Artificial Photosynthesis: A photovoltaic perspective Joel Ager - PowerPoint PPT Presentation

Artificial Photosynthesis: A photovoltaic perspective Joel Ager Joint Center for Artificial Photosynthesis and Materials Sciences Division Lawrence Berkeley National Laboratory UCB EECS Solid State Seminar Berkeley, CA December 9, 2011 Ager, EECS Seminar, 1/27/12-1

Acknowledgment and Disclaimer Acknowledgment: This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Ager, EECS Seminar, 1/27/12-2

What is “artificial photosynthesis”? Why might it be of interest? What does this logo mean? Ager, EECS Seminar, 1/27/12-3

What is Carbon Cycle 2.0? • This Energy & Environment initiative defines an overarching Lab mission directed at the most important and challenging issue facing mankind Ager, EECS Seminar, 1/27/12-4

Carbon Cycle 1.0: Natural Carbon Cycle 0.15 Gt C/yr Transfer rate from geologic reservoirs to surface: 0.15 Gt C/yr Ager, EECS Seminar, 1/27/12-5

Current open-ended C cycle Future balanced C cycle Carbon Cycle 1.x (2011 AD) Carbon Cycle 2.0 (2100 AD?) Transfer rate from geologic reservoirs Goal: 2x to 3x more energy production but driven by burning fossil fuels = 9 Gt C/yr with less than 1/3 of 2010 C emissions Ager, EECS Seminar, 1/27/12-6

LBNL Research – Carbon Cycle 2.0 Initiative Ager, EECS Seminar, 1/27/12-7

Let’s look at the energy landscape Ager, EECS Seminar, 1/27/12-8

Look at all that “fossil fuel” 86% fossil fuels US budget ~ 100 Quads 1 TW x 1 year = 30 Quads Units are Quads = 10 15 BTU ~ 10 18 joule (EJ) Ager, EECS Seminar, 1/27/12-9

Solar, in perspective Solar 0.006 Quads = 1.7 TW-hr 5 MWp solar farm Diablo Canyon Nuclear Power Plant 2 x 1100 MW reactors Ran at 90% capacity in 2006: 18 TW-hr Altamont Wind Turbines 576 MW capacity, 125 MW on average 1.1 TW-hr yearly average Ager, EECS Seminar, 1/27/12-10

Fossil fuel use and consequences • Photosynthesis fixed 3 gigatons carbon/year on average in 2000-2008 ? Wikipedia Global Carbon Project Ager, EECS Seminar, 1/27/12-11

Is there a particular fossil fuel which would be good to replace? Ager, EECS Seminar, 1/27/12-12

Which line is the fattest? 86% fossil fuels US budget ~ 100 Quads 1 TW x 1 year = 30 Quads Units are Quads = 10 15 BTU ~ 10 18 joule (EJ) Ager, EECS Seminar, 1/27/12-13

Why fossil fuels are so good for transportation 15 gallons of gasoline is 1800 MJ Need to run a 20% efficient solar panel (2m x 10m) for 2 weeks Less volume Weighs less Ager, EECS Seminar, 1/27/12-14

With the exception of nuclear and geothermal, the sun was the source of “our” energy Ager, EECS Seminar, 1/27/12-15

Natural Photosynthesis • Old photosynthesis: fossil fuels – Convenient but finite – Impacts of CO 2 emission • Current photosynthesis: biofuels – Scalable – Not as efficient as we would like ca. 0.5% energy conversion efficiency – How much fuel can we generate this way? Ager, EECS Seminar, 1/27/12-16

What is “artificial photosynthesis”? Why might it be of interest? What does this logo mean? Ager, EECS Seminar, 1/27/12-17

Simple picture of natural photosynthesis Plants (also algae and cyanobacteria) perform synthetic redox chemistry with two red photons, using the reduction products to build plant mass and releasing the oxidation product (O 2 ) into the air Adapted from Photosystem II (Springer, 2005) Ager, EECS Seminar, 1/27/12-18

In artificial photosynthesis we want to do the same thing as the natural system, but more efficiently catalyst for the reduction reaction (Nature uses Fe CH 3 O H O H complex for H 2 production) CH 3 H Light Capture O H electron –hole pairs generated O C O here with sufficient voltage (e.g. 1.23 eV + overpotential) to drive reactions. O C O O H H O O O O catalyst for the oxidation Courtesy of Freefoto.com reaction (plants use Mn complex) Membrane keeps oxidation and reduction products separated (to avoid reverse reactions) but allows H + transport Ager, EECS Seminar, 1/27/12-19

