A scheme of the synthesis procedure: Figure S 1 Active phase deposition scheme: Stage I impregnation of carbonaceous material with cobalt-base precursor solution - NH3 solution, stage II hydrothermal synthesis in an autoclave, stage III material recovery - centrifugation, lyophilisation and drying.; CNF plasma treatment optimisation based on XPS surface characterisation: Figure S 2 C 1s XPS spectra CNF650 with assigned components for the reference and plasma-modified materials: A) 0.1 min, B) 1 min, C) 5 min, D) 10 min, E) 15 min of plasma, and F) reference; Table S 1 ID/IG ratios for CNF650 samples after cobalt deposition.Figure S 3 A) Experimental and theoretical Co3O4 loadings on CNF650, B) calculated deposition yield. Figure S 4 SEM images of cobalt-doped CNF samples. A) CNF-005M Co, B) CNF-PL-005M Co, C) CNF-05M Co, D) CNF-PL-05M Co, E) CNF-1M Co, F) CNF-PL-1M Co. Figure S 5 Chronoamperometric tests of CNF-based composite electrocatalysts. Figure S 6 Data point for determination of double layer capacitance values. Figure S 7 Nyquist plots of the EIS data for A) CNF-005M Co, B) CNF-PL-005M Co, C) CNF-05M Co, D) CNF-PL-05M Co, E) CNF-1M Co, F) CNF-PL-1M Co.S8. Figure S 8 Bode plots of the EIS data shown in Figure 8 and Figure S 7: A) CNF-005M Co, B) CNF-PL-005M Co, C) CNF-01M Co, D) CNF-PL-01M Co, E) CNF-05M Co, F) CNF-PL-05M Co, G) CNF-1M Co, H) CNF-PL-1M Co. S9 Figure S 9 OER overpotential plotted against an average RCT value (before and after test). Figure S 10 Ten-cycle LSV stability tests up to 2.0 V vs RHE (non-iR-corrected) for A) reference CNF material, B) CNF-005M Co, C) CNF-01M Co, D) CNF-05M Co, E) CNF-1M Co, F) CNF-PL-005M Co, G) CNF-PL-01M Co, H) CNF-PL-05M Co, I) CNF-PL-1M Co.

Raman spectroscopy was used to examine the influence of cobalt deposition on the structural properties of CNF support, described by the degree of graphitisation, defined as ID/IG, Table S 1. For the CNF carbon materials, the theoretical masses of Co3O4 resulting from the precursor concentrations used were determined and then converted as % content against the mass of the whole sample, i.e. the mass of carbon weighed for the individual synthesis and the mass of spinel that formed on it. The results are shown in Figure S 3. Chronoamperometric measurements were performed to determine double layer capacitancefree current flux values in OER potential range (Figure S 5). The CDL values were determined for the cyclic voltammetry sweep rates of 2, 4, 6, 8, 10 and 12 mVs–1. The difference in current intensities was determined for the symmetric half of the potential, i.e. the average of the start and end potentials of the resulting voltammetry for cycles 3, 6, 9, 12, 15 and 18. The capacitances of the electrical double layer were determined as half the value of the slope of the straight line (Figure S 6). The highest CDL value is characterised by the sample synthesised with a 0.1 M precursor solution without plasma modification and the lowest by the CNF650-15 reference material with a CDL of 182.9 μF. After an initial increase of CDL values with the increase of cobalt concentration from 0.01 to 0.05 M, a decrease in CDL values is observed as the concentration of cobalt precursor increases.--Under a Creative commons license BY-NC-ND 4.0. https://creativecommons.org/licenses/by-nc-nd/4.0/

This study was financially supported by the National Science Centre, Poland, project number 2020/37/B/ST5/01876. The study was carried out using research infrastructure funded by the European Union in the framework of the Smart Growth Operational Programme, Measure 4.2; Grant No. POIR.04.02.00-00-D001/20, “ATOMIN 2.0 – Center for materials research on ATOMic scale for the INnovative economy”.

Peer reviewed