Electroreduction of Carbon Dioxide to Methane Enabled By Molybdenum Carbide Nanocatalyst

The electrochemical conversion of carbon dioxide to value-added products powered by renewable energies is potentially cost-effective and green method of synthesizing hydrocarbon fuels. It is also of significant interest as a strategy to reduce the concentration of atmospheric CO 2 and close anthropogenic carbon cycle. An overarching challenge for this technology is developing an inexpensive and earth-abundant catalyst with high activity, stability and selectivity toward hydrocarbon fuels such as methane (CH 4 ), methanol (CH 3 OH) and ethylene (C 2 H 4 ). Among all type of heterogenous catalysts used for carbon dioxide reduction reaction, only copper-based catalysts have shown the ability to form hydrocarbon fuels. But they possess too low reaction rate and high overpotentials to justify their use for large-scale applications. Here, we are presenting an earth-abundant nanostructured molybdenum carbide nanoflakes (Mo 2 C NFs) as a highly effective catalyst for electrochemical CO 2 reduction reaction. The Mo 2 C NFs were synthesized using a facile colloidal chemistry method followed by liquid exfoliation. The electrocatalytic performance of Mo 2 C NFs were carried out in a custom-made two-compartment three-electrode electrochemical cell using CO 2 saturated water like buffer electrolyte and compared with Cu nanoparticles (Cu NPs) which is the conventional catalysts for electrochemical CO 2 reduction reaction. The linear sweep voltammetry (LSV) results for Mo 2 C NFs and Cu NPs indicate that at the potential of -1.25 V vs RHE, a total current density of -138.2 mA/cm 2 was obtained for Mo 2 C NFs while the Cu NPs show a total current density of -44.9 mA/cm 2 at the same applied potential suggesting higher activity of Mo 2 C NFs. Our selectivity analysis, product formation faradaic efficiency (FE) measurements, show a CH 4 formation onset potential of -0.45 V vs RHE for Mo 2 C NFs which is 500 mV less than that of Cu NPs (-0.95 V vs RHE) at identical experimental conditions. At this potential (-0.45 V vs RHE), a CH 4 formation efficiency of 36.12% is recorded for Mo 2 C NFs that further reaches to its maximum to 51.73% at a potential of -0.65 V vs RHE while Cu NPs remain inactive for CH 4 formation up to a potential of -0.95 V vs RHE, confirming higher CH 4 formation selectivity of Mo 2 C NFs at low potentials. The results also indicate H 2 , CO and C 2 H 4 production FEs of 7%, 36% and 2%, respectively, as the side products for Mo 2 C NFs at the potential of -0.65 V vs RHE. Moreover, our turnover frequency (TOF) calculation, actual CH 4 formation activity per active site, exhibits a CH 4 formation TOF of 0.4868 s -1 for Mo 2 C NFs that is approximately 500-times higher than Cu NFs (0.001 s -1 ) at the potential of -0.95 V vs RHE. We also performed different characterization methods such as X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Raman spectroscopy and Scanning Transmission Electron Microscopy (STEM) to determine the structural and electronic properties as well as the origin of this high catalytic activity of the Mo 2 C NFs. The highly active and inexpensive catalyst found by this study makes it a promising candidate for effective electrochemical reduction of CO 2 to CH 4 that can work with renewable energy resources such as solar or wind to address ever-increasing energy demands in a sustainable pathway.