Electrochemical codeposition of nickel oxide and polyaniline

Nickel oxide (NiO_x) and polyaniline (PAni) were electrocodeposited from NiSO₄ and aniline through cyclic voltammetric scans to afford PAni–NiO_x composite film at controlled pH environment. The electrochemical activities of the film were investigated by cyclic voltammetry in 0.1 M NaOH and 0.1 M H₂SO₄, respectively. Typical redox couples of PAni in 0.1 M H₂SO₄ appeared at approximately 0.2 and 0.4 V vs. saturated calomel electrode (SCE); Ni(II)/Ni(III) redox couple was observed at approximately 0.4 V vs. SCE in 0.1 M NaOH. The morphologies and elemental components of the films were inspected by scanning electron microscopy and energy dispersive X-ray diffraction. The stability of nickel oxide in the films was found to be enhanced against acidic environments. Electrochemical catalytic behavior of NiO_x within the composite film was conserved and demonstrated by catalytic oxidation of methanol and ethanol.     

Structural and Thermal Characteristics of Nickel Dioxide Derived from LiNiO2

We report the structural and thermal characteristics of highly delithiated (lithium extracted) compounds Li_xNiO₂, which can be called “nickel dioxide.” We obtained Li_0.10NiO₂ and Li_0.04NiO₂ by treating LiNiO₂ with sulfuric acid. Both products contained phases with NiO₂ stacking similar to cadmium chloride (O3-type), but the latter also included a phase with NiO₂ stacking similar to cadmium iodide (O1-type). We examined their thermal behavior using high temperature X-ray diffraction analysis together with thermogravimetric analysis and found that novel polymorphs, with similar chemical compositions but different structures, were obtained by heating them at appropriate temperatures. We discuss these results together with those for Li_xNiO₂ obtained by electrochemical delithiation. We also report acid-treated products derived from Li_0.93Ni_1.07O₂.     

NixCo3‐xO4 Nanoneedle Arrays Grown on Ni Foam as an Efficient Bifunctional Electrocatalyst for Full Water Splitting

Developing highly active, stable and robust electrocatalysts based on earth‐abundant elements for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is important for many renewable energy conversion processes. Herein, Ni_xCo_3‐xO₄ nanoneedle arrays grown on 3D porous nickel foam (NF) was synthesized as a bifunctional electrocatalyst with OER and HER activity for full water splitting. Benefiting from the advantageous structure, the composite exhibits superior OER activity with an overpotential of 320 mV achieving the current density of 10 mA cm^?2. An exceptional HER activity is also acquired with an overpotential of 170 mV at the current density of 10 mA cm^?2. Furthermore, the catalyst also shows the superior activity and stability for 20 h when used in the overall water splitting cell. Thus, the hierarchical 3D structure composed of the 1D nanoneedle structure in Ni_xCo_3‐xO₄/NF represents an avenue to design and develop highly active and bifunctional electrocatalysts for promising energy conversion.     

Electrocatalytic Hydrogen Production by a Nickel(II) Complex with a Phosphinopyridyl Ligand

A novel nickel(II) complex [Ni(L)₂Cl]Cl with a bidentate phosphinopyridyl ligand 6‐((diphenylphosphino)methyl)pyridin‐2‐amine (L) was synthesized as a metal‐complex catalyst for hydrogen production from protons. The ligand can stabilize a low Ni oxidation state and has an amine base as a proton transfer site. The X‐ray structure analysis revealed a distorted square‐pyramidal Ni^II complex with two bidentate L ligands in a trans arrangement in the equatorial plane and a chloride anion at the apex. Electrochemical measurements with the Ni^II complex in MeCN indicate a higher rate of hydrogen production under weak acid conditions using acetic acid as the proton source. The catalytic current increases with the stepwise addition of protons, and the turnover frequency is 8400 s^?1 in 0.1 m [NBu₄][ClO₄]/MeCN in the presence of acetic acid (290 equiv) at an overpotential of circa 590 mV.     

