Rotating Ring Disk Electrode Measurements Research Articles

Oxygen Reduction Reaction (ORR) of Pt/C Standard in Different Electrolyte Solutions and Terbium(III) Monoporphyrinato Complex

Electrocatalytic parameters of a Pt/C standard and a sample of terbium(III) monoporphyrinato were investigated in different solutions. N electron transfer, Tafel slope, Eonset, and overpotential of the catalyst of Pt/C in different solutions were calculated and analyzed using a rotating ring disk electrode (RRDE) in 0.5 M H2SO4, 0.1 M HClO4 and 0.1 M NaOH. In the RRDE measurements, a bipotentiostat at a potential range of 1.03 to 0.05 V vs RHE (Ering = 1.2 V vs RHE) with a scan rate of 5 mV/s and rotation rates of 200, 400, 900, 1600 and 2500 rpm was used. Hereafter, the test of terbium(III) monoporphyrinato compound formulated in [Tb(TPP)(cyclen)]Cl (TPP = 5,10,15,20-tetraphenylporphyrinato; cyclen = 1,4,7,10-tetraazacyclododecane) as a candidate material for ORR electrocatalyst was also done. The results showed that the measurement of Pt/C standards was satisfactory according to the literature for all parameters with the n electron transfer close to 4 in all electrolytes media. [Tb(TPP)(cyclen)]Cl had an n electron transfer value of 2.38, suggesting that the [Tb(TPP)(cyclen)]Cl compound has less potential for ORR catalysts.

Size dependent electrocatalytic activities of h-BN for oxygen reduction reaction to water.

Electrocatalytic activities for the oxygen reduction reaction (ORR) of Au electrodes modified by as prepared and size selected (0.45-1.0, 0.22-0.45, and 0.1-0.22 µm) h-BN nanosheet (BNNS), which is an insulator, were examined in O2 saturated 0.5M H2SO4 solution. The overpotential was reduced by all the BNNS modifications, and the smaller the size, the smaller the overpotential for ORR, i.e., the larger the ORR activity, in this size range. The overpotential was reduced by as much as ∼330 mV compared to a bare Au electrode by modifying the Au surface by the BNNS of the smallest size range (0.1-0.22 µm). The overpotential at this electrode was only 80 mV more than that at the Pt electrode. Both the rotation disk electrode experiments with Koutecky-Levich analysis and rotating ring disk electrode measurements showed that more than 80% of oxygen is reduced to water via the four-electron process at this electrode. These results strongly suggest and theoretical density functional theory calculations support that the ORR active sites are located at the edges of BNNS islands adsorbed on Au(111). The decrease in size of BNNS islands results in an effective increase in the number of the catalytically active sites and, hence, in the increase in the catalytic activity of the BNNS/Au(111) system for ORR.

Effect of aggregate size and film quality on the electrochemical properties of non-noble metal catalysts in rotating ring disk electrode measurements

Non-noble metal catalysts (NNMCs) are promising candidates to replace the high-cost Pt-based catalysts required to catalyze the oxygen reduction reaction (ORR) in the cathode of proton exchange membrane fuel cells (PEMFCs), and their electrochemical properties (i.e., ORR-activity and H2O vs. H2O2 selectivity) are commonly studied using rotating ring disk electrode (RRDE) measurements. However, such RRDE measurements are often performed using high NNMC-loadings that can lead to underestimated H2O2-yields. These effects can be curtailed by using lower loadings entailing the preparation of thin films whose homogeneity reportedly impacts the RRDE-results observed for Pt-based catalyst, but of unknown effects in the NNMC field. To shed light on this matter, in this study we investigated the effect of film thickness and quality on the electrochemical behavior of three NNMCs featuring different aggregate sizes ranging from ≈ 5 μm to ≈ 100 nm. The RRDE-results displayed a significant enhancement in the ORR limiting current of the NNMC with a smaller particle size that we tied to its improved mass transport properties. Moreover, improving the thin film quality led to a ≈ 2-fold enhancement of the H2O2-yield (vs. a similarly loaded, yet non-homogeneous NNMC-film) that demonstrates the importance of this parameter for the ORR-selectivity trends inferred from these RRDE measurements.

A Selective Copper Based Oxygen Reduction Catalyst for the Electrochemical Synthesis of H2O2 at Neutral pH.

