Electrode Measurements Research Articles

2H–MoS2 nanoflowers based high energy density solid state supercapacitor

Metal dichalcogenides nanostructures are observed as suitable materials for storage applications. Herein, we report three dimensional MoS2 nanoflowers with hexagonal (2H) phase for supercapacitor application. The 2H–MoS2 nanoflowers based supercapacitor electrodes exhibit a high specific capacitance of 382 Fg-1 at a current density of 1 Ag-1 in three electrodes measurements with 1 M Na2SO4 aqueous electrolyte. The symmetric solid state supercapacitor device with 2H–MoS2 nanoflowers electrodes and PVA-Na2SO4 electrolyte exhibits an outstanding cell performance with an energy density of 16.4 Wh kg−1 (measured at a constant current density of 0.2 A g−1) and an excellent capacitance retention of 97.5% after 4000 cycles. The outstanding performance of 2H–MoS2 nanoflowers based electrodes can be attributed to the 3D flower like architecture with large number of exposed edges of different petals and their wettability due to oxygen containing functional groups.

Single fibre electrode measurements – A versatile strategy for assessing the non-uniform kinetics at carbon felt electrodes

Redox flow batteries (RFBs) are a promising technology for the storage of intermittent renewable energy. Graphite and carbon felts electrodes are commonly used in RFBs as they typically exhibit good conductivity, high liquid permeability, high surface area and are relatively inexpensive. The electrochemical performance of carbon felts is commonly tested by calculating the energy efficiency for charge-discharge cycles of electrochemical cells containing the felt electrodes. This method whilst useful does not provide a direct measure of the kinetics of the redox reactions at the electrode surface. Furthermore, the standard electrochemical methods (e.g. cyclic voltammetry) commonly used to determine the electron transfer kinetics cannot be easily applied to carbon felt electrodes due to the complex mass transport behaviour within the porous felts. In this work we have shown that kinetics at carbon felts can be determined by performing cyclic voltammetry on single carbon fibre electrodes extracted from the carbon felt. This approach avoids the complex mass transport behaviour while also providing a method to determine the distribution of rate constants within a carbon felt electrode. By combining this measured rate constant distribution with the void size distribution of the carbon felt, the cyclic voltammetry of carbon felt electrodes can be successfully simulated over a wide range of sweep rates. Importantly, this approach highlights the necessity of accounting for the rate constant distribution within carbon felt electrodes.

Electrochemical oxidation of a complex mixture of phenolic compounds in the base media using PbO2-GNRs anodes

A novel anode based on PbO2-GNRs nanocomposites was synthesized in order to investigate the simultaneous electrochemical oxidation of phenolic compounds in a mixture of methyl and chloro substituted phenols in alkaline solution (pH = 8.3). The crystal structure, morphology and composition of the synthesized materials were characterized using X-ray powder diffraction (XRPD), field emission scanning electron microscopy with energy dispersive X-ray spectrometer (FESEM-EDX) and transmission electron microscopy (TEM). FESEM images confirmed the presence of PbO2 nanoparticles on the PbO2-GNRs composite. The cyclic voltammetry measurements of the PbO2-GNRs electrode in 0.1 M Na2SO4 electrolyte indicated the electrochemical oxidation of phenol via indirect mechanism by hydroxyl radicals. Electrolytic oxidation of phenolic compounds was performed under potentiostatic conditions at potentials 2.3 V and 3 V. Removal efficiency of phenolic compounds increased with increasing the time of electrolysis and applied potential, up to 78% for 300 min and potential of 3 V. GC–MS chromatograms confirmed that mechanism of phenolic compounds degradation is based on their dechlorination and demethylation followed by the process of aromatic ring degradation. Newly synthesized PbO2-GNRs as electrode material could be successfully implemented in wastewater treatment of phenolic compounds by the electrochemical oxidation process.

A systematic review of endovascular stent-electrode arrays, a minimally invasive approach to brain-machine interfaces.

