Aarti Tiwari

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.

Nitrogen containing carbon spheres as an efficient electrocatalyst for oxygen reduction: Microelectrochemical investigation and visualization

The oxygen reduction reaction (ORR) in low temperature fuel cells limits the obtainable output triggering the necessity of screening the employable catalysts precisely. A nitrogen-containing carbon sphere (NCS) catalyst was synthesized using a soft template and was studied for the oxygen reduction reaction. The NCS catalyst was shown to have superior activity with an onset potential of 0.985 V (vs. RHE) and a diffusion-limited current density of 6.16 mA cm−2. The kinetic insight from the rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) revealed an overall 4e− reduction by a (2 + 2) electron pathway with a rate constant of 4.6 × 10−4 cm s−1 at the onset potential. A fast screening of the localized electrochemical activity using sample generation-tip collection (SG-TC) in combination with a chronoamperometric multiple pulse technique was applied in the present study to trace the formation, evolution and conversion of intermediate species using a Pt ultramicroelectrode. The local catalytic activity of the NCS catalyst was successfully visualized by scanning electrochemical microscopy (SECM) in highly alkaline medium.

Facet-Controlled Morphology of Cobalt Disulfide towards Enhanced Oxygen Reduction Reaction

Catalytic activity has a significant contribution from exposed facets which are prominently correlated with morphology. It has been witnessed that even a small refinement in morphology alters the catalytic activity over several folds which is a key towards catalyst design. The present study thereby explores the design of octahedral CoS2 crystals with the aim of exposing highly active facets for the oxygen reduction reaction (ORR) which were carefully synthesized to incorporate carbon inherently. One-pot hydrothermal synthesis was employed using trisodium citrate and sodium thiosulfate. The resultant crystals of octahedral CoS2 with exposed {111} and {220} planes were revealed by HR-TEM and XRD studies. Detailed structure–activity insight regarding ORR was obtained using rotating ring disk electrode and electrochemical quartz crystal microbalance (EQCM) analysis and facet controlled octahedral CoS2 was shown to have the 50 fold increase in activity relative to other variants in an acidic medium and is comparable to a state-of-the-art Pt/C (20%) catalyst. The local catalytic activity of the CoS2 catalyst was visualized by the redox-competition mode of scanning electrochemical microscopy (RC-SECM).

Electrocatalytic Borohydride Oxidation by Supported Tungsten Oxide Nanoclusters Towards Direct Borohydride Fuel Cells

Tungsten oxide supported over mesoporous carbon rich in nitrogen was analysed for electrochemical oxidation of borohydride as a potential candidate for anodic catalyst in direct borohydride fuel cell. The fabrication of composite catalyst included a hard template method employing SBA-15 with optimal tungsten content. This non-noble catalyst with a minimal tungsten oxide loading of 5 % over the mesoporous support was found to exhibit appreciable current density with an onset potential comparable to noble metals. Besides reliable activity the composite also exhibited significant stability under highly alkaline conditions necessary to keep a check over the competent and notorious borohydride hydrolysis reaction occurring simultaneously. The effect of temperature was also studied by performing borohydride oxidation at various temperatures and the catalyst was found to support the activity even at elevated temperatures.