Interfacial Electronic Modulation of Mo5 N6 /Ni3 S2 Heterojunction Array Boosts Electrocatalytic Alkaline Overall Water Splitting.

Bifunctional electrocatalysts with excellent activity and durability are highly desirable for alkaline overall water splitting, yet remain a significant challenge. In this contribution, palm-like Mo5 N6 /Ni3 S2 heterojunction arrays anchored in conductive Ni foam (denoted as Mo5 N6 -Ni3 S2 HNPs/NF) are developed. Benefiting from the optimized electronic structure configuration, hierarchical branched structure and abundant heterogeneous interfaces, the as-synthesized Mo5 N6 -Ni3 S2 HNPs/NF electrode exhibits remarkably stable bifunctional electrocatalytic activity in 1 m KOH solution. It only requires ultralow overpotentials of 59 and 190 mV to deliver a current density of 10 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 m KOH solution, respectively. Importantly, the overall water splitting electrolyzer assembled by Mo5 N6 -Ni3 S2 HNPs/NF exhibits an exceptionally low cell voltage (1.48 V@10 mA cm-2 ) and outstanding durability, surpassing most of the reported Ni-based bifunctional materials. Density functional theory (DFT) further confirms the heterostructure can optimize the Gibbs free energies of H and O-containing intermediates (OH, O, OOH) during HER and OER processes, thereby accelerating the catalytic kinetics of electrochemical water splitting. The findings provide a new design strategy toward low-cost and excellent catalysts for overall water splitting.

Phase-Engineered Robust Cocatalyst of NiFe Bicarbonate with Pt Dopant for Enhanced Photocatalytic Water Splitting.

Exploring efficient cocatalysts capable of accelerating surface catalytic reaction is of great significance for the development of solar-driven hydrogen production. Herein, on the basis of NiFe hydroxide, we developed a series of Pt doped NiFe-based cocatalysts to promote the photocatalytic hydrogen production of graphitic carbon nitride (g-C3 N4 ). We find that the Pt doping can trigger phase reconstruction of NiFe hydroxide and lead to the formation of NiFe bicarbonate, which displays higher catalytic activity toward hydrogen evolution reaction (HER). The Pt doped NiFe bicarbonate modified g-C3 N4 shows excellent photocatalytic activity with H2 evolution rate up to 100 µmol/h, which is more than 300 times that of pristine g-C3 N4 . The experimental and calculation results demonstrate that the greatly improved photocatalytic HER activity of g-C3 N4 is not only due to the efficient carrier separation, but also attributed to the accelerated HER kinetics. Our work may provide guidance for designing novel and superior photocatalysts.

Phase transition in V-doped cobalt hydroxide for a superior alkaline hydrogen evolution reaction.

Herein, V-doped cobalt hydroxides grown on carbon cloth (V-Co(OH)2/CC) were prepared via hydrothermal method. The incorporation of V can trigger phase transition and tune the local electronic structure of Co(OH)2, thereby improving the intrinsic alkaline HER activity. We find that the V-Co(OH)2 dominated by ß-Co(OH)2 exhibits excellent HER activity with only 83 mV overpotential at a current density of 10 mA cm-2, which outperforms most reported hydroxide-based catalysts and even surpasses the commercial Pt/C at large current density (>160 mA cm-2).

Three-dimensional mesoporous sandwich-like g-C3N4-interconnected CuCo2O4 nanowires arrays as ultrastable anode for fast lithium storage.

