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.

An electrodes performance for catalytic CO 2 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 2 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 2 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 2 conversion into fuels.Intermittent renewable electricity can be stored in the energy-dense bonds of chemical fuels by the electrochemical reduction of CO 2 .[1-4] However, the low efficiencies and high costs of current CO 2 -to-fuels technologies have impeded widespread commercial deployment.[4] A principle impediment to the development of practical CO 2 -to-fuels devices is the lack of efficient and selective catalysts for the multielectron, multi-proton reduction of CO 2 . CO 2 reduction is most practically carried out in aqueous electrolytes, in which the reduction of protons to H 2 often outcompetes CO 2 -tofuels conversion, eroding reaction selectivity. Thus, a key requirement for any viable catalyst is the ability to preferentially activate CO 2 over H + , despite the relative kinetic difficulty of the former process. The importance of this initial selectivity determining step is highlighted on Ag and Au surfaces, [3,5] which principally generate CO [Eq. (1)] and H 2 [Eq. (2)] as per the following electrochemical half reactions:Although the CO produced [Eq.(1)] can be further reduced to a wide array of higher order carbonaceous products on Cu metal surfaces, the initial kinetic branching ratio between CO and H 2 production places an upper limit on overall CO 2 -to-fuels selectivity. [3,[5][6][7][8] Despite the central role of this initial kinetic branch point, general materials design principles for realizing selective CO 2 over H + conversion remain scarce, thereby impeding systematic development of CO 2 -to-fuels catalysts. [3] As electrocatalysis is an interfacial phenomenon, the relative rates of CO 2 and H + activation will be dictated by both the intrinsic selectivity of surface active sites as well as the local concentration of reaction partners involved in the rate-controlling steps of each pathway. While the bulk concentration of all relevant chemical species can be easily measured and varied by changing, for example, the pH, buffer strength, or CO 2 partial pressure, the local concentration of these species at ...

We studied the role of different Se precursors for PbSe nanorod (NR) synthesis, focusing on phosphine chemistry to understand precursor decomposition. After characterizing the morphology of PbSe nanocrystals (NCs) and NRs with absorption spectra and TEM analyses, we used 31P NMR to correlate morphology with precursor decomposition during synthesis. While spherical PbSe NCs can be produced with a trioctylphosphine selenide (TOPSe)-based synthesis even at low temperatures (50−60 °C) or without free phosphine, PbSe NRs are more sensitive to their reaction conditions. At lower temperatures, tris(diethylamino)phosphine selenide (TDPSe) does not show any evidence of Se precursor decomposition, and the presence of amine-based free phosphine in the Se precursor affects the morphology of PbSe NRs dramatically. Further TGA-MS analysis implies that TDP accelerates precursor decomposition and morphology evolution by releasing amine species. A control experiment that added amine into both TOPSe and TDPSe with no free phosphine-based reactions shows amine species enhance the attachment process and morphology change.

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