Electrochemical Reduction of  CO 2 at Intentionally Oxidized Copper Electrodes

Abstract

We observed CO2 reduction to CH3OH at various oxidized copper electrodes at 22~ The electrode types included anodized Cu foil, Cu foil thermally oxidized in air, and air-oxidized Cu electrodeposited on anodized or air-oxidized Ti foil. The highest rates to date, 1 • 10 4 mol cm -2 h -1 (geometrical area), were found using anodized Cu in 0.5M KHCO3, pH = 7.6, and -1.9 V (SCE). Faradaic yields for CH3OH depended on the current and reached about 240%. The onset potential for CH3OH formation was near -0.4 V (SCE). Hydrogen gas and small amounts of CO were also formed. A mechanism is proposed involving chemical reduction steps up to HCOad, followed by three hydiogenation steps to CH3OH. Absorption of oxygen by Cure is implied. The stability of p-Cu20 semiconductor films is discussed in terms of breakdown currents d/le to band-to-band tunneling. The major impurities deposited on the Cu electrode during electrolysis are Zn and Fe at several atom percent each. An 8% level of C1 was found, originating from the HC1 etch used to remove the native oxide before thermal oxidation.

Production of methanol from CO2 electroreduction at Cu2O and Cu2O/ZnO-based electrodes in aqueous solution

“…Interestingly, these values are remarkably higher than those obtained with the Cu plate, independently of the applied potential. This fact is in good agreement with previous findings where surfaces with copper oxides have shown higher activity for the reduction of protons than the Cu (0) sites for CO2 electroreduction to CH3OH, as demonstrated by Frese et al [15] where the electrochemical performance of Cu-based electrodes (i.e Cu foil, Cu foil thermally oxidized in air and air-oxidized Cu electrodeposited on anodized or airoxidized Ti foil) was compared, or as recently reported by Lan et al [23], where a larger amount of CH3OH was obtained for Cu(core)/CuO(shell) catalysts in 1 M KHCO3 at -1.72 V vs. Ag/AgCl than that amount obtained at a Cu foil.…”

“…Finally, Table 4 compares the production of CH3OH, r, and efficiency, FE, at Cu2O (1) and Cu2O/ZnO (1:1)-based electrodes prepared in this study with previous results reported in literature for Cu-based surfaces, when different supporting electrolytes and cell configurations are applied [11,[15][16][17][18][19][20][21][22][23]. Further advances are still required in order to develop catalyst materials with optimal performance for practical applications.…”

“…[17,38], and also probably related by the crystallization of Cu2O catalyst, which may be accelerated by water produced along with CH3OH [39]. Moreover, the agglomeration of particles and probable defects in the surface region occurred during the preparation of the electrodes would likely assist tunnelling and thus destabilize the Cu2O surface [15], which may be the cause of the shortcoming of electrochemical CO2 reduction performance.…”

“…Unfortunately, among the many electrocatalytic materials previously investigated for CO2 reduction, none of them are both efficient and selective, and the most common products are CO and HCOOH, which are not as readily used as fuels as hydrocarbons or alcohols are [10]. Among the studied materials, copper (Cu), molybdenum (Mo) and ruthenium (Ru), as well as their mixtures and oxidized forms [11,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], have been reported to be the most active materials for the electrochemical transformation of CO2 to CH3OH. Cu is the only known material capable of producing mixtures of chemicals potentially interesting for industrial applications at high reaction rates over sustainable periods of time [31][32][33][34].…”

Liquid Hydrocarbon Production from CO2: Recent Development in Metal‐Based Electrocatalysis

“…[13] However,a major breakthrough was achieved by Frese in the 1990s, who reportedt he finding that Cu oxides can playamajor role in stabilizing the adsorbed CO to form adsorbed CHO that can be furtherr educed to form CH 3 OH. [167,185] Although the catalytic resultso btained by Frese were not feasible, (the faradaic efficiencies obtained for CH 3 OH formation were 240 %), the conclusion of the work paved the way for future research into CH 3 OH, as it is now understood that Cu + species play the major role in binding CHO species, whichdetermines the product selectivity for CH 3 OH. This knowledgew as invoked by Le et al who reported an electrodeposited Cu 2 Ot hin film that yielded aF E CH 3 OH value of 33 %a tÀ0.4 V. [168,186] This ascription was also supported by an oxide-derived Cu/C catalyst that demonstrated ah igh FE CH 3 OH of 40 %a ta na pplied potential of only À0.3 V ( Figure8).…”