National Renewable Energy Laboratory ... · Operated for the U.S. Department of Energy by Midwest Research Institute • Battelle • Bechtel National Renewable Energy LaboratoryOne

Operated for the U.S. Department of Energy by Midwest Research Institute • Battelle • Bechtel

National Renewable Energy Laboratory

Photoelectrochemical Water SplittingNREL Principal Investigator: John A. Turner

Current staffHeli Wang, PostdocTodd Deutsch, University of Colorado (PhD Student)John Einspahr, Colorado School of Mines (SS Student)Jennifer Leisch, Colorado School of Mines (PhD Student)Ken Menningen, University of Wisconsin (Sabbatical)

Recent PastJoe Beach, Colorado School of Mines (PhD, Graduated Dec 2001) Lara Kjeldsen, Colorado School of Mines (MS Student - graduated 2003)Scott Warren, Whitman College (SS - now at Cornell)Ken Varner, (SS - North Carolina State)

OthersVladimir Aroutiounian, University of Yerevan, Armenia (CRDF, ISTC)Arturo Fernández, Centro de Investigación en Energía-UNAM, Mexico


National Renewable Energy LaboratoryPhotoelectrochemical Conversion

Goals and Objectives

The goal of this research is to develop a stable, cost effective,photoelectrochemical based system that will split water using sunlight as the only energy input, with a solar-to-hydrogen efficiency of 10% with a 10-year lifetime. Our objectives are:

Identify and characterize new semiconductor materials that have appropriatebandgaps and are stable in aqueous solutions.

Study multijunction semiconductor systems for higher efficiency water splitting.

Develop techniques for the energetic control of the semiconductor electrolyte interphase.

Developing techniques for the preparation of transparent catalytic coatingsand their application to semiconductor surfaces.

Identify environmental factors (e.g., pH, ionic strength, solution composition, etc.) that affect the energetics of the semiconductor, the properties of the catalysts, and the stability of the semiconductor.

Band Edges of p- and n-TypeSemiconductors Immersed in Aqueous Electrolytes to Form Liquid Junctions

Econduction

Evalence

p-type2H2O + 2e– = 2OH– + H

2O2

Econduction

Evalence

n-type H2O + 2h+ = 2H+ + 1/2 O2H2

P158-A199107



Material and Energetic Criteria

•Band Gap (Eg) must be at least 1.6-1.7 eV

•Band Edges must straddle H2O redox potentials

•Fast charge transfer

•Stable in aqueous solution

1.23 eV1.6-1.7 eV Counter

Electrode

All must be satisfied simultaneously

Eg

H2O/H2

H2O/O2

p-typeSemiconductor



Timeline: 1991 to present

Cumulative Federal Funding: $4500K (~0.75 FTE + postdoc, average)



Project Status • Current status:

– 12.4% efficiency multi-junction system, 20 hour lifetime.

– H2 Cost: >$13/kg

• Targets:– 10% solar-to-hydrogen

efficiency, 10-year lifetime– H2 Cost: $3/kg


National Renewable Energy LaboratoryGallium Indium Phosphide/Electrolyte System

Used to gain a fundamental understanding of semiconductor/electrolyte junctions

Eg = 1.83 eV

Band edges are 0.2-0.4 V too

negative

Band edges are pH sensitive

(-)

(+)

E CB

Energy p-GaInP2

H2O/H2

E VB

E F

H2O/O2



World Record Photoelectrolysis Device Science, April 17 1998.

Experimental Cell

•Direct water electrolysis.

•Unique tandem (PV/PEC) design.