The goal of this study was to systematically review the feasibility and safety of minimally invasive neurovascular approaches to brain-machine interfaces (BMIs). A systematic literature review was performed using the PubMed database for studies published between 1986 and 2019. All studies assessing endovascular neural interfaces were included. Additional studies were selected based on review of references of selected articles and review articles. Of the 53 total articles identified in the original literature search, 12 studies were ultimately selected. An additional 10 articles were included from other sources, resulting in a total of 22 studies included in this systematic review. This includes primarily preclinical studies comparing endovascular electrode recordings with subdural and epidural electrodes, as well as studies evaluating stent-electrode gauge and material type. In addition, several clinical studies are also included. Endovascular stent-electrode arrays provide a minimally invasive approach to BMIs. Stent-electrode placement has been shown to be both efficacious and safe, although further data are necessary to draw comparisons between subdural and epidural electrode measurements given the heterogeneity of the studies included. Greater access to deep-seated brain regions is now more feasible with stent-electrode arrays; however, further validation is needed in large clinical trials to optimize this neural interface. This includes the determination of ideal electrode material type, venous versus arterial approaches, the feasibility of deep brain stimulation, and more streamlined computational decoding techniques.

Electrochemically Generated Interfacial pH Change: Application to Signal‐Triggered Molecule Release

AbstractElectron transfer processes during redox reactions are frequently accompanied with protonation/deprotonation processes, thus changing H+/OH− concentrations. When the redox reactions proceed at electrode surfaces, being electrochemically processed, changes in local interfacial pH are possible, particularly when the electrolyte solution is not strongly buffered. The pH gradient can be produced in the diffusion layer and its thickness depends on the rate of electrochemical process, which is measured as the current density, diffusion rates, and buffer capacity. While conventional techniques for measuring pH values are not applicable to a very thin diffusional layer, special methods have been developed for the interfacial pH measurements, including scanning electrochemical microscopy (SECM), rotating ring‐disk electrode (RRDE) measurements, various optical methods, particularly using confocal fluorescent microscopy, and others. Dramatic pH changes proceeding in a near‐electrode layer have been reported for H2O reduction/oxidation, O2 reduction, and some other electrochemical electron/proton transfer processes. These pH changes can be used to trigger some other physical processes, particularly (bio)molecule release processes from modified electrodes, which can be destabilized upon the electrochemically generated pH changes, thus releasing entrapped/loaded target molecules. This review‐type article overviews the formation and analysis of locally produced interfacial pH changes and their use for electrochemically triggered (bio)molecule release. The paper briefly reviews the research area, then concentrating on the systems designed recently by the authors.

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

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

Laser-scribed graphene (LSG) as new electrode material for impedance-based cellular assays

Impedance-based monitoring of cell-based assays has evolved to a multi-functional tool in fundamental and applied biomedical research. The majority of studies rely on gold-films as the electrode material which serves as growth surface and electrode at a time. Besides all its favorable properties like inertness, biocompatibility and superior electrochemical characteristics, gold-film electrodes in contact to cell culture medium show a capacitance of the electrode/electrolyte interface that may become limiting for the sensitivity of the readout. This study describes the use of laser-scribed graphene (LSG) as an alternative electrode material in impedance-based cell monitoring. LSG electrodes are prepared from commercial polyimide foils by simple CO2-laser-induced carbonization. The resulting electrodes show a 25times larger interface capacitance than standard gold-film electrodes due to their foam-like surface topography. Furthermore, LSG-electrodes are (i) highly compatible with cell attachment, (ii) easy to prepare in customized geometries from μm to cm with tunable surface topography and (iii) accessible by roll-to-roll production lines. We conclude from time- and frequency dependent impedance measurements of cell-covered LSG electrodes in direct comparison to the reference material gold that LSG provides an enormous potential to improve the sensitivity of impedance-based monitoring and electric-field mediated manipulation of adherent cells.

Validation of 1D porous electrode theory using steady-State measurements of flooded electrodes at variable electrolyte compositions

Steady-state measurements were performed using flow-through cells and electrolyte solutions spanning a wide range of concentrations of ferrous/ferric ions as active species and supporting salts (hydrochloric acid, sodium chloride, cesium chloride). Voltage losses obtained from steady-state polarization curves and impedance measurements were analyzed using Newman’s porous electrode model. The results showed that the model accurately predicts the electrode performance for commonly used concentrated electrolyte solutions but deviates substantially for low levels of supporting electrolyte (particularly for sodium chloride). Effects of mass transport and diffuse double layer effects are discussed to account for the nonlinear voltage losses.

Fe3O4@N-Doped Interconnected Hierarchical Porous Carbon and Its 3D Integrated Electrode for Oxygen Reduction in Acidic Media.

