Polished Glassy Carbon Electrode Research Articles

Glassy Carbon Electrodes Decorated with HgO/CNT Nanocomposite and Modified with a Conducting Polymer Matrix for Enzyme‐Free Ascorbic Acid Detection

AbstractIn an alkaline solution, mercuric oxide decorated carbon nanotube nanocomposites (HgO.CNT NCs) were manufactured utilizing an uncomplicated wet‐chemical approach. Ultra‐Violet spectroscopy (UV‐Visible), Fourier Transform Infra‐red spectroscopy (FTIR), Powder X‐ray Diffraction (XRD), X‐ray Photo‐electron Spectroscopy (XPS), and Field Emission Scanning Emission Microscopy (FESEM), X‐ray Electron Dispersive Spectroscopy (XEDS) techniques were applied to examine the HgO.CNT NCs that had been prepared. Ascorbic acid sensor was developed upon light coating of HgO.CNT NCs on polished Glassy carbon electrode (GCE) modified with nafion (Nf). Analytical performance of the selected ascorbic acid sensor was achieved such as sensitivity, Limit of quantification (LOQ), Limit of detection (LOD), Linear dynamic range (LDR), extended reliability, interference effect experimentation, and actual sample inspection using a consistent current‐voltage approach. Calibration curve of the suggested sensor was observed to be linear (R2=0.9957) throughout a broad range of ascorbic acid concentrations (100.0 pM∼100.0 mM) at a voltage of 0.8 V. The calibration curve yielded analytical parameters such as sensitivity (632.91 pAμM−1cm−2), LOQ (255.0 pM), LOD (76.50 pM), and LDR (100.0 pM ∼1.0 mM) for the proposed ascorbic acid sensor (GCE/HgO.CNT NCs/Nf). The wet‐chemical synthesis of HgO.CNT NCs is a good development of nanocomposite‐oriented sensor advancement in medical sciences with non‐enzymatic detection of biological substances. This proposed sensor applied to selective ascorbic acid detection in biological samples and obtained logical outcomes.

Characterizing the Impact of the Oxygen Reduction Reaction Mechanism and Current Distribution on Mineral Deposit Structure and Composition

In marine environments, cathodic protection is applied to metallic structures to prevent corrosion through polarization. The cathodic current causes secondary mineral deposition reactions, whose makeup, phase, and existence are dependent on the pH at the interface. These deposits can cause a significant increase in the interfacial impedance through electrolytic exclusion and obstructions to diffusion. Two factors contributing to the pH at a surface are current distributions across a surface and the reaction mechanism mediating the current. This work looks at the influence of these two factors on the make-up, phase and surface impedance impacts of mineral deposits on electrode surfaces. The role of Oxygen Reduction Reaction (ORR) mechanism and current distribution on the formation of Ca and Mg deposits on platinum and 316L stainless steel were assessed using a combination of rotating disc electrode (RDE), electrochemical impedance spectroscopy (EIS), and surface analysis techniques.In order to verify the material and potential dependent ORR mechanisms on platinum, glassy carbon, and 316L stainless steel, a series of RDE experiments were conducted. The platinum electrode and glassy carbon electrodes were shown to have excellent agreement with the predicted Levich behavior for a 4-electron ORR mechanism and a 2-electron ORR mechanism respectively, for all of the potentials in the mass transport limiting current range for ORR. However, the 316L stainless steels samples exhibited a deviation in the predicted Levich response at lower cathodic overpotentials in the ORR limiting current region. This behavior has been hypothesized to be caused by a reduction in the iron oxide layer present on the stainless-steel surface, which in turn results in an increase in ORR efficiency at more cathodic overpotentials. This transition in ORR efficiency may result in a change in the interfacial pH gradient, and impact the resultant mineral deposit structure and composition. In this work, four different ORR mechanism conditions were evaluated in terms of mineral deposit formation potential: polished platinum electrodes, polished glassy carbon electrodes, polished 316L stainless steel electrodes, and pre-reduced 316L stainless steel electrodes.Current distribution was evaluated in a fabricated, macro scale scanning probe system. These experiments assessed Pt and 316L as a 7 cm diameter disk working electrode, surrounded by a platinized ring counter electrode under quiescent conditions. Current distribution was observed via electric field measurement and surface potential mapping. The electrodes were polarized in ASTM D1141 artificial seawater, and the growth of the deposit was monitored in situ using EIS. The structure of the deposits on each material was evaluated by confocal profilometry and scanning electron microscopy with energy dispersive x-ray spectroscopy to determine structure and chemical composition. Mineral deposits were evaluated spatially and correlated to RDE work via an equipotential model. The work presented here provides insight into the contribution of ORR mechanism and current distribution across several size scales, on mineral deposit formation on materials with both ideal and hybrid ORR activity.

