Rotating Disk Electrode Method Research Articles

Electrocatalysis of oxygen reduction on iron- and cobalt-containing nitrogen-doped carbon nanotubes in acid media

Iron- and cobalt-containing nitrogen-doped carbon catalysts based on multi-walled carbon nanotubes are prepared using pyrolysis of dicyandiamide and CoCl2 or FeCl3 at 800°C. The electroreduction of oxygen is studied in 0.5M H2SO4 solution by the rotating disk electrode method and the surface morphology and composition of the catalysts are characterised by scanning and transmission electron microscopies and X-ray photoelectron spectroscopy. The as-prepared catalysts show mediocre electrocatalytic activity for oxygen reduction reaction (ORR), which increases as a result of acid treatment and second pyrolysis. The electrocatalytic activity of Fe-containing catalyst towards the ORR surpasses that of Co-based material and it supports a 4-electron reduction of O2. Co-containing catalyst, in turn, shows higher stability. Both catalysts are highly methanol tolerant in acid media. The transition metal-containing N-doped carbon materials are promising cathode catalysts for low-temperature fuel cells.

Preparation of Pt/Carbon-Sphere As Cathode Catalyst of PEFC

Polymer electrolyte fuel cell (PEFC) is emerged as one of the candidates for automotive and residential power sources because has relatively small and lightweight and works under 100ºC. The electrocatalyst of PEFC consists of platinum nanoparticles, which size is several nanometers, and activated carbon as catalyst support. Catalyst support requires to two things at least; one is good electron conductivity, second one is high surface area in order to achieve dispersing of platinum particles. Activated carbon is cheap and meets two requirements needed as catalyst support. Therefore, activated carbon is widely used as catalyst support in PEFC. However, activated carbon is corroded under the start-up/shutdown conditions reaching the high potential region. Carbon corrosion leads to detachment and aggregation of platinum nanoparticles. These phenomena mean catalyst degradation, and are directly linked to decrease of cell performance. In addition, platinum utilization is not high in case of using activated carbon as catalyst support. This attributes that platinum nanoparticles is accommodated by complex pore structure of activated carbon. For realization of wide-spread commercialization of PEFC, inhibition of catalyst degradation and improvement of platinum utilization are necessary to build up the low cost system. Recently, graphitization carbon is focused as corrosive-resistant carbon support. Graphene, in particular, has high stability of thermodynamics and good electron conductivity. We succeeded already a preparation of carbon sphere composed from graphite oxide nanosheets (GOns) wall. GOns could reduce by the reductant, such as hydrazine and hydrogen, and is given electron conductivity by reduction treatments. Moreover, our carbon sphere is expected to decrease the substance diffusion resistance of oxygen and water, due to it has homogeneous size. In this study, platinum nanoparticles supported on carbon sphere, and its activity of oxygen reduction reaction and durability were evaluated. Graphite oxide was synthesized by typical Hummers method. GOns colloid solution was obtained by shaking GO with 0.01 wt% TBAOH aqueous solution. Preparation method of carbon sphere was briefly described below. Polymer spheres were dispersed in cationic polymer aqueous solution and washed with ultrapure water. After that, their polymer spheres were mixed with GOns colloid solution. The above procedure was repeated five times. Platinum supported carbon sphere was prepared by a traditional impregnation method. Platinum hexahydrate ethanol solution was added to a carbon sphere dispersed with ethanol. And then, this mixed solution was dried at 60ºC. A dried powder containing platinum complex was reduced at 200ºC under the vacuum conditions. Its morphology and crystalline structure were observed SEM and XRD, raman spectroscopy. Electrochemical measurement was conducted by rotating disk electrode method. A working electrode was prepared by thin film method. Briefly, test catalyst was dispersed in mixed solution of ultrapure water and isopropyl alcohol. And then catalyst dispersion solution was dropped 10 µL on the mirror-polished glassy carbon electrode and dried at 60°C. Carbon sphere as catalyst support was kept sphere shape after platinum deposition treatments. This platinum supported catalyst may form by stacked GOns wall and polymer sphere core. Because raman peaks attributed polymer sphere were not observed, but D- and G-band attributed GOns were observed. In diffraction peaks of XRD, clear peaks were observed at ~40° and ~45°. These diffraction peaks are attributed to Pt (111) and Pt (200) phases. On the other hand, diffraction peaks of polymer sphere were not observed. In addition, oxygen reduction activity and hydrogen adsorption/desorption current of platinum were also confirm by electrochemical measurement using rotating disk electrode method. Thus, we succeeded synthesis of platinum supported carbon sphere catalyst as new electrocatalyst. Its activity of oxygen reduction reaction and durability will be reported at the venue.

