Important questions

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Biomimetic approaches and role of biological
processes as paradigms for solar to fuel LBNL
Workshop Solar to Fuel - Future Challenges and
Solutions 28-29 March 2005
Important questions Can bio-inspired constructs
play a role in large scale solar energy
conversion? Provide models for the capture and
transformation of solar energy? Or, does the
nature of biological energy conversion preclude
it serving as a paradigm large scale energy
production to meet human needs?
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Global energy flow
Is biological energy conversion sufficiently
large scale to be relevant?
Approximately 4 x1021 J of chemical energy stored
in photosynthetic biomass per year. Power is
about 125 TW
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Solar energy conversion
Non-biological
Biological
Photoinduced electron transfer
reaction centers (molecular-level photovoltaics,
emf)
photovoltaics
emf
membrane distribution
H
H
wire distribution
other electrical work

• Transducers for
• synthesis work
• mechanical work
• transport work
• driving complex non-linear
• processes

Halophilic Archaea, bacterioplankton
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Bio-inspired catalysts for sustainable large
scale energy production and conversion
Photosynthetic organisms provide myriad examples
of catalysis including several essential redox
ones that operate with essentially no
overpotential. These include 2H2O 4H
4e- O2 oxygen evolving
complex H2 2H 2e-

hydrogenase O2 4H 4e- 4H(pumped) 2H2O
4H(pumped) complex 4 With these
three enzymes nature has provided the basic
paradigms for fuel cell operation and
regeneration of hydrogen and oxygen.
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Questions regarding artificial photosynthesis,
water oxidation, oxygen reduction and hydrogen
production.
Why doesnt complex 4, cytochrome c oxygen
oxido-reductase, operate in reverse? O2 4H
4e- 4H(pumped) 2H2O 4H(pumped)
complex 4 Can the oxygen evolving complex (oec)
operate in reverse? 2H2O 4H 4e- O2