Chiral voltammetric sensor for tryptophan enantiomers by using a self-assembled multiwalled carbon nanotubes/polyaniline/sodium alginate composite.

Due to the crucial role of amino acids in life sciences and pharmaceutics, identification of optical amino acid molecules is of great significance. In this study, the two materials (CNT and PANI) were combined together to obtain the magnification of electrochemical signal by substrate material (CNT/PANI). Then a self-assembled multiwalled carbon nanotubes/polyaniline/sodium alginate (CNT/PANI/SA) nanocomposite with chiral sites and conductive material was synthesized as the electrochemical sensing interface. Next, a novel electrochemical sensing interface was fabricated via modifying the as-prepared chiral material on a polished glassy carbon electrode (CNT/PANI/SA/GCE) for precisely, efficiently, and rapidly differentiation of tryptophan (Trp) enantiomers. It was observed that CNT/PANI/SA/GCE showed desirable stereoselective recognition effect in the variety of signal strength to peak current (Ip) to the different optical activity of Trp enantiomers. In the case of optimal conditions, the peak current ratio in the solution of l-Trp and d-Trp (ID /IL ) was observed to be 2.1 at CNT/PANI/SA/GCE by differential pulse voltammogram (DPV). UV-visible spectroscopy further showed that CNT/PANI/SA had a greater binding energy to l-Trp. Also different factors affecting the enantioselectivity of CNT/PANI/SA/GCE, such as the incubation time, pH, and dropcoating volume of CNT/PANI/SA were optimized. Moreover, the proposed CNT/PANI/SA/GCE showed excellent specific stereoselectivity and anti-interference ability. Besides, the proposed chiral sensing platform can be effectively applied in real samples to detect Trp enantiomers sensitively. This work inspires us a new path for the preparation of substrate material with excellent electrical conductivity, as well as extend its application potential in chiral recognition.

Ultrathin Metal–Organic Framework Nanosheets-Derived Yolk–Shell Ni 0.85 Se@NC with Rich Se-Vacancies for Enhanced Water Electrolysis

We present a controlled fabrication of selective ultrathin metal–organic framework (MOF) nanosheets as preassembling platforms, yolk–shell structured with a few-layered N-doped carbon (NC) shell-en...

Icosahedral Cobalt Bis(dicarbollide) Ions with Bridge Linker and Their Electrochemistry in Aqueous Media

Boron cluster compounds (BCC) proved recently promising properties for use in drug design and molecular probes. Their applications in medicine are inherently connected with use of water-based environments. Up to now, the BCC were predominantly characterized in non-aqueous milieus. Therefore, we decided to perform electrochemical research of BCC in water-based electrolytes. We have focused on several boron clusters with structures that follow our previous study on icosahedral cobalt bis(dicarbollide) ions with a central coordination Co(III) atom in phosphate buffers, where we have found the best response for bridge type of exo-skeletal substituent. Therefore, in present study, we decided to focus on the promising structures, in which two 11 vertexes C2B9 dicarbollide ligands are interconnected via short bridge. Such structures may contain side chains usable for bioconjugation experiments.For our electrochemical experiments, we have used phosphate buffers of different pHs (2-10) and polished glassy carbon electrode. As the bridge structure served these exo-skeletal substituents: sulfur, amino group, tolylene, chlorpehenylene, phenylene, xylylene, byphenylene, phosphoric groups.The effect of various exo-skeletally substituted cobaltacarboranes (differing in type and length of the side chain) on the electrochemical response was studied. The results show that all parameters, the peak shape, the potential position and the peak current density height are significantly altered by the endo- and exo- skeletal substituents. Within the range of studied substitution, we were able to detect the signals in a broad potential range from E = -1.38 V to E = 1.73 V. From this follows clear possibilities to tune the position of two potential types in the electrochemical window that alternatively correspond either to the oxidation of the core structure of boron polyhedra or the central Co(III). The results also show that the electrochemical methods represent another tool for fast tracking the aggregation phenomena connected with non-classical amphiphilic behavior of boron clusters in aqueous solutions

Phospholipid modified glassy carbon electrode for determination of chili peppers pungency by ex-situ extraction voltammetry

Objective determination of chili peppers pungency, for which capsaicinoids (CPDs), predominantly capsaicin (CA) and dihydrocapsaicin (DHC) are responsible, belongs to frequently solved tasks in food and medicinal analysis. In this manuscript, application of ex-situ extractive stripping voltammetry (ExSV) utilizing a phospholipid modified glassy carbon electrode is described.The obtained analytical parameters, e.g., linear dynamic range from 3 to 60 mg L−1, limit of detection 2.1 mg L−1, proved to be fully comparable with those achieved using the direct voltammetric method on a polished glassy carbon electrode and reference HPLC/UV. Although the sensitivity of ExSV was slightly lower compared to the direct method, ExSV provided better selectivity to CPDs due to their lipophilic interactions with the phospholipid layer. Hydrophilic components of the chili pepper samples were not extracted into the phospholipid layer and therefore did not interfere. The developed method was tested on the determination of CPDs in different chili peppers: Foxta, Madras Chilli, Habanero, Carolina Reaper, Bird's eye, Bhut Jolokia, Naga Jolokia extract, and Habanero extract.Furthermore, the electrochemical reactions of CPDs in acidic aqueous-methanolic media were characterized using an electrochemical cell hyphenated with mass spectrometer and the main oxidation products were identified.

