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Development and Optimization of Cost Effective Material Systems For Photoelectrochemical Hydrogen Production
Eric W. McFarlandDepartment of Chemical EngineeringUniversity of California, Santa Barbara
May 19, 2009
Project ID # PDP_01_McFarland
This presentation does not contain any proprietary, confidential, or otherwise restricted information
Overview
• Start date September 1, 2004• End date May 31, 2010• 90% Complete
• Total project funding– DOE share $ 894k – Contractor share $ 223k
• Funding for FY08 $498k• Funding for FY09 $0
Timeline
Budget
BarriersTechnical Barriers Addressed
• (Y) Materials Efficiency• (Z) Materials Durability• (AA) PEC Device• (AB) Bulk Material Synthesis
Technical Targets2013 DOE PEC
• Solar-to-Hydrogen Conversion Efficiency >8%– Bandgap ~ 1.7-2.2 eV
• Lifetime > 1000 hours UCSB
• Scalable to produce hydrogen at a cost less than PV-electrolysis
RelevanceThere are no known material systems that are sufficiently efficient, inexpensive, and massively
scalable that might realistically be used for the large scale, cost-effective production of hydrogen, or any chemical fuel, from sunlight.
• Task #1. With a focus on abundant and non-toxic elements, develop improved materials for solar photon absorption using high throughput methods and new syntheses.
• Task #2. Utilize high-throughput screening to identify candidate materials with a threshold efficiency and stability that , with optimization, might meet the DOE performance and stability targets.
• Task #3. Explore the effects of morphology on the PEC material system efficiency making use of nanostructures to minimize charge carrier path lengths and maximize reactive surface area.
• Task #4. Explore processing and synthesis parameters to optimize efficiency through increased conductivity and minimized charge trapping and surface recombination of selected materials.
• Task #5. Identify and minimize electrokinetic limits by synthesis of appropriate electrocatalysts compatible with the host, electrolyte, and reactant/product properties.
• Task #6. Develop a complete, “photoelectrochemical unit”, combining material absorption, charge transport, stability, and electrokinetic design features.
• Tasks #7, #8, and #9: Evaluate conceptual model reactor systems, theoretical and practical economic potential of alternative redox reactions, estimate hydrogen production costs.
Objectives and Tasks for Total Project
New Candidate Material Discovery: High Throughput
Synthesis and ScreeningAbsorption Edge
< 2.2 eV ?
Synthesis ofOptimal Compositions
and StructuresSolar Photon-electron Current
Conversion Efficiency> 10%
Solar/HydrogenEfficiency
> 8% @ 10000 hours
Processing, Surface, Electrolyte, and
Electrokinetic Optimization
Optimized Large Scale Reactor System Techno-economics
Hydrogen Cost Estimate
< $5/Kg
Yes
No
Yes
No
Yes
No
YesSuccess!
No
20 nm20 nm
Approach/Selection Criteria
Material-Class SynopsisIron Oxide
FeFeFe
FeFeFe
Spin allowede- conduction
Conduction within (001) planes is 4 order of magnitude higher than parallel to [001].
Spin forbiddene- conduction
Bandgap ~ 2 eV (40% solar light absorption).Abundant and inexpensiveHigh Stability in Electrolytes (pH>3)
Carrier TransportValence Band EdgeWater Oxidation Kinetics
Low optical absorption
promise challenges
•23 different dopant species have been tested (MxFe2-xO3)•XPS only detected Fe3+ for all samples (Pt, Cr, Mo)• No (catalytic) Pt0 observed near the surface• Pt4+ , Mo6+, Cr4+ enhance the photo-electrochemical performance •No phase segregation was observed by XRD
Doped Fe2O3
Technical Accomplishments and ProgressNew materials.
• Theory Guided Synthesis of Doped and Mixed Oxides.• Comparison of high-throughput syntheses and screening directly with theory
provides basis for additional syntheses (proposed Al and mixed donor/acceptors).
