(3cf) Electrochemistry of Oxygen with Protons and Lithium and Their Role in Fuel Cells and Lithium-Air Batteries

With a growing energy demand, an energy economy based on cheap and sustainable energy sources such as solar and wind is likely necessary to power the planet in the coming years.   However, the issue of the intermittent availability of these renewable energy sources needs energy efficient storage options. The coupling of renewable sources to energy efficient energy conversion devices would also be necessary to make efficient utilization of the renewable energy.  Batteries and fuel cells stand at the forefront of energy storage and utilization options.  I will discuss my research work in the areas of fuel cells and lithium-air batteries.

The electrification of road transportation has been severely limited by the limited capacity of the current batteries and there has been a recent push towards more energy dense batteries beyond Li-ion batteries.  Of all the possibilites, Li-air is the most promising with a potential 5-10x gain in energy density.  These batteries can be exploited in two configurations: (a) non-aqueuous and (b) aqueous.  I will outline my work in both of these battery configurations.

There are many technical hurdles facing the non-aqueous Li-air batteries before they can become practical.  Using a combined experimental and theoretical approach, we have identified two key fundamental issues involved in lithium-air batteries - (a) the origin of the sudden death involved in Li-air batteries and (b) the presence of carbonate impurities and their role in affecting the rechargability of these batteries.  Using non-equilibrium transport calculations, we show that the origin of this ‘sudden death’ in flat electrode cells is due to the limitation imposed by electron transport to support the required electrochemistry. Using a simple metal-insulator-metal model, we show that discharge products beyond > 10 nm, have serious transport limitations and this leads to a bias rise across the film[1].

The fundamental issue during recharging the battery is associated with the presence of an impurity carbonate layer which has been recently identified experimentally. We develop a model incorporating details from the recent experiments and conclusively show that the origin of potential rise during recharge of the lithium-air batteries is due to a rising concentration of carbonate impurities at the Li₂O₂-electrolyte interface. This rise in the overpotential leads ultimately to the decomposition of the electrolyte and limits the rechargability of lithium-air batteries[2].

In the case of aqueous Li-air battery, although the inherent chemistry is more rechargable than the non-aqueous Li-air battery, fundamental issues associated with the discharge product and the need for precious metal-catalysts limit the practicality of this battery.  We develop the first continuum-scale model for the aqueous Li-air battery identifying some of the key performance limitations of these batteries[3].

On the avenue of energy utilization, fuel cells are an attractive option.  However, the electrochemistry associated with reduction of oxygen with protons has been severely limiting the performance of fuel cells.  In this regard, we have develop a detailed first-principles based mechanism of oxygen reduction reaction on (111) and (100) facet of Pt which is the best elemental catalyst for oxygen reduction[4,5].  Using the detailed mechanism, we address the effect of particle size and support on the electrocatalytic activity of oxygen reduction on Pt nanoparticles using a combination of first-principles DFT calculations and Dynamic Monte Carlo simulations.  We show that tuning the metal-support inteface could have a drastic effect on the electrocatalytic activity[6].  We have unified this analysis and shown that the activity trends on low-index facets of (111) and (100) follow a universal volcano[7].

This analysis has been used to understand effects of nanostructuring Pt monolayers on top of Rh(111).  We show that a 3D Pt particle on top of Rh(111) has enhanced activity compared to a 2D monolayer of Pt on top of Rh(111)[8].  This design principle of nanostructing has been used to carry out a computational search for new candidate materials and one of them was identified which has been tested experimentally.   This catalyst shows a higher specific activity than state-of-the-art TKK Pt/C catalyst.

Doctoral advisors: J. K. Nørskov and T. F. Jaramillo, Stanford University

As part of my PhD, I have been invovled in use and development of advanced computational methodologies in the exciting new area of 'engineering the new tools of scientific discovery'.  I have gained expertise in first-principles based calculations for thermodynamics of reactions on surfaces, hybrid functionals for describing electronic structure of oxide materials, non-equilibrium transport calculations for calculating conductivity of various interfaces.  I have developed Monte-Carlo based codes to examine complex geometric effects.  I also have a firm grasp of contiuum scale modelling with a detailed system model developed for a realistic battery.  A mix of engineering skills along with a deep fundamental understanding of physics of chemistry of surface processes and interface phenomenon puts me in a unique position to critically assess and develop the advanced energy-storage and conversion devices of the future.

Relevant publications:

[1] V. Viswanathan, K. Thygesen, J.S. Hummelshøj, J. K. Nørskov, G. Girishkumar, B.D. McCloskey, A. Luntz, J. Chem. Phys., 135, 214704 (2011).

[2] B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov and A. C. Luntz, J. Phys. Chem. Lett., (2012), 3, 997-1001.

[3] P. Albertus and V. Viswanathan, J. Electrochem. Soc. (in submission)

[4] V. Viswanathan, H. A. Hansen, J. Rossmeisl, T. Jaramillo, H. Pitsch,and J. K. Nørskov, J. Phys. Chem. C, (2012) 116 (7), 4698-4704.

[5] B.C. Han, V. Viswanathan, and H. Pitsch, J. Phys. Chem. C, (2012) 116 (10), 6174-6183.

[6] V. Viswanathan and F. Wang, Nanoscale, (2012) (accepted).

[7] V. Viswanathan, H. A. Hansen, J. Rossmeisl, and J. K. Nørskov, ACS Catalysis (2012) (submitted).

[8] D. Friebel, V. Viswanathan, D. J. Miller, A. Larsen, T. Anniyev, H. Ogasawara, J. K. Nørskov, and A. Nilsson, J. Am. Chem. Soc. (2012) (accepted).