Electrocatalytic Activity Of Novel Binary Modified Platinum Surfaces With Cobalt

and Nickel Oxides Nanostructured Electrodes Towards Formic Acid Oxidation:

Direct Formic Acid Fuel Cells (DFAFCs)By

Gumaa Ali M. El-Nagar)Assistant Lecturer(

Chemistry Department, Faculty of Science, Cairo University, Egypt.

June 12, 2014June 12, 2014

Renewable Energy resources: Fuel Cells

Outlines

World Energy Crisis & Fuel Cells (FCs)

Direct Formic Acid Fuel cell (DFAFCs)

Conclusions

Experimental

Results and Discussions:

Formic Acid Oxidation (FAO)

World Energy CrisisWorld Energy Crisis

Energy one of major problem face humans today, Exist in all of our daily life

Why We NeedNeed Alternative Energy Resources?

Development of any country depends on its consumption of Energy

More than 75% of Energy comes from Fossil Fuels; Non-renewable, Fossil Fuels; Non-renewable, Rapidly depletion, Resulted in Climate ChangeRapidly depletion, Resulted in Climate Change

Due to the depletion of petroleum-based energy resources and Its environmental impact, limitations and climate

change (Green house effect)

There is a growing awareness of the need for basic and applied energy research

CO2 Emission & Climate Change

FCs technologies have received much attention in recent years owing to their broad range of Benefits (e.g., Energy security, Environmental benefits and domestic economy):

high efficiencies and low emissions Fuel flexibility (use of diverse, domestic fuels, including clean and

renewable fuels)

Intense research has focused on alternative energy technologies that can reduce the dependence on fossil fuels and its pollution

Fuel Cells (FCs)Market

FCs expected to replace the fossil fuel-based energy sources to provide electric power daily-live activities (portable, stationary and mobile applications)

Fuel Cells Applications

Fuel Cells (FCs)Fuel Cells (FCs)

FCs have several types, distinguished from each other by the used materials (e.g., electrolyte & charged species that it transports)

Two types that received the most attention in recent years are Proton-Exchange Membrane Fuel Cells (PEMFCs) and Solid Oxide Fuel Cells (SOFC).

PEMFC is one of the most widely researched fuel cell technologies because it offers several advantages;Easily transported and storedIts low-temperature operation, high power density, fast start-upsystem robustness, and low emissions have ensured that the majority of motor manufacturers are actively pursuing PEMFC research and development.

DFAFCs are a promising solution to provide electricity for mobile and portable applications due to there advantages over Hydrogen (H) and methanol (M) FCs:

HFCs were limited by difficulties with hydrogen storage and transport

MFCs suffered from inherent toxicity, and slow oxidation kinetics and high crossover through Nafion-based membranes

FA non-Flammable, Non-Toxic and has a smaller crossover flux through Nafion membrane

Thinner membranes in DFAFCs, this is highly desirable for the design of compact portable power systems

DFAFCs have a higher theoretical open-circuit potential (1.40 V) than that of hydrogen fuel cells (1.23 V) and MFCs (1.21 V)

Direct Formic Acid Fuel Cells (DFAFCs)Direct Formic Acid Fuel Cells (DFAFCs)

Commercialization of FCsCommercialization of FCs

The world-wide commercialization of FCs has not yet come Two greatest barriers hamper further development in FCs are

durability (of Nafion membrane) and cost electrodes, Pt (limited resources and expensive).

Understanding of all the Understanding of all the electrocatalytic activity electrocatalytic activity and and mechanisticmechanistic of the reaction at Pt electrodes is the key to of the reaction at Pt electrodes is the key to reduce the Pt amount reduce the Pt amount or or replaced it with other non-precious metalreplaced it with other non-precious metal

Formic Acid Oxidation at NiOx and CoOx nano-structured Pt-Based electrodes

Note that, FAO essential anodic reaction on DFAFC

Formic Acid OxidationFormic Acid Oxidation

FAO on Pt has dual pathway mechanism: the direct oxidation, dehydrogenation of FA molecule to CO2 at a

low anodic potential (desireddesired), while formate anion serves as the reactive intermediate

the indirect (dehydration) pathway, the adsorption of the dehydration product of HCOO (i.e., CO) at low potential domain and its oxidation at a higher potential domain ( undesired, undesired, Poising Electrode surfacePoising Electrode surface)

CO2 + 2H+ + 2e

Kdirect

Reactive intermediate

HCOOHH

2 OK

poisoning COad

koxCO2 + 2H+ + 2e

H2Ok OH

 

The CVs in 0.3M HCOOH (pH=3.5) for bare Pt electrode at 100 mVs-1

Ratio between direct and indirect peaks give degree of electrocatalytic activity

As this ratio increase, Direct pathway is favorable and High electrocatalytic activity obtained

Ratio between forward and backward peaks give degree of poising (CO Tolerance)

As close to one means high CO tolerance or less poising occurred

Formic Acid Oxidation (FAO)

E / mV vs. Ag/AgCl/KCl (sat.)