DFT calculations performed by Y. Yan et. al. through
NREL collaboration initiated in 2008
Pure Pt doped iron oxide
FY2007-2008 work
Variable Concentrations of Al Doped α-Fe2O3
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.8
0.0
0.8
1.7
2.5
3.3
Ph
otoc
urre
nt (m
A/c
m2 )
Voltage vs. Ag/AgCl
0% Al in electrodeposited bath 2% Al in electrodeposited bath 6% Al in electrodeposited bath 20% Al in electrodeposited bath 40% Al in electrodeposited bath
400 450 500 550 6000.0
2.0
4.0
6.0
8.0
10.0
IP
CE
(%)
Wavelength (nm)
0 V vs Ag/AgCl--0%Al 0.2 V vs Ag/AgCl--0% Al 0.4 V vs Ag/AgCl--0% Al 0 V vs Ag/AgCl--10% Al 0.2 V vs Ag/AgCl--10% Al 0.4 V vs Ag/AgCl--10% Al
0 10 20 30 400
2
4
6
8
10
% (Al/(Al+Fe)) in Electrolyte
% (A
l/(A
l+Fe
)) in
sam
ple
• Best performance obtained ~0.3 -0.5% Al doping
• Higher % Al phase segregation to the surface
• Iso- valent doping improvement due to lattice contraction (predicted by
NREL collaborators)
Technical Accomplishments and ProgressNew materials.
N2
N2
Fe(CO)5
Ti precursor
O2Heater/T Controller
Substrate 350‐400 °CFTO
Ti:Fe2O3
Electrochemical synthesis of Ti:Fe2O3films were disappointing, new water-free synthesis route developed, 3x improvement in IPCE.
Technical Accomplishments and ProgressNew materials.
350 400 450 500 550 600 65002468
101214161820
IPC
E
Wavelength (nm)
0.2 V vs Ag/AgCl 0.4 V vs Ag/AgCl
200 300 400 500 600 700 800
Ordered mesoporous Fe2O3 Fe2O3 from Fe(NO3)3.9H2O decomposition Worm-hole mesoporous Fe2O3
Abs
orba
nce
(a.u
.)Wavelength (nm)
Worm-hole mesoporous Mesoporous Hematite
30 40 50 60 70
Ordered mesoporous hematite worm-hole mesoporous hematite
Inte
nsity
(a.u
.)
2 Theta (deg.)
Generalized syntheses developed for high surface area PEC materials: reduced overpotential for the same photon flux;
High pore volume: capability of heterogeneous nanoparticles, e.g. Pt, Au NPs
Both nanoparticles and mesoporous nanoparticles have high surface area and short transport length
Indirect direct bandgap transition due to wall effects
Ordered mesoporous
Technical Accomplishments and ProgressNew Morphologies
Hybrid PEC “Nanoreactors” - semiconductor/metal/oxide heterostructuresLow cost, scalable, solution phase synthesis of: co-catalyst/SC@encapsulant
Broadly applicablesynthesis route
Co-catalyst• Pt, Au, Ag, Pd, etc.• Virtually any nanoparticle
Support• Fe2O3, TiO2, ZnO, SiO2,
Al2O3, CeO2• Any metal oxide
Encapsulant• SiO2, ZrO2, TiO2, Al2O3Au
SiO2
Fe2O3
30 nm SiO2 10 nm SiO2 6 nm SiO2
Electrocatalystprotrusion
Au/α-Fe2O3@SiO2 FY2007-2008 work
Technical Accomplishments and ProgressComplete PEC “Systems”
•Hematite nanocubes synthesized•Pt nanoparticle electrocatalysts deposited by
photoreduction.•SiO2 encapsulated for stability.• Hydrogen production efficiency under
evaluation.
