Conductivity of solution due to total ions present in per unit volume of solution is known as specific conductivity. Specific conductivity decreases due to decrease in the number of ions per unit volume. We add water to aqueous solution, number of ions present in per unit volume decreases.

Standard hydrogen electrode (SHE) is the reference electrode whose electrode potential is taken to be zero. The electrode potential of other electrodes are measured with respect to it .

Cell reaction represented in the question is composed of two half cell reactions. These reactions are as follows

At anode $\mathrm{Cu} \longrightarrow \mathrm{Cu}^{2+}+2 e^{-}$

At cathode $\mathrm{Cl}_2+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{Cl}^{-}$

Copper is getting oxidised at anode. $\mathrm{Cl}_2$ is getting reduced at cathode.

$$\begin{aligned} \mathrm{Zn}+\mathrm{Cu}^{2+} \longrightarrow \mathrm{Zn}^{2+}+\mathrm{Cu} & \\ E_{\text {cell }} & =E_{\text {cell }}^{\circ}-\frac{0.0591}{2} \log \left[\frac{\mathrm{Zn}^{2+}}{\mathrm{Cu}^{2+}}\right] \\ E_{\text {cell }}^{\circ} & =E_{\text {cell }}^{\circ}+\frac{0.0591}{2} \log \left[\frac{\mathrm{Cu}^{2+}}{\mathrm{Zn}^{2+}}\right] \end{aligned}$$

According to this equation $E_{\text {cell }}^{\circ}$ is directly dependent on concentration of $\mathrm{Cu}^{2+}$ and inversely dependent upon concentration of $\mathrm{Zn}^{2+}$ ions.

$E_{\text {cell }}$ decreases when concentration of $\mathrm{Zn}^{2+}$ ions is increased.

Primary batteries contain a limited amount of reactants and are discharged when the reactants have been consumed. Secondary batteries can be recharged but it take a long time. Fuel cell run continuously as long as the reactants are supplied to it and products are removed continuously.