NixCo1-x@NixCo1-xO/NCNT as Trifunctional ORR, OER, and HER Electrocatalysts and its Application in a Zn-Air Battery.

The efficient electrochemical conversion and storage devices can be boosted by the development of cost-effective and durable electrocatalysts. However, simultaneous in-depth understanding of the reaction mechanism is also required. Herein, we report the preparation, characterization, and electrochemical activities of bimetallic NixCo1-x NPs and core-shell NixCo1-x@NixCo1-xO NPs stabilized on N-doped carbon nanotubes (NCNTs). The electrocatalyst is derived from a bimetallic MOF {[Ni0.5Co0.5(bpe)2(N(CN)2)](N(CN)2)·(5H2O)}n (1) via pyrolysis followed by calcination. Pyrolysis of the bimetallic MOF gives rise to bimetallic nanoparticles stabilized on NCNTs, which, when subsequently calcined, leads to the formation of a core-shell structure with a semiconducting oxide shell (NixCo1-xO) encapsulating the NixCo1-x bimetallic NP core. Detailed evaluation of the electrocatalytic performance of NixCo1-x@NixCo1-xO/NCNT proves its worth as a bifunctional catalyst with 380 mV overpotential for oxygen evolution reaction at 10 mA cm-2 current density and 0.87 V (vs RHE) onset for oxygen reduction reaction in the alkaline medium. Additionally, the prepared electrocatalyst efficiently catalyzes the hydrogen evolution reaction with a nominal overpotential of 74 mV (vs RHE) for reaching 10 mA cm-2 current density in acidic medium. The practical applicability of this catalyst is further upheld in the fabrication of a zinc-air battery having high specific capacity with high round-trip efficiency and adequate cycle life. DFT calculations establish that the structure of NixCo1-x@NixCo1-xO/NCNT is crucial for its electrochemical activity since it has the threefold advantages of cooperative charge transfer from Co to Ni, synergistic relationship between the conductive alloy core and semiconducting oxide shell, and a highly conductive N-doped CNT matrix.

Unravelling faradaic electrochemical efficiencies over Fe/Co spinel metal oxides using surface spectroscopy and microscopy techniques.

Cobalt and iron metal-based oxide catalysts play a significant role in energy devices. To unravel some interesting parameters, we have synthesized metal oxides of cobalt and iron (i.e. Fe2O3, Co3O4, Co2FeO4 and CoFe2O4), and measured the effect of the valence band structure, morphology, size and defects in the nanoparticles towards the electrocatalytic hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). The compositional variations in the cobalt and iron precursors significantly alter the particle size from 60 to <10 nm and simultaneously the shape of the particles (cubic and spherical). The Tauc plot obtained from the solution phase ultraviolet (UV) spectra of the nanoparticles showed band gaps of 2.2, 2.3, 2.5 and 2.8 eV for Fe2O3, Co3O4, Co2FeO4 and CoFe2O4, respectively. Further, the valence band structure and work function analysis using ultraviolet photoelectron spectroscopy (UPS) and core level X-ray photoelectron spectroscopy (XPS) analyses provided better structural insight into metal oxide catalysts. In the Co3O4 system, the valence band structure favors the HER and Fe2O3 favors the OER. The composites Co2FeO4 and CoFe2O4 show a significant change in their core level (O 1s, Co 2p and Fe 2p spectra) and valence band structure. Co3O4 shows an overpotential of 370 mV against 416 mV for Fe2O3 at a current density of 2 mA cm-2 for the HER. Similarly, Fe2O3 shows an overpotential of 410 mV against the 435 mV for Co3O4 at a current density of 10 mA cm-2 for the OER. However, for the ORR, Co3O4 shows 70 mV improvement in the half-wave potential against Fe2O3. The composites (Co2FeO4 and CoFe2O4) display better performance compared to their respective parent oxide systems (i.e., Co3O4 and Fe2O3, respectively) in terms of the ORR half-wave potential, which can be attributed to the presence of the oxygen vacancies over the surface in these systems. This was further corroborated in density functional theory (DFT) simulations, wherein the oxygen vacancy formation on the surface of CoFe2O4(001) was calculated to be significantly lower (∼50 kJ mol-1) compared to Co3O4 (001). The band diagram of the nanoparticles constructed from the various spectroscopic measurements with work function and band gap provides in-depth understanding of the electrocatalytic process.

