Eli Yablonovitch & Joseph Thurakal,

Materials Sciences Division, LBNL

UC Berkeley

Electrically Created Fuels

Materials For Renewable Energy, Lecture 2

Erice, Italy

July 24, 2012

2010

>15GW

Cost per peak Watt for Solar Panels

Date

$10

$1

$0.30

X

X

1981 2011 2020

40GW installed cumulative

1TW installed

cumulative

subsidies will no

longer be needed

Year

24

26

28

30

32

34

0.8 1 1.2 1.4 1.6

GaAs

33.5%

26.4%

Theory

Previous Record (2010) Voc=1.03V

GaAs, theory & expt.

25.1% record (1990- 2007)

~30%

Alta Devices 28.8% Voc=1.12V (2012)

e-

h+

h h h

25.1% efficiency

1990-2007

h h h hg

hg

e-

h+

28.8% efficiency

2011-2012

For solar cells at 25%,

good electron-hole transport is already a given.

Further improvements of efficiency above 25%

are all about the photon management!

A good solar cell has to be a good LED!

Counter-intuitively, the solar cell performs best

when there is

maximum external fluorescence yield ext.

http://arxiv.org/abs/1106.1603

Latest flat plate

results from

Alta Devices, Inc.

Expected to reach

29.5% single

junction shortly,

and

34% dual junction,

eventually.

p-Al0.2Ga0.8As

n-GaAs Eg=1.4eV

n-Al0.5In0.5P

n+-Al0.5In0.5P Eg~2.35eV

p+-Al0.5In0.5P Eg~2.35eV

p-Ga0.5In0.5P Eg~1.9eV

n-Al0.5In0.5P Eg~2.35eV

n-Ga0.5In0.5P Eg~1.9eV

Tunnel

Contact

GaAs

VOC=1.1V

Solar Cell p-GaAs Eg=1.4eV

Ga0.5In0.5P

VOC=1.5V

Solar Cell

Tunnel

Contact

GaAs

VOC=1.1V

Solar Cell

Ga0.5In0.5P

VOC=1.5V

Solar Cell

Dual Junction Series-Connected Tandem Solar Cell

h h

All Lattice-Matched ~34% efficiency should be possible.

Courtesy of

Alta Devices,

Inc.

5. What are the top competing technologies?

a. c-Si ~ 15%-23% in production

90% market share

b. flat-plate GaAs ~ 28.8% in R&D

Alta Devices Inc.

c. Concentrators ~ 43.5% in R&D

triple-junction III-V Solar Junction Inc.

-Summer: More hours of daylight. Sun is higher in the sky

-Winter: Shorter days, increased cloudiness, and sun is lower.

Need for seasonal, long term energy storage.

Time

Sun

ligh

t

Summer Sunlight Cycle: Longer Days

Sun

ligh

t

Winter Sunlight Cycle: Shorter Days

Time

Therefore storable fuel, not batteries are needed.

The starting point for electrically created fuel is usually water splitting:

2H2O(l) O2(g)+ 2H2(g) V1.23Volts

Pearl Gas to Liquids (GTL) Plant from Shell – Largest GTL Plant in the world

Oryx GTL Plant from Sasol

Gases to Liquid Fuels Fischer-Tropsch; Now a commercial reality:

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methane octane gasoline

CH4 C8H18

actual reaction 8CO+17H2 C8H18+ 8H2O

syngas

Fischer-Tropsch needs a source of carbon, for us CO2

Hydrogen and CO2 First Step: Inverse Water Gas Shift Reaction

8H2 + 8CO2 8CO + 8H2O H = 37 kJ/mole (Mildly Endothermic)

H = -1344 kJ/mole @873K, 100atm

(Exothermic)

Final Step:

SynGas octane gasoline

8CO+17H2 C8H18+ 8H2O But there are many other ways to create fuel

from electricity:

Fischer-Tropsch is being proposed for the USA, due to our low CH4 prices.

Creating liquid fuels requires high pressure ~100atm. fundamental Fischer-Tropsch reaction:

But the reaction is basically exothermic!