A sensitive and selective electrochemical sensor for the simultaneous determination of trace Cd2+ and Pb2+

A neoteric sensor for the detection of Cd2+ and Pb2+ has been investigated. The sensor was investigated based on a glutathione/gold nanoparticles/amino-reduced graphene oxide (GSH/AuNPs/NH2-rGO) nanocomposite, which was decorated on a polished glassy carbon electrode. The characterization analysis of the nanocomposite was performed with scanning electron microscopy, electrochemical techniques, and FTIR. The results indicated that GSH/AuNPs/NH2-rGO/GCE exhibited excellent analytical performance in detecting Cd2+ and Pb2+ owing to its large surface area, numerous functional groups, and good electrical conductivity. Under optimized experimental conditions, the stripping peak currents were linear with the concentrations of Cd2+ and Pb2+ in the scope of 1–120 μg/L with the detection limits of 0.09 μg/L for Cd2+ and 0.38 μg/L for Pb2+ (S/N = 3). Also, the electrochemical sensor displayed excellent selectivity and reproducibility. Hence, this research revealed that GSH/AuNPs/NH2-rGO nanocomposite was a promising sensing material for the detection of Cd2+ and Pb2+.

New Electrochemical Sensor for Hg(II) Trace Detection in Natural Waters: Electrode Functionalization with Gold Nanoparticles and Diazonium Salts

Mercury (Hg) represents a growing environmental and health major concern. It originates from natural sources and mainly from anthropogenic processes and it is widely distributed in the natural media. According to the last Global Mercury Assessment (2013), annual global emissions from both sources were estimated to be from 5,000 to 8,000 metric tons per year. Among the different mercury species released to the environment, methylmercury (MeHg) is considered as the most toxic form due to its ability to bioaccumulate, being then threatening even at very low concentrations.Its presence depends onHg(II) bioavailability and global amount.This explains the urgent need to ensure a continuous Hg(II) monitoring. Many efforts have been made in order to develop reliable systems able to deliver quick data and to comply with low detection limits, in accordance with the threshold value delivered by the World Health Organization (1µg L-1/ 5 nM). Spectroscopic techniques such as CV-AFS and CV-AAS are routinely used for Hg(II) determination. Although these methods can afford good sensitivity and low concentrations determination, they require sample preparation step, complex procedures and expensive material, which limits their use for on-site measurements. In this context, electrochemical sensors present excellent candidates for in situ Hg(II) trace analysis, taking in account their numerous advantages compared to spectroscopic techniques: easier handling, simple procedure, low energy consuming, low cost material and portability. In this work, we will propose a new electrochemical approach aiming to conceive and optimize an electrochemical Hg(II) sensor. The method consists in the functionalization of a glassy carbon electrode (GC) with gold nanoparticles (AuNPs) and Diazonium Salts. The main idea is to combine the interesting properties of both AuNPs and Diazonium salts. AuNPs were chosen for their electrocatalytic effect, large surface area, mass transport enhancement and for the strong affinity to mercury which will improve the sensor sensitivity. On the other hand, diazonium salts are used to improve the sensor stability by anchoring the AuNPs to the GC surface. First, nanometricorganic layer were grafted of the polished GC surface, by electrochemical reduction of 1.5 mM4-thiophenol diazonium (SH) using Constant Potential Electrolysis (CPE) in 0.1 M HCl solution at -0.55 V for 300 seconds. Electrochemical characterization performed by Cyclic Voltammetry (CV) and redox probes (ferricyanide and hexaamineruthenium(III)) revealed a total suppression of the signal, confirming the formation of a continuous blocking layer. This was confirmed by Atomic Force Microscopy (AFM)used to estimate the layer thickness, which was found to be 4 nm. Second, AuNPs were electrodeposited, for the first time, onto the diazonium multilayer by CPE in NaNO3 solution containing 0.25 mM HAuCl4for 300 seconds. Once more, redox probes were used to characterize the resulting interface and a total signal restauration and enhancement was observed after AuNPs electrodeposition, which highlights the effective AuNPs onto the organic layers. Field emission gun scanning microscopy (FEG-SEM) was used to provided further evidence and to quantify particle size and density of the AuNPs deposits. Both size and density are dependent on the CPE duration. Small homogeneous AuNPs with 27±3 nm average diameter and 158 NPs/µm2density were observed when the CPE was carried out during 300 seconds, while larger particles with 63±6 nm average diameter and lower density (63 NPs/µm2) were obtained when a longer CPE duration (600 seconds) is used. Finally, AuNPs were activated by cyclic voltammetry in H2SO4 prior to Hg(II) detection in order to homogenize the surface and to rearrange the crystallographic plans of the AuNPs. Herein, the well-known gold oxides reduction peak was observed and used to calculate the electroactive surface area (ESA) of the functionlized electrode. The electrochemical response of the final generated GC/SH/AuNPs interface towards Hg(II) was recorded by Square Wave Anodic Stripping Voltammetry (SWASV) in 0.01 M HCl solution containing different amounts of Hg(II). The SWASV procedure consists on the Hg(II) preconcentration at the electrode surface followed by the preconcentrated Hg(0) reoxidation in Hg(II). Under optimized conditions, and for a preconcentration time of 300 seconds, a well-defined peak, corresponding to Hg(0) reoxidation, was observed around 0.5 V/ECS. The sensor showed a linearity range from 1 up to 10 nM and allowed to reach picomolar level. The stability in HCl, phosphate buffer and air was also studied over several weeks: Once a week, the activation procedure was performed and followed by Hg(II) determination in order to evaluate the analytical performances of the sensor over time. Finally, Hg(II) detection assays were conducted in natural water samples collected from different sampling points. Figure 1