oec Can the catalytically active sites of redox
enzymes be assembled in artificial constructs and
electrically coupled to electrodes? Sufficient
density of catalytic sites on electrodes to make
real-life energy fluxes possible? 1
amp/cm2 See Basic Research Needs for Hydrogen
Production, Storage, and Use. The workshop
report is available as a 3 MB pdf file on the BES
website  http//www.sc.doe.gov/bes/hydrogen.pdf
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Characteristics of biological catalytic activity
slower, molecular recognition near
thermodynamic limit efficiency highly specific,
molecular recognition robustness through
replacement/self repair
Characteristics of present day human-engineered
catalytic activity faster sacrifice
efficiency for speed (overpotential) less
specific robustness inherent (but some easily
poisoned)
Evolution of bio-inspired catalysis includes
elements from both sides
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But, perhaps the most important characteristic of
biological catalysts is that
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Biological catalyst do not come wired to
electrodes
Nature does not use metallic conductors and emf
in either synthesis or energy-yielding processes
(in the sense that human do). A molecule - metal
interface must be made. Molecular wire, redox
relay shuttle, conducting polymer, redox
hydrogel, or other means of electrically
connecting catalytic site to electrode.
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Schematic of wiring enzyme with relay or directly
to electrode
E
Mox/Mred
0.82 V - EoM
EoM
O2
water
0.82 V
Perfect electrode
SHE
O.82 V
E
Low beta molecular wire at low bias connecting
E to electrode Electron tunneling 10 pA current
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Methods of wiring redox enzyme to electrode
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Wiring with a rotaxane molecular shuttle
Katz et al., Angew. Chem. Int. Ed. 43, 3292-3300
(2004)
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Amine oxidase wired to gold electrode
Hess et al., J. Am. Chem. Soc. 125, 7156-7157
(2003)
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Parameters for bio-catalyzed O2 reduction at fuel
cell cathode
Catalytic site
4H
O2
2H2O
Molecule - metal interface. Molecular wire, redox
relay shuttle, conducting polymer, redox
hydrogel, or other means of electrically
connecting catalytic site to electrode
4e-
Cathode
Metallic conductor to RL, current at least 1
A/cm2
Current will depend on Number of
sites/cm2 Turnover number/site Carrying capacity
of interface
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Cu ions at the active site of phenoxazinone
synthase, a multicopper oxidase
Max footprint 9 nm2 (suppose 3x3nm
squares) 1x1013 sites/cm2 102 s-1
turnover? (what limits turnover?) 1x1015
turnovers/cm2/s 4x1015 electrons/s cm2/s (4
e-/turnover) About 700 µA/cm2 As water
oxidizer Solar driver at AM1.5 20
mA/cm2 1.25x1017 e-/cm2 Turnover appears rate
limiting
O2 4e- 4H ? 2H2O 2H2O ? 4H 4e- O2 ?
1 nm
2 nm
Francisco and Allen, 2005
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Carrying capacity of interface How much current
can be pushed through a molecular wire
In single molecule conducting AFM studies of
conducting polymers and molecules with low Beta,
currents of about 10 pA are observed at low
bias. 10 pA corresponds to 6x107 e-s-1 This
easily exceeds by orders of magnitude the
turnover number of any enzymes under
consideration. Therefore, kcat limits
current. J. Am. Chem. Soc. 127,11384-1385 (2005)
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Consider bio-inspired catalysts for improved
fuel cells
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Two fuel cells, same cathodic rxn
H2/air fuel cell 2H2 O2 ?
2H2O Conversion of electrochemical potential to
work meeting human needs with modest efficiency
( 50). Not so good cathode
Mitochondrion as a fuel cell 2NADH O2 ?
2NAD 2H2O Conversion of electrochemical
potential to biochemical work with high (gt 90?)
efficiency Good cathode
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Voltage Loss Contributions - H2/Air
(H2/air (s2/2) at 150kPa, 80C, and 100RH -
0.4mgPt-cathode/cm2)
Source H. Gastieger, GM Fuel Cell Division
Thanks to Frank DiSalvo
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Enzymatic reduction of O2 to H2O
And some questions that come up 1) Current
density 2) V loss to overpotential 3)
Availability of enzyme 4) Assembly on
electrode 5) Robustness
S. C. Barton et al., J. Am. Chem. Soc., 123, 5802
(2001)
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Proposed mechanism for O2 reduction by a
multicopper oxidase
There is a lot of chemistry going on here - no
wonder it is slow
S. C. Barton, et al., Chem. Rev., 104, 4867-4886
(2004)
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Schematic of the overpotential problem
S. C. Barton, et al., Chem. Rev., 104, 4867-4886
(2004)
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Example of a small scale biofuel cell using the
mitochondrial cathodic reaction
Examples of small (energy) scale devices using
biocatalysis include Adam Hellers glucose
sensor. Many examples in literature of working
systems. Very small scale - µW - power
production.
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A fuel cell anode without Pt?
Less complicated chemistry and Pt works well,
but, it there enough of it? Can the H2/H
reaction be catalyzed by Fe?
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A synthetic active site mimic of iron-only
hydrogenase - a bio-inspired anode
Active site of all-iron hydrogenase
Synthetic analogue
Synthetic analogue shows catalytic H reduction
on vitreous carbon electrode
Tard et al., (Pickett), Nature, 433, 610 (2005)
NV 433, 589 (2005)
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Structure of the synthetic active site mimic of
iron-only hydrogenase from DFT calculations
H reduction on a vitreous carbon electrode at
200 mV more positive than electrode alone.
Tard et al., (Pickett), Nature, 433, 610 (2005)
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Mimicking Hydrogenase
Synthetic model of active site of an Fe-only
hydrogenase
Thomas B. Rauchfuss, et al., J. Am. Chem. Soc.,
2001, 123, 9476
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Solar energy conversion
Non-biological
Biological
Photoinduced electron transfer
reaction centers (molecular-level photovoltaics,
emf)
photovoltaics
emf
Separate charge
membrane distribution
H
H
wire distribution
other electrical work

• Transducers for
• synthesis work
• mechanical work
• transport work
• driving complex non-linear
• processes

Halophilic Archaea, bacterioplankton
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Photosynthetic reaction centers
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Energetics and electron transport pathways of PS
N.B.
Thanks to B Blankenship
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Artificial reaction centers

• Basis is photoinduced electron transfer
• Minimum requirements
• Donor chromophore (P)
• Suitable electron acceptor (A)
• Electronic coupling
• Useful systems require more complexity
• -Secondary donor or acceptor

hn
P-A ? 1P-A
1P-A ?? P-A
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A carotenoporphyrin-fullerene triad
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Light energy stored as electrochemical energy
The best C-P-C60 triads Yield of charge
separated state 100 Stored energy 1.0
electron volt Lifetime hundreds of ns at room
temp. 1 microsecond at 8K
C ?? -P-C60??

-
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Energy levels for artificial reaction centers.
Nature views D.-P-A.- as redox potential, not
as a source of emf. Subsequent energy conserving
processes are based on redox chemsitry. Nature
does not use emf to drive synthesis.
Apparently more energy stored here than at this
point in time in reaction centers
0
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Solar energy conversion
Non-biological
Biological
Photoinduced electron transfer
reaction centers (molecular-level photovoltaics,
emf)
photovoltaics
emf
membrane distribution
This is what is really needed
H
H
wire distribution
other electrical work

• Transducers for
• synthesis work
• mechanical work
• transport work
• driving complex non-linear
• processes

