Unraveling\nthe Role\nof Different Acids in Subcritical\nWater Treatment of Spent Lithium-Ion Battery for Photocatalytic Hydrogen\nProduction

The worsening conditions of global\nwarming caused by\nfossil fuels\nstill remains the hottest issue to date. Hydrogen, as one of the most\nprospective renewable energies, was extensively developed to replace\nfossil energy sources. Besides, the transition to an electrification\nsystem has led to a massive use of lithium-ion batteries (LIB), which\ncannot be missed because of the huge amount of waste that can harm\nour environment. Rather than disposing of spent LIBs, this work utilized\nthe spent LIBs, performed subcritical water treatment using different\nacids (HNO<sub>3</sub>, H<sub>2</sub>SO<sub>4</sub>, HCl, and ascorbic\nacid), and applied them in photocatalytic hydrogen evolution reaction\n(HER). In brief, subcritical water treatment using HCl (LIB-C) demonstrated\nthe most optimum photocatalytic HER activities, with an evolved hydrogen\namount of 28.7 μmol within 5 h of solar-light irradiation in\na 10% ethanolic solution. It was found that the reduction of metal\ncompounds (Co and Mn) to lower valences occurred in LIB-C samples,\nwhich are preferred for photocatalytic water reduction to generate\nhydrogen because low valences of metals are more available to be oxidized\nto higher valences, Co<sup>2+</sup> to Co<sup>3+</sup> and Mn<sup>2+</sup> to Mn<sup>3+</sup> or Mn<sup>4+</sup>.