Accelerated Durability Tests for Pt-Nanoclusters Modified Carbide Derived Carbons Catalysts in Acidic Solution

Pt-nanoparticles activated porous carbon composite catalysts are widely used in PEMFC applications. The instability of these catalysts in harsh fuel cell working conditions is the main problem hindering the large-scale application of the fuel cell technology in commercial and energy recuperation systems [1]. Hierarchical microporous-mesoporous carbon supports C(Mo2C) were prepared from molybdenum carbide using high temperature chlorination method at different synthesis temperatures from 600 °C to 1000 °C. The Pt-nanoparticles were deposited onto the carbon support using the NaHB4method [2,3]. The X-ray diffraction, low temperature nitrogen sorption, thermogravimetric analysis (TGA) and high-resolution scanning electron microscopy (HRSEM) methods were used to characterize the catalysts materials. The medium size of the Pt-nanoparticles deposited was 4.5±0.6 nm. Corrosion of the Pt-C(Mo2C) catalysts were evaluated using an accelerated durability test (ADT) method (in oxygen (99.999%, AGA) saturated 0.5 M H2SO4 aqueous solution, from 0.58 V to 0.98 V vs SHE, at potential scan rate 50 mV s−1) [4]. The oxygen electroreduction reaction kinetics and the value of electrochemically active surface area (ECA) of the catalysts were determined after 10, 100, 1000, 3000, 10 000, 20 000 and 30 000 cycles. Electrochemical impedance spectra were obtained at 0.9 V vs SHE within ac frequencies from 0.01 to 10 000 Hz to determine the changes of the electrolyte resistance and capacitance values during the ADT. The results show that only 20% decrease in electrode capacitance and 15% increases in series resistance takes place during 30,000 potential cycles. The values of ECA were estimated in the solution saturated with nitrogen (99.999%, AGA) using the cyclic voltammetry at different potential scan rates from 30 to 1000 mV s−1. The rotating disc electrode method was used to evaluate the oxygen electroreduction kinetics in solution saturated with oxygen at rotation rate 1600 rev min−1 and at potential scan rate 20 mV s−1within the region of potentials from 1.03 V to 0.03 V vs SHE. Based on the cyclic voltammetry and rotating disk electrode data the main decrease in ECA took place during the first 1000 cycles and thereafter the ECA value stabilized. The half wave potential values depend weakly on the number of cycles (up to 30,000) measured. The content of Pt in the catalyst was determined before and after the ADT using the electrochemical dissolution of platinum from the electrode in 6 M HCl solution. The average weight percentage of Pt in different catalyst materials was 71.8±1.7. The content of Pt in the catalysts before the ADT was in a good agreement with the TGA data.

