Understanding the Effect of Monomeric Iridium(III/IV) Aquo Complexes on the Photoelectrochemistry of IrO_x·nH₂O-Catalyzed Water-Splitting Systems

Abstract

Soluble, monomeric Ir(III/IV) complexes strongly affect the photoelectrochemical performance of IrO(x)·nH2O-catalyzed photoanodes for the oxygen evolution reaction (OER). The synthesis of IrO(x)·nH2O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through monomeric intermediates that were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions, the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)5(H2O)](2-) and [Ir(OH)6](2-), respectively. These monomeric anions strongly adsorb onto IrO(x)·nH2O colloids but can be removed by precipitation of the colloids with isopropanol. The monomeric anions strongly adsorb onto TiO2, and they promote the adsorption of ligand-free IrO(x)·nH2O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode redox cycle and thus dramatically degrades the photoelectrochemical performance of the cell. The growth of a dense TiO2 barrier layer prevents access of soluble monomeric anions to the interface between the oxide semiconductor and the electrode back contact (a fluorinated tin oxide transparent conductor) and leads to improved photoanode performance. Purified IrO(x)·nH2O colloids, which contain no adsorbed monomer, give improved performance at the same electrodes. These results explain earlier observations that IrO(x)·nH2O catalysts can dramatically degrade the performance of metal oxide photoanodes for the OER reaction.

Charge Transfer Stabilization of Late Transition Metal Oxide Nanoparticles on a Layered Niobate Support

“…First, the IrO x ·nH 2 O colloidal solution was purified as previously reported to remove the monomeric anions from the surface of the nanoparticles. 32 Interestingly, there was no measurable heat of interaction between these purified particles and TBA 0.24 H 0.76 Ca 2 Nb 3 O 10 nanosheets. Next, the heat of interaction between the monomeric anions and nanosheets was measured and found to be −83 ± 17 kJ mol −1 .…”

“…Therefore, we use the interaction heat of the monomeric anions in plotting the periodic trends below (Figure 3). It should be noted that alkaline solutions of the monomer in equilibrium with air contain both Ir III and Ir IV forms of the monomer ([Ir(OH) 5 (H 2 O)] 2− and [Ir(OH) 6 ] 2− , respectively), but EPR experiments show that the predominant form is Ir III , 32 and therefore we refer to the equilibrium mixture of anions simply as [Ir(OH) 5 (H 2 O)] 2− .…”

“…Purified IrO x ·nH 2 O nanoparticles were obtained by precipitating the above product with double volume isopropyl alcohol (IPA) and re-suspending the precipitate in deionized water. 32 A more dilute 0.5 mmol L −1 iridium monomer solution (which contains little or no colloidal IrO x ·nH 2 O) was synthesized by the addition of 0.025 mmol K 2 IrCl 6 to 50.0 mL of 0.100 mmol L −1 TBA + OH − , heating the solution to 70 °C until the solution turned colorless, and then being cooled immediately in an ice bath. 32 The iridium species were deposited by adding 1.6 mL of the 20 mmol L −1 solution to 50.0 mL of the nanosheet solution to achieve a mass fraction of 0.05.…”

Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

“…The Journal of Physical Chemistry C Article Following these promising data, we wished to explore the effect of the iridium cocatalyst on photocurrent and charge recombination in the air-treated films. While neither of the airtreated samples exhibited any large-scale photocurrent response (overlapping solid and dashed lines in Figure 1a), the peak current of the Ir V /Ir IV reduction wave 77,86 for sample A−Ir (inset of Figure 1a) was observed to decrease when the sample was illuminated during the reverse scan. On the next cycle, when the film was scanned in the dark again, the peak current would increase back to its original value.…”

Microwave‐Assisted Synthesis of Stable and Highly Active Ir Oxohydroxides for Electrochemical Oxidation of Water

“…[21,22] Accordingly,t oi nvestigate the influence of the initial KOH/Ir ratio, as eries of sevenI rb ased catalysts were produced by MW-assistedh ydrothermalt reatment at 250 8C, from precursor solutionsc ontaining varying initial molar KOH/Ir ratios from 1:1t o1 00:1 (1:1, 4:1, 5:1, 7:1, 10:1, 50:1, 100:1)w hilem aintaining the total Ir concentrationa t 10 À2 mol L À1 .F igure S1 (Supporting Information) shows the relevant synthesis parameters monitored during the MW-assisted synthesis. [21,22] Accordingly,t oi nvestigate the influence of the initial KOH/Ir ratio, as eries of sevenI rb ased catalysts were produced by MW-assistedh ydrothermalt reatment at 250 8C, from precursor solutionsc ontaining varying initial molar KOH/Ir ratios from 1:1t o1 00:1 (1:1, 4:1, 5:1, 7:1, 10:1, 50:1, 100:1)w hilem aintaining the total Ir concentrationa t 10 À2 mol L À1 .F igure S1 (Supporting Information) shows the relevant synthesis parameters monitored during the MW-assisted synthesis.…”