59th Annual Report on Research 2014

1.      OBJECTIVES: Recent years has seen a significant impetus to address global environmental and energy challenges. Essential technological innovations include incorporating renewable energy sources and energy storage into the complex system of producing and distributing electricity through a smart grid.¹ Redox flow batteries promise to be a cheap and sustainable alternative for large scale energy storage.² Because of the intermittent nature of many renewable energy sources (e.g. wind and solar), developing a storage system to provide a reliable and stable source of power is essential and remains a significant challenge. Graphite felt (GF) electrodes are primarily used in redox flow batteries to provide high surface area for reaction, high conductivity, and high energy at a reasonably low cost. However, as of today, performance and durability of these electrodes are a significant concern. Additionally, the kinetics of the redox reactions which occur on the surface of these electrodes is poorly understood, and is inseparable from competing transport limitations. The primary objective of the project is to develop a detailed transport model to understand the kinetics of felt electrodes in a Thin Film Rotating Disk Electrode (TFRDE) set-up. The project also aims to develop a model to examine the surface heterogeneity and carbon corrosion that can lead to electrode damage and battery failure under high operating voltages. The insight gained from detailed interfacial studies of the GF electrode will provide insight on the effects of various surface modification protocols reported in the literature and suggest new pathways for electrode performance and durability improvement.      

2.      RESEARCH FINDINGS To understand the surface kinetics of graphite felt electrodes, physics based model for the TFRDE setup was developed taking into account, fluid flow and material balances. Rotating disk electrode technique is a very useful and powerful technique used commonly to study surface electrochemical reaction kinetics. The diffusion-controlled limiting current on the RDE surface is given analytically for a steady state condition by Levich equation:                                                                                                                         where i_L is the limiting current, n is the number of electrons transferred in the half-cell reaction, F is the Faraday constant, A is the electrode area, D is the diffusion coefficient, w is the angular rotation rate of the electrode, v is the kinematic viscosity, and C is the concentration of the reactant. For the TFRDE, calculation of limiting current by the Levich equation does not match the experimental data. This is because of the difference in flow and concentration fields as electrolyte is flowing inside the porous space in the electrode film and the reaction occurs on the surface of the fibers of porous electrode film. At present, competing transport and kinetic mechanisms are not properly understood. Therefore detailed studies based on simulations are required to characterize the reactions. To address these issues, the physics based model is developed. Fig. 1 shows the modeling domain in 2 D consisting of the porous disk and electrolyte. Fig. 2 presents the spatial concentration of species for low and high rotation rates. At low rates, the species react on the surface of the disk, as the concentration boundary layer is formed near the surface of the porous disk. But at higher rpms, ingression of concentration boundary layer was observed resulting in perfusion of bulk electrolyte into the porous felt. Fig. 3 shows the variation of total current in the film with rotation rate as a function of thickness of the film. At low rotation rates (< 634 rpm), a straight line or Levich-like behavior was noticed similar to characteristics of activation –controlled reaction region. But at higher rates of rotation (634 to 3791 rpm), limiting current results deviate from such behavior and also show dependence on thickness of electrode film. Moreover, in that zone the current increases very rapidly as visible from the sharp rising slope, a characteristic similar to mass transport-controlled region. In this region, Levich like behavior is affected because of the competing effects of diffusive and convective mass transfer. At very high rotation rates (3791 to 12000 rpm), total current hits a limiting value depending on electrode thickness mainly due to finite rate of felt kinetics, a characteristic typical to the kinetics-controlled region.    

Fig. 1. (a) Arrow plot for the velocity field in the 2D Porous RDE in both porous electrode and bulk electrolyte domain at 12,000 rpm (b) Inset of velocity field in the porous felt region and its vicinity.      

Fig. 2. Spatial variation of V ³⁺ species concentration (mol/m³) for low and high rotation rates with 0.75 mm thick porous felt   (A. 634 rpm & B. 3791 rpm)

Fig. 3.Variation of total current in the disk with rotation speed (rpm) as a function of film thickness.

3.      OUTCOMES This research project has yielded interesting results but further studies are required till our results are publishable. The PI just moved from WUStL to UW and has requested that the grant stays at WUStL (a no-cost extension will be requested) to make progress in the project and to fund the graduate student at WUStL. In addition, we plan to additional experiments to get relevant data for validation.  

4.      FUTURE RESEARCH The long term research plans include (i) study the effect of operating condition on the durability of the flow batteries (ii) use kinetic Monte-Carlo simulation techniques to understand and model the corrosion of carbon and the effect of surface heterogeneity on the electrochemical performance. In addition, we have initiated developing models for flow batteries and we will include the modified kinetics from this work into the same.   References 1.      J. Eyer, G. Corey, Energy Storage for the Electricity Grid: Benefits and Market Potential Assessment Guide, in:  Sandia Report, Sandia National Laboratories, Albuquerque, NM, 2010.

2.      T. Nguyen, R. Savinell, Flow Batteries, in:  The Electrochemical Society Interface, vol. 19, The Electrochemical Society, New Jersey, USA, Fall 2010, pp. 54-56.