Rotating Ring Disk Electrode Research Articles

Oxygen Reduction Reaction Kinetics at the K-O2 Cell Cathode

The K-KO2 alkali metal-oxygen cell chemistry exhibits a very high theoretical specific energy of 935 Wh/kg1 . The free energy of formation of KO2 is -239.4 KJ mol-1 (compared to -570.8 KJ mol-1 for Li2O2 in a Li-O2 cell) and further, it is thermodynamically stable. However, these cells have been reported to exhibit a steep drop off in capacity following the first cycle1 with the reported state-of-the-art specific energy being 100 Wh/kgKO2 2. Reasons for the steep decline in capacity include: 1) lack of selectivity at the cathode (peroxide/carbonate/hydroxide formation in addition to the desired superoxide) resulting in lower reversibility1, and 2) reactions with the electrolyte solvent (ether/carbonate) to form undesirable byproducts (which then form insulating layers on the K anode3). The mitigation of the first issue requires a fundamental understanding of the oxygen reduction reaction (ORR) at the cathode of these cells. Herein, a rotating ring-disk electrode (RRDE) investigation of the K-O2 ORR is presented. The voltage window for the desired one electron reduction from K to KO2 was identified and a gap of 0.3V was measured between this reaction and the subsequent second electron transfer to produce K2O2, in line with the 0.28V difference in their E0 values. Subsequent cyclic voltammograms (CV) and linear sweep voltammograms (LSV) were performed within the KO2 voltage window (-0.9V to -1.3V vs. Ag/Ag+). Calculations of the number of electrons in this window using the Nicholson-Shain and Koutecky-Levich relationships on the CVs and LSVs respectively confirmed the formation of KO2 as the product in a one electron process. Nevertheless, changes in the iD/iR plots at more cathodic potentials, with the plots displaying typical one step and multi-step behaviors4 respectively at different potential regimes, indicated that the mechanism involved more than the single electron transfer step. It was hypothesized that given favorable binding orientation on the surface, chemical disproportionation of KO2 occurs to form K2O2. The RRDE data was fitted to kinetic models incorporating one -step and multi-step reactions in the appropriate voltage regions and the various elementary reaction rate constants were calculated. The rate constant for the first electron transfer reaction to form KO2 was found to be similar in both models, with a higher desorption rate for the KO2 at lower potentials (closer to the limiting region). The formation rate of K2O2 is predicted to increase closer to the limiting region as the surface coverage of KO2 increases. Thus, the chemical disproportionation reaction to form K2O2 is identified as yet another confounding factor in the practical realization of K-O2 cells and partially explains the observations regarding the inverse correlation between charging/discharging rate and the capacity of the cells2.

(Invited) Platinum Group Metal-Free Oxygen Reduction Electrocatalysts: Structure-to-Property Relationships and Design Directions

