The global urgency to reduce greenhouse gas emissions has accelerated the use of critical raw materials, creating a pressing need to manage the increasing volume of end-of-life lithium-ion batteries (LIBs). Repurposing these elements, especially those derived from the cathode active material, offers a crucial approach to minimising both environmental impact and the consumption of critical raw materials, which are essential to a circular economy. Ni-Co-Mn-based materials are commonly employed as cathode active materials due to their energy density, power capability, stability and
safety1. In parallel with the push toward a circular economy for batteries, the demand for H₂ production has been rapidly increasing and receiving more attention since it can be used as a feedstock and energy carrier for diverse applications. Electrochemical water splitting coupled with a renewable energy source for preparing green hydrogen provides great potential to achieve carbon neutrality. The development of high-performance OER and HER catalysts is the key aspect of water electrolysis. Pt-based electrocatalysts are usually employed to catalyse HER with an overpotential as low as ≈30 mV². However, its high cost and low availability continuously prompt the discovery of substituents that can present comparable catalytic activities. Transition metal catalysts could be considered a good alternative as are more affordable and sustainable. This study aims to use nickel (Ni), cobalt (Co), and manganese (Mn) obtained from battery cathode recycling as electrocatalysts for HER. Ternary transition metals compounds (NiCoMn) were obtained by hydrometallurgical processes for the treatment of waste lithium-ion batteries (based on lithium, nickel, cobalt, and manganese oxide cathodes) and hydrothermal synthesis followed by H₂ reduction.

The electrochemical performances of synthesised NiCoMn materials and NiCoMn obtained from cathode recycling were tested for HER in alkaline media. Carbon black was also incorporated in the matrix of ternary transition metal compounds to promote synergy between the ternary compound and the highly conductive carbon material. These results demonstrated the synergistic effect between the three metals; the resulting NiCoMn_200 °C material performs better than the sum of its parts, showing improved activity, faster kinetics (lower Tafel slopes), and better long-term stability compared to single-element catalysts. Additionally, Co oxides sites could be mainly responsible for this activity and stability performances. The detailed electrochemical characterisation revealed that the reduction of the NiCoMn under H₂ flow at 200 °C forms an amorphous/crystalline structure of NiCoMn, exhibits markedly improved HER performance in alkaline media, when compared to samples without temperature treatment and those treated at higher temperatures. These optimised catalysts afford overpotentials of -329 mV, exhibiting not only apparent but also excellent intrinsic HER activities.