Solar Energy-driven Photoelectrochemical Conversion Using Earth-Abundant Materials

Gengfeng Zheng

Fudan University

2015-02-02, UCSB

TeraWatts, TeraGrams, TeraLiters Workshop on Challenges and Opportunities for Sustainable Production of Chemicals and Fuels beyond the Shale Gale

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Acknowledgements

Collaborators:

Dongyuan Zhao (Fudan, Chemistry)

Wenbin Cai (Fudan, Chemistry)

Xingao Gong (Fudan, Physics)

Min Jiang (Fudan, Life Sciences)

Zhongqin Yang (Fudan, Physics)

Song Jin (U. Wisconsin, Chemistry)

Rong Fan (Yale, BioMedEng)

Ahmed Elzatahry (King Saud Univ.)

Students:

Jing Tang (PhD)

Biao Kong (PhD)

Yongcheng Wang (MS)

Peimei Da (PhD)

Yuhang Wang (PhD)

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TeraWatt Scale Global Electricity Consumption

Substantial global energy demand, cost and environment footprints.

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Research Goal

Research in synthesis of new nanomaterials/structures and developing novel physical measurement methods for a variety of opportunities for catalysis,

photoconversion, energy storage and biointerface.

Material Structure

Electronic Structure

Interfaces

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Solar Energy-driven Artificial Photosynthesis

• Semiconductor particles as photo harvester

• Needs co-catalysts (e.g., Pt, MoS2, and Co3O4)

• Return reaction of H2O need to be prevented

• e- or h+ scavengers can be used (electrolytes) to generate only O2 or H2.

Osterloh and Parkinson, MRS Bulletin, 2011, 36, 17A. Kudo and Y. Miseki. Chem. Soc. Rev. 2009, 38, 253-278

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Semiconductor Photoelectrochemical (PEC) Conversion

Water reduction

Water oxidation

Grätzel et al. Science 2014, 345, 1593

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Earth-Abundant Materials in the Earth Crust

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Sustainable Solar Energy Conversion

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Design of Semiconductor Heterostructure & Interface

ħωφe-

ECB

EVB

EF

Surface Chemistry (catalyst,

sensitizer, receptor…)

Band Gap Tuning

EF,Redox

DOx

DRede-

Electrolyte

Ene

rgy

Semiconductor Heterostructure & Interface

e−

e− e− e−

h+h+ h+

Charge Carrier Density

Charge Transfer

Electrochem& Kinetics

Goals: Rational design/synthesis of material structures & interface for enhanced photocatalysis.

Utilizing charge transport behavior for energy conversion and probing molecule interfaces.

ηtotal = ηabsorption × ηseparation × ηtransfer

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Research Design

Bandgap Tuning

Charge Carrier Density

Charge Transfer

ηtotal = ηabsorption × ηseparation × ηtransfer

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Semiconductor Materials for PEC

Walukiewicz, Physica B 302-303, 123 (2001)Van de Walle, Nature 423, 626 (2003)Peidong Yang, Chem. Mater. 26, 415-422 (2014)

band gap, band alignment, conductivity, stability, cost, ...

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Mesoporous Fe2O3 Nanopyramid-Au NP Heterostructure

Mesoporous Fe2O3 nanopyramid-Au nanoparticle heterostructure:

Mesoporous Fe2O3 nanopyramids are formed by an interfacial oriented growth of Prussian blue nanocubes w/o template, and subsequent calcination. AuNPs (5 nm) are then sputtered.

Kong B, Zheng GF*, Zhao DY*, et al. J. Am. Chem. Soc., 2014, 136, 6822.

Kong B, Zheng GF*, Zhao DY*, et al. Angew. Chem. Int. Ed., 2014, 53, 2888.

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Mesoporous Fe2O3 Nanopyramid-Au NP Heterostructure

Features:

Mesoporous nanopyramid structure with Au NPs

High surface area (~ 175 m2/g)

Large mesopore size (~ 20 nm)

Excellent flexibility

Scalability (inch to meter)

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Fe2O3 Nanopyramids-Au NPs as LSP-Enhanced PEC

The integration of plasmonic Au NPs with Fe2O3 nanopyramids enable localized surface plasmon (LSP) for PEC conversion, leading to ~6- and ~83-fold increase of photocurrent under solar light and visible light illumination, respectively.

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Simultaneous PEC Conversion & Energy Storage

TiO2 – Ni(OH)2

Si – Pt

Wang YC, Zheng GF*, et al. Nano Lett., 2014, 14, 3668.

Wang YH, Xia YY*, Zheng GF*, et al. Nano Lett., 2014, 14, 1080.

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Simultaneous PEC Conversion & Energy Storage

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Research Design

Bandgap Tuning

Charge Carrier Density

Charge Transfer

ηtotal = ηabsorption × ηseparation × ηtransfer

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Simultaneous Etching & W-Doping of TiO2 NWs

Post-doping of TiO2 NWs:

Conventional post-doping methods require high processing temperatures.

