A Component Approach to H2O Splitting: Hydrogen Evolution at Positive Potentials

- An NSF CBC dedicated to the discovery of solar fuels -

http://www.caltechmitsolarpower.caltech.edu/Powering the Planet

H. Gray J. WinklerJ. Peters N. Lewis B. Brunschwig D. NoceraC. Cummins

A Component Approach to H2O Splitting: Hydrogen Evolution at Positive Potentials

- An NSF CBC dedicated to the discovery of solar fuels -

http://www.caltechmitsolarpower.caltech.edu/Powering the Planet

2000 2010 2020 2030 2040 2050

¢ /Kwh

Year

5

20

Harry & Jay’s Energy Price Predictions

H. B. Gray 100th Birthday

SOLAR

OIL

Harry

retires !

H. Gray

The photosynthetic apparatus is the ultimate Rube Goldberg apparatus

H2O / CO2 / nutrientsHow? How?

Why not make robust synthetic analogues of these kinds of catalysts?

Rational synthesis has a long way to go. We don’t know how to control each step!

Perhaps every detail counts? Or can we distill out the essential chemical components?

The catalytic active sites within biological fuel-forming enzymescan be extremely fascinating inorganic clusters

The so-called ‘Biomimetic approach’

2H+ + 2e- = H2

E0 = -420 mV (pH = 7)

Fe-only H2ase

Darensbourg et. al., JACS, 2001

E ~ -1.6 V vs. SCE in H2OCollin, Sauvage et al.

E ~ -1.2 V (M = Ru)E ~ -1.6 V (M = Os)

vs. SCE in THFCollman et al.

Some functional H2 evolving catalysts

E ~ -1.5 V (Ni) and -1.6 V (Co) in H2O/CH3CN vs. SCEEisenberg et al.

E ~ -1.15 V vs. SCE in H2OKoelle et al.

E ~ -1.6 V (both) vs. SCE in DMSOSaveant et al.

Why not just use a Pt-electrode?

Pt

e-H+ H2

E° ~ 0 V (NHE)defined at 1 M H+ in H2O

Making H2 from protons and electrons

H+ + e- → H• E° ~ - 2 V vs SCE

We need to avoid the 1-electron reduction step……reduce H+ by two electrons to generate stabilized H-

Mn + H+ → [Mn+2(H)]+

1.89 x 106 mol Pt, per yearThis is how much

Pt we make

3 x 1017 kJ, per year This is how muchfossil fuel energy we need

1 x 1015 mol H2, per year We need this muchH2 for all fossil fuel

5 mol H2, per mol Pt, per sec This is the crude magnitude ofH2 production per Pt per sec

200 g of Pt need to produce 112 liters of H2 per second,and this would need to happen 10 hours a day, 365 days

a year, using 10 years worth of today’s Pt supply, assuming every available Pt atom is catalytically active

Is there enough available Pt on Earth to act as the H2-evolving cathode for a global solar fuels system

that would account for all needed fossil fuel?

Is it possible to prepare a synthetic, abundant metal complexthat can catalyze both H2 evolution and oxidation,i.e., a complex that functions at essentially thethermodynamic potential of the given system?

We’ve set out to address the following question:

Making H2 from protons and electrons

H+ + e- → H• E° ~ - 2 V vs SCE

We need to avoid the 1-electron reduction step……reduce H+ by two electrons to generate stabilized H-

Mn + H+ → [Mn+2(H)]+

Dr. Xile Hu

CrCl2 + HCl → CrCl2+ + ½ H2

EspensonInorg Chem 1986

slow catalysis in waterslow catalysis in waterraterate--limiting limiting Cr(II)Cr(II)∙∙∙∙∙∙Co(II)Co(II)→→Cr(III)Cr(III)∙∙∙∙∙∙Co(ICo(I) step) step

Eo {Cr(II)/(III)} = -0.415 V, Eo {Co(II)/(I)} = - 0.43 V vs. NHE

The thermodynamics looked particularly interesting for this system:

Our approach:Study molecular H2 evolving complexes that emphasize function

Uphill by ~ 15 mV

H2 evolution using TsOH·H2O as the acid

TsOH·H2O in CH3CN, pKa = 8

0.3 mM catalyst 0.3 mM catalyst

1。5 mM

4。5 mM

9 mM 9 mM

The system is completely CO tolerant !

