The Challenge of Mass Transport Resistance in

Catalyst Layers and Its Impact to Performance

at High Current Density

Di-Jia Liu

Argonne National Laboratory

Presented at DOE CATALYSIS WORKING GROUP MEETING

Wednesday, July 27, 2016

Argonne National Laboratory

Mass Transport Limited Region in Fuel Cells

2

0

0.5

1.0

Theoretical Voltage

Mas

s Tr

ansp

ort

Lim

ited

Reg

ion

Ohmic Region

Current Density K

inet

ic R

egio

n

Ce

ll V

olt

age

Fuel Cell Polarization Curves

Availability → Accessibility → Activity

Reaction Temperature

Cata

lyti

c c

on

vers

ion

Kinetic

Regime

Intra-

particulate

Diffusion

Regime

Bulk

Diffusion

Regime

Heterogeneous Catalytic Conversion

Key Factors Hampering Fuel Cell Performance at

High (or low) Current Domain

Lack of Activity – low turn-over-frequency (TOF)

Lack of Accessibility – active site not near triple-phase boundary

Lack of Accessibility – mass transport resistance

Lack of Availability – insufficient active site density

Lack of Reactant – mass transfer rate < conversion rate

3

Examples of Performance Differences at High Current

Region between PGM and PGM-free Catalyst Electrodes

4

0 500 1000 1500 2000 2500 3000 3500 4000

0.2

0.4

0.6

0.8

1.0

Non-PGM

BASF

Non-PGM

BASF

Current Density (mA cm-2)

Ce

ll p

ote

nti

al (V

)

0

200

400

600

800

1000

1200

Po

wer

Den

sit

y (

mW

cm

-2)

One-bar Oxygen

0 200 400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

non-PGM

BASF

NON-PGM

BASF

Current Density (mA cm-2)

Ce

ll P

ote

nti

al (V

)

0

100

200

300

400

500

600

700

Po

wer

Den

sit

y (

mW

cm

-2)

One-bar Air

Condition: Pair or PO2= PH2 = 1 bar (back pressure = 7.3 psig) fully humidified; T = 80 °C; N-211 membrane; 5 cm2 MEA; PGM-free cathode catalyst = 3.5 mg/cm2, anode catalyst = 0.3 mgPt/cm2.

Commercial BASF MEA, cathode catalyst = 0.35 mgPt/cm2 , Anode catalyst = 0.2 mgPt/cm2

Challenges in Catalytic Structure using Conventional

Carbon Support

5

Deff = D/; = porosity

= (path l/straight l)2

Gas diffusion inside electrode

1/Deff = 1/Dmicro + 1/Dmeso + 1/Dmacro

Macro Meso

Micro

Catalyst

Micro→meso→macro tiered structure held by vdW force

Mass transfer through propagation of different porosities (Knudsen vs. molecular diffusions)

Charge/heat transfer through particle contact percolation

Challenges in Electrode Architecture using

Conventional Carbon Support

6

Conventional Membrane Electrode Assembly

Electrolyte Anode

GDL Cathode

GDL

H2 e-

e-

H+ H2

e-

e-

O2

H2O

H2

Nonoyama, et. al. JES 158 (4) B416, (2011)

7

Aligned Carbon Nanotube (ACNT) as Catalytic

Support for PEMFC

ACNT MEA

Electrolyte Anode Cathode

H + O2

e-

H2

e-

H+

H2O

Better catalyst utilization

Better support stability

Better electrical & thermal conductivity

Better water management

Better mass transport

Functionalized ACNT as PGM-free catalyst

Advantages of ACNT based MEA

J. Yang, G. Goenaga, A. Call and D.-J. Liu*, Electrochem. Solid-State Lett, (2010) 13 (6) B55-B57,

J. Yang, D.-J. Liu*, N. Kariuki and L, X. Chen, Chem. Comm. 3 (2008) 329 – 331

D.-J. Liu*, J. Yang, N. Kariuki, G. Goenaga, A. Call, and D. Myers, (2008), ECS Trans, 16 (2) 1123-1129

