Fuel cell potential for the electrolysis of ammonia
You can see my detailed response on the sign of electrode potentials here:
Still taught to reverse oxidation half cells in electrochemistry?

To summarize, electrochemists all over the world have decided to quote all electrode potentials as reduction potentials. This tug of war of signs between the so-called European convention and American convention of electrochemistry for more than a century, resulted in this decision. Basically, the current convention is the same as that of Ostwald.