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Abstract
• DEFCs: alternative energy sources recently
• Emreging DEFC technology has challenges
• Many improvements have been made.
• Yet, there are deep needs for addressing
current challenges.
Introduction
• Direct Alcohol Fuel Cells DAFCs are from the Alkaline Fuel Cells AFCs family
• AFCs give higher energy density than PEMFC
• Non-noble metal catalysts can be used in AFCs
• DAFCs: (methano, ethanol, ethelyne glycol, 2-propanol)
• DAFCs use both alkaline (electrooxidation ) and acitic (CO2 , performenace ) media.
DAFCs challenges • Poor peformenace electrocatalysts (Low T)
• Anode surface poisoning (intermediates CO)
• Some cells: acidic & alkaline media(1.14 V)
DMFC vs DEFC
• Sluggish reactions kinetics for methanol oxid.
• Methanol crossover through nafion membrane
• Anode poisoning by CO
• Ethanol: less toxic
• Ethanol: higher energy density
• Ethanol: agriculture biomass products
• Ethanol: lower crossover rate
Direct ethanol fuel cell
DEFC challenges- crossover
• Crossover: the permeation of ethanol from the anode through the electrolyte membrane to the cathode.
• Crossover effect: cathode potential and cathode depolarization, reducing cell efficiency
• Crossover occurs when acetic acid, CO 2 &acetaldehyde (%) > O2 (%) in cathode.
Effect of current density on the crossover rate at different
temperatures and different ethanol concentrations
The plot of ethanol
crossover rate
versus ethanol
concentration with
different
temperature and
different helium
flow rate
Challenges= slow kinetics
• Its deduced the best DEFC performenace temperature is 90 C
Challenges = heat management • Temperature = performenace
• Ethanol conversion with current & T
The effect of operating
discharge cell current and
temperature on ethanol
conversion
Challenges= water management
• Cathode reaction: the major water source & ethanol dilution in the anode
• Water can generate cell resistence (performenace) (management needed)
• water can be removed through the cathode or transferred to the anode & eleminated
• Water uptake from polymer membrane: (T, disscoiation, counter ions type, elasticity, hydrophobicity
Typical water distribution in alkaline DEFC
• contineous flow field
• Hydrophibic filters
• Cathode flooding
Solutions thought
Challenges: durability & stability • According to MEA coditions
• Some research: 60h concluding the catalysts
aggolimeration and cathode flooding are the
major causes of degredation
• Ethanol is not giving the desirable
performenace
• Pd can replace PtRu catalyst
• Breaking C-C bond is obstacle to form CO2
Challenges: fabrication & design
The cell components Anode Gas Difusion layer GDL, Anode catalyst layer, Electrolyte membrane, Cathode catalyst layer, & Cathode GDL
Two alternative
routes always
used for (MEA)
preparation: a)
fixing the
catalyst layer
directly onto the
membrane &b)
the separate
electrode method
Schematic presentation of the detailed electrode preparation procedures
(a) the conventional method
7/2
6/2
01
2
18
(b) the decal transfer method
Good membrane should have:
• High proton conductivity • Low electron conductivity • Resistant to oxidation • Low fuel crossover • Adequate mechanical, thermal & chemical
stability • Good water water management
Electrooxidation
Pathways ethanol in
alkaline media
Reaction pathways DEFC using Pt in acidic media
Cathode catalysts
• Ag-W2 C, Pd, Pt-Ru
• Pt-Co/C, Pt-Pd/C
• At MEA foam layer of (Ni-Cr)
Performance ranking of PtRuNi/C, PtSnNi/C, PtRu/C & PtSn/C in DEFC
DEFC applications
DEFC applications