Rotating Disk Electrode Experiments Research Articles

Effect of oxidizing treatment on electrocatalytic activity of boron-doped amorphous carbon thin films

Boron-doped amorphous carbon (BDAC) thin films with a regular oxygen reduction reaction (ORR) catalytic activity were synthesized in a hot filament chemical vapor deposition device using a mixture of CH4 and H2 as a gas source and B2O3 as a boron source and then oxidized in air at 380–470 °C for 15–75 min. Scanning electron microscope, transmission electron microscope, Raman spectroscopy, X-ray photoelectron spectroscopy, and electrochemical tests were used to characterize the physical and electrochemical properties of the BDAC catalysts. It was concluded that the BDAC catalyst oxidized at 450 °C for 45 min showed the best ORR catalytic activity in alkaline medium. The oxygen reduction potential and the transfer electron number n, respectively, are − 0.286 V versus Ag/AgCl and 3.24 from the rotating disk electrode experiments. The treated carbon film has better methanol resistance and stability than the commercial Pt/C catalyst.

(Invited) Electrolyte Oxidation Limits the Life of Alkaline Membrane Water Electrolyzer

Alkaline anion exchange membrane (AEM) water electrolyzers paid significant attention due to the cost benefits over proton exchange membrane electrolyzers. The durability of AEM electrolyzers is one of the critical requirements for the commercially viable systems. We have investigated that the possible performance loss over time is related to the oxidation of polymeric binders used for AEM electrolyzers. In this presentation, the oxidative degradation of the phenyl group is reported using the commercial Pt/C, IrO2 and house-made perovskite oxide catalyst. Rotating disk electrode experiments exhibited that the oxidation of phenyl group on the oxygen evolution reaction (OER) catalysts is pronounced: The oxidation rate of commercial Pt/C and IrO2 catalysts was > 11% at 1.6 V vs. RHE over the 5-hour chronoamperometric test, while the house-made LA0.85Sr0.15CoO3 catalyst perovskite oxide catalyst exhibited 4.1%. Density functional theory calculations show that the adsorption of the phenyl on the perovskite oxide catalyst is significantly smaller, suggesting that the electrolyte oxidation is related to the adsorption properties of the phenyl group.

Carbon-supported Fe/Mn-based perovskite-type oxides boost oxygen reduction in bioelectrochemical systems

Bimetallic Fe-Mn oxides supported on graphene oxide (GO/FeMnO3) and carbon Vulcan (C/FeMnO3) were synthesized via a low-cost and facile chemical/thermal method. As revealed by SEM images, Fe-Mn oxide nanoparticles were homogeneously dispersed on the surface of the carbon nanostructure; the formation of a prevailing bimetallic FeMnO3 phase with perovskite structure was clearly indicated by X-ray diffraction and confirmed by X-ray photoelectron spectroscopy. Both composites, GO/FeMnO3 and C/FeMnO3, displayed good catalytic activity towards the oxygen reduction reaction (ORR) in neutral environment, as indicated by cyclic voltammetry, hydrodynamic voltammetry with rotating disk electrode experiments, and electrochemical impedance spectroscopy. In particular, higher porosity and more accessible surface area of C/FeMnO3 lead to a faster ORR as compared to that of GO/FeMnO3. When assembled at the cathode side of a single-chamber air-cathode microbial fuel cell (MFC), C/FeMnO3 showed higher performance than that obtained with a Pt-based MFC, taken as reference, in terms of voltage and power generation, The application of a stability protocol, developed to simulate MFC operating conditions, also demonstrated an excellent temperature-cycling resistance of C/FeMnO3, hence indicating its suitability to substitute Pt at MFC cathodes.

