Enhancing Full Water-Splitting Performance of Transition Metal Bifunctional Electrocatalysts in Alkaline Solutions by Tailoring CeO₂–Transition Metal Oxides–Ni Nanointerfaces

Rational design of highly efficient bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is critical for sustainable energy conversion. Herein, motivated by the high activity of OER catalyst on water dissociation that is the rate-determining step of alkaline HER, a bifunctional catalyst of metallic nickel-decorated transition metal oxide nanosheets vertically grown on ceria film (ceria/Ni-TMO) is synthesized by composition controlling and surface engineering. Because of the idealized electronic structure of the active centers and the abundance of such sites, as well as a synergistic effect between the carbon cloth/ceria film and the in situ formed TMO/Ni nanoparticles, the as-synthesized ceria/Ni-TMO exhibited long-time stability and a low cell voltage of 1.58 V at 10 mA/cm² when applied as both the cathode and anode in alkaline solutions. Moreover, it is the first time that pH-independent four-proton-coupled-electron-transfer processes and multiple adsorption–desorption processes were found to occur at the interfaces of ceria/TMO and Ni/TMO in a single catalyst for catalyzing OER and HER, respectively

Close-Packed Colloidal SiO₂ as a Nanoreactor: Generalized Synthesis of Metal Oxide Mesoporous Single Crystals and Mesocrystals

We report a generalized “immobilized crystallization in silica nanoreactor” (ICSR) strategy for the synthesis of an extensive series of well-defined and high-quality metal oxide mesoporous single crystals (SnO₂, TiO₂, and CeO₂ MSCs) and mesoporous mesocrystals (CeO₂ and ZrO₂ MMCs) with varying morphologies, sizes, and phases. Close-packed colloidal SiO₂ is used as a nanoreactor, a peculiar reaction medium in which immobilized nucleation and crystallization have been systematically studied. High hydrophilicity of residual Si-OH groups facilitates surface adsorption and pore filling of precursor solution, leading to spontaneous nucleation and subsequent crystal growth in the reactor template. The silica template merely serves a faithful negative replication without interfering in the crystallization process but with an added advantage of avoiding crystal aggregation without the need for surfactant. The universality of the ICSR strategy is demonstrated by synthesizing MSCs and MMCs of different materials and different pore sizes. The great value of the as-obtained MSCs and MMCs is exemplified by a case study on the conspicuous gas-sensing activities of the SnO₂ MSCs. With 3D-connected mesopores and a single-crystalline framework, the highest gas-sensing activity is achieved when high-energy facets are maximally exposed. Overall, this work has provided insights and strategies for the rational fabrication of MSC and MMC materials and opened unprecedented opportunities for studying their structure–property relationship

Oriented Calcite Micropillars and Prisms Formed through Aggregation and Recrystallization of Poly(Acrylic Acid) Stabilized Nanoparticles

Though calcium carbonate crystals with various morphologies have been successfully fabricated via bioinspired methods, the mechanism underlying crystallization of one-dimensional (1D) calcite microstructures along defined crystallographic axes is poorly understood. In this paper, we first show that by combining the effects of poly­(acrylic acid) (PAA) and calcite substrates we can direct the formation of calcite through an intermediate complex of PAA and Ca²⁺ into oriented calcite micropillars with {104} faceted coaligned platelike subunits. Moreover, in situ AFM studies under different conditions than those used in bulk experiments also lead to formation of 1D calcite microstructures. With a slight change in conditions, arrays of oriented calcite prisms with triangular cross sections are formed on calcite substrates. Though distinct in morphology, these pillars and prisms form in a similar way via anisotropic nanoparticle aggregation, growth, fusion, and reorganization. The results may provide new insights into mechanisms of biomineralization

Cobalt-Embedded Nitrogen Doped Carbon Nanotubes: A Bifunctional Catalyst for Oxygen Electrode Reactions in a Wide pH Range

Electrocatalysts for the oxygen reduction and evolution reactions (ORR/OER) are often functionally separated, meaning that they are only proficient at one of the tasks. Here we report a high-performance bifunctional catalyst for both ORR and OER in both alkaline and neutral media, which is made of cobalt-embedded nitrogen doped carbon nanotubes. In OER, it shows an overpotential of 200 mV in 0.1 M KOH and 300 mV in neutral media, while the current density reaches 50 mA cm^–2 in alkaline media and 10 mA cm^–2 in neutral media at overpotential of 300 mV. In ORR, it is on par with Pt/C in both alkaline and neutral media in terms of overpotential, but its stability is superior. Further study demonstrated that the high performance can be attributed to the coordination of N to Co and the concomitant structural defects arising from the transformation of cobalt-phthalocyanine precursor

Exploratory Study of Zn_<i>x</i>PbO_<i>y</i> Photoelectrodes for Unassisted Overall Solar Water Splitting