Platinum group metal-free (PGM-free) electrocatalysts draw much attention these days as low-temperature (polymer electrolyte) fuel cell technology is approaching its broad market introduction. Among several competing chemistries, catalysts based on metal-nitrogen-carbon (M-N-C) materials system are by far the most popular choice. UNM have developed and commercialized catalysts from this class obtained by pyrolysis of N-containing organic precursor (N and C source) and transition metal salt (Fe, Co, Mn or combination of more than one metal). The pyrolysis is carried after integration of the precursors with a sacrificial support (usually mono-dispersed silica powder) through ball-milling. After the pyrolysis, the silica support is removed by leaching with HF. The catalysts are usually re-pyrolyzed and a second acid wash is carried out to remove remaining of metallic, oxide or carbide particles from the nano-composite. The resulting catalysts show high graphitic content (exceeding 80%) and atomically dispersed transition metal (usually less than 1%). Both nitrogen and transition metal moieties are integrated into the carbonaceous structure as defects in graphene. Design for performance of this type of catalysts depends on an understanding the nature and of these transition metal and nitrogen sites and the ways they participate in the oxygen reduction reaction (ORR). The structure-to-property relationships in these materials will be discussed here through correlating of theirs observed electrocatalytic activity in Rotating Ring-Disk Electrode (RRDE) and performance in single Membrane Electrode Assembly (MEA) polymer electrolyte fuel cells with results of X-ray photoelectron (XPS), X-ray absorption (XAS) and Mössbauer spectroscopy charaterization. We have introduced Density Functional Theory (DFT) calculations of core level binding energy shifts as a powerful tool for interpretation of XPS spectra of such catalysts.1-3 A combination of XPS and first principles calculations of nitrogen-containing model electrocatalysts4-5 was further used to elucidate the chemical nature of the nitrogen defects. We will discuss here how hydrogenation and protonation of different nitrogen-containing defects play a major role in ORR activity of the PGM-free catalysts across different pH ranges. This paper will place a particular emphasis on elucidating the role of nitrogen- and Fe-N-containing in-plane and edge defects on the acid-base steps in ORR catalysis and thus regulation PGM-free catalysts reactivity and selectivity. The nature of Fe-N-containing in-plane and edge defects pre-determines their association with the micro-pores (edge defects) or with the extended graphene surface (in-plane defects). This sets the stage for a “first order” dependence of chemistry of the active site and the morphology of the carbonaceous material, which is the catalysts’ backbone (see the Figure for conceptual illustration). We have shown that these correlations are more pronounced when MEA performance data are being used. In this case, we observe a strong anti-correlation of the overall performance with the “stacking number” of the graphitic domains in these catalysts, which lead us to conclude that in-plane defects are the predominant contributors to catalytic activity in fuel cells.6 Further discussion will bring the importance of these two classes of active sites in different modes of MEA performance loss and will serve as a guide for both active and durable PGM-free Polymer Electrolyte Fuel Cell (PEMFC) cathode catalysts and catalyst layer design. K. Artyushkova, et al., Chem. Commun., 49 (2013) 2539-2541S. Kabir, et al., Phys. Chem. Chem. Phys., 17 (2015) 17785-17789S. Kabir, et al., Surf. Interface Anal., 48 (2016) 293-300I. Matanovic, et al., J. Phys. Chem. C, 120 (2016) 29225-29232K. Artyushkova, et al., J. Phys. Chem. C, 121 (2017) 2836–2843M. Workman, et al., ACS Energy Letters, 2 (2017) 1489-1493 Figure 1

N/S-Me (Fe, Ni) Doped Porous Carbon Derived from Metal–Organic Frameworks As Efficient Electrocatalysts for Oxygen Reduction Reactions

In the past few decades, polymer electrolyte membrane fuel cells (PEMFCs), as a kind of clean, efficient energy sources, which is considered to be the most promising solution to solve the increasingly serious energy crisis and reduce the environmental pollution, have experienced continuously increasing attention1. Oxygen reduction reaction (ORR) in fuel cells plays a decisive role in determination of performance, and electrocatalysts with high-performance ORR are essential for practical applications. However, the commercialization of PEMFCs has long been hindered by the utilized expensive Pt-based catalyst for cathodic ORR2 3. To solve the bottleneck, non-platinum-group-metal electrocatalysts with low cost, high activity and durability have been intensively pursued as alternatives to Pt for the ORR4. Recent years witnessed the dramatic interest in using nitrogen/sulfer-doped porous carbon as one of the promising ORR catalysts. Herein, we present N/S-Me (Fe, Ni) doped porous carbon derived from a metal–organic frameworks MOF-67 (ZIF-N/S-Me-PC) as highly efficient ORR electrocatalysts. The catalytic ORR activities of ZIF-N/S-Me-PC are studied by linear sweep voltammetry (LSV), employing rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques to quantitatively obtain the ORR kinetic constants and the catalysts' possible reaction mechanisms. In comparison to the commercial Pt/C (20%), the electrocatalyst ZIF-N/S-Me-PC exhibits a pronounced positive shift in Eonset. In addition, it also demonstrates excellent long-term stability and durability during the 5000-cycle continuing-oxygen-supply (COS) accelerating durability tests (ADTs). Catalytic ORR activities were thoroughly examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), low-temperature N2 adsorption and X-ray photoelectron spectroscopy (XPS). References 1 M.J. Wu, E.G. Zhang, Q.P. Guo, Y.Z. Wang, J.L. Qiao, K.X. Li, P.C. Pei, Appl. Energy, 175, 468 (2016). 2 Q.W. Tang, L.M. Wang, M.J. Wu, N.N. Xu, L. Jiang, J.L. Qiao, J. Power Sources, 365, 348 (2017). 3 M. Sahoo, S. Ramaprabhu, Energy, 119, 1075 (2017). 4 M.J. Wu, Q.W. Tang, F. Dong, Z.Y. Bai, L. Zhang, J.L. Qiao, J. Catal, 352 , 208 (2017).

