The cause of residual capacity in nickel oxyhydroxide electrodes

Understanding the Oxidative Properties of Nickel Oxyhydroxide in Alcohol Oxidation Reactions

The NiOOH electrode is commonly used in electrochemical alcohol oxidations. Yet understanding the reaction mechanism is far from trivial. In many cases, the difficulty lies in the decoupling of the overlapping influence of chemical and electrochemical factors that not only govern the reaction pathway but also the crystal structure of the in situ formed oxyhydroxide. Here, we use a different approach to understand this system: we start with synthesizing pure forms of the two oxyhydroxides, β-NiOOH and γ-NiOOH. Then, using the oxidative dehydrogenation of three typical alcohols as the model reactions, we examine the reactivity and selectivity of each oxyhydroxide. While solvent has a clear effect on the reaction rate of β-NiOOH, the observed selectivity was found to be unaffected and remained over 95% for the dehydrogenation of both primary and secondary alcohols to aldehydes and ketones, respectively. Yet, high concentration of OH^- in aqueous solvent promoted the preferential conversion of benzyl alcohol to benzoic acid. Thus, the formation of carboxylic compounds in the electrochemical oxidation without alkaline electrolyte is more likely to follow the direct electrochemical oxidation pathway. Overoxidation of NiOOH from the β- to γ-phase will affect the selectivity but not the reactivity with a sustained >95% conversion. The mechanistic examinations comprising kinetic isotope effects, Hammett analysis, and spin trapping studies reveal that benzyl alcohol is oxidatively dehydrogenated to benzaldehyde via two consecutive hydrogen atom transfer steps. This work offers the unique oxidative and catalytic properties of NiOOH in alcohol oxidation reactions, shedding light on the mechanistic understanding of the electrochemical alcohol conversion using NiOOH-based electrodes.

Experimental Study of Heat Generation Rate during Discharge of LiFePO4 Pouch Cells of Different Nominal Capacities and Thickness

High manufacturing cost and thermal stability of Li-ion battery cells are currently the two main deterrents to prolific demand for electric vehicles. A plausible solution to this issue is a modular/scalable battery thermal management system (TMS). A modular TMS can ensure thermal reliability for battery cells of different capacities and size without needing major structural revision besides facilitating mass-production. However, understanding the relationship of heat generation rates with cell capacity and thickness is essential for developing a scalable TMS. The present paper discusses results derived from an experimental investigation undertaken with this purpose. Heat generation rates for LiFePO4 pouch cells of different nominal capacities are measured at discharge rates of 0.33C, 1C and 3C in ambient temperatures ranging between −10 and 50 °C using a custom-designed calorimeter. It is observed that heat generation rates of the LiFePO4 pouch cells become independent of their nominal capacity and thickness if the ambient temperature is regulated at 35 °C. In ambient temperatures lower than 35 °C though, the thin battery cells are found to be generating heat at rates greater than those of thick battery cells and vice-versa at temperatures over 35 °C for all discharge rates.

Memory effect in a lithium-ion battery

Memory effects are well known to users of nickel-cadmium and nickel-metal-hydride batteries. If these batteries are recharged repeatedly after being only partially discharged, they gradually lose usable capacity owing to a reduced working voltage. Lithium-ion batteries, in contrast, are considered to have no memory effect. Here we report a memory effect in LiFePO4-one of the materials used for the positive electrode in Li-ion batteries-that appears already after only one cycle of partial charge and discharge. We characterize this memory effect of LiFePO4 and explain its connection to the particle-by-particle charge/discharge model. This effect is important for most battery uses, as the slight voltage change it causes can lead to substantial miscalculations in estimating the state of charge of batteries.

Oxygen Plasma Preparation of Nickel Oxide Layers

Nickel oxide thin films form and grow when Nickel foils are exposed to an inductively coupled low pressure oxygen plasma. The resulting oxide (NiO) is identified by near-grazing X-ray diffraction and by FTIR specular reflectance spectroscopy methods. The thickness of the NiO layer increases with exposure time.