Rotating Ring Disk Electrode Technique Research Articles

Alkali metal cations at work: Enhancing CO2 electroreduction to CO on ZnO nanorods

Oxide-derived zinc, an abundant and cost-effective electrode material alternative to gold and silver, has exhibited high activity for the electroreduction of CO2 to CO (CO2RR). However, the impact of cations on the CO2RR activity remains unexplored. In this study, we investigated the effect of three alkali metal cations (Cs+, K+, and Li+) on the CO2RR over a ZnO nanorod electrode, using a rotating ring disk electrode (RRDE) technique. Cyclic voltammetry of the Pt ring in the RRDE tip reveals a consistent trend in the buffering capacity of these cations (Cs+ > K+ > Li+), as evidenced by the availability of OH⁻ ions for CO electrooxidation and hydrogen oxidation reactions (HOR). Cs+ showed a pronounced effect on the activity of the CO2RR to CO since it can regulate OH⁻ concentration and maintain the local pH. This study addresses the ongoing debate concerning the effects of cations; interfacial electric field and buffering capacity; and elucidates the relationship among electrolyte/cation, local pH dynamics, and the CO2RR activity towards CO over the active oxide-derived Zn electrodes, all derived from the fast and facile RRDE technique.

Benchmarking Local pH of Electrocatalysts for Water Electrolysis Using the Rotating Ring-Disk Electrode Technique

Water electrolysis is an important route for production of green hydrogen and remains a field of active research in pursuit of improved electrocatalytic activity and durability. Operating with a near-neutral feed is critical for this technology to be more widely adapted, because it allows the direct use of various water sources and imposes less stringent requirements on the components of the electrochemical cell.During water electrolysis, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) involving protons and hydroxides can experience a significant change in local pH at the surface of the catalyst, especially in near-neutral bulk conditions. This change in local pH is usually dictated by the reaction conditions such as electrolyte composition and mass transport within the system. Notably, there has been an increase in development of catalysts for water electrolysis with the ability to modulate their local pH in recent years. 1–6 However, comparing and quantifying the local pH modulation effects of these catalysts is challenging due to varying experimental techniques and protocols. The rotating ring-disk electrode (RRDE) with a potentiometric pH sensing ring has been presented as a promising and facile tool for in-situ local pH detection. 3,4,7,8 The local pH at the disk is estimated from the ring pH through solving the convective-diffusion equation under the well-defined mass transport conditions of an RRDE system. 8 In this study, we have investigated the local pH of iridium oxide during OER in both unbuffered and buffered electrolytes under near-neutral to alkaline bulk conditions using the RRDE technique. Our results provide benchmarks for future studies on the local pH modulation effects of catalysts and insights into the impact of electrolyte composition on local pH. In combination with mass transport modelling, we further examined the limitations of this technique and provide suggestions for best practices. References X. Liu et al., Appl. Catal. B Environ., 331, 122715 (2023).Z. Li et al., Angew. Chemie Int. Ed., 62, e202217815 (2023).J. Guo et al., Nat. Energy, 8, 264–272 (2023).X. Zheng et al., Nat. Commun. 2023 141, 14, 1–13 (2023).X. Wang, C. Xu, M. Jaroniec, Y. Zheng, and S. Z. Qiao, Nat. Commun. 2019 101, 10, 1–8 (2019).H. Tan et al., Nat. Commun. 2022 131, 13, 1–9 (2022).Y. Yokoyama et al., Chem. Lett., 49, 195–198 (2020).W. J. Albery and E. J. Calvo, J. Chem. SOC., Faraday Trans. 1, 79, 2583–2596 (1983). Figure 1

Electrochemically Reduced Graphene Oxide Supported Palladium-Cobalt Alloy Nanoparticles as Highly Efficient Electrocatalyst for Oxygen Reduction Reaction

