Ni Molar Ratio Research Articles

Physicochemical and corrosion properties of sono-electrodeposited Cu-Ni thin films

Cu-Ni thin films were electroplated from a bath containing fixed amount of Ni and three different concentration of copper in order to make Cu:Ni molar ratio of 1:10, 1:15 and 1:20 in static and ultrasonic condition. Electrochemical characterization i.e. cyclic voltammetry (CV) was done to know the reduction potential range for Cu-Ni co-deposition. Corrosion study revealed that films synthesized under static condition had better corrosion protective behavior as compared to films in ultrasonic condition. X-ray diffraction (XRD) patterns were recorded and it showed that alloying was better in presence of ultrasound. Surface morphology was examined by scanning electron microscopy, atomic force microscopy and composition analysis was done by energy dispersive spectroscopy (EDS) studies. Both morphology and composition showed a strong dependence on bath concentration and ultrasonic irradiation. EDS analysis showed that the films are Ni rich in static condition and Cu rich in presence of ultrasound. Hardness was tested by Micro-Vickers hardness tester where films in presence of ultrasound were found to have better hardness value due to refined grain size. Thickness and residual stress of the films were measured by Stylus surface profilometer. Film thickness varied from 1.98–2.16 μm in static condition which got increased to 2.69–3.59 μm in presence of sonication for equal period of deposition. Residual stress analysis reported a stress value ranging from 172.43 to 273.18 GPa in static condition which showed a remarkable decrease to 35.25–82.34 GPa in ultrasonic condition.

Ni removal from aqueous solutions by chemical reduction: Impact of pH and pe in the presence of citrate

The chemical precipitation of Ni ions from industrial wastewater at alkaline pH values creates waste chemical sludge (e.g., Ni(OH)2). We herein focused on Ni removal via chemical reduction using dithionite, by converting Ni(II) to its elemental or other valuable forms. Without the presence of a chelator (e.g., citrate), the nickel reduction efficiency increased with increasing dithionite:Ni molar ratio, reaching ⿼99% at ratios above 3:1. The effect of pH on Ni reduction was in agreement with the standard redox potentials (pe0) of dithionite, which became more negative with an increase in pH leading to greater Ni reduction efficiencies. With the formation of Ni-citrate chelates, however, the Ni reduction deteriorated. Elevated pH and temperature improved nickel reduction, due to the greater reducing power of dithionite. The optimal pH value for Ni(II) reduction was found to be ⿼8. Injecting Cu seed particles enhanced the rate and amount of Ni reduced. NiS and Ni3S2 were identified in the crystal of the resulting solids by X-ray crystallography, and the presence of elemental Ni was explained by X-ray photoelectron spectroscopy. The chemical reduction of actual printed circuit board wastewater with the dithionite:Ni(II) molar ratio dose of 12:1 retrieved ⿼99% nickel after 30-min reaction at 40°C.

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In this study, an amorphous silica was functionalized with aminosilane, N-((3-trimethoxysilyl)propyl)ethylenediamine (2NS) and the late transition metal catalysts including ((DME)NiBr2 and PdCl2(COD)) were subsequently immobilized on the func- tionalized amorphous silica for norbornene polymerization. Effects of the polymerization temperature, polymerization time, Al/Ni molar ratio, and type of co-catalyst on norbornene polymerization were investigated. Unsupported late transition metal catalysts did not show any activities for norbornene polymerization. However, the SiO2/2NS/Ni catayst with MAO system, with increasing polymerization temperature, increased the polymerization activity and decreased the molecular weight of the polynorbornene (PNB). Furthermore, the catalyst when increasing polymerization temperature caused the decrease in both the polymerization activity and molecular weight of PNB. This confirmed that the stability of SiO2/2NS/Ni at a high temperature was greater than that of SiO2/2NS/Pd. Also the longer polymerization time resulted in the higher conversion of norbornene for both catalysts. When the Al : Ni molar ratio was 1000 : 1, the highest activity (15.3 kg-PNB/ (µmol-Ni*hr)) but lowest molecular weight (Mn = 124,000 g/mol) of PNB were achieved. Also SiO2/2NS/Ni catalyst with borate/TEAL resulted in di- minishing the polymerization activity and molecular weight of PNB with increasing the polymerization temperature.

