N. Rajalakshmi

Nitrogen doped graphene nanoplatelets as catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell

Graphene nanoplatelets have been synthesized by thermal exfoliation of graphitic oxide and nitrogen doped graphene nanoplatelets have been obtained by nitrogen plasma treatment. Graphene nanoplatelets and nitrogen doped graphene nanoplatelets have been used as a catalyst support for platinum nanoparticles for oxygen reduction reactions in proton exchange membrane fuel cells. Platinum nanoparticles were dispersed over these support materials using the conventional chemical reduction technique. The morphology and structure of the graphene based powder samples were studied using X-ray diffraction, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. A full cell was constructed with platinum loaded nitrogen doped graphene nanoplatelets and the results have been compared with platinum loaded graphene nanoplatelets. A maximum power density of 440 and 390 mW cm−2 has been obtained with platinum loaded nitrogen doped graphene and platinum loaded graphene nanoplatelets as ORR catalysts respectively. Nitrogen plasma treatment created pyrrolic nitrogen defects, which act as good anchoring sites for the deposition of platinum nanoparticles. The improved performance of fuel cells with N-G as catalyst supports can be attributed to the increased electrical conductivity and improved carbon–catalyst binding.

Synthesis of graphene-multiwalled carbon nanotubes hybrid nanostructure by strengthened electrostatic interaction and its lithium ion battery application

We report a novel way of synthesizing graphene-carbon nanotube hybrid nanostructure as an anode for lithium (Li) ion batteries. For this, graphene was prepared by the solar exfoliation of graphite oxide, while multiwalled carbon nanotubes (MWNTs) were prepared by the chemical vapor deposition method. The graphene–MWNT hybrid nanostructure was synthesized by first modifying graphene surface using a cationic polyelectrolyte and MWNT surface with acid functionalization. The hybrid structure was obtained by homogeneous mixing of chemically modified graphene and MWNT constituents. This hybrid nanostructure exhibits higher specific capacity and cyclic stability. The strengthened electrostatic interaction between the positively charged surface of graphene sheets and the negatively charged surface of MWNTs prevents the restacking of graphene sheets that provides a highly accessible area and short diffusion path length for Li-ions. The higher electrical conductivity of MWNTs promotes an easier movement of the electrons within the electrode. The present synthesis scheme recommends a new pathway for large-scale production of novel hybrid carbon nanomaterials for energy storage applications and underlines the importance of preparation routes followed for synthesizing nanomaterials.

Hydrogen storage in carbon nanotubes and related materials

Adsorption of hydrogen at 300 K has been investigated on well-characterized samples of carbon nanotubes, besides carbon fibres by taking care to avoid many of the pitfalls generally encountered in such measurements. The nanotube samples include single- and multi-walled nanotubes prepared by different methods, as well as aligned bundles of multi-walled nanotubes. The effect of acid treatment of the nanotubes has been examined. A maximum adsorption of ca. 3.7 wt% is found with aligned multi-walled nanotubes. Electrochemical hydrogen storage measurements have also been carried out on the nanotube samples and the results are similar to those found by gas adsorption measurements.

Electrochemical investigation of single-walled carbon nanotubes for hydrogen storage

Abstract Electrodes made of purified and open single walled carbon nanotubes behave like metal hydride electrodes in Ni–MH batteries, showing high electrochemical reversible charging capacity up to 800 mAh g −1 corresponding to a hydrogen storage capacity of 2.9 wt% compared to known AB 5 , AB 2 metal hydride electrodes.

Nanostructured Pt Dispersed on Graphene-Multiwalled Carbon Nanotube Hybrid Nanomaterials as Electrocatalyst for PEMFC

Nanostructured platinum dispersed on functionalized graphene and functionalized multiwalled carbon nanotube [Pt/(f-G-f-MWNT)] hybrid nanomaterials, a unique combination of three-, two, and one-dimensional structures, were used as an electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC). Electrochemical studies performed on Pt/(f-G-f-MWNT) composite materials by varying the ratio of the composition of f-G and f-MWNT for the investigation of the electrochemical active surface area (ECSA) have resulted in an ECSA as high as 108 m 2 /g for the Pt dispersed on nanocomposite containing equal proportions of f-G and f-MWNT. Polarization graphs for the ORR reaction in PEMFC with Pt/(f-G-f-MWNT) as an electrocatalyst resulted in the best performance of 540 mW/cm 2 for the Pt/(50 wt % f-G + 50 wt % f-MWNT) cathode catalyst, agreeing with the electrochemical active surface area of Pt, due to good accessibility and uniform dispersion of the nanostructured Pt catalyst dispersed on the f-G-f-MWNT catalyst support, making them a suitable electrocatalyst for advanced PEMFC.

