Non-Precious Electrodes for Practical Alkaline Water Electrolysis

Water electrolysis is a promising approach to hydrogen production from renewable energy sources. Alkaline water electrolyzers allow using non-noble and low cost materials. An analysis of common assumptions and experimental conditions (low concentrations, low temperature, low current densities and short-term experiments) found in the literature is reported. The steps to estimate the reaction overpotentials for hydrogen and oxygen reaction are reported and discussed. The results of some of the most investigated electrocatalysts, namely from the iron group elements (iron, nickel and cobalt) and chromium are reported. Past findings and recent progress in the development of efficient anode and cathode materials appropriate for large-scale water electrolysis are presented. The experimental work is done involving the direct-current electrolysis of highly concentrated potassium hydroxide solutions at temperatures between 30 and 100°C which are closer to industrial applications than what usually found in literature. Stable cell components and a good performance was achieved using as a cathode Raney nickel and stainless steel 316L as an anode by means of a monopolar cell at 75°C which ran for one month at 300 mA cm?2. Finally, the proposed catalysts show a total kinetic overpotential of circa 550 mV at 75 °C and 1 A cm?2.Fil: Colli, Alejandro Nicolás. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; ArgentinaFil: Girault, Hubert. Ecole Polytechnique Federale de Lausanne; FranciaFil: Battistel, Alberto. Ecole Polytechnique Federale de Lausanne; Franci

On the Analysis of Non-stationary Impedance Spectra

In this work we study the possibility to analyze the non-stationary impedance spectra by employing standard equivalent circuits. For this purpose, the dynamic multi-frequency analysis (DMFA) is introduced and compared with a set of consecutive stationary impedance spectra. In order to prove the hypothesis, the data are obtained by the simulation in the time domain of the electron transfer process between an electrode and a free-diffusing redox couple in solution. During the simulation, the system is perturbed with a cyclic voltammetry super-imposed to a small multisine perturbation (DMFA) or with a series of stationary impedance spectroscopies. Also, a new fitting algorithm, which takes into account the correlation between consecutive spectra, is proposed and tested. Although the Randles circuit can be used to fit both dynamic and stationary impedance spectra, the values of the fitting parameters are different and depend on the direction of the scan and on the rate. This is related to the influence of the diffusion profile on the fitting parameters

Efficiency improvement of an all-vanadium redox flow battery by harvesting low-grade heat

Redox flow batteries (RFBs) are rugged systems, which can withstand several thousand cycles and last many years. However, they suffer from low energy density, low power density, and low efficiency. Integrating a Thermally Regenerative Electrochemical Cycle (TREC) into the RFB, it is possible to mitigate some of these drawbacks. The TREC takes advantage of the temperature dependence of the cell voltage to convert heat directly into electrical energy. Here, the performance increase of a TREC-RFB is investigated using two kinds of all-vanadium electrolyte chemistries: one containing a typical concentration of sulfuric acid and one containing a large excess of hydrochloric acid. The results show that the energy density of the system was increased by 1.3Wh L−1 and 0.8Wh L−1, respectively and the overall energy efficiency also increased by 9 and 5 percentage points, respectively. The integration of the heat exchangers necessary to change the battery temperature is readily facilitated by the design of the redox flow battery, which already utilizes fluid circulation loops

Electrochemical impedance spectroscopy beyond linearity and stationarity - a critical review

Electrochemical impedance spectroscopy (EIS) is a widely used experimental technique for characterising materials and electrode reactions by observing their frequency-dependent impedance. Classical EIS measurements require the electrochemical process to behave as a linear time-invariant system. However, electrochemical processes do not naturally satisfy this assumption: the relation between voltage and current is inherently nonlinear and evolves over time. Examples include the corrosion of metal substrates and the cycling of Li-ion batteries. As such, classical EIS only offers models linearised at specific operating points. During the last decade, solutions were developed for estimating nonlinear and time-varying impedances, contributing to more general models. In this paper, we review the concept of impedance beyond linearity and stationarity, and detail different methods to estimate this from measured current and voltage data, with emphasis on frequency domain approaches using multisine excitation. In addition to a mathematical discussion, we measure and provide examples demonstrating impedance estimation for a Li-ion battery, beyond linearity and stationarity, both while resting and while charging

Redox Solid Energy Boosters for Flow Batteries: Polyaniline as a Case Study

In this work, the viability of polyaniline as a solid charge storage material in an aqueous redox-mediated flow battery is investigated. Fe(III/II) and V(IV/III) were identified as potential redox mediators to target the emeraldine-pernigraniline and leucoemeraldine-emeraldine redox transitions of the polymer. An indirect chemical cycling method was developed and used to investigate the charging/discharging of the polymer by the selected redox mediators. With Fe(III/II) as a redox mediator, respectable specific capacity and cycling stability were demonstrated over 25 cycles. V(IV/III) was deemed unsuitable as a redox mediator, due to rather poor kinetics. When added to the electrolyte tanks of a complete flow battery, a conductive composite of polyaniline and carbon black provided a significant improvement in capacity, exhibiting a specific capacity of 64.8 mA h g−1 at a current density of 38.5 mA cm−2. This represents a three-fold improvement in volumetric capacity, compared with the electrolyte alone. Moreover, the addition of the polyaniline composite was observed to lower the average potential at the positive electrode, providing a considerable improvement in voltage efficiency. This work demonstrates the potential of utilizing redox mediators to enable bulk solid-phase charge storage in the tanks of aqueous redox flow batterie

