Xuefeng Long

Nitrogen-doped mesoporous carbon nanosheet/carbon nanotube hybrids as metal-free bi-functional electrocatalysts for water oxidation and oxygen reduction

The development of metal-free catalysts for efficient catalysis of both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is extremely desirable in energy technologies. Herein, nitrogen-doped mesoporous carbon nanosheet/carbon nanotube (CNT) hybrids have been synthesized by the pyrolysis of glucose, urea and CNTs. Impressively, in 0.1 M KOH, the resulting hybrids afford remarkable OER activities with a low onset potential (1.50 V vs. RHE) and an exceptional over-potential (only 320 mV at 10 mA cm−2). Moreover, the same hybrids show comparable catalytic performance but better durability compared to the benchmark Pt/C (20 wt%) catalyst for ORR. The achieved ultrahigh catalytic performance of the hybrids originates from their large specific surface area (594.1 m2 g−1), high content percentage of N doping (8.5 wt%), and mesoporous structure, which leads to fully exposed active sites, improved mass/electron transport capability, easy adsorption/release of oxygen gas bubbles, and high structural stability. This work also provides a novel concept for fabricating heteroatom doped porous carbonaceous materials with integrated and improved catalytic performance for advanced applications.

Ultrafine Co2P nanoparticles encapsulated in nitrogen and phosphorus dual-doped porous carbon nanosheet/carbon nanotube hybrids: high-performance bifunctional electrocatalysts for overall water splitting

The development of active, robust, and nonprecious electrocatalysts for both the oxygen evolution reaction and hydrogen evolution reaction (OER and HER) is highly crucial and challenging. Herein, ultrafine Co2P nanoparticles (NPs) encapsulated in nitrogen and phosphorus dual-doped porous carbon nanosheet/carbon nanotube hybrids are prepared via a straightforward pyrolysis method. Impressively, the hybrids exhibit remarkable catalytic performance for both the OER and HER in 1.0 M KOH solution, with a current density of 10 mA cm−2 at low over-potentials of 280 mV for the OER and 154 mV for the HER, respectively. More importantly, when fabricated as an alkaline electrolyzer, the hybrids afford 10 mA cm−2 at a cell voltage of 1.64 V with strong stability, rivalling the integrated performance of a commercial IrO2 and Pt catalyst couple. The achieved ultrahigh catalytic performance can be attributed to the nitrogen and phosphorus dual-doped carbon nanosheets, carbon-encapsulated ultrafine Co2P NPs, high conductivity of incorporated carbon nanotubes, large surface area (199.94 m2 g−1), interpenetrated macro-/mesoporous structure, and the strong synergistic effect among these factors.

Mesoporous titanium dioxide coating on gold modified silica nanotubes: a tube-in-tube titanium nanostructure for visible-light photocatalysts

A novel tube-in-tube structured titanium dioxide (TiO2) based visible-light photocatalyst with non-metal doping and plasmonic metal decoration was fabricated using a well-controlled programmed synthesis method and was characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), N2 adsorption/desorption, X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy. The as-obtained tube-in-tube structure is composed of inner mesoporous silica nanotubes with high specific surface area and an outer layer of anatase TiO2 nanocrystals with considerable visible-light activity. For the photocatalytic degradation of rhodamine B (RhB) in aqueous solution, the photocatalyst showed superior photocatalytic activities compared with commercial TiO2 and nanometer-sized photocatalyst Degussa P25. The strategy is simple, but efficient, and can be extended to the synthesis of other multifunctional composites. It has opened a new pathway for the construction of hetero-nanocomposites with high activity and durability, which would serve as excellent models in catalytic systems of both theoretical and practical interest.

Coaxial ultrathin Co1−yFeyOx nanosheet coating on carbon nanotubes for water oxidation with excellent activity

The rational design of high-performance and non-precious metal electrocatalysts is highly important for energy technologies. Herein, for the first time, a coaxial ultrathin Co1−yFeyOx nanosheet coating on carbon nanotubes (denoted as Co1−yFeyOx/CNTs) is prepared by a one-step pyrolysis method. The fabrication procedure is ultrafast and uncomplicated, furthermore, it does not need any reducing agent, alkali, or surfactant. In 1.0 M KOH, the obtained optimal hybrids Co0.8Fe0.2Ox/CNTs25 wt% catalyze oxygen evolution reactions (OER) with a very sharp onset potential (∼1.45 V) and an exceptional over-potential (0.28 V, at 10 mA cm−2) for more than 14 h, benefiting from the abundant active sites of ultrathin Co0.8Fe0.2Ox nanosheets, the hierarchical tubular architecture, and the strong cooperative effect among Co, Fe and CNTs. Remarkably, the excellent activity and durability of Co0.8Fe0.2Ox/CNTs25 wt% for the OER is superior to commercial RuO2 and many other highly active precious-metal/transition-metal catalysts reported to date. The design concept for tubular iron-group binary metal nanosheet hybrids creates new pathways for energy technologies.