Electrochemical preparation of silicon nanowires from porous Ni/SiO2 blocks in molten CaCl2

Silicon nanowires (SiNWs) with diameter distributions ranging from 80 to 350 nm were prepared by electrochemical reduction of Ni/SiO2 in molten CaCl2. The effect of the content of nickel additives on the morphology of produced silicon was investigated. Large quantities of SiNWs are obtained by the electrochemical reduction of Ni/SiO2 blocks with SiO2 to Ni molar ratio of 20 and 10. Nickel additives repress the growth of irregular branches and promote longitudinal growth of SiNWs. Wire morphologies and surfaces are influenced by the electrolysis temperature. SiNWs become thicker with the increase of the electrolysis temperature. The optimum temperature to prepare single crystal SiNWs with high aspect ratio and extraordinary surface quality seems to be 1173 K. The amorphous layer of the silicon nanowire is thinner compared to the SiNWs obtained from the pure SiO2 pellets. The produced SiNWs show a photoluminescence emission peak at about 758 nm at room temperature. This work demonstrates the potentiality for the electrochemical reduction process to obtain large quantities of SiNWs with high quality. Silicon nanowires (SiNWs) were prepared by the electrochemical reduction of Ni/SiO2 blocks in molten CaCl2. The effects of the content of nickel additives and the electrolysis temperature on the morphology of produced silicon were investigated. Nickel additives promote the oriented growth of SiNWs and the SiNWs become thicker with the increase of electrolysis temperature.

Corrosion performance of superhydrophobic nickel stearate/nickel hydroxide thin films on aluminum alloy by a simple one-step electrodeposition process

Superhydrophobic thin films are fabricated on chemically cleaned aluminum alloy substrates by a one-step electrodeposition process in the ethanolic solution containing Ni2 + ions and stearic acid (SA) under applied DC voltage. Morphological features observed by scanning electron microscopy (SEM), indicate that the thin films are composed of cauliflower-like structures with micro-nano scale particles. The chemical compositions of these films are characterized using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), which indicate that these films consist of low energy nickel stearate (Ni(SA)2). A schematic model of the electrodeposition process of Ni(SA)2 is also presented. Wetting analysis demonstrates that the surface of the film is superhydrophobic with a water contact angle (CA) of 160 ± 1° with contact angle hysteresis (CAH) of 2.1 ± 1°, which is obtained under optimum electrodeposition conditions based on a 0.4 molar ratio of Ni2 +/SA solution. The corrosion resistance properties calculated from the Tafel curve of superhydrophobic surfaces prepared in this solution are 280 times larger than the chemically cleaned aluminum alloy substrate. Furthermore, electrochemical impedance spectroscopy (EIS) analysis demonstrates that the charge transfer resistance is 1.63 × 107 Ω·cm2 for superhydrophobic films (Ni2 +/SA = 0.4) compared to that of chemically cleaned aluminum alloy substrates (1.56 × 103 Ω·cm2). Therefore, the superhydrophobic film of Ni(SA)2 on the chemically cleaned aluminum alloy substrate improves the corrosion resistance performance of aluminum substrates in corrosive environments.

Effect of Acidity, Basicity and ZrO2 Phases of Cu–Ni/ZrO2 Catalysts on the Direct Synthesis of Diethyl Carbonate from CO2 and Ethanol

Cu–Ni/ZrO2 catalysts with different Cu:Ni molar ratios were synthesized and tested in the direct synthesis of diethyl carbonate from carbon dioxide and ethanol. The effect of Cu:Ni molar ratio on the acidity and basicity of the catalysts was investigated using NH3 and CO2-TPD, respectively. Experimental results revealed that Cu:Ni-3:1/ZrO2 and Cu:Ni-2:1/ZrO2 samples exhibited the largest acidity and basicity, as well as the highest yield to diethyl carbonate. In particular, these catalysts displayed very-high selectivity (i.e., larger than 95 %) and activity (i.e., equilibrium conversions are attained). Additionally, XRD and Raman studies showed that structural and crystalline properties of ZrO2 were modified with Cu:Ni ratios of 3:1 and 2:1 (Cu:Ni-1:1/ZrO2, Cu:Ni-1:2/ZrO2) which was related to the improved catalytic activity in comparison to Cu:Ni-1:1/ZrO2, Cu:Ni-1:2/ZrO2, Cu:Ni-1:3/ZrO2 and ZrO2.

