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In this work, a nitrogen-doped CuWO4 (N-CuWO4) photoanode for photoelectrochemical (PEC) water oxidation was synthesized by a mixed metal-imidazole casting (MiMIC) method with 1-butylimidazole (BIm). The N-CuWO4 electrode achieved PEC water oxidation at a wavelength of 540 nm that is longer than that of the film (neat-CuWO4) prepared without BIm by 40 nm due to the N-doping, which was conscientiously characterized by spectroscopic and theoretical investigations. The N-CuWO4 surface was compactly covered with worm-like particles of 100–500 nm diameter, which is responsible for the rigid adherence of the N-CuWO4 film onto the substrate. The N-CuWO4 film showed anisotropic crystallization of triclinic CuWO4 with predominant growth of the (010) and (100) planes, in contrast to the isotropic crystallization observed for the neat-CuWO4 film. The N-CuWO4 electrode demonstrated superior performance for PEC water oxidation with an incident photon-to-electron conversion efficiency (IPCE) of 5.6% contributed by a charge separation efficiency (ηsep) of 12.3% and a catalytic efficiency (ηcat) of 51.9% (at 420 nm and 1.23 V), a faradaic efficiency (FEO2) of 97% for O2 evolution, and considerable stability for 40 h, which are advantageously comparable to those of the state-of-the-art CuWO4-based photoanodes. The water oxidation rate constant (kO2 = 1.6 × 102 s−1) at the surface for the N-CuWO4 electrode was higher than that (6.8 s−1) for the neat-CuWO4 electrode by 2 orders of magnitude, which is responsible for the high IPCE and ηcat of the N-CuWO4 electrode. The higher kO2 value for the N-CuWO4 electrode is ascribed to the higher active site on the (100) facet for water oxidation at the surface. [ABSTRACT FROM AUTHOR]

Development of efficient and robust electrocatalysts for the hydrogen evolution reaction (HER) is of great importance for H2 production via water splitting which is one of the most promising sustainable and eco-friendly approaches for H2 production with zero CO2 emission. Herein we report a Pt/NiOx composite film deposited on a nickel foam (NF) electrode via a mixed metal-imidazole casting (MiMIC) method using a precursor suspension containing H2PtCl6 in mixed solvents of methanol/1-methylimidazole (MeIm) (3 : 1). The Pt/NiOx composite film (denoted as Pt-film(w)) was fully characterized by possible spectroscopic techniques, compared with the Pt-deposited film (denoted as Pt-film(w/o)) prepared without MeIm in a similar manner. Pt-film(w) is composed of well-dispersed Pt nanoparticles incorporated into the co-deposited NiOx layer on the NF surface, in sharp contrast with Pt nanoparticles mostly formed as an elongated cluster into the deposited layer for Pt-film(w/o). X-ray photoelectron spectroscopic (XPS) data indicate the single state of PtIV on the surface of Pt-film(w) but the mixed state of PtIV and Pt0 on the Pt-film(w/o) surface. Presumably, the PtIV state of the Pt nanoparticle surface is stabilized due to interaction with nickel oxoes of the NiOx layer for both films. Pt-film(w) showed extremely efficient HER performance with the lowest overpotentials of only η10 = 4.2 and η100 = 27 mV among the state-of-the-art HER electrocatalysts under alkaline conditions, so far. The mass activity (0.42 A cm−2 mgPt−1) of Pt-film(w) at η = 50 mV is 60 times higher than that of the commercial 10 wt% Pt/C film on the NF electrode as a commonly used HER electrocatalyst. The extremely stable HER electrocatalysis was first demonstrated at a high current density of −100 mA cm−2 for 60 h under alkaline conditions. This study opens up a new avenue towards practical application of Pt-film(w) as an efficient HER cathode to large-scale and commercial systems for water splitting. [ABSTRACT FROM AUTHOR]

