Impedance analysis for hydrogen adsorption pseudocapacitance and electrochemically active surface area of Pt electrode

Mesoporous Acridinium-Based Covalent Organic Framework for Long-lived Charge-Separated Exciton Mediated Photocatalytic [4+2] Annulation

Readily tuneable porosity and redox properties of covalent organic frameworks (COFs) result in highly customizable photocatalysts featuring extended electronic delocalization. However, fast charge recombination in COFs severely limits their photocatalytic activities. Herein a new mode of COF photocatalyst design strategy to introduce systematic trap states is programmed, which aids the formation and stabilization of long-lived charge-separated excitons. Installing cationic acridinium functionality in a pristine electron-rich triphenylamine COF via postsynthetic modification resulted in a semiconducting photocatalytic donor-acceptor dyad network that performed rapid and efficient oxidative Diels-Alder type [4+2] annulation of styrenes and alkynes to fused aromatic compounds under the atmospheric condition in good to excellent yields. Large mesopores of ≈4 nm diameter ensured efficient mass flow within the COF channel. It is confirmed that the catalytic performance of COF originates from the ultra-stable charge-separated excitons of 1.9 nm diameter with no apparent radiative charge-recombination pathway, endorsing almost a million times better photo-response and catalysis than the state-of-the-art.

Voltammetric and galvanostatic methods for measuring hydrogen crossover in fuel cell

Hydrogen crossover rate is an important indicator for characterizing the membrane degradation and failure in proton exchange membrane fuel cell. Several electrochemical methods have been applied to quantify it. But most of established methods are too rough to support follow-up applications. In this paper, a systematic and consistent theoretical foundation for electrochemical measurements of hydrogen crossover is established for the first time. Different electrochemical processes occurring throughout the courses of applying potentiostatic or galvanostatic excitations on fuel cell are clarified, and the linear current-voltage behavior observed in the steady-state voltammogram is reinterpreted. On this basis, we propose a modified galvanostatic charging method with high practicality to achieve accurate electrochemical measurement of hydrogen crossover, and the validity of this method is fully verified. This research provides an explicit framework for implementation of galvanostatic charging method and offers deeper insights into the principles of electrochemical methods for measuring hydrogen crossover.

Resistance Separation of Polymer Electrolyte Membrane Fuel Cell by Polarization Curve and Electrochemical Impedance Spectroscopy

The separation of resistances during their measurement is important because it helps to identify contributors in polymer electrolyte membrane (PEM) fuel cell performance. The major methodologies for separating the resistances are electrochemical impedance spectroscopy (EIS) and polarization curves. In addition, an equivalent circuit was selected for EIS analysis. Although the equivalent circuit of PEM fuel cells has been extensively studied, less attention has been paid to the separation of resistances, including protonic resistance in the cathode catalyst layer (CCL). In this study, polarization curve and EIS analyses were conducted to separate resistances considering the charge transfer resistance, mass transport resistance, high frequency resistance, and protonic resistance in the CCL. A general solution was mathematically derived using the recursion formula. Consequently, resistances were separated and analyzed with respect to variations in relative humidity in the entire current density region. In the case of ohmic resistance, high frequency resistance was almost constant in the main operating load range (0.038–0.050 Ω cm2), while protonic resistance in the CCL exhibited sensitivity (0.025–0.082 Ω cm2) owing to oxygen diffusion and water content.

Ultra-low Impedance Graphene Microelectrodes with High Optical Transparency for Simultaneous Deep 2-photon Imaging in Transgenic Mice

The last decades have witnessed substantial progress in optical technologies revolutionizing our ability to record and manipulate neural activity in genetically modified animal models. Meanwhile, human studies mostly rely on electrophysiological recordings of cortical potentials, which cannot be inferred from optical recordings, leading to a gap between our understanding of dynamics of microscale populations and brain-scale neural activity. By enabling concurrent integration of electrical and optical modalities, transparent graphene microelectrodes can close this gap. However, the high impedance of graphene constitutes a big challenge towards the widespread use of this technology. Here, we experimentally demonstrate that this high impedance of graphene microelectrodes is fundamentally limited by quantum capacitance. We overcome this quantum capacitance limit by creating a parallel conduction path using platinum nanoparticles. We achieve a 100 times reduction in graphene electrode impedance, while maintaining the high optical transparency crucial for deep 2-photon microscopy. Using a transgenic mouse model, we demonstrate simultaneous electrical recording of cortical activity with high fidelity while imaging calcium signals at various cortical depths right beneath the transparent microelectrodes. Multimodal analysis of Ca2+ spikes and cortical surface potentials offers unique opportunities to bridge our understanding of cellular dynamics and brain-scale neural activity.

