Terbium has atomic number 65. It forms a 3+ ion and also a 4+ ion…

The 3+ ion of a lanthanoid is $[\text{Xe}]\,4f^{n}$ with $n = Z - 57$. Unpaired electrons: $n$ up to 7, then $14 - n$. Spin-only moment: $\mu = \sqrt{n(n+2)}\ \text{BM}$. An empty, half-full or full 4f subshell ($4f^0$, $4f^7$, $4f^{14}$) makes an ion unusually stable. That explains $\text{Ce}^{4+}$, $\text{Tb}^{4+}$, $\text{Eu}^{2+}$ and $\text{Yb}^{2+}$. For a cell, $\Delta G^\circ = -nFE^\circ$ with $F = 96.5\ \text{kJ/(V mol)}$.