Synthesis and OER activity of NiO coatings with micelle-templated mesopore structure

Mesoporous WCx Films with NiO-Protected Surface: Highly Active Electrocatalysts for the Alkaline Oxygen Evolution Reaction

Metal carbides are promising materials for electrocatalytic reactions such as water electrolysis. However, for application in catalysis for the oxygen evolution reaction (OER), protection against oxidative corrosion, a high surface area with facile electrolyte access, and control over the exposed active surface sites are highly desirable. This study concerns a new method for the synthesis of porous tungsten carbide films with template-controlled porosity that are surface-modified with thin layers of nickel oxide (NiO) to obtain active and stable OER catalysts. The method relies on the synthesis of soft-templated mesoporous tungsten oxide (mp. WOx ) films, a pseudomorphic transformation into mesoporous tungsten carbide (mp. WCx ), and a subsequent shape-conformal deposition of finely dispersed NiO species by atomic layer deposition (ALD). As theoretically predicted by density functional theory (DFT) calculations, the highly conductive carbide support promotes the conversion of Ni2+ into Ni3+ , leading to remarkably improved utilization of OER-active sites in alkaline medium. The obtained Ni mass-specific activity is about 280 times that of mesoporous NiOx (mp. NiOx ) films. The NiO-coated WCx catalyst achieves an outstanding mass-specific activity of 1989 A gNi -1 in a rotating-disc electrode (RDE) setup at 25 °C using 0.1 m KOH as the electrolyte.

Influence of Phase Composition and Pretreatment on the Conversion of Iron Oxides into Iron Carbides in Syngas Atmospheres

CO2 Fischer–Tropsch synthesis (CO2–FTS) is a promising technology enabling conversion of CO2 into valuable chemical feedstocks via hydrogenation. Iron–based CO2–FTS catalysts are known for their high activities and selectivities towards the formation of higher hydrocarbons. Importantly, iron carbides are the presumed active phase strongly associated with the formation of higher hydrocarbons. Yet, many factors such as reaction temperature, atmosphere, and pressure can lead to complex transformations between different oxide and/or carbide phases, which, in turn, alter selectivity. Thus, understanding the mechanism and kinetics of carbide formation remains challenging. We propose model–type iron oxide films of controlled nanostructure and phase composition as model materials to study carbide formation in syngas atmospheres. In the present work, different iron oxide precursor films with controlled phase composition (hematite, ferrihydrite, maghemite, maghemite/magnetite) and ordered mesoporosity are synthesized using the evaporation–induced self–assembly (EISA) approach. The model materials are then exposed to a controlled atmosphere of CO/H2 at 300 °C. Physicochemical analysis of the treated materials indicates that all oxides convert into carbides with a core–shell structure. The structure appears to consist of crystalline carbide cores surrounded by a partially oxidized carbide shell of low crystallinity. Larger crystallites in the original iron oxide result in larger carbide cores. The presented simple route for the synthesis and analysis of soft–templated iron carbide films will enable the elucidation of the dynamics of the oxide to carbide transformation in future work.

Self-Assembled Two-Dimensional NiO/CeO2 Heterostructure Rich in Oxygen Vacancies as Efficient Bifunctional Electrocatalyst for Alkaline Hydrogen Evolution and Oxygen Evolution

The development of high-efficiency bifunctional electrocatalysts toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline surroundings is essential and challenging for the large-scale generation of clean hydrogen. Herein, a novel self-assembled two-dimensional (2 D) NiO/CeO₂ heterostructure (HS) consisting of NiO and CeO₂ nanocrystals is prepared through a facile two-step approach, and utilized as an enhanced bifunctional electrocatalyst for the HER and OER under alkaline conditions. It is concluded that this 2 D NiO/CeO₂ HS, rich in oxygen vacancies, demonstrates attractive electrocatalytic properties for both the HER and OER in 1 m KOH, including low onset overpotential (η₁ ), η₁₀ and Tafel slope, excellent durability, as well as large active surface area. Therefore, the self-assembled 2 D NiO/CeO₂ HS is believed to be an efficient bifunctional electrocatalyst toward the HER and OER.

Oxygen-Evolution Activity of p-n Heterojunction NiO-SnO2 Ceramic on Ti Substrate Fabricated Using a Simple Layer-by-Layer Method

To expand the application of p-n heterojunction NiO-SnO₂ ceramic materials from gas sensors and photoelectrocatalysts to oxygen-evolution reaction (OER) catalysts, we fabricated two NiO-SnO₂ ceramics on a Ti plate (NSCTs) using a simple layer-by-layer method. The prepared NSCTs (NSCT-480 and NSCT-600) were characterized and analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance ultraviolet-visible spectroscopy (DRUV-vis), and X-ray photoelectron spectroscopy (XPS). The OER activity and stability were measured by linear sweep voltammetry, cyclic voltammetry, chronoamperometry, amperometric i-t curve, and chronopotentiometry in a 1.0 mol/L NaOH solution at normal temperature and pressure. After 500 cycles, the lower overpotential (η = 194 mV at 1 mA/cm²) indicated that NSCT-600 offered adequate performance as an OER electrocatalyst. Moreover, the changes observed with cyclic voltammetry, SEM, XRD, and XPS during the OER test revealed that the redox cycle of Ni²⁺/Ni³⁺, morphology, and crystal faces of NiO and SnO₂ were three critical factors. The data proved that the NiO-SnO₂ ceramic is a stable OER electrocatalyst. The results of this study will provide a guide for the design and fabrication of p-n heterojunction metal-oxide ceramic electrocatalysts with a high OER performance.

An Fe-V@NiO heterostructure electrocatalyst towards the oxygen evolution reaction

The development of a nonprecious and Earth-abundant electrocatalyst with high electrocatalytic activity for the oxygen evolution reaction (OER) is an emerging hot issue and remains a grand challenge. In the present work, we proposed a facile strategy to construct ultrathin NiO nanosheets decorated with Fe-V nanoparticles on nickel foam (Fe-V@NiO/NF) for use as an OER electrocatalyst. Due to the 3D rational configuration, the Fe-V@NiO/NF with a heterostructure shows excellent electrocatalytic activity towards the OER. Interestingly, it is found that in situ oxidation by galvanostatic electrolysis in alkaline solution is beneficial to enhance the OER performance. After 10 h of electrolysis, a current density of 50 mA cm^-2 is achieved at a low overpotential of 271.1 mV. This is because during the in situ oxidation process, iron and vanadium ions insert into the NiO lattice and lead to the generation of highly active α-FeOOH and an amorphous (oxy)-hydroxide layer. Additionally, the charge transfer resistance dramatically reduces with the prolonging of oxidation time.

Improved photoelectrochemical performance of GaN nanopillar photoanodes

In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.