## 1 Introduction  
The idea behind the research presented here took shape some years ago after discussions about the enormous resources of hydrogen sulfide accumulated in the Black Sea waters at huge depths. Hydrogen sulfide was detected in the Black Sea at 150 m depth reaching 22 mg dm\({}^{-3}\) in the deepest water, i.e., 2200 m [1]. Its amount was estimated to be about 4.6 billion tons [2] with annual additions of at least 5 million tons [3]. Having in mind that the enthalpy of oxidation of hydrogen sulfide to sulfate is 833.6 kJ/mole at 25 \({}^{\circ}\)C, one can calculate that the total amount of hydrogen sulfide for the Black Sea is equivalent to 33,000 TWh electric energy. The annual increase due to 5 million tons is equivalent to 36 TWh. This amount is comparable to the annual electricity demand of Bulgaria. A comparison of thermodynamic data to the energy capacity of some better known fuels can be found in Table 1.  
One can see that hydrogen sulfide has mass energy capacity comparable to all typical energy sources, except hydrogen. There have been different ideas for hydrogen sulfide utilization. Efforts have been made to split hydrogen sulfide into hydrogen and sulfur and to recover the produced hydrogen as an energy source [3; 4; 5; 6; 7; 8; 9; 10]. The setback was the higher amount of energy that would be needed to split hydrogen sulfide as compared to the energy yield resulting from the produced hydrogen. Another setback would be the waste of sulfur obtained in colloidal state. Hydrogen sulfide destruction by plasma method was also proposed [11], as well as a process and equipment for H\({}_{2}\)S extraction from sea water [12]. In a recent paper, [PERSON] et al. [13] proposed an electro-catalytic membrane reactor for hydrogen sulfide oxidation in aqueous media using solid electrolyte membrane [13]. There are some studies on hydrogen sulfide processing in solid oxide fuel cells (SOFC) operating in gaseous phase at very high temperatures giving sulfur and sulfur dioxide as final products. Those are the works of [PERSON] and [PERSON] [14] and [PERSON] et al. [15]. There had also been other efforts to use hydrogen sulfide in a fuel cells in gaseous phase [16], but elemental sulfur proved to be the only product of oxidation. Although high current and power density was obtained, the fuel cells in question prove unsuitable for treatment of aqueous solutions. Our idea is to use hydrogen sulfide directly as electric energy generating electromotive force in a specially constructed sulfide-driven fuel cell (SDFC) [17; 18]. The principle of this type of fuel cell is shown in Figure 1, the stages of the proposed technology being: (1) pumping sea water from adequate depths (e.g., 1000 m) to a rig on the sea surface where the equipment is installed; (2) enrichment to achieve suitable sulfide inlet concentration: and (3) passing the enriched sulfide solution through the fuel cell. The advantages of the proposed technology from environmental and operational point of view are demonstrated in Table 2. \begin{table}
\begin{tabular}{c c} \hline
**Fuel** & **Energy Capacity, G/\(\hbar\)** \\ \hline Oil equivalent & 41.86 \\ Hydrogen & 143.02 \\ Methane & 56.46 \\ Ethanol & 26.8 \\ Methanol & 19.9 \\ Hydrogen sulfide & 24.52 \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the energy capacity of some known energy sources to that of hydrogen sulfide.