Reduced-Graphene-Oxide-Based Hybrid Supports for Platinum Catalysts Active at Low Loadings during Oxygen Reduction

Hybrid systems composed of the reduced-graphene-oxide supported gold, iridium or bimetallic (AuIr) nanoparticles (at loadings typically below 5 µg cm-2) have been considered as active matrices for platinum catalysts utilized at low loadings (<20 µg cm-2) during the reduction of oxygen in mostly acid but also alkaline media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared both by the electrochemical and chemical reductionapproaches in which the pre-reduced Keggin-type phosphomolybdate heteropolyblue species act as the reducing agent for the HAuCl4 precursor. Polyoxmetallate (PMo12O40 3-) adsorbates or “capping ligands” tend to stabilize gold (iridium or bimetallic) nanoparticle deposits, facilitate their dispersion and attachment to carbon supports. Indeed, it is apparent from the independent diagnostic voltammetric experiments (in 0.5 mol dm-3 H2SO4) that heteropolymolybdates form readily stable adsorbates on nanostructures of gold, irridium and carbon (reduced graphene oxide and Vulcan). It is reasonable to expect that the polyoxometallate-assisted nucleation of gold has occurred in the proximity of oxygenated defects existing on carbon substrates. Under conditions of the electrochemical experiments performed in 0.1 mol dm-3 KOH) the phosphomolybdate adsorbates are removed from the interface because they undergo dissolution and decomposition in alkaline medium. High electrocatalytic activity of the reduced-graphene oxide-supported catalytic systems toward the reduction of oxygen is demonstrated using chronoamperometry and gas-diffusion electrode, in addition to the conventional and rotating ring-disk electrode (RDE) voltammetry. Among important issues is the presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). When using the reduced graphene oxide carriers for Au (Ir or AuIr) nanostructures together with Pt nanoparticles, the resulting catalytic systems have exhibited typically higher (certainly not lower) O2-reduction currents (relative to those recorded at conventional Vulcan-supported Pt at the same loading) in acid medium (0.5 M H2SO4). The RDE data are consistent with the lower formation of the hydrogen peroxide intermediate (<1% at potentials 0.6 V, or lower, vs RHE). Furthermore, the long-term durability of this family of catalysts should be appreciated.

Rotating Ring-Disk Electrode Study of the Electrochemical Dehalogenation of Iodinated Contrast Media

