Microbial Fuel Cells (MFCs): - AZ Water Association

Microbial Fuel Cells (MFCs):
Generating Electricity, Hydrogen and Chemicals from Wastewater
César I. Torres
(cit@asu.edu)
AZ Water Conference
May 5, 2011

2
MXC Team: César I. Torres, Bruce E. Rittmann,
Rosa Krajmalnik-Brown, Andrew Kato Marcus,
Prathap Parameswaran, Sudeep Popat, Joseph
Miceli, Jon Badalamenti, Bradley Lusk, Abdul
Hasam, Tyon Bry.
Other collaborators: Dae Wook Kang, Anca
Delgado, Junseok Chae, Seokheun Choi, Martina
Hausner, Laura Berthiaume, Greg Wanger, Yuri
Gorby.
All EB team.
Plus support from: ASU Biohydrogen Initiative,
Open Cel, Office of Naval Research.

3
Why Microbial Fuel Cells?
• Chemical fuel cells are based on metal catalysts
(expensive).
• They are affected by impurities in the fuel/air
• The reactions that can be catalyzed are rather
simple:
–H2→H + + e- –CH 3 OH + H 2 O→ CO 2 + 6H + + 6e -
• They are very efficient, more efficient than
combustion processes.
• They reliably produce high power densities.
H 2
H +
A
n
o
d
e
e -

4
Why Microbial Fuel Cells
• Microbial fuel cells take advantage of
the diversity of microbial enzymes.
– Anything anaerobic bacteria can consume
will be converted into power in an MFC. Biofilm
• This is what makes an MFC unique:
Their capability to catalyze complex
reactions and obtain energy from
diverse chemical bonds!
e -
A
n
o
d
e
e -

Our target wastes
• We focus on wastewater and animal wastes,
applying microbial fuel cell technologies.
• Our target is to maximize energy capture from
these waste streams.

• Anode Respiring Bacteria (ARB)
ARB to produce electricity
– Convert organic substrates and wastes into
electrical energy
• Excellent way to “concentrate” and transform
the energy
– A one-step process to obtain useful energy
from bacterial processes – higher efficiency.
– A new form of anaerobic respiration that
allows the fast consumption of BOD in
water.
Biofilm
e -
A
n
o
d
e
e -

7
ARB in the Biofilm Anode
• Microbial nanowires are thought to be the main way
ARB transfer electrons to a solid anode.

8
Finding ARB in nature
• So far, few ARB isolates are able to produce high current
densities, we look for more ARB capable of doing this.
Carolina, PR
Superior, AZ
Current Density (A/m 2 )
Current Density (A/m 2 )
10
8
6
4
2
0
0 2 4 6
Time (days)
8 10 12
10
8
6
4
2
0
New media
0 5 10 15 20
Time (days)

9
Biofilm Transport Limitation
• We develop models to characterize how much electrical
current is expected from a particular waste.
• PCBIOFILM allow us to calculate the limit of substrate/H +
transport limitation.
Alk 0 (mgCaCO 3/L)
5000
4000
3000
2000
1000
0
Donor Limitation
0 100 200 300
0
400
Effluent BOD (mg/L)
20
15
10
5
Current Density (A/m 2 )

10
Phototrophs as ARB
• If EET is a ubiquitous process carried out by bacteria,
some phototrophs must be able to perform EET.
– Donate electrons to an anode at lower potentials,
– Accept electrons from a cathode to fix CO2.

Microbial Fuel Cell (MFC)
e- donor half reaction: CH3COO - 0.29 V
- + 3 H2O → CO2 + HCO -
3 + 8H + + 8e- e - acceptor half reaction: 2 O 2 + 8H + + 8e - → 4 H 2 O 0.81 V
Net reaction: CH 3 COO - + 2O 2 → CO 2 + HCO 3 - + H2 O
Electrical power
generation
1.10 V
CH3COO M
e
m
b
r
a
n
H2O e
The reaction potential drives all biological, chemical, and electrochemical
processes in MFC => typical recovered potentials are 0.3 - 0.6 V
-
¼O2 Air
e- H + Anode Cathode
+

Microbial Electrolysis Cell (MEC)
e- donor half reaction: CH3COO - 0.29 V
- + 3 H2O → CO2 + HCO -
3 + 8H + + 8e- e - acceptor half reaction: 8H + + 8e - → 4 H 2 -0.41 V
Net reaction: CH 3 COO - + 3H 2 O → CO 2 + HCO 3 - + 4 H2
Anode
CH 3 COO -
H + +
e -
Due to the various types of electrochemical cells, we term this
technology MXCs
M
e
m
b
r
a
n
e
H 2
-0.12 V
H 2 gas
production
Cathode

