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Steve Dennison & Chris Carver Chemical Engineering Imperial College

Hydrogen Generation Using a Photoelectrochemical Reactor: Materials Assessment and Reactor Development. Steve Dennison & Chris Carver Chemical Engineering Imperial College. Photoelectrolysis of water. Requires 1.23 V: thermodynamic value from  G 0 = -237 kJmol -1 .

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Steve Dennison & Chris Carver Chemical Engineering Imperial College

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  1. Hydrogen Generation Using a Photoelectrochemical Reactor:Materials Assessment and Reactor Development Steve Dennison & Chris Carver Chemical Engineering Imperial College

  2. Photoelectrolysis of water Requires 1.23 V: thermodynamic value from G0 = -237 kJmol-1. Equivalent to a photon of wavelength ~1000 nm

  3. The semiconductor-electrolyte interface

  4. The semiconductor-electrolyte interface 2 Band Bending e- Separation between Fermi energy and Conduction band edge e- H+ / H2 Thermodynamic Potential of Water: h O2 / H2O h+ Overpotential for O2 evolution

  5. The semiconductor-electrolyte interface 3 0.4V 0. 3 V E f + H / H 2 An ideal semiconductor for water-splitting has band gap of: ~2.6eV 1.23 V O / H O 2 2 0.4V

  6. Choosing the semiconductor • It must be an OXIDE • Stability/insolubility in aggressive media • Stability under conditions of oxygen evolution

  7. Candidate Materials • TiO2: Eg ~ 3.0-3.2 eV (410-385 nm) • Fe2O3: Eg ~ 2.2 eV (>565 nm) • WO3: Eg ~ 2.6 eV (475 nm)

  8. TiO2 Fe2O3 WO3 Match to Solar Spectrum

  9. Fe2O3: typical photoresponse

  10. Fe2O3: voltammetry under illumination

  11. Fe2O3: photocurrent transients @ +0.6V

  12. Fe2O3: photocurrent transients @ +0.6V

  13. Fe2O3: photocurrent transients @ +0.1V

  14. Findings for Fe2O3 • Preliminary (and not concluded yet) • In the absence of MeOH see cathodic “dark” current, even at 0.6 V. • As applied potential is decreased, the photocurrent becomes more transient • As applied potential is decreased the cathodic “dark” current increases (relative to the photocurrent)

  15. WO3: recent work • Photocurrent observed (poor efficiency) • Enhancement with oxygen evolution catalyst (electrodeposited IrO2) not realised • Further detailed electrochemical analysis underway (plus SEM/TEM, XRD, etc.)

  16. Christopher Carver Dr Klaus Hellgardt SOLAR HYDROGEN: Photoelectrochemical Reactor Design

  17. PROJECT OBJECTIVES • Design flexible test-bed reactor • 10 x 10cm photoanode • Photon absorption • Quantum efficiency • High mass transfer rate coefficients • Separate hydrogen and oxygen • Hydrogen production experiments • Semiconductor material • Electrode configuration

  18. WORK TO DATE

  19. SEMICONDUCTOR MATERIAL titanium PVDF LIGHT TRANSMISSION Good absorption Stable in alkali Recombination ELECTRODE CONFIGURATION Fe2O3 ELECTROLYTE DISTRIBUTION TiO2 GAS SEPARATION WO3 Stable in acid/alkali UV absorption only HYDROGEN EXTRACTION REACTOR CONSTRUCTION Good efficiency Stable in acid

  20. quartz SEMICONDUCTOR MATERIAL LIGHT TRANSMISSION ELECTRODE CONFIGURATION ELECTROLYTE DISTRIBUTION GAS SEPARATION HYDROGEN EXTRACTION REACTOR CONSTRUCTION

  21. Butler-Volmer equation Current density (A/m2) Kinetic control SEMICONDUCTOR MATERIAL Increasing mass transport rate LIGHT TRANSMISSION ELECTRODE CONFIGURATION Transport control ELECTROLYTE DISTRIBUTION Overpotential (V) GAS SEPARATION HYDROGEN EXTRACTION REACTOR CONSTRUCTION

  22. SEMICONDUCTOR MATERIAL LIGHT TRANSMISSION ELECTRODE CONFIGURATION CATHODE ELECTROLYTE DISTRIBUTION GAS SEPARATION PHOTO-ANODE HYDROGEN EXTRACTION REACTOR CONSTRUCTION REACTOR CONSTRUCTION

  23. PRELIMINARY RESULTS

  24. PC SOLAR SIMULATOR PEC REACTOR POTENTIOMETER PUMP RESERVOIR / H2 COLLECTION

  25. PHOTOANODE PHOTOANODE ELECTRODE CONFIGURATION LIGHT quartz window membrane cathode electrolyte

  26. FUTURE WORK

  27. PHOTOANODE ELECTRODE CONFIGURATION LIGHT quartz window membrane cathode electrolyte

  28. SUMMARY FUTURE WORK

  29. THANK YOU QUESTIONS?

  30. Electrolyte Flow (with H2 or O2) Mesh Cathode Membrane Fluid Chamber Fluid Chamber Electrolyte Inflow Photo-Anode

  31. Mesh Cathode (Conductor) (2) Diffusion 2e (3) Kinetics H2 Fluid Flow+ Diffusion Electrolyte Flow (if laminar) 2H+ (1) Diffusion (4?) Membrane Fluid Flow+ Diffusion (1) H2O O2 + 2H+ (3) Kinetics Photo-Anode Absorption, Diffusion (2), Band Bending + (2) Absorption, α e- h+

  32. Choosing the semiconductor • Absolute levels of the electronic levels in the semiconductor: • Defined by the electron affinity • Require EA ~ 3.7 eV

  33. Fe2O3: NaOH-H2O

  34. Fe2O3: NaOH-H2O/MeOH (80:20)

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