Iodinated contrast media (ICM) occur widely in the environment and are difficult to remove by conventional water treatment methods because of ICMs’ hydrophilicity and resistance to biodegradation. ICMs have also been shown to be precursors of toxic reaction products, notably iodine-containing trihalomethanes and haloacetic acids. This study examined the performance of an alternative method of ICM degradation, notably their electrochemical (EC) reduction. ICMs used in this study were exemplified by iopamidol and diatrizoate. The method of rotating ring-disc electrode (RRDE) was used to elucidate rates and mechanisms of the EC reactions of the selected ICMs. Experiments were carried at varying hydrodynamic conditions, concentrations of iopamidol, diatrizoate, natural organic matter (NOM) and model compounds (resorcinol, catechol, guaiacol) which were used to examine interactions between products of the EC reduction of ICMs and halogenation-active species. The data showed that iopamidol and diatrizoate were EC-reduced at potentials < -0.45 V vs. s.c.e. In the range of potentials -0.65 to -0.85 V their reduction was mass transfer-controlled. The presence of NOM and model compounds did not affect the EC reduction of iopamidol and diatrizoate but active iodine species formed as a result of the EC-induced transformations of these ICMs reacted readily with NOM and model compounds. These data provide more insight into the nature of generation of iodine-containing by-products in the case of reductive degradation of ICMs.

Nitrogen Doped Short-Length Carbon Nanofiber Supported Cobalt Oxides for Oxygen Reduction Reaction and Evolution Reaction Catalysts

High demand of energy, environmental problems, and energy crisis have brought great attentions in renewable and environment-friendly energy resources(1). Among various candidate energy sources, electrochemistry based energy storage and harvesting devices such as rechargeable batteries, supercapacitors, utilized regenerative fuel cells(URFCs), and solar cells have received great interests (1, 2). Particularly, URFCs which include fuel cells and water electrolyzers are getting a big spotlight in the renewable energy fields due to their high efficiency and clean properties(3, 4). The key technology for URFCs is to develop the cost-effective, durable, and high catalytic active catalysts for oxygen reduction(ORR) and evolution reactions(OER) in order to decrease the overpotential and improve efficiency of cells (5). Ideal bifunctional ORR and OER catalysts are Pt, RuO, and IrO for URFCs(6-8). However, precious metal-based catalysts are inadequate in terms of price competitiveness and limited durability to commercialize the URFCs(9). Recently, modification of cobalt oxides nanostructures with various dopants have reported to reduce the overpotential with high OER and ORR performances (10). Furthermore, nitrogen-doped graphene oxide is used to further enhance the electronic conductivity with durable mechanical property, because nitrogen sites can function as electrochemical active sites(11). In this work, several short-length N-doped carbon nanofibers(N-CNF) are introduced to maximize electrochemical active sites of catalysts. The shortened N-CNF catalysts are synthesized by the hydrothermal method. The physicochemical properties of the N-CNF-supported cobalt oxides nanostructures are investigated by various tools such as X-ray diffraction, scanning electron microscope, Brunauer Emmett Teller, and transmission electron microscope. To analyze electrochemical activities, rotating ring disk electrode system is used with an Hg/HgO, a platinum wire, and a 0.1M KOH solution for reference, counter, and electrolyte respectively. The electrochemical characteristics are measured by linear sweep voltammetry at a scan rate of 5 mVs-1 for OER (1.2V–1.7V) and ORR (0.2V–1.2V). To analyze the long-term stability for OER and ORR, the accelerated degradation tests are performed at a scan rate of 200 mVs-1. References Wang H-F, Tang C, Zhang Q. Template growth of nitrogen-doped mesoporous graphene on metal oxides and its use as a metal-free bifunctional electrocatalyst for oxygen reduction and evolution reactions. Catalysis Today. 2017. Kushwaha HS, Halder A, Thomas P, Vaish R. CaCu3Ti4O12: A Bifunctional Perovskite Electrocatalyst for Oxygen Evolution and Reduction Reaction in Alkaline Medium. Electrochimica Acta. 2017;252(Supplement C):532-40. Bau VM, Bo XJ, Guo LP. Nitrogen-doped cobalt nanoparticles/nitrogen-doped plate-like ordered mesoporous carbons composites as noble-metal free electrocatalysts for oxygen reduction reaction. J Energy Chem. 2017;26(1):63-71. Zhang X, Ding P, Sun Y, Li X, Li H, Guo J. CoMoS3.13 nanosheets grafted on B, N co-doped graphene nanotubes as bifunctional catalyst for efficient water electrolysis. Journal of Alloys and Compounds. 2018;731(Supplement C):403-10. Yang J, Fujigaya T, Nakashima N. Decorating unoxidized-carbon nanotubes with homogeneous Ni-Co spinel nanocrystals show superior performance for oxygen evolution/reduction reactions. Scientific Reports. 2017;7. Song W, Ren Z, Chen SY, Meng Y, Biswas S, Nandi P, et al. Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction. ACS Applied Materials and Interfaces. 2016;8(32):20802-13. Elumeeva K, Masa J, Tietz F, Yang F, Xia W, Muhler M, et al. A Simple Approach towards High-Performance Perovskite-Based Bifunctional Oxygen Electrocatalysts. ChemElectroChem. 2016;3(1):138-43. Lee DU, Park MG, Park HW, Seo MH, Ismayilov V, Ahmed R, et al. Highly active Co-doped LaMnO3 perovskite oxide and N-doped carbon nanotube hybrid bi-functional catalyst for rechargeable zinc-air batteries. Electrochem Commun. 2015;60:38-41. Zhao BT, Zhang L, Zhen DX, Yoo S, Ding Y, Chen DC, et al. A tailored double perovskite nanofiber catalyst enables ultrafast oxygen evolution. Nat Commun. 2017;8. So IS, Kim NI, Cho SH, Kim YR, Yoo J, Seo Y, et al. Enhancement of Bifunctional Activity of the Hybrid Catalyst of Hollow-Net Structure Co3O4 and Carbon Nanotubes. J Electrochem Soc. 2016;163(11):F3041-F50. Kim NI, Afzal RA, Choi SR, Lee SW, Ahn D, Bhattacharjee S, et al. Highly active and durable nitrogen doped-reduced graphene oxide/double perovskite bifunctional hybrid catalysts. Journal of Materials Chemistry A. 2017;5(25):13019-31.

