Tharamani C. Nagaiah

Metal-free and electrocatalytically active nitrogen-doped carbon nanotubes synthesized by coating with polyaniline

Nitrogen doping of multi-walled carbon nanotubes (CNTs) was achieved by the carbonization of a polyaniline (PANI) coating. First, the CNTs were partially oxidized with KMnO4 to obtain oxygen-containing functional groups. Depending on the KMnO4 loading, thin layers of birnessite-type MnO2 (10 wt% and 30 wt%) were obtained by subsequent thermal decomposition. CNT-supported MnO2 was then used for the oxidative polymerization of aniline in acidic solution, and the resulting PANI-coated CNTs were finally heated at 550 degrees C and 850 degrees C in inert gas. The samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. A thin layer of carbonized PANI was observed on the CNT surface, and the surface nitrogen concentration of samples prepared from 30% MnO2 was found to amount to 7.6 at% and 3.8 at% after carbonization at 550 degrees C and 850 degrees C, respectively. These CNTs with nitrogen-containing shell were further studied by electrochemical impedance spectroscopy and used as catalysts for the oxygen reduction reaction. The sample synthesized from 30 wt% MnO2 followed by carbonization at 850 degrees C showed the best electrochemical performance indicating efficient nitrogen doping.

Mesoporous Nitrogen‐Rich Carbon Materials as Catalysts for the Oxygen Reduction Reaction in Alkaline Solution

ORR MNC, FTW! Mesoporous nitrogen-rich carbon (MNC) materials are synthesized by using polymer-loaded SBA-15 pyrolyzed at different temperatures. The activity and stability of the catalysts in the oxygen reduction reaction (ORR) are investigated by using cyclic voltammetry and rotating-disk electrode measurements. The MNC material pyrolyzed at 800 °C exhibits a high electrocatalytic activity towards the ORR in alkaline medium.

Electrochemically deposited Pd-Pt and Pd-Au codeposits on graphite electrodes for electrocatalytic H2O2 reduction.

Improved electrocatalytic activity and selectivity for the reduction of H2O2 were obtained by electrodepositing Pd-Pt and Pd-Au on spectrographic graphite from solutions containing salts of the two metals at varying ratio. The electrocatalytic activity of the resulting binary codeposits for H2O2 reduction was evaluated by means of the redox-competition mode of scanning electrochemical microscopy (SECM) and voltammetric methods. In a potential range from 0 to -600 mV (vs. Ag/AgCl/3 M KCl) at pH 7.0 in 0.1 M phosphate citrate buffer, the electrocatalytic activity of both Pd-Pt and Pd-Au codeposits was substantially improved as compared with the identically deposited single metals suggesting an electrocatalytic synergy of the codeposits. Pd-Pt and Pd-Au codeposits were characterized by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Codepositing with Au caused a change of hedgehog-like shaped Pd nanoparticles into cauliflower-like nanoparticles with the particle size decreasing with increasing Au concentration. Codepositing Pd with Pt caused the formation of oblong structures with the size initially increasing with increasing Pt content. However, the particle size decreases with further increase in Pt concentration. The improved electrocatalytic capability for H2O2 reduction of the Pd-Pt electrodeposits on graphite was further demonstrated by immobilizing glucose oxidase as a basis for the development of an interference-free amperometric glucose biosensor.

Visualization of the local catalytic activity of electrodeposited Pt-Ag catalysts for oxygen reduction by means of SECM.

