Nanomaterials for Advanced Photocatalytic Plastic Conversion.

As the disposal of waste plastic emerges as a societal problem, photocatalytic waste plastic conversion is attracting significant attention. Ultimately, for a sustainable future, the development of an eco-friendly plastic conversion technology is essential for breaking away from the current plastic use environment. Compared to conventional methods, photocatalysis can be a more environmentally friendly option for waste plastic reprocessing because it uses sunlight as an energy source under ambient temperature and pressure. In addition to this, waste plastics can be upcycled (i.e., converted into useful chemicals or fuels) to enhance their original value via photocatalytic methods. Among various strategies for improving the efficiency of the photocatalytic method, nanomaterials have played a pivotal role in suppressing charge recombination. Hence, in recent years, attempts have been made to introduce nanomaterials/nanostructures into photocatalytic plastic conversion on the basis of advances in material-based studies using simple photocatalysts. In line with this trend, the present review examines the nanomaterials/nanostructures that have been recently developed for photocatalytic plastic conversion and discusses the direction of future development.

Highly Conformal Deposition of an Ultrathin FeOOH Layer on a Hematite Nanostructure for Efficient Solar Water Splitting.

An ultrathin (ca. 2 nm) amorphous FeOOH overlayer was deposited conformally on a hematite nanostructure by a simple solution-based precipitation method, to generate an oxygen evolution cocatalyst for efficient solar water splitting. This uniform and highly conformal coating of the ultrathin metal oxyhydroxide is rare and is distinguished from the layers prepared by other conventional methods. With the FeOOH overlayer as the cocatalyst, the water oxidation photocurrent of hematite increased by a factor of approximately two and the onset potential shifted in the cathodic direction by 0.12 V under 1 sun illumination. The enhanced performance was attributed to the improved water oxidation kinetics and the passivation of the surface states of the hematite.

Phosphorus-Modified Palladium and Tungsten Carbide/Mesoporous Carbon Composite for Hydrogen Oxidation Reaction of Proton Exchange Membrane Fuel Cells.

A composite material of tungsten carbide and mesoporous carbon was synthesized by the sol-gel polycondensation of resorcinol and formaldehyde, using cetyltrimethylammonium bromide as a surfactant and Ludox HS-40 as a porogen, and served as a support for Pd-based electrodes. Phosphorus-modified Pd particles were deposited onto the support using an NH3-mediated polyol reduction method facilitated by sodium hypophosphite. Remarkably small Pd nanoparticles with a diameter of ca. 4 nm were formed by the phosphorus modification. Owing to the high dispersion of Pd and its strong interaction with tungsten carbide, the Pd nanoparticles embedded in the tungsten carbide/mesoporous carbon composite exhibited a hydrogen oxidation activity approximately twice as high as that of the commercial Pt/C catalyst under the anode reaction conditions of proton exchange membrane fuel cells.

Exploring the source of ammonia generation in electrochemical nitrogen reduction using niobium nitride.

In this study, niobium nitride (NbN) is prepared via the urea-glass route by annealing a mixture of NbCl5 and urea at 650 °C under a flow of N2, and is used as a catalyst for the electrochemical nitrogen reduction reaction (NRR). The as-prepared NbN exhibits a maximum production rate of 5.46 × 10-10 mol s-1 cm-2 at -0.6 V vs. RHE, along with an apparent FE of 16.33% at -0.3 V vs. RHE. In addition, the leaching of NbN is confirmed by ICP-OES, where the leached amount of Nb is almost identical to the amount of N measured by UV-vis. Moreover, 1H NMR experiments are performed using 15N2 as the feeder gas; the dominant detection of 14NH4+ peaks strongly suggests that the produced NH3 originates from the leaching of NbN rather than via an electrocatalytic process. Hence, for a comprehensive understanding of NH3 generation, especially when utilizing transition metal nitride (TMN)-based NRR catalysts, a thorough investigation employing multiple analytical methods is imperative.

