1. 1 216 th ECS Meeting: October 8, 2009 Fe 2 O 3 Photoanodes for Hydrogen Production Using Solar Energy S. Dennison, K. Hellgardt, G.H. Kelsall, Department of Chemical Engineering Imperial College London, SW7 2AZ, UK s.dennison@imperial.ac.uk

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 H + / H 2 O 2 / H 2 O Thermodynamic Potential of Water: h e-e- h+h+ e-e- Separation between Fermi energy and Conduction band edge Band Bending Overpotential for O 2 evolution 3

5. Energy requirement for Photoelectrolysis of water An ideal semiconductor for water-splitting has band gap of ca. 2.6eV H + / H 2 O 2 2 O 1.5V 0.0. 3V 0.4V E f 4

6. Candidate Materials TiO 2 : E g ~ 3.0-3.2 eV (410-385 nm) Fe 2 O 3 : E g ~ 2.2 eV (>565 nm) WO 3 : E g ~ 2.6 eV (475 nm) 5

7. Fe 2 O 3 : range of stability 6

8. Production of Fe 2 O 3 Photoelectrodes CVD: Fe(CO) 5 + tetraethoxysilane (Si-dopant ) Spray pyrolysis: FeCl 2 + SnCl 4 Ultrasonic spray pyrolysis: Fe(acac) 3 + ~1% Nb 7

9. Fe 2 O 3 electrochemistry 0.1M NaOH/Water; 0.01 Vs -1 ; Black: dark; Red: illuminated @ 450nm 8

10. Fe 2 O 3 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 C SC -2 vs. electrode potential: gradient proportional to donor density (N D ) intercept = flatband potential 10

12. Fe 2 O 3 electrochemistry Modulation frequency: 10KHz V mod = 0.005 V 11

13. Impedance analysis From Mott-Schottky plots: N D > 5 x10 19 cm -3 E FB = -0.55 V vs SCE (water) = -0.35 V vs SCE (water-methanol) 12

14. Fe 2 O 3 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. Fe 2 O 3 electrochemistry: photocurrent transients Water 450nm; Chop @ 3 Hz Potential: 0.6 V 14

16. Fe 2 O 3 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 FeO 4 2-  FeO 4 2- is unstable and decomposes according to: 16  Oxidation of Fe 2 O 3 to FeO 4 2- is possible  This reaction would generate a net cathodic current  CH 3 OH would suppress formation of FeO 4 2-

18. Fe 2 O 3 : range of stability – including CH 3 OH 17

19. Fe 2 O 3 photoelectrochemistry: summary Surface state (reduced by CH 3 OH?) 18

20. Possible nature of surface state Derives from surface Fe 3 O 4  Formed by reduction of Fe 2 O 3  Reactive Fe 3+ at the surface:  Reduced chemically or electrochemically 19

21. Modelling Fe 2 O 3 Photoresponse k maj k min k0k0 h 20

22. Modelling Fe 2 O 3 Photoresponse Gärtner photoresponse: Steady-state photocurrent given by: Peter et al., J Electroanal Chem, 1984, 165, 29 21

23. Data input to model  N D = 10 20 cm -3   = 2.2 x 10 5 cm -1  I 0 = 10 14 cm -2   = 50  k p = 10 -6 cm -2 s -1  k n = 2 x 10 -8 cm -2 s -1  k 0 = 10 3 cm s -1  n 0 = 10 21 cm -3  Ns = 10 12 cm -2  Es = 0.7 eV 22

24. Initial modelling results 23

25. Depletion Layer Model for Fe 2 O 3 k maj k min k0k0 h kSkS 24

26. Conclusions Spray pyrolysed Fe 2 O 3 demonstrates: Poor efficiency (V onset ca. 0.7 V from V fb ) Surface states from photoelectrochemically generated »FeO 4 2- »Fe 3 O 4 Modelling approximates some observed behaviour 25

27. Future Work Develop Fe 2 O 3 deposition methods Refine model Add surface state mediated charge transfer Apply to Fe 2 O 3 from other deposition methods Improvements to Fe 2 O 3 : surface catalysis? 26