High voltage on downstream O2 sensor after driving, but low voltage on idle

After driving on the highway, my downstream O2 sensor voltage was a steady 0.8V, which is high.

On the other hand, when idling before the drive, it was a steady 0.3V.

Is it normal for the downstream O2 sensor voltage to be so high after driving? Does this mean I have a bad catalytic converter, or is there another explanation?