Pt-Ag nanoparticle co-deposits with different Pt-Ag ratios were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and investigated for their catalytic activity in electrocatalytic oxygen reduction by using cyclic voltammetry (CV), rotating disc electrode (RDE) and scanning electrochemical microscopy (SECM) in 0.1 M phosphate buffer (pH 7.0). The atomic composition of the Pt-Ag co-deposits was studied by means of energy-dispersive X-ray analysis (EDAX). In combination with X-ray diffraction (XRD), the presence of partly alloyed Pt and Ag on the GC surface was confirmed. Scanning electron microscopy (SEM) images indicate that the prepared Pt-Ag catalyst particles are homogenously dispersed over the GC surface. Their size and morphology depend on their composition. The electrocatalytic activity of Pt-Ag deposits with high Pt content was the highest, exceeding even that of electrodeposited Pt as evaluated by quantitative RDE analysis. The redox competition mode of scanning electrochemical microscopy (RC-SECM) was successfully used to visualize the local catalytic activity of the deposited Pt-Ag particles. Semi-quantitative assessment of the SECM results confirmed the same order of activity of the different catalysts as the RDE investigations.

Pt−Ag Catalysts as Cathode Material for Oxygen-Depolarized Electrodes in Hydrochloric Acid Electrolysis

Pt-Ag nanoparticles were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and tested using cyclic voltammetry and scanning electrochemical microscopy (SECM) with respect to their possible use as catalyst material for oxygen reduction in 400 mM HCl solution. For comparison, a Pt catalyst was investigated under similar conditions. The redox competition mode of scanning electrochemical microscopy (RC-SECM) was adapted to the specific conditions caused by the presence of Cl(-) ions and used to visualize the local catalytic activity of the Pt-Ag deposits. Similarly prepared Pt deposits were shown to dissolve underneath the SECM tip. Pt-Ag composites showed improved long-term stability toward oxygen reduction as compared with Pt even under multiple switching off to open-circuit potential in 400 mM HCl.

Rh–RhSx nanoparticles grafted on functionalized carbon nanotubes as catalyst for the oxygen reduction reaction

Rhodium–rhodium sulfide nanoparticles supported on multi-walled carbon nanotubes (CNTs) were synthesized via a multi-step colloid route. The CNTs were first exposed to nitric acid to generate oxygen-containing functional groups, and then treated with thionyl chloride to generate acyl chloride groups. The grafting of thiol groups was subsequently carried out by reaction with 4-aminothiophenol. Colloidal rhodium nanoparticles were synthesized using rhodium chloride as metal source, sodium citrate as stabilizer, and sodium borohydride as reducing agent. The immobilization of the generated colloidal rhodium nanoparticles was achieved by adding the thiolated CNTs to the colloidal suspension. All these steps were monitored by X-ray photoelectron spectroscopy, which disclosed the presence of rhodium sulfide, whereas metallic rhodium was detected by X-ray diffraction, suggesting that the nanoparticles probably consist of a metallic Rh core covered by a sulfide layer. Scanning and transmission electron microscopy studies showed that the diameter of the catalyst particles was about 7 nm even at high Rh loadings. Rotating disc electrode measurements and cyclic voltammetry were employed to test the electrocatalytic activity in the oxygen reduction reaction in hydrochloric acid. Among all the synthesized catalysts with different rhodium loadings (4.3–21.9%), the 16.1% rhodium catalyst was found to be the most active catalyst. In comparison to the commercial E-TEK Pt/C catalyst, the 16.1% catalyst displayed a higher electrochemical stability in the highly corrosive electrolyte, as determined by stability tests with frequent current interruptions.

Nitrogen-doped carbon nanotubes for sensitive and selective determination of heavy metals

The present study attempts to demonstrate the potential of nitrogen-containing carbonaceous materials towards electrochemical determination of heavy metal ions. For this purpose, nitrogen-doped carbon nanotubes (N-CNTs) have been synthesized from post treatment of oxidized carbon nanotubes (O-CNTs) in the presence of NH3 at three different temperatures of 200, 400 and 600 °C to analyse their relevance in heavy metal ions determination. The electrocatalytic activity of these materials towards determination of Cd, Pb and Cu metal ions was examined by means of the square wave stripping voltammetry (SWSV) technique which demonstrates the suitability of all temperature variants for the desired purpose. Eventually, N-CNTs treated at 600 °C (N-CNT 600) displayed comparatively better sensitivity towards the investigated heavy metal ions (Cd, Pb and Cu) as compared to the other temperature variants of the N-CNTs (N-CNT 200 and N-CNT 400). The role of nitrogen groups was assessed in comparison with O-CNTs as it eliminates the requirement of the pre-concentration step by efficient complexation with heavy metal ions. The peak current for the Pb and Cd metal ions increased linearly in the concentration range of 0.01 μM–70 μM and 0.1 μM–100 μM respectively (individual as well as simultaneous determination). The limit of detection (LOD) was calculated to be 0.001 μM and 0.005 μM (3σ method) for the simultaneous determination of Pb and Cd metal ions respectively. Furthermore, a real time application of a N-CNT 600 modified GCE was probed by electrochemical detection of Cd and Pb metal ions in tap water and ground water.