Inspite of their impressive high theoretical capacity as Lithium-ion batteries (LIBs) anodes, spinel transition-metal oxides (TMOs) suffer serious volume expansion, aggregation and the pulverization of crystal structures during lithiation/delithiation, and this process severely restrict their industrial application. Multi-dimensional morphological engineering of spinel TMO nanostructures is an effective way to solve this issue. In this work, using facile hydrothermal synthetic methods, spinel CuCo2O4 nanowires arrays are synthesized and supported on g-C3N4 nanosheets, thus forming a unique sandwich-like interconnected three-dimensional mesoporous structure containing high amount of void spaces. Addition of g-C3N4 nanosheets to CuCo2O4 nanowire arrays may shorten the Li+ diffusion distance and electron transfer pathway, and may also provide more active sites for Li+ diffusion into electrolyte and buffer for the volume expansion and aggregation of CuCo2O4. As a LIB anode material, CuCo2O4@g-C3N4 shows initial lithiation capacity of 840.6 mAh g-1, and capacity retention of 641.2 mAh g-1 after 60 cycles at the current density of 0.1 A g-1 and 499.2 mAh g-1 after 40 cycles at high current of 1 A g-1, which is significantly better than value of pure CuCo2O4 nanowires. This work affords a new way to tackle the problem of volume expansion of high capacity spinel TMO anode materials using g-C3N4 nanosheets as buffering agent.

ZnO nanorod arrays grown on g-C3N4 micro-sheets for enhanced visible light photocatalytic H2 evolution.

The designed synthesis of noble-metal-free photocatalysts with hierarchical heteroassemblies in a facile, mild and eco-friendly way becomes more and more important, because we can explore the novel properties and applications of novel heterostructures via this method. Herein we report a two-step aqueous strategy for novel hierarchical heterostructures of ZnO nanorod (NR) arrays grown on graphitic carbon nitride (g-C3N4). The novel g-C3N4/ZnO NR heterostructures that integrate g-C3N4 and ZnO NR via high-quality g-C3N4-ZnO heterojunctions have beneficial properties such as high specific surface area (SSA), open spatial architecture, good electronic conductivity, and effective charge transfer interfaces, and are promising in many related areas such as water splitting, solar cells, etc. As a noble-metal-free and visible-light-responsive photocatalytic material, a typical g-C3N4/ZnO NR photocatalytic system exhibits enhanced photocatalytic activity toward H2 evolution, almost 3.5 times higher than that of pure g-C3N4. The superior photocatalytic property can be ascribed to the synergistic effect of the unique g-C3N4/ZnO NR heterostructures.

Mg-porphyrin complex doped divinylbenzene based porous organic polymers (POPs) as highly efficient heterogeneous catalysts for the conversion of CO2 to cyclic carbonates.

A series of Mg-porphyrin complex doped divinylbenzene (DVB) based porous organic polymers (POPs) were systematically afforded through the method of free radical polymerization under solvothermal conditions. These POP catalysts have physical advantages of high surface areas, hierarchical pore structures, high thermal stability and spatially separated active Mg-porphyrin sites, which lead to very high efficiency in the conversion of CO2 to cyclic carbonates with the aid of tetra-n-butyl ammonium bromide (TBAB) as a nucleophile. The effect of the doping ratio (Mg-porphyrin complex to DVB) on catalytic efficiency was studied and discussed, and the detrimental embedding effect was found. The effects of reaction temperature and pressure on catalytic activity as well as other epoxide substrates were also examined fully. More importantly, under very mild conditions (30 °C, 0.1 MPa CO2), a considerable turnover number (TON) value of 1800 was obtained. The heterogeneous POP catalyst can be easily recovered and reused 10 times without loss of activity.

Chemical Foaming Coupled Self-Etching: A Multiscale Processing Strategy for Ultrahigh-Surface-Area Carbon Aerogels.

Due to the unique structure, carbon aerogels have always shown great potential for multifunctional applications. At present, it is highly desirable but remains challenging to tailor the microstructures with respect to porosity and specific surface area to further expand its significance. A facile chemical foaming coupled self-etching strategy is developed for multiscale processing of carbon aerogels. The strategy is directly realized via the pyrolysis of a multifunctional precursor (pentaerythritol melamine phosphate) without any special drying process and multiple steps. In the micrometer scale, the macroporous scaffold structures with interconnected and strutted carbon nanosheets are built up by chemical foaming from decomposition of melamine, whereas the meso/microporous nanosheets are formed via self-etching by phosphorus-containing species. The delicately hierarchical structures and record-breaking specific surface area of 2668.4 m2 g-1 render the obtained carbon aerogels great potentials for absorption (324.1-593.6 g g-1 of absorption capacities for varied organic solvents) and energy storage (338 F g-1 of specific capacitance). The construction of such novel carbon nanoarchitecture will also shed light on the design and synthesis of multifunctional materials.

