*4.2. Ni-Based Ions and Nanoparticles*

Similarly, nickel ions or Ni-based nanoparticles are another widely studied substance that can significantly enhance BioH2 production in dark fermentation. The mechanisms between Ni-ion/Ni-based nanoparticles and Fe-ion/Fe-based nanoparticles are largely identical but with minor differences. The key mechanisms for Ni include (a) facilitating the synthesis of [FeNi] hydrogenase [42], (b) improving the activity of ferredoxin oxidoreductase [43], and (c) Ni NPs controlling the concentration of Ni2+ at the optimum level. In addition, it is worth noting that [NiFe] hydrogenase exists in more bacteria than [FeFe] hydrogenase. Therefore, Ni can promote H2-producing bacteria in the dark fermentation process to a certain extent [44].

#### **5. Results**
