Fundamental Challenges in Solar to Fuel Conversion aka Improving on Photosynthesis

38 Pages·2014·14.71 MB·English

Fundamental Challenges in Solar to Fuel Conversion
aka Improving on Photosynthesis

Joel Ager
Joint Center for Artificial Photosynthesis
Materials Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, CA, USA
February 4, 2014
NERSC User Meeting
Berkeley, CA
The Joint Center for Artificial Photosynthesis is a DOE Energy Innovation Hub,
supported by the Office of Science of the U.S. Department of Energy Ager, NERSC, 2/4/14 - 1
What is “artificial photosynthesis”?

What is photosynthesis?

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Natural photosynthesis

with an energy level diagram

Plants (also algae and cyanobacteria)
perform synthetic redox chemistry with two
red photons, using the reduction products to
build plant mass and releasing the oxidation
product (O ) into the air
2
Adapted from Photosystem II (Springer, 2005) Ager, NERSC, 2/4/14 - 3
What is “artificial photosynthesis?
•  Same basic idea as natural photosynthesis
–  Use sunlight
–  Synthesize a (useful) chemical product
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Is it hard to do?
Solar to fuel energetics do not look too difficult…
Water splitting half reactions
Reduction: 2H+ + 2e- -> 2H
2
Oxidation: H O + 2h+ -> 1/2O + 2H+
2 2
Overall: H O -> 1/2O + H
2 2 2
ΔG = +237 kJ/mol, 1.23 eV/electron
CO energetics are similar
2
•  Observation
ΔGo ΔEo λmax
Reaction (kJ mol-1) n (eV) (nm) –  The money making reaction is
_______________________________________________________________________________________________
H O → H + ½ O 237 2 1.23 611 reduction
2 2 2
CO2 + H2O → HCOOH + ½ O2 270 2 1.40 564 •  So why are oxidizing water?
CO + H O → HCHO + O 519 4 1.34 579
2 2 2
–  Where else are we going to get
CO + 2H O → CH OH + 3/2 O 702 6 1.21 617
2 2 3 2
Gt-equivalents of electrons?
CO + 2H O → CH + 2O 818 8 1.06 667
2 2 4 2

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The voltage
requirements are a
little tougher than
one might think
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Thermodynamics vs. Kinetics
Use water splitting as a model system, CO reduction is similar
2
Reduction: 2H+ + 2e- -> 2H
2
Oxidation: H O + 2h+ -> 1/2O + 2H+
2 2
Overall: H O -> 1/2O + H
2 2 2
ΔG = +237 kJ/mol, 1.23 eV/electron,
at 1.23 V the forward and reverse rates are equal

Therefore "Overpotentials" needed to drive reaction at an appreciable rate
0.6
 V
 overpoten-al

Smolinka, T. Water electrolysis,
Encyclopedia of Electrochemical
Power Sources, 394-413 (2009)
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Approach
ΔG
 =
 +237
 kJ/mol,
 1.23
 eV/electron

Photoanode
  Photocathode

A
 photocathode
 and
 photoanode
 linked
 in
 series
 analogous
 to

Photosystem
 I
 and
 Photosystem
 II
 of
 natural
 photosynthesis

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Why aren’t we doing artificial photosynthesis on a
large scale right now?

The individual components exist…
Alkali-based H generator PEM-based system
2
1 m3 H /hour
2
0.1 m3 H /hour
2
PV power getting close to grid parity
Smolinka, T. Water electrolysis,
Encyclopedia of Electrochemical
But…
Power Sources, 394-413 (2009)
Not the cheapest way to make H
2
H is not used in our current energy cycle
2
So…
Solar to H not practical (yet) on any commercially
2
interesting or ecologically relevant scale
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There are lab-scale demos
12% STH 18% STH ~3% STH
Turner et al. (1998) Licht et al. (2000) Nocera et al. (2011)
Pt/pn-GaAs//p-GaInP/Pt RuO /pn-AlGaAs//pn-Si/Pt black Co-Pi/3J-a-Si/NiMoZn
2
But…
A really attractive and integrated
combination of efficiency, stability,
~5% STH
and scalability has yet to be
van de Krol et al. (2013)
demonstrated
Co-Pi/BiVO //2J-a-Si/Pt wire
4
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