Electrocatalytic Activity Of Novel Binary Modified Platinum Surfaces With Cobalt and Nickel Oxides...
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Transcript of Electrocatalytic Activity Of Novel Binary Modified Platinum Surfaces With Cobalt and Nickel Oxides...
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)
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.