Peidong Yang Department of Chemistry

University of California, Berkeley Materials Science Division

Lawrence Berkeley National Lab

Artificial Photosynthesis: Challenges & Strategies

It is about environment

• Total worldwide power consumption was ~15 terawatts with 80 to 90 percent derived from the combustion of fossil fuels.

•Currently low percentage of renewable energy in world-wide energy portfolio

• Pollution, Global warming, CO2 emission, Health

US: cut carbon emissions 26-28% on 2005 levels by 2025. China: make “best efforts” to peak emissions before 2030; increase the share of

non-fossil fuels energy consumption to around 20% by 2030.

• Solar Cell: Solar energy to electricity, requires energy storage solution.

• Artificial Photosynthesis: Solar energy directly to chemical energy, solving the energy conversion and storage problems in one integrated system.

Harvesting Solar Energy: Solar to fuel

Carbon-Neutral Solution

Artificial Photosynthesis

Hydrogen Methanol, Ethanol

Gasoline CO2, H2O

Solar Fuels Generation Reactions Reaction

ΔGo (kJ mol-1)

n

ΔEo (eV)

____________________________________________________________________________

H2O → H2 + ½ O2 237 2 1.23

CO2 + H2O → HCOOH + ½ O2 270 2 1.40

CO2 + H2O → HCHO + O2 519 4 1.34

CO2 + 2H2O → CH3OH + 3/2 O2 702 6 1.21

CO2 + 2H2O → CH4 + 2O2 818 8 1.06

Require the invention of new photoactive materials that accomplish the combined tasks of light harvesting, charge separation, and compartmentalized chemical transformation.

Solar-to-Fuel Conversion efficiency Cost-effective Earth-Abundant elements Less energy-intensive processes

Solar Water Splitting with single bandgap light absorber

Fujishima & Honda, Nature 238, 37 - 38 (07 July 1972).

Photosynthesis

Light Capture

Light Capture

Oxidation Catalysis

Reduction Catalysis

Nature’s Functional Solar-Fuel Generation System

Arthur J. Nozik, Photochemical Diodes, Applied Physics Letters 30, 567-569 (1977). C. Liu, N. P. Dasgupta, P. Yang, Chem. Mater., 26, 415 (2014)

Photochemical diodes: Dual absorber Concept

Two light absorbers vs. One

e-/s.nm2

1200

900

600

300

• Earth-abundant, Low-cost • Photoelectrochemically stable • Small-bandgap • Suitable CBM & VBM

The Matching Problem

~10-20 mA/cm2

~600-1200 e/s.nm2 Cathode-anode current matching Catalyst TOF -Photocurrent matching

C. Liu, N. P. Dasgupta, P. Yang, Chem. Mater., 26, 415 (2014)

Single optical path

e-/s.nm2

1200

900

600

300

Planar Electrodes

Nanowire Arrays (D=500nm, L=25 µm)

e-/s.nm2

12

9

6

3

High surface area semiconductor nanowire arrays with large carrier mobility as photoelectrodes

Nanowire arrays enable the stacking of catalysts in third dimension, effectively relax the stringent needs for

catalysts with high TOF, also lower the overpotential.

-0.6 -0.3 0.0 0.3 0.6 0.9-350

-300

-250

-200

-150

-100

-50

0

Dark 75mW/cm2

Curr

ent (

pA)

Bias vs NHE (V)

Boron doped (~1018) Length=8.5μm ,Diameter=650nm

VLS Single Nanowire Photocathode

Y. Su, J. Tang, Unpublished Results

Current Density ~6-12 electron.s-1.nm-2

sampling entire nanowire surface.

Final Products: Hydrogen Methanol …

The Helios Design (~2003) “Semiconductor nanowire array: potential substrates for

photocatalysis and photovoltaics”, Y. Wu, H. Yan, P. Yang, Topics in Catalysis, 19, 197, 2002.