Oxygen Reduction on Graphene-Based Materials of Different Origin

Graphene, the two-dimensional carbon nanomaterial with interesting properties, is still under an intense focus of research. The traditional method to obtain a high-quality graphene is by mechanical exfoliation. However, it has several drawbacks. For example, it is very time consuming method in addition to the low productivity. To overcome those problems, various alternative methods for graphene synthesis have been developed. Currently, one of the most extensively used method to produce graphene is to first, exfoliate graphene oxide (GO) from graphite oxide by solution-based processes followed by reduction of GO by chemical, thermal or UV-assisted approaches in order to obtain a reduced GO (rGO), which should have similar properties to pristine graphene. Also, an electrochemical exfoliation of graphite for the production of graphene has attracted considerable interest due to its high productivity as well as simplicity.1 The use of graphene-based nanomaterials as electrocatalysts for oxygen reduction reaction (ORR) occurring at the cathode of fuel cells and metal-air batteries, has received a great attention.2 Numerous studies have been carried out to prepare graphene-based materials doped with heteroatoms or non-precious metals with improved electrocatalytic activity toward the ORR. However, comparative studies on the ORR on pristine graphene of different origin are still insufficient. Based on the literature, the electrochemical behavior of various graphene-based materials (e.g. CVD-grown graphene, rGO, etc.) toward the ORR depends on the origin of the graphene.3,4 The main purpose of this work is to study the effect of graphene-based materials of different origin on the ORR activity in alkaline medium. Herein, two commercially available graphene samples and pre-synthesized rGO were selected. rGO samples were prepared from GO which in turn was synthesized using two different approaches: modified Hummer´s method and electrochemical exfoliation. Since GO is the starting material of rGO, glassy carbon (GC) electrodes were coated with GO under exactly the same conditions as were prepared with commercially available graphene samples and rGO for comparison purpose. The materials were studied through various characterization techniques such as transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. In the literature, there are only a few reports in which OH‒ ionomer has been used instead of Nafion ionomer as a binder during the catalyst ink preparation. Therefore, it was of special interest to study the effect of OH‒ ionomer on the ORR process as well. For comparison purposes, the catalyst ink made from N,N-dimethylformamide (DMF) was also used. To evaluate the catalytic activity of these graphene-based materials for ORR in O2-saturated 0.1 M KOH solution, several techniques including linear sweep voltammetry and the rotating disk electrode method were applied. Since the rotating ring-disk electrode (RRDE) method gives additional information about intermediate peroxide species (OOH– in the alkaline solution) formed during the ORR process, hence, in some experiments, RRDE method was employed as well. Firstly, the results revealed that the ORR activity depended on the origin of the catalyst material. In more specific, the electrocatalytic behavior of commercially available graphene-based electrodes toward the ORR was higher than that of the rGO-modified GC electrode. In addition, the graphene-based electrodes prepared from the suspension in DMF showed better ORR activity than those made using the suspensions in 2-propanol containing OH‒ ionomer, referring to the inhibiting effect of the ionomer. However, the results also showed that the amount of OH‒ionomer in the catalyst layer should be optimized. Otherwise, the electrocatalytic activity of the catalyst material toward the ORR was significantly reduced. It was also clear that the electrochemical reduction of oxygen on all the studied materials followed a two-step ORR process with the formation of peroxide as an intermediate.

Development of Novel Hierarchically Microporous-Mesoporous Carbon and Carbon Nanospheres Based Materials and PEMFC Single Cells

Microporous-mesoporous carbon materials synthesized from molybdenum and tungsten carbides by high-temperature chlorination method were used as supports for catalysts activated with platinum nanoclusters. The characteristics obtained have been compared with those for carbon material synthesized from D-glucose by hydrothermal carbonization followed by additional chemical activation with CO2. The scanning electron microscopy, X-ray diffraction, Raman spectroscopy, high resolution transmission electron microscopy and low temperature N2 and Ar as well as CO2 adsorption methods were used to characterize the structure of the materials prepared. The materials exhibited high specific surface area and remarkable differences in their microporous-mesoporous structure. Platinum has been deposited onto the porous carbon materials in the form of nanoparticles or agglomerates of nanoparticles. All catalysts (studied by cyclic voltammetry and rotating disc electrode methods) showed high activity towards oxygen reduction and hydrogen adsorption from 0.1 M KOH and 0.5 M H2SO4 solutions.

