Rotating Ring Disk Electrode Study Research Articles

Rotating Disk Electrode Studies of Benzaldehyde Oxidation on Group Ib Metals

Water electrolysis is a green means of producing hydrogen gas to be used as a fuel or reactant in key industrial processes such as ammonia production. The conventional system of oxygen evolution (OER) and hydrogen evolution (HER) suffers from slow kinetics at the anode and a high cell voltage limited by thermodynamics. One solution is to replace OER with another reaction such as an organic oxidation reaction that has a lower equilibrium potential and produces a valuable organic byproduct at the anode. Recent work has demonstrated that biomass-derived aldehydes such as furfural and 5-hydroxymethylfurfural can be selectively oxidized to their corresponding carboxylates and H2 on Cu-based anodes at low potentials (~0.1 VRHE) in alkaline media. In alkaline media, a hydroxide ion adds to the carbonyl carbon of an aldehyde molecule forming a gem diolate in solution phase. It is argued that dehydrogenation becomes more facile due to the weaker C-H bond of the gem diolate compared to the aldehyde form of the molecule. This may enable oxidation without the need for coadsorbed-OH. Additionally, aldehyde oxidation is only active on Au, Ag, and Cu electrodes at higher pHs.The aim of this work is to identify the conditions under which low potential oxidation of aldehydes and high anodic Faradaic efficiency to H2 occurs, namely, electrode material and applied potential. To avoid polymerization side reactions of furans in alkaline media, we studied benzaldehyde as a model compound which has also been shown to exhibit low potential oxidation and anodic H2. The activity (rate of aldehyde consumption) and selectivity (to anodic H2 versus H2O) were compared for the group Ib metals (Au, Ag, Cu), each of which demonstrates a relatively early onset potential (<0.4 VRHE) and generates anodic H2 at certain potentials. Rotating disk electrode studies with Koutecky-Levich analysis were conducted to separate mass transfer effects from kinetic current to compare the intrinsic activity of each electrode. Additionally, the average number electrons transferred per aldehyde molecule consumed was estimated from Koutecky-Levich analysis; the one-electron pathway combines the aldehyde hydrogens to H2 while the two-electron pathway requires an additional electron per aldehyde molecule to oxidize the hydrogen from the surface to form H2O. Rotating ring disk electrode studies were used to further validate the anodic H2 generation as a Pt ring was still able to oxidize H2 despite the presence of benzaldehyde. This was used as an additional means of estimating the selectivity to H2 as a function of potential.CVs for benzaldehyde oxidation on each metal are shown in Figure a. Peak current followed the order of Au > Ag >> Cu. Au reached a mass transfer limiting current, and Ag also showed an RPM dependence but did not reach full mass transfer limitations. Cu was the least active and did not show any mass transfer dependence. On the other hand, Cu has the earliest onset potential followed by Ag then Au. The average number of electrons transferred per aldehyde molecule according to RRDE measurements is shown in Figure b, where one electron represents H2 and two electrons represents H-oxidation to H2O. Cu had 100% selectivity to H2 over its entire potential window of activity (up to 0.45 VRHE). Au and Ag have high selectivity to H2 at low potentials but decreases to near zero H2 production in the range of 0.6-0.8 VRHE; the decrease in selectivity to H2 on Ag occurs slightly earlier than Au (~0.1 V). H2 selectivity is related to the inability of each metal to oxidize H to H2O at low potentials. Figure 1