-200 0 200 400 600

I / m

A c

m-2

0

5

10

15

20

25

Direct PathwayFA oxidize to CO2

Indirect PathwayCoad oxidize to CO2

Problem of CO-Poisoning Modification of Pt surface by foreign metals and/or metal oxides

may overcome the CO poisoningThree approaches used: Electronic EffectElectronic Effect: addition of another metal to Pt which modify its

electronic structure in away to disfavor the CO adsorption(e.g., PtPd)

Bifunctional Effect:Bifunctional Effect: addition of metal oxides can easily provide oxygen atoms to facilitate the oxidation removal of CO at low potential domain (e.g., MnOx, RuOX, NiOxe.g., MnOx, RuOX, NiOx)

Third-body effect:Third-body effect: Utilizes the fact that three adjacent Pt sites is necessary for CO

adsorption Interruption of the contiguity by a surface modifier such as gold

nanoparticles (AuNPs) can overcame the CO poisoning (e.g., Pt/AuPt/Au)

Experimental

Measurements: The electrocatalytic measurements was performed in a conventional two compartment three electrode glass cell.

All measurements were performed at room temperature (25±1◦C) using an EG&G potentiostat (model 273A) operated with Echem 270 software.

Electrode preparations

The most familiar binder-free used for the preparation of nanoparticles

It is a facile technique which results in the direct attachment of the nanoparticles to the substrate

The facile control of the characteristics of nano-materials (e.g., size, crystallographic orientation, mass, thickness and morphology) by adjusting the operating conditions and bath chemistry

Electrochemical Methods: Electrochemical Methods: using an EG&G potentiostat (model 273A) operated with Echem 270 software:

Fabrication with PtNPs PtNPs were electrodeposited from 0.1 M H2SO4 containing

1.0 mM H2PtCl6

Potential step electrolysis from 1 to 0.1 V vs. Ag/AgCl for 120 s resulting in the electrodeposition of 3.3 μg of Pt (estimated from the charge of the i-t curve)

Grained shape structure with average particle size 40 nm

Homogenously covered surface

FE-SEM image of PtNPs

Fabrication with nano-NiOx:

Modification was achieved in two sequential steps: The first involved electrodeposition of metallic nickel from an aqueous solution of 0.1 M acetate buffer solution (ABS, pH=4.0) containing 1 mM Ni(NO3)2 by a constant potential electrolysis at −1V vs. Ag/AgCl

It relation of the cathodic deposition of metallic nickel on GC electrode

FE-SEM image of the electrodeposited metallic nickel on GC substrate

Dendritic shape structure with average particle size ca. 35 nm

On the second step, the metallic Ni was passivated (oxidized) in 0.1 M phosphate buffer solution (PBS, pH=7) by cycling the potential between−0.5 and 1 V vs. Ag/AgCl/KCl(sat) for 10 cycles at 200 mV/s.

CVs of the passivation of the electrodeposited metallic nickel on in 0.1 M PBS at 100 V s-1

FE-SEM micrographs of the passivated Nickel

Aggregation, average particle size increased to 80 nm

Fabrication with CoOx

Electrodeposition took place in phosphate buffer solution (PBS with pH = 7.0) containing 1mM CoCl2

Potential was cycled from 1.2 V and − 1.1 V vs. Ag/AgCl/KCl (sat.) at 100 mVs-1

Spongy Porous structure

FE-SEM image CoOx/Pt/GC

Formic Acid Oxidation At NiOx modified Pt/GC electrode NiOx/Pt/GC

Results and Discussions

Formic Acid Oxidation At Binary NiOx and CoOx modified Pt/GC electrode: Stability Issue

Material Characterizations

SEM image NiOx-Pt/GC Flower-like structure

FAO at NiOx/Pt/GC electrode

SEM image passivated nickel, NiOX (Inset Metallic Nickel)

SEM image PtNPs

(a)

XRD pattern for NiOx/GC shows NiOOH/Ni(OH)2 phases present

EDX for Pt/GC and NiOx-Pt/GC electrodes

Kev

0 2 4 6 8 10 12 14

Inte

nsity

0

500

1000

1500

2000

2500

3000

3 min, NiOX/Pt/GCPt/GC

C

O

Pt

PtPt

Ni

Table 1 Bulk composition of Pt/GC catalyst

Element Atomic content,

At% ,.