Facile Synthesis of High Performance Platinum-Cerium Oxide Nanocatalysts for Methanol Oxidation

In this study, platinum-cerium oxide nanoparticles synthesised from CeNO3 and H2PtCl6 solutions in ethylene glycol using microwave heating method, exhibited high activity towards methanol electrooxidation. The materials were electrochemically characterised using cyclic voltammetry and rotating disk electrode methods, and physically characterised using X-Ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-Ray spectroscopy (SEM-EDX). The methodology of the synthesis of the materials was straightforward and resource efficient. SEM-EDX data indicated that the synthesised nanocatalysts have a very uniform dispersion on the catalyst support material, and a narrow particle size distribution. The synthesised nanocatalysts had a low overpotential towards methanol electrooxidation and high currents for methanol and carbon monoxide oxidation.

Composites of a Prussian Blue Analogue and Gelatin‐Derived Nitrogen‐Doped Carbon‐Supported Porous Spinel Oxides as Electrocatalysts for a Zn–Air Battery

To date, most studies have focused only on the interaction between oxygen and the catalyst, with the intention of minimizing the mass‐transfer resistance by using the rotating disk electrode (RDE) method, which is based on the forced‐convection theory. To begin with, in order to increase the reaction rate, the oxygen should be able to reach the active sites of the catalyst readily (mass transfer). Next, a moderate (i.e., not too strong or weak) interaction (kinetics) should be maintained between the oxygen molecules and the catalyst, in order to allow for better adsorption and desorption. Therefore, these two factors should be taken into consideration when designing electrocatalysts for oxygen reduction. Further, there is bound to be a demand for large‐scale metal‐air batteries in the future. With these goals in mind, in this study, a facile and scalable method is developed for fabricating metal‐air batteries based on the fact that the Prussian blue analogue Mn3[Co(CN)6]2•nH2O and gelatin‐coated Ketjenblack carbon thermally decompose at 400 °C in air (i.e., without requiring high‐temperature pyrolysis under inert conditions) to form porous spinel oxides and N‐doped carbon materials. The intrinsic kinetics characteristics and the overall performance of the resulting catalysts are evaluated using the RDE method and a Zn‐air full cell, respectively.

Re-examination of the Pt Particle Size Effect on the Oxygen Reduction Reaction for Ultrathin Uniform Pt/C Catalyst Layers without Influence from Nafion

The platinum ‘particle size effect’ on the oxygen reduction reaction (ORR) has been re-evaluated using commercial Pt/C catalysts (2–10nm Pt particle) and polycrystalline Pt (poly-Pt) in 0.1M HClO4 with a rotating disk electrode method. Nafion-free catalyst layers were employed to obtain specific activities (SA) that were not perturbed (suppressed) by sulfonate anion adsorption/blocking. By using ultrathin uniform catalyst layers, O2 diffusion limitation was minimized as confirmed from the high SAs of our supported catalysts that were comparable to unsupported sputtered Pt having controlled sizes. The specific activity (SA) steeply increased for the particle sizes in the range ∼2–10nm (0.8–1.8mA/cm2Pt at 0.9V vs. RHE) and plateaued over ∼10nm to 2.7mA/cm2Pt for bulk poly-Pt. On the basis of the activity trend for the range of particle sizes studied, it appears that the effect of carbon support on activity is negligible. The experimental results and the concomitant profile of SA vs. particle size was found to be in an agreement to a truncated octahedral particle model that assumes active terrace sites.

Coating Pt-Ni Octahedra with Ultrathin Pt Shells to Enhance the Durability without Compromising the Activity toward Oxygen Reduction.

We describe a new strategy to enhance the catalytic durability of Pt-Ni octahedral nanocrystals in the oxygen reduction reaction (ORR) by conformally depositing an ultrathin Pt shell on the surface. The Pt-Ni octahedra were synthesized according to a protocol reported previously and then employed directly as seeds for the conformal deposition of ultrathin Pt shells by introducing a Pt precursor dropwise at 200 °C. The amount of Pt precursor was adjusted relative to the number of Pt-Ni octahedra involved to obtain Pt-Ni@Pt1.5L octahedra of 12 nm in edge length for the systematic evaluation of their chemical stability and catalytic durability compared to Pt-Ni octahedra. Specifically, we compared the elemental compositions of the octahedra before and after treatment with acetic and sulfuric acids. We also examined their electrocatalytic stability toward the ORR through an accelerated durability test by using a rotating disk electrode method. Even after treatment with sulfuric acid for 24 h, the Pt-Ni@Pt1.5L octahedra maintained their original Ni content, whereas 11 % of the Ni was lost from the Pt-Ni octahedra. After 10 000 cycles of ORR, the mass activity of the Pt-Ni octahedra decreased by 75 %, whereas the Pt-Ni@Pt1.5L octahedra only showed a 25 % reduction.