Modulating Metal‐Free and Non‐Enzymatic Electrocatalytic Activity of sp2 Carbons Towards H2O2 Reduction by a Facile and Low‐Temperature Electrochemical Approach

AbstractDesigning a self‐supported and binder‐free electrocatalyst for electrochemical detection of bio‐analytes is an intriguing area of research on electrochemical sensors. Herein, glassy carbon GC and graphene (Gr) were transformed to self‐supported and binder‐free electrocatalysts (N−GCE and N−Gr) for metal‐free and non‐enzymatic H2O2 detection by a simple electrochemical incorporation of nitrogen on their surfaces. The presence of nitrogen and its functional groups in N−GCE and N−Gr was confirmed by Raman, X‐ray photoelectron spectroscopy and cyclic voltammetry. We found that the nitrogen incorporated surfaces, N−GCE and N−Gr show enhanced electrocatalytic activity compared to the oxygen incorporated surfaces, O−GCE and O−Gr, and plain GCE and Gr. The apparent electron transfer rate constant, ka of N−GCE and N−Gr increased significantly after nitrogen incorporation. The N−GCE and N−Gr show enhanced electrochemical performance towards H2O2 in terms of sensitivity, selectivity, stability, and reusability. The rate determining step of H2O2 reduction at N−GCE and N−Gr was determined by rotating disc electrode experiments. Also, N−GCE and N−Gr are shown to be potential candidates for the detection of H2O2 in real samples like urine and milk. The hidden potential of the method reported is that it can be extended to design many novel electrocatalytic materials simply by selecting a proper allotrope of carbon from the wide list of carbon allotropes.

Effect of Environment Variables and Material Properties on the Limiting Current Density for Oxygen Reduction

For aircraft alloys that have received surface treatments, primers, and topcoats to control self-corrosion due to precipitate phases, high-strength fasteners made of stainless steel or titanium alloys are the most obvious point of attack due to galvanic mismatches between the materials. However, the electrolytes that form from atmospheric processes can differ widely in composition and thickness. Dissolved oxygen concentrations vary due to temperature and salinity changes in the electrolyte. Recent attempts to model atmospheric corrosion have suggested that, at high salt loadings on the surface, electrolytes can form that have water layer thicknesses that appear as bulk electrolytes in terms of oxygen transport1. Experiments using rotating disk electrode (RDE) measurements and polarization curve data,2 have suggested an upper bound of approximately 800 mm on the diffusion layer thickness for oxygen that can be expected to form in a thin film of water. At distances beyond this thickness, convective flow is expected to dominate oxygen transport. In this report, we compare the results of RDE experiments that simulate thin film behavior on different substrate materials in different electrolytes with polarization data obtained under actual electrolyte thin films equilibrated at different relative humidity values, and bulk polarization measurements, as shown in Figure 1, in order to determine the effects of ohmic drop on the measurements and isolate relevant kinetic parameters. Isolating these effects is important in order to accurately model the corrosion currents arising from galvanic couples in which the oxides at which the reduction reactions are occurring can be changing in response to the changes in the electrolyte as the environment changes. Figure 1. Bulk cathodic polarization curves in 0.6M NaCl at 25◦C for Pt, UNS S31600, and UNS R56400. This work was sponsored by the Office of Naval Research, ONR, under grant/contract number N0001418WX00826; the views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy or the U.S. government. References 1 Van den Steen, N., Simillion, H., Dolgikh, O., Terryn, H. & Deconinck, J. An integrated modeling approach for atmospheric corrosion in presence of a varying electrolyte film. Electrochimica Acta 187, 714-723, doi:10.1016/j.electacta.2015.11.010 (2016). 2 Liu, C., Srinivasan, J. & Kelly, R. G. Electrolyte Film Thickness Effects on the Cathodic Current Availability in a Galvanic Couple. J Electrochem Soc 164, C845-C855, doi:10.1149/2.1641713jes (2017). Figure 1

Nanoscale Engineering of Efficient Oxygen Reduction Electrocatalysts By Tailoring the Local Chemical Environment of Pt Surface Sites

