B2 - Strategic development for practical application / Research on nanomaterials for high performance PEFCs

1

Makoto Uchida

Fuel Cell Nanomaterials Center, University of Yamanashi, Japan

B2 - Strategic development for practical application /

Research on nanomaterials for high performance PEFCs

WORLD OF ENERGY SOLUTIONS,

f-cell – The fuel cell | 13th forum for producers and users,

SEPTEMBER 30 – OCTOBER 2, 2013

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Motivation



In order to reduce the Pt loading without serious loss of cell

performance, it has been proposed that maximizing the Pt

utilization is necessary.



It has been reported that

the utilization of Pt (

U

) reaches

60-80%

according to a conventional evaluation method. On

the other hand, it has been shown that it is necessary to

reduce the Pt loading of the MEA, according to industry

requirements, to

1/10, for cost reduction

.



If this information were correct, it would meen that there

would be

little room for improvement

in reducing the Pt

loading.



However, we believe that there are

no clear indices to

3 Good condition <10% Non coated 30-50% Pt Carbon black membrane Binder (ionomer) Low O₂ permeability Low H+ conductance Binder thick Binder thin U_Pt (from CV) Ef_Pt (from ORR)

M. Lee, M. Uchida, H. Yano, D.A. Tryk, H. Uchida, M. Watanabe, Electrochimica Acta, 55 (2010) 8504. M. Lee, M. Uchida, D.A. Tryk, H. Uchida,

M. Watanabe, Electrochimica Acta, 55 (2011) 4783.

 The Ef_Pt indicates the extent to which Pt particles exist in the catalyst layer in a

good condition of binder coverage.

 The value of Ef_Pt was found to be 10% or less under actual operating conditions.  This value shows the ratio of catalyst particles existing in an effective reaction

environment during actual operation.

 We have much room for improvement for the design of catalyst layers.

We proposed a new evaluation method for the effectiveness of Pt (Ef_Pt) under actual operating conditions.

This parameter is termed the Ef_Pt and is defined as the ratio [1] of mass activity

(MA) in the MEA to the maximum obtainable mass activity (MA_max) at high potential for the same catalyst, as estimated by use of the channel flow double electrode (CFDE) technique.

EfPt = MA/MAmax [1]

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Analysis of strong metal support interactions

Improvement of Pt ORR activity 1. alloying 2. skin layer

Improvement of ionomer condition in CL 1. support materials design

2. ionomer design

Scenarios of catalyst layer development for high

Ef

MEAs

Conventional CL

Highly dispersed ionomer

Design plans for high

performance catalyst layer

2D/3D distribution analysis of Pt & ionomer in the agglomerate

Improvement of ionomer coverage on Pt in the

primary particle

2D/3D distribution analysis of Pt & ionomer in the primary particle

Achievement of the ideal CL

Pt alloy Improvement of support materials Others 1. Durability 2. Water management 3. Gas transport/diffusivity etc. ⇒ improvement of Ef_Pt primary particles of carbon black (CB) PFSI ionomer Pt agglomerates

= flocks of secondary particles of CB aggregates

= secondary particles of CB (carbon structure)

Improvement of Ef_Pt in the primary particle 1. graphitized carbon 2. conductive ceramic

primary pores (mesopores)

Nano pores

M. Uchida, Y. Aoyama, N. Eda, A. Ohta, Journal of The Electrochemical Society 142 (1995) 4143.

M. Uchida, Y. Fukuoka, Y. Sugawara, N. Eda, A. Ohta, Journal of the Electrochemical Society 143 (1996) 2245.

M. Uchida, Y.-C. Park, K. Kakinuma, H. Yano, D. A. Tryk, T. Kamino, H. Uchida, M. Watanabe, Phys. Chem. Chem. Phys., 2013, 15, 11236-11247

100 101 102 103 104 105 106

0 40 60 80

Number of potential step cycles, N

5

20 c-Pt/CB c-Pt/GCB c-Pt/GCB-HT n-Pt/GCB

Plot of the progress of ECA degradation for c-Pt/CB, c-Pt/GCB, c-Pt/GCB-HT, and n-Pt/GCB catalysts as a function of the log of N.

• M. Lee, M. Uchida, K. Okaya, H. Uchida, and M. Watanabe, Electrochemistry, 79, 381 (2011).

• M. Hara, M. Lee, C.-H. Liu, B.-H. Chen, Y. Yamashita, M. Uchida, H. Uchida, and M. Watanabe, Electrochim. Acta, 70, 171 (2012)

• Y-C. Park, K. Kakinuma, M. Uchida, D. A. Tryk, T. Kamino, H. Uchida, M. Watanabe, Electrochimica Acta, 91, 195 (2013)

The ECSA values decreased in the order:

c-Pt/CB >> c-Pt/GCB > n-Pt/GCB(50w%) > c-Pt/ GCB-HT

The essential factors for maintenance of ECSA values

and cell performance during durability testing are (1) the high corrosion resistance of the support material

(for graphitized carbon black, GCB), (2) uniform dispersion of the Pt nanoparticles on the support (for n-Pt/GCB), (3) a slightly increased Pt particle size by heat treatment (for c-Pt/GCB-HT).