Reduced Graphene Oxide-Supported Bimetallic M-Platinum (M: Co, Ni, Cu) Alloy Nanoparticles for Hydrogen Evolution Reaction

Hydrogen is one of the most promising energy carriers, especially if produced from renewable energy sources, replacing fossil fuels for both portable and stationary applications [1]. Although hydrogen is one of the most abundant elements on Earth, it does not exist in nature in molecular form. Currently, most of the hydrogen is produced from fossil fuel-based technologies, such as gas reforming. Only 5% of worldwide hydrogen production comes from water electrolysis. The main problem of this process is the high overpotentials necessary to split the water molecule into hydrogen and oxygen and, consequently, the high energy consumption. To overcome this problem, different electrode materials such as Pt, Ni, Co, Ir and Rh, have been tested as cathodes for water electrolyzers. Pt has high activity and good stability for the hydrogen evolution reaction (HER), but its limited resources and high price prevent its use on industrial scale [2]. Alkaline water electrolysis is a mature technology that typically operates at temperatures around 60 – 80 ºC and uses cathodes based on Ni, Raney Ni and Co [3]. These cathodes are less expensive than Pt, but show good activity (in alkaline media) and corrosion resistance. A common approach used in electrocatalysis to minimize the utilization of precious metals involves employing an electrocatalyst support material. The support material has several functions, as to increase the catalytic electrochemically surface area, to stabilize and to accommodate the metal particles and also to increase the catalyst’s conductivity [4-6]. Carbon-based supports are the mostly used, as they can offer all the aforementioned properties [6]. The present work combines the advantages of using highly active Pt together with low cost transition metals, like Ni, Co and Cu, with the employment of reduced graphene oxide (rGO) as an efficient carbon-based electrocatalyst support. Thus, Pt and three different PtM alloys supported on rGO, i.e., Pt/rGO, PtCo/rGO, PtCu/rGO and PtNi/rGO, were tested as electrocatalysts for hydrogen production by alkaline water electrolysis. rGO has adequate features to enhance and stabilize the metal particles, with rGO-supported catalysts having recently demonstrated promising results in alkaline media [7]. To prepare the working electrodes, 5 mg of each of the fours electrocatalysts was dispersed in 125 µL of 5 wt.% solution of polyvinylidene fluoride (PVDF, Alfa Aesar) in N-methyl-2-pyrrolidone (NMP, Sigma Aldrich) followed by ultrasonic treatment for ca. 30 min. The working electrodes were prepared by pipetting 80 μL of the corresponding catalytic ink onto a polished glassy carbon (GC) electrode (A = 4 cm2). The electrodes were dried at 80 ºC for 4 hours. A conventional three-electrode setup was used for the electrochemical characterization. The rGO-supported PtM (M = Co, Cu, Ni) electrode was used as working electrode, the counter electrode was a Pt mesh (Johnson Matthey) of 25 cm2 area, and a calomel electrode (Hanna Instruments, HI-5412, 3.5 M KCl) was used as the reference. All recorded potentials were converted to RHE scale using the equation ERHE = Ecal + 0.250 + 0.059 pH. The evaluation of HER kinetics at the PtM/rGO composites was done by linear scan voltammetry measurements at scan rate of 1 mV s-1, starting from the open circuit potential up to -0.17 V in 8 M KOH (AnalaR NORMAPUR, 87 wt.%) solutions. Higher current densities were achieved for the PtM/rGO electrocatalysts, in comparison with the non-alloyed material (Pt/rGO). It was also observed that increasing the temperature up to 65 ºC leads to a substantial increase of HER current densities at all electrocatalysts. Furthermore, long-term stability of the four electrocatalysts’ activity for HER was evaluated by chronoamperometry studies at 25 and 65 ºC, revealing better stability at lower temperature. It is shown that PtM/rGO nanocomposites are good candidates for application as novel electrocatalysts for the HER in alkaline media. [1] J.A.S.B. Cardoso, B. Šljukić, M. Erdem, C.A.C. Sequeira, D.M.F. Santos, Catalysts 50, 8 (2018). [2] D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Macciò, A. Saccone, J.L. Figueiredo, Energy 50, 486 (2013). [3] D.S.P. Cardoso, L. Amaral, D.M.F. Santos, B. Šljukić, C.A.C. Sequeira, D. Macciò, A. Saccone, Int. J. Hydrogen Energy 40, 4295 (2015). [4] M. Martins, B. Šljukić, C.A.C. Sequeira, O. Metin, M. Erdem, T. Şener, D.M.F. Santos, Int. J. Hydrogen Energy 41, 10914 (2016). [5] R.C.P. Oliveira, V. Vasić, D.M.F. Santos, B. Babić, R. Hercigonja, C.A.C. Sequeira, B. Šljukić, Electrochim. Acta 269, 517 (2018). [6] E. Antolini, Appl. Catal. B 88, 1 (2009). [7] M. Martins, B. Šljukić, O. Metin, M. Sevim, C.A.C. Sequeira, T. Şener, D.M.F. Santos, J. Alloys Compd. 718, 204 (2017).