The rational design of electrode structure with catalysts adequately utilized is of vital importance for future fuel cells. Herein, a novel 3D oriented wholly integrated electrode comprising core–shell Fe3O4@N‐doped‐C (Fe3O4@NC) nanoparticles embedded into N‐doped ordered interconnected hierarchical porous carbon (denoted as Fe3O4@NC/NHPC) is developed for the oxygen reduction reaction (ORR). The as‐prepared catalyst possesses novel structure and efficient active sites. In rotating disk electrode measurements, the Fe3O4@NC/NHPC exhibits almost identical ORR electrocatalytic activity, superior durability, and much better methanol tolerance compared with the commercial Pt/C in acidic media. To the authors’ knowledge, this is among the best non‐precious‐metal ORR catalysts reported so far. Importantly, the Fe3O4@NC/NHPC is successfully in situ assembled onto carbon paper by the electrophoresis method to obtain a well‐designed 3D‐ordered electrode. With improved mass transfer and maximized active sites for ORR, the 3D‐oriented wholly integrated electrode shows superior performance to the one fabricated by the traditional method.

Solid-electrolyte interphase nucleation and growth on carbonaceous negative electrodes for Li-ion batteries visualized with in situ atomic force microscopy

Li-ion battery performance and life cycle strongly depend on a passivation layer called solid-electrolyte interphase (SEI). Its structure and composition are studied in great details, while its formation process remains elusive due to difficulty of in situ measurements of battery electrodes. Here we provide a facile methodology for in situ atomic force microscopy (AFM) measurements of SEI formation on cross-sectioned composite battery electrodes allowing for direct observations of SEI formation on various types of carbonaceous negative electrode materials for Li-ion batteries. Using this approach, we observed SEI nucleation and growth on highly oriented pyrolytic graphite (HOPG), MesoCarbon MicroBeads (MCMB) graphite, and non-graphitizable amorphous carbon (hard carbon). Besides the details of the formation mechanism, the electrical and mechanical properties of the SEI layers were assessed. The comparative observations revealed that the electrode potentials for SEI formation differ depending on the nature of the electrode material, whereas the adhesion of SEI to the electrode surface clearly correlates with the surface roughness of the electrode. Finally, the same approach applied to a positive LiNi1/3Mn1/3Co1/3O2 electrode did not reveal any signature of cathodic SEI thus demonstrating fundamental differences in the stabilization mechanisms of the negative and positive electrodes in Li-ion batteries.

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

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

(Invited) Electrocatalytic Ammonia Oxidation

Much of the global effort toward producing solar fuels has involved splitting water to obtain molecular hydrogen. Unfortunately, hydrogen gas has a relatively low energy-per-volume, and extreme cryogenic temperature and pressure is needed to bring hydrogen to a condensed state. An interesting choice for a hydrogen storage medium that circumvents these setbacks is ammonia. Synthesis of ammonia (largely via the Haber-Bosch process) is already the second-largest chemical industrial process in the world, and as a result the infrastructure for ammonia storage and transport already exists. Ammonia can be easily liquefied under 10 bar of pressure which leads to a three-fold higher energy density than hydrogen gas in state-of-the-art carbon fiber cylinders. One significant bottleneck limiting the use of ammonia for hydrogen storage is the inability to efficiently extract hydrogen from ammonia on-demand.Electrolysis offers a convenient route to decompose ammonia into nitrogen and hydrogen, which potentially only requires 0.1 V. However, there is a lack of fundamental understanding of the reactions involved in the electrolysis. In this talk, the electrolysis of liquid ammonia with heterogeneous catalysts, as well as ammonia dissolved in non-aqueous solvents, using two and three electrode measurements, will be presented.1,2 The possible mechanisms of the anodic and cathodic reactions will be discussed, as well as the cause and magnitude of the related overpotentials which limit the efficiency. Further, while there have been multiple reports on heterogeneous catalysts for electrocatalytic oxidation of NH3, with limited success, homogenous molecular catalysts have been non-existent until now.3 A transition metal complex, [Ru(tpy)(dmabpy)NH3]2+ (tpy = 2,2′:6′,2′′-terpyridine, dmabpy = 4,4'-dimethylamino-2,2'-bipyridine), will be presented which shows the first example of electrocatalytic activity in presence of ammonia at room temperature and ambient pressure. Gas chromatography measurements reveals production of N2 and H2 in a 1:3 ratio following bulk electrolysis of ammonia. Absorption measurements following reaction of [Ru(tpy)(dmabpy)NH3]3+ with one equivalent of ammonia clearly show regeneration of [Ru(tpy)(dmabpy)NH3]2+ which provides evidence of redox disproportionation to produce a Ru(IV) imido species as an intermediate in formation of the N-N bond. Nitrogen isotope labeling experiments reveal incorporation of the coordinated amine with free ammonia in solution to produce dinitrogen. Results from cyclic voltammetry, stopped-flow, NMR and rotating ring-disc measurements will also be presented. From these combined results, a tentative mechanism will be presented and the implications discussed.1) Little, D.J., Smith, M.R., Hamann, T.W.; “Electrolysis of Liquid Ammonia for Hydrogen Generation” Energy & Environmental Science, 2015, 8, 2775 – 27812) Little, D.J., Edwards, D., Smith, M.R., Hamann, T.W.; “As Precious as Platinum: Iron Nitride for Electrocatalytic Oxidation of Liquid Ammonia” ACS Applied Materials & Interfaces 2017, 9 (19), 16228 – 162353) Habib-Zadeh, F., Miller, S.L., Hamann, T.W., Smith, M.R.; “Homogenous Electro-Catalytic Oxidation of Ammonia to N2 Under Mild Conditions” Proceedings of the National Academy of Science 2019, 116 (8) 2849 – 2853

