He Fu

MOF derived catalysts for electrochemical oxygen reduction

Developing noble metal free catalysts for the oxygen reduction reaction (ORR) is of critical importance for the production of low cost polymer electrolyte membrane fuel cells. In this paper, metal organic frameworks (MOFs) are used as precursors to synthesize ORR catalysts via pyrolysis in an inert atmosphere. The ORR performance is found to be closely associated with the metal/ligand combination in MOFs. The Co-imidazole based MOF (ZIF-67) derived catalyst exhibits the best ORR activity in both alkaline and acidic electrolytes. The Co cations coordinated by the aromatic nitrogen ligands in ZIF-67 may assist the formation of ORR active sites in the derived catalyst. The best ORR performance is obtained when the porosity of the derived catalyst is maximized, by optimizing the pyrolysis temperature and the acid leaching process. The performance of the best MOF derived catalyst is comparable to that of Pt/C in both alkaline and acidic electrolytes.

Noble Metal-Free Oxygen Reduction Reaction Catalysts Derived from Prussian Blue Nanocrystals Dispersed in Polyaniline

A highly efficient noble-metal-free catalyst for the oxygen reduction reaction (ORR) is derived from a composite of polyaniline (PANI) and Prussian blue analogue (PBA, Co3[Fe(CN)6]2) by pyrolysis. The composite consists of 2-5 nm PBA nanocrystals homogeneously dispersed in PANI. During the pyrolysis, the PBA nanocrystals serve as both the template for the pore formation and the precursor for the ORR active sites, which results in a nanoporous structure strongly coupled with the ORR active sites. The catalyst exhibits superior ORR performance in both alkaline and acidic electrolyte, comparable to that of the commercial Pt/C with 20 wt % Pt loading.

An efficient Co–N–C oxygen reduction catalyst with highly dispersed Co sites derived from a ZnCo bimetallic zeolitic imidazolate framework

In this work we report a highly efficient Co based catalyst for the oxygen reduction reaction (ORR) with highly dispersed Co sites on N-doped carbon. The catalyst is derived from a ZnCo bimetallic metal organic framework (MOF) by heat treatment in an inert atmosphere at 1000 °C. Zn is simultaneously eliminated during the pyrolysis due to its high volatility at high temperature, yielding a highly porous structure with homogeneous Co loading. Another effect of Zn is to disperse Co in the MOF precursor, which effectively inhibits the aggregation of Co after pyrolysis. The best ORR performance is achieved when 5% Zn is substituted by Co in the MOF precursor. The resulting catalyst shows a high half wave potential of 0.90 V vs. reversible hydrogen electrode in 0.1 M KOH solution, which is mainly attributed to the high dispersion of the ORR active Co sites.

In situ hybridization of LiNH2–LiH–Mg(BH4)2 nano-composites: intermediate and optimized hydrogenation properties

Nano-composites of LiNH(2)-LiH-xMg(BH(4))(2) (0 ≤ x ≤ 2) were prepared by plasma metal reaction followed by a nucleation growth method. Highly reactive LiNH(2)-LiH hollow nanoparticles offered a favorable nucleus during a precipitation process of liquid Mg(BH(4))(2)·OEt(2). The electron microscopy results suggested that more than 90% of the obtained nano-composites were in the range 200-400 nm. Because of the short diffusion distance and ternary mixture self-catalyzing effect, this material possesses enhanced hydrogen (de)sorption attributes, including facile low-temperature kinetics, impure gases attenuation and partial reversibility. The optimal hydrogen storage properties were found at the composition of LiNH(2)-LiH-0.5Mg(BH(4))(2), which was tentatively attributed to a Li(4)(NH(2))(2)(BH(4))(2) intermediate. 5.3 wt% hydrogen desorption could be recorded at 150 °C, with the first 2.2 wt% release being reversible. This work suggests that controlled in situ hybridization combined with formula optimization can improve hydrogen storage properties.

Excellent hydrogen sorption kinetics of thick Mg–Pd films under mild conditions by tailoring their structures

500 nm thick Mg–Pd films with different structures were prepared and their hydrogen storage properties were investigated. Results indicated that thin Ti interlayers in the Mg bulk film could significantly improve the hydrogen sorption kinetics and reversibility, offering an effective way to improve the hydrogen storage properties of thick Mg-based films.

