Shu-Feng Zhao

Polyethylenimine promoted electrocatalytic reduction of CO2 to CO in aqueous medium by graphene-supported amorphous molybdenum sulphide

Efficiently and selectively converting CO2 to value-added carbon compounds remains a major challenge in sustainable energy research. In this paper, we report the synthesis of a cost-effective catalyst, i.e. amorphous molybdenum sulphide on a polyethylenimine modified reduced graphene oxide substrate, for electrocatalytically reducing CO2 into CO in CO2 saturated aqueous NaHCO3 medium with high efficiency and selectivity. The catalyst is capable of producing CO at overpotentials as low as 140 mV and reaches a maximum faradaic efficiency of 85.1% at an overpotential of 540 mV. At an overpotential of 290 mV with respect to the formation of CO, it catalyzes the formation of syngas with high stability. Detailed investigations reveal that PEI works as a co-catalyst by providing a synergetic effect with MoSx.

Simplifying the Evaluation of Graphene Modified Electrode Performance Using Rotating Disk Electrode Voltammetry

Graphene modified electrodes have been fabricated by electrodeposition from an aqueous graphene oxide solution onto conducting Pt, Au, glassy carbon, and indium tin dioxide substrates. Detailed investigations of the electrochemistry of the [Ru(NH(3))(6)](3+/2+) and [Fe(CN)(6)](3-/4-) and hydroquinone and uric acid oxidation processes have been undertaken at glassy carbon and graphene modified glassy carbon electrodes using transient cyclic voltammetry at a stationary electrode and near steady-state voltammetry at a rotating disk electrode. Comparisons of the data with simulation suggest that the transient voltammetric characteristics at graphene modified electrodes contain a significant contribution from thin layer and surface confined processes. Consequently, interpretations based solely on mass transport by semi-infinite linear diffusion may result in incorrect conclusions on the activity of the graphene modified electrode. In contrast, steady-state voltammetry at a rotating disk electrode affords a much simpler method for the evaluation of the performance of graphene modified electrode since the relative importance of the thin layer and surface confined processes are substantially diminished and mass transport is dominated by convection. Application of the rotated electrode approach with carbon nanotube modified electrodes also should lead to simplification of data analysis in this environment.

Electrooxidation of Ethanol and Methanol Using the Molecular Catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10.).

Highly efficient electrocatalytic oxidation of ethanol and methanol has been achieved using the ruthenium-containing polyoxometalate molecular catalyst, [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-)). Voltammetric studies with dissolved and surface-confined forms of [1(γ-SiW10O36)2](10-) suggest that the oxidized forms of 1 can act as active catalysts for alcohol oxidation in both aqueous (over a wide pH range covering acidic, neutral, and alkaline) and alcohol media. Under these conditions, the initial form of 1 also exhibits considerable reactivity, especially in neutral solution containing 1.0 M NaNO3. To identify the oxidation products, preparative scale bulk electrolysis experiments were undertaken. The products detected by NMR, gas chromatography (GC), and GC-mass spectrometry from oxidation of ethanol are 1,1-diethoxyethane and ethyl acetate formed from condensation of acetaldehyde or acetic acid with excess ethanol. Similarly, the oxidation of methanol generates formaldehyde and formic acid which then condense with methanol to form dimethoxymethane and methyl formate, respectively. These results demonstrate that electrocatalytic oxidation of ethanol and methanol occurs via two- and four-electron oxidation processes to yield aldehydes and acids. The total faradaic efficiencies of electrocatalytic oxidation of both alcohols exceed 94%. The numbers of aldehyde and acid products per catalyst were also calculated and compared with the literature reported values. The results suggest that 1 is one of the most active molecular electrocatalysts for methanol and ethanol oxidation.

Electrocarboxylation of acetophenone in ionic liquids: the influence of proton availability on product distribution

The electroreduction of acetophenone has been investigated in two dry ionic liquids (1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([BMMIM][BF4]; [H2O] = 9.2 mM) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyrd][TFSI]; [H2O] = 1.0 mM)) under both N2 and CO2 atmospheres using transient cyclic voltammetry, near steady-state voltammetry, the bulk electrolysis technique and numerical simulations. The proton availability in both solvents is low. In these dry ionic liquids under a N2 atmosphere, the sole reduction product detected is a dimer. The rate constants for dimer formation determined by comparison of experiment and simulation are 5.0 × 104 M−1 s−1 and 4.0 × 103 M−1 s−1, in [BMMIM][BF4] and [BMPyrd][TFSI], respectively. In dry [BMMIM][BF4] under a CO2 atmosphere, the products of the electroreduction of acetophenone are mixtures of 2-hydroxy-2-phenylpropionic acid, 1-phenylethanol and dimers. By contrast, the major reduction product in dry [BMPyrd][TFSI] is 2-hydroxy-2-phenylpropionic acid, suggesting that this ionic liquid is a suitable medium for electrocarboxylation. In water saturated [BMPyrd][TFSI] ([H2O] = 0.63 M), dimers are the major products under both N2 and CO2 atmospheres. The dimerization rate constant determined for this reaction under a N2 atmosphere was 1.0 × 107 M−1 s−1; more than three orders of magnitude higher than that found in dry [BMPyrd][TFSI]. Presumably strong interactions between the acetophenone radical anions and water through an extensive hydrogen bonding network lead to a larger degree of charge delocalization and thus favour dimer formation.

