(218e) Gas Evolution In a Flow-Assisted Zinc-Nickel Oxide Battery

Zinc-based rechargeable batteries are one
of the most attractive electrical energy storage systems due to its advantages
such as low cost and green. On the other hand, short cycle life due to internal
short circuit is the most critical disadvantage. We have carried out
experiments in flow-assisted zinc-nickel oxide batteries, and concluded that
flowing electrolyte contributes to the modification of zinc morphology and
extend the battery cycle life significantly. More than 1500 cycles have been
obtained while keeping the Coulombic efficiency at or above 90% by utilizing a
deep-discharge reconditioning step every 15 cycles [1].

                The
loss of Coulombic efficiency is attributed to remaining zinc on anodes on
discharge, hydrogen and oxygen evolution occurring as parasitic reaction, and
reasons such as corrosion in the sintered nickel electrodes. The extent of gas
evolution is determined by the balance of thermodynamics and kinetics of the
hydrogen and oxygen, and those of the main reduction-oxidation reactions in the
battery. We have conducted experiments on gas evolution in a sealed zinc-nickel
oxide battery for with and without flow assisted cases. The effects of flowing
electrolyte on gas evolution were quantitatively evaluated and the impacts on
battery performances were discussed.

                Metal
zinc as negative (anode) electrode was deposited on polished nickel coated
copper sheets as substrate. Sintered nickel oxide electrode was used as the
cathode. The test cell was a prismatic cell which has a capacity of 2.2 Ah.
Zinc oxide at a concentration of 60 g L^-1 was initially dissolved
into 45 wt% potassium hydroxide (KOH) solution as electrolyte. The electrolyte
flows from the bottom to the top of the cell at an average velocity of 3 mm s^-1
in the flowing case. Evolved gases were accumulated in the head space of the
cell. The concentration of the oxygen in the head space was measured by an
oxygen sensor. The internal pressure of the cell was measured by a pressure
transducer which is also attached to the head space of the cell. Galvanostatic
battery cycling experiments were carried out for a charge and discharge rates
of 1C and 0.5C, respectively. Charge was terminated when a battery cell was
fully charged to the full rated capacity of the sintered nickel oxide
electrodes, and discharge was terminated when the cell voltage dropped to 1.0
V.

                The
results show that, on charge, hydrogen is evolved from zinc (anode) side.
Because of the formation of concentration boundary layer and the lower
concentration of zinc (zincate) ion near the zinc anodes in the non-flowing
case, hydrogen evolution tends to occur more instead of zinc electrodeposition.
On the other hand, on discharge, hydrogen can theoretically be evolved from the
cathode (nickel oxide). However, from a thermodynamic point of view, hydrogen
evolution should not start until the cell voltage drops much lower, near 0 V.
Other possibilities are zinc corrosion and that hydrogen evolved on charging is
released from voids within the zinc electrodeposit during
discharging. Time elapsed images of the zinc anode in a cycle showed that the
evolved bubbles on charging, which are presumed to be hydrogen, do not detach
from the surface on charging and detach on discharging. Also, there are quite a
few bubbles that appear as zinc dissolves on discharging. From a thermodynamic
point of view, these bubbles should not be oxygen and, in fact, oxygen in the
cell doesn't increase on discharging. Therefore, these are expected to be
hydrogen which is generated on charging.

                On
the other hand, xygen is drastically evolved toward
the end of charge for both the cases. This oxygen evolution is from the
positive (nickel oxide) electrodes and this phenomenon in zinc–nickel oxide
batteries has been reported in many previous works. The results also show that
oxygen is evolved more in the non-flowing case than the flowing case. Though we
did not compare the differences of crystal structure of the nickel oxide
electrodes between the flowing and non-flowing cases, considering that oxygen
evolution basically occurs on overcharging, and that electrolyte (KOH)
concentrations affect the performance of sintered nickel oxide electrodes,
oxygen evolution tends to occur more than the reaction between nickel oxide and
hydroxide ion in the non-flowing case, because of the concentration gradient of
the hydroxide ion at the electrodes surfaces. On discharging, oxygen is not
evolved for both the cases, and the curves rather show a tendency of slight
decrease of oxygen in the cell. This indicates that oxygen is slightly
recombined on discharging, probably by the reduction on the positive plates.

[1] Y. Ito, M. Nyce, R. Plivelich, M. Klein, D. Steingart, S. Banerjee, J Power
Sources, 196, 2340-2345 (2011).