Pd-Co alloy nanoparticles were synthesized on electrochemically reduced graphene oxide (ErGO) for electrocatalytic oxygen reduction reaction (ORR) in alkaline media. The morphological structure, chemical composition, and crystallinity of the afforded ErGO/Pd-Co catalyst were examined by various techniques, including transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analysis. The Pd-Co alloy nanoparticles were evenly distributed on the ErGO sheet and had a narrow size distribution, with an average particle size of 4.25 nm. The ORR activity of the as-prepared ErGO/Pd-Co alloy catalyst was evaluated by cyclic voltammetry, linear sweep voltammetry, rotating disk electrode, and rotating ring disk electrode techniques in an alkaline electrolyte. The ErGO/Pd-Co catalyst exhibits a very high electrocatalytic activity in ORR with the mass activity more than 2.0 times of the commercial 20% Pt/C due to the synergistic catalytic effect of both ErGO and Pd-Co nanoparticles. Electrocatalytic kinetics investigation shows that, the ORR on the ErGO/Pd-Co catalyst proceeds through four electrons transferred mechanism with negligible amount of peroxide species. Moreover, the ErGO/Pd-Co shows an excellent methanol tolerance than the commercial 20% Pt/C.

High-Performance Perovskite Bifunctional Electrocatalysts for Oxygen Reduction Reaction and Oxygen Evolution Reaction

Perovskite structured earth-abundant metal oxides are attractive candidates as bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline electrolytes. Herein, we present a series of perovskite electrocatalysts (La0.65Sr0.3)zFeO3−δ with different A-site cation deficiencies through a simple and scalable solid-state synthesis method. Their electrocatalytic activities toward ORR and OER are studied by rotating disk electrode and rotating-ring disk electrode techniques in an alkaline electrolyte. Two obvious oxygen reduction potential regions are first reported in this work for these perovskites; the electron transfer number during ORR in the first oxygen reduction potential region is ∼2 and that in the second region is ∼4, indicating two different ORR mechanisms in these two potential regions. Among these perovskites, (La0.65Sr0.3)1.00FeO2.95 exhibits the highest electrocatalytic activity for ORR in the first ORR potential region with an onset potential of 0.72 V, a potential of 0.6 V at −1 mA cm–2, and a Tafel slope of 60.4 mV dec–1 in 0.1 M KOH. For OER in the alkaline electrolyte, (La0.65Sr0.3)0.89FeO2.9 with the highest A-site cation deficiency is proved to be the most active catalyst among these perovskites, which is mainly caused by its highest surface oxygen vacancy content. As a result, our work could provide guidance in the further development of easily prepared, scalable, and earth-abundant perovskite metal oxides.

A new spin on electrochemistry in the undergraduate lab

Abstract The growing interest in electrochemistry over recent years has sparked an increase in the popularity of various electrochemical techniques, including more advanced methods, that have previously been overlooked in academia and industry. This makes comprehensive hands-on experience in electrochemistry a highly demanded addition to chemistry graduates. However, many students do not receive sufficient training in the theory and experimental design to confidently use and apply various electrochemical techniques throughout their undergraduate, and sometimes even in graduate studies. Here we summarize the theory and practical applications for both rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) techniques. The different modes of operation of rotating ring disk voltammetry, methodologies of data analysis and interpretation as well as the scope of the information that can be extracted from the RDE/RRDE are discussed. Proposed modifications of the laboratory curriculum will allow students to examine and learn valuable information about the reactions on the surface of the electrode/liquid interface. This information will allow chemists to confidently use RDE and RRDE techniques for a wide range of research and development targets. Furthermore, incorporating these techniques into existing chemistry laboratories will help chemistry educators to enrich the undergraduate chemistry curriculum and improve students’ learning outcomes.

The Chalkboard: A Positive Spin on Corrosion Measurements: The Rotating Ring-Disk Electrode

Within the inventory of highly sensitive, selective, and broadly useful electroanalytical techniques available nowadays resides a very clever approach to the measurement of reaction rates, including those of certain non electrochemical reactions, and the simultaneous quantitative identification of reaction products, including short-lived intermediates. Conceived by Alexander Frumkin in 1958 and first built by Lev Nekrasov, the rotating ring-disk electrode (RRDE) was a spin-off of the rotating disk electrode (RDE) for which Benjamin Levich developed the hydrodynamic equations defining the electrochemical response. A fascinating account of the development of the RRDE technique by Soviet, American, and British electrochemists during and despite the Cold War can be found in an earlier issue of Interface.