Effect of Electrochemical Treatment of Glassy Carbon Electrodes on Electrode Surface and VanadiumIV-VanadiumV Kinetics

The VIV-VV electrode kinetics of polished and non-polished glassy carbon electrodes were monitored during repeated cathodic and anodic treated. Although anodic treatment initially enhances the electrode kinetics, repeated treatment leads to the reverse: i.e. the kinetics is inhibited by anodic treatment and enhanced by cathodic treatment. The kinetics of the system was investigated using cyclic voltammetry (CV) and impedance spectroscopy (EIS) and the two techniques show strong agreement. The enhancement factor between the activated (i.e. cathodically treated) and deactivated (i.e. anodically treated) electrodes was as high as 11.5. Scanning electron microscopy (SEM) shows that the electrode surface is stable with only limited cracking. CV analysis in sulfuric acid shows very stable values for current density on anodically treated electrodes but a proportional relationship between current density and treatment number on cathodically treated electrodes suggesting functionalization of the system through repeated reduction and oxidation of the electrodes. These functionalized sites may be easily reduced or oxidized allowing toggling between the activated and deactivated states of VIV-VV electrode kinetics.

Highly Active Transition Metal and Nitrogen Co-Doped Carbon Nanotube Based Cathode Catalysts 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. Maximum power density with H2/O2 gases at ambient pressure was ~115 mWcm-2. The fuel cell perfomance of the Co/N-MWCNT catalyst was also compared with the perfomance of the commercial Pt/C and cobalt phthalocyanine modified MWCNT catalysts (Fig.1).

A simple electrochemical platform for detection of nitrobenzene in water samples using an alumina polished glassy carbon electrode

In this work, we report a selective electrochemical sensing of nitrobenzene (NB) using an alumina (γ-Al2O3) polished glassy carbon electrode (GCE) for the first time. The scanning electron microscopy studies confirm the presence of alumina particles on the GCE surface. X-ray photoelectron spectroscopy studies reveal that the utilized alumina is γ-Al2O3. The alumina polished GCE shows an enhanced sensitivity and lower overpotential toward the reduction of NB compared to unpolished GCE. The differential pulse voltammetry response was used for the determination of NB and it shows that the reduction peak current of NB is linearly proportional to the concentrations of NB ranging from 0.5 to 145.5μM. The limit of detection is found to be 0.15μM based on 3σ. The fabricated electrode exhibits its appropriate selectivity towards NB in the presence of a range of nitro compounds and metal ions. The good practicality of the sensor in various water samples reveals that it can be a promising electrode material for practical applications. In addition, the proposed NB sensor is simple and cost effective one when compared with previously reported NB sensors in the literature.

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.

