Showing papers by "Choong-Nyeon Park published in 2012"
TL;DR: In this article, the simple/cost-effective hydriding combustion synthesis (HCS) was used to prepare highly active Mg-based-samples and its effects on the micro-structural and hydrogen storage properties were determined.
11 citations
04 Jun 2012
TL;DR: In this article, NAFION was used as a separator between the electrode and electrolyte allowing only H ions to flow across it in the battery applications, avoiding the surface corrosion of Mg based electrodes.
Abstract: Magnesium-based hydrogen storage alloys have attracted much interest because of their high hydrogen storage capacity, low weight, low material cost and easy accessibility [1]. However, their hydrides are too stable to be used at room temperatures and around atmospheric pressure. Mg–Ni mixed in some ranges using ball-milling and refined to an amorphous state exhibits high rate of H2 absorption/desorption kinetics. Ball milling the alloy (ingots prepared by arc-melting) in H2 is the most widely used method to refine the grains of hydrides; however it is time consuming, expensive and energy-inefficient. To produce hydrogen storage alloys, especially for magnesium-based alloys hydriding combustion synthesis (HCS) was firstly proposed by Akiyama et al in 1997 [2]. HCS can yield ultrafine product in a combined single-step synthesis of metal alloys with its hydriding, and possess the advantages like short processing time, high product purity and low energy requirement. However, when these Mg based alloys with high hydrogen storage capacities are used in electrochemical application, such as Ni-MH batteries, it shows very poor cycle-ability, due to formation of Mg(OH)2 on its surfaces [3]. To achieve the stability for Mg based alloys in electrolytes for battery application, there have been various reports related to its structural modification. However, structural modifications are likely to decrease its overall storage capacity. In present research, we propose an alternative solution to this problem by coating the Mg based electrode by NAFION. The NAFION is well known for its proton exchange properties and due this property it can be used as a separator between the electrode and electrolyte allowing only H ions to flow across it in the battery applications, avoiding the surface corrosion of Mg based electrodes. The Mg95-Ni5 alloys have been prepared using fast, simple and cost-effective hydrating combustion synthesis (HCS)[4]. Prepared alloys were characterized for their micro-structural and electrochemical properties. The microcrystalline Mg and Ni were mixed in acetone using ultra-sonication treatment and dried-mixture was allowed to undergo the thermal profile in specially designed reactor. The thermal profile comprise the heating the reactor to 590°C for 5min in H2 gas at 2MPa and cooling to 400°C which is maintained for 4h. The prepared samples are mixed with additives to fabricate the electrode to test its discharge capacities using electrochemical cell testing method. Before testing the fabricated electrode, the electrodes with contacts were dip-coated in NAFION solution for different dipping time and dipping cycles. Fig. 1 shows the XRD pattern of the HCS prepared Mg-Ni alloy and cross-sectional SEM images of NAFION dip-coated Mg-Ni alloy for different dipping time. The prepared alloy is not completely hydrided as the XRD pattern shows the peaks corresponding to unreacted Mg. By analysing the XRD intensity of different phase peaks we obtained the phase percentages which confirm that more than 70% of initial Mg is converted to β-MgH2 phase. The coating thickness of NAFION coated MgH2-Ni alloy was increased with increasing dipping times, which may limit the H penetration through the coating and may lead to decreased capacity. However, surface morphology of NAFION film prepared with 2 min dipping time was porous and therefore to achieve the uniform and pore-free surface the dipping cycles were repeated and were tested for electro-chemical properties.