Xiao Xie

Cobalt phosphate nanoparticles decorated with nitrogen-doped carbon layers as highly active and stable electrocatalysts for the oxygen evolution reaction

One promising approach to the production of clean hydrogen energy from electrochemical water splitting mainly relies on the successful development of earth-abundant, highly efficient and stable electrocatalysts for the oxygen evolution reaction (OER). Herein, we report the synthesis of robust cobalt phosphate nanoparticles (NPs) decorated with nitrogen-doped carbon layers (denoted as Co3(PO4)2@N-C) using O-phospho-DL-serine as both phosphate and carbon sources by hydrothermal treatment. The obtained Co3(PO4)2@N-C catalyst exhibits a remarkable electrocatalytic performance for the OER in alkaline media. A current density of 10 mA cm−2 is generated at a overpotential of only 317 mV with a small Tafel slope of 62 mV per decade in 1 M KOH electrolyte, which is even superior to those of state-of-the-art noble metal catalysts such as benchmark IrO2 catalysts. Notably, the Co3(PO4)2@N-C electrode shows excellent stability evaluated by 1000 potential cycles and operation with a high current density at a fixed potential for 8 h, which is highly desirable for a promising electrocatalyst. The excellent activity can be attributed to the unique network structure of materials, a large number of active sites and good conductivity under catalytic conditions. Our findings imply the possibility for the development of robust and cost-efficient cobalt phosphate as a promising candidate to replace high-cost and scarce noble metal catalysts for electrochemical water splitting.

The synergistic effect of metallic molybdenum dioxide nanoparticle decorated graphene as an active electrocatalyst for an enhanced hydrogen evolution reaction

As a durable and renewable fuel, hydrogen has attracted a huge amount of global interest in its production via different routes. Among these methods, the electrocatalytic hydrogen evolution reaction (HER) is one of the most promising ways for low-cost hydrogen production in the future. In this work, a simple redox hydrothermal method has been developed to fabricate a noble-metal-free MoO2/rGO composite for a highly efficient HER. GO nanosheets provide oxygen-containing functional groups for precursor attachment, and restrict growth of MoO2 nanoparticles (NPs) with a small size due to the space confinement effect among GO layers as well. Benefitting from a synergistic effect between metallic MoO2 NPs and graphene, the obtained MoO2/rGO composite exhibits excellent HER activity with a small onset overpotential of 190 mV, a large cathodic current density, and a small Tafel slope of 49 mV per decade, while MoO2 NPs or rGO itself is not a very efficient HER catalyst. Additionally, the MoO2/rGO composite displays good stability after 1000 potential cycles under both acidic and alkaline conditions. Dramatically improved HER activity and excellent stability are attributed to small size, more active sites, high conductivity and a synergistic effect of MoO2 NPs and graphene. The development of the MoO2/rGO composite as an enhanced active HER catalyst broadens the members of Mo-based HER catalysts and provides an insight into the design and synthesis of other noble-metal-free materials for the cost-effective and environmentally friendly catalyst in electrochemical hydrogen production.

P doped molybdenum dioxide on Mo foil with high electrocatalytic activity for the hydrogen evolution reaction

As a clean and renewable energy carrier, hydrogen generation has attracted much interest and the electrocatalytic hydrogen evolution reaction (HER) is one of the most promising ways of low-cost hydrogen production in the future. In this work, we report the fabrication of noble-metal-free P doped MoO2 nanoparticles (NPs) on Mo foil as electrodes for highly efficient HER. Benefiting from a strong interaction between P doped MoO2 NPs and Mo foil as a current collector, the obtained electrode exhibits excellent HER activity with a small onset overpotential of 80 mV, a large cathodic current density of 10 mA cm−2 at 135 mV and a small Tafel slope of 62 mV per decade, much better than MoO2-based catalysts. Additionally, a P doped MoO2 film/Mo foil electrode displays good stability even after 2000 potential cycles in acidic media. The development of a novel route to prepare P doped MoO2 on Mo foil as an active HER catalyst broadens the insight of designing noble- metal-free HER efficient catalysts with cost-effective and environmentally friendly advantages.

