Green conversion of spent alkaline battery materials to value-added materials

Abstract

This study aims to investigate eco-friendly strategies for recovering and functionalizing spent zinc oxide (ZnO) and spent manganese oxide (MnOx) from waste alkaline batteries. In detail, after extraction from waste alkaline batteries, spent ZnO and spent MnOx served as secondary metal resources to synthesize value-added materials. A solvent-minimized mechanochemical method was utilized to convert spent ZnO into metal-organic frameworks (MOFs) materials including zeolitic imidazolate framework–8 (ZIF–8) and metal-organic framework–74 (MOF–74). The synthesized ZIF–8 was subjected to capture hazardous iodine vapor within a short exposure duration, exhibiting industrial competitive iodine capture capability. The structural evolutions including long-range order and short-range order changes during the iodine capture process were monitored using structural characterization techniques. ZIF–8 synthesized from spent ZnO was revealed to structurally capture iodine, avoiding the release of iodine vapor into the environment. Additionally, spent ZnO and spent MnOx were transformed simultaneously into ZIF–8@Mn3O4 composites via a mechanochemical route. The ZIF–8@Mn3O4 composites exhibited superior bisphenol A degradation capability compared to the untreated battery material. In the composites, Mn3O4 plays a crucial catalytic role in bisphenol A (BPA) degradation, while its catalytic effect is more pronounced in the presence of ZIF–8 material. Moreover, spent ZnO was more prone to be converted into MOF–74 than commercial ZnO under identical mechanochemical conditions, and the MOF–74 synthesized from spent ZnO exhibited superior CO2 adsorption capability than those synthesized from commercial ZnO. The transformation of cost-free ZnO into value-added MOF–74 via a green and scalable method demonstrated the great potential in the reutilization of waste alkaline batteries.

Element-doped strategy was applied to reconstruct the structure of spent MnOx to achieve waste valorization. Ca2+ and Al3+ ions were successfully incorporated into MnOx, respectively, through thermal sintering schemes. The structural evolutions upon doping with different contents of CaCO3 and Al2O3 were thoroughly studied. In Ca-doped system, marokite phase CaMn2O4 was revealed to has a certain solid solubility to form Ca1-xMn2O4- (x=0, 0.05, 0.10, 0.15), in which calcium deficiency and oxygen vacancy were simultaneously introduced. The samples Ca1-xMn2O4- exhibited better oxygen evolution reaction (OER) catalytic activities over MnOx (MnO2 and Mn3O4), and Ca0.95Mn2O4- has the highest activity among Ca1-xMn2O4-, which might result from the less numbers of O around Mn and the shortest Mn-O. In Al-doped system, mono-tetragonal structure in pristine Mn3O4 gradually transformed to nano-heterogeneous phase upon Al2O3 doping contents from 5% to 25%. The sample doing with 15% of Al2O3 exhibited superior OER catalytic activity among the Al-doped samples. At last, a self-consumed strategy was proposed that spent MnOx acted as a stable immobilized form to stabilize different amounts of spent ZnO, achieving a reduced metal leaching performance. Overall, this work provides several cost-effective methods to achieve waste alkaline battery materials valorization.