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Bottlenecks in Biofuel Cell Research

Han YanSupervisor: Dr. Gu Xu

Date: Jan 28th,2009

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Outline

Basics of Biofuel Cells

Newly found problem: pH gradient

Major obstacles

Proposed solutions

Our experiments

Results & a strange phenomenon

Summary and future work

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Basics of Biofuel Cells: Hydrogen Fuel Cells

PEM Commerciallyavailable at 1.0 W/cm2

Advantageso High efficiency

o Simple structure

o Clean/Quiet

Major obstacleso H2 production

o H2 storage

o Expensive catalyst

Rabaey et al. 2003

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Basics of Biofuel Cells: Introduction

Why Biofuel Cells

Fuel: renewable carbohydrates, such as sugars and alcohols, even waste water

Catalysts: microorganisms or enzymes, renewable

Milder operation temperature and neutral pH

Biofuel cells use biocatalysts for the conversion of chemical energy to electrical energy

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Power density(W/cm2)

Microbial fuel cell

0.43×10-3

Enzymatic fuel cell

0.46×10-3

PEM fuel cell

1.0

Developed since 1980s, but still in infancy, Why?103-104 times lower than PEM fuel cells in power density

Logan et.al,2006

Basics of Biofuel Cells: Current Status

Slow biological reaction

Demonstration SONY Biofuel Cell formp3 player

www.sony.net

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Basics of Biofuel Cells: Description

•Two types of Biofuel cells are currently being researched1) Microbial Type: microbials as catalyst in anode, difficult e- transfer

2) Enzyme Type: enzymes as catalyst in anode, sensitive to environment

Our Focus

Liu et al. 2004

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Basics of Biofuel Cells: Future Applications

Electricity from wastewater treatment (microbial)

Implanted medical sensor

(enzyme)Vielstich et al., 2003

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Levels of Analyses

Variable loadings/Start up Transient Behavior

Overpotentials Steady State Kinetics

Theoretical Energy Output Thermodynamics

Favorable ? Bio/Electro-Chemistry

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Level 1: Bio/Eletro-Chemistry

Anode: 2β-D-glucose → 2δ-gluconolactone +4H++4e-

Cathode: O2 + 4H++4e- →2 H2O

Overall: 2β-D-glucose +O2 → 2δ-gluconolactone +2 H2O

β-D-glucose δ-D-gluconolactone

http://www.chemblink.com/products/90-80-2.htmhttp://www.scientificpsychic.com/fitness/carbohydrates.html

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Level 2: ThermodynamicsElectromotive force (EMF):

At 25 ºC, pH 7, ionic strength 0.1M:ΔGfa of β-D-glucose (aq) : -429.08 kJ/molΔGfb of δ-gluconolactone (aq) : -500.26 kJ/molΔGfc of H2O (l) : -156.05 kJ/molΔGfr= ΔGfc +ΔGfb –ΔGfa=-227.23 KJ/mol

Theoretical Er = 1.18 V

nFG

E fr

Δ−= r

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ηact

ηohm

ηconc

Typical IV curve

Power output = Vcell (Cell Voltage) X Icell (Cell Current)

IV curve decides the power output, characterized by 3 kinetics losses (overpotentials)

1. Activation Overpotential (ηact)

• Ohmic Overpotential (ηohm)

• Concentration Overpotential (ηconc)

Level 3: Overpotentials

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Level 3: OverpotentialsActivation Overpotential(ηact)Governed by Tafel equation:

ΔVact=A log (i/i0)Caused by redox reaction at electrode/electrolyte interface

ηact= a log (i) +dMinimized by

1. roughness of electrode2. i0 by temperature,

efficiency of catalyst or concentration of reactant

K. Muller, "Journal of the Research Institute for Catalysis, Hokkaido University", Vol. 17, pp 54-75, 1969.

Real Curve

Best-fit line

A, a, b are constants

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Ohmic Overpotential (ηohm)1) ηohm= b i b=overall resistance2) b is caused by electrode, ions/electrons migration in

electrolyte, wires and all contact

Concentration Overpotential ( ηconc )ηconc= m exp(ni)

Therefore: Total voltage output is expressed as:

E=Er-(ηact+ ηohm+ ηconc)

E=Er-a log (i) –b (i)- m exp(ni)

Level 3: Overpotentials

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1. Glucose → gluconolactone +H++ e- by E1.

