Crystal Engineering of MOF-Derived Bimetallic Oxide Solid Solution Anchored with Au Nanoparticles for Photocatalytic CO2 Reduction to Syngas and C2 Hydrocarbons.

Considering that CO2 reduction is mostly a multielectron reaction, it is necessary for the photocatalysts to integrate multiple catalytic sites and cooperate synergistically to achieve efficient photocatalytic CO2 reduction to various products, such as C2 hydrocarbons. Herein, through crystal engineering, we designed and constructed a metal-organic framework-derived Zr/Ti bimetallic oxide solid solution support, which was confirmed by X-ray diffraction, electron microscopy and X-ray absorption spectroscopy. After anchoring Au nanoparticles, the composite photocatalyst exhibited excellent performances toward photocatalytic CO2 reduction to syngas (H2 and CO production rates of 271.6 and 260.6 µmol g-1 h-1) and even C2 hydrocarbons (C2H4 and C2H6 production rates of 6.80 and 4.05 µmol g-1 h-1). According to the control experiments and theoretical calculations, the strong interaction between bimetallic oxide solid solution support and Au nanoparticles was found to be beneficial for binding intermediates and reducing CO2 reduction, highlighting the synergy effect of the catalytic system with multiple active sites.

Highly sensitive surface acoustic wave biosensor for the detection of Hg2+ based on the thymine-Hg2+-thymine structure.

The detection of mercury ions (Hg2+) with an ultralow concentration is of great significance for the treatment of heavy metal pollution in industrial waste water and the monitoring of drinking water quality. In this study, a simple, sensitive, and portable surface acoustic wave (SAW) biosensor for the determination of Hg2+ concentration was developed. First, a DNA fragment with a specific binding capacity for Hg2+ was chemically adsorbed on to the sensitive region through forming Au-S bonds with the SAW biosensor. The DNA probe could then form a thymine-Hg2+-thymine (T-Hg2+-T) complex with strong affinity for Hg2+ after immersion in a test solution containing Hg2+, resulting in a significant change in the response frequency of the device. The linear detection range of the device was 10 pM to 1 nM, while the limit of detection (LOD) was as low as 6.3 pM. Furthermore, the SAW biosensor exhibited excellent selectivity to Hg2+ compared with that of interfering ions, e.g., Ag+, Ba2+, Cu2+, Zn2+, Mn2+, Fe3+, Ca2+, and Na+. The results provide a new strategy for the preparation of portable devices that can monitor toxic heavy metal ions with high sensitivity and selectivity.

Two-Dimensional Graphitic Carbon Nitride for Improving the Performance of Organic Solar Cells.

Organic solar cells (OSCs) have attracted lots of attention owing to their low cost, lightweight, and flexibility properties. Nowadays, the performance of OSCs is continuously improving with the development of active layer materials. However, the traditional hole transport layer (HTL) material Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) presents insufficient conductivity and rapid degradation, which decreases the efficiency and stability of OSCs. To conquer the challenge, the two-dimensional (2D) graphitic carbon nitride (g-C3N4) nanomaterials incorporated into the PEDOT:PSS as hybrid HTL are reported. The addition of g-C3N4 into PEDOT:PSS enables the thickness of the HTL to decrease for enhancing the transmittance of the film and increase the conductivity of PEDOT:PSS. Thus, the device exhibts improved charge transport and suppressed carrier recombination, leading to the increase in short-circuit current density and power conversion efficiency of the devices. This work demonstrates that the incorporation of 2D g-C3N4 into PEDOT:PSS for D18:Y6 and PM6:L8-BO-based OSCs can significantly improve the device efficiency to 17.48% and 18.47% with the enhancement of 7.04% and 8.46%, respectively.

Electrochemical Preparation of Crystalline Hydrous Iridium Oxide and Its Use in Oxygen Evolution Catalysis.

