Production of a nickel-based catalyst for urea electrooxidation using spent batteries as raw material: Electrochemical synthesis and implications from a circular economy stand-point

In this work, we leached the electrochemically active powder (black mass) from spent Ni-MH batteries in an aqueous solution containing citric acid and hydrogen peroxide. The obtained multimetallic solution was chemically analyzed and its speciation simulated via thermodynamic software. Electrolysis of the leachate rendered a new metal coating containing Ni, Zn and Co on a copper substrate. The electrochemical stability of the alloy-coated electrode was evaluated in a 1 M KOH solution through cyclic voltammetry analysis and compared against a pure‑nickel electrode standard. Moreover, the catalytic capacity of the alloyed electrode was studied by cyclic voltammetry and chronoamperometric analysis using a mixture of 1 M KOH and 0.3 M urea. During the electrooxidation of urea, the Ni-Zn-Co coated electrode exhibited better performance than the pure‑nickel catalytic electrode used as standard, with a current response 1.8 times higher than pure nickel and 9.4% lower energy consumption than pure nickel. This promising result opens the possibility for targeting the manufacturing of catalytic electrodes as a viable end-user of the recovered metal values contained in spent batteries. As a result, a conceptual process flow diagram was devised for this novel valorization procedure. The application of these alloyed electrodes for the remediation of urea-containing water was conceptually proposed, as well. From a circular economy standpoint, this work describes an industrial ecosystem involving the life cycle of Ni-MH batteries, the electroplating industrial process, and the environmental services. Consequently, an innovative and symbiotic materials flow was depicted for wastewater treatment based on electrooxidation procedures.