ARTIFICIAL SOLAR FUELS GENERATORS

Transcription

1 ARTIFICIAL SOLAR FUELS GENERATORS Rachel Segalman Acting Division Director, Materials Science Division Lawrence Berkeley National Laboratories And Professor of Chemical Engineering, UC Berkeley

2 NATURE STORES SOLAR ENERGY IN CHEMICAL BONDS RESIDENCE TIME HUNDREDS OF MILLIONS OF YEARS

3 HOW TO UTILIZE SOLAR ENERGY SHORTCUT THE CYCLE? NATURAL PHOTOSYNTHESIS CURRENT TECHNOLOGY CONCENTRATED SOLAR hν Sugar (fuel) H 2 O + CO 2 + energy C 6 H 12 O 6 + O 2 Theoretical max efficiency = 11% Photosynthetic efficiency of 3 6% Excess absorbed light heat Heating a fluid to run a thermal cycle similar to conventional power generation Power generation varies with time /kwh Image source:

4 PHOTOVOLTAIC -- DIRECT SOLAR TO ELECTRICITY Efficiencies improving (up to 40%) All photovoltaic and solar-thermal systems require development of energy storage for load balancing

5 NATURE STORES SOLAR ENERGY IN CHEMICAL BONDS NATURAL PHOTOSYNTHESIS ARTIFICIAL PHOTOSYNTHESIS It is time to build an actual artificial photosynthetic system, to learn what works and what does not work, and thereby set the stage for making it work better Melvin Calvin (1961 Nobel Laureate)

6 CAPTURING AND STORING SOLAR ENERGY INTO FUELS Scalable solar-fuel generators pose many challenges: Development of cost-effective components and processes (high efficiency, earth-abundant, continuous and stable fuel production) Component integration (light-capture, catalysis, mass transport)

7 CAPTURING AND STORING SOLAR ENERGY INTO FUELS First demonstration of photoelectrochemical water splitting (TiO 2 ) Fujishima and Honda. Nature, 1972, 238, High solar-hydrogen conversion efficiency (expensive PV components) Khaselev and Turner, Science, 1998, 280, Artificial leaf with earthabundant components (up to 7% solar-fuel efficiency) Reece et al. Science, 2011, 334,

8 GENERAL APPROACHES TO ARTIFICIAL PHOTOSYNTHESIS DISCRETE PHOTOVOLTAIC WIRED TO ELECTROLYZER SOLAR-FUEL GENERATING PARTICLE DISPERSIONS INTEGRATED PHOTOELECTROCHEMICAL SOLAR-FUEL GENERATOR H 2 Catalyst H 2 O O 2 Semiconductor nanoparticle Advantages: Operational system has already been demonstrated with 18% efficiency. 1 Challenges: Demonstrated system demands expensive components; lack of integration further reduces cost efficiency. Advantages: Offers a simple architecture with the potential for low materials cost. Challenges: Cogeneration of fuel and oxidizer pose operational safety issues. Advantages: Potentially lower component costs than a discrete system with reduced complexity. Challenges: Requires that semiconductor, catalysts, and membranes operate efficiently under identical conditions. 1 G. Peharz, F. Dirmouth, and U. Wittstadt Int. J. of Hydrogen Energy 2007, 32, (DOI: /j.ijhydene )

9 INTEGRATED ARTIFICIAL PHOTOSYNTHESIS REQUIRES MULTIPLE SYSTEM DEVELOPMENT

10 PROCESSES INVOLVED IN A SOLAR-FUEL MEMBRANE Membrane Catalyst hν 2H + + 2e - H 2 Photovoltaic H + H + H 2 O 2H + + 1/2O 2 H 2 O + 2h +

11 Gas diffusion Rate H + diffusion Rate CAN WE DESIGN AN EFFICIENT DEVICE? Membrane hν Reduction rate 2H + + 2e - H 2 H 2 O 2H + + 1/2O 2 H 2 O + 2h + Oxidation rate

12 MODEL PROTOTYPE REQUIREMENTS Input literature material properties Output operating current, voltage, crossover current mem,l V L R mem mem,r s,h 2 s,o 2 PV V R > 6 μm > 60 μm Nafion proton conductivity is in 3 orders of mag in excess Decreased gas permeation strongly impacts final design Alan Berger and John Newman

