Nanomaterials for Portable Power

Sanjeev Mukerjee (Chemistry), Nurcan Bac (Chemical Engineering) and Srinivas Sridhar (Physics)

The last two decades has seen tremendous strides towards portability of electronic devices. Though initially fueled by consumer electronics such as walkman etc., the focus has shifted to providing serious communication and computation capabilities in a small portable format. This revolution has been enabled primarily by enormous surge in the micro-electronic field providing us with a complete switch to a digital world. However in order to sustain these increasingly complex devices better power sources are required, which eliminate down time and have higher gravimetric and volumetric energy densities. The current and future power sources for these devices are based on electrochemical energy storage (batteries, ultra-capacitors etc) and or conversion (fuel cells).

It is paramount therefore for these electrochemical devices to keep pace with the advancement in miniaturization in the micro-electronic field. The present state of the art in these devices however relies on fundamentals, which have remained unchanged since the last fifty years. At a most fundamental level all these electrochemical devices depend on efficient charge transfer at the electrochemical interface. However the current state of the art is almost always based on a random dispersion of a reaction center in a conductive medium, which is rendered to provide efficient electronic and ionic pathway. In the case of energy storage (such as in secondary batteries, i.e., Li-ion batteries), these reaction centers are either intercalation compounds or nano-segregated materials which enable alloying and de-alloying. For energy storage, these are electro-catalysts, which enable better kinetics and selectivity to oxidation of the fuel and reduction of an oxidant. In all cases this involves a carbon support imbibed with an ion conducting agent.

The research in progress at Northeastern University is committed to providing a paradigm shift in thinking in order to enable better interfacial design for efficient charge transfer. There is a very strong multidisciplinary team present in various departments spread over both College of Arts and Sciences as well as in Engineering. This multidisciplinary research group draws on the expertise of three faculty members with rather different research expertise and is committed to providing a paradigm shift in thinking in order to enable better interfacial design for efficient charge transfer in Proton Exchange Membrane Fuel cells.

The principal thrust area is in Nanomaterials synthesis and spectroscopy. The science of nanocluster synthesis and its application as electrocatalysts and intercalation compounds is central research thrust in Professor Mukerjee's lab. Professor Bac's lab specializes in the synthesis and characterization of polymer membranes for gas separation. This effort is complimented by Professor Mukerjee's group, which is involved in development of novel proton conducting membranes possessing higher glass transition temperature as compared to current state of the art, based on perfluorinated sulfonic acid (Nafion type), with the ultimate goal of maintaining proton conductivity at lower relative humidity's. The aim is to understand at a fundamental level the dynamics of charge transfer at the surface (in the case of electrocatalysts) and in the bulk (such as in the case of intercalation compounds) as well as a molecular level understanding of transport processes in polymer membranes. Prospect of probing an electrochemical interface in situ, under actual operating conditions, and ability to map both the substrate (in terms of its electronic and short range atomic order) as well as the substrate-adsorbate interactions has tremendous technological implications. Synchrotron based far infrared and X-ray techniques (X-ray absorption, and scattering) offer such an opportunity as a consequence of the unique characteristics of the synchrotron source. These include higher: intensity (10⁴ higher), collimation, polarization, and pulse time structure enabling true in situ interfacial measurements. While far infrared spectroscopy is an emerging technique, x-ray absorption and scattering have recently evolved as a true in situ probe for electrochemical interface with both model and commercially relevant nano dispersed materials. The overall approach being pursued at Northeastern University is to combine these synchrotron based X-ray and far infrared spectroscopy, with novel materials synthesis for developing better fuel cell electrocatalysis and intercalation compounds.

Sridhar's group has a strong background in impedance measurement and analysis which has been used for measurements of bound and free water in electrolytes; these measurements form an important input for testing the computational approach.