electrically created fuels materials for renewable energy ... · oc =1.1v solar cell p-gaas e g...
TRANSCRIPT
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.