ChE 6505, Biochemical Engineering A. K. M. Kazi Aurnob, Arnab Mutsuddy, Abdullah Al Moinee Course Teacher: Dr. Shoeb Ahmed ChE, BUET

1. Microbial Solar Cells: Applying
Photosynthetic and Electrochemically
Active Organisms
ChE 6505
Biochemical Engineering
Department of Chemical Engineering
Bangladesh University of Engineering & Technology

2. Reviewed Papers
[1] Timmers. R., 2012, “Electricity generation by living plants in a plantmicrobial fuel cell”, Ph.D.
thesis, Chapter-1, pp. 3-39 Wageningen University, The Netherlands.
&
[2] Strik. David. P.B.T.B., Timmers. R.A., Helder. M., Steinbusch. Kirsten. J.J., Hamelers. Hubertus.
V.M., Buisman. Cees. J.N., 2011, “Microbial solar cells: applying photosynthetic and electrochemically
active organisms,” Trends in Biotechnology, 29(1), pp. 41-49.

3. Microbial Fuel Cell (MFC)
MFC is a bio-electrochemical device that can harness the microbial cell respiration to
generate energy by supplying electrons in the cell by their physiochemical activities (e.g.
metabolism & cell respiration) .
[1] Kuman, R., Singh, L. and Zularisam, A.W. (2016). Exoelectrogens: Recent advances in molecular drivers involve in extracellular electron
transfer and strategies used to improve it for microbial fuel cell applications. Renewable and Sustainable Energy Reviews, 56, 1322-1336 .
Figure 1 : Microbial Fuel Cell [1]

4. ❑ Microbial solar cells (MSCs) are recently developed technologies that utilize solar energy to produce
electricity or chemicals. MSC is a solar assisted MFC (Microbial Fuel Cell).
❑ MSCs use photoautotrophic microorganisms or higher plants to harvest solar energy, and use
electrochemically active microorganisms in the bioelectrochemical system to generate electrical current.
Microbial Solar Cell (MSCs)
Figure 2 : Model of Solar assisted
Microbial Fuel Cell [2]
[2] Wang, H. Qian, F. Li, Yat., 2014, “ Solar assisted microbial cell for bioelectricity and chemical fuel generation,” Nano Energy, 8, pp. 264-273.
❑ Green plants and photosynthetic bacteria are
photoautotrophs. Such organisms derive their energy
for food synthesis from light and are capable of using
carbon dioxide as their principal source of carbon.
❑ Higher plants denote the plants of relatively
complex or advanced characteristics, especially
vascular plants including flowering plants.

5. Principles and performance of MSCs
The basic principles of MSCs,
(i) photosynthesis;
6CO2+6H2O = C6H12O6+6O2
(ii) transport of organic matter to the anode compartment;
(iii) anodic oxidation of organic matter by electrochemically active
bacteria;
C6H12O6+12H2O = 6HCO3
-+ 30H+ +24e-
(iv) cathodic reduction of oxygen.
6O2+ 24H++ 24e- = 12H2O
Figure 3 : Model of a Microbial Solar Cell [3]
[3] Strik. David. P.B.T.B., Timmers. R.A., Helder. M., Steinbusch. Kirsten. J.J., Hamelers. Hubertus. V.M., Buisman. Cees. J.N., 2011, “Microbial solar cells: applying
photosynthetic and electrochemically active organisms,” Trends in Biotechnology, 29(1), pp. 41-49.

6. Benthic Microbial Solar Cell
The benthic zone is the ecological region at the lowest level of a
body of water such as an ocean or a lake, including the sediment
surface and some sub-surface layers. Organisms living in this
zone are called benthos.
Figure 4: Schematic depiction of simplified mechanism of
power generation by the BMFC/BMSC [4]
1: Biofilm catalyzed anode reaction
2: Biofilm catalyzed cathode reaction
3: Fermentative reaction,
4: Microbial oxygen barrier.
[4] Malik, S. Drott, E., Grisdela, P., Lee, J., Lee, C., Lowy, Daniel A., Gray, S., Tender, Leonard M., 2009, “A self-assembling self-reparing photoelectrochemical solar cell,”
Energy Environmental Science, 2, pp. 292-298.

