Anqi Zhao

MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen-Doped Carbon Matrix for High-Performance Bifunctional Oxygen Electrodes†

Reversible interconversion of water into H2 and O2, and the recombination of H2 and O2 to H2O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn3O4 and Co3O4 nanoparticles embedded in nitrogen-doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N4 macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M-N(x) coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1 M) than those for RuO2, IrO2, Pt, NiO, Mn3O4, and Co3O4, thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date.

Spinel Mn–Co Oxide in N-Doped Carbon Nanotubes as a Bifunctional Electrocatalyst Synthesized by Oxidative Cutting

The notorious instability of non-precious-metal catalysts for oxygen reduction and evolution is by far the single unresolved impediment for their practical applications. We have designed highly stable and active bifunctional catalysts for reversible oxygen electrodes by oxidative thermal scission, where we concurrently rupture nitrogen-doped carbon nanotubes and oxidize Co and Mn nanoparticles buried inside them to form spinel Mn-Co oxide nanoparticles partially embedded in the nanotubes. Impressively high dual activity for oxygen reduction and evolution is achieved using these catalysts, surpassing those of Pt/C, RuO2, and IrO2 and thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries and reversible fuel cells, among others, for a sustainable and green energy future.

Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction

Abstract Insufficient electrochemical stability is a major challenge for carbon materials in oxygen reduction reaction (ORR) due to carbon corrosion and insufficient metal-support interactions. In this work, titania is explored as an alternative support for Pt catalysts. Oxygen deficient titania samples including TiO 2– x and TiO 2– x N y were obtained by thermal treatment of anatase TiO 2 under flowing H 2 and NH 3 , respectively. Pt nanoparticles were deposited on the titania by a modified ethylene glycol method. The samples were characterized by N 2 -physisorption, X-ray diffraction and X-ray photoelectron spectroscopy. The ORR activity and long-term stability of supported Pt catalysts were evaluated using linear sweep voltammetry and chronoamperometry in 0.1 mol/L HClO 4 . Pt/TiO 2– x and Pt/TiO 2– x N y showed higher ORR activities than Pt/TiO 2 as indicated by higher onset potentials. Oxygen deficiency in TiO 2– x and TiO 2– x N y contributed to the high ORR activity due to enhanced charge transfer, as disclosed by electrochemical impedance spectroscopy studies. Electrochemical stability studies revealed that Pt/TiO 2– x exhibited a higher stability with a lower current decay rate than commercial Pt/C, which can be attributed to the stable oxide support and strong interaction between Pt nanoparticles and the oxygen-deficient TiO 2– x support.

The formation mechanism of cobalt silicide on silica from Co(SiCl3)(CO)4 by in situ Fourier transform infrared spectroscopy.

Silica supported CoSi particles were synthesized by metal organic chemical vapor deposition of the Co(SiCl(3))(CO)(4) precursor carried in hydrogen at atmospheric pressure and moderate temperature in a fluidized bed reactor. In contrast, CoCl(2) supported on silica was formed by using argon as the carrier gas. The samples were characterized by X-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, and thermogravimetric/differential thermogravimetric analysis. The precursor Co(SiCl(3))(CO)(4) reacted with the hydroxyl groups of amorphous silica via loss of HCl and introduced cobalt species onto the surface. The decomposition mechanism of the supported precursor on silica was investigated by in situ Fourier transform infrared spectroscopy from room temperature to 300 °C in a hydrogen or argon atmosphere. The results showed that CO and HCl elimination occurred in a hydrogen atmosphere, while only CO elimination occurred in Ar. All of the results showed that it was possible to prepare supported CoSi at lower temperatures via changing the carrier gas.

A novel approach to synthesize highly selective nickel silicide catalysts for phenylacetylene semihydrogenation

Abstract Nickel silicides (NiSi x ) have been prepared by carbon template for nanostructured NiO and further silicidization with SiH 4 /H 2 at relatively low temperature and atmospheric pressure. The results showed that the formation of nickel silicides involves the following sequence, Ni (cubic) → Ni 2 Si (orthorhombic) → NiSi (orthorhombic) → NiSi 2 (cubic), with increasing temperatures. The as-prepared nickel silicides showed above 92% selectivity to styrene in the semihydrogenation of phenylacetylene due to the electronic and geometrical effects derived from the addition of Si into Ni particles.