Electrochemical CO2 Reduction at Surface Modified Silver Nanoparticles

CO2 capture and transformation into chemical fuels presents a useful carbon-neutral energy source to curb the effects of global warming if the electrocatalyst used is highly active, exhibits sufficient product selectivity, and is energetically efficient. Metal nanoparticles (NPs) have emerged as a promising class of catalysts for the electrochemical CO2 reduction reaction (CO2RR) and their product selectivity are expected to be further improved through surface modification with different ligands. The primary objective of this work is to compare the use of Scanning Electrochemical Microscopy (SECM) and Rotating Ring Disk Electrode (RRDE) to study CO2 reduction at silver nanoparticles (Ag NPs) modified with amino acids (cysteine and tryptophan) and Ag NPs synthesized by a citrate reduction method. RRDE studies showed the onset potential for CO2RR was earlier for both the Ag NPs with cysteine and Ag NPs with tryptophan than those with citrate. SECM showed earlier onset potentials for CO2RR than the RRDE experiment for every type of Ag NP as well as higher sensitivity in detecting the major CO2RR products such as formate and CO. SECM also showed less hydrogen formation for the Ag NPs with cysteine and Ag NPs with tryptophan.

Embedded Platinum–Cobalt Nanoalloys in Biomass-Derived Laser-Induced Graphene as Stable, Air-Breathing Cathodes for Zinc–Air Batteries

Fuel cells and metal–air batteries hold significant promise to help decarbonize transportation and the electricity grid. To encourage widespread adoption of these technologies, improvements to their air-breathing cathodes are required, which address problems such as the high cost of platinum (Pt) catalysts used to boost the kinetics of the oxygen reduction reaction (ORR) as well as the stability of Pt/C interfaces over long-term cycling. In this paper, we demonstrate a facile approach to reduce Pt content to less than 2 wt % by interfacing Pt with CoOx as well-dispersed nanoparticles entrapped within a highly conductive laser-induced graphene (LIG) matrix. Laser-induced carbonization of polymerized furfural alcohol preloaded with Co and Pt precursors resulted in the formation of a mixture of spherical nanoalloys PtCoOx and core (CoOx)–shell (Pt) structures. This LIG-PtCoOx electrode exhibited a low onset and half-wave potential in alkaline media, which closely approached a benchmark Pt/C. The effectiveness of LIG-PtCoOx was demonstrated by its performance in rotating disk and rotating ring disk electrode studies versus commercial Pt/C with the same concentration of the catalyst, which resulted in 4-fold greater mass activity and more than 6-fold higher specific activity, which are reflected in a high turnover frequency (TOF). The resulting material was tested as an air–cathode for zinc (Zn)–air batteries leading to improved stability (118 h of operation) and rechargeability (0.75 V voltage gap), exhibiting a higher peak power density compared to batteries assembled with the commercial benchmark Pt/C cathodes with similar composition.

A rotating ring disc electrode study of photo(electro)catalyst for nitrogen fixation.

There are numerous reports of photo(electro)catalysts demonstrating activity for nitrogen reduction to ammonia and a few reports of photo(electro)catalysts demonstrating activity for nitrogen oxidation to nitric acid. However, progress in advancing solar-to-fertilizer applications is slow, due in part to the pace of catalyst screening. Most evaluations of photo(electro)catalysts activity occur using batch reactors. This is because common product analyses require accumulation of ammonia or nitric acid in the reactor to overcome instrument detection limits. The primary aim here is to examine the use of an electroanalytical method, rotating ring disk electrode voltammetry (RRDE), to detect ammonia produced by a nitrogen fixing photo(electro)catalyst. To examine the potential for RRDE, we investigated a photo(electro)catalyst known to reduce nitrogen to ammonia (titania), while varying the applied electrochemical potential and degree of illumination on the disk. We show that the observed ammonia oxidation at the ring electrode corresponds strongly with ammonia measurements obtained from the bulk electrolyte. Indicating that RRDE may be effective for catalyst screening. The chief limitation of this approach is the need for an alkaline electrolyte. In addition, this approach does not rule out the presence of adventitious ammonia.