Preparation of Ni—W aromatic hydrocarbon hydrogenation catalysts by breaking reverse emulsions or suspensions of a precursor in hydrocarbon feedstock

Reverse emulsions (hydrocarbon feedstock)/(aqueous precursor solution) for synthesizing aromatic hydrocarbon hydrogenation catalysts have been prepared using the water-soluble salts (NH4)2WS4 and Ni(NO3)2 · 6 H2O as precursors. It has been found that the optimum stabilizer for the emulsions is the nonionic surfactant SPAN-80. The resulting emulsions exhibit low catalytic activity in the hydrogenation of aromatic hydrocarbons; it has been shown that the activity of the systems in hydrodearomatization reactions is adversely affected by the presence of water. A procedure for preparing suspensions of solid precursor particles in a hydrocarbon feedstock by the removal of water from the resulting reverse emulsions has been developed. The catalytic activity of the resulting suspensions in the hydrogenation of aromatic hydrocarbons has been studied using naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene as examples. It has been shown that the catalytic activity of the suspensions is higher than that of the reverse emulsions of the same composition. It has been found that the feedstock should be subjected to additional sulfurization with elemental sulfur for further sulfiding the surface of the nickel—tungsten catalyst. The W: Ni molar ratio of 1 : 1 has been found to be optimum.

Synthesis of nickel–tungsten sulfide hydrodearomatization catalysts by the decomposition of oil-soluble precursors

Nickel–tungsten sulfide catalysts for the hydrogenation of aromatic hydrocarbons have been prepared by the in situ decomposition of an oil-soluble tungsten hexacarbonyl precursor in a hydrocarbon feedstock using oil-soluble nickel salt nickel(II) 2-ethylhexanoate as a source of nickel. The in situ synthesized Ni–W–S catalyst has been characterized by X-ray photoelectron spectroscopy. The activity of the resulting catalysts has been studied in the hydrogenation of bicyclic aromatic hydrocarbons and dibenzothiophene conversion in a batch reactor at a temperature of 350°C and a hydrogen pressure of 5.0 MPa. It has been shown that the optimum W : Ni molar ratio is 1 : 2. Using the example of the hydrofining of feedstock with high sulfur and aromatics contents, it has been shown that the synthesized catalyst exhibits high activity in the hydrogenation of aromatic hydrocarbons.

Controllable synthesis and electromagnetic interference shielding properties of magnetic CoNi alloy nanoparticles coated on biocarbon nanofibers

CoNi alloy nanoparticles with different molar ratios of Co:Ni were uniformly coated on the surface of biocarbon nanofibers (BCF) by electroless plating. Transmission electron microscopy, X-ray diffraction and vibration sample magnetometry were employed to characterize the morphologies, phase structures and magnetic properties of the coated BCF. The permittivity behavior of coated BCF-paraffin wax matrix composites was studied in the frequency range of 8.2–12.4 GHz by vector network analysis. Magnetic measurements showed that the saturation magnetizations increased linearly with increasing Co content in CoNi deposits and that the coercivity is insensitive towards the Co content. Of the four synthesized BCF with various Co:Ni molar ratios, the BCF with Co:Ni molar ratio of 1:2 displayed higher electromagnetic interference shielding efficiencies of 30.7 dB (99.91 % attenuation) due to its higher permittivity.

Electrochemical Dealloying of Bimetallic ORR Nanoparticle Catalysts at Constant Electrode Potentials

Dealloyed, that is, selectively leached Pt-based oxygen reduction reaction (ORR) nanoparticle catalysts have demonstrated previously unachieved initial reactivity and performance durability in single cell of PEM fuel cells. Dealloying is typically achieved using free corrosion in acid or electrochemical cycling. Here, we explore dealloying at constant electrode potentials of PtNi3 bimetallic alloy nanoparticles at 5–7 nm and >20 nm size ranges. We investigate how Ni dissolution at four distinct electrode potentials affects the composition, morphology, and surface roughness of the resulting dealloyed catalysts. The electrode potentials cover the hydrogen-adsorption, the double layer region, and the oxygenate adsorption region to examine whether adlayers affect the characteristics of the dealloyed catalysts. We show that large alloy nanoparticles invariably dealloy into porous nanoparticles with a relatively low Ni molar ratio of 0.4, while the smaller size particles show non-porous solid core-shell structures with a monotonic dependence between Ni at% and potential. We provide evidence that the dealloyed catalyst surface is strongly influenced by the presence/absence of adsorbed adlayers of hydrogen or oxygenates. In particular, data suggest that adsorbate adlayers modify the balance between Ni dissolution and Pt surface diffusion during the dealloying process resulting in rougher catalyst surfaces with enhanced surface area values.