Preparation and characterization of zwitterion-substituted lignin/Nafion composite membranes.

In this study, a novel zwitterion-substituted lignin (ZL) containing amino and sulfonic acid groups was synthesized, and ZL/Nafion composite membranes were fabricated as proton exchange membranes. Kraft lignin was modified using an aminosilane and 1,3-propanesultone via a continuous grafting reaction to provide zwitterionic moieties. Chemical structural analyses confirmed the successful introduction of the zwitterion moiety into lignin. In particular, the surface charge of ZL is positive in an acidic medium and negative in a basic medium, suggesting that ZL is a zwitterionic material. ZL was incorporated into a Nafion membrane to enhance its ion exchange capacity, thermal stability, and hydrophilicity. The proton conductivity of ZL/Nafion 0.5 %, 151.0 mS/cm, was 55.3 % higher than that of unmodified ML (methanol-soluble lignin)/Nafion 0.5 % (97.2 mS/cm), indicating that the zwitterion moiety of ZL enhances the proton transport ability. In addition, oxidative stability evaluation confirmed that ZL/Nafion 2 % was chemically more durable than pure Nafion. This confirmed that using lignin as a membrane additive yielded positive results in terms of chemical durability and oxidation stability in Nafion. Therefore, ZL is expected to be utilized as a multifunctional additive and exhibits the potential for fuel cell applications.

Molybdenum Carbide Anchored on N,S Co-Doped Carbon Composite Derived from Lignosulfonate as a High Performance Electrocatalyst for Hydrogen Evolution Reaction.

A composite of Mo2C nanoparticles dispersed onto a nitrogen and sulfur co-doped carbon scaffold (Mo2C/N,S-C) was prepared by a simple and environmentally friendly method of one-pot annealing of MoCl5, urea, and lignosulfonate under a N2 atmosphere at 700 °C. Lignosulfonate, a by-product of the sulfite pulping process, was employed as a feedstock to fabricate the S-doped carbon scaffold and carbide simultaneously, and urea acted as a nitrogen source for N-doping to carbon. The as-prepared Mo2C/N,S-C catalyst showed high performance for the hydrogen evolution reaction (HER), with a small overpotential of 105 mV at 10 mAcm-2, and good stability for 3000 cycles. The improved HER performance of the Mo2C/N,S-C originated from the interplay between the highly active Mo2C nanoparticles and the N,S co-doped carbon scaffold with its high electrical conductivity and large surface area. Furthermore, N,S co-doping to carbon improved the hydrophilicity of the catalyst surface, thus further enhancing the HER activity.

Nanostructured Iron Sulfide/N, S Dual-Doped Carbon Nanotube-Graphene Composites as Efficient Electrocatalysts for Oxygen Reduction Reaction.

Nanostructured FeS dispersed onto N, S dual-doped carbon nanotube-graphene composite support (FeS/N,S:CNT-GR) was prepared by a simple synthetic method. Annealing an ethanol slurry of Fe precursor, thiourea, carbon nanotube, and graphene oxide at 973 K under N2 atmosphere and subsequent acid treatment produced FeS nanoparticles distributed onto the N, S-doped carbon nanotube-graphene support. The synthesized FeS/N,S:CNT-GR catalyst exhibited significantly enhanced electrochemical performance in the oxygen reduction reaction (ORR) compared with bare FeS, FeS/N,S:GR, and FeS/N,S:CNT with a small half-wave potential (0.827 V) in an alkaline electrolyte. The improved ORR performance, comparable to that of commercial Pt/C, could be attributed to synergy between the small FeS nanoparticles with a high activity and the N, S-doped carbon nanotube-graphene composite support providing high electrical conductivity, large surface area, and additional active sites.

Direct One-Step Growth of Bimetallic Ni2Mo3N on Ni Foam as an Efficient Oxygen Evolution Electrocatalyst.

