Ms. Elena Praznikov - M.Sc. Candidate
20/04/2025
אודיטוריום ע"ש דויד וואנג, בניין מידן, קומה 3
13:30
Nickel (oxy)hydroxide electrodes are used as cathodes in rechargeable alkaline batteries and as anodes in alkaline water electrolysis. Their charge storage and oxygen evolution properties are combined to decouple water electrolysis in an electrochemical and thermally-activated chemical (ETAC) cycle which produces hydrogen and oxygen in different stages.
This work studies the phase transitions that occur upon electrode deposition, aging and activation and cyclic operation in charge and thermally activated self-discharge ETAC cycles. The phase composition was analyzed after these processes by ex-situ XRD and the oxidation state of nickel was analyzed by iodometric titration. The as-deposited Alpha-Ni(OH)2 transformed to Beta-Ni(OH)2 after aging and farther activated. The activated Beta-Ni(OH)2 electrodes were charged to Beta-NiOOH by limiting the maximum potential of CV oxidation peak (about 1.43 V vs. RHE). Without setting this potential limit, overcharging to a mixture of Gamma-NiOOH and Beta-NiOOH took place. Charged Beta-NiOOH electrodes could hardly be self-discharged, whereas overcharged Gamma-NiOOH was self-discharged to metastable Alpha-Ni(OH)2 that converted to Beta-Ni(OH)2 by aging in the concentrated alkaline electrolyte. Our observations shed light on the ETAC cycle, revealing that charged Beta-NiOOH electrodes hardly self-discharge during short-duration TAC stages (several minutes) and they need to be overcharged to Gamma-NiOOH in order to achieve a high capacity utilization in ETAC cycles.