(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

Influence of Polycation Composition on Electrochemical Film Formation.

The effect of polyelectrolyte composition on the electrodeposition onto platinum is investigated using a counterion switching approach. Film formation of preformed polyelectrolytes is triggered by oxidation of hexacyanoferrates(II) (ferrocyanide), leading to polyelectrolyte complexes, which are physically crosslinked by hexacyanoferrate(III) (ferricyanide) ions due to preferential ferricyanide/polycation interactions. In this study, the electrodeposition of three different linear polyelectrolytes, namely quaternized poly[2-(dimethylamino)ethyl methacrylate] (i.e., poly{[2-(methacryloyloxy)ethyl]trimethylammonium chloride}; PMOTAC), quaternized poly[2-(dimethylamino)ethyl acrylate] (i.e., poly{[2-(acryloyloxy)ethyl]trimethylammonium chloride}; POTAC), quaternized poly[N-(3-dimethylaminopropyl)methacrylamide] (i.e., poly{[3-(methacrylamido)propyl]trimethylammonium chloride}; PMAPTAC) and different statistical copolymers of these polyelectrolytes with N-(3-aminopropyl)methacrylamide (APMA), are studied. Hydrodynamic voltammetry utilizing a rotating ring disk electrode (RRDE) shows the highest deposition efficiency DE for PMOTAC over PMAPTAC and over POTAC. Increasing incorporation of APMA weakens the preferred interaction of the quaternized units with the hexacyanoferrate(III) ions. At a sufficient APMA content, electrodeposition can thus be prevented. Additional electrochemical quartz crystal microbalance measurements reveal the formation of rigid polyelectrolyte films being highly crosslinked by the hexacyanoferrate(III) ions. Results indicate a different degree of water incorporation into these polyelectrolyte films. Hence, by adjusting the polycation composition, film properties can be tuned, while different chemistries can be incorporated into these electrodeposited thin hydrogel films.

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.

