(647d) Electrodeposition of Thick Zinc Layers in a Microfluidic Parallel Plate Flow Channel

Evolution of the electrical grid to incorporate renewable generation by methods such as wind and solar will require grid level storage capability. Grid level energy storage will allow electrical load leveling and accommodate the periodic nature of wind and sunlight. The need for secondary batteries of MW or larger size has renewed interest in zinc as a rechargeable energy storage medium. Alkaline zinc battery systems avoid the drawbacks of lithium-based batteries?resource limitation and catastrophic reactivity?while providing higher energy density than lead-acid batteries.^1,2 Zinc is water compatible, inexpensive, stable, non-toxic, and has a low formal potential, making it an ideal basis material for a battery anode. However, the use of plated zinc metal anodes faces longstanding challenges due to the tendency of zinc to form high aspect ratio protrusions during electrodepostion, such as mossy protrusions, forked ramifications, and dendrites. Such formations can bridge the cell gap and cause shorting, material loss, and failure of the battery.

Suppression of surface roughness and dendritic morphologies in metal-anode secondary batteries is of critical importance. Even in situations when protrusions are not formed on initial battery charging, they form on successive charging cycles, when metal is redeposited at the negative electrode.³ Growing metal layers have been observed in situ previously, but generally under conditions of stagnant electrolyte or in geometries that were not analogous to those in a zinc metal battery.^4-6 The microfluidic electrochemical platform is well suited for this work, as mass transport is a major factor in the morphology of electrodeposited metals.^5,7 In microfluidic electrochemical systems, diffusion layers are well-defined at the working electrode interface, electrolyte compositions can be rapidly changed in situ, and visual imaging of the electrode surfaces is straightforward.⁸

A single-use microfluidic electrochemical cell was designed, intended for in situ monitoring of a metal layer electrodeposited from a flowing electrolyte stream, with working and counter electrodes in a lateral configuration and separated by a flow channel 90 um in height. A ZnO-KOH zincate electrolyte was passed over the electrodes at varying flowrates. Current-potential relationships in this cell were verified to be in accord with those recorded in a 3-electrode microfluidic electrochemical cell, which was reported previously.^8,9

Zinc was electrowon at the working electrode, with oxygen gas generation at the counter electrode. As the zinc layers grew, morphology was observed to be a function of deposition current, as has been found previously.¹⁰ At low current densities a dark, porous or ?mossy? zinc was formed. At higher current, morphology changed to a dense, crystalline zinc. At electrolyte flowrates of 0.3 and 3 cm/s, this transition was at a cell potential of -2.4 V, with current density higher in the case with more rapid flowrate. As the thickness of the zinc layer increased, the dense mode invariably lead to ramification, with protrusions shorting the cell eventually. Coulometry was used to evaluate the efficiency of zinc packing into a parallel plate system. This packing efficiency was a function of morphology, as compact zinc is more dense than porous zinc; however, ramifications can greatly decrease bulk density by forming rapid paths to the counter electrode. It was assumed that a flowrate of 0.3 cm/s or below was practical for modeling a zinc battery, as the power required for electrolyte pumping results in a loss of battery round-trip efficiency.

At high deposition rates, increasing electrolyte flow did not eliminate zinc protrusions, but increased their density on the electrode, increasing the total charge stored in the zinc layer. Rapid, in situ image analysis was also used to observe the current distribution on the developing electrode surface as a function of time. While ramifications may initiate in regimes of mass transport control, as the zinc layer thickens the ramification tips transition to a regime of kinetic control, and local current densities are independent of flowrate.

References

1. F. R. Mclarnon and E. J. Cairns, J Electrochem Soc, 138, 645 (1991). 2. S. I. Smedley and X. G. Zhang, J Power Sources, 165, 897 (2007). 3. Y. Ito and S. Banerjee, in AIChE 2009 Annual Meeting, Nashville, TN (2009). 4. D. Barkey, P. Garik, E. Benjacob, B. Miller and B. Orr, J Electrochem Soc, 139, 1044 (1992). 5. O. Crowther and A. C. West, J Electrochem Soc, 155, A806 (2008). 6. Y. Oren and U. Landau, Electrochim Acta, 27, 739 (1982). 7. D. P. Barkey, R. H. Muller and C. W. Tobias, J Electrochem Soc, 136, 2199 (1989). 8. J. W. Gallaway, M. J. Willey and A. C. West, J Electrochem Soc, 156, D146 (2009). 9. M. J. Willey and A. C. West, Electrochem Solid St, 9, E17 (2006). 10. R. D. Naybour, J Electrochem Soc, 116, 520 (1969).