Melvin Calvin, 1982 : It is time to build an actual artificial photosynthetic system, to learn what works and what doesn’t work, and thereby set the stage for making it work better Photosynthesis Artificial Photosynthesis There are some challenges – otherwise we would already be doing it Ager, EECS Seminar, 1/27/12-20

There are challenges at all length scales S y s t km-scale e m / d e s i g n / p r o c e s s l e v e l Flow channel building blocks m-scale D e v i c e / p h cm-scale y s i c s l e v e l mm-scale Scalablility and Solar Fuels Generator Sustainability Analysis Prototypes nm-scale Scale-Up from Mesoscale to Macroscale Emergent Phenomena on Mesoscale Integration of Components Nanoscale to Macroscale Earth-abundant light absorbers and low-overpotential Photoelectrochemical Membranes catalysts ( Homogeneous; Heterogeneous; “Hybrid”) Ager, EECS Seminar, 1/27/12-21

Joint Center for Artificial Photosynthesis • Initiated July, 2010 • Eight Partners • Two DOE National Laboratories ( LBNL , SLAC) • Six Research Universities ( Caltech , UCB, Stanford, UCSB, UCI, UCSB) • Start-up company approach with highly focused research agenda Ager, EECS Seminar, 1/27/12-22

JCAP Strategic Structure Ager, EECS Seminar, 1/27/12-23

What is "Light Capture and Conversion"? Answer: The photovoltaic heart of the fuel generating system, delivering photo-generated electrons and holes to the redox catalysts at the chemical potentials required to perform the desired synthetic chemistry Ager, EECS Seminar, 1/27/12-24

Redox chemistry and current continuity Water splitting half reactions 2H + + 2e - -> 2H 2 Reduction: H 2 O + 2h + -> 1/2O 2 + 2H + Oxidation: Overall: H 2 O -> 1/2O 2 + H 2 ∆ G = +237 kJ/mol, 1.23 eV/electron CO 2 energetics are similar • Observation � G o � E o � max (kJ mol -1 ) Reaction n (eV) (nm) – The money making reaction _______________________________________________________________________________________________ H 2 O → H 2 + ½ O 2 237 2 1.23 611 is reduction CO 2 + H 2 O → HCOOH + ½ O 2 270 2 1.40 564 • So why are oxidizing CO 2 + H 2 O → HCHO + O 2 519 4 1.34 579 water? CO 2 + 2H 2 O → CH 3 OH + 3/2 O 2 702 6 1.21 617 CO 2 + 2H 2 O → CH 4 + 2O 2 818 8 1.06 667 – Where else are we going to get Gt-equivalents of electrons? Ager, EECS Seminar, 1/27/12-25

The voltage requirements are a little tougher than one might think Ager, EECS Seminar, 1/27/12-26

Thermodynamics vs. Kinetics Use water splitting as a model system, CO 2 reduction is similar Reduction: 2H + + 2e - -> 2H 2 H 2 O + 2h + -> 1/2O 2 + 2H + Oxidation: Overall: H 2 O -> 1/2O 2 + H 2 ∆ G = +237 kJ/mol, 1.23 eV/electron But "Overpotentials" needed to drive reaction at an appreciable rate 0.6 V overpotential for Pt García-Valverde et al. , Int. J. Hydrogen Energy 33 5352 (2008) Ager, EECS Seminar, 1/27/12-27

The absolute band positions matter Aligning with the redox potentials… • Conduction band edge has to be higher than the potential for the reduction reaction • Valence band edge has to be lower than the potential for the oxidation reaction Very important: Stability, especially for the photoanode (holes) Osterloh, Chem. Mater. 20 35 (2008) Ager, EECS Seminar, 1/27/12-28

Can regular solar cells do it? Ager, EECS Seminar, 1/27/12-29

PV technology is aimed at maximum efficiency Declining PV efficiency but higher voltage Shockley-Queisser limit, JAP 32 510-519 (1961) Ager, EECS Seminar, 1/27/12-30

Most single junction cells Not enough voltage These voltages look interesting… Ager, EECS Seminar, 1/27/12-31

Can a single photon do it? Ager, EECS Seminar, 1/27/12-32

Wide bandgap oxides work But efficiency is poor Only some of these have stoiochiometric products without 260 references! bias or other tricks But TiO 2 , SrTiO 3 , etc. do work… Highest quantum efficiency for NaTaO 3 -based system 56% QE at 270 nm (E g ~ 4.1 eV) Kato et al. , JACS (2003) Ager, EECS Seminar, 1/27/12-33

Ok, what about two photons (like the natural system)? Ager, EECS Seminar, 1/27/12-34