Even the most stable Ir-based oxides inevitably encounter a severe degradation problem during the oxygen evolution reaction (OER) in acid, resulting in quick formation of amorphous IrOx layers on the catalyst surface. Unfortunately, there is still a lack of fundamental understanding of such hydrous IrOx layers, including the atomic arrangement, key active structure, compositions, chemical stability, and so on. In this work, we demonstrate an electrochemical strategy to prepare two types of protonated iridium oxides with well-defined crystalline structures: one possesses a 2D layered structure (denoted as α-HxIrO3) and the other consists of 3D interconnected polymorphs (denoted as ß-HxIrO3). Both protonated iridium oxides demonstrate superior electrochemical stabilities with 6 times suppressed Ir dissolution comparing to the initial Li2IrO3 and rutile IrO2. It is hypothesized that the enriched protons and fast diffusions in these two protonated HxIrO3 crystal oxides may promote surface structural stability by suppressing the formation of high-valence Ir species at the solid-liquid interfaces during OER. Overall, the results of this work shed light on the role of proton dynamics toward the OER processes on the catalyst surface in acid media.

Chloride removal from flue gas desulfurization wastewater through Friedel's salt precipitation method: A review.

As a high efficiency method for chloride removal, Friedel's salt precipitation (FSP) method has attracted much attention in zero liquid discharge (ZLD) of flue gas desulfurization (FGD) wastewater. This review provides comprehensive knowledge of FSP method for chloride removal through analysis of the evolution, reaction mechanisms and influential factors, and describes the recent research progress. FSP method is a cost-efficient technology to remove chloride from saline wastewater by adding lime and aluminate. Chloride ions react with the precipitants by adsorption or/and ion exchange to form Friedel's salt, which is affected by the reaction conditions including reaction time, temperature, interferential ions, etc. The effluent of this process can be reused as the makeup water of desulfurization tower, and the dechloridation precipitates can be reclaimed as adsorption materials and sludge conditioners. That can not only offset a fraction of the treatment cost, but also avoid secondary pollution, so ZLD of FGD wastewater can be achieved. This paper summarizes the deficiencies and potential improvement measures of FSP method. We believe this technology is a promising way to achieve ZLD of FGD wastewater and other wastewater containing chloride, and expect FSP method would become more mature and be widely applied in hypersaline wastewater treatment in the foreseeable future.

Investigation of the electrocatalytic mechanisms of urea oxidation reaction on the surface of transition metal oxides.

Urea oxidation reaction (UOR) has been widely considered as an alternative anodic reaction to water oxidation for the green production of hydrogen fuel. Due to the high catalytic activity of transition metal oxides towards UOR, various strategies have been developed to improve their syntheses and catalytic properties. However, little is known about the underlying mechanisms of UOR on catalyst surface. In this work, three transition metal oxides, including NiO, Co3O4, and Fe2O3 are investigated as model catalysts. Through analyzing the electrochemical properties by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and operando Raman spectroscopy, it is revealed that NiO has a unique high catalytic activity towards UOR due to simultaneous formation of a thin layer of oxyhydroxide species above 1.40 V vs. RHE in alkaline media. In addition, density functional theory (DFT) calculations further suggest that the adsorption of urea molecules is largely affected by surface interactions resulting in different space configurations, which impose large influences on the consecutive deprotonation and NN formation processes. Overall, results of this work point to the subtle adsorption - kinetics relationship in UOR and highlight the importance of the interfacial electronic interactions on catalyst surface.

Enhanced light harvesting and charge separation of carbon and oxygen co-doped carbon nitride as excellent photocatalyst for hydrogen evolution reaction.

Solar-driven water splitting has been regarded as a promising strategy for renewable hydrogen production. Among many semiconductor photocatalysts, graphitic carbon nitride (g-C3N4) has received tremendous attention due to its two-dimensional structure, appropriate band gap and decent photocatalytic activity. However, it suffers severe charge recombination problems, affecting its practical performance. In this work, we demonstrated that dual heteroatoms (C and O) doped g-C3N4 can exhibit about 3 times higher catalytic performance for hydrogen evolution than that of the normal g-C3N4 with a hydrogen evolution rate reaching 2595.4 umol g-1h-1 and an apparent quantum efficiency at 420 nm of 16.6%. The heteroatoms (C and O) doped g-C3N4 photocatalyst also exhibited superior removal performance when removing Rhodamine B (RhB) . X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (ssNMR) and X-ray absorption near-edge structure (XANES) spectroscopy reveal that the carbon and oxygen dopants replace the sp2 nitrogen and bridging N atom, respectively. DFT calculations demonstrate the codoping of carbon and oxygen- induced the generation of mid-gap state, leading to the improvement of light harvesting and charge separation.