J. Yang and D.-J. Liu*, Carbon 45 (2007) 2843–2854.

D.-J. Liu, J. Yang and D. Gosztola, ECS Transc. (2007) 5(1) 147-154

Straight Spiral

Branched Wavy

8

Process of Fabricating ACNT as Catalyst Support of

MEA

Fabricating ACNT as catalyst support in MEA

“Method of fabricating electrode catalyst layers with directionally oriented carbon support for proton exchange membrane fuel cell”, Liu

& Yang, US Patent 8,137,858

“Catalytic Membranes for Fuel Cells”, Liu & Yang, US Patent 7,927,748

“Aligned carbon nanotube with electro-catalytic activity for oxygen reduction reaction”. Liu, Yang & Wang, US Patent 7767616

“Method of fabricating electrode catalyst layers with directionally oriented carbon support for proton exchange membrane fuel cells”,

Liu, Yang & Wang, US Patent 7,758,921

Forming MEA Growing ACNT Seeding Catalyst

9

Catalyzing ACNT with Pt via Dry and Wet Chemistries

-1.0E-01

-8.0E-02

-6.0E-02

-4.0E-02

-2.0E-02

0.0E+00

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

E (V vs SHE)

I / A

mg

-1 P

t

Commercial Pt/C

Pt/ACNT-4c

TEM Study RDE Study

50 nm

Pt/ACNT prepared through CVD process demonstrated very good

electrochemical activity even with highly hydrophobic surface

Bifunctional solvents needed to avoid nanotube bundling

CVD processes enable >20 wt% Pt incorporated in ACNT

Highly dispersed Pt with particle sizes from 2 nm ~6 nm was observed

Relatively uniform distribution through depth of ACNT layer was observed

10

Cross-section Characterization of ACNT-MEA

SEM Images of ACNT-MEA

ACNT Cathode Layer

Nafion® Membrane Layer

Ink-based Anode Layer

11

Single Cell Test

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Current Density, A/cm2

Cel

l V

olt

ag

e, V

olt

s

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Po

wer

Den

sity

, W

/cm

2

Argonne ACNT-MEA

Benchmark - MEA

Single cell test demonstrated improved polarization using ACNT based MEA

5 cm2 Single Cell, T = 75 C,

PH2= PAir=1 Bar,

H2 flow=100 ml/min,

Air flow=400 ml/min

Support Stability Significantly Improved during

Potential Cycling

12

0

20

40

60

80

100

0 100 200 300

Potential cycles

Per

cen

t of

EC

SA

as

of

ori

gin

al,

% ACNT MEARef MEA

Cell cycling potentials 0.5 V to 1.4 V

Improved electrocatalytic surface area retention is due to more stable graphitic ACNT

13

Synthesize TM/N/C active site over carbon nanotubes

ACNT as PGM-free Catalysts

0 1 2 3 4 5 6

ACNT-1

ACNT-2

R (Å)

FT

(

·k2)

Fe-Fe

Fe-N

0 1 2 3 4 5 6

ACNT-1

ACNT-2

R (Å)

FT

(

·k2)

Fe-Fe

Fe-N

A. Titov, P. Zapol, P. Kral, D.-J. Liu, & L. Curtiss, J. Phys. Chem. C. 113, 21629 (2009)

Study N-ligated TM active site structure by XAS and DFT calculation

Shell N R (Å)

Fe-N 4.2 1.95

+ NH3 M +

CH3

CH3 N

N

M

N

N

TM precursor

HC’s N-dopant Active site CNT

J. Yang, D.-J. Liu, N. Kariuki, L. Chen, Chem. Comm. 3, 329 (2008)

14

Probing Active Site Redox Mechanism by In Situ X-ray

Absorption Spectroscopy

It

X-ray

I0

If

APS

Wo

rkin

g

Re

fere

nce

Co

un

ter

In-Situ Electrochemical Cell

O2

D.-J. Liu, J. Yang and D. Gosztola, ECS Transc. 5(1) 147 (2007).