•12.4% Solar-to-hydrogen


National Renewable Energy LaboratoryComparison of p-GaInP2 and PEC/PV device

-1.5 -1.0 -0.5 0.0 0.5

-160

-120

-80

3 M H 2SO4

1 : p-GaInP2(Pt)/TJ/GaAs

2 : p-GaInP2(Pt)

Cur

rent

Den

sity

,mA/

cm

, ~ 11.6 suns

0

-402

2 1

Voltage versus Pt

The GaAs integrated PV cell compensates for the energetic mismatch – but expensive



Additional Approaches for BandedgeMismatch and Stability Issues

-1.0

0.0

1.0

2.0

-1.0

0.0

1.0

2.0

CB

VB

CB

VB

α -Fe2O

3p-GaInP

2

µA

e

h

e

h

E 0(H2/H

2O)

E0(H2O/O

2)

-2.0-2.0

b

Lower cost Fe2O3 electrode is a possibility, but…..


National Renewable Energy Laboratory ….While the System splits water, the efficiency is very low.

The photocurrent is limited by response of the iron oxide. Work is continuing in collaboration with other researchers (as part of our IEA efforts) to discover better materials.

Hematite nanorods in 0.5 M KNO3 ~1 W/cm2 GaInP2 in 0.5 M KNO3 ~1 W/cm2


National Renewable Energy LaboratoryBand Edge Engineering

The goal is to shift the band edges by surface modification to provide the proper energetic overlap.

EE

++

--Negative charges move the bandedges negative.

H2O/H2

H2O/O2

CB

VB

CB

VB

Modification

Positive charges move the bandedges positive.


National Renewable Energy LaboratoryThe work involved two series of Metallo-

PorphyrinsInsoluble Soluble

Tetra(N-Methyl-4-Pyridyl)porphyrin

~TMPyP(4)~Octaethyl Porphyrin

~OEP~

Transition metal in center of porphyrin maycombine favorable properties of band edge shifting and charge transfer.



Band Edge Engineering - GaInP2VFB vs. pH for RuCl3 + RuOEP Treated GaInP2 Electrode

H2O Oxidation Potential

The band edges here should be able to split water…..but no water splitting observed.



Metal Ion CatalysisAll show bandedge migration

Flatband Potential Migration, V

Cur

rent

Den

sity

,mA/

cm2



Band edge migration is due to negative charges accumulating at the surface – catalysis is not

sufficient – work continues….. (-)

(+) Dark

e-

h+

H2/H2O H2/H2O H2/H2O

O2/H2O O2/H2O O2/H2O

Increasing Light Intensity



Mott-Schottky Plot of Fe(III)TMPyP(2), pH 7

-5.E+13

0.E+00

5.E+13

1.E+14

2.E+14

2.E+14

3.E+14

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Potential, V vs. SCE

1/C2

Treated with FeTMPyP(2)Vfb = 1.195R2 = 0.9953

UntreatedVfb = 0.221R2 = 0.9996

The soluble porphyrins show large band edge shifts, but why the large change in slope…indicates a higher doping density…?


National Renewable Energy LaboratorypH testing shows even more interesting

behavior



Mott-Schottky Plot for VO(IV)TMPyP(4), pH 7

This phenomenon is not unique to the iron system.Electron density is being drawn from the near surface of the semiconductor,

but the mechanism is unclear…

UntreatedVfb = 0.220R2 = 0.9996

Treated withVOTMPyP(4)Vfb = 1.076R2 = 0.9966

-5.E+13

0.E+00

5.E+13

1.E+14

2.E+14

2.E+14

3.E+14

3.E+14

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Potential, V vs. SCE

1/C2



Photocurrent time profile for PEC/PV Water-Splitting device, showing current decay due to corrosion.

0 500 1000 1500

-140

-120

-100

-80

-60

-40

Efficiency(%) = (120mA/cm2*1.23v/1190 mW/cm )*1002 = 12.4%

Cur

rent

Den

sity

,mA/

cm2

Time, sec

0 5 10 15 20

-100

-50

CD

,mA/

cm2

Time, hours

We are looking for materials with inherently greater stability.