Separation and recovery of metal values from leach liquor of waste lithium nickel cobalt manganese oxide based cathodes

In this study, a hydro-metallurgical process has been proposed to separate and recover valuable metals from citric acid leach solution of waste lithium nickel cobalt manganese oxide based cathodes. Nickel, cobalt, manganese and lithium were effectively separated and recycled by a combined method of selective precipitation and solvent extraction. First, about 98% of nickel was selectively precipitated by dimethylglyoxime reagent under conditions of equilibrium pH of 6 and molar ratio of Ni2+ to dimethylglyoxime of 0.5, with little loss of other metals. Then about 97% of cobalt was precipitated by ammonium oxalate and recovered as CoC2O4·2H2O under conditions of equilibrium pH of 6, reaction temperature of 55°C and molar ratio of C2O42+ to Co2+ of 1.2. The recovery of manganese was conducted by solvent extraction method using D2EHPA with 70–75% saponification rate. About 97% of manganese was extracted under optimal experimental conditions and recovered as MnSO4 after stripped with sulfuric acid solution. The McCabe–Thiele isotherm indicated that three extraction stages were needed for complete extraction of manganese at specific extraction condition. Finally, about 89% of lithium was precipitated and recovered as Li3PO4 by 0.5molL−1 Na3PO4 solution. This combined hydro-metallurgical recovery process is a relatively simple and environment-friendly route, which all metal values in the leach liquor can be effectively separated and recovered and both dimethylglyoxime and D2EHPA can be recycled and re-used.

One-step solution combustion synthesis of cobalt–nickel oxides/C/Ni/CNTs nanocomposites as electrochemical capacitors electrode materials

With Co(NO3)2·6H2O (oxidizer and Co source), Ni(NO3)2·6H2O (oxidizer and Ni source) and citric acid (fuel) as starting materials, cobalt–nickel oxides/C/Ni ternary nanocomposites have been synthesized by a scalable, one-step solution combustion process at only 300 °C within 30 min in air. In these composites, the metallic nickel and amorphous carbon (conductive phases) were in situ formed by the reduction of Ni2+ and carbonization of the excess citric acid during combustion, respectively. Experimental results indicated that the fuel:oxidant and Co:Ni molar ratios in precursor solution showed strong influences on the phase composition, morphology and electrochemical performance of products. With the increase of the fuel dosage, the products transformed from well-crystallized cubic NiO/Ni to Ni (nickel-related phases), then to relatively amorphous Ni/NiO and finally NiO. Electrochemical tests indicated that the optimized product showed a high specific capacitance of 446 F g−1 at 1 A g−1 (or 280 F g−1 at 10 A g−1) with a Co:Ni:C6H8O7 molar ratio of 4:5:86/9. Significantly, besides its mild experimental conditions, the method could be used to prepare cobalt–nickel oxides/C/Ni/CNTs quarternary nanocomposites by simply adding acid-treated CNTs into precursor combustion solution. Thanks to the high electrical conduction of CNTs, the specific capacitance could be further improved up to 579 F g−1 at 1 A g−1, or 350 F g−1 at 10 A g−1.

Synthesis and characterization of first‐ and second‐generation polyamide pyridylimine nickel dihalide metallodendrimers and their uses as catalysts for ethylene polymerization

AbstractFirst‐ and second‐generation pyridylimine‐terminated dendrimeric ligands were prepared by the reaction of the corresponding amine‐terminated aromatic polyamide dendrimers with 2‐acetylpyridine. The pyridylimine terminal groups were used as bidentate N,N ligands of nickel halide to prepare the corresponding first‐generation and second‐generation nickel dihalide metallodendrimers C1 and C2, respectively. The synthesized dendrimers and metallodendrimers were characterized by elemental and spectral analyses. C1 and C2 were evaluated as catalyst precursors for ethylene oligomerization after being activated with methylaluminoxane (MAO) and diethylaluminum chloride (Et2AlCl) under 1 atm and 5 atm pressure of ethylene. In both cases, the use of 1 atm or 5 atm pressure of ethylene and a 1500:1 Al:Ni molar ratio for C1 and C2 resulted in high catalytic activities toward ethylene polymerization. Upon activation with MAO and Et2AlCl, C1 exhibited promising activities toward ethylene polymerization and produced linear chain structures that were associated with high density polyethylene. In contrast, C2 produced a polymer with the branching nature of low density polyethylene under similar conditions. © 2014 Society of Chemical Industry

Sn–Ni/MWCNT nanocomposite negative electrodes for Li-ion batteries: The effect of Sn:Ni molar ratio