H2O2 is a bulk chemical used as “green” alternative in a variety of applications, but has an energy and waste intensive production method. The electrochemical O2 reduction to H2O2 is viable alternative with examples of the direct production of up to 20% H2O2 solutions. In that respect, we found that the dinuclear complex Cu2(btmpa) (6,6’‐bis[[bis(2‐pyridylmethyl)amino]methyl]‐2,2’‐bipyridine) reduces O2 to H2O2 with a selectivity up to 90 % according to single linear sweep rotating ring disk electrode measurements. Microbalance experiments showed that complex reduction leads to surface adsorption thereby increasing the catalytic current. More importantly, we kept a high Faradaic efficiency for H2O2 between 60 and 70 % over the course of 2 h of amperometry by introducing high potential intervals to strip deposited copper (depCu). This is the first example of extensive studies into the long term electrochemical O2 to H2O2 reduction by a molecular complex which allowed to retain the high intrinsic selectivity of Cu2(btmpa) towards H2O2 production leading to relevant levels of H2O2.

Compositionally Tuned Trimetallic Thiospinel Catalysts for Enhanced Electrosynthesis of Hydrogen Peroxide and Built-In Hydroxyl Radical Generation

On-site electrochemical production of hydrogen peroxide (H2O2), an oxidant and disinfectant with growing demand, could be realized through the selective two-electron oxygen reduction reaction (2e– ORR), but the widespread adoption of this method depends on robust and efficient electrocatalysts. Current catalysts have been limited by cost or toxicity and lack well-defined structures that can facilitate systematic tuning of activity and selectivity. Here, we demonstrate a series of CuCo2–xNixS4 (0 ≤ x ≤ 1.2) thiospinel catalysts for 2e– ORR with variable compositions that can be synthesized via hydrothermal conversion. Rotating ring disk electrode measurements show that these catalysts have high selectivity for 2e– ORR (>60%) and that their activity can be improved by increasing the nickel content without compromising selectivity. An acid treatment step is critical prior to employing the optimized CuCo0.8Ni1.2S4 catalyst for bulk electrosynthesis of H2O2 in 0.05 M H2SO4 solution. Various structural analyses, including synchrotron X-ray spectroscopy, confirm that the catalysts retain the spinel structure after acid treatment and H2O2 electrosynthesis. The acid treatment likely leaches the soluble copper species from the as-synthesized catalysts that would catalyze an electro-Fenton process to consume H2O2, generate hydroxyl radicals, and therefore prevent the accumulation of H2O2. This work demonstrates a general strategy for systematic tuning of metal compound catalysts for practical H2O2 electrosynthesis and facile generation of hydroxyl radicals.

Copper single-atoms embedded in 2D graphitic carbon nitride for the CO2 reduction

We report the study of two-dimensional graphitic carbon nitride (GCN) functionalized with copper single atoms as a catalyst for the reduction of CO2 (CO2RR). The correct GCN structure, as well as the adsorption sites and the coordination of the Cu atoms, was carefully determined by combining experimental techniques, such as X-ray diffraction, transmission electron microscopy, X-ray absorption, and X-ray photoemission spectroscopy, with DFT theoretical calculations. The CO2RR products in KHCO3 and phosphate buffer solutions were determined by rotating ring disk electrode measurements and confirmed by 1H-NMR and gas chromatography. Formate was the only liquid product obtained in bicarbonate solution, whereas only hydrogen was obtained in phosphate solution. Finally, we demonstrated that GCN is a promising substrate able to stabilize metal atoms, since the characterization of the Cu-GCN system after the electrochemical work did not show the aggregation of the copper atoms.

Mixed Diffusion‐Kinetic Control of H2O2 Oxidation at an Oxide‐Covered Platinum Electrode in Alkaline Electrolyte: Implications for Oxygen Electroreduction Studies with a Rotating Ring Disk Electrode

AbstractThe rotating ring disk electrode (RRDE) with a Pt ring is widely applied for oxygen reduction reaction (ORR) studies, and it is generally believed that oxidation of H2O2 at the Pt ring is controlled by diffusion. Indeed, H2O2 oxidation at bare platinum is a diffusion‐controlled reaction. However, when Pt ring is held at 1.2 VRHE, it is being covered with surface oxide, and H2O2 oxidation turns into a reaction under mixed diffusion‐kinetic control. We show that, for H2O2 oxidation at an oxide‐covered Pt ring in 0.1 M KOH, the ratio of the empirical ring current to corresponding ring current under pure diffusion control varies monotonically with rotation rate. At 1.2 VRHE, the ratio is equal to 79.9 % and 61.4 % at 400 rpm and 2500 rpm, respectively. We demonstrate that this deviation prevents the correct diagnosis of ORR mechanism and leads to overestimated electron transfer number values. To eliminate the error in RRDE measurements, we employed a procedure based on the Koutecký‐Levich method, which, to our best knowledge, has not been reported yet.

