Shannon Boettcher – Electrochemical Energy Storage 1 Electrochemical Energy Storage: Design Principles for Oxygen Electrocatalysts and Aqueous Supercapacitors Shannon W. Boettcher Asst. Prof. of Chemistry University of Oregon Eugene, USA Boettcher Solar Materials and Electrochemistry Laboratory
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Shannon Boettcher – Electrochemical Energy Storage 1
Electrochemical Energy Storage: Design Principles for Oxygen Electrocatalysts
and Aqueous Supercapacitors
Shannon W. Boettcher Asst. Prof. of Chemistry University of Oregon Eugene, USA
Boettcher Solar Materials and Electrochemistry Laboratory
Motivation: Powering the planet
Solar is the only renewable source capable of providing 20-50 TW of power worldwide.
Shannon Boettcher – Electrochemical Energy Storage 2
Shannon Boettcher – Electrochemical Energy Storage 3
Cost of solar energy must be reduced. We must store that energy.
Solar Electricity > 11 ¢ per kWh (sunny climate, large installation) Industrial Electricity ~ 5-10 ¢ per kWh
Solar Energy Challenges
Shannon Boettcher – Electrochemical Energy Storage 4
Solar Materials and Electrochemistry Lab solar fuels electrochemical capacitors
scalable III-V semiconductors for PV
2H2O O2
O Ni ?
OER active site design?
interface theory
aq. low-T synthesis of functional films precise precursors
Fe
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Solar fuels synthesis using semiconductors and electrocatalysts
solution n-type SC
solution p-type SC
Walter, M.; Warren, E.; McKone, J.; Boettcher, S. W.; Qixi, M.; Santori, L.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473.
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Oxygen electrocatalysis has broad importance
solar water splitting
air-breathing batteries
fuel-cells (ORR)
http://protononsite.com/
large scale electrolysis
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From Wang et al., Electrochimica Acta, 2005, 50, 2059–2064
Need for well-defined systems and clean measurements
Smith, R.D.L. et al. Science, 2013, 340, 60.
Designed for maximum current per geometric area Dark colored – poorly suited for integrating in solar fuels systems
What is the role of composition, conductivity, and porosity? What is the surface-active component? Complicated!
O2
h+
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How do we design well-defined catalysts? What determines their activity?
Kerisha Williams
James Ranney
(Dr.) Lena Trotochaud
Team Catalyst c. 2011
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Solution-processed ultra-thin film catalysts
Advantages for fundamental study:
• Catalyst electrical conductivity (largely) irrelevant • Film composition controlled
exactly by precursor solution • Mass known • Surface area controlled • Facile gas and ion transport
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Initial target: Ni-Co-O
1. Trasatti, S.; Lodi, G. In Electrodes of Conductive Metallic Oxides; Trasatti, S., Ed.; Elsevier: Amsterdam, 1981; Vol. B, p 521-626. 2. Rasiyah, P.; Tseung, A. C. C. A mechanistic study of oxygen evolution on NiCo2O4 J. Electrochem.Soc. 1983, 130,2384-2386.
Mixing Co and Ni oxides reported = better performance. electronic? chemical? morphological?
* Indicates bonded to the surface H2O → *OH → *O + H2O → *OOH → O2
* Indicates bonded to the surface (see Rossmeisl, Norskov, etc.)
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Oxygen evolution with NixCo1-xOy films
1Rasiyah, P.; Tseung, A. C. C. A mechanistic study of oxygen evolution on NiCo2O4 J. Electrochem.Soc. 1983, 130,2384-2386.
Performance increases with increasing Ni content. No synergistic effect apparent. Apparent activity of “plain NiO” very high. Why?
- steady-state (> 15-30 min/step), 1 M KOH (99.999%) -Hg|HgO 1 M KOH reference (0.929 V vs. RHE at pH 14 or 0.112 V vs. NHE) - Rs 2-3 Ω via AC impedance
NiCo2O4 20 μm film,1 pH 14
Teff = 2-3 nm Performance increases with increasing Ni content. No synergistic effect apparent. Apparent activity of “plain NiO” very high. Why?
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Samples containing NiO evolve as a function of time
in-situ formation of a NiOOH?
?
Corrigan, D. A.; Bendert,. J. Electrochem. Soc. 1988, 135, C156-C156.
NiOx
1M 99.999% KOH, 20 mV s-1
0.3 V vs. Hg|HgO
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XPS analysis confirms transition to Ni(OH)2/NiOOH
Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S.; McIntyre, N. S. New interpretations of XPS spectra of nickel metal and oxides. Surf. Sci. 2006, 600, 1771-1779.
