The synthesis of hydrocarbons via electroreduction of CO 2 is an attractive approach to store energy generated from intermittent renewable sources of electricity (e.g., solar) through formation of the high-energy C-C and C-H bonds of reduced carbon compounds (1, 2). Establishment of such processes also represents a critical step toward the sustainable production of carbonbased commodity chemicals and energy-rich liquid fuels from nonpetroleum resources (3). Despite the promise that such strategies hold, facilitating the rapid, selective, and efficient electrosynthesis of multicarbon products from CO 2 is an inherently difficult proposition. Part of the challenge stems from the fact that CO 2 reduction half-reactions that generate value-added C 1 and multicarbon products take place within a narrow potential window that is less than 0.5 V wide ( Fig. 1).As a result of the reaction landscape illustrated in Fig. 1, it is virtually impossible to target a given CO 2 reduction product based purely on thermodynamic considerations. For instance, electrochemical reduction of CO 2 to ethylene occurs at 0.08 V vs. reversible hydrogen electrode (RHE). Accordingly, any electrochemical process that targets this product must be run at a potential at which production of other species such as ethane (E°= 0.14 V) and methane (E°= 0.17 V) is also thermodynamically feasible. Moreover, kinetics associated with CO 2 reduction reactions (CO 2 RRs) can often be sluggish. This is particularly true for CO 2 RR processes that generate reduced products with C-C bonds, which require application of modest overpotentials of at least 400 to 500 mV. In practice, electrochemical hydrocarbon evolution ultimately requires cathodic potentials that are more negative than -0.5 V vs. RHE, which ultimately brings all of the CO 2 couples of Fig. 1 into play. Matters are further complicated by the fact that each CO 2 reduction couple requires both eand H + equivalents. As a result, the kinetically facile reduction of protons to hydrogen gas (E°= 0.0 V) represents a competitive cathodic process that must be suppressed for efficient and selective hydrocarbon evolution to be realized.Each CO 2 reduction half-reaction requires multiple eand H + equivalents that are most logically provided by the oxidation of water (E°= 1.23 V). Accordingly, electrochemical cells (ECs) for sustainable hydrocarbon synthesis must juxtapose cathode and anode catalysts that can manage the demanding multielectron proton-coupled electron transfer reactions attendant to CO 2 reduction and H 2 O oxidation, respectively. Moreover, since production of hydrocarbons from Potential versus RHE (V) Fig. 1. Equilibrium potentials for reductive and oxidative half-reactions relevant to the synthesis of hydrocarbons from carbon dioxide, water, and sunlight. Electron and proton equivalents generated via water oxidation at the anode of an EC can be utilized for fuel-forming cathodic processes to generate hydrogen gas or a broad array of reduced carbon products. Promoting the efficient and selective...
The electrochemical reduction of CO 2 into reduced carbon compounds is a compelling strategy to sustainably synthesize fuels and commodity chemicals using renewable energy sources. Although promising post-transition metal electrocatalysts have been developed from monometallic thin films, limited effort has been dedicated to the use of trimetallic alloys for CO 2 electrocatalysis. In this work, we have explored the electrocatalytic effect of alloying Sn, Pb, and Bi, which are all reported as being individually active catalysts for CO 2 reduction to different extents. It is demonstrated that the well-known soldering alloy Bi 50 Sn 22 Pb 28 (Rose's metal = RM) promotes the conversion of CO 2 to CO in MeCN electrolyte containing millimolar concentrations of the ionic liquid additive [BMIM]OTf. Planar RM electrocatalysts evolve CO with Faradaic efficiencies as high as 95% with average geometric current densities of j tot = 3−10 mA/cm 2 . XPS analysis shows evidence for the accumulation of metal oxides on the RM surface following CO 2 electrocatalysis, which may influence the activity of the alloy.
Electrodeposited composite film electrodes prepared from electroplating baths with varying ratios of Ag+ and Sn2+ triflates were studied to understand how the performance of such materials varies as a function of composition. X-ray photoelectron spectroscopy (XPS) analysis confirms that for each composite, Ag existed in the metallic (Ag0) state, while Sn was mainly oxidized (Sn2+/4+). The AgSn composite films studied herein are therefore best considered as AgSnO x cathodes with varying ratios of Ag0/Sn2+/4+. These systems were assessed as CO2 reduction reaction (CO2RR) electrocatalysts and were found to promote the 2e–/2H+ reductions to deliver CO and HCOOH with fast kinetics and high efficiencies from electrolyte solutions containing the protic organic cation [DBU–H]+ (i.e., protonated 1,8-diazabicyclo[5.4.0]undec-7-ene). While Sn-rich composite films showed poor selectivities for CO vs HCO2H, a significant increase in CO vs HCO2H selectivity (up to 99%) was achieved for composite film electrodes in which the Ag content ranged from 25 to 75%. Tuning the ratio of Ag0 to SnO x delivered composite films that support quantitative current efficiencies for generation of CO with geometric current densities approaching 30 mA/cm2 at applied overpotentials that are less than 750 mV were realized. Additionally, electrochemical impedance spectroscopy (EIS) coupled with analysis of the distribution of relaxation times (DRT) was used to better understand factors important to the composites’ activity under CO2RR conditions. Probing the dynamics with DRT analysis revealed that multiple processes relating to both adsorption and diffusion-controlled events are important to the activity of the electrocatalysts considered in this work. The collection of electroanalytical investigations suggest that synergistic interactions between Ag and SnO x give rise to rough films that support enhanced CO2RR kinetics and that mixing of Ag with SnO x enhances the efficacy of adsorption and stabilization of reduced CO2 intermediates and [DBU–H]+ cations to facilitate CO evolution at the cathode/electrolyte interface.
15 new quaternary Zintl phases have been synthesized by solid-state reactions from the respective elements, and their structures have been determined by single-crystal X-ray diffraction. Na 3 E 3 TrPn 4 (E = Ca, Sr, Eu; Tr = Al, Ga, In; Pn = P, As, Sb) crystallize in the hexagonal crystal system with the non-centrosymmetric space group P6 3 mc (No. 186). The structure represents a variant of the K 6 HgS 4 structure type (Pearson index hP22) and features [TrPn 4 ] 9tetrahedral units, surrounded by Na + and Ca 2+ , Sr 2+ , Eu 2+ cations. The nominal formula rationalization [Na + ] 3 [E 2+ ] 3 [TrPn 4 ] 9follows the octet rule, suggesting closed-shell configurations for all atoms and intrinsic semiconducting behavior. However, structure refinements for several members hint at disorder and mixing of cations that potentially counteract the optimal valence electron count.
Sequential infiltration synthesis (SIS) is a vapor phase synthesis technique with potential to exert precise control over metal oxyhydroxide incorporation into polymer scaffolds. We observe strong size-dependent properties of InOx(OH)y few-atom clusters deposited with variable SIS cycle numbers within a polymethylmethacrylate (PMMA) matrix. Infrared spectroscopy and ultraviolet-visible absorption spectroscopy reveal that the metal atom coordination and optical properties of the clusters depend on the number of SIS cycles performed as well as the choice of processing parameters. The incorporation of indium oxyhydroxide in PMMA via SIS presents an opportunity to improve the CO2 absorption capacity and gas selectivity of inexpensive polymers.
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