7. Microbial Solar Cell (MSC) : Plant Microbial Fuel Cell (PMFC)
❑ MSCs with living higher plants are called plant microbial fuel cells
(PMFCs) .
❑ In PMFCs, plant roots directly fuel the electrochemically active
bacteria at the anode by excreting rhizodeposits.
❑ Rhizodeposition of plant roots is the excretion of organic compounds
into the soil, including sugars, organic acids, polymeric
carbohydrates, enzymes and dead-cell material.
❑ The rhizodeposits account for approximately 20–40% of plant
photosynthetic productivity, and these compounds can be degraded by
a mixture of microorganisms.
❑ When the plant is growing with its roots in the MFC, electricity is
continuously generated in situ.
❑ The first published PMFC study estimated that net power generation
of 21 GJ ha -1year-1 or 67 mW/m2 is theoretically possible under
Western European (i.e. Netherlands, Belgium and France) climate
conditions.
Figure 4: Plant Microbial Fuel Cell [5]
[5] Timmers. R., 2012, “Electricity generation by living plants in a plantmicrobial fuel cell”, Ph.D. thesis, Chapter-1, pp. 3-39 Wageningen University, The Netherlands.

8. Microbial Solar Cell (MSC) : Plant Microbial Fuel Cell (PMFC)
Figure 6 : Schematic overview of the partial potential losses
in the plant microbial fuel cell [6]
❑ The internal potential loss (Eint) of the PMFC can be
represented as a series of the anode (Ea), membrane (Em),
and cathode potential loss (Ec) in an equivalent circuit.
❑ The membrane potential loss consists of the ionic potential
losses (Eionic) and the transport potential losses (Et).
[6] Timmers. R., 2012, “Electricity generation by living plants in a plantmicrobial fuel cell”, Ph.D. thesis, Chapter-1, pp. 3-39 Wageningen University, The Netherlands.

9. Biofilms of Anode
Figure 7: The biofilm life cycle [8]
[8] Muhsin, J., Tasneem, U., Hussain, T., Andleed, Saadia., 2015, “Bacterial Biofilm: Its Composition, Formation and Role in Human Infections,” Research & Reviews: Journal of
Microbiology & Biotechnology.
❑ Biofilms are complex, self-organized consortia of
microorganisms that produce a functional, protective matrix of
biomolecules.
❑ Physically, the structure of a biofilm can be described as an
entangled polymer network which grows and changes under the
effect of gradients of nutrients, cell differentiation, bacterial
motion, and interaction with the environment. [7]
[7] Mazza, M.G., 2016, “The physics of biofilms – an introduction,” Journal of Physics D: Applied Physics, 49(20).
Figure 8: The biofilm in MSC [9]
[9] Lee, H., Choi, S., 2016, “A micro-sized bio-solar cell for self-sustaining power generation,” Lab Chip, 15, pp. 391-398.

10. ❑ Solar energy is converted to electricity by growing a phototrophic
biofilm on the anode of a fuel cell.
❑ All studies to date have used mixed microbe populations, which
probably includes electrochemically active bacteria.
❑ MSCs with self-organizing phototrophic biofilms containing
Chlorophyta and/or Cyanophyta and can operate for sustained
periods of more than 20 days .
Microbial Solar Cell with phototropic biofilms
❑ In some applications, transfer of electron from the microorganism
to the anode can be enhanced using cyanobacterium such as
Synechocytis, which can form electrically conductive nanowire
under carbondioxide limitation and excess light
❑ Maximum theoretical power density of 61 mW/m2, experimentally
measured maximum average value was 7 mW/m2

11. Microbial Solar Cell with photobioreactors
❑ Microbial solar cells can use photobioreactors to
harvest solar energy via photosynthetic
microorganism such as algae
❑ Such a system includes a photobioreactor, a
microbial fuel cell and an anaerobic digestor. The
digester pretreats the metabolites before
supplying it to MFC.
❑ With a photosynthetic efficiency of 15%, and
MFC energy recovery of 29%, theoretical
estimation of maximum PD is 2806 mW/m2
❑ Best results have been achieved with Chlorella with a photosynthetic efficiency of 6.3% and a
power conversion efficiency of only 0.04%, where the average PD was 14 mW/m2 which is
only 0.5% of theoretical maximum.
❑ Upto 10 w/m2 is needed for mixing and removal of oxygen. Thus, for the current state-of-the-
art, MSC with photobioreactor produces no net electricity.

