2, Korea Institute of Science and Technology, Seoul, , Korea (the Republic of)
Electrocatalytic conversion of CO2 into long-chain hydrocarbon represents important research directions in adding value to CO2-based chemicals and realizing its practical application. Long chain hydrocarbons may change current fossil fuel-based industry in that those chemicals have similar energy density with gasoline, high compatibility with current infrastructure. However, most of the electrocatalysts produce C1, C2, and C3 chemicals, and methods for producing long chain hydrocarbons are not available thus far. Cu electrode is a good candidate for producing long chain hydrocarbon because several ketones and aldehyde compounds involved in aldol condensation have been proposed as reaction intermediates for the electrochemical CO2 reduction. Although Cu electrode is the only reported catalyst that can generate such key intermediate species, it preferentially produces C2H4, C2H5OH like C2 chemicals as its final product, rather than inducing C2 dimerization. It is assumed that the currently used Cu electrode is not suitable for holding the enol tautomer of aldehyde or ketones due to its moderate affinity for oxygen. Utilization of Cu binary alloy can be one effective strategy to overcome such limitations by incorporating high oxygen affinity elements into the structure. Indeed, with Cu alloy metal catalyst, superior efficiency and new products that were not expected in a pure metal electrode have been reported recently. However, we realized that the underlying principles of their outstanding performance have not been fully addressed. In particular, possible phase segregation with concurrent composition changes, which has been widely observed in the field of metallurgy, has not been considered at all. Moreover, surface-exposed metals can easily form oxide species, which is another pivotal factor that determines overall catalytic properties. Here, to clarify the alloying effect which determines the reduction of CO2 to long-chain hydrocarbons, the current understanding of the structural-property relationships of various alloy systems was discussed. In order to expect the alloying effect from the alloy structure, we proposed a possible microstructure and naturally occurring surface oxide species from the viewpoint of the thermodynamic stability of the alloy structure and element mixing. Inspired from the thermodynamic database, we developed a new Cu binary alloy catalyst. Remarkably, by mixing copper with high oxygen affinity metal, we observed value-added hydrocarbon products which have never been reported in Cu based system. In this manner, we expect the basic principle of material science can guide us to develop new binary alloy catalysts to further improve CO2 conversion and, ultimately, achieve long chain hydrocarbon production.