Electroreduction of Carbon Dioxide to Methane Enabled By Molybdenum Carbide Nanocatalyst
Authors: Esmaeilirad Mohammadreza , Kondori Alireza , Ruiz Belmonte Andrés , Asadi Mohammad 
Published: December 2020

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 CO2 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 (CH4), methanol (CH3OH) and ethylene (C2H4). 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 (Mo2C NFs) as a highly effective catalyst for electrochemical CO2 reduction reaction. The Mo2C NFs were synthesized using a facile colloidal chemistry method followed by liquid exfoliation. The electrocatalytic performance of Mo2C NFs were carried out in a custom-made two-compartment three-electrode electrochemical cell using CO2 saturated water like buffer electrolyte and compared with Cu nanoparticles (Cu NPs) which is the conventional catalysts for electrochemical CO2 reduction reaction. The linear sweep voltammetry (LSV) results for Mo2C NFs and Cu NPs indicate that at the potential of -1.25 V vs RHE, a total current density of -138.2 mA/cm2 was obtained for Mo2C NFs while the Cu NPs show a total current density of -44.9 mA/cm2
at the same applied potential suggesting higher activity of Mo2C NFs. Our selectivity analysis, product formation faradaic efficiency (FE) measurements, show a CH4 formation onset potential of -0.45 V vs RHE for Mo2C 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 CH4 formation efficiency of 36.12% is recorded for Mo2C 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 CH4 formation up to a potential of -0.95 V vs RHE, confirming higher CH4 formation selectivity of Mo2C NFs at low potentials. The results also indicate H2, CO and C2H4 production FEs of 7%, 36% and 2%, respectively, as the side products for Mo2C NFs at the potential of -0.65 V vs RHE. Moreover, our turnover frequency (TOF) calculation, actual CH4 formation activity per active site, exhibits a CH4 formation TOF of 0.4868 s-1 for Mo2C 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 Mo2C NFs. The highly active and inexpensive catalyst found by this study makes it a promising candidate for effective electrochemical reduction of CO2 to CH4 that can work with renewable energy resources such as solar or wind to address ever-increasing energy demands in a sustainable pathway.