Role of the Carbon Support on the Oxygen Reduction and Evolution Activities in LaNiO3 Composite Electrodes in Alkaline Solution

Metal–air batteries and fuel cells show a great deal of promise in advancing low-cost, high-energy-density charge storage solutions for sustainable energy applications. To improve the activities and stabilities of electrocatalysts for the critical oxygen reduction and evolution reactions (ORR and OER, respectively), a greater understanding is needed of the catalyst/carbon interactions and carbon stability. Herein, we report how LaNiO3 (LNO) supported on nitrogen-doped carbon nanotubes (N-CNT) made from a high-yield synthesis lowers the overpotential for both the OER and ORR markedly to enable a low bifunctional window of 0.81 V at only a 51 μg cm–2 mass loading. Furthermore, the addition of LNO to the N-CNTs improves the galvanostatic stability for the OER by almost 2 orders of magnitude. The nanoscale geometries of the perovskites and the CNTs enhance the number of metal–support and charge transfer interactions and thus the activity. We use rotating ring disk electrodes (RRDEs) combined with Tafel slope ...

Evaluation of H2O2 electrogeneration and decolorization of Orange II azo dye using tungsten oxide nanoparticle-modified carbon

In this paper, we report on the development of a new catalyst for H2O2 electrogeneration for the preparation of a gas diffusion electrode (GDE) for water treatment by electrochemical advanced oxidation processes (EAOPs). Catalysts based on tungsten oxide nanoparticles on Vulcan XC72 and Printex 6L carbons were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), contact angle measurements and oxygen reduction reaction (ORR) assays using a rotating ring-disk electrode (RRDE) system. The WO2.72/Vulcan XC72 (1% w/w of W) catalyst was used for the construction of a GDE due to its notable performance in H2O2 electrogeneration, its good current efficiency for H2O2 electrogeneration and its lower energy consumption. Thus, the WO2.72/Vulcan XC72 GDE cathode was combined with a Pt or boron-doped diamond (BDD) anode for the decolorization of 0.260 mM Orange II azo dye solutions by EAOPs. The decolorization efficiency was in the following sequence: photodegradation (PD) < Pt/anodic oxidation (AO) ≈ BDD/AO < BDD/electro-Fenton (EF) ≈ BDD/photoelectro-Fenton (PEF) < Pt/EF < Pt/PEF. The Orange II decolorization efficiency by Pt/PEF was equal to or higher than that reported in similar studies. Moreover, the results showed that an Orange II decolorization percentage of 77% in the PEF and EF processes with a Pt anode was due to the WO2.72/Vulcan XC72 cathode, showing that this catalyst is a promising candidate for application in EAOPs.

Investigating products of titanium dissolution in the presence of fluoride ions with dual dynamic voltammetry

&lt;p class="PaperAbstract"&gt;&lt;span lang="EN-US"&gt;Products of titanium dissolution in the active state are investigated in fluoride contain­ing 1 M H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; solutions. The novel method of dual dynamic voltammetry, applied to Ti disk/Pt ring rotating ring–disk electrodes, is utilized for the simultaneous detection of different dissolution products. Potential regions where certain products (primarily, Ti&lt;sup&gt;III&lt;/sup&gt; and H&lt;sub&gt;2&lt;/sub&gt;) are formed are identified by a 3D electrochemical map constructed based on the ring–disk electrode measurements. Besides dissolution in the form of Ti&lt;sup&gt;III&lt;/sup&gt; species and hydrogen evolution, the formation and prompt oxidation of Ti&lt;sup&gt;II&lt;/sup&gt; can also be presumed under the applied conditions.&lt;/span&gt;&lt;/p&gt;

High-performance Platinum-free oxygen reduction reaction and hydrogen oxidation reaction catalyst in polymer electrolyte membrane fuel cell

