Electrodeposition of Pt and Gd from the Same Ionic Liquid

Abstract

Platinum rare earth alloys show several times higher electrocatalytic activity than pure platinum while still maintaining an excellent stability [1-3]. This has been demonstrated for polycrystalline bulk materials [1, 3], for nanoparticles prepared in a cluster source [2] and for thin sputtered films [4, 5]. Once exposed to aqueous solutions, dealloying leads to formation of a several layers thick Pt skin that is under compressive strain, causing the activity enhancement [1]. The controlled preparation of such alloys using scalable techniques is difficult due to the high reactivity of the rare earth elements [5]. Electrochemical deposition from ionic liquids should be a possibility [6]. The electrodeposition of a number of rare earth elements has been reported in the literature [7-10], even though not in all cases clear proof has been given that a pure metal has been obtained. Several studies showed that in TFSI based ionic liquids rare earth electrochemistry can be complicated, and that not always a metal can be easily achieved [11]. Our own experiments – usually applying the electrochemical quartz crystal microbalance technique - on the electrodeposition of yttrium from two ionic liquids did not lead to the desired metal deposition [6], while there was indication for deposition of lanthanum [6] and gadolinium from BMP TFSI using TFSI precursors. However, the deposition was very slow, and independent proof for actual metal deposition was not obtained. Motivated by a study reporting lanthanum electrodeposition from butylmethylimidazolium dicyanamide (BMIm DCA) at elevated temperatures [10], we decided to attempt the gadolinium deposition from this ionic liquid. While experiments at room temperature failed due to passivation of the counter electrode, the deposition worked well at 60°C using GdCl3 as precursor. Under these conditions a thick metal greyish layer was obtained that in contact with air rapidly reacted and became whitish. The electrodeposition of Pt has been accomplished in different ionic liquids in literature [12-14], and we also managed to deposit Pt from two ionic liquids using a –still water containing – H2PtCl6 precursor [6]. In this work we discuss the results obtained for Pt deposition using different water-free precursors in BMIm DCA, namely Pt(acac)2, PtCl2 and PtCl4, and in part using additives to control complex formation. From Pt(acac)2, electrodeposition of Pt was not achieved at all. Addition of Pt(acac)2 to a GdCl3 electrolyte did not result in alloy formation, but the electrodeposition of Gd was blocked as well. Using PtCl2 and PtCl4, indication for successful Pt deposition was obtained. The role of complex formation on Pt deposition and alloy deposition will be elaborated in this contribution. The project leading to this application has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 700127. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation Programme and Hydrogen Europe and N.ERGHY.