Influence of Stress Distribution on the Ionic Conductivity of a Sulfide All-Solid-State Lithium-Ion Battery

Introduction, Objective, MethodsIt is widely known that the ionic conductivity of a sulfide based solid electrolyte (SE) is enhanced with pressurization[1]. In order to take advantage of this property, sulfide all-solid-state lithium-ion battery (SASSLiB) is expected to be pressurized. The electrodes of SASSLiB are composite material constituted by different substances such as SE and active material (AM). Hence, when pressurized, a non-uniform stress distribution forms within the electrodes due to the unique Young’s moduli of the constituting substances[2]. Considering the pressure dependence of ionic conductivity observed in SE, a non-uniform stress distribution is likely to affect the net ionic conductivity of the electrodes.Objective of this study is to elucidate the effect of stress distribution on the ion transportation characteristics of an electrode layer in SASSLiB by combining experiment and numerical calculation. The experiment includes internal visualization by x-ray CT and electrochemical measurement of a simplified electrode consisting SE (Li10GeP2S12)[3] and electrochemically stable zirconia. To separate the effect of stress distribution from morphological factors such as volumetric fraction and tortuosity, structural data collected from x-ray CT was used to quantify the volume of non-SE regions and used to quantify tortuosity by numerical calculation employing random-walk algorithm. Additionally, voxel-finite-element-stress calculation and electric field calculation was combined to perform ion transportation simulation. To elaborate, stress distribution was expressed in two different calculation models: isotropic and anisotropic. The isotropic model assumes that the ionic conductivity at a particular voxel is dependent on the minimum principal stress value in all x, y, z directions. The anisotropic model assumes that the ionic conduction occurs anisotropically with different ionic conductivity corresponding to the voxel’s perpendicular stresses in x, y, z direction. Furthermore, the ion transportation simulation was conducted for cases with SE’s Young’s modulus set to 0.3 GPa and 21.7 GPa. The former value is experimentally measured under low pressure condition, and the latter is the value obtained by first principle calculation[4]. We have assumed that the true value of unknown and pressure dependent Young’s modulus of SE lies in between these two values.Result and DiscussionThe ionic conductivity of a composite made by SE and zirconia was significantly lower than that of SE-only sample. Comparing the ionic conductivities obtained experimentally and numerically, the discrepancy was large when only void ratio and tortuosity were considered. When stress distribution was additionally considered, discrepancy drastically decreased. Fig.1 and Fig.2 show the magnitude of discrepancy between experimental and numerical results when composite was under 100 MPa of pressure, and are results obtained by isotropic model and anisotropic model respectively. The points plotted are relative conductivity with respect to a mixture with 0% zirconia, and the points represented are results from the composite with zirconia fraction varying from 19% to 67% from left to right. The dotted line represents a complete alignment of experimental and numerical results, and closer a value is to this line, smaller the discrepancy. It is apparent from the two figures that in both models that there is a significant offset between the line of complete alignment and the case stress distribution was not considered. On the other hand, the line of complete alignment lies in between the results from numerical models obtained with respective Young’s modulus. This validates our assumption that the true Young’s modulus lies in between the values mentioned earlier. Additionally, this implies that by accounting for the non-linearity of stress-strain relationship of SE, the discrepancy between the line of complete alignment and the numerical results can further be decreased.The discrepancy between the line of complete alignment and the results excluding stress distribution increases as the volumetric fraction of zirconia increases. Additionally, the discrepancy between the numerical results of two different Young’s moduli increases as zirconia fraction increases. These increasing discrepancies with the increased zirconia fraction can be attributed to the decreasing stress in SE, instigating decrease of ionic conductivity, and in turn, widening the discrepancy.ConclusionPresence of a material with different Young’s modulus than that of SE alters the ion transportation characteristics of an electrode layer by forming a non-uniform stress distribution. And the impact of such stress distribution is much greater as the volumetric ratio of materials with high Young’s modulus increase. Therefore, stress distribution is an important factor to consider to accurately evaluate the ion transportation characteristics in the electrode layers of SASSLiB, especially as volume of non-SE materials, such as AM, increases. Figure 1