Sulfate modified g-C3N4 with enhanced photocatalytic activity towards hydrogen evolution: the role of sulfate in photocatalysis.

Sulfate modified graphitic carbon nitride (g-C3N4) was prepared by simple co-pyrolysis of dicyandiamide and ammonium sulfate, and shows seven times higher photocatalytic activity towards hydrogen production than pristine g-C3N4. The origin of its improved photocatalytic activity was comprehensively investigated, and it was found that there are two kinds of sulfate (strongly adsorbed sulfate and a weakly adsorbed one) in the modified sample, both of which play important but slightly different roles in the photocatalysis. Compared to the strongly adsorbed one, the weakly adsorbed sulfate is more beneficial for charge separation and thus promotes more electrons to participate in the photocatalytic reaction. By applying the above synthesis method, most sulfate in our best photocatalyst exists as weakly adsorbed species, which is confirmed by advanced characterization techniques as well as DFT calculations. The increased number of electrons and improved charge separation, which are induced by the weakly adsorbed sulfate, are key to boosting the photocatalytic activity of g-C3N4. Hence, this work provides comprehensive insights into the effect of sulfate on the photocatalytic activity of g-C3N4, which help in the design of more efficient photocatalysts by suitable surface modification.

Cation insertion to break the activity/stability relationship for highly active oxygen evolution reaction catalyst.

The production of hydrogen at a large scale by the environmentally-friendly electrolysis process is currently hampered by the slow kinetics of the oxygen evolution reaction (OER). We report a solid electrocatalyst α-Li2IrO3 which upon oxidation/delithiation chemically reacts with water to form a hydrated birnessite phase, the OER activity of which is five times greater than its non-reacted counterpart. This reaction enlists a bulk redox process during which hydrated potassium ions from the alkaline electrolyte are inserted into the structure while water is oxidized and oxygen evolved. This singular charge balance process for which the electrocatalyst is solid but the reaction is homogeneous in nature allows stabilizing the surface of the catalyst while ensuring stable OER performances, thus breaking the activity/stability tradeoff normally encountered for OER catalysts.

An all-nanosheet OER/ORR bifunctional electrocatalyst for both aprotic and aqueous Li-O2 batteries.

Rechargeable lithium-oxygen (Li-O2) batteries are receiving intense interest because of their high energy density. A highly efficient catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a key factor influencing the performance of Li-O2 batteries. In this work, a facile synthesis of an all-nanosheet architecture electrocatalyst constructed from a monolayer ruthenium dioxide (RuO2) nanosheet with a nitrogen doped sulfonated graphene nanosheet (RuO2-NS-GNS) has been developed for Li-O2 batteries. This complex catalyst displays excellent activity towards the ORR and OER in both aprotic and aqueous Li-O2 batteries. A low overpotential around 1.0 V during the discharge/recharge process is obtained for the aprotic Li-O2 battery with RuO2-NS-GNS. Meanwhile, linear sweep voltammetry curves show that the OER and ORR potentials are 1.45 V and 0.81 V in an alkaline solution (1 M LiOH-5 M LiNO3) for RuO2-NS-GNS, respectively. Both aprotic and aqueous Li-O2 batteries with RuO2-NS-GNS exhibit stable cyclability and low round-trip overpotential without obvious degradation at a limited specific capacity of 1000 mA g-1. The advanced electrochemical performance of RuO2-NS-GNS in both aprotic and aqueous Li-O2 batteries can be attributed to the increased catalytic sites and synergistic effect arising from RuO2 and NS-GNS nanosheets.

Determining the Facile Routes for Oxygen Evolution Reaction by In Situ Probing of Li-O2 Cells with Conformal Li2O2 Films.

An ongoing challenge with lithium-oxygen (Li-O2) batteries is in deciphering the oxygen evolution reaction (OER) process related to the slow decomposition of the insulating lithium peroxide (Li2O2). Herein, we shed light on the behavior of Li2O2 oxidation by exploiting various in situ imaging, gas analysis, and electrochemical methods. At the low potentials 3.2-3.7 V (vs Li/Li+), OER is exclusive to the thinner parts of the Li2O2 deposits, which leads to O2 gas evolution, followed by the concomitant release of superoxide species. At higher potentials, OER initiates at the sidewalls of the thicker Li2O2. The succeeding lateral decomposition of Li2O2 indicates the preferential Li+ and charge transport occurring at the sidewalls of Li2O2. To ameliorate the OER rate, we also investigate an alternative approach of rerouting charge carriers by using soluble redox mediators. Our in situ probes provide insights into the favorable charge-transport routes that can aid in promoting Li2O2 decomposition.

Phosphate Ion Functionalization of Perovskite Surfaces for Enhanced Oxygen Evolution Reaction.

Recent findings revealed that surface oxygen can participate in the oxygen evolution reaction (OER) for the most active catalysts, which eventually triggers a new mechanism for which the deprotonation of surface intermediates limits the OER activity. We propose in this work a "dual strategy" in which tuning the electronic properties of the oxide, such as La1-xSrxCoO3-δ, can be dissociated from the use of surface functionalization with phosphate ion groups (Pi) that enhances the interfacial proton transfer. Results show that the Pi functionalized La0.5Sr0.5CoO3-δ gives rise to a significant enhancement of the OER activity when compared to La0.5Sr0.5CoO3-δ and LaCoO3. We further demonstrate that the Pi surface functionalization selectivity enhances the activity when the OER kinetics is limited by the proton transfer. Finally, this work suggests that tuning the catalytic activity by such a "dual approach" may be a new and largely unexplored avenue for the design of novel high-performance catalysts.

Chemical Recognition of Active Oxygen Species on the Surface of Oxygen Evolution Reaction Electrocatalysts.

Owing to the transient nature of the intermediates formed during the oxygen evolution reaction (OER) on the surface of transition metal oxides, their nature remains largely elusive by the means of simple techniques. The use of chemical probes is proposed, which, owing to their specific affinities towards different oxygen species, unravel the role played by these species on the OER mechanism. For that, tetraalkylammonium (TAA) cations, previously known for their surfactant properties, are introduced, which interact with the active oxygen sites and modify the hydrogen bond network on the surface of OER catalysts. Combining chemical probes with isotopic and pH-dependent measurements, it is further demonstrated that the introduction of iron into amorphous Ni oxyhydroxide films used as model catalysts deeply modifies the proton exchange properties, and therefore the OER mechanism and activity.

Chemical vs Electrochemical Formation of Li2CO3 as a Discharge Product in Li-O2/CO2 Batteries by Controlling the Superoxide Intermediate.

The Li-O2/CO2 battery with high capacity has recently been proposed as a new protocol to convert CO2. However, the fundamental mechanism for the reaction still remains hazy. Here, we investigated the discharge processes of Li-O2/CO2 (70%/30%) batteries in two solvents, dimethyl sulfoxide (DMSO) and 1,2-dimethoxyethane (DME). During discharge, both solvents initially show the reduction of oxygen. However, afterward, the solvent affects the reaction pathways of superoxide species by solvating Li+ with different strength, depending on the so-called donor number. More precisely, the initial formation of CO4•- is favored in DMSO at the expense of lithium superoxide formation that we observed in DME. Despite the different intermediate processes, X-ray diffraction showed that Li2CO3 was the final discharge product in both solvents. Moreover, we observed that CO2 cannot be reduced within the electrochemical stability window of DMSO and DME.

Unexpected Li2O2 Film Growth on Carbon Nanotube Electrodes with CeO2 Nanoparticles in Li-O2 Batteries.

In lithium-oxygen (Li-O2) batteries, it is believed that lithium peroxide (Li2O2) electrochemically forms thin films with thicknesses less than 10 nm resulting in capacity restrictions due to limitations in charge transport. Here we show unexpected Li2O2 film growth with thicknesses of ∼60 nm on a three-dimensional carbon nanotube (CNT) electrode incorporated with cerium dioxide (ceria) nanoparticles (CeO2 NPs). The CeO2 NPs favor Li2O2 surface nucleation owing to their strong binding toward reactive oxygen species (e.g., O2 and LiO2). The subsequent film growth results in thicknesses of ∼40 nm (at cutoff potential of 2.2 V vs Li/Li(+)), which further increases up to ∼60 nm with the addition of trace amounts of H2O that enhances the solution free energy. This suggests the involvement of solvated superoxide species (LiO2(sol)) that precipitates on the existing Li2O2 films to form thicker films via disproportionation. By comparing toroidal Li2O2 formed solely from LiO2(sol), the thick Li2O2 films formed from surface-mediated nucleation/thin-film growth following by LiO2(sol) deposition provides the benefits of higher reversibility and rapid surface decomposition during recharge.

Highly alloyed PtRu nanoparticles confined in porous carbon structure as a durable electrocatalyst for methanol oxidation.

The state-of-the-art carbon-supported PtRu catalysts are widely used as the anode catalysts in polymer electrolyte fuel cells (PEMFCs) but suffer from instability issues. Severe ruthenium dissolution occurring at potentials higher than 0.5 V vs NHE would result in a loss of catalytic activity of PtRu hence a worse performance of the fuel cell. In this work, we report an ultrastable PtRu electrocatalyst for methanol oxidation by confining highly alloyed PtRu nanoparticles in a hierarchical porous carbon structure. The structural characteristics, e.g., the surface composition and the morphology evolution, of the catalyst during the accelerated degradation test were characterized by the Cu-stripping voltammetry and the TEM/SEM observations. From the various characterization results, it is revealed that both the high alloying degree and the pore confinement of PtRu nanoalloys play significant roles in suppressing the degradation processes, including Ru dissolution and particle agglomeration/migration. This report provides an opportunity for efficient design and fabrication of highly stable bimetallic or trimetallic electrocatalysts in a large variety of applications.

Three-dimensional ordered macroporous MnO2/carbon nanocomposites as high-performance electrodes for asymmetric supercapacitors.

MnO2/carbon composites with ultrathin MnO2 nanofibers (diameter of 5-10 nm) uniformly deposited on three dimensional ordered macroporous (3DOM) carbon frameworks were fabricated via a self-limiting redox process. The MnO2 nanofibers provide a large surface area for charge storage, whereas the 3DOM carbon serves as a desirable supporting material providing rapid ion and electron transport through the composite electrodes. Cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) were used to characterize the capacitive performance of these composites. Optimization of the composition results in a composite with 57 wt% MnO2 content, which gives both a high specific capacitance (234 F g(-1) at a discharge current of 0.1 A g(-1)) and good rate capability (52% retention of the capacitance at 5 A g(-1)). An asymmetric supercapacitor was fabricated by assembling the optimized MnO2/carbon composite as the positive electrode and 3DOM carbon as the negative electrode. The asymmetric supercapacitor exhibits superior electrochemical performances, which can be reversibly charged and discharged at a maximum cell voltage of 2.0 V in 1.0 M Na2SO4 aqueous electrolyte, delivering both high energy density (30.2 W h kg(-1)) and power density (14.5 kW kg(-1)). Additionally, the asymmetric supercapacitor exhibits an excellent cycle life, with 95% capacitance retained after 1000 cycles.

Uniform dispersion of 1 : 1 PtRu nanoparticles in ordered mesoporous carbon for improved methanol oxidation.

PtRu nanoparticles dispersed in CMK3 mesoporous carbons have been prepared via a CPDM (carbonization over poly-furfuryl alcohol-protected dispersed mixed metals) method. The as-synthesized CMK3 supported PtRu nanoparticles are characterized using tomography and cross-sectional TEM analysis and are compared against those synthesized by the conventional ethylene glycol (EG) method. The atomic ratio of Pt : Ru, which has an essential role on methanol oxidation, is found to be consistent at the nanometer scale. The good dispersion and uniform composition of PtRu nanoparticles result in improved methanol oxidation performance including higher methanol oxidation current and long-term stability.