The document discusses the potential of solar fuels as a sustainable energy source that can address energy security and carbon emissions through artificial photosynthesis. It highlights key issues such as the cost of catalysts, thermodynamic barriers, and the need for scalable technology. Current research from notable scientists aims to improve efficiency and affordability to make solar fuels viable for widespread use.

2. Outline
1. OVERVIEW
2. STATE OFTHE PLANET
3. HOW IT WORKS
4. KEY ISSUES
5. CURRENT RESEARCH
6. SCALABILITY
7. ROLE OF CHEMISTRY
8. FUTURE POSSIBILITIES
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3. 1. Overview
• Question of the day:
• What energy-producing technology can be envisioned
today that will last for millennia and can be implemented in
developing countries ?
• Answer…

4. …Solar fuels
• Potential to solve two major problems:
• Energy security
• Carbon emissions
• The key is to make energy-dense
chemical fuel with minimal carbon
emissions.

5. Motivation is all around us
Real-life leaves prove that sunlight can be converted into fuel
using only common elements
Q: Can humankind imitate this process to rescue the planet
from global warming?

6. Fuel Cell in Reverse
• For solar fuel, sunlight provides driving force in the fuel-forming direction.
• For traditional fuel cells, fuels like hydrogen drive the production of electricity.

7. 2 principle elements
• Collectors to convert solar
energy to electrical energy,
and
• Electrolyzers that use the
electrical energy to split water
into H2 and O2
Industrial electrolyzer
made by Norwegian-
German company
GHW
PV cells made by Sanyo
used in Universal Studios
theme park in Singapore

8. 2. State of the planet

9. Projected GrowthTrends
World Population (Billions) 6.145 9.4 10.4
Energy Consumption (TW/yr) 13.5 27.6 43
CO2 Emissions (GtC/yr) 6.57 11 13.3
2001 2050 2100
2000 2050 2100
Population
Energy
Consumption
CO2 Emissions
Year
Year

10. Co2 is stubborn
• In absence of geoengineering, the effects on environment caused by CO2
over next 40 years will persist globally for 500-2,000 years
• Atmospheric CO2 levels were between 210-300 ppm for last 420,000 years
• We are hoping to stabilize it in the 550-650 ppm range
• In order to do this, by 2050 we would need as much carbon neutral power
as the amount of total energy produced today

11. Carbon-neutral Power Options
• Three main options
• Nuclear fission
• Clean coal with carbon capture and storage
• Renewable sources of energy
• The technology must start now and maintain a similar growth rate
• Probably too late for nuclear fission and carbon capture
technologies
• Look to renewables!

12. The sun has potential…
• The Sun is by far the largest exploitable source of energy
• “The Sun pours more energy onto the Earth every hour than
humankind uses in a year” –Nathan Lewis
• But what about when the sun goes down? Storage?

13. Store energy in chemical bonds
• Use the Sun to churn out fuel that we can burn
• To power cars,
• To create heat,
• To generate electricity,
• And that we can store for use
when the Sun goes down.

14. 3. How it works

15. Natural Photosynthesis
• Stores solar energy as fuel by
rearranging the chemical bonds of
water to form O2 and NADPH, which is
nature’s form of H2
• Later in the process, NADPH is used
to form glucose, which is a sugar and
a main basis for energy in most
organisms
Glucose

16. Artificial Photosynthesis
imitation is the sincerest form of flattery
222
2
22
22
244
442
OHOH
HeH
OeHOH





• Two spatially separated electrodes
coated with catalysts placed in water
• Sunlight creates a wireless current
that sparks the reactions below
• Cathode produced hydrogen, and
anode produces oxygen
Anode (oxidation)
Cathode (reduction)
Overall reaction H2

17. Energy Diagram
• Energy of light photon, E = hv, is absorbed at anode with help of catalyst
• Charge separation:
• Electron (e-) jumps to higher band
• Hole (h+) is left behind
•
•
• E
AnodeCathode

18. Turner’s 1998
prototype
•It works!
•Built by John Turner in 1998
•Overall 12.4% solar to hydrogen
efficiency, which is about 12x as
efficient as a leaf
• But…
•Lifespan of only about 20 hours
•Used expensive platinum as catalyst
•Cost roughly $10,000/cm2
Hydrogen bubbles