Single optical path

• Nanowire arrays as photoelectrodes • Nanowire arrays for solar cell (DSC). • The nanowire array concept also inspired many of recent solar

fuel generator designs. Law, M., Greene, L. et al. Nature Mater. 4, 455 (2005). M. Law et al. J. Phys. Chem. B, 110, 22652 (2006).

Artificial Photosynthesis: Materials Challenges

Photoanode materials critical part of

the solar Fuels Technology.

Catalysts with higher TOF, and Lower overpotential.

Photocathode materials for Water & CO2 reduction.

Ion-conductive membrane gas impermeable, mechanically stable,

optically transparent .

Photocathode for Water Reduction: Silicon Nanowire Array

Si wire array as photocathode

High surface area photocathode, can be decorated with Pt or MoS2, CoSx nanoclusters.

22

ALD of Catalytic Pt Clusters on Silicon Nanowire Photocathodes

• Atomic layer deposition (ALD) of Pt allows for quantitative control of Pt loading in the sub-monolayer regime, with a conformal surface coating

• 1 cycle of Pt provides nanoparticles with diameters < 1 nm, with a surface mass loading of ~13 ng/cm2

• HER activity can be controlled by varying the ALD cycle number, demonstrating the lower limits of Pt loading on high surface area electrodes.

N. P. Dasgupta et al., J. Am. Chem. Soc. 135, 12932 (2013)

18

16

14

12

10

8

6

4

2

0

%

of T

otal

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Diameter (nm)

1x 3x 10x

Photocathode: Silicon nanowire array

JSun et al, J. Am. Chem. Soc. 133, 19306, 2011 C. Liu, J. Sun, J. Tang, P. Yang, Nano. Lett, 12, 5407, 2012.

GaP Nanowire Photocathode for water & CO2 reduction

• Surfactant-free solution process • Low temperature • Large scale production • Tunable doping (e.g. Zn precursor) • Applicable to InP, InGaP alloys

Colloidal Processable GaP Nanowire Photocathode

large scale production

Processable J. Sun et al, J. Am. Chem. Soc. 133, 19306, 2011

C. Liu, J. Sun, J. Tang, P. Yang, Nano. Lett, 12, 5407, 2012.

Si, 1.1eV, 39 mA/cm2

InP, 1.34eV, 31 mA/cm2

WO3, 2.7eV, 3.3 mA/cm2 BiVO4, 2.4eV, 6.3 mA/cm2

GaP, 2.3eV, 7.4 mA/cm2 Fe2O3, 2.2eV, 8.9 mA/cm2

Rutile, 3.0eV, 1.4 mA/cm2

Anatase, 3.2eV, 0.8 mA/cm2

ZnO, 3.4eV, 0.5 mA/cm2

UV region: GaAs, 1.43eV, 28 mA/cm2

WSe2, 1.2eV, 35 mA/cm2

WS2, 1.51eV, 25 mA/cm2

400 nm 750 nm

Cu2O, 2.1eV, 10.7 mA/cm2

Also: Cu2S 1.2eV; CdTe 1.5eV; GeS 1.7eV; Zn3P2 1.7 eV; CdS 2.4eV CIGS & CZTS: 1.0~1.7 eV;

CdSe, 1.7eV, 20 mA/cm2

x-axis: energy of photon (eV) y-axis: current density (mA/cm2) Based on 100mW/cm2 AM1.5G solar spectrum

1 mA/cm2

62 electron per nm2 per sec

Macroscopic: linked-electrode configuration ~0.35 mA/cm2 under one-sun illumination (AM 1.5G). Overall efficiency: 0.21%. Faradic efficiency: 91%.

Band gap:

Si: 1.1 eV

Theoretical efficiency: 0.86% (one-sun illumination)

TiO2 (Rutile): 3.0 eV

Si NW

TiO2 NW

A Fully Integrated Nanosystem

Factors considered:

Sluggish O2 evolution process on TiO2

Inadequate light-absorption of TiO2 Fast charge transport of Si

Length-scale of depletion layer thickness of individual components

Artificial Photosynthetic Nanosystem

C. Liu et al, Unpublished 2012

IrOx clusters on TiO2 nanowires

Artificial Photosynthetic Nanosystem

C. Liu et al, Unpublished 2012

Artificial Photosynthetic Nanosystem

C. Liu et al, Nano Lett 2013

Artificial Photosynthetic Nanosystem

Current Matching between Electrodes

M. G. Walter, Chem. Rev., 2010, 110, 6446

Photoanode represents one of the biggest bottlenecks for artificial photosynthesis. We need discovery of

photo-oxidatively stable anodes with high photovoltage and high photocurrent.