0.0

0.4

0.8

1.2

1.6

7.100 7.110 7.120 7.130 7.140 7.150 7.160

E (keV)

Norm

ali

zed

Ab

sorp

tio

n

E = 1.0 volt

E = 0.55 volt

E = 0.0 volt

1s 3d

1s 4p

-4

-2

0

2

4

6

8

-10010030050070090011001300

E/mV (vs. NHE)

i/m

A

Ar Saturated

O2 Saturated

TM oxidation state and coordination structure under different polarization

potentials revealed O2 binding and conversion kinetics during ORR

R (Å) 0 1 2 3 4 5 6

RD

F A

mp

litu

de

(a

.u.)

E = 1.0 v

E = 0.55 v

E = 0.0 v

Fe-N/O bindings

Fe electronic transitions

NN

Fe+3

NN

NN

Fe+2

NN

O=O

e-H+

NN

Fe+2

NN

O=OH

e-H+

Step 1 Step 2

Design Non-PGM Catalyst using “Support-free” Precursors

15

Catalytic Activity Turn-Over-Freq. x Site Density

• Different transition metals & organic ligands

• Different metal-ligand coordination

• Carbon “Support-free”

• High & uniformly distributed active site density

Conventional ANL’s MOF/POP approach

From carbon supported to “support-free” catalyst

Achievable Current Density @ 0.8 V

(Cathode loading @ 4mg∙cm-2 / 1 bar O2)

- 170 mA/cm2 for 1% Fe, or - 340 mA/cm2 for 2% Fe loading

Critical assumption: a) TM site atomically dispersed & fully utilized b) TOF = 1/10 of Pt (2.5 e-/site.s)

* Gasteiger, et al. Applied Catalysis B: Environmental 56 (2005) 9 R. Jasinski, Nature, (1964) Ma, Goenaga, Call and Liu,

Chemistry: A Euro. J (2011)

“Reality-check” on Non-PGM Activity

i (A/cm2) = 1.6x10-19 x TOF (e-/site∙s) x SD (cm-3) x τ (cm)* From 2D to 3D - Breaking away from 50-year tradition of supported

square-planar precursor

16

MOFs as Precursors for non-PGM Catalysts

“Non-Platinum Group Metal Electrocatalysts Using Metal Organic Framework Materials and Method of Preparation”, D.-J. Liu, S. Ma, G. Goenage, US Patent 8,835,343

.

Synthesis of MOF-based non-PGM Catalyst (Why “burn a perfectly good crystal”?)

TM SBU

MOF Catalyst

Solvothermal Reaction

Heat Activation

Organic Ligand

Advantages of MOF based non-PGM Catalyst

– Highest precursor density for active site conversion

– Well-defined coordination between metal (SBU) & ligand

– Porous 3-D structure with high SSA and uniform micropores

– Large selection of existing MOF compositions

ANL Non-PGM Catalyst IP Portfolio

Developed Fe/N/C decorated CNT catalyst for ORR and demonstrated FeN4 active site structure

– Chem. Comm. 3, 329 (2008) – US Patent 7,927,748 – US Patent 7767616 – US Patent 8,137,858 – US Patent 7,758,921

Developed metal-organic framework (MOF) based non-PGM catalyst and started “support-free” approach

– Chemistry: A European Journal, 17 2063 (2011)

– US Patent 8,835,343 – US Patent Application 20150180045

Developed binary MOF non-PGM catalyst, achieved measured volumetric current density and power density

– Chemical Science, 3 (11), 3200 (2012)

Aligned Carbon

Nanotube

Co-Zeolitic Imidazolate

Framework (ZIF)

Fe-ZIF/ZIF-8 binary MOF

System

ANL Non-PGM Catalyst IPs (continued)