Alumina Polished Glassy Carbon Electrode as a Simple Electrode for Lower Potential Electrochemical Detection of Dopamine in its Sub‐micromolar Level

AbstractThe electrochemical behaviour of dopamine (DA) at a cleaned and alumina polished glassy carbon electrode (GCE) was studied using cyclic voltammetry (CV). The CV studies revealed that alumina polished GCE (AGCE) shows an enhanced oxidation peak current response with 217 mV negative potential shift towards DA than that of cleaned GCE. The differential pulse voltammetry result shows that the AGCE detects the DA in the linear concentration ranges from 0.15 to 25.25 µmol L−1. The limit of detection was calculated as 0.046 µmol L−1 with a sensitivity of 3.74 µA µmol L−1 cm−2 for the determination of DA. The fabricated AGCE shows a satisfactory selectivity, practicality along with appreciable repeatability and reproducibility.

Efficient Fe, Co-Containing, Nitrogen-Doped Carbon Nanomaterials for Oxygen Reduction Reaction

The sluggish kinetics of the oxygen reduction reaction (ORR) is generally considered to be the main obstacle in producing viable polymer electrolyte membrane fuel cells. As a solution, noble metal-containing catalysts are traditionally used. However, the higher activity of Pt-based catalyst materials comes at a cost – they are unstable, limited in supply and expensive, which severely inhibits their usage in commercial applications. In recent years, nitrogen-doping of carbon materials has emerged as a way of achieving highly active materials that are without these limitations.1,2 There is a large body of evidence that N-doped nanocarbons are active catalysts for ORR in alkaline solution, however, in acid media these materials show only modest activity.3 N-containing carbon materials can be used as cathode catalysts for alkaline membrane fuel cells.4 It has been reported that the presence of transition metals such as Co or Fe further enhances the electrocatalytic properties of N-doped carbons.5 In this work, a simple synthesis method for attaining transition metal and nitrogen-doped carbon nanomaterials with high activity towards the oxygen reduction reaction is demonstrated. Both multi-walled carbon nanotubes (MWCNT) treated in a concentrated HNO3-H2SO4 mixture and graphene oxide (GO) synthesized via the modified Hummers’ method were used as carbon supports. These carbon nanomaterials were first dispersed in ethanol by sonication, after which dicyandiamide as the nitrogen source and a Fe or 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 either Nafion or Tokuyama OH− ionomer AS-04. The rotating disk electrode (RDE) measurements were performed in 0.1 M KOH and 0.5 M H2SO4 solutions. The stability and methanol tolerance of the catalysts were also tested using RDE. X-ray photoelectron spectroscopy was used to characterize the surface composition of the materials. In alkaline conditions, all of the transition metal and N-doped nanocarbons showed excellent activity for oxygen electroreduction, in particular those containing Co. Among the carbon supports, MWCNTs were shown to produce the catalysts of higher activity. The number of electrons transferred per O2 molecule (n) calculated from the Koutecky–Levich equation was close to 4 for all the Co-containing catalysts studied. In acid solution, iron-containing materials exhibited medium activity while cobalt-containing materials were determined to be highly active. Methanol tolerance tests were carried out by measuring activities in 0.1 M KOH solution containing 3 M MeOH, with all catalysts displaying exceptional immunity toward methanol crossover and the stability of all catalysts was monitored over 1000 potential cycles in alkaline conditions, with a minimal loss in activity. Fe- and Co-containing nitrogen-doped carbon nanomaterials can be considered as promising candidates for polymer electrolyte fuel cell cathode catalysts as they are stable and highly active toward oxygen reduction. Furthermore, their excellent methanol tolerance renders them immune from crossover effect and they could thus also be used in direct methanol fuel cells. Work is in progress to test these materials in fuel cell conditions.

Cyclic Voltammetry Studies of H-Bond Complex of a p-Phenylenediamine-Based Urea with 1,8-Naphthyridine As the Proton-Coupled Electron Transfer Guest with Platinum and Glassy Carbon As Working Electrodes

