I asked a question previously on here but still have some confusion. To briefly summarize, I made a voltaic cell for a school project with 0.1 M copper nitrate and varying concentrations of aluminum, zinc and nickel nitrates. My results were fairly consistent with the theoretical values from the Nernst equation for zinc, however, the aluminum and nickel cells produced substantially less voltage than their theoretical values. The answer I got from my previous question explained how the low voltage in the aluminum cell was a result of its oxide layer.
However, I still had some confusion over why there were differences between aluminum and nickel, and zinc. Firstly, it's my understanding that a metal's ability to form an oxide layer is related to its reactivity. This would explain aluminum's tendency to form such a strong oxide layer however, zinc has a higher reactivity than nickel so wouldn't it also form an oxide layer and its outputted voltage would be lowered similarly.
My next explanation for the differences between aluminum and nickel, and zinc was related to the ability of the oxides to either react or dissolve in the nitrate solutions, however, I found that nickel oxide is soluble in acids and aluminum oxide reacts with acids.
I was also wondering how I even got a voltage in the aluminum cell if the oxide layer does not allow any aluminum ions to be transferred.
I would appreciate some guidance from the chemistry wizards because at this point I am stumped.