Overall water-splitting efficiency

n+p-Si

p-InP

n-BiVO4 *

n-TiO2

1 2 3

4

1 2 3 4

Max. efficiency 0.7% 1.1% 1.5% 3.0%

* T. W. Kim, K. Choi, Science, 2014, 343, 990-994

39

Key Interfaces in Solar-to-Fuel Conversion

Inorganic-molecule

Semiconductor-semiconductor

Semiconductor-catalyst

Oxygen evolution reaction (OER) & ohmic contact

e H2O

O2

Hydrogen evolution reaction (HER)

h

h

e

H2O

H2

h

e

Si Pt ?

h IrOx

? H2O

O2

e

h

Si

TiO2 ?

System Components Interfaces

1 μm

Nanowire-Bacteria Bio-hybrids Feeding microbes with electrons for CO2 reduction

A Materials Biology Approach to CO2 Reduction

C. Liu, J. Tang, H. M. Chen, B. Liu, P. Yang, Nano Lett., 2013, 13, 2989

Artificial photosynthesis from hybrid materials/biological CO2 catalysts

Acetogensis, Harold L. Drake, Chapman & Hall, 1994 D. R. Lovley, et. al., mBio, 2010, 1, e00103-10

Light-Harvesting Materials Biological CO2 Catalysts

pH = 0

Bacteria attached on nanowires with high selectivity.

Patterned SiNWs array - VLS growth - Diameter: 300-400nm - Length: 3-6um - Passivation: SiO2 Bacteria: MR-1 Shewanella oneidensis

H. Jeong, Nano Lett, 2013

New Oxide-Coated Si Nanowire Arrays are Biocompatible

Chong Liu, 2014

Living bacteria can adhere and grow on nanowire arrays in pH 7 water

5 µm

1 µm

1 µm

Nanowires Can Protect Bacteria from Oxygen

Nanowire electrode allows operation of strict anaerobes in headspace of atmospheric O2 (20%) while not affecting mass transport of CO2

Some ORR catalyst is required to efficiently remove O2 in nanowire cathode

Chong Liu 2014

Sporomusa ovata

Unassisted Solar CO2 Reduction by Hybrid Catalyst

Light source: 100mW/cm2 AM 1.5G, 300W Xenon lamp

2 CO2 + 2 H2O CH3CO2H + O2 hν

Integrated nanowire/bacterial catalyst can drive pure solar CO2 fixation

E0 = 1.09 V

Efficiency = 0.38%

Chong Liu, Eva Nichols, Joe Gallagher 2014

Sporomusa ovata

P. Yang, M. Chang, C. Chang, 2014

Towards systems materials engineering

Dual light absorber Configuration -Integrated photoanode/cathode, mimicking photosynthesis in Nature.

Efficiency ~0.5% for fully integrated system has been demonstrated.

Photocathode materials: ~700mV; ~30 mA/cm2

Photoanode materials : ~1V; >10 mA/cm2

Catalysts on high surface area, high charge mobility semiconductor support with higher TOF, and Lower overpotential:

> 10 electrons/s.nm2

< 100 meV? for water reduction, oxidation & CO2 reduction

P. Yang, J. Tarascon, Nature Mater., 11, 560, 2012.

Chen et al. Science, 343, 1339,2014

Bimetallic Catalysts: synergistic geometric and electronic effects for CO2 reduction

D. Kim et al. Nature Comm, 5, 4948, 2014

Chemical Fuels; Drug Intermediates; Biopolymers….

Gas Station of the Future

Acknowledgements