19. 4. Key issues
• Cost of Catalysts
• Thermodynamic Barriers
• Corrosion

20. Cost of catalysts
• Commercial PV cells contain expensive silicon (Si) crystals
• Electrolyzers use platinum (Pt), which costs $1,500 an ounce
• At these prices, maybe alright for the military, but not to power civilization
• Look to cheap minerals for
catalysts, like iron, cobalt, or
manganese

21. Thermodynamic barriers
• Lack of efficient light
absorption
• Energetics - Matching
band energies with
reactions
• Electron-hole pair
recombination
Energy Diagram
light

22. Corrosion
• Water splitting reaction is highly corrosive
• The oxidizing power causes electrodes to degrade
• Same with natural photosynthesis, but plants can rebuild
• Turner’s cell lasted only 20 hours

23. 5. CURRENT RESEARCH
• Many researchers are trying to make this technology more efficient, affordable,
and more durable
• Two notable researchers are Nathan Lewis and Daniel Nocera
Nathan Lewis, Caltech Daniel Nocera, MIT

24. Improving the collector
• Lewis has devised a collector made of silicon nanowires embedded in a
transparent plastic film
• Practical ability to roll and unroll like a blanket
• The light to electric energy efficiency of nanowires at 3% is much less than
the 20% of commercial solar cells
• But it might be acceptable if cheap enough

25. Finding a better catalyst
• In 2008, Nocera hit on an inexpensive combination of phosphate and cobalt
that can catalyze the production of O2
• Used an electrode made of inert indium tin oxide in phosphate-buffered
water containing cobalt ions
• Many similarities to natural photosynthesis
• Catalyst that forms in situ from earth-abundant materials
• Generates O2 in neutral water under ambient conditions
• Highlights a new era of exploration

26. 6. scalability
Must be able to scale up cheaply into thin flexible solar-fuel films that roll off
high-speed production lines the way newsprint does

27. What about costs?
• As for cost, it would have to be as cheap as wall-to-wall carpeting, less
than $1 per sq. foot
• “We need to think potato chips, not silicon chips” – HarryAtwater, Jr.,
Caltech

28. 7. Role of chemistry

29. 8. Possibilities for the future

30. How ambitious are we?
• Could produce pure water for the municipal water supply
• Pure water is a by-product of burning hydrogen
• Could use ocean water to create hydrogen, then burn the hydrogen at a
power plant to produce electricity for the grid and clean water
• It’s a win-win!
• In theory, could combine Sun, water and basic atmospheric gases like carbon
dioxide, nitrogen, and oxygen to create
• Not only fuels, electricity, and pure water, but also
• Polymers , food, and almost everything else we need!

31. How close are we?
• Will we soon be cooking up hydrogen for their cars using
affordable backyard equipment?
• Many solar-fuel experts maintain that
the research has decades to go
• Considering the challenges, they might be
right

32. Sources
• Oscar Khaselev & JohnTurner, A Monolithic Photovoltaic-Photoelectrochemical Device
for Hydrogen Production viaWaterSplitting, SCIENCE, April 17, 1998, at 425-27.
• JohnTurner, A Realizable Renewable Energy Future, SCIENCE, July 30, 1999, at 687-89.
• Antonio Regalado, Reinventing the Leaf:The ultimate fuel may come not from corn or
algae but directly from the sun itself, SCIENTIFIC AMERICAN, October 2010, at 32-35.
• Matthew Kanan & Daniel Nocera, In Situ Formation of anOxygen-EvolvingCatalyst in
NeutralWaterContaining Phosphate andCo2+ , SCIENCE, Aug. 22, 2008, at 1072-75.
• Harry Gray, Powering the Planet with Solar Fuel, NatureChemistry,April 2009, at 7.
• Nathan Lewis & Daniel Nocera, Powering the Planet:ChemicalChallenges inSolar
EnergyUtilization, PNAS, Oct. 24, 2006, at 15729-35.
• $122 Million Granted to Solar Fuel Research,CALFINDER (July 26, 2010),
http://solar.calfinder.com/blog /solar-research/122-million-solar-fuel-research/ .
• Solar Fuel StartingUp, CALFINDER (April 30, 2010),
http://solar.calfinder.com/blog/news/solar-fuel-starting-up/
• Sunlight Advances Hydrogen-ProductionTechnology, ENERGY INNOVATIONS: SCIENCE AND
TECHNOLOGY, Winter 2010 (published by National Renewable Energy Laboratory).
• Original Presentation by Philip Hof
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