Reprint of "Rotating ring-disc electrode measurements for the quantitative electrokinetic investigation of the V3+-reduction at modified carbon electrodes"

Thin film rotating-ring disc electrode (RRDE) technique is exploited to quantify the parasitic hydrogen evolution reaction (HER) competing with the desired V3+-reduction at surface modified carbon nanoparticles for application as electrocatalysts in the negative half-cell of vanadium redox-flow batteries (VRFB). Carbon based electrode materials are derived from standard Vulcan XC-72 carbon, treated by chemical surface etching techniques proposed for carbon felt-electrodes in the literature. Additional electrochemical characterization is performed using stationary cyclic voltammetry (CV) followed by fitting of CV data, Fourier-transform alternating-current cyclic voltammetry (FT-ACCV) and electrochemical impedance spectroscopy (EIS) followed by distribution of relaxation times (DRT) analysis. To our knowledge the present paper is the first study using the RRDE technique for separating HER and V3+-reduction reactions. It is demonstrated that the ratio of HER to V3+-reduction significantly depends on the chemical pretreatment of the carbon electrodes and that the V3+-reduction proceeds at an optimum rate at E − ERHE = −0.45 V. Separating the HER from the V3+-reduction also allows us to provide highly accurate values for the diffusion coefficient of the V3+-ion in sulfuric acid solutions.

Rotating ring-disc electrode measurements for the quantitative electrokinetic investigation of the V3+-reduction at modified carbon electrodes

Thin film rotating-ring disc electrode (RRDE) technique is exploited to quantify the parasitic hydrogen evolution reaction (HER) competing with the desired V3+-reduction at surface modified carbon nanoparticles for application as electrocatalysts in the negative half-cell of vanadium redox-flow batteries (VRFB). Carbon based electrode materials are derived from standard Vulcan XC-72 carbon, treated by chemical surface etching techniques proposed for carbon felt-electrodes in the literature. Additional electrochemical characterization is performed using stationary cyclic voltammetry (CV) followed by fitting of CV data, Fourier-transform alternating-current cyclic voltammetry (FT-ACCV) and electrochemical impedance spectroscopy (EIS) followed by distribution of relaxation times (DRT) analysis. To our knowledge the present paper is the first study using the RRDE technique for separating HER and V3+-reduction reactions. It is demonstrated that the ratio of HER to V3+-reduction significantly depends on the chemical pretreatment of the carbon electrodes and that the V3+-reduction proceeds at an optimum rate at E − ERHE = −0.45 V. Separating the HER from the V3+-reduction also allows us to provide highly accurate values for the diffusion coefficient of the V3+-ion in sulfuric acid solutions.

Unraveling Complex Electrode Processes by Differential Electrochemical Mass Spectrometry and the Rotating Ring-Disk Electrode Technique

The competition between the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) on a polycrystalline Pt (pc-Pt) electrode in weakly acidic solutions (pH ≈ 3) under the condition when both reactions are thermodynamically allowed is investigated using the rotating ring disk electrode (RRDE) technique and differential electrochemical mass spectrometry (DEMS). The partial currents of HER and ORR producing H2O2 or H2O are quantified. The results show that (i) ORR occurs preferentially in a diffusion-limited manner (either limited by the mass transfer of O2 or H+) and only the excess H+ not being consumed by the ORR can be converted to Had and H2 and (ii) at potentials negative of the onset of Had formation, the H2O2 production rate first increases and then decreases with the increasing overpotential. Possible origins for the H2O2 yield in the HER region are suggested. Present results demonstrate that combining DEMS and RRDE techniques can be powerful for deriving the intrinsic kinetics of individual processes in a complex electrochemical system.

Bifunctional Oxygen Reduction and Evolution Activity in Brownmillerites Ca2Fe(1-x)Co x O5.

