Electrifying HCOOH synthesis from CO2 building blocks over Cu-Bi nanorod arrays.

Precise electrochemical synthesis of commodity chemicals and fuels from CO2 building blocks provides a promising route to close the anthropogenic carbon cycle, in which renewable but intermittent electricity could be stored within the greenhouse gas molecules. Here, we report state-of-the-art CO2-to-HCOOH valorization performance over a multiscale optimized Cu-Bi cathodic architecture, delivering a formate Faradaic efficiency exceeding 95% within an aqueous electrolyzer, a C-basis HCOOH purity above 99.8% within a solid-state electrolyzer operated at 100 mA cm-2 for 200 h and an energy efficiency of 39.2%, as well as a tunable aqueous HCOOH concentration ranging from 2.7 to 92.1 wt%. Via a combined two-dimensional reaction phase diagram and finite element analysis, we highlight the role of local geometries of Cu and Bi in branching the adsorption strength for key intermediates like *COOH and *OCHO for CO2 reduction, while the crystal orbital Hamiltonian population analysis rationalizes the vital contribution from moderate binding strength of η2(O,O)-OCHO on Cu-doped Bi surface in promoting HCOOH electrosynthesis. The findings of this study not only shed light on the tuning knobs for precise CO2 valorization, but also provide a different research paradigm for advancing the activity and selectivity optimization in a broad range of electrosynthetic systems.

Zn-Sb Bimetallic Electrocatalyst Enhances the Conversion of CO2 to Formate.

It was discovered the bimetallic Zn-Sb nanoparticles supported on carbon nanotubes can electrocatalyze CO2 into formate efficiently, comprising the best performance to date for Sb-based catalysts under moderate overpotential. This project was accomplished by a versatile two-step alcoholysis precipitation strategy with tunable Zn : Sb ratios, and the performance of optimized Zn2.33 Sb0.67 O4 was locked in the electrochemical CO2 reduction reaction. During the subsequent electrolysis, the mixed phases of metallic Zn and Sb served as active centers. The Zn-Sb heterostructure and the electron relocation were confirmed. By means of interactions and possible additional binding sites for reaction intermediate *OCHO, the material displayed different catalytic properties from either Zn or Sb, and was selective for formate up to 92%, which was ca. 6.1 times or 4.6 times than that of each single component. The encouraging results highlight the power of the interaction between binary metallic components to synergistically electrocatalyze CO2 conversion.

ZnSn nanocatalyst: Ultra-high formate selectivity from CO2 electrochemical reduction and the structure evolution effect.

The introduction of tin (Sn) into Zn-based catalyst can change its intrinsic properties of the electrochemically reduction of CO2 to CO, obtaining a high formate yield. The electron transfer from Zn to Sn lowers down the d-band center of Sn, leading to a more reliable surface adsorption of the *OCHO intermediate and high formate selectivity. The obtained ZnSn catalyst enables formate formation with a drastically boosted Faradaic efficiency (FE) up to 94%, which is 2.04 and 1.34 times of pure Zn and Sn foils, respectively, indicating a synergistic effect between Zn and Sn. During the electrochemical CO2 reduction reaction (eCO2RR) process, the morphology of the ZnSn catalyst evolved from nanoparticles to nanosheets, nanoneedles and collapsed structures, corresponding to the activation, stabilization and decay stages, respectively. This study provides a facile and controllable approach for the construction of novel bimetallic catalyst favoring formate selectivity based on the synergistic effect.

Boosting carbon monoxide production during CO2 reduction reaction via Cu-Sb2O3 interface cooperation.

Development of multiple-component catalyst materials is a new trend in electrochemical CO2 reduction reaction (eCO2RR). A new type of metal-oxide interaction is reported here to improve carbon monoxide production via synergistic effect between the CO2-to hydrocarbon selective metal material and CO2-to hydrogen generation oxide material. Cu/Sb2O3 material originates from the hetero-structured CuO/Sb2O3 by a facile two-step hydrolysis and precipitation method, cooperative to inhibit hydrogen evolution or methane product, achieving CO Faradaic efficiency to 92% in CO2 saturated KCl electrolyte at -0.99 V with good stability. The formation of a stable *COOH intermediate by electronic and geometric effects via Cu and Sb2O3 are responsible to promote CO selectivity. Cu-Sb2O3 interface interaction also destabilizes the adsorption *H as well, an intermediate for H2 evolution. This study proposes a versatile design strategy for construction and utilization of metal-oxide interface for eCO2RR.

Active sites of copper-complex catalytic materials for electrochemical carbon dioxide reduction.

