Electrodeposition of Rhenium-Iron Alloy from Water-in-Salt Electrolytes

Abstract

Rhenium is a refractory metal and has distinguished properties, including excellent wear resistance, superior tensile strength, high creep-rupture, and resistivity in elevated temperatures [1]. More interestingly, it is also a superconductor at cryogenic temperatures. Electrodeposited superconducting materials can be used to fabricate superconducting circuits for cryogenic quantum devices [2,3]. As rhenium oxide is often inevitably formed during rhenium deposition, which prevents further reduction of itself into rhenium metal, highly acidic electrolytes are typically required for rhenium electrodeposition [4]. A significant amount of proton in the electrolyte, however, exacerbates hydrogen evolution reaction, resulting in not only a low current efficiency but also hydrogen incorporation into the metal deposit. The latter often, causes cracks and pin holes in the deposited films. Water-in-salt electrolyte is an aqueous solution with super high concentration of salt, e.g. LiCl, where the hydration of salt depletes or significantly decreases free water molecules, thus disrupting the hydrogen bond network, and suppressing proton reduction. In our previous work, a water-in-salt electrolyte of super high concentration of LiCl was applied to electrodeposit rhenium and rhenium-cobalt alloy to mitigate hydrogen evolution reaction and film cracks [5,6].In this presentation, the study has been extended to rhenium-iron alloy. While the rate decrease of hydrogen evolution reaction upon the use of water-in-salt electrolyte was expected, a surprisingly strong enhancement of Re deposition rate in the presence of Fe was also observed. Figure 1 shows the partial current densities of rhenium during alloy deposition, where it not only showed more than 10x deposition rate compared with an elemental Re water-in-salt electrolyte.The morphology and crystallographic structure of ReFe films and the effect of annealing will be characterized with microscopic and diffraction techniques and will be discussed in conjunction with pure metal films. Superconductivity and the effect of alloying on the critical temperature will also be discussed in detail in the talk. Figure 1 . The rhenium partial current in the deposited films from different applied potential from four electrolytes with 0, 10, 20, and 35 mM iron.