## 3 Results and Discussion
### Study with Different Membranes  
In this work three types of membranes have been tested as separators in the fuel cell. Two of them are strictly anion-exchange ones (Fumapem* FFA-3-PK-75 (OH\({}^{-}\)form) and Neosepta* AFN). The third one, Celgard 3501 has been found suitable to conducting either protons or OH\({}^{-}\) anions. The experiments have been carried out in a rectangular fuel cell with a cross-section of 100 sq. cm. The polarization curves with the power density data are shown in Figure 13a,b. All three membranes show similar results with a slight superiority of the Fumapem one. Figure 12: Effect of direct aeration in the cathode space, 6 L/min; 241 mg dm\({}^{-3}\) sulfide solution. Continuous process. Membrane Celgard 3501. Active membrane area—7 sq.cm. Feed flow rate—0.2 dm\({}^{3}\)/h. Anode volume 80 cm\({}^{3}\). (\(\bullet\))—cell voltage; (\(\Circle\)) power density. Temperature 20 \({}^{\circ}\)C.  
Figure 13: Comparison of the polarization curves (**a**) and the power densities (**b**) for three different membranes. Sulfide concentration—256 g dm\({}^{-3}\). Supporting electrolyte—16 g dm\({}^{-3}\). Active electrode and membrane surface—100 sq.cm. Feed flow rate—0.2 dm\({}^{3}\)/h. (\(\Circle\)), solid line—Celgard 3501; (\(\bullet\)), solid line—Neosepta*AFN; (\(\square\)), dashed line - Fumapem* FFA-3-PK-75. Importantly, this type of membranes is characterized by their ionic conductivity \(\sigma\). It is reciprocal to the specific material resistance. The temperature dependence of the membranes has the following exponential form [31]:  
\[\sigma T\sim\exp(-Ea/RT),\,[\sigma T]=\text{mS-cm}^{-1}\;\text{K} \tag{5}\]  
The conductivity of the tested membranes was estimated from the polarization curves, taking the total Ohmic resistance of the fuel cell and then calculating its specific conductivity at a certain temperature. Of course, such calculation gives the total fuel cell resistance including the solution resistance, as well as the one of the electrodes, due to mass transfer, etc. However, in our case it also gives an idea about the applicability of each membrane. A comparison of the membrane conductivities with those obtained for solid membranes operating in fuel cells with hydrogen and oxygen is gaseous phase is shown in Figure 14 in an Arrhenius logarithmic plot. (The reference data are taken from Reference [30], pp. 154-196). They deal with gadolinium doped ceria and ytta doped zirconia for the temperature range of 550-1250 \({}^{\circ}\)C.  
Although our experiments were carried out at ambient temperature the overall cell conductivity for all of the tested membranes seems better compared to the solid membranes used for gaseous phase fuel cells. The estimated overall fuel cell conductivities for the three membranes are 0.4 \(\div\) 2.8 mS-cm\({}^{-1}\) for the Celgard 3501 membrane; 7.7 mS-cm\({}^{-1}\) for the Fumapem membrane and 1.9 mS-cm\({}^{-1}\) for the Neosepta membrane. As a reference, the specific electric conductivity of sodium chloride (supporting electrolyte in our case) is about 67 mS-cm\({}^{-1}\) for the concentration used [33]. The durability of the tested membranes was assessed by measuring the overall cell conductivities in time. For the Celgard 3501 membrane this conductivity decreased from 2.8 to 0.4 mS.cm\({}^{-1}\) after forty experiments. The other two membranes were sensitive to alkaline media, which explained why their conductivity decreased more rapidly. Thus, the specific cell conductivity for the Fumapem membrane dropped from 35 to 0.4 mS-cm\({}^{-1}\), being 7.7 mS-cm\({}^{-1}\) on the average. The optimum pH range for the Neosepta membrane is from 0 to 10. It means that pH values at the edge or out of this range might damage the membrane. That is why, in this type of membrane, repeated runs lead to worse results for cell conductivity. In the case of Neosepta membrane the estimated overpotential was 4 mV, whereas for the other two it was approximately 1 mV.