Restructuring-induced catalytic activity is an intriguing phenomenon of fundamental importance to rational design of high-performance catalyst materials. We study three copper-complex materials for electrocatalytic carbon dioxide reduction. Among them, the copper(II) phthalocyanine exhibits by far the highest activity for yielding methane with a Faradaic efficiency of 66% and a partial current density of 13 mA cm-2 at the potential of - 1.06 V versus the reversible hydrogen electrode. Utilizing in-situ and operando X-ray absorption spectroscopy, we find that under the working conditions copper(II) phthalocyanine undergoes reversible structural and oxidation state changes to form ~ 2 nm metallic copper clusters, which catalyzes the carbon dioxide-to-methane conversion. Density functional calculations rationalize the restructuring behavior and attribute the reversibility to the strong divalent metal ion-ligand coordination in the copper(II) phthalocyanine molecular structure and the small size of the generated copper clusters under the reaction conditions.

Coupled Metal/Oxide Catalysts with Tunable Product Selectivity for Electrocatalytic CO2 Reduction.

One major challenge to the electrochemical conversion of CO2 to useful fuels and chemical products is the lack of efficient catalysts that can selectively direct the reaction to one desirable product and avoid the other possible side products. Making use of strong metal/oxide interactions has recently been demonstrated to be effective in enhancing electrocatalysis in the liquid phase. Here, we report one of the first systematic studies on composition-dependent influences of metal/oxide interactions on electrocatalytic CO2 reduction, utilizing Cu/SnOx heterostructured nanoparticles supported on carbon nanotubes (CNTs) as a model catalyst system. By adjusting the Cu/Sn ratio in the catalyst material structure, we can tune the products of the CO2 electrocatalytic reduction reaction from hydrocarbon-favorable to CO-selective to formic acid-dominant. In the Cu-rich regime, SnOx dramatically alters the catalytic behavior of Cu. The Cu/SnOx-CNT catalyst containing 6.2% of SnOx converts CO2 to CO with a high faradaic efficiency (FE) of 89% and a jCO of 11.3 mA·cm-2 at -0.99 V versus reversible hydrogen electrode, in stark contrast to the Cu-CNT catalyst on which ethylene and methane are the main products for CO2 reduction. In the Sn-rich regime, Cu modifies the catalytic properties of SnOx. The Cu/SnOx-CNT catalyst containing 30.2% of SnOx reduces CO2 to formic acid with an FE of 77% and a jHCOOH of 4.0 mA·cm-2 at -0.99 V, outperforming the SnOx-CNT catalyst which only converts CO2 to formic acid in an FE of 48%.

Self-Cleaning Catalyst Electrodes for Stabilized CO2 Reduction to Hydrocarbons.

A surface-restructuring strategy is presented that involves self-cleaning Cu catalyst electrodes with unprecedented catalytic stability toward CO2 reduction. Under the working conditions, the Pd atoms pre-deposited on Cu surface induce continuous morphological and compositional restructuring of the Cu surface, which constantly refreshes the catalyst surface and thus maintains the catalytic properties for CO2 reduction to hydrocarbons. The Pd-decorated Cu electrode can catalyze CO2 reduction with relatively stable selectivity and current density for up to 16 h, which is one of the best catalytic durability performances among all Cu electrocatalysts for effective CO2 conversion to hydrocarbons. The generality of this approach of utilizing foreign metal atoms to induce surface restructuring toward stabilizing Cu catalyst electrodes against deactivation by carbonaceous species accumulation in CO2 reduction is further demonstrated by replacing Pd with Rh.

Adsorption configuration of sodium 2-quinoxalinecarboxylate on iron substrate: Investigation by in situ SERS, XPS and theoretical calculation.

The adsorption geometry of sodium 2-quinoxalinecarboxylate (2-QC) on iron surface was investigated by in situ surface-enhanced Raman scattering spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS) measurements. The density functional theory (DFT) calculations predicted that 2-QC ion was a highly efficient inhibitor and N as well as O atoms were the possible adsorption centers, and theoretically offered the Raman-active band position and intensity. Potential-dependent SERS results suggested that the 2-QC strongly bonded to the iron surface via the lone pair electrons of the two O atoms of the carboxylate group in a bidentate configuration with a vertical orientation at more positive potentials; However, at -1.0 V, only one O atom of the carboxylate and the neighboring N(1) atom (or very close to surface) adsorbed on the iron surface forming an unidentate configuration with a titled orientation. The ions did not remain on the iron surface at more negative potentials.

Exploring electrosorption at iron electrode with in situ surface-enhanced infrared absorption spectroscopy.