12. MSC with marine coastal ecosystem
❑ Nearly half the world’s primary productivity occurs in the ocean
which is the basis of the marine food web. 85% of the
photosynthetically derived carbon is consumed by the organisms
in water, with the remainder is deposited in the underlying
marine sediments.
❑ MFCs can be used to harness the power from coastal and deep
ocean sediments and planktons without any fuel supplement or
inoculation.
❑ An anode buried in marine sediments, electrically connected to a
cathode in the overlying water, generates current. Continuous,
uninterrupted power generation has been observed in every
organic-rich sediment at a power densities of 28 mW/m2.
❑ In more recent studies, chambered MFC systems with marine sediments have been developed at
laboratory scale that achieved power densities of 140 mW/m2.
❑ Pre-inoculated electrodes may increase coulombic efficiencies of these MFC systems.

13. Challenges in improving energy recovery
with MSC
■ The power density of the output of an MSC system has to be increased to obtain a cost-
effective process.
■ All the MSC systems that have been developed are still in laboratory scale and they
were not designed for scale-up.
■ Data available for major processes are insufficient. There are no measured data available
on the coulombic efficiencies of MFCs.
■ Several measures have to be taken to increase the power density of MSCs, such as:
I. Increasing substrate flux by pretreatment.
II. Decreasing the oxidation state of organic matter derived from photosynthetic
organism
III. Decreasing the pH gradient resistance which occurs due to acid production at
anode and alkaline production at cathode.

14. Comparison of PMFCs with other
sources of renewable energy
■ PMFCs may serves as an alternative to other sources of renewable energy such as
photovoltaic solar panels and wind turbines.
■ Although photovoltaic solar panels and wind turbines can achieve higher power yields,
PMFCs can increase aesthetic value and biodiversity.
■ Maximum estimated power yield of PMFCs is 1.6 MW/km2, which is 5-7.7 MW/km2 for
wind turbines and 4.5-7.5 MW/km2 under Western European conditions.
■ Environmental impact of wind turbines and solar panels such as avian mortality, noise,
electromagnetic interference, loss of biodiversity and use of polluting material is subject to
societal debate.
■ PMFCs can offer an opportunity for electricity generation while sustaining the environment
where wind turbines and solar panels are not desirable.

15. Some Background : Fuel Cell
■ Unlimited Power (sort of)
– Do not run down
– Do not need recharging
– Continuous discharging
■ Hydrogen fuel cell
– Hydrogen gas is the fuel
– It is separated at the anode into protons and electrons
– Electrons travel through the external circuit
– H+ ions pass through the electrolyte to the O2 and forms H2O

16. Testing a fuel cell: Polarization Curves
■ Displays voltage output (V) of the fuel cell against the applied current density
(A/cm2)
■ Fuel Cells don’t behave ideally
[http://www.fuelcellstore.com/blog-section/polarization-curves]

17. Why?
■ At lower power densities, potential drops due to activation polarization
– Kinetic loss due to slow reaction of O2 at the cathode side
– Even Pt catalyst is not efficient enough
■ At moderate power densities, ohmic loss also starts to take place
– Due to the cell resistance
– Mainly the membrane resitance
■ At higher power densities, concentration polarization takes place
– Due to low mass transfer of the reactants to the catalyst side
– Could be due to low porosity of the electrode, or due to water flooding. (back
diffusion)

18. Polarization Curve for a typical MFC
■ Theoretical max potential, Ecell, max, can be
calculated using the Nernst Equation
– R : Molar Gas Constant
– F : Faraday’s Constant
– T : Temperature
– n : number of electrons in the
reaction
– Q : Reaction Quotient

19. Polarization Curve for a typical MFC
■ Open cell potential, EOCP, is less
than Ecell,max because of internal
losses in the fuel cell
■ Shows linear relationship in the mid
range current density

20. Challenges in Energy Recovery
■ How can the power density (PD) be increased in order to have a cost-effective MSC?
– Cannot be answered as no big-scale fuel cell available
– No data available either
■ However MSC performance can be improved by lowering the internal resistances.