A Self-Formed Stable PbI2/NiOx Interface with Increased Ni3+ Centers for Perovskite Photovoltaics

Nickel oxide (NiO_x) is a promising hole transport material (HTM) for perovskite photovoltaics owing to its chemical stability and low cost. However, most NiO_x based solar cells deliver relatively weak performance because of its insufficient electrical property and interfacial contact. In this work, a self-formed PbI₂/NiO_x interface was developed to stabilize the Ni³⁺ centers, which was beneficial for electrical transport and band alignment in perovskite solar cells. Combined with ultraviolet ozone treatment ( UVO ) and sequential deposition, the density of Ni³⁺ centers was greatly increased and could be stabilized by the PbI₂ interface. These merits contributed coordinately to the fast hole extraction and low energy loss across the PbI₂/NiO_x interface, yielding power conversion efficiencies (PCEs) of over 19 %.     

A Hybrid Co Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO2 Reduction in Water

Associating a metal‐based catalyst to a carbon‐based nanomaterial is a promising approach for the production of solar fuels from CO₂. Upon appending a Co^II quaterpyridine complex [Co(qpy)]²⁺ at the surface of multi‐walled carbon nanotubes, CO₂ conversion into CO was realized in water at pH 7.3 with 100 % catalytic selectivity and 100 % Faradaic efficiency, at low catalyst loading and reduced overpotential. A current density of 0.94 mA cm^?2 was reached at ?0.35 V vs. RHE (240 mV overpotential), and 9.3 mA cm^?2 could be sustained for hours at only 340 mV overpotential with excellent catalyst stability (89 095 catalytic cycles in 4.5 h), while 19.9 mA cm^?2 was met at 440 mV overpotential. Such a hybrid material combines the high selectivity of a homogeneous molecular catalyst to the robustness of a heterogeneous material. Catalytic performances compare well with those of noble‐metal‐based nano‐electrocatalysts and atomically dispersed metal atoms in carbon matrices.     

Synthesis of Cu-Modified Nickel Oxide Nanocrystals and Their Applications as Hole-Injection layers for Quantum-Dot Light-Emitting Diodes

Solution-processed NiO_x thin films have been applied as hole-injection layers (HILs) in quantum-dot light-emitting diodes (QLEDs). The commonly used NiO_x HILs are prepared by the precursor-based route, which requires high annealing temperatures of over 275 °C to in situ convert the precursors into oxide films. Such high processing temperatures of NiO_x HILs hinder their applications in flexible devices. Herein, we report a low-temperature approach based on Cu-modified NiO_x (NiO_x-Cu) nanocrystals to prepare HILs. A simple post-synthetic surface-modification step, which anchors the copper agents onto the surfaces of oxide nanocrystals, is developed to improve the electrical conductivity of the low-temperature-processed (135 °C) oxide-nanocrystal thin films. In consequence, QLEDs based on the NiO_x-Cu HILs exhibit an external quantum efficiency of 17.5 % and a T₉₅ operational lifetime of ∼2,800 h at an initial brightness of 1,000 cd m⁻², meeting the commercialization requirements for display applications. The results shed light on the potential of using NiO_x-Cu HILs for realizing high-performance flexible QLEDs.     

Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation

We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni²⁺ ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 atomic %)‐intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA cm^?2. This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst β‐Ni(OH)₂ (η≈550 mV). Molecular dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chemistry in the spatially confined interlayer region.     