Dioxygen Reduction by Nitrogen-Rich Mesoporous Carbon bearing Electrodeposited Silver Particles

The electrocatalytic activity of silver particles supported on nitrogen‐containing mesoporous carbon materials (NMC) was studied with a view to dioxygen reduction. Silver particles with controlled size and shape on the NMC were synthesized by a double‐pulse electrodeposition technique. Changing the nucleation and growth potential as well as their growth times resulted in varied morphologies of the electrodeposited silver. The effect of the supporting material itself on the activity was assessed by employing variably pyrolyzed NMC supports. The associated kinetic parameters were evaluated by hydrodynamic measurements in alkaline medium. The dioxygen reduction activity was associated with the available surface area, which was determined by double‐layer capacitance measurements. The stability of the catalyst was tested by chronoamperometric measurements both in the presence and absence of methanol. Elemental components were identified by X‐ray diffraction and X‐ray photoelectron spectroscopy techniques.

Tuning the MnWO4 morphology and its electrocatalytic activity towards oxygen reduction reaction

The present study explores the morphological evolution and structure–activity correlation of manganese tungstate (MnWO4), a new electrocatalyst towards alkaline oxygen reduction reaction (ORR). Morphological tuning was performed by a complexation–precipitation approach employing one-pot hydrothermal synthesis and the obtained material was characterized using XRD, FT-IR and XPS. A peculiar bird-feather (BF) like morphology was obtained by optimizing the structure directing agent (SDA) concentration, reaction temperature and time. Its ORR reactivity was studied by performing macro and micro-electrochemical analysis suggesting a prominent (2 + 2) e− pathway having an onset potential of 0.99 V (vs. RHE). Furthermore, the role of the SDA in generating electrocatalytically active sites was mapped by performing comparative scanning electrochemical microscopy imaging (SECM) in the redox-competition (RC) mode. The obtained surface plots correlated well with the rotating ring-disk electrode (RRDE) measurements in response to distinct sample (MnWO4) potentials ranging from kinetic-limited to mass-transfer limited domains. The morphologically optimized MnWO4 was further found to be an efficient electrocatalytic competitor against Pt/C (20%), an expensive and limited noble-metal catalyst.

Ultrasensitive and Highly Selective Electrochemical Detection of Dopamine Using Poly(ionic liquids)–Cobalt Polyoxometalate/CNT Composite

A novel sandwich polyoxometalate (POM) Na12[WCo3(H2O)2(CoW9O34)2] and poly(vinylimidazolium) cation [PVIM+] in combination with nitrogen-doped carbon nanotubes (NCNTs) was developed for a highly selective and ultrasensitive detection of dopamine. Conductively efficient heterogenization of Co5POM catalyst by PVIM over NCNTs provides the synergy between PVIM–POM catalyst and NCNTs as a conductive support which enhances the electron transport at the electrode/electrolyte interface and eliminates the interference of ascorbic acid (AA) at physiological pH (7.4). The novel PVIM–Co5POM/NCNT composite demonstrates a superior selectivity and sensitivity with a lowest detection limit of 500 pM (0.0005 μM) and a wide linear detection range of 0.0005–600 μM even in the presence of higher concentration of AA (500 μM).