Is the terminal potential difference always greater than its emf?

No, in general, the potential difference across a battery is smaller than its electromotive force. But, when an electric current flows through a battery in such a way that the direction of current is opposite to the direction of the emf of the battery, the potential difference across the battery is greater than its emf. In such a case, the current flows from the positive to the negative terminal inside the battery (charging of the cell). The relationship between the potential difference of a battery and its emf is given by

E = V + (-I)r, where r is the internal resistance of the battery

E = V - Ir For r ≠ 0, then E < V. At this condition a part of the emf is used in overcoming the internal resistance of the cell.