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By Gumaa Ali M. El-Nagar (Assistant Lecturer)

Renewable Energy resources: Fuel Cells. By Gumaa Ali M. El-Nagar (Assistant Lecturer) Chemistry Department, Faculty of Science, Cairo University, Egypt. June 12, 2014.

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By Gumaa Ali M. El-Nagar (Assistant Lecturer)

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  1. Renewable Energy resources: Fuel Cells By Gumaa Ali M. El-Nagar (Assistant Lecturer) Chemistry Department, Faculty of Science, Cairo University, Egypt. June 12, 2014 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)

  2. Outlines • World Energy Crisis & Fuel Cells (FCs) • Direct Formic Acid Fuel cell (DFAFCs) • Formic Acid Oxidation (FAO) • Experimental • Results and Discussions: • Conclusions

  3. World Energy Crisis Why We NeedAlternative EnergyResources? • Energy one of major problem face humans today, Exist in all of our daily life • Development of any country depends on its consumption of Energy • More than 75% of Energy comes from Fossil Fuels; Non-renewable, Rapidly depletion, Resulted in Climate Change

  4. CO2 Emission & Climate Change • Due to the depletion of petroleum-based energyresources • 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

  5. Fuel Cells (FCs)Market • Intense research has focused on alternative energy technologies that can reduce the dependence on fossil fuels and its pollution • FCstechnologies 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)

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

  7. 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 MembraneFuel 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 stored • Its low-temperature operation, high power density, fast start-up • system robustness, and low emissions have ensured that the majority of motor manufacturers are actively pursuing PEMFC research and development.

  8. Direct Formic Acid Fuel Cells (DFAFCs) • DFAFCsare a promising solution to provide electricity for mobile and portable applications due to there advantages over Hydrogen (H) and methanol (M) FCs: • HFCswere 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 throughNafion membrane • Thinner membranes in DFAFCs, this is highly desirable for the design of compact portable power systems • DFAFCshave a higher theoretical open-circuit potential(1.40 V) than that of hydrogen fuel cells (1.23 V) and MFCs (1.21 V)

  9. Commercialization of FCs • The world-wide commercialization of FCs has not yet come • Two greatest barriers hamperfurther development in FCs are durability (of Nafion membrane) and cost electrodes, Pt (limited resources and expensive). • Understanding of all the electrocatalytic activity and mechanistic of the reaction at Pt electrodes is the key to reduce the Pt amount or replaced it with other non-precious metal

  10. Formic Acid Oxidation at NiOx and CoOx nano-structured Pt-Based electrodes Note that, FAO essential anodic reaction on DFAFC

  11. Formic Acid Oxidation CO2 + 2H+ + 2e Kdirect Reactive intermediate HCOOH H2O kOH H2O • FAO on Pt has dual pathway mechanism: • the direct oxidation, dehydrogenation of FA molecule to CO2 at a low anodic potential (desired), while formate anion serves as the reactive intermediate Kpoisoning COad CO2 + 2H+ + 2e kox • 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, Poising Electrode surface)

  12. Formic Acid Oxidation (FAO) • Ratio between direct and indirect peaks give degree of electrocatalytic activity • As this ratio increase, Direct pathway is favorable and High electrocatalytic activity obtained Indirect Pathway Coad oxidize to CO2 Direct Pathway FA oxidize to CO2 The CVs in 0.3M HCOOH (pH=3.5) for bare Pt electrode at 100 mVs-1 • Ratio between forward and backward peaks give degree of poising (CO Tolerance) • As close to one means high CO tolerance or less poising occurred

  13. Problem of CO-Poisoning • Modification of Pt surface by foreign metals and/or metal oxides may overcome the CO poisoning • Three approaches used: • Electronic Effect:addition of another metal to Pt which modify its electronic structure in away to disfavor the CO adsorption(e.g., PtPd) • 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, NiOx) • 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/Au)

  14. 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.

  15. Electrode preparations Electrochemical Methods: using an EG&G potentiostat (model 273A) operated with Echem 270 software: • 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

  16. Fabrication with PtNPs • PtNPs were electrodeposited from 0.1 M H2SO4 containing 1.0 mM H2PtCl6 • Potential step electrolysisfrom 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

  17. 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 Dendritic shape structure with average particle size ca. 35 nm It relation of the cathodic deposition of metallic nickel on GC electrode FE-SEM image of the electrodeposited metallic nickel on GC substrate

  18. 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. Aggregation, average particle size increased to 80 nm 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

  19. 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

  20. Results and Discussions • Formic Acid Oxidation At NiOx modified Pt/GC electrode NiOx/Pt/GC • Formic Acid Oxidation At Binary NiOx and CoOx modified Pt/GC electrode: Stability Issue

  21. FAO at NiOx/Pt/GC electrode • Material Characterizations SEM image PtNPs SEM image passivated nickel, NiOX (Inset Metallic Nickel) • SEM image NiOx-Pt/GC • Flower-like structure

  22. (a) Pt C O Pt Pt EDX for Pt/GC and NiOx-Pt/GC electrodes Ni 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. C(002) NiOOH Pt(200) Pt(111) Pt(311) Pt(220) XRD pattern for, shows Face Centered the cubic structure XRD pattern for NiOx/GC shows NiOOH/Ni(OH)2 phases present

  23. Electrochemical Characterizations Had/des PtO formation region PtO reduction peak CV of GC Pt/GC Alkaline medium at 100 mV /s CV of GC Pt bare Alkaline medium at 100 mV /s NiOOH and Ni(OH)2 transformation peak couple • Nickel deposition resulted in decrease in PtO reduction peak and Had/des peak CV of GC NiOx-Pt/GC Alkaline medium at 100 mV /s

  24. Electrocatalytic activity towards FAO: Ipd/ Ipind = 0.1 Id/Ib =0.04 Neither GC nor NiOx/GC electrodes has any catalytic activity towards FAO Ipd/ Ipind = 0.3 Id/Ib =0.7 • FAO at Pt/GC electrode • FAO at Pt bulk electrode • PtNPs curial component for FAO and has superior activity than Pt bulk • NiOx modified electrode has high electrocatalytic activity and high CO tolerance Id/Ib =1.0 • FAO at NiOX-Pt/GC, indirect peak completely disappeared • FAO at NiOx/Pt/GC electrode

  25. 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

  26. b • Stability (A) a 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 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 • 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)

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

  28. Electrochemical Characterizations • Deposition of CoOx and/or NiOx resulted in decrease in PtO reduction, PtO formation and Had/des peaks e CoO2 CoOOH Co(OH)2 d • Two peaks couples appear for CoOx transformations and one peak for NiOx transformation Co(OH)2 CoOOH CoO2 c • When CoOx deposited first and then NiOx only one peak couple appeared with potential in-between NiOx and CoOx transformations peak b 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

  29. Material Characterizations: Flower-like structure Spongy-like structure Nano-rod network Structure Alloy-formed 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

  30. Electrocatalytic activity towards FA • CoOx modified electrodes has more electrocatalytic activity than NiOx modified electrodes with same surface coverage d c • Binary modified CoOx and NiOx resulted in synergistic effect ..significant enhancement b a 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

  31. 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) CO stripping at Pt/GC, NiOx/Pt/GC and NiOx-CoOx/Pt/GC in 0.5M Na2SO4 measured at 50 mV s-1

  32. Stability d c b a CVs obtained at CoOx/NiOx/Pt/GC electrode before and after I-t measurements 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. • 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

  33. 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/Ipind 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.

  34. 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.

  35. Thank You for Your Attention

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