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

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

The Oxygen Reduction Reaction Kinetics on a Platinum /Graphene Surface

The oxygen reduction reaction (ORR) is rate-limiting in polymer electrolyte fuel cells over a wide range of overpotentials. To improve our understanding of the underlying physics and chemistry of ORR, we created a model system with atomically-ordered interfaces that allows us to distinguish between bulk and electronic effects on catalytic behavior. Specifically, we developed a facile way of growing thin, epitaxial Pt nanostructures on graphene, which serves as both growth substrate and electrode. Rotating disc electrode measurements showed a high specific activity relative to platinum on glassy carbon. The enhanced ORR activity was observed despite consistent measurements that show a small electrochemically-active surface area arising from the fact that virtually all carbon atoms in monolayer graphene are surface atoms. This talk will address the reasons for the increase of ORR activity in view of the new ground state of atomically thin platinum on graphene.

Unifying Concepts in (Electro)Catalysis for Understanding the Direct Synthesis of H2O2

Hydrogen peroxide (H2O2) is a sustainable and selective oxidant that is currently produced via the costly anthraquinone process from H2 and O2 gas.1,2 On-site production of H2O2 for various industrial applications would benefit from the direct synthesis of H2O2 using alternative highly selective and highly active catalysts. Recently, the mechanism of H2O2 production on metal catalysts was suggested to occur via an electron transfer between H2 and O2 mediated by individually addressable electrocatalytic steps.3 Understanding the manifestations of this electrocatalytic paradigm on (electro)catalysts would help design more selective and active H2O2 catalysts and create new opportunities for designing heterogeneous catalysts using electrochemical principles. Here, we present an electrochemical model based on mixed potential theory for explaining and predicting the performance of H2O2 catalysts and its verification using high pressure electrochemistry, scanning electrochemical microscopy, rotating ring-disk electrode measurements, and matched electrochemical and heterogeneous catalysis reactor measurements. In our scheme, the H2 oxidation reaction (HOR) and the O2 reduction reaction (ORR) are treated as distinct half-cell reactions, allowing us to conveniently use electrochemical methods to determine reaction descriptors such as Tafel slopes and the number of electrons n transferred during the ORR. Several classes of catalysts were assessed towards their selectivity to form H2O2 via electrocatalysis and direct synthesis. Electrochemical kinetic parameters were used to obtain measures that relate the partial pressures of H2/O2 in direct synthesis to an equivalent operating potential. This operating potential was then used to predict the selectivity observed during thermochemical synthesis. We further generated guidelines to predict the characteristics of highly active catalysts.Our model accurately predicted aspects of heterogeneous catalysis starting from purely electrochemical concepts. With the ability to directly predict H2O2 selectivities from electrochemical measurements, it is possible to rapidly screen materials to discover new low-cost and selective direct synthesis catalysts for H2O2 and for other reactions of interest. We hope the results presented here will seed new approaches for the discovery of heterogeneous catalysts using readily accessible electrochemical measurements. 1. Goor, G.; et al., 2007. Hydrogen Peroxide. Ullmann's Encyclopedia of Industrial Chemistry.2. García-Serna, J.; et al. Green Chem. 2014, 16, 2320–2343.3. Wilson, N. M.; et al. J. Am. Chem. Soc. 2016, 138, 574-586.