Since Ni is used to behave as a buffer component in the Sn-based anode materials for the Li-ion batteries, it is aimed to reveal the optimum Sn:Ni ratio to reduce the electrode pulverization emanated from volume increase during the charge/discharge process. MWCNTs were also co-deposited from the suspended MWCNT in the electrolyte to increase buffering effect and conductivity. To reduce irreversible capacity and improve cycle performance of tin electrodes, Sn–Ni/MWCNT three components nanocomposite electrodes were prepared with different Sn:Ni ratio by pulse electrodeposition method using copper substrate as current collector. The morphology and the structures of the Sn–Ni/MWCNT nanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD). Raman spectroscopy was used to determine the existence of MWCNT in the Sn–Ni matrix. The electrochemical performance of Sn–Ni/MWNT nanocomposites studied by charge/discharge tests and cyclic voltammetry experiments. Results showed that increasing the amount of co-deposited Ni has a strong effect on the electrochemical performances. The best electrochemical results were obtained in the nanocomposite electrodes with Ni content of 29 wt.%

Nickel-cobalt nanostructures coated reduced graphene oxide nanocomposite electrode for nonenzymatic glucose biosensing

Nickel-cobalt nanostructures (Ni-Co NSs) electrodeposited on reduced graphene oxide (RGO)-modified glassy carbon electrode (GCE) was prepared and used for highly sensitive glucose detection. RGO nanosheets were firstly assembled onto GCE surface by π–π interaction and then Ni-Co NSs were constructed on RGO/GCE by dynamic potential scan. The electrochemical and electrocatalytic behaviors of the Ni-Co NSs/RGO/GCE toward glucose oxidation were evaluated by cyclic voltammograms, chronoamperometry and amperometric method. The effects of some factors related to the fabrication of Ni-Co NSs/RGO/GCE, such as potential scan number and the molar ratio of Ni2+/Co2+ in a solution, on the catalytic performance of the Ni-Co NSs/RGO/GCE were also explored. The results showed that the Ni-Co NSs/RGO/GCE exhibited the best catalytic activity at the potential scan number of 20 and the Ni2+/Co2+ molar ratio of 1:1. The glucose concentration in the range of 10μM to 2.65mM linearly depended on the catalytic current (r=0.9967, n=17). The sensitivity was 1773.61μAcm−2mM−1, and the detection limit was 3.79μM (S/N=3). This high catalytic activity, good sensitivity and stability of the Ni-Co NSs/RGO/GCE sensor opened up a new kind of hybrid materials in electrochemical detection of glucose.

Spherical and radiate Ni particles prepared by the tartrate precipitation and thermal decomposition method

Nickel tartrate precursor particles were synthesized by the liquid phase precipitation method in an ethanol-water-ammonia mixed solution, with tartaric acid and using nickel chlorate as raw materials, with the pH value controlled at 4.0, and the temperature controlled at 50 °C. Nickel particles with complicated morphology were prepared by the decomposition of nickel taratrate precursor particles at temperatures of 360, 380 and 400 °C, respectively. The study of infrared spectroscopy (IR) indicated that the product was pure nickel tartrate. The studies of the atomic absorption spectrometry (AAS) and organic elemental analysis (OEA) indicated that the molar ratio of Ni2+ to (C4H4O6)2− is close to 1:1. The studies of the differential scanning calorimeter and thermo-gravimetric analysis (DSC-TG) indicated that the chemical formula Ni2(C4H4O6)2·5H2O was confirmed. The studies of X-ray diffractions (XRD) indicated that the silvery white metal powders were pure Ni, with a face-centered cubic crystal structure. The images of scanning electron microscopy (SEM) showed that the morphology of metal Ni particles was obvious spherical and radiate. The diameter of nickel tartrate particles was about 60 μm, which consisted of many nanolathes; and the diameter of metal Ni particles was about 30 μm, which consisted of many lathes about 0.5 μm in thickness.

Microwave-absorbing properties of CoNi nanoparticles

The CoNi alloy nanoparticles with different molar ratios of Co:Ni were prepared by the liquid-phase reduction, and the morphology, chemical composition and structure of the particles were investigated by field-emission scanning electron microscopy and X-ray powder diffraction. The complex permittivity and permeability of the nanoparticles were investigated by network analyzer in the frequency range of 1–18 GHz. The reflection loss was calculated based on the determined complex permittivity and permeability. The field-emission scanning electron microscopy results showed that all samples with different Co:Ni molar ratios were spherical and evenly distributed with the diameter ranging from 60 to 200 nm. Compared with other samples with different Co:Ni molar ratios, the sample with Co:Ni molar ratio of 1:2 displayed higher microwave-absorbing properties due to the higher imaginary parts of the complex permittivity and permeability.