The oxygen reduction reaction (ORR) is the major source of overpotential loss in low-temperature fuel cells. Expensive, Pt-based materials have been found to be the most effective catalysts, but exploration of alternatives has been hampered by stability constraints at the typical operating conditions of low pH and high potential. I will discuss our studies of elementary mechanism of ORR on various metal electrodes using kinetic and micro-kinetic analysis of reaction pathways and quantum chemical calculations. These studies allowed us to identify the elementary steps and molecular descriptors that govern the rate of ORR. Using these performance descriptors we have been able to identify families of Pt and Ag-based alloys that exhibit superior ORR performance is acid and base respectively. We have synthesized these alloys to demonstrate the superior ORR activity with rotating disk electrode experiments. We have also performed thorough structural characterization of the bulk and surface properties with a combination of cyclic voltammetry, x-ray diffraction, and electron microscopy with spatially resolved energy-dispersive x-ray spectroscopy and electron energy loss spectroscopy. Xin, H.; Linic, S., J. Chem. Phys. 2016 144 (23), 234704Van Cleve T, Moniri, S, Linic, S, 2016, ACS Catalysis 7 (1), 17-24Moniri S., Van Cleve T., Linic S., Journal of Catalysis 345, 1-10Holewinski and Linic. J. Electrochem. Soc. 159, (2012).Adam Holewinski, Juan-Carlos Idrobo, Suljo Linic. Nature Chemistry , 6, 828-834, 2014.

In-Depth Study of Zn Electrode Passivation in Alkaline Solutions for Zn Batteries

Recently, increasing demand for energy storage options has rekindled work on Zn based alkaline batteries, particularly Zn-Air battery systems. Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost, Zn is abundant in the earth’s crust and aqueous alkaline electrolytes are cheap and easy to work with, have a low toxicity and high chemical stability associated with them. To make Zn-air batteries viable for large scale use, issues associated with Zn electrode passivation need to be addressed that lead to a loss of performance during cycling. During the oxidation of Zn to discharge the battery, a passive layer of ZnO is formed on the electrode. This reduces the discharge current possible. Additionally, passivation is linked to electrode morphology changes between cycles, producing inconsistent performance and a reduction in capacity of the battery. Here we describe the current knowledge about Zn passivation and recent work done to better understand Zn passivation in alkaline solutions. Zn passivation is not well understood. There are multiple mechanism of passivation proposed and studies often have presented different results and conclusions despite using the same experimental conditions. Recently there have been calls in the literature for new in-depth studies that clear up the confusion present in the understanding of Zn passivation and dissolution processes in general[1, 2]. The Electrochemical Quartz Crystal Microbalance (EQCM) is a technique which can observe mass changes on an electrode at the order of 15 nanograms. The change in mass observed can be correlated to the charge passed at a given point during an experiment to give a value of mass change per mole of electron passed. This can then be used to determine the equivalent mass of the species being added to or removed to or from the electrode during an electrochemical reaction. EQCM experiments in KOH solutions of various concentrations and saturated to different levels with Zn have been conducted to observe the process by which the passivation layer forms on the surface and the process by which it is removed. Cyclic voltammogram (CV) experiments show that peaks associated with passivation removal give values of 8 to 16 grams per mole electron which corresponds well with the transition of ZnO or Zn(OH)2 to Zn metal on the surface. The transition of the passive layer to active Zn metal on the surface may contribute to electrode shape change. During the anodic dissolution part of the CV sweep, initially values near 32 grams per mole of electron are observed in the EQCM plots, corresponding to direct removal of Zn in a 2-electron reaction. The mass to charge ratios subsequently transition to values near 49 grams per mole electron which would indicate that ZnO or Zn(OH)2 is being formed and removed from the surface. This suggests that the formation of the passivating Zn species is potential dependent and is not solely precipitation of a film on the electrode surface. Linear sweep voltammetry and chronoamperometry experiments will be discussed as well as the impacts of additives to the solution on the electrochemistry of the electrode. Additionally, rotating disc electrode experiments were conducted using a Zn electrode in KOH solutions of various molarities saturated with Zn to understand the mass transport dependence of the passivation behavior. Changes in the location and height of peaks associated with passivation with rotation speed and sweep rate show a mass transport dependence on when passivation occurs but not on the removal of the passivation layer. Dissolution steps were conducted using Zn plate electrodes with different electrolyte concentrations and saturations of Zn at various potentials to observe major influencing factors in the passivation layer formation. These showed that passivation is heavily dependent on the amount of Zn in the solution as well as the potential applied. We conclude that the formation of the passive layer may depend on the ‘escape’ of the ZnO and Zn(OH)­2 species formed during dissolution from the electrode surface. This will be discussed in the context of the various proposed passivation mechanisms and will lead to some thoughts about improved electrode design. Acknowledgement We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). Bockelmann, M., et al., Electrochemical characterization and mathematical modeling of zinc passivation in alkaline solutions: A review. Electrochimica Acta, 2017Mainar, A., et al., Alkaline aqueous electrolytes for secondary zinc-air batteries: an overview. International Journal of Energy Research, 2016 Figure 1

