Shape-Controlled Synthesis of Nickel Carbide for Electrocatalysis of the Hydrogen Evolution Reaction
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Transition metal carbides (TMCs) could serve as precious metal-free electrocatalysts for the hydrogen evolution reaction (HER). Specifically, nickel is considered the most promising transition metal to replace platinum as a HER catalyst. In this thesis, we report the solution phase synthesis of hierarchical nickel carbide nanocrystals (NCs) in which shape was controlled by tuning the solvent environment, particularly varying the 1-octadecene (ODE) concentration. This provides further insight into the “dandelion” shaped nickel carbide nanocrystals previously reported by our group. We carried out the hydrogen evolution reaction (HER) using different sizes of nickel carbide flowers under various conditions. Experiments testing heat treatment of the catalyst under N 2 and NH 3 revealed that nickel carbides annealed under NH 3 at 150 °C provided increased HER activity. For different sizes of nickel carbide structures, the medium flowers and full-blown flowers exhibited similar HER results at a current density of 20 mA/cm 2 , even though the full-blown flowers have higher surface area. This reminds us that the conductivity of nickel carbide flowers might affect the catalytic effects. To improve electrical conductivity of the catalysts, we mixed different carbons like graphene, multiwall carbon nanotubes and carbon black particles with nickel carbides. The electrochemical results illustrate that nickel carbides mixed with BP 2000 carbon black provided the best HER results while also being economical. Building on this result, we next focused on synthesizing particles with intrinsically better conductivity. Inspired by good electrical properties of MXene materials arising from their 2D structure, we synthesized 2D nickel carbide snowflakes. With a better conductivity, we expect these new snowflake structures may exhibit even better HER activity than the Ni3C dandelions. Further electrochemical tests will be carried out in our group to test this hypothesis.