(Invited) Aqueous Electrochemical Reduction Approaches for Functionalized Surfaces
Authors: Herrala Reima , Cui Linfan , Wilson Benjamin , Yliniemi Kirsi , Lundström Mari 
Published: December 2024

To meet the demands of the green transition and mitigate climate change there is an increasingly urgent need for a wide range of crucial raw materials. Global resource scarcity, criticality, and supply disruption due to material shortfalls or geopolitical instability is driving efforts towards an inclusive, multi-materials circular economy with the objective of providing a more sustainable future. Within the domain of metals production, recent focus has been on development of new technological approaches that maximize the recovery of valuable materials, whilst simultaneously aiming at the reduction of the associated environmental impacts. In addition, the shift towards the increased use of lower grade input materials - either because of reduced ore quality or inclusion of secondary raw materials sources as feedstocks - often means there is a broader range of impurities within metallurgical process streams.
In the case of industrial hydrometallurgical base metals—Cu, Ni, Zn—production methods like leaching, precipitation, and electrochemical reduction (electrorefining/electrowinning) impurities can include valuable metals like silver, gold, or platinum at such limited concentrations that conventional methods of recovery are often not economically viable. Moreover, the nature of hydrometallurgy process solution is that it is often contain highly concentrated levels of base metal (10’s g/l) - relative to the impurity materials (ppm/ppb) - that excludes the selective extraction of the valuable metals by conventional separation steps like solvent extraction, precipitation, cementation, or ion exchange. Consequently, there exists a need for alternative methods that can efficiently remove such low-level elements from complex solutions for recirculation or re-use.
Over the last decade, two related electrochemical approaches have emerged as possible ways not only for the recovery of metals at low concentrations, but also for the creation of surfaces with additional functionality. The Electrodeposition-Redox Replacement (EDRR) methodology [1] exploits the cathodic reduction of less noble metal ions to electrodeposit a solid layer on an electrode surface by application of a constant or pulsed potential or constant current. This ED process is then followed by a period when the system is allowed to relax to its open circuit potential (OCP), which results in the spontaneous redox replacement (RR) of the deposited metal layer with more noble ions from the solution. These two steps - which can also be considered as sequential electrowinning-cementation - can be tailored to salvage different metals or produce different surfaces depending on the predominating base metal and the type of impurity within solution. For example, EDRR has been utilized to recover metals including Ag (from Zn process solutions), Au (from Cl leach solutions) and Te (from metallurgical industrial waste) as well as create surfaces with catalytic, anti-corrosion and analytical applications [1].
More recently, the related Electrochemically-assisted Aqueous Reduction (EAR) method has emerged as an alternative approach to selectively recover Au from high concentration Cu chloride-based solutions[2]. In contrast to EDRR, this method uses a current or potential that is pulsed in order to reduce the copper ions in the solution proximal to the electrode from Cu(II) to Cu(I). This generated monovalent copper can then reduce the Au(III) or Au(I) present in solution due to their nobility difference. In situ studies performed with EQCM-D have allowed the influence of solution concentration changes and applied electrochemical parameters on the overall recovery process to be determined. Additionally, through the appropriate selection of the system boundaries, electrode surfaces decorated by gold nanoparticles could be created demonstrating that EAR can be used to electrochemically prepare noble metal surfaces with the potential for additional functionalities like electrochemical ethanol oxidation from complex process solutions.
Overall, both EDRR and EAR offer alternative and efficient methods that allow not only for the recovery of trace levels of valuable metals from under exploited hydrometallurgical solutions, but these approaches also provide the potential to directly create materials with a range of surface functionalized materials. Furthermore, EDRR is already being exploited on an industrial scale [3] to enhance energy efficiency, improve materials circularity and process economics in a range of applications.

Acknowledgments

This work was supported by the Research Council of Finland project EARMetal (#339979 and #342080). The RawMatTERS Finland Infrastructure (RAMI) also funded by the Research Council of Finland and based at Aalto University is acknowledged.

References

[1] L. Cui, K. Yliniemi, J. Vapaavuori, M. Lundström, “Recent developments of electrodeposition-redox replacement in metal recovery and functional materials: A review” Chemical Engineering Journal 465, (2023) 142737.
[2] R. Herrala, Z. Wang, J. Vapaavuori, M. Lundström, K. Yliniemi, “Recovery of Gold as Nanoparticles from Gold-Poor Au-Cu-Cl Solutions” J. Phys. Chem. C, 127 (2023) 16099–16109
[3] https://elmery.fi/