Correlation of Chemical and Electrochemical Catalysis-Importance of Half Reactions: The Case of Catalytic Oxidation of Ferrous Sulfate by Molecular Oxygen

Catalytic oxidation of ferrous sulfate with molecular oxygen in 0.5 M H2SO4 has been studied on carbon supported platinum catalyst. Analysis of this catalytic chemical reaction as two distinct electrochemical half reactions proceeding on the catalyst surface is presented. Further, it is proposed that the catalytic chemical reaction can be studied based on mixed potential theory. Pertinent results for the proposal are presented in terms of conversions to ferric ions and the potential acquired by the electrode when immersed in an oxygen saturated solution of ferrous sulfate (mixed potential). The two individual half reactions have been studied separately to obtain key electrochemical parameters through rotating disk electrode experiments. A theoretical approach is also presented that would enable analysis of the catalytic chemical reaction through few electrochemical parameters. The results obtained clearly demonstrate that a greater understanding of various mechanistic aspects of a catalytic chemical reaction can be achieved if it is studied as two or more distinct electrochemical half reactions connected through an electron transfer mechanism.

Highly active platinum supported on Mo-doped titanium nanotubes suboxide (Pt/TNTS-Mo) electrocatalyst for oxygen reduction reaction in PEMFC

Abstract In this study, an innovative carbon-free electrocatalyst for oxygen reduction reaction was synthesized via deposition of platinum on titanium nanotube suboxide (trititanium pentoxide nanotubes, TNTS) obtained from titania doped with molybdenum (Pt/TNTS-Mo). The TNTS-Mo support was synthesized in autoclave, while the Pt/TNTS-Mo using the polyol method. The carbon-free support and the Pt-based catalyst were fully characterized via rotating disk electrode (RDE) technique and polymer exchange membrane fuel cell (PEMFC) station by preparing a membrane electrode assembly (MEA) with Nafion® 115, loaded with 0.35 mgPt cm−2 of Pt/TNTS-Mo at the cathode, and 0.35 mgPt cm−2 of commercial Pt/C (E-TEK) at the anode. In RDE experiments, the Pt/TNTS-Mo exhibited low overpotential and remarkable electroactivity toward oxygen reduction reaction (ORR: mass activity of 110.7 mA mgPt−1 at 0.9 V vs SHE). Moreover, the Pt/TNTS-Mo demonstrated excellent stability. Tests in a 25 cm2 single cell PEMFC resulted to maximum power density of 0.52 W cm−2.

On site formation of N-doped carbon nanofibers, an efficient electrocatalyst for fuel cell applications

The development of non-noble electrocatalysts for low temperature fuel cells is essential for a broader use of hydrogen technologies. In this study, nitrogen doped carbon nanofibers are obtained without post-treatments; the effect of the carbonization temperature on the physicochemical properties of the carbon nanofibers was studied by X-Ray diffraction, Raman spectroscopy, Nitrogen adsorption and X-ray photoelectron spectroscopy. The calcination temperature has a profound effect on the carbon nanofiber properties, which, influences their electrocatalytic activity towards the oxygen reduction reaction in alkaline media. Unlike previous studies, the formation of Nitrogen doped Carbon Nanofibers in a single step without post-treatments or special atmospheres simplify the production method. It was found that higher calcination temperature results in better graphitization process, along with a greater formation of meso and microporosities; moreover, higher calcination temperatures induces a higher amount of nitrogen as dopant in quaternary-N positions, promoting a higher catalytic activity towards the oxygen reduction reaction, analyzed by Cyclic voltammetry and Rotating disk electrode experiments.