Interconnected Phosphorus and Nitrogen Codoped Porous Exfoliated Carbon Nanosheets for High-Rate Supercapacitors.

Carbon-based supercapacitors have high power density and long cycle life; however, they are known to suffer from problems related to low energy density and high inner resistance. Here, we report a novel hierarchically porous functional carbon that is made up of interconnected exfoliated carbon nanosheets with thickness of a few nanometers. Notably, these porous carbon nanosheets are doped with abundant nitrogen (N) dopants in the basal plane and phosphorus (P) functional groups at the edge of the graphene lattice. The specific surface chemistry and pore structure of the synthesized sample, combined with its large specific surface area, make it a high-performance active material for supercapacitor electrode. The obtained supercapacitor made with the optimized sample showed a high specific capacitance (265 F g-1 at 0.5 A g-1) as well as long-term stability (94% capacitance retention after 5000 cycles). Particularly, the enhanced electrochemical characteristics were maintained even at high electrode mass loading (time constant (τ0) is 1.10 s for an electrode mass loading of 12.38 mg cm-2 compared to 1.61 s for a mass loading of 4.17 mg cm-2 for commercial activated carbon), which is important for a high packing factor of the capacitor.

Efficient oxygen reduction electrocatalyst based on edge-nitrogen-rich graphene nanoplatelets: toward a large-scale synthesis.

The large-scale synthesis of nitrogen doped graphene (N-graphene) with high oxygen reduction reaction (ORR) performance has received a lot of attention recently. In this work, we have developed a facile and economical procedure for mass production of edge-nitrogen-rich graphene nanoplatelets (ENR-GNPs) by a combined process of ball milling of graphite powder (GP) in the presence of melamine and subsequent heat treatment. It is found that the ball milling process can not only crack and exfoliate pristine GP into edge-expanded nanoplatelets but also mechanically activate GP to generate appropriate locations for N-doping. Analysis results indicate that the doped N atoms mainly locate on the edge of the graphitic matrix, which contains ca. 3.1 at.% nitrogen content and can be well-dispersed in aqueous to form multilayer nanoplatelets. The as-prepared ENR-GNPs electrocatalyst exhibits highly electrocatalytic activity for ORR due to the synergetic effects of edge-N-doping and nanosized platelets. Besides, the stability and methanol tolerance of ENR-GNPs are superior to that of the commercial Pt/C catalyst, which makes the nanoplatelets a promising candidate for fuel cell cathode catalysts. The present approach opens up the possibility for simple and mass production of N-graphene based electrocatalysts in practice.

Catalyst-free synthesis of crumpled boron and nitrogen co-doped graphite layers with tunable bond structure for oxygen reduction reaction.

Two-dimensional materials based on ternary system of B, C and N are useful ranging from electric devices to catalysis. The bonding arrangement within these BCN nanosheets largely determines their electronic structure and thus chemical and (or) physical properties, yet it remains a challenge to manipulate their bond structures in a convenient and controlled manner. Recently, we developed a synthetic protocol for the synthesis of crumpled BCN nanosheets with tunable B and N bond structure using urea, boric acid and polyethylene glycol (PEG) as precursors. By carefully selecting the synthesis condition, we can tune the structure of BCN sheets from s-BCN with B and N bond together to h-BCN with B and N homogenously dispersed in BCN sheets. Detailed experiments suggest that the final bond structure of B and N in graphene depends on the preferentially doped N structure in BCN nanosheets. When N substituted the in-plane carbon atom with all its electrons configured into the π electron system of graphene, it facilitates the formation of h-BCN with B and N in separated state. On the contrary, when nitrogen substituted the edge-plane carbon with the nitrogen dopant surrounded with the lone electron pairs, it benefits for the formation of B-N structure. Specially, the compound riched with h-BCN shows excellent ORR performance in alkaline solution due to the synergistic effect between B and N, while s-BCN dominant BCN shows graphite-like activity for ORR, suggesting the intrinsic properties differences of BCN nanosheets with different dopants bond arrangement.

