Electrochemical Study of Zr from Anodic Dissolution using a Consumable Zirconium Nitride Anode
Authors: Solem Cathrine Kyung Won , Wang Zhaohui , Osen Karen Sende , Martinez Ana Maria 
Published: December 2024

The use of catalysts in the chemical industry is key to a clean energy transition, as catalysts are essential for the oxygen reduction reaction (ORR) for fuel cell and metal-air battery technologies. Today, it is common to use catalysts based on Platinum Group Metals (PGMs), e.g., Pt and Pd, but this comes with a high cost as well as the use of strategic raw materials defined by the European Union. It has therefore been investigated other more abundant and cheaper materials.
Nitride-based materials have been studied and identified as promising catalysts for the chemical industry, and it has been reported that a zirconium nitride (ZrN)-based catalyst had an even greater performance as a catalyst in alkaline ORR than the state-of-the-art Pt-based catalysts, thus, being a promising replacement for future catalysts. It is, however, important to include how new and upcoming catalysts are to be recycled, as the chemical industry is dependent on a reliable access to the materials to keep up with the green transition that electrochemical technology is a central part of.
Through the FIREFLY project, the present work is focusing on how electrochemical technologies can be used to recover metals from spent catalysts from the chemical industry. Novel electrochemical processes using consumable anodes have been reported for other materials, such as TiOxCy and TiN, but not for ZrN. The present work has therefore focused on the use of ZrN as a consumable anode in an electrochemical recovery of Zr metal.
A mechanically stable ZrN anode was made from synthetic ZrN powder. The anodic dissolution of Zr ions has been demonstrated in an equimolar NaCl-KCl molten chloride mixture at 1000 K, by potentiostatic electrolysis using a molybdenum (Mo) cathode and an Ag/AgCl reference electrode as illustrated in Figure 1. Cyclic voltammetry (CV) of the molten chloride was performed before and after electrolysis, using a tungsten (W) or glassy carbon (GC) working electrode and a graphite counter electrode.
It was successfully demonstrated that Zr species were dissolved into the molten chloride mixture as presented in the insert in Figure 1. The CV results showed a two-step electrochemical process, which is believed to be related to the reduction Zr (IV) to Zr (II), and from Zr (II) to Zr metal. Analysis of the voltammetric curves obtained at different sweep rates showed diffusion controlled electrochemical reactions. The diffusion coefficients were calculated and correlated well with those reported in the literature. Post-mortem analyses were carried out of a molten chloride bath sample and of the ZrN anode. Inductively Couple Plasma-Mass Spectrometry (ICP-MS), Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS), and X-Ray Diffraction (XRD) analyses confirmed the presence of Zr species in the bath and Zr-Cl-N-O complexes at the ZrN anode surface, which verifies a successful electrochemical consumption through anodic dissolution.

Acknowledgements: This work has been funded by the European Union’s Horizon Europe research and innovation programme under Grant Agreement no. 101091715.
Figure 1 CVs obtained on a W working electrode before (black graph) and after anodic dissolution of a ZrN anode in an electrochemical cell using a 3-electrode set-up as shown in the insert. The A’/A (purple curve) and the B’/B (green curve) electrochemical systems were related to Zr(IV)/Zr(II) and Zr(II)/Zr, respectively. Sweep rate 200 mV/s.

Figure 1