Simultaneous etching (NH2OH∙HCl) and W-doping (Na2WO4) can dope W atoms into deeper layer of NWs.

Wang YC, Zheng GF*, et al. ACS Nano, 2013, 7, 9375.

Xu M, Da PM, Zheng GF*, et al., Nano Lett. 2012, 12, 1503.

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Enhanced Carrier Density and Charge Transfer

Samples EFB (V) vs. Ag/AgCl Nd / 1018 cm-3

pristine TiO2 NW −0.89 3.86

doped TiO2 NW −0.87 2.06

etched TiO2 NW −0.58 1.36

dual etched/doped TiO2 NW −0.60 5.04

DFT simulation shows W 5d states exist in the bandgap and is close to the conduction band edge (Ti 3d), leading to the enhanced charge excitation and carrier density.

Flatband potential (EFB), Charge carrier density (Nd)Mott-Schottky Plot

DFT Simulation

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Dual Etched & W-doped TiO2 NWs for Enhanced PEC

Dual etched/W-doped NWs: Substantial PEC activity enhancement compared to the pristine, the etch-only, and the doped-only TiO2 NWs.

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Simultaneous Etching and Reducing of WO3 Nanoplates

Substantial etching and reducing of WO3 nanoplates allows for forming more reduced W5+ ions via a facile solution process, leading to enhanced charge carrier densities.

Li WJ, Da PM, Zheng GF*, et al. ACS Nano, 2014, 8, 11770.

Peng Z, Jia DS, Zheng GF*, et al. Adv. Energy Mater., 2015, 5, 1402031.

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Dual Etched & Reduced WO3 for Enhanced PEC

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Research Design

Bandgap Tuning

Charge Carrier Density

Charge Transfer

ηtotal = ηabsorption × ηseparation × ηtransfer

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Reduced Mesoporous Co3O4 NWs for Water Oxidation

NaBH4 reduction of mesoporous Co3O4 NWs:

The enhanced surface area of mesoporous Co3O4 NWs allows for more efficient NaBH4 solution reduction of Co3O4 NWs, leading to higher oxygen vacancy density.

Wang YC, Yang ZQ*, Zheng GF*, et al. Adv. Energy Mater., 2014, 4, 1400696.

Wang YC, Yang ZQ*, Zheng GF*, et al. Small, 2014, 10, 4967.

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Reduced Mesoporous Co3O4 NWs for Water Oxidation

Chemically reduced mesoporous Co3O4 NWs:

Current: 13.1 mA/cm2 at 1.65 V vs RHE, (7-fold of pristine Co3O4, also higher than IrO2)

Onset V: 1.52 V vs RHE, (50 mV ahead of pristine Co3O4, but 100 mV higher than IrO2)

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Tuning of Charge Carriers by Reduction/Oxidation

NaBH4 reduction of TiO2 NWs:

Oxygen vacancies in the reduced TiO2 cause defect states in the band structure and result in enhanced carrier density and conductivity.

Wang YC, Yang ZQ*, Zheng GF*, et al. Adv. Energy Mater., 2014, 4, 1400696.

Wang YC, Yang ZQ*, Zheng GF*, et al. Small, 2014, 10, 4967.

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Co3O4 Nanosheet/Nanotube as OER Catalyst

Ultrathin CoOx nanosheets that are further assembled into a nanotube structure.

Highly active Co2+ electronic structure for efficient OER at the atomic scale, ultrahigh surface area (371 m2·g-1) for interfacial electrochemical reaction at the nanoscale, and enhanced transport of charge and electrolyte over CoOx nanotube building blocks at the microscale.

Peng Z, Jia DS, Zheng GF*, et al. Adv. Energy Mater., 2015, 5, 1402031.

Wang YC, Cai WB*, Zheng GF*, et al. Adv. Science, 2015, in press.

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Co3O4 Nanosheet/Nanotube as OER Catalyst

Low onset potential of ~1.46 V vs. RHE, high current density of 51.2 mA·cm-2 at 1.65 V vs. RHE, and a Tafel slope of 75 mV·dec-

1.

1.5 V for full water splitting.

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Mass Production and Wide Applicability

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Properties of Fe2O3 Frameworks

1.2 kg production

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3.8 m

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Research Goal

Research in synthesis of new nanomaterials/structures and developing novel physical measurement methods can open a variety of opportunities for

catalysis, photoconversion, energy storage and biointerface.

Review: Wang YL, Zheng GF*, et al. Adv. Mater. 2013, 25, 5177.

Review: Tang J, Li J, Zheng GF*, et al. Chem. Eur. J. 2015, in press.

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Thank You !

Shanghai – Seen from a Clear Sky