Sergey Kalyuzhnyi

Department of Chemical Enzymology, Chemistry Faculty Moscow State University, 119992, Moscow, Russia

Biomass Fuel Cells

•Basic principles of fuel cell

Content

•Enzymatic fuel cell

•Biomass fuel cell:

•Limitations of chemical fuel cell

– how it works?

– performance

– problems

– perspectives

Basic principles of fuel cell (FC)•Related to battery: both convert chemical energy into electricity

•Battery: the chemical energy has to be stored beforehand

•FC only operates when it is supplied from external sources

•Fundamental mechanism: inverse water hydrolysis reaction

Anode: 2H 2 4H+ + 4e-

Cathode:

4e - + 4H + + O 2 2H 2O

Net reaction:

2H2 + O2 2H2O

Fuel cell Operating temp., °C Specific power, kW/l

Alkaline (AFC) 60-100 <0.3

Polymer electrolyte (PEPC, PEMFC, SPFC)

80-100 0.2-1

Phosphoric Acid (PAFC) 160-200 <0.1

Direct methanol (DMFC) 60-120 <0.1

Molted carbonate (MCFC) 600-700 <0.1

Solid Oxide (SOFC) 900-1000 <0.1

Fuel cell technologies

Hydrogen-oxygen PEFC(data of US Department of Energy )

Parameters Modern state Goal for 2008

Specific power, W/kg 200 550

Efficiency, % 45 55

Work time, months 1.4 7

СО inhibition, ppm 100 1000

Capital cost, $/kW 200 35

Pt expense, g/kW 20 less

Temperature, oC ~80 same

• Cost is the major hurdle

Limitations of large chemical FC

• The most widely marketed FC - 4,500 $/kW

• Diesel generators – 800-1,500 $/kW

• Natural gas turbines - even less!

• The goal of US DOE – to cut costs for FC to 400 $/kW by 2010

• Cost as well

Limitations of small chemical FC

•Pt-based:

50 kW (<$ 10 000)

$10,000 – only engine!

– poisoning with CO, H2S etc.

– low fuel versatility (H2, CH3OH)

– cost & shortage of Pt

Dynamic of Pt cost & its availability

1960 1970 1980 1990 20000

2

4

6

8

10

12

14

16

18

20

22

Co

st o

f P

t /

US

$ g

-1

year

Annual production:

180 tons

In 2000: 57.5 mln. cars

50 kW engines 5750 tons Pt

Reforming gas (H2): 12.5 % of CO

Pt electrodes: under 0.1% CO activity irreversibly decreases 100 times after 10 min

Hydrogenase el-ds: -not sensitive up to 1% of CO;-reversibly restore activity after inhibition;

- catalyst is renewable

Poisoning by fuel impurities

Н2O2

Immobilised oxydase (laccase)

Immobilised hydrogenase

Electric current

Solid polymer electrolyte

Enzymatic fuel cell (indirect bioFC)

Power density – till 40 W/m2

Specific power - till 6 kW/l

Theoretical specific power - till 20 kW/l

Problems of enzymatic fuel cells

• No any full scale implementation– Cost (pure enzymes are expensive)

– Stability of enzymes (inactivation, inhibition)

• Strong need in further R&D:

– Genetic engineering for improvement of enzyme properties & development of stable large-scale source of enzymes

– Improvement of electrode compartments (mass-transfer, new methods of enzyme immobilization)

– Low fuel versatility (enzymes are too specific)

Microbial fuel cell (MFC)

Electron

tran

sport

chain

of cell

CO2

Mox

Mred

An

ode

Solid

electrolyte

Cath

ode

V

O2

H2O

H+

Organics

Electron

• MFC – mimic of biological system in which bacteria do not directly transfer their produced electrons to their characteristic acceptors• MFC could be mediator–less (e.g., external cytochromes like in Shewanella putrefaciens or Geobacter sulfurreducens)

History & current developments of MFC

• Pioneering research: Potter (1912), Cohen (1931), Allen (1972) - inefficient

• The first viable MFC – Bennetto et al., 1984

Current interest on the following types of prokaryotes:

• Yeast-driven MFC (Reed & Nagodawithana, 1991)

– Heterotrophs (Delaney et al., 1984)

– Photoheterotrophs (Tsujimura et al., 2001)

– Sediment (Tender et al., 2002)

• Gas-generators (additionally - heat production)

• Reforming (conversion to H2) + H2-O2 fuel cell

Indirect (via biogas)

Direct (without biogas)

• Sulphate reducing fuel cell (sulphide is mediator)

• Mediator-less MFC (direct transfer of electrons from cells)

Electricity from anaerobic digestion

• Mediator MFC

Sulphate reducing fuel cell

Biological reaction: SO42- + 2CH2O S2- + 2CO2 + 2H2O

Anode reaction: S2- + 4H2O SO42- + 8H+ + 8e

SR

B cells

Organics

CO2

SO42-

S2-

An

ode

Solid

electrolyte

Cath

ode

V

O2

H2O

H+

Electron

Cathode reaction: 2O2 + 8H+ + 8e 4H2O

Performance of MFCMicrobe (fuel) Power

density, W/m2

Coulombic efficiency,

%

Reference

Sediment (Pt or graphite)

Mixed population (decay organics)

0.01 ND Reimers et al., 2001

Mediator-less (graphite felt)

Shewanella putre-faciens (starch WW)

0.012 ND Gil et al., 2003

Mediator-less (graphite)

Geobacter sulpfur-reducens (acetate)

0.016 96.8 Bond & Lovley, 2003

Mediator, photo (felt carbon)

Synechococcus sp. (light)

0.3-0.4 2.5-4.0 (light yield)

Tsujimura et al., 2001

Mediator-less (graphite + MnO2)

Activated sludge (glucose)

0.7 ND Park & Zeikus, 2003

Mediator-less (graphite)

Mixed population (glucose)

3.6 89 Rabaey et al., 2003

Sulphate reducing (graphite+Co(OH)2

Mixed population (sugar WW)

150 (short-term)

ND Habermann & Pommer, 1991

• Anode efficiency (harmonization of biological & anode reactions)

- Inhibition of biological activity (pH, products)

- Good mediators are toxic, stable binding to the electrode surface is difficult to achieve

- Biofilm formation on electrode (hardly controlled)

- Mass transfer limitations

• Proton transport (membranes are costly – 100$/m2)

• Cathode efficiency: overpotential, H2O2 production (the same as for chemical FC), biocathodes are possible

Problems of Microbial FC

• Due broad fuel versatility - not only energy production but waste(water) treatment too!

• No any full scale implementation

Perspectives of MFC

Load,

kg COD/m3/d

Efficiency Power density, kW/m3

UASB-reactor 10 0.85*0.38=0.32 0.5

Best lab MFC 3 0.65 0.54

Monolayer porous electrode*

10 0.7 1.5

Mediated MFC* 32 0.7 4.7

*Calculations of Bert Hamelers (Wageningen University)

Gastrorobot• Literally: robot with a stomach (food powered machine)

• Goal – to create bioelectrochemical machine that derives all the operational power by tapping the energy of real food digestion, using microorganisms as biocatalysts

The challenges of gastrorobotics:

– Foraging (food location & identification)

– Harvesting (food gathering)

– Mastication (chewing)

– Ingestion (swallowing)

– Digestion (energy extraction) - MFC

– Defecation (waste removal)

“Gastronome”: a prototype MFC powered robot

Wilkinson (2000), University of South Florida

Thank you!