Technical Accomplishments and ProgressNew Morphologies
350 400 450 500 550 600
0
2
4
6
8
10
12Zero bias
NaOH Glucose Fructose Inositol
IPC
E (%
)
Wavelength (nm)0 20 40 60 80 100 120
-1400
-1200
-1000
-800
-600
-400
-200
0
200
Zero Bias
Phot
ocur
rent
(μA/
cm2 )
Time (s)
NaOH Sucrose Myo-Inositol Galactose Maltose Lactose Cellobiose Fructose Glucose
Technical Accomplishments and ProgressImproved Electro‐kinetics: Alternative Oxidants
•Probes efficiency in absence of ORR•??? Hematite below HER• Eliminates large ORR overpotential.•Municipal wastewater and biomass
potential feedstocks
Zero bias
1 M NaOH + 0.02 M Poly alcohol 1 Sun
350 400 450 500 550 6000
2
4
6
8
10
12
14
16
18Zero Bias
NaOH Glycerol Erythritol Xylitol
IPC
E (%
)
Wavelength (nm)
•Poly-alcohols showed an increased performance as compared to methanol•Increased IPCE ~ 17 % was achieved with Xylitol•Increased in performance could be obtained if better OER catalyst could be found•H2 production and byproducts of redox reaction to be studied
Technical Accomplishments and ProgressImproved Electro‐kinetics: Alternative Oxidants on Ti doped Fe2O3
• 3B group have direct band gap.• Effective masses are small, so
conduction should be better.• Alloying group 3A and 3B could be
used to reduce the band gap• Many possible substitutions for the
R group in CuRO2Possibly abundant and inexpensive
Stability under illuminationHigh recombination ratesValence and conduction Band positionWater Oxidation Kinetics
promise challenges
Cu+
O2-
Cr3+
Material-Class SynopsisCu Delafossites
CuLaO2
Inspired byDFT results from
NREL, M. Huda et al
Technology & Barriers:
Barrier Challenges Strengths
Y.MaterialsEfficiency
Due to the instability of Cu+ under ambient conditions, photogenerated holes are easily trapped by the Cu+ , which leads to the decomposition of the structure and decrease of the e-/h+ transport efficiency.
A narrow bandgap (~1.3 eV) makes the delafossiteabsorb most of visible light. Doping of divalent ions, e.g. Mg2+, will significantly further increase the electron conductivity.
Z.MaterialsDurability
Stability of CuCrO2 (bulk and mesoporous) under illumination in strong base solution suffers from its decomposition into spinel CuCr2O4 and dissolution of copper ions into solution.
The doping with Mg2+ significantly increase the electron conductivity in the crystals by a factor of ~1000 upon a doping of 5%. The Mg2+ doped mesoporous CuCrO2 has been shown to be stable under a long time illumination (159 hours) from the results of TEM and XPS.
AA. PEC Device and System Auxiliary Material.
Synthesis of mesoporous delafossite structure has some limitation in extending to other materials beside CuCrO2 due to the reaction of metal ions with silica template at high temperature. Further research into less expensive nanoparticles, applicable to other delafossite structures, is also a major key issue.
The silica template applied in the synthesis produced mesoporous nanoparticles with a high surface area , thin wall thickness, and reltivelygood crystallization. The nanocasting method can be applied in the synthesis of CuLaO2, CuGaO2, CuFeO2, and their sulfide and selenides.
AB.BulkMaterialsSynthesis
The obtained material powder is rough and difficult to be redispered in the electrolyte.
Materials are inexpensive and easy to synthesize.
Mesoporous CuCrO2 delafossite structures, SA=90 m2/g
30 40 50 60 70 80
200
400
600
800
1000
1200
1400
1600
1800
Inte
nsity
(a.u
.)
2 θ (degree)
(a) N2 adsorption/desorption isotherm (b) pore size distribution
• Hydrogen production demonstrated under visible illumination in powders, quantification in progress.
• High conductivity, Doping with Mg2+
increases conductivity by a factor of 1000
• Indirect bandgap ~1.3 eV
Technical Accomplishments and ProgressNew materials.
