For redox questions:
Weakest OA: Zn2+
Conventional cell diagram: Zn | Zn2+ || Co2+ | Co
Half equation: Zn -> Zn2+ + 2e-
Cobalt species: Co3+, oxidation product is oxygen, then reaction
Negative electrode: H2 + 2OH- -> 2H2O + 2e-
Positive electrode: 4e- + O2 + 2H2O -> 4OH-
Overall reaction: 2H2 + O2 -> 2H2O
Graph of emf: just a straight horizontal line