Pt-Ni/WC Alloy Nanorods Arrays as ORR Catalyst for PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) among the other types of fuel cells technology are attractive power sources especially for electric vehicle applications. Even though there is ongoing progress and plausible prospects of PEMFCs, there are several challenges that need to be overcome to make them successfully and economically viable. One of the most important concerns in a PEMFC is the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) that requires an expensive platinum (Pt) catalyst with high weight loadings (~0.3 mg/cm2) to meet the power requirements, which is mostly responsible for the high cost of PEMFCs. Therefore, reduction in the major cost of the system can be achieved by increasing the ORR kinetics of platinum-based catalyst and by more efficient utilization of the Pt component. In addition, carbon support used in conventional Pt/C catalyst present problems including oxidation of carbon, formation of peroxide species causing degradation of polymeric membrane, and separation from the ionomer over time leading to loss of catalyst effectiveness. For this purpose, we developed vertically aligned nanorod arrays of tungsten carbide (WC) as a support and coated them with platinum-nickel (Pt-Ni) alloy shell. A glancing angle deposition (GLAD) technique was used to fabricate the WC nanorod arrays and the shell comprised of different atomic ratios of Pt:Ni prepared by a small angel deposition (SAD) technique for conformal thin film coating. We utilized a multi-source sputter deposition unit that is capable of performing GLAD and SAD in the same chamber. In our design, WC nanorods offer advantages including higher electron conductivity, thermal and chemically stability, compared to the conventional carbon support. In addition, GLAD WC nanorods are coated with SAD Pt-Ni thin film in the same deposition unit without breaking the vacuum allowing to minimize surface oxidation resulting in decreased contact resistance between Pt-Ni and WC. Moreover, Pt-Ni alloy shell can achieve superior ORR activity due to expected electronic and geometric effects. Pt-Ni/WC nanorods were deposited on glassy carbon electrodes and as well as on silicon substrates for evaluation of their electrocatalytic ORR activity and physical properties. Cyclic voltammetry and rotating disk electrode experiments were performed in a 0.1 M HClO4 electrolyte at room temperature to characterize the ORR activity and activity stability of Pt-Ni/WC nanorods. Scanning electron microscopy and X-ray diffraction methods were utilized to study morphology and crystallographic properties, respectively.

Building Thin Film Model Materials to Study Active Sites of PGM-Free Electrocatalyst for ORR

Replacement of noble metal catalysts for oxygen reduction reaction (ORR) with transition metals can have a profound impact on clean and economical energy production. Although non-precious metals have been extensively studied as ORR electrocatalysts, there has been a long debate about what active site is responsible for the ORR and what the catalytic mechanism is. The mystery of the chemical nature of the active sites hinders the development of efficient and stable ORR catalyst. The biggest obstacle to resolving the mechanism is the fact that the active sites are buried in the bulk, porous catalyst and the components in the real catalyst are complicated. Here, we propose a thin film model approach to study the active sites of FeN thin film, deposited on top of glassy carbon by magnetron sputtering. Rutherford backscattering, X-ray diffraction and Raman spectroscopy were carried out to characterize bulk films and to determine deposition conditions that result in Fe tetrahedrally coordinated with nitrogen. The significance of this study is that this is the first time active sites are intentionally exposed to the surface and can be directly characterized by x-ray photoelectron spectroscopy (XPS). Bonding between Fe-N in the bulk film and N-C at the interface between FeN and glassy carbon have been confirmed by XPS. Rotating disk electrode experiments have been carried out to measure the ORR activity of Fe-N, C-N, Fe-C the Fe-N-C exposed surfaces by looking at thin Fe-N, C-N, Fe-C, and Fe-N-C films. This study will also provide answers to the controversy about whether carbon, nitrogen or iron is essential in the ORR catalyst.

