posted on 2020-03-23, 15:34authored byDuhan Zhang, Alexander J. Warren, Gaojin Li, Zhu Cheng, Xiaoxing Han, Qing Zhao, Xiaotun Liu, Yue Deng, Lynden A. Archer
Strong
polarization of the ion distribution in liquid electrolytes
subjected to potential differences exceeding the thermal voltage, VT = kT/e,
produces a hydrodynamic instability termed electroconvection at ion-selective
interfaces. Electroconvection is desirable in some situations (e.g.,
electrodialysis) because it promotes mixing in a stagnant electrolyte
layer, enhancing the ion flux at a fixed potential difference. It
is undesirable in others (e.g., electrodeposition of metals) where
early experiments show that convective fluid rolls associated with
hydrodynamic instability in bounded electrolytes produce preferential
metal deposition at localized regions on an electrode. Such localized
deposition drives the family of morphological instabilities loosely
termed dendritic electrodeposition. We experimentally investigate
the effect of ultrahigh molecular weight polymer additives on the
onset conditions and physical characteristics of both instabilities.
Direct observations of electrodeposit morphology and tracer particle
motions used in tandem with indirect electrokinetic measurements reveal
that even at moderate concentrations, the polymer additives have a
large beneficial effect in extending the range of electric potentials
where stable electrodeposition is observed. Additionally, we report
that at polymer concentrations above the entanglement threshold, high
molecular weight polymers impart elasticity to liquid electrolytes
which dampen electroconvective flow at a cation-selective interface
but have at most a minimal effect on the bulk ionic conductivity of
the liquid.