The development of affordable electrocatalysts that can drive the reduction of CO2 to CO with high selectivity, efficiency, and large current densities is a critical step on the path to production of liquid carbon-based fuels. In this work, we show that inexpensive triflate salts of Sn(2+), Pb(2+), Bi(3+), and Sb(3+) can be used as precursors for the electrodeposition of CO2 reduction cathode materials from MeCN solutions, providing a general and facile electrodeposition strategy, which streamlines catalyst synthesis. The ability of these four platforms to drive the formation of CO from CO2 in the presence of [BMIM]OTf was probed. The electrochemically prepared Sn and Bi catalysts proved to be highly active, selective, and robust platforms for CO evolution, with partial current densities of jCO = 5-8 mA/cm(2) at applied overpotentials of η < 250 mV. By contrast, the electrodeposited Pb and Sb catalysts do not promote rapid CO generation with the same level of selectivity. The Pb material is only ∼10% as active as the Sn and Bi systems at an applied potential of E = -1.95 V and is rapidly passivated during catalysis. The Sb-comprised cathode material shows no activity for conversion of CO2 to CO under analogous conditions. When taken together, this work demonstrates that 1,3-dialkylimidazoliums can promote CO production, but only when used in combination with an appropriately chosen electrocatalyst material. More broadly, these results suggest that the interactions between CO2, the imidazolium promoter, and the cathode surface are all critical to the observed catalysis.
Two new 2,2’-bipyridine (bpy) derivatives containing ancillary BODIPY chromophores attached at the 5- and 5’-positions (BB3) or 6- and 6’-positions (BB4) were prepared and characterized. In this work, the basic photophysics, electrochemistry and electrogenerated chemiluminescence (ECL) of BB3 and BB4 are compared with those previously reported for a related bpy-BODIPY derivative (BB2) (J. Phys. Chem. C 2011, 115, 17993–18001). Cyclic voltammetry revealed that BB3 and BB4 display reversible 2e− oxidation and reduction waves, which consist of two closely spaced (50 – 70 mV) 1e− events. This redox behavior is consistent with the frontier molecular orbitals calculated for BB3 and BB4 and indicates that the 2,2’-bipyridine spacer of each bpy- BODIPY homologue does not facilitate efficient electronic communication between the tethered indacene units. In the presence of a coreactant such as tri-n-propylamine (TPA) or benzoyl peroxide (BPO), BB3 and BB4 exhibit strong ECL and produce spectra that are very similar to their corresponding photoluminescence profiles. The ECL signal obtained under annihilation conditions, however, is significantly different and is characterized by two distinct bands. One of these bands is centered at ~570 nm and is attributed to emission via an S- or T-route. The second band, occurs at longer wavelengths and is centered around ~740 nm. The shape and concentration dependence of this long-wavelength ECL signal is not indicative of emission from an excimer or aggregate, but rather is suggests that a new emissive species is formed from the bpy-BODIPY luminophores during the annihilation process.
The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethylbiladiene tetrapyrrole bearing ancillary pentafluorophenyl groups at the 5- and 15-meso positions (DMBil1) is presented. This nonmacrocyclic tetrapyrrole platform is robust and can serve as an excellent ligand scaffold for Zn2+ and Cu2+ centers. X-ray diffraction studies conducted for DMBil1 along with the corresponding Zn[DMBil1] and Cu[DMBil1] complexes show that this ligand scaffold binds a single metal ion within the tetrapyrrole core. Additionally, electrochemical experiments revealed that all three of the aforementioned compounds display an interesting redox chemistry as the DMBil1 framework can be both oxidized and reduced by two electrons. Spectroscopic and photophysical experiments carried out for DMBil1, Zn[DMBil1], and Cu[DMBil1] provide a basic picture of the electronic properties of these platforms. All three biladiene derivatives strongly absorb light in the visible region and are weakly emissive. The ability of these compounds to sensitize the formation of 1O2 at wavelengths longer than 500 nm was probed. Both the free base and Zn2+ 10,10-dimethylbiladiene architectures show modest efficiencies for 1O2 sensitization. The combination of structural, electrochemical, and photophysical data detailed herein provides a basis for the design of additional biladiene constructs for the activation of O2 and other small molecules.
Real-time changes in the composition and structure of bismuth electrodes used for catalytic conversion of CO2 into CO were examined via X-ray absorption spectroscopy (including XANES and EXAFS), electrochemical quartz crystal microbalance (EQCM), and in situ X-ray reflectivity (XR). Measurements were performed with bismuth electrodes immersed in acetonitrile (MeCN) solutions containing a 1-butyl-3-methylimidazolium ([BMIM]+) ionic liquid promoter or electrochemically inactive tetrabutylammonium supporting electrolytes (TBAPF6 and TBAOTf). Altogether, these measurements show that bismuth electrodes are originally a mixture of bismuth oxides (including Bi2O3) and metallic bismuth (Bi0) and that the reduction of oxidized bismuth species to Bi0 is fully achieved under potentials at which CO2 activation takes place. Furthermore, EQCM measurements conducted during cyclic voltammetry revealed that a bismuth-coated quartz crystal exhibits significant shifts in resistance (ΔR) prior to the onset of CO2 reduction near −1.75 V vs Ag/AgCl and pronounced hysteresis in frequency (Δf) and ΔR, which suggests significant changes in roughness or viscosity at the Bi/[BMIM]+ solution interface. In situ XR performed on rhombohedral Bi (001) oriented films indicates that extensive restructuring of the bismuth film cathodes takes place upon polarization to potentials more negative than −1.6 V vs Ag/AgCl, which is characterized by a decrease of the Bi (001) Bragg peak intensity of ≥50% in [BMIM]OTf solutions in the presence and absence of CO2. Over 90% of the reflectivity is recovered during the anodic half-scan, suggesting that the structural changes are mostly reversible. In contrast, such a phenomenon is not observed for thin Bi (001) oriented films in solutions of tetrabutylammonium salts that do not promote CO2 reduction. Overall, these results highlight that Bi electrodes undergo significant potential-dependent chemical and structural transformations in the presence of [BMIM]+-based electrolytes, including the reduction of bismuth oxide to bismuth metal and changes in roughness and near-surface viscosity.
Controlling the molecular topology of electrode–catalyst interfaces is a critical factor in engineering devices with specific electron transport kinetics and catalytic efficiencies. As such, the development of rational methods for the modular construction of tailorable electrode surfaces with robust molecular wires (MWs) exhibiting well-defined molecular topologies, conductivities and morphologies is critical to the evolution and implementation of electrochemical arrays for sensing and catalysis. In response to this need, we have established modular on-surface Sonogashira and Glaser cross-coupling processes to synthetically install arrays of ferrocene-capped MWs onto electrochemically functionalized surfaces. These methods are of comparable convenience and efficiency to more commonly employed Huisgen methods. Furthermore, unlike the Huisgen reaction, this new surface functionalization chemistry generates modified electrodes that do not contain unwanted ancillary metal binding sites, while allowing the bridge between the ferrocenyl moiety and electrode surface to be synthetically tailored. Electrochemical and surface analytical characterization of these platforms demonstrate that the linker topology and connectivity influences the ferrocene redox potential and the kinetics of charge transport at the interface.
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