General Pyrolysis for High-Loading Transition Metal Single Atoms on 2D-Nitro-Oxygeneous Carbon as Efficient ORR Electrocatalysts.

Single-atom catalysts (SACs) possess the potential to involve the merits of both homogeneous and heterogeneous catalysts altogether and thus have gained considerable attention. However, the large-scale synthesis of SACs with rich isolate-metal sites by simple and low-cost strategies has remained challenging. In this work, we report a facile one-step pyrolysis that automatically produces SACs with high metal loading (5.2-15.9 wt %) supported on two-dimensional nitro-oxygenated carbon (M1-2D-NOC) without using any solvents and sacrificial templates. The method is also generic to various transition metals and can be scaled up to several grams based on the capacity of the containers and furnaces. The high density of active sites with N/O coordination geometry endows them with impressive catalytic activities and stability, as demonstrated in the oxygen reduction reaction (ORR). For example, Fe1-2D-NOC exhibits an onset potential of 0.985 V vs RHE, a half-wave potential of 0.826 V, and a Tafel slope of -40.860 mV/dec. Combining the theoretical and experimental studies, the high ORR activity could be attributed its unique FeO-N3O structure, which facilitates effective charge transfer between the surface and the intermediates along the reaction, and uniform dispersion of this active site on thin 2D nanocarbon supports that maximize the exposure to the reactants.

Tunable Metal-Free Imidazole-Benzimidazole Electrocatalysts for Oxygen Reduction in Aqueous Solutions.

A series of metal-free imidazole-benzimidazole catalysts (ImBenz-H, ImBenz-NO2 , ImBenz-OCH3 ) for oxygen reduction reaction (ORR) were prepared. We demonstrate that the electrocatalytic O2 reduction by ImBenz-NO2 with the electron-withdrawing group showed high selectivity toward H2 O with the number of electrons transferred (n=3.7) in a neutral aqueous solution. The highest ORR selectivity toward H2 O2 was achieved using ImBenz-H (n=2.4) in an alkaline solution. Electrochemical studies of reaction kinetics disclosed that the highest turnover frequencies were obtained from ImBenz-H in both neutral and alkaline aqueous solutions. The results prove that the ORR selectivity is tunable by modulating the substituent of the ImBenz catalysts. Furthermore, DFT calculations suggested that the ORR mechanism of ImBenz-H involves the electron transfer from imidazole-benzimidazole to O2 resulting in the formation of H2 O2 which supports the redox active properties of the catalysts ImBenz.

New Folding 2D-Layered Nitro-Oxygenated Carbon Containing Ultra High-Loading Copper Single Atoms.

The discoveries of 2D nanomaterials have made huge impacts on the scientific community. Their unique properties unlock new technologies and bring significant advances to diverse applications. Herein, an unprecedented 2D-stacked material consisting of copper (Cu) on nitro-oxygenated carbon is disclosed. Unlike any known 2D stacked structures that are usually constructed by stacking of separate 2D layers, this material forms a continuously folded 2D-stacked structure. Interestingly, advanced characterizations indicate that Cu atoms inside the structure are in an atomically-dispersed form with extraordinarily high Cu loading up to 15.9 ± 1.2 wt.%, which is among the highest reported metal loading for single-atom catalysts on 2D supports. Facile exfoliation results in thin 2D nanosheets that maximize the exposure of the unique active sites (two neighboring Cu single atoms), leading to impressive catalytic performance, as demonstrated in the electrochemical oxygen reduction reaction.

Conductive Co-triazole metal-organic framework exploited as an oxygen evolution electrocatalyst.

A Co-triazole metal-organic framework (Co-trz) endowed with electrical conductivity was synthesized effortlessly via a microwave-based method. Providing a high density of catalytic centers with electrically conductive features, as suggested by DFT calculations, the framework exhibited a low overpotential for the oxygen evolution reaction (OER) with good kinetics. A mechanistic reaction pathway was proposed based on monitoring alterations in the oxidation state and local coordination environment of Co centers upon the occurrence of the OER. Due to its performance and its chemical and electrochemical robustness, the framework was highlighted as a promising MOF electrocatalyst for the OER.

Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide.

Electrochemical production of hydrogen peroxide (H2O2) from water oxidation could provide a very attractive route to locally produce a chemically valuable product from an abundant resource. Herein using density functional theory calculations, we predict trends in activity for water oxidation towards H2O2 evolution on four different metal oxides, i.e., WO3, SnO2, TiO2 and BiVO4. The density functional theory predicted trend for H2O2 evolution is further confirmed by our experimental measurements. Moreover, we identify that BiVO4 has the best H2O2 generation amount of those oxides and can achieve a Faraday efficiency of about 98% for H2O2 production.Producing hydrogen peroxide via electrochemical oxidation of water is an attractive route to this valuable product. Here the authors theoretically and experimentally investigate hydrogen peroxide production activity trends for a range of metal oxides and identify the optimal bias ranges for high Faraday efficiencies.

Tandem Core-Shell Si-Ta3N5 Photoanodes for Photoelectrochemical Water Splitting.

Nanostructured core-shell Si-Ta3N5 photoanodes were designed and synthesized to overcome charge transport limitations of Ta3N5 for photoelectrochemical water splitting. The core-shell devices were fabricated by atomic layer deposition of amorphous Ta2O5 onto nanostructured Si and subsequent nitridation to crystalline Ta3N5. Nanostructuring with a thin shell of Ta3N5 results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta3N5 shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta3N5 shells, indicating minority carrier diffusion lengths on the order of tens of nanometers. The fabrication of a heterostructure based on a semiconducting, n-type Si core produced a tandem photoanode with a photocurrent onset shifted to lower potentials by 200 mV. CoTiOx and NiOx water oxidation cocatalysts were deposited onto the Si-Ta3N5 to yield active photoanodes that with NiOx retained 50-60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable Ta3N5 photoanodes reported to date.

Materials for solar fuels and chemicals.

The conversion of sunlight into fuels and chemicals is an attractive prospect for the storage of renewable energy, and photoelectrocatalytic technologies represent a pathway by which solar fuels might be realized. However, there are numerous scientific challenges in developing these technologies. These include finding suitable materials for the absorption of incident photons, developing more efficient catalysts for both water splitting and the production of fuels, and understanding how interfaces between catalysts, photoabsorbers and electrolytes can be designed to minimize losses and resist degradation. In this Review, we highlight recent milestones in these areas and some key scientific challenges remaining between the current state of the art and a technology that can effectively convert sunlight into fuels and chemicals.

Mapping Photoelectrochemical Current Distribution at Nanoscale Dimensions on Morphologically Controlled BiVO4.

We develop a method that can be used to qualitatively map photocurrent on photoelectrode surfaces, and show its utility for morphologically controlled W-doped BiVO4. The method is based on the deliberate photoinduced sintering of Au NPs, a photon-driven process that indicates oxidation with nanoscale-resolution. This strategy allows us to identify the active regions on W-doped BiVO4 photoelectrodes, and we observe a strong dependence of photoactivity on the electrode morphology, controlled by varying the relative humidity during the sol-gel fabrication process. We find that photoelectrode morphologies that exhibit the most evenly distributed Au sintering are those that yield the highest photoelectrochemical (PEC) activity. Understanding the correlation between electrode morphology and PEC activity is essential for designing structured semiconductors for PEC water splitting.