Rapid Characterization of Oxygen-Evolving Electrocatalyst Spot Arrays by the Substrate Generation/Tip Collection Mode of Scanning Electrochemical Microscopy with Decreased O-2 Diffusion Layer Overlap

A simple approach for the screening of oxygen evolution reaction (OER) electrocatalyst arrays by scanning electrochemical microscopy (SECM) in the substrate generation/tip collection (SG/TC) mode is described. The methodology is based on the application of a series (9-10 replicates) of double-potential steps to a catalytically active substrate electrode, which is switched between potentials where it displays OER activity and inactivity. With an SECM tip coaligned to a given electrocatalyst spot, the dual potential step is applied for a relatively short time in order to restrict the growth of the resulting O2 diffusion layer. The SECM is then able to measure the O2 produced while the potential sequence prevents the overlap of the diffusion layer from neighboring spots. With this approach, each spot of material in an array of Ir:Sn oxide compositions (disk shaped, about 150 μm radius) was examined independently at a constant distance. The method was tested for a series of oxygen evolution catalysts made of SnO2-IrO2 mixtures, with compositions varying between Ir:Sn 100:0 to Ir:Sn 0:100. Optimal conditions for avoiding overlapping of the diffusion profiles generated at each spot of the substrate were evaluated by digital simulation. The results obtained for the activity of SnO2-IrO2 mixtures using this new technique were validated by comparison to reported results using SECM and other technique

Development of the intermodulated differential immittance spectroscopy for electrochemical analysis

In der Doktorarbeit wird eine neue elektrochemische Technik, die Intermodulated Differential Immitance Spectroscopy (IDIS) eingeführt, charakterisiert und angewendet. IDIS basiert auf dem Phänomen der Intermodulation, welche entsteht, wenn zwei periodische Stimuli mit unterschiedlichen Frequenzen in einem nicht-linearen System miteinander interagieren. Die Technik wurde anhand einer Diode getestet und für die Untersuchung eines reversiblen Redoxpaares in Lösung angewendet. Im Gegensatz zur elektrochemischen Impedanzspektroskopie wurden kinetische Eigenschaften und Massentransportparameter direkt quantifiziert. Die Intermodulation wurde ebenfalls für die Untersuchung des makrokinetischen Effekts der Gasentwicklung während der Sauerstoffentwicklung angewendet. Verschiedene Einflüsse der periodisch wachsenden und entweichenden Gasblasen auf die Größe der elektrochemisch aktiven Elektrodenoberfläche sowie auf die Durchmischung der Lösung wurden so identifiziert und unterschieden.This doctoral thesis introduced, characterized, and applied a new electrochemical technique, the Intermodulated Differential Immittance Spectroscopy (IDIS). The IDIS was based on the phenomenon of the intermodulation which appears when two periodic stimuli with different frequency interact in a nonlinear system. The technique was tested on a diode and applied to the study of a reversible redox couple in solution in order to quantify the kinetic and mass transport parameters of the reaction which were not directly accessible via impedance spectroscopy. The intermodulation was also applied to the study of the macrokinetic effect of gas bubbles on oxygen evolution reaction. It was possible to distinguish the various influences of the bubbles as the variation of the active electrodic area and the stirring of the solution given by the bubbles growing and departure

Recent trends in thermoelectrochemical cells and thermally regenerative batteries

Given the ever-growing awareness on global warming, much interest has focused on new and effective ways to manage energy, especially by harvesting and exploiting low-temperature heat sources, ubiquitous in the modern environment. Here, the holy grail is the direct conversion of heat into electricity especially using thermoelectric devices, and in this contribution, we focus on thermoelectrochemical systems.We give a brief overview of the most common thermally regenerative electrochemical cells developed nowadays with a short overview of their thermodynamic derivation, and we collect some of the most recent results in terms of their thermoelectrochemical properties, in particular, their temperature coefficients. We see that although the most used redox couples are based on Fe3+/Fe2+ and their derivates, thermodiffusion effects and other entropy-related phenomena are attracting the attention of the scientific community and boosting astonishing results. On the other hand, thermally regenerative batteries are emerging, showing modest performance.</p

Numerical Analysis of the Localization of Pulmonary Nodules during Thoracoscopic Surgery by Ultra-Wideband Radio Technology

Worldwide, lung cancer is one of the most common causes of cancer-related death. Detected by computer tomography, it is usually removed through thoracoscopic surgery. During the surgery the lung collapses requiring some strategies to track or localize the new position of the lesion. This is particularly challenging in the case of minimally invasive surgeries when mechanical palpation is not possible. Here we undertake a preliminary study with numerical analysis of an ultra-wideband (UWB) radio technology which can be employed directly during thoracoscopic surgery to localize deep solitary pulmonary nodules. This study was conducted through Finite Difference Time Domain (FDTD) simulations, where a spherical target mimicking a nodule located between 1 and 6 cm of depth and an UWB pulse at several frequencies between 0.5 and 5 GHz was used for localization. This investigation quantifies the influence of several parameters, such frequency, lesion depth, and number of acquisitions, on the final confocal image used to locate a cancer in the lung tissue. We also provide extensive discussion on several artifacts that appear in the images. The results show that the cancer localization was possible at operational frequencies below 1 GHz and for deep nodules (&gt;5 cm), while at lower depths and higher frequencies several artifacts hindered its detection