Enhanced electrolytic generation of oxygen gas at binary nickel oxide–cobalt oxide nanoparticle-modified electrodes

This study addresses the enhancement of the oxygen evolution reaction (OER) on glassy carbon, Au, and Pt electrodes modified with binary catalysts composed of nickel oxide nanoparticles (nano-NiO_x) and cobalt oxide nanoparticles (nano-CoO_x). Binary NiO_x/CoO_x-modified electrodes (with NiO_x initially deposited) show a high catalytic activity and a marked stability which far exceeds that obtained at the individual oxide-modified electrodes. This enhancement is demonstrated by a marked negative shift (more than ca. 600 mV) in the onset potential of the OER compared to that obtained at the unmodified electrodes. The modified electrodes show a significantly higher long-term stability, over a period of 5 h of continuous electrolysis, without any significant loss of activity towards the OER in alkaline medium. The influence of the solution pH, the loading level, and sequence of deposition of each oxide on the electrocatalytic activity of the modified electrodes is addressed with an aim to maximize the catalytic activity of the modified electrodes towards the OER. SEM imaging is used to disclose the size and morphology of the fabricated nano-NiO_x and nano-CoO_x binary catalysts at the electrode surface.     

非晶态NiP基催化剂的制备及电催化析氢性能研究

以NaH₂PO₂和Ni₂SO₄为磷源和镍源,使用一锅法合成了非晶态NiP合金及其碳纳米（乙炔黑和石墨烯）复合催化剂。用透射电子显微镜（TEM）、X射线衍射仪（XRD）、热重分析（TGA）、电感耦合等离子体光谱仪（ICP）分别对催化剂性能和组成进行了表征和分析。通过线性扫描伏安对催化剂在酸性和碱性条件下的析氢性能进行了评价,研究结果表明,非晶态NiP/还原氧化石墨烯复合催化剂（NiP/RGO）展现出优异的电催化性能。在0.5 mol/L H₂SO₄中的起始过电位为89.0 mV,塔菲尔斜率为135.1 mV/decade;在1 mol/L NaOH中,起始过电位为116.1 mV,塔菲尔斜率为122.4 mV/decade,这与商业化Pt黑催化剂很接近。500次循环以后,催化剂活性没有明显下降,表明该催化剂具有良好的稳定性。该研究提供了一种简单可行的制备非贵金属磷化物方法用于电催化析氢反应。     

Nanoporous Graphene with Single‐Atom Nickel Dopants: An Efficient and Stable Catalyst for Electrochemical Hydrogen Production

Single‐atom nickel dopants anchored to three‐dimensional nanoporous graphene can be used as catalysts of the hydrogen evolution reaction (HER) in acidic solutions. In contrast to conventional nickel‐based catalysts and graphene, this material shows superior HER catalysis with a low overpotential of approximately 50 mV and a Tafel slope of 45 mV dec^?1 in 0.5 M H₂SO₄ solution, together with excellent cycling stability. Experimental and theoretical investigations suggest that the unusual catalytic performance of this catalyst is due to sp–d orbital charge transfer between the Ni dopants and the surrounding carbon atoms. The resultant local structure with empty C–Ni hybrid orbitals is catalytically active and electrochemically stable.     

Highly Efficient Proline Ester‐based Nickel Catalysts for Michael Addition of Thiophenols to α,β‐Enones

Two N₃O₂ pentadentate ligands, BMPP and BPPP, were prepared for synthesizing highly efficient nickel catalysts, [Ni(BMPP)(CH₃CN)](ClO₄)₂ ( 1 ) and [Ni(BPPP)(CH₃CN)](BPh₄)(ClO₄) ( 2 ), for thia‐Michael addition of thiophenols to α,β‐enones. X‐ray structures of 1 and 2 revealed that a labile CH₃CN molecule was bound to the nickel center of the catalysts. ESI‐MS spectroscopy indicated that thiolate replaced the bound CH₃CN molecule and coordinated to the nickel center during the catalytic cycle.     