Durability of various Pt catalysts under

simulated start-up/shut-down cycling

TEM images of the CB support: (a) initial and (b) after durability test.

(a) initial _{(b) after}

5 E CS A / m 2 g -1

Protocol of a start-stop cycle test

1 cycle = 1 min.

v

ol

tage

6

 In the present research,

we have focused on elucidating the effect of

(1) the Pt particle distributions on both

the exterior and interior surfaces of various carbon particles,

(2) new catalyst preparation methods for optimal control of the

distribution of both Pt particle size and location

(nanocapsule method),

(3) the effects of these various catalysts on the Pt utilization, mass

activity and the effectiveness of Pt for MEAs.

(4) the new improvement of Pt ORR activity by alloying and Pt skin layer formation in our basic research.

(5) the effects of our technologies on the effectiveness of Pt and the

ability to meet NEDO targets.

STEM & SEM system

HD-2700 Hitachi Hitech Co.Ltd.

Resolution: 0.78Å

Accelerating voltage : 80 kV

3D holder

In the TEM images, Pt particles are observed on both the exterior and interior surface of the carbon particles as black dots.

In the SEM images, Pt particles are observed only on the exterior surface of the carbon particles as white dots.

5nm 5nm

From the comparison of these images, the presence of Pt catalysts in nanopores was confirmed.

Scanning Transmission Electron Microscopic Observation of Carbon-Supported Pt Catalysts SEM TEM Pt Carbon c-Pt/GCB-HT 7 3D-TEM

Particle size

(nm) number (%) Pt particle surface area (%) Pt particle

CB exterior 2.5±0.6 48.6 62.0

CB interior 1.9±0.5 51.4 38.0

49% of the Pt particles existed on the carbon exterior surface and 51% existed in the interior of the carbon particles. Surface areas of Pt particles on the exterior

and in the interior were 62% and 38%, respectively.

SEM(exterior) and STEM(exterior and the interior) images of the c-Pt/CB catalyst for both the front(0o_{) and back (180}o_).

0° 0° 180° 180°

Pt particle distribution of c-Pt/CB

Pt particle distribution of commercial graphitized carbon

supported Pt catalyst (c-Pt/GCB)

SEM(exterior) and STEM(exterior and the interior) images of the c-Pt/GCB catalyst for both the front(0o_{) and back (180}o_).

Pt particles existed mainly on the exterior of the GCB. The Pt particles in nanopores were smaller than those on the exterior of GCB.

The number of nanopores decreased due to graphitization.

Particle size

(nm) number (%) Pt particle surface area (%) Pt particle

CB exterior 4.3±1.1 72.3 81.9 CB interior 3.3±0.8 27.7 18.1 9 P ar ti cl e n u m b er fr eq u en cy ( % ) Exterior (total 72%) Interior (total 28%) S u rf ace ar ea / m 2 Exterior (total 82%) Interior (total 18%) Particle size / nm S(s)_Pt: 69m2 _g-1 S(i)Pt: 15m2 g-1

Pt particle distribution of c-Pt/GCB-HT

SEM(exterior) and STEM(exterior and the interior) images of the c-Pt/GCB-HT catalyst

for both the front(0o_{) and back (180}o_).

10 All of the Pt particles existed on the carbon exterior surface. Internal Pt has aggregated with the external Pt as a result of the heat treatment.

Particle size (nm) _{surface area (%)} Pt particle

CB exterior 7.35±1.91 100 CB interior - N.D. 0° 180° SE SE 0° 180° TE TE Exterior (total 100%) P ar ti cl e n u m b er fr eq u en cy ( % ) S u rf ace ar ea / m 2 Exterior (total 100%) S(s)_Pt: 38m2 _g-1 Particle size / nm

