Iron sulfide was synthesized for 4 h, 7 h and 12 h by a hydrothermal process directly on stainless steel current collectors. The synthesis time determined the material morphology and electrochemical response. The shortest synthesis time promoted the formation of randomly oriented nanowires that evolved to nanosheets decorated with nanoflakes, organized in a cuboidal-like morphology upon longer synthesis times. XRD, Raman, FTIR and XPS investigations confirmed the formation of FeSx. The electrochemical activity was studied in a potential window ranging from −0.95 to 0 V and the material obtained after 7 h of synthesis stored the maximum specific capacitance of 730 mF cm−2 at the current density of 1 mA cm−2. This material also retained approximately 34% of its initial capacitance at 10 mA cm−2 and showed very good cycling stability, keeping around 95% of the specific capacitance after 2000 galvanostatic charge-discharge (GCD) cycles. The kinetic analysis of the electrochemical results revealed the predominance of diffusional controlled processes. An asymmetric cell was assembled using FeSx as negative electrode and carbon nanofoam (CNF) as positive electrode. The FeSx||CNF cell showed enhanced capacitive response in a potential window of 1.8 V in 1 M Na2SO4 electrolyte and delivered specific capacitance of 236 mF cm−2 at 0.5 mA cm−2 with good rate capability. The FeSx||CNF cell stored maximum energy density of 0.11 mW h cm−2 at the power density of 0.45 mW cm−2. The cell showed very good stability by retaining 83% of the initial capacitance after 2000 cycles of consecutive charge discharge.