0.32 mM catalystwith added HCl

0.0 -0.3 -0.6 -0.9 -1.2

0

8

16

24

32

40

C

urre

nt / µA

E vs. SCE / V

0.32 mM 1 + 0.34 mM HCl + 2.7 mM HCl

CV of 0.32 mM of [Co(dmgBF2)2(CH3CN)2] recorded in CO saturated CH3CN in the presence of HCl. Electrolyte: 0.1 M TBAClO4.

0.34 mM

2.7 mM

Cp2Co

CH3CN

CO

sparge

ν(CO) = 2015 cm-1

Examining steric and electronic factors for cobalt catalyzed hydrogen evolution

Eo(CoII/I) = -0.55 V Eo(CoII/I) = -0.28 V

Eo(CoII/I) = -0.38 V Eo(CoII/I) = -0.25 V Eo(CoII/I) = -0.15 V

N/A in CH3CN

Eo(CoII/I) = -0.08 V Eo(CoII/I) = -0.36 V Eo(CoII/I) = -0.20 V

H2 No H2

Cobalt systems of these general types look quite promising.They facilitate cathodic H2 evolution at very modest overpotentials.

How do they work?

Molecular systems allow detailed structure/function studies,invaluable with regard to mechanistic insight

e- (using Cp2Co) H+

Digital Simulation of Common Electrochemical Experiments M. Rudolph; http://www.digielch.de/

1. Co(III) + e → Co(II) Eo ~ 0.2 V 2. Co(II) + e → Co(I) Eo = -0.55 VMonometallic Pathway:3. Co(I) + H+ → Co(III)-H

K3 = ? k3 = ?4. Co(III)-H + H+ → Co(III) + H2

K4 = ? k4 = ?5. Co(III) + Co(I) → Co(II) + Co(II)

K5 = 1x1012, k5 = 1x1010 M-1 s-1

Bimetallic Pathway:3. Co(I) + H+ → Co(III)-H

K3 = ? k3 = ?4. Co(III)-H + Co(III)-H → 2Co(II) + H2

K4 = ? k4 = ?

experimental monometallic bimetallic

experimental monometallic bimetallic

Evidence for a Co-H intermediate?

Eo (CoIII-H/CoII-H) ~ -1 V

H+

H+H+

H+

H+ H+

glassy carbon electrode

ET (very fast)

H+

H+H+

H+

H+ H+

solv

solv

solv

glassy carbon electrode

H+

H+H+

H+

H+ H+

solv

solv

solv

glassy carbon electrode

Protonation (fast ~ 105 M-1 s-1)

H+

H+

glassy carbon electrode

H+

H+H+

H+

H+ H+

glassy carbon electrode

H+H+

H+

H+H+

ET (very fast)

H+H+

H+

H+ H+

solv

solv

solv

glassy carbon electrode

H+

H+

H+

H+

H+H+

H+

H+H+

H+

H+ H+

solv

solv

solv

glassy carbon electrode

H+

H+

H+

H+

H+H+

H+

H+

H+H+

H+ H+

solvsolv

solv

glassy carbon electrode

H+ H+

H+

H+

H+H+

H+

Protonation (slow ~ 500 M-1 s-1)

How can we shift the Co(II/I) redox potentialanodically without decreasing the basicity of

the Co(I) species that is generated?

These systems catalyze H2 evolution at unusuallypositive potentials, but we’re not there quite yet.

