 64 -

-5.0 -4.0 -3.0 -2.0 -1.0 0.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

i/ mA cm-2

E/ V vs. RHE Pt/Ti_0.9Ta_0.1O_2-δ

(a)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

E/ V vs. RHE Pt/Ti_0.8Ta_0.2O_2-δ

400℃

600℃

800℃

900℃

400℃ 600℃

800℃ 900℃

0 20 40 60 80 100

200 400 600 800 1000

MA@0.9V/ AgPt-1

Heat treatment temperature (℃) (b)

0 100 200 300 400 500 600

200 400 600 800 1000

SA@0.9V/ μAcmPt-2

Heat treatment temperature (℃) (c)

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ

Figure 3-7 (a) Hydrodynamic voltammograms and relationship between the (b) MA@0.9 V, (c) SA@0.9 V and the heat treatment temperature for Pt/Ti1-xTaxO2–δ.

- 65 -

Figure 3-8 The outermost surface compositions of the Pt particles in the Pt/Ti0.9Ta0.1O2–δ

estimated by STEM-EDX.

- 66 -

-2.0 -1.0 0.0 1.0 2.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

i / m A cm

-2

E / V vs. H

₂

anode

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

H2/N2 H2/O2 H2/N2 H2/O2 H2/N2 H2/O2 Pt/GCB Pt/Ti0.9Ta0.1TiO2-δ Pt/Ta0.2-TiO2

Cell resistance/ Ω cm2

H₂/N₂ H₂/O₂ H₂/N₂ H₂/O₂ H₂/N₂ H₂/O₂ Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/GCB

Figure 3-9 Cyclic voltammograms for Pt/Ti0.9Ta0.1O2–δ, Pt/Ti0.8Ta0.2O2–δ and Pt/GCB measured at 65 °C in N2 (cathode) and H2 (anode) with 100 % RH; potential sweep rate 10 mV s⁻¹, H2

flow rate 0.3 dm³ min⁻¹, N2 flow rate 0.3 dm³ min⁻¹, ambient pressure.

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/GCB

Figure 3-10 Cell resistances for Pt/GCB cathode cell, Pt/Ti_0.9Ta_0.1O_2–δ cathode cell and Pt/Ti0.8Ta0.2O2–δ cathode cell measured at 80 °C in H2 (anode)/N2 (cathode) and H2

(anode)/O₂ (cathode) with 100 % RH.

- 67 - 0

10 20 30 40 50

0 1000 2000 3000 4000 ECSA / m2 g-1

Number of cycles,N

1 10 100 1000

Current Density / mA cm^-2 (b) H₂/air

0.2 0.4 0.6 0.8 1.0

1 10 100 1000

Cell Voltage (IR-free) / V

Current Density / mA cm^-2 (a) H₂/O₂

Figure 3-11 IR-free (a) H2/O2 and (b) H2/air polarization curves of Pt/Ti0.9Ta0.1O2–δ cathode cell (solid circles), Pt/Ti0.8Ta0.2O2–δ cathode cell (solid triangles) and Pt/GCB cathode cell (open circles) at 80 °C and 53 % RH ; H₂ flow rate 0.390 dm³ min⁻¹, O₂ flow rate 1.706 dm³ min⁻¹, ambient pressure.

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/GCB

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/GCB

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/GCB

Figure 3-12 Plots of the ECSA of Pt/Ti_0.9Ta_0.1O_2–δ (solid circles), Pt/Ti_0.8Ta_0.2O_2–δ (solid triangles) and Pt/GCB (open circles) as a function of the number of the potential cycles measured at 65 °C in H2 (anode) and N2 (cathode) with 100 % RH; potential sweep rate 10 mV s⁻¹, H₂ flow rate 0.3 dm³ min⁻¹, N₂ flow rate 0.3 dm³ min⁻¹, ambient pressure.

- 68 - 0

5 10 15 20

H2/N2 H2/O2 H2/N2 H2/O2 H2/N2 H2/O2 Pt/GCB Pt/Ti0.9Ta0.1TiO2-δ Pt/Ta0.2-TiO2

Cell resistance/ Ω cm2

H₂/N₂ H₂/O₂ H₂/N₂ H₂/O₂ H₂/N₂ H₂/O₂ Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/GCB

0.2 0.4 0.6 0.8 1.0

1 10 100 1000

Cell Voltage (IR-free) / V

Current Density / mA cm^-2

Figure 3-14 IR-free H2/O2 polarization curves of Pt/Ti0.9Ta0.1O2–δ cathode cell (solid circles), Pt/Ti_0.8Ta_0.2O_2–δ cathode cell (solid triangles) and Pt/GCB cathode cell (open circles) after 3000 cycles at 80 °C and 53 % RH ; H2 flow rate 0.390 dm³ min⁻¹, O2 flow rate 1.706 dm³ min⁻¹, ambient pressure.

Pt/Ti_0.9Ta_0.1O_2-δ Pt/Ti_0.8Ta_0.2O_2-δ Pt/GCB

Figure 3-13 Cell resistances for Pt/GCB cathode cell, Pt/Ti_0.9Ta_0.1O_2–δ cathode cell and Pt/Ti0.8Ta0.2O2–δ cathode cell after 3000 cycles measured at 80 °C in H2 (anode)/N2 (cathode) and H₂ (anode)/O₂ (cathode) with 100 % RH.

0.13 0.14

- 69 -

Intensity / a.u.

21 23

25 27

29 31

Intensity / a.u.

Binding energy / eV (c) Ta 4f before

(d) Ta 4f after

Ta⁵⁺: 79.6 atom%

Ta²⁺: 17.2 atom%

Ta⁺: 3.2 atom%

Ta⁵⁺: 85.0 atom%

Ta²⁺: 8.6 atom%

Ta⁺: 6.4 atom%

Pt/Ti_0.9Ta_0.1O_2-δ

Intensity / a.u.

452 454

456 458

460 462

Intensity / a.u.

Binding energy / eV Ti⁴⁺: 66.5 atom%

(a) Ti 2p before

(b) Ti 2p after

Ti³⁺: 14.4 atom%

Ti⁰: 19.1 atom%

Ti⁴⁺: 72.7 atom%

Ti³⁺: 9.0 atom%

Ti⁰: 18.3 atom%

Pt/Ti_0.9Ta_0.1O_2-δ

Figure 3-15 XPS spectra of Ti 2p and Ta 4f regions for the Pt/Ti0.9Ta0.1O2–δ and the Pt/Ti_0.9Ta_0.1O_2–δ cathode catalyst layer before and after the potential cycling.

452 454

456 458

460 462

Intensity / a.u.

Binding energy / eV

Intensity / a.u.

Ti⁴⁺: 69.2 atom%

(e) Ti 2p before

(f) Ti 2p after

Ti³⁺: 17.4 atom%

Ti⁰: 13.4 atom%

Ti⁴⁺: 79.7 atom%

Ti³⁺: 10.4 atom%

Ti⁰: 9.9 atom%

Pt/Ti_0.8Ta_0.2O_2-δ

21 23

25 27

29 31

Intensity / a.u.

Binding energy / eV

Intensity / a.u.

(g) Ta 4f before

(h) Ta 4f after

Ta⁵⁺: 88.4 atom%

Ta²⁺: 8.4 atom%

Ta⁺: 3.2 atom%

Ta⁵⁺: 89.4 atom%

Ta²⁺: 7.3 atom%

Ta⁺: 3.3 atom%

Pt/Ti_0.8Ta_0.2O_2-δ

- 70 -

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