2. e- from E1 --M1-- anode --external circuit --cathode --M2 --E2.

conductivenonconductive

Basics of Biofuel Cells: Electron Transfer

Electron transfer is a cause of 2 overpotentials:

1. Ohmic

2. Activation

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Newly Found Problem: pH gradient

Gil et al. (2003) observed a decreasing pH in the anode chamber and an increasing pH in the cathode chamber of a two-chamber microbial fuel cell using Nafion membrane.

Rozendal et al. (2006) observed mainly cation species other than protons were responsible for the transport of positive charge through Nafion membrane in a microbial fuel cell, which resulted in an increased pH in the cathode chamber and a decreased cell performance.

Gil et al. 2003

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Bad Consequences of pH Gradient (1)Na+ in Buffer replace H+ in NafionNa+ is from buffer solution that is used to stabilize the neutral condition Na+ caused water loss in Nafion results in low H+ transferH+ transfer in Nafion is suppressedPoisoning of Nafion (sulfonated tetrafluorethylene )The performance of cell therefore is hindered

1) J. St-Pierre et al in J. New Mat. Electrochem. Systems(2000) explained the Na+ and H+ behavior in PEM fuel cell

2) Rozendal et al. in Environ. Sci. Technol (2006) found large amount of Na+ in Cathode

3) Chae et al. in Energy and Fuels (2008) proved Rozendal’s point by EDX

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Neutral pHBuffer Solution

Na+ H+

Nafion

H+

Anolyte

CNa+

CH+106

Bad Consequences of pH Gradient (2)

No paper has systematically discussed the problem yet

3 4 5 6 7 8 9 10 110

20

40

60

80

100

Rela

tive

activ

ity, %

pH value

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Our ExperimentMediator: FMCA (ferrocene monocarboxylic acid )

Enzyme: GOx (Glucose Oxidase)

Single compartment structure, air cathode in first step for simplicityBiofuel in liquid anodeSimilar to PEM fuel Cell but using enzyme as catalyst

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Our Set Up

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pH Drop in Anode Voltage Degradation

pH Problem Confirmed

Current

Voltage

pH drop has been Confirmed by our own experiment

6.7

6.1

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Proposed Solutions

1. Other cation membranes with H+ selectivity

2. Membraneless Structure

3. Using a buffer that contains no Na+

4. Anion Exchange Membranes (AEM)

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Possibility 1 : Selective Membranes

Unfortunately, such membrane has not been invented or discovered so far

Such ideal membrane remains as a challenge to biochemists

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Possibility 2 : MembranelessCause direct oxidation and short circuit within the cell

Introduced to minimize cost in waste water treatment

Low efficiency (oxidation)

Much more sophisticated method to immobilize catalyst on electrodes

Selective enzymes are used as well

A expensive alternative

Anode: 2β-D-glucose → 2δ-gluconolactone +4H++4e-

Cathode: O2 + 4H++4e- →2 H2O

Numerous discussions about membraneless

• Liu et al. 2004, Environ. Sci. Technol. 38, 4040-4046

• Logan et al. Environ. Sci. Technol. 2004, 38, 2281-2285

• Logan et al. Journal of Power Source, 2008, 179, 274-279

Liu et al. 2004

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Possibility 3: Na+-free Buffer

0.0 0.5 1.0 1.5 2.0 2.50.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Current density,mA/cm2

Vol

tage

, V

Anode, DI water Anode, 0.1M Phosphate buffer Anode, 0.1M Tris buffer

0.0 0.5 1.0 1.5 2.0 2.50.0

0.1

0.2

0.3

0.4

Anode

Cathode

Pote

ntia

l, V

Current density,mA/cm2

DI water 0.1M Phosphate buffer 0.1M Tris buffer

pH =7.05

pH =7.23

pH =4.45

pH =7.42

The limited H+

transport and electron transport in Tris buffer caused the increase of the anode and cathode overpotentials

Tris buffer contains no alkali or alkaline earth metal or transitional metal cations.