State-of-the-art catalysts for oxygen reduction and evolution reactions (ORR and OER), which form the basis of advanced fuel cell applications, are based on noble metals such as Pt and Ir. However, high cost and scarcity of noble metals have led to an increased demand of earth-abundant metal oxide catalysts, especially for bifunctional activity in ORR and OER. The fact that Pt and Ir or C, the cost-effective alternatives suggested, do not display satisfactory bifunctional activity has also helped in turning the interest to metal oxides which are stable under both ORR and OER conditions. Brownmillerite A2B2O5 type oxides are promising as bifunctional oxygen electrocatalysts because of intrinsic structural features, viz., oxygen vacancy and catalytic activity of the B-site transition metal. In this study, Co-doped Ca2Fe2O5 compounds are synthesized by the solid state method and structurally analyzed by Rietveld refinement of powder X-ray diffraction data. The compound Ca2Fe2O5, crystallizing in the Pcmn space group has alternative FeO4 tetrahedral and FeO6 octahedral layers. Its Co-doped analogue, Ca2Fe1.75Co0.25O5, also crystallizes in the same space group with both tetrahedral and octahedral Fe positions substituted with Co. However, Ca2FeCoO5 in the Pbcm space group shows interlayer ordering with Co-rich octahedra connected to Fe-rich tetrahedra and vice versa. Oxygen bifunctional activities of these catalysts are monitored by rotating disc electrode and rotating ring disc electrode techniques in alkaline media. A close analysis of the ORR and OER was conducted through comparison of various parameters such as onset potential, current density, halfwave potential, and other kinetic parameters, which suggests that the presence of Co in the B site aids in achieving better bifunctional activity and bulk conductivity. In addition, Co(II)/Co(III) redox systems and their comparative concentrations also play a decisive role in enhancing the activity.

La0.7Sr0.3Mn1–xNixO3−δElectrocatalysts for the Four-Electron Oxygen Reduction Reaction in Concentrated Alkaline Media

Electrocatalytic activity for the four-electron-transfer oxygen reduction reaction (ORR) was examined with perovskite-type La0.7Sr0.3Mn1–xNixO3—δ (x = 0–0.5) compounds in concentrated KOH. Electrochemical measurements by means of a rotating ring–disk electrode technique revealed that 10% substitution of Mn by Ni atoms significantly improved ORR activity whereas further Ni substitution significantly decreased the catalytic performance. Moreover, La0.7Sr0.3Mn0.9Ni0.1O3—δ could achieve the ideal reaction electron number (∼4) without the aid of carbon additives, whereas compounds with x > 0.1 achieved a number less than 4 in the absence of carbon cocatalysts, attributed to lowered reaction rates related to two-electron reduction from oxygen to peroxide. Combined with the results of X-ray photoelectron spectroscopy, the pronounced catalytic activity of La0.7Sr0.3Mn0.9Ni0.1O3—δ could be correlated with the population of Mn3+ states with (eg)1 configuration.

Oxygenic Photoreactivity in Photosystem II Studied by Rotating Ring Disk Electrochemistry.

Protein film photoelectrochemistry has previously been used to monitor the activity of photosystem II, the water-plastoquinone photooxidoreductase, but the mechanistic information attainable from a three-electrode setup has remained limited. Here we introduce the four-electrode rotating ring disk electrode technique for quantifying light-driven reaction kinetics and mechanistic pathways in real time at the enzyme–electrode interface. This setup allows us to study photochemical H2O oxidation in photosystem II and to gain an in-depth understanding of pathways that generate reactive oxygen species. The results show that photosystem II reacts with O2 through two main pathways that both involve a superoxide intermediate to produce H2O2. The first pathway involves the established chlorophyll triplet-mediated formation of singlet oxygen, which is followed by its reduction to superoxide at the electrode surface. The second pathway is specific for the enzyme/electrode interface: an exposed antenna chlorophyll is sufficiently close to the electrode for rapid injection of an electron to form a highly reducing chlorophyll anion, which reacts with O2 in solution to produce O2•–. Incomplete H2O oxidation does not significantly contribute to reactive oxygen formation in our conditions. The rotating ring disk electrode technique allows the chemical reactivity of photosystem II to be studied electrochemically and opens several avenues for future investigation.