Nickel foam supported porous copper oxide catalysts with noble metal-like activity for aqueous phase reactions.

Contiguous metal foams offer a multitude of advantages over conventional powders as supports for nanostructured heterogeneous catalysts; most critically a preformed 3-D porous framework ensuring full directional coverage of supported catalyst, and intrinsic ease of handling and recyclability. Nonetheless, metal foams remain comparatively underused in thermal catalysis compared to more conventional supports such as amorphous carbon, metal oxides, zeolites and more recently MOFs. Herein, we demonstrate a facile preparation of highly-reactive, robust, and easy to handle Ni foam-supported Cu-based metal catalysts. The highly sustainable synthesis requires no specialized equipment, no surfactants or additive redox reagents, uses water as solvent, and CuCl2(H2O)2 as precursor. The resulting material seeds as well-separated micro-crystalline Cu2(OH)3Cl evenly covering the Ni foam. Calcination above 400 °C transforms the Cu2(OH)3Cl to highly porous CuO. All materials display promising activity towards the reduction of 4-nitrophenol and methyl orange. Notably, our leading CuO-based material displays 4-nitrophenol reduction activity comparable with very reactive precious-metal based systems. Recyclability studies highlight the intrinsic ease of handling for the Ni foam support, and our results point to a very robust, highly recyclable catalyst system.

[MoS4]2--Intercalated NiCo-Layered Double Hydroxide Nanospikes: An Efficiently Synergized Material for Urine To Direct H2 Generation.

Substituting the energy-uphill water oxidation half-cell with readily oxidizable urea-rich urine, a ground-breaking bridge is constructed, combining the energy-efficient hydrogen generation and environmental protection. Hence, designing a robust multifunctional electrocatalyst is desirable for widespread implementation of this waste to fuel technology. In this context, here, we report a simple tuning of the electrocatalytically favorable characteristics of NiCo-layered double hydroxide by introducing [MoS4]2- in its interlayer space. The [MoS4]2- insertion as well as its effect on the electronic structure tuning is thoroughly studied via X-ray photoelectron spectroscopy in combination with electrochemical analysis. This insertion induces overall electronic structure tuning of the hydroxide layer in such a way that the designed catalyst exhibited favorable kinetics toward all the required reactions of hydrogen generation. This is why our homemade catalyst, when utilized both as a cathode and anode to fabricate a urea electrolyzer, required a mere ∼1.37 V cell potential to generate sufficient H2 by reaching the benchmark 10 mA cm-2 in 1 M KOH/0.33 M urea along with long-lasting catalytic efficiency. Other indispensable reason of selecting [MoS4]2- is its high-valent nature making the catalyst highly selective and insensitive to common catalyst-poisoning toxins of urine. This is experimentally supported by performing the real urine electrolysis, where the nanospike-covered Ni foam-based catalyst showed a performance similar to that of synthetic urea, offering its industrial value. Other intuition of selecting [MoS4]2- was to provide a ligand-based mechanism for hydrogen evolution half-cell [hydrogen evolution reaction (HER)] to preclude the HER-competing oxygen reduction. Another crucial point of our work is its potential to avoid the mixing of two explosive product gases, that is, H2 and O2.

Iron Catalyzed Hydroformylation of Alkenes under Mild Conditions: Evidence of an Fe(II) Catalyzed Process.