Use of in Situ Synchrotron Techniques to Probe the Oxidized Surface of Molybdenum Nitride Oxygen Reduction Electrocatalysis

The development of active and stable earth-abundant catalysts for the oxygen reduction reaction (ORR) is needed for widespread, economic development of fuel cell technologies. Designing and optimizing these non-platinum group metals is challenging, however, because they are susceptible to composition and structure changes, including dissolution, oxidation, and corrosion, both in air and under reaction conditions. To identify the active surface, and thus understand the properties that affect activity, the catalyst surface must be characterized in situ. Herein, we utilize a grazing incidence electrochemical cell to investigate in situ composition and morphology changes of a molybdenum nitride (Mo-N) thin film catalyst using grazing incidence x-ray absorption spectroscopy (GI-XAS) and x-ray reflectivity (XRR). In rotating ring disk electrode measurements, we find that the activity, selectivity, stability, and capacitance of the Mo-N catalyst is dependent on the maximum potential to which it has been exposed. Specifically, the overpotential required to reach -2 mA cm-2 geo decreases by over 90 mV when the maximum potential is increased from 0.3 to 0.8 V vs RHE (Figure 1). Because the Mo-N oxidizes rapidly in air, ex situ characterization methods including x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry can provide only limited insight into these in situ catalyst changes. Using in situ GI-XAS measurements at applied potentials between 0.3 and 0.9 V vs RHE, however, we are able to determine that the surface of the film oxidizes and becomes more amorphous when exposed to increasingly higher potentials (Figure 1). Furthermore, the surface remains oxidized on the order of several hours when returned to “ORR relevant potentials” (< 0.6 V vs RHE), indicating that this surface-oxidized nitride is the active surface for ORR. Using in situ XRR measurements, we find that there is no change in surface roughness at potentials below 0.7 V vs RHE, but the film roughens significantly at 0.8 V vs RHE, correlating with ex situ measurements of Mo dissolution at this potential (Figure 1). We therefore conclude that the intrinsic activity of the Mo-N catalyst increases when exposed to potentials up to 0.7 V vs RHE, while above that potential activity enhancements are due to the exposure of more active sites through dissolution. The in situ electrochemical surface-sensitive x-ray characterization as used here is a promising methodology for understanding and leveraging surface dynamics to improve the performance of non-traditional catalysts.Figure 1. RRDE measurements in O2-saturated 0.1M HClO4 at 1600 rpm and schematics illustrating how the catalyst composition and morphology change with applied potential. Figure 1

