Hydrous iridium oxides (IrO x ) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox-state kinetics of IrO x electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir3.x+, Ir4+, and Ir4.y+ are identified spectroscopically, and their concentrations are quantified as a function of applied potential. The generation of Ir4.y+ states is found to be the potential-determining step for catalytic water oxidation, while H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerates from ∼20 to <0.5 s with increasing applied potential above 1.3 V versus reversible hydrogen electrode [i.e., turnover frequencies (TOFs) per active Ir state increasing from 0.05 to 2 s–1]. In contrast, the reaction kinetics for H2O2 is found to be almost independent of the applied potential (increasing from 0.1 to 0.3 s–1 over a wider potential window), indicative of a first-order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.y+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a unique kinetic insight into IrO x electrocatalytic reaction mechanisms, indicating a first-order reaction mechanism for H2O2 oxidation driven by Ir4+ states and a higher-order reaction mechanism involving the cooperative interaction of multiple Ir4.y+ states for water oxidation.

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir 2 (pyalc) 2 (H 2 O) 4 -(μ-O)] 2+ (Ir Molecular , pyalc = 2-(2′pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrO x ) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)−Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for Ir Molecular contrasting with potential-dependent kinetics for IrO x . This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of Ir Molecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.

CuInSe 2 (CISe) lattice widens the bandgap from 1.04 eV [5,6] up to 1.68 eV [7] in pure CuGaSe 2 , the change affecting the conduction band and leaving the valence band largely unaffected. [8-10] Since 1994, several studies have focused on optimising device efficiency by adjusting the Ga-concentration profile in the CIGSe layer [3,11,12] nowadays reaching record device efficiencies surpassing 23%. [13] A double Ga-gradient profile (Ga-rich/Ga-poor/Ga-rich) is commonly implemented in high-efficiency CIGSe solar cells. [3,12,14] The gradient in the conduction band assists in driving electrons (minority carriers in p-type CIGSe) towards the space charge region (SCR) and the heterojunction with the n-type CdS layer. [15] The resulting decrease in electron density near the molybdenum (Mo) back contact has been shown to suppress recombination losses, [16-18] notably associated with interfacial recombination at the CIGSe/Mo junction, [14] thereby significantly increasing the device open-circuit voltage (V OC). [19-21] Optimisation of the CIGSe film thickness, composition and Ga-gradient profile has largely been carried out by monitoring improvements in device efficiency, with only limited knowledge of the underlying dynamics and diffusion of the minority carriers to the n-contact. In particular, minority carrier mobility and driftdiffusion times in high-efficiency CIGSe solar cells are important parameters to quantify performance losses in state-of-the-art devices. A number of studies report carrier mobility in CISe and CIGSe measured with a variety of techniques as summarised in Table 1. However, the reported mobility values show variations of several orders of magnitude. Furthermore, only a few studies investigated device-relevant Ga-graded CIGSe layers, instead, focusing on simpler, ungraded absorbers with poorer performance. Combining time-resolved photoluminescence (TRPL) spectroscopy and numerical simulations, Weiss et al. extracted minority carrier mobilities between 32 and 45 cm 2 V −1 s −1 in Gafree CISe absorbers, however for back-graded CIGSe (GGI ratio increasing from 0 to 0.28 towards the back) only a lower limit of 8.3 cm 2 V −1 s −1 could be evidenced. [22] Kuciauskas et al. carried out TRPL studies on a typical Ga gradient device and estimated a minority carrier mobility of 55-230 cm 2 V −1 s −1 in the SCR near the CdS/CIGSe interface, [23] but did not address electron transport across the Ga-gradient towards the back contact region. Though transient absorption spectroscopy (TAS) has been utilized in the fields of organic and hybrid organic-inorganic Cu(In,Ga)Se 2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post-deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga...

Using transient spectroelectrochemical techniques, we investigate multiply reduced states of molecular catalysts on titania photoelectrodes as a function of the applied bias and the light intensity.

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