Halophilic Archaea, bacterioplankton
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Contrast of bio-catalysis with human-engineered
catalysis. Mainstream energy-transducing redox
processes
Biological
Human engineered
Living organisms use FeS centers, Fe, Cu, Mn and
sometimes Ni Catalysis often involves covalent
intermediates with catalytic sites having
distinct 3-dimensional architecture. A necessary
feature of enzymatic catalytic mechanisms for
lowering ?G C-C bond cleavage facile.
Pathways to synthesize meOH, etOH, CH4 from
CO2
Carbon, Pt with alloys and intermetalic
compounds, efforts span periodic table Emphasis
on surface structure. No good catalysts
for C-C bond cleavage in context of low temp fuel
cell Demonstrated using enzymes in small scale
systems
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Platinum vs. PtBi
Pt
(111) plane Pt-Pt 2.77 Å
(001) plane Pt-Pt 4.32 Å
Thanks to Frank DiSalvo
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Alloys vs. Ordered Intermetallics
2 Electrocatalytic Oxidation of Formic Acid at
an Ordered Intermetallic PtBi Surface, E.
Casado-Rivera, Z. Gál, A.C.D. Angelo, C. Lind,
F.J. DiSalvo, and H.D. Abruña, Chem. Phys. Chem.
4, 193-199 (2003)
Thanks to Frank DiSalvo
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Metals that can be purchased or can be easily
synthesized as alkoxides, ethylhanoates, MOEEAAs
  Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga
Ge As Se Y Zr Nb Mo Ru Rh Pd Ag
Cd In Sn Sb Te La Ta Re
Ir Pt Hg Tl Pb Bi Take home
message synthetic tools to prepare nanoparticles
of almost any intermetallic compound are now
available
Thanks to Frank DiSalvo
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Photochemical oxidation of water by band gap
illumination of a semiconductor
Zou et al., Nature 414, 625-627 (2001)
First reported for TiO2 in Nature 238, 37-38
(1972)
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Structure of the oxygen evolving complex
Ferreira et al. Science 2004
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Model of the oxygen evolving complex
Britt et al., BBA 1655, 158-171 (2004)
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Model for water oxidation by the OEC
Britt et al., BBA 1655, 158-171 (2004)
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Equal time to the east coast
Mn(V)oxo set up for nucleophilic attack on the
electropositive oxygen by nearby water
coordinated by Ca
McEvoy and Brudvig PCCP 6, 4754-4763 (2004)
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Electrolysis of water - anode side
Using Si PV cells, 3 in series are necessary to
provide the voltage to overcome the overpotential
associated with removing electrons from water
using available catalysts. Commercial
electrolyzers operate at 1.7-1.9 V. In PSII
electrons are smoothly removed from water with
an oxidant of about 1 V (vs. NHE).
Key Question Can the natural water oxidation
system (PSII OEC), which oxidizes water at near
the thermodynamic potential, be forced to run
faster? OEC never wired to an electrode or driven
electrochemically. At 1A/cm2, and an area of 100
nm2 per site (arbitrary), each site would need a
turnover of 6x106 s-1. In nature the turnover
number is about 1X103 s-1. Possibilities
rough surface, but factor of 102 at most.
improve catalytic turnover by factors of 103 to
106??? Hard to imagine given what is
known about the chemical steps in the mechanism.
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Energetics of water oxidation with H2 formation
Can 4 one photon processes both oxidize water and
reduce protons to H2?
2H 2e- H2
0.42
Well, could be. The reducing side works. In
photosynthesis the OEC smoothly oxidizes water to
O2 by removing 4 e- using an oxidant that is only
1 V.
4H 4e- O2 2H2O
0.82
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CH4 synthesis from H2 CO2 and methanol
Energy input from ion gradient
For the oxidative branch (up arrows) 4CH3OH ?
3CH4 CO2 2H2O ?G0 - 106 kJ/mole CH4 4H2
consumed in reductive branch (down arrows)
Deppenmeier, J Bioenergetics and Biomembranes
36, 55-64 (2004)
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Conclusion Bio-inspired energy-converting
processes can by imagined
The milk cow model Engineer organism to express
excess designer enzymes that can be harvested for
human use. These would be renewable biocatalyst
(even the natural system fails every 10 minutes).
Must think in terms of land area for both TW of
solar and land area to grow the bacteria, algae
and plants to provide the enzymes. Can the
active site of key enzymes be mimicked and be
made robust? Can the mainstream redox enzymes be
driven backwards? Engineer ones that can.
Wiring to active site is not rate limiting and
tunneling is not hard on the molecules. Main
stream 1 A/cm2 chemistry to electricity is hard .
Depends on what is discovered for turnover rates
when one substrate is a metallic source of
either electrons or holes. Enzymes not designed
by Nature to react with metallic conductors. Can
turnover rate be increased? Engineering
biocatalysts for better performance.
3-dimensional binding sites likely fundamentally
slower than reactions at surfaces. High level
of organization required for processes that
couple redox to protonmotive force. Do not limit
bioinspired constructs to main stream energy
processing. Niche applications add up. It is a
hard problem.