Nitrogen-doped carbide-derived carbon/carbon nanotube composites as cathode catalysts for anion exchange membrane fuel cell application

Nitrogen-doped carbide-derived carbon/carbon nanotube (CDC/CNT) composites were prepared and employed for the first time as a cathode electrocatalyst for the anion exchange membrane fuel cell (AEMFC). The CDC/CNT composites were doped with nitrogen using high temperature pyrolysis in the presence of different nitrogen precursors. In the rotating disk electrode measurements all N-CDC/CNT catalysts showed good electrocatalytic activity for oxygen reduction reaction (ORR) in alkaline solution with the half-wave potential (E1/2) around –0.25 V vs SCE. Moreover, the materials showed high tolerance to methanol and excellent stability after 10,000 potential cycles with a negative shift in E1/2 of 10 and 14 mV, respectively. In AEMFC testing employing hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI) anion exchange membrane, N-CDC/CNT as a cathode catalyst exhibited very good performance with the peak power density of 310 mW cm–2. It can be concluded that the N-CDC/CNT composites are promising cathode catalysts for AEMFCs and alkaline direct methanol fuel cells.

Iridic oxide nanoparticles grown in situ on BCN nanotubes as highly efficient dual electrocatalyst for rechargeable lithium-O2 batteries

Abstract Oxygen cathode catalysts can significantly address the issues faced by Li-O2 battery. In this research, a composite of IrO2 nanoparticles grown in situ on BCN nanotubes (IrO2@BCNNTs) has been synthesized by facile hydrothermal method, which is initially fabricated as cathode catalyst for Li−O2 battery. The results indicate that IrO2@BCNNTs nanocomposite has a better effect on improving the actual discharge capacity, voltage gap and cyclability of Li−O2 battery. In addition, it is also demonstrated that the IrO2@BCNNTs composite exhibits bifunctional characteristics for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) through rotating disk electrode (RDE) measurements. The excellent performances of the synthesized catalyst may be attributed to the unique interconnected tubular structure and strong synergistic effect, which can provide more charged sites and defect sites and then facilitate reversible Li2O2 formation and decomposition. Therefore, it is promising for applying the rational design of the bifunctional catalyst to Li−O2 battery.

X-ray photoelectron spectroscopy and rotating disk electrode measurements of smooth sputtered Fe-N-C films

Electrocatalysts for the oxygen reduction reaction (ORR) based on complexes of iron and nitrogen in a carbon matrix (Fe-N-C) are a promising alternative to platinum group metal (PGM) based catalysts in polymer electrolyte membrane (PEM) fuel cells. Further improvements of Fe-N-C catalysts would benefit from model thin film studies of activity and stability of catalytic sites, but synthesis of Fe-N-C model thin films is challenging. Here we report on synthesis and characterization of Fe-N-C thin films produced by co-sputtering iron and carbon in a reactive nitrogen atmosphere onto removable glassy carbon rotating disk electrode (RDE) tips. Scanning electron microscopy (SEM) measurements indicate that the Fe-N-C films deposited at high temperature are smoother than the films annealed at high temperature. Electrocatalytic activity measured on the thin Fe-N-C films is greater for both high-temperature samples than for the room-temperature sample. From the analysis of X-ray photoelectron spectroscopy (XPS) data, exposure of the films to high temperatures results in increased graphitization of the carbon within the Fe-N-C films, and increased relative amount of graphitic and hydrogenated nitrogen species. Overall, the results of this study demonstrate the feasibility of a thin film model system approach for studying active sites in PGM-free catalysts.

Combinatorial Studies of Palladium-Based Oxygen Reduction Electrocatalysts for Alkaline Fuel Cells.

Hydrogen fuel cells have emerged as promising, potentially renewable energy-based, energy conversion technologies for powering electric vehicles. However, the sluggish oxygen reduction reaction (ORR) at the cathode has remained a longstanding challenge and requires the design of nonplatinum electrocatalysts with high activity and, ideally, low cost. Here, we present a combinatorial study of Pd-Cu thin-film electrodes with well-defined composition and structures, prepared by magnetron sputtering, as a fast method for assessing the ORR activity of binary alloys. This represents a facile catalyst screening method, using replaceable glassy carbon disk electrodes, which enables the rapid and reliable evaluation of ORR activity using standard rotating disk electrode (RDE) measurements. Among nine Pd-Cu alloys, Pd50Cu50 was identified as the most promising composition for the ORR and employed as a target for nanoparticle synthesis. The PdCu nanoparticles, supported on carbon, achieved a mass-specific and surface-specific activity, 3 and 2.5 times, respectively, as high asPd/Cin 1 M KOH. PdCu/C further exhibited an impressive durability with only 3 and 13 mV negative shifts in the half-wave potential after 20000 and 100000 potential cycles, respectively. The combinatorial approach guiding the nanoparticle synthesis, described herein, provides an optimized high-throughput screening method for other binary or ternary alloys as fuel cell electrocatalysts.