A Wide Look at MXene-Based CO₂ Reduction Electrocatalysts: Pioneering Pathways for Green Formaldehyde Production

Transition metal carbides, oxy-carbides, nitrides, and carbonitrides (MXenes) constitute an ever-growing class of two-dimensional materials with unique properties, including high conductivity and surface area as well as versatile and tunable surface chemistry.^1–3 Such a wide-ranging array of properties primes MXenes for various applications and, hence, provides them with the potential to tackle some of the most pressing challenges faced by our planet. Climate change is one such challenge, and the electrochemical CO₂ reduction reaction (CO₂RR) provides a potent pathway to alleviate the effects of CO₂ emission on the environment.⁴The maturation of electrochemical CO₂RR, however, will require the development of electrocatalysts that maximize the value gained from converting the CO₂ feedstock. For this reason, we have developed a novel class of MXene-based electrocatalysts that leverage the unique properties of MXenes to convert CO₂ to formaldehyde. Formaldehyde is a key ingredient in manufacturing many value-added products, including resins, coatings, and vehicle components.⁵ However, formaldehyde is not typically generated with traditional electrocatalysts, indicating that the herein introduced MXene catalysts drive CO₂RR through new reaction pathways. This work highlights the behavior of a wide array of MXenes: ranging from single-layered traditional MXenes (Ti₃C₂T<i>_x</i>, Mo₂TiC₂T<i>_x</i>, and W₂TiC₂T<i>_x</i>) to multi-layered MXene/metal heterostructures (Ti₃C₂T<i>_x</i>/CuM, M = Ag, Sn, Zn, Ru, Ni, Fe). The Ti₃C₂T<i>_x</i>/CuM heterostructures were developed through a novel electroless deposition of the bimetals onto the surface of MXenes by oxidizing the Ti moieties of the Ti₃C₂T<i>_x</i> MXene. The reduction potential of the adsorbed metals dictated this process, with the strongly oxidizing metals reaching their lowest oxidation states while the weakly oxidizing metals were only partially reduced. In all cases, formaldehyde was generated and the highest efficiencies were achieved at low, industrially relevant, cell potentials (between -1.4 V to -2.2 V). These results, as they pertain to pioneering green formaldehyde production and favorability for industry adoption, warrant further exploration of the MXene-based CO₂RR electrocatalysts.<br/><br/><b>Acknowledgements</b><br/>This material is based upon work supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-06CH11357. This research used resources of the Center for Nanoscale Materials, U.S. Department of Energy (DOE) Office of Science user facilities operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.<br/><br/><b>References:</b><br/>1. Gogotsi, Y. & Huang, Q. MXenes: Two-Dimensional Building Blocks for Future Materials and Devices. <i>ACS Nano</i> <b>15</b>, 5775–5780 (2021).<br/>2. Michalowski, P. P. <i>et al.</i> Oxycarbide MXenes and MAX phases identification using monoatomic layer-by-layer analysis with ultralow-energy secondary-ion mass spectrometry. <i>Nat. Nanotechnol.</i> <b>17</b>, 1192–1197 (2022).<br/>3. Zhou, C. <i>et al.</i> Hybrid organic–inorganic two-dimensional metal carbide MXenes with amido- and imido-terminated surfaces. <i>Nat. Chem.</i> 1–8 (2023) doi:10.1038/s41557-023-01288-w.<br/>4. Kuhl, K. P. <i>et al.</i> Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces. <i>J. Am. Chem. Soc.</i> <b>136</b>, 14107–14113 (2014).<br/>5. Reuss, G., Disteldorf, W., Gamer, A. O. & Hilt, A. Formaldehyde. in <i>Ullmann’s Encyclopedia of Industrial Chemistry</i> (ed. Wiley-VCH Verlag GmbH & Co. KGaA) a11_619 (Wiley-VCH Verlag GmbH & Co. KGaA, 2000). doi:10.1002/14356007.a11_619.