A Solvent-Controlled Oxidation Mechanism of Li2O2 in Lithium-Oxygen Batteries

Summary Long-unresolved discrepancies in the oxidation mechanism of lithium peroxide (Li2O2) impede electrode/electrolyte development for Li-O2 batteries. In this study, we report direct evidence of the formation of soluble LiO2 upon the oxidation of Li2O2 and reveal a strong solvent-controlled Li2O2-oxidation reaction mechanism. We exploit a thin-film rotating ring-disk electrode to show that soluble LiO2 is generated when oxidizing Li2O2 in high-donicity solvent but is absent in low-donicity glyme-based solvent. Synchrotron-based X-ray absorption near-edge structure spectroscopy further proved the presence of LiO2 upon Li2O2 oxidation in high-donicity solvent but not in low-donicity solvent. We show that preferential formation of soluble LiO2 in the high-donicity solvent could account for poor cycling stability, which suggests that strategies that bypass the formation of soluble LiO2 upon Li2O2 oxidation are critical. Our work offers new insights in resolving the discrepancy in Li-O2 charging mechanism and design strategies to achieve efficient and long-life Li-O2 batteries.

Synthesis and Characterization of Nitrogen-Doped Ordered Mesoporous Hollow Carbon Spheres for the ORR

This work presents the synthesis by a silica template-based method of metal-free electrocatalyst for the Oxygen Reduction Reaction (ORR) for alkaline fuel cells. Nitrogen-doped Ordered Mesoporous Hollow Carbon Spheres (N-OMHCS) was obtained starting from a SiO2 template. The N-OMHCS material has been functionalized with methanol using the intermittent microwave heating technique to produce the N-OMHCS-f electrocatalyst. Then, the catalytic activity of N-OMHCS and N-OMHCS-f electrocatalysts were evaluated for the ORR in 0.5 mol L-1 KOH, with the rotating ring-disk electrode technique (RRDE). The electrochemical results indicate that N-OMHCS-f has higher catalytic activity for the ORR than N-OMHCS, whit an onset potential of 0.81 and 0.82 V vs. RHE and a limiting current of -3.6 and -2.3 mA cm-2, respectively. On the other hand, both electrocatalysts produce about the same amount of H2O2 (between 14 and 26 %). The catalytic activity of N-OMHCS and N-OMHCS-f can be attributed to the nitrogen species on their surface and their porous structure, which provides a triple phase region promoting the adsorption-desorption of O2.

Synthesis and Characterization of Nitrogen-Doped Ordered Mesoporous Hollow Carbon Spheres for the ORR

This work presents the synthesis by a hydrothermal procedure of metal-free nitrogen-doped Ordered Mesoporous Hollow Carbon Spheres (N-OMHCS) starting from a SiO2 template. N-OMHCS powders have been functionalized with methanol using the intermittent microwave heating technique to produce the N-OMHCS-f sample. Then, N-OMHCS and N-OMHCS-f have been evaluated as cathode electrocatalysts for the Oxygen Reduction Reaction (ORR) in 0.5 mol L-1 KOH, with the rotating ring-disk electrode technique (RRDE). The morphology of N-OMHCS and N-OMHCS-f is that of hollow porous spherical particles of carbon of 280 nm in diameter. The chemical composition analysis shows that N-OMHCS-f is composed of basically C and O. The electrochemical results indicate that N-OMHCS-f are active for the ORR, with an onset potential of 0.81 and 0.82 V/RHE and a limiting current of -3.6 and -2.3 mA cm-2, respectively. On the other hand, both electrocatalysts produce about the same amount of H2O2 (between 14 and 26 %). Meanwhile, the number of electrons transferred at the electrocatalysts is between 3.6 and 3.8, which is close to a 4e- transfer mechanism. The catalytic activity of N-OMHCS and N-OMHCS-f can be attributed to their high specific surface area and porous structure, which provides a triple phase region promoting the adsorption-desorption of O2.