Metal (metal = Fe, Co), N codoped nanoporous carbon for efficient electrochemical oxygen reduction

Metal, N codoped nanoporous carbon (N–M–nC, M = Fe, Co) is prepared by in situ incorporation of the metal during the formation of the nanoporous carbon skeleton followed by NH3 treatment. The samples exhibit superior catalytic performance for the oxygen reduction reaction (ORR) in alkaline electrolytes. M, N codoping shows a synergic effect with improved ORR performance compared to the sample with only nitrogen dopant (N–nC), in the order of N–Fe–nC > N–Co–nC > N–nC, indicating that the M–N synergic effect is critical for high ORR performance in alkaline electrolyte. A detailed structural characterization of the catalysts is carried out, which suggests that the improved ORR performance should be attributed to the formation of active sites with M–N bonding. Other structural differences, including surface area, porosity and carbon structure, play a minor role. The performance of the N–Fe–nC sample is comparable to that of commercial Pt/C, including more positive onset and halfwave potential, comparable saturation current density and a dominant four-electron pathway, which suggests that nanoporous carbon can serve as an ideal platform for developing high performance ORR catalysts via proper doping.

Preparation and Dehydrogenation Properties of Lithium Hydrazidobis(borane) (LiNH(BH3)NH2BH3)

LiNH(BH3)NH2BH3, the first example of metal-substituted hydrazine bisborane (HBB), is synthesized via the reaction between HBB and n-butyllithium in ether solution. (11)B NMR and Fourier transform infrared spectroscopy indicate a new structure, in which one of the N-H bonds is replaced by a N-Li bond. The X-ray diffraction pattern of the product also indicates the formation of a new crystal structure. This compound releases hydrogen at 126 and 170 °C with satisfactory purity and exhibits superior hydrogen storage properties compared with HBB. Differential scanning calorimetry measurement suggests the dehydrogenation reaction of this compound is less exothermic than that of HBB.

Promoted hydrogen release from alkali metal borohydrides in ionic liquids

The use of alkali metal borohydrides for hydrogen storage has long been restricted by high dehydrogenation temperature and large endothermic dehydrogenation enthalpy. Here we report that the dehydrogenation properties of NaBH4 and LiBH4 can be significantly improved by the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimNTf2). The borohydrides form homogeneous solutions in bmimNTf2, which release more than 70% of theoretical hydrogen below 180 °C, significantly lower than that in the solid state (370 °C for LiBH4 and 500 °C for NaBH4). The dehydrogenation reactions become highly exothermic in the IL, which is in contrast to the highly endothermic process in their solid states. The drastically changed dehydrogenation behaviour in IL is attributed to the destabilization of borohydrides due to the more favorable charge transfer from BH4− to the cation in the IL, which is in line with the established stability rule of metal borohydrides. The IL remains unchanged after dehydrogenation, which provides the possibility of its repeated use.

Hydrogen generation from reactions of hydrides with hydrated solids in the solid state

The reaction of water with hydrides (hydrolysis reaction) is very attractive for onsite hydrogen generation. Instead of using liquid water like in most cases, we demonstrate that hydrogen generation from hydrolysis reactions can also occur in the solid state. By simply heating mixtures of hydrated solids and hydrides, hydrogen generation is readily achieved through the recombination from the protonic hydrogen in the hydrated solids and the hydridic hydrogen in the hydride. The composites 5CaH2 + Na4P2O7·10H2O and NaBH4 + H2C2O4·2H2O give attainable gravimetric hydrogen storage capacity of 2.76% at 40 °C and 2.79% at 70 °C with rapid response, respectively. The dehydrogenation temperature can be controlled by the dehydration temperature of the hydrated solids. This innovative hydrogen generation approach provides a temperature activated, easy to control solution for onsite hydrogen generation.

2-Aminoimidazole borohydride as a hydrogen carrier

2-Aminoimidazole borohydride (Im-NH2BH4) is synthesized via the reaction between 2-aminoimidazole hemisulfate ((Im-NH2)2SO4) and sodium borohydride by a simple ball milling method. This compound holds theoretical hydrogen capacity of 8.1 wt%. It is designed based on the strategy of destabilizating BH4− using a large conjugated cation, Im-NH2+ and maximizing the protonic-hydridic hydrogen interaction by balancing their numbers. Thermal dehydrogenation analyses demonstrate that it can release about 3.2 wt% hydrogen (Na2SO4 included) without gaseous impurities below 320 °C. The decomposition process has three exothermal steps and the onset dehydrogenation temperature is 50.5 °C. These results indicate that the above strategy is effective. The dehydrogenation process of this compound is discussed and the product at 320 °C is proposed to be C3N3H3BH.