## 3 Results and Discussion
### Electrochemical Analysis  
Figure 4 shows the CV performances of the electrodes prepared by using sHEO NPs, sHEO NFs, C/sHEO NFs, and C/rHEO NFs as active materials. The curves are recorded in 0.1 mol L\({}^{-1}\) NaCl solution, with sweep rates from 5 to 100 mVs\({}^{-1}\) in the potential range from \(-1\) to 0.2 V vs. Ag/AgCl. At lower scan rates, they are featured by a quasi-rectangular shape and exhibit no obvious redox peak. This suggests a purely capacitive behavior due  
Figure 3: Micro-Raman spectra of the samples. to the perfect EDL formation at the electrode-solution interface. With increasing scan rate, the shape of the CV curves slightly deviates from rectangular and tends to become oval. The specific capacitances, as calculated according to Equation (1) from the current-voltage curves, are plotted in Figure 4e as functions of scan rate. For each electrode, the value of \(C\) diminishes with increasing scan rate, as expected [22; 39; 51]. By working at low scan speeds, the salt ions have sufficient time to diffuse into the internal pores of the electrodes, which is crucial for the EDL to form. Conversely, when the scan rate increases,  
Figure 4: (**a-c**) CV curves of (**a**) sHEO NPs, (**b**) sHEO NFs, (**c**) C/sHEO composite NFs, and (**d**) C/rHEO composite NFs and (**e**) corresponding specific capacitance as a function of potential scan rate. salt ions do not have enough time to move and accumulate in the internal pores, and, as a result, the accessible area becomes smaller, and the EDL formation remains incomplete. In the 5-100 mV s\({}^{-1}\) scan rate range, the CDI performance, as monitored by the specific capacitance, improves in the order C/rHEO NFs (8-32 F g\({}^{-1}\)) \(\cong\) sHEO NPs (9-32 F g\({}^{-1}\)) \(<\) sHEO NFs (8-43 F g\({}^{-1}\)) \(<\) C/sHEO NFs (12-66 F g\({}^{-1}\)). The improvement is in the same order as the average size of the HEO grains, namely C/rHEO NFs (250 nm) \(\cong\) sHEO NPs (240 nm) \(<\) sHEO NFs (200 nm) \(<\) C/sHEO NFs (110 nm). Of course, smaller-sized grains result in larger exposed surface area available for the ion adsorption. The best and the worst capacitive performances pertain to C/sHEO NFs and C/rHEO NFs, respectively. A previous study on common LEO NFs and C/LEO composite NFs [39] has evidenced the relevant role played by morphology of the composite NFs in determining their electrochemical behavior. In C/rHEO NFs, the larger-sized rHEO grains protrude out of the fibrous C matrix (Figure 1d); this is reflected in a detrimental reduction of the contact between the two active material components. On the contrary, in C/sHEO NFs, the smaller-sized sHEO grains are in intimate contact with the host C NFs; this produces a positive effect in terms of overall specific capacitance, which benefits also from the smaller size of the sHEO grains. Figure S1 compares the specific capacitance of the best performing C/sHEO NFs with that which was previously measured in the C/LEO composite NFs [39]. At 5 mV s\({}^{-1}\), in 0.1 M NaCl aqueous solution, C/(Cr\({}_{0.2}\)Mn\({}_{0.2}\)Fe\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\))\({}_{3}\)O\({}_{4}\) NFs exhibit specific capacitance comparable to that of C/Fe\({}_{3}\)O\({}_{4}\) NFs obtained from a spinable solution with a higher total metal amount (57.7 vs. 38.5 wt% relative to PAN). This finding indicates that the use of HEOs in place of the common LEOs allows for the reduction of the total metal content in the composites with consequent lowering of the material production costs, which represent the major cost component of the CDI processes [12]. At 100 mV s\({}^{-1}\) scan rate, the specific capacitance of C/(Cr\({}_{0.2}\)Mn\({}_{0.2}\)Fe\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\))\({}_{3}\)O\({}_{4}\) NFs _exceeds_ that of C/MnO\({}_{\rm X}\) NFs obtained from a spinnable solution with the _same_ total metal amount in the precursor solution, proving that, for fixed metal load, the C/HEO NFs outperform the C/LEO NFs.