Weight content,

Wt% ,.

Measurements error ,

%

C K17.7517.75±0.0445

O K3.651.15±0.0025

Pt L21.1581.10±0.7149

Total100.00100-

Table 2 Bulk composition of NiOx/Pt/GC catalyst

Element Atomic content,

At% ,.

Weight content, Wt% ,.

Measurements error% ,

C K71.0315.27±0.0653

O K5.131.62±0.0009

Pt L15.7965.89±0.4666

NiK7.0515.220.0087

Total100.00100-

XRD pattern for, shows Face Centered the cubic structure

2

20 40 60 80

Cou

nts

200

400

600

800

1000Pt/GCNiOx/Pt/GC

C(002)

Pt(111)

Pt(311)

Pt(200)

Pt(220)

NiOOH

Peaks of NiOx-Pt/GC shifted to lower angles Assuming alloy formation between Pt and NiOx shift can be attributed to the difference in atomic size.

Electrochemical Characterizations

E / mV vs. Ag/AgCl/KCl (sat.)

-800 -600 -400 -200 0 200 400 600

I /

mA

cm-2

-1

0

1

2

CV of GC NiOx-Pt/GC Alkaline medium at 100 mV /s

NiOOH and Ni(OH)2 transformation peak couple

E / mV vs. Ag/AgCl/KCl (sat.)

-800 -600 -400 -200 0 200 400 600

I /

mA

cm

-2

-1.5

-1.0

-0.5

0.0

0.5

1.0

CV of GC Pt bare Alkaline medium at 100 mV /s

E / mV vs. Ag/AgCl/KCl (sat.)

-800 -600 -400 -200 0 200 400 600

I /

mA

cm

-2

-1.0

-0.5

0.0

0.5

CV of GC Pt/GC Alkaline medium at 100 mV /s

Nickel deposition resulted in decrease in PtO reduction peak and Had/des peak

Had/des PtO formation region

PtO reduction peak

Electrocatalytic activity towards FAO:

FAO at NiOX-Pt/GC, indirect peak completely disappeared

PtNPs curial component for FAO and has superior activity than Pt bulk

NiOx modified electrode has high electrocatalytic activity and high CO tolerance

E / mV vs. Ag/AgCl/KCl (sat.)

-200 0 200 400 600

I / m

A c

m-2

0

5

10

15

20

25

FAO at Pt bulk electrode

E / mV vs. Ag/AgCl/KCl (sat.)

-400 -200 0 200 400 600

I / m

A c

m-2

0

1

2

3

FAO at Pt/GC electrode

E / mV vs. Ag/AgCl/KCl (sat.)

-400 -200 0 200 400 600

I /

mA

cm-2

-5

0

5

10

15

20

25

FAO at NiOx/Pt/GC electrode

Ipd/ Ip

ind = 0.1Id/Ib =0.04

Neither GC nor NiOx/GC electrodes has any catalytic activity towards FAO

Ipd/ Ip

ind = 0.3Id/Ib =0.7

Id/Ib =1.0

CO Stripping, Role of NiOX Same amount of CO

formed at the two electrodes

CO stripping peak at NiOx modified electrode shifted to more negative potential

NiOx nano-structured catalyze CO at low potential (bi-functional effect)

CO stripping experiment at Pt/GC and NiOx/Pt/GC. The poisonous species was adsorbed from 0.5M FA and the poison stripping as conducted at 100mVs−1 in 0.5M H2SO4

E / mV vs. Ag/AgCl/KCl (sat.)

-200 0 200 400 600 800 1000

I / m

A c

m-2

0

1

2

3

4

5

6

7

PtGCNiOx/PtGC

Stability

CVs response obtained NiOx/Pt/GC before (solid line-black), after ageing (dashing red line) for 5 and after ageing (dashing green line)15 hours in FA solution with pH 3.5 at + 0.3 V in 0.5 M KOH with scan rate 0.1 V s-1

Decrease in I-t curve may be due to dissolution of NiOX, PtNPS or CO poising

From Figure B real area of PtNPs not changed that mean NiOx good attached on surface

But NiOx transformation peak decrease with time which explain decrease in I-t curve (deactivation of active NiOOH phase)

