ABSTRACT
This work shows how hydrophobicity and porosity can be incorporated into copper catalyst layers (CLs) for the efficient electroreduction of CO (CORR) in a flow cell. Oxide-derived (OD) Cu catalysts are synthesized using K+ and Cs+ as templates, termed respectively as OD-Cu-K and OD-Cu-Cs. CLs, assembled from OD-Cu-K and OD-Cu-Cs, exhibit enhanced CORR performance compared to "unmodified" OD-Cu CL. OD-Cu-Cs can notably reduce CO to C2+ products with Faradaic efficiencies (FE) as high as 96% (or 4% FE H2). During CO electrolysis at -3000 mA cm-2 (-0.73 V vs reversible hydrogen electrode), C2+ products and the alcohols are formed with respective current densities of -2804 and -1205 mA cm- 2. The mesopores in the OD-Cu-Cs CL act as barriers against electrolyte flooding. Contact angle measurements confirm the CL's hydrophobicity ranking: OD-Cu-Cs > OD-Cu-K > OD-Cu. The enhanced hydrophobicity of a catalyst is proposed to allow more triple-phase (CO-electrolyte-catalyst) interfaces to be available for CORR. This study shows how the pore size-hydrophobicity relationship can be harvested to guide the design of a less-is-more Cu electrode, which can attain high CORR current density and selectivity, without the additional use of hydrophobic polytetrafluoroethylene particles or dopants, such as Ag.
ABSTRACT
A cause of losses in energy and carbon conversion efficiencies during the electrochemical CO2 reduction reaction (eCO2RR) can be attributed to the formation of carbonates (CO32-), which is generally considered to be an electrochemically inert species. Herein, using in situ Raman spectroscopy, liquid chromatography, 1H nuclear magnetic resonance spectroscopy, 13C and deuterium isotope labeling, and density functional theory simulations, we show that carbonate intermediates are adsorbed on a copper electrode during eCO2RR in KHCO3 electrolyte from 0.2 to -1.0 VRHE. These intermediates can be reduced to formate at -0.4 VRHE and more negative potentials. This finding is supported by our observation of formate from the reduction of Cu2(CO3)(OH)2. Pulse electrolysis on a copper electrode immersed in a N2-purged K2CO3 electrolyte was also performed. We found that the carbonate anions therein could be first adsorbed at -0.05 VRHE and then directly reduced to formate at -0.5 VRHE (overpotential of 0.28 V) with a Faradaic efficiency of 0.61%. The nature of the active sites generating the adsorbed carbonate species and the mechanism for the pulse-enabled reduction of carbonate to formate were elucidated. Our findings reveal how carbonates are directly reduced to a high-value product such as formate and open a potential pathway to mitigate carbonate formation during eCO2RR.
ABSTRACT
Cu catalysts are most apt for reducing CO(2) to multi-carbon products in aqueous electrolytes. To enhance the product yield, we can increase the overpotential and the catalyst mass loading. However, these approaches can cause inadequate mass transport of CO(2) to the catalytic sites, which will then lead to H2 evolution dominating the product selectivity. Herein, we use a MgAl LDH nanosheet 'house-of-cards' scaffold to disperse CuO-derived Cu (OD-Cu). With this support-catalyst design, at -0.7â VRHE , CO could be reduced to C2+ products with a current density (jC2+ ) of -1251â mA cm-2 . This is 14× that of the jC2+ shown by unsupported OD-Cu. The current densities of C2+ alcohols and C2 H4 were also high at -369 and -816â mA cm-2 respectively. We propose that the porosity of the LDH nanosheet scaffold enhances CO diffusion through the Cu sites. The CO reduction rate can thus be increased, while minimizing H2 evolution, even when high catalyst loadings and large overpotentials are used.
ABSTRACT
Electrocatalytic carbon monoxide reduction has been previously reported to yield a range of carbonaceous products including alcohols, hydrocarbons and carboxylic acids. However, esters, an important family of organic compounds, have not been formed. Herein, we report the electrosynthesis of C3 to C6 acetate esters (H3 C-(C=O)-O-R) from carbon monoxide using copper catalysts in a membrane electrode assembly cell. Ethyl acetate and propyl acetate could be produced with an unprecedented total Faradaic efficiency (FE) of â¼22 % and with a current density of up to -55â mA cm-2 , alongside minor quantities of methyl acetate and butyl acetate. The esters are produced via the addition reaction of ethenone (H2 C=C=O) and alcohols produced during CO reduction. We show that the near water-free reaction conditions and the high local pH play key roles in the formation of the esters.