Electrochemical Cycling‐Induced Amorphization of Cobalt(II,III) Oxide for Stable High Surface Area Oxygen Evolution Electrocatalysts

One-step recovery of cobalt from ammonia-ammonium carbonate system via pressurized ammonia distillation

Ammonia leaching of Co-bearing resources has attracted attention due to its selectivity to cobalt and mild leaching conditions. However, in ammonia leach liquor, cobalt and ammonia are complexed stably, which undoubtedly increases the difficulty of preparing cobalt products from the solution. In this study, ammonia distillation process was proposed and studied to recover cobalt from NH₃-(NH₄)₂CO₃ system. First, the E-pH diagram of Co-NH₃-CO₃^2--H₂O system was drawn, and the possibility of preparing cobalt products from the solution was discussed. Then, by comparing atmospheric and pressurized ammonia distillation processes, it was found that the microspherical Co₃O₄ product can be obtained directly through pressurized ammonia distillation. Furtherly, the effects of parameters on this process and the formation mechanism of Co₃O₄ were investigated systematically. Over 99% of cobalt could be recovered in one step under optimal conditions. Finally, CoCO₃ products with different morphologies were also obtained directly by adding the reducing agent during pressurized ammonia distillation, and the cobalt recovery rate was hardly affected. The evaporated ammonia and residual solution can be recycled. This work realizes the one-step preparation of cobalt products and provides a new perspective on cobalt recovery from ammoniacal solution.

Isovalent anion-induced electrochemical activity of doped Co3V2O8 for oxygen evolution reaction application

The activity of an OER electrocatalyst is a strong function of the reaction kinetics at the active sites, which can be influenced by catalytic engineering (e.g., heterostructure, doping, and the addition of cocatalysts). Herein, we report the improved reaction kinetics of cobalt oxide for the OER via the addition of high valence vanadium and thereafter doping with sulphur (S-Co₃V₂O₈). The addition of vanadium increases the oxygen vacancy while the doping of sulphur increases the electronic conductivity of the electrocatalyst. The synergic effect of the oxygen vacancy and electronic conductivity increases the activity of S-Co₃V₂O₈. Furthermore, S-Co₃V₂O₈ showed the least Tafel slope, which showed the activity enhancement towards the oxygen evolution reaction. Moreover, the underlying reaction mechanism is explored by electrochemical impedance spectroscopy, which reveals that the ratio of polarisation resistance to double-layer capacitance is minimum for S-Co₃V₂O₈, indicating the highest activity.

Amorphous nickel tungstate films prepared by SILAR method for electrocatalytic oxygen evolution reaction

Development of electrocatalyst using facile way from non-noble metal compounds with high efficiency for effective water electrolysis is highly demanding for production of hydrogen energy. Nickel based electrocatalysts were currently developed for electrochemical water oxidation in alkaline pH. Herein, amorphous nickel tungstate (NiWO₄) was synthesized using the facile successive ionic layer adsorption and reaction method. The films were characterized by X-ray diffraction, Raman spectroscopy, Fourier transfer infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy techniques. The electrochemical analysis showed 315 mV of overpotential at 100 mA cm^-2 with lowest Tafel slope of 32 mV dec^-1 for oxygen evolution reaction (OER) making films of NiWO₄ compatible towards electrocatalysis of water in alkaline media. The chronopotentiometry measurements at 100 mA cm^-2 over 24 h showed 97% retention of OER activity. The electrochemical active surface area (ECSA) of NW120 film was 25.5 cm^-2.

Climbing with support: scaling the volcano relationship through support–electrocatalyst interactions in electrodeposited RuO2 for the oxygen evolution reaction

The interfacial charge transfer and support-induced electrocatalyst faceting in thin catalysts enable ‘climbing up’ the volcano map for OER electrocatalysts. The conductivity of the support determines the OER activity of thick catalysts.

Design of iso-material heterostructures of TiO2via seed mediated growth and arrested phase transitions

Stabilization of different morphologies of iso-material native/non-native heterostructures is important for electron-hole separation in the context of photo-electrochemical and opto-electronic devices. In this regard, we explore the stabilities of different morphologies of rutile ("native", ground state phase) and anatase ("non-native" phase) TiO₂ heterostructures through (1) seed-mediated growth and (2) a thermally induced arrested phase transition synthesis protocol. Furthermore, the experimental results are analyzed through a combination of Density Functional Tight Binding (DFTB) and Finite Element Model (FEM) methods. During the seed-mediated growth, anatase is grown over a polydispersed and polycrystalline rutile core through thermal treatment yielding core-shell, Janus and yolk-shell iso-material heterostructures as observed from HRTEM. The arrested phase transition of anatase to rutile at different annealing temperatures yields rutile crystals in the subsurface region of the anatase and rutile/core-thin anatase/shell heterostructures but does not yield a Janus structure. Small particles that can be modeled via DFTB computations suggest that: (1) a heterostructure of the rutile/core-anatase/shell is energetically more stable than the anatase/core-rutile/shell or any other Janus configuration, (2) the off-centered rutile/core-anatase shell is more favorable to the mid-centered rutile/core-anatase shell and (3) Janus heterostructures can be stabilized when the mass ratio of the rutile seed to anatase overgrowth is high. FEM simulations, performed to evaluate the importance of stress relaxation in bicrystalline materials without defects, suggest that Janus structures can be stabilized in larger particles. The present studies add to the heuristics available for synthesizing iso-material heterostructures.