Enhanced Oxygen Evolution Reaction Activity of Nanoporous SnO2/Fe2O3/IrO2 Thin Film Composite Electrodes with Ultralow Noble Metal Loading
Résumé
sources is inalienable. Among renewable energy resources, the sun is the most striking candidate with its capability of satisfying the world's annual total energy consumption with less than an hour of solar energy. [1] However, solar energy utilization faces a major challenge in addition to the efficient capture and conversion of solar radiation itself, namely, the storage and transportation of the energy produced. [1,2] This latter aspect can be integrated into the former in the form of the photoelec-trochemical (PEC) splitting of water into dioxygen and dihydrogen. This reaction offers the possibility to convert and store solar energy in form of chemical bonds directly. [3] Here, the oxidation of H 2 O to O 2 has proven to be the more demanding half-reaction. [4] Among a great variety of suitable photoanode materials, iron oxide (Fe 2 O 3) stands out as an abundant and inexpensive potential semiconductor and catalyst. [5] Fe 2 O 3 also offers a favorable bandgap of 2.0-2.2 eV, [6] a valence band energy position sufficiently positive for the water oxidation reaction (2.4-2.7 V vs normal hydrogen electrode (NHE)), [7] and chemical stability in neutral and moderately basic pH. [6c,8] However, its slow oxygen evolution kinetics, [9] poor electrical conductivity (σ ≈ 10 −12 Ω −1 m −1), [10] relatively large light absorption depth (α −1 = 118 nm for a wavelength of λ = 550 nm), [11] short hole diffusion length (2-4 nm), [6c,12] and its correlated high rate of charge carrier recombination [9,13] limit the PEC anode efficiency. Among the diverse approaches that have been applied to overcome these drawbacks, we will focus on the following three. 1) Efforts to nanostructure photoanodes and simultaneously reduce the iron oxide thickness have improved the water oxidation performance. [13a,14] Porosity in a macroscopically thick Fe 2 O 3 layer enhances light collection while each individual structure may be microscopically thin in order to balance the development of an electrostatic field (depletion region) with a short charge carrier collection distance. [15] 2) Enhanced effici-encies can also be achieved by introducing a conductive scaffold that collects and transports the photoexcited electrons and thereby avoids charge carrier recombination. [15d,16] 3) Fe 2 O 3 surfaces can be further modified with a particularly proficient water oxidation cocatalyst. [17] The lower kinetic barrier of the cocatalyst A conductive SnO 2 layer and small quantities of IrO 2 surface cocatalyst enhance the catalytic efficiency of nanoporous Fe 2 O 3 electrodes in the oxygen evolution reaction at neutral pH. Anodic alumina templates are therefore coated with thin layers of SnO 2 , Fe 2 O 3 , and IrO 2 by atomic layer deposition. In the first step, the Fe 2 O 3 electrode is modified with a conductive SnO 2 layer and submitted to different postdeposition thermal treatments in order to maximize its catalytic performance. The combination of steady-state electrolysis, electro-chemical impedance spectroscopy, X-ray crystallography, and X-ray photoelec-tron spectroscopy demonstrates that catalytic turnover and e − extraction are most efficient if both layers are amorphous in nature. In the second step, small quantities of IrO 2 with extremely low iridium loading of 7.5 µg cm −2 are coated on the electrode surface. These electrodes reveal favorable long-term stability over at least 15 h and achieve maximized steady-state current densities of 0.57 ± 0.05 mA cm −2 at η = 0.38 V and pH 7 (1.36 ± 0.10 mA cm −2 at η = 0.48 V) in dark conditions. This architecture enables charge carrier separation and reduces the photoelectrochemical water oxidation onset by 300 mV with respect to pure Fe 2 O 3 electrodes of identical geometry.
Origine : Publication financée par une institution
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