Amorphous MOF Introduced N‑Doped Graphene: An Efficient and Versatile Electrocatalyst for Zinc–Air Battery and Water Splitting

Recently, developing metal–organic framework (MOF) derived carbon-based electrocatalysts has become more and more popular for large-scale application of renewable energy devices. However, the rational conversion of MOFs into a versatile platform for high-efficiency catalyst is still very challenging. Moreover, the relationship between the crystallinity of MOF precursor and the catalytic activity of resultant carbon-based catalyst is still not well-understood. In this work, a strategy for the synthesis of sheet-like mesoporous nitrogen-doped graphene (MNG) derived from amorphous MOFs is demonstrated. The amorphous MOF derived MNG showed much higher catalytic activity than that of nitrogen-doped carbon (MNC) derived from highly crystallized MOFs. This rationally designed MNG catalyst served as a multifunctional electrode in a zinc–air battery and a water splitting device, both of which showed electrocatalytic performance superior to those of platinum group metal (PGM) catalysts. The characterization analysis confirmed that the enhanced activity of amorphous MOF derived MNG was primarily attributed to the optimal properties of electronic conductivity, graphitization degree, and high specific surface area

Phosphorus and Aluminum Codoped Porous NiO Nanosheets as Highly Efficient Electrocatalysts for Overall Water Splitting

We present a facile way to fabricate phosphorus and aluminum codoped nickel oxide-based nanosheets by using layered double hydroxide (AlNi-LDH) as precursors, which showed an overall water-splitting performance in alkaline solution. The codoping of phosphorus and aluminum into nickel oxide nanosheets leads to an optimum balance among surface chemical state, electrochemically active surface area, and density of active sites. As a result, it can afford a current density of 100 mA cm^–2 at the overpotential of 310 mV for oxygen evolution reaction (OER) and a current density of 10 mA cm^–2 at the overpotential of 138 mV for hydrogen evolution reaction (HER) in 1 M KOH. When it was used as a bifunctional catalyst in a two-electrode water-splitting device, a potential of 1.56 V was achieved at the current density of 10 mA cm^–2

Nickel Sulfide Freestanding Holey Films as Air-Breathing Electrodes for Flexible Zn–Air Batteries

In this work, a combination of bottom-up electrochemical deposition and top-down electrochemical etching strategies followed by a subsequent sulfuration treatment was employed to rationally synthesize a nickel sulfide (NiS_<i>x</i>) freestanding holey film (FHF). Owing to the holey structure along with the optimal electrochemically active surface area and active sites, the as-prepared NiS_<i>x</i> FHF showed an impressive bifunctional electrocatalytic performance toward both oxygen evolution and reduction reactions. The holey and freestanding features provide the NiS_<i>x</i> FHF with promising characteristics to be used as an ideal air-breathing cathode in flexible Zn–air batteries (ZABs). As a proof-of-concept, the rationally designed NiS_<i>x</i> FHF achieved remarkable rechargeability and flexibility in a ZAB configuration

DataSheet_1_Field determination of nitrate in seawater using a novel on-line coppered cadmium column: A comparison study with the vanadium reduction method.docx

Nitrate is the main form of dissolved inorganic nitrogen, playing an important role in both marine biogeochemical research and water environment management. In this work, the most commonly used coppered cadmium column was modified and a novel on-line Cu/Cd column with a spiral structure coupled with a de-bubbling device and syringe-type filter was developed. With the advantages of convenience, portability, stability, and high reduction efficiency, the interference of air bubbles in the column could easily be avoided. Based on the classic Griess reaction, a simple reverse flow injection system coupled with a novel Cu/Cd column and custom-made flow cell was established for the field spectrophotometric determination of nitrate in seawater. The effects of certain reaction parameters—including the reagent concentration, flow rate, length of the Cu/Cd column, and salinity—were investigated, optimized, and compared with pure water, with an approximate 9% increase in the sensitivity of seawater samples. This method exhibited a detection limit of 0.03 μmol/L, with a relative standard deviation of 0.6%, and the working range was 20 μmol/L before dilution. Compared to the referred vanadium reduction method based on the same flow system, the proposed method showed significant advantages including sensitivity and reproducibility. No significant difference was observed between the analytical seawater sample results obtained by the proposed and reference methods. Furthermore, the proposed method was validated by the first class of National Certified Reference Materials and successfully applied to the nitrate determination of Wenling coastal seawater (Zhejiang, China).</p

Understanding Synergism of Cobalt Metal and Copper Oxide toward Highly Efficient Electrocatalytic Oxygen Evolution

Understanding the synergism of bimetallic transition metal (TM)-based catalysts for oxygen evolution reaction (OER) is very difficult because it is complicated to identify the surface active sites in a bimetal system. Herein, we rationally designed Cu oxide (CuOx) nanoarray film (NF) as an example to investigate the synergism and doping effects of iron group metals on OER. This is an advantage because CuOx is electrocatalytically inert and oxidatively stable, which is much better than carbon-based platforms. Especially, cobalt (Co) shows a much stronger synergism as compared with nickel (Ni) and iron (Fe). By introducing Co into the inert CuOx NFs, the Co active sites can be correlated to the OER activity by rationally regulating the morphology of CuOx NFs. In addition, the phase transformation from Cu2O to CuO occurs during the OER testing, further boosting the OER activity of Co-doped CuOx NF due to the hybridization change of Co active site. As a result, the Co-doped CuOx NF with 0.30 at. % Co (denoted as Co0.30CuOx) shows a remarkable OER activity (an overpotential of 0.29 V at 10 mA cm–2) in basic solution, superior to those of the state-of-the-art OER catalysts. Both experimental and computational studies indicate that the introduction of Co-dopant in CuOx changes the rate-limiting step from M-OHads → M-Oads to M-Oads → M-OOHads and decreases the theoretical onset potential by 0.31 V. The optimal concentration of Co-dopant in CuOx nanocrystals renders the favorable surface properties for the electron transfer, the adsorption, and desorption of OER-relevant intermediates. Moreover, the small size of CuOx nanocrystals contributes to the large electrochemically active surface area, which enables the sufficient Co active sites to the electrolyte