Surface-enhanced infrared absorption spectroscopy (SEIRAS) in attenuated total reflection (ATR) configuration has been extended to the Fe electrode/electrolyte interface in neutral and weakly acidic solutions for the first time. The SEIRA-active Fe film electrode was obtained through a potentiostatic electrodeposition of a virtually pinhole-free 40 nm-thick Fe overfilm onto a 60 nm-thick Au underfilm chemically predeposited on the reflecting plane of an ATR Si prism. The infrared absorption for CO adlayer at the Fe film electrode measured with ATR-SEIRAS was enhanced by a factor of larger than 34, as compared to that at a Fe bulk electrode with external infrared absorption spectroscopy in the literature. More importantly, the unipolar band shape enabled the reliable determination of the Stark tuning rates of CO adlayer at Fe electrode. In situ ATR-SEIRAS was also applied to study the electrosorption of a typical corrosion inhibitor benzotriazole (BTAH) on Fe electrode as a function of potential, providing additional spectral information at positive potentials in support of the formation of a polymer-like surface complex Fe(II)(m)(BTA)(n) as the corrosion-resistant layer.

Attenuated total reflection surface-enhanced infrared absorption spectroscopy at a cobalt electrode.

In situ surface-enhanced infrared absorption spectroscopy (SEIRAS) in attenuated total reflection (ATR) configuration has been extended to a Co electrode fabricated by potentiostatic deposition of a 50-nm-thick Co overlayer onto a Au underlayer chemically preformed on the reflecting plane of an ATR Si hemi-cylindrical prism. The as-prepared Co-on-Au film was characterized with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The AFM images of the films before and after Co coating revealed island structures facilitating the SEIRA effect with Co nanoparticles much smaller than the underlying Au ones. The XPS spectrum did not contain any characteristic peaks related to Au, suggestive of a virtually pinhole-free nature of the Co overlayer. The voltammetric response of the as-prepared films in phosphate buffer solution (PBS, pH 6.9) was characteristic of a polycrystalline bulk Co electrode. Normally directed unipolar bands were found for surface probe CO molecules on Co surfaces in the PBS with their major band (CO(L)) intensity being one order of magnitude higher than that obtained with conventional IR reflection-absorption spectroscopy (IRRAS). By taking advantage of the higher detection sensitivity, the bands for linearly bonded CO (CO(L)) at 1965-2005 cm(-1) and the multi-bonded (CO(M)) band at 1845-1875 cm(-1) were clearly detected with their Stark tuning rates being 59 and 63 cm(-1) x V(-1), respectively, which would be otherwise unobtainable with the conventional IRRAS in the neutral solution.

Seeded-growth approach to fabrication of silver nanoparticle films on silicon for electrochemical ATR surface-enhanced IR absorption spectroscopy.

Ag nanoparticle films (simplified as nanofilms hereafter) on Si for electrochemical ATR surface enhanced IR absorption spectroscopy (ATR-SEIRAS) have been successfully fabricated by using chemical deposition, which incorporates initial embedding of Ag seeds on the reflecting plane of an ATR Si prism and subsequent chemical plating of conductive and SEIRA-active Ag nanofilms. Two alternative methods for embedding initial Ag seeds have been developed: one is based on self-assembly of Ag colloids on an aminosilanized Si surface, whereas the other the reduction of Ag+ in a HF-containing solution. A modified silver-mirror reaction was employed for further growth of Ag seeds into Ag nanofilm electrodes with a theoretically average thickness of 40-50 nm. Both Ag seeds and as-deposited Ag nanofilms display island structure morphologies facilitating SEIRA, as revealed by AFM imaging. The cyclic voltammetric feature of the as-prepared Ag nanofilm electrodes is close to that of a polycrystalline bulk Ag electrode. With thiocyanate as a surface probe, enhancement factors of ca. 50-80 were estimated for the as-deposited Ag nanofilms as compared to a mechanically polished Ag electrode in the conventional IRAS after reasonable calibration of surface roughness factor, incident angles, surface coverage, and polarization states. As a preliminary example for extended application, the pyridine adsorption configuration at an as-deposited Ag electrode was re-examined by ATR-SEIRAS. The results revealed that pyridine molecules are bound via N end to the Ag electrode with its ring plane perpendicular or slightly tilted to the local surface without rotating its C2 axis about the surface normal, consistent with the conclusion drawn by SERS in the literature.

In situ surface-enhanced IR absorption spectroscopy on CO adducts of iron protoporphyrin IX self-assembled on a Au electrode.

The surface coordination chemistry of carbon monoxide with the reduced form (Fe(II)PP) of iron(III) protoporphyrin IX (Fe(III)PP) monolayer self-assembled on a Au electrode in 0.1 M HClO4 was studied for the first time by using in situ ATR-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Both mono- and biscarbonyl adducts [simplified as Fe(II)(CO)PP and Fe(II)(CO)2PP, respectively] were detected, depending on the history of potential control. Initially, the Fe(II)(CO)PP predominates, and the intermediate transition potential for the conversion of Fe(II)(CO)PP to Fe(III)PP and CO was spectrally determined to be ca. 0.09 V (vs SCE). The ratio of Fe(II)(CO)2PP and Fe(II)(CO)PP increases after a potential excursion to a sufficiently positive value. Fe(II)(CO)2PP is much more stable against its electro-oxidation to Fe(III)PP than its counterpart Fe(II)(CO)PP with increasing potential. The observed change of coordination properties may be ascribed to an irreversible structural reorganization of the FePP adlayer caused by the potential excursion.