Earth abundant, first row transition metals offer a cheap and sustainable alternative to the rare and precious metals. However, utilization of first row metals in catalysis requires harsh reaction conditions, suffers from limited activity, and fails to tolerate functional groups. Reported here is a highly efficient iron catalyzed hydroformylation of alkenes under mild conditions. This protocol operates at 10-30 bar syngas pressure below 100 °C, utilizes readily available ligands, and applies to an array of olefins. Thus, the iron precursor [HFe(CO)4]-[Ph3PNPPh3]+ (1) in the presence of triphenyl phosphine catalyzes the hydroformylation of 1-hexene (S2), 1-octene (S1), 1-decene (S3), 1-dodecene (S4), 1-octadecene (S5), trimethoxy(vinyl)silane (S6), trimethyl(vinyl)silane (S7), cardanol (S8), 2,3-dihydrofuran (S9), allyl malonic acid (S10), styrene (S11), 4-methylstyrene (S12), 4- iBu-styrene (S13), 4- tBu-styrene (S14), 4-methoxy styrene (S15), 4-acetoxy styrene (S16), 4-bromo styrene (S17), 4-chloro styrene (S18), 4-vinylbenzonitrile (S19), 4-vinylbenzoic acid (S20), and allyl benzene (S21) to corresponding aldehydes in good to excellent yields. Both electron donating and electron withdrawing substituents could be tolerated and excellent conversions were obtained for S11-S20. Remarkably, the addition of 1 mol % acetic acid promotes the reaction to completion within 16-24 h. Detailed mechanistic investigations revealed in situ formation of an iron-dihydride complex [H2Fe(CO)2(PPh3)2] (A) as an active catalytic species. This finding was further supported by cyclic voltammetry investigations and intermediacy of an Fe(0)-Fe(II) species was established. Combined experimental and computational investigations support the existence of an iron-dihydride as the catalyst resting state, which then follows a Fe(II) based catalytic cycle to produce aldehyde.

Realizing High Capacitance and Rate Capability in Polyaniline by Enhancing the Electrochemical Surface Area through Induction of Superhydrophilicity.

Polyaniline (PANI) as a pseudocapacitive material has very high theoretical capacitance of 2000 F g-1. However, its practical capacitance has been limited by low electrochemical surface area (ESA) and unfavorable wettability toward aqueous electrolytes. This work deals with a strategy wherein the high ESA of PANI has been achieved by the induction of superhydrophilicity together with the alignment of PANI exclusively on the surface of carbon fibers as a thin layer to form a hybrid assembly. Superhydrophilicity is induced by electrochemical functionalization of the Toray carbon paper, which further induces superhydrophilicity to the electrodeposited PANI layer on the paper, thereby ensuring a high electrode-electrolyte interface. The Toray paper is electrochemically functionalized by the anodization method, which generates a highly active electrochemical surface as well as greater wettability (superhydrophilic) of the carbon fibers. Because of the strong interaction of anilinium chloride with the hydrophilic carbon surface, PANI is polymerized exclusively over the surface of the fibers without any appreciable aggregation or agglomeration of the polymer. The PANI-Toray paper assembly in the solid-state prototype supercapacitor can provide a high gravimetric capacitance of 1335 F g-1 as well as a high areal capacitance of 1.3 F cm-2 at a current density of 10 A g-1. The device also exhibits high rate capability, delivering 1217 F g-1 at a current density of 50 A g-1 and a high energy density of 30 W h kg-1 at a power density of 2 kW kg-1.

Cobalt Ferrite Bearing Nitrogen-Doped Reduced Graphene Oxide Layers Spatially Separated with Microporous Carbon as Efficient Oxygen Reduction Electrocatalyst.

The present work discloses how high-quality dispersion of fine particles of cobalt ferrite (CF) could be attained on nitrogen-doped reduced graphene oxide (CF/N-rGO) and how this material in association with a microporous carbon phase could deliver significantly enhanced activity toward electrochemical oxygen reduction reaction (ORR). Our study indicates that the microporous carbon phase plays a critical role in spatially separating the layers of CF/N-rGO and in creating a favorable atmosphere to ensure the seamless distribution of the reactants to the active sites located on CF/N-rGO. In terms of the ORR current density, the heat-treated hybrid catalyst at 150 °C (CF/N-rGO-150) is found to be clearly outperforming (7.4 ± 0.5 mA/cm(2)) the state-of-the-art 20 wt % Pt-supported carbon catalyst (PtC) (5.4 ± 0.5 mA/cm(2)). The mass activity and stability of CF-N-rGO-150 are distinctly superior to PtC even after 5000 electrochemical cycles. As a realistic system level exploration of the catalyst, testing of a primary zinc-air battery could be demonstrated using CF/N-rGO-150 as the cathode catalyst. The battery is giving a galvanostatic discharge time of 15 h at a discharge current density of 20 mA/cm(2) and a specific capacity of ∼630 mAh g(-1) in 6 M KOH by using a Zn foil as the anode. Distinctly, the battery performance of this system is found to be superior to that of PtC in less concentrated KOH solution as the electrolyte.