Direct Ethanol Fuel Cells DEFCs: Review

A. M. Sheikh

ahmad.elsheikh@hotmail.com

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