An anisotropically crystallized CuBi2O4 (CuBi2O4(w)) film adhering rigidly on a fluorine doped tin oxide (FTO) electrode was prepared by a mixed metal-imidazole casting (MiMIC) method using 1-methylimidazole (MeIm). MeIm acts as a binder among metal oxide nanoparticles to result in tightly interconnected angular nanoparticles, as well as a structure-directing agent for growth of the anisotropically crystallized CuBi2O4 film. The photoelectrochemical (PEC) oxygen reduction reaction (ORR) at the CuBi2O4(w) electrode was investigated to compare with the electrode (CuBi2O4(w/o)) prepared without MeIm in a similar manner. The photocurrent for the ORR at the CuBi2O4(w) electrode under O2 in liner sweep voltammetry (LSV) was generated at an onset potential (Eon) of 0.96 V vs. RHE. The photocurrent decreased immediately for the CuBi2O4(w) electrode in chronoamperometry (CA) under Ar, being ascribable to reductive decomposition of the CuBi2O4(w) film. However, under O2, the photocurrent remained for 1 h (4% decrease) due to the ORR proceeding stably at the CuBi2O4(w) film. The high selectivity of the ORR to produce water was confirmed at the CuBi2O4(w) electrode, despite 8.0% hydrogen peroxide production after 1 h of photoelectrolysis under O2. The IPCE value (21.0%) at 440 nm and 0.41 V vs. RHE for the ORR at the CuBi2O4(w) electrode was 2.2-fold higher than that (9.6%) at the CuBi2O4(w/o) electrode. The photoelectrochemical impedance spectroscopic (PEIS) measurement suggested the faster ORR at the surface of the CuBi2O4(w) electrode, likely resulting from the anisotropic crystallization of tetragonal CuBi2O4, which is responsible for the high IPCE and stability for the PEC ORR at the CuBi2O4(w) electrode. [ABSTRACT FROM AUTHOR]

A Pt nanoparticle (Pt(w -MI)) film was prepared on a fluorine-doped tin oxide (FTO) conducting substrate from a precursor solution containing H 2 PtCl 6 in a mixed solvent of methanol/1-methyimidasol (MI) (3:1 v/v) by a mixed metal-imidazole casting (MiMIC) method. The Pt(w -MI) film is the uniformly dense morphology composed of compacted and well-connected Pt nanoparticles compared to the hollow morphology for the Pt(w/o -MI) film fabricated without MI. The Pt(w -MI)/FTO cathode showed efficient and durable HER activities, generating a current density of 10 mA cm−2 at an overpotential (η 10) of 198 mV, a Tafel slope of 128 mV dec−1, and a Pt mass activity of 0.11 A cm−2 mg Pt −1 at 200 mV in the alkaline medium (1.0 M KOH solutions, pH = 14.0), which are substantially higher than the Pt(w/o -MI)/FTO and the commercially available Pt-loaded carbon (Pt/C) on the FTO cathodes. The mechanism of the efficient HER for the Pt(w -MI)/FTO cathode was investigated by an electrochemical impedance spectroscopic (EIS) method. The bulk charge transport in the Pt(w -MI) film is faster than that in the Pt(w/o -MI) film, ascribed to the compacted and well-connected Pt nanoparticles for Pt(w -MI). Furthermore, the surface proton reduction for Pt(w -MI) is also faster than that for Pt(w/o -MI), affording the efficient HER activity for Pt(w -MI) compared to Pt(w/o -MI). [Display omitted] • A Pt film was prepared on FTO by a mixed metal-imidazole casting (MiMIC) method. • The MiMIC-prepared Pt film showed superior HER performance compared to Pt/C. • Mechanistic insights into HER of the film were provided by EIS analysis. [ABSTRACT FROM AUTHOR]

A unique transformation of WO 3 nanowires (NW-WO 3 ) into hexagonal prisms (HP-WO 3 ) was demonstrated by tuning the temperature of the (N 2 H 4 )WO 3 precursor suspension prepared from tungstic acid and hydrazine as a structure-directing agent. The precursor preparation at 20 °C followed by calcination at 550 °C produced NW-WO 3 nanocrystals ( ca. <100 nm width, 3-5 μm length) with anisotropic growth of monoclinic WO 3 crystals to (002) and (200) planes and a polycrystalline character with randomly oriented crystallites in the lateral face of nanowires. The precursor preparation at 45 °C followed by calcination at 550 °C produced HP-WO 3 nanocrystals ( ca. 500-1000 nm diameter) with preferentially exposed (002) and (020) facets on the top-flat and side-rectangle surfaces, respectively, of hexagonal prismatic WO 3 nanocrystals with a single-crystalline character. The HP-WO 3 electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO 3 electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO 3 was 3.1-fold higher than 15% for the NW-WO 3 electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO 3 layer with the unidirectional nanowire structure is more efficient than that through the HP-WO 3 layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO 3 electrode is more efficient than the NW-WO 3 electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO 3 electrode. The efficient water oxidation reaction at the surface for the HP-WO 3 electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO 3 to suppress the electron-hole recombination at the surface.