Reactions in which both electron and proton are transferred, called proton-coupled electron transfer (PCET) reactions, are essential to many of the fundamental chemical processes of life. While PCET reactions have been well studied, it is only relatively recently that the role of H-bonding intermediates has been examined. The role that H-bonding can play in PCET was demonstrated by studies in the Smith group with U(H)H, a p-phenylenediamine-based urea which undergoes a reversible oxidation in methylene chloride.1 The overall oxidation of U(H)H is a 1 e- transfer with a surprising 2 e-/ 1 H+ mechanism where removal of the first electron produces a radical cation with an acidic NH proton that is then removed by the dimethylamino group from another U(H)H. This leads to removal of a second electron and creates the final products: the protonated electroinactive species, HU(H)H+, and the doubly oxidized form, U(H)+. Since this corresponds to 2 e- per 2 ureas, the process appears to be a net 1 e-. The interesting observation is that on the return scan two distinct pathways are observed. At the more negative potential, one pathway proceeds via the reduction of the U(H)+. This path dominates at low concentrations and fast scan rates. We hypothesize that the other path, which occurs at less negative potential, involves reduction of a H-bond complex formed between HU(H)H+ and U(H)+, leading to an overall electrochemically reversible process. In this study, cyclic voltammetry of U(H)H in the presence of 1,8-naphthyridine has been examined. The addition of naphthyridine results in an increase in current height and the appearance of a second oxidation wave at a more positive potentials . The increase in current is due to competition for the acidic proton between naphthyridine and the dimethylamino group of another U(H)H. NMR titration analysis show that naphthyridine H-bonds to the reduced U(H)H meaning naphthyridine blocks the H-bond sites from another U(H)H. Interestingly, the shape and behavior of the more positive oxidation wave alters depending on the electrode material. We hypothesize that the change in the CV wave is due to electrode fouling on the platinum (Pt) and poorly polished glassy carbon (GC) electrodes. With a carefully polished GC electrode, which minimizes the effect of electrode fouling, the observed second oxidation wave shifts slightly to more negative potentials and appears to merge with the first oxidation wave as the concentration of naphthyridine increases. This behavior reflects a mechanism that is similar to that of U(H)H by itself except the dimethylamino group of the other urea is replaced with naphthyridine. Therefore, the appearance of the second oxidation wave is due to oxidation of the H-bonding complex between the radical cation urea, U(H)H+, and naphthyridine, which is accompanied by proton transfer to naphthyridine. 1. Clare, L. A.; Pham A. T.; Magdaleno, F.; Acosta, J.; Woods, J. E.; Cooksy, A. L.; Smith, D. K. J. Am. Chem. Soc., 2013, 135 (50), 18930–18941.

X-ray Photoelectron Spectroscopy (XPS) Investigation of Alkylation of Glassy Carbon Electrodes Via Reduction of Primary Alkyl Halides

Alkylation of glassy carbon electrodes during the reduction of primary alkyl halides presents a problem for electrochemical experiments involving this class of compounds. Details of the grafting of primary alkyl moieties onto the surface of a glassy carbon electrode have come to light as a result of recent research conducted by Simonet and co-workers.1–6 These findings as well as work from our laboratory have revealed that surface alkylation results during an electrolysis of primary alkyl halides. In a controlled-potential electrolysis, this effect can be especially problematic if the actual reduction potential for a chosen compound shifts to a value that is more negative than the targeted potential. To understand further the effects and mechanism of this surface alkylation, we have employed X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry to examine and characterize glassy carbon and reticulated vitreous carbon electrodes before and after their surfaces have been alkylated via the reduction of alkyl halides.Shown in Figure 1 are cyclic voltammograms for the reduction of 1-iodooctane at a glassy carbon electrode in acetonitrile (MeCN) containing 0.050 M tetramethylammonium tetrafluoroborate (TMABF4). Between the scans shown in the figure, the electrode was not polished; instead, grafting of octyl moieties onto the electrode surface was allowed to continue with each successive scan until a limiting state was achieved, as evidenced by little or no further negative shift of the cathodic peak potential. In addition, we have investigated the effects of this surface alkylation on the redox behavior of some common electron-transfer mediator systems involving 1,2,4,5-tetracyanobenzene, 4-nitropyridine N-oxide, nickel(II) salen, and cobalt(II) salen, with significant changes being observed. As an example, the effects of a surface-octylated glassy carbon electrode on the redox behavior of 1,2,4,5-tetracyanobenzene are depicted in Figure 2.A previously proposed mechanism for surface alkylation suggests that sp2-hybridized carbon atoms act as carbanions when electrons are forced to the surface of the electrode at negative potentials, thereby promoting an SN2-type attack on primary alkyl halides to produce grafted alkyl moieties.3 To investigate this process further, the surfaces of glassy carbon and reticulated vitreous carbon electrodes have been examined by means of XPS before and after the electrochemical reduction of perfluorobutyl iodide (C4F9I) in DMF containing 0.10 M tetra-n-butylammonium tetrafluoroborate (TBABF4). It is noteworthy that cyclic voltammograms for the reduction of C4F9I are similar to those shown in Figure 1 for 1-iodooctane, an observation indicating that perfluorobutyl moieties are grafted onto the surface of a glassy carbon cathode at negative potentials. Using perfluorobutyl iodide as the substrate for electrochemical experiments involving XPS allowed us to distinguish carbon signals for grafted perfluorobutyl groups from the sp2 and sp3 carbon signals for the glassy carbon surface itself. Our XPS results support the proposed mechanism, with the data showing a conversion of sp2 to sp3 surface carbon atoms. A freshly polished glassy carbon electrode from our laboratory was found to have a sp2:sp3 surface-carbon ratio of approximately 4:1. After cyclic voltammetric experiments involving the reduction of C4F9I, the sp2 component of the surface carbons was diminished, with the resulting sp2:sp3 ratio being approximately 1:1, a result consistent with the proposed grafting mechanism. Signals corresponding to the presence of carbon bonded to fluorine (CF2 and CF3 moieties) were also observed, with no iodine present, all findings in accord with the grafting of C4F9radicals onto the surface of the electrode. References Simonet, J. Electrochem. Commun. 2011, 13, 107–110.Jouikov, V.; Simonet, J. Appl. Electrochem. 2012, 42, 527–537.Jouikov, V.; Simonet, J. Langmuir 2012, 28, 931–938.Poizot, P.; Durand-Drouhin, O.; Lejeune, M.; Simonet, J. Carbon 2012, 50, 73–83.Jouikov, V.; Simonet, J. J. Electrochem. Soc. 2013, 160, G3008–G3013.Hui, F.; Noël, J.-M.; Poizot, P.; Hapiot, P.; Simonet, J. Langmuir 2011, 27, 5119–5125.