(Invited) Electrocatalytic Activity Towards ORR and Stability of Binary and Ternary Catalysts Based on Pt and Cu

The catalytic activity of platinum can be enhanced by modifying Pt-NPs with transition metal atoms such as Ni, Fe, Co, Cr, Cu [1,2]. But, it has also been proposed that the addition of non-noble transition metals could involve a lower stability than pure platinum due to their dissolution [3,4] leading further to a loss of performances in PEMFC. Other authors have shown that the dealloying of the nanoparticle surface, leading to a core-shell structure having a Pt rich surface, could enhance the catalytic activity of binary or ternary Pt-based systems. On the one hand, gold was proposed to prevent platinum dissolution by raising the Pt oxidation potential [5,6], a property which could be interesting to enhance the stability of core-shell Pt-based catalysts with Pt rich surface. Moreover, it has also been shown that PtAu-based catalysts could exhibit similar or even higher catalytic activity towards ORR than pure platinum [7–9]. On the other hand, recent DFT calculation identified that copper containing Pt-based catalysts, such as PtAuCu core-shell structures, as displaying high expected ORR activity and also a stabilizing effect from the Au-Cu interaction [10]. For these reasons, PtxCu1-x/C and ternary PtxAuyCuz/C catalysts were synthesized by a microemulsion method and characterized by TGA, TEM, XRD, AAS, XPS and cyclic voltammetry in order to assess their metal loading, their microstructure and their bulk and surface compositions. The electrocatalytic activity towards ORR, the selectivity and the stability of catalysts were studied in acidic electrolyte using the rotating disc and the rotating ring disc electrode techniques. Acknowledgement: The financial support of the European Commission under the FP7 Fuel Cells and Hydrogen Joint Technology Initiative grant agreement FP7-2012-JTI-FCH-325327 for the SMARTCat project is gratefully acknowledged.

(Invited) Identifying the Forefront of Electrocatalytic Oxygen Evolution Reaction: Electronic Double Layer

Developing a fundamental understanding of oxygen evolution reaction (OER) is essential to advance state-of-the-art energy conversion and storage technologies such as electrolysis. Different from acidic conditions, non-noble metal catalysts, such as Ni and Co based oxides, can be used for the OER in alkaline media to reduce cost significantly1. As a heterogeneous reaction, OER occurs at the interface formed between the catalyst layer surface and adjacent liquid electrolyte. The interaction of adsorbed OH- ions with the charged catalyst interface forms an electronic double layer (EDL) for charge storage and transfer. In principle, the EDL microstructure and chemical state governed by the interfacial electric potential has a significant influence on electrocatalytic performance of OER2. EDL thickness is determined mostly by the interactive force between catalyst and ionic species and is generally a few hundred angstroms. Due to its reaction nature and extreme thickness, real time in-situ visualization of the EDL microstructure and electrochemical behaviors is not possible. Therefore, developing effective electrochemical diagnostic tools is imperative to analyze EDL performance. This current work is focused on designing electrochemical experiments to characterize various interfacial behaviors during alkaline OER. It is found that the catalyst activity measured by traditional linear sweep voltammetry method depends strongly on the applied potential scan rate. While noble-metal oxide IrO2 shows enhanced activity as scan rate increases, opposite behavior is observed for non-noble transition-metal oxide, such as NiCo2O4. Further characterizations by thin-film rotating ring disk electrode technique reveal that this reversed trend can be ascribed to their intrinsic structural and chemical features, as well as drastic reconstruction of EDL microstructure during OER. Compared with IrO2, NiCo2O4 catalyst shows higher resistances of OH-, O2 and electron transport, capacitive and pseudocapacitive charging process, and electrocatalytic OER. These combined resistances of NiCo2O4 catalyst prolong the time required to reach steady state OER performance in response to potential perturbation. Furthermore, results from chronoamperometry demonstrate that the continuous EDL reconstruction of NiCo2O4 is favorable for OH- adsorption and OER. In contrast, continuous EDL reconstruction of IrO2 is harmful for OER due to Ir dissolution and Ir5+ formation. Based on the collective electrochemical observations, three new insights are gained: (1) how to reliably characterize OER catalyst performance, (2) how to evaluate EDL structure and its interfacial behaviours, and (3) how to demonstrate EDL modification during continuous electrochemical operation. References Varcoe JR, et al. Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 7, 3135-3191 (2014).Li G, Anderson L, Chen Y, Pan M, Chuang P. A. New insights into evaluating catalyst activity and stability of oxygen evolution reactions in alkaline media. Sustainable Energy & Fuels, (2017), DOI:10.1039/C7SE00337D Figure 1

