Photovoltage Effects of Sintered IrO₂ Nanoparticle Catalysts in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Abstract

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize high surface area TiO2 electrodes functionalized with light absorbing sensitizers and water oxidation catalysts. Because water splitting requires vectorial electron transfer from the catalyst to the sensitizer to the TiO2 surface, attaching both sensitizer and catalyst to TiO2 in the correct sequence and stabilizing them under photoelectrochemical conditions has been a challenging problem. Rutile-phase IrO2 nanoparticles can be deposited directly on the TiO2 electrode by adsorbing citrate-capped amorphous IrO x and then sintering at 450 °C. Electrodes functionalized with these nanocrystalline particles show higher activity than those made from ligand-capped amorphous IrO x without sintering. In the WS-DSPEC, the Coulombic efficiency for oxygen evolution from the sintered nanoparticle photoelectrodes was near unity. The loading of colloidal IrO x and IrO2 particles onto the porous TiO2 electrodes was quantified by neutron activation analysis. Photovoltage measurements suggest that at high catalyst loading the dominant charge recombination pathway is from photoinjected electrons to IrO2.

Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells

“…For WS-DSPECs, the photovoltage is defined as the difference between the potential of water oxidation (+0.63 V vs. Ag/AgCl at pH 6.8) and the Fermi level of the TiO 2 under illumination (19). We previously determined the optimal IrO 2 loading to be 0.5 pmol/cm 2 (19).…”

“…For WS-DSPECs, the photovoltage is defined as the difference between the potential of water oxidation (+0.63 V vs. Ag/AgCl at pH 6.8) and the Fermi level of the TiO 2 under illumination (19). We previously determined the optimal IrO 2 loading to be 0.5 pmol/cm 2 (19). At this loading, we measured open-circuit photovoltages of 1.07 ± 0.01 V and 1.11 ± 0.03 V with the [Ru(bpy) 2 (4,4′-(PO 3 H 2 ) 2 bpy)] sensitizer (19,33).…”

“…IrO 2 -catalyzed TiO 2 photoanodes were made as described elsewhere (19). An aqueous paste of anatase TiO 2 nanoparticles was applied to an 8-Ω/cm 2 fluorine-doped tin oxide-coated glass slide via a doctor blading technique.…”

“…The voltage decay was monitored on an oscilloscope (Tektronix TDS 540A). The current efficiency for oxygen generation at the photoanodes was measured using a calibrated Pt collector electrode as previously described (18,19). For each value reported, standard errors were calculated from measurements of six replicate samples.…”

“…Less attention has been paid to the development of earth-abundant sensitizers-an important problem for WS-DSPECs where absorber-to-catalyst ratios can exceed 1,000:1. Ruthenium polypyridyl sensitizers are most commonly used in WS-DSPECs (10,(16)(17)(18)(19), although Moore et al (20) demonstrated a WS-DSPEC sensitized with a zinc porphyrin that produced modest photocurrent (∼30 μA/cm 2 ). Organic sensitizers (containing C, H, N, and O) are earth abundant and offer the possibility of being low cost.…”