Humidification studies on polymer electrolyte membrane fuel cell

Two methods of humidifying the anode gas, namely, external and membrane humidification, for a polymer electrolyte membrane fuel (PEMFC) cell are explained. It is found that the water of solvation of protons decreases with increase in the current density and the electrode area. This is due to insufficient external humidification. In a membrane-based humidification, an optimum set of parameters, such as gas flow rate, area and type of the membrane, must be chosen to achieve effective humidification. The present study examines the dependence of water pick-up by hydrogen on the temperature, area and thickness of the membrane in membrane humidification. Since the performance of the fuel cell is dependent more on hydrogen humidification than on oxygen humidification, the scope of the work is restricted to the humidification of hydrogen using Nafion® membrane. An examination is made on the dependence of water pick-up by hydrogen in membrane humidification on the temperature, area and thickness of the membrane. The dependence of fuel cell performance on membrane humidification and external humidification in the anode gas is also considered.

Evaluation of current distribution in a proton exchange membrane fuel cell by segmented cell approach

A technique is developed for the determination of the distribution of current density in operating fuel cells using a segmented cell structure concept. Real-time current density distribution data are presented and it is shown that they can contribute to an improved understanding of the reactant distribution over the active fuel cell area, for optimization of fuel cell performance. The technique also offers an alternative to a commercial polymer electrolyte membrane fuel cell stack in the small power range. When stacking several segmented structure electrodes, the resulting volume can be considerably reduced compared with a state-of-the-art stack.

Development of polymer electrolyte membrane fuel cell stack

Abstract The proton exchange membrane fuel cell (PEMFC) is one of the strongest contenders as a power source for space, electric vehicle and domestic applications. Since 1988 intensive research is being carried out at our centre to develop PEMFCs. The main RandD activities are: (i) to develop a method for the electrode preparation (ii) to enhance platinum utilisation using low platinum loading and (iii) to design multicell stacks. The results of RandD development of the above activities are discussed in this paper.

Solar exfoliated graphene–carbon nanotube hybrid nano composites as efficient catalyst supports for proton exchange membrane fuel cells

Ultra thin graphene–multi walled carbon nanotube composites prepared by a solar exfoliation technique have been explored as catalyst supports for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). Pt nanoparticles were dispersed on a solar exfoliated graphene-functionalized multi walled carbon nanotube (sG-fMWNT) hybrid nanocomposite, which exhibits higher electrocatalytic activity for ORR than Pt dispersed functionalized solar graphene (f-sG) catalyst support. The single cell PEMFC measurements give maximum power densities of 355 mW cm−2 and 675 mW cm−2 with sG and sG-f MWNT respectively. The improved performance in the power density with the sG-f MWNT fuel cell can be ascribed to the synergistic effect of 1D MWNT and 2D graphene in sG-f MWNT as well as its high electrochemical surface area. The inclusion of MWNT bridges the defects for electron transfer besides increasing the basal spacing between graphene sheets. The good performance and possibility of high throughput production of sG-f MWNT makes this material a promising catalyst support for PEMFC.

Analysis of Flow Maldistribution of Fuel and Oxidant in a PEMFC

The flow of fuel and oxidant through a PEMFC is analyzed for prediction of maldistribution. Flow distribution of both fuel and oxidant from the port to the individual cells critically control the performance of a PEMFC stack in combination. The distribution of fluids was simulated by analytical approach utilizing flow channeling model of a manifold. A detailed numerical modeling is also carried out considering flow in each cell between the electrodes as flow through an equivalent porous medium offering identical resistance. The results show a close match between the analytical and numerical results. The parametric study reveals that flow rate and port size plays major role determining maldistribution of the fluids, which can be considerably skewed when large numbers of cells are stacked for larger power output.