Xiaogang Hao

Nanostructured catalysts for electrochemical water splitting: current state and prospects

Hydrogen is an ideal candidate for the replacement of fossil fuels in the future due to zero emission of carbonaceous species during its utilization. Water electrolysis is a dependable link of primary renewable energy and stable hydrogen energy. In this work, the fundamentals of water electrolysis, current popular electrocatalysts developed for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in liquid electrolyte water electrolysis are reviewed. The main HER catalysts include noble metals, non-noble metals and composites, noble metal-free alloys, metal carbides, chalcogenides, phosphides and metal-free materials while the OER catalysts are focused on efficient Co-based, Ni-based materials and layered double hydroxide (LDH) materials. The strategies to improve catalytic activity, long-term durability and endurance to electrochemical erosion are introduced. The main challenges and future prospects for the further development of electrodes for water electrolysis are discussed. It is expected to give guidance for the development of novel low-cost nanostructured electrocatalysts for electrochemical water splitting.

Separation of low concentration of cesium ion from wastewater by electrochemically switched ion exchange method: Experimental adsorption kinetics analysis

A series of experiments were performed to evaluate the continuous separation of cesium based on an electrochemically switched ion exchange (ESIX) process using a diaphragm-isolated reactor with two identical nickel hexacyanoferrate/porous three-dimensional carbon felt (NiHCF/PTCF) electrodes as working electrodes. The effects of applied potential, initial concentrations and pH values of the simulation solutions on the adsorption of cesium ion were investigated. The adsorption rate of cesium ion in the ESIX process was fitted by a pseudo-first-order reaction model. The experiments revealed that the introduction of applied potential on the electrodes greatly enhanced the adsorption/desorption rate of Cs(+) and increased the separation efficiency. H(3)O(+) was found to play a dual role of electrolyte and competitor, and the adsorption rate constant showed a curve diversification with an increase in pH value. Also, it was found that the electrochemically switched adsorption process of Cs(+) by NiHCF/PTCF electrodes proceeded in two main steps, i.e., an ESIX step with a fast adsorption rate and an ion diffusion step with a slow diffusion rate. Meanwhile, the NiHCF/PTCF film electrode showed adsorption selectivity for Cs(+) in preference to Na(+).

A novel potential-responsive ion exchange film system for heavy metal removal

In this study, we report on a novel potential-responsive hybrid film system for heavy metal removal, which was composed of a layered α-zirconium phosphate (α-ZrP) nanosheet and a conducting polyaniline (PANI)-intercalated chain that functioned as a proton pumping element. The proposed system features a quick responsive uptake/release rate that was about 10 times the adsorption/desorption rate with an open circuit. The potential-responsive film system exhibited a high electroactivity because of an acidic micro-environment for the nitrogen atoms in the PANI chains due to protons available on the electronegative α-ZrP nanosheets and their ion exchangeability. The uniform and dense hybrid film was formed on platinum electrodes using a one-step electrosynthesis based on unipolar pulse electrodeposition (UPED). By simply switching the operating potential applied to the electrode, heavy metal uptake onto the film from aqueous solutions and metal release for film regeneration were accomplished. This film system is shown to be a promising alternative to conventional techniques for heavy metal capture from wastewater.

Ultrastable coaxial cable-like superhydrophobic mesh with self-adaption effect: facile synthesis and oil/water separation application

A coaxial cable-like stainless steel wire@conducting polymer (polyaniline or polypyrrole) with special hierarchical micro–nanoscale roughness was successfully synthesized using a facile, inexpensive and controlled pulse electro-polymerization method (PPM). Conjugated with stearoyl chloride (SC) through an acylation reaction, the obtained meshes exhibit both superhydrophobic and superoleophilic properties with a water contact angle of 154° and an oil contact angle of 0°, which was successfully applied in separating a series of oil/water mixtures with a separation efficiency of over 98%. The separation efficiency also remains high even after 25 reuse cycles (99.90% for a hexane/water mixture). More importantly, owing to their self-adaption aging protection and antioxidant properties, the obtained meshes retain ultrastable superhydrophobicity and high separation efficiency in long-term storage, ultrasonic treatment, and even under extreme environment conditions of strong acidic, alkaline, hyperhaline and oxidizing solutions. In addition, electrochemical measurements indicate that the corrosion dynamic rate of the superhydrophobic mesh is 1/60 lower than that of bare stainless steel mesh. Therefore, this work provides a novel method to fabricate a high quality superhydrophobic and superoleophilic mesh for effective oil/water separation.

