B. Seger and I. ChorkendorffCenter for Individual Nanoparticle Functionality (CINF)

Department of Physics, DTU

Design Parameters for 2-Photon Water Splitting

Why Photoelectrolysis

• Solar irradiation produces 69,000TW just on land.• For a sustainable future we need an energy source that give us

~14 TW of power (28TW by 2050).• To make 28TW of photoabsorbers, we can only use earth

abundant materials.

-Vesborg and Jaramillo, RSC Advances, 2012

Technoeconomic Analysis

James et. al., 2009, Technoeconomic Analysis of Photoelectrochemical (PEC) Hydrogen Production, DOE contract # GS-10F-009J

Advantages of a 2-Photon Device

2.3 eV

- M. Weber and M. Dignam, Int. J. Hydrogen Energ., 1986

• A 2 photon device allows us to use lower bandgap materials thus we can capture more of the solar spectrum.

• By using a 2 photon device we can potentially improve the efficiency from ~10% to 29%.

• It has been found that the bandgaps of the 2 photoabsorbers should be 1.7 eV and 1.0 eV

• Fundamentally the large bandgap (LBG) must come before the small bandgap (SBG) photoabsorber.

Optical Absorption Properties

• The question is what side is the photoanode and what side is the photocathode?

For a photoanode:– Valence band > 1.6V vs. RHE– Stable during O2 evolution

For a photocathode:– Conduction band < -0.05V vs. RHE @ pH=0– Conduction band < -0.15V vs. RHE @ pH=14– Stable during H2 evolution

General Conditions• Large Bandgap = 1.5 ≤ EGap ≤ 2.1• Small Bandgap = 0.9 ≤ EGap ≤ 1.5• pH= 0 or 14 (to minimize ohmic losses)

Using Computational Screening• High throughput screening was used to look for potential

candidates.• 2,400 candidates were investigated from the Materials Project

Genome. (http://cmr.fysik.dtu.dk)• The parameters we used are as followed:

Potential Candidates (at pH=0)

6 Candidate 11 Candidates 8 Candidates 2 Candidates

AuClO, Co(ReO4)2, Cr2Ag2O7, CuRhO2,Mg(BiO3)2, Zn(RhO2)2,

As2Os, As2Ru, CdTe, FeSbS, GeAs, GeAs2, KCuSe, MoSe2

NaTiCuS3, SnSe,Te2Mo

CdSe, Cs2Ni3S4, InSe, WSe2, NaPt2Se3 , SbIrS,

Bi2Pt2O7, HfNBr

-Seger et al., Submitted

Bi2Pt2O7, HfBrN,PtO2

Potential Candidates (at pH=14)

16 Candidates 2 Candidates 1 Candidate 3 Candidates

Ag3VO4, AuClO, Au2O3, Ba2FeMoO6, Bi4O7 , Ca(RhO2)2, CdHgO2, Cd(RhO2)2, Cd2SnO4, Co(ReO4)2, Cr2Ag2O7, CuRhO2, Mg(BiO3)2, LaRhO3, LiBiO3, Zn(RhO2)2

Ca3(CoO3)2, LaRhO3

NaPt2Se3

Big Problems

• Why is finding the right photoabsorbers so hard?– Answer: Photoabsorber stability.

• Can we eliminate the stability problem?– Yes, with corrosion resistant protection layers.

• Potential Materials- Metals, Semiconductors, Insulators

Catalyst Issues

• Redox catalyst can interfere with light absorption.• H2 evolution catalyst have 109 better current exchange

densities than O2 evolution catalysts.• Mitigating redox catalyst light absorption clearly favors

Design 2.

Trotochaud, JPC Letters, 2013

Solar Irradiation

Redox reaction Redox reaction

Wired Photoabsorbers

Large Band Gap Wires

Small Band Gap Wires

Catalysts on Pillared Devices

• Structured devices means the small bandgap protection layer can’t absorb light.

Catalyst can’t absorb light

Catalysts can’t absorb red light

Metal Protection Layers

Metal may work Metal may work

• Issue: Metals absorb a lot of light.– Solution: Metals can only be used on small bandgap side.

• Issue: They interfere with the photovoltage/bandbending.– Solution: Create a p-n junction.

• Issue: Many metals convert to oxides at their surface.– Solution: Make this oxide works as a catalyst (favors O2 evolution

catalysts).

Metal won’t work Metal won’t work

Oxidized metal may work

Semiconductor Protection Layers• Issue: They aren’t very conductive.

– Solution: Protection layers can be ~50nm or less.• Issue: They may absorb some light.

– Solution: Use large bandgap semiconductors (bandgap > 3.0 eV).• Issue: Bandbending may prevent electron transfer.

– Solution: Align the bands properly. This takes a little work.

Semiconductor Protection Layers• At the Photoanode:

– The semiconductor transfers holes through the valence band (p-type).– The valence band needs to be near the O2 evolution potential.

• At the Photocathode:– The semiconductor transfers electrons through the conduction band (n-type).– The conduction band needs to be near the H2 evolution potential.

TiO2,Nb2O5

Potential Candidates (at pH = 0)

0 Candidates 0 CandidatesMoS2 , TiO2, Nb2O5

• All cathode protection layers have been tested in our labs- Laursen et al., 2013, PCCP, Seger et al., 2013, JACS

Long Term Stability

• TiO2 protected a Si photocathode (Design 1) for 30 days with no noticeable degradation.

• Performance decrease after 30 days was due to catalyst detachment or contaminants in the electrolyte.

• Redeposition of catalyst after 30 days brought back original performance.

