Imperial College London ©1 Enhancing Photoelectrode Performance with Nanoparticulate Electrocatalysts P. Bumroongsakulsawat, S. Dennison, K. Hellgardt, G. Kelsall Dept of Chemical Engineering, Imperial College, LONDON SW7 2AZ e:

Photoelectrolysis of water Requires 1.23 V (equivalent to a photon of wavelength ~1000 nm)

Energy Requirements for Photoelectrolysis 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

Candidate Materials –TiO 2 : Eg ~ eV ( nm) –Fe 2 O 3 : Eg ~ 2.2 eV (>565 nm) –WO 3 : Eg ~ 2.6 eV (475 nm)

Stability of WO 3 in aqueous media

Fabrication of WO 3 thin films From H 2 WO 4 : –“Electrodeposition”: potential cycling: -0.4 to +0.8 V vs. SCE 1 –“Doctor blading”: using stabilised H 2 WO 4 sol 2  Both annealed: 15 min at 550°C 1 Kulesza and Faulkner, J Electroanal Chem, 1988, 248, Santato et al., J Amer Chem Soc, 2001, 123, 10639

Dark electrochemistry of WO 3 (1)

Dark electrochemistry of WO 3 (2): Impedance (Mott-Schottky) E fb ~ 0.1 V vs. SCE 1M H 2 SO 4 Modulation frequency: 10 kHz

Measured band-edge potentials of WO 3

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, 1258

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

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

Ir Electrodeposition – Cycle 5 Vitreous Carbon electrode 10 mM IrCl 3 /0.5 M KCl Sweep rate: 0.01 Vs -1 *

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

Stability of IrO 2

Electrodeposition Conditions Ir: –Nucleation: 5 ms at V vs. SCE –Deposition: at -0.5 ± 0.05 V vs. SCE IrO 2 : –Deposition: at V vs SCE (Electrodeposition or electrophoretic deposition?) On 5-layer, doctor-bladed WO 3

Ir Electrodeposition on WO 3 WO 3 electrode: 10 mM IrCl 3 /0.5 M KCl, pH ~ V vs SCE for 60 s

Ir: Effect of deposition time 100 s: charge equiv. to ~15 monolayers for V vs SCE 60 s: charge equiv. to ~12 monolayers for V vs SCE 0.1 s: ~10 monolayers.

IrO 2 Electrodeposition on WO 3 WO 3 electrode: H 2 IrCl 3 /(COOH) 2 /K 2 CO 3, pH V vs SCE for 300 s

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

Capacitance-Potential: IrO 2 -coated WO 3 1M H 2 SO 4 Modulation frequency: 10 kHz

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) irreversible damage of the WO 3 (MS data). b) deposition of excessive quantities of Ir/IrO 2 (?)

Future Work Determine whether this is this a viable approach: –Investigate deposition of sub-monolayer Ir/IrO 2 –Investigate control of Ir solution species –Further characterisation of Ir/IrO 2 deposits