Facile preparation of ion-imprinted composite film for selective electrochemical removal of nickel(II) ions.

A facile unipolar pulse electropolymerization (UPEP) technique is successfully applied for the preparation of ion-imprinted composite film composed of ferricyanide-embedded conductive polypyrrole (FCN/PPy) for the selective electrochemical removal of heavy metal ions from wastewater. The imprinted heavy metal ions are found to be easily removed in situ from the growing film only by tactfully applying potential oscillation due to the unstable coordination of FCN to the imprinted ions. The obtained Ni(2+) ion-imprinted FCN/PPy composite film shows fast uptake/release ability for the removal of Ni(2+) ions from aqueous solution, and the adsorption equilibrium time is less than 50 s. The ion exchange capacity reaches 1.298 mmol g(-1) and retains 93.5% of its initial value even after 1000 uptake/release cycles. Separation factors of 6.3, 5.6, and 6.2 for Ni(2+)/Ca(2+), Ni(2+)/K(+), and Ni(2+)/Na(+), respectively, are obtained. These characteristics are attributed to the high identification capability of the ion-imprinted composite film for the target ions and the dual driving forces resulting from both PPy and FCN during the redox process. It is expected that the present method can be used for simple preparation of other ion-imprinted composite films for the separation and recovery of target heavy metal ions as well.

Electrochemical synthesis of flower shaped morphology MOFs in an ionic liquid system and their electrocatalytic application to the hydrogen evolution reaction

An important functional material, MOF-5, with unique flower shaped morphology, which is usually synthesized through hydrothermal or solvothermal methods at high temperature and pressure with high energy consumption, was successfully prepared by a mild in situ electrochemical synthesis method in a tunable ionic liquid (IL) system. In the reaction, H2BDC (BDC = 1,4-benzene-dicarboxylate) was chosen as the organic ligand, and the ionic liquid was Bmim (Bmim = 1-butyl-3-methylimidazole) bromine which functioned as a templating agent. The π–π stacking interaction between the imidazole groups, and the ionic band between the Zn2+ and Cl−, cause the directional arrangement of the MOF-5 crystal. Results show that the reaction results in a more perfect MOF-5 crystalline phase in comparison to other methods. The product, MOF-5(IL), presents a distinctive flower shaped morphology with a diameter of about 10 microns, and possesses a homogeneous morphology, stable structure and high thermal stability (up to 380 °C in N2 atmosphere). The electrochemical reaction in the ionic liquid Bmim bromine is a quasi-reversible redox reaction. The cyclic voltammetric curve of the MOF-5(IL) modified carbon paste electrode (CPE) illustrates that the flower shaped MOF-5(IL) has a better ability to catalyze the hydrogen evolution reaction than cubic MOF-5 prepared by other methods. The electrochemical method in the ionic liquid system can also be used to synthesize other MOF materials and nanomaterials by changing the metal ions, ligands and ionic liquid types.

Selective catalytic conversion of bio-oil over high-silica zeolites.

Four high silica zeolites, i.e., HSZ-385, 890, 960, and 990 were utilized for the selective catalytic conversion of bio-oil from Fallopia japonica to certain chemicals in a fixed-bed reactor. The Beta-type HSZ-960 zeolite showed the highest selectivity to hydrocarbons, especially to aromatics as well as PAH compounds with the lowest unwanted chemicals while HSZ-890 showed high selectivity to aromatics. NH3-Temperature Programmed Desorption (TPD) analysis indicated that different amounts of acid sites in different zeolites determined the catalytic activity for the oxygen removal from bio-oil, in which the acid sites at low temperature (LT) region gave more contribution within the utilized temperature region. The reusability test of HSZ-960 showed the stability of hydrocarbons yield at higher temperature due to the significant contribution of coke gasification which assisted further deoxygenation of bio-oil. These results provide a guidance to select suitable zeolite catalysts for the upgrading of bio-oil in a practical process.