The integration of polymer electrolyte membrane fuel cell (PEMFC) stack into vehicles necessitates the replacement of high-priced platinum (Pt)-based electrocatalyst, which contributes to about 45% of the cost of the stack. The implementation of high-performance and durable Pt metal-free catalyst for both oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) could significantly enable large-scale commercialization of fuel cell–powered vehicles. Towards this goal, a simple, scalable, single-step synthesis method was adopted to develop palladium-cobalt alloy supported on nitrogen-doped reduced graphene oxide (Pd3Co/NG) nanocomposite. Rotating ring-disk electrode (RRDE) studies for the electrochemical activity towards ORR indicates that ORR proceeds via nearly four-electron mechanism. Besides, the mass activity of Pd3Co/NG shows an enhancement of 1.6 times compared to that of Pd/NG. The full fuel cell measurements were carried out using Pd3Co/NG at the anode, cathode in conjunction with Pt/C and simultaneously at both anode and cathode. A maximum power density of 68 mW/cm2 is accomplished from the simultaneous use of Pd3Co/NG as both anode and cathode electrocatalyst with individual loading of 0.5 mg/cm2 at 60 °C without any backpressure. To the best of our knowledge, the present study is the first of its kind of a fully non-Pt based PEM full cell.

Electrochemical reductive dehalogenation of iodine-containing contrast agent pharmaceuticals: Examination of reactions of diatrizoate and iopamidol using the method of rotating ring-disc electrode (RRDE)

AbstractThis study examined the electrochemical (EC) reduction of iodinated contrast media (ICM) exemplified by iopamidol and diatrizoate. The method of rotating ring-disc electrode (RRDE) was used to elucidate rates and mechanisms of the EC reactions of the selected ICMs. Experiments were carried at varying hydrodynamic conditions, concentrations of iopamidol, diatrizoate, natural organic matter (NOM) and model compounds (resorcinol, catechol, guaiacol) which were used to examine interactions between products of the EC reduction of ICMs and halogenation-active species. The data showed that iopamidol and diatrizoate were EC-reduced at potentials < −0.45 V vs. s.c.e. In the range of potentials −0.65 to −0.85 V their reduction was mass transfer-controlled. The presence of NOM and model compounds did not affect the EC reduction of iopamidol and diatrizoate but active iodine species formed as a result of the EC-induced transformations of these ICMs reacted readily with NOM and model compounds. These data provide more insight into the nature of generation of iodine-containing by-products in the case of reductive degradation of ICMs.

Monodisperse Pd nanoparticles assembled on reduced graphene oxide-Fe3O4 nanocomposites as electrocatalysts for borohydride fuel cells

5 nm palladium nanoparticles (Pd NPs) are synthesized and assembled on reduced graphene oxide-iron oxide nanocomposite (rGO-Fe3O4) to be used in oxygen reduction reaction (ORR) and borohydride oxidation reaction (BOR) studies in alkaline media. The structure and morphology of the resulting Pd/rGO-Fe3O4 hybrid material are evaluated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) analyses. The electrochemical behavior of Pd/rGO-Fe3O4 hybrid material for the ORR and BOR is investigated by voltammetry with rotating disk and rotating ring disk electrode and electrochemical impedance spectroscopy, enabling evaluation of the number of exchanged electrons, Tafel slope, exchange current density and activation energy. The results reveal that ORR at Pd/rGO-Fe3O4 proceeds as a 2-electron process with Tafel slope of 0.133 V dec−1, while BOR proceeds as a 5.6-electron process with Tafel slope of 0.350 V dec−1 and exchange current density of 1.38 mA cm−2. The BOR activation energy was found to be 12.4 kJ mol−1. Overall, this study demonstrates the good efficiency of Pd/rGO-Fe3O4 hybrid material for BOR.