Franks and Nevin, 2010
MXCs – Current Applications

14
CH3COO- CH3COO- H + H + + +
e- e- M
e
m
b
r
a
n
e
Electrical power
generation
¼O 2
H 2 O 2
Air
Cathode
MFCs – WW Treatment
• Our approach to wastewater
treatment is to use an MFC to
produce hydrogen peroxide (H 2 O 2 ).
• Peroxide can be used to
disinfect drinking water, plus the
wastewater effluent.
• It could be a key component to
achieve water reuse remote
locations.
e- donor half reaction: CH3COO - 0.29 V
- + 3 H2O → CO2 + HCO -
3 + 8H + + 8e- e - acceptor half reaction: 4 O 2 + 8H + + 8e - → 4 H 2 O 2 0.28 V
Net reaction: CH 3 COO - + 3 H 2 O + 4 O 2 → CO 2 + HCO 3 - + H2 O 2
0.57 V

Microbial Fuel Cell (MFC)
e- donor half reaction: CH3COO - 0.29 V
- + 3 H2O → CO2 + HCO -
3 + 8H + + 8e- e - acceptor half reaction: 4 O 2 + 8H + + 8e - → 4 H 2 O 2 0.28 V
A
Net reaction: CH 3 COO - + 3 H 2 O + 4 O 2 → CO 2 + HCO 3 - + H2 O 2
Organic
waste
CO 2
e- e- A
n
o
d
e
M
e
m
b
r
a
n
e
Electrical power
generation
OH- OH- C
a
t
h
o
d
e
O 2
H H2O 2O 2
e- e- A
n
o
d
e
Organic
waste
CO 2
H 2 O 2
0.57 V
Electrical power
generation
C
a
t
h
o
d
e
O 2
Diffusion
Layer

• An important consideration when building MXCs
is the need for a conductive biofilm support.
H +
Engineering MXCs
– Carbon fibers can be used because of their high
surface area and conductivity.
A
n
o
d
e

Engineering MXCs
• The engineering design can help increase MXC efficiency.
• An MXC is a hybrid between a biological reactor and an
electrochemical cell: careful design must accommodate
both considerations.
• A 125mL MEC produces
~0.2 A and 2.1 L-H 2 /day.
– This equals to
17.5 L-H 2 /L-day
Inner
cathode
H 2 gas out
AEM
Graphite or
SS fiber
anode
Anode liquid
outer flow

Engineering MXCs
• Despite the improved engineering approach, wastewater
feeds show other transport limitations that lead to lower
current densities.
Current density (A/m 2 )
50
40
30
20
10
0
Ac
-1
Real wastewater (WW)
Current density
Applied voltage
WW1 WW2 WW3 WW4
0 50 100 150 200
Time (day)
Acetate-2
8
6
4
2
0
Applied voltage (V)

19
MXC design
• Maximize efficiency and optimize waste consumption.
• Develop prototypes to be tested in the field.
Anode
liquid
AEM and
Pt/C Cathode
Anode Chamber-
Carbon or SS fiber
Cathode
Chamber-
Air/CO 2 gas
Cathode Air flow
Z" (Imaginary), ohm
2.5
2.0
1.5
1.0
0.5
0.0
0 2 4 6 8 10 12 14
‐0.5
Z' (Real), ohm

20
Summary
• ARB are fast consumers of BOD, potentially faster than
aerobic organisms.
• The main challenges in MXC implementation lie within the
engineering design.
• MXCs are currently tested for field-scale applications and
this will lead to implementation in the near future.
• A fundamental understanding of the chemical and biological
processes is needed to design a feasible and efficient MXC.

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Torres Lab: César I. Torres, Sudeep Popat, Joseph
Miceli, Bradley Lusk, Jon Badalamenti, Abdul
Hamdan, Dongwon Ki.
Other collaborators: Bruce E. Rittmann, Rosa
Krajmalnik-Brown, Prathap Parameswaran,
Daewook Kang, Anca Delgado, Nicole Hansmeier,
Junseok Chae, Seokheun Choi, Martina Hausner,
Laura Berthiaume, Greg Wanger, Yuri Gorby.
Plus support from: Office of Naval Research, ASU
Biohydrogen Initiative.