A Component Approach to H2O Splitting: Hydrogen Evolution at Positive Potentials

- An NSF CBC dedicated to the discovery of solar fuels -

http://www.caltechmitsolarpower.caltech.edu/Powering the Planet

Acknowledgments

Xile HuNathan S. Lewis

Bruce S. BrunschwigHarry Gray & Jay Winkler

Brandi M. Cossairt (SURF)

Chao Li (MS)Alex Sessions (MS)

$$$ NSF-CBC (Powering the Planet)

E-catalysis by a Pt-Electrode in CH3CN with Acids

Fc/Fc+

-0.58 V

-0.26 V

H2 evolution catalysis using Pt electrode (BAS)

Using a freshly platinized Ptelectrode under 1 atm H2V = 0.0 V for 10 mM HCl

V = -0.12 V for 10 mM CF3COOHvs SCE

Powering the Planet

H. Gray

J. Winkler

J. Peters

N. Lewis B. Brunschwig

D. Nocera

C. Cummins

The photosynthetic apparatus is the ultimate Rube Goldberg apparatus

H2O / CO2

Photosynthesis

H2 evolution in water:

Eo’ for CoII/I: - 0.73 V; E1/2 : -0.73 V

PH = 2, SHE = - 0.36 vs. SCE

Eo’ for CoII/I: - 0.69 V; E1/2 : -0.90 V

16

Summary:

MLn :I.

II.

16Hu X.; Brunschwig B.S.; Lewis N.S.; Peters, J.C. Submitted, 2006.

A Component Approach to H2O Splitting: Hydrogen Evolution at Positive Potentials

- An NSF CBC dedicated to the discovery of solar fuels -

Harry Gray (PI)Daniel NoceraNate LewisJay WinklerKit Cummins

Bruce Brunschwig

http://www.caltechmitsolarpower.caltech.edu/Powering the Planet

0.1 M [nBu4N][ClO4]; Scan rate: 100 mV / s-1; Working electrode: glassy carbon

HCl inCH3CN

CF3COOHin CH3CN

0.78 mM

3.1 mM

6.1 mM9.2 mM

1.5 mM catalyst 0.34 mM catalyst

pKa ~ 8.6 pKa ~ 12.7

first prepared by Schrauzer

5 mM

14 mM

31 mM

Getting started…replace the CrCl2 reductant by an applied voltage

Faradaic yield: ~ 95%50 equiv HCl (2 h, -0.75 V)

E-chem for [Co(dmgBF2)2(CH3CN)2] with TBACl in Acetonitrile

Electrolyte: 0.1 M TBAClO4Electrode: working, Glassy Carbon; reference, Ag/AgNO3 (0.01 M); auxiliary, Pt wire

Scan rate: 100 mV/s

EE** ((Co(IICo(II)/(I)) = )/(I)) = --0.81 V0.81 V

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2-6

-4

-2

0

2

4

6

8

Cur

rent

/ µA

E vs. SCE

0.22 mM Co(dmgBF2)2 + 3mM TBACl + 15 mM TBACl + 68 mM TBACl

0.1 M [nBu4N][ClO4]; Scan rate: 100 mV / s-1; Working electrode: glassy carbon

pKa ~ 0.1

HCl inCH3CNpKa ~ 8.6

HBF4 inCH3CN

1.7 mM

7.0 mM12.2 mM

0.5 mM catalyst 0.16 mM catalyst

0.25 mM

0.5 mM

Faradaic yield: ~ 95%50 equiv (HCl, 1 h, -0.5 V)

One very simplified mechanistic proposal

Biologicalinspiration

But do we need to structurally mimic nature?

The world produced ~ 370 met tons of Pt, or 3.7 x 105 kg, per year (in 2000)

Is there enough available Pt on Earth to act as the H2-evolving cathode for a global solar fuels system

that accounts for all needed fossil fuel?

Let’s crudely consider the order of magnitude problem

3.7 x 108 g Pt 1 mol Pt

195.1 g Pt1.89 x 106 mol Pt, per year

How much energy do we need from a fossil fuel source?

The total mean energy budget for 1998 was ~ 383 Quads.1 Quad ~ 1.1 x 1015 KJ.

1.1 x 1015 KJQuad

383 Quads4 x 1017 KJ, per year

80% fossil fuel

3 x 1017 KJ, per year

This is how muchenergy we need

This is how muchPt we make

3 x 1017 KJyear

If the fossil fuel is to be H2, how much H2 do we need?