A simple and economical synthetic route for direct one-step growth of bimetallic Ni2Mo3N nanoparticles on Ni foam substrate (Ni2Mo3N/NF) and its catalytic performance during an oxygen evolution reaction (OER) are reported. The Ni2Mo3N/NF catalyst was obtained by annealing a mixture of a Mo precursor, Ni foam, and urea at 600 °C under N2 flow using one-pot synthesis. Moreover, the Ni2Mo3N/NF exhibited high OER activity with low overpotential values (336.38 mV at 50 mA cm-2 and 392.49 mV at 100 mA cm-2) and good stability for 5 h in Fe-purified alkaline electrolyte. The Ni2Mo3N nanoparticle surfaces converted into amorphous surface oxide species during the OER, which might be attributed to the OER activity.

Electrode Degradation in Lithium-Ion Batteries.

Although Li-ion batteries have emerged as the battery of choice for electric vehicles and large-scale smart grids, significant research efforts are devoted to identifying materials that offer higher energy density, longer cycle life, lower cost, and/or improved safety compared to those of conventional Li-ion batteries based on intercalation electrodes. By moving beyond intercalation chemistry, gravimetric capacities that are 2-5 times higher than that of conventional intercalation materials (e.g., LiCoO2 and graphite) can be achieved. The transition to higher-capacity electrode materials in commercial applications is complicated by several factors. This Review highlights the developments of electrode materials and characterization tools for rechargeable lithium-ion batteries, with a focus on the structural and electrochemical degradation mechanisms that plague these systems.

Cu4SnS4-Rich Nanomaterials for Thin-Film Lithium Batteries with Enhanced Conversion Reaction.

Through a simple gelation-solvothermal method with graphene oxide as the additive, a Cu4SnS4-rich composite of nanoparticles and nanotubes is synthesized and applied for thin and flexible Li-metal batteries. Unlike the Cu2SnS3-rich electrode, the Cu4SnS4-rich electrode cycles stably with an enhanced conversion capacity of ∼416 mAh g-1 (∼52% of total capacity) after 200 cycles. The lithiation/delithiation mechanisms of Cu-Sn-S electrodes and the voltage ranges of conversion and alloying reactions are informed by in situ X-ray diffraction tests. The conversion process of three Cu-Sn-S compounds is compared by density functional theory (DFT) calculations based on three algorithms, elucidating the enhanced conversion stability and superior diffusion kinetics of Cu4SnS4 electrodes. The reaction pathway of Cu-Sn-S electrodes and the root cause for the unstable capacity are revealed by in situ/ex situ characterizations, DFT calculations, and various electrochemical tests. This work provides insight into developing energy materials and power devices based on multiple lithiation mechanisms.

Self-Assembled Cu-Sn-S Nanotubes with High (De)Lithiation Performance.

Through a gelation-solvothermal method without heteroadditives, Cu-Sn-S composites self-assemble to form nanotubes, sub-nanotubes, and nanoparticles. The nanotubes with a Cu3-4SnS4 core and Cu2SnS3 shell can tolerate long cycles of expansion/contraction upon lithiation/delithiation, retaining a charge capacity of 774 mAh g-1 after 200 cycles with a high initial Coulombic efficiency of 82.5%. The importance of the Cu component for mitigation of the volume expansion and structural evolution upon lithiation is informed by density functional theory calculations. The self-generated template and calculated results can inspire the design of analogous Cu-M-S (M = metal) nanotubes for lithium batteries or other energy storage systems.

Facile Synthesis of Ge/N-Doped Carbon Spheres with Varying Nitrogen Content for Lithium Ion Battery Anodes.