Highly Active Transition Metal-Nitrogen Co-Doped Carbon Nanotube Based Cathode Catalyst for Anion Exchange Membrane Fuel Cells

Low cost and high durability are considered as the main challenges in mass scale commercialization of the low temperature fuel cells. Anion exchange membrane fuel cells (AEMFC’s) have attracted tremendous attention because of their promising characteristics to overcome some of the critical problems associated with proton exchange membrane fuel cells. The main advantages of AEMFC´s are possibility of using cheaper non-noble metal catalyst, lower crossover of cationic redox couples, facile reaction kinetics. In recent years, nitrogen-doping of carbon materials has been used to achieve active non-platinum catalyst.1,2 However it has been reported that the presence of transition metals such as Co or Fe can further enhances the electrocatalytic properties of N-doped carbons.3 The main goal of this study was to develop highly active transition metal-nitrogen co-doped catalyst as alternative cathode material to Pt/C in AMFC. A simple synthesis method was used to receive nitrogen doped multi-walled carbon nanotube supported CoCl2 catalyst material (Co/N-MWCNT). MWCNT treated in a concentrated HNO3-H2SO4 mixture were used as carbon support. MWCNTs were first dispersed in ethanol by sonication, after which dicyandiamide as the nitrogen source and Co salt were added along with a dispersing agent. The mixture was then heat-treated at 800 °C for 2 h in a tube furnace. Polished glassy carbon electrodes were modified with a dispersion containing the resulting materials and Tokuyama OH− ionomer AS-04. The rotating disk electrode (RDE) measurements were performed in 0.1 M KOH. The stability of the catalysts were also tested using RDE method. The structural and compositional properties of the catalyst material were defined by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The oxygen reduction reaction (ORR) has been studied on Co/N/MWCNT electrocatalyst modified GC electrodes in 0.1 M KOH using the RDE method. The fuel cell performance of Co/N/MWCNT cathode catalyst material was investigated by fabricating membrane-electrode assemblies (MEAs) using Tokuyama membrane (# A201). The RDE results indicated excellent electrocatalytic properties of Co/N-MWCNT material toward oxygen electroreduction in alkaline media. The comparative study of Co/N-MWCNT with commercial Pt/C catalyst showed fair catalytic activity of our non-platinum catalyst in 0.1M KOH and significant enhancement of the ORR was observed in comparison to unmodified MWCNTs. The number of electrons transferred per oxygen molecule (n) at different potentials for the Co/N-MWCNT modified GC electrode was calculated from the KouteckyLevich equation. The value of n for the Co/N-MWCNT material was 4 over the whole range of potentials studied. The fuel cell performance was investigated with Co/N-MWCNT cathode catalyst and Tokuyama A201 anion exchange membrane. AEMFC performance was evaluated and compared at different temperatures up to 50 ⁰C (Fig.1). Maximum power density with H2/O2 gases at ambient pressure was ~115 mWcm-2.