New Catalysts with high activity and durability

Size control of Pt-based catalysts by the nanocapsule method

50 wt% Pt/C

We have succeeded in

controlling the Pt

particle size simply by changing the molar ratio of metal precursor to surfactant (M/S) in the preparation. M/S 金属担持率 (TG分析) 粒径 (wt%) dXRD (nm) d_STEM (nm) 0.1 46.5 2.0 2.0 ± 0.2 0.5 48.8 2.9 3.1 ± 0.3 1.0 48.7 4.5 4.5 ± 0.4 M/S 金属担持率 (TG分析) 粒径 (wt%) dXRD (nm) d_STEM (nm) 0.1 46.5 2.0 2.0 ± 0.2 0.5 48.8 2.9 3.1 ± 0.3 1.0 48.7 4.5 4.5 ± 0.4 Metal loaded (by TG) Particle size Monodisperse Pt/GCB GCB diphenyl ether 1,2-hecadecanediol Nano-capsule space enclosing Pt(acac)₂ Pt nanoparticle Reduction Loading

on carbon (to remove organic moieties) Heat-treatment

M/S=0.1 0 2 4 6 8 0 20 40 60 80 Particle size / nm Fr e que nc y, % 0 2 4 6 8 0 20 40 60 80 Particle size / nm Fr e que nc y, % 0 2 4 6 8 0 20 40 60 80 Particle size / nm Fr e que nc y, % d_STEM = 2.0 nm M/S=0.5 M/S=1.0 d_STEM = 3.1 nm d_STEM = 4.5 nm 11

H. Yano, M. Kataoka, H. Yamashita, H. Uchida and M. Watanabe, Langmuir, 23, 6438 (2007). H. Yano, T. Akiyama, H. Uchida and M. Watanabe, Energy Environ. Sci., 3, 1511 (2010), H. Yano, T. Akiyama, M. Watanabe and H. Uchida, J. Electroanal. Chem., 668, 137 (2013).

Pt particle distribution of n-Pt/GCB

SEM(exterior) and STEM(exterior and the interior) images of the n-Pt/GCB (30w%) catalyst

for both the front(0o_{) and back (180}o_).

12 All of the Pt particles existed only on the carbon exterior surface. Uniform size distribution of Pt was also controlled

by the nanocapsule method.

Particle size

(nm) surface area (%) Pt particle

CB exterior 3.04±0.60 100 CB interior - N.D. 0° 0° 180° 180° SE SE TE TE ZC P ar ti cl e n u m b er fr eq u en cy ( % ) S u rf ace ar ea / m 2 Particle size / nm Exterior (total 100%) Exterior (total 100%) S(s)_Pt: 88m2 _g-1 Particle size / nm

ECSA/m2_g Pt-1 100 81 73 0 20 40 60 80 100 13

U

,

%

channel flow double electrode

(30o_C) *heat treatment RDE (30o_{C) (40}MEA o_C) Pt surface area on CB exterior 62% Pt surface area in CB interior 38%

Difficult for the electrolyte to contact with all interior Pt.

Using our ink preparation process, most of the exterior Pt is in contact with ionomer.

Nanopores

in the carbon support should be

eliminated

for the

improvement not only of

durability

but also of

U

for the MEA

.

Relationship between Pt utilization (U_Pt) and Pt location on CB (c-Pt/CB)

U

,

%

a: effect of decreasing carbon nanopores

c: effect of optimizing ionomer

b: effect of both Pt location on the exterior

surface of CB and uniformity of Pt particle size

Improvement of

U

for MEA

To improve not only

durability

but also the

U

of the MEA,

First,

selecting the graphitized carbon support;

Second,

using the nanocapsule method;

Finally,

optimizing the ionomer.

14 0 20 40 60 80 100 1 2 3 4 5 c-Pt/ c-Pt/ c-Pt/ n-Pt/ n-Pt/ CB GCB GCB-HT GCB GCB Nafion Aquivion EW1100 EW980 0 20 40 60 80 100 1 2 3 4 5 EC SA /m 2 g Pt -1 RDE MEA c-Pt/ c-Pt/ c-Pt/ n-Pt/ n-Pt/ CB GCB GCB-HT GCB GCB Nafion Aquivion EW1100 EW980 (a)

15 0 20 40 60 80 100 120 140 160 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 c-Pt/ c-Pt/ c-Pt/ n-Pt/ n-Pt/ CB GCB GCB-HT GCB GCB Nafion Aquivion EW1100 EW980 Ef Pt vs . M A ma x o f P t/C B , % Ma ss Act ivi ty / Ag Pt -1 at 0. 85V 80o_{C, Air (1atm.), 100%RH, Pt 0.05mg/cm}2

Improvement of Pt effectiveness (

Ef

) for MEA

improved durability by GCB

Improvement was achieved by selection of the

graphitized carbon

support,

using the

nanocapsule method,

and

optimizing the ionomer.

The effect can be attributed to the improvement of

the distribution of both the size and the location of the Pt particles.

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Optimizing by low EW ionomer

Improvement of Pt ORR activity by alloying & skin layer

Achievement of the ideal CL

Pt alloy

Decreasing of nano pore by graphitized carbon

Nano pores

Preferential loading to the carbon support surface by nanocapsule method

 Our n-Pt₃Co/GCB prepared by the nanocapsule method exhibited the highest MA_k up to 70oC as catalyst activity in the CFDE evaluation. However, its durability was insufficient because the nonuniform Pt-skin layer cannot suppress the dissolution of the underlying alloy.