Utilization of calcium sulfide derived from waste gypsum board for metal-containing wastewater treatment

To achieve recycling of waste gypsum boards (CaSO4), production of calcium sulfide by reductive decomposition of CaSO4 was investigated and the decomposition product of CaS was used as a sulfuration agent in the treatment of nickel-containing simulated wastewater. At the end of the experiment, residual nickel concentration and filtration characteristics of the slurry formed by adding CaS to the nickel-containing wastewater were investigated. Regarding the reductive decomposition of CaSO4, it was found that CaS could be effectively generated by CaSO4 reductive decomposition with graphite and the CaS content in the final product was greater than 80% when the reductive decomposition was carried out for one hour at a temperature of 1273 K, under N2 atmosphere. Then, in the sulfuration treatment of nickel-containing wastewater with CaS, the applicability of CaS as a sulfuration agent was confirmed and the nickel concentration in the simulated wastewater was reduced from 100 mg-Ni l-1 to a value below 1.0 mg l-1, when the CaS was added to the simulated wastewater at CaS to Ni molar ratio of 1.3. Moreover, sulfide slurry obtained by adding CaS was characterized by lower average specific filtration resistance and compressibility coefficient, compared to hydroxide slurry formed by the conventional precipitation treatment.

Acetic acid gas sensors based on Ni2+ doped ZnO nanorods prepared by using the solvothermal method

Ni2+-doped ZnO nanorods with different doping concentrations are prepared via the solvothermal method. The doped ZnO nanorods are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The amount of Ni2+ ions that enter the lattice of ZnO increases with increasing the Ni2+/Zn2+ molar ratio when the molar ratio of Ni2+/Zn2+ in the starting solution is lower than 3% and does not change obviously if the mole ratio of Ni2+/Zn2+ in the starting solution is in the range of 3–10 mol%. The effect of Ni2+ doping on the gas-sensing properties is investigated. The results reveal that the amount of Ni2+ has a great influence on the response (Ra/Rg) and the gas-sensing selectivity. The sensor based on 1 mol % Ni2+ doped ZnO nanorods (120 °C, 10 h) exhibits a high response to acetic acid vapor, in particular, the responses to 0.001 ppm and 0.01 ppm acetic acid vapor reach 1.6 and 2, respectively. The response time and the recovery time for 0.001 ppm acetic acid are only 4 s and 27 s, respectively.

Speciation of nickel (II) chloride complexes in hydrothermal fluids: In situ XAS study

In situ XAS data of Ni(II) chloride solutions with various salinities (0–7.68m) were collected to investigate the stoichiometry and geometry of Ni(II) chloride complexes from room temperature up to 369°C at 400bar, and to 434°C at 600bar. Increasing temperature and/or salinity results in a change in the coordination of the Ni(II) chlorocomplexes from octahedral to (distorted) tetrahedral. Octahedral species predominate within the whole salinity range at room temperature and up to ~200°C, and tetrahedral species become significant beyond this temperature. At 369°C and 400bar, octahedral species remain in equilibrium with tetrahedral species, and the number of chloride ligands in the first coordination shell depends on the Cl:Ni molar ratio. EXAFS refinements and ab initio XANES calculations show that the highest order chlorocomplex present over the investigated pressure, temperature and composition ranges is the distorted tetrahedral complex [NiCl3(H2O)]−, which predominates in the highest Cl concentration (7.68m) solution at 434°C and 600bar. A quantitative thermodynamic analysis of the XANES and EXAFS data reveals that the XAS data are consistent with the speciation models derived from recent UV-Vis spectrophotometric measurements (25°C–250°C, 100bar; Liu et al., 2012b) and the high temperature, high pressure solubility experiments of Lin and Popp (1984) and Fahlquist and Popp (1989), if the NiCl2(aq) species exists in both octahedral ([NiCl2(H2O)4](aq)) and tetrahedral ([NiCl2(H2O)2](aq)) forms, with the ratio of octahedral to tetrahedral decreasing at high temperature (>200°C). The new XAS data show that the octahedral to tetrahedral transition in Ni(II) chloride complexes occurs at higher temperature and/or salinity than the corresponding Co(II) complexes (Liu et al., 2011).