The Role of Heat Treatment in Enhanced Activity of Manganese Oxides for the Oxygen Reduction and Evolution Reactions

In this study we use ex-situ Mn L-edge X-ray absorption spectroscopy and electrochemical characterization to understand the relationship between surface properties of five differently prepared manganese oxides (MnOx) electrocatalysts and their activity for the oxygen reduction and evolution reactions (ORR, OER). We prepare the five catalysts by electrodepositing MnOx on polished glassy carbon electrodes and heat treating them under different conditions: no heat treatment (“no ht”), 250°C, 350°C, 450°C, and 500°C. We find that “no ht” catalyst, despite having the lowest (pseudo)capacitive current, is capable of the largest changes in Mn oxidation state, as the applied potential switches from an ORR to an OER relevant value. Heat treatment at 450°C or 500°C, leading to a significant increase in (pseudo)capacitance, is on the other hand associated with only minor changes in the surface oxidation state. Investigation of (pseudo)capacitive behavior of the same MnOx thin films electrodeposited on a high surface area rather than polished carbon support reveals that the observed differences in pseudocapactive current can be explained by changes in the surface area of the electrocatalysts. Characterization of the electrocatalytic activity of the thin films demonstrates that the catalysts exposed to the two highest temperatures are associated with enhanced bifunctional activity for the ORR and the OER. The enhancement in the ORR activity is attributed to the formation of a Mn2O3 phase, while the enhancement in the OER activity is primarily explained by the increase in the electrochemical surface area of the catalyst.

Square wave anodic stripping voltammetry determination of lead based on the Hg(II) immobilized graphene oxide composite film as an enhanced sensing platform

In present paper, Hg(II) modified graphene oxide (Hg(II)/GO) was firstly prepared by reaction of graphene oxide (GO) with Hg(II). The Hg(II)/GO dispersed in a Nafion solution was coated on the polished glassy carbon electrode (GCE) for fabricating the enhanced electrochemical sensing platform. And then Hg(II)/GO modified GCE was held at a negative potential (−1.0V) to reduce the coordinated Hg(II) to Hg forming Hg nanodroplets, which was been used as the sensing platform to determine Pb2+ by square wave anodic stripping voltammetry (SWASV). In comparison with the Hg(II) immobilized MWCNT graphite electrode, the response signal of our electrode was obviously increased owing to the unique structure of GO. The linear calibration curves ranged from 5ngL−1 to 70ngL−1 and 0.1μgL−1 to 10μgL−1 for Pb2+. The detection limit (S/N=3) was estimated to be around 0.13ngL−1 for Pb2+. The practical application of Hg(II)/GO modified GCE was also evaluated by the detection of Pb2+ in different water samples, the results suggest that the sensor is very promising for practical applications.

Voltammetry of nanomolar leveled environmental hazards on the polymer/CNT coated electrodes

4-tert-Octylphenol is toxic even at nanomolar levels but widely used as an intermediate substance. The detection methods for 4-tert-octylphenol at nanomolar levels are however missing in literature. We investigate in this paper the voltammetric response and electrochemical detection of nanomolar leveled 4-tert-octylphenol in water. A polymer/CNT film coated electrode was applied. The electrode was prepared by firstly coating a polished and clean glassy carbon electrode with a CNT film and then covering the CNT film coated electrode with a conducting film from l-lysine. On this electrode, 4-tert-octylphenol is electrochemically oxidized at 0.60V vs. SCE. This irreversible oxidation process is controlled by adsorption. The anodic peak current is proportional to the concentration of 4-tert-octlyphenol in the range of 6.5–20nM. The detection limit is 0.5nM. This electrode has been applied for the detection of 4-tert-octylphenol in lake and river water. The relative standard derivation (RSD) of these measurements is less than 4.3%.