Facile preparation of electroactive amorphous α-ZrP/PANI hybrid film for potential-triggered adsorption of Pb2+ ions

An electroactive hybrid film composed of amorphous α-zirconium phosphate and polyaniline (α-ZrP/PANI) is controllably synthesized on carbon nanotubes (CNTs) modified Au electrodes in aqueous solution by cyclic voltammetry method. Electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM) and X-ray power diffraction (XRD) analysis are applied for the evaluation of the synthesis process. It is found that the exfoliated amorphous α-ZrP nanosheets are well dispersed in PANI and the hydrolysis of α-ZrP is successfully suppressed by controlling the exfoliation temperature and adding appropriate supporting electrolyte. The insertion/release of heavy metals into/from the film is reversibly controlled by a potential-triggered mechanism. Herein, α-ZrP, a weak solid acid, can provide an acidic micro-environment for PANI to promote the electroactivity in neutral aqueous solutions. Especially, the hybrid film shows excellent potential-triggered adsorption of Pb(2+) ion due to the selective complexation of Pb(2+) ion with oxygen derived from P-O-H of α-ZrP. Also, it shows long-term cycle stability and rapid potential-responsive adsorption/desorption rate. This kind of novel hybrid film is expected to be a promising potential-triggered ESIX material for separation and recovery of heavy metal ions from wastewater.

Highly-efficient steam reforming of methanol over copper modified molybdenum carbide

Cu doped molybdenum carbide (Cu–MoxCy) catalysts were prepared by carburization of Cu doped molybdenum oxide (Cu–MoO3) using a temperature-programmed reaction with a 20% CH4–H2 mixture at 700 °C. Phase transition of the prepared molybdenum carbide was found to be related to the doping amount of Cu: with the increase in the doping amount of Cu/Mo molar ratio of 1.6/98.4 to 10/90, the cubic α-MoC1−x phase increased in the catalyst, but with the continued increase of the doping amount to a Cu/Mo molar ratio of 15/85, the α-MoC1−x phase began to decrease, and when the Cu doping amount reached a Cu/Mo molar ratio of 25/75, the α-MoC1−x phase became very weak and mainly hexagonal β-Mo2C phase was found in the catalysts. TEM images indicated that carbon growth on the surface of Cu occurred during the carburization process in the case of high Cu doping. Steam reforming of methanol (SRM) over the Cu–MoxCy catalyst was investigated at a temperature range of 200–400 °C. It is found that Cu–MoxCy catalyst with Cu/Mo molar ratios in the range of 1.6/98.4–10/90 showed high catalytic activity as well as long-term stability. X-ray photoelectron spectroscopy analysis indicated the coexistence of CuI and CuII species on the surface of the molybdenum carbide. The existence of CuI could result in high activity for methanol conversion and high stability, which might result from the strong interaction between Cu and Mo2C support.

Electroactive ion exchange materials: current status in synthesis, applications and future prospects

Electroactive ion exchange materials (EIXMs) with unique electrochemically switched ion exchange function have been extensively applied in various fields including high value-added ion recovery, toxic ion removal, energy storage devices and electrochemical ion sensors. In particular, the charge and discharge of EIXMs can be electrochemically controlled by an external electric field or by a redox agent while the charge balance is compensated via the ion exchange with electrolyte solution. To date, the discovered EIXMs mainly involve inorganic compounds with mixed-valence transition metals, organic conducting polymers and organic–inorganic hybrid materials. Compared with their bulk forms, nanostructured EIXMs have aroused considerable research interest recently owing to their unique properties such as larger surface area, better ion diffusion path and excellent electron transfer property. In this article, the design principles and synthetic routes for the development of various nanostructured EIXMs and their applications in the fields of selective ion separation, supercapacitors and electrochemical ion sensors are reviewed. In addition, the main challenges and future prospects for the further development of high-performance EIXMs are discussed.