(Invited) Electrochemical Carbon Dioxide Reduction on Titanium Nitride Mxenes

Carbon dioxide (CO 2 ) is one of the leading causes of environmental issues, including global warming, with a major path of release being the burning of fossil fuels as energy sources. For this reason, the capture and reduction of CO 2 to value-added chemicals and fuels via electrocatalysis is on the forefront of the 21 st century grand challenges, but the technology is facing a severe low selectivity towards higher hydrocarbon products. Among the various catalysts explored for CO 2 reduction reaction (CO 2 RR), two-dimensional (2D) carbides and nitrides based on early transition metals are gaining much attention due to their unique properties including high surface area, high electronic conductivity, and tunable surface chemistries. Compared to carbide MXenes, nitrides have been less explored despite their superior properties. In this work, we leverage the high electronic conductivity of 2D titanium nitride MXenes to tune the selectivity of CO 2 RR towards higher hydrocarbon products. Specifically, we report on an oxygen-assisted molten salt fluoride etching method to produce large-lateral, few-to-single layer Ti 4 N 3 and Ti 2 N nitride MXene flakes, and how varying the Ti:N ratio influences the activity and selectivity of CO 2 RR electrocatalysis. On these nitride MXenes, we observe carbon monoxide as the main product with high selectivity values. To take advantage of this key intermediate for C 2+ products, we deposit single atom Cu particles on the surface of the MXene to foster dimerization. This approach led to the formation of higher order carbonaceous products. We found that the selectivity can be tuned by changing the applied current. We also utilize in-situ/operando spectroelectrochemical techniques, including Raman and Fourier-transform infrared (FTIR) spectroscopies, to elucidate the mechanism of product formation on these MXene materials. These findings will be of great interest to the scientific community and ultimately to the broad society as they report on an efficient catalyst design for transforming CO 2 into value-added fuels and chemicals.