## 3 Results and Discussion  
### Initial Sulfate Concentrations  
The electrochemical data were compared to those from the chemical analyses for different redox reactions with a different number of exchanged electrons, cf. Table 3. In addition, they were compared to the results of qualitative chemical analyses. It appeared that there was a range of optimum sulfide concentrations in which the current and power densities have maximum values. This is the range of 200-300 mg dm\({}^{-3}\) sulfide concentration. At lower concentrations the electrochemical potential of the tested solution is low, although the sulfide to sulfate conversion is satisfactory. At higher concentrations parallel and not desired reactions like polysulfide and thio-compounds formation occur leading to reactions with a small number of exchanged electrons and therefore to a low current yield. These results are demonstrated in Figure 7. The superiority of the concentration of 240 mg dm\({}^{-3}\) is evident. Another interesting observation was that the pH of the outlet flow was lower than that of the inlet sulfide solution. The same effect was observed for the batch experiments, where the pH dropped during the process, signifying a pH drops inside the fuel cell owing to consumption of hydroxyl ions, i.e., the cell operation is accompanied by OH\({}^{-}\) exchange through the separation membrane, cf. Figure 1b. The polarization curve for a continuously operating SDFC is shown in Figure 8a. The time profile of a cell discharging at 10 Ohms external resistance is demonstrated in Figure 8b. The sulfide consumption calculated from the chemical analytical data is considered as 100% and is compared to the ones calculated by [PERSON]'s law from the measured electric currents for different numbers of exchanged electrons (Figure 9). In this case the fuel cell operated as a continuous stirred tank reactor (CSTR). In the specific situation, if SDFC operates with complete sulfide-to-sulfate oxidation the cell efficiency is 34%; if the product is only sulfite, it is 46%; if the oxidation product is only thiosulfate, the efficiency is 69%. Options of total sulfide-to-sulfur oxidation (with two exchanged electrons) and polysulfide formation (one exchanged electron) are unrealistic because the calculated current efficiencies are much higher than 100%. The open circuit potential in this case was 0.622 V. The qualitative analyses of the outlet samples show that there are predominantly sulfite and sulfate anions. At higher initial concentrations of sulfide (1000 mg/L and above) thiosulfate and polysulfide were also observed. Of course, many redox reactions, including various numbers of exchanged electrons, are possible. However, the voltammetric data show that the maximum power density is attained at an electromotive force of about 0.3 V corresponding to efficiency of 29% with respect to the theoretical value (1.08 V), i.e., almost coinciding with the current efficiency for sulfide to sulfate oxidation. Another interesting  
Figure 7: Polarization curves for three different inlet sulfide concentrations. Membrane Celgard 3501 with 7 sq. cm membrane area. Feed flow rate 0.2 dm\({}^{3}\)/h. Symbols: (\(\bullet\)), solid line—70 mg dm\({}^{-3}\); (\(\Circuit\)), solid line—240 mg dm\({}^{-3}\); (\(\square\)), dashed line—1100 mg dm\({}^{-3}\). Supporting electrolyte—16 mg dm\({}^{-3}\) NaCl. Anode space volume—800 cm\({}^{3}\). (**a**) Polarization curve; (**b**) power density vs. current density. A comparison of current densities as demonstrated in this presentation, to data supplied by other authors can be found in Table 4. The comparison shows modest values in comparison to the ones measured for a fuel cell with gas oxygen electrode and 3 M NaOH as supporting electrolyte in the anode compartment [23]. This is explained by the fact that the processes in the present study are in liquid phase, with solute diffusivities of a much lower value than in gaseous phase. Hence the higher mass transfer resistances.