Development of cavity microelectrode devices and their uses in various research fields

Evaluating Fe-Site and Vacancy Dependent Intrinsic Activity of NiFe Layered Double Hydroxides through Cavity Microelectrodes

We report evaluating the intrinsic activity of Ni(OH)₂, NiFe layered double hydroxides (LDHs), and NiFe-LDH having vacancies for oxygen evolution reaction (OER) by the use of cavity microelectrodes (CMEs) with controllable mass loading. The number of active Ni sites (N_Ni-sites) ranging from 1 × 10¹² to 6 × 10¹² is quantitatively correlated with OER current, which reveals that the introduction of Fe-sites and vacancies increases the turnover frequency (TOF) from 0.027 to 0.118 and 0.165 s^-1, respectively. Electrochemical surface area (ECSA) is further quantitatively correlated with N_Ni-sites, which indicates that N_Ni-sites per unit ECSA (N_Ni-per-ECSA) is decreased by the introduction of Fe-sites and vacancies. Therefore, the difference of OER current per unit ECSA (J_ECSA) is reduced compared with that of TOF. The results demonstrate that CMEs provide a good platform to evaluate intrinsic activity with TOF, N_Ni-per-ECSA, and J_ECSA more reasonably.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Bimetallic Cu-Rh Nanoparticles on Diazonium-Modified Carbon Powders for the Electrocatalytic Reduction of Nitrates

4-Benzenethiol-functionalized high-surface-area graphite powder was prepared and decorated with bimetallic Cu_100-xRh_x nanoparticles (NPs) to serve as electrocatalysts for the reduction of nitrates. In the first step, the HSAG powder was grafted with in-situ-generated diazonium compounds from 4-aminothiophenol (ATP) in an acidic medium using NaNO₂ for the diazotization process. The surface composition was tuned using different initial quantities of ATP. The surface XPS-determined S/C atomic ratio was found to increase stepwise with the initial quantity of amine. In a second step, the grafted and untreated HSAG powders were decorated with Cu_100-xRh_x NPs by a wet chemical method and the elemental composition of the end composites was assessed by EDS-SEM and ICP, whereas TEM and EDS-TEM served to characterize the NP morphology and their composition on the nanometer scale. In all cases, the NP size was invariably found to be ∼1.7 nm but with a size distribution becoming narrower under an increasing grafting rate and the global composition enriched in copper. Voltammetry was performed with a cavity microelectrode to evaluate the electrocatalytic performance of the composites for nitrate reduction. Increasing diazonium grafting led to a progressive reduction of the peak current intensity and a shift of the peak potentials toward cathodic values. The maximum intensity was obtained for 0.005 μmol of diazonium salt per mg of HSAG, with a gain of 40% in comparison to the best untreated sample. This improvement and a change in the voltammogram characteristics after grafting seem to result from modifications of the local composition at the level of NPs that differ from the global composition. This work conclusively shows that diazonium surface modification is important not only in attaching electrocatalytic NPs to carbon supports but also in providing a narrower size distribution of the electrocatalysts together with finely tuned catalytic properties.

Characterization of LiCoO2 nanoparticle suspensions by single collision events

Transient electrochemical experiments associated with the collisions between hydrothermally synthesized LiCoO₂ (LCO) nanoparticles/aggregates of different sizes and a polarized gold ultramicroelectrode (UME) were used as a new additive-free analytical tool applied to characterize Li ion insertion compounds.

New Application of Waste Citrus Maxima Peel-Derived Carbon as an Oxygen Electrode Material for Lithium Oxygen Batteries

Recently, lithium oxygen battery has become a promising candidate to satisfy the current large-energy-storage devices demand because of its amazing theoretical energy density. However, it still faces problems such as poor reversibility and short cycle life. Here, citrus maxima peel (CMP) was used as a precursor to prepare activated and Fe-loading carbon (CMPACs and CMPACs-Fe, respectively) via pyrolysis in nitrogen atmosphere at 900 °C, in which KOH was added as an activator. Electrochemical measurements show that CMPAC-based Li-O₂ battery possesses high specific capacity of 7800 mA h/g, steady cycling performance of 466 cycles with a corresponding Coulombic efficiency of 92.5%, good rate capability, and reversibility. Besides, CMPACs-Fe-based O₂ electrode delivers even lower overpotential in both charge and discharge processes. We conclude that these excellent electrochemical performances of CMPACs and CMPACs-Fe-based O₂ electrode benefit from their cellular porous structure, plenty of active sites, and large specific surface area (900 and 768 m²/g), which suggest that these biomass-derived porous carbons might become promising candidates to achieve efficient lithium oxygen battery.

Carbon Capsules of Ionic Liquid for Enhanced Performance of Electrochemical Double-Layer Capacitors

Ion accessibility, large surface area, and complete wetting of a carbonaceous electrode by the electrolyte are crucial for high-performance electrochemical double-layer capacitors. Herein, we report a facile and scalable method to prepare electrode-electrolyte hybrid materials, where an ionic liquid (IL) electrolyte is encapsulated within a shell of reduced graphene oxide (rGO) nanosheets as the active electrode material (called rGO-IL capsules). These structures were templated using a Pickering emulsion consisting of a dispersed phase of 1-methyl-3-butylimidazolium hexafluorophosphate ([bmim][PF₆]) and a continuous water phase; graphene oxide nanosheets were used as the surfactant, and interfacial polymerization yielded polyurea that bound the nanosheets together to form the capsule shell. This method prevents the aggregation and restacking of GO nanosheets and allows wetting of the materials by IL. The chemical composition, thermal properties, morphology, and electrochemical behavior of these new hybrid architectures are fully characterized. Specific capacitances of 80 F g^-1 at 18 °C and 127 F g^-1 at 60 °C were achieved at a scan rate of 10 mV s^-1 for symmetric coin cells of rGO-IL capsules. These architected materials have higher capacitance at low temperature (18 °C) across many scan rates (10-500 mV s^-1) compared with analogous cells with the porous carbon YP-50. These results demonstrate a distinct and important methodology to enhance the performance of electrochemical double-layer capacitors by incorporating electrolyte and carbon material together during synthesis.

On-Line Monitoring the Growth of E. coli or HeLa Cells Using an Annular Microelectrode Piezoelectric Biosensor

Biological information is obtained from the interaction between the series detection electrode and the organism or the physical field of biological cultures in the non-mass responsive piezoelectric biosensor. Therefore, electric parameter of the electrode will affect the biosensor signal. The electric field distribution of the microelectrode used in this study was simulated using the COMSOL Multiphysics analytical tool. This process showed that the electric field spatial distribution is affected by the width of the electrode finger or the space between the electrodes. In addition, the characteristic response of the piezoelectric sensor constructed serially with an annular microelectrode was tested and applied for the continuous detection of Escherichia coli culture or HeLa cell culture. Results indicated that the piezoelectric biosensor with an annular microelectrode meets the requirements for the real-time detection of E. coli or HeLa cells in culture. Moreover, this kind of piezoelectric biosensor is more sensitive than the sensor with an interdigital microelectrode. Thus, the piezoelectric biosensor acts as an effective analysis tool for acquiring online cell or microbial culture information.