Year: 2026
Type: preprint
Abstract: Nickel oxyhydroxide electrodes undergo spontaneous self-discharge that is strongly accelerated at elevated temperature. Although detrimental to alkaline batteries, this reaction is central to electrochemical–thermally activated chemical (E‑TAC) water splitting. The process operates in two distinct steps: electrochemical charging of Ni(OH)2 in cold (~25 °C) alkaline electrolyte and hydrogen generation at the cathode, followed by spontaneous regeneration of the charged NiOOH anode in hot (~80–100 °C) alkaline electrolyte, with concurrent oxygen evolution. A key unresolved question is how the charged-phase composition of NiOOH affects this thermally activated self-discharge. Here, we prepare phase-pure cobalt-doped β-NiOOH and γ-NiOOH electrodes using tailored electrochemical charging protocols and compare their short-term self-discharge in hot alkaline electrolyte simulating the TAC step in the E-TAC cycle. X-ray diffraction and iodometric titration show that phase-pure β-NiOOH undergoes little or no measurable structural or oxidation-state change after 5 min in 5 M KOH at 80 or 100 °C. In contrast, phase-pure γ-NiOOH exhibits substantial self-discharge, corresponding to 43-46% of the charge transferred during the preceding charging step, accompanied by partial transformation to α-Ni(OH)2. These results demonstrate that thermally activated self-discharge in charged nickel hydroxide electrodes is strongly phase-dependent under E-TAC-relevant conditions. More broadly, they show that γ-NiOOH, which is usually regarded as an undesirable overcharged phase in alkaline batteries, is closely associated with rapid chemical regeneration and therefore should be considered a key design variable in E-TAC anodes.
Source: ChemRxiv
Language: English
Field: Energy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7212009866