Noble metal-free Fe[sbnd]N-CNFs as an efficient electrocatalyst for oxygen reduction reaction

Carbonaceous materials containing non-precious metal atoms and doped with nitrogen have enthralled stunning attention in the field of electrochemical energy conversion systems. Herein, we demonstrated a facile method to fabricate iron and nitrogen doped carbon nanofiber (FeN-CNFs) catalyst material from ferric chloride and interfacial synthesized polyaniline (PANI) nanofibers, by carbonization process in an inert atmosphere at 800 °C. Further, synthesized material was characterized by elemental analysis and X-ray photoelectron spectroscopy (XPS) that confirms the presence of FeN bonds. The structural and morphological features are studied using various microscopy and spectroscopy techniques. The oxygen reduction reaction (ORR) activity of synthesized catalyst materials was examined by rotating disk electrode experiments in 0.1 M KOH. Among all these synthesized materials FeN-CNFs material showed enhanced ORR activity regarding current density and onset potential. Also, FeN-CNFs catalyst exhibited tolerance to methanol and durability in comparison to commercial Pt/C catalyst. The superior performance of FeN-CNFs may be attributed due to the introduction of Fe and formation of FeN bond in catalyst material.

Probing Transition-Metal Silicides as PGM-Free Catalysts for Hydrogen Oxidation and Evolution in Acidic Medium

In this experimental study, we investigate various transition-metal silicides as platinum-group-metal-(PGM)-free electrocatalysts for the hydrogen oxidation reaction (HOR), and for the hydrogen evolution reaction (HER) in acidic environment for the first time. Using cyclic voltammetry in 0.1 M HClO4, we first demonstrate that the tested materials exhibit sufficient stability against dissolution in the relevant potential window. Further, we determine the HOR and HER activities for Mo, W, Ta, Ni and Mo-Ni silicides in rotating disk electrode experiments. In conclusion, for the HOR only Ni2Si shows limited activity, and the HER activity of the investigated silicides is considerably lower compared to other PGM-free HER catalysts reported in the literature.

Two-Dimensional Nanoframes As Bifunctional Oxygen Electrodes for Unitized Regenerative Fuel Cells

Proton exchange membrane (PEM) fuel cells provide high efficiencies and clean power generation, however to enable wide scale adoption approaches are needed to reduce costs, improve durability, and provide on-demand hydrogen generation. Unitized regenerative fuel cells (URFCs) that combine power generation with fuel/oxidant generation within a single system can reduce costs and lower weight and volume compared with separate fuel cell and electrolyzer systems. Improved bifunctional oxygen electrodes (BOEs) that have lower overpotentials for oxygen reduction reaction (ORR) and oxygen evolution reactions (OER), higher stabilities for extended operating times, and lower costs are needed for URFCs. For extended durability, non-carbon-supported catalysts are required for BOEs since carbon oxidation occurs at high positive potentials leading to significant performance degradation. Our group has demonstrated metallic two-dimensional (2D) nanoframes can be derived from controlled temperature/atmosphere treatments of metal hydroxide nanosheets. Metallic Ni-Pt 2D nanoframes exhibit a unique porous nanoarchitecture that allows the use of unsupported (carbon-free) electrodes that have shown significantly higher ORR activities and stabilities compared with Pt/C. Incorporation of Ir and/or Ru in an integrated 2D NiPtxMy (M=Ru, Ir) nanoframe was investigated to obtain bifunctional (ORR and OER) electrocatalyst nanoarchitectures. Electrochemical analysis of unsupported metallic 2D nanoframes using rotating disc electrode experiments shows that incorporating Ni within the structure significantly improves both ORR and OER activities. Further, we have investigated using transition metal oxide-supported catalysts to reduce the precious metal loading while maintaining high activity and stability. The ability to control the structure and properties of 2D nanoframes provides a route to develop bifunctional electrocatalysts with high activity, extended durability, and lower cost.

An Approach Toward Replacing Vanadium: A Single Organic Molecule for the Anode and Cathode of an Aqueous Redox-Flow Battery.