O 1s Ni 2p
NiO
Ni(OH)2 Ni2O3
NiO
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Co substitution suppresses transformation to oxyhydroxide
1M 99.999% KOH, 20 mV s-1
Alloying with Co suppresses formation of Ni(OH)2 / NiOOH during conditioning.
sample e- per metal e- per Ni CoOx 0.04 n/a
Ni0.25Co0.75Ox 0.07 0.27 Ni0.5Co0.5Ox 0.31 0.63
Ni0.75Co0.25Ox 0.61 0.82 NiOx 1.0 1.0
CV curves collected after 6 hr conditioning: # Ni measured with QCM
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In-situ phase transformation
Rock Salt (NiO) catalysis limited to surface
Brucite/Hydrotalcite (e.g. M(OH)2/MOOH) catalysis throughout bulk – “3D”
Spinel (e.g. Co3O4) catalysis limited to surface
X
Active structure:
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Ni(OH)2 electrochemistry studied extensively for alkaline batteries
Corrigan, D.A. J. Electrochem. Soc. 1987, 134, 377. Corrigan, D.A.; Bendert, R.M. J. Electrochem. Soc. 1989, 136, 723.
Fe is incorporated into electrochemically conditioned
NiOOH films
Dennis Corrigan: – Ni(OH)2 films cathodically
electrodeposited • Fe increases OER activity
– Fe added intentionally – or Fe impurities in
electrolyte
Electrochemically Conditioned Sample
EPMA Fe/(Ni+Co)
XPS Fe/(Ni+Co)
CoOx 0 0 Ni0.5Co0.5Ox 0.034 0.11
NiOx 0.18 0.14 NiOx, electrolyzed
KOH 0.035 0.079
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Thin-film OER catalyst quantitative comparison using an EQCM
sample
η @ J = 1 mA cm-2
(mV)
Tafel Slope (mV
dec-1) A g-1 TOF
(sec-1)
MnOx 512 49 ± 3 1.3 0.0003
FeOx 409 51 ± 3 4.5 0.0009
CoOx 395 49 ± 1 7.6 0.0016
IrOx 381 42 ± 1 24.2 0.014 (Fe)
Ni0.5Co0.5Ox 321 35 ± 2 273 0.056
NiOx (Fe) 300 29 ± 0.4 773 0.17
Fe0.1Ni0.9 Ox 297 30 ± 1 1009 0.21
at η = 300 mV
• Fe:NiOOH >10x more active an IrO2 and >100x more active than CoOx • Highest activity OER known in basic media. Why???
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• Merits (vs. EDLC) − electrolyte weight as active component − increased capacity / specific energy − use of aqueous electrolytes with high solubility (limited by low V) • Challenges − self-discharge − maintain cycleability and high power
separator
current collector
+ −
Lu, Beguin, Frackowiak, Supercapacitor: Materials, Systems and Applications, Wiley, 2013
Concept: Redox-enhanced supercapacitor
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Redox electrolyte “design principles”
• use aqueous electrolyte (safety, cost) • need different redox processes at negative and positive electrodes • formal potentials near solvent window (energy) • high solubility (energy) • fast kinetics (power) • stable (cyclability) • design for slow self discharge??
separator
current collector
+ −
Lu, Beguin, Frackowiak, Supercapacitor: Materials, Systems and Applications, Wiley, 2013
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Potential redox couple species
MV = methyl viologen
Need: - high solubility - large ΔV btw. couples - fast kinetics - slow self discharge
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Three electrode cell design
Epoxy insulating layer
Glassy carbon (current collector)
Ni bar N2 purge
10 mg per electrode, ~220 um thick - Activated carbon prepared by standard
CO2 activation process - Electrode pellet - AC (85 %) : PTFE (10 %) : Acetylene black (5 wt.%)
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Control inert electrolyte
C = Q/V energy = 1/2CV2
Charging 1 A/g
Discharging −1 A/g
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Halogen electrolytes for positive electrode
Three electrode Swage-lock cell used to monitor both electrode processes simultaneously.
capacitive
faradaic
3I- → I3- + 2e- 3Br- → Br3- + 2e-
charging at 1 A/g
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Self-discharge dynamics
• I- and Br- show slow self-discharge rate (specific absorption) • Fe(CN)6
4-/Fe(CN)63- redox couple shows fast self-discharge