18

Developed non-PGM catalyst using porous organic polymer (POP), achieved peak power density of 730 mW/cm2

– Angew. Chem. Int. Ed. 52 (32), 8349 (2013) – US Patent 9012344 – US Patent Application 20150194681

Developed “one-pot” synthesis approach for low-cost production of a broader range of MOF-base non-PGM catalyst

– Advanced Materials, 26, 1093, (2014) – US Patent Application 20130273461 – US Patent Application 20150056536

Developed the first nano-network non-PGM catalyst, achieved peak power density of 870 mW/cm2

– Proceedings of National Academy of Sciences, 112(34), 10629 (2015)

– US Patent 9,350,026

Fe/Co–POP

“One-pot” Synthesized

ZIFs

Nanofibrous Network

Catalytic Mass/Charge Transport Improvement in

Graphtic Nanonetwork Architecture

19

Conventional Support O2

H+

MOF Catalyst

Nanofibrous Support

O2

H+

Nano- network

Electrospinning Conversion to catalyst

Fuel cell fabrication e-

H+

O2 H2O

Process of Preparing PGM-free Catalyst with Porous

Nano-network Architecture

J. Shui, C. Chen, L. R. Grabstanowicz, D. Zhao and D.-J. Liu, Proceedings of National Academy of Sciences, U. S. A. 2015, vol. 112, no. 34, 10629 D.-J. Liu, J. Shui, C. Chen, “Nanofibrous electrocatalysts”, US Patent 9,350,026

Unique Nanofiber Morphology – High Surface Area

Nearly Exclusively Distributed in Micropore Region

21

Black Pearl

Ketjen Black

Fe/N/CF

BET (m2 g-1) 1432 798 809

Surface Area of Pores ≤ 2nm (m2 g-1)

867 398 780

Surface Area of Pores ≤ 50nm (m2 g-1)

960 507 788

S≤2nm/S≤50nm 90% 78% 99%

Total Volume in Pores ≤ 80 nm (cm³ g-1)

0.955 0.508 0.330

Volume of Pores ≤ 2nm (cm³ g-1)

0.371 0.167 0.271

Volume of Pores ≤ 50nm (cm³ g-1)

0.766 0.463 0.323

V≤2nm/V≤50nm 48% 36% 84%

High specific surface area (BET = 700 ~ 1251 m2/g)

High micro-porosity (Micro-pore vol./Total pore vol. > 80%)

Excellent PGM-free Catalytic Activity Measured by RDE

22

E1/2 = 0.82 V

Ne 4

New Electrode Architecture Led to Very Good

Performance at Single Cell Levels

23

Nanofibrous Network Morphology is Sensitive to

Composition of Electro-spin Mixture

24

14% 0%

39% 24%

Morphology Impacts Electrode Architecture and

Fuel Cell Performance

25

New Nanofibrous Network Showed Promise for

Durability Improvement

26

RDE shows excellent activity retention under multi-potential (0.6 to 1.0 V)

cycling in acidic media

Fuel cell accelerated stress test (AST) shows similar decay rate to that of

commercial Pt/C MEA

Prospects of Porous Nanonetwork as New Support

for PGM Catalyst

Efficient mass transport

Better thermal/electronic conductivity

Stable graphitic network backbone

High specific surface area

Tailored surface compositions to promote catalyst-support interaction

27

Acknowledgement

28

Fuel Cell Research - Shengqian Ma, Dan Zhao, Shengwen Yuan, Jianglan Shui, Gabriel Goenaga, Junbing Yang, Chen Chen, Heather Barkholtz, Lina Chong, Lauran Grabstanowicz, Alex Mason, Brianna Reprogle, Sean Comment, Zachary Kaiser, Debbie Myers

US DOE, US DOE Office of Fuel Cell Technologies and Office of Science. The use of Advanced Photon Source and Electron Microscopy Center are supported by Office of Science, U. S. Department of Energy under Contract DE–AC02–06CH11357.