By combining a viologen unit and a 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO) radical in one single combi‐molecule, an artificial bipolar redox‐active material, 1‐(4‐(((1‐oxyl‐2,2,6,6‐tetramethylpiperidin‐4‐yl)oxy)carbonyl)benzyl)‐1′‐methyl‐[4,4′‐bipyridine]‐1,1′‐diium‐chloride (VIOTEMP), was created that can serve as both the anode (−0.49 V) and cathode (0.67 V vs. Ag/AgCl) in a water‐based redox‐flow battery. While it mimics the redox states of flow battery metals like vanadium, the novel aqueous electrolyte does not require strongly acidic media and is best operated at pH 4. The electrochemical properties of VIOTEMP were investigated by using cyclic voltammetry, rotating disc electrode experiments, and spectroelectrochemical methods. A redox‐flow battery was built and the suitability of the material for both electrodes was demonstrated through a polarity‐inversion experiment. Thus, an organic aqueous electrolyte system being safe in case of cross contamination is presented.

Activity and durability of Pt-Ni nanocage electocatalysts in proton exchange membrane fuel cells

A Ni-rich Pt-Ni alloy was synthesized via a solvothermal method and transformed into platinum-nickel nanocage catalysts (PNCs) by applying a two-phase corrosion process. The catalysts were physically and electrochemically characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and both cyclic and linear sweep voltammetry using a rotating disk electrode (RDE). During the RDE testing, the half-wave potential of the PNC was 30mV higher compared to that of a commercial Pt/C catalyst for the oxygen reduction reaction (ORR). The RDE experiments showed that the specific and mass activity of the PNC was 2 and 4 times greater than Pt/C, respectively, at 0.9V. Catalyst-coated membranes (CCMs) were fabricated with the Pt-Ni nanocages using an automated air-assisted cylindrical liquid jet sprayer system. The PNC CCMs were loaded into proton exchange membrane fuel cells (PEMFC) for activity and stability testing. The CCMs showed no obvious Pt and Ni dissolution and redeposition in the membrane even after 30K cycles. The performance and electrochemically active surface area (ECSA) retention of the PNC was far superior to commercial Pt/C, just short of the US Department of Energy (DOE) 2020 targets, suggesting that Pt-alloy nanocages are very promising candidates for high-performing commercial PEMFCs.

Rde Kinetic Study of V(IV)/V(V) and V(II)/V(III) Reactions on Thermally Activated and Non-Activated Carbon Felts

Graphite felt is the most common electrode material used at both the cathode and the anode in vanadium redox flow batteries (VRFBs). Graphite felt exhibits good activity towards the redox reactions, has a wide operational potential range, and exhibits stability in acidic environments. A variety of studies have been conducted to investigate the effects of different felt pre-treatment methods for on VRFB performance (1, 2). Basically the pre-treatment methods can be categorized into oxidation and reduction of the graphite felts. Even though this field has been extensively studied by using different techniques, there is still some controversy. It is commonly accepted that chemical/electrochemical oxidation of graphite felts increases their activity towards V(IV)/V(V) reactions (3, 4). However, there are few studies reporting the opposite result (5, 6). It is therefore necessary to perform a fundamental study of the kinetics of vanadium species redox reactions on activated and non-activated graphite felts. The felt material (non-activated or activated) was ground using a mortar and mixed with epoxy resin to form a paste (33wt% of epoxy). The paste was pressed (8000 kg/cm2) in a stainless steel mold until it hardened, and the resultant electrode was employed as the disk in the rotating disk electrode experiments (RDE). To our best knowledge, this is the first RDE study of vanadium reactions on non-activated and activated graphite felts. The results and trends obtained in the RDE studies were verified by performing vanadium flow battery experiments. The presentation will discuss how the RDE disk was made, present the kinetic data of non-activated graphite felt and thermally activated (7) graphite felt for both V(IV)/V(V) and V(II)/V(III) redox reactions. The kinetic analysis will be accompanied with the analysis of the carbon felts by scanning electron microscopy, X-ray photoelectron spectroscopy, and charge/discharge curves in an all-vanadium redox flow battery.