13 SYSTEM LEVEL: MICROSCALE INTEGRATION

14 INTEGRATING COMPONENTS: EARLY-STAGE PROTOTYPES Prototypes: 100 cm 2 scale (large component integration) Test beds: small scale, flexible, tunable and controllable

15 ADVANTAGES OF A MICROFLUIDIC PLATFORM Modular Design Easy to probe various materials for light capture, catalysis, proton conduction/gas separation Architecture Tunability Optimization of architecture and operating conditions is fairly simple Quantitative measurements of performance Simple setup to measure gas production output

16 BUILDING A MICROFLUIDIC ELECTROLYZER Glass Cover Nafion 117 Membrane 177 μm 2H + H 2 O ½ O 2 +2H + Pt 2e - 20 μm 2H + 2e - Pt H 2 15 μm Si 50 μm SU μm 19 Interdigitated electrodes inside channels Miguel A. Modestino et al. Phys. Chem. Chem. Phys., 2013, ASAP Microfabrication by Camilo Diaz, Rafael Gomez

17 GAS SEPARATION ACROSS MICROCHANNELS Miguel A. Modestino et al. Phys. Chem. Chem. Phys., 2013, ASAP

18 Potential [V] Potential [V] ELECTROCHEMICAL PERFORMANCE Current density [A/m 2 ] J = 175 A/m2 J = 88 A/m Time [hrs] Electrochemical performance matches modeling results Stable operation of devices >10 hrs at current densities relevant to solar-fuel generators Miguel A. Modestino et al. Phys. Chem. Chem. Phys., 2013, ASAP Modeling by Sophia Haussener

19 OTHER ACHIEVABLE CONFIGURATIONS Asymmetric catalysts electrolyzer Facile electrodeposition of catalysts Cathode: NiMo, Pt Anode: CoPi, FeNi Inclusion of photoanodes/ photocathodes can lead to photoelectrolyzer Microfluidic solar-fuel generator Pattern commercial Si triplejunction PV to build enough potential for splitting (>2 V) Pattern catalysts at each electrode of the PV substrate Flow electrolyte, shine light, and get H 2! SU-8 ITO electrodes SU-8 Anode Cathode SiO 2 Si ITO Triple-junction PV S.S. Electrode

20 SYSTEM LEVEL INTEGRATION: SCALE-UP CONSIDERATIONS Stability of solar hydrogen generators under electrolytes

21 DEVICE OPERATION UNDER ACIDIC ELECTROLYTES Component corrosion in acid electrolytes After 21 minutes After 40 minutes After 72 hours

22 ONLY AT MODERATE PH THE SYSTEM IS STABLE Reece et al. Science, 2011, 334,

23 WHAT IS THE PROBLEM WITH BUFFERED ELECTROLYTES? Net accumulations of M + Net depletion of M +

24 Potential Increase [V] UNSTABLE OPERATION OF PEC DEVICE UNDER BUFFERED ELECTROLYTES Time [hrs] Hydrogen evolution side basic Oxygen evolution side acidic Devices stops after 2 hrs

25 FINDING A SOLUTION FOR CONTINUOUS GENERATION: RECIRCULATING STREAMS Net accumulations of M + Net depletion of M + Requirements: Maintain low concentration differences Avoid significant product losses through crossover Modestino, Haussener, LBNL Invention disclosure 2012

26 IDENTIFYING THE WINDOW FOR OPERATION

27 Lets build this!

28 Potential Increase [V] ph AND CHEMICAL ENGINEERING WORKS! 5 4 No Circulation 1 ml/min 2 ml/min ma/cm ml/min 1 ml/min No Circulation Time [hrs] Time [hrs] Stable operation for > solar-cycle with low recirculation Concentration difference is reduced with recirculation rate

29 J [ma/cm 2 ] CONTINUOUS SOLAR DRIVEN WATER SPLITTING 2.5 Continuous Recirculation Time [hrs]

30 INTEGRATED SOLUTION FOR CONTINUOUS SOLAR H 2 GENERATION

31 ACKNOWLEDGEMENTS JCAP Miguel Modestino, Karl Walczak, Sophia Haussener (EPFL), Joel Ager, Alan Berger, John Newman, and Carl Koval.