# Selected Aspects of Designing Modular PEMFC Stacks as Power Sources for Unmanned Aerial Vehicles
## 3 Results  
### Electrical Tests of the Designed Two-Module 2-kW PEMFC Stack  
In accordance with the requirements of the electrical propulsion system, it was necessary to maintain operation of the PEMFC stack within the voltage range 28-38 V. The dimensions and combined weight of the two modular stacks were limited by the construction and dimension of the UAV's fuselage. Our design of the 2-kW PEMFC stack met the requirements of the electrical propulsion system; it proved efficacious to construct it from smaller modules. The elaborated stack was assembled from two 1-kW modules, each consisting of 44 single MEAs. The two 1-kW modules, electrically connected in parallel, possessed two independent microcontroller system units (MCU), one for each module. Prior to the connection of modules (1) and (2) as one 2000-W power source (I), the U-I and P-I curves were recorded for each module independently. Figure 6 presents the voltage (U)-current (I) and power (P)-current (I) curves recorded for modules (1) and (2). The obtained voltage (U)-current (I) and power (P)-current (I) curves indicated that the two constructed 1-kW modules exhibited similar electrical parameters. Nominal power output P\({}_{\text{nom}}\) was higher than the assumed level of 1000 W, equaling P\({}_{\text{m1}}\) = 1152 W and P\({}_{\text{m2}}\) = 1087 W for modules (1) and (2), respectively. This data indicated that both modules could be applied via parallel electrical connections to obtain total nominal power of 2 kW. Electrochemical impedance spectroscopy (EIS) was also applied to the study of variations in the electrical resistance of 1-kW module (1) or (2) prior to its electrical connection in parallel to the designed 2-kW PEMFC stack. The measurements were performed under different electrical loads. Figure 7a,b presents the Z'-Z\" diagrams for modules (1) and (2). The EIS spectra recorded for PEMFC modules can be numerically fitted by the equivalent circuit shown in Figure 7c. The high frequency intercept part (intersection with the real axis in impedance spectrum) reflects the ohmic resistance (R\({}_{\Omega}\)) of PEMFC module (1) or (2); the arc at medium frequency reflects the combination of charge transfer (R\({}_{\text{ct}}\)) due to the oxygen reduction reaction and double layer capacitance within the catalyst layer, which is considered to be a constant phase element (CPE\({}_{1}\)), the arc at low frequency reflects the mass transport resistance of oxygen in the catalyst layer (R\({}_{\text{m}}\)) and the associated constant phase element (CPE\({}_{2}\)). The anodic polarization is very small and can be ignored. Thus,  
Figure 6: Voltage (U)-current (I) and power (P)-current (I) curves recorded for independent 1-kW modules 1 (Um1, Pm1) and 2 (Um2, Pm2) prior to electrical connection in parallel. the obtained impedance spectrum mainly reflects the cathode polarization. The described equivalent electrical circuit is often applied to EIS analysis of PEMFC stacks [43; 44; 45]. An ohmic resistance \(\mathrm{R_{\Omega}}\), charge transfer resistance \(\mathrm{R_{ct}}\), \(\mathrm{R_{m}}\)-mass transport resistance and total electrical resistance \(\mathrm{R_{t}}\) calculated as a sum of (\(\mathrm{R_{ct}}+\mathrm{R_{m}}+\mathrm{R_{\Omega}}\)) (Figure 8a,b) decreased along with an increase in the electrical load. One possible explanation for this phenomenon is the gradual humidification of the Nafion-polymer electrolyte. An increase in humidity level due to the operation of the PEMFC module with a higher electrical load caused the presence of water in the fuel cell area. This reduced the total electrical resistance of the PEMFC stack and improved the kinetics of the electrode processes (\(\mathrm{R_{ct}}\)) and mass transport (\(\mathrm{R_{m}}\)) in the PEMFC modules due to an increase in temperature. These statements are in close agreement with data obtained from U-I curves.