A hydrogen-oxygen fuel cell at 298 K is first run with pure $\mathrm{O}{}_2$ at $1\mathrm{bar}$ and then with air, in which the partial pressure of $\mathrm{O}{}_2$ is $0.21\mathrm{bar}$. Hydrogen is at $1\mathrm{bar}$ in both cases and the product is liquid water. Taking $\frac{2.303RT}{F}=0.0591\mathrm{V}$ and $\log 2.1=0.322$, the change in the open-circuit EMF on switching to air is

1. Aa decrease of about $10\mathrm{mV}$
2. Ba decrease of about $40\mathrm{mV}$
3. Can increase of about $10\mathrm{mV}$
4. Dno change, since the cathode reaction is the same