Influence of Ammonia Contamination on HT-PEM Fuel Cell Platinum Catalyst

High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are considered as one of the promising types of power sources for automobile and stationary application. They have several advantageous characteristics, including high power density and efficiency, high tolerance against CO poisoning as well as zero-emission when using hydrogen and air as fuel and oxidant, respectively[1]. In the last decades HT-PEM fuel cells have been investigated extensively with the aim to introduce them into the market for commercial use.Gas impurities are one of the factors which can limit their durability and life time. Among them ammonia is a feed stream gas impurity which may contaminate the cathode as well as the anode of fuel cells. The main sources of ammonia traces come from hydrogen fuel produced from ammonia and from reforming of fossil fuel that contain species of nitrogen compounds[2]. Moreover, traces of ammonia in ambient air can occur in agricultural areas. The presence of ammonia traces was shown to decrease the performance of low temperature PEM fuel cells [3]. The ammonia species can effect on hydrogen oxidation reaction as well as oxygen reduction reaction affecting the catalyst and leading to reduced efficiency of fuel cells.[3] To the best of our knowledge, the effect of ammonia contamination on HT-PEM fuel cells and in particular on the carbon-supported platinum catalyst has not been fully explored yet. Therefore, the first aim of the present work is to investigate the effect of NH4 + poisoning on carbon-supported platinum in 0.5 mol L-1 H3PO4 in order to simulate the environment of HT-PEMFCs. The oxygen reduction reaction was studied in the presence of 10 ppm NH4 + ions using the rotating ring disk electrode (RRDE). In particular, activity of the Pt/C catalyst was determined before and after poisoning to evaluate the degradation. The effect of contamination on the performance loss, reduction of electrochemical active area and increase of H2O2 formation has been defined by RRDE measurements. In the second part, a commercially available membrane electrode assembly (MEA) was exposed to 10 ppm NH3 inside the air stream using a HT-PEM single cell test bench and applying a constant load of 0.3 A cm-2. Polarization curves and electrochemical impedance spectroscopy were used to investigate changes of the MEA performance. The RRDE study in H3PO4 electrolyte of the catalyst provides in combination with HT-PEM fuel cell testing an increased understanding of the NH3 impact on HT-PEMFC systems.Figure: CVs of Pt/C catalyst in 0.5 M H3PO4 at 50 mV/s in the potential range 0.05 – 1.5 V vs RHE in the absence and in the presence of 10 and 100 ppm NH4 +, respectively.[1] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang, J. Shen, J. Power Sources 2007, 165, 739-756.[2] X. Zhang, U. Pasaogullari, T. Molter, Int. J. Hydrogen Energy 2009, 34, 9188-9194.[3] F. A. Uribe, S. Gottesfeld, T. A. Zawodzinski, J. Electrochem. Soc. 2002, 149, A293-A296; R. Halseid, P. J. S. Vie, R. Tunold, J. Power Sources 2006, 154, 343-350. Figure 1

Influence of Ligand Denticity and Flexibility on the Molecular Copper Mediated Oxygen Reduction Reaction.

To date, the copper complex with the tris(2-pyridylmethyl)amine (tmpa) ligand (Cu-tmpa) catalyzes the ORR with the highest reported turnover frequency (TOF) for any molecular copper catalyst. To gain insight into the importance of the tetradentate nature and high flexibility of the tmpa ligand for efficient four-electron ORR catalysis, the redox and electrocatalytic ORR behavior of the copper complexes of 2,2′:6′,2″-terpyridine (terpy) and bis(2-pyridylmethyl)amine (bmpa) (Cu-terpy and Cu-bmpa, respectively) were investigated in the present study. With a combination of cyclic voltammetry and rotating ring disk electrode measurements, we demonstrate that the presence of the terpy and bmpa ligands results in a decrease in catalytic ORR activity and an increase in Faradaic efficiency for H2O2 production. The lower catalytic activity is shown to be the result of a stabilization of the CuI state of the complex compared to the earlier reported Cu-tmpa catalyst. This stabilization is most likely caused by the lower electron donating character of the tridentate terpy and bmpa ligands compared to the tetradentate tmpa ligand. The Laviron plots of the redox behavior of Cu-terpy and Cu-bmpa indicated that the formation of the ORR active catalyst involves relatively slow electron transfer kinetics which is caused by the inability of Cu-terpy and Cu-bmpa to form the preferred tetrahedral coordination geometry for a CuI complex easily. Our study illustrates that both the tetradentate nature of the tmpa ligand and the ability of Cu-tmpa to form the preferred tetrahedral coordination geometry for a CuI complex are of utmost importance for ORR catalysis with very high catalytic rates.

Influence of Ammonia Contamination on HT-PEM Fuel Cell Platinum Catalyst

In this work the effect of NH3 contamination on platinum catalyst has been investigated in the frame of a high temperature polymer electrolyte membrane single fuel cell as well as rotating ring disk electrode measurements (RRDE) using phosphoric acid electrolyte. In the first part single cell operation in presence of 10 ppm NH3 in the cathode gas combined with electrochemical impedance spectroscopy was performed with a fuel cell test bench. In the second part of work RRDE experiments were accomplished in the presence of 10 and 100 ppm of NH4H2PO4 contamination in 0.5 mol L-1 H3PO4. It was revealed that platinum catalyst is strongly affected by ammonia contamination while the increasing ammonium concentrations reduce the electrochemical surface area (ECSA) of the Pt catalyst. These observed losses of ECSA were associated to adsorbed nitrogen compounds on the catalyst surface.