One possibility is nitride materials, an example of which is p-GaN

10 -8 10 -7 10 -6 10 -5 10-4 10-3

-1.0

-0.5

0.0

0.5

1.0

dark

under illum ination

icathod ic

icathod ic

ianodic

ianodicp-G aN0.1 M KO H

E (V

) vs

SCE

log i(log(A /cm 2))

0 1000 2000 3000 4000

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

6.0x10-5

7.0x10-5

8.0x10-5

9.0x10-5

1 M KOH pH 7 buffer 3 M H2SO4

Cur

rent

den

sity

(A/c

m2 )

Time (s)

It shows excellent stability…

…and water splitting capability

But…3.0eV bandgap

GaAs1.4 eV

GaAs1.4 eV

GaAs1.4 eV

GaInP1.8 eV

GaInP1.8 eV

GaInP1.8 eV

Ge0.7 eV

New1.0 eV

Future generationInproduction

New1.0 eV

4 5 6 7 8 91

2 3 4

Energy (eV)

Calculated efficiencies (ideal)500X AM1.5D: 36% 47% 52%one sun AM0: 31% 38% 41%

These materials are also interesting for high efficiency

Solar Cells

Lighting

Adding indium lowers the bandgap



Other material possibilities include…..

New materials - Preliminary Investigation of GaAsPN for PEC Water Splitting Systems

with Professor Carl Koval University of Colorado at Boulder

Indirect Photoresponse ME087-2

y = 0.0439x - 0.0772R2 = 0.9843Eg=1.76 eV

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

1.5E+00 1.6E+00 1.7E+00 1.8E+00 1.9E+00 2.0E+00 2.1E+00 2.2E+00 2.3E+00

Energy (eV)

Direct Photoresponse ME087-2

y = 9E-08x - 2E-07R2 = 0.9926Eg=1.88 eV

0

0.000000005

0.00000001

0.000000015

0.00000002

0.000000025

0.00000003

1.5E+00 1.6E+00 1.7E+00 1.8E+00 1.9E+00 2.0E+00 2.1E+00 2.2E+00 2.3E+00

Energy (eV)

Electrode Direct Eg Indirect EgME085-2 1.74 eV 1.69 eVME085-3 1.75 eV 1.67 eVME086-1 1.66 eV 1.59 eVME086-2 1.69 eV 1.58 eVME087-1 1.88 eV 1.75 eVME087-2 1.88 eV 1.76 eV



A long shot, but could be low cost….

New Materials - Band Gap Requirements for Electrodeposited CIGSSe

• Dependence on sulfur content noted.

• However, this is convoluted by varying concentrations of other elements

Possible low-cost thin-film material



Results for a-Si samples with Metal-ion Catalysts.

Another possible low-cost thin-film material

e-

α-Si Cell #3 p-i-n

α-Si Cell #2 p-i-n

α-Si Cell #1 p-i-n

α-SiCEg

1.8 eV

1.6 eV

1.4 eV



Summary of ongoing work for FY2003in blue

GaInP2 - NREL (fundamental understanding)GaPN - NREL (high efficiency, stability)InGaN - SVT Associates, NREL, (high efficiency, stability)CuInGaSeS - UNAM (Mexico), NREL (Low cost)Multi-junction Amorphous Silicon - University of Toledo and ECD (Low cost)Energetics

Band edge controlCatalysisSurface studies

+ +



Plans for future work• Continue materials research,

discovery and development.– InxGa1-xN – GaPN– a-Si– CuInGaSSe– a-SiN (coating for stability)– Others…

• Energetics– Band-edge engineering

• Development fundamental understanding of surface interaction.

• Try treatment on other materials– Catalysis– Surface studies



Collaborations and Interactions

In the USColorado School of MinesUniversity of ColoradoUniversity of Wisconsin (Whitewater)GM, Astropower (proposed)Others working in this area: FSEC, Duquesne

Outside of the USSwitzerland, Mexico, Armenia, Sweden, Japan



Plans

• Continue materials development.– Nitrides– CuGaInSSe– a-Si

• Band-edge engineering– Development fundamental understanding of surface

interaction.– Try treatment on other materials

• Increase industry interactions

Documents

National Renewable Energy Laboratory ... · Operated for the U.S. Department of Energy by Midwest Research Institute • Battelle • Bechtel National Renewable Energy LaboratoryOne