Extending in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy to ni electrodes.

Surface-enhanced infrared absorption spectroscopy (SEIRAS) in the attenuated-total-reflection configuration (ATR-SEIRAS) has been applied for the first time to Ni electrodes. SEIRA-active Ni electrodes were obtained through initial chemical deposition of a 60-nm-thick Au underlayer on the reflecting plane of an ATR Si prism followed by potentiostatic electrodeposition of a 40-nm-thick Ni overlayer in a modified Watt's electrolyte. The Ni nanoparticle film thus obtained exhibited exceptionally enhanced IR absorption for the surface probe molecule CO while maintaining unipolar and normally directed bands. With the advantages of ATR-SEIRAS, free H2O molecules coadsorbed with CO at the Ni electrode were revealed, and their role in the electrooxidation of the CO adlayer at the Ni electrode is discussed. In addition, the conversion of bridge to linearly bonded CO at Ni electrode in a neutral solution was clearly identified upon electrooxidation of the CO adlayer. ATR-SEIRAS was also used to characterize the adsorption configuration of a pyridine adlayer at the Ni electrode. Both A1 and B1 modes of adsorbed pyridine were detected with comparably large intensities, essentially maintaining the spectral feature of pyridine molecules rather than that of "alpha-pyridyl species", which strongly suggests an "edge-tilted pyridine" configuration present at the Ni electrode, a configuration intermediate between the "end-on pyridine" and "edge-on alpha-pyridyl" adsorption modes reported in the literature.

Ubiquitous strategy for probing ATR surface-enhanced infrared absorption at platinum group metal-electrolyte interfaces.

A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanoparticle films (simplified as nanofilms hereafter) for in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study of electrochemical interfaces is presented, which incorporates an initial chemical deposition of a gold nanofilm on the basal plane of a silicon prism with the subsequent electrodepostion of desired platinum group metal overlayers. Galvanostatic electrodeposition of Pt, Rh, and Pd from phosphate or perchloric acid electrolytes, or potentiostatic electrodeposition of Ru from a sulfuric acid electrolyte, yields sufficiently "pinhole-free" overlayers as evidenced by electrochemical and spectroscopic characterizations. The Pt group metal nanofilms thus obtained exhibit strongly enhanced IR absorption. In contrast to the corresponding metal films electrochemically deposited directly on glassy carbon and bulk metal electrodes, the observed enhanced absorption for the probe molecule CO exhibits normal unipolar band shapes. Scanning tunneling microscopic (STM) images reveal that fine nanoparticles of Pt group metals are deposited around wavy and stepped bunches of Au nanoparticles of relatively large sizes. This ubiquitous strategy is expected to open a wide avenue for extending ATR surface-enhanced IR absorption spectroscopy to explore molecular adsorption and reactions on technologically important transition metals, as exemplified by successful real-time spectroscopic and electrochemical monitoring of the oxidation of CO at Pd and that of methanol at Pt nanofilm electrodes. The spectral features of free water molecules coadsorbed with CO on Pt, Pd, Rh, and Ru are also discussed.

Tunable surface-enhanced infrared absorption on au nanofilms on si fabricated by self-assembly and growth of colloidal particles.

Au colloids were used to fabricate nanoscale-tunable Au nanofilms on silicon for surface-enhanced IR absorption bases in both ambient and electrochemical environments. This wet process incorporates the self-assembly of colloidal Au monolayer using 3-aminopropyl trimethoxysilane as the organic coupler with subsequent chemical plating in an Au(III)/hydroxylamine solution. FTIR spectroscopy in transmission mode of the probe species SCN- was used to evaluate the apparent surface enhancement in IR absorption of 2D Au colloid arrays and chemically plated Au particles. The nanostructure of Au films was examined by atomic force microscopy. The IR and AFM results show that the apparent surface enhancement factor (1-2 orders of magnitude) increases with increasing sizes and/or contact, and the severe aggregation of Au nanoparticles may cause the bipolar band shape. Cyclic voltammetry on the Au nanofilm obtained by the above nucleation and growth strategy exhibits a feasible electrochemical stability and behavior. In situ ATR-FTIR measurement of p-nitrobenzoic acid adsorption demonstrates that the as-grown Au film yields rather promising surface enhancement as well.