(Invited) Efficient Electrocatalysis of Oxygen Reduction with Metallo-Corroles

The most active non-precious metal catalysts (NPMCs) for oxygen reduction (ORR) to date are the pyrolyzed catalysts, inspired from Heme-like complexes. These are usually composed of iron and/or cobalt coordinated by nitrogens, claimed to resemble the structure of porphyrins and phthalocynines, on the surface of a carbon support. Unfortunately, the exact structure of the catalytic sites remains a mystery and the solution for this conundrum seems be almost impossible. The advantages of using such catalysts are: (1) their high activity and (2) low price, which is derived from the cost of their precursors. The disadvantages are: (1) low durability compared to precious metal catalysts, and (2) their unknown structure, which limits their further improvement in order to obtain both the activity and durability benchmarks needed to become good alternatives for precious metals. One of the most promising options to resolve these issues is to find non-pyrolyzed molecular non-precious metal catalysts for ORR that could be tuned to match the necessary requirements. Recently, Metallo-corroles, a relatively new family of molecular catalysts was reported to have very good potential as non-precious metal catalysts for ORR. The electro-reduction of oxygen was investigated, using a series of first row transition metal β-pyrrole-brominated 5,10,15-tris(pentafluorophenyl)-corroles [M(tpfcBr8), M=Mn, Fe, Co, Ni and Cu] as catalysts in alkaline solutions. The kinetics of the oxygen reduction reaction (ORR) on these catalysts was evaluated using the rotating disk electrode method in a solution of 0.1M KOH. These corroles were adsorbed on a high surface area carbon powder (BP2000) prior to electrochemical measurements, to create a unique composite material. The comparison between the corroles with different metal centers showed a favorable catalytic performance of the ORR in the case of Fe and Co Corroles. The mechanism by which these corroles catalyze the ORR was studied by the rotating ring disk electrode (RRDE) technique, showing that in the case of alkaline solutions, there is a favorable 4 electron transfer mechanism. When comparing the performance of these catalysts in alkaline solutions, to their performance in acidic media, it was clear that the former was substantially higher, both in the onset potentials of the ORR and in the overall kinetic parameters.

(Invited) Limitations in Electrochemically Active Biofilms

Almost a decade has passed since the rediscovery of microbial fuel cell research, yet the power generation of these devices has not advanced. This is mainly due to the research being focused solely on the improvement of power generation rather than on a fundamental understanding of electron transfer processes and their limitations in the biofilms grown on the anode and cathode. The chemical and electrochemical gradients in these biofilms play a critical role in electron transfer rates between cells and a solid electron acceptor or donor. Our research group has developed and used novel microelectrodes and in situ Nuclear Magnetic Resonance (NMR) imaging techniques to investigate electron transfer mechanisms in biofilms. The newly developed microelectrodes were successfully operated on polarized surfaces. Both NMR and microelectrode technologies allowed us to quantify metabolic and chemical variations in biofilms while the cells respired on polarized electrode surfaces. We combined our techniques with two new techniques – rotating disk electrode (RDE) and electrochemical quartz crystal balance (eQCM). The addition of both these techniques allowed us to differentiate mass transfer limitations from intra-biofilm electron transfer processes. When we attempted to identify limitations in anodic biofilms, first, we addressed the importance of local acetate concentrations within G. sulfurreducens biofilms using a novel acetate microelectrode which has better resolution than NMR. The microelectrode work confirmed our previous NMR findings and generated profiles on the order of µM. We should note that the microelectrode method developed is significantly more economical to use than NMR and can facilitate rapid in situ measurements of acetate in anoxic biofilms. Second, we integrated our previous RDE method with our eQCM methods to show that, on both a per current basis and a per mass basis, biofilm capacitance is a better indicator of biofilm performance than conductance.In particular, we compared the capacitance/conductance relationship of G. sulfurreducens biofilms to those of both polyaniline films and diffusing flavin systems. The results indicated that the tandem increase in capacitance and conductance is unique to the extracellular electron transfer mechanisms operating in G. sulfurreducens biofilms. Our local potential and current measurements indicated that the upper layers of the biofilms are always under a reducing condition. This knowledge also confirmed that the amount of cytochromes in the biofilm is not high enough or the biofilm conductivity is not sufficient to generate an oxidizing condition near the top of a biofilm. Finally, we extended the role of excess surface area in the scale-up of bioelectrochemical systems by examining biomass accumulation under this condition. We found that biomass continues to accumulate on the excess surface area electrodes with net arithmetic growth rates and acetate consumption during steady-state current. The linear growth rates constrained by ion transport limitations that we found may be important factors in understanding the current generation using high-surface-area 3D electrodes with advection. In conclusion, we demonstrated the importance of the metabolic inactivity of layers within G. sulfurreducens biofilms using microelectrodes and NMR. In addition, we verified these findings using electrochemical impedance spectroscopy (EIS) by demonstrating the pseudocapacitive behavior of biofilm impedance. We also demonstrated the feasibility of directly measuring biofilm inefficiencies via intra-biofilm electron transfer rates and local current depth profiles.

