## 3 Results and Discussion
### Stack and Consecutive Fuel Cell Performance  
The results for polarization curves obtained for parallel two-stack fuel cells and two consecutively coupled ones are shown in Figure 15a,b, respectively. The parallel coupling of the fuel cells in a stack (Figure 15a) leads to somewhat higher current and power density compared to the consecutively coupled cells (cf. Figure 15b), with residual thiosulfate and polysulfide in the outlet flow. However, at the outlet solution of the second cell of the consecutively coupled fuel cells only sulfate and a small amount of sulfite were detected. In this case complete consumption of sulfide was registered.  
For those two experiments the following averaged fuel cell efficiency was estimated (according to Equation (4)). For the stack of two parallel fuel cells: \(\varepsilon=0.78\); for the consecutively coupled fuel cells: \(\varepsilon=0.36\).  
As demonstrated by Table 5, the comparison of selected experiments characteristics differing in cell geometry, size, type of aeration and flow organization showed best results in current and power density for the small cylindrical cell. In this case best sulfide consumption was also observed.  
Direct aeration shows better performance compared to the aeration by Venturi ejector with comparable cell efficiency.  
Figure 15: Polarization curves and power density for two types of fuel cell organization. (**a**) two parallely coupled fuel cells as a stack; (**b**) two consecutively coupled fuel cells. Continuous process. Membrane Celgard 3501. Active membrane and electrode area—650 sq. cm. Inlet sulfide concentration 236 g dm\({}^{-3}\). Supporting electrolyte—16 g dm\({}^{-3}\). Feed flow rate—0.4 dm\({}^{3}\)/h for each of the parallel cells (**a**) and for the consecutively coupled ones (**b**). Anode volume 800 cm\({}^{3}\) for each cell. Temperature 20 \({}^{\circ}\)C. (\(\bullet\))—cell voltage; (\(\odot\)) power density.  
The stack and cascade assemblies give comparable current and power density values, the cell efficiency for the parallel cell assembly is two times higher than the cascade one.  
Theoretically 1 cubic meter of marine water per second containing 20 g m\({}^{-3}\) sulfide will yield power of 500 kW. However, the estimated current and power density values in our case are much lower than the ones accepted for traditional hydrogen/oxygen PEMFC at 350 K (i.e., about 0.1 A/cm\({}^{2}\) for hydrogen and 10\({}^{-4}\) A/cm\({}^{2}\) for oxygen reduction) and area-specific resistance ASR = 0.01 \(\Omega\) cm\({}^{2}\) (cf. [30], p. 198). One must keep in mind that traditional cells operate at high temperatures and in gaseous phase, where the diffusivities and the reaction rates are much higher. However, a power density of 10 W/m\({}^{2}\) will lead to 1 kW per cubic meter, which seems to be attractive for small scale practical applications. An increase of the current density to 100 A/m\({}^{2}\) will lead to power density of 50 W/m\({}^{2}\) at voltage cell efficiency of 50%.  
Efforts should be made therefore to enhance electrode reactions and minimize parasitic reactions in the bulk; as well as to reduce the overall cell resistance.