a component approach to h2o splitting: hydrogen evolution...
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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
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Year
5
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Harry & Jay’s Energy Price Predictions
H. B. Gray 100th Birthday
SOLAR
OIL
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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
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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 …
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