400 600 800 1000 1200
0.2
0.4
0.6
0.8
1.0
Bulk CuCrO2
Mesoporous Mg:CuCrO2
Mesoporous CuCrO2
Abs
orba
nce
(%)
Wavelength (nm)
Mesoporous CuCrO2 delafossite structures show activity for hydrogen production in Na2S + NaOH electrolyte
1020
3040
5002468
10
12
14
16
18
Bulk MesopCuCrO2 MesopMgCuCrO2
H2 e
volu
tion
μL/h
Illumination hours
Hydrogen production rate of bulk, mesoporous CuCrO2, and Mg2+ doped mesoporous CuCrO2 (5 mg powder in 15 mL NaOH and Na2S solution). Mesoporous delafossite shows an efficiency > x10 bulk material, however, λ < 400nm, required.
Bulk CuCrO2CuCrO2Mg: CuCrO2
Technical Accomplishments and ProgressNew materials.
Active for H2
Not active for H2
3
0
2
4
6
8
10
12
0.5 3
16.5 19 59
61.5
121.5
123
125.5
146.5
149
3
5
1H2 evolution uL/h
I = 8 mW/cm2
I = 20 mW/cm2
Bulk Mesoporous CuCrO2
Mesoporous Mg: CuCrO2
Cu metal nanorod
Mg doped CuCrO2 shows the highest chemical stability over 149 hours of illumination.
Became nanoparticles
Photocorrosion: MesoporousCuCrO2 delafossite structures
Technical Accomplishments and ProgressNew materials.
Cu+ oxidized into Cu2+ according to XPS, such reaction is proposed:2CuCrO2+ H2O CuCr2O4+CuO + H2
970 960 950 940 930 9203000
4000
5000
6000
Inte
nsity
(a.u
.)
Binding energy (eV)
CPS_CuCrO2_0h_Cu/2 CPS_CuCrO2_2h_Cu/8 CPS_CuCr2O4_0h_Cu/14
Cu 2p3/2
Cu 2p1/2
XPS results on Photo-oxidation of Cu+ in bulk and mesoporous delafossite structures
XPS of control and illuminated delafossiteI‐1: Mg:CuCrO2, remained 100% Cu(II) I‐2: Mesoporous CuCrO2, remained 58%I‐3: Bulk CuCrO2, remained 0%
02468
I‐1 I‐2 I‐3
H2 evolution from 5 mg CuCrO2 18.5 hours (uL/h)
100% 58% 0%
Technical Accomplishments and ProgressNew materials.
Stability for Cu(II) Cu(I)Cu (I) Cu(II )
CuLaO2 CuGaO2• Recent success in synthesisof several new Delafossites, though not all pure phase materials.
• Have set-up necessary systems to rapidly create other combinations in the same class.
• PEC performance screening in progress. CuFeO2
Technical Accomplishments and ProgressNew materials.
CuLaO2 +La2O3CuLaO2 +La2O3
CuGaS2SnS • Recent success in synthesisof new sulfides.
• H2S furnace able to process safely multiple samples.
• PEC performance screening in progress.
Technical Accomplishments and ProgressNew materials.
CuGaS2
CuGaSe2
CuGaSe2
20 40 60 80300
350
400
450
500
550
600
650
Inte
nsity
(a.u
.)
2 Θ
WSe2
WSe2
2 0 3 0 4 0 5 0 6 0 7 0 8 00
2 0 0
4 0 0
6 0 0
Inte
nsity
(a.u
.)
2 Θ
C u In S e 2
Mesoporous CuInSe2
• Recent success in synthesisof new selenides by wet
chemical route amenable to scale-up.
• PEC performance screening in progress.
Technical Accomplishments and ProgressNew materials.
Mesoporous CuCr2O4 spinel structures active as powders for hydrogen production in Na2S + NaOH electrolyte only under UV illumination
1.0 1.5 2.0 2.5
Inte
nsity
(a.u
.)
