Electropolishing Valve Metals with a Sulfuric Acid-Methanol Electrolyte at Low Temperature

This study reports the electropolishing Ti and Nb metals using a fluoride-free electrolyte of sulfuric acid and methanol at low temperature (-70°C) without prior treatment. A fluoride-free electrolyte provides a less hazardous and more environmentally friendly option for electropolishing procedure. Experimental studies are presented on electropolishing with sulfuric acid electrolyte, which provides high quality macro- and micro-smoothing of the metal surfaces. Optimal conditions yielded leveling and brightening of the surface of Ti and Nb metals beyond that of the currently utilized electropolishing procedures with fluoride-containing electrolytes. The root mean squared roughness (Rq) from atomic force microscopy (AFM) analysis was 1.64 and 0.49 nm for Ti and Nb, respectively. Lower temperature experiments led to noticeable kinetic effects, indicated by a dramatic drop in current densities and the expansion of the steady-state current density plateau in anodic polarization curves. In addition, the voltage range of the current plateau expanded with increasing acid concentration. Surface characterization of Ti and Nb metals after polishing provided evidence of salt film formation. In addition, these metals were used as substrates in the formation of nanostructured metal oxides. The overall quality of the polishing led to a dramatic improvement in the uniformity of the nanostructures

Electrochemically Induced Amorphous-to-Rock-Salt Phase Transformation in Niobium Oxide Electrode for Li-Ion Batteries

Intercalation-type metal oxides are promising negative electrode materials for safe rechargeable lithium-ion batteries due to the reduced risk of Li plating at low voltages. Nevertheless, their lower energy and power density along with cycling instability remain bottlenecks for their implementation, especially for fast-charging applications. Here, we report a nanostructured rock-salt Nb2O5 electrode formed through an amorphous-to-crystalline transformation during repeated electrochemical cycling with Li+. This electrode can reversibly cycle three lithiums per Nb2O5, corresponding to a capacity of 269 mAh g−1 at 20 mA g−1, and retains a capacity of 191 mAh g−1 at a high rate of 1 A g−1. It exhibits superb cycling stability with a capacity of 225 mAh g−1 at 200 mA g−1 for 400 cycles, and a Coulombic efficiency of 99.93%. We attribute the enhanced performance to the cubic rock-salt framework, which promotes low-energy migration paths. Our work suggests that inducing crystallization of amorphous nanomaterials through electrochemical cycling is a promising avenue for creating unconventional high-performance metal oxide electrode materials

Amorphous to Cubic Nanoporous Niobium Oxide Electrodes for Lithium-Ion Batteries

New electrode materials are needed to produce the next generation of electrochemical energy storage (EES) devices. In the case of amorphous metal oxides, lower energy and power densities have been reported, in general. Nevertheless, amorphous oxides should not be overlooked as a potential electrode material. We report here on the electrochemically driven amorphous-to-crystalline phase transformation of an anodically grown nano-channeled niobium oxide (NCNO) electrode. The crystalline NCNO has exhibited improvements in stability and rate capability compared to its amorphous form. This work aims to gain a fundamental understanding of the structural evolution of the metal oxide and charge storage and transport mechanism

Nanoporous Niobium Oxide Electrode for Sodium-Ion Batteries

The growing demand for renewable energy, like solar and wind, places an increasing need for large-scale energy storage systems. These systems are needed due to the intermittent nature of renewable energy sources. Li-ion batteries (LIB) have been selected to perform this task for its high energy storage. However, Li is relatively rare and has various obstacles to its production. Therefore, a more abundant and less expensive alternative is appealing. Sodium-ion batteries (SIB) has been considered a potential candidate for its abundance, low cost, and sustainability. Unfortunately, there are difficulties to overcome when implementing SIB. Sodium (Na) has a higher mass, larger ionic radius, and lower mobility compared to Lithium (Li). These properties cause a reduction in cycle stability, lower energy output, and increased stress/strain in the electrodes structure\u27s inability to support the difference between the two ions. Therefore, finding a new intercalation host that is capable of supporting the transfer of Na+ becomes paramount. This work explores the formation and use of niobium oxide as an anode material for SIBs. Anodization and temperature conditions were tuned to produce a variety of pore sizes with differing morphologies. Initial results indicate that the amorphous material formed at 30V performed at the highest capacity and is further explored herein

Nanoporous Niobium Oxide as an Anode for Na-ion Batteries

Solar, wind, and other renewable energy sources tend to be intermittent, and thus large-scale energy storage is needed to fully utilize them. While sodium-ion batteries currently fall short of the energy density of the leading lithium-ion technologies, they are a potentially cost-effective alternative, since sodium is more abundant but is chemically similar to lithium. Additionally, for stationary applications cost is a much larger driving factor than for mobile applications. However, improvements are needed to increase the stability and reliability of sodium-ion batteries before they become a legitimate option. Nanostructured metal oxides such as nanotube arrays are promising for use in anodes due to their high surface area and ability to withstand the volume changes that accompany repeated Na+ insertion/extraction during battery cycling. Niobium oxide is one such material, but research into its use in sodium-ion batteries is limited. Nanoporous niobium oxide films were synthesized via anodization of niobium foil, where the morphology was modified by changing the anodization voltage and the crystallinity was modified using heat treatments. The films were characterized with SEM and XRD, then cycled in half-cells with sodium foil counter electrodes to assess their electrochemical behavior

Nanostructured Niobium Oxide for Sodium and Potassium-Ion Batteries

The battery is a key component to the development and implementation of renewable energy. Wind and sunlight are intermittent energy sources. Batteries offer the ability to store this energy and distribute it during high demand. Currently, Lithium-ion battery technology leads the market in performance. However, lithium-based technologies face challenges in availability and cost. The ability to use sodium or potassium as an alternative ion source could negate these challenges. However, sodium and potassium-ion batteries are inherently difficult to produce due to their larger ionic size, weight, and lower mobility. Finding electrode materials that are capable of coping with these properties is a challenge. This study experiments on the use of nano-channeled niobium oxide (NCNO) as an anode for sodium and potassium-ion batteries. As a ceramic, niobium oxide is a prospective candidate as an electrode material. The porous nanostructure of NCNO provides more surface area for electrochemical reactions to take place. Additionally, amorphous NCNO’s can be crystallized to change the kinetics and cell performance of the battery. Samples of NCNO were electrochemically tested in sodium and potassium-ion half-cells. Through this work we will learn about the electrochemical performance of this material based on the rate capability, life cycle, and capacity observed

Electropolishing Valve Metals with Sulfuric Acid-Methanol Electrolyte

To develop uniform oxide nano-structures on the surface of valve metals, via anodization, it is desirable to start with a polished surface. Electropolishing is a common method to produce highly polished surfaces. However, common procedures utilize toxic, fluoride containing electrolytes. This study reports on a novel method for electropolishing titanium and niobium, in a sulfuric acid/methanol electrolyte, at low temperature (-70 oC). Electropolishing at low temperature has a significant effect on reaction kinetics. Experiments show an expansion of the steady-state current density plateau of anodic polarization curves. Additionally, increasing the sulfuric acid concentration led to broadening of the current density plateau. Optimization of conditions produced a root mean squared roughness of 1.64 nm and 0.49 nm for titanium and niobium, respectively. An improvement over results obtained with fluorine-containing electrolytes. We believe it is possible to apply this method to other valve metals, like zirconium and tantalum. Preliminary experiments with zirconium have shown a brightening and smoothing of the surface. However, there is further work required to optimize results with this metal. Additionally, we show that polished valve metal surfaces produce more uniform nano-structures, formed via anodization