H = -1344 kJ/mole @873K, 100atm

(Exothermic)

use the exothermicity of

octane formation to create

steam to drive pumps for

the high pressure!

steam-driven pump for high pressure

8CO(g)+17H2(g) C8H18(g)+ 8H2O(g)

Also use the exothermicity of partial

methane oxidation

Electrolyze Club Soda Producing Methane, Methanol, Formic Acid, Ethylene… :

K.P. Kuhl, E.R. Cave, N. David, T.F. Jaramillo. “New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces,” Energy & Environmental Science. pp. 7050-7059 (2012)

-1.2 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1

Volts vs. RHE

Electrode

% Y

ield

Idea: Electrolyze Glucose (Gatorade) to Hexane?

No: creates Sorbitol and only at very high potentials: 8mA/cm2 at 4-5V.

Hexane: C6H14

Glucose: C6H12O6 Sorbitol: C6H14O6

Y. Tang et al. “Application of hydrogen-storage alloy electrode in electrochemical reduction of glucose,” Journal of Power Sources. vol 130. pp. 56-60. (2004).

Li et al. “Integrated Electromicrobial Conversion of CO2 to Higher Alcohols” Science. vol. 335, pg. 1596 (2012)

Electrolyzing Club Soda CO2 HCOOH (Formic Acid)=(H2+CO2) takes almost the same voltage as H2 generation.

Feed the Formic Acid to microbes, which can create butanol enzymatically.

Mild enzymatic conditions usually are too slow, necessitating substantial capital cost. Go back to Fischer-Tropsch.

Upgrade Biofuel: Can use electrolytically generated H2 to upgrade biofuels:

lignocellulosic materials

(levulinic acid)

(valeric acid)

Ethyl valerate (biofuels)

(γ-valerolactone)

Example: Valeric Acid Biofuels

Lange et al. “Valeric Biofuels: A Platform of Cellulosic Transportation Fuels,” Angew. Chem. Int. Ed. vol 49. pp. 4479-4483. (2010).

Since solar electricity only has a 25% duty factor, the capital cost of the electrolyzer dominates total cost. Downstream fuel processing sub-systems can run 24 hours a day.

25% Duty Factor of the Electrolyzer Su

nlig

ht

Time

Sunlight Cycle

Pearl Gas to Liquids (GTL) Plant from Shell – Largest GTL Plant in the world

Oryx GTL Plant from Sasol

Gases to Liquid Fuels Fischer-Tropsch; Runs 24 hours a day is already economic.

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actual reaction 8CO+17H2 C8H18+ 8H2O

syngas

But Electrolyzer runs only 6 hours/day.

Separator

Anode

Cathode

SEM image of a polymer exchange membrane fuel cell Cost should be <$1/Watt

Kim et al. “The effects of Nafion ionomer content in PEMFC MEAs prepared by catalyst-coated membrane

(CCM) spraying method,” International Journal of Hydrogen Energy. vol 35 pp. 2119-2126 (2010)

electrolyzer structure looks like a solar cell—low cost

60

m

250 m High Performance Silicon

Solar Cell SunPower Inc.

Cost is ~$1/Watt and going down

Separator

Anode

Cathode

Cost reduction by increasing current density, up to 1Amp/cm2

33 smaller area.

Reduce electrolyzer cost by running at higher current density than solar cells.

High Performance GaAs Solar Cells

0.03Amp/cm2

Platinum Catalyst Cost

• Cost of Pt is about 0.5 cents per Watt of power output in a fuel cell. • Goal of 50 cents per Watt, so catalyst is about 1% of final cost. • Platinum has average thickness of about 60nm on the electrode ~0.1mg/cm2.

TEM of Pt catalyst on carbon black. Black dots in image are the Platinum particles

T. Yoshitake et al. “Preparation of fine platinum catalyst supported on a single-wall carbon nanohorns for fuel cell application,” Physica B. vol 323. pp 124-126. (2002).

Fuel Cell Animation:

The Electrolyzer is the inverse of the Fuel Cell!