Insulating Boron Nitride Nanosheet on Inert Gold Electrode As a Novel Efficient Electrocatalyst for Oxygen Reduction Reaction

Large overpotential for oxygen reduction reaction (ORR) is one of the most serious problems in the development of fuel cells. Although Pt based electrocatalysts are known to be most efficient, they have several problems such as high cost, less abundance, poor stability, and still sluggish kinetics. Many efforts have been made to find alternative catalysts and N- and B-doped carbon materials have been demonstrated to be effective metal free ORR catalysts and their ORR activity is further increased by co-doping of B and N atoms. If all carbon atoms in graphene are substituted by B and N atoms, hexagonal boron nitride (h-BN) monolayer, which has geometric structure similar to the graphene, as an extreme case, is obtained. Although BN is an insulator with a wide band gap (5.8eV), our recent theoretical studies showed that the band gap of h-BN monolayer can be considerably reduced by B- and N- vacancy and impurity defects as well as by interaction with metal substrate and BN on appropriate substrates can be used as an ORR catalyst.1-3 In the present study, electrocatalytic activity of various forms of BN on Au(111) for ORR is investigated both theoretically and experimentally. DFT calculations for BN/Au(111) predicted the possible ORR activity of BN/Au(111)4 and electrocatalytic activities of various types of BN, i.e., spin coated BN nanotube (BNNT), BN nanosheet (BNNS) and sputter deposited BN, on Au electrodes as well as those of BNNS modified glassy carbon (GC) and Pt electrodes for ORR were examined.4 ,5 The overpotential for ORR at Au electrode was reduced by ca. 100, ca. 270, and ca.150 mV by spin coating of the dispersion of BNNT and liquid exfoliated BNNS, and sputter deposition of BN, respectively. Oxygen is reduced only to H2O2 by 2-electron process as at Au electrode in all cases. The reason why the highest activity was obtained by the BNNS modification is attributed to the presence of B-and/or N-edge structures. If the edge plays important role for higher activity, one would expect higher ORR activity by increasing the BN – Au interaction. We have attempted to increase the interaction by depositing gold nanoparticles (AuNP) on BNNS, which is then placed on a Au electrode (Au-BNNS/Au). The decoration of BNNS with AuNP not only reduces the overpotential for ORR further by ca. 50 mV, but also opens a 4-electron reduction route to water.6 Both rotating disk electrode experiments with Koutecky-Levich analysis and rotating ring disk electrode measurements show that more than 50% of oxygen is reduced to water via 4-electron process at Au-BNNS/Au electrode while less than 20 and 10 % of oxygen are reduced to water at the BNNS/Au and bare Au electrodes, respectively. Theoretical analysis of free energy profiles for ORR at the BN monolayer with and without Au8 cluster placed on Au(111) show significant stabilization of adsorbed oxygen atom by the Au8 cluster, opening a 4-electron reduction pathway.6 1. A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, PCCP, 2013, 15, 2809. 2. A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, JPC C, 2013, 117, 21359. 3. A. Lyalin, A. Nakayama, K. Uosaki, and T. Taketsugu, Top. Cat., 2014, 57, 1032. 4. K. Uosaki, G. Elumalai, H. Noguchi, T. Masuda, A. Lyalin, A. Nakayama, and T. Taketsugu, J. Amer. Chem. Soc., 2014, 136, 6542. 5. G. Elumalai, H. Noguchi, and K. Uosaki, PCCP, 2014, 16, 13755. 6. G. Elumalai, H. Noguchi, A. Lyalin, T. Taketsugu, and K. Uosaki, Electrochem. Comm., 2016, 66, 53.

Highly active and selective nickel molybdenum catalysts for direct hydrazine fuel cell

Carbon-supported NiMo catalysts (NiMo/C) were synthesized and investigated for the electrooxidation of hydrazine. Their morphology and composition were determined with physicochemical characterizations (TEM-EDS, XPS and XRD) and further bridged to their electrocatalytic activity. The electrochemical performances measured in rotating disk electrode experiments reached 7.68±0.96 kAgmetal−1 for hydrazine electrooxidation at E=0.4V vs. RHE for the carbon-supported NiMo (9:1)/C. This value is amongst the highest reported in the literature. The large activity of the carbon-supported NiMo (9:1)/C catalyst was ascribed to the effect of a low but non-negligible (<15 at. %) molybdenum content, as molybdenum atoms stabilize the hydrazine N-N bond, thereby preventing the chemical decomposition of hydrazine into ammonia, and improving the catalyst selectivity towards the complete (and desired) hydrazine oxidation into N2 gas.