Cheng-Zong Yuan

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.

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.

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.

Direct growth of cobalt-rich cobalt phosphide catalysts on cobalt foil: an efficient and self-supported bifunctional electrode for overall water splitting in alkaline media

The design of high-efficiency, economical and self-supported bifunctional electrodes for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is extremely crucial to renewable energy conversion processes, yet remains a long and arduous task. Here, we report the first example of cobalt-rich cobalt phosphide catalysts directly grown on cobalt foil (denoted as Co2P/Co-foil) as a novel non-noble metal and integrated electrode by one-step phosphorization of a pre-oxidized Co foil. Owing to the intrinsic catalytic properties of cobalt-rich cobalt phosphide and the intimate contact between Co2P and highly conductive Co foil, the resulting Co2P/Co-foil electrode exhibits excellent catalytic performances for both the HER and OER in basic solution, affording a current density of 10 mA cm−2 at low overpotentials of 157 mV for the HER and 319 mV for the OER, respectively. More importantly, these electrodes can be directly employed as both the anode and cathode in an alkaline electrolyzer, showing noble metal-like water splitting performances and long-term stability. Density functional theory (DFT) calculations suggest that the sites on the top of the P atoms in Co2P are the most active sites for the HER. This work would open an exciting new avenue to synthesize other metal-rich metal phosphide catalysts on conductive metal foil as self-supported electrodes using this facile, cost-effective and easy scale-up fabrication method for overall water splitting.

Bimetallic phosphide hollow nanocubes derived from a prussian-blue-analog used as high-performance catalysts for the oxygen evolution reaction

The development of efficient and stable electrocatalysts for the oxygen evolution reaction (OER) based on earth-abundant materials is of significance to enable water splitting as a feasible source of alternative energy. Metal–organic frameworks (MOFs) have been intensively employed as the templates/precursors to synthesize catalysts with hollow structures for various energy-related applications. In this study, using MOF (Ni–Fe prussian-blue-analog) as a template and precursor, a novel and promising bimetallic phosphide catalyst (Ni0.62Fe0.38)2P was obtained. Benefiting from synergistic effect between the Ni and Fe species, well-defined architecture and high surface area the as-made (Ni0.62Fe0.38)2P hollow nanocubes show a remarkable electrocatalytic performance for the OER in 1 M KOH electrolyte with a low overpotential of only 290 mV at a current density of 10 mA cm−2 and a small Tafel slope of 44 mV per decade, which even surpass the benchmark IrO2 catalyst. Moreover, the (Ni0.62Fe0.38)2P hollow nanocubes exhibit good long-term stability. This facile and novel route to prepare bimetallic phosphide hollow nanocubes as active OER catalysts broadens the scope for designing other noble-metal-free OER efficient catalysts for electrochemical water splitting in the future.

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.

Bare Cd1-xZnxS ZB/WZ Heterophase Nanojunctions for Visible Light Photocatalytic Hydrogen Production with High Efficiency.

In this work, we report the synthesis of Cd1-xZnxS zinc blende/wurtzite (ZB/WZ) heterophase nanojunctions with highly efficient charge separation by a solvothermal method in a mixed solution of diethylenetriamine (DETA) and distilled water. l-Cysteine was selected as a sulfur source and a protecting ligand for stabilization of the ZB/WZ homojunction. The optimal ternary chalcogenide Cd0.7Zn0.3S elongated nanocrystals (NCs) without any cocatalyst loading show very high visible light photocatalytic activity with H2 production efficiency of 3.13 mmol h(-1) and an apparent quantum efficiency of 65.7% at 420 nm. This is one of the best visible light photocatalysts ever reported for photocatalytic hydrogen production without any cocatalysts. The charge separation efficiency, having a critical role in enhancing photocatalytic activity for hydrogen production, was significantly improved. Highly efficient charge separation with a prolonged carrier lifetime is driven by the internal electrostatic field originating from the type-II staggered band alignment at the ZB/WZ junctions, as confirmed by steady and time-resolved photoluminescence spectra. Further, the strong binding between the l-cysteine ligand and Cd1-xZnxS elongated nanocrystals protects and stabilizes NCs; the l-cysteine ligand at the interface could trap holes from Cd1-xZnxS NCs, while photogenerated electrons transfer to Cd1-xZnxS catalytic sites for proton reduction. Our results demonstrate that Cd1-xZnxS ZB/WZ heterophase junctions stabilized by l-cysteine molecules can effectively separate charge carriers and achieve highly visible light photocatalytic hydrogen production. The present study provides a new insight into the design and fabrication of advanced materials with homojunction structures for photocatalytic applications and optoelectronic devices.

One-Step Growth of Iron–Nickel Bimetallic Nanoparticles on FeNi Alloy Foils: Highly Efficient Advanced Electrodes for the Oxygen Evolution Reaction

Electrochemical water splitting is an important process to produce hydrogen and oxygen for energy storage and conversion devices. However, it is often restricted by the oxygen evolution reaction (OER) due to its sluggish kinetics. To overcome the problem, precious metal oxide-based electrocatalysts, such as RuO2 and IrO2, are widely used. The lack of availability and the high cost of precious metals compel researchers to find other resources for the development of cost-effective, environmentally friendly, earth-abundant, nonprecious electrocatalysts for OER. Such catalysts should have high OER performance and good stability in comparison to those of available commercial precious metal-based electrocatalysts. Herein, we report an inexpensive fabrication of bimetallic iron-nickel nanoparticles on FeNi-foil (FeNi4.34@FeNi-foil) as an integrated OER electrode using a one-step calcination process. FeNi4.34@FeNi-foil obtained at 900 °C shows superior OER activity in alkaline solution with an overpotential as low as 283 mV to achieve a current density of 10 mA cm-2 and a small Tafel slope of 53 mV dec-1. The high performance and durability of the as-prepared nonprecious metal electrode even exceeds those of the available commercial RuO2 and IrO2 catalysts, showing great potential in replacing the expensive noble metal-based electrocatalysts for OER.

Supramolecular polymers-derived nonmetal N, S-codoped carbon nanosheets for efficient oxygen reduction reaction

The rational design and fine synthesis of highly efficient and cost-effective electrocatalysts for oxygen reduction reaction (ORR) is crucial for the wide application of fuel cells (FCs). In this work, we select a novel nitrogen and sulfur-rich supramolecular polymer as a precursor for in situ, large scale and controlled synthesis of nitrogen and sulfur dual doped carbon (N, S–C) nanosheets as a catalyst for ORR. The supramolecular polymer MTCA particles are quickly self-assembled via triple-hydrogen-bonding between melamine (M) and trithiocyanuric acid (TCA). The uniform distribution and high content of nitrogen and sulfur in the polymer is beneficial to the high and homogeneous doping in the produced self-supporting catalyst after pyrolysis. When evaluated as an electrocatalyst, the catalyst pyrolyzed at 800 °C (N, S–C/800) shows a superior ORR activity in alkaline solution. Furthermore, the N, S–C/800 catalyst exhibits superb durability and immunity towards methanol crossover. This metal-free, cost-effective and highly efficient ORR catalyst will find wide potential applications in fuel cells.

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.