1 216 th ECS Meeting: October 8, 2009 Comparison of Inexpensive Photoanode Materials for Hydrogen Production Using Solar Energy N.Cook, R. Gallen S. Dennison, K. Hellgardt, G.H. Kelsall, Department of Chemical Engineering Imperial College London, SW7 2AZ, UK

14 TW Energy Gap by 2050! 1

H 2 as sustainable energy carrier? 2 adapted and modified from J.A.Turner, Science 285, 687(1999)

Plugging the energy gap (14TW) Combined area of black dots would provide total world energy demand 3

Solar Hydrogen at Imperial  £4.2M EPSRC sponsored project (5 years)  Chemical Engineering, Chemistry, Biology, Earth Science and Engineering  Approx researchers at any one time  2 strands: Biophotolysis and Photoelectrochemistry  Chemical Engineering to develop devices and reactors and technology for scale-up and scale out 4

Application  Targets:  Biophotolytic H 2 : £5.00/kg;  Photoelectrolytic H 2 : £2.50/kg  Fuel Cell Operation  Distributed Market 5

Candidate Materials TiO 2 : E g ~ eV ( nm) Fe 2 O 3 : E g ~ 2.2 eV (>565 nm) WO 3 : E g ~ 2.6 eV (475 nm) 6

Stability of Fe 2 O 3 7

Stability of TiO 2 8

Stability of WO 3 9

Photoelectrolysis – Materials Evaluation  Photocurrent Spectroscopy  Photo-electrochemical activity of photo-anodes based on transition metal oxides (Fe, W, Ti)  Fe-based system needs bias but otherwise promising (& cheap) 10

WO 3 : further investigations From H 2 WO 4 : Electrodeposition: potential cycling V vs. SCE 1 “Doctor blading”: using stabilised H 2 WO 4 sol 2  Both annealed: 15 min at 550°C 1 Santato et al., J Amer Chem Soc, 2001, 123, Kulesza and Faulkner, J Electroanal Chem, 1988, 248,

Measured band-edge potentials of WO 3 12

Ir/IrO 2 Electrodeposition Ir: From “IrCl 3,aq ” : E 0 = V vs NHE 1 Convert to IrO 2 by electrochemical oxidation 2 IrO 2 : From [IrCl 6 ] 3- pH 10.5/galvanostatic deposition 3 1 Munoz and Lewerenz, J Electrochem Soc, 2009, 156, D184 2 Elzanowska et al. Electrochim Acta, 2008, 53, Marzouk, Anal Chem, 2003, 75,

Ir Electrodeposition – Cycle 1 Vitreous carbon electrode: 10 mM IrCl 3 /0.5 M KCl Sweep rate: 0.01 Vs -1 Ir nucleation 14

Ir Electrodeposition – Selected Cycles Vitreous Carbon electrode 10 mM IrCl 3 /0.5 M KCl Sweep rate: 0.01 Vs -1 15

IrO 2 Electrodeposition H 2 IrCl 6 + (COOH) 2 (pH 10.5, K 2 CO 3 ) Sweep rate: 0.01Vs -1 16

Effect of IrO 2 on WO 3 Photoresponse 1M H 2 SO 4 Sweep rate: 0.01 Vs -1 17

Mott-Schottky analysis following IrO 2 -coating 1M H 2 SO 4 Modulation frequency: 10 kHz 18

Conclusions The electrodeposition of Ir and IrO 2 is interesting! Deposition of Ir & IrO 2 onto WO 3 results in loss of photoelectrochemical O 2 evolution activity. This is due to: a) deposition of excessive quantities of Ir/IrO 2 b) irreversible damage of the WO 3 (MS data). 19

Design and Development of a Photoelectrochemical Reactor Key criteria: Optimising illumination of photoelectrode Optimising fluid and current distributions Product separation Minimising bubble formation Materials (of construction) selection 20

Photoelectrolytic Reactor Design 21

Photoelectrolytic Reactor Design 22

Photoelectrolytic Reactor Design 23

Photoelectrolytic Reactor Design 24

Photoelectrolytic Reactor Design 25

Photoelectrolytic Reactor Performance 26

Photoelectrolytic Reactor: conclusions Main contributing factors to response: Photoanode material quality Cathode gauze too coarse Large illumination losses (mirror, etc.) 27

Future Work Materials fabrication: WO 3 and Fe 2 O 3 Photoelectrochemical reactor: Photoanode material quality Reduce shading by cathode Hydrogen measurement and collection Fully develop reactor model 28