Unprecedented dependence of the oxygen reduction activity on Co content at Pt Skin/Pt–Co(111) single crystal electrodes

The kinetically controlled area-specific activities, jk, for the oxygen reduction reaction (ORR) were evaluated at well-defined Pt-skin/Pt100−xCox(111) single crystal electrodes as a function of Co content by use of the rotating disk electrode method. The Pt-skin layer formed by annealing under a pure H2 atmosphere was clarified to consist of atomically flat terraces with the same atomic arrangement as the underlying crystal. At 25 atom%-Co, the jk value reached a maximum as high as 3.0mAcm−2 at 0.9V vs. RHE, which is ca. 25 times larger than that on pure Pt(111). Such an enhanced ORR activity of Pt-skin/Pt–Co(111) electrodes cannot be explained on the basis of decreased OH poisoning, because the surface oxidation charge densities measured at 0.9V in N2-purged solution were nearly constant.

Relationships between Atomic Level Surface Structure and Stability/Activity of Platinum Surface Atoms in Aqueous Environments

The development of alternative energy systems for the clean production, storage, and conversion of energy is strongly dependent on our ability to understand, at atomic molecular levels, the functional links between the activity and stability of electrochemical interfaces. Whereas structure–activity relationships are rapidly evolving, the corresponding structure–stability relationships are still missing. This is primarily because there is no adequate experimental approach capable of monitoring the stability of well-defined single crystals in situ. Here, by utilizing the power of inductively coupled plasma mass spectrometry (ICP-MS) connected to a stationary probe and coupling this technique to the rotating disk electrode method, it was possible to simultaneously measure the dissolution rates of surface atoms (as low as 0.4 pg cm–2 s–1) and correlate them with the kinetic rates of electrochemical reactions in real time. Making use of this unique probe, it was possible to establish almost “atom by atom” stru...

Oxygen reduction reaction on carbon-supported palladium nanocubes in alkaline media

Carbon-supported Pd nanocubes with the size of 30, 10 and 7nm were prepared and their electrocatalytic activity towards the oxygen reduction reaction (ORR) in alkaline solution was studied. For comparison carbon-supported spherical Pd nanoparticles and commercial Pd/C catalyst were used. The catalysts were characterised by transmission electron microscopy, electro-oxidation of carbon monoxide and cyclic voltammetry and the ORR activity was evaluated using the rotating disk electrode method. The ORR on all studied Pd/C catalysts proceeded via four-electron pathway where the rate-limiting step was the transfer of the first electron to O2 molecule. The specific activity of Pd nanocubes was more than two times higher than that of spherical Pd nanoparticles and increased with increasing the particle size.

Synthesis of PtM (M=Co, Ni)/Reduced Graphene Oxide Nanocomposites as Electrocatalysts for the Oxygen Reduction Reaction.

A series of PtM (M=Co, Ni)/reduced graphene oxide (rG-O) nanocomposites were successfully synthesized through a facile hydrothermal method. The as-synthesized nanocomposites were characterized using transmission electron microscopy and high-resolution transmission electron microscopy, X-ray diffraction, inductively coupled plasma-atomic emission spectrometer, and X-ray photoelectron spectroscopy. The electrochemical performance and oxygen reduction reaction (ORR) activity of PtM/rG-O nanocomposites were evaluated using cyclic voltammetry and the rotating disk electrode method. The results show that the addition of the reductant (1,2-hexadecanediol, HAD) in the reaction system slightly improved the ORR activity of PtM/rG-O nanocomposites with a negligible influence on the size and morphology of alloy NPs. Furthermore, PtNi/rG-O nanocomposites displayed the higher electrochemical stability than PtCo/rG-O nanocomposites. These results provide a facile strategy for the synthesis of Pt-based alloy NPs/rG-O nanocomposites for applications in catalysis and energy-related processes.

