A water oxidation side reaction on a photoelectrochemical charging supercapacitor is completely suppressed by controlling the thickness of a capacitive material.

Semiconductor/Faradaic layer/liquid junctions have been widely used in solar energy conversion and storage devices. However, the charge transfer mechanism of these junctions is still unclear, which leads to inconsistent results and low performance of these devices in previous studies. Herein, by using Fe 2 O 3 and Ni(OH) 2 as models, we precisely control the interface structure between the semiconductor and the Faradaic layer and investigate the charge transfer mechanism in the semiconductor/ Faradaic layer/liquid junction. The results suggest that the short circuit severely restricts the performance of the junction for both solar water splitting cells and solar charging supercapacitors. More importantly, we also find that the charge-discharge potential window of a Faradaic material sensitively depends on the energy band positions of a semiconductor, which provides a new way to adjust the potential window of a Faradaic material. These new insights offer guidance to design high-performance devices for solar energy conversion and storage.

Iron-based (oxy)hydroxides are especially attractive electrocatalysts for the oxygen evolution reaction (OER) owing to their earth abundance, low cost, and nontoxicity. However, poor OER kinetics on the surface restricts the performance of the FeOOH electrocatalyst. Herein, a highly efficient and stable Ni(OH) /β-like FeOOH electrocatalyst is obtained by facile electroactivation treatment. The activated Ni(OH) /β-like FeOOH sample indicates an overpotential of 300 mV at 10 mA cm for the OER, and no clear current decay after 50 h of testing; this is comparable to the most efficient nickel- and cobalt-based electrocatalysts on planar substrates. Furthermore, studies suggest that β-like FeOOH plays a key role in remarkably enhancing the performance during the electroactivation process owing to its metastable tunnel structure with a lower barrier for interface diffusion of Ni ions between the bilayer electrocatalyst. This study develops a new strategy to explore efficient and low-cost electrocatalysts and deepens understanding of bilayer electrocatalysts for the OER.

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carrier generation. [1,2] This has led to sig nificant interest in plasmonic nanostruc tures for photocatalysis, either through local heat generation or as a photo sensitizer. [3,4] Materials in the family of plasmonic transition metal nitrides (e.g., TiN, HfN, NbN, WN) feature high ther momechanical robustness and recently have been proposed for applications requiring extreme operating conditions, such as photothermal catalysis or solar thermophoto voltaics. [5] These materials have high melting points and demonstrate high temperature durability, chemical sta bility, and corrosion resistance, while pre senting an optical response similar to Au or Ag plasmonic nanostructures. [5,6] With a strong response in the visible range, high mechanical hardness, low material cost, [6][7][8][9] and outstanding performance in electro chemical reactions, [10][11][12] the photophysics of these materials requires further research. In the following, we will briefly review the current level of understanding of the photophysics of noble metal plasmonic particles, followed by a discussion on transition metal nitride plasmonic nanoparticles.Light absorption and heat generation by noble metal nano particles can be summarized as follows: first, the local surface plasmon resonance (LSPR) is excited, which lasts several fs and decays by nonradiative dephasing through Landau damp ening (1-100 fs). This process generates hot carriers at regions with high optical absorption (hotspots), the hot carriers subse quently decay by electron-electron scattering (1-100 fs) followed by electron-phonon coupling (0.1-10 ps). Ultimately, phonons dissipate heat to the surroundings (1-10 ns). [4,13,14] There is increasing interest in hot carrier processes, chemical reac tions induced by them, and determining whether the observed changes in chemical reactions are due to lattice heating or hot charge carriers. [15,16] Since elementary chemical transformations typically occur on a 1-100 ps timescale, [17] it is essential to characterize the light induced carrier dynamics and thermal relaxation of plasmonic systems that consist of nonnoble metal materials. Recent studies have shown that in particular hafnium nitride (HfN) performs well at converting light into heat through thermoplas monic relaxation. [18,19] This efficient lightinduced heating likely stems from a less negative real permittivity (ε′) and a higher imaginary permittivity (ε″) of HfN relative to noble metals, leading to a lossy plasmonic response accompanied by lower There is great interest in the development of alternatives to noble metals for plasmonic nanostructures. Transition metal nitrides are promising due to their robust refractory properties. However, the photophysics of these nanostructures, particularly the hot carrier dynamics and photothermal response on ultrafast timescales, are not well understood. This limits their implementation in applications such as photothermal catalysis or solar thermophotovoltaics. In this study, the light-induced relaxation processes in water-dispersed Hf...

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Photoelectrochemical (PEC) cells containing photocathodes based on functionalized NiO show a promising solar-to-hydrogen conversion efficiency. Here, we present mechanistic understanding of the photoinduced charge transfer processes occurring at the photocathode/electrolyte interface. We demonstrate via advanced photophysical characterization that surface hydroxyl groups formed at the NiO/water interface not only promote photoinduced hole transfer from the dye into NiO, but also enhance the rate of charge recombination. Both processes are significantly slower when the photocathode is exposed to dry acetonitrile, while in air an intermediate behavior is observed. These data suggest that highly efficient devices can be developed by balancing the quantity of surface hydroxyl groups of NiO, and presumably of other p-type metal oxide semiconductors.

Dye-sensitized photoelectrochemical (DSPEC) water splitting is an attractive approach to convert and store solar energy into chemical bonds. However, the solar conversion efficiency of a DSPEC cell is typically low due to a poor performance of the photocathode. Here, we demonstrate that Cu-doping improves the performance of a functionalized NiO-based photocathode significantly. Femtosecond transient absorption experiments show longer-lived photoinduced charge separation for the Cu:NiO-based photocathode relative to the undoped analogue. We present a photophysical model that distinguishes between surface and bulk charge recombination, with the first process (∼10 ps) occurring more than 1 order of magnitude faster than the latter. The longer-lived photoinduced charge separation in the Cu:NiO-based photocathode likely originates from less dominant surface recombination and an increased probability for holes to escape into the bulk and to be transported to the electrical contact of the photocathode. Cu-doping of NiO shows promise to suppress detrimental surface charge recombination and to realize more efficient photocathodes.