Research on going…

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Last Solution: Anion MembraneSlightly different bio-electrochemistry

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Last Solution: Anion Membrane

OH-

Work by Others:Rozendal et al. in Environ. Sci. Technol. (2007) and (2008): single-chamber air-cathode microbial electrolysis cell (MEC) with Nafion and Fumasep (AEM)-Objective=compare 2 membranes in H2production, nonbiofuel cell system

Logan et al. in Environ. Sci. Technol. (2009) reviewed all microbial electrolysis methodsCall et al. in Environ. Sci. Technol. (2009) described a new microbial electrolysis cell…

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pH gradient problem: Resolved?

Nafion AEM

0 20 40 60 80 100 120 140 160 1806.0

6.1

6.2

6.3

6.4

6.5

6.6

6.7

6.8

6.9

7.0

Nafion pH

pH

Time (hour)0 20 40 60 80 100 120 140 160 180

6.0

6.1

6.2

6.3

6.4

6.5

6.6

6.7

6.8

6.9

7.0

AEM pH

pH

Time (hour)

(Measurement error: + 0.02 pH)

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But: Much Lower Power Output!

0.00 0.01 0.02 0.03 0.04

0.00

0.05

0.10

0.15

0.20

0.25

Nafion CMI-7000 EXCELLION Fumasep Reinforced Fumasep

Volta

ge (V

)

Current Density (mA/cm2)0.00 0.01 0.02 0.03 0.04

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

Nafion CMI-7000 EXCELLION Fumasep Reinforced Fumasep

Pow

er D

ensi

ty (

mW

/cm

2 )

Current Density (mA/cm2)

(Measurement error: + 0.02 pH)

(Measurement error: + 5 %)

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Why AEM has a lower power?

Description Thickness (mm)

Resistance *(Ω)

Nafion Sulfonated tetrafluorethylene

0.2133 <0.034

Fumasep crosslinked polypropylene

substrate0.0355 <0.106

Fumasep (Reinforced)

0.0812 <0.245

CMI-7000 Gel polystyrene crosslinked with divinylbenzene

0.4622 <4.820

EXCELLION I-200 polypropylene binder from a

solventless method0.3530 0.803-1.606

* With effective electrode area of 6.24 cm2

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Summary of Anion Membrane BFC

Higher resistance is one of the causes for its low power densityOther unknown causes are also possibleSeems to be a brighter way against the bottleneckTransient behavior of OH- in cathode is still not fully understood

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Also Found: A Strange Phenomenon (1)

• Flat/decreasing voltage and current are normal for ordinary cell/battery

• Abnormal performance for biofuel cell

• All enzymatic type experiments show this behaviour including Nafion and all Anion Membranes

• More obvious in tests involving large resistors

normal

Abnormal

0 200 400 600 800 10000.20

0.25

0.30

0.35

0.40

0.45

0.50

Volta

ge (V

)

Cur

rent

(mA

)

Time (s)

0.00

0.01

0.02

0.03

0.04

0.05

0 200 400 600 800 10000.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

Volta

ge (V

)

Cur

rent

(mA

)

Time (s)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Nafion

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A Strange Phenomenon (2)

Possible Patterns:1. Stable at a certain value

for each cell2. Large time interval

between runs3. Large resistors

difference between runs4. Mixed 2 or more effects

More stable with less or no interruption between each run

0 100 200 300 400 5000.03

0.06

0.09

0.12

0.15

Volta

ge (V

)

Cur

rent

(mA

)

Time (s)

0.060

0.062

0.064

0.066

0.068

0.070

0 50 100 150 200 250 300 350 4000.04

0.08

0.12

0.16

0.20

0.24

Volta

ge (V

)

Cur

rent

(mA

)

Time (s)

0.003

0.006

0.009

0.012

0.015

Normal

Abnormal

AEM

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www.phy-astr.gsu.edu

A Strange Phenomenon (3)

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Reminds us of capacitanceProposed mechanism

1. Double layers on interface2. In parallel or in series?3. Charge accumulated on double layer in open

circuit4. Long time of charge/discharge in bio-involved

reaction

A Strange Phenomenon (3)

Other proposed mechanism???

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Summary/Further Work

The pH problem caused by buffer is critical

No systematic analysis is available

Tris buffer method can be studied further by exploring similar buffer solution

AEM seems to be a possible solution, and should be studied further by comparing individual current/power behaviour

Further understanding of the strange phenomenon will contribute to transient behavior on the electrolyte/electrode interface

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Thanks!