Effects of fulvic acids on the electrochemical reactions and mass transfer properties of organic cation toluidine blue: Results of measurements by the method of rotating ring-disc electrode

This study examined effects of aquatic and soil natural organic matter (NOM) exemplified by standard Suwannee River fulvic acid (SRFA) and Pahokee Peat fulvic acid (PPFA), respectively, on the electrochemical (EC) reactivity and mass transfer properties of the cationic organic probe toluidine blue (TB) that forms complexes with NOM. EC measurements that were carried out using the method of rotating ring-disc electrode (RRDE) showed that for disc potentials below −0.4 V vs. the standard Ag/AgCl reference electrode, TB molecules undergo EC reduction accompanied by the formation of EC-active products that undergo oxidation at the ring electrode. EC reactions of TB in the range of potentials −0.2 to −0.4 V were determined to involve free TB+ cations and TB species adsorbed on the electrode surface. The EC reduction of TB species at the disc potentials < −0.4 V was controlled by the mass transfer of the free TB+ cations and TB/NOM complexes to the electrode surface. Formation of TB/NOM complexes caused the mass transfer-controlled TB currents to undergo a consistent decrease. The observed changes were correlated with the extent of TB/NOM complexation and decreases of the diffusion coefficients of TB/NOM complexes that have higher molecular weights (MW) than the free cations. Properties of the intermediates formed upon the reduction of TB+ cations were also affected by NOM. These results demonstrate that RRDE measurements of EC reactions of TB or possibly other EC active probes allow probing the complexation of EC-active organic species with NOM and mass transfer properties of NOM complexes and ultimately NOM itself.

The influence of hydrogen sulphide contamination on platinum catalyst used in polymer electrolyte membrane fuel cells during potential cycling at 0.05–1.05 V vs RHE: An RRDE study

The effect of H2S contamination on platinum catalyst has been investigated in terms of rotating ring disk electrode measurements (RRDE) in 0.1 M HClO4. The electrochemical surface area (ECSA) was determined by hydrogen underpotential deposition (HUPD) and CO stripping methods before and after accelerated stress tests (AST). The observed reduced losses of ECSA of the catalyst in the presence of H2S were associated to adsorbed sulphuric compounds on the catalyst surface which changed electrochemical characteristics of the materials surface. The RRDE experiments revealed that for oxygen reduction reaction (ORR) mass and specific activities were essentially decreased after AST with H2S contamination which was attributed to the interstitial defect on platinum atom sites due to adsorbed sulphuric compounds. Identical location and high resolution transmission electron microscopy (TEM) analysis have revealed only slightly different catalyst surface morphology and particle sizes before and after the AST indicating rather atomic scale deterioration of the platinum catalyst surface due to the adsorbed sulphur species.

Prussian-blue-modified reduced-graphene-oxide as active support for Pt nanoparticles during oxygen electroreduction in acid medium

Abstract Platinum nanoparticles have been deposited onto hybrid carriers composed of iron hexacyanoferrate (Prussian Blue) pretreated or functionalized reduced graphene oxide (rGO), and the resulting system’s electrocatalytic activity has been investigated during oxygen reduction reaction in acid solution. Physicochemical properties of the Pt nanoparticle-containing materials have been obtained using transmission electron microscopy, X-ray diffraction and various electrochemical diagnostic techniques including cyclic voltammetry and rotating ring-disk electrode (RRDE) voltammetry. Although functionalization of rGO carriers with Prussian Blue, followed by the system’s thermal treatment at 300°C, does not change activity of Pt catalytic centers in terms of the electroreduction potential of oxygen in 0.5 mol dm−3 H2SO4 (under rotating disk voltammetric conditions), the presence of cyanide-bridged iron ionic sites tends to decrease formation of the undesirable hydrogen peroxide intermediate, as demonstrated by monitoring ring currents during RRDE measurements. The results imply usefulness of PB-modified rGO supports for dispersed Pt nanoparticles at the low loading of 10 μg cm−2 in the oxygen reduction electrocatalysis. In addition to the possible metal-support activating interactions, the mechanism, in which oxygen reduction is initiated at Pt centers and the hydrogen peroxide intermediate is reductively decomposed at reactive PB-modified rGO supports, is feasible.

