Sustainable Enhanced Sodium-Ion Storage at Subzero Temperature with LiF Integration.

Though layered sodium oxide materials are identified as promising cathodes in sodium-ion batteries, biphasic P3/O3 depicts improved electrochemical performance and structural stability. Herein, a coexistent P3/O3 biphasic cathode material was synthesized with "LiF" integration, verified with X-ray diffraction and Rietveld refinement analysis. Furthermore, the presence of Li and F was deduced by inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy dispersive X-ray spectroscopy (EDS). The biphasic P3/O3 cathode displayed an excellent capacity retention of 85% after 100 cycles (0.2C/30 mA g-1) at room temperature and 94% at -20 °C after 100 cycles (0.1C/15 mA g-1) with superior rate capability as compared to the pristine cathode. Furthermore, a full cell comprising a hard carbon anode and a biphasic cathode with 1 M NaPF6 electrolyte displayed excellent cyclic stabilities at a wider temperature range of -20 to 50 °C (with the energy density of 151.48 Wh kg-1) due to the enhanced structural stability, alleviated Jahn-Teller distortions, and rapid Na+ kinetics facilitating Na+ motion at various temperatures in sodium-ion batteries. The detailed post-characterization studies revealed that the incorporation of LiF accounts for facile Na+ kinetics, boosting the overall Na storage.

Ethylene glycol-mediated one-pot synthesis of Fe incorporated α-Ni(OH)2 nanosheets with enhanced intrinsic electrocatalytic activity and long-term stability for alkaline water oxidation.

Sustainable electrocatalytic water splitting stipulates the development of cheap, efficient and stable electrocatalysts to promote comparatively sluggish oxygen evolution reaction. We have synthesized iron-incorporated pure phase α-nickel hydroxide, Ni0.8Fe0.2(OH)2 electrocatalyst utilizing N,N,N',N'-Tetramethylethane-1,2-diamine (TMEDA) and ethylene glycol (EG) following a simple one-pot synthesis process. PXRD and FTIR data suggest that the intercalation of EG in the interlayer spacing promotes amorphousness of the material. FESEM and TEM analyses suggest that the catalyst possesses hierarchical sheet-like morphology and BET measurements indicated the surface area of 50 m2 g-1 with high mesoporosity. Electrochemical studies suggest that Ni0.8Fe0.2(OH)2 prepared using water-EG mixture is the most efficient electrocatalyst for OER activity as it requires only 258 mV overpotential (considering backward LSV) on a glassy carbon electrode to achieve the benchmark current density of 10 mA cm-2geo. Additionally, the catalyst shows remarkable long-term stability for up to 7 days. The efficiency of Ni0.8Fe0.2(OH)2 electrocatalyst is reflected in its low Tafel slope (43 mV dec-1) and high OER faradaic efficiency (93%). The enhanced activity is attributed to the increase in the interlayer spacing due to the intercalation of EG into the material, which facilitates the transport of ions during the OER process. The overall improved catalytic property is due to the enhanced ionic mobility, controllable textural property, higher per-site activity and increased conductivity for the Ni0.8Fe0.2(OH)2 catalytic network.

Nickel-cobalt oxalate as an efficient non-precious electrocatalyst for an improved alkaline oxygen evolution reaction.

The quest for developing next-generation non-precious electrocatalysts has risen in recent times. Herein, we have designed and developed a low cost electrocatalyst by a ligand-assisted synthetic strategy in an aqueous medium. An oxalate ligand-assisted non-oxide electrocatalyst was developed by a simple wet-chemical technique for alkaline water oxidation application. The synthetic parameters for the preparation of nickel-cobalt oxalate (Ni2.5Co5C2O4) were optimized, such as the metal precursor (Ni/Co) ratio, oxalic acid amount, reaction temperature, and time. Microstructural analysis revealed a mesoporous block-like architecture for nickel-cobalt oxalate (Ni2.5Co5C2O4). The required overpotential of Ni2.5Co5C2O4 for the alkaline oxygen evolution reaction (OER) was found to be 330 mV for achieving 10 mA cmgeo -2, which is superior to that of NiC2O4, CoC2O4, NiCo2O4 and the state-of-the-art RuO2. The splendid performance of Ni2.5Co5C2O4 was further verified by its low charge transfer resistance, impressive stability performance, and 87% faradaic efficiency in alkaline medium (pH = 14). The improved electrochemical activity was further attributed to double layer capacitance (C dl), which indefinitely divulged the inferiority of NiCo2O4 compared to Ni2.5Co5C2O4 for the alkaline oxygen evolution reaction (OER). The obtained proton reaction order (ρ RHE) was about 0.80, thus indicating the proton decoupled electron transfer (PDET) mechanism for OER in alkaline medium. Post-catalytic investigation revealed the formation of a flake-like porous nanostructure, indicating distinct transformation in morphology during the alkaline OER process. Further, XPS analysis demonstrated complete oxidation of Ni2+ and Co2+ centres into Ni3+ and Co3+, respectively under high oxidation potential, thereby indicating active site formation throughout the microstructural network. Additionally, from BET-normalised LSV investigation, the intrinsic activity of Ni2.5Co5C2O4 was also found to be higher than that of NiCo2O4. Finally, Ni2.5Co5C2O4 delivered a TOF value of around 3.28 × 10-3 s-1, which is 5.56 fold that of NiCo2O4 for the alkaline OER process. This report highlights the unique benefit of Ni2.5Co5C2O4 over NiCo2O4 for the alkaline OER. The structure-catalytic property relationship was further elucidated using density functional theory (DFT) study. To the best of our knowledge, nickel-cobalt oxalate (Ni2.5Co5C2O4) was introduced for the first time as a non-precious non-oxide electrocatalyst for alkaline OER application.

