Synthesis of graphitic nanofibers and carbon nanotubes by catalytic chemical vapor deposition method on nickel chloride alcogel for high oxygen evolution reaction activity in alkaline media

Abstract

The carbonaceous materials containing high density of active sites are desirable to develop efficient oxygen evolution reaction electrocatalyst for the production of hydrogen gas in alkaline water electrolyzer. In this work we report facile synthesis method to produce graphitic nanofibers and carbon nanotubes by catalytic chemical vapor deposition method at 650 °C temperature on nickel chloride alcogel catalyst for efficient oxygen evolution reaction activity in alkaline media. It was found that relatively low molar concentration of nickel i.e. 0.333 molar ratio with respect to tetra ethylene ortho silane in the alcogel resulted into predominantly carbon nanotubes formation with the diameter in the range 15–45 nm, while higher concentration of nickel in the alcogel i.e. 0.495 molar ratio with respect to tetra ethylene ortho silane yielded the predominant graphitic nanofibers of the diameter 40–100 nm. The results showed that nickel particles of relatively bigger diameter exceeding 50 nm preferably produce graphitic nanofibers while smaller particles allow the formation of carbon nanotubes during in-situ catalytic chemical vapor​ deposition process. Moreover, the oxygen evolution reaction activities of synthesized carbon nanomaterials were compared with one another and it was found that the sample containing predominantly graphitic nanofibers outperformed the carbon nanotubes and commercial Ir/C (20 wt % Ir) electrocatalysts in OER electrocatalysis by exhibiting overpotential just 280 mV at current density of 10 mA/cm2. The better oxygen evolution reaction activity exhibited by graphitic nanofibers was attributed to the exposed aligned graphene layers at an angle to fiber axis constituting large number of edges, which might have served as active sites and facilitated the adsorption/desorption of hydroxide ions intermediates in alkaline media for subsequent oxygen evolution.