H2O selectivity of carbon alloy catalysts for

PEMFC cathode

1

July 7,2014

Naokatsu Kannari, Takafumi Suzuki and Jun-ichi Ozaki

The 4th German-Japanese Joint Symposium on Carbon materials

Gunma University

2

Outline

・・・・Introduction

・・・・Results and discussion

・・・・Conclusion

― Deference of the selectivity between Co-NSCC and Fe-NSCC

― Reaction pathway of oxygen reaction reaction

― Nanoshell-containing carbon

・・・・Experimental

― Proton exchange membrane fuel cell

3

Introduction

cathodeanode

+

H+

H2

2H+

2e-

H2O

1/2O2

2H+

2e-

O2

H2

Proton

exchange

membrane

e- e-

Proton exchange membrane fuel cell (PEMFC)

Advantages

・high energy density

・low working temperature

・cleanliness

Problems The cathode requires more platinum than the anode,

because the cathode reaction is quite slow

Exploration of the non-platinum cathode catalysts is needed

to replace the costly platinum catalyst

– Proton exchange membrane fuel cell –

←Oxygen Reduction Reaction (ORR)

Introduction

Non-platinum cathode catalyst

for PEMFCCarbon alloy catalyst

① Nanoshell structure ② Heteroatoms doping

nanoshell

20ｰ50 nm

Carbon alloy catalyst

Highly active catalysts for ORR4

5

Introduction

Starting polymer Transition metal complex

Carbonization

TEM image of NSCC

ex.) Co or Fe phthalocyanines

Preparation and structure of nanoshell-containing carbon (NSCC)

–Nanoshell-containing carbon as an ORR catalyst –

NSCCs show ORR activityPromising candidate

for the cathode catalyst

Model structure of nanoshell (NS)

20-50 nm

Hollow spherical shape,

whose wall is composed of

graphitic layers

Reaction pathways of ORR

6

Nanoshell forming catalyst %H2O

Fe High

Co low

Strong oxidant to damage the polymer electrolyte and the catalyst

Need to develop ORR catalysts

with no or low H2O2 formation

What is the

controlling factor

for the selectivity?

Our previous study

①: Direct 4-electron pathway

②+③: 2+2-electron pathway

O2 H2O

H2O2

4e-

2e-2e-

①

② ③

Objective of this study

7

To clarify the factors determining the H2O

selectivity of NSCCs

8

Experimental

M-NSCC (M = Co or Fe)

Carbonization

Ball-milling

Acid washing

In a nitrogen stream,

Heating rate：10℃/ min

Temperature：800 (Fe)

or 1000℃ (Co)

Time：1 h

Carbonization conditions

Precursor

Phenol-formaldehyde resin(PF) Metallophthalocyanine（Fe or Co：3 wt%）

OH

**

n

–Preparation of NSCCs –

9

Evaluation of ORR activity and selectivity

O2

Ox Red OxReference

electrode

Working

electrode

Counter

electrode

Disk electrodeDetection of ORR current

Ring electrode

Detection of H2O2 formation

Rotating ring-disk electrode (RRDE) method

Working electrode0 0.2 0.4 0.6 0.8 1

-0.6

-0.4

-0.2

0

Potential / V vs. NHE

Curr

en

t /

mA

Disc current : ID

Ring current : I R

Calculation of the selectivity

to H2O from O2 (%H2O) by

both disk and ring currents

10

Sample EO2

(V vs. RHE)

%H2O

(%)

Fe-NSCC 0.78 86

Co-NSCC 0.78 59

0 0.2 0.4 0.6 0.8 1-0.4

-0.3

-0.2

-0.1

0

Potential / V vs. RHE

Curr

ent

/ m

A

0

0.05

Ring

Disk

Fe-

NSCC

Co-

NSCC

EO2: Potential at -10 µA cm-2

EO2 ・・・ Fe-NSCC = Co-NSCC

%H2O ・・・ Fe-NSCC > Co-NSCC

Selectivity depended on the

types of nanoshell forming

catalysts, Fe or Co.

ORR activities and selectivies of NSCCs

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Possible explanations for the Fe-NSCC’s high H2O-

selectivity compared to Co-NSCC’s

① Direct 4-electron pathway

② 2+2-electron pathway

③ H2O2 decomposition by disproportionation reaction

O2 H2O

H2O2

4e-

2e- 2e-

Fe-NSCC promotes ….

(difficult to occur in carbon catalysts )

O2 + H2O

No

electron

transfer

12

0 0.2 0.4 0.6-0.6

-0.4

-0.2

0

Curr

ent

density /

mA

cm

-2

Potential / V vs. RHE

Fe-NSCC

Co-NSCC

Fe-NSCC shows higher catalytic activity

for H2O2 reduction than Co-NSCC

Electrolyte

Counter electrode

Reference electrode

Scanning region

Scanning speed

Rotating speed

Conditions of H2O2 reduction

： 1 mM H2O2 / 0.5 M H2SO4

: Glass-like carbon

: Reversible hydrogen electrode

: 1 〜 0 V vs. RHE

: 1 mV/s

: 1500 rpm

0.167

0.067

2.5 times

larger

0

0.04

0.08

0.12

0.16

0.2

Fe-NSCC Co-NSCC| ｊｊ ｊｊ|

0.5

V[m

A/

cm

2]

H2O2 reduction activity

O2 H2O

H2O2

4e-

2e- 2e-

13

How dose Fe-NSCC promote the H2O2 reduction?

Possible controlling factors

① Surface metal species

② Surface chemical structure

14

Surface metal species

710720730740

Inte

nsity

Binding Energy [eV]

100 cps

780790800810

Binding Energy [eV]

Inte

nsity

100 cps

Fe 2p XPS spectrum

of Fe-NSCCCo 2p XPS spectrum

of Co-NSCC

Sample M/C (M = Fe or Co) H2O2

Reduction activity

Fe-NSCC 0 High

Co-NSCC 0.002 low

Metal species is

not the

controlling

factor

15

Fe-NSCC showed a

redox peak at ca. 0.6 V

Electrolyte

Counter electrode

Reference electrode

Scanning range

Scanning speed

Conditions for CV

: 0.5 M H2SO4

: Glass-like carbon

: Reversible hydrogen

electrode (RHE)

: 1 〜〜〜〜 0 V vs. RHE

: 50 mV/s

0 0.5 1-1

0

1

Curr

ent

density /

mA

cm

-2

Potential / V vs. RHE

Fe-NSCC

Co-

NSCC

Electrochemically active species on NSCC

The redox species should be

introduced by the addition of Fe, which

does not mean the direct action of Fe

Quinone groups are the candidates

for the redox species

=

=

-

-

Ox

RedOH

OH O

O

Possible factor for

the promotion of H2O2 reduction

16

Red

Ox

H2O2

H2O

2H+,

2e-

From

electrode

From

electrolyte

Ox/Red H2O/H2O2

0.6 V 1.76 VPotential / V

H2O2/O2

0.7 V

2H+,

2e-

Formation

of H2O2

The redox cycle

with the redox

potential at 0.6 V

promoted H2O2

reduction

Mechanism for the promotion of H2O2 reduction

by the redox species

17

Conclusion

In this study, we investigated the controlling factor for the

ORR selectivity of NSCCs.

・Fe-NSCC shows the higher selectivity than Co-NSCC

Comparison of Fe-NSCC and Co-NSCC

the redox species at 0.6 V

The redox species promotes the selectivity of the ORR

the higher H2O2 reduction activity than Co-NSCC