Assuming we’ll burn the H2 via combustion with O2, making water as the byproduct

∆Hcomb(H2) = -300 KJ/molWe get this muchenergy if we burn H2

300 KJ

mol H21 x 1015 mol H2, per year

We need this muchH2 per year

How much H2 does each atom of Pt need to produce per second?

1 x 1015 mol H2

year

year

2 x 107 mol Pt

10 year supply of Pt

5 x 107 mol H2, per mol PtEach mol Pt mustmake this much H2

per year

5 x 107 mol H2

mol Pt • year

Now let’s assume a device operating at full efficiency for 10 hours per day, 365 days per year

1.3 x 107 seconds per year

1 x 107 sec

year5 mol H2, per mol Pt, per sec

This is the crude magnitude ofH2 production per Pt per sec

Why now?

1) We know a lot about the biological systems1) high resolution structures2) molecular biology and genetics3) spectroscopy4) electron-transfer

2) Synthesis and catalysis1) Vastly improved methods for synthesis

and characterization2) Rational design in homogeneous and

materials chemistry has advanced a great deal.

3) High through-put approaches and multi-reaction assemblies have been realized.

3) We don’t have a choice!

103 (km)103 (Km)4*1035*104 (TsOH)-0.35[Co(TimMe)(CH3CN)2]2+ (3’)

8*104 (kbi)105 (Kbi)8*103105 (TsOH)-0.38[Co(TimMe)Br2]+ (3)

-0.28

-0.55

Eo for CoII/I / V

500 (km)103 (Km)500100 (TsOH)Co(dpgBF2)2 (2)

4*105 (kbi)105 (Kbi)105107 (TsOH)Co(dmgBF2)2 (1)

kbi or km/M-1s-1K(bi) or K(m)/M-1s-1

k(H)/M-1s-1K(H)/M-1s-1

H. Co(I) + H+ → Co(III)-H KH = ? kH = ?Bi. Co(III)-H + Co(III)-H → 2Co(II) + H2 Kbi = ? kbi = ? M. Co(III)-H + H+ → Co(III) + H2 Km = ? km = ?

Kinetics of H2 evolution : Summary

14

Simulation: DigiElch

Bimetallic Pathway:Monometallic Pathway:

1. Co(III) + e → Co(II) Eo ~ 0.2 V 2. Co(II) + e → Co(I) Eo = -0.55 V3. Co(I) + H+ → Co(III)-H

K3 = 107 M-1 k3 = 1E5 M-1s-1

4. Co(III)-H + Co(III)-H → 2Co(II) + H2K4 = 103 M-1 k4 = 4E5 M-1s-1

Bimetallic Pathway Dominate!Espenson et al. J. Am. Chem. Soc.1978, 100, 129-133.

Co(III)-H + Co(III)-H → 2Co(II) + H2Kb = 8 M-1 kb = 1.7E4 M-1s-1

Co(III)-H + H+ → Co(III) + H2kH = 0.42 M-1s-1 12

Fe

O

BOH

Relation of OEC and Hangman for Water Assembly and

Activation

H

MnIVMnIVMnIVMnV

O

OCa

O

OO

Cl

O

[Co(TimMe)Br2]+ and [Co(TimMe)(CH3CN)2]3+

3Eo(CoII/I) = -0.38 V Eo(CoII/I) = -0.36 V3’

Condition: 0.7 mM catalyst, 9mM TsOH (pKa ~ 8.0 in CH3CN)

First prepared by Busch et al.

Faradaic yield: 90~100%20 equiv TsOH (30 min, -0.58 V)

ip = FSCpo(DkCs

o)1/2

Faradayconstant

Electrodesurface area

Bulk concof catalyst Diffusion

constantReaction

rate constantBulk conc

of substrate

Plateaucurrent

0.0 -0.2 -0.4 -0.6 -0.8 -1.0

0

8

16

24

32

40

Cur

rent

/ µA

E vs. SCE

0.34 mM Co(dmgBF2)2 + 5 mM CF3COOH + 14 mM CF3COOH + 21 mM CF3COOH + 26 mM CF3COOH + 31 mM CF3COOH

ip

Correlation between Rates of Catalysis and Reduction Potential

CoII/I

Tosic acid

the mechanism …

10