 The newly developed Pt_2ML-PtCo_(2nm)/GCB exhibited an MA_k value ca. 2.2 times larger than that for c-Pt/CB. The Pt skin layer of the new catalyst is very uniform.

Next, we will improve the ORR activity of the MEA by alloying & forming a uniform skin layer.

20 40 60 80 100 0 1000 2000 M a ss ac ti vi ty ( 0. 8 5 V ), MA k / A g -1 Temperature / oC n-Pt₃Co/GCB n-Pt/GCB Pt_2ML-PtCo_(2nm)/GCB c-Pt/CB -3000 O2saturated 0.1 M HClO4

Temperature Dependence of

Kinetically-Controlled Mass Activities (MA_k) at 0.85 V

K. Okaya, H.Yano, K. Kakinuma, M.Watanabe, and Hiroyuki Uchida, ACS Appl. Mater. Interfaces, 4 , 6982 (2012)

 We improved the MEA performance by using the following technologies:

(i) Thinning the CL, (ii) Uniform dispersion of Pt, (iii) Increase of Pt interparticle

distance, and (iv) Increase of ion exchange capacity of the ionomer (lower EW).

The Ef_Pt increased from 3.2% to 14.5% by these effects.

 This will be improved by 17.7% vs. the value of a commercial alloy catalyst by (v)

Alloying & (vi) Formation of a uniform skin layer.

 Compared with the NEDO target for mass power of the stack (10 Wmg-1 = 0.1

g_Pt/kW), we achieved higher performance by these effects in the middle & high

humidity range of 50 to 100% RH and 80 o_{C. (0.07～0.08 g/kW).}

 We will also meet the NEDO target at low humidity (30% RH) by alloying & forming a

uniform Pt skin layer using the nanocapsule method. 12.8

12.2

i. Thinning of CL

ii. Uniform dispersion

iii. Increasing of inter-particle distance iv. Ion exchange capacity of the ionomer

3.2 80o_{C, Air/H} 2, 30%RH, Uo:40%,Uf:70% Pt/CB 0.5mg/cm2 50w% EW1100 nPt/GCB 0.05mg/cm2 30w% EW980 0 10 20 Ef Pt vs. M Am ax of P t/CB , % Pt/CB 0.04mg/cm2 50w% EW1100 17.7 Pt₃Co/CB 0.04mg/cm2 50w% EW1100 v. Alloying &

vi. Uniform skin layer

nPt/GCB 0.05mg/cm2 30w% EW700 14.5 20 40 60 80 100 0 5 10 15 Pt/CB-50wt%, LSC:Nafion EW1100 nPt/GCB-30wt%, SSC:Asahikasei EW560 nPt/GCB-30wt%, SSC:Aquivion EW700 nPt/GCB-30wt%, SSC:Aquivion EW980 nPt/GCB-30wt%, LSC:Nafion EW1100 Relative humidity (%) M as s p ow er / W m g -1 Pt

Target of NEDO project (30%RH)

Cathode: Pt 0.05mg/cm2_{, 80}o_{C, Air/H} 2, Uo:40%,Uf:70%, 100kPa In v . S p ec if ic p ow er / g Pt kW -1 0.10 0.40 0.13 0.20 0.08

Improvement of MEA vs. NEDO Target

18



From the comparison of TEM, SEM and 3D images, the

presence of Pt

catalyst particles in nanopores

was confirmed.



The

nanopores

in the carbon support

should be eliminated for

the

improvement not only of the

durability

but also of the

Pt utilization for

the MEA.



We improved MEA performance by using the following technologies:

i. Thinning the CL,

ii. Making a uniform dispersion of Pt,

iii. Increasing the Pt interparticle distance

and

iv. Increasing the ion exchange capacity of the ionomer

.



Compared with

the NEDO target for mass power of the stack (10 W

mg

_{= 0.1 g}

Pt

/kW)

, we achieved

higher performance by these effects

at middle & high humidity (50 to 100% RH) and 80

_{C. (0.07}

_～

_0.08

g/kW).



We will also realize the NEDO target at low humidity (30% RH)

by

alloying & forming a uniform Pt skin layer using the nanocapsule method.

M. Uchida, Y.-C. Park, K. Kakinuma, H. Yano, D. A. Tryk, T. Kamino,

H. Uchida, M. Watanabe, Phys. Chem. Chem. Phys., 2013, 15, 11236-11247

Summary and future plans

19

This work was supported by funds for the “HiPer-FC Project” of

the New Energy and Industrial Technology Development

Organization (NEDO) of Japan.