Direct upcycling of Mn-rich residues into γ-MnOOH for efficient Alkaline OER

The growing stream of end-of-life lithium-ion batteries from electronic and mobility products has created a pressing need to manage solid residues generated during battery-recycling operations. In ammonia-based leaching flowsheets used for selective critical metal recovery, manganese-rich solid residues (e.g., MnCO₃ or ammonium manganese sulfite/sulfate) are intentionally formed to avoid complex purification. Yet they are often retained as low-value waste due to impurities from unreacted cores. Here, we demonstrate a waste-to-resource pathway that bypasses solution phase recovery by upcycling manganese-rich residues directly into a γ-MnOOH alkaline oxygen evolution catalyst. The residue with MnCO₃ as the main precursor delivers the best performance, with an overpotential of 457.65 mV at 100 mA cm^-2 and a Tafel slope of 60.37 mV dec^-1, demonstrating a favorable comparison with commercial noble metal and other benchmarks using commercial transition metal salts as raw materials. Structure-activity analysis attributes the performance to the preferential expression of the high-index facet. This direct solid-state route avoids reagent-grade manganese precursors, eliminates gypsum-forming steps, and cuts life cycle economic and energy demand. The work illustrates a practical waste-to-resource strategy that strengthens manganese security and advances circular economy goals in clean energy technologies.