The simple fabrication of composites of germanium nanoparticles dispersed on nitrogen-doped carbon nanospheres (Ge/NC) of varying nitrogen content and their performance in lithium ion battery anodes are reported. A heavily nitrogen-doped carbon gel was formed by condensing m-phenylenediamine with formaldehyde (PF-gel); a less heavily N-doped gel was formed by condensing resorcinol and m-phenylenediamine with formaldehyde (RPF-gel); and an undoped gel was formed by condensing resorcinol with formaldehyde (RF-gel). Pyrolises of the gels with GeCl4 at 750 °C produced nanocrystalline Ge composites with 7.5 atom % N-doped carbon, termed Ge/NC (PF), with 3.9% N-doped carbon, termed Ge/NC (RPF) and undoped carbon, termed Ge/C (RF). The heavily N-doped Ge/NC (PF) anode retained a reversible capacity of 684 mAhg-1 at a specific current of 0.2 Ag-1 after 200 cycles, versus 337 mAhg-1 retained by anode made with Ge/NC (RPF) and 278 mAhg-1 retained by anode made with undoped Ge/C (RF). At a specific current 2.0 Ag-1, the capacity of the Ge/NC (PF) anode was 472 mAhg-1, versus the 210 mAhg-1 of the Ge/NC (RPF) anode and 83 mAhg-1 of the Ge/C (RF) anode. The enhanced performance of the Ge/NC (PF) anode is attributed to the better electrical conductivity of Ge/NC (PF) and to the higher density of Li+ binding defects in its N-doped carbon.

A highly active and stable palladium catalyst on a g-C3N4 support for direct formic acid synthesis under neutral conditions.

Graphitic carbon nitride (g-C3N4) is applied as a support of the Pd catalyst for direct HCOOH synthesis by CO2 hydrogenation under neutral conditions. The high CO2 affinity of g-C3N4 is responsible for the enhanced catalytic activity and stability relative to the inert support such as a carbon nanotube.

Simple Synthesis of Nanocrystalline Tin Sulfide/N-Doped Reduced Graphene Oxide Composites as Lithium Ion Battery Anodes.

Composites of nitrogen-doped reduced graphene oxide (NRGO) and nanocrystalline tin sulfides were synthesized, and their performance as lithium ion battery anodes was evaluated. Following the first cycle the composite consisted of Li2S/LixSn/NRGO. The conductive NRGO cushions the stress associated with the expansion of lithiation of Sn, and the noncycling Li2S increases the residual Coulombic capacity of the cycled anode because (a) Sn domains in the composite formed of unsupported SnS2 expand only by 63% while those in the composite formed of unsupported SnS expand by 91% and (b) Li percolates rapidly at the boundary between the Li2S and LixSn nanodomains. The best cycling SnS2/NRGO-derived composite retained a specific capacity of 562 mAh g-1 at the 200th cycle at 0.2 A g-1 rate.

Nanostructure-Preserved Hematite Thin Film for Efficient Solar Water Splitting.

High-temperature annealing above 700 °C improves the activity of photoelectrochemical water oxidation by hematite photoanodes by increasing its crystallinity. Yet, it brings severe agglomeration of nanostructured hematite thin films and deteriorates electrical conductivity of the transparent conducting oxide (TCO) substrate. We report here that the nanostructure of the hematite and the conductivity of TCO could be preserved, while the high crystallinity is attained, by hybrid microwave annealing (HMA) utilizing a graphite susceptor for efficient microwave absorption. Thus, the hematite thin-film photoanodes treated by HMA record 2 times higher water oxidation photocurrents compared to a conventional thermal-annealed photoanode. The enhanced performance can be attributed to the synergistic effect of a smaller feature size of nanostructure-preserved hematite and a good electrical conductivity of TCO. The method could be generally applied to the fabrication of efficient photoelectrodes with small feature sizes and high crystallinity, which have been mutually conflicting requirements with conventional thermal annealing processes.

Tree branch-shaped cupric oxide for highly effective photoelectrochemical water reduction.

