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Catalysts 2020, 10, 1287 10 of 25 the length and density of Cu nanowire which is linked to the increased local pH within the nanowire arrays. Ethanol with a very low FE nearly 4% was produced at −1.1 V vs. RHE on Cu catalyst when the nanowire length increased to 7.3 μm or more. Jiao’s group fabricated a nanoporous Cu catalyst through the annealing of Cu(OH)2 nanorods and the electrochemical reduction of the nanoporous CuO [53]. When the porous Cu was integrated into a CO2 flow cell electrolyzer with 1 M KOH as the electrolyte, it exhibits a high FE of 17% towards ethanol at the current density of 653 mA·cm−2 and the potential of −0.67 V vs. RHE. This kind of porous structure facilitates rapid gas transport across the electrode–electrolyte interface especially at high current densities. Similarly, the Cu catalysts synthesized by electrochemical oxidation-reduction cycling of Cu foil can electroreduce CO2 to ethanol with an increased FE up to 18% at −1.0 V vs. RHE in 0.1 M CsHCO3 [54]. Using the sol-gel Cu2(OH)3Cl as the precursor, Sargent’s group presented an electro-redeposition method to prepare the Cu catalysts with controlled morphologies and oxidation states [55]. At −1.1 V vs. RHE, the electro-redeposited Cu catalyst exhibited a FE of 12% for ethanol product. Loiudice et al. reported the highest FE for ethanol (around 10%) achieved on Cu nanocrystal cubes with 24 nm edge length by tuning nanocrystal spheres (7.5 nm and 27 nm) to nanocrystal cubes (24 nm, 44 nm, and 63 nm) [56]. Overall, the cube-shaped Cu was more intrinsically active than the spheres, and smaller nanocrystals showed higher activity for the same morphology. Additionally, Jiang and co-workers tuned the facet exposure on Cu foil by the metal ion battery cycling method [57]. The 100-cycled Cu nanocube catalyst with exposed (100) facets exhibits a six-fold improvement in C2+ to C1 product ratio compared with the polished Cu foil and an ethanol FE of 13% at −0.95 V vs. RHE. Additionally, the inclusion of a grain boundary into active sites of Cu-based electrocatalysts has been considered to improve the selectivity of electrocatalytic CO2 reduction towards multi-carbon products. In a recent report [58], the grain boundary can be controllably grown and enriched in electrodeposited Cu by using the poly (vinylpyrrolidone) additive. The obtained grain-boundary-rich metallic Cu was able to convert CO2 to ethanol with a high FE of 32% and a partial current density of −45 mA·cm−2 at −1.3 V vs. RHE in a flow cell, which is superior to the electrodeposited Cu without grain boundary. Oxide-derived copper (OD-Cu) has been discovered as a simple method to improve the intrinsic catalytic properties towards C2+ formation owing to the introduction of Cu+ species on the surface [45,59,85]. A recent report of thick Cu2O-film-derived Cu catalysts achieved a higher FE of ethanol at lower overpotential than that on thin OD-Cu films, which can be attributed to the higher content of Cu+ species [86]. Yeo’s group systematically tuned the FE of ethanol by changing the thickness of the deposited Cu2O overlayers. The highest FE of 16% for ethanol formation was achieved on 3.6 μm film in 0.1 M KHCO3 electrolyte at −0.99 V vs. RHE [45]. These systems have verified the promotion of ethanol production by Cu+ species. However, the resultant Cu+ species are prone to being reduced to Cu0 under CO2 reduction conditions, especially at the high applied reducing potentials required to produce ethanol [87]. Therefore, research efforts have been done to stabilize the Cu+ species during CO2 redution. For instance, a 3D dendritic Cu-Cu2O oxide composite was developed by in situ reduction in an electrodeposited copper complex on Cu substrate to keep the Cu+/Cu0 ratio unchanged during CO2 reduction reaction, which resulted in a high FE of 32% for ethanol formation [59]. Yu’s group recently reported that the nanocavities in the multihollow Cu2O can confine carbon intermediates formed in situ, which, in turn, covers the local catalyst surface and thereby stabilized Cu+ species [60]. At the potential of −0.61 V vs. RHE in 2 M KOH, this catalyst yields a maximum ethanol FE of 27% and delivers a high current density of −320 mA·cm−2 in a flow cell system (Figure 4).PDF Image | Advances in Clean Fuel Ethanol Production from CO2 Reduction
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