I-t obtained during FAO at (a) nano-Pt/GC and (b) nano-NiOx/nano-Pt/GC in 0.3 M HCOOH (pH 3.5) at a potential of +0.3 V vs. Ag/AgCl

t / h

0 2 4 6 8 10 12 14 16

I / m

A c

m-2

0

2

4

6

8

ba

(A)

Formic Acid Oxidation At Binary NiOx and CoOx modified Pt/GC electrodes: Stability

Issue

CVs of (a) bare Pt, (b)Pt/GC,(c) NiOx/Pt/GC, (d) CoOx/Pt/GC and (e) NiOx-CoOx/ Pt/GC electrodes in 0.5 M KOH at a scan rate of 100 mV s−1

E / mV vs. Ag/AgCl /KCl(sat.)

-800 -600 -400 -200 0 200 400 600

I /

mA

cm

-2

-2

0

2

4

6

8

10

12

ab

c

d

e

Co(OH)2CoOOH CoO2

Co(OH)2 CoOOH CoO2

Electrochemical Characterizations Deposition of CoOx

and/or NiOx resulted in decrease in PtO reduction, PtO formation and Had/des peaks

Two peaks couples appear for CoOx transformations and one peak for NiOx transformation

When CoOx deposited first and then NiOx only one peak couple appeared with potential in-between NiOx and CoOx transformations peak

FE-SEM micrographs obtained for (a) NiOx/Pt/GC, (b) CoOx/Pt/GC, (c) NiOx-CoOx/Pt/GC, and (d) CoOx-NiOx/Pt/GC electrodes

Material Characterizations:

Flower-like structure

Spongy-like structure

Nano-rod networkStructureAlloy-formed

Electrocatalytic activity towards FA

FAO at (a) unmodified Pt/GC, (b) NiOx/Pt/GC, (c) CoOx/Pt/GC and (d) NiOx-CoOx/ Pt/GC, scan rate of 0.1 V s−1

E / mV vs. Ag/AgCl/KCl (sat.)

-200 0 200 400 600 800 1000

a

b

c

d

CoOx modified electrodes has more electrocatalytic activity than NiOx modified electrodes with same surface coverage

Binary modified CoOx and NiOx resulted in synergistic effect ..significant enhancement

E / mV vs. Ag/AgCl/KCl (sat.)

-200 0 200 400 600 800 1000

I / m

A c

m-2

0

1

2

3

4

5

6

7

PtGCNiOx/PtGCNiOx-CoOx/PtGC

CO stripping at Pt/GC, NiOx/Pt/GC and NiOx-CoOx/Pt/GC in 0.5M Na2SO4 measured at 50 mV s-

1

CO Stripping Amount of CO formed at

the three electrodes is the same

NiOx and CoOx oxides shifted the CO oxidation peak to more negative potential

CoOx enhanced FAO via catalyze CO oxidation at low potential (Bifunctional effect)

Stability

I-t obtained at (a) PtGC, (b) NiOx/PtGC, (c) CoOx/PtGC, and (d) NiOx-CoOx/PtGC in 0.3 M FA solution (pH 3.5) at a potential of +0.3 V.

t / h

2 4 6 8 10 12 14

I / m

A c

m-2

0

1

2

3

4

5

6

7

ab

c

d

E / mV vs. Ag/AgCl /KCl(sat.)

-800 -600 -400 -200 0 200 400

I /

mA

cm

-2

-1.0

-0.5

0.0

0.5

1.0

1.5

CVs obtained at CoOx/NiOx/Pt/GC electrode before and after I-t measurements

As clearly seen from I-t curves CoOx modified electrodes has high catalytic activity and stability towards FAO

Presence of CoOx increase the stability of NiOOH/Ni(OH)2 transformation

Conclusions A novel nano-CoOx and/or nano-NiOx modified Pt catalyst for the

direct electrooxidation of FA was developed

This modification resulted in a superb enhancement of the direct oxidation pathway of FA to CO2.

The ratio Ipd/Ip

ind increased about 75 and 50 times upon modifying the Pt substrate with a nano-CoOx and nano-NiOx, respectively

This reflects that the direct dehydrogenation pathway has become preferential for the FA oxidation.

Nickel oxide (in the NiOOH phase) and cobalt oxide (in the CoOOH phase) are believed to provide mediate the oxidation scheme of FA in such a way that facilitate the charge transfer

NiOx and CoOx catalyze CO at low potential (Bi-functional effect)

The prepared catalyst exhibits satisfactory stability and reproducibility for 15 hours of continues electrolysis, which makes it attractive as anode in DFAFCs and applications.

Thank You for Your Attention