Highly efficient tree branch-shaped CuO photocathodes are fabricated using the hybrid microwave annealing process with a silicon susceptor within 10 minutes. The unique hierarchical, one-dimensional structure provides more facile charge transport, larger surface areas, and increased crystallinity and crystal ordering with less defects compared to irregular-shaped CuO prepared by conventional thermal annealing. As a result, the photocathode fabricated with the tree branch-shaped CuO produces an unprecedently high photocurrent density of -4.4 mA cm(-2) at 0 VRHE under AM 1.5 G simulated sunlight compared to -1.44 mA cm(-2) observed for a photocathode fabricated by thermal annealing. It is also confirmed that stoichiometric hydrogen and oxygen are produced from photoelectrochemical water splitting on the tree branch-shaped CuO photocathode and a platinum anode.

Highly active and stable hydrogen evolution electrocatalysts based on molybdenum compounds on carbon nanotube-graphene hybrid support.

Highly active and stable electrocatalysts for hydrogen evolution have been developed on the basis of molybdenum compounds (Mo2C, Mo2N, and MoS2) on carbon nanotube (CNT)-graphene hybrid support via a modified urea-glass route. By a simple modification of synthetic variables, the final phases are easily controlled from carbide, nitride to sulfide with homogeneous dispersion of nanocrystals on the CNT-graphene support. Among the prepared catalysts, Mo2C/CNT-graphene shows the highest activity for hydrogen evolution reaction with a small onset overpotential of 62 mV and Tafel slope of 58 mV/dec as well as an excellent stability in acid media. Such enhanced catalytic activity may originate from its low hydrogen binding energy and high conductivity. Moreover, the CNT-graphene hybrid support plays crucial roles to enhance the activity of molybdenum compounds by alleviating aggregation of the nanocrystals, providing a large area to contact with electrolyte, and facilitating the electron transfer.

Fabrication of graphene-based electrode in less than a minute through hybrid microwave annealing.

Highly efficient and stable MoS2 nanocrystals on graphene sheets (MoS2/GR) are synthesized via a hybrid microwave annealing process. Through only 45 second-irradiation using a household microwave oven equipped with a graphite susceptor, crystallization of MoS2 and thermal reduction of graphene oxide into graphene are achieved, indicating that our synthetic method is ultrafast and energy-economic. Graphene plays a crucial role as an excellent microwave absorber as well as an ideal support material that mediates the growth of MoS2 nanocrystals. The formed MoS2/GR electrocatalyst exhibits high activity of hydrogen evolution reaction with small onset overpotential of 0.1 V and Tafel slope of 50 mV per decade together with an excellent stability in acid media. Thus our hybrid microwave annealing could be an efficient generic method to fabricate various graphene-based hybrid electric materials for broad applications.

TiN nanoparticles on CNT-graphene hybrid support as noble-metal-free counter electrode for quantum-dot-sensitized solar cells.

The development of an efficient noble-metal-free counter electrode is crucial for possible applications of quantum-dot-sensitized solar cells (QDSSCs). Herein, we present TiN nanoparticles on a carbon nanotube (CNT)-graphene hybrid support as a noble-metal-free counter electrode for QDSSCs employing a polysulfide electrolyte. The resulting TiN/CNT-graphene possesses an extremely high surface roughness, a good metal-support interaction, and less aggregation relative to unsupported TiN; it also has superior solar power conversion efficiency (4.13 %) when applying a metal mask, which is much higher than that of the state-of-the-art Au electrode (3.35 %). Based on electrochemical impedance spectroscopy measurements, the enhancement is ascribed to a synergistic effect between TiN nanoparticles and the CNT-graphene hybrid, the roles of which are to provide active sites for the reduction of polysulfide ions and electron pathways to TiN nanoparticles, respectively. The combination of graphene and CNTs leads to a favorable morphology that prevents stacking of graphene or bundling of CNTs, which maximizes the contact of the support with TiN nanoparticles and improves electron-transfer capability relative to either carbon material alone.