2 θ (degree)
(a)
20 30 40 50 60 70
Inte
nsity
2 θ (degree)
(b)
Technical Accomplishments and ProgressNew materials.
Mesoporous CuCr2O4 spinel active as powders for hydrogen production in Na2S + NaOH electrolyte only under UV illumination
Electrodeposition of NiFe Electrocatalysts
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-3
-2
-1
0
1
2
3
4
5 Fe
2O
3 Control
Fe2O
3 Ni(II)-Fe(III)
Fe2O3 Ni(II)-Fe(II) Fe2O3 Ni(II)
Cur
rent
Den
sity
(mA
/cm
2 )
Applied Voltage (vs Ag/AgCl)
(a)
Sample
mV onset vs Ag/AgCl
αIo [A/cm2]α
Fe2O3 Control 650 0.68 2.1-E7
NiFe (III) 560 0.59 7.4E-7
NiFe (II) 490 0.39 2.0E-7
Ni 490 0.44 2.1E-7
0 40 80 120 160-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
Cur
rent
Den
isty
(mA
/cm
2 )
Time (s)
α-Fe2O3 Control α-Fe2O3 Td=90 S α-Fe2O3 Td=30 S α-Fe2O3 Td=120 S α-Fe2O3 Td=60 S
(c)
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-2
-1
0
1
2
3
4
5
-0.4 -0.2 0.0-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Applied Voltage (vs Ag/AgCl)
Cur
rent
Den
sity
(mA/
cm2 )
Fe2O3 Control Fe2O3 Ni(II)-Fe(III)
(b)
•NiFe(III) improved the PEC performance by as much as 4.5 times as compared to the control sample.•NiFe(II) and Ni had a decrease in the PEC performance•The optimum loading for the NiFe(III) electrocatalyst corresponded Tdep ~90 sec.
Electrochem. Comm. 2009 (in Press)
Technical Accomplishments and ProgressImproved Electro-kinetics
0.5 1.0 1.5 2.0 2.5
0.0
1.2
2.5
3.7
Cur
rent
Den
sity
(mA
/cm
2 )
Applied bias vs Ag/AgCl
WO3
WO3- RuO2 micells0.33 M H3PO4
100 mW/cm2
•WO3 is greatly hindered by Oxygen Evolution reaction (a)•Fe2O3 is greatly improved by NiFe or RuO2 electrocatalyst deposition•No improvement observed by: RuO2 nanoparticles deposition on WO3(c) or electrocatalyst deposition on Fe2O3 (d)
a) b)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.0
1.0
2.0
3.0
4.0
Cur
rent
Den
sity
(mA
/cm
2 )
Applied Bias Vs Ag/AgCl
Fe2O3
Fe2O3 25% Ni Micells Fe2O3 50% Ni Micells Fe2O3 75% Ni Micells Fe2O3 100% Ni Micells Fe2O3 RuO2 Micells
1M NaOH
c) d)
0.8 1.2 1.6 2.0 2.4
0.0
0.2
0.4
0.6
0.8
1.0
Applied Bias (vs Ag/AgCl)
Cur
rent
(mA
)
FTO FTO- RuO2
WO3 no catalyst0.33M H3PO4 Ar degassed
-0.2 0.0 0.2 0.4 0.6 0.8
0.0
1.0
2.0
3.0
4.0
Applied Bias Vs Ag/AgCl
Cur
rent
Den
sity
(mA
/cm
2 )
Fe2O3
Fe2O3 25% Ni Micells Fe2O3 50% Ni Micells Fe2O3 75% Ni Micells Fe2O3 100% Ni Micells Fe2O3 RuO2 Micells
1M NaOH
Dark IV curves
PEC performance
Technical Accomplishments and ProgressImproved Electro‐kinetics
Electronegative surface species shifts conduction bandElectrons energy relative to H2/H+ redox level
0 1000 2000 3000
-900-800-700-600-500-400-300-200-100
0
iv
iii
ii
Phot
ocur
rent
(μA
/cm
2 )
Time (s)
i
i: Ti-doped sample without CoF3 treatment in NaOH solution
ii: Ti-doped sample with CoF3 treatment in NaOH solution
iii: Ti-doped sample without CoF3 treatment in NaOH+Glucose solution
iv: Ti-doped sample with CoF3 treatment in NaOH+Glucose solution
•Record efficiency for a True Zero bias hematite photoelectrode•New method applicable to Fe2O3 to shift the VB/CB
Chem. Comm. 2009 (in press)
Technical Accomplishments and ProgressProcessing and synthesis
Summary• New PEC Materials
– Theory guided synthesis and characterization of additional doped iron oxides supports theoretical predictions that the Mott‐Insulator symmetries must be broken to improve conductivity. HTE of Al doped hematite identified 0.3‐0.5 % as optimal, IPCE increased 4‐5x over controls . Initial results of donor/acceptor doping predicted by theory, disappointing.
