216 th ECS Meeting: October 8, 2009

1. Fe2O3 Photoanodesfor Hydrogen Production Using Solar Energy S. Dennison, K. Hellgardt, G.H. Kelsall, Department of Chemical EngineeringImperial College London, SW7 2AZ, UK s.dennison@imperial.ac.uk 216th ECS Meeting: October 8, 2009

2. Project Objectives • Solar-powered hydrogen generation systems: • Biophotolysis • Photoelectrolysis • Assessment of materials for photoelectrodes 1

3. Photoelectrolysis of water Requires > 1.5 V (  < ca. 830 nm) 2

4. Energy requirements for Photoelectrolysis of water 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 3

5. Energy requirement for Photoelectrolysis of water 0.4V 0. 3 V E f + H / H 2 An ideal semiconductor for water-splitting has band gap of ca. 2.6eV 1.5 V O / H O 2 2 0.4V 4

6. 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) 5

7. Fe2O3: range of stability 6

8. Production of Fe2O3 Photoelectrodes • CVD: • Fe(CO)5 + tetraethoxysilane (Si-dopant) • Spray pyrolysis: • FeCl2 + SnCl4 • Ultrasonic spray pyrolysis: • Fe(acac)3 + ~1% Nb 7

9. Fe2O3 electrochemistry 0.1M NaOH/Water; 0.01 Vs-1; Black: dark; Red: illuminated @ 450nm 8

10. Fe2O3 electrochemistry 0.1M NaOH/Water-MeOH 80:20; Scan rate: 0.01 Vs-1 Black: dark; Red: illuminated @ 450nm 9

11. Impedance analysis • Impedance analysis in the dark (Mott-Schottky) • Plot of CSC-2 vs. electrode potential: • gradient proportional to donor density (ND) • intercept = flatband potential 10

12. Fe2O3 electrochemistry Modulation frequency: 10KHz Vmod = 0.005 V 11

13. Impedance analysis • From Mott-Schottky plots: • ND > 5 x1019 cm-3 • EFB = -0.55 V vs SCE (water) = -0.35 V vs SCE (water-methanol) 12

14. Fe2O3 electrochemistry: illuminated Chopped Illum (87 Hz) @ 450nm Scan rate: 0.01 Vs-1; 0.1M NaOH Red: Water Blue: Water-MeOH 80:20 13

15. Fe2O3 electrochemistry: photocurrent transients Water 450nm; Chop @ 3 Hz Potential: 0.6 V 14

16. Fe2O3 electrochemistry: photocurrent transients Water-MeOH 80:20 450nm Chop @ 3 Hz Potential: 0.6 V 15

17. Source of apparent dark reduction reaction • From photochemically generated FeO42- • FeO42- is unstable and decomposes according to: • Oxidation of Fe2O3 to FeO42- is possible • This reaction would generate a net cathodic current • CH3OH would suppress formation of FeO42- 16

18. Fe2O3: range of stability – including CH3OH 17

19. Fe2O3 photoelectrochemistry: summary Surface state (reduced by CH3OH?) 18

20. Derives from surface Fe3O4 Formed by reduction of Fe2O3 Reactive Fe3+ at the surface: Reduced chemically or electrochemically Possible nature of surface state 19

21. Modelling Fe2O3 Photoresponse hn kmin kmaj k0 20

22. Modelling Fe2O3 Photoresponse • Gärtner photoresponse: • Steady-state photocurrent given by: Peter et al., J Electroanal Chem, 1984, 165, 29 21

23. Data input to model • ND = 1020 cm-3 •  = 2.2 x 105 cm-1 • I0 = 1014 cm-2 •  = 50 • kp = 10-6 cm-2s-1 • kn = 2 x 10-8 cm-2s-1 • k0 = 103 cm s-1 • n0 = 1021 cm-3 • Ns = 1012 cm-2 • Es = 0.7 eV 22

24. Initial modelling results 23

25. Depletion Layer Model for Fe2O3 hn kmin kS kmaj k0 24

26. Conclusions • Spray pyrolysed Fe2O3 demonstrates: • Poor efficiency (Vonset ca. 0.7 V from Vfb) • Surface states from photoelectrochemically generated • FeO42- • Fe3O4 • Modelling approximates some observed behaviour 25

27. Future Work • Develop Fe2O3 deposition methods • Refine model • Add surface state mediated charge transfer • Apply to Fe2O3 from other deposition methods • Improvements to Fe2O3: surface catalysis? 26