Remarkable Sensitivity of the Electrochemical Reduction of Benzophenone to Proton Availability in Ionic Liquids

The reduction of benzophenone was investigated in five different ionic liquids by using transient cyclic voltammetry, near steady-state voltammetry, and numerical simulation. Two reversible, well-resolved one-electron-reduction processes were observed in dry (≤20 ppm water, ca. 1 mM)) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrd][NTf(2)]) and 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([Bmpipd][NTf(2)]), which did not contain any readily available proton source. Upon addition of water, the second process became chemically irreversible and shifted to a more positive potential by approximately 600 mV; moreover, the two reduction processes merged into a single two-electron proton-coupled process when about 0.6 M H(2)O was present. This large dependence of potential on water content, which was not observed in molecular solvents (electrolyte), was explained by a reaction mechanism that incorporated protonation and hydrogen-bonding interactions of the benzophenone dianion with as many as seven water molecules. In the three imidazolium-based ionic liquids used herein, the first benzophenone-reduction process was again reversible, whilst the second reduction process became chemically irreversible owing to the availability of the C2-H imidazolium protons in these ionic liquids. The reversible potentials for benzophenone reduction were remarkably independent of the identity of the ionic liquids, thereby implying either weak interactions with the ionic liquids or relatively insignificant differences in the levels of ion-pairing. Thus, the magnitude of the separation of the potentials of the reversible first and irreversible second reduction processes mainly reflected the proton availability from either the ionic liquid itself or from adventitious water. Consequently, voltammetric reduction of benzophenone provides a sensitive tool for the determination of proton availability in ionic liquids.

A unique proton coupled electron transfer pathway for electrochemical reduction of acetophenone in the ionic liquid [BMIM][BF4] under a carbon dioxide atmosphere

The mechanism of electrochemical reduction of acetophenone in 1-butyl-3-methylimidazolium tetrafluroborate ([BMIM][BF4]) under nitrogen (N2) and carbon dioxide (CO2) atmospheres have been investigated using transient voltammetry, steady-state voltammetry, bulk electrolysis and numerical simulation. Under a N2 atmosphere, acetophenone undergoes a one-electron reduction to the radical anion followed by rapid dimerization reactions with an apparent rate constant of 1.0 × 106 M−1s−1. In contrast, under a CO2 atmosphere, the electrochemical reduction of acetophenone is an overall two-electron transfer chemically irreversible process with the final electrolysis product being 1-phenylethanol, instead of the anticipated 2-hydroxy-2-phenylpropionic acid resulting from an electrocarboxylation reaction. A proton coupled electron transfer pathway leading to the formation of 1-phenylethanol requires the presence of a sufficiently strong proton donor which is not available in neat [BMIM][BF4]. However, the presence of CO2 enhances the C-2 hydrogen donating ability of [BMIM]+ due to strong complex formation between the deprotonated form of [BMIM]+, N-heterocyclic carbene, and CO2, resulting in a thermodynamically favorable proton coupled electron transfer pathway.

Voltammetry of Adhered Microparticles in Contact with Ionic Liquids: Principles and Applications

Although many compounds possess a high thermodynamic level of solubility in ionic liquids, the rate of their dissolution in this type of media is frequently extremely slow. As a result, it can take several hours to prepare the solutions needed for electrochemical studies, even when the dissolution process is assisted by sonication or heating. Under these conditions, voltammetric studies with adhered microparticles are advantageous, allowing parameters such as formal potentials and electrode kinetics of electroactive species to be investigated without lengthy and tedious solution preparation. This chapter focuses on reviewing the attributes of voltammetry of adhered microparticles in contact with ionic liquids method for determining the thermodynamics and kinetic properties of electroactive species exhibiting a diverse range of electrode-reaction mechanisms. The conditions under which the adhered microparticle method gives voltammetric data analogous to that observed when working in the dissolved state are outlined, and the results obtained when applied to oxidation of ferrocene and its derivatives in a range of ionic liquids are surveyed. Additionally, quantitative measurements on electrode processes exhibiting more complex mechanisms, such as electron-transfer with a coupled first-order homogeneous process (oxidation of cis-[Mn-(CN)(CO)2{P(OPh)3} (Ph2PCH2PPh2)]) or with unstable reactants, intermediates, or products (polyoxometalate reduction in the “distillable ionic liquid” DIMCARB, which is largely composed of N,N-dimethylammonium, N′,N′-dimethylcarbamate and the corresponding N′,N′-dimethylcarbamic acid) and benzophenone reduction in “wet” 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide are considered. Use of a microchemical thin-layer configuration, which involves immersing a microparticle-modified electrode with a thin film of adhered hydrophobic ionic liquid into aqueous solution is considered, along with application of this technique to the reversible oxidation of trans-[Mn-(CN)(CO)2{P(OPh)3}(Ph2PCH2PPh2)]. In summary, the voltammetry of adhered microparticles is demonstrated to provide a powerful electroanalytical tool which has great versatility in ionic liquids, under conditions where conventional dissolved-state voltammetry is sometimes difficult or impractical.