– Theory inspired synthesis and initial characterization of new, CuMO2 Delafossites completed for M=Cr,Fe,Ga,La. Now attempting electrode configurations.
– Several new oxides (CuCr2O4 ), sulfides (SnS, CuGaS2), and selenides (WSe2,CuInSe2 , CuGaSe2 ) synthesized, PEC characterization in progress.
• Optimization of Material Performance– Surface modification with F to shift conduction band‐edge of Ti‐doped hematite
produced record zero bias (two electrode) efficiency for hematite ~ 1% at 450 nm.
– NiFe electrocatalysts have shown an improvement of ~ 4X on the performance
– Biomass analogues have been successfully photo decomposed with increased performance 15 times or higher than NaOH.
• New Structures– Developed generalized synthesis routes to integrated absorber/catalyst/coating PEC
systems. Will defer further work to focus on new absorber material identification.
Future Work(focus on finding and optimizing solar spectrum absorber consisting of earth abundant materials)
• Synthesis and Screening of New Materials:– Iron based oxides (to make go/no‐go decision on iron oxides in 2009)
• High‐throughput investigation of Fe‐Ti up to 40% Ti.• Thin‐Film doped iron oxides using organometallic pyrolysis (Si, Ti,C, doping guided by theory)
– Copper based oxides• Complete screening of delafossite oxides materials
– Attempt pure phase Cu(Fe)O2 , Cu(La)O2– Delafossite synthesis as films to allow electrochemical analysis.
– Begin initial exploration of Sulfides, Phosphides, and Selenides• Solution synthesis of Cu(Ga)Se2 , Cu(La)S2 , SnS , WSe2 , SnP• Develop High‐Throughput Synthesis Methods.
– High‐throughput screening and selected physical, electronic, and photoelectrochemical characterization of new host materials. Quantitative analysis of photocurrents and photovoltages and indirect measurement of hydrogen production activity and flatband potential. Selected use of Mott‐Schottky analysis and direct bulk H2 yields.
– Detailed analysis of selected candidates including carrier lifetime analysis by femtosecond transient absorption spectroscopy; internal quantum efficiency (IQE) characterization, and Raman.
• New Structures– Thin films and Mesoporous supports (ITO, WO3, TiO2) with nanometer scale doped iron oxide inclusions. – Solution phase synthesis of p‐n junctions, Cu(M)O2 /TiO2
30
Collaborations
• DOE H2 Program
– Directed Technologies (Participated in PEC System Analysis)
– Standard PEC testing group discussion
– Yanfa Yan, NREL (Theoretical Calculations)
– Eric Miller, University of Hawaii (Electrocatalysts for WO3)
– Clemens Heske, UNLV (Characterization of Fe2O3)
– Tom Jaramillo, Stanford (round‐robin testing)
• M. Gräztel, Ecole polytechnique fédérale de Lausanne ( Fe2O3 )
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