## 2 Experimental Part
### Fuel Cell Design  
Three different fuel cells were tested. The first one consisted of two co-centric glass cylinders with i.d. of 3 cm and o.d. of 3.8 cm for the inner one and i.d. 3.8 and 5 cm for the annular space between the cylinders. The two compartments were separated by a membrane with diameter of 3 cm and area of 7 sq. cm. Graphite rods of spectroscopic purity were used as electrodes. In order to increase the cathode surface the space of the inner cylinder was packed with granulated activated carbon (Fujikasui, Tokyo, Japan) with a specific area of 680 m\({}^{2}\) g\({}^{-1}\) and particle size 4 to 6 mm. The re-distribution of the air bubbles by the bed of carbon particles has additional beneficial effect on the oxygen mass transfer in the cathode space. The images of the used activated carbon at different magnifications are shown in Figure 3a-d. A transmission electron microscope JEOL JEM V-200 (JEOL, Tokyo, Japan) with scanning attachment EM-ASID-3D was used.  
Air was blown in the cathode space with a flow rate of 6 L/min. The useful volume of the anode compartment was 80 mL and of the cathode one, 40 mL. The experiments in this cell were carried out in a batch or continuous mode. The electric current and the cell voltage were measured and monitored during the runs. Samples from the anode space were taken periodically and analyzed quantitatively for sulfide and qualitatively for sulfate, sulfite, thiosulfate, and polysulfide. Preliminary cyclic voltammetry carried out at different temperatures (15, 20, 23, and 25 \({}^{\circ}\)C) has shown that there are no significant differences in the obtained electrochemical data for this temperature range. The experiments were therefore mostly performed at 20 \({}^{\circ}\)C.  
Three different membranes were tested as separators: Celgard 3501, Fumapem* FAA-3-PK-75 (OH\({}^{-}\)form), and Neosepta* AFN. The latter two are typical anion exchange ones, whereas Celgard 3501, made out of polypropylene, conducts both H\({}^{+}\) and OH\({}^{-}\) ions. The membranes have been selected because of their relatively high ionic conductivity [31].  
Various sulfide concentrations were tested starting with 10 mg dm\({}^{-3}\) and reaching to 2000 mg dm\({}^{-3}\). The initial pH values of the feed solutions varied between 7.9 (for lower sulfide concentrations, from 60 mg dm\({}^{-3}\)) up to pH 12.65 for 1000 mg dm\({}^{-3}\) sulfide concentration.  
Figure 3: \(Cont.\)  
Two other fuel cell constructions were also tested. The first was assembled as a two-cell stack designed to operate simultaneously or separately. The electrodes were rectangular plates of sintered graphite (manufactured by Cometech OOD, Sofia, Bulgaria) size 500 \(\times\) 130 \(\times\) 5 mm with a cross-section area of 650 sq.cm. The SEM images of the sintered graphite at different magnifications are shown in Figure 4. They were taken by the same equipment as in Figure 3.  
The SEM energy dispersive spectroscopy investigation was performed at 15 kV acceleration voltages using a ZEISS EVO LS25 (Carl Zeiss, Oberkochen, Germany) scanning electron microscope equipped with an EDAX Trident analytical system for energy dispersive X-ray analysis. The energy dispersive X-ray (EDX) spectrum of the sintered graphite shows silicon as a main contamination, whereas the other elements were in very low concentrations, cf. Figure 5.  
The void volume of the entire electrode space was 800 mL. In this case, a Celgard 3501 membrane with the same cross-section was used as a separator.  
Figure 4: SEM images at different magnifications of sintered graphite electrode. (**a**) magnification \(\times\)1000; (**b**) magnification \(\times\)5000.  
Figure 3: SEM images at different magnifications of granulated activated carbon used in the cathode area. (**a**) Magnification \(\times\)500; (**b**) magnification \(\times\)1000; (**c**) magnification \(\times\)2000; (**d**) magnification \(\times\)5000.