0 5 10 15 20 25 30

-24

-20

-16

-12

-8

-4

0

Pho

tocu

rren

t m

A/cm

2 )

Time (days)

ALD 100nm TiO2 /5nm Ti/n+p Si(Vacuum annealed at 400°C for 1 hour) Ran at +300mV vs. NHE

0.1 0.2 0.3 0.4 0.5-24

-20

-16

-12

-8

-4

0

4

Phot

ocur

rent

(mA

/cm

2 )

Voltage (V vs. RHE)

100nm TiO2 /5nm Ti/n+p Si(Vacuum annealed at 400 C for 1 hour)

Initially After 1 Day After 30 Days Replatinize

(after 30 days)

(Seger et. al., RSC Advances, 2013)

Potential Candidates (at pH = 14)

TiO2 TiO2NiOCa4PdO6

BPNiOCa4PdO6

• Cathode protection layers have been tested in our labs.• We have found no literature on any of the anode protection layers

except for BP.

Insulating Protection Layers

• Issue: Insulators aren’t conductive– Solution: Electronically tunnel using very thin layers (~2 nm)

• Issue: Can a film 2nm thin be pinhole free?– Maybe.

• Issue: Is a 2nm thick layer mechanically durable enough?– Maybe.

Protection Layers Summary

• Metals– Great for protecting the small bandgap.

• Semiconductors:– Viable candidates for cathodic protection layers.– Untested candidates for anodic protection layers.

• Insulators- Works, but a gamble with long term stability.

Computational Screening for Protected Photoabsorbers

General Conditions

• Large Bandgap = 1.5 ≤ EGap ≤ 2.1

• Small Bandgap = 0.9 ≤ EGap ≤ 1.5

• The protection layers make stability irrelevant.• If the photoabsorber isn’t in contact with the electrolyte, the bands

may become de-pinned.• Thus conduction/valence band levels may become a non-issue• This means the same photoabsorbers can work in both Design 1 and

Design 2.

Candidates

205

249

Protected Photoabsorbers Candidates

• The table below shows only earth-abundant candidates.• There are an order of magnitude more candidates than the

unprotected case.• This allows for more stringent conditions to be placed on

photoabsorber candidates.

DesignScreening

Parameters# of

Candidates

SBG 0.9 ≤ EG ≤ 1.5 51

BaAs2, BaCaSn, Ba2Cu(PO4)2, Ba2FeMoO6, Ba3(Si2P3)2, BaLaI4, Ba3P4, CaBaSi, Ca3(CoO3)2, Ca2Si, Ca3SiO, CoAsS, CuCl2, CuP2, FeS2, FeSbS, K2Mo6S6, KNbS2, KPb, KSnAs, KZnAs, LaAs2, LaZnAsO, LaZnPO, LaS2, MgP4, MnP4, Na4FeO3, Na4FeO4, NaNbS2, NaNiO2, Na3Sb, NaSnP, NaTiCuS3, NaTiS2, NaZnP, NbFeSb, NbI3, Si, SnS, Sr2As2, Sr3As4, Sr3SbN, SrCaSi, SrCaSn, SrLaI4, Sr(ZnP)2, V(S2)2, Zn2Cu(AsO4)2, ZrBr3, ZrCl3

LBG 1.5 ≤ EG ≤ 2.1 50

B, BP, BaCu2SnS4, Ba(MgSb)2, BaP3, Ba4Sb2O, Ba2ZnN2, Ca3AlAs3, Ca(BC)2, Ca3(BN2)N, Ca(MgSb)2, Ca Na10Sn12, Ca3VN3, Ca(ZnP)2, CoBr2, CuSbS2, Cu2O, Cu3VS4, FeBr2, FeSO4, Fe(SiP)4, I2, K3As, K2Ni3S4, K4P6, K3Na2SnAs3, K2NiAs2, KSb, KV(CuS2)2, KZnP, KCuZrS3, MgAs4, NaCuO2, NaNbN2, NaP, NaSbS2, Nb6F15, NbI5, SnZrS3, SrP, Sr3P4, SrPbO3, TiBrN, TiI4, TiNCl, Sn2TiO4, WBr6, ZnSiAs2, ZrCl2, Zr2SN2,

Conclusions

• Without protection layers, there are major limitations on material choices for either design.

• With protection layers, Design 2 is clearly favored. • Our screening showed protecting the photoabsorbers greatly

increased the potential number of candidates.

Water SplittingPEC vs. PV+ Electrolyzer

• Why build 2 devices, when you can build one?• Why have the photogenerated electrons transfer meters, when

you can have it transfer micro-meters?• With a 1-photon PV, you have lower efficiency.• With a 2-photon device, optimal PV bandgaps = optimal PEC

bandgaps.

Reasons PEC is a better method

2-Photon Optimum Bandgap

Bandgap Small Large

PV 1.12 1.84

PEC 1.03 1.67

Acknowledgements

• The following people made this work possible– Ivano Castelli (Computational Screening)– Ib Chorkendorff, Peter C. Vesborg, Ole Hansen, Karsten Jacobsen

(Project Leaders)

• We most importantly must thank the CASE, CINF and CAMD grants for funding this research.

3 g/cm2 Pt

Do we have enough Pt, for photochemical water splitting?

• The best fuel cell anodes get 5x107 W/Kg of Pt with little voltage loss (<100mV). (See Figure)

• Currently the world produces 1.8x105 kg/year of Pt.• This means we have 15 TW/year potentially of Pt production.• Assuming 20% Capacity factor,

we can still produce 3 TW/year. • If water splitting devices last 20

years, and we need 15TW of energy, we will only use ½ of all the Pt currently produced.

• Pt expenses will be1.30$/kW using current prices.

Schwanitz et al., Electrocat, 2011