Carbon nanotube/S–N–C nanohybrids as high performance bifunctional electrocatalysts for both oxygen reduction and evolution reactions

Exploring high performance bifunctional electrocatalysts for efficient oxygen reduction and evolution reactions is of crucial importance for sustainable energy conversion and storage devices including rechargeable metal–air batteries and fuel cells. In this work, one-dimensional (1D) cable-like multiwall carbon nanotube/S, N co-doped carbon nanodot (MWCNT@S–N–C) hybrids were synthesized by annealing multiwall carbon nanotubes/polythiophene (MWCNT@Pth) in the NH3 atmosphere. The as-prepared catalysts exhibit an outstanding oxygen reduction reaction (ORR) activity (an unusual high limiting current density of 7.62 mA cm−2), strong immunity towards methanol crossover and long-term stability. Moreover, the obtained catalysts also display a higher activity toward oxygen evolution reaction (OER) compared to the state-of-the-art IrO2 catalyst, demonstrating excellent overall bi-catalytic performance. This superior electrocatalytic activity arises from synergistic chemical coupling effects between S and N, excellent reactant transport provided by mesoporous structures and a high charge transfer rate driven by 1D MWCNTs, thereby generating a high performance bi-functional electrocatalyst for both ORR and OER. This is the first time that S, N co-doped carbon materials have been investigated as effective bifunctional catalysts, which not only provides a further insight into the electrocatalytic mechanism, but also provides a new approach for the design and fabrication of metal-free bi-functional oxygen catalysts with low cost and high efficiencies in electrochemical energy conversion.

Metallic 1T-LixMoS2 Cocatalyst Significantly Enhanced the Photocatalytic H2 Evolution over Cd0.5Zn0.5S Nanocrystals under Visible Light Irradiation

In the present work, metallic 1T-LixMoS2 is utilized as a novel cocatalyst for Cd0.5Zn0.5S photocatalyst. The obtained LixMoS2/Cd0.5Zn0.5S hybrids show excellent photocatalytic performance for H2 generation from aqueous solution containing Na2S and Na2SO3 under splitting visible light illumination (λ ≥ 420 nm) without precious metal cocatalysts. It turns out that a certain amount of intercalating Li(+) ions ultimately drives the transition of MoS2 crystal from semiconductor triagonal phase (2H phase) to metallic phase (1T phase). The distinct properties of 1T-LixMoS2 promote the efficient separation of photoexcited electrons and holes when used as cocatalyst for Cd0.5Zn0.5S photocatalyst. As compared to 2H-MoS2 nanosheets only having edge active sites, photoinduced electrons not only transfer to the edge sites of 1T-LixMoS2, but also to the plane active sites of 1T-LixMoS2 nanosheets. The content of LixMoS2 in hybrid photocatalysts influences the photocatalytic activity. The optimal 1T-LixMoS2 (1.0 wt %)/Cd0.5Zn0.5S nanojunctions display the best activity for hydrogen production, achieving a hydrogen evolution rate of 769.9 μmol h(-1), with no use of noble metal loading, which is about 3.5 times higher than that of sole Cd0.5Zn0.5S, and 2 times higher than that of 2H-MoS2 (1.0 wt %)/Cd0.5Zn0.5S samples. Our results demonstrate that Li(+)-intercalated MoS2 nanosheets with high conductivity, high densities of active sites, low cost, and environmental friendliness are a prominent H2 evolution cocatalyst that might substitute for noble metal for potential hydrogen energy applications.