A complete photoelectrochemical (PEC) water splitting system requires a photocathode and a photoanode to host water oxidation and reduction reactions, respectively. It is thus important to search for efficient photoelectrodes capable of full water splitting. Herein, we report on an exploratory study of a new photoelectrode family of Zn_<i>x</i>PbO_<i>y</i>ZnPbO₃ and Zn₂PbO₄similarly synthesized by a simple and economical method and shown to be a promising photocathode (p-type semiconductor) and photoanode (n-type semiconductor), respectively. From PEC measurements, the bare ZnPbO₃ photocathode achieved a photocurrent density of −0.94 mA/cm² at 0 V versus reversible hydrogen electrode (RHE), whereas the pristine Zn₂PbO₄ photoanode delivered a photocurrent density of 0.51 mA/cm² at 1.23 V versus RHE. By depositing suitable cocatalysts onto the photoelectrodes established above, we also demonstrated unassisted overall PEC water splitting, a rare case, if any, wherein a single material system is compositionally engineered for either of the photoelectrodes

CuCl₂/FeCl₃ Bimetallic Photocatalyst for Sustainable Ethylene Production from Ethanol via Recoverable Redox Cycles

Photocatalytic conversions of ethanol to valuable chemicals are significant organic synthesis reactions. Herein, we developed a CuCl2/FeCl3 bimetallic photocatalyst for sustainable dehydration of ethanol to ethylene by recoverable redox cycles. The selectivity of ethylene was 98.3% for CuCl2/FeCl3, which is much higher than that of CuCl2 (34.5%) and FeCl3 (86.5%). Due to the ligand-to-metal charge transfer (LMCT) process involved in generating the liquid products, the CuCl2/FeCl3 catalyst will be reduced to CuCl/FeCl2. Oxygen (O2) is required for the recovery of CuCl2/FeCl3 to avoid exhaustion. The soluble Fe3+/Fe2+ redox species deliver catalyst regeneration properties more efficiently than single metal couples, making a series of redox reactions (Cu2+/Cu+, Fe3+/Fe2+, and O2/ethanol couples) recyclable with synergistic effects. A flow reactor was designed to facilitate the continuous production of ethylene. The understanding of bimetallic synergism and consecutive reactions promotes the industrial application process of photocatalytic organic reactions

Origin of the Different Photoelectrochemical Performance of Mesoporous BiVO₄ Photoanodes between the BiVO₄ and the FTO Side Illumination

Understanding charge separation and charge transport in mesoporous semiconductor films is crucial to designing high efficiency photoelectrochemical water splitting cells. In the present work, we systematically study the origin of the higher photoelectrochemical performance of mesoporous BiVO₄ film under FTO-side illumination (F-illumination) than that under BiVO₄-side illumination (B-illumination). Via intensity-modulated photocurrent spectroscopy in conjunction with modeling simulation of electron diffusion inside mesoporous BiVO₄ films with different thicknesses, we find that the F-illumination is more tolerant to recombination than the B-illumination, leading to a higher charge separation efficiency of the former. Specifically, we have identified a trap-free electron transport region of BiVO₄ vicinal to the FTO substrate and a trap-limited transport region farther away under F-illumination, whereas only a trap-limited transport exists under B-illumination. Simulated results accord well with the experimental data and further provide a deep insight of the detailed electron transport behavior: it is the higher electron density in the region proximal to the FTO under F-illumination that has led to the greater recombination tolerance than under B-illumination. Such a photogenerated electron transport characteristic in mesoporous films is expected to be common for other semiconductors and will inspire practicle strategies for designing high efficiency semiconductor nanostructure-based photoelectrochemical devices

Metallic Iron–Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media

We report on the synthesis of iron-nickel sulfide (INS) ultrathin nanosheets by topotactic conversion from a hydroxide precursor. The INS nanosheets exhibit excellent activity and stability in strong acidic solutions as a hydrogen evolution reaction (HER) catalyst, lending an attractive alternative to the Pt catalyst. The metallic α-INS nanosheets show an even lower overpotential of 105 mV at 10 mA/cm² and a smaller Tafel slope of 40 mV/dec. With the help of DFT calculations, the high specific surface area, facile ion transport and charge transfer, abundant electrochemical active sites, suitable H⁺ adsorption, and H₂ formation kinetics and energetics are proposed to contribute to the high activity of the INS ultrathin nanosheets toward HER

CuCl₂/FeCl₃ Bimetallic Photocatalyst for Sustainable Ethylene Production from Ethanol via Recoverable Redox Cycles

Photocatalytic conversions of ethanol to valuable chemicals are significant organic synthesis reactions. Herein, we developed a CuCl2/FeCl3 bimetallic photocatalyst for sustainable dehydration of ethanol to ethylene by recoverable redox cycles. The selectivity of ethylene was 98.3% for CuCl2/FeCl3, which is much higher than that of CuCl2 (34.5%) and FeCl3 (86.5%). Due to the ligand-to-metal charge transfer (LMCT) process involved in generating the liquid products, the CuCl2/FeCl3 catalyst will be reduced to CuCl/FeCl2. Oxygen (O2) is required for the recovery of CuCl2/FeCl3 to avoid exhaustion. The soluble Fe3+/Fe2+ redox species deliver catalyst regeneration properties more efficiently than single metal couples, making a series of redox reactions (Cu2+/Cu+, Fe3+/Fe2+, and O2/ethanol couples) recyclable with synergistic effects. A flow reactor was designed to facilitate the continuous production of ethylene. The understanding of bimetallic synergism and consecutive reactions promotes the industrial application process of photocatalytic organic reactions