## 2 Experimental  
### Materials and Synthesis Routes  
Chromium (III) acetate hydroxide, Cr\({}_{3}\)(OH)\({}_{2}\)(CH\({}_{3}\)COO)\({}_{7}\) (purity: 98%; CAS No. 39430-51-8, Sigma Aldrich, St. Louis, MO, USA), manganese(II) acetate tetrahydrate, Mn(CH\({}_{3}\)COO)\({}_{2}\)\(\cdot\)4H\({}_{2}\)O (purity: 99%; CAS No. 6156-78-1, Sigma Aldrich), iron (II) acetate, Fe(CH\({}_{3}\)COO)\({}_{2}\) (purity 95%; CAS No. 3094-87-9 Sigma Aldrich), cobalt (II) acetate tetrahydrate, Co(CH\({}_{3}\)COO)\({}_{2}\)\(\cdot\)4H\({}_{2}\)O (purity: 99%; CAS No. 6147-53-1, Sigma Aldrich), and nickel (II) acetate tetrahydrate, Ni(CH\({}_{3}\)COO)\({}_{2}\)\(\cdot\)4H\({}_{2}\)O (purity: 98%; CAS No. 6018-89-9, Sigma Aldrich) are utilized as precursors for the preparation of CrMnFeCoNi-HEO (sHEO) in the form of nanoparticles (NPs) and nanofibers (NFs). sHEO NPs are produced by classical sol-gel method (SG), as already described in detail [31]. Citric acid monohydrate, HOC(COOH)(CH\({}_{2}\)COOH)\({}_{2}\cdot\)H\({}_{2}\)O (purity: 98%; CAS No. 5949-29-1, Sigma Aldrich) is used as a complexing agent. Stoichiometric amounts of Cr, Mn, Fe, Co, and Ni are dissolved in water (30 g), one at a time; then, citric acid (3.5 g) is added under continuous stirring for 1 h at 90 \({}^{\circ}\)C. After drying at 80 \({}^{\circ}\)C overnight, the as-formed gel is calcined in static air at 350 \({}^{\circ}\)C for 2 h, 500 \({}^{\circ}\)C for 2 h, 800 \({}^{\circ}\)C for 2 h, and 900 \({}^{\circ}\)C for 2 h.  
sHEO NPs are produced by electrospinning (ES), following the procedure illustrated in detail elsewhere [31]. Polyacrylonitrile (PAN), (C\({}_{3}\)H\({}_{3}\)N)\({}_{n}\) (average molecular weight: 150,000 g mol\({}^{-1}\); purity: 99.9%; CAS No. 25014-41-9, Sigma Aldrich), and N,N-dimethylformamide (DMF), HCON(CH\({}_{3}\))\({}_{2}\) (anhydrous: 99.8%; CAS No. 68-12-2, Sigma Aldrich) are used for the preparation of the polymer/solvent solution. After complete dissolution of 7.1 wt% PAN in DMF, the five metallic salts, containing stoichiometric amounts of Cr, Mn, Fe, Co, and Ni, are added, one at a time, to the solution under continuous stirring. The total amount of metals is 38.5 wt% relative to PAN. After loading into a 20 mL syringe equipped with a 40 mm long 0.8 mm gauge stainless steel needle, the homogeneous solution, fed at 23.5 \(\mu\)L min\({}^{-1}\) rate, is electrospun by using a CH-01 Electro-spinner 2.0 (Linari Engineering s.r.l.). ES is carried out at 25 \(\pm\) 1 \({}^{\circ}\)C and 40-45% relative air humidity under a DC voltage of 15 kV applied between needle and the 15 cm distant grounded collector. After drying overnight and peeling from the collector, the as-spun membrane is calcined at 700 \({}^{\circ}\)C for 2 h and 900 \({}^{\circ}\)C for 2 h in static air. C/sHEO composite NFs are obtained from the same spinnable solution and ES conditions as sHEO NFs by a different thermal treatment, namely stabilization in air at 225 \({}^{\circ}\)C for 2 h [36] and subsequent carbonization in inert atmosphere at 700 \({}^{\circ}\)C for 2 h and 900 \({}^{\circ}\)C for 2 h.  
Magnesium (II) acetate tetrahydrate, Mg(CH\({}_{3}\)COO)\({}_{2}\cdot\)4H\({}_{2}\)O (purity: 98%; CAS No. 16674-78-5, Sigma Aldrich), cobalt (II) acetate tetrahydrate, Co(CH\({}_{3}\)COO)\({}_{2}\cdot\)4H\({}_{2}\)O (purity: 99%; CAS No. 6147-53-1, Sigma Aldrich), nickel (II) acetate tetrahydrate, Ni(CH\({}_{3}\)COO)\({}_{2}\cdot\)4H\({}_{2}\)O (purity: 98%; CAS No. 6018-89-9, Sigma Aldrich), copper (II) acetate, Cu(CH\({}_{3}\)COO)\({}_{2}\), (purity: 98%; CAS No. 142-71-2, Sigma Aldrich), and zinc (II) acetate dihydrate, Zn(CH\({}_{3}\)COO)\({}_{2}\cdot\)2H\({}_{2}\)O (purity: 98%; CAS No. 5970-45-6, Fischer Scientific, Hampton, NH, USA) are utilized as precursors for the preparation of C/MgCoNiCuZn-HEO composite NFs by following the same procedure as for C/CrMnFeCoNi-HEO NFs.