A Novel Magnetically Recoverable Ni-CeO2–x/Pd Nanocatalyst with Superior Catalytic Performance for Hydrogenation of Styrene and 4-Nitrophenol

Metal/support nanocatalysts consisting of various metals and metal oxides not only retain the basic properties of each component but also exhibit higher catalytic activity due to their synergistic effects. Herein, we report the creation of a highly efficient, long-lasting, and magnetic recyclable catalyst, composed of magnetic nickel (Ni) nanoparticles (NPs), active Pd NPs, and oxygen-deficient CeO2-x support. These hybrid nanostructures composed of oxygen deficient CeO2-x and active metal nanoparticles could effectively facilitate diffusion of reactant molecules and active site exposure that can dramatically accelerate the reaction rate. Impressively, the rate constant k and k/m of 4-nitrophenol reduction over 61 wt % Ni-CeO2-x/0.1 wt % Pd catalyst are 0.0479 s-1 and 2.1 × 104 min-1 g-1, respectively, and the reaction conversion shows negligible decline even after 20 cycles. Meanwhile, the optimal 61 wt % Ni-CeO2-x/3 wt % Pd catalyst manifests remarkable catalytic activity toward styrene hydrogenation with a high TOF of 6827 molstyrene molPd-1 h-1 and a selective conversion of 100% to ethylbenzene even after eight cycles. The strong metal-support interaction (SMSI) between Ni NPs, Pd NPs, and oxygen-deficient CeO2-x support is beneficial for superior catalytic efficiency and stability toward hydrogenation of styrene and 4-nitrophenol. Moreover, Ni species could boost the catalytic activity of Pd due to their synergistic effect and strengthen the interaction between reactant and catalyst, which seems responsible for the great enhancement of catalytic activity. Our findings provide a new perspective to develop other high-performance and magnetically recoverable nanocatalysts, which would be widely applied to a variety of catalytic reactions.

Synergistic effect of graphene and multi-walled carbon nanotubes composite supported Pd nanocubes on enhancing catalytic activity for electro-oxidation of formic acid

The selectivity and sensitivity of a support material can highly improve the catalytic performance of known catalysts. As an excellent electron transfer material and having intercalation characteristics, reduced graphene oxide/multiwalled carbon nanotubes (rGO/MWCNTs) composite provides a synergistic effect on enhancing the electrocatalytic performance of direct formic acid fuel cells. Herein, we report the synthesis of palladium nanocubes (NCs) supported on rGO/MWCNTs composite, rGO and MWCNTs. The electrocatalytic performance for the formic acid oxidation reaction (FAOR) is tested by detailed electrochemical techniques such as cyclic voltametry (CV), chronoamperometery (CA) and electrochemical impedence spectroscopy (EIS) for all supported Pd-NCs catalysts and the results were compared with unsupported Pd-NCs. A significant, systematic and desired improvement in the activity of the FAOR is found for the Pd-NCs/rGO/MWCNTs catalyst. The order of activity is observed to be Pd-NCs < Pd-NCs/MWCNTs < Pd-NCs/rGO < Pd-NCs/rGO/MWCNTs. The results can be attributed to the synergistic effect induced by the hybrid support material on enhancing the activity of the Pd-NCs catalyst.

In situ redox deposition of palladium nanoparticles on oxygen-deficient tungsten oxide as efficient hydrogenation catalysts

Noble metal/metal oxide support hybrid materials have attracted tremendous interest due to their wide applications in catalysis. Herein, we have developed a novel and surfactant-free method to prepare Pd/WO3−x composite materials with clean surfaces. Oxygen-vacancy-rich WO3−x nanowires (NWs) provide free electrons to reduce Pd2+, and surface-clean Pd nanoparticles (NPs) directly grow on WO3−x surfaces through an in situ redox reaction between reductive WO3−x and metal salt precursor (Na2PdCl4) in aqueous solution. The as-obtained Pd/WO3−x nanocomposites show excellent catalytic activities for the hydrogenation of 4-nitrophenol (4-NP) and styrene. The apparent rate constant for 4-NP reduction is 0.045 s−1, over the Pd/WO3−x catalyst. The turnover frequency (TOF) value for styrene hydrogenation is 1074.5 h−1, thus, exhibiting high catalytic performance. Moreover, the obtained Pd/WO3−x catalyst exhibits good stability. Oxygen vacancies in WO3−x NWs can accelerate electron transport and promote hydrogen adsorption and dissociation on the surface of the catalyst. The strong interaction between Pd NPs and WO3−x support contributes to the excellent performance. Our work provides a novel and simple strategy to directly fabricate other-noble metal NP loaded oxygen-deficient metal oxides as highly efficient catalysts for chemical transformation.