Photosensitization study of cage-like metal complexes with hexadentate ligands, tptn and tpen. Part 1. Photochemistry and spectroelectrochemistry of iron(II), cobalt(III) and nickel(II) complexes

Two hexadentate ligands, N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,3-propanediamine (tptn) and N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (tpen), were used to prepare cage-like metal complexes of Fe(II), Co(III) and Ni(II). As expected, these complexes are photochemically inert. The electrochemical behaviour of these complexes was studied by spectroelectrochemical methods. The iron(II) complexes exhibit chemical irreversibility on reduction and the nickel(II) complexes show reversible reduction behaviour. The potential of cobalt(III) complexes as photosensitizers is discussed.     

Water Oxidation Catalysis Beginning with CuCo2S4: Investigation of the True Electrochemically Driven Catalyst

The exploitation of efficient and stable water oxidation catalysts is a pressing challenge to solve the energy crisis. Herein, flower‐like CuCo₂S₄ microspheres were successfully synthesized and used as an effective water oxidation catalyst. CuCo₂S₄/NF (NF=nickel foam) affords electrocatalytic water oxidation activity, with a current density of 20 mA cm^?2 at a low overpotential of 260 mV. The overpotential value is lower than that of benchmark RuO₂/NF (overpotential of 340 mV at a current density of 20 mA cm^?2). The water oxidation activity increases linearly before nonlinearly improving with increasing pH; this indicates that the substrate changes from water to hydroxyl. The CuCo₂S₄/NF catalyst is demonstrated to be a real water oxidation catalyst based on diverse experiments.     

Electrochromic nickel oxide films made by reactive r.f. sputtering from different targets

We describe the preparation and optical–electrical characterization of thin electrochromic layers based on nickel oxide (NiO_x). These layers were deposited by reactive radiofrequency (r.f.) sputtering from nickel or nickel oxide targets, maintaining the r.f. power and varying the oxygen flow in a gas mixture of Ar and O₂ from 5% up to 30%. The Ni/O ratio in the deposited thin films was determined by Rutherford backscattering spectroscopy (RBS) and the microstructure was investigated by x‐ray diffraction. The deposition rate was found to be strongly dependent on the type of target used. The electrochromic behaviour in aqueous alkaline electrolyte (0.1 N KOH solution) was investigated by electrochemical cyclization. Samples deposited by the NiO target exhibited the lowest N/O ratio (0.5) and the highest value of intercalated charge (～4.9 × 10^?2 mC cm^?2 nm^?1). Among the same samples the highest value of the intercalated charge was found for those deposited at low oxygen flow, but these samples exhibit the smallest reversible changes in optical absorption in the wavelength range 330–1200 nm. Samples deposited by the nickel target do not exhibit significant variations in the value of the exchanged charge; the measured Ni/O ratio indicates a stoichiometry closer to NiO. We also observed the switching behaviour by analysing the films in cyclic time mode at a fixed wavelength. The commutation response time is estimated by measurement under the application of a step waveform potential. Copyright © 2004 John Wiley & Sons, Ltd.     

Self‐Supported Cu3P Nanowire Arrays as an Integrated High‐Performance Three‐Dimensional Cathode for Generating Hydrogen from Water

Searching for inexpensive hydrogen evolution reaction (HER) electrocatalysts with high activity has attracted considerable research interest in the past years. Reported herein is the topotactic fabrication of self‐supported Cu₃P nanowire arrays on commercial porous copper foam (Cu₃P NW/CF) from its Cu(OH)₂ NW/CF precursor by a low‐temperature phosphidation reaction. Remarkably, as an integrated three‐dimensional hydrogen‐evolving cathode operating in acidic electrolytes, Cu₃P NW/CF maintains its activity for at least 25 hours and exhibits an onset overpotential of 62 mV, a Tafel slope of 67 mV dec^?1, and a Faradaic efficiency close to 100 %. Catalytic current density can approach 10 mA cm^?2 at an overpotential of 143 mV.     

