Unraveling the Role of Different Acids in Subcritical Water Treatment of Spent Lithium-Ion Battery for Photocatalytic Hydrogen Production

The worsening conditions of global warming caused by fossil fuels still remains the hottest issue to date. Hydrogen, as one of the most prospective renewable energies, was extensively developed to replace fossil energy sources. Besides, the transition to an electrification system has led to a massive use of lithium-ion batteries (LIB), which cannot be missed because of the huge amount of waste that can harm our environment. Rather than disposing of spent LIBs, this work utilized the spent LIBs, performed subcritical water treatment using different acids (HNO₃, H₂SO₄, HCl, and ascorbic acid), and applied them in photocatalytic hydrogen evolution reaction (HER). In brief, subcritical water treatment using HCl (LIB-C) demonstrated the most optimum photocatalytic HER activities, with an evolved hydrogen amount of 28.7 μmol within 5 h of solar-light irradiation in a 10% ethanolic solution. It was found that the reduction of metal compounds (Co and Mn) to lower valences occurred in LIB-C samples, which are preferred for photocatalytic water reduction to generate hydrogen because low valences of metals are more available to be oxidized to higher valences, Co²⁺ to Co³⁺ and Mn²⁺ to Mn³⁺ or Mn⁴⁺,.