Development of Nanosized Mn3O4-Co3O4 on Multiwalled Carbon Nanotubes for Cathode Catalyst in Urea Fuel Cell

Double-oxide Mn3O4-Co3O4 nanoparticles were synthesized and anchored on multiwalled carbon nanotubes (MWCNTs) via a single-step solvothermal method. The largest specific area (99.82 m2g−1) of the catalyst was confirmed via a nitrogen adsorption isotherm. Furthermore, the uniform coating of the Mn3O4-Co3O4 nanoparticles on the surface of the MWCNTs was observed via scanning electron microscopy and transmission electron microscopy; the uniform coating provided an effective transport pathway during the electrocatalytic activities. The rotating disk electrode and rotating ring disk electrode measurements indicated that the electron transfer number was 3.96 and the evolution of H2O2 was 2%. In addition, the Mn3O4-Co3O4/MWCNT catalyst did not undergo urea poisoning and remained stable in an alkaline solution. Conversely, commercial Pt/C could not withstand urea poisoning for long. The performance cell achieved a power density of 0.4226 mW cm−2 at 50 °C. Therefore, Mn3O4-Co3O4/MWCNT is an efficient and inexpensive noble-metal-free cathodic catalyst for direct urea fuel cells.

One-Pot Synthesis of Pt-Free 3D-Catalyst for PEMFC

Pt-free catalysts for Oxygen Reduction Reaction (ORR) are broadly studied for Proton Exchange Membrane Fuel Cells (PEMFC) in order to widely develop hydrogen as a fuel for individual transportation. To reduce the performance gap between noble and non-noble catalysts, kinetics and thermodynamics are to be considered. The kinetics of the reaction is influenced by both the turn over frequency of the catalytic site and the density of said sites in the materials. The diffusion of gaseous reactants and liquid water produced is equally important as it modifies the thermodynamic of the reaction. For a long time now, Fe-C-N materials have been identified as the most promising Pt-free electrocatalysts for ORR despite the limited knowledge on the actual structure of the involved sites. Several approaches are currently studied to produce materials with high active site density and high porosity. Different carbon structures are modified to include active sites. The most reported and promising routes are Metal Organic Frameworks, polymers on carbon nanoparticles and silica templated organic structures.In this new approach, carbon cryogels are chosen for their tunability and high specific surface area. Porous Fe-C-N catalysts are synthesized in one step by a modified resorcinol-formaldehyde aerogel method. Gelation, drying and pyrolysis steps are carefully studied and optimized to obtain a high density of accessible catalytic sites. The porosity of the formed materials is measured with nitrogen adsorption. Values of the Brunauer Emmett Teller (BET) surface area up to 800 m2/g are obtained. The impact of different synthesis parameters on catalytic activity is studied. To assess the catalyst’s performances toward ORR, rotating ring Disk Electrode measurements as well as Fuel Cell polarization curves will be reported.

Designing Aqueous Organic Electrolytes for Zinc–Air Batteries: Method, Simulation, and Validation

AbstractAqueous zinc–air batteries (ZABs) are a low‐cost, safe, and sustainable technology for stationary energy storage. ZABs with pH‐buffered near‐neutral electrolytes have the potential for longer lifetime compared to traditional alkaline ZABs due to the slower absorption of carbonates at nonalkaline pH values. However, existing near‐neutral electrolytes often contain halide salts, which are corrosive and threaten the precipitation of ZnO as the dominant discharge product. This paper presents a method for designing halide‐free aqueous ZAB electrolytes using thermodynamic descriptors to computationally screen components. The dynamic performance of a ZAB with one possible halide‐free aqueous electrolyte based on organic salts is simulated using an advanced method of continuum modeling, and the results are validated by experiments. X‐ray diffraction, scanning electron microscopy, and energy dispersive X‐ray spectroscopy measurements of Zn electrodes show that ZnO is the dominant discharge product, and operando pH measurements confirm the stability of the electrolyte pH during cell cycling. Long‐term full cell cycling tests are performed, and rotating ring disk electrode measurements elucidate the mechanism of oxygen reduction reaction and oxygen evolution reaction. The analysis shows that aqueous electrolytes containing organic salts could be a promising field of research for zinc‐based batteries, due to their Zn2+ chelating and pH buffering properties. The remaining challenges including the electrochemical stability of the electrolyte components are discussed.