Hybrid Lithium Polysulfide Flow Batteries for Large Scale Energy Storage

Lithium-sulfur battery chemistry has garnered global attention as a promising next-generation energy storage technology due to its significantly higher theoretical capacity (450 Wh/kg) compared to lithium-ion (265 Wh/kg), and the fact that its elemental components are green, safe and abundant[1]. As opposed to lithium-ion, the cathode solution chemistry is rich, as elemental sulfur forms polysulfide chains during discharge which can transport and deposit on the metallic lithium anode during a dissolution-migration-deposition “shuttle” mechanism which in effect a) cause a constant internal shorting current proportional to the transport of polysulfides and b) cause a build-up of lithium- and sulfur-rich solid-electrolyte interphase (SEI) on the anode which irreversibly passivates the lithium metal anode. This effect must be supressed at all costs in conventional lithium-sulfur batteries, and is achieved by encouraging rapid precipitation of Li2S salts by the use of low-donor number solvents for the electrolyte such as diglyme (DME) and dioxolane (DOL).However, polysulfide chains (Li2Sx, 3 ≤ x ≤ 8) have great potential as redox couples due to their stable, successive multistep redox behaviour and have been successfully demonstrated in hybrid redox-flow battery configurations[2], in particular enabled by lithium nitrate as an additive to the catholyte that forms a stable SEI on the lithium metal surface that greatly reduces the polysulfide deposition. The lithium-polysulfide redox flow battery in theory far outstrips current state of the art vanadium redox flow batteries due to the higher capacity density in the catholyte (50-150 Wh/L vs 30 Wh/L), and the energy dense lithium metal[2]. However, the solubility of polysulfides decrease with chain length and depth of discharge, and high polarity, high donor number solvents that can enable high polysulfide concentrations[3] are typically far more reactive towards lithium metal[4]. Moreover lithium nitrate have little effect as anode protectant in this class of solvents compared to low donor number, low polarity solvents such as DME and DOL, and the polysulfide reduction pathway is dependent on the stabilising property of the solvent[5].In collaboration with our commercial partner StorTera under the Faraday Institute, we have developed novel techniques for catholyte analysis. We show the role of nitrate consumption rate on protection of the anode, and the relative corrosive rate of lithium in a high polarity, high donor number class solvent (DMSO) versus conventional low polarity, low donor number class solvent (DOL/DME). Further we explore avenues to protect metallic lithium in highly concentrated polysulfide catholyte that enables large-scale energy storage that surpasses lithium-ion and vanadium redox flow batteries for cost, safety, serviceability and environmental impact. Such factors will be key for commercial deployment, in particular suitable for developing countries where microgrids for remote communities rely on intermittent renewable power supply. Zhang, G., Zhang, Z. W., Peng, H. J., Huang, J. Q. & Zhang, Q. A Toolbox for Lithium–Sulfur Battery Research: Methods and Protocols. Small Methods 1, 1–32 (2017).Yang, Y., Zheng, G. & Cui, Y. A membrane-free lithium/polysulfide semi-liquid battery for large-scale energy storage. Energy Environ. Sci. 6, 1552–1558 (2013).Pan, H. et al. On the Way Toward Understanding Solution Chemistry of Lithium Polysulfides for High Energy Li-S Redox Flow Batteries. Adv. Energy Mater. 5, (2015).Gupta, A., Bhargav, A. & Manthiram, A. Highly Solvating Electrolytes for Lithium–Sulfur Batteries. Adv. Energy Mater. 9, 1–9 (2019).Lu, Y. C., He, Q. & Gasteiger, H. A. Probing the lithium-sulfur redox reactions: A rotating-ring disk electrode study. J. Phys. Chem. C 118, 5733–5741 (2014).

Vacancy‐induced LaMnO3 Perovskite as Bifunctional Air‐breathing Electrode for Rechargeable Lithium‐Air Battery

AbstractDevelopment of cost‐effective efficient bifunctional electrocatalysts is indispensable for commercialization of metal‐air batteries. Here, vacancy‐induced nanostructured LaMnO3 perovskite was generated by a facile hydrothermal method and explored as a bifunctional air‐breathing electrode for rechargeable Lithium‐Air battery. Materials characterizations confirmed the formation of the nanostructured LaMnO3 with anion and cation vacancies. The LaMnO3 electrocatalyst on rotating ring disc electrode study showed excellent oxygen reduction reaction and oxygen evolution reaction activities in 0.1 M KOH electrolyte. The oxygen vacancies associated with the LaMnO3 acted as catalyst centre and promoted reaction rate for the ORR and OER. Laboratory prototype CR‐2032 air‐coin cell was fabricated using the LaMnO3 as air‐breathing electrode, resulted open circuit voltage 3.3 V for more than 5 days and high discharge capacity of 3100 mA h g−1 at current density of 50 mA g−1. Further, once charged air‐coin cell could power an LED for more than 24 h continuously.