Advanced oxygen reduction electrocatalyst based on nitrogen-doped graphene derived from edible sugar and urea.

Designing and fabricating advanced oxygen reduction reaction (ORR) electrocatalysts is critical importance for the sake of promoting widespread application of fuel cells. In this work, we report that nitrogen-doped graphene (NG), synthesized via one-step pyrolysis of naturally available sugar in the presence of urea, can serve as metal-free ORR catalyst with excellent electrocatalytic activity, outstanding methanol crossover resistance as well as long-term operation stability in alkaline medium. The resultant NG1000 (annealed at 1000 °C) exhibits a high kinetic current density of 21.33 mA/cm(2) at -0.25 V (vs Ag/AgCl) in O2-saturated 0.1 M KOH electrolyte, compared with 16.01 mA/cm(2) at -0.25 V for commercial 20 wt % Pt/C catalyst. Notably, the NG1000 possesses comparable ORR half-wave potential to Pt/C. The effects of pyrolysis temperature on the physical prosperity and ORR performance of NG are also investigated. The obtained results demonstrate that high activation temperature (1000 °C) results in low nitrogen doping level, high graphitization degree, enhanced electrical conductivity, and high surface area and pore volume, which make a synergetic contribution to enhancing the ORR performance for NG.

NiCo2S4@graphene as a bifunctional electrocatalyst for oxygen reduction and evolution reactions.

Here, the hybrid of NiCo2S4 nanoparticles grown on graphene in situ is first described as an effective bifunctional nonprecious electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the alkaline medium. NiCo2S4@N/S-rGO was synthesized by a one-pot solvothermal strategy using Co(OAc)2, Ni(OAc)2, thiourea, and graphene oxide as precursors and ethylene glycol as the dispersing agent; simultaneously, traces of nitrogen and sulfur were double-doped into the reduced graphene oxide (rGO) in the forms of pyrrolic-N, pyridinic-N, and thiophenic-S, which are often desirable for metal-free ORR catalysts. In comparison with commercial Pt/C catalyst, NiCo2S4@N/S-rGO shows less reduction activity, much better durability, and superior methanol tolerance toward ORR in 0.1 M KOH; it reveals higher activity toward OER in both KOH electrolyte and phosphate buffer at pH 7.0. NiCo2S4@graphene demonstrated excellent overall bicatalytic performance, and importantly, it suggests a novel kind of promising nonprecious bifunctional catalyst in the related renewable energy devices.

Identifying the active site in nitrogen-doped graphene for the VO2+/VO2(+) redox reaction.

Nitrogen-doped graphene sheets (NGS), synthesized by annealing graphite oxide (GO) with urea at 700-1050 °C, were studied as positive electrodes in a vanadium redox flow battery. The NGS, in particular annealed at 900 °C, exhibited excellent catalytic performance in terms of electron transfer (ET) resistance (4.74 ± 0.51 and 7.27 ± 0.42 Ω for the anodic process and cathodic process, respectively) and reversibility (ΔE = 100 mV, Ipa/Ipc = 1.38 at a scan rate of 50 mV s(-1)). Detailed research confirms that not the nitrogen doping level but the nitrogen type in the graphene sheets determines the catalytic activity. Among four types of nitrogen species doped into the graphene lattice including pyridinic-N, pyrrolic-N, quaternary nitrogen, and oxidic-N, quaternary nitrogen is verified as a catalytic active center for the [VO](2+)/[VO2](+) couple reaction. A mechanism is proposed to explain the electrocatalytic performance of NGS for the [VO](2+)/[VO2](+) couple reaction. The possible formation of a N-V transitional bonding state, which facilitates the ET between the outer electrode and reactant ions, is a key step for its high catalytic activity.