IR Drop

Nafion Resistivity: 10 Ω-cm

Super-capacitor electrolyte: 16 Ω-cm

1M NaCl electrolyte: 8 Ω-cm

These are typical resistivities, that we can’t change much.

The separator thickness should be < 50m

Nafion Membrane

Anode

Cathode

SEM image of a polymer exchange

membrane fuel cell

Kim et al. “The effedts of Nafion ionomer content in PEMFC MEAs prepared by catalyst-cated membrane (CCM) spraying method,” International Journal of Hydrogen Energy. vol 35 pp. 2119-2126 (2010)

Polymer Electrolyte Membrane (PEM) Fuel Cells/Electrolysers

Frano Barbir, PEM electrolysis for production of hydrogen from renewable energy sources, Solar Energy, Volume 78, Issue 5, May 2005, Pages 661-669.

60

m

Alkaline Electrolysis Cell

Over-

Potential

The small over-potentials would require >200C which would require high pressure water which would have high capital cost.

Nickel electrode—Nickel catalyst

H2/air operation at 80 °C

H.A. Gasteiger, J.E. Panels, S.G. Yan, Dependence of PEM fuel cell performance on catalyst loading, Journal of Power Sources, Volume 127, Issues 1–2, 10 March 2004, Pages 162-171.

Polymer Electrolyte Membrane Fuel Cell IV-Curve

Close to ambient pressure and temperature:

Under-Potential

requires better catalysts

Molten Carbonate Fuel Cell

H. Morita et al. “Performance analysis of molten carbonate fuel cell using a Li/Na electrolyte,” Journal of Power Sources. vol. 112. pp. 509-518 (2002).

Solid Oxide Fuel Cell

Commercial System Costs approximately $5/W

O= ions move through a ceramic electrolyte at 800C.

Direct Methanol Fuel Cell producing carbonated H2O:

P.L Antonucci, A.S Aricò, P Cretı̀, E Ramunni, V Antonucci, “Investigation of a direct methanol fuel cell based on a composite Nafion®-silica electrolyte for high temperature operation,” Solid State Ionics, Volume 125, Issues 1–4, October 1999, Pages 431-437.

Example of Intermediate States Causing Over-potential: Creating Methane Electrically:

CO2 + 2H2O CH4 + 2O2

BUT: Possible Intermediate Reaction Sequence:

Methane

CO2 + H2O HCOOH + 1/2O2

1.06V

1.21V

Formic Acid

HCOOH CH2O + 1/2O2 Formaldehyde

1.53V

CH2O + H2O CH3OH + 1/2O2 0.72V

Methanol

CH3OH CH4 + 1/2O2 0.77V

(Source of Over-potential) Formic Acid

Formaldehyde

Methanol Methane

ΔG Po

ten

tial

En

ergy

Reaction Co-ordinate

{ Sources of Over-potential: Activation Energy

Reaction without Activation Energy

Po

ten

tial

En

ergy

Reaction Co-ordinate

{ ΔG

} Activation Energy = Over-potential

Activation Energy

Over-potential supplied to compensate for Activation Energy

Separator

Anode

Cathode

polymer exchange membrane fuel cell Cost should be <$1/Watt

electrolyzer structure looks like a solar cell—low cost

60

m

250 m

High Performance Silicon Solar Cell

SunPower Inc. Cost is ~$1/Watt and going down

Cost reduction by increasing current density, up to 1Amp/cm2

25 smaller area.

J=0.04Amp/cm2

Conclusions: 1. Use a catalyst, it doesn’t add much to the cost.

2. Run at a current density 0.03A/cm2 < J < 1A/cm2.

3. The Electrolyzer should cost the same as the

solar cell, <50cents/Watt,

but is allowed to be more expensive per cm2.

4. Over-potential is not a show-stopper, since solar

electricity is becoming cheaper.

5. Run the Electrolyzer at <100C, to reduce capital cost.

6. Fischer-Tropsch is OK, but there is surprise potential in

the direct electrolytic production of fuels, methanol, etc.