Strategies for Modulating the Catalytic Activity and Selectivity of Manganese Antimonates for the Oxygen Reduction Reaction

Strategies for improving the performance of nonprecious metal catalysts for the oxygen reduction reaction (ORR) can facilitate the cost-effective deployment of fuel cell devices. Electrocatalyst performance is typically improved via two approaches: increasing the number of active sites and increasing the intrinsic activity of the active site. Herein, we utilize these two methods of improving performance for MnSb2O6, which we have recently shown to be a promising ORR catalyst due to improvements in per-metal-site activity in the antimonate framework. First, electrode engineering is used to investigate the role of mass and conductive support loading in the observed ORR performance and selectivity. The apparent 2-electron selectivity is found to decrease with increases in mass and/or conductive support loading, indicating that rotating ring disk electrode studies do not necessarily measure the intrinsic selectivity of the catalyst. Second, theoretical calculations are used to identify Cr, Fe, and Ni as promising first-row transition metals for improving the intrinsic activity of MnSb2O6. Experimentally, the addition of Cr results in 3-fold and 2-fold increases in the mass and specific activities at 0.7 V vs the reversible hydrogen electrode, respectively. This enhancement is attributed to the modulation of the active site structure and Mn oxidation state with the addition of Cr. Through these studies, we gain insight into the intrinsic and extrinsic factors that govern ORR performance.

Potentiometric Rotating Ring Disk Electrode Study of Interfacial pH during CO2 Reduction and H2 Generation in Neutral and Weakly Acidic Media

Potentiometric Rotating Ring Disk Electrode Study of Interfacial pH during CO₂ Reduction and H₂ Generation in Neutral and Weakly Acidic Media

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

Emodin Scavenging of Superoxide Radical Includes π-π Interaction. X-Ray Crystal Structure, Hydrodynamic Voltammetry and Theoretical Studies.

The naturally occurring anthraquinone emodin is found in many plants that have been part of traditional Chinese medicine (TCM) for thousands of years. Recent pharmacological studies suggest that emodin might be a valuable therapeutic option for the treatment of various diseases. We describe the antioxidant effects of emodin on the superoxide radical. Our techniques include X-ray crystallography, density functional theory (DFT), and a recently developed cyclovoltammetry improvement, the rotating ring-disk electrode (RRDE) method. X-ray results show offset π–π stacking of emodin units in the crystal, and this type of interaction is supported by the DFT, which indicates one superoxide interacting via π–π stacking with the quinone moiety, by transferring one electron to the ring, and inducing some quinone aromatization. The second superoxide seems to form a stable complex after interacting with the H(hydroxyl) in position 3 of emodin. We show that one molecule of emodin sequesters two molecules of superoxide: one forming a complex with H(hydroxyl) in position 3, and the other due to π–π oxidation of superoxide and emodin ring reduction. We conclude that emodin is a very strong antioxidant. Color variation in the voltaic cell was observed during the RRDE study. This was analyzed and explained using a mini-grid gold electrode for UV-Vis spectroscopy in the voltaic cell.