Nitrogen-doped carbon nanoflower with superior ORR performance in both alkaline and acidic electrolyte and enhanced durability

In this work, three new types of nitrogen-doped carbon nanoflower (NCNF) are synthesized by a template method. The resultant carbon was extensively investigated with the transmission electron microscopy (TEM), X-ray diffraction (XRD), Nitrogen adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS), and electrochemical methods. Firstly, the carbon precursor, viz. pyrrole, aniline and phenanthroline, is found to yield a considerable effect on the morphology, which can be attributed to their dynamic motion in the template during the synthesis. All of the NCNF materials possess well-developed 3D pore structure and high specific surface area, which are beneficial for the mass transfer of the reactant and accessibility of the active sites toward oxygen reduction reaction (ORR). Secondly, the optimal catalyst NCNF-PHEN-900, which acquires the largest specific surface area (1039 m2 g−1), exhibits a superior ORR performance in both alkaline and acidic media. NCNF-PHEN-900 exceeds the commercial Pt/C with 30-mV halfwave potential (E1/2) surpassing in alkaline media and is comparable to Pt/C in acidic media (E1/2 = 0.76 V). Lastly, NCNF-PHEN-900 outperforms Pt/C on both durability and methanol tolerance in both alkaline and acidic media. And the rotating ring-disk electrode (RRDE) test indicates NCNF-PHEN-900 is highly selective to a four-electron transfer pathway in both alkaline (electron transfer number = 3.98) and acidic (electron transfer number = 3.97) media. This work sheds light on the importance of large specific surface area, well-developed pore structure and dopant configuration on electrocatalysis, offering a conjoint viewpoint of developing metal-free catalysts for the ORR.

A CeO2 modified phenylenediamine-based Fe/N/C with enhanced durability/stability as non-precious metal catalyst for oxygen reduction reaction

The poor durability/stability of non-precious metal catalysts (NPMCs) hampers their substitution for Pt-based catalysts. Here, phenylenediamine-based Fe/N/C catalysts modified with CeO2 were synthesized by a hydrothermal method to eliminate hydrogen peroxide (H2O2), thereby improving the durability/stability of the catalyst. Oxygen reduction reaction (ORR) and catalytic durability/stability were evaluated by linear sweep voltammetry and accelerated durability tests (ADTs). The results show that the yield of H2O2 evaluated by rotating ring disk electrode (RRDE) measurement decreased obviously when the CeO2 mass ratio in catalyst is optimal. The loss of current density at 0.8 V of catalyst with 6 wt% CeO2 and catalyst without CeO2 were 51.1% and 91.5%, respectively. The addition of CeO2 can improve the durability/stability of Fe/N/C catalysts obviously due to the elimination of H2O2 during ORR.

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.

The Electrochemistry of Copper Release from Stainless Steels and Its Role in Localized Corrosion

In this investigation, the role of copper in MnS dissolution was studied in a series of lab-made austenitic stainless steels (SS) with varying Cu content. The base composition of these samples was that of SS 303 and the Cu content was varied between 0.02 and 0.80 wt%. In potentiodynamic polarization experiments, it was found that Cu deposition passivated the MnS inclusions in all except the 0.02 wt% specimen. The critical potential for this passivation, from potentiostatic experiments, was found to be associated with the onset of metastable pitting. The “apparent” pitting potential in the specimen with 0.02 wt% Cu content was approximately 200 mV more negative than the other specimens. This apparent potential was attributed to MnS dissolution, due to a lack of Cu deposition/passivation, and not pitting. With respect to pit repassivation, at concentrations equal to and greater than 0.2 wt% Cu, repassivation potentials were on the order or +0.10 V SCE. In comparison, for the 0.02 wt% Cu specimen, the repassivation potential was less than the OCP (−0.125 V vs. Ag/AgCl) indicating Cu reduction inside the pit plays a role in the measured repassivation potential. Cu release was quantified using a rotating ring disk electrode. In these experiments, oxidation peaks for Cu(I) and Cu(II) were detected. In these experiments Cu was released at low potentials and low, passive, current densities but the resulting near surface concentrations of Cu(I) (0.02 mM) were insufficient to passivate MnS.