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Age loss contributions h2 air

“On the arid lands there will spring up industrial colonies without smoke and without smokestacks; forests of glass tubes will extend over the plains and glass buildings will rise everywhere; inside of these will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry and mankind is.”

Giacomo Ciamician

Science36, 385 (1912)


Age loss contributions h2 air

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

emf

pmf

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


Age loss contributions h2 air

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.


Age loss contributions h2 air

Questions regarding artificial photosynthesis, water

oxidation, oxygen reduction and hydrogen production.

Why doesn’t 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


Age loss contributions h2 air

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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O2

O2

water

water

Schematic of wiring enzyme with relay or directly to electrode

Mox/Mred

E

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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Calculated optimal electron transfer rates

log ket = 13 - (1.2 - 0.8r)(R -3.6)

Dutton and coworkers Nature,402, 47–52 (1999)

Nature, 355, 796–802 (1992)


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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


Age loss contributions h2 air

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)


Age loss contributions h2 air

Consider bio-inspired catalysts for improved fuel cells


Age loss contributions h2 air

Two fuel cells, same cathodic rxn

Mitochondrion as a fuel cell

2NADH + O2 2NAD+ + 2H2O

Conversion of electrochemical

potential to biochemical work

with high (> 90%?) efficiency

Good cathode

H2/air fuel cell

2H2 + O2 2H2O

Conversion of electrochemical potential to work meeting human needs with modest efficiency (~ 50%).

Not so good cathode


Voltage loss contributions h 2 air

Eloss at 1.5 A/cm2:

400 mV (68%)

70 mV (12%)

120 mV (20%)

major losses due to poor cathode kinetics (ORR)

 minor losses by ohmic resistance (50% RH+,membrane, 50% Rcontact)

 significant voltage/power-density via FF/DM optimization (mass-tx)

Voltage Loss Contributions - H2/Air

(H2/air (s=2/2) at 150kPa, 80C, and 100%RH - 0.4mgPt-cathode/cm2)

Thanks to Frank DiSalvo

Source: H. Gastieger, GM Fuel Cell Division


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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 Heller’s 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); N&V 433, 589 (2005)


Age loss contributions h2 air

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)


Mimicking hydrogenase

Mimicking Hydrogenase

Synthetic model of active site of an Fe-only hydrogenase

Thomas B. Rauchfuss, et al., J. Am. Chem. Soc., 2001, 123, 9476


Age loss contributions h2 air

Solar energy conversion

emf

pmf

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


Photosynthetic reaction centers

Photosynthetic reaction centers


Energetics and electron transport pathways of ps

Energetics and electron transport pathways of PS

N.B.

Thanks to B Blankenship


Age loss contributions h2 air

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•–


A carotenoporphyrin fullerene triad

A carotenoporphyrin-fullerene triad


Age loss contributions h2 air

Light energy stored as electrochemical energy

Dipole moment ~160 D

Smirnov, S. N.; Liddell, P. A.; Vlassiouk, I. V.; Teslja, A.; Kuciauskas,D.; Braun,C. L.; Moore, A. L.; Moore, T. A.; and Gust, D. J. Phys. Chem. A, 2003, 107, 7567-7573

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

•+

•-


Age loss contributions h2 air

Energy levels for artificial reaction centers.

e

V

2

.

0

1

D

-

P

-

A

1

.

8

1

.

6

+

-

D

-

P

-

A

1

.

4

+

-

D

-

P

-

A

1

.

2

1

.

0

0

.

8

0

.

6

0

.

4

0

.

2

D

-

P

-

A

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


Age loss contributions h2 air

Solar energy conversion

emf

pmf

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


Age loss contributions h2 air

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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PtBi

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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Ordered Intermetallic e.g. BiPt

Alloy; e.g. Pt/Ru (1:1)

(A)

(B)

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


Age loss contributions h2 air

Metals that can be purchased or can be easily synthesized as alkoxides, ethylhanoates, MOEEAAs:

ScTi VCr Mn Fe Co Ni Cu Zn GaGe As Se

Y ZrNbMoRuRh Pd Ag Cd In Sn Sb Te

La Ta Re IrPt Hg Tl Pb Bi

Take home message: synthetic tools to prepare nanoparticles of almost any intermetallic compound are now available

Thanks to Frank DiSalvo


Age loss contributions h2 air

Photochemical oxidation of water by band gap illumination of a semiconductor

First reported for TiO2 in Nature238, 37-38 (1972)

Zou et al., Nature414, 625-627 (2001)


Age loss contributions h2 air

Structure of the oxygen evolving complex

Ferreira et al. Science 2004


Age loss contributions h2 air

Model of the oxygen evolving complex

Britt et al., BBA1655, 158-171 (2004)


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Model for water oxidation by the OEC

Britt et al., BBA1655, 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 PCCP6, 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.


Age loss contributions h2 air

NHE

– 0.93

NAD+ + e- NAD•

P+ + e-P*

– 0.67

– 0.63

TiO2 ECB(FTO, pH8)

SnO2 ECB(ITO, pH7)

– 0.07

0.92

NADH •+ + e- NADH

P+ + e-P

1.23

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- + O22H2O

0.82


Age loss contributions h2 air

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)


Age loss contributions h2 air

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.


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