Carbon materials as additives to the OER catalysts: RRDE study of carbon corrosion at high anodic potentials

Carbon materials are widely applied as conductive additives in studies of the oxygen evolution reaction (OER) in alkaline media catalyzed by transition metal oxides. In this work we investigate the anodic behavior of three representative carbon materials: furnace black (Vulcan XC-72R), acetylene black, and pyrolytic carbon of the Sibunit family (Sibunit-152), in 1 M NaOH at high potentials (ranging from the OER onset and up to ca. 2 V vs. RHE) in the time span from several minutes to several hours. We apply the rotating ring-disk electrode (RRDE) to separate the OER current from the carbon corrosion current. We then use transmission electron microscopy (TEM) to visualize changes in the carbon morphology resulting from corrosion. Finally, we study the OER performance of composite electrodes comprising carbon materials mixed with a La0.5Sr0.5Mn0.5Co0.5O3−δ perovskite OER catalyst, and discuss possible influence of the oxide on the carbon corrosion.

Conductive additives for oxide-based OER catalysts: A comparative RRDE study of carbon and silver in alkaline medium

The objective of this work is to explore whether carbon materials, which are usually utilized as additives for improving the electronic conductivity of oxide-based oxygen evolution reaction (OER) catalysts, could be replaced by silver nanoparticles. We integrate La0.75Sr0.25Mn0.5Co0.5O3−δ, a perovskite with a limited electronic conductivity, in electrode compositions with either carbon materials (pyrolytic carbon of the Sibunit family – Sibunit-152, furnace black – Vulcan XC-72R, and Acetylene black) or silver nanoparticles, and investigate their OER activity in 1 M NaOH solution. We apply the rotating ring-disk electrode (RRDE) technique to separate the OER currents from those originating from corrosion of the additives under anodic polarization. We conclude that certain carbon materials would be better suited for use as conductive additives for oxide-based OER catalysts than silver particles.

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.

L-Cysteine oxidation studied by rotating ring disk electrodes: Verification of reaction intermediates

l-Cysteine (l-Cys) is an amino acid of high interest in the electrochemical field since it is one of the five electrochemically active amino acids. Due to the S atom on its side chain, this molecule presents the highest amount of possible oxidation states. The rotating ring disk electrode (RRDE) technique is a very useful method for the investigation of complex mechanisms, such as l-Cys oxidation, as it allows the identification of some intermediates. By rotating disk electrode (RDE), it was possible to observe that the adsorption of l-Cys onto Pt electrodes is influenced by mass transport. The chemical reaction was found to be reversible, while the charge transfer irreversible. The RRDE study facilitated the proposal of an electrochemical-chemical-electrochemical mechanism. The ring current was observed to be dependent on the quadratic of the potential applied to the disk electrode combined with the value of the rotation rate in a soft-model algorithm. The product of this chemical step was oxidized in two steps on the ring, with the first one including the generation of an adsorbed intermediate as the rate-limiting step.

Graphene-Supported Cobalt(III) Complex of a Tetraamidomacrocyclic Ligand for Oxygen Reduction Reaction

Polymer electrolyte membrane fuel cells (PEMFCs) are at the cusp of providing large scale energy solutions, yet challenges in developing high performance, durable, and cost effective platinum catalyst alternatives continue to impede commercialization efforts. A graphene-supported cobalt(III) catalyst nanocomposite was prepared and investigated for the first time as a potential cathode material for PEMFCs. The material was characterized using a variety of microscopy and spectroscopy techniques, and the electrochemical performance was assessed using voltammetry equipment. A peak potential at − 0.088 V versus standard hydrogen electrode was observed by cyclic voltammetry during oxygen reduction reaction (ORR). The material was found to reduce oxygen via a four-electron process in both acidic and alkaline pH conditions, with rotating disk electrode and rotating ring disk electrode studies revealing the ORR to occur via 3.60 and 3.86 electrons at pH 2, respectively. A rate constant of 9.78 × 106 mol− 1 s− 1 was observed for the cobalt(III) catalyst/graphene complex in acidic conditions, and a mechanism has been proposed based on these results. An economic, non-precious cobalt(III) complex supported on graphene successfully reduced oxygen at a high rate in cathodic fuel cell applications.

