Binary heap removal peculiar potential function analysis
Amortized analysis relies on $\sum_{i=1}^n \overline{C_i} = \sum_{i=1}^n C_i+\phi_n-\phi_0$ being an upper bound for $\sum_{i=1}^n C_i$. This requires $\phi_n$ to be at least as large as $\phi_0$. Your potential function does not satisfy this requirement, aince $\phi$ will get smaller with each step.