Fast kinetics of magnesium monochloride cations in interlayer-expanded titanium disulfide for magnesium rechargeable batteries

Magnesium rechargeable batteries potentially offer high-energy density, safety, and low cost due to the ability to employ divalent, dendrite-free, and earth-abundant magnesium metal anode. Despite recent progress, further development remains stagnated mainly due to the sluggish scission of magnesium-chloride bond and slow diffusion of divalent magnesium cations in cathodes. Here we report a battery chemistry that utilizes magnesium monochloride cations in expanded titanium disulfide. Combined theoretical modeling, spectroscopic analysis, and electrochemical study reveal fast diffusion kinetics of magnesium monochloride cations without scission of magnesium-chloride bond. The battery demonstrates the reversible intercalation of 1 and 1.7 magnesium monochloride cations per titanium at 25 and 60 °C, respectively, corresponding to up to 400 mAh g⁻¹ capacity based on the mass of titanium disulfide. The large capacity accompanies with excellent rate and cycling performances even at room temperature, opening up possibilities for a variety of effective intercalation hosts for multivalent-ion batteries.

Magnesium rechargeable batteries potentially offer high-energy density, safety, and low cost. Here the authors show a battery that reversibly intercalates magnesium monochloride cations with excellent rate and cycle performances in addition to the large capacity.

Sorption of Pyrene by Clay Minerals Coated with Dissolved Organic Matter (DOM) from Landfill Leachate

Interactions of dissolved organic matter (DOM) from landfill leachate with clays could affect significantly the fate of hydrophobic organic compounds (HOCs) in soils. The complexes of montmorillonite (MT) and kaolinite (KL) with DOM extracted from landfill leachate were prepared under controlled conditions, termed CMT and CKL, respectively. The bare clays and their complexes were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), thermogravimetry (TG), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Batch experiments were designed to investigate the sorption behavior of pyrene onto the clays in the presence of DOM. The maximum sorption capacities of pyrene on MT, CMT, KL, and CKL were 22.18, 38.96, 42.00, and 44.99 μg·g−1, respectively, at the initial concentration of 1000 ± 150 μg·L−1. The sorption isotherms of pyrene by the bare clays followed the Henry model well, whereas the Freundlich sorption isotherm provided a better fit to the equilibrium data of the sorption by the complexes. The role of montmorillonite and kaolinite complexes with DOM in the retention of pyrene in soils was different. This may be due to the different crystal structures between montmorillonite and kaolinite.

Hydrogen nanobubble at normal hydrogen electrode

Electrochemically formed hydrogen nanobubbles at a platinum rotating disk electrode (RDE) were detected by re-oxidation charge. The dissolution time course of the hydrogen nanobubbles was measured by AFM tapping topography under open-circuit conditions at stationary platinum and gold single-crystal electrodes. The bubble dissolution at platinum was much faster than that at gold because two types of diffusion, bulk and surface diffusion, proceeded at the platinum surface, whereas surface diffusion was prohibited at the gold electrode. These findings indicated that the electrochemical reaction of normal hydrogen electrode partly proceeded heterogeneously on the three-phase boundary around the hydrogen nanobubble.

Impedance Spectroscopic Investigation of Proton Conductivity in Nafion Using Transient Electrochemical Atomic Force Microscopy (AFM)

Spatially resolved impedance spectroscopy of a Nafion polyelectrolyte membrane is performed employing a conductive and Pt-coated tip of an atomic force microscope as a point-like contact and electrode. The experiment is conducted by perturbing the system by a rectangular voltage step and measuring the incurred current, followed by Fourier transformation and plotting the impedance against the frequency in a conventional Bode diagram. To test the potential and limitations of this novel method, we present a feasibility study using an identical hydrogen atmosphere at a well-defined relative humidity on both sides of the membrane. It is demonstrated that good quality impedance spectra are obtained in a frequency range of 0.2–1000 Hz. The extracted polarization curves exhibit a maximum current which cannot be explained by typical diffusion effects. Simulation based on equivalent circuits requires a Nernst element for restricted diffusion in the membrane which suggests that this effect is based on the potential dependence of the electrolyte resistance in the high overpotential region.

Corrosion of tungsten microelectrodes used in neural recording applications

In neuroprosthetic applications, long-term electrode viability is necessary for robust recording of the activity of neural populations used for generating communication and control signals. The corrosion of tungsten microwire electrodes used for intracortical recording applications was analyzed in a controlled bench-top study and compared to the corrosion of tungsten microwires used in an in vivo study. Two electrolytes were investigated for the bench-top electrochemical analysis: 0.9% phosphate buffered saline (PBS) and 0.9% PBS containing 30 mM of hydrogen peroxide. The oxidation and reduction reactions responsible for corrosion were found by measurement of the open circuit potential and analysis of Pourbaix diagrams. Dissolution of tungsten to form the tungstic ion was found to be the corrosion mechanism. The corrosion rate was estimated from the polarization resistance, which was extrapolated from the electrochemical impedance spectroscopy data. The results show that tungsten microwires in an electrolyte of PBS have a corrosion rate of 300-700 μm/yr. The corrosion rate for tungsten microwires in an electrolyte containing PBS and 30 mM H₂O₂ is accelerated to 10,000-20,000 μm/yr. The corrosion rate was found to be controlled by the concentration of the reacting species in the cathodic reaction (e.g. O₂ and H₂O₂). The in vivo corrosion rate, averaged over the duration of implantation, was estimated to be 100 μm/yr. The reduced in vivo corrosion rate as compared to the bench-top rate is attributed to decreased rate of oxygen diffusion caused by the presence of a biological film and a reduced concentration of available oxygen in the brain.