Comprehensive Understanding of Electrode Processes in PEMFCs Exposed to Various Pollutants Using Spatial AC-Impedance Spectroscopy

The success of recent deployments of automotive and stationary sources of energy utilizing proton exchange membrane fuel cells (PEMFCs) relies on a high performance under a broad range of environmental conditions. However, PEMFCs using Pt-based catalysts as electrode materials were shown to be very sensitive to many fuel and air contaminants, which typically originate from natural as well as anthropogenic emission sources [1]. The knowledge of the electrochemical behavior of impurities and their intermediates on Pt is a principal requirement for understanding their impacts on the oxygen reduction reaction (ORR), fuel cell performance, mitigation and recovery procedures. AC-impedance is a powerful in-situ technique for determination and characterization of different processes in electrochemical systems. In this work, PEMFC electrode reactions occurring under exposure to various airborne contaminants (SO2, NO2, VOCs) were studied by spatial AC-impedance, which gives locally resolved data over the electrode area, while a single cell only provides average values of measured parameters. The experimental work was performed using a GRandalytics test station, a segmented cell system and a commercially available 100 cm2 membrane/electrode assembly (MEA) [2]. The MEA was operated under galvanostatic control of the total cell current. Many airborne impurities including inorganic and organic compounds (SO2, NO2, propylene, acetylene, acetonitrile, isopropanol, benzene, naphthalene, bromomethane, methyl methacrylate, etc) were studied. The electrochemical properties of the selected compounds are typically potential dependent. This should be taken into account when considering the ORR which occurs in parallel with contaminant chemical and electrochemical transformations. The effects of the studied pollutants on fuel cell impedance can be separated into three major groups: 1) the impurity does not significantly affect charge and mass transfer resistances, 2) the impurity increases charge and mass transfer resistances and, 3) the impurity leads to the appearance of a low frequency inductance. The first type of behavior was found when the cathode was poisoned by isopropanol and SO2 (at a steady state condition) (Fig. 1 a and c). Rotating ring/disk electrode (RRDE) data showed that isopropanol and SO2 (at a sulfur coverage below 0.3) do not significantly change the ORR mechanism on Pt, which proceeds via a 4-electron pathway [3, 4]. Contaminant adsorption on Pt decreases the electrochemical surface area (ECSA) and the exchange current density (i0) but it does not noticeably affect intrinsic Pt properties and ORR kinetics. AC-impedance is not sensitive to i0 variations at high current operation and it does not detect any changes. However, a decrease in ECSA and i0lead to a performance loss. An increase in impedance response was observed for the majority of the studied contaminants and it is connected with an increase in the Tafel slope, a decrease in the proton conductivity and possible diffusion limitations due to foreign species adsorption on Pt (Fig. 1 b). RRDE studies clearly demonstrated that these contaminants cause an increase in the Tafel slope for ORR and noticeable H2O2 production, indicating a shift in O2 reduction to combined 4- and 2-electron mechanisms [3]. This observation implies that the adsorbed contaminant not only decreases the ECSA but also alters the Pt intrinsic activity due to strong electronic interactions. The first and the second AC-impedance behavior groups are usually observed at potentials which are favorable to contaminant oxidation via electrochemical and chemical pathways. A low-frequency inductance in AC-impedance spectra indicates electrochemical reactions involving adsorbed contaminant species and proceeding with the formation of intermediates on the Pt surface. Moreover, such an inductance means that the current signal follows a perturbation with a phase delay due to the slow relaxation of adsorbate coverage compared to the ORR [5]. The inductive loop was detected for PEMFCs exposed to C2H2, SO2, CH3CN and naphthalene at potentials where the electroreduction of contaminants can occur (Fig. 1 c). A detailed discussion of AC-impedance data in connection to ORR mechanisms, contaminant reactions and poisoning propagation along the cathode channel length will be presented. In addition, anode and cathode poisoning processes will be compared. ACKNOWLEDGEMENTS The author gratefully acknowledges funding from ONR (N00014-12-1-0496) and DOE EERE (DE-EE0000467), and Hawaiian Electric Company support. The author thanks G. Randolf for system operation support and A. Kulikovsky for discussion. REFERENCES Y. Zhai, J. St-Pierre, M. Angelo, ECS Trans., 50(2), 635 (2012).T.V. Reshetenko, K. Artyushkova, J. St-Pierre, J. Power Sources, 342, 135 (2017).J. St-Pierre et al., in DOE Hydrogen and Fuel Cells Program - 2014 Annual Progress Report, DOE/GO-102014-4504, November 2014, pp. V-133-V-139.Y. Garsany, O. Baturina, K. Swider-Lyons, J. Electrochem. Soc., 154, B670 (2007).D.A. Harrington, B.E. Conway, Electrochim. Acta, 32, 1703 (1987) Figure 1