Arginine‐Containing Ligands Enhance H2 Oxidation Catalyst Performance

Hydrogenase enzymes use Ni and Fe to oxidize H₂ at high turnover frequencies (TOF) (up to 10 000 s^?1) and low overpotentials (<100 mV). In comparison, the fastest reported synthetic electrocatalyst, [Ni^II(P^Cy₂N^tBu₂)₂]²⁺, oxidizes H₂ at 60 s^?1 in MeCN under 1 atm H₂ with an unoptimized overpotential of ca. 500 mV using triethylamine as a base. ¹ Here we show that a structured outer coordination sphere in a Ni electrocatalyst enhances H₂ oxidation activity: [Ni^II(P^Cy₂N^Arg₂)₂]⁸⁺ (Arg=arginine) has a TOF of 210 s^?1 in water with high energy efficiency (180 mV overpotential) under 1 atm H₂, and 144 000 s^?1 (460 mV overpotential) under 133 atm H₂. The complex is active from pH 0–14 and is faster at low pH, the most relevant condition for fuel cells. The arginine substituents increase TOF and may engage in an intramolecular guanidinium interaction that assists in H₂ activation, while the COOH groups facilitate rapid proton movement. These results emphasize the critical role of features beyond the active site in achieving fast, efficient catalysis.     

Fe‐Doped Ni3C Nanodots in N‐Doped Carbon Nanosheets for Efficient Hydrogen‐Evolution and Oxygen‐Evolution Electrocatalysis

Uniform Ni₃C nanodots dispersed in ultrathin N‐doped carbon nanosheets were successfully prepared by carburization of the two dimensional (2D) nickel cyanide coordination polymer precursors. The Ni₃C based nanosheets have lateral length of about 200 nm and thickness of 10 nm. When doped with Fe, the Ni₃C based nanosheets exhibited outstanding electrocatalytic properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For example, 2 at % Fe (atomic percent) doped Ni₃C nanosheets depict a low overpotential (292 mV) and a small Tafel slope (41.3 mV dec⁻¹) for HER in KOH solution. An outstanding OER catalytic property is also achieved with a low overpotential of 275 mV and a small Tafel slope of 62 mV dec⁻¹ in KOH solution. Such nanodot‐incorporated 2D hybrid structures can serve as an efficient bifunctional electrocatalyst for overall water splitting.     

Development of nano IrO<Subscript>2</Subscript> composite-reinforced nickel–phosphorous electrodes for hydrogen evolution reaction

Electroless and electroplated nickel electrodes are extensively used for hydrogen evolution reaction (HER). In the present work, TiO₂-supported IrO₂ mixed oxide composite was prepared and used to reinforce Ni–P electroless plates to be used as catalytic electrodes for HER. The electrodes exhibited high electrocatalytic activity when the electrodes were used for HER. All the parameters including particle size of the catalyst, surface roughness, and surface active sites were studied. The particle size of the IrO₂ catalyst in the mixed oxide was found to have high influence on the catalytic activity of the electrodes. Low overpotential as low as 70 mV at a current density of 200 mA cm⁻² was achieved with the mixed oxide-reinforced Ni–P electrodes.     

Super-Oxidizing Covalent Triazine Framework Electrocatalyst for Two-Electron Water Oxidation to H2O2

Electrochemical two-electron water oxidation (2e WOR) is gaining surging research traction for sustainable hydrogen peroxide production. However, the strong oxidizing environment and thermodynamically competitive side-reaction (4e WOR) posit as thresholds for the 2e WOR. We herein report a custom-crafted covalent triazine network possessing strong oxidizing properties as a proof-of-concept metal-free functional organic network electrocatalyst for catalyzing 2e WOR. As the first-of-its-kind, the material shows a maximum of 89.9 % Faradaic Efficiency and 1428 μmol/h/cm² H₂O₂ production rate at 3.0 V bias potential (vs reversible hydrogen electrode, RHE), which are either better or comparable to the state-of-the-art electrocatalysts. We have experimentally confirmed a stepwise 2e WOR mechanism which was further computationally endorsed by density functional theory studies.