Core–shell Mg–Ni carbonate supercapacitor materials by concurrent Ni recovery and CO2 mineralization

Abstract

Quickly increasing CO₂ concentrations and rising demands for nickel (Ni) urge us to develop new efficient Ni recovery methods from unconventional resources, such as mafic and ultramafic low-grade ores, while simultaneously reducing CO₂ concentrations. Concurrent CO₂ mineralization and Ni recovery offer great potential for addressing both environmental and resource challenges efficiently. However, the formation of Ni carbonate in the presence of major alkaline earth cations that widely coexist (i.e., Mg) during CO₂ mineralization remains unclear, complicating the product separation and utilization. Here, we investigated the formation of Mg and Ni carbonates at 180 °C and a CO₂ pressure of 100 bar. The results demonstrated that a core–shell structure of Ni–Mg carbonates was formed, where magnesite (MgCO₃) constituted the core and gaspéite (NiCO₃) formed the shell (MgCO₃@NiCO₃). This structure arose because magnesite crystallized faster than gaspéite, serving as a substrate that promoted the nucleation of gaspéite, as supported by the thermodynamic calculation of interfacial energies. However, the core–shell structure of Ni and Mg coprecipitates may still hinder the selective recovery of Ni from Mg. To address this challenge, we showed that the MgCO₃@NiCO₃ product can be directly used as a supercapacitor electrode without further treatment. The cyclic voltammetry (CV) results revealed that the composite had a capacitance comparable to that of pure NiCO₃ and a long recyclability. This study provides insights into carbonate mineral coprecipitation, applicable to Ni recovery from sources (e.g., electronic waste and silicate minerals), and introduces novel electrode materials to be used in catalysts and ion batteries, making the overall process more sustainable.