A Rotating Ring Disk Electrode Study to Detect the Acceleration Effect of SPS and MPS in the Presence of Chloride

A Rotating Ring Disk Electrode Study to Detect the Acceleration Effect of SPS and MPS in the Presence of Chloride

Oxygen Evolution at Manganite Perovskite Ruddlesden-Popper Type Particles: Trends of Activity on Structure, Valence and Covalence.

An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite Ca1−xPrxMnO3 and the first-order RP-system Ca2−xPrxMnO4 at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO6 network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the eg↑ pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.

A Rotating Ring Disk Electrode Study of the Stability in Different Electrolytes for Lithium-Oxygen Batteries

The electrolyte formula (especially, the composition of organic solvent) is well-known to influence the electrochemical performance of Li-O2 batteries. The electrolyte instability in the battery can be attributed to its reaction with the superoxide radical (O2 • −) produced at the battery’s cathode during discharge. In this work, we have employed the rotating ring disk electrode (RRDE) technique to study the stability of various organic solvents against O2 • − by the quantification of the electrolyte decomposition rate constant (k) for the reaction between various electrolytes and O2 • −. Moreover, RRDE transients are also applied to estimate O2 solubility and diffusion rates of O2 and O2 • − coefficients ( and ) at different electrolytes.

Optimization of Perovskite Oxide/Carbon Composites for Oxygen Reduction Reaction in Alkaline Media

Non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) present an opportunity to improve the industrial feasibility of low temperature fuel cells by replacing expensive precious metal catalysts. However, improvements in performance of NPMCs are needed before they can realize widespread commercial adoption. Since the catalyst is projected to be the largest single contributor to the overall cost of the fuel cell, this area has seen heavy research activity. Many NPMC chemistries have been studied for the ORR in alkaline media, with perovskite oxides (of the form ABO3) being of particular interest due to the large variety of possible compositions that form perovskite structures, and the ability to tune material properties through doping of the A and B sites.1 Optimization of the bulk and surface chemistry to maximize intrinsic catalytic activity is of major importance to drive the development of perovskite oxide NPMCs. Synthesis strategies that balance surface chemistry and surface area optimization are necessary to improve the active site density while maintaining high catalytic activity. In addition, formation of effective perovskite oxide/carbon composites is essential to catalyze the ORR by a 2 step, 4 electron (2 per step) mechanism.2 This work focuses on the development of high performance perovskite/carbon composites by tuning surface chemistry, surface area, and optimizing the ratio of perovskite oxide to carbon. Through an aerogel synthesis process, a high surface area Ca0.9La0.1Al0.1Mn0.9O3- δ perovskite oxide catalyst was produced. Calcination temperature was varied between 500 and 1000C to study the interplay between phase purity and the catalyst surface area. Rotating disk electrode (RDE) studies showed that the optimum balance between phase, purity, surface composition and surface area is obtained using calcination at 800C. Rotating ring disk electrode studies (RRDE) were used to evaluate the effect of perovskite oxide to carbon ratio with further investigations in membrane electrode assemblies (MEAs) also demonstrating a significant impact on the performance (Figure 1). Additional improvements in performance were realized by varying the catalyst loading and using carbon functionalized with nitrogen.