Operando Identification of Liquid Intermediates in Lithium–Sulfur Batteries via Transmission UV–vis Spectroscopy

Lithium-sulfur (Li-S) batteries are facing various challenges with regards to performance and durability, and further improvements require a better understanding of the fundamental working mechanisms, including an identification of the reaction intermediates in an operating Li-S battery. In this study, we present an operando transmission UV–vis spectro-electrochemical cell design that employs a conventional sulfur/carbon composite electrode, propose a comprehensive peak assignment for polysulfides in DOL:DME-based electrolyte, and finally identify the liquid intermediates in the discharging process of an operating Li-S cell. Here, we propose for the first time a meta-stable polysulfide species (S32−) that is present at substantial concentrations during the 2nd discharge plateau in a Li-S battery. We identify the S32− species that are the reduction product of S42−, as deducted from the analysis of the obtained operando UV–vis spectra along with the transferred charge, and confirmed by rotating ring disk electrode measurements for the reduction of a solution with a nominal Li2S4 stoichiometry. Furthermore, our operando results provide insight into the potential-dependent stability of different S-species and the rate-limiting (electro)chemical steps during discharging. Finally, we propose a viable reaction pathway of how S8 is electrochemically reduced to Li2S2/Li2S based on our operando results as well as that reported in the literature.

Electrochemical reduction mechanism of NbF5 and NbCl5 in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate

The electrochemical reduction mechanism of niobium was studied in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([BMP][OTf]) containing niobium halides (NbCl5 and NbF5). The influence of the electrolyte composition, temperature, and nature of the substrate were systematically investigated. In situ electrochemical techniques, such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical quartz crystal microbalance (EQCM) and rotating ring disc electrode (RRDE), were used, besides the ex situ characterization techniques scanning electron microscopy (SEM) and vibrational spectroscopy. The number of transferred electrons was estimated from the half width of the cathodic peaks in the DPV measurements. Furthermore, the product of the diffusion coefficient and the number of transferred electrons at the power of 3/2 were evaluated from RRDE measurements to be approx. 3 × 10−10 m2/s. Thicker and better adherent niobium-based deposits were obtained from NbCl5 in [BMP][OTf] than from NbF5 in [BMP][OTf]. EQCM measurements combined with DPV indicated the potential region where niobium-based deposits could be obtained as well as their stoichiometry.Vibrational spectroscopy reveals that NbF5 and NbCl5 interact differently with [BMP][OTf]. The spectra show that in the case of NbF5 – [BMP][OTf] only niobium(V) anionic species [NbF6A]2- and [NbF5A]- (where A = [OTf]-) are formed, while in NbCl5 – [BMP][OTf] both niobium(V) and niobium(IV) anionic species are present. At low NbCl5 concentrations, [NbCl4A2]2- and [NbCl4A2]- are obtained, while at high NbCl5 concentration, [NbCl5A]2- and [NbCl5A]- are found. The electrochemical niobium reduction mechanism depends on temperature and on the anionic species that are involved in the reduction process. In the case of NbCl5 – [BMP][OTf], at high cathodic potentials the formation of subvalent Nb-clusters (possibly Nb6Cl14) was observed.

Carbon Nanofiber-Supported Fe3O4 nanoestructures As Efficient Electrocatalyst for the Oxygen Reduction Reaction

Fe3O4 nanostructures were supported on carbon nanofiber synthesized at two different temperatures (600 and 700 °C) and tested as cathode catalyts for oxygen reduction reaction (ORR) in alkaline medium. Briefly, an iron oxide precursor, Fe(NO3)3.9H2O, is impregnated in carbon nanofiber (CNF 600 and CNF 700), followed by heat-treatment at 400 ° C for 4 h under Ar. The structure and morphology of the catalyst was evaluated by x-ray diffraction, raman spectroscopy scanning electron microscopy and nitrogen sorption isotherms. The electrocatalytic activity towards ORR was investigated by cyclic and linear voltammetries in 0.1 mol.L-1 KOH. Analysis of polarization curves showed the electrocatalytic activity of the materials towards oxygen reduction expressed by on-set potentials which Fe3O4/CNF 600 (+0.68 V vs RHE) presents more positive potential than Fe3O4/CNF 700 (+0.65 V vs RHE) and mass activity at +0.55 V vs RHE with values of 25.4 A.g-1 (Fe3O4/CNF 600) and 14.4 A.g-1 (Fe3O4/CNF 700). Nevertheless, the variation of number of exchage electrons (n) and peroxide ion percentage (%H2O-) with applied voltage indicate a direct oxygen reduction starts (4 electrons pathway) at more positive potential in Fe3O4/CNF 700 (+0.21 V vs RHE) than Fe3O4/CNF 600 (+0.12 V vs RHE). These results were obtained by indirect calculation using the Koutecky-Levich equation and confirmed by direct determination of ion peroxide intermediate formation with rotating-ring disk electrode measurements. Figure 1