presented by:
Ch. Venkateswara Rao

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SEARCH FOR A VIABLE OXYGEN REDUCTION ELECTRODE
Importance of electrochemical reduction of oxygen
í Fuel Cells
í Metal-Air batteries
í Industrial electrolytic processes
Fuel Cells
Direct Energy Conversion Vs. Indirect Technology
Batteries/Internal Combustion Engines/Fuel Cells
Batteries
- Needs recharging
- Toxic chemicals
- Low energy density
Internal combustion engines
- Carnot limitations
- Moving parts and hence friction
- Noisy
Fuel Cells – Advantages
• Efficiency
í Cleanliness
í Unique operating characteristics
í Planning flexibility
í Reliability
í Future development potntial
Difficulties in PEMFC & DMFC
 Sluggish oxygen reduction kinetics
 Methanol crossover (in DMFC)
Electrocatalysts
Essential criteria for choosing an electrocatalyst for oxygen reduction
 High oxygen adsorption capacity
 Structural stability during oxygen adsorption and reduction
 Stability in electrolyte medium
 Ability to decompose H2O2
 High conductivity
 Tolerance to CH3OH (in DMFC)
 Low cost
Noble metal based electrocatalysts
Pt
í Pt alloys
-- PtFe, PtCo, PtNi, PtCr
í Carbon supported Pt and its alloys
Why Pt ?
High work function ( 4.6 eV )
í Ability to catalyze the reduction of oxygen
í Good resistance to corrosion and dissolution
í High exchange current density
Why carbon as an electrode support ?
Chemical properties
- Good corrosion resistance
- Available in high purity
- Forms intercalation compounds
Electrical Properties
- Good Conductivity
Mechanical Properties
- Dimensionally & Mechanically stable
- Low modulus of elasticity
- Light weight & adequate strength
- Availability in variety of physical Structures
- Easily fabricated into Composite Structures
í Most promising Electrocatalyst – 20 wt% Pt/C
Difficulties with Pt
í Slow ORR due to the formation of –OH species at +0.8 V
Development of mixed potential (in DMFC)
Scarce and expensive