Inception of Co3O4 as Microstructural Support to Promote Alkaline Oxygen Evolution Reaction for Co0.85Se/Co9Se8 Network.

Developing electrocatalysts with abundant active sites is a substantial challenge to reduce the overpotential requirement for the alkaline oxygen evolution reaction (OER). In this work, we have aimed to improve the catalytic activity of cobalt selenides by growing them over the self-supported Co3O4 microrods. Initially, Co3O4 microrods were synthesized through annealing of an as-prepared cobalt oxalate precursor. The subsequent selenization of Co3O4 resulted in the formation of a grainy rodlike Co3O4/Co0.85Se/Co9Se8 network. The structural and morphological analysis reveals the presence of Co3O4 even after the selenization treatment where the cobalt selenide nanograins are randomly covered over the Co3O4 support. The resultant electrode shows superior electrocatalytic activity toward OER in alkaline medium by delivering a benchmark current density of 10 mA/cm2geo at an overpotential of 330 mV. As a comparison, we have developed Co0.85Se/Co9Se8 under similar conditions and evaluated its OER activity. This material consumes an overpotential of 360 mV to deliver the benchmark current density, which signifies the role of the Co3O4 support to improve the electrocatalytic activity of Co0.85Se/Co9Se8. Despite having a low TOF value for Co3O4/Co0.85Se/Co9Se8 (0.0076 s-1) compared to Co0.85Se/Co9Se8 (0.0102 s-1), the improved catalytic activity of Co3O4/Co0.85Se/Co9Se8 is attributed to the presence of a higher number of active sites rather than the improved per site activity. This is further supported from the Cdl (double layer capacitance) measurements where Co3O4/Co0.85Se/Co9Se8 and Co0.85Se/Co9Se8 tender Cdl values of about 8.19 and 1.08 mF/cm2, respectively, after electrochemical precondition. As-prepared Co3O4/Co0.85Se/Co9Se8 also manifests rapid kinetics (low Tafel slope ∼ 91 mV/dec), long-term stability, low charge-transfer resistance, and 82% Faradaic efficiency for alkaline electrocatalysis (pH = 14). Furthermore, the proton reaction order (ρRHE) is found to be 0.65, indicating a proton decoupled electron transfer (PDET) mechanism for alkaline OER. Thus, the Co3O4 support helps in the exposure of more catalytic sites of Co0.85Se/Co9Se8 to deliver the improved catalytic activities in alkaline medium.

Effect of Intrinsic Properties of Anions on the Electrocatalytic Activity of NiCo2O4 and NiCo2O x S4-x Grown by Chemical Bath Deposition.

Electrochemical water (H2O) splitting is one of the most promising technologies for energy storage by hydrogen (H2) generation but suffers from the requirement of high overpotential in the anodic half-reaction (oxygen evolution), which is a four-electron process. Though transition-metal oxides and oxysulfides are increasingly researched and used as oxygen evolution electrocatalysts, the bases of their differential activities are not properly understood. In this article, we have synthesized NiCo2O4 and NiCo2O x S4-x by a chemical bath deposition technique, and the latter has shown better oxygen evolution performance, both in terms of stability and activity, under alkaline conditions. Comprehensive analysis through time-dependent cyclic voltammetry, microscopy, and elemental analysis reveal that the higher activity of NiCo2O x S4-x may be attributed to the lower metal-sulfur bond energy that facilitates the activation process to form the active metal hydroxide/oxyhydroxide species, higher electrochemically active surface area, higher pore diameter and rugged morphology that prevents corrosion. This work provides significant insights on the advantages of sulfur-containing materials as electrochemical precatalysts over their oxide counterparts for oxygen evolution reaction.