Synthesis of metastable chromium carbide nanomaterials and their electrocatalytic activity for the hydrogen evolution reaction.

Transition metal carbides including chromium, molybdenum, and tungsten are of particular interest as renewable energy catalysts due to their low cost and abundance. While several single metal carbide systems form multiple phases with different compositions and crystal structures, most of these materials are not as well studied due to their limited synthetic approaches and instability. By taking advantage of a low temperature salt flux synthetic method, these unique phases can be more easily synthesized and separated as phase pure materials. As an example, Chromium carbide forms five different crystal structures including three common phases, Cr3C2, Cr7C3, and Cr23C6, and two less studied phases, Cr2C and CrC. All five compounds were synthesized using the salt flux method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrocatalytic testing for the hydrogen evolution reaction (HER). This low temperature method allows for routine access to multiple compounds in complex phase diagrams and separation of each phase synthetically. This represents a significant step forward in synthesizing rare phases like rocksalt CrC and hexagonal Cr2C and allows for investigation into their potential catalytic properties and future applications.

Formation mechanism of nanostructured metal carbides via salt-flux synthesis.

Nanostructured metal carbides are of particular interest because of their potential as high surface area, low-cost catalysts. By taking advantage of a salt-flux synthesis method, multiple carbide compounds were synthesized at low temperatures providing a pathway to nanosized materials. To better understand the reaction mechanism, vanadium carbide (V8C7) synthesis was monitored by quenching samples at 100 °C intervals and analyzed by multiple spectroscopic methods. The reaction was determined to occur through the formation of metal halide and acetylide carbide intermediates, which were repeatedly observed by X-ray diffraction and further supported by IR and Raman spectroscopies. Control experiments were also employed to further verify this mechanism of formation by using different salt compositions and a solid-state metathesis reaction. The reaction mechanism was also verified by applying these techniques to other metal carbide systems, which produced similar intermediate compounds.