Unraveling the Correlation between Solvent Properties and Sulfur Redox Behavior in Lithium-Sulfur Batteries

Systematic understanding of how solvent property influences Li-S redox chemistry is required to develop an effective electrolyte for Li-S batteries. In this study, we investigate the correlation between solvent property and Li-S redox chemistry in nine non-aqueous electrolyte solvents that cover a wide range of three main solvent physiochemical properties, namely dielectric constant (ɛ), Gutmann donor number (DN), and acceptor number (AN). We exploit various analytical techniques including cyclic voltammetry, rotating ring disk electrode technique, UV-Vis spectroscopy and galvanostatic measurement in a two-compartment cell. We show that the potential of S8-reduction increases with increasing AN and that the polysulfide-reduction/oxidation is strongly influenced by the DN. The common discrepancy in the literature on the role of dielectric constant and donor number is addressed by examining the redox reactions, polysulfide stability, and the effect of salt concentration in acetonitrile - a solvent with high dielectric constant and low DN. We show that the DN is the primary descriptor for polysulfide redox reactions, as it controls the effective charge density of the solvated cation (Li+), which affects the stability of polysulfides with different charge density via Pearson's Hard Soft Acid Base theory.

Rotating Ring Disk Electrode for Monitoring the Oxygen Release at High Potentials in Li-Rich Layered Oxides

Li-rich layered oxides show a staggering capacity that relies on cumulative cationic and anionic redox processes. However, their practical applications are plagued by roadblocks dealing with large hysteresis, capacity fade and irreversible oxygen loss during the first charge that causes undesirable structural changes. Hence, the first step to screen the Li-rich layered oxides is the identification of this gas release phenomenon and its better understanding. Online electrochemical mass spectrometry (OEMS) is presently used for the elucidation of gas evolution, but its usage is lengthy and far to be routine. Herein we propose the utilization of the simple rotating ring disc electrode (RRDE) technique for the identification of O2 release phenomenon and hence the quick screening of Li-rich layered oxides. We have evaluated the feasibility of RRDE to monitor the O2 generation by conducting studies on Li-half cells having various Li-rich layered oxides as cathodes and found that our results nicely compare with those obtained by OEMS, hence demonstrating the validity of RRDE technique. The proposed RRDE approach which offers high sensitivity for the identification of O2 release while being fast and simple should greatly help to advance the understanding of oxygen redox processes and promote the design of new materials.

A Chronoamperometric Differential Electrochemical Mass Spectroscopy Study on the Growth of Li2O2 and Its Effect on the Mechanism of Oxygen Reduction in Dimethylsulfoxide Based Electrolytes

We report on potential step experiments into the oxygen reduction region in combination with differential mass spectroscopy (DEMS) and the rotating ring disc electrode technique (RRDE). We can thus distinguish between the one electron reduction path leading to superoxide and the 2 electron process leading to lithium peroxide. The evolution of the mass spectroscopic response in the DEMS experiments and evolution of the current at the ring electrode discloses that the deposition of lithiumperoxide hampers the capacity of the electrode for the consecutive transition of two electrons. The latter is required to form Li2O2 on a direct electrochemical pathway. Instead, superoxide becomes the main product of oxygen reduction. A proportionality between coverage with Li2O2 and the share of oxygen reduced to superoxide is observed at platinum. The effect of the Li2O2-deposit is less severe at large overpotentials. However, completion of a single monolayer of Li2O2 on platinum and two monolayers on gold inhibit further oxygen reduction. We suggest that growth of larger Li2O2 structures observed by other authors is caused by a chemical rout to Li2O2 via superoxide disproportionation. Experimental evidence for the latter reaction is provided.In addition, we observe a non-monotonic faradaic current transient typical for nucleation after steps to certain potentials at the gold electrode and ascribe this to the formation of 2D Li2O2. This implies that nuclei of Li2O2 have to form before the solid phase can grow uninhibited. Evidently the involved surface energies cause a crystallisation overpotential and therefore impair the formation of solid Li2O2.