Electrochemical Determination of Dopamine on a Glassy Carbon Electrode Modified by Using Nanostructure Ruthenium Oxide Hexacyanoferrate/Ruthenium Hexacyanoferrate Thin Film

Application of mixed ruthenium oxide hexacyanoferrate/ruthenium hexacyanoferrate glassy carbon electrode for electrochemical determination of dopamine (DA) is described for the first time. The overlapped voltammetric oxidation potentials of ascorbic acid (AA) and DA are separated and shifted to more facile direction, +170 and +320 mV versus Ag/AgCl, respectively. Voltammetric response of the electrode toward the DA showed a dynamic calibration curve with two linear parts, from 0.50 to 10.00 μM and 25.00 to 550.00 μM DA, and a detection limit of 0.195 μM. The sensitivity (0.2917 μA/μM) and detection limit (0.195×10−7 μMDA) of this electrode are 21 times higher and 11.5 times lower than those found in our previous paper. The sensor response of 9.95 μMDA was not affected by 5.0 mM of glucose, 4.5 mM of fructose, 0.58 mM of sucrose, 0.28 mM of cystine, 0.25 mM of ascorbic acid, 79.60 μM of cysteine, and 49.70 μM of uric acid and urea. The fabricated sensor was successfully tested for determination of DA in injection medicine and human blood plasma samples.

Electrostatic and electrochemical nature of liquid-gated electric-double-layer transistors based on oxide semiconductors

The electric-double-layer (EDL) formed at liquid/solid interfaces provides a broad and interdisciplinary attraction in terms of electrochemistry, photochemistry, catalysts, energy storage, and electronics because of the large interfacial capacitance coupling and its ability for high-density charge accumulation. Much effort has recently been devoted to the fundamental understanding and practical applications of such highly charged EDL interfaces. However, the intrinsic nature of the EDL charging, whether it is electrostatics or electrochemistry, and how to distinguish them are far from clear. Here, by combining electrical transport measurements with electrochemical impedance spectroscopy (EIS), we studied the charging mechanisms of highly charged EDL interfaces between an ionic liquid and oxide semiconductor, ZnO. The direct measure for mobile carriers from the Hall effect agreed well with that from the capacitance-voltage integration at 1 Hz, implying that the pseudocapacitance does not contribute to carrier transport at EDL interfaces. The temperature-frequency mapping of EIS was further demonstrated as a "phase diagram" to distinguish the electrostatic or electrochemical nature of such highly charged EDL interfaces with densities of up to 8 × 10(14) cm(-2), providing a guideline for electric-field-induced electronic phenomena and a simple method for distinguishing electrostatic and electrochemical charging in EDLTs not only based on a specific oxide semiconductor, ZnO, but also commonly applicable to all types of EDL interfaces with extremely high-density carrier accumulation.

Revelation of multiple underlying RuO2 redox processes associated with pseudocapacitance and electrocatalysis

Advances in basic knowledge relevant to the pseudocapacitive and electrocatalytic properties of RuO(2) materials require a detailed understanding of the redox chemistry that occurs at the electrode interface. Although several redox processes have been identified via dc cyclic voltammograms derived from surface-confined RuO(2) materials, mechanistic details remain limited because the faradaic signals of interest are heavily masked by the background current. Here, it is shown that the underlying electron transfer reactions associated with the VI to II oxidation states of surface-confined RuO(2) materials in acidic medium are far more accessible in the background current free fourth and higher harmonic components available via large-amplitude Fourier transformed ac voltammetry. Enhanced resolution and sensitivity to both electron transfer and protonation processes and discrimination against solvent and background capacitance are achieved so that the Ru(V) to Ru(VI) process can be studied for the first time. Thus, kinetic and thermodynamic information relevant to each ruthenium redox level is readily deduced. The relative rate of electron transfer and the impact of protonation associated with Ru(VI) to Ru(II) redox processes are found to depend on the nature of the RuO(2) materials (extent of crystallinity and hydration) and concentration of sulfuric acid electrolyte. In the electrocatalytic oxidation of glucose in alkaline medium, access to the underlying electron transfer processes allows ready detection of the redox couple associated with the catalysis. Thus, application of an advanced ac electroanalytical technique is shown to provide the methodology for enhancing our understanding of the charge transfer processes of RuO(2), relevant to pseudocapacitance and electrocatalysis.