Gold inverse opal (Au--IO) thin films are active for CO 2 reduction to CO with high efficiency at modest over--potentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by ten fold with increasing porous film thickness while CO evo--lution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au--IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal meso--porosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish elec--trode mesostructuring as a complementary method for tun--ing selectivity in CO 2 --to--fuels catalysis.The electroreduction of carbon dioxide is a promising meth--od for storing intermittent renewable electricity in energy dense carbonaceous fuels. 1--4 However, the high cost and low efficiency of electrochemical CO 2 reduction (CDR) has pre--vented this technology from reaching economic viability. 4 CDR is most practically achieved in aqueous electrolytes, in which the more kinetically facile reduction of protons to H 2 often outcompetes CO 2 reduction, eroding reaction selectivi--ty. Indeed, the paucity of general materials design principles for selectively inhibiting the hydrogen evolution reaction (HER) impedes the systematic development of improved CDR catalysts. 1 Recently, numerous nanostructured metals have been shown to catalyze CO 2 reduction with improved selectivity relative to planar polycrystalline foils. For example gold, copper, and lead films prepared by electrochemical reduction of copper, gold, and lead oxides, respectively, display high CDR selectiv--ity at low overpotentials. 5--7Likewise, de--alloyed porous Ag films 8 and carbon--supported Au nanoparticle 9 --11 and nan--owire electrodes 12 have been shown to catalyze the reduction of CO 2 to CO with high selectivity. This enhanced selectivity may arise from increases in the specific (surface area normal--ized) activity for CDR and/or from a decrease in specific ac--tivity for HER. For oxide--derived gold, evidence points to both effects, 13 whereas for oxide--derived Cu and Pb, specific HER activity have been shown to diminish more dramatically than CDR activity, giving rise to enhanced selectivity for the latter. 5,7 In general, selectivity differences have been attribut--ed to the intrinsic selectivity of the active sites in the materi--al. However, observations of thickness--dependent product selectivity for electrodeposited porous copper thin films 14 suggest that mass transport effects may also play a role in determining product selectivity. For example, when consid--ering CO 2 reduction catalyzed by Au, which generates CO and H 2 predominantly, both the desired reaction (eq. 1) and H 2 evolution (eq. 2) consume protons,necessitating the formation of a pH gradient at the electrode surface irrespective of the product...
We show that bicarbonate is neither a general acid nor a reaction partner in the rate-limiting step of electrochemical CO reduction catalysis mediated by planar polycrystalline Au surfaces. We formulate microkinetic models and propose diagnostic criteria to distinguish the role of bicarbonate. Comparing these models with the observed zero-order dependence in bicarbonate and simulated interfacial concentration gradients, we conclude that bicarbonate is not a general acid cocatalyst. Instead, it acts as a viable proton donor past the rate-limiting step and a sluggish buffer that maintains the bulk but not local pH in CO-saturated aqueous electrolytes.
An electrode's performance for catalytic CO conversion to fuels is a complex convolution of surface structure and transport effects. Using well-defined mesostructured silver inverse opal (Ag-IO) electrodes, it is demonstrated that mesostructure-induced transport limitations alone serve to increase the turnover frequency for CO activation per unit area, while simultaneously improving reaction selectivity. The specific activity for catalyzed CO evolution systematically rises by three-fold and the specific activity for catalyzed H evolution systematically declines by ten-fold with increasing mesostructural roughness of Ag-IOs. By exploiting the compounding influence of both of these effects, we demonstrate that mesostructure, rather than surface structure, can be used to tune CO evolution selectivity from less than 5 % to more than 80 %. These results establish electrode mesostructuring as a powerful complementary tool for tuning both catalyst selectivity and efficiency for CO conversion into fuels.
The dynamics of carbon monoxide on Cu surfaces was investigated during CO reduction, providing insight into the mechanism leading to the formation of hydrogen, methane, and ethylene, the three key products in the electrochemical reduction of CO . Reaction order experiments were conducted at low temperature in an ethanol medium affording high solubility and surface-affinity for carbon monoxide. Surprisingly, the methane production rate is suppressed by increasing the pressure of CO, whereas ethylene production remains largely unaffected. The data show that CH and H production are linked through a common H intermediate and that methane is formed through reactions among adsorbed H and CO, which are in direct competition with each other for surface sites. The data exclude the participation of solution species in rate-limiting steps, highlighting the importance of increasing surface recombination rates for efficient fuel synthesis.
Correlating the current/voltage response of an electrode to the intrinsic properties of the active material requires knowledge of the electrochemically active surface area (ECSA), a parameter that is often unknown and overlooked, particularly for highly nanostructured electrodes. Here we demonstrate the power of nonaqueous electrochemical double layer capacitance (DLC) to provide reasonable estimates of the ECSA across 17 diverse materials spanning metals, conductive oxides, and chalcogenides. Whereas data recorded in aqueous electrolytes generate a wide range of areal specific capacitance values (7-63 μF/real cm), nearly all materials examined display an areal specific capacitance of 11 ± 5 μF/real cm when measured in weakly coordinating KPF/MeCN electrolytes. By minimizing ion transfer reactions that convolute accurate DLC measurements, we establish a robust methodology for quantifying ECSA, enabling more accurate structure-function correlations.
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