21. Challenges in Energy Recovery
■ Increase substrate flux from photosynthetic to electrochemically active organisms
– MSCs such as PMFC, can be substrate limited
– Flux can be increased by having increased rhizodeposition
– Choosing plant type
– Successful maximization could increase PD tenfolds

22. Challenges in Energy Recovery
■ Decrease the oxidation state of the organic matter
– The electrons that can be derived from the electron donor depends on the
individual oxidation states of the substance
– By controlling the electron donor mobilized by the photosynthetic organisms,
and thus the OS of the electron donor, energy recovery can be improved in the
MSC.
– Several plants increase the release of low molecular weight compounds, such
as sugars, amino acids, etc. under iron or zinc limitation conditions. Similar
methods can be utilized to control plant exudation of more reduced
compounds.

23. Challenges in Energy Recovery
■ Increase organic matter oxidation at the anode
– Plant roots excrete oxygen, which is an electron acceptor, into the anode
compartment
– Electron acceptors at the anode side can take up the electron derived from the
electrochemically active bacteria
– These electrons can’t travel to the cathode
– Decrease in power output
– Can be improved by decreasing the average root length (oxygen introduction
into the rhizosphere decreases with root length)

24. Challenges in Energy Recovery
■ Decrease pH gradient resistance of the cell
– Accumulation of H+ ions, which transfers to the cathode side, increasing the
pH of the cathode side.
– This creates pH gradient resistance (Nernst Equation / Le Chatelier’s)
– Although the pH of rhizospheres of the plants are slightly acidic, decreasing
the pH can increase the energy recovery.
– Use of buffer has been suggested
– But cost could outweigh the net production of power

25. Challenges in Energy Recovery
■ Decrease the transport and ionic resistance of the fuel cell
– Transport resistance is the greatest fraction of the total internal resistance
– Cations should travel from the anode to the cathode
– Mixing of the anolyte or circulation of the catholyte over the anolyte will break
down the concentration gradient of the cations and anions
– Thus decrease the transport resistance

26. Challenges in Energy Recovery
■ Decrease the anode and cathode resistance
– Increase the surface area of the anode and cathode
– Oxygen reduction on graphite shows poor performance due to charge and
mass transfer resistance
– Air-cathodes can be used to reduce resistance
– Bio-cathodes can also be used. They catalyze the production of oxygen and
other electron acceptors, such as manganese or iron.

27. Challenges in Energy Recovery
■ Decrease the energy input of the MSCs with photobioreactors or with marine
ecosystems
– MFC with seawater from marine ecosystems are limited by very dilute electron
donors
– These are concentrated for higher power output
– MSCs that use a photobioreactor or MSCs with marine ecosystems require
energy of 6-10 W/m2 for processing the electron donor
– Lowering this energy input would increase the new energy production.

28. Prospects and Future
■ MSC can produce fuels and chemicals beside electricity (methane, ethanol, etc.)*
■ PMFCs can be incorporated into landscapes and urban areas
■ Both photosynthetic and electrochemical reactions are carried out by continuously
growing population of microorganisms. Self-sustainable.
■ Doesn’t need costly or toxic catalysts, no risk of pollution.
■ Organic materials (from the photosynthetic parts) accumulate in the MSC, so
electricity can be generated in the dark
■ Integrated PMFCs can add value to other applications (such as rice paddy fields,
wastewater treatment, etc). Extra organic matter from these processes can be used
for energy production.
*Hamelers, H.V.M. et al. (2010) New applications and performance of
bioelectrochemical systems. Appl. Microbiol. Biotechnol. 85, 1673–1685

29. ChE 6505
Biochemical Engineering
THANK YOU
Microbial Solar Cells: Applying
Photosynthetic and Electrochemically
Active Organisms

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