Facile regrowth of Mg-Fe2O3/P-Fe2O3 homojunction photoelectrode for efficient solar water oxidation

The photoelectrochemical (PEC) water splitting performance of hematite-based photoanodes is still far from their theoretical value due to the ultrafast charge recombination in bulk and on the surface. In response, we developed a regrowth strategy to fabricate a photoanode with metallic Mg-doped α-Fe2O3 coating on nonmetallic P-doped α-Fe2O3 nanorods (Mg-Fe2O3/P-Fe2O3 NRs). This homojunction structure fulfils the requirements of both high charge separation efficiency and favorable band alignment, showing a high photocurrent density (4.7-fold higher than pristine Fe2O3) and low onset potential of 0.68 VRHE. Experimental and density functional theory (DFT) results reveal that the origin of such superior PEC performance is P-doping and Mg-Fe2O3 coating, which efficiently eliminate lattice mismatching, improve the conductivity of hematite, and simultaneously promote the photogenic carriers separation by the built-in electric field. This promising design may open an avenue to fabricating various efficient homojunction photoanodes for practical PEC water splitting.

Ultrafine CoPS nanoparticles encapsulated in N, P, and S tri-doped porous carbon as an efficient bifunctional water splitting electrocatalyst in both acid and alkaline solutions

Searching for highly efficient, stable and non-noble metal electrocatalysts for the hydrogen/oxygen evolution reaction (HER/OER) is crucial to renewable-energy conversion systems. Cobalt phosphosulfide (CoPS) is regarded as a promising electrocatalyst for the HER in acid solution. Nonetheless, there are no studies of its HER and OER activity in basic solution. Herein, we engineer a novel and highly efficient water splitting electrocatalyst of ultrafine CoPS nanoparticles (NPs) encapsulated in N, P, and S tri-doped porous carbon (CoPS@NPS-C) through metal–organic frameworks (MOFs) and commercial carbon black (XC-72) as precursors. The abundant and evenly distributed metals of MOFs form ultrafine CoPS NPs, and the rich N organic ligands of MOFs and XC-72 are shaped into the NPS-C to protect CoPS NPs leading to long-term stability in water-splitting applications. Meanwhile, the strong interaction between CoPS and NPS-C in this special structure not only ensures the high activity of the HER in acid solution and the OER in alkaline solution, but also maintains good HER performance in alkaline solution. The results display that the novel structure electrocatalyst of CoPS@NPS-C has great potential in water-splitting applications.

NiO Nanoparticles Anchored on Phosphorus-Doped α-Fe2O3 Nanoarrays: An Efficient Hole Extraction p-n Heterojunction Photoanode for Water Oxidation

The photoelectrochemical (PEC) water-splitting efficiency of a hematite-based photoanode is still far from the theoretical value due to its poor surface reaction kinetics and high density of surface trapping states. To solve these drawbacks, a photoanode consisting of NiO nanoparticles anchored on a gradient phosphorus-doped α-Fe2 O3 nanorod (NR) array (NiO/P-α-Fe2 O3 ) was fabricated to achieve optimal light absorption and charge separation, as well as rapid surface reaction kinetics. Specifically, a photoanode with the NR array structure allowed a high mass-transport rate to be achieved, while phosphorus doping effectively decreased the number of surface trapping sites and improved the electrical conductivity of α-Fe2 O3 . Furthermore, the p-n junction that forms between NiO and P-α-Fe2 O3 can further improve the PEC performance due to efficient hole extraction and the water oxidization catalytic activity of NiO. Consequently, the NiO/P-α-Fe2 O3 NR photoanode produced a high photocurrent density of 2.08 mA cm-2 at 1.23 V versus a reversible hydrogen electrode and a 110 mV cathodic shift of the onset potential. This rational design of structure offers a new perspective in exploring high-performance PEC photoanodes.

Dual Modification of a BiVO4 Photoanode for Enhanced Photoelectrochemical Performance

Bismuth vanadate (BiVO4 ) has triggered extensive interest in photoelectrochemical (PEC) water splitting, owing to its narrow band gap and sufficiently positive valence band. However, some defects still exist to block the solar utilization efficiency and hydrogen evolution kinetics. Herein, the NiMoO4 semiconductor is combined with a BiVO4 photoanode, for the first time, and excellent PEC performance is achieved on the basis of heterojunction formation and favorable conductivity of NiMoO4 . In addition, it has been demonstrated that NiMoO4 promotes the light absorption ability, charge separation efficiency, and surface charge-transfer efficiency comprehensively. To further improve the photoconversion efficiency, cobalt phosphate, as an oxygen evolution reaction cocatalyst, is deposited on the above electrode and achieves a much enhanced utilization efficiency of 1.18 %, with a photocurrent density of 5.3 mA cm-2 at 1.23 V versus a reversible hydrogen electrode; this exceeds most results reported to date. This rational and unique photoanode construction provides a new thread for the photoelectrode designation.

Heterojunction and Oxygen Vacancy Modification Strategies of ZnO Nanorod Arrays Photoanode for Enhanced Photoelectrochemical Water Splitting

Application of ZnO in the field of photoelectrochemical water splitting is limited because of its wide-band-gap and high recombination rate. Herein is reported the design of an efficient ZnO photoanode deposited with CoOx nanoparticles to achieve a heterojunction and oxygen vacancies. The CoOx nanoparticles with abundant oxygen vacancies were anchored onto the nanorod arrays by spin coating and calcination followed by a solvothermal treatment. CoOx nanoparticles serve the dual function of forming a p-n heterojunction to facilitate the separation of photogenerated carriers, and act as a cocatalyst to decrease water oxidation barrier. Finally, oxygen vacancies increase the number of active redox sites and act as hole traps, enabling their migration to the electrode/electrolyte interface. The composite photoanode exhibits a high incident photon-to-current conversion efficiency (76.7 % at 350 nm), which is twice that of pristine ZnO, and a photoconversion efficiency of 0.68 % (0.73 V versus RHE). The current approach can be expanded to fabricate other efficient photocatalysts.