Electrochemical properties of gold and glassy carbon electrodes electrografted with an anthraquinone diazonium compound using the rotating disc electrode method

The RDE method was combined with the electrografting procedure to prepare thick AQ films on Au and glassy carbon electrodes.

Investigation of the Passivation Properties of the Solid Electrolyte Interphase Using a Soluble Redox Couple

The solid electrolyte interphase (SEI) that forms at carbonaceous anodes makes Li-ion battery a viable technology because it inhibits solvent-decomposition reactions. However, passivation is never complete and SEI “leakage” appears as the main contributor to Li-ion battery aging. There has been a great deal of experiments focusing on the chemical analysis of SEIs over the past decades. Still, a direct evaluation of their passive character has not been much regarded. In this work, SEIs formed cathodically on glassy carbon electrodes are characterized using the rotating disk electrode method and ferrocene/ferrocenium as a redox shuttle, as originally proposed in Tang and Newman [M. Tang and J. Newman, This journal, 158(5), A530-A536 (2011)]. A comprehensive model is developed that accounts for transport of soluble redox species across the diffusion layer and the porous SEI as well as charge-transfer kinetics at the modified electrode surface. From a model analysis of electrodes with SEIs formed in various conditions, values of SEI porosity and effective rate constant of ferrocenium reduction are derived and discussed. First attempts are conducted to extend the method to SEIs formed at graphite composite electrodes. Preliminary results suggest SEIs are less passivating than those on glassy carbon.

C(Mo2C) and Pt–C(Mo2C) based mixed catalysts for oxygen reduction reaction

The mixed catalysts were prepared by mechanical mixing of the synthesized 60 wt.% Pt–C(Mo2C) (with average Pt nanoparticle size ~ 5 nm) with unmodified carbon powder (synthesized from molybdenum carbide (Mo2C)) to receive the following compositions of the catalysts: 1:1, 1:2, 1:5 60 wt.% Pt–C(Mo2C):C(Mo2C), respectively. The electrochemical characteristics for various micro- and mesoporous carbon based catalysts have been established by cyclic voltammetry and rotating disk electrode methods. Oxygen electroreduction characteristics for the mixed catalysts were compared with the 60 wt.% Pt–C(Mo2C) electrode and with respective 1:1 20 wt.% Pt–C(Mo2C):C(Mo2C) data. Experiments in KOH aqueous solution show that the additionally added carbon powder plays a weak role in Pt–C(Mo2C):C(Mo2C) mixed catalysts increasing the capacitive current densities and shifting the half-wave potential toward more negative values.

Rationalizing the Influence of the Mn(IV)/Mn(III) Red-Ox Transition on the Electrocatalytic Activity of Manganese Oxides in the Oxygen Reduction Reaction

Knowledge on the mechanisms of oxygen reduction reaction (ORR) and descriptors linking the catalytic activity to the structural and electronic properties of transition metal oxides enable rational design of more efficient catalysts. In this work ORR electrocatalysis was studied on a set of single and complex Mn(III) oxides with a rotating disc electrode method and cyclic voltammetry. We discovered an exponential increase of the specific electrocatalytic activity with the potential of the surface Mn(IV)/Mn(III) red-ox couple, suggesting the latter as a new descriptor for the ORR electrocatalysis. The observed dependence is rationalized using a simple mean-field kinetic model considering availability of the Mn(III) centers and adsorbate-adsorbate interactions. We demonstrate an unprecedented activity of Mn2O3, ca. 40 times exceeding that of MnOOH and correlate the catalytic activity of Mn oxides to their crystal structure.