MWCNTs and carbon onions grown by CVD method on nickel-cobalt alloy nanocomposites prepared via novel alcogel electrolysis technique and its oxygen evolution reaction application

Novel nanocatalyst designs to synthesize carbon based electrocatalyst are highly desirable for the production of sustainable energy technologies like hydrogen through electrochemical water splitting. Herein, we report alcogel electrolysis technique to prepare nickel cobalt alloy nanoparticles in silica matrix and then growth of carbon nanostructured material by chemical vapour deposition (CVD) method for its application towards electrochemical catalysis of oxygen evolution reaction. The growth conditions of carbon nanostructures specially multiwalled carbon nanotubes (MWCNTs) and carbon onions are tuned by controlling the temperature of CVD. It is found that MWCNTs at temperature of 650 degrees C are preferentially formed while carbon onions are formed predominantly at 750 degrees C. The onion like carbon formed on Ni-Co alloy nanostructures at temperature of 750 degrees C is attributed to over growth of NiCo alloy nanoparticles achievable at higher temperatures. The diameter of MWCNTs were found to be in the range of 5-25 nm while carbon onions in the range 50-100 nm as revealed by TEM studies after etching silica particles by HF. The oxygen evolution reaction activity of the sample synthesized at 750 degrees C was found to be inferior than exhibited by MWCNTs formed at 650 degrees C which was possibly due to the presence of bigger size carbon onions having a less surface area. The overpotential exhibited by NiCo-MWCNTs is 340 mV which is just 35 mV higher than bench mark Ir/C catalyst. The better OER performance exhibited by NiCo-MWCNTs is devoted to encapsulated nickel-cobalt alloy particles by well graphitized carbon layers, defective side walls of nanotubes and oxygenated functional groups (C=O, C-O) which are proven active sites for OER.