Graphitic Nitrogen Is Responsible for Oxygen Electroreduction on Nitrogen-Doped Carbons in Alkaline Electrolytes: Insights from Activity Attenuation Studies and Theoretical Calculations

To date, controversies remain in the unambiguous identification of the active sites in N-doped carbons for oxygen reduction reaction (ORR). In the present study, prolonged potential cycling was conducted on three N-doped carbons in O₂-saturated 0.1 M KOH aqueous solution, where apparent attenuation of the ORR activity was observed, within the context of limiting current and onset potential. The attenuation trend of the limiting current was closely correlated with the diminishing content of graphitic N, as manifested in X-ray photoelectron spectroscopy measurements and Mott–Schottky analysis. In addition, the specific activity per graphitic N was found to be almost invariant within a wide range of potentials during prolonged potential cycling for all three model catalysts, in good agreement with theoretical prediction, whereas no such a correlation was observed with pyrrolic or pyridinic N. Density functional theory calculations showed that the first-electron reduction, which is a rate-determining step for the 4e^– ORR process, on carbon atoms adjacent to graphitic N, exhibited a much smaller Gibbs free-energy change than that on carbons neighboring pyrrolic or pyridinic N. These results strongly suggest that graphitic N is responsible for the ORR activity of N-doped carbons in alkaline electrolytes. Results in the present work may offer a generic, effective paradigm in the determination of catalytic active sites in heteroatom-doped carbons and be exploited as a fundamental framework for the rational design and engineering of effective carbon catalysts

Polymer-Capped Sulfur Copolymers as Lithium–Sulfur Battery Cathode: Enhanced Performance by Combined Contributions of Physical and Chemical Confinements

Flexible polymers show high potential applications in rechargeable lithium–sulfur (Li–S) batteries for their capability of confining sulfur diffusion and tolerance to large volume expansion during lithiation. Herein, sulfur is copolymerized with 3-butylthiophene via radical polymerization by heating the mixture of both components at controlled temperatures. Further capping of the thus-synthesized copolymer CP­(S3BT) with highly conductive PEDOT:PSS thin film substantially enhances the electrical conductivity. With the resulting polymer hybrids as the cathode material, a Li–S battery is constructed which shows an initial discharge capacity of 1362 mA h g^–1 at 0.1 C and a reversible capacity of 631 mA h g^–1 even at 5 C. Moreover, the polymer cathode exhibits a high capacity of 682 mA h g^–1 after 500 charge–discharge cycles at 1 C with 99.947% retention per cycle. The remarkable performance is attributed to the synergetic effects of (i) high conductivity resulting from both the conducting blocks of poly­(3-butylthiophene) (P3BT) and PEDOT:PSS capping layer, (ii) physical confinement of polysulfides by P3BT segments and PEDOT:PSS capping layers, and (iii) chemical confinement resulting from the high density of chemical bonds between sulfur and 3-butylthiophene. The results may offer a new paradigm in the development of efficient and stable polymer cathodes for high performance Li–S batteries

Mesoporous N‑Doped Carbons Prepared with Thermally Removable Nanoparticle Templates: An Efficient Electrocatalyst for Oxygen Reduction Reaction

Thermally removable nanoparticle templates were used for the fabrication of self-supported N-doped mesoporous carbons with a trace amount of Fe (Fe-N/C). Experimentally Fe-N/C was prepared by pyrolysis of poly­(2-fluoroaniline) (P2FANI) containing a number of FeO­(OH) nanorods that were prepared by a one-pot hydrothermal synthesis and homogeneously distributed within the polymer matrix. The FeO­(OH) nanocrystals acted as rigid templates to prevent the collapse of P2FANI during the carbonization process, where a mesoporous skeleton was formed with a medium surface area of about 400 m²/g. Subsequent thermal treatments at elevated temperatures led to the decomposition and evaporation of the FeO­(OH) nanocrystals and the formation of mesoporous carbons with the surface area markedly enhanced to 934.8 m²/g. Electrochemical measurements revealed that the resulting mesoporous carbons exhibited apparent electrocatalytic activity for oxygen reduction reactions (ORR), and the one prepared at 800 °C (Fe-N/C-800) was the best among the series, with a more positive onset potential (+0.98 V vs RHE), higher diffusion-limited current, higher selectivity (number of electron transfer <i>n</i> > 3.95 at +0.75 V vs RHE), much higher stability, and stronger tolerance against methanol crossover than commercial Pt/C catalysts in a 0.1 M KOH solution. The remarkable ORR performance was attributed to the high surface area and sufficient exposure of electrocatalytically active sites that arose primarily from N-doped carbons with minor contributions from Fe-containing species

Increasing Iridium Oxide Activity for the Oxygen Evolution Reaction with Hafnium Modification

Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrOx) due to its superior performance and excellent stability. However, large scale applications using IrOx are impractical due to its low abundance and high cost. Herein, we report a highly active hafnium-modified iridium oxide (IrHfxOy) electrocatalyst for OER. The IrHfxOy electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrOx electrocatalyst. The highest intrinsic mass activity of the IrHfxOy catalyst in acid conditions was calculated as 